Microcracking and Time-Dependent Strains in High Strength Concrete

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Title: Microcracking and Time-Dependent Strains in High Strength Concrete

Author(s): Ali S. Ngab, Floyd O. Slate, and Arthur H. Nilson

Publication: Journal Proceedings

Volume: 78

Issue: 4

Appears on pages(s): 262-268

Keywords: compressive strength; creep properties; deformation; high strength concretes; loads (forces); microcracking; shrinkage; specimens; strains; stresses; stress-strain relationships.

Date: 7/1/1981

Abstract:
The relationship between time-dependent deformation and in ternal microcracking of high strength concrete was investigated experimentally. Direct comparison was made to the behavior of normal strength concrete subjected to similar relative uniaxial compressive stresses and under the same environmental conditions. Sealed and unsealed specimens were analyzed for microcracking after they were subjected to short-term loading, to shrinkage, and to sustained loading. Results confirm that microcracking, always present even in unloaded specimens, is increased by short-term loading, shrinkage, and sustained loading. However, the amount of cracking, as well as the increase relative to the initial state, is significantly less in high strength concrete than in normal strength material. The amount of creep strain associated with internal cracking in high strength concrete is negligible, whereas such creep is significant in normal strength concrete, particularly at high stresses. The research also indicates that the ratio of the sustained load strength to the short-term strength is higher for high strength than for normal strength concrete. This also can be explained in terms of differences in microcracking. Time-dependent engineering properties for high strength concrete, such as creep coefficient, specific creep, and shrinkage characteristics, are reported in a separate paper.