A design and specification guide for Scotland s road authorities to facilitate the use of recycled and secondary aggregates

Transcription

1 Creating markets for recycled resources A design and specification guide for Scotland s road authorities to facilitate the use of recycled and secondary aggregates Research Report: Aggregates Section 4: Case Studies 1. Stabilisation of landfill material for road construction 2. HBM road base, sub base and capping containing crushed concrete 3. The use of cement stabilised foamed bitumen mixture as base and binder layers in various road schemes 4. Recycled asphalt planings and soil as a sub base material Project code: AGG0073 Promoted by: SCOTS (Society of Chief Officers for Transportation in Scotland) Published by: The Waste & Resources Action Programme The Old Academy, 21 Horse Fair, Banbury, Oxon OX16 0AH Tel: Fax: WRAP Business Helpline: Freephone: Date: January 2006 ISBN:

2 No. 1: Stabilisation of landfill material for road construction 1 Background 1.1 Project information Name Rainham Landfill Access Road Phase 1 Region London Borough of Havering Type of Project Pavement Reconstruction Date 2003 Client Cleanaway Ltd Contractor Fitzpatrick Contractors Ltd Consultant Engineer Bureau Veritas Consulting Specialist Consultant Pavement Technology Limited (PTL) 1.2 Project Details The project consisted of 900m of pavement reconstruction that provides the main access route into Cleanaway s Rainham Landfill Site. Being over former marshland, the existing pavement was showing some signs of unacceptable deformation and distress. The design life was approximately 5.5msa, based on remaining landfill capacity. It was established at an early stage that potential on-site material sources were present, that if stabilised could provide the following: A higher performing base construction compared with unbound sub-base; Reduced costs associated with using reclaimed materials from site compared with importing virgin aggregates. In addition to material costs, these would also include transport and aggregate levy. The ground conditions were such that a large amount of conventional unbound sub-base material would be required to provide an adequate support structure for laying asphalt. 2

3 The approach was to assess all available potential materials available within the landfill site, including the existing roadway. Four materials were appraised for their ability to create hydraulically bound materials. The volume and variability of available materials also had to be considered. 1.3 Local Materials Figure 1 overleaf shows the area, around the Cleanaway Landfill site, where potential construction materials were sampled. A description of these materials is listed in Table 1 below. Table 1: A description of material within the site Sample A B C D E F Description Stockpile of incinerator bottom ash (IBA), which was screened to comply with the grading needed for 6F1 material. Stockpiled IBA. Settlement lagoon silt arising from dredgings from the River Thames placed by Port of London Authority. Two samples were taken as gradings were ostensibly very different, firstly from the surface and secondly from a level approximately 2.5m depth. This sample was removed from a stockpile of sand from a process to reclaim the 40mm diameter aggregate from the silt lagoon. This sample is comprised of granite fines, passing the 75µm sieve. Recycled asphalt planings (RAP) from the existing pavement. 3

4 Figure 1: Plan showing the areas, around the Cleanaway Landfill site, where sample materials were taken 4

5 1.4 Mix Design Visual inspection of the materials in Table 1 indicated that material A and D were granular and consisted of fine and coarse aggregate whilst samples B, C, and E were very fine material with plastic properties. More detailed appraisal of samples A (IBA), D (sand with some gravel) and F (RAP) was completed, consisting of a laboratory testing programme on a series of four mixtures. In each case, the binder was ordinary Portland cement (OPC) and mixtures were compacted at optimum moisture content (OMC). Mixture details are as follows: 1. Material A only, a. 96% material + 4% OPC; b. 92% material + 8% OPC; 2. Material D only, a. 96% material + 4% OPC; b. 92% material + 8% OPC; 3. Material A and Material D in a 50-50% blend a. 96% material + 4% OPC; b. 92% material + 8% OPC; 4. Recycled Asphalt Planing (RAP) only; a. 92% RAP + 3% OPC + 5% PFA; b. 89% RAP + 6% OPC + 5% PFA; 1.5 Pavement Design Initially, a falling weight deflectometer (FWD) survey was completed on the existing pavement to examine the condition of each layer and the subgrade stiffness. As a result, some existing fill materials were left in-situ to provide a more competent formation and a compaction platform for the RAP. PTL used an analytical design approach to reduce the foundation layer using hydraulically bound recycled asphalt planings (RAP), compared with unbound sub-base. This is estimated to have reduced costs by 30% compared with the equivalent flexible design using imported unbound sub-base. 2 Construction specification and related details 2.1 Specification Target moisture content was OMC ±1.5% with no values exceeding the optimum value by ±2%. Minimum mean dry mass density had to be equal to 95% of the refusal density with no individual value below 93% of the refusal density. The mean layer thickness was specified as the design value ±10mm with no value falling 20mm below the target. A tight control is required to maintain the added cement, (target ± 1%) RAP compressive strength at 7 days of curing was specified as 7.0MPa mean with no value below 5.0MPa. 2.2 Construction The reconstruction process was as follows: 1. Plane off existing asphalt. 2. Soft areas were stabilised within the unbound sub-base layer 5

