Core Drilling Method

The core-drilling method (CDM) is a technique for measuring in-situ stress in concrete structures. A small hole is drilled into the structure, and the deformations in the vicinity of the hole are measured and related via elasticity theory to the stress. The method is similar to the ASTM hole-drilling strain-gauge method excepting that displacements rather than strains are the measured quantities. Displacement measurements in the current work are performed using 3D digital image correlation. The theoretical basis for the CDM hinges on the classical elasticity solution for the displacements in the vicinity of a small hole in an infinite, thin plate comprised of a linear-elastic, isotropic and homogenous material and loaded with simple stress states. Factors such as differential shrinkage, swelling induced during wet drilling, and the inclusion of steel reinforcement and coarse stone aggregate violate some of these assumptions and necessitate strategies for incorporation of their effects in an overall evaluation for stress.

Displacements around a core hole that has been exposed to water as part of the drilling process
(the swelling that occurs in the vicinity of the hole is caused by water absorption)

Photograph of a concrete plate and associated core after coring
(visible is the pattern applied as part of the digital image correlation process)

Displacement output from third-party vendor digital image correlation software during a core-drilling method test.

Plot of displacements measured around a core hole during an experimental test.
Displacements were measured with digital image correlation

Mohr's Circle plot of experimental results showing the need for correction to stresses measured and the effectiveness of the proposed correction. In this case the results were distorted due to water absorption during the test.