Laboratory and Field Evaluation of Porous Asphalt Mixes

Transcription

1 1 st International Conference on Transportation Infrastructure and Materials (ICTIM 2016) ISBN: Laboratory and Field Evaluation of Porous Asphalt Mixes Jian-Shiuh Chen 1, Shih-Hsiu Wei 2 and Han-Chang Ho 3 1 Professor, National Cheng Kung University, Department of Civil Engineering, Tainan 701, Taiwan; jishchen@mail.ncku.edu.tw 2 Ph.D. candidate, National Cheng Kung University, Department of Civil Engineering, Tainan 701, Taiwan; n @mail.ncku.edu.tw 3 Ph.D. candidate, National Cheng Kung University, Department of Civil Engineering, Tainan 701, Taiwan; n @mail.ncku.edu.tw ABSTRACT: Porous asphalt concrete (PAC) generally has strongly open-graded aggregate gradations to yield high air voids, and is mainly applied to a surface drainage layer on high-speed trafficked highway pavements. Tests including durability and resistance to drain-down were conducted to determine the mechanical properties of PAC mixtures in the laboratory. The use of polymer-modified binder was shown to minimize the abrasion loss and enhance the durability of the PAC mixture. A test road was constructed to monitor the performance of PAC pavements based on the laboratory results. Surveys for pavement performance were conducted at scheduled intervals. Test results indicated that using polymer-modified binders instead of traditional binder improved drainability of PAC. Friction characteristics appeared to have no correlations with the binder type. INTRODUCTION Drainability is one of the main characteristics of porous asphalt concrete (PAC) mixtures and is the primary reason for using these mixtures around the world (Donavan 2014; Lyons and Putman 2013; Shirke and Shuler 2009). PAC is an open-graded mixture placed at the surface of asphalt pavements to produce benefits in terms of safety, economy, and the environment. The air voids of PAC are created by gap grading the coarse aggregates and either eliminating or minimizing the volume of fine aggregates in the mixture to form a network of interconnected pores within the material. PAC is an environmentally friendly road material using advanced technology in pavement design. PAC applied to the surface layers usually has an air void content of 20 percent. Due to higher proportions of coarse aggregates and lower sand content, interconnected air voids are created which, in wet weather, drain the moisture through a series of water channels inside PAC. This drainage system prevents aquaplaning on the road surface and improves visibility. The high porosity of PAC also reduces traffic induced noise emissions significantly (Huurman et al. 2010; Masad et al. 2004). Although the use of PAC mixtures provides various advantages due to the high air voids content and the large permeability, PAC can also pose some disadvantages, such as reduced performance, high construction costs, winter maintenance problems, and limited structural contribution (Bendtsen and Andersen 2004; Suresha et al. 2010). Reduced performance of PAC mixtures is associated with reduced durability and functionality (i.e., drainability and noise reduction effectiveness), due to raveling and clogging, respectively. In addition, for pavement structure design, PAC mixtures are typically considered to have limited structural contribution (Estakhri et al. 2008; Martin et al. 2014). 273

