Advanced interface testing of grids in asphalt pavements

Nowadays, road pavements are subjected to steadily increasing traffic volumes generating accelerated functional and structural distresses that require frequent and expensive maintenance. On the basis of such needs, in recent years, practical applications and theoretical studies have proved that the service life of flexible pavements can be extended by installing geosynthetic reinforcements. In particular, grids can be placed at the interface of bituminous layers for both new constructions and rehabilitation of existing pavements, in order to improve repeated loading and rutting resistance and to prevent or delay reflective cracking. However, the presence of an interlayer reinforcement may also hinder the full transmission of horizontal shear stress between asphalt layers (debonding effect), penalizing the overall efficiency of the pavement system. For the above-mentioned reasons, both laboratory and in situ investigation are needed in order to better understand the real role played by the grid reinforcement. The achievement of such objective is the main goal of the RILEM TC 237-SIB/TG4 that carried out an interlaboratory experiment focused on the “Advanced Interface Testing of Geogrids in Asphalt Pavements”. In this context, the participating laboratories were involved with a twofold objective: to compare the predictive effectiveness of different experimental approaches and to analyze the behavior of different grid types. For this purpose, two experimental reinforced pavement sections were realized with the same materials and construction techniques. The first pavement section was used to prepare samples for the interlaboratory experiment, the second one was specifically designed and instrumented to analyze the field performance of the grids under heavy traffic conditions. The objective is the characterization of the mechanical behavior of grid reinforced interfaces in asphalt concrete pavements using different test methodologies and the analysis of the relationship between laboratory test results and actual field performance. To this purpose, the laboratory research activities were based on the analysis and comparison of the results obtained following specific testing protocols proposed by the participating laboratories that combine performance-based tests (e.g. interlayer shear tests, static and dynamic bending tests, tensile-bending tests), in order to investigate the overall behavior of double-layered asphalt systems. The role of the instrumented pavement section was complementary and oriented towards an improvement in the existing design and testing approaches. Such goal was attained by analyzing the actual stress-strain response of grid-reinforced systems under vehicular loads, also monitoring the natural and induced field cracking evolution. Despite the variety of the testing equipment and protocols adopted by the participating laboratories, all test results were consistent. Moreover, such experimental results contributed, together with the data analysis collected on the instrumented pavement section, to the correct understanding of the grids performance that were characterized by specific peculiarities making them appropriate for different applications.