Targeting aggregate marginality from a chemical and materials perspective to improve pavement performance

Abstract

Unbound granular aggregates, due to a comparatively lower cost in comparison to bound pavement layers, are extensively used in the construction of road pavements. Thus, it is not surprising that a careful testing and selection of these materials is required to ensure a longlasting pavement life that avoids early and costly maintenance intervention. In order to appropriately evaluate the road aggregates, a series of standardised laboratory tests are undertaken to aid appropriate decision making prior to construction phases beginning. Generally, it is assumed if properly constructed, that the selection of aggregates based on these tests largely guarantees a satisfactory performance of them in the structure of pavements. Based on the results of a number of laboratory tests, aggregates have typically been categorized into two groups; premium and marginal. Where marginal aggregates are excluded from being used in high-trafficked pavement sections or State Highways in New Zealand. However, the depletion of high-quality aggregates, in conjunction with the significant cost of transport haul distance to site and its adverse environmental effects, has led road agencies in New Zealand to consider using locally available aggregates that have a more marginal quality. Due to their inherent mineralogical properties, marginal aggregates are usually more susceptible to adverse environmental conditions of the pavement structure. Therefore, their engineering characteristics can be progressively affected by the in-service weathering processes. Generally, because of the initial mechanical crushing, strong compaction, continuous in-service traffic loading, and direct interaction with the surrounding environment, the process of weathering can be significant in unbound aggregates. This topic is of even higher relevance for New Zealand, where there is a high demand for unbound aggregates and its young geologically sourced aggregates are generally more susceptible to the diverse environmental and climatic conditions. Although weathering processes may happen in a relatively short time and can cause ageing and deterioration of the structure of aggregates, a quantitative evaluation of its effect on road aggregates over the lifetime of the pavement is usually disregarded and properties of aggregates are assumed to be constant over time and location. In order to better understand the weathering process of aggregates and achieve a more efficient use of them this research develops a field and laboratory- based experimental methodology (as follows). Two sources of andesitic unbound aggregates are used in this study, one source of aggregates is characterised to be of high quality and the other of more marginal quality according to the local standard New Zealand Transport Agency (NZTA) M4 specification. - Field-based experiment: The chemical and physical degradation of road aggregates within quarries are considered as an observable analogue of in-situ weathering of aggregates in the pavement structure. A variety of analytical methods, including thin section petrography, X-Ray Fluorescence (XRF), and X-ray Diffraction (XRD) reveal the short-term (less than 15 years) in-situ weathering of aggregates. Overall, the immediate loss of bases and production of clay minerals are the notable trend of chemical weathering of selected aggregates. Moreover, thin-section analysis reveals changes in the textural features of aggregates during the weathering. - Lab-based experiment (loose aggregates): In order to make the current New Zealand standard Weathering Quality Index (WQI) testing a better representative of in-service condition of unbound aggregates a series of modifications are proposed to this testing methodology. These modifications include improved wetting and drying processes, varied characteristics of the weathering solution, and a testing of the wider gradation of aggregates. Compared to standard WQI test, the results show 200% and 70% increase in CI value and 130% and 230% increase in WCI results of produced fines in premium and marginal aggregates, respectively. Thus, it is believed that the modified WQI test method better represents the potential in-field pavement weathering of andesitic road aggregates.in harsh environmental conditions. - Lab-based experiment (loose aggregates): Water is usually known to be a key factor in the performance of the structural elements of road pavements. To gain more insight into the water-aggregate interactions of UGM, a testing methodology is devised to determine the real-time appetite of aggregates for water in various environmental conditions. It is concluded that the water absorption of road aggregates depends upon the temperature and composition of the solution. Due to the increase in the water temperature, 160% and 100% increase in the absorbed water is reported for premium and marginal aggregates, respectively. Furthermore, it is concluded that the standard water absorption testing methodology generally under-estimates the appetite of aggregates to absorb water. In extreme cases, the proposed testing methodology results in 130% higher water absorption compared to the standard test. - Lab-based experiment (compacted aggregates): The effect of Wetting and Drying (WD) cycles on the performance of compacted aggregates are investigated by conducting California Bearing Ratio (CBR) alongside Repeated Load Triaxial (RLT) tests. The results show that W-D can affect the properties of fines in the compacted specimens and controls the swelling behavior of marginal aggregates. W-D cycles result in approximately 40% increase in CI and WCI results of produced fines in both sources of aggregates. The W-D can also have an appreciable effect on the permanent deformation, resilient modulus and CBR values of specimens. 160% increase in permanent deformation is reported for soaked-conditioned premium aggregates, whereas marginal aggregates fail in the last stage of the loading. Overall, a thorough investigation of andesitic aggregate weatherability has been undertaken based upon the fundamental testing techniques required during the selection procedures of aggregates to ensure engineering life performance. This investigation includes an understanding of the ageing properties of the available aggregates and the local environmental condition where the aggregates are to be placed. This can be quite important for marginal aggregates, whose properties are usually more susceptible to environmental conditions. Furthermore, the Pass/fail criteria of the traditional testing methodologies do not adequately consider the durability of road aggregates in the construction, maintenance and environmental conditions that an aggregate is placed. It is believed that a grey-scale labelling (i.e. non-binary pass/fail status) of aggregates would be more effective in describing the materials’ natural variability and statistical distribution to subsequently characterise an aggregates performance given the associated environmental and climatic conditions of the target location.