The behaviour of long tension reinforcement laps

Abstract

Over time, the length of reinforcement laps required by design standards has increased significantly. By way of illustration, fib Model Code 2010 can require over twice the lap length required by the superseded UK code BS 8110-1:1997. The need for this increase is debatable since, outside the laboratory, there is no evidence that laps designed to BS 8110-1:1997 are unsafe. This paper describes an experimental programme undertaken to compare the failure modes of beams with laps of varying length loaded in four-point bending. The tested laps were classified as ‘short’, ‘long’ or ‘very long’, with long laps just sufficient to develop reinforcement yield. The very long laps were between 1·5 and 2·0 times the length of the long laps. The tested laps were between bars of equal as well as mixed diameter, with diameters in the range 16–25 mm. Instrumentation included strain gauges and digital image correlation, which was used to record crack development. Bond failure was very sudden and brittle in the short laps. The failure modes of both long and very long laps were ductile due to flexural reinforcement yield. However, bond failure occurred subsequent to yield in long laps including at least one 25 mm diameter bar.