Asphalt surfacing to bridge decks

Transcription

1 Asphalt surfacing to bridge decks Prepared for SSR Directorate (Highways Infrastructure) Pavement Engineering Team, Highways Agency J C Nicholls, R W Jordan and K E Hassan TRL Report TRL655

2 First Published 2006 ISSN ISBN X Copyright TRL Limited This report has been produced by TRL Limited, under/as part of a contract placed by the Highways Agency. Any views expressed in it are not necessarily those of the Agency. TRL is committed to optimising energy efficiency, reducing waste and promoting recycling and re-use. In support of these environmental goals, this report has been printed on recycled paper, comprising 100% post-consumer waste, manufactured using a TCF (totally chlorine free) process. ii

3 CONTENTS Page Executive Summary 1 1 Introduction 3 2 Questionnaire Format Responses 3 3 Laboratory test programme Asphalt mixture properties Objectives and test programme Air voids content Permeability Indirect Tensile Stiffness Modulus Wheel-tracking Review of asphalt results Asphalt flexibility Objectives and test programme Indirect tensile fatigue test point bending fatigue test Semi-circular bending test Asphalt bending test Discussion Waterproofing system properties Objectives and test programme Torque bond tests Interface permeability 16 4 Considerations for specification of surfacing for bridge decks General approach Drainage Drainage requirements Removal of water Sub-surface drainage Waterproofing system Bond to asphalt Membrane stiffness Effect of laying and compaction temperatures on the waterproofing system 19 iii

4 Page 4.4 Asphalt properties Deformation resistance Texture depth Skid resistance Flexibility and fatigue Permeability and air voids content Protection for waterproofing systems Joints and interface between layers General Joints Bond between layers Bond to waterproofing system Permeability of interface between asphalt and waterproofing system 23 5 Conclusions 24 6 Acknowledgements 24 7 References 25 Appendix A: HA draft notes for bridge-deck overlays 27 Appendix B: Questionnaire 28 Appendix C: Permeability tests 32 Appendix D: Semi-circular bending test 33 Abstract 40 Related publications 40 iv

5 Executive Summary The current requirements in the Specification for Highway Works require waterproofing systems on concrete bridge decks to be overlaid with a 20 mm thick sand asphalt protection layer and binder and surface courses so that the total thickness of the three layers is 120 mm. These requirements include that the material that directly overlays the waterproofing system should be sand asphalt as well as considerations about the bond of the surfacing to the waterproofing system and the sub-surface drainage. In service, it has been found that the performance of surfacing on concrete bridges is generally satisfactory if the total thickness of the asphalt layers is at least 120 mm. However, the total thickness on some bridges has to be reduced for practical and/or economic reasons and, in such cases, a number of premature failures have occurred when the asphalt has broken up and potholes have developed. Therefore, HA commissioned TRL to develop a specification for the asphalt surfacing on bridge decks that is suitable even when the surfacing has a total thickness of less than 120 mm. The objective of this work was to investigate parameters that would allow the development of a specification for surfacings on concrete bridges that enhances the probability of achieving reasonable durability when they are less than the standard thickness. The research has included a literature search, a questionnaire and laboratory test programmes. The literature search provided little useful information. The results from the questionnaire, sent to individuals and companies involved in the manufacture and laying of the current materials used on bridge decks, were disappointing, with only eight responses received. The laboratory test programme of asphalt mixtures that have been, or could be, used for surfacing bridge decks has identified some differences in their properties that have been used as the basis for the specification. A supplementary test programme was also undertaken to look at tests for measuring the flexibility of asphalt materials, an important property for both bridge deck surfacings (particular those with a thin deck) and other situations with relative thin pavements over soft substrates. Also, tests were undertaken on composite samples of concrete, waterproofing and asphalt to assess the bond achieved. From the tests on a series of twelve asphalt mixtures, mastic asphalt was found to have the most suitable air voids content, permeability and stiffness modulus properties and was fifth at wheel-tracking, making it the best material overall for these tests. However, mastic asphalt is a relatively expensive mixture, a factor that cannot be excluded. A dense 0/10 SMA was the next best despite having the 8 th highest air voids content whilst an open 0/10 SMA was the worst, showing that the precise mixture design can be critical. The remaining mixtures showed relatively similar overall ratings, but with the sand carpet only ranked 8 th overall. Nevertheless, when considering the appropriate materials, the choice is often a trade off between properties and they will not usually have equal ranking. Material optimisation is usually avoiding any excessively adverse property rather than getting the best performance in all. The flexibility test programme involved applying four different tests to three control mixtures and six stone mastic asphalt mixtures. The tests undertaken were the indirect tensile fatigue test, the 4-point bending fatigue test at low temperature and low frequency, the semi-circular bending test from the Netherlands and an asphalt bending test from China. The results obtained from the test programme are reviewed together with theoretical considerations and the semi-circular bending test is identified as the most promising. This test was found to be practical, equivalent to a controlled stress fatigue test and ranks materials similarly to binder content, a known component of flexibility if other things are constant. It is suggested that initially values of 3.0 N/mm 2 and 19 N/mm 3/2 could be set as the minimum values required for tensile strength and fracture toughness, respectively. These limits are considered practical because they were exceeded by the majority of the mixtures tested. The results from the torque bond test on specimens including waterproofing systems without secondary compaction varied by an order of magnitude, with the two waterproofing systems used ranking the surfacing materials differently. In all cases, the failures were at the interface between the waterproofing membrane and the tack/bond coat, so the failure stresses were more dependent on the properties of the waterproofing systems than the asphalt mixtures. For one waterproofing system, the highest failure stresses were measured with a tack coat and sand carpet whilst, for the other waterproofing system, the failure stresses differed for two different tack coats. The high values for mastic asphalt were probably due to the high temperature at which the material was laid and compacted and at which the tack/bond coat was activated. Based on these findings and other considerations, various additions and changes to the Design Manual for Roads and Bridges, Specification for Highway Works and Notes for Guidance on the Specification for Highway Works have been proposed. The main changes include:! Sub-surface drainage is emphasised.! Bond requirements are strengthened.! Deformation requirements are specified for all mixtures within 100 mm of the surface.! Maximum air voids content limits on all asphalt mixtures. Aspects that were not fully covered are permeability testing of the asphalt and at the interfaces. Potential tests have been identified that could be developed for standardisation if these aspects are considered critical. The asphalt permeability can be covered by the surrogate of air voids content, but this approach is more difficult at interfaces where more than one material is involved. 1

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7 1 Introduction The current clause in the Specification for Highway Works (MCHW 1) requires waterproofing systems on concrete bridge decks to be overlaid with a 20 mm thick sand asphalt protection layer and binder and surface courses so that the total thickness of the three layers is 120 mm. A previous research project by TRL for the Highways Agency (HA) found that the performance of surfacing on concrete bridges is generally satisfactory if the total thickness of the asphalt layers is at least 120 mm. However, the total thickness on some bridges has to be reduced for practical and/or economic reasons and, in such cases, a number of premature failures have occurred when the asphalt has broken up and potholes have developed. The failures have been attributed to a number of factors, including:! The accumulation of sub-surface water in the asphalt.! Poor bond of the asphalt layers to the waterproofing system.! Low compressive modulus of the waterproofing system.! Low stiffness modulus of the asphalt layers. Therefore, HA commissioned TRL to develop a specification for the asphalt surfacing on bridge decks that is suitable even when the surfacing has a total thickness of less than 120 mm. The specification is intended to develop further the existing HA advice for bridge deck surfacing, which is reproduced as Appendix A. The research has included a literature search, questionnaire and two laboratory test programmes. The results from the literature search and the questionnaire to those that manufactured and laid the current materials used on bridge decks were disappointing, with only eight responses received from the questionnaire. However, the initial laboratory test programme of asphalt mixtures that have been, or could be, used for surfacing bridge decks has identified some differences in their properties that have been used as the basis for the specification. The second laboratory test programme identified a test method for assessing the flexibility of asphalt materials. 2 Questionnaire 2.1 Format A questionnaire was issued to individuals and companies involved in the manufacture and laying of the current materials used on bridge decks in order to ascertain if they were aware of any deficiencies in the present systems and, probably more importantly, whether they considered that some new materials, or combination of materials, could produce better results. It was anticipated that the replies would focus on practical aspects. There was a deliberate policy to keep the questionnaire relatively short but to allow recipients to extend their thoughts if they wished. The questionnaire, which is reproduced as Section B.1 of Appendix B, was sent to a total of 32 people from 23 different organisations. 2.2 Responses The reaction to the questionnaire was very limited with only eight responses received, of which only four contained useful comments. The contents of the constructive replies are given in Section B.2 of Appendix B. However, any conclusions from the survey can only be regarded as indicative. The replies were not consistent other than that the aim should be to produce surfacings with a service life of about 15 years. This aim is consistent with the top end of the durability found for thin surfacing systems in conventional situations (Nicholls and Carswell, 2004) given that the surfacing will need to be maintained at, if anything, a higher standard than typically on a highway. Nevertheless, it is an indication of what engineers, let alone the driving public, expect. One reply indicated reservations about currently used materials but the respondent did not think anything else currently available would produce a practical alternative that was economic. A second respondent offered epoxy asphalt, but that tends to be an expensive product that presents considerable risks if there is any distance between the batching plant and the site. It is, therefore, only appropriate for important and/or conveniently located bridges, although it was for such site that the responder put it forward. Another respondent did identify red sand carpet as a particular material that should be replaced. This replacement was already being considered because the limited deformation resistance it provides will become more of a liability as the thickness of material over it reduces. Nevertheless, some material with relatively small aggregate particles will be needed to protect some types of the waterproofing systems from being punctured. 3 Laboratory test programme 3.1 Asphalt mixture properties Objectives and test programme The object of the initial laboratory test programme on asphalt mixtures was to identify the values achieved by various materials in tests used to measure the properties assumed to be relevant to bridge deck surfacings. These properties are air voids content, permeability, stiffness and deformation resistance. Air voids content and permeability were investigated because drainage of water is particularly important on bridges whereas the stiffness and deformation resistance are required as standard properties needed for all surfacings. Flexibility was not included within this programme because there are no accepted tests to define the property, but is covered separately (Section 3.2). The results were required in order to be able to specify achievable levels of performance for future surfacing. The known limitations on the site performance of some of the materials for specific parameters can be used to ensure that the requirements do not allow it to be used where it would not perform satisfactorily. 3

