Title:
Bond of Epoxy-Coated Reinforcement: Cover, Casting Position, Slump, and Consolidation
Author(s):
Hossain Hadje-Ghaffari, Oan Chul Choi, David Darwin, and Steven L. McCabe
Publication:
Structural Journal
Volume:
91
Issue:
1
Appears on pages(s):
59-68
Keywords:
bond (reinforcement to concrete); deformed reinforcement; coatings; epoxy coated reinforcement; pullout tests; reinforcing steels; structural engineering; Structural Research
DOI:
10.14359/4481
Date:
1/1/1994
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 36-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 factors for epoxy-coated bars. Epoxy coatings significantly reduce bond strength. However, the extent of the reduction is less than that used to select the development length modification factors in the 1989 ACI Building Code and the 1989 AASHTO Bridge Specifications for bars with cover much less than 3 bar diameters (d b) or a clear spacing much less than 6d b. 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 much less than 3d b and a clear spacing much greater than 6d b when combined with the 0.8 factor for bars with a clear spacing much greater than 5d b (ACI) or a center-to-center spacing much greater than 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 cast in low-slump concrete. B/T increases significantly for 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.