The effect of lateral pressure on anchorage bond in lightweight aggregate concrete

This research examines the effect of lateral pressures on the bond characteristics of both plain and deformed reinforcing bars in lightweight aggregate (Lytag) concrete. Two common bond tests, the cube pull-out and the semi-beam test, were modified in order that lateral pressures could be applied to the bond specimens. The major variables studied were the magnitude of the lateral pressure, the bar diameter, the length of embedment and the concrete strength. The results of over 200 bond tests show that for round bars the application of a lateral pressure close to the cube-strength of the concrete can result in an increase in pull-out load of as much as 260% and that for similar sized deformed bars the corresponding increase is about 75%. The different bond mechanisms for round and deformed bars were highlighted by the failure modes of the bond specimens and this difference in behaviour is reflected in the theoretical work by a frictional bond strength criterion for round bars and a splitting or shearing criterion for deformed bars. As part of the experimental work a reinforcing bar was fully strain-gauged, enabling the strain distribution along the bar to be monitored for various combinations of lateral pressure and pull-out loads. The bond stress was found to be uniformly distributed along the bar for pull-out loads greater than about half the ultimate and the strain distribution relatively unaffected by increasing lateral pressure. Finally, to test the conclusions from the bond pull-out tests in a practical situation, a total of sixty-four lightweight concrete deep beams, with varying anchorage lengths, bearing block sizes and web reinforcement were tested. The results of these tests confirm that the high bearing stresses that occur at the supports of deep beams do have significant positive effects on the anchorage bond that can be developed. A method is proposed to take into account the enhanced bond strengths that occur over the supports of deep beams with span/depth ratios of less than 2.