AN INVESTIGATION OF CEMENT COATING FOR NATURAL AGGREGATES IN POROUS ASPHALT BITUMINOUS MATERIALS Abbas M. ABBAS 1

Transcription

1 AN INVESTIGATION OF CEMENT COATING FOR NATURAL AGGREGATES IN POROUS ASPHALT BITUMINOUS MATERIALS Abbas M. ABBAS 1 School of Built Environment, Liverpool John Moores, Byrom Street, Liverpool L3 3AF, UK ABSTRACT Shape, texture, and angularity are among the properties of aggregates that have a significant effect on the performance of road pavement, and as there is a huge amount of natural aggregate all over the world having a limited usage in pavement construction industry, due to its smooth, rounded macrostructure, consequently, there is a need to develop the shape properties of this natural aggregate to fulfill the never ending increasing demand for new materials worldwide. In this study, cement paste was used to coat natural aggregate (namely Croxden) to observe its effect on the main properties of aggregates and compare that with a control mix (namely Arcow). This paper investigates the relationships between Rutting of Porous Asphalt (namely open graded surface course) after the addition of cement paste to natural aggregates, with that of a control mix, using aggregate gradation specified by the British Standard for Asphalt Concrete (AC 14mm open surf) had been determined at the optimum bitumen content, of two levels of bituminous binder, (100/150 pen and 40/60 pen). Rut depths have been evaluated using the Wheel tracking Machines (large and small scales). Results have revealed that there are very encouraging results in the coated aggregate performance. Keywords: Cement Coating, Natural Aggregate, Porous Asphalt, Rutting, Wheel Tracking 1. INTRODUCTION Pavements are a conglomeration of materials. These materials, their associated properties, and their interactions determine the properties of the resultant pavement. Thus a good understanding of these materials, how they are characterised and how they perform, is fundamental to understanding pavement. Throughout the transportation industry there is a continuous need to improve the quality of the materials used for the construction of flexible pavements. These pavements constitute a very large part of the transportation network facility, therefore the performance and durability of bituminous material needs constantly upgrading to meet the socio-economic and environmental demands of the transportation network users. Bituminous pavements consist mainly from graded crushed rock aggregates, sands and asphalt binders. In many countries there are insufficient supplies of crushed aggregates suitable for high stability, low deformation bituminous road surfacing. Often these countries need to import expensive aggregates to meet their construction needs. It follows then, that road construction and maintenance can become a burden

2 on local and national economies. Poorer quality bituminous surfacing made from uncrushed natural gravel or reclaimed waste materials are no longer acceptable by road engineers, for they can fail under the stresses applied by modern vehicles (Fwa, 2006). The use of these materials could therefore be costly in repair bills, and in the resulting traffic delays. It can be seen therefore that there is a need to improve the performance of gravels and recycled materials so they can be used in a wider range of applications. This will allow the imbalance of quarried (crushed) aggregate, versus gravels and recycled material to be redressed. 2. MATERIALS AND METHODS 2.1 Materials Increased aggregate angularity is well known to increase the resistance to permanent deformation in bituminous mixtures; this phenomenon is documented in references (Brannan et al, 1991) and road stone cement coating was not new. In 1973 Schmidt(Schmidt, 1973)showed that adding dry cement at any percentage of less than 10% of the total weight of crushed rock aggregates in the mix will produce an enhanced strength properties of the mix. It can be seen therefore, that cement coating to crushed rock appears to have many advantages over untreated aggregates. However natural aggregates was not treated with wet cement coating and used in road pavement which is the subject of this study and mainly to prove that the applying of coating paste developed at LJMU is capable for upgrading natural aggregates for use in road pavements and remove or ease the restriction implies currently by road engineers on the use of un treated natural aggregates in bituminous mixtures. Fig.1: Macrostructure of Croxden aggregates before and after the cement coating

