Page 266 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
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a few handfuls, he should request the contractor to bring this to the notice of
the supplier. He need not reject the load out of hand, but it will do no harm to
let the supplier know the aggregates are being watched. If a load contains
numerous weak stones or several pieces of clay, it should be rejected.
Diagnosing whether an aggregate is likely to give rise to alkali-silica reac-
tion (which can cause expansion and disruption of concrete in a few years in
the presence of moisture) requires specialist knowledge. The most practical
approach for the engineer is to ask the supplier if his aggregate has been tested
for this; if not, structures built some years previously with the aggregate
should be checked for signs of cracking due to alkali-silica reaction. Guidance
and precautions are set out in certain publications (References 1 and 2), but if it
is proposed to use an aggregate not used before, the site staff should refer the
problem to the engineer.
Second, choose tested cement. The same principle applies to cement as
with the choice of aggregates; find the supplier of cement to other jobs and
request a recent test certificate. Troubles can start when imported cement has
to be used or cement from a variety of suppliers. Overseas it is not unusual for
a small contractor to buy his cement a few bags at a time from the local bazaar.
Testing such cement on site before any concrete is placed in an important part
of a structure is essential. BS 12 provides methods for testing the compressive
strengths of 1:3 mortar cubes or 1:2:4 concrete cubes but, if this is difficult to
arrange, the flexural test mentioned in Section 19.3 can be applied on site.
Third, ensure reasonably graded aggregates. In delivery and stockpiling of
coarse aggregate there is a tendency for the mix to segregate, the larger mater-
ial remaining on top. Care has to be taken to ensure that certain batches are
not made up from all the coarsest material and others from most of the fines.
Crushed rock often has a considerable amount of dust in it, although this does
not normally present a problem one does not want a batch made up mostly
from dust and fines taken from the bottom of a stockpile.
Fourth, use washed aggregates. Unwashed aggregates suitable for concret-
ing are rare: they are usually comprised of crushed clean homogeneous rock.
Sometimes a river sand is supplied unwashed – it being assumed that the
sand has already been ‘washed’ by the river. This should not be accepted as a
fact, since a river also carries silts and clays. Sea-bed or beach sands must be
washed in fresh water to remove the salt from them.
Fifth, achieve the right workability. Mechanical mixers are seldom at fault
with regard to mixing, and hand-mixing can also be quite satisfactory; but it
is the water content of a mix that requires the most vigilant attention. The site
engineer should never let ‘slop’ be produced. Although the slump test and the
compacting factor test are useful in defining the degree of stiffness of a mix, in
practice judging the water content of a mix ‘by eye’ is both necessary and pos-
sible. The right sort of mix should look stiff as it comes out of the mixer or
when turned over by hand on mixing boards. It should stand as a ‘heap’ and
not as a ‘pool’ of concrete. When a shovel is thrust into such a pile, the shovel-
cut should remain open for some minutes. Such a mix will look quite different
after it is discharged and worked into some wet concrete already placed.