8 The materials tested had to represent those that have been, or could be, used on bridge decks. The mixtures that were considered covered the range of potential mixture types that might be used on bridge decks whilst not being too extensive. The mixtures included a variety of hot rolled asphalts (HRA), dense bitumen macadams (DBM), stone mastic asphalts (SMA) and mastic asphalts. From these, the following twelve mixtures were selected for studying in the initial test programme:! Mastic asphalt BS 1447 (BSI, 1988).! 0 % 0/2 HRA (Sand carpet) Column 6/1 in BS 594 (BSI, 2002).! 30 % 0/10 HRA surface Column 6/3 in BS 594 course (BSI, 2002).! 35 % 0/14 HRA surface Column 3/3 in BS 594 course (BSI, 2002).! 55 % 0/14 HRA base Column 2/2 in BS 594 (roadbase), binder course (BSI, 2002). and regulating course! 55 % 0/10 HRA base Column 2/1 in BS 594 (roadbase), binder course (BSI, 2002). and regulating course! 0/20 DBM binder course Tables 15 and 16 in BS 4987 (BSI, 2001).! 0/6 SMA Table 2, column 3 of pren (CEN, 2000a).! 0/10 SMA (open) Table 2, column 5 of pren (CEN, 2000a).! 0/10 SMA (dense) Table 2, column 5 of pren (CEN, 2000a).! 0/14 SMA (open) Table 2, column 6 of pren (CEN, 2000a).! 0/14 SMA(dense) Table 2, column 6 of pren (CEN, 2000a). For convenience, the sieve sizes current at the start of the work were used to define the gradings for the first seven mixtures rather than those implemented in Similarly, the SMA mixtures were derived from gradings given in the draft for development of the harmonised European Standard (CEN, 2000a) after conversion to the old sieve sizes. The target gradings, based on the mid point of the envelopes for most of the mixtures but on the envelope boundaries for the target gradings of 0/10 and 0/20 SMA mixtures to give the permitted extremes, are given in Table 3.1. Four 300 mm by 300 mm by 50 mm thick slabs were mixed to BS EN (CEN, 2004a) and compacted by roller-compactor to BS EN (CEN, 2003a) for each of the mixtures. Five 100 mm diameter cores were then cut from two of the slabs of each material in accordance with BS EN (CEN, 2000b). One core from each cored slab was tested, consecutively, for bulk density to BS EN (CEN, 2002a), indirect tensile stiffness modulus at 20 C and then at 40 C to BS DD 213 (BSI, 1993) and maximum density to BS EN (CEN, 2002b). The air voids contents were then calculated in accordance with BS EN (CEN, 2003b) using the bulk density and maximum densities. Table 3.1 Composition of mixtures 30% 35% 55% 55% Mastic Sand 0/10 0/14 0/14 0/10 Type asphalt carpet HRA HRA HRA HRA 28 mm 20 mm mm mm mm mm mm mm mm mm mm Binder content Binder grade 15 pen 50 pen 50 pen 50 pen 100 pen 100 pen 0/10 0/10 0/14 0/14 0/20 0/6 SMA SMA SMA SMA Type DBM SMA (open)* (dense) (open) (dense) 28 mm mm mm mm mm mm mm mm mm mm mm Binder content Binder grade 100 pen 50 pen 50 pen 50 pen 50 pen 50 pen * Also 0/10 PMB SMA (open) mixture except for binder grade for the flexibility programme (Section 3.2). A separate core from the cored slab was tested for bulk density and the air voids content calculated in accordance with BS EN (CEN, 2003b) using the maximum density for the previous core from the same slab. The permeability of the cores was then measured using the TRL permeability cell, as described in Section C.1 of Appendix C. The remaining two slabs of each mixture were tested for wheel-tracking in accordance with BS EN (CEN, 2003c) using Procedure A with the small size device, one slab at 45 ºC and the other at 60 ºC Air voids content The results of the density and air voids content measurements are given in Table 3.2, where ITSM and Permeability refer to the other test carried out on the samples on which the measurements were made. The bulk densities measured varied within a range of 2.18 to 2.46 Mg/m³ and the maximum densities within a range of 2.27 to 2.51 Mg/m³. These values are of limited value in themselves because they will be more dependent on the density of the aggregate than the properties of the asphalt. Such variation is hidden for this investigation because aggregate from the same source was used for all mixtures. However, the combination of bulk and maximum density to produce air voids content is 4

9 Table 3.2 Air voids content results Sample Bulk density Max density Air voids content Material ID ITSM Permeability ITSM ITSM Permeability Mastic asphalt A % 0.4 % B % -0.4 % Mean % 0/2 HRA A % 2.1 % (Sand carpet) B % 3.9 % Mean % 30 % 0/10 HRA A % 1.4 % B % 2.5 % Mean % 35 % 0/14 HRA A % 4.7 % B % 2.7 % Mean % 55 % 0/14 HRA A % 2.7 % B % 3.7 % Mean % 55 % 0/10 HRA A % 2.4 % B % 1.9 % Mean % 0/20 DBM A % 3.0 % B % 2.8 % Mean % 0/6 SMA A % B % Mean % 0/10 SMA A % 8.3 % (open) B % 8.7 % Mean % 0/10 SMA A % 2.8 % (dense) B % 3.6 % Mean % 0/14 SMA A % 7.2 % (open) B % 7.1 % Mean % 0/14 SMA A % 3.0 % (dense) B % 2.6 % Mean % significant. For the mixtures tested, the air voids content ranged from zero for the mastic asphalt to 8.5 % for the 0/10 SMA (open) mixtures. The width of the range and the ranking of mixtures within it are consistent with what was expected, with the denser, more binder rich mixtures having less air voids content than the more open mixtures. The relationships between the binder content of the various mixtures and their densities are shown in Figure 3.1 and that between the binder content and air voids content in Figure 3.2. Included on the figures are also linear trend lines, the equations for which are given as Equations (3.1) to (3.3). ρ bulk = b c (R 2 = 0.34)...(3.1) ρ max = b c (R 2 = 0.84)...(3.2) ν air = b c (R 2 = 0.014)...(3.3) where: ρ bulk = bulk density (Mg/m³) ρ max = maximum density (Mg/m³) ν air = air voids content (%) b c = binder content (%) There is little correlation between binder content and air voids content because the binder contents are selected based on the mixture type and aggregate skeleton. However, the correlation between binder content and maximum density is surprisingly robust Permeability The permeability testing was carried out on asphalt cores (100 mm diameter and height between 70 mm and 100 mm). Air was applied at the desired pressure (P) and the flow 5

10 Density (Mg/m³) Max density Bulk density Binder content (%) Figure 3.1 Relationship between binder content and density 10.0 Air voids content (%) Binder content (%) Figure 3.2 Relationship between binder content and air voids content rate was calculated using a bubble flowmeter with diameter, D f, length, L f, and the average flow time, t average. The flow rates were used for the calculation of the air permeabilities, as described in Section C.1 of Appendix C, and the results are given in Table 3.3. The permeability ranking of the different asphalt mixtures is shown in Table 3.4. The lowest permeability values were obtained from the mastic asphalt mixture, which exhibited the lowest air voids content of around 0 %. The permeability of the HRA mixtures varied over a range of two orders of magnitude for air voids content between 2.3 % and 5.4 %. The permeability and air voids content of the sand carpet and the 50 % stone HRA mixtures were lower than those of the 30 % and 35 % stone HRA mixtures. In fact, for the same aggregate size (0/14), increasing the aggregate content from 35 % to 55 % reduced the air voids content from 5.4 % to 2.4 % and the permeability by approximately two orders of magnitude. The DBM and dense SMA mixtures gave permeability values within the lower band of HRA. In contrast, the high air voids content of the 0/6 and open SMA mixtures with >5 % air voids contents resulted in high permeability values. These values were out of the limit of the flowmeter used with the TRL permeability cell. The results in Table 3.4 shows that the permeability ranking, from low to high, of the different asphalt mixtures is generally as follows: mastic asphalt, HRA, DBM, dense SMA and open SMA. The most obvious outlier is 35 % 0/14 HRA, which has a higher air voids content and, to a greater extent, permeability than the other HRA mixtures. Nevertheless, this ranking is quite similar to that reported earlier (Daines, 1995). It is important to highlight that the permeability results are obtained from laboratory specimens with high quality control, which may not represent the actual values of the same mixtures constructed on site. However, the ranking is expected to be the same. Figure 3.3, with the data points plotted on a previously found relationship (Zoorob, 1999), shows the influence of air voids content on the permeability of asphalt. Whilst there are not adequate data to establish a reliable relationship, particularly in terms of mixtures with high air voids contents, a similar trend can be fitted to that found for concrete. Regardless of the mixture type, low permeability values are obtained when the air voids content is 4 % or below and the permeability increases rapidly for air voids contents above 5 %. In the absence of a specification limit for permeability, it appears reasonable to use the air voids content as an indicator for permeability Indirect Tensile Stiffness Modulus The results of the Indirect Tensile Stiffness Modulus (ITSM) test are given in Table

11 Table 3.3 Permeability results Sample Flow meter Permeability (10-17 m2) Df Lf P t average Material ID (mm) (mm) (bar) (s) Sample Mean Mastic asphalt A B % 0/2 HRA A B % 0/10 HRA A B % 0/14 HRA A B % 0/14 HRA A B % 0/10 HRA A B /20 DBM A Permeable 3.41 B /6 SMA A Permeable Permeable B Permeable 0/10 SMA (open) A Permeable Permeable B Permeable 0/10 SMA (dense) A B /14 SMA (open) A Permeable Permeable B Permeable 0/14 SMA (dense) A B Table 3.4 Permeability ranking of the asphalt mixtures Air voids Air permeability Mixture content (%) (10-17 m2) Mastic asphalt < HRA DBM Dense SMA Open SMA Permeable Air permeability (10-17 m 2 ) Air voids (%) Figure 3.3 Influence of air voids content on the permeability of asphalt Table 3.5 ITSM results 20 C (GPa) 40 C (GPa) Material A B Mean A B Mean Mastic asphalt /2 HRA % 0/10 HRA % 0/14 HRA % 0/14 HRA % 0/10 HRA /20 DBM /6 SMA /10 SMA (open) /10 SMA (dense) /14 SMA (open) /14 SMA (dense) The ITSM of the mixtures ranged from 1.0 GPa to 5.8 GPa at 20 C and 0.09 GPa to 0.55 GPa at 40 C. However, the results for mastic asphalt are effectively outliers, with the range reducing to 1.0 GPa to 2.2 GPa at 20 C and 0.09 GPa to 0.27 GPa at 40 C without them. The high value for mastic asphalt is probably due to the high binder content giving relative high tensile strength. Ignoring mastic asphalt, the lack of any correlation between ITSM and air voids content is shown in Figure 3.4. The plotted trend line at 20 C, including mastic asphalt, has a correlation coefficient of R² = 0.23, but the value reduces to when the mastic asphalt is omitted. 7

12 6.0 Indirect tensile stiffness Modulus (kpa) o C 40 o C Air voids content (%) Figure 3.4 Stiffness modulus variation with air voids content and temperature The effect of temperature on ITSM is also shown in Figure 3.4. The ratio of ITSM at 20 C to ITSM at 40 C ranged from 6.9 to 15.4 for the mixtures tested with the ranking order being marginally different for the two temperatures, as shown in Table 3.6. Table 3.6 Ranking of ITSM results ITSM (GPa) ITSM 20 C Mixture 20 C 40 C 20 C 40 C Mean / 40 C Mastic asphalt % 0/14 HRA % 0/10 HRA /10 SMA (dense) = /20 DBM /6 SMA = /14 SMA (dense) = % 0/10 HRA = /2 HRA = /14 SMA (open) = % 0/14 HRA = /10 SMA (open) The ranking orders are similar for the two temperatures, but not identical. The top three materials are the same in both with the material that has the greatest change in ranking being 55 % 0/10 HRA, which is 11 th at 20 C but 5 th = at 40 C. The differences demonstrate that the choice of test temperature will be important if the asphalt stiffness needs to be a parameter when specifying the stiffness of the waterproofing system Wheel-tracking The results of the wheel tracking test are given in Table 3.7 with the plots of the tests undertaken at 45 ºC and 60 ºC been shown in Figure 3.5 and Figure 3.6, respectively. The plots for the tests at 60 ºC show dramatically that the HRA mixtures, with the possible exception of the 35 % 0/14 HRA, were less deformation resistant than the DBM and SMA mixtures. At the lower temperature of 45 ºC, the plots show that 0/2 HRA (sand carpet) is significantly the Table 3.7 Wheel tracking test results 45 ºC 60 ºC Wheel Wheel tracking Rut tracking Rut rate depth rate depth Overall Material (µm/cycle) (mm) (µm/cycle) (mm) ranking 0/10 SMA (dense) /14 SMA (open) /6 SMA /10 SMA (open) Mastic asphalt /20 DBM /14 SMA (dense) % 0/14 HRA % 0/10 HRA % 0/14 HRA % 0/10 HRA % 0/2 HRA worst at deformation resistance with the other HRA mixtures in a band between the sand carpet and the other, almost non-deforming mixtures. Therefore, sand carpets are not suitable for use in the top 100 mm of surfacing whilst other HRA mixtures should be designed against deformation to SHW clause 943 (which was not the case for the trialled mixtures) for use in that region. An inconsistency among the mixtures is the 0/14 SMA (dense) at 45 ºC, in which the rate of deformation started high before reducing considerably so that the rut depth is in the range for the HRA mixtures tested but the wheel tracking rate is only marginally higher than the other SMA mixtures. Therefore, this result is assumed to be an outlier caused by excessive bedding in at the beginning of the test Review of asphalt results The rankings from the various tests can be combined to produce a rough overall ranking as in Table 3.8. However, when considering the appropriate materials, the choice is often a trade off between these properties and they will not usually have equal ranking. Material optimisation is usually avoiding any excessively adverse property rather than getting the best performance in all. Nevertheless, mastic 8

13 Rut 45 o C (mm) Number of load cycles 0% 0/2 HRA 30% 0/10 HRA 35% 0/14 HRA 55% 0/14 HRA 55% 0/10 HRA 0/20 DBM 0/6 SMA 0/10 SMA open 0/10 SMA dense 0/14 SMA open 0/14 SMA dense Mastic Asphalt Figure 3.5 Plot of wheel-tracking 45 o C Rut 60 o C (mm) Number of load cycles 0% 0/2 HRA 30% 0/10 HRA 35% 0/14 HRA 55% 0/14 HRA 55% 0/10 HRA 0/20 DBM 0/6 SMA 0/10 SMA open 0/10 SMA dense 0/14 SMA open 0/14 SMA dense Mastic Asphalt Figure 3.6 Plot of wheel-tracking 60 o C 9

14 Table 3.8 Overall rankings for asphalt properties Air voids Perme Stiffness Wheel Sum of Test content -ability modulus -tracking rankings Mastic asphalt /10 SMA (dense) /20 DBM % 0/10 HRA % 0/10 HRA /6 SMA 7 11= % 0/14 HRA % 0/14 HRA /2 HRA /14 SMA (open) 11 11= /14 SMA (dense) /10 SMA (open) 12 11= asphalt is demonstrated to be the best material for all the properties other than deformation resistance, when it was still in the top half. However, mastic asphalt is a relative expensive mixture, a factor that cannot be excluded. The maximum density (and, to a lesser extent, bulk density) correlated well with the binder content, but not with the air voids content because the binder content was selected based on the mixture type and aggregate skeleton. Therefore, assuming air voids content is considered to be indicative of permeability, the binder content cannot be used as a surrogate for permeability without reference to the mixture type. 3.2 Asphalt flexibility Objectives and test programme Flexible asphalt has obvious advantages for use in situations where there is potential movement. Such situations include roads on soft foundations as well as bridge decks. However, there are currently no test requirements for flexibility in either the British Standards for asphalt or in the British Board of Agrément (BBA) Highway Authorities Product Approval Scheme (HAPAS) for thin surfacing systems. The inclusion of such a test as an option in the BBA/HAPAS scheme has been requested by representatives from the County Surveyors Society. Because of the synergy with the bridge deck issue, the Highways Agency extended the scope of this work to include a programme of tests to identify a suitable test and typical values for the property. The test programme devised consisted of undertaking four different tests that were proposed by members of BBA-HAPAS Specialist Group 3 on a series of asphalt mixtures. The known ranking for the properties required of some of the mixtures tested was hoped to provide an insight into which of the tests had potential to assess the flexibility of asphalt mixtures. From the mixtures used in the asphalt material trials (Section 3.1.1), the mastic asphalt, 30 % 0/10 HRA and 55 % 0/14 HRA mixtures were selected as the control mixtures for the flexibility programme with the SMA mixtures, including a variant of the 0/10 PMB SMA (open) mixture that used styrene-butadiene-styrene block co-polymer (SBS) modified binder instead of fibres, as the trial materials. The test methods undertaken on the nine mixtures were: a Indirect tensile fatigue test. b 4-point bending fatigue test. c Semi-circular bending test. d Asphalt bending test. The indirect tensile fatigue tests were carried out in accordance with Annex E of BS EN (CEN, 2004b) at 20 C except that the deformations were measured vertically rather than horizontally because Cooper Research Technology Limited, manufacturer of the fatigue equipment used, do not offer a horizontal deflection measurement system. The specimens had to go through the test two or three times before they fractured because the test stops at a deflection of 8 mm. The results are the cumulative number of cycles and deflections combined. The 4-point bending fatigue tests were carried out in accordance with Annex D of BS EN (CEN, 2004b) with a loading device capable of imparting a fixed adjustable amplitude up to ± 5 mm with a frequency of 6 cycles/h at a temperature of 5 ºC and a sample size of 300 mm 40 mm 40 mm. The semi-circular bending tests were carried out in accordance with Appendix D, repeated on separate samples with and without a notch, at a temperature of 15 C and a deformation rate of 5.1 mm/min. The draft method is based on a method developed in the Netherlands and has recently been put forward for standardisation as a European test method to assess the crack propagation property of asphalt. The asphalt bending tests were carried out in accordance with Appendix E at temperatures of 0 C and 15 C and a deformation rate of 50 mm/min. The draft method is based on PRC Requirement T (Peoples Republic of China, 1993) except that, for practicality, the deformation was measured at the top of the specimen rather than the bottom. During the tests, it appeared that compression of the specimens was minimal (also evidenced by the low test loads achieved before failure) which would imply very similar deformations at the bottom and the top of the specimens Indirect tensile fatigue test The mean fatigue lives of the various asphalt mixtures tested for different applied tensile stresses are summarised in Table 3.9. An approximate ranking of the mixtures has been included. These values are plotted on Figure 3.7, from where the rough ranking order can be seen point bending fatigue test The mean fatigue lives in terms of time of the various asphalt mixtures tested are summarised in Table An approximate ranking of the mixtures has been included Semi-circular bending test The values calculated for the tensile strength (from the unnotched samples) and the fracture toughness (from the notched samples) of the asphalt mixtures tested are given in Table An approximate ranking of the mixtures has 10

15 Table 3.9 Summary of mean fatigue lives in ITFT Applied tensile stress (kpa) Material Ranking Mastic asphalt 155, ,000 5, /14 SMA(dense) 176,000 11,900 6,810 2= 0/10 SMA (dense) 162,000 15,600 7,250 2= 30 % 0/10 HRA 117,000 16,900 6,470 2= 0/6 SMA 94,300 12,500 5,130 2= 0/14 SMA (open) 64,200 9,440 2, /10 SMA (open) 43,200 20,200 4,670 7= 55 % 0/14 HRA 95,000 19,400 3,100 7= 0/10 PMB SMA (open) 27,000 5,620 1, Log (Fracture life) Applied tensile stress (kpa) 30 % 0/10 HRA 55 % 0/14 HRA Mastic asphalt 0/6 SMA 0/10 SMA (open) 0/10 PMB SMA (open) 0/10 SMA (dense) 0/14 SMA (open) 0/14 SMA (dense) Figure 3.7 Plot of mean fatigue lives Table 3.10 Summary of fatigue lives in 4-point bending Table 3.11 Summary of semi-circular bending test results Fatigue life (h) for sample: Tensile strength Fracture toughness Material Mean Ranking 0/10 PMB SMA (open) % 0/14 HRA = 0/6 SMA = 0/10 SMA (open) % 0/10 HRA /10 SMA (dense) /14 SMA (dense) = 0/14 SMA (open) = Mastic asphalt Combined Material (N/mm²) Ranking (N/mm 3/2 ) Ranking ranking Mastic asphalt % 0/10 HRA = 2 0/14 SMA (dense) = = 3 0/14 SMA (open) = = 4 0/6 SMA = = 5= 0/10 SMA (dense) = = 5= 0/10 SMA (open) % 0/14 HRA = = 0/10 PMB SMA (open) = = 11

16 been included for both properties, which were similar, together with an overall ranking by rounding the average of the two rankings Asphalt bending test The values calculated for the bending strength, maximum bending strain and bending stiffness modulus of the asphalt mixtures tested are given in Table 3.12 for a test temperature of 0 ºC and in Table 3.13 for a test temperature of 15 ºC. An approximate ranking of the mixtures has been included for each property plus an overall ranking combining them for each temperature. Table 3.12 Summary of asphalt bending test 0 ºC Bending Max m Bending strength bending strain stiff. modulus Value Ran- Ran- Value Ran- Material (MPa) king Value king (GPa) king 0/14 SMA (dense) = = /10 PMB SMA (open) 8.3 5= = = 30 % 0/10 HRA = = 0/10 SMA (dense) 8.4 5= = = 0/10 SMA (open) = = 55 % 0/14 HRA = = 0/14 SMA (open) 7.8 7= = Mastic asphalt 8.0 7= /6 SMA A plot of the ranking of the six different measures (three properties at two temperatures) is shown in Figure 3.8. The order of the materials has been revised in accordance with their average ranking from the bending strength and bending stiffness modulus at both temperatures to assist the comparison. Table 3.13 Summary of asphalt bending test 15 ºC Bending Max m Bending strength bending strain stiff. modulus Value Ran- Ran- Value Ran- Material (MPa) king Value king (GPa) king 0/14 SMA (dense) /10 SMA (dense) = = 348 2= 0/14 SMA (open) = 331 2= 0/10 PMB SMA (open) = = Mastic asphalt = = % 0/10 HRA = = 0/10 SMA (open) = = 228 6= 55 % 0/14 HRA = /6 SMA = 72 9 The bending strength and bending stiffness modulus rankings are in approximately the reverse order of the rankings for maximum bending strain. An overall ranking can be derived by averaging the rankings for the bending strength and bending stiffness modulus and the inverse (i.e. 10 minus the actual ranking) of the ranking for the maximum bending strain, as shown in Table Discussion Practicality The complexity of the potential flexibility tests differ. The ITFT took considerably longer than the other tests because the fatigue life has to be between 10 3 and 10 6 cycles with each cycle lasting 0.5 s (0.1 s loading time and 0.4 s rest period), giving a total duration of between 8 min and 6 days continuous working for a single determination with three determinations required per test. Of the other three tests, the four-point bending fatigue test was the most complex to set up whilst the semi-circular bending and asphalt bending tests were both the simplest to set up and quickest to perform Ranking /6 SMA 0/10 SMA (open) Mastic asphalt Material 55 % 0/14 HRA 0/14 SMA (open) 0/10 PMB SMA (o) 30 % 0/10 HRA 0/10 SMA (dense) 0/14 SMA (dense) Max bending 15 o C Max bending 0 o C Bending 15 o C Bending 0 o C Bending stiffness 15 o C Bending stiffness 0 o C Test Figure 3.8 Relative rankings of the properties measured in the asphalt bending test 12

17 Table 3.14 Development of an overall ranking for the asphalt bending test Ranking Max Bending bending Bending stiff. strain Material 0ºC 15ºC 0ºC 15ºC 0ºC 15ºC Overall 0/14 SMA (dense) 6= 9 1= /10 SMA (dense) 6= 7= 5= 3= 2= 2= 2 0/10 PMB SMA (open) 6= 4= 5= 3= 2= 4 3 0/14 SMA (open) 5 4= 7= 2 5= 2= 4= 30 % 0/10 HRA 4 3 1= 3= 2= 6= 4= Mastic asphalt 3 7= 7= 6= 8 5 6= 0/10 SMA (open) 6= 4= 9 6= 5= 6= 6= 55 % 0/14 HRA 2 1= 1= 8 5= 8 8 0/6 SMA 1 1= Comparison of rankings The overall rankings derived from each of the four tests are compared in Table 3.15 with the ranking plotted in Figure 3.9. Table 3.15 Ranking order by different tests 4-point Semi- Asphalt Material ITFT bending circular bending 0/14 SMA(dense) 2= 7= 3 1 0/10 SMA (dense) 2= 6 5= 2 0/10 PMB SMA (open) 9 1 8= 3 30 % 0/10 HRA 2= 5 2 4= 0/14 SMA (open) 6 7= 4 4= Mastic asphalt = 0/10 SMA (open) 7= 4 7 6= 55 % 0/14 HRA 7= 2= 8= 8 0/6 SMA 2= 2= 5= 9 The mastic asphalt and 0/10 PMB SMA tend to be the extremes on the rankings, certainly for the first three tests although, compared to the ITFT and semi-circular bending tests, the order is reversed for the 4-point bending and, to a lesser extent, the asphalt bending test. The reason is that the tests undertaken measure different properties at different temperatures. The properties and temperatures are:! ITFT: Fatigue life under controlled stress (force) conditions at 20 ºC.! 4-point bending: Fatigue life under controlled strain (displacement) conditions at 5 ºC.! Semi-circular bending: Tensile strength and/or fracture toughness at 15 ºC.! Asphalt bending: The bending strength, maximum bending strain and/or bending stiffness modulus at both 0 ºC and 15 ºC. Controlled stress and controlled strain tests are known to result in reversed rankings. Therefore, it can be assumed that the tensile strength and the fracture toughness in the semi-circular bending test are aligned to constant stress rather than constant strain conditions. Figure 3.10 shows the ranking orders when the materials are listed in order of their semi-circular bending test results whilst Figure 3.11 shows them when ordered for the 4-point bending test. Both these figures show that there a consistency in the order for three of the tests (even if the 4-point bending test order has to be reversed), but that the overall ranking derived from the asphalt bending test shows no consistency with the others. Plotting the individual rankings from the asphalt bending test on graphs with the same axes as Figure 3.10 and Figure 3.11 did not produce any obvious equivalence. Therefore, the properties derived from the asphalt bending test are not considered to be well correlated with those from the other three tests. Mastic asphalt could be expected to be the most flexible because of its high binder content, although that binder is Ranking % 10 HRA 55% 14 HRA Mastic asphalt Material 0/6 SMA 0/10 SMA (o) 0/10 PMB SMA 0/10 SMA (d) 0/14 SMA (o) 0/14 SMA (d) Asphalt bending Semi-circular 4-point bending ITFT Test Figure 3.9 Plot of observed rankings for each test 13

18 Ranking Mastic asphalt 30 % 0/10 HRA 0/14 SMA (dense) 0/14 SMA (open) 0/6 SMA 0/10 SMA (dense) 0/10 SMA (open) 55 % 0/14 HRA 0/10 PMB SMA (open) ITFT 4-point Semi-circular Asphalt bending Figure 3.10 Relative rankings in order of semi-circular bending ranking Ranking /10 PMB SMA 55 % 0/14 HRA 0/6 SMA 0/10 SMA (open) 30 % 0/10 HRA 0/10 SMA (dense) 0/14 SMA (open) 0/14 SMA (dense) Mastic asphalt ITFT 4-point Semi-circular Asphalt bending Figure 3.11 Relative rankings in order of 4-point bending ranking harder than that for other mixtures which could lead to less flexibility, particularly at colder temperatures. If the binder content is the overwhelming criteria for flexibility, then the ranking would be as follows: 1 Mastic asphalt (8.0 % binder) 2 30 % 0/10 HRA (7.8 % binder) 3 0/6 SMA (7.7 % binder) 4 0/14 SMA (dense) (7.6 % binder) 5 0/14 SMA (open) (7.4 % binder) 6 0/10 SMA (dense) (6.9 % binder) 7= 0/10 SMA (open) (6.7 % binder) 7= 0/10 PMB SMA (open) (6.7 % binder) 9 55 % 0/14 HRA (6.5 % binder) This ranking is very close to that obtained with the semicircular bending test. The differences are that 0/6 SMA was moved up two places and 55 % 0/14 HRA swapped places with 0/10 PMB SMA (open). These differences could be due to an improvement in flexibility with smaller aggregate size. What is unexpected is that the styrene-butadiene-styrene (SBS) modified-binder did not enhance the flexibility of the mixture compared to the fibres, as measured by the semicircular bending test. A possible for this lack of enhancement is that the SBS binder was used as a replacement rather than designing a new mixture with SBS binder. The binder content ranking was not consistent with that obtained from the 4-point bending test Theoretical considerations There is a need for flexibility in bridge-deck surfacing and surfacings over weak substrates if they are to overcome slow but repeated movements, particularly at low temperatures. Therefore, the concept of the 4-point bending test at 5 ºC appears be appropriate. However, the need for controlled stress or control strain conditions in that test depend on the overall structure constant strain conditions are indicative of thick, stiff pavements (where other layers will take more of the load when the surfacing weakens with fatigue) while constant strain conditions are indicative of thinner pavements (where the deflections will increase as the material weakens and flexes to resist the applied force without much help from other layers). Therefore, any test for flexibility should be based on controlled stress rather than controlled strain, implying the ITFT rather than the 14

19 4-point bending fatigue test or, by default, the semi-circular test. However, the ITFT takes considerable time and, therefore, is not particularly practical. Therefore, the recommendation from theoretical considerations aligned to the results of the test programme is to use the semi-circular bending test. The median value of results obtained were a tensile strength of 3.07 N/mm 2 and a fracture toughness of N/mm 3/2, so suggested initial limits that are known to be attainable are a tensile strength of 3.0 N/mm 2 and a fracture toughness of 19 N/mm 3/ Waterproofing system properties Objectives and test programme The objective of the laboratory study of waterproofing systems was to assess the variation of, and typical values for, the bond strength between them and possible overlaying asphalt materials. It was hoped that the knowledge would be useful in setting realistic specification limitations which become more important as the thickness of the layers is reduced. In addition, the opportunity was taken to investigate the interlayer permeability because water will always find the weakest link, and that is usually the joints. A pavement structure will not be impermeable, even when constructed of impermeable materials, if the horizontal and/or vertical joints between those materials are permeable. Eight 300 mm by 300 mm by 55 mm thick blocks were manufactured using C40 concrete and waterproofing systems applied. Two different membranes were used, each with two different tack coats. The waterproofing systems were then overlaid with 40 mm depth of asphalt, compacted using a roller compactor to give an overall depth of around 100 mm (block plus waterproofing plus surfacing). The mixtures used were mastic asphalt, 0/2 HRA (sand carpet), 30 % 0/10 HRA and 0/10 SMA (dense) from Section such that there was one slab with each of the combinations of waterproofing manufacturer and asphalt type. Both waterproofing manufacturers offer at least two different tack/bond coats for different types of asphalt, so they selected tack/bond coats that were deemed most suitable for these mixtures. Two cores were taken from each composite specimen and tested for torque bond in accordance with the HAPAS thin surfacing guidelines (BBA, 2000). The holes were then refilled and the composite specimens re-compacted for 250 passes with the roller compacter set at 1 bar in order to try to simulate trafficking before two further cores were taken and tested for torque bond. Each composite specimen was sliced vertically down the centre to provide 300 mm by 100 mm cut faces with the waterproofing system along the centre. These faces were then tested for permeability at the interface between asphalt and the waterproofing system using apparatus developed for measuring the initial surface absorption of concrete (BSI, 1996), as shown in Section C.2 of Appendix C Torque bond tests The results of the initial torque-bond test are given in Table 3.16 together with the results after simulated trafficking in Table In order to estimate the ultimate shear stress of a failure surface from a torque bond test, assumptions must be made concerning the stress-strain curve of the material at the failure surface. If the material yields so the shear stress at failure is uniform across the failure surface, the maximum shear stress can be calculated from the theoretical relationship given in Equation (3.4) T τ = π d 3 where: = shear stress in N/mm² T = maximum torque in N.m d = diameter in mm. (3.4) Table 3.16 Torque-bond results before simulated trafficking Waterproofing system Diameter Height Max. torque Shear stress Failure Membrane Tack coat Surfacing type ID (mm) (mm) (N.m) (N/mm²) location B S Mastic asphalt B R 0 % 0/2 HRA B S 30 % 0/10 HRA B S 0/10 SMA (dense) A P Mastic asphalt A P 0 % 0/2 HRA A Q 30 % 0/10 HRA A Q 0/10 SMA (dense) Location of failure 1: Debonded at interface between tack/bond coat and waterproofing membrane. 15

20 Table 3.17 Torque-bond results after simulated trafficking Waterproofing system Diameter Max. torque Shear stress Failure Membrane Tack coat Surfacing type ID (mm) (N.m) (N/mm²) location B S Mastic asphalt B R 0 % 0/2 HRA B S 30 % 0/10 HRA B S 0/10 SMA (dense) A P Mastic asphalt A P 0 % 0/2 HRA A Q 30 % 0/10 HRA A Q 0/10 SMA (dense) Location of failure 1: Debonded at interface between tack/bond coat and waterproofing membrane. Location of failure 2: Material debonded during trafficking. If the stress is proportional to the strain at the failure surface, the shear stress at failure will increase from zero at the centre to a maximum at the outer diameter. The maximum stress will be given by Equation (3.5) T τ = π d 3. (3.5) In practice, the stress-strain curve of the material at the failure surface will fall somewhere between the fully elastic and fully plastic behaviour strain hardening is unlikely. Therefore, the true shear stress will be within the range calculated by Equations (3.4) and (3.5) that are shown in Table 3.16 and Table Of the initial results, the torque bond varied considerably by an order of magnitude with the two waterproofing systems giving different rankings for the different materials. For all materials other than 30 % 0/10 HRA, the results were higher for Membrane A systems than for Membrane B systems. Overall, the results can be ranked as follows: 1 Membrane A and tack coat P with (c.300 N.m) mastic asphalt. 2 Membrane A and tack coat P with (c.200 N.m) sand carpet. 3= Membrane B and tack coat R with (c.130 N.m) sand carpet, and Membrane A and tack coat Q with 0/10 SMA. 5= Membrane B and tack coat S with (c.60 N.m) 30 % 0/10 HRA, and Membrane B and tack coat S with mastic asphalt. 7= Membrane B and tack coat S with (c.30 N.m) 0/10 SMA, and Membrane A and tack coat Q with 30 % 0/10 HRA. According to BD 47/99 (DMRB 2.3.4), the minimum shear bond strength of an asphalt layer to a bridge deck waterproofing system should be 0.2 MPa at 23 ºC. Table 3.16 shows that, for each membrane, the failure stresses before simulated trafficking exceeded the minimum for three mixtures but were below it for the fourth mixture. In all cases, the failures were at the interface between the waterproofing membrane and the tack/bond coat, so the failure stresses were more dependent on the properties of the waterproofing systems than the asphalt mixtures. For the Membrane B systems, the highest failure stresses were measured with the tack coat R tack coat and sand carpet. For the Membrane A systems, the failure stresses were higher for tack coat P than for tack coat Q. The high values for mastic asphalt were probably due to the high temperature at which the material was laid and compacted and at which the tack/bond coat was activated. The additional passes of the roller compactor at elevated temperature to simulate trafficking did not prove successful, with debonding occurring, particularly for the Membrane B systems, rather than the gain in adhesion. Therefore, the results after simulated trafficking have been ignored from any analysis. However, it should be noted that most of the samples that debonded had a low shear stress before trafficking Interface permeability The permeability at the interface between the asphalt surfacing and the waterproofing was carried out on the following three composite systems with the results being given in Table 3.18: 16

