Shrinkage stresses in concrete using a new material parameter

Abstract

A new concept of αhygro, referred to as the coefficient of moisture contraction, is introduced. It is similar philosophically to that of the coefficient of thermal expansion αtherm. The introduction of αhygro makes stress computations due to moisture movement rather convenient, much in the same manner as thermal stress computations. Loss of moisture ΔM can be obtained from the associated boundary value problem governed by Fick's law (analogous to change of temperature ΔT from the associated heat transfer problem), followed by computations of free shrinkage strain using the relationship εsh  =  αhygro×ΔM, where εsh is the free shrinkage strain, αhygro is the coefficient of moisture contraction and ΔM is the moisture loss percentage. The associated stress build-up is found by using standard finite-element software that has a temperature loading module built in. This approach would obviate the need to measure shrinkage strain in the laboratory as is current standard practice. Instead, one would merely use established data bank values of αhygro obtained under standard conditions. Repair engineers would have to establish αhygro for their own concrete materials based on the test method outlined in this paper. In the present paper, αhygro values are determined for normal and self-compacting concretes using constituent materials from the Eastern Province region of Saudi Arabia and are shown to have a form that is invariant of specimen size and temperature exposure conditions. Stresses due to restrained shrinkage are also computed for a typical patch repair problem using commercially available multiphysics software Ansys.