The influence of casting position and depth of concrete pour on bond strength

Abstract Bond strength of steel bars cast near the top of a concrete pour may be lower than that of bars cast close to the bottom due to an increase in water content, to consolidation of fluid concrete below the bar, or to both. Current design codes typically require increases of 20%–40% in anchorage and lap lengths in “poor” bond conditions to allow for this. The influence of casting position has generally been reported as the ratio of bond strength of bars cast near the top of a pour to that of bars cast near the bottom in the same pour. However, an increase in water content toward the top of a pour can only come about through a corresponding reduction lower down, which will increase the bond of bars near the bottom. The lower bond strength reported for top bars in deeper pours may thus be a consequence of both a reduction in bond near the top of the pour and an increase near the bottom. Four series of tests have been conducted with different concrete mixes, each series containing three section depths. Bond of top cast bars did not decrease further once depth concrete below the bar exceeded 400 mm, but bond of bottom cast bars did increase in deeper pours. Ignoring the increased bond of bottom bars thus overestimates the effect of a poor casting position. Implications for design codes are discussed.