Nonlinear creep of early-age cement mortar assessed by minutes-long flexural test

Abstract

Nonlinear creep normally defined as the time-dependent deformation of materials under high stress condition, is highly coupled with the damage condition. The microstructure evolution of early-age cementitious materials due to the cement hydration influences the accurate measurement of nonlinear creep. Besides, the commonly-used assumption in the uniaxial creep test that the shrinkage of the loaded concrete is the same as that of the unloaded concrete is questionable for the high stress case. To reduce the influence of cement hydration and shrinkage on the measured results, a minutes-long flexural test was designed in this study to investigate the nonlinear creep of cement mortar with w/c ratio of 0.45 at the ages of 7 and 28 days under drying condition with relative humidity of 70 %. The maximum stress applied to the mortar beam was set as 30 %, 40 %, 70 %, 85 %, and 90 % of the flexural strength. The results show that cement mortar subjected to less than 40 % of the flexural strength exhibits a linear creep property, while a significant nonlinear creep is observed when the stress level is greater than 70 %. The influence of curing time on the creep rupture time seems more significant than that of the stress level. The nonlinear creep coefficient of early-age cement mortar decreases gradually and tends to converge to a constant if it is not ruptured during the minutes-long creep test, which cannot be characterized by the existing models where the nonlinear creep coefficient depends only on the stress level. A new model is proposed in this study with the function form of η(t)=η0+ke-xt, which is capable of capturing the evolution of the nonlinear creep coefficient of early-age cement mortar with good accuracy.