21 ! Mastic Asphalt Membrane A and tack coat P! 0 % 0/2 HRA Membrane A and tack coat P! 0/10 SMA (dense) Membrane A and tack coat Q Table 3.18 Flow rate at interface between asphalt and waterproofing Average flow rate (ml/m2/s) Time Mastic 0% 0/10 SMA (min) asphalt 0/2 HRA (dense) Mean The test specimens were taken from the surfaced concrete blocks used for the torque bond tests. The diameter of the test areas was 85 mm and they were approximately 25 mm deep. The measured flow rate was dependent on the permeability of the concrete, the permeability of the interface between the asphalt and the waterproofing system, and the permeability of the asphalt within the test section. Because the permeability of the concrete was low, most of the flow was through the interface and the asphalt itself. A very low flow rate was measured on the mastic asphalt specimen because both the interface and the asphalt were of low permeability. The flow rate was, on average, about 160 times higher on the sand asphalt specimen than on the mastic asphalt specimen. Because of the nature of sand asphalt, it is unlikely that the permeability of the interface was significantly different to that of the asphalt itself. However, it is possible that the permeability of the base of the layer of sand asphalt was less than that at mid-layer or at the top of the layer, the latter having an influence on the results shown in Table 3.3. The flow rate was, on average, about 2700 times higher on the 0/10 (dense) SMA specimen than on the mastic asphalt specimen. Because the waterproofing membrane was overlaid with a thick layer of tack coat Q, the interface permeability should not have been significantly higher than that of the SMA. Therefore, most of the flow should have been through the SMA itself. 4 Considerations for specification of surfacing for bridge decks 4.1 General approach The Specification for the surfacing on bridge decks must include the requirements normally applied to asphalt surfacings, e.g. skidding resistance, resistance to deformation and surface regularity, as well as further criteria specific to asphalt on bridges. The specific criteria likely to have the greatest effect on the durability of the surfacing on bridges concern the following properties:! The permeability of the asphalt and the control and removal of water.! The bond of the asphalt to the waterproofing system.! The resistance of the asphalt to fatigue cracking.! The compatibility with the waterproofing system. 4.2 Drainage Drainage requirements Whereas sub-surface water can permeate downwards through several bound and unbound layers as it flows towards the sub-surface drainage systems on pavements, sub-surface water on bridges can only flow downwards as far as the waterproofing system. It must then flow horizontally towards any sub-surface drainage systems that are located at the low points and may be well over ten metres away from where the water entered the surfacing. Therefore, drainage and the movement of water through asphalt have a greater impact on the durability of asphalt on bridge decks than on pavements. Clause 4.1 of BD 47/99 (DMRB 2.3.4) requires surface water to be removed from bridge decks by the provision of falls and suitable surface drainage systems. Clause 4.2 also requires sub-surface drainage where natural drainage is not possible. Some expansion joints provide sub-surface drainage by incorporating 20 mm square slotted drainage channels that run across the carriageway. However, these drains can become clogged over a period of time so they no longer drain away the water quickly enough. Throughdeck drains at the low points are considered to be a more effective means of sub-surface drainage. Whilst these may be installed on new bridges, they are not always installed on old bridges during maintenance works unless specific problems have been encountered. Therefore, Specifications for the asphalt surfacing on bridge decks should be ensure:! Minimum accumulation of water by identifying suitable requirements for the asphalt used in each layer.! Efficient surface and sub-surface drainage systems to facilitate drainage, aided by longitudinal gradients and crossfalls of the bridge deck and the overlaying asphalt, as are currently specified Removal of water The accumulation of water within an asphalt layer, whether on a bridge or a pavement, has a detrimental effect in areas trafficked by heavy good vehicles. This is because wheel loading induces high hydrostatic pressures in saturated asphalt that are sufficient to weaken the layer and break it up so that potholes are formed. Also, the pressures have an adverse effect on the bond between layers and, in particular, the bond of asphalt to bridge deck waterproofing systems and, hence, on the structural integrity of the asphalt layers on bridge decks. There have been a number of failures of the surfacing on bridge decks where water has accumulated in the asphalt, particularly on 17

22 the high side of certain types of expansion joint that form a barrier to the flow of water. Many of these failures have occurred after periods of heavy rain. The main criterion influencing the penetration and movement of water is the permeability both of the asphalt itself (see also Section 4.4.5) and of the interface between layers (see also Section 4.5.5). Permeability is a measure of the connectivity of pores/voids within the binder matrix, aggregate and the interface between them. The void content at the interface between the asphalt layers and the waterproofing system is dependent on the characteristics of the asphalt layer and the waterproofing system, so they have been considered together Sub-surface drainage If the surface course material is permeable (see Section 4.4.5), significant amounts of water could reach the lower asphalt layer(s) so that the potential for water to enter these layer(s) through air voids, defects and improperly sealed joints and reach the waterproofing system will be greater. Therefore, to minimise the risk of premature failure, an edge of carriageway drainage system should be provided to drain the full depth of a permeable surface course. Furthermore, the risk should be reduced further by specifying a surface course system which incorporates a thick bond coat that when applied to the lower asphalt layer helps to seal it. 4.3 Waterproofing system Bond to asphalt According to Clause B4.2 (l) of BD 47/99 (DMRB 2.3.4), the minimum tensile bond of an asphalt layer to a bridge deck waterproofing system should be 0.1 MPa at 23 ºC. According to Clause B4.2 (k), the minimum shear bond should be 0.2 MPa at -10 ºC and 23 ºC, and 0.1 MPa at 40 ºC. However, because BD 47/99 requires the total thickness of the asphalt layers to be 120 mm, it is implied that these bond strength requirements are applicable for surfacing of this standard thickness. The bond of asphalt to a waterproofing system is dependent on the characteristics of both the waterproofing system and the asphalt, e.g. the adhesive and cohesive properties of the membrane, tack coat and asphalt, not just the thickness of the tack coat. It is also dependent on the temperature at which the asphalt is laid and compacted and whether any tack coat is activated. The bond strength requirements in BD 47/99 (DMRB 2.3.4) refer to the initial bond, but Clause of the Specification for Highway Works (MCHW 1) states that: The additional protection layer 1 or surfacing laid on the waterproofing system shall be fully bonded to the system for the life of the system. The bond is prone to failure in service when the surfacing became saturated and high hydrostatic pressures are generated by wheels of heavy goods vehicles. Surfacing of 1 The additional protection layer is specified in the DMRB and SHW as a 20 mm layer of sand asphalt. standard thickness with a sand asphalt layer performs well even if, apparently, it may be poorly bonded to the waterproofing system. However, surfacing of less than standard thickness has not been durable when it has not remained firmly bonded. In these circumstances, water has accumulated on the waterproofing system and weakened/ failed the bond of the surfacing to the waterproofing system and/or the surfacing itself. The durability of the surfacing appears to be more reliant on the bond when the thickness, and hence dead weight, of the surfacing is low. Where the surfacing is less than 120 mm thick, the bond strength requirements are much higher in other countries compared to those in the UK. For example, the current Japan Highway Public Corporation specification requires waterproofing systems to be overlaid with asphalt of total thickness 75 mm. The 35 mm thick layer directly overlaying the waterproofing system is SMA with a 0/5 mm aggregate size. The 40 mm thick surface course is a drainage (porous asphalt) layer with a 0/13 mm aggregate size. Performance tests on waterproofing systems include tests to measure the shear and tensile bond strength of the weakest interface of specimens comprising a concrete block, the waterproofing system and an asphalt layer. The shear bond strength must not be less than 0.8 MPa at -10 C and 0.15 MPa at 20 C. The tensile bond strength must not be less than 1.2 MPa at -10 C and 0.6 MPa at 20 C. The current German specification requires waterproofing systems on major roads to be overlaid with surfacing of total thickness from 70 mm to 80 mm. A 35 mm thick protective layer of Gussasphalt with a 0/8 mm aggregate size directly overlays the waterproofing system. The surface course is 35 mm to 45 mm thick. The tensile bond strength of the weakest interface of specimens comprising a concrete block and the waterproofing system must not be less than 0.7 MPa at 8 C and 0.4 MPa at 23 C. The shear bond strength of the weakest interface of specimens comprising a concrete block, the waterproofing system and an asphalt layer must not be less than 0.15 MPa at 23 C when the shear force is applied at an angle of 15º to the plane of the specimen. Therefore, it is proposed that the minimum tensile and shear bond requirements are increased from the values given in BD 47/99 (DMRB 2.3.4) when the surfacing is less than 120 mm thick. Furthermore, if the surfacing is 120 mm or more thick, higher bond strength requirements should be specified if the waterproofing system is not overlaid with an additional protective layer of sand asphalt, because of the increased risk of water accumulating on the membrane and weakening the bond. The higher bond strength should not be achievable unless the contact area between the asphalt and waterproofing system is high, i.e. there are few voids at the interface. To ensure that the bond is not susceptible to the presence of water or varies with time, the bond strength should be measured both before and after exposure to water Membrane stiffness Although well-designed pavements should not suffer fatigue cracking, there has been concern that the surfacing overlaying waterproofing systems on bridge decks can be susceptible to fatigue when the asphalt layers are thin. 18

23 The susceptibility of asphalt layers to fatigue is dependent on the strains induced in the asphalt and its fatigue properties. For a given wheel load, the strains induced are dependent on the thickness and stiffness properties of the waterproofing system, the stiffness properties of each asphalt layer, the combined thickness of the layers, and the bond of the asphalt layers to each other and to the waterproofing system. In addition, cracking of the upper asphalt layers can be induced because of the break-up of the lower layers by high hydrostatic pressures when they become saturated. The strains induced by wheel loading are enhanced by the local and global deformation of the substrate to which the waterproofing system and asphalt are applied. The deformation of the substrate of steel bridge decks (the deck plate) is significant. However, concrete bridge decks have a local stiffness that is considerably higher than that of steel decks, so strains induced by the local deformation of the substrate are relatively low. Similarly, strains induced by the global deformation of decks are considered to be lower that those due to the other factors listed above. The stiffness moduli and Poisson s ratio of the membranes of waterproofing systems have been measured at different temperatures. The membranes were found to behave elastically or visco-elastically, with one type stiffer at lower temperatures and the other type stiffer at higher temperatures at the strains and strain rates encountered on bridge decks. A series of finite element analyses is being undertaken to determine the significance of the properties of the membrane on the strains induced in the asphalt layers overlaying waterproofing systems on bridge decks. The findings, yet to be published, indicate the susceptibility to fatigue of surfacing overlaying different types of waterproofing system and the significance of the stiffness properties of the waterproofing membrane Effect of laying and compaction temperatures on the waterproofing system The DMRB and SHW specify requirements that are intended to:! prevent waterproofing systems from being damaged when they are overlaid with hot asphalt; and! ensure that asphalt remains bonded to the waterproofing systems over their service life. Clauses B4.2 (i) and (j) of BD 47/99 (DMRB 2.3.4) require waterproofing systems to pass tests that simulate the conditions when they are overlaid with hot asphalt materials. The test specified in Clause B4.2 (j) assesses the effects of high temperatures encountered during surfacing on the crack bridging ability of the waterproofing membrane by overlaying it with hot material to achieve a temperature of 145 C on its surface. Clause B4.2 (i) assesses the resistance to aggregate indentation during the compaction of the asphalt. Currently, all membranes must be permanently indented by no more than half their thickness after a force of 500 N has been applied by an aggregate indentor heated to a temperature of 80 C. This test is designed to simulate the compaction of sand asphalt. If systems are to be overlaid with mixtures containing larger sized aggregates, the membrane must pass the tests with the aggregate indentator heated to 125 C. Not all of the waterproofing systems currently registered for use on Highways Agency bridges have passed this test at 125 C and can be overlaid with coarse mixtures. Allied to the aggregate indentation requirements are those in Clause of the SHW (MCHW 1), which states that: With the exception of sand asphalt carpet, bituminous materials with a temperature greater than 125 C shall not be deposited on a bridge deck waterproofing system unless adequate precautions are taken to avoid heat damage in accordance with a good industry practice. A maximum temperature of 145 C is permitted for sand asphalt carpet. The BBA Roads and Bridges Agrément Certificates for the different waterproofing systems currently registered include the following statements: Temperature of the APL or HRA surfacing when applied should exceed the minimum reactivation temperature of 100 C required for the tack coat R tack coat. The rolling temperature of the surfacing must not fall below the minimum reactivation temperature of 85 C required for Tack coat P, and 90 C for Tack coat SA Temperature of the surfacing when applied should exceed the minimum reactivation temperature of 80 C required for Britdex MDP Tack Coat. Temperature of the APL when applied should be as specified in BS 594-1: 1992 and BS 594-2: Clause of the SHW (MCHW 1) states that: The additional protective layer of surfacing laid on the waterproofing system shall be full bonded to the system for the life of the system. The bond shall be achieved by either: (i) the binder within the directly applied additional protective layer of surfacing; or (ii) a separate tack coat details of which are given on the BBA Roads and Bridges Agrément Certificate. Where the tack coat is of the type activated by the heat of the succeeding bituminous layer the temperature of this layer shall be sufficient to ensure adhesion. When a layer of asphalt is laid onto a substrate such as a waterproofing system, the base of the layer cools rapidly as heat is transferred from the layer into the waterproofing system and concrete substrate. After a short period of time, the temperature of the waterproofing system will have risen so that it is similar to that of the base of the layer. However, the waterproofing system will then cool as heat is lost by conduction to the concrete substrate and also by convection and radiation through the top of the layer. The asphalt layer must be laid at a sufficiently high temperature so that the rolling/compaction temperature is 19

