Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 53

instead of the specified salt solution for the latter test. It is to be noted that
the use of pure water will give a different result with the clay taking lon-
ger to settle and giving a higher reading. The important point to realise is
that the test only establishes whether organic impurity is present and not
whether it is deleterious. The colour test can fail due to the presence of a
few pieces of organic matter, such as small twigs or other vegetation, that
are too few and too localised to have any significant effect on strength (but
could produce a visual defect on a surface).
Sands failing the colour test should then be tested for setting time and
initial strength development. If they are satisfactory in these respects, it
is unlikely that there will be any long-term problems (although another
problem encountered has been of sands that automatically entrain air due
to natural lignin).
A common effect of organic impurity (if there is any effect) is retarding
or preventing chemical set. If there is no ill effect on strength up to 28 days,
then the sand is satisfactory. There may be a strength reduction at 1 to
7 days but no loss of strength at 28 days, which may or may not be satis-
factory for particular applications. There may be implications, with early
strength loss, of setting time extension and consequent surface finishing
problems for slabs.
For organic impurity evaluation, comparative mortar cubes should have
the same water/cement ratio, not the same workability.
Natural impurities are not the only kind and there have been instances
of accidental contamination, especially with sugar. One example was
of a barge used to transport sand after transporting a load of bulk raw
sugar, one result of this was to cause a large floor slab in a multistory
building not to set for several days. It takes very little sugar to cause
a retardation problem. In one instance a concrete strength problem
later traced to employees emptying the dregs of their morning tea onto
the sand pile of a small manual batching plant. They obviously liked
sweet tea!
Rivaling the frequency of occurrence of all the aforementioned chemi-
cal issues with aggregates combined in the authors' experience has been
the frequency of multiple dosing of retarding admixtures. This is outside
the scope of the topic, but it has provided more examples of concrete that
has eventually proved quite satisfactory after taking several days to set.
The message here is not to panic too early. If a sample sets after being in
boiling water overnight (inside a plastic bag of course) then the concrete
in the structure will set eventually. The question is whether it will develop
serious settlement cracks in the interim due to prolonged bleeding, or to
water soaking into formwork, or escaping at joints that are not watertight.
It is certainly important to cover the concrete with plastic sheeting or wet
hessian to stop it from drying out.