Civil Engineering Reference
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surface during curing. This does not require the continuous operation of
sprinklers, and timers may be used based on the weather conditions and the
estimated rate of evaporation. When spraying is carried out intermittently,
burlaps or other similar materials should be used to prevent the drying out
of the exposed concrete surface. This is because alternate cycles of drying
and wetting can cause surface cracking. The relatively high water supply
requirement is the main disadvantage of the spraying method. Closed-
system sprinklers capable of collecting and recycling the water used can
save a lot of water and are therefore recommended. The effectiveness of wet
curing using sprinklers is highly affected by windy conditions. In such con-
ditions, adequate supervision is required to ensure that the entire exposed
surface of the concrete is maintained moist without alternate wetting and
drying of any part of the concrete being cured.
Covering the concrete surface with saturated coverings such as burlap,
cotton mats, rugs, or any other moisture-retaining materials is another com-
mon wet curing method that can be applied to both horizontal and verti-
cal concrete surfaces. The requirements for burlap and burlap-polyethylene
sheeting suitable for curing are specified, respectively, in the AASHTO
(American Association of State Highway and Transportation Officials)
M182 and ASTM C171 standards. The coverings should be free of any
substance that may be harmful to concrete, and new coverings should be
rinsed with water to remove soluble substances. Before placement of cover-
ings on the surface of concrete, it should be ensured that concrete has hard-
ened sufficiently such that any damage to the surface of concrete is unlikely.
Also, the coverings are to be supplied with sufficient water to maintain
the required water content in the concrete during curing. Wet coverings of
sawdust, sand, and earth are also effective for the curing of concrete and
are used in small jobs. In such cases, a layer approximately 50 mm thick
of wet covering materials should be evenly distributed over the prewetted
surface of the concrete and maintained wet continuously. The main disad-
vantage of using wet sawdust, sand, or earth coverings is the possibility of
staining the concrete surface.
In all these wet curing methods, it is important that the supply of water
should start at least 1 hour before the initial set, that is, before the end of
the induction period. This is because maintaining a continuous supply of
water, which facilitates the ingress and transport of free water to the large
capillary pores present in the concrete, is essential for the free water to
reach the unhydrated cement particles. A delay in the provision of suffi-
cient free water may result in the development of menisci in capillary pores,
which render the penetration of water into the capillary pores more diffi-
cult. The importance of this issue increases with a decrease in the w/c ratio
of the concrete being cured.
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