Civil Engineering Project Management (4th Edition)

248 Civil Engineering Project Management
Such simple errors are not unusual and must be guarded against because they
cause much perplexity and waste of time trying to discover the cause of a bad
test result. The concrete must be fully compacted in the mould, which is kept
under damp sacking until the next day when the mould can be removed and
the cube marked for identity. It is then best stored in water at ‘room temperature’
for curing until sent to the test laboratory. If poor cube test results appear
on consecutive batches, an error in the cement content of batches may be
suspected, or else the quality of the cement itself.
Honeycombing is most usually caused by inadequate vibration or rodding
of the concrete adjacent to the face of formwork. Sometimes too harsh a mix
is used so there are insufficient fines to fill the trapped interstices between
coarse aggregate and formwork, or the larger stones cause local arching. Sand
runs – patches of sandy concrete on a wall surface which can be scraped away
with a knife – can be due to over-vibration near a leaking joint in the formwork
which allows cement and water to pass out of the mix. One simple, and
not infrequent, cause of poor concrete is use of the wrong mix due to a ‘failure
of communication’ with the batching plant operator or ready-mix supplier.
An experienced concreting foreman should be able to detect a ‘wrong mix’ the
moment it is discharged.
19.8 Site checks on concrete quality
The defect of cube and beam tests on concrete is that results cannot be known
until some days after the concrete has been placed. If weak concrete appears
to have been placed in a structure a difficult situation arises. The resident
engineer can ask for the offending concrete to be demolished and re-built but
this may pose such difficulty and delay that the decision ought not to be made
on site without first discussing the problem with the engineer. The action
taken depends upon how far the strength of the concrete falls short of the
required strength, the load-bearing function of the under-strength concrete,
and whether some alternative exists which does not involve breaking out the
faulty concrete.
Frequent site checks of concrete quality can help to avoid such problems.
Section 19.6 has already indicated that the water content of a mix can easily be
judged by eye; and if the quality of the aggregate stocks held on site is kept
under reasonable supervision, defects arising from aggregate quality or water
content are unlikely to arise. Thus it is to the batching plant, and more particularly
to the cement content, that checks should be directed.
One of the simplest on-the-spot tests which can be conducted is the density
of freshly made concrete. This should be at least 2350 kg/m 3 (147 lb/ft 3 ) for a
C20 mix and 2390 kg/m 3 (149 lb/ft 3 ) for a C30 mix on the assumption that the
relative density of the aggregate is 2.65. The trial concrete mixes, however,
should have revealed the typical densities expected for various grades of
mix. The density can be obtained by filling and weighing an 0.015 m 3 (0.5 ft 3 )