Shrinkage-Cracking Characteristics of Structural Lightweight Concrete

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Title: Shrinkage-Cracking Characteristics of Structural Lightweight Concrete

Author(s): Robert G. McKeen and W. B. Ledbetter

Publication: Journal Proceedings

Volume: 67

Issue: 10

Appears on pages(s): 769-777

Keywords: cement content; coarse aggregates;concretes; cracking (fracturing) ; curing; evaporation; gravel; lightweight aggregates; lightweight aggregate concretes;reseach;shrinkage;tensile strength;volume change.

Date: 10/1/1970

Abstract:
Tests were conducted to determine the effect of coarse aggregate type, cement content, and curing environment on unrestrained volume changes and restrained shrinkage cracking behavior. Measurements of restrained shrinkage stress and ultimate tensile strength were made for purposes of comparing these properties with the above characteristics. Two synthetic lightweight aggregates, commercially produced in Texas and having widely different saturation characteristics, were used along with a natural siliceous river gravel as coarse aggregates. After a 5 day initial moist curing period, the specimens were placed in four different curing environments. Unrestrained volume changes were measured on standard type specimens, 3 x 3 x 11.25 in. (7.5 x 7.5 x 28.1 cm). Cracking was evaluated as the number of cracks occurring on a specimen 4 x 4 x 48 in. ( IO x IO x 120 cm) with a #8 reinforcing bar through the center. Using concrete made with one of the lightweight coarse aggregates, specimens were cast for measurement of restrained shrinkage stress. After 60 days, these specimens were loaded to concrete’ failure in tension and ultimate tensile strength was determined. The effective or usable tensile strength was greatly affected by the curing environment. Results indicated that both unrestrained shrinkage and concrete water loss relate to restrained shrinkage stress. Unrestrained shrinkage did not indicate cracking tendency while water loss provided an indication of cracking tendency.