Civil Engineering Reference
In-Depth Information
piles places severe restraints on the type of concrete that may be used. In general it is
preferable to ensure the pile shaft is free from defects by using a highly workable mix,
than to specify one with a low water/cement ratio in the expectation of producing a
concrete of high durability. Using concrete with too low a workability is a major cause
of defects in bored pile construction (Figure 7.10).
It is not practicable to vibrate concrete in a pile shaft, hence reliance is placed on
the energy of the falling stream of concrete to do this. The free-fall method of placing
concrete will result in the segregation of the concrete if the mix is unsuitable.
In general terms concrete suitable for constructing cast-in-place piles should be free
flowing yet cohesive; the mixes are designed to be 'self-compacting'. The slump test
is quite adequate to assess the former property; however, cohesiveness or resistance
to segregation is difficult to define. If available, rounded aggregates should be chosen
and a reasonably high sand content adopted. Plasticizers may be used to improve
the workability of the mix with advantage. Experiments with super-plasticizers have
Figure 7.10
The effect of low slump concrete. (Photographs courtesy of CIRIA, London.)
