Elastic Compatibility and the Behavior of Concrete
T. W. Bremner and T. A. Holm
Appears on pages(s):
aggregates; concretes; elastic properties; lightweight aggregate con-cretes;lightweight aggregates; microcracking; modulus of elasticity; stiffness;strength; stress concentration.
Concrete is considered a particulate composite in which the coarse aggregate acts as a spherical inclusion in an infinite matrix of mortar. Equations developed by Goodier are used to calculate the stresses that develop in and around the aggregate inclusions. With ordinary concrete, large stresses develop at the interface as a result of the large difference in elastic modulus between the aggregate and the matrix. With structural lightweight concrete, the elastic modulus of the inclusion (expanded aggregate) is similar to that of the matrix, resulting in signtficantly lower stress concentrations developing at the aggregate-matrix interface. Elastically matched constituents will result in lower levels of the stress at the interface and less microcracking. Evidence is presented to support the view that more uniform distribution of stress within lightweight concrete will compensate for the lower particle strength of the expanded aggregate. Incorporation of entrained air is shown to promote elastic mismatching in ordinary concrete because it increases the difference in stiffness between the inclusion and the matrix. With structural lightweight concretes, entrained air lowers the stiffness of the mortar matrix so that it approaches the stiffness of the expanded aggregates, resulting in a more uniform stress distribution. At the point of elastic compatibility of the two phases, external loading will develop a stress concentration factor approaching unity and the stress at the interface perpendicular to the direction of the applied load will approach zero. In addition to contributing to efficient strength development, reduction in micro-cracking should substantially reduce the ingress of water and chlorides into the concrete as the paths of easy flow are minimized.