Investigating the Layer-Wise Hydration Process in Recycled Concrete Aggregates Under Laboratory Conditions

Abstract: Application of recycle concrete aggregates (RCAs) in pavements has been identified as one of the key strategies in reducing waste, conserve natural resources and create more eco-friendly infrastructures. An adequate amount of research has been carried out in the recent past to study the geotechnical properties of RCAs, when utilized as a replacement of virgin aggregate (VA) in granular layers of the pavements. These studies have found that RCAs exhibit enhanced stiffness in the field and therefore, suggest them as a viable substitute for VAs. The better performance of RCAs is primarily linked with the hydration of cementitious materials in its matrix. These reactions are known for their exothermic nature and therefore, studying the quantity and duration of heat released during its formation could be significant. To investigate this, a hydration test setup was established in the laboratory to analyze the layer-wise hydration process in RCAs. The setup involved simulating rainwater supply from above to induce hydration in RCA layers. Three sample boxes were prepared: one containing coarse RCA, another with fine RCA, and a third serving as a control with limestone. Moisture sensors and thermocouples were installed at depths of 6.35 cm (2.5 in), 14 cm (5.5 in), 21.5 cm (8.5 in), and 30.5 cm (12 in) to monitor temperature changes and moisture variations. Samples from different depths were also collected for X-ray Diffraction (XRD) analysis to determine the extent of hydration products and later, develop their relationship to temperature measurements. The results indicated that RCAs experienced layer-wise hydration after multiple rainfall events, with notable temperature differences along depth. By examining these temperature variations in relation to the formation of hydration products at different depths, this study aims to establish a clear connection between heat release and hydration products development within RCAs during rainfall. These insights will improve understanding of the layer-wise hydration process and the resulting increase in stiffness of RCAs in pavements. In future, this information could also contribute towards optimizing RCA layer thicknesses to achieve desired stiffness properties, representing an important step toward more effective use of RCAs in pavement engineering.