3 Natural aggregates represented by a Croxden aggregate(plates 1, & 2) that has a rounded, smooth macrostructure (Fig.1) was coated with a layer of cement paste, ordinary Portland cement mixed with water /cement ratio of 0.4 as it was found to be the optimum. Cement paste was representing 10% of the aggregate dry weight. Aggregates sizes retained on 4mm sieve and bigger were coated with cement paste, as smaller particles were found to be difficult to coat as they were sticking together and making small agglomerations, and that will affect the overall mix gradation. In order to have a strong adhesion between the cement paste and the smooth surface of the aggregates sufficient hydration time should be allowed, the designated period for this was 28 days with continuous curing of the coated aggregates at 20 0 C. (Paul K. et al, 1994).Table 1 lists the Permissible Gradation limits according to (BS EN pt 7, 2006) and (BS PD 6691, 2007) and the chosen gradation. Table 1: Permissible Gradation limits and the chosen gradation (BS EN pt 7, 2006; BS PD 6691, 2007) Two types of bituminous binder were used for sample preparation 100/150 and 40/ penetration grades, having specific gravities of g/cm and 1.033g/cm respectively. 2.2 Preparation of porous asphalt samples Cement coated aggregates are to be regarded as a new material, as if they were brought from a quarry.therefore usual tests, carried out on any aggregate evaluation for pavement construction, are performed on it as well. Results of the sieve analysis have shown that, the aggregate blend became courser than before coating due to the shielding coat of the aggregates; this means that the required blend should not rely on the sieve gradation analysis done before the coating process has taking place. Samples of sizes (500x180x50) mm and (305x305x50)mm were made and compacted by a suitable Compactor to an equivalent level of 50 blows / face generated by a Marshall compaction machine, to represent an ESAL of (10E4 10E6) kn (i.e. moderate traffic loading conditions). (Brannan et al, 1991, Colorado DoT, 2007).

4 The bulk specific gravity of the mixes was obtained from the compaction data according to (BS EN pt 6, 2003), while calculating Gmb, dimensional analysis was done, i.e. Gmb was used for calculation of air voids. This was because the mixtures were so coarse that the bulk specific gravity experiment would not have yielded appropriate value of Gmb (Colorado DOT, 2007). Two samples were tested simultaneously by applying a repetitive force of (5,000 N) while a rubber tyre inflated to (0.6 MPa) passes across the slabs for 30,000 cycles (60,000 passes). Test temperature was fixed at 45 0 C, as the testing temperature ranges between 30 C & 60 C (BS EN pt 22, 2003). The deformation of the slabs was first measured at 1,000 cycles at room temperature before heat was applied to obtain an initial testing surface (Datum). The slabs were then heated for four hours for conditioning. At the end of 100, 300, 1000,3000,10000 and cycles, the depth of deformation was again measured and reported in mm as an average of the fifteen measurements that were taken across the two slabs. The outcomes of these tests were compared with a control mix samples prepared specifically for this test, namely Arcow aggregate asphalt mix (Plate 3). Plate 1: Croxden Aggregates before and after washing Croxden (Unwashed) Croxden (Washed) Plate 2: Croxden Aggregates after coating Plate 3: Arcow Aggregates

5 2.3 Mixtures evaluations Evaluating the effect of coating on natural aggregate mixes of porous asphalt ( namely open graded surface course, designated as AC 14 open surf 40/60 & AC 14 open surf 100/150 (BS IAN 102/07, 2007)), were made confirming to the British Standard (BS EN pt 35, 2004). All mixes have the same gradation as in (table 1). The optimum asphalt contents, determined by the Marshall Test method of mix design(bs EN pt 34, 2004), were found to be as shown in (table 2). In general, it is known that any increase in asphalt contents provides higher plastics flow susceptibility, This high plastic susceptibility may lead to higher permanent deformation, due to too much asphalt binder in the mix that causes the loss of internal friction between aggregate particles and results in the loads being carried by the asphalt rather than the aggregate structure (Roberts Freddy L, 1991). Type of Mix Binder 100/150 pen (%) Binder 40/60 pen (%) Arcow Uncoated Croxden Coated Croxden Table 2: Optimum Binder contents of total mix corresponding to each mix The permanent deformation test was performed by the wheel tracking large device (BS EN pt 22, 2003). Test was performed at 45 0 C. Although cement coated mixture has higher asphalt contents than the uncoated Croxden mixtures (table 2), the loss of internal friction between aggregate particles was observed in uncoated Croxden mixtures, as it failed the test where the highest deformation happened. In this study, optimum asphalt contents for cement coated mixtures were higher than the Arcow and the uncoated Croxden mixtures. Cement coated mixtures revealed more resistance to permanent deformation than the uncoated mixtures. It has been noted that cement paste contributes to adhesion ability to deformation resistance (Fig.2).

