Assessing the Bond Strength of Hot Mix Asphalt Pavement for Wearing and Binder Courses

This study investigated the use of a shear box device to measure the bond condition between two layers of hot mix asphalt pavement: the wearing course and the binder course. The wearing course analysed was a Malaysian dense-graded asphaltic concrete mixture of nominal maximum aggregate 10 mm in size (AC10), which was applied over the binder course of another dense-graded asphaltic concrete mixture, AC28. A range of bond conditions was investigated by selecting various asphalt emulsions, application rates, and wearing course thicknesses based on the Malaysian standards of specification. Test results showed that interface shear strength increased as tack coat application rates and wearing course thicknesses increased. Among the tested asphalt emulsion types, a modified asphalt emulsion called RS2KL provided the highest shear resistance. Findings also show that a binder’s complex shear modulus elastic portion (G*/sin?) can affect interface shear strength for thin mixes at low rates of tack coat application.

Asphalt pavement; Asphalt emulsion; Bituminous layer; Bond strength; Tack coat

Modern asphalt pavements are engineering structures normally constructed in several layers to withstand traffic loadings. As a layered structure, the performance of the hot mix asphalt overlay relies considerably on the bond strength between the contact interfaces of adjacent layers. Tack coat is typically applied between the interfaces of the surfacing layers to provide the necessary adhesion and enable the overall structure to behave monolithically. A tack coat can either be straight or cutback asphalt; however, it usually is in the form of emulsified asphalt because of environmental concerns. Emulsified asphalt is considered to be ‘greener’ because it can be applied at ambient temperatures with no heating required. However, pavement failures related to bond strength, such as slippage, delamination, and the formation of shallow potholes, have been reported (Muench & Moomaw, 2008; Alhaji & Alhassan, 2018). This study focused on the laboratory assessment of shear bond strength between two layers of Malaysian dense-graded hot mix asphalt: the wearing and binder courses. The interface shear strength was also correlated to the rheological properties of the asphalt emulsion residue to determine their possible relationship. Testing was carried out using a shear box to measure the interface properties. Over the years, the number of commercialized vehicles has gradually grown. This phenomenon has also increased the standard equivalent axle loads, which are harmful to existing pavements. The consequence is a series of reported road deteriorations and distresses that affects overall driving quality (Lee et al., 2013). Numerous studies on material properties have been conducted to increase the performance of the overall mix; however, the performance of the bonding between pavement layers has often been overlooked (Puri, 2017). In Malaysia, road deteriorations due to interface shear strength remain high and alarming. Hence, interface bonding must be given due attention because it can be equally important in the performance of a constructed pavement.

Research in the area of pavement’s interface bond strength developed rapidly as early as the late 1970s when Uzan et al. (1978) and Leutner (1979) first worked on the shear mechanism and studied the shear bond strength of a two-jointed interface. Uzan et al. (1978) examined the effects of the tack coat application rate, test temperature, and magnitude of confinement pressure on interlayer properties using a shear box. They identified a relationship between interface shear strength, which is parallel to the decreasing test temperature and increasing confinement pressure, a finding verified further by different researchers (Canestrari et al., 2005; West et al., 2005; Chen & Huang, 2010). Their testing indicated that an optimum application rate of tack coat would produce maximum shear resistance. The optimum tack coat application rate identified (from the total of the emulsion weight) was 0.49 L/m² at 25°C and 0.97 l/m² at 55°C for dense-graded and open-graded types of mixture, respectively. A review of the literature shows that different factors were identified as contributing to pavement bond strength. The identified factors are worth studying so that pavement bond strength can progress further, particularly the strength of local, Malaysian materials that comply with local designs and specifications. Therefore, this study investigated behaviours at the pavement interface using the Malaysian asphalt mixture design and locally available tack coats. The study also attempted to correlate interface bond strength with rheological properties of the tack coat using laboratory-prepared specimens.

Interface shear bond strength increased with an increase in the tack coat application rate, regardless of the type of tack coat used. In most test combinations, interface shear strength was also found to increase as the thickness of the wearing course increased. The difference in interface shear strength was especially significant when comparing the 35 mm specimen to the 50 and 65 mm specimens. However, the difference between the 50 and 65 mm specimens was marginal. Interface shear strength was observed to have a good correlation with the complex shear modulus elastic portion (G*/sin?) of the binder for the 35 and 50 mm specimens. This observation indicates that a binder’s G*/sin? value can affect interface shear strength only for thin mixes at low tack coat application rates. Therefore, it is important to have good properties of bitumen emulsion (i.e. the right proportion of residual bitumen content with a low application rate within the specification), especially for the application of tack coat for a thin surfacing or thin asphalt layer. Future research in this area should focus on increasing the shearing speed of the tests to assess the effect of speed’s consequences, particularly increased temperature, on pavement’s interface bond strength.

The authors would like to express their gratitude to associated staff members at the Malaysia Ministry of Higher Education (MOHE) and the Universiti Teknologi Malaysia for the research grants (GUP Tier 1 Vote Q.J130000.2522.17H68) that financially supported this research project.