6 3. RAP was used as a lower base layer after stabilisation with cement mm asphalt layers were used as an upper base and surface layers. Details of the layer system are shown in Table 2. Table 2: Layer system for the reconstruction of the existing road 40mm 170mm 150mm 200mm or greater 600mm or greater Surface layer Upper base layer (or 50mm binder course and 120mm base layer) Cement stabilised RAP or material D with 6 percent OPC, during construction, CBR >60% Existing Sub-base layer (stabilised any soft areas) CBR >30% Existing capping layer with CBR value of 20% or greater 6

7 (a) (b) (c) Figure 2: (a) Hydraulically bound recycled aggregate (b) Hydraulically bound secondary aggregate (c) Finished pavement construction 7

8 3 Performance results Before the final surfacing, the completed pavement was assessed, cored, and surveyed using a FWD. It was established that the RAP layer was performing better than anticipated at the design stage. This enabled the surface course of asphalt to be replaced with a micro-surfacing to act as a sealant and to provide skidresistance, thereby giving further cost-savings. This was because the improved RAP stiffness meant that a reduced total asphalt thickness was possible. Recent pavement engineering research has focused on interlayer-bonding as de-bonding had been found to reduce pavement design life. Whilst a fully-bonded pavement was found to give a design life well in excess of the design requirement, the sensitivity to de-bonding was also analysed. This analysis demonstrated that reduced interlayer bonding would still achieve the required design life. Indications are that the bond between the RAP and the asphalt was the most important. The pavement is now two years old and no sign of structural deterioration has been found. 4 Conclusions The project demonstrated that stabilised asphalt planings reclaimed from pavement reconstruction performed better than imported granular sub-base. Moreover, this was found to be a more sustainable and cheaper option. The RAP also gave structural benefits, enabling reduced asphalt thickness. It is important to ensure that the pavement layers are fully bonded, otherwise design life can be much reduced. 8

9 No. 2: HBM road base, sub base and capping containing crushed concrete (RCA) 1 Background 1.1 Project information Name Rainham Landfill Site (Phase 2) Region Type of Project London Borough of Havering Landfill access road Date 2004 Client Contractor Consultant Engineer Specialist Consultant Cleanaway Ltd Fitzpatrick Contractors Ltd Bureau Veritas Consulting Pavement Technology Limited (PTL) 1.2 The project This project formed the second phase of the access road into Cleanaway s Rainham Landfill Site. This section was completely new build across former landfill. Unlike the existing access road, this section follows the site perimeter rather than through areas of ongoing landfilling operations. The new section of road would therefore provide additional void space which will extend landfill operations for approximately six years. Total road length was approximately 1.3km. 1.3 Pavement Design Design traffic loading was for 5.5msa, as for Phase 1. Unlike Phase 1, there was no on-site source of aggregate for a stabilised base. Reasons for this included the following: Insufficient incinerator bottom ash (IBA) quantities available; It was subsequently found that large scale extraction from the silt lagoons would require an Environmental Impact Assessment (EIA) which would unduly delay the programme. To obtain sufficient recycled material, the Contractor was provided with a landfill tax exempt area within the landfill site to stockpile and crush concrete arisings from the local area. This enabled gradual material stockpiling, thereby avoiding having to find large quantities at relatively short notice. This approach was considered to obtain raw materials at a more competitive price. Recycled concrete aggregate (RCA) thus obtained, was to be use as a hydraulically bound base with a flexible overlay. 9

10 Being a former landfill, ground conditions were potentially highly variable. Some landfill areas date back to the late 19 th Century and contain a high proportion of ash and these areas were found to be relatively competent. More recent landfill areas with high plastics content were found to be of much reduced strength. A constraint was to avoid excavation and handling of waste on the following grounds: Health and safety risk to operatives involved; Disposal to landfill, thereby losing the client further void; The vertical alignment was therefore chosen to avoid any unnecessary excavation with local hollows filled with recycled concrete aggregate from the site stockpiles. This excavation constraint meant that the pavement had to be constructed on whatever formation prevailed. An investigation into the risk of variability was examined using dynamic compaction which also gave some ground improvement at the same time. 2 Construction specification and related detail 2.1 Specification The majority of the site was constructed with a flexible composite pavement structure consisting of an asphalt surface course, asphalt base and two hydraulically bound base layers laid directly onto formation. The dynamic compaction identified a small section where ground conditions were considered too poor to risk fully flexible construction. Moreover, the waste at this section had a high proportion of plastics rendering the ground unsuitable for normal types of ground improvement. This section was constructed as a rigid composite pavement consisting of an asphalt surface course, continuous reinforced concrete pavement (CRCP) and hydraulically bound material contained within a geocell. The construction for each pavement type is shown in Tables 1 and 2 below. Table 1: Flexible composite pavement construction 30mm 145mm 300mm 400mm SMA Surface layer Upper base layer HDM50 Cement-stabilised RCA lower base layer The long term stiffness modulus of this layer is a minimum of 500MPa RCA (6F1) with the top 300mm cement-stabilised. The long term stiffness modulus of this layer is a minimum of 300MPa when stabilised and 80MPa where unbound. Design subgrade CBR = 2% 10

