Civil Engineering Reference
In-Depth Information
design strength with less cement (and therefore a higher water to cement
ratio). Whereas, previously, concrete tended to gain in strength significantly
after 28 days, with the newer cements that strength gain was much lower.
The end result of this was concrete that carbonated rapidly, resulting in
the onset of reinforcement corrosion much sooner than expected. There
has also been a tendency for contractors to leave the procurement of the
concrete to the buying department. Left with a free hand, they will choose
the cheapest concrete possible - typically with about a 50 mm slump (S1
consistence). This concrete may be totally unsuitable to place in the works,
where a higher workability may well be required, so the temptation to add
some water on site to improve placeability is huge. The result, again, is a
higher than anticipated water to cement ratio and lower durability. The
correct procedure, of course, is to decide on an appropriate set of concretes
for the different parts of a contract, with appropriate workability in each
case. These can then be called off as required, and the temptation to add
water avoided!
Designers can also improve the chances of a durable structure by avoiding
placing drip details directly under a bar (or at least providing additional cover
