Date of Award

Fall 1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Applied Mathematics - (Ph.D.)

Department

Civil and Environmental Engineering

First Advisor

Methi Wecharatana

Second Advisor

William R. Spillers

Third Advisor

Farhad Ansari

Fourth Advisor

Jay N. Meegoda

Fifth Advisor

Priyantha Liyanage Perera

Abstract

Strength of concrete is normally measured using the standard cylinder or cube. The measured strength is used for design. The accuracy of concrete strength is frequently challenged, particularly in large concrete structures where size effect of the test specimens is attributed for the differences. Many nondestructive tests were developed to evaluate concrete strengths. In recent years, it was obvious that these tests are unreliable. As the infrastructure decays, more nondestructive tests are required to evaluate the existing structures.

The Break Off Test is a recently developed nondestructive test. Although substantial amount of experimental investigations have been carried out on this test, no in-depth theoretical evaluation has yet been done to date.

In this study the behavior of the break off test specimen was investigated and the potential theoretical basis of this test explored.

Based on linear elastic fracture mechanics, a model to predict the compressive strength of concrete-manometer reading relationship of the break off tester was proposed and compared with experimental results with good correlation. Both flexural and shear failure modes were considered and the effect of aggregate interlock was investigated.

The stress distribution of the deep-beam cantilever core was obtained using finite elements. It also confirmed the experimentally established minimum thickness of structural members for which this test method could be used. The study also found that the American Concrete Institute's recommendation on the modulus of rupture is an extremely conservative value, especially for members with widths less than 6". The modulus of rupture of a rectangular beam is different from that observed from a circular cross section such as the break off test specimen. These findings strengthen the concerns over the size effects on various recommended concrete strength parameters. In this study, new modulus of rupture values were suggested for small rectangular beams and members with circular cross sections.

The study confirmed the existence of a theoretical basis for the break off test and showed that it can be a simple and reliable nondestructive test for measuring the compressive strength of concrete.

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