24 high enough to activate the tack coat and form a dense layer. Table 4.1 and Table 4.2 show, respectively, the times after laying for 20 mm and 40 mm thick layers to reach a given temperature at mid-layer that were estimated using the method by Nicholls and Daines (1993). The temperature at the base of a layer (i.e. at the tack coat) could be about 10 C below the mid-layer temperature. Therefore, the current Specification gives little time to complete compaction at a sufficiently high temperature when the activation temperature of the tack/bond coat is 100 C, especially when the asphalt layer is only 20 mm thick and is laid at 145 C. There is more time to complete the compaction of a 40 mm layer, even if it is laid at only 125 C. Much more time would be available if mixtures containing coarse aggregates were laid at temperatures higher than 125 C, which probably already happens on a number of bridges. The implication from the aggregate indentation test is that rolling should not occur when the temperature at the waterproofing system is greater than 125 C. Therefore, the Specification could be changed to prevent rolling when the mid-layer temperature is 125 C (rather than specify a laying temperature of 125 C). However, because the temperature at the waterproofing system is lower than that at mid-layer, rolling could be permitted when the temperature at the waterproofing system is 125 C or lower, or rolling could be permitted Table 4.1 Effect of environmental conditions on time available to compact a 20 mm thick layer Time after laying to Wind reach given temperature Air speed at Laying at mid-layer (min) temperature 2 m height temperature ( C) (km/h) ( C) 120 C 110 C 100 C Table 4.2 Effect of environmental conditions on time available to compact a 40 mm thick layer Wind Air speed Laying Time after laying to reach given temper- at 2 m temper- temperature at mid-layer (min) ature height ature ( C) (km/h) ( C) 120 C 110 C 100 C when the mid-layer temperature is, say, 135 C or less. Clearly, the contractor should provide details of the temperature at the tack/bond coat above which compaction should be completed or, preferably, the wording in the BBA Roads and Bridges Agrément Certificates should be changed accordingly. The above may appear to be unnecessarily complicated, but it is important because the bond of asphalt to waterproofing systems tends to increase with the rolling temperature. An aggregate indentation test could be carried out at a temperature up to a maximum of 145 C, the temperature at which the crack bridging ability of the waterproofing membrane is assessed, if it is necessary to compact asphalt when the waterproofing membrane is at such temperatures. If the surfacing is likely to produce higher temperatures at the waterproofing system, it would be necessary to carry out a crack bridging test during which such temperatures were induced during the thermal shock preconditioning phase. 4.4 Asphalt properties Deformation resistance Deformation resistance of the component materials of a pavement is more important for those materials nearest the surface where the loads caused by traffic are highest. The loads are distributed by the pavement layers so that they reduce with the thickness of overlying pavement. The approach used to be to consider only the surfacing layers, with a more stringent requirement on the surface course than on the binder course. However, with the introduction of proprietary thin surfacings and hence thinner surface courses, it becomes more logical to apply the stringent requirement for deformation resistance to composite samples for the top 50 mm and the less stringent requirement to the next 50 mm. It is proposed to take this approach for bridge deck surfacings. Requirements for deformation resistance should become more important as the overall thickness of the surfacing layers is reduced. Using the conventional construction approach, if in thin layers, the red sand carpet would come more within the critical depth. Sand carpet is not particularly deformation resistant, being the worst of the materials tested (Section 3.1.5). Therefore, it is proposed to apply the current surface course requirements (as defined in tables NG 9/28 and NG 9/29 of the Notes for Guidance on the Specification for Highway Works (MCHW 2)) for deformation resistance to (50 ± 10) mm thick samples of each material type that occurs in the top 50 mm. In addition, it is proposed to apply the requirements at the next level down to (50 ± 10) mm thick samples of materials that occur in the next 50 mm of the pavement. Where materials are nominally laid at a thickness less than 40 mm, it is proposed to permit the testing of composite samples made up of more than one different layers to be used in the pavement provided no part layer has a thickness less than twice its maximum aggregate size. 20

25 4.4.2 Texture depth Texture depth is a standard requirement for the surface course of high-speed trunk roads, and the requirements of clause 921 of the Specification for Highway Works (MCHW 1) should be applied when appropriate for the specification of surfacings to bridge decks Skid resistance Although it is only referenced explicitly in clause 918 (slurry surfacing), 919 (surface dressing), 922 (surface dressing), 938 (porous asphalt) and 942 (thin surface course systems) of the Specification for Highway Works (MCHW 1), the skid resistance of the surface course is defined by the polished stone value of the coarse aggregate as laid out in Appendix 7/1 of the job specification. Advice on values to specify in Appendix 7/1 is given in Advice Note HD 36/99 (DMRB 7.5.1). This approach should be used for the specification of surfacings for bridge decks Flexibility and fatigue Flexibility has been identified as an important factor over bridge decks (Section 4.3.2). However, there are doubts about the current fatigue tests for asphalt because they do not provide a consistent ranking. In particular, the choice between constant stress and constant strain can reverse the order. For bridge decks, it is assumed that constant strain will apply because the support is so much stiffer than the asphalt. The obvious fatigue tests are the indirect tensile, four-point bending and two-point bending, the first having had some use in the UK whilst the latter two are the methods that will be available for CE marking the property when the European standards are implemented. However, none of these tests are currently used for routine testing of asphalt in the UK. An alternative to fatigue is flexibility. The research into a possible test for flexibility identified the semi-circular bending test from several put forward. This test was found to be practical, equivalent to a controlled stress fatigue test and ranks materials similarly to binder content, a known component of flexibility if other things are constant. The test is currently being considered for European standardisation as a measure of crack propagation. For the BBA HAPAS scheme for thin surfacings, it was suggested that initially the values of 3.0 N/mm² and 19 N/mm 3/2 could be set as the minimum values required for tensile strength and fracture toughness, respectively. These limits were considered practical because the majority of the results obtained from trial mixtures exceeded them. Whilst it would be preferable to have fatigue and/or flexibility requirements that get more severe as the asphalt thickness gets less, there is currently no agreement on the test to be used or the limits to be achieved. Therefore, it is proposed not to set any fatigue requirements at this time, but to consider latter inclusion if subsequent research identifies more precisely what is appropriate Permeability and air voids content Previous research at TRL has shown that the layer of asphalt directly overlaying the waterproofing system must be impermeable if surfacing on bridge decks is to remain durable. If the lower asphalt layers on bridge decks are permeable, any water that passes through the surface course through the air voids in the layer and any defects (cracks and/or improper sealing around joints) will percolate down towards the waterproofing system while flowing across the deck towards the low points. If the water encounters a barrier on its way, such as an expansion joint or a less permeable area, it will accumulate in the asphalt until it is drained by a sub-surface drainage system. Though-deck drains will be required at all low points on the deck, including large hollows and depressions that are not free draining. Furthermore, the lower asphalt layer will need to remain sufficiently permeable throughout its service life to drain the sub-surface water quickly in the heaviest periods of rain. However, premature failures have occurred on a number of bridges when the lower asphalt layer has been permeable, even when there has been some sub-surface drainage (Section 4.2.3), because high hydrostatic pressures have been generated by the wheels of heavy goods vehicles in the surfacing when saturated. Therefore, the lower asphalt layer should be impermeable and sub-surface drainage should be provided to drain the small amounts of water that will inevitably enter the layer and permeate down to the waterproofing system. In spite of the great importance of permeability in relation to the durability performance bridges, there is currently no permeability specification to control the quality of asphalt surfacing. This deficiency may be attributed to the controversy regarding the repeatability and reproducibility of permeability tests using different techniques and the relatively long time required to produce results. A new European Standard, EN (CEN, 2004c), describes a method for measuring the permeability of porous asphalt to water, but this method would be timeconsuming for partially permeable and unsuitable for impermeable asphalt mixtures. Permeability testing has received more attention from the concrete industry and has been widely used within the durability specifications of major concrete constructions, such as the Jubilee Line Extension Project in the UK. Hassan and Cabrera (1997) demonstrated that the permeability of concrete is dependent on many factors that are all related to the volume of open pores, as the permeability increases with higher volume of open porosity. A concrete with a volumetric proportion of open pores of less than 4 % exhibited low permeability values, whereas above 5 % there is a very rapid increase in permeability. This change indicates that an interconnected system of pores is established when the volume of open pores is greater than about 3 % or 4 %. A similar trend of permeability results is obtained from the asphalt mixtures, as shown in Figure 3.3. Regardless of the mixture type, low permeability values are obtained when the air voids content is 4 % or below and the permeability increases rapidly for air voids contents above 5 %. This hypothesis is supported from results of other investigations (Zoorob et al., 1999). The permeability test, under a pressure gradient, is an easy test and takes a short time to be conducted. The test results are found reliable in ranking the permeability of 21