2 To maintain the benefits of a PAC layer, a mix design system that produces mixtures that are both functional and durable is required. Functionality of PAC mixtures includes properties related to drainability and surface friction. Drainability and noise reduction effectiveness are the main functional properties of PAC mixtures that justify using these mixtures as surface layers in asphalt pavements (Fwa et al. 1999; Masad et al. 2004). However, raveling is the distress most frequently reported as the cause of failure in PAC mixtures (Isenring et al. 1990; Sprinkel et al. 2015). The engineering properties of PAC mixtures could be lower than those of the conventional densegraded asphalt mixtures. High air voids created inside PAC mixtures could lead to a potential loss of cohesion and less resistance to disintegration in the mix. Using modified binders and adding additives are the common methods to improve the performance of porous asphalt concrete. However, the effects of all these materials are two-sided: when they improve the mixture s performance in one aspect, they might decrease its performance in the other aspect. There is a need to investigate the effect of binder types for PAC on the functionality and durability of these mixtures. The main objective of this study focuses on evaluating the engineering properties of PAC mixtures, and assessing PAC performance according to field measurements. MATERIALS Bitumen is one of the most important factors affecting the performance of porous asphalt concrete. Three binders commonly used in Taiwan for PAC pavements were included in this study as follows: conventional bitumen (CB), polymer -modified bitumen (PB) and high-viscosity bitumen (HB). While PB was modified by the addition of polymers to basic bitumen, HB was a particularly-manufactured binder represented by a high absolute viscosity value at 60 as listed in Table 1. The mixing and compaction temperatures for the CB binder were selected corresponding to 0.17 Pa.s and 0.28 Pa.s viscosities, respectively. For PB and HB, the mixing and rolling temperatures were recommended by binder manufactures. The properties of limestone coarse and fine aggregate were summarized in Table 2. Coarse aggregate for a PAC mixture must be strong to carry the imposed loads because coarse aggregate plays an important role in carrying the traffic loads for PAC mixes. The LA abrasion value of coarse aggregate should be less than 30% to possess sufficient toughness. In addition, flat and elongated proportions must be limited to be a minimum value, and fracture faces are required to provide a coarse aggregate structure with high internal friction. Cellulose fibers were used in porous asphalt mixtures to prevent drain-down as well as to improve its strength and durability. These fibers were added at 0.3% by weight of asphalt mixture. Aggregate gradation listed in Table 3 is for each of the following three types of asphalt binders allowed for PAC mixtures: CB, PB and HB. CB and PB mixtures need to be mixed with a minimum of 0.3% cellulose fibers by weight of total mix. The 19- mm maximum aggregate size gradation was gapped on the 4.75-mm sieve. The percentage of mineral fillers was 5 percent. Open-graded mixtures that are designed to have a minimum of 18 percent air voids are considered PAC. A target air void content of 20 percent was selected to enhance environmental benefits of PAC mixtures. The job mix formula was decided using the Marshall mix design method with 274

3 specimens prepared by applying 50 blows on each faces. The range of allowable binder content was determined using the following two properties: abrasion and draindown tests. The asphalt binder content was decided to be 5.0% for all PAC mixtures. Table 1. Properties of Bitumen. Properties CB PB HB Penetration (25 C, 0.1 mm) Softening point (ºC) Solubility (%) Viscosity (60 C, poise) 3,200 15, ,200 RTFOT Viscosity (60 C, poise) 8,900 20, ,200 Penetration ratio (25 C, %) Loss of mass (%) Table 2. Properties of Aggregate. Types Properties Test results Coarse LA abrasion (%) 18 Aggregate Soundness (%) 4 Flat & elongated (%) 3:1 8 5:1 4 Absorption (%) 1.2 Fracture faces (%) one 100 Fine Aggregate two 96 Soundness (%) 4 Sand equivalent value (%) 82 Table 3. Gradation of Porous Asphalt Mixtures. Results Sieve Size (mm) Specification Passing (%)

4 LABORATORY TESTS Laboratory tests including drain-down and durability were conducted to investigate the engineering properties of PAC mixtures. The testing data presented in this study are the average values of three replicates. Evaluation of Drain-Down The binder drainage test, combined with determination of volumetric properties, was used to evaluate the potential of binders draining down from the aggregate in accordance with ASTM D6390. In addition, this test could be used to determine the target binder content for PAC mixtures to maximize binder content so as to optimize durability while eliminating possible binder drainage. A sample of loose asphalt mixture to be tested was prepared and placed in a wire basket. The sample and the container were heated in a forced draft oven for one hour at 165 C. At the end of one hour, the basket containing the sample was removed from the oven along with the container and the mass of the plate or container was determined. The amount of draindown was then calculated. Fig. 1 indicates that the drain-down increases with increasing the binder content. Each bar represents the average of three tests with the variation ticks indicating one standard deviation of the tests. A maximum drain-down of 0.3% by weight of total mix is used as the limiting value for determining acceptable performance. Porous asphalt mixtures without fibers have significantly more drain-down. At 0.3% cellulous fibers by weight of mixture, drain-down is reduced to be about 0.1% at the asphalt content of 5%. This suggests that fibers help hold the asphalt binder to the aggregate structure of a porous asphalt mix. The amount of binder loss of the PB mix is also lower than that of the CB mix. Reduction in drain-down could be attributed to the increase in the viscosity of polymer-modified asphalt as well as the addition of fibers. The use of fibers appears to greatly reduce the potential for drain-down; more so than does polymer modification as shown in Fig. 1. In addition, the HB binder can be used in porous asphalt mixtures with little potential for drain-down problems even without the addition of fibers. Figure 1. Laboratory results of drain-down test. 276

