INVESTIGATING THE EFFECT OF MOISTURE ON CLIMATE VULNERABLE PAVEMENT BASE AGGREGATES

Abstract

In view of alarming projections of increased climate water stressors, and considering the subsequent effect on the integrity and performance of pavement systems, there is a need for solutions to reduce the effect by making our pavements more sustainable and resilient. Given the increased frequency of heavy rainfall, there is a high probability that pavement base aggregates in the field, particularly in coastal regions, become saturated and remain so for an extended period of time. The increase in moisture content can reduce the mechanical properties, resilient modulus (MR) and shear strength, of base materials. In several states, climate vulnerable rocks, such as limestone and limerock, are the primary rock types crushed and used in roadway construction as pavement base aggregates. In recent years, the number of requests for approval of locally mined limerock in Alabama has increased due to the potential economic and environmental benefits associated with its use. However, the potential moisture susceptibility of these types of rock is a concern that has not been fully addressed, as it can impact the pavement long-term performance. In this study, a reliable adaptive approach using a bottom-loading device for laboratory MR testing was used to evaluate the effect of moisture, over a broad spectrum of moisture conditions ranging from a very dry state to complete saturation, on the mechanical behavior of climate vulnerable pavement base aggregates. Limestone (three sources) and limerock (four sources) were the two aggregate types used and their differences were studied in depth because distinguishing between them can be challenging and there is limited information available on this comparison in the literature. The laboratory tests results showed that the MR and the triaxial shear maximum strength of limestone and limerock increased significantly with decreasing moisture content when drying from the optimum moisture content. When saturated, limestone and limerock aggregates mostly experienced a reduction in their MR. This reduction was significant for aggregates that contained a large number of particles finer than 4.75 mm and/or 0.075 mm and had a high content of clay-like materials or plastic fines. Furthermore, the factors contributing to the impact of moisture were identified and a practical and cost-effective method was proposed to evaluate the moisture susceptibility of unbound base materials for quality assurance purposes. The results demonstrated that the fine aggregate content, fines content, plastic fines content, and fine aggregate angularity most influence the moisture susceptibility of base aggregates when moisture content exceeds the optimum moisture content. A threshold chart was developed for these influencing properties and a set of rapid tests was recommended to serve as moisture susceptibility evaluation methods during material selection and quality assurance testing in the field and laboratory. A case study confirmed the validity and reliability of the threshold chart. Finally, the existing constitutive models used to predict the MR of pavement base aggregates at different moisture levels during pavement design were evaluated to determine their accuracy at saturation. The models were found to be more accurate and even conservative when predicting the MR of saturated base aggregates with low moisture susceptibility. However, the models were erroneous in their prediction and overestimated the MR at saturation of highly moisture susceptible pavement base aggregates. A method was proposed to improve the accuracy of the prediction models for saturated base aggregates with high moisture susceptibility. The findings of this study help advance the state-of-the-art knowledge of the moisture effect on pavement materials. They are valuable for pavement design and construction material selection to achieve more sustainable and climate-resilient pavements. The proposed methods can help ensure that the unbound base materials selected for construction have the ability to retain their stiffness or recover from the effects of climatic events in a timely and efficient manner.