Civil Engineering Reference
In-Depth Information
described in a paper from the Structural Faults and Repair Conference
(Grantham, 1993)
Cement content
It is a fundamental requirement of good-quality concrete that it contains an
adequate cement content, or more precisely, a sufficiently low water/cement
ratio, to provide adequate durability for the intended exposure conditions.
In the absence of chemical admixtures, a certain amount of water is required
to provide an adequate workability; essentially to simply lubricate the
aggregate particles and the cement. To achieve the desired water/cement
ratio, the amount of cement required is therefore automatically defined. This
can be altered only by changing the physical properties of the aggregate, or
by the addition of a water-reducing admixture.
If the cement content is too low (i.e. the water/cement ratio too high),
the concrete will be attacked by the weather and be liable to freeze-thaw
damage and the effects of carbonation. If the cement content is too high,
heat of hydration can cause thermal cracking in large pours, the risk of
shrinkage increases (because of the higher water content) making curing
doubly important, and, if a high-alkali cement is used, the risk of ASR
increases with susceptible aggregates and DEF is more likely to occur.
T
EST
METHODS
The test to determine the cement content of concrete is given in BS 1881 :
Part 124 : 1988 (BSI, 1988). It requires the crushed concrete to be extracted
with dilute acid and dilute alkali solution to remove the cement. The extract
