The behaviour of long tension reinforcement laps

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 By way of illustration, fib Model Code 2010 can require over twice the lap length required by the superseded UK code BS8110:1997. The need for this increase is debatable since, outside the laboratory, there is no evidence that laps designed to BS8110 are unsafe. The paper describes an experimental programme which was undertaken to compare failure modes of beams with laps of varying length loaded in four point bending. Tested laps are classified as “short”, “long” and “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. Tested laps were between bars of equal as well as mixed diameter with diameters ranging between 16 mm and 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 “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