Cement Paste, Mortar and Concrete Under Monotonic, Sustained and Cyclic Loading

Abstract

The behavior of saturated specimens of cement paste and mortar under monotonic, sustained and cyclic loading, is compated to that of concrete at water - cement ratios of 0.5 and 0.7. Specimen age, at testing, ranges from 27 to 29 days. For monotonic loading, the behavior of each material is described in terms of peak stress, strain at peak stress, and initial modulus of elasticity. For sustained loading, the behavior is described in terms of creep strain as a function of stress - strength ratio and time under load. Mathematical relationships ate developed on the sustained load response to estimate the cumulative static creep for a cyclic test. Cyclic test results ate exatnined in terms of strain at 15 seconds, the difference between the strain at 15 seconds and the peak strain for a given cycle (cyclic strain), the estimated creep strain for a cyclic test (equivalent creep, based on sustained load test results), the difference between cyclic strain and equivalent creep (cyclic action strain), and the change in secant unloading modulus (a measute of material damage). The equivalent creep duting a cyclic test is used to distinguish between cyclic strain and cyclic action strain, which may include accelerated creep strain as well as strain related to tnicrocracking. Cyclic action strain is correlated with change in modulus of elasticity to determine the extent to which these strains ate the result of damage. Monotonic test results show that for the materials used in this study, at a given water - cement ratio, cement paste has a higher strength and strain capacity than do the corresponding mortat and concrete, while mortat and concrete have a higher initial stiffness than cement paste. Sitnilatly, mortat has a higher strength and strain capacity than the corresponding concrete, but has approximately the same initial stiffness. The sustained load test results show that over a four hour period, creep strain increases nonlinearly with increasing stress - strength ratio. At the same stress - strength ratio, total strain and creep strain accumulate more rapidly for cement paste than for mortar and more rapidly for mortar than for concrete. The cyclic test results show that for cyclic tests with a maximum stress - strength ratio greater than 0.6f', cyclically loaded cement paste, mortar and concrete exhibit larger strains than similar materials exposed to a sustained load equal to the mean cyclic stress. For the load regimes studied, maximum cyclic stress appears to have a much greater impact on the cyclic action strain and change in stiffness than the mean cyclic stress or the cyclic stress range. The overall damage, as measured by the cyclic action strain and change in secant unloading modulus, in mortar in concrete is similar, suggesting that the behavior of concrete under cyclic loading is dominated by its mortar constituent. Under monotonic, sustained and cyclic loading, the behavior of mortar more closely resembles that of concrete than it does cement paste.