Basic piling methods - Piling Engineering - page 70

3.4.6 Concrete mixes for bored piles

The integrity of the pile shaft is of paramount importance, and the concreting mixes
and methods that have evolved for bored piles are directed towards this, as opposed
to the high-strength concrete necessary for pre-cast piles or structural work above
ground.
This prerequisite has led to the adoption of highly workable mixes, and the 'total
collapse' mix for tremied piles has been mentioned. In order to ensure that the concrete
flows between the reinforcing bars with ease, and into the interstices of the soil, a high-
slump, self-compacting mix is called for. A minimum cement content of 300 kg/m 3 is
generally employed, increasing to 400 kg/m 3 at slumps greater than 150mm, with
a corresponding increase in fine aggregate content to maintain the cohesion of the
mix. Three mixes recommended by the Federation of Piling Specialists are given in
Table 3.9. However, these are now superseded by CEN EN 1534 European Standards
which quote minimum cement contents of 325 kg/m 3 under dry placement conditions
and 375 kg/m 3 under submerged conditions, both at a water-cement ratio of 0.6. Other
requirements are quoted with respect to slump according to the placing conditions,
i.e. slumps between 130 to 180mm and >180mm depending on whether the concrete
is placed in a dry bore or tremied.

3.4.7 Reinforcement for bored piles

For piles loaded in compression alone, it is generally only necessary to reinforce the
shaft to a depth of 2m greater than the depth of temporary casing, to prevent any
tendency for concrete lifting when pulling the casing. Piles subject to tension or lat-
eral forces and eccentric loading (possibly by being out of position or out of plumb)
do however require reinforcement suitable to cope with these forces. Typical nominal
reinforcement for piles in compression is shown for guidance in Table 3.10. The restric-
tions that apply to the use of this table have to be carefully considered in any particular
application.
It should be noted that the minimum amounts of reinforcement to comply with
European Standards are prescribed by CEN EN 1536. The assembled cage should be
sufficiently strong to sustain lifting and lowering into the pile bore without perma-
nent distortion or displacement of bars and in addition bars should not be so densely
packed that concrete aggregate cannot pass freely between them. Hoop reinforcement

Table 3.9 Recommended concrete slumps for cast-in-place piles (based on Federation of Piling
Specialists' Specification, for cast-in-place piling)

Piling mix Typical
slump (mm)

Conditions of use

A 125 Poured into water-free unlined bore. Widely spaced reinforcement leaving

ample room for free movement of the concrete between bars.

B 150 Where reinforcement is not spaced widely enough to give free movement of

concrete between bars. Where cur-off level of concrete is within casing.
Where pile diameter is < 600mm

C > 175 Where concrete is to be placed by tremie under water or bentonite in slurry.