Behavior of Geosynthetic-Interlayered Asphalt Overlays

V, Vinay Kumar and Saride, Sireesh (2018) Behavior of Geosynthetic-Interlayered Asphalt Overlays. PhD thesis, Indian Institute of Technology, Hyderabad.

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Abstract

Distressed pavements, due to rutting and fatigue failures, lead to a huge discomfort to the road users and warrant periodic maintenance operations. The conventional rehabilitation program adopted by the various transportation agencies is to provide a hot mix asphalt (HMA) overlay of sufficient thickness to patch-up the existing cracks on the pavement surface. However, the overlay system is regarded as an interim solution, because within a transient duration of their construction, the discontinuities and cracks prevailing in the existing pavements reflect into the overlays and this phenomenon is known as reflective cracking. Reflective cracking is a very complex phenomenon, as the influencing factors are innumerous, including the thickness of overlay, traffic conditions, weather conditions, subgrade conditions and the condition of the existing distressed pavement, hence, there is no unique solution to arrest them completely. Some of the important solutions available in practice to retard the rate of cracking are providing a stress absorbing membrane interlayer (SAMI), paving fabrics, geosynthetic reinforcement (geogrids and geo-composites), chip-seal treatment, and saw and seal treatment techniques. Among these methods, providing a geosynthetic interlayer at the interface of old pavement and new overlay is regarded to be an effective solution to minimize the reflection cracks for a substantial performance period. The interlayers provide functions like reinforcement, stress relief and moisture barrier in the pavement system and enhance their performance duration. However, the mechanisms involved in inhibiting the crack propagation and the quantitative assessment of the fatigue life expectation from these interlayers are not completely understood. It can be clearly understood from the existing literature that the performance of geosynthetic interlayers was studied under small-scale two-layered beam fatigue tests. However, a limited number of studies are available on the evaluation of the actual performance of the geosynthetic reinforced asphalt overlays under field conditions. It is also observed that the existing understanding is based on a new asphalt overlay on a new pavement layer, as opposed to a new overlay on a distressed pavement. In this context, the present study attempts to develop an understanding on the performance of an old distressed pavement treated with a new overlay with and without geosynthetic interlayers. In addition, an attempt has also been made to understand the mechanism involved in resisting crack propagation into overlays and develop a model to predict the service life of geosynthetic reinforced asphalt overlays. To meet these objectives, a four-stage experimental program was designed in which, four different types of geosynthetic-interlayers viz. a woven geo-jute mat (GJ), a bi-axial polypropylene grid (PP), a polyester grid coated with polymer modified binder coating (PET) and a glass-grid composite (GGC) were considered. These materials were chosen based on their material composition, aperture sizes, bonding ability, and working mechanical and tensile properties. During the first stage, the flexural fatigue behavior of geosynthetic reinforced HMA overlay placed on a distressed pavement layer, which was extruded from an existing highway during a rehabilitation activity, were studied with the help of a flexural fatigue (four-point bending) test. The tests were performed under a load controlled mode, in which a repeated load corresponding to the equivalent single axle contact pressure of 550 kPa was maintained. The effectiveness of geosynthetic-interlayers in resisting the crack propagation into the overlays was studied by varying the crack depth (viz. no crack, 25 mm crack and 40 mm crack) in the old pavement (bottom) layer. Further, to understand the flexural fatigue behavior along with the crack initiation and propagation phases in the beam specimens, a two-dimensional non-contact optical imaging technique known as digital image correlation (DIC) was adopted. Results indicated that all the interlayers employed retarded the rate of reflection crack propagation into the overlays and among them, GGC interlayers have shown a superior performance, irrespective of crack depth. The DIC technic was effective in providing the crack evolution and propagation patterns along with the corresponding strains mobilized in the specimens. These tensile strains and crack propagation characteristics are crucial in predicting the service life of the overlays. In addition, it is also important to estimate the energy dissipated during the crack propagation or energy required to propagate the cracks, which was addressed through direct tensile strength tests (DTT) in the second stage of the testing program. The DIC technic was adopted to understand the crack propagation patterns and the corresponding strains mobilized in the specimen during the cracking process. Results indicated that the cracking resistance potential of geosynthetic reinforced specimens was consistently higher than the unreinforced specimens, irrespective of specimen conditioning temperature (20 C, 30 C and 40 C). The cracking resistance potential of specimens was higher at low temperature (20 C), irrespective of interlayer type. The DTT results also indicated a possible delamination of pavement layers at the interface. During the third stage, the delamination of pavement layers was addressed by quantifying the interfacial bond strengths when overlays are reinforced with geosynthetic-interlayers with the help of interface shear strength (ISS) and the adhesion tensile strength (ATT) tests. The interface shear strength between the old and new pavement layers was determined with the help of a large-scale interface shear test. The ISS test results indicated a reduction in interface shear strength by about 17% - 46% in the geosynthetic reinforced interface conditions. The ATT tests conducted at different specimen conditioning temperatures (20 C, 30 C and 40 C) indicated that the bond strength was higher at low temperature, irrespective of the interlayer type. From ISS and ATT results, it was observed that the PET interlayers have a higher interface bond strength. During the final stage, to understand the actual behavior of geosynthetic reinforced overlays in a field scenario repeated load (fatigue) tests were performed on large-scale pavement sections prepared in a laboratory test tank of dimensions 1 m (length) × 1 m (width) × and 1 m (depth). A continuous haversine type load pattern at a frequency of 1 Hz was applied to replicate the live moving traffic with an equivalent single axle contact pressure of 550 kPa. The performance of geosynthetic reinforced overlay sections was evaluated with the help of performance indicators like traffic benefit ratio (TBR) and rut depth reduction (RDR) factors. Results indicated that the presence of the geosynthetic-interlayers have improved the performance of asphalt overlays with a maximum TBR of 20 at a cumulative rut depth of 5 mm and a highest RDR of 55% at 100,000 load cycles for GGC reinforced section. A 50% and 30% reductions, respectively in surface deformation and vertical contact pressure at the interface of subgrade and base layers were witnessed after 100,000 load cycles. Finally, a simple model to estimate the service life of geosynthetic reinforced asphalt overlays from the beam fatigue test results is proposed. The beam fatigue and large-scale fatigue test results of GGC and PET reinforced specimens are considered for the model development, due to their superior performance compared to the other interlayers. It is proposed to conduct a simple beam fatigue test to obtain the normalized complex modulus × cycles (NCM) and beam fatigue life (Nb) to predict the actual service life of the overlays. The proposed model with a high coefficient of determination has demonstrated to estimate the service life of overlays from an independent test data. Overall, for a vertical deformation of about 7.5-8 mm, it is estimated that the GGC and PET interlayers would give a service life of 4.1 and 2.9 years, respectively in real field scenario, against a service life of 0.5 years in control sections.

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