26 various asphalt mixtures and, therefore, could be adopted in the durability specifications of asphalt surfacing to bridge decks. However, there is still no agreement about the test method, the associated limits of permeability to be specified or the precision of the results. In the absence of a generally accepted standard permeability test, it appears reasonable to use the air voids content as an indicator for permeability. Any specified maximum values for air voids contents, as a surrogate for permeability, should depend on the presence and effectiveness of sub-surface drainage. If there is adequate drainage that can be properly maintained, a design air voids contents of up to 6 % could be acceptable for asphalt layers other than that directly over the waterproofing system whereas, for the bottom layer and where there is no adequate sub-surface drainage, an air voids contents of not more than 4 % should be required. Both requirements can be met by all of the materials tested (Section 3.1.2) other than the open SMAs and the 35 % 0/10 HRA (despite 35 % 0/14 HRA having been used successfully on bridges in the past), the latter being the only one to fall between the two proposed limits. If those limits are used for the mean, a tolerance of 2 % should be included to give maxima on individual readings of 8 % and 6 %, respectively. It would be ideal to use the individual maxima on site, but it is appreciated that it is best not to take cores when impermeability is sought Protection for waterproofing systems The current Specification requires waterproofing systems to be overlaid with a 20 mm thick additional protection layer of sand asphalt (0/2 HRA). Because the layer has low resistance to deformation, it performs satisfactorily only if it is overlaid with sufficient thickness of deformation resistant surfacing. Therefore, it should be omitted when the overlaying surfacing is less than 100 mm thick. However, the benefits of laying sand asphalt, even when it is overlaid with 100 mm or more of surfacing, have been questioned by many in the industry. The original intention was to lay the sand asphalt immediately after waterproofing in order to protect the waterproofing system before and during surfacing operations. However, a number of waterproofing systems have passed the aggregate indentation test at 125 ºC (see Section 4.3.3) and do not require the protection of sand asphalt provided due care is taken before and during surfacing operations. Appendix B to BD 47/99 (DMRB 7.2.3) requires that all permitted waterproofing systems are tested for aggregate indentation at 40 C and 80 C with a compliance requirement that the indentation after a recovery period shall not exceed 50 % of the initial system. Waterproofing systems may be overlaid with asphalt containing coarse aggregates of maximum aggregate size greater than that found in sand asphalt if they have passed an aggregate indentation test at 125 C. Not all of the registered systems have passed the test at 125 C, which could disadvantage them in the marketplace, particularly when a specification requires that the asphalt directly overlaying the waterproofing system should contain large sized coarse aggregate, reducing the number of waterproofing systems that can comply. The material itself and the interface between the material and waterproofing system would need to be impermeable. Also, the resistance of the waterproofing system to aggregate indentation when overlaid with the material would need to be demonstrated. The use of red oxide in the sand carpet could be reproduced in the alternative mixtures if required. However, there have been hearsay reports that often the colour has been lost by the time the material is exposed and, with a thinner overall surfacing, the protective layer will be reached sooner so that greater care will be needed. It has been found that the red sand carpet cannot be used reliably as an indicator layer to enable a bridge to be resurfaced without it being re-waterproofed, which was its main purpose. Therefore, the requirement to use red oxide should become optional. 4.5 Joints and interface between layers General The use of impermeable asphalt layers will be inadequate without ensuring proper sealing at any details, interfaces or joints between the rips, and any variations in permeability associated with defects such as cracks. The relevant details are kerbs, parapets and expansion joints. The relevant interfaces are between the asphalt layers and between the lowest asphalt layer and the waterproofing system Joints The joints between adjacent rips of asphalt should be sealed effectively in order to prevent an easy path for water to flow vertically through an asphalt layer, as set out in both BS and BS However, joints that are fully sealed will prevent the flow of water horizontally across a bridge so provision must be made so there is no danger that water can accumulate in a layer, which could lead to high hydrostatic pressures being generated by the wheels of heavy goods vehicles. However, given the intention to ensure that the materials used are relatively impermeable (Section 4.4.5), it is proposed to require all cold vertical joints to be painted with bitumen before being laid against Bond between layers The bond between layers has become more important with the use of thin surface course layers because the absence of the weight supplied by a thick layer needs to be replaced by positive adhesion with the layer below in order to minimise the potential for de-bonding. However, there is no generally accepted method for measuring the property in-situ on a routine basis. There are two aspects to bond, the adhesion achieved by the binder present and the aggregate interlock created by texture of the underlying material being filled by particles from the overlay. Different test methods that have been proposed to measure bond are affected differently by the two aspects, with pull-off tests (as used for bridge waterproofing and high-friction surfacing system) ignoring aggregate interlock whilst shear tests (as used for thin surfacing systems) incorporate that aspect. Provided they are not saturated, the adhesion generally improves with 22

27 trafficking because the two layers are pressed together with each wheel-pass, so the timing of any measurements of the property is also important. Bond, based on a torque shear test, is a required property for thin surfacing systems in order to obtain a Highway Authorities Product Approval Scheme (HAPAS) certificate from the British Board of Agrément (BBA). However, there is no pass/fail rating under the current guidelines (BBA, 2000), only the need to report the result. The test, which is carried out after between 28 and 56 days of trafficking, is intended as a type test because of the disruption caused by closing the road to obtain the samples so soon after being re-opened. However, the results recorded to date typically vary between 400 and 1500 kpa, so that a specification requirement for 400 kpa would allow all currently certified thin surfacing systems to be used whereas a limit of, say, 700 kpa would exclude some of those that have performed least well in the test. If a limit is set, any non-proprietary surfacing would also need to be tested before it was permitted for use on bridge deck overlays. The limits proposed will be dependent on the depth of the interface with 700 kpa if within 20 mm of the surface, reducing to 400 kpa at 50 mm and no requirement below that. The tests should be performed with the relevant tack or bond coat to be used on site with the appropriate materials as substrate and overlay. Given the variable approach, it is proposed here to raise the general requirement, of either a tack or bond coat between each layer, to a requirement for bond coats at all interfaces. This approach does not affect the current bond requirements for waterproofing systems or thin surfacing systems in order to obtain their BBA-HAPAS certificates Bond to waterproofing system According to clause of the Specification for Highway Works (MCHW 1), a waterproofing system needs to have a BBA Roads and Bridges certificate in order to be permitted for use on trunk road bridges. Part of the laboratory test procedure to gain certification is to check the bond between the waterproofing system and the underlying concrete substrate by means of a tensile adhesion (pull-off) test and to check the bond between waterproofing system and the overlying asphalt by means of shear-adhesion and tensile bond tests. These tests are described in Appendix B to BD 47/99 (DMRB 7.2.3). There are limits for the tests, with minima of:! 0.3 N/mm² at -10 C, 0.3 N/mm² at 23 C and 0.2 N/mm² at 40 C for the tensile adhesion test on the waterproofing system to concrete substrate interface;! 0.2 N/mm² at -10 C, 0.2 N/mm² at 23 C and 0.1 N/mm² at 40 C for the shear adhesion test on the surfacing to waterproofing system interface; and! 0.1 N/mm² at 23 C for the tensile bond test on the surfacing to waterproofing system interface. These limits apply to overlaying asphalt of minimum thickness 120 mm where the asphalt directly overlaying the waterproofing system is 0 % 0/2 mm HRA (sand asphalt) or 50 % 0/10 HRA. However, higher limits are considered necessary when mixtures containing coarse aggregates directly overlays the waterproofing system. Also, higher limits are appropriate for thicknesses less than 120 mm, and the proposal is to increase them in steps of 30 mm down to 60 mm. Any total depth of surfacing less than 60 mm is regarded as a special case that will require expert advice. The resultant limits are given in Table 4.3. Table 4.3 Bonding limits for waterproofing systems when overlaid by coarse mixtures Surfacing thickness 120 mm <120 mm; 90 mm < 90 mm; 60 mm Tensile adhesion -10 C 0.30 N/mm² 0.50 N/mm² C 0.30 N/mm² 0.50 N/mm² C 0.20 N/mm² 0.30 N/mm² 0.30 N/mm² Shear adhesion -10 C 0.30 N/mm² 0.30 N/mm² C 0.30 N/mm² 0.30 N/mm² C 0.10 N/mm² 0.15 N/mm² 0.15 N/mm² Tensile bond 23 C 0.40 N/mm² 0.45 N/mm² 0.50 N/mm² Permeability of interface between asphalt and waterproofing system Most waterproofing systems comprise a primer to optimise the bond of the system to the concrete substrate, a membrane to prevent water and chlorides from reaching the concrete, and a tack/bond coat to optimise the bond of the overlaying asphalt to the system. When asphalt with coarse aggregates is compacted onto a hard surface, the bulk of the material may have a low void content but there may be large voids at the base of the layer. The voids tend to be larger with larger aggregate sizes and with higher proportions of coarse aggregate. Above a certain proportion of coarse aggregate, the voids may be interconnecting. When there are voids at the interface between the lowest asphalt layer and the waterproofing system, water can accumulate and there is a risk of premature failure. Therefore, the void content at the interface should be low and voids should not be interconnecting. The permeability of the interface between the waterproofing system and the asphalt is dependent on the properties of both the asphalt and the tack/bond coat. Whereas a tack/bond coat for a waterproofing system may comprise more than one layer, the upper layer that is in contact with the asphalt and which aggregates can penetrate can be described as thin (generally <0.2 mm) or thick (generally >1.0 mm). When the tack/bond coat is thin, an asphalt layer with a small aggregate size and low proportion of coarse aggregate will yield no large interconnecting voids. However, any mixture that contains large aggregates will result in some voids where water may accumulate and a reduction in contact area between the mixture and the waterproofing system. As discussed below, a reduction in the contact area may adversely affect the tensile bond of the asphalt to the waterproofing system. A thick tack/bond coat can (partially) fill the voids at the base of an asphalt layer with coarse aggregates and, 23

28 thereby, limit the accumulation of water and interconnecting voids and, potentially, improve the tensile adhesion. Therefore, a waterproofing system incorporating a thick tack/bond coat should be specified for mixtures with coarse aggregates. The tack/bond coat must not be too thick otherwise bleeding of the excess binder through the overlying asphalt layer may occur during its laying and compaction. Also, the asphalt layer will be more susceptible to fatigue if it floats on a thick layer of soft tack/bond coat. Ideally, the coarse aggregates should almost fully penetrate the tack/bond coat as the tack/bond coat material fills the voids at the base of the layer. Therefore, the specification needs to ensure that the lower asphalt layer overlaying the waterproofing system is impermeable. The mixture design and type of tack/bond coat should ensure that the air voids contents at the interface between the waterproofing system and the base of the layer is low without interconnecting voids (see Section 3.3.3). To minimise the amount of water that can enter the lower asphalt layers, the full depth of permeable surface courses should be drained at the edges. If this is not possible, an impermeable surface course should be laid that directs water to a suitable surface drainage system. These requirements are covered in Section However, frequent changes in surface texture and road noise associated with a change in surface course materials from bridge to pavement should be avoided. The total thickness of surfacings on bridges may range from 40 mm to over 120 mm. Such surfacings may require one, two or three asphalt layers, each with a different material. On most bridges, there are small variations in the level of the deck that can be accommodated without varying the number of asphalt layers. However, it has been necessary on a few bridges to vary the number of layers, requiring some degree of compromise in the regions where the number changes. Under these circumstances, the lower layer directly overlaying the waterproofing system should be of reasonably uniform thickness and of the same impermeable material. Changes in thickness should be accommodated in the upper layers, with tapered layers trimmed so material too thin to have been compacted sufficiently is removed. 5 Conclusions From the tests on a series of twelve asphalt mixtures, mastic asphalt was found to have the most suitable air voids content, permeability and stiffness modulus properties and was fifth at wheel-tracking, making it the best material overall for these tests. However, mastic asphalt is a relatively expensive mixture, a factor that cannot be excluded. The dense 0/10 SMA was the next best despite having the 8 th highest air voids content whilst the open 0/10 SMA was the worst, showing that the precise mixture design can be critical. The remaining mixtures showed relatively similar overall ratings, but with the sand carpet only being ranked 8 th out of 12 overall. Nevertheless, when considering the appropriate materials, the choice is often a trade off between properties and they will not usually have equal ranking. Material optimisation is usually avoiding any excessively adverse property rather than getting the best performance in all. The initial results from the torque bond test varied by an order of magnitude, with the two waterproofing systems ranking the surfacing materials differently. In all cases, the failures were at the interface between the waterproofing membrane and the tack/bond coat, so the failure stresses were more dependent on the properties of the waterproofing systems than the asphalt mixtures. For Membrane B systems, the highest failure stresses were measured with tack coat R and sand carpet. For Membrane A systems, the failure stresses were higher for tack coat P than for tack coat Q. The high values for mastic asphalt were probably due to the high temperature at which the material was laid and compacted and at which the tack/ bond coat was activated. Based on these findings and other considerations, various additions and changes to the Design Manual for Roads and Bridges, Specification for Highway Works and Notes for Guidance on the Specification for Highway Works have been proposed. The main changes include:! sub-surface drainage is emphasised;! bond requirements are strengthened;! deformation requirements are specified for all mixtures within 100 mm of the surface; and! maximum air voids content limits on all asphalt mixtures. Aspects that were not fully covered are permeability testing of the asphalt and at the interfaces. Potential tests have been identified that could be developed for standardisation if these aspects are considered critical. The asphalt permeability can be covered by the surrogate of air voids content, but this is more difficult at interfaces where more than one material is involved. For flexibility, the test selected from those put forward is the semi-circular bending test. This test was found to be practical, equivalent to a controlled stress fatigue test and ranks materials similarly to binder content, a known component of flexibility if other things are constant. It is suggested that initially the values of 3.0 N/mm² and 19 N/mm 3/2 should be set as the minimum values required for tensile strength and fracture toughness, respectively. These limits are considered practical because the majority of the results obtained exceeded them. 6 Acknowledgements The work described in this report was carried out in the Infrastructure and Environment Division of TRL Limited. The authors are grateful to Kevin Green, Mario Patchett and Jon Harper, who carried out all the laboratory testing that underpin this work, and to Val Atkinson, who carried out the quality review and auditing of this report. Particular thanks are given to the two manufacturers who applied their waterproofing systems to concrete samples and to the four positive responders to the questionnaire. 24

