Bond of Epoxy-Coated Reinforcement: Cover, Casting Position, Slump, and Consolidation

Abstract

The effects of concrete cover, casting position, concrete slump, and degree of consolidation on the reduction in bond strength between reinforcing bars and concrete caused by epoxy coating are described. Tests include beam-end specimens containing No. 5, No. 6, No. 8 and No. 11 (16, 19, 25, and 46 mm) bars. Bottom-cast and top-cast bars with 1, 2, or 3 bar diameters of cover are evaluated. Concrete slump ranges from 2 1/4 to 8 in. (55 to 205 mm). some specimens containing high slump concrete are not vibrated. The results of the study are used to develop improved development length modification facts for epoxy-coated bars. Epoxy coatings significantly reduce bond strength. However, the extent of the reduction is less than used to select the development length modification factors in the 1989 ACI Building Code and the 1989 AASHTO Bridge Specifications for bars with cover < 3 bar diameters (db) or a clear spacing < 6 db. The development length modification factor can be reduced from 1.5 to 1.35 for these bars. The relative bond strength of epoxy-coated reinforcement increases as cover increases. As a result, the development length modification factor of 1.2 in ACI 318-89 is realistic for epoxy-coated bars with a cover > 3 db and a clear space > 5 db (ACI) or a center-to-center spacing > 6 in. (AASHTO). However, the modification factor of 1.15 in the 1989 AASHTO Bridge Specifications is slightly unconservative for these bars and should be modified. The ratio of the bond strength of bottom-cast bars to the bond strength of top-cast bars, B/T, is about the same for coated and uncoated bars and decreases slightly for coated bars as slump increases. As a result, the upper limit on the product of the epoxy-coating factor and the top-bar factor can be reduced from 1.7 to 1.5. A lack of vibration has a negative effect on the bond strength of both coated and uncoated reinforcement in high slump concrete. This is the third in a series of papers describing research at the University of Kansas on epoxy-coated reinforcement. The research is aimed at gaining a better understanding of the bond of epoxy-coated reinforcement to concrete and developing design procedures that accurately reflect the changes in bond strength caused by epoxy coating.