Experimental and numerical development of the core-drilling method for the nondestructive evaluation of in-situ stresses in concrete structures.

摘要

The core-drilling method (CDM) is a technique to assess the in-situ stresses in concrete structures. In the method a circular core hole is cut into the concrete in a structure and the surface displacements that occur as a result are measured. These displacements are used to estimate the in-situ stresses through elasticity theory.; The current research investigates the effects on the CDM of three issues that were previously unexplored: (1) core-drilling water induced swelling displacements; (2) differential concrete shrinkage; and (3) steel reinforcement proximate to a cored hole in the concrete. These issues were probed analytically and numerically with the finite element method and other techniques, and experimentally in core-drilling tests of concrete plates. Displacements in the experiments were measured with digital image correlation.; The relationships relating in-situ stresses to relieved displacements proposed by previous researchers accurately describe the behavior that occurs in the CDM, and digital image correlation is an acceptable measurement technique for this application. The effects of swelling displacements, shrinkage, and reinforcement must be considered in the calculation of in-situ stresses to obtain acceptable accuracy. The average error in the experiments dropped from 28.4% to 9.5% if these factors were addressed. Any of the three factors may influence calculated in-situ stresses, depending on the condition and history of the interrogated concrete structure. Parameters that determine which of these factors are important are the age, sorptivity and thickness of the interrogated concrete element, relative humidity, and the size and proximity of reinforcement. Absorption of drilling water by the concrete around a core hole causes swelling of this concrete and swelling displacements. These displacements introduce fictitious apparent stresses that appear primarily as hydrostatic tension. An approach to correct for these apparent stresses was developed. The apparent stresses from differential shrinkage also appear as hydrostatic tension. Differential shrinkage does not significantly affect the CDM except in certain circumstances. Proximate reinforcement causes a significant under-prediction in stress if the reinforcement is neglected, however the effect reduces significantly with increasing concrete cover or increasing clear spacing to the nearest bar.