Mineralogical analyses of a subgrade soil underneath a concrete pavement suffering from distress in Hawai‘i

X-ray Diffraction (XRD) was conducted to identify cementitious (Calcite and Anhydrite) and other minerals present in a subgrade soil underneath a distressed concrete pavement. X-ray Computed Tomography (XCT) was performed on the undisturbed subgrade to study changes in the soil structure before and after a “load-and-soak” episode and to measure the collapse strain. Soils below pavements are known to be moist due to a phenomenon known as the canopy effect, whereby moisture accumulates on the underside of pavements. Concrete pavements are also known to curl downwards when hot and upwards when cool during daily temperature cycles. This induces stress changes in the moist subgrade soils. XCT imaging, performed first at its natural water content and then again after loading to represent the thermal-induced pavement stress and soaking for 72 h, revealed that the soil contains relatively large voids and fissures. Upon loading and soaking, the collapse strain was about 13%. This is larger than the 6% load-and-soak collapse strain measured in an oedometer test on a similar soil but under a minimal seating stress. Thus about 6% of the collapse strain resulted from dissolution of cementing agents. The remaining 7% collapse is due to closure of naturally occurring voids and fissures, compression of the clay upon application of stress and achievement of proper contact between the soil and the wall of the sample holder. The mineralogical analyses and XCT imaging provide a valuable methodology into assessing the collapse mechanisms in soils with a similar geological setting.