Understanding the stress-dependent resilient modulus of cement-treated aggregate via discrete element modeling

Cement treated aggregate (CTA) is the primary material for constructing the base layer of highway pavements in China, and its resilient modulus has been proven notably stress-dependent. Existing testing specifications have limitations, being restricted to uniaxial tests or trial loading sequence fail to clearly differentiate between bulk and shear stress. This leads to uncertainty and high variability in the parameters of resilient modulus prediction models. Additionally, the microscopic mechanisms underlying the stress-dependent resilient modulus of CTA are yet insufficiently understood. In this study, unconfined compressive strength tests (UCS) and repeated load triaxial tests (RLT) were conducted on CTA at three supersulfate cement contents. A dynamic triaxial loading scheme was proposed to independently address the effects of bulk and shear stresses on the resilient modulus. Results indicate that CTA exhibits stress-dependent nonlinear characteristics, with its resilient modulus positively correlated with bulk stress and negatively correlated with octahedral shear stress. Among the three stress- dependent models compared, the NCHRP1-28A prediction model demonstrated the highest accuracy. Using the discrete element method (DEM), it was revealed that the stress nonlinearity of CTA arose from more tensile contacts at the microscale between aggregate particles under shear stresses, leading to reduced friction between particles and a subsequently reduced resilient modulus. In comparison, under bulk stress, more compressive contacts were detected between aggregate particles at the microscale, improving interlocking and friction between the particles.