The effects of placement conditions on the quality of concrete in large-diameter bored piles

In South Africa, concrete in large-diameter bored piles is generally placed by discharging a high-flow concrete mix directly from the truck mixer and allowing the concrete to fall freely to the base of the pile hole. While certain site practices have been used by piling contractors for years, many engineers are not convinced of their acceptability. In order to assess the effects of free-fall concrete placement, a series of tests were undertaken in which the properties of concrete placed in this manner were compared with the properties of conventionally placed concrete. The tests included an assessment of the effect of water and spoil in the pile hole at the time of casting, as well as poor placement techniques. The results of this investigation indicate that casting of concrete in 50 mm and 400 mm of water in the bottom of the pile hole significantly reduced the compressive strength by approximately 50% and 80%, respectively. Furthermore, the effect of spoil at the bottom of the pile hole was dependent on the amount of water present. Finally, a separate investigation, at a bridge site, indicated the free-fall placement technique to be at least as effective as the tremie technique.