Experimental results on the local bond behavior of bars in bundles

When a reinforced concrete structure is highly loaded, the use of two, three or four bundled bars instead of individual ones could reduce or even avoid the reinforcement congestion, allowing easier placing and compaction of concrete. However, there is still a lack in knowledge of the fundamental phenomena related to the bond behaviour of bars in bundles. Therefore, design code rules for anchorages and splices differ significantly among international Standards (Eurocode 2, Model Code 2010[9-1] and ACI 318-19[9-2]). The present paper reports the results of more than 100 pull-out tests with short embedded length with the aim of comparing the local bond behaviour of bundles with that of corresponding notional individual bars. Both equivalent area and minimum perimeter criteria are evaluated. Experimental results show that both criteria are suitable methods for evaluating the bearing capacity of bundled bar anchorages, even if equivalent area criterion seems to be slightly more conservative. The experimental results provide also information on the bursting forces generated by the wedge action of the ribs which clearly increases with bar diameter. Finally, experimental results are compared with design rules for anchorage strength of bundle bars as prescribed by fib Model Code 2010.