Civil Engineering Reference
In-Depth Information
Plastic cracking
Cracks due to this phenomenon appear within the first two hours after
placing and are of two distinct types:
1
Plastic settlement cracks: Typically found in columns, deep beams or
walls. The problem tends to occur with high water/cement ratio concretes
which have suffered from bleeding. The concrete literally 'hangs up' on
the steel slumping between it, with cracks forming over the line of the
steel. Caught early enough, re-vibration of the concrete can repair the
damage while the concrete is still plastic.
Figure 1.16
shows a core taken
through a plastic settlement crack in a car park. The damage that the
easy pathway for chlorides has caused can easily be seen.
2
Plastic shrinkage cracks: More common in flat exposed slabs. This can
occur anywhere where the rate of loss of moisture due to evaporation
exceeds the rate of bleeding. Not surprisingly, it is more of a problem in
Note: Neither of these should be confused with drying shrinkage cracks which only
occur after a considerable time (though they may start as cracks from another cause).