6 Comparison of the Rut Resistance of different Mixes tested by the Large Wheel Tracker machine Rut Resistance UNCOATED CROXDEN COATED CROXDEN ARCOW Types of Mixes Fig.2: Rut resistance comparison of three mixes tested by Large Wheel tracker 45 0 C (with 100/150 pen asphalt binder) A wheel tracking machine (with small device) was used to perform permanent deformation testing on square samples of (305x305x50mm) dimensions to assess the resistance to rutting of the three types of mixes under conditions which simulate the effect of traffic (BS EN pt 22, 2003). A loaded wheel tracks a single sample under specified conditions of load (700N) and speed (53 passes/min). Two different temperatures (30 & 45 0 C) were used respectively for testing and the development of the rut profile was monitored continuously during the test and plotted. The outcomes of the second wheel tracking machine have also shown improvement in the cement coated aggregates performance in regards to the permanent deformation in comparison to the control mix and the uncoated aggregate mixture (Fig. 3 & 4). 16 AverageRut Depths of mixes with 100/ C & 45 0 C ( small scale wheel tracker) 14 Rut Depth (mm) Uncoated Croxden Coated Croxden Arcow C Fig.3: Small wheel tracker testing machine results (with 100/150 pen asphalt binder)

7 16 Average Rut Depths of mixes with 40/ C & 45 0 C (small scale wheel tracker) Rut Depth (mm) Uncoated Croxden Coated Croxden C Fig.4: Small wheel tracker testing machine results (with 40/60 pen asphalt binder) 3. Conclusions From these tests it was concluded that the addition of cement paste to natural aggregate has enhanced its resistance against plastic deformation (rutting). Therefore it can be postulated that coating can also enhance the performance of reputable aggregates further and by using coated aggregates the mixes produced will perform better in terms of stability under the action of traffic and environmental conditions. This will be presented in another paper. REFERENCES BRANNAN, C. J., JEON, Y.W. PERRY, L.M. AND CURTIS, C.W "Adsorption Behaviour of Asphalt Models and Asphalts on Siliceous and Calcareous Aggregates". T. R. (Ed.). BS EN PT 6 (2003) "Bituminous mixtures. Test methods for hot mix asphalt, Determination of bulk density of bituminous specimens". B. S. (Ed.). BS EN PT 22 (2003) bituminous mixtures. Test methods for hot mix asphalt. Wheel tracking. B. S. (Ed.). BS EN PT 34 (2004) bituminous mixtures. Test methods for hot mix asphalt. Marshall Test. B. S. (Ed.). BS EN PT 35 (2004) bituminous mixtures. Test methods for hot mix asphalt. Laboratory mixing. B. S. (Ed.). BS EN PT 7 (2006) Bituminous Mixtures Material Specifications Part 7 Porous Asphalt.

8 BS IAN 102/07 (2007) The Introduction of BS EN Standards for Bituminous Mixtures site supervision aspects, Interim Advice Note. BS PD 6691 (2007) Guidance on the use of BS EN Bituminous mixtures - Material specifications. B. S. (Ed.). COLORADO DOT (2007) French Rut Testing of Compacted Bituminous Mixtures. Colorado D0T. CURTIS C.W. (B), C., D.J. JEON, Y.W., KIGGUNDU, B.M., (1992) Adsorption of Model Asphalt Functionalities, AC-20, and Oxidized Asphalts on Aggregate Surfaces. Transportation Research Record, FWA, T. F. (2006) The Handbook of highway engineering, Taylor & Francis Group. JAMIESON, I. L. J., D.R. AND MOULTHROP, J.S, (1993) Advances in the Understanding of Binder-Aggregate Adhesion and Resistance to Stripping. Highways and Transportation Journal,, KUENNEN, T. (1996) Open-graded mixes: better the second time around. American City & County. PAUL K. et al, J. F. L. (1994) Significance of Tests and Properties of Concrete and Concrete-making ASTM International. ROBERTS FREDDY L, K. P., BROWN ER, LEE D-Y, KENNEDY TW. (1991.) hot mix asphalt materials, mixture design and construction.. Lanham (MD). SCHMIDT, R. J., SANTUCCI, L.E. CONYNE, L.D. (1973) Performance Characteristics of Cement-Modified Asphalt Emulsion Mixes. AAPT.