Civil Engineering Project Management (4th Edition)

Site concreting and reinforcement 247
As soon as it is worked with shovels or vibrated, it will settle and appear to
flow into and become part of the previously placed concrete.
The same characteristic makes it possible to judge the water content by
noticing what happens if the freshly mixed concrete is carried in a dumper
hopper to the point of discharge. The ‘heap’ of stiff concrete discharged from
the mixer to the dumper hopper will appear to change to a pool of concrete
as the dumper bumps its way round the usual site roads. When the dumper
hopper is tipped, however, the concrete discharged should again appear stiff.
But if, in transport, the concrete slops as a semi-fluid over the side of the
dumper hopper, this shows too much water has been added. A simple density
test on freshly mixed concrete (see Section 19.8) may assist in finding if the
mix has too much water.
Sixth, ram the concrete well in place. Properly shovelled, rodded, or vibrated,
the concrete should be seen to fill the corners of shuttering and to easily wrap
around the reinforcing bars. When hand shovelling or rodding is adopted, it is
scarcely possible to over-compact the concrete. But when mechanical vibrators
are used the vibration should not be so prolonged as to produce a watery mix
on the surface. Vibrators of the poker immersion type should be kept moving
slowly in and out of the concrete. They should not be withdrawn quickly or they
may leave an unfilled hole in the concrete; nor should they be left vibrating
continuously in one location. Where vibrators are used, it is necessary for the
contractor also to have available suitable hand rammers in case the vibrators
break down in the middle of a pour.
Seventh, ensure the mix has sufficient cement in it. Normally contractors
will use a little more cement than is theoretically necessary and this is helpful
since batches of concrete vary. But if a contractor becomes too keen on cutting
the cement to the bare minimum, a number of the cube crushing tests may fail
to reach the required strength, and much delay may be caused by conducting
the investigations required to seek out the cause.
19.7 Some causes of unsatisfactory concrete test results
The two most common kinds of failure are:
• failure to get the required strength, the concrete being otherwise apparently
good;
• structural failures, such as honeycombing, sandy patches, and cracking.
Failure to get the right strength in cubes taken from a concrete pour can sometimes
have a very simple cause. Among such causes are the following:
• the cube was not compacted properly;
• it was left out all night in hard frost or dried out in hot sun;
• there was a mix-up of cubes and a 7-day old cube was tested on the
assumption it was 28 days old;
• the cube was taken from the wrong mix.