11 Table 2: Layer system for the construction of the flexible rigid pavement 30mm 255mm SMA Surface layer Continuous reinforced concrete pavement (CRCP) 100mm 200mm 100mm Cement stabilised RCA (6F1) designed for stiffness modulus of 200MPa Cement stabilised RCA placed within a 240 x 240 x 200mm Geocell The design stiffness modulus is 60 to 100MPa Class 6F1 capping comprising RCA with minimum stiffness modulus of 30MPa The subgrade is consisted of up to 7m of waste with a high plastics content. CBR was not considered valid and stiffness was too low to measure with a lightweight deflectometer. In both cases, the lower stabilised layer s primary role was to act as a compaction platform for the upper stabilised layer. Figure 1: Grading of sub-formation 11

12 3 Stabilised Mix Design and Performance As the RCA played a significant part in the construction of this project, this material was assessed in the laboratory prior to the design. This assessment was carried out not only to optimise the mixture but to obtain pavement design parameters. Two mixture designs were considered; 4 and 6% OPC content. The following parameters were assessed for each mixture: Particle size distribution. Optimum moisture content (OMC). 150mm cubes specimens to carry out compressive strength tests, and 100 x 100 x 500mm beams specimens to carry out flexural strength tests. Both groups of specimens were prepared with three levels of moisture content. These levels of moisture were OMC value, OMC + 3% and OMC 2%. Determination of the compressive strength of specimens sealed cured for 7, 14 and 28 days. Determination of the compressive strength of specimens sealed cured for 14 days and then cured in water for another 14 days and compared with specimens sealed cured for 28 days Determination of flexural strength of specimens sealed cured for 14 days. 3.1 Particle size distribution The particle size distribution of the RCA was determined in accordance with British Standard BS EN The material grading was found to satisfy the requirements of a Class 6F1 capping material and closely satisfies the requirements for a CBM2 as specified in the Specification for Highway Work (SHW) Series 1000 Clause 1037, Table 10/12. A typical grading curve and the specification requirement for 6F1 and CBM2 are shown in Figure 1 below. 12

13 CBM2 series Nov F1 Series 1000-Nov.2003 Grading used in the investigation mixed Passing (%) Sieve size (mm) Figure 2: Particle Size Distribution of material C1 and C Optimum moisture content (OMC) Optimum moisture content (OMC) results are shown in Figures 3 and 4 for 4 and 6% cement respectively. The OMC for both mixtures was found to be 11 percent by mass of the dry aggregate-cement blend. In any mix design, it is important to confirm OMC at the time of compaction and therefore the correct binder content needs to be included. 13

14 Density (kg/m3) %cement Dry density 1800 Bulk density Moisture content (%) Figure 3: Optimum moisture content with 4% cement Density (kg/m 3 ) %cement Dry Density Bulk Density Moisture content (%) Figure 4: Optimum moisture content with 6% cement 14

15 3.3 Compressive strength Specimens were prepared at three moisture contents: 11% (OMC), 14% (OMC+3%) and 9% (onc-2%). The specimens were tested after sealed curing for periods of 7, 14 and 28 days, as well as specimens sealed cured for 14 days and then 14 days cured soaked in water. The compressive strength test was carried out in accordance with BS 1881 part 115: The densities of all tested specimens were calculated by volume using the measurements of weight and the dimensions of each specimen. The test results of compressive strength values and the bulk density of the specimens are shown in Tables 3. Table 3: Test results, compressive and flexural strength Curing time days Compressive strength (MPa) at different moisture content OMC - 2% OMC OMC + 3% OMC - 2% OMC OMC + 3% Cement content = 4 percent Cement content = 6 percent 7 sealed sealed sealed sealed + 14 soaked in water Retained percentage Bulk density (gram/cm 3 ) 7 sealed sealed sealed sealed + 14 soaked in water Flexural strength (kpa) 14 sealed 324 to to 2800 Optimum moisture content (OMC) was found to be around 11 percent The compressive strength values of the 6% mixture was found to be less sensitive to the variations in moisture content compared with the 4% mixture. Also, the 6% mixture showed a high retained value of compressive strength. It is important that curing conditions are as consistent as possible with site conditions because compressive strength improves significantly when cured in water. The compressive strength value after 7 days curing for mixture with 6 percent cement will satisfy the requirements for CBM Flexural Strength Beam specimens 100mm x 100mm x 500mm were prepared for both mixtures, 4 and 6% cement content. Specimens were prepared at three moisture contents: 11% (OMC), 14% (OMC+3%) and 9% (OMC-2%). These specimens were sealed cured for 14 days and then tested to determine the flexural strength. The 15

16 process of sample preparation, curing and testing was carried out in accordance with BS EN 12390:2000, parts 1, 2 and 5. The testing was carried out at a loading rate of 0.04 to 0.06MPa/s using two point loading system. These tests results are tabulated in Tables 3 above. Flexural strength for the 4% cement mixture varied from 324kPa to 1318kPa, whilst the value for the mixture with 6 percent cement was found to vary from 1470kPa to 2800kPa. This means that the flexure strength of the 6% mixture is much improved over the 4% mixture. Flexural strength is a more important pavement design parameter that compressive strength so it is important to keep this as high as reasonably possible. The large flexural strength improvement using the 6% mix was the reason for choosing this mix for use on the works. Figure 5: Core of the cement stabilised RCA 4 Conclusions RCA with grading within the Class 6F1 envelope has been used successfully to construct 5.5msa pavement design over a very poor subgrade. Material assessment is very important and should be carried out prior to detailed design. The design should be based on the laboratory assessment of any stabilised material whilst being mindful of the risk that the site performance is always below the laboratory values. The design procedure should follow closed loop approach. The closed loop approach is as follow: o o o o o Material assessment, Structural design, Construction specification, Site trial assessment (if necessary), Sampling and testing during construction 16

