170 7 Poor Construction Methods
not done, very low temperatures may be avoided by insulating the member appro-
priately. Heaters are also used under the insulated members (tarps) after ponding the
concrete surface.
Delay form removal: Evaporation of water from vertical members like beams
and columns is largely prevented by the forms, which should be left in place as long
as possible by periodically spraying them with water. