John Aidoo

Deterioration of FRP-to-Concrete Bond under Failure Loading

Characteristics of FRP-to-concrete bond are observed to deteriorate under cyclic loads. Deterioration has been observed under load conditions which result in a stress range in the bonded FRP of only 4% of the material capacity. Current design recommendations and “accepted practice” often neglect this detrimental effect. The clear effects of bond deterioration under fatigue loading are demonstrated for moderate-scale laboratory specimens and in full-scale specimens recovered from a forty-year old interstate highway bridge. The results suggest additional limit states that must be considered when even relatively small fatigue loads are present.

Effect of Variations in Practice of ASTM D7522 Standard Pull-Off Test for FRP-Concrete Interfaces

The recently promulgated ASTM D7522 Standard Test Method for Pull-Off Strength for FRP Bonded to Concrete Substrate provides a method for quality control and acceptance testing of fiber reinforced polymer (FRP) systems adhesively bonded to a concrete substrate. In the process of developing ASTM D7522, a number of comments and questions were received—some were incorporated as best practices and some were tabled, requiring further investigation. The present study addresses the latter issues and thereby directs future amendments of the ASTM D7522 standard method. Specifically, this study addresses the effects of specimen shape, the effects of variation in specimen cut depth, methods of cutting square specimens, and the impact of retesting specimens that do not fail during their initial test. It is shown that square specimens should be permitted although their results may not be directly compared with round specimens and they may have some other practical considerations affecting their use. Additionally, it is shown that cutting too deeply into the concrete substrate results in lower values of pull-off strength.

Towards a Standard Test Method for Assessing FRP-to-Concrete Bond Characteristics

It is well established that debonding of adhesive-applied FRP retrofit materials from their concrete substrate often is a critical limit state. This paper adopts the use of a simple concrete beam specimen similar to that used to determine the modulus of rupture of concrete. To achieve controlled debonding behaviour, the beam is notched to represent cracked concrete. This paper addresses specific issues associated with standardising such a test specimen including a) the width of the FRP relative to the concrete substrate width; b) the geometry of the induced notch; and c) the effect of providing an initially unbonded region in the vicinity of the notch. Conclusions provide recommended specimen geometry which satisfies the objectives of such a standardised test.