17 o Testing post-construction to confirm that the design requirement are met Careful assessment of very poor subgrade; always use more than one method of assessment. It is often misleading information may be obtained from some apparatus. RCA can be treated with hydraulic binder with different dosages to achieve different levels of performance. 5 References Figure 6: Finished road Manual of contract documents for Highway Works. Volume 1 Specification for Highway Works BS EN Tests for geometrical properties of aggregates. Determination of particle size distribution. Sieving method BS :1986 Testing concrete. Part 115 Specification for compression testing machines for concrete. BS EN : Testing hardened concrete. Part 1 Shape dimensions and other requirements for specimens and moulds. BS EN : Testing hardened concrete. Part 2 Making and curing specimens for strength tests. BS EN : Testing hardened concrete. Part 3 Flexural strength of test specimens. 17

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19 No. 3: The use of cement stabilised foamed bitumen mixture as base and binder layers in various road schemes 6 Background 6.1 Project information Name Region Type of Project Refurbishment of Church Hill Street Glenrothes Road Construction Date Client Contractor Stabilisation Contractor Specialist Consultant Fife Council Tilcon Contracting (now Tarmac) Linear Quarry Products (LQP) Advanced Pavement Technology Centre (now Pavement Technology Limited) 6.2 The project A cement stabilised foamed bitumen base (DURAFOAM) was used in the refurbishment of Church Hill Street. The client had previous experience of recycling operations and chose to use recycled material in preference to conventional materials. Repairs to part of the area reconstructed using conventional materials showed significant signs of distress and full depth failure after only 18 months. It was agreed with the client that the entire asphaltic layers would be removed and replaced with a cement stabilised foamed bitumen mixed with recycled asphalt planings (RAP) used as a base layer along with conventional surfacing layers. It was envisaged that the removal of the existing granular sub-base layer, with some large boulders, would be detrimental to the performance of the pavement. The sub-base CBR value was found to be around 30 percent. For some locations the CBR value fell below 20 percent which can be attributed to the disturbances caused by the testing excavation. The decision was made to leave the sub-base layer in place as the performance of the sub-base would be enhanced significantly once the construction was completed due to the confinement. The subgrade was found to have CBR values of 2 to 5 percent. The busy town centre route is a feeder road for HGV traffic to a large town centre shopping mall and multistory car park. The section of road repaired was between two roundabouts and was effectively a 4 lane carriageway. With a bus stop on either side it tended to operate as two lanes given the high frequency of PSV traffic approximately every 15 minutes from the main bus terminus 500 metres away. The roadway was constructed with the following layer thickness: 40±5mm Stone mastic asphalt as surface course 19

20 70 ±5mm Dense Bitumen Macadam as binder course 125±15mm Cement stabilised foamed bitumen base layer c200mm minimum existing granular layer as sub-base (a) (b) Figure 1: (a) Laying material using an auger (b) Cement Stabilised foamed bitumen mixture 7 Construction specification and related details 7.1 Specification The material Supplier (LQP) provided the cement stabilised foamed bitumen base which was designed in a previous programme of work. The details of the mixture design are as follows: The target moisture content was 6.5 percent, based on 85 percent of the Optimum moisture content. The foamed bitumen content = 3.5 ±0.5 percent Cement content = 1.5 ±0.5 percent Pulverised Fuel Ash (PFA) = 5 ±1 percent Aggregate (RAP) content = 90 ±1 percent. To insure a particle size distribution that matched grading Zone A (TRL 611), it was found necessary to replace 4 to 8 percent of the RAP with crushed rock fine aggregate. The material ingredients were mixed in LQP s mobile plant whilst the material was laid by Tilcon Contracting. Tilcon Contracting used a combination of a 10t deadweight roller and a 10t PTR to maximise compaction on the relatively soft base. Materials were transported from Blantyre due to the lack of suitable ground for temporary recycling operation in the busy town centre. The contract involved the supply of 1000t of cement enhanced foam bitumen macadam for base construction (Figure 1). Traffic was kept off the surface until the conventional hot laid binder course was constructed. The surface course was SMA. 20