29 7 References British Board of Agrément (2000). Guidelines document for the assessment and certification of thin surfacing systems for highways. BBA-HAPAS SG3/98/169, Working Draft 3. Watfird: British Board of Agrément. British Standards Institution (1988). Specification for mastic asphalt (limestone fine aggregate) for roads, footways and paving in building. BS 1447: London: British Standards Institution. British Standards Institution (1993). Methods for determination of the indirect tensile stiffness modulus of bituminous mixtures. British Standard Draft for Development DD 213: London: British Standards Institution. British Standards Institution (1996). Testing concrete Recommendations for the determination of the initial surface absorption of concrete. BS : London: British Standards Institution. British Standards Institution (2001). Coated macadam (asphalt concrete) for roads and other paved areas Part 1: Specification for constituent materials and asphalt mixtures. BS : London: British Standards Institution. British Standards Institution (2002). Hot rolled asphalt for roads and other paved areas Part 1: Specification for constituent materials and asphalt mixtures. BS 594-1: London: British Standards Institution. Comité Européen de Normalisation (2000a). Bituminous mixtures Material specification. Part 5, Stone mastic asphalt. Draft BS EN , DPC No. 00/100954DC. London: British Standards Institution. Comité Européen de Normalisation (2000b). Bituminous mixtures Test methods for hot mix asphalt Part 27: Sampling. BS EN : London: British Standards Institution. Comité Européen de Normalisation (2002a). Bituminous mixtures Test methods for hot mix asphalt Part 7: Determination of bulk density of bituminous specimens by gamma rays. BS EN : London: British Standards Institution. Comité Européen de Normalisation (2002b). Bituminous mixtures Test methods for hot mix asphalt Part 5: Determination of the maximum density. BS EN : London: British Standards Institution. Comité Européen de Normalisation (2003a). Bituminous mixtures Test methods for hot mix asphalt Part 33: Specimen prepared by roller compactor. BS EN : London: British Standards Institution. Comité Européen de Normalisation (2003b). Bituminous mixtures Test methods for hot mix asphalt Part 8: Determination of void characteristics of bituminous specimens. BS EN : London: British Standards Institution. Comité Européen de Normalisation (2003c). Bituminous mixtures Test methods for hot mix asphalt Part 22: Wheel tracking. BS EN : London: British Standards Institution. Comité Européen de Normalisation (2004a). Bituminous mixtures Test methods for hot mix asphalt Part 35: Laboratory mixing. BS EN : London: British Standards Institution. Comité Européen de Normalisation (2004b). Bituminous mixtures Test methods for hot mix asphalt Part 24: Resistance to fatigue. BS EN : 2004 British Standards Institution, London. Comité Européen de Normalisation (2004c). Bituminous mixtures Test methods. Part 19, Permeability of porous asphalt specimen. BS EN : London: British Standards Institution. Daines M E (1994). Tests for voids and compaction in rolled asphalt surfacings. Project Report PR78. Wokingham: TRL. Grube H and C D Lawrence (1984). Permeability of concrete to oxygen. Proceedings of the RILEM Seminar on Durability of concrete structures under normal outdoor exposure. Hanover, pp Hassan K E and J G Cabrera (1997). Controlling the quality of concrete by measuring its permeability. Proceedings 13th International Conference on Building Materials, Volume 3. Weimar-Germany, pp. 3/005-3/0017. The Highways Agency, Scottish Development Department, The National Assembly for Wales and The Department for Regional Development Northern Ireland. Manual of Contract Documents for Highway Works. London: The Stationery Office: Volume 1: Specification for Highway Works (MCHW 1). Volume 2: Notes for Guidance on the Specification for Highway Works (MCHW 2). Highways Agency, Scottish Executive Development Department, National Assembly for Wales and Department for Regional Development, Northern Ireland. Design Manual for Roads and Bridges. London: The Stationery Office: BD 47/99: Waterproofing and surfacing of concrete bridge decks (DMRB 2.3.4) HD 36/99: Surfacing materials for new and maintenance construction (DMRB 7.5.1) 25

30 Nicholls J C and Carswell I (2004). Durability of thin asphalt surfacing systems: Part 2: Findings after 3 years. TRL Report TRL606. Wokingham: TRL. Nicholls J C and Daines M E (1993). Acceptable weather conditions for laying bituminous materials. Project Report PR13. Wokingham: TRL. Peoples Republic of China (1993). Complete ultimate strain at a low temperature 10 ºC: > 6 x 10-3; Asphalt Bending Test. PRC Requirement T Zoorob S E, Cabrera J G and Suparma L B (1999). A gas permeability method for controlling quality of dense bituminous composites. Proceedings of the 3rd European Symposium on Performance and durability of bituminous materials and hydraulic stabilised composites. pp Leeds. 26

31 Appendix A: HA draft notes for bridge-deck overlays A.1 Sub-surface drainage All blacktop leaks to some extent, through joints etc and the use of macadam bases with void contents that may be (at worst) up to 7 or 8 % and more porous quiet surfacings may well exacerbate this. Advice is given in DMRB Vol 2 Section 3 Part 4 BD 47/99 Chapter 4 Drainage. In paragraph 4.2 on sub-surface drainage, to quote: Bituminous surfacing is porous and can retain surface water. Where the geometry of the deck or deck movement joints prevents this water from draining naturally through surface drainage, sub-surface drains shall be provided. Advice on sub-surface water drainage is given in BA 47 (DMRB 2.3.5) The term surfacing in this instance is referring to the bituminous overlay above the waterproofing not just the surfacing course. Further information is given in TRL Application Guide 33 Water Management for Durable Bridges in the drainage section. Other useful references are BD 33/94 and BA 26/94 both concerning expansion joints and covering their drainage. Bridge-deck overlays (the surface course and base layers) should always be regarded as porous and drainage should be provided below the overlay at low points over the bridge deck waterproofing. Edge drainage should be provided at joints where compaction is likely to be least efficient and sub-surface drainage installed where interstitial ponding may occur. There have been one or two instances of fretting in thin surfacings at such locations, caused by poor compaction. It is important to note that this may not be on the bridge deck side of the joint, depending on the gradient. A.2 Bond to the waterproof membrane Another issue is the bond of the overlay to the waterproofing membrane. This tends to be relatively low with the modern waterproofing systems now in use and may be disrupted by pressures generated under traffic if the overlay becomes saturated. There have been instances where this appears to have occurred; in particular where the blacktop overlay, usually in the past surfaced with HRA, is less than the standard 120 mm in thickness. A.3 The thin surface course It is government policy to use quiet surfacings on all trunk roads in England, including motorways. The capacity of these proprietary systems to waterproof the base layers below often appears to depend more on the bond or tack coat applied than the apparent porosity of the surface course. Anecdotal evidence suggests that thin surfacing systems with an open texture laid on a heavy polymer modified bond coat can be more effective at sealing and waterproofing the base layers than thicker, less open systems (SMAs) laid on a thin tack coat. A.4 Sub-standard overlay thickness A bridge-deck overlay is more vulnerable the thinner it is. Modern waterproofing systems are very effective at waterproofing. However they are resilient and although they are designed to be applied in very thin layers, concrete bridge decks to which they are applied are often quite rough. This can result in thicker areas of membrane, increasing the resilience. Premature failures have occurred with both hot rolled asphalt and stone mastic asphalt due at least in part to fatigue of a sub-standard thickness overlay. Such failures are always associated with debonding of the overlay and it has not been possible to determine the primary cause of these failures poor bond, water saturation and traffic generated pressure, or fatigue. It is likely that each plays some part. A.5 Actions to be considered before applying for a departure from standards to use a sub-standard bituminous overlay less than 120 mm in thickness i Re-assessment of the structure to maximise the thickness of the bituminous overlay. ii Specification of the highest modulus approved proprietary waterproofing system obtainable. (note: the need for this will diminish with increasing thickness of overlay and where the thickness of the membrane can be kept to a minimum.) iii Omission of the sand asphalt layer above the waterproofing system. iv Specification of a waterproofing system which includes the provision of a bond coat for the asphalt overlay. Where the waterproofing system offers alternative bond promoting treatments, the specification of the treatment claimed to provide the maximum bond shall be specified. Any proprietary bond coat between the waterproofing system and the overlay shall be a tackfree material, such that it does not adhere to tyres of vehicles delivering asphalt to the paver. v Provision of sub-surface, edge and joint drainage as appropriate, to reduce or eliminate water pressure under traffic. vi Specification of a paver-laid hot rolled asphalt layer or layers, containing an elastomeric polymer and an appropriate aggregate size, to form the overlay binder course, all in accordance with SHW Clause 943, (but omitting any coring over the bridge-deck!). vii Specification of a thin surface course system which incorporates a heavy elastomeric-polymer modified bond coat. The risk of premature failure of sub-standard thickness bituminous overlays on bridge decks is considerable. For further advice or clarification, please contact James Gallagher or John Williams of SSR RDS Highways Infrastructure Group. James Gallagher: Tel: (GTN ) John Williams: Tel: (GTN ) or (GTN ) 27

32 Appendix B: Questionnaire 28

33 29

34 30

35 31

36 Appendix C: Permeability tests C.1 TRL permeability test The TRL permeability cell (Figure C.1) is similar to that developed by the Cement and Concrete Association (C&CA, since renamed the British Cement Association, BCA) (Grube and Lawrence, 1984) and Leeds University (Hassan and Cabrera, 1997) for concrete under differential pressure techniques. Measurement of air permeability was conducted by sealing the curved surface of 100 mm diameter cores and applying a desired pressure on one side of the specimen. The pressure gradient across the specimen results in a flow, which is measured at the other side using a flowmeter. The test duration is quite short, less than 30 min, depending on the air voids content and the continuity of voids of the tested asphalt. Figure C.1 The TRL permeability cell The intrinsic oxygen permeability can be determined from the measurements of flow rate according to the modified D Arcy s equation, as given in Equation (C.1). 2νηLP2 K = A P P 2 2 ( 1 2 ) where: K = intrinsic air permeability (m²) ν = flow rate (m³/sec).. (C.1) η = viscosity of air ( N.s/m² at 20 C) L = length of the specimen (m) A = cross-sectional area of the specimen (m²) P 1 = inlet absolute applied (gauge) pressure (bar) P 2 = outlet pressure at which the flow rate is measured (bar), usually 1 bar C.2 Interface permeability test The water permeability at the interface between the asphalt surfacing and the waterproofing system was measured using the apparatus described in BS : 1996 for measuring the initial surface absorption of concrete. The principle of the test is to determine the time taken for a quantity of water to flow through a calibrated glass tube onto a given area of the test specimens. The composite slabs (concrete, waterproofing and asphalt) were sliced vertically down the centre to provide cut faces specimens with the waterproofing system along the centre. The test cap was sealed into the sliced specimens using silicon rubber and a clamp mechanism, as shown in Figure C.2. Water was then introduced through the reservoir, funnel, to fill the test cap. The amount of water leaking through the sliced specimens was recorded periodically to determine the flow rate per unit area of the test specimen. Figure C.2 The initial surface absorption test for measuring permeability at the interface 32