Bonding performance to concrete: Epoxy-coated bars vs. galvanized bars 

Based on the findings of CEB Bulletin 211 the bonding of galvanized bars to concrete is lower in early age because of hydrogen release when zinc reacts with calcium hydroxide in concrete and presence of hydrogen tend to decrease the bond strength between galvanized bars and concrete. Though bonding will increase with time till the full bond strength of un-galvanized bars is attained. 

For epoxy-coated bars there is a 20% decrease in bond strength forbars placed at the bottom of concrete sections whereas for bars placed on the top there isn't any major difference in bond compared with uncoated bars.