5 Evaluation of Durability The Cantabro abrasion test measures the breakdown of a compacted PAC specimen after the completion of a specified number of revolutions, and is used to evaluate the resistance of PAC mixtures to disintegration and raveling. The test method entailed compacting a PAC mix compacted by 50 blows on each site at the optimum asphalt content, allowing the specimen to cool to a test temperature of 25 C, weighing the specimen to the nearest 0.1 g, and then placing the specimen into a Los Angles Abrasion machine without the charge of steel spheres. The Los Angles Abrasion machine was operated for 300 revolutions at a rate of 30 rpm. After 300 revolutions, the specimen is removed and again weighed to the nearest 0.1g. The weight loss during this process is defined as the Cantabro abrasion value. Fig. 2 shows that the CB mix has the highest abrasion loss, and the HB mix has the lowest abrasion loss. A maximum weight loss of 20% is specified for PAC. As compared with the CB binder, the use of the PB binder reduces the weight loss by 2% to 6%. The use of polymer-modified asphalt is shown to make a significant difference in results for the Cantabro weight loss test. Weight loss (%) Figure 2. Laboratory results of abrasion test. FIELD PERFORMANCE Monitoring in-service pavements is one of the best methods for gaining data on how PAC pavements will perform over time under real environmental and traffic conditions. Accordingly, after obtaining laboratory test results, the Taiwan Area National Freeway Bureau constructed a test road with typical construction equipment and operations in The PAC pavements with CB+fiber, PB+fiber and HB test sections are located adjacent to each other on an urban roadway, with no exits among them. These three mixtures were used to build test sections 3000 m long with each section of 1000 m. This test road has an average traffic volume of 36,000 vehicles per day with about 15% truck traffic. Surveys for pavement performance including drainability and skidding resistance were conducted at scheduled intervals. 277

6 Drainability The Public Construction Commission (PCC) in Taiwan specifies a minimum field drainability value, measured in terms of specific water volume within an outflow time. The outflow time of a specific water volume can be considered as an indication of the mean rate of water discharge. Although these measurements are often referred to as field permeability, the outflow time cannot be used to calculate the coefficient of permeability since the area and direction of flow during the test are not controlled. Nevertheless, the outflow time is a useful parameter to compare the performance in terms of drainability for different mixtures or that of a specific mixture under different compaction conditions or stages or in different project locations. Fig. 3 shows the drainability of the porous asphalt wearing course was almost the same in the beginning, but decreases over time in general. The reduction rate appears to be binder specific with the HB section having the best drainability. Both CB+fiber and PB+fiber sections had lower drainability as compared with the HB section. After approximate eight years of service, a slight filling up of the PAC layer occurred for the HB section. The water evacuation time measured with the field permeameter had gone from 1,399 to 1,180 ml per 15 seconds. The slight reduction had no appreciable effects on the efficient of water draining and the avoidance of splashing. Drainability (ml/15sec) Friction Figure 3. Field results of drainability measurements (mo=month). The British Pendulum Tester was performed to measure pavement skid resistance according to ASTM E303. The British Pendulum number (BPN) values were all adjusted by the exact pavement temperature at measurement to an equivalent BPN value at 20 C. The measurement of the friction showed an initial BPN of 61 to 62 as demonstrated in Fig. 4. Skid resistance was relatively low just after construction 278