21 7.2 Compaction Generally the base layer is an important structural element of a pavement. It is required to spread the wheel load so that the underlying materials are not over-stressed. The construction of this layer will depend on many factors such as: material properties and performance; the laying and compaction procedure used; and, the properties of the underlying material. Foamed bituminous materials require a high energy of compaction to consolidate the layer, encourage aggregate interlock, and encourage adhesion of the bitumen to the aggregate. In cases where the underlying layers lack stiffness the base layer will exhibit a density gradient through the depth of the layer. The lower part of the layer will exhibit a lower density than the top of the layer; therefore the dynamic stiffness modulus will be smaller at the bottom of the layer compared with the top of the layer. This gradient of dynamic stiffness modulus can be reduced when the underlying material is stabilised and the layer is well compacted. Any differences in the dynamic stiffness modulus through the depth of the base should be taken into consideration when designing or analysing a structure. This has been considered with the design of Church Street. A density and dynamic stiffness modulus gradient within a material layer can be accounted for in the laboratory by preparing specimens with different levels of compaction energy. Specimens are prepared by applying different numbers of Marshall hammer blows. 7.3 Effect of moisture Moisture content has a significant influence on the performance of foamed bitumen stabilised aggregate mixtures. The moisture content affects the distribution of the foamed bitumen within mixture ingredients; it also influences the compaction process and the curing time. High moisture content will lead to high void content which will influence the mechanical properties of a pavement layer, and consequently the performance of a pavement structure. During compaction a foamed bitumen mixture is in essence a moist granular material, the moisture content of a mixture has a significant effect on the density that can be achieved. Each residual one-percent by mass of moisture within a mixture converts to a two-percent voids content (a two-percent bulking of a mixture). It is critically important that the moisture content to achieve maximum density is defined. It should be recognised that maximum density and the moisture content to achieve maximum density of a mixture are related. These in turn are related to the energy of compaction applied to the mixture. A higher maximum density and lower moisture content to achieve maximum density occur when a higher level of compaction energy is applied to a mixture. Target moisture content is always below (say 85 percent) the optimum moisture content of the material ingredient excluding the foamed bitumen. The foamed bitumen will compensate for this reduction. 8 Lessons Learnt Since August 1999 the reconstruction using DURAFOAM has surpassed all expectations and out performed the previous reconstruction in the area which used conventional materials. Cores taken at 18 months showed a continuing increase in stiffness values. The pavement design check carried out using the data gathered from this contract verified that the life expectancy was consistent with the initial design of between 1.3 and 3.2msa minimum life. This was based on the initial performance of the pavement layer system. Certainly the continuous curing of the base will enhance the pavement life over time. The roadway construction success can be attributed to the programme of testing accomplished prior to the roadway construction. A site trial was carried out at Church Street prior to undertaking the refurbishment work. Two panels (1.5 metre by 2 metres by 300 mm deep) were filled with the same cement stabilised, foamed bituminous material as used in the preliminary laboratory investigation. The panels were filled in three layers; each layer was compacted using a wacker plate. A thin surface layer of hot bituminous material was applied to the foamed bitumen base material in each panel. The site trial panels were laid on a warm summer day. The two panels were dry-cored by Heriot-Watt University at an age of 10 days. Tests carried out on cores, 21

22 material taken from site trial pits and material taken from the main construction (as quality control) led to the following finding: 8.1 Influence of layer compaction Cores obtained from the site trial were tested for dynamic stiffness modulus, moisture content and density and void content. Each core was cut to form three sub-layers: top, middle and bottom. Measurements were made of the dynamic stiffness modulus, dry mass density and moisture content for each sub-layer and for each core. The data with the cores was found to be compatible with the data generated in the laboratory. The trial panel data confirmed the following. There was a density gradient through the depth of the cores, with a higher density value at the top and lower density value at the bottom of cores. The dynamic stiffness modulus was proportional to the density and therefore developed a similar gradient through the depth of the cores. The test results were found to be in agreement with the data obtained from laboratory fabricated specimens prepared from the same material and sealed-cured with cling film at the same age after compaction. Directly comparing the data from site trial panel cores with laboratory prepared specimens is valid. The reasoning for this is the fact that both materials came from one series of plant batches, and both materials were compacted dynamically. Also, the warm dry weather following the laying of the trial panels would have assisted in making the curing regime of material in the trial panels similar to that of material in the laboratory site data, compacted by the wacker Whacker 1800 laboratory data sealed-curing 60 blows 75 blows Stiffness modulus (MPa) Av erage bottom layer 30 blows 40 blows Average middle layer 50 blows Av erage top layer Dry mass density (g/ml) Figure 2: Density versus stiffness modulus at age 10 days Figure 2 shows the relationship between the site data and the sealed-cured laboratory data. The average densities of the top, middle and bottom of cores were plotted against the corresponding values of dynamic stiffness modulus. Figure 10 also shows corresponding values of dry mass density and dynamic stiffness modulus data gathered from samples subjected to different levels of laboratory compaction energy. Dry mass density and dynamic stiffness modulus data defined at the bottom of cores was lower than values obtained with 30 blows of compaction in the laboratory. However, dry mass density and dynamic stiffness modulus data from the middle of cores lay between the dry mass density and dynamic stiffness modulus data gathered from 30 and 40 blows compaction in the laboratory. The dry mass density and dynamic stiffness modulus found at the 22