Alhaji, M.M., Alhassan, M., 2018. Effect of Reclaimed Asphalt Pavement Stabilization on the Microstructure and Strength of Black Cotton Soil. International Journal of Technology, Volume 9(4), pp. 727–736

ASTM D36, 2012. Standard Test Method for Softening Point of Bitumen (Ring-and-Ball Apparatus). ASTM International, West Conshohocken, PA

ASTM D4402, 2012. Standard Test Method for Viscosity Determination of Asphalt at Elevated Temperatures using a Rotational Viscometer. ASTM International, West Conshohocken, PA

ASTM D5, 2013. Standard Test Method for Penetration of Bituminous Materials. ASTM International, West Conshohocken, PA

ASTM D6926, 2016. Standard Practice for Preparation of Asphalt Mixture Specimens using Marshall Apparatus. West Conshocken, PA

ASTM D6997, 2012. Standard Test Method for Distillation of Emulsified Asphalt. ASTM International, West Conshohocken, PA

ASTM D7175, 2008. Standard Test Method for Determining the Rheological Properties of Asphalt Binder using a Dynamic Shear Rheometer. ASTM International, West Conshohocken, PA

ASTM D7402, 2009. Standard Practice for Identifying Cationic Emulsified Asphalts. ASTM International, West Conshohocken, PA

ASTM D7496, 2017. Standard Test Method for Viscosity of Emulsified Asphalt by Saybolt Furol Viscometer. ASTM International, West Conshohocken, PA

Bae, A., Mohammad, L.N., Elseifi, M.A., Button, J., Patel, N., 2010. Effects of Temperature on Interface Shear Strength of Emulsified Tack Coats and Its Relationship to Rheological Properties. Journal of the Transportation Research Board, Volume 2180, pp. 102–109

Canestrari, F., Ferrotti, G., Partl, M., Santagata, E., 2005. Advanced Testing and Characterization of Interlayer Shear Resistance. Journal of the Transportation Research Board, Volume 1929, pp. 69–78

Chen, J.S., Huang, C.C., 2010. Effect of Surface Characteristics on Bonding Properties of Bituminous Tack Coat. Journal of the Transportation Research Board, Volume 2180, pp. 142–149

Choi, Y., Collop, A., Airey, G., Elliot, R., 2005. Comparison between Interface Properties Measured using the Leutner Test and the Torque Test. Journal of the Association of Asphalt Paving Technologists, Volume 74E, pp. 309–316

Collop, A., Thom, N., Sangiorgi, C., 2003. Assessment of Bond Condition using the Leutner Shear Test. In: Proceedings of the Institution of Civil Engineers - Transport, Volume 156, pp. 211–217

Hakim, B., 1997. An Improved Back-calculation Method to Predict Flexible Pavement Layers Moduli and Bonding Condition between Wearing Course and Base Course. Liverpool John Moores University

Lee, J., Ahn, H., Shah, A., Sommer, K., 2013. Investigation of Delamination on I-65 in Indiana. Journal of Testing and Evaluation, Volume 41(5), pp. 745–753

Leutner, R., 1979. Untersuchung des Schichtenverbundes beim bituminösen Oberbau [Investigation of the Adhesion of Bituminous Pavements]. Bitumen, Volume 41, pp. 81–94

Muench, S.T., Moomaw, T., 2008. De-bonding of Hot mix Asphalt Pavements in Washington State: An Initial Investigation. Technical report. Washington 146 State Department of Transportation, Office of Research & Library Services: Transportation Northwest Regional Center X (TransNow)

Public Works Department (PWD) of Malaysia, 2008. Standard Specification for Road Works, Section 4 – Flexible Pavement. Jabatan Kerja Raya, Kuala Lumpur, Malaysia

Puri, A., 2017. Developing Curve of Displacement Factor for Determination of Additional Modulus of Subgrade Reaction on Nailed-slab Pavement System. International Journal of Technology, Volume 8(1), pp. 124–133

Sholar, G.A., Page, G.C., Musselman, J.A., Upshaw, P.B., Moseley, H.L., 2004. Preliminary Investigation of a Test Method to Evaluate Bond Strength of Bituminous Tack Coats. Journal of the Association of Asphalt Paving Technologists, Volume 73, pp. 771–806

Uzan, J., Livneh, M., Eshed, Y., 1978. Investigation of Adhesion Properties between Asphaltic-Concrete Layers. Journal of the Association of Asphalt Paving Technologists, Volume 47, pp. 495–521

West, R.C., Zhang, J., Moore, J., 2005. Evaluation of Bond Strength between Pavement Layers. National Centre for Asphalt Technology, Auburn University, Alabama

Yaacob, H., Chang, F.L., Rosli Hainin, M., Jaya, R.P., 2014. Curing of Asphalt Emulsified Tack Coat Subjected to Malaysian Weather Conditions. Journal of Materials in Civil Engineering, Volume 27, pp. 1–10

Yaacob, H., Hainin, M.R., Safuan, A., Chang, F.L., 2014. Single Face Compaction on Laboratory Marshall Specimen Towards Satisfactory Degree of Compaction and Thickness. Sains Malaysiana, Volume 43(2), pp. 29–302