Effects of stress-history on the microstructure and creep properties of maturing concrete
Creators: name: Mullick, Ajoy Kumar
Publisher: name: University of Calgary
Year: 1972
Subjects: subject: TA 440 M84 1972 Microfilm
subject: Creep of concrete
subject: Concrete - Testing

description: Bibliography: p. 272-280.
description: This investigation is concerned with a study of the effects of stress history and the state of hydration on the creep and creep recovery of maturing concrete. Modifications in the creep properties result from the microstructural changes in the cement paste wh ich accompany deformations under sustained load. Such microstructural changes are too small for dir ect observation. Therefore logical hypotheses regarding the extent and nature of such microstructural changes were drawn from the macroscopic creep responses of concrete subjected to variable stress conditions. The creep potential of concrete is governed by the state of hydration in the cement paste. The influence of this state and subsequent changes in it on long-term creep was investigated and parameters related to degree of hydration which might logically be used to normalise the effects of age and continuing hydration on creep were examined. Creep and creep recovery under constant as well as time-variab le uniaxial compressive stresses were investigated. The tests were carried out under saturated conditions at 70F. The same mix proportions were used throughout. A hydraulic loading device was adopted for creep tests. In variable-stress tests, the magnitude and duration of stresses were varied, so that the specimens had different stress histories. The effect of stress history was evaluated by comparing the creep of preloaded and previously unloaded specimens. Both sets of specimens had attained an identical state of hydration at the time of final loading, so that differences in creep response were due to stress history. The influence of state of hydration at the time of loading and subsequent changes therein on long-term creep was investigated by using constant stress creep tests started at various ages. The degree of hydration at different ( ages was determined on cement paste samples , having the same water-cement ratio as in the concrete mix. Test results showed that stress history affected creep and creep recovery in a well-defined way. Creep of preloaded specimens under subsequent loading was more greatly reduced, the more severe the stress history. Such reduction in creep results from microstructural changes in the cement paste, caused by dissolution and recrystallisation of sol id hydrates, irreversible diffusion of hindered adsorbed layers (including interlayer hydrate water) and formation of new sol id to sol id contacts etc. during previous load . The contribution of these processes to total creep during reloading becomes less significant as the microstructure attains a new orientation of greater stability. Irrecoverable creep was shown to result from such microstructural changes in addition to that caused by continuing hydration. The amount of irrecoverable creep was an indicator of stress history and could be used to estimate the modified creep response under subsequent stress programmes. Creep during reloading occurred in more reversible manner. The dependence on stress history of irrecoverable creep during reloading substantiates the contention that such microstructural changes do take place. Constant stress tests showed that the state of hydration at the time of loading, which governs the creep potential, is related to the amount of cement hydrated. Products of further hydration reactions interact with the gel formed at the time of loading. The long-term characteristic creep was augmented by the increase in the volume of the gel and was generally higher, the ear lie r the age of loading. The modification in creep caused by continuing hydration was closely related to the increase in gel-concentration factor.