23 top of cores was found to be similar to the dry mass density and dynamic stiffness modulus data obtained with 60-blows compaction in the laboratory. The compaction process was found to have a very profound impact on the performance of the foamed bitumen material. Using the data gathered in this investigation a relationship between the dynamic stiffness modulus with dry mass density has been plotted using the dynamic stiffness modulus of the specimens compacted by 75 blows with the Marshall hammer as data. The data gathered from air-cured and sealed-cured specimens compacted with 30 blows, 40 blows, 50 blows, 60 blows and 75 blows were referenced against data gathered with specimens compacted by 75 blows. Dry mass density and dynamic stiffness modulus data gathered with cores from the two trial panels were referenced against the dry mass density and dynamic stiffness modulus of sealed-cured specimens of the same material and compacted with 75 blows by Marshall hammer. The three sets of data are plotted in Figure 3; a straight-line relationship is established. This reveals how significant the process of densification of the material is. This has shown that for each one percent drop in dry mass density there is a drop of around 7 percent in dynamic stiffness modulus value. The dynamic stiffness modulus loses 50 percent of its value with a 7 percent drop in dry mass density. Table 1 summarised the findings. 100 Stiffness modulus as percentage of stiffness modulus of specimens prepared with 75 blows Air-curing, laboratory prepared specimens Sealed-curing, laboratory prepared specimens Site trial data, compared to the sealed-cured specimens Dry mass density as percentage of the dry mass density of specimens made with 75 blows Figure 3: Shift factor for the determination of stiffness modulus from specimens compacted with 75 blows by Marshall Hammer at any age of curing 23

24 Table 1: Shift factor to determine dynamic stiffness modulus from the density with 75 blows by Marshall hammer Percentage of density with 75 blows by Marshall hammer Shift factor for dynamic stiffness modulus Influence of variation in moisture content Data gathered from this investigation consistently exhibits the fact that the initial moisture content (at time of compaction) plays a major role in controlling the maximum dry mass density achieved, and therefore controls the value of dynamic stiffness modulus achieved. A relationship between the initial moisture content and dry mass density has been plotted. Data from 13 supplied samples have been plotted in Figure 4. This relationship shows that as the moisture content increases the dry mass density decreases. It may be concluded from this evidence that the initial moisture content of material should always be kept as low as practically possible. Higher moisture content not only decreases the dry mass density and the dynamic stiffness modulus of a mixture - they also create difficulties with the compaction process through material adhering to the steel rolls of a dead-weight or vibrating roller. It is suggested that the target moisture content with foamed bituminous mixtures is 1.5 percent to 2 percent below the optimum moisture content value. Care must be taken to control the moisture content of stockpiles of recycled asphalt planings used in foamed bitumen mixtures; stockpiles may have a level of moisture content exceeding the optimum moisture content of the designed mixture. 24

25 1.8 Laboratory prepared specimens (different age) 1.6 Site 1, cores specimens Site 2, cores specimens 1.4 Stiffness modulus ratio The initial moisture content above or below the target value Figure 4: The relationship between stiffness modulus ratio and initial moisture content 25

26 No. 4: Recycled asphalt planings and soil as a sub base material 9 Background 9.1 Project information Name Region Type of Project Stabilisation trials with different binders Tayside Distribution park for heavy vehicles Date October 2004 April 2005 Client Contractor Specialist Consultant ScotAsh Ltd Tayside Contracts Pavement Technology Limited (PTL) 9.2 The project The aim of this project was to appraise different binders to stabilise a mixture consisting of Recycled Asphalt Planings (RAP) and contaminated soil. The stabilised layer serves as a foundation platform for pavements with heavy loading. The pavement will serve as heavy goods parking areas for a distribution centre in Tayside. The blend of RAP and contaminated soil provide a mix that is unlikely to appeal to a material engineer even for general fill or capping layers. The only way to use this material is through stabilisation with a hydraulic binder. The hydraulic binder not only will solidify any contaminated material and remove any plastic action of the soil; but will provide a mixture which will serve as a medium to high performance sub-base layer. ScotAsh Limited provided the hydraulic binders. They are blended with different proportions of pulverised fuel ash (PFA) and cement, and a proprietary hydraulic binder (Proprietary A). The stabilisation process aimed to maximise the performances of the PFA and the RAP/soil material. Mixtures modified with the cement exhibited an increase in performance as the dosage increased. However, one mixture modified with cement and the Proprietary A has shown significant performance improvement. Test results from both the laboratory and the site trial indicated that the recycled material can perform well as a stabilised material (Class 3 foundation, TRL611). However mixtures made with Proprietary A, cement and PFA have shown promising results indicating that the material has the potential to qualify as a Class 4 foundation (TRL611). Class four foundations are the highest class within the new design standard. This mixture consisted of 90 percent recycled material with the addition of PFA, Proprietary A and Cement at the percentages of 5 percent, 3.5 percent and 1.5 percent respectively. For this mixture, there was further room for improvement in the performance by increasing the cement percentage from 1.5 percent to 2 or 2.5 percent whilst reducing the Proprietary A and PFA accordingly. 26

27 9.3 What the site trial achieved The work presented in this case study has transformed a blend of materials, hardly suitable for capping, to a high quality sub base and maybe a base layer. The material developed in this project has the potential to be used for low-traffic low-cost roads provided that the surface is protected either with a surface course or with a double surface dressing system. The appraisal is based on data gathered from laboratory investigation and site trials. 10 Construction specification and related details 10.1 The site o The granular material was obtained from the Tayside Depot (Figure 1). The material was pulverised. recycled asphalt planings (RAP) and soil. Figure 1: Pulverised material at Tayside Depot o o o Granular material from Tayside site was brought to the laboratory to be stabilised using a combination of various binders: PFA Cement (finely ground) Proprietary A (Proprietary hydraulic product). The aim of adding the above powders was to change a material only suitable as a capping layer for road construction into a class3 or 4 foundation layer (TRL611). The particle size distribution was carried out on the granular material using the wet sieve method (BS , 1990). Test results are shown in Figure 2. 27

28 Passing (%) Sieve Size (mm) Zone A TRL611 Zone B TRL611 Zone C TRL611 Raw material alone With 6% PFA/Cement (40/60) With 10% PFA/Cement (40/60) With15% PFA/Cement (40/60) Figure 2: Particles size distributions The new specification for recycled and secondary aggregate proposed by the TRL611 report provided three zones (A, B and C) for particle size distribution. Zone C provides coarse grading whilst zone B provides fine grading. Zones C and B overlap to form zone A as an average grading. Mixing the raw material with a range of powder blends (6 to 15 percent) showed that the material lay within Zone C of TRL611 which is the coarse zone. A coarser zone, generally, requires less optimum moisture content to achieve maximum dry density than the equivalent material with fine grading. o Full compaction of stabilised soil is only achieved by optimising the moisture content of the mixture. The test was carried out in accordance with BS1377 part4: The specimens were compacted using a vibrating hammer within a CBR mould. Thee tests were carried out all with 10 percent powder. The differences were in the blend of the hydraulic powder proportion. The optimum moisture content with a powder content of 10 percent was found to be between 7.9 and 8.2 percent, for three different combinations of powder component. The results demonstrated that the effect of powder type is marginal. However experience has shown that the powder percentage has more profound effect on the 28

29 OMC value. It is expected that for mixtures with lower or higher percentages of powder, the OMC values may decrease and increase accordingly Mixture preparation All materials in the laboratory were prepared using a high shear paddle mixture. Material on site was stabilised by spreading pre-mixed bags of 60/40 PFA/ cement along the site (Figure 2) and mixing it into the material using a HOWARD HK30 Harrow (Figure 3). Figure 2: Distribution of powder on site 29

30 (a) Figure 3: (a) Powder was mixed in to the pulverised material using a HOWARD HK30 harrow (b) Shape of harrow blades A tight quality control regime was adopted by fabricating cubes and beam specimens in situ and in the laboratory. The compaction of all beams and cubes in the laboratory and on site were carried out using a Kango hammer. Cylinders were compacted in the laboratory using a gyrator machine, on site they were compacted using a Kango hammer. Laboratory prepared materials were seal cured under ambient temperature. Site data prepared materials were cured outside to simulate site conditions. (b) 30

31 11 Performance assessment A series of laboratory performance parameters were defined; these were: 11.1 Compressive strength gain with time: A compressive strength test is one of the most important evaluations of any cementitious material; the strength parameter is used as a quality control tool as well as a fundamental property of a cementitious material. Compressive strength testing was carried out on all mixtures. The tests are destructive. The tests were carried out in accordance with BS EN 12390: Part 3: Compressive strength is influenced by two important parameters: the powder content and the moisture content. An increase in the powder content certainly increases the compressive strength provided the moisture content during the compaction is kept close to the optimum value. Any increase in moisture content above the optimum had a detrimental effect on the material performance even though the binder powder content was increased. Generally compressive strength at age of 28 days of 3.5MPa is achieved with binder content of 10 percent in which the PFA represent around 6 to 7 percent. Certainly decreasing the PFA content to 5 percent and replacing it with another binder such as Cement, increases the compressive strength to 5.8MPa Flexural and Indirect tensile strength Tensile strength is a significant property of any material forming a pavement layer, particularly with cementitious mixtures. Flexural strength relates to the ability of a brittle material to resist cracking as a result of tensile strains generated by the flexing of the pavement structure. The flexural and indirect tensile strength tests were carried out in accordance of BS EN 12390: Part 5 and 6: Tensile strength, as is the case for compressive strength, is influenced by the powder content and the moisture content. At optimum moisture content an increase in the powder content certainly increases the tensile strength. Test results of specimens consisting of 10 percent binder content have shown flexural strength to be between c1.0mpa to 1.8MPa and indirect tensile strength between 0.65MPa to c1.2mpa. The high value is achieved when the PFA value is reduced to 5 percent and 5 percent of the Proprietary A and the cement Dynamic stiffness modulus The test evaluates the load-spread ability of a material forming a pavement layer. The test is carried out in controlled strain mode to determine the dynamic stiffness modulus where a specimen of 100mm diameter is exposed to an indirect tensile strain of 5 microns. This small deformation level is considered to be within the elastic range of a mixture. A Poisson s ratio of 0.35 was assumed for the material. The test is non-destructive; specimens can be tested at different curing times or different material temperatures. Tests were carried out in accordance with the BS, DD 213: 1993 using a Nottingham Asphalt Tester (NAT). The dynamic stiffness modulus defined using the NAT is described as Indirect Tensile Stiffness Modulus (ITSM). The influence of the cement on the dynamic stiffness modulus is significant. Almost each one percent of cement translated into a minimum 1000MPa in dynamic stiffness modulus. Also, the cement helped the mixture to retain a high percentage of the stiffness modulus value when soaked in water or frozen for 24 hours. All mixtures have PFA; PFA will slowly hydrate with the cement by-product or lime; and then enhance the mixtures performances. Mixtures with Proprietary A and no cement exhibited relatively high stiffness modulus as well as having the potential of improving with curing time. However, these mixtures were found to be more susceptible to water immersion and seriously deteriorated when subjected to one cycle of freeze. Mixtures with PROPRIETARY A and no cement have to be protected from the frost effect Density and moisture content Density and moisture measurement are important parameters and should accompany all the above tests: compressive, tensile and stiffness modulus. The dry mass density and the moisture reflect the compaction status and then the material performance. Low compaction energy or high moisture content or both, decrease the dry mass density. Laboratory measurements generally obtained values between c1.9mg/m 3 to c2.1mg/m 3 31