7 because of asphalt binder film coating the aggregate at the pavement surface. As a consequence of the disappearance of the binder film covering the surface of the aggregate, skid resistance was improved after PAC open to traffic. After twelve months, it was already 64 and after twenty four months 65. A BPN of 45 is considered sufficient and safe for highway pavements. According to test results shown in Fig. 4, porous asphalt layers provide good wet weather friction. The BPN value does not appear to be correlated with the binder type of the PAC mixture mo 12 mo 18 mo 24 mo 32 mo 36 mo 40 mo 48 mo 54 mo 62 mo 66 mo 72 mo 78 mo 82 mo 88 mo Min = 45 BPN Value CB+fiber PB+fiber HB Figure 4. Field results of friction measurements (mo=month). CONCLUSIONS This paper is to evaluate the engineering properties of porous asphalt concrete in laboratory, and compare field performance of PAC pavements constructed using three different types of asphalt. Conclusions subsequently provided are based on the analysis of limited testing results obtained from laboratory and field. Mixtures without fibers or polymers showed greater drain-down than those with additives. The use of polymeric bitumen made it easier to obtain a greater resistance to drain-down, particularly for the HB mix. The wear losses in the Cantabro test were lower when using polymeric bitumen compared with a pure binder. The combined use of polymer modified binder and fiber minimized the abrasion loss and, thus, increased durability of the mixture. The drainability of the polymer-modified asphalt sections was higher than that of the traditional asphalt section. Clogging of pavement pores was likely to occur in the section using traditional asphalt and resulted in the reduction in drainability. PAC pavement surfaces provided good wet weather friction resistance once the asphalt binder film worn from the aggregate. 279

8 ACKNOWLEDGMENTS The authors are very grateful for the Ministry and Science and Technology and Taiwan Area National Expressway Engineering Bureau, and the Taiwan Area National Freeway Bureau providing financial and field supports to complete this project. REFERENCES Bendtsen, H., and B. Andersen (2004). Thin open layers as noise reducing pavements. Report 135, Danish Road Institute, Road Directorate, Ministry of Transport, Denmark. Donavan, P.R. (2014). Effect of porous pavement on wayside traffic noise levels. Transportation Research Record 2403, Transportation Research Board, Washington, D.C., Estakhri, C.K., A. E. Alvarez, and A. Epps Martin (2008). Guidelines on construction and maintenance of porous friction courses in Texas. Report No , Texas Transportation Institute, Texas A&M University, College Station. Fwa, T.F., S.A. Tan, and Y.K. Guwe (1999). Laboratory evaluation of clogging potential of porous asphalt mixtures. Transportation Research Record 1681, Transportation Research Board, Washington, D.C., Huurman, R.M., Mo, L., and Woldekidan, M.F. (2010). Unravelling porous asphalt concrete towards a mechanistic material design tool. Road Materials and Pavement Design, Vol.11(3): Isenring, T., H. Köster, and I. Scazziga (1990). Experiences with porous asphalt in Switzerland. Transportation Research Record 1265, Transportation Research Board, Washington, DC, Lyons, K. R. and B.J. Putman (2013). Laboratory evaluation of stabilizing methods for porous asphalt mixtures. Construction and Building Materials, Vol.49(1): Martin, W.D., B.J. Putman, and A.I. Neptune (2014). Influence of aggregate gradation on clogging characteristics of porous asphalt mixtures. Journal of Materials in Civil Engineering, Vol.26(5): Masad, E., Birgisson, B., Al-Omari, A., and Cooley, A. (2004). Analytical derivation of permeability and numerical simulation of fluid flow in hot-mix asphalt. Journal of Materials in Civil Engineering, Vol.16(5): Public Construction Commission. (2014). Porous Asphalt Concrete. PCC Designation: 02798, Executive Yuan, Taipei, Taiwan. (in Chinese) Shirke, N.A. and Shuler, S. (2009). Cleaning porous pavements using a reverse flush process. Journal of Transportation Engineering, Vol.135(11): Sprinkel, M.M., K.K. McGhee, and E.D. de León Izeppi (2015). Virginia s experience with high-friction surface treatments. Transportation Research Record 2481, Transportation Research Board, Washington, D.C., Suresha, S. N., V. George, and A.U. Ravi Shankar (2010). Effect of aggregate gradations on properties of porous friction course mixes. Materials and Structures, Vol.43(6):