Page 82 - Masonry and Concrete

Concrete Construction Techniques
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CONCRETE CONSTRUCTION TECHNIQUES

Plastic shrinkage cracking is usually associated with hot-weather
concreting. It is caused by rapid evaporation of surface moisture from
a slab or other flatwork. The procedures recommended for hot-weather
concreting will alleviate the possibility of plastic shrinkage cracking.
Dusting is the wearing away of hardened concrete surfaces under
traffic. Dusting is caused by mixes with too much water, segregation
during the placement and consolidation of the concrete, dirty aggre-
gate, applying water to the concrete surface during finishing opera-
tions, or premature or prolonged finishing operations which cause the
formation of a weak surface layer called laitance. Laitance is a white or
light gray substance which appears on the surface of concrete after it is
consolidated and finished and which consists of water, cement, and
fine sand or silt particles. Laitance prevents good bond of subsequent
layers of concrete and adhesion of other materials to the concrete such
as finish flooring. In an exposed slab, laitance will scale and dust off
after the floor is in use, and it can contribute to hairline cracking and
checking. Excessive amounts of rock dust, silt, clay and other similar
materials can also contribute to laitance. The same measures that are
used to reduce bleeding will also reduce the occurrence of laitance.
Scaling is the flaking or peeling away of a thin layer of cement mor-
tar on the surface of concrete. The aggregate below is usually clearly
exposed in patchy areas and often stands out from the remaining sur-
1
face. Scaling can be paper thin or as deep as 4 in. One type of scaling
is caused by the same things that cause dusting and laitance: mixes
with too much water, segregation during the placement and consoli-
dation of the concrete, applying water to the concrete surface during
finishing operations, or premature or prolonged finishing operations.
Another type of scaling is caused by the use of deicing salts on non-air-
entrained concrete, and can be prevented by the use of air-entrained
cement or air-entraining admixtures.
In false set, concrete appears to set or harden after only a few min-
utes. This is a temporary condition caused by hydration of unstable
gypsum (calcium sulfate) in the cement. It usually disappears with
prolonged mixing or remixing and is generally not a problem with
ready-mixed concrete. Do not add more water. After a few more min-
utes, with or without additional mixing, false set will usually disap-
pear on its own.

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