32 whilst cores from site exhibited values between c1.9mg/m 3 to c2.1mg/m 3. The dry mass density of cores taken from site was found to be around c1.89mg/m 3 to c2.05mg/m Definition of the performance of hydraulically stabilised material The long term performance of any hydraulically treated material is extremely critical. Hydraulically treated aggregate can be divided into three categories: 1. Long term bound layer: Can work as a base layer within a composite pavement structure. This material requires after curing to have relatively high compressive strength not less than 15MPa with flexural strength not less than 2.5MPa 2. Short term bound layer: Developed into cracked layer equivalent to well interlocked aggregate system. This material will have a compressive strength not less than 10MPa with flexural strength between 1.0 to 2.5MPa. This material can perform as a lower base layer with two stage life (before and after cracking) or as a class 2, 3 and 4 foundation type (TRL 611). 3. Stabilised layer: Cracked layer well interlocked aggregate system. This material will have a relatively low compressive strength (5MPa) with a flexural strength less than 1.0MPa. This material will be qualified to work as a class 2 or 3 foundation type. 12 Lesson learnt The process of stabilisation in pavement construction is to maximise the performance of an aggregate to enable it to deal with structural and environmental demands. In this project the aim was to maximise the performance of recycled aggregate which could not be used in pavement construction or used as low performance layer. The stabilisation protocol is governed by the determination of several key factors: particle size distribution, chemical stability, optimisation of water and powder content. This in turn will enable key performance parameters to be quantified during testing of the various mix designs. Material type and powder content as well as the quality of the construction play a significant part in determining the performance of treated aggregate. The most significant parameters can be summarised in the choice of binder and binder content, the compaction energy and the level of moisture content during the construction. Pavement protection from an adverse environmental condition in the first 48 hours is essential. The treated material can be used for low volume roads or footpaths normally constructed with unbound aggregate systems. Compressive strength data was analysed using the profile of the optimum moisture content, the dry mass density and the influence of the dry mass density on compressive strength. The gathered data was adjusted to take into account the fluctuation in moisture content and the fluctuation in the compaction energy. The adjusted compressive strength data versus the powder content of 60/40 PFA/ cement was plotted and shown in Figure 4. The relationship is found to be linear, therefore increases in the powder content cause an increase in the compressive strength. This relationship is valid for this particular material. Unfortunately for this range of mixtures there is no data for 28 days curing. However, the addition of cement allows most of the hydration to take place during the early life. There may be some additional enhancement in performance with time as PFA usually enhances the performance after a long curing period. 32

33 9 8 Based on Laboratory curing 7 Compressive strength (MPa) CBM1 at 7 days curing CBM2 at 7 days curing After 14 days sealed curing (60/40 PFA/Cement) After 7 days sealed curing (60/40 PFA/Cement) After 3 days sealed curing (60/40 PFA/Cement) Dosage of powder PFA/Cement (60/40) Figure 4: Adjusted compressive strength versus powder content Test results from both the laboratory and the site trial indicated that the recycled material can perform well as a stabilised material (Class 3 foundation, TRL611). However mixtures made with Proprietary A, cement and PFA have shown promising results indicating that the material has the potential to qualify as a Class 4 foundation (TRL611). There is room for some improvement in the performance by increasing the cement percentage to 2 or 2.5 whilst reducing the Proprietary A and PFA accordingly. However, the work presented in this report has transformed an aggregate system, only suitable for capping, to a high quality foundation and maybe a base layer. The material developed in this project has the potential to be used for low-traffic low-cost roads provided that the surface is protected either with a surface course or with a double surface dressing system. 13 References Merrill D., Nunn M. and Carswell I, A guide to the use and specification of cold recycled materials for the maintenance of road pavements, TRL Report TRL 611, BS : Methods of test for soils for civil engineering purposes. Part 2 Classification Tests. BS : Methods of test for soils for civil engineering purposes. Part 4 Compaction-related Tests. BS EN : Testing hardened concrete. Part 3 Compressive strength of test specimens. BS EN : Testing hardened concrete. Part 3 Flexural strength of test specimens. BS EN : Testing hardened concrete. Part 3 Tensile splitting strength of test specimens. BS DD : Method for the determination of the indirect tensile stiffness modulus of bituminous mixtures. 33

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