Differential shrinkage effects in the core-drilling method

Abstract

The core-drilling method is a technique that is used to determine stresses in concrete. In the method, a small hole is drilled into the concrete, and the resulting displacements are related to stresses present in the structure via elasticity theory. One source of complication when applying the method is the presence of differential shrinkage stresses. The exterior of a concrete specimen dries and shrinks faster than the interior, thus causing tension stresses on the exposed faces, and compressive stresses on the interior. These stresses tend to dissipate with time as the moisture profile becomes more uniform. However, for relatively young concrete specimens these stresses can be significant. In the current work, the moisture distribution and corresponding differential shrinkage stresses are analytically predicted in concrete plate specimens by solving a non-linear diffusion equation with appropriate boundary conditions. These differential shrinkage stresses are then evaluated numerically in the core-drilling method. It is shown that these differential shrinkage stresses can result in large discrepancies in applied versus measured stresses, especially for concrete specimens that are particularly young (less than 3 years of age), particularly thick (greater than 150 mm thick), or cast/stored in a very dry environment. Application of the procedure detailed herein to the results of a previous hole drilling study in concrete plates improved measured accuracy from 47% relative error to 34% error if it is assumed that the previous plates were stored in a 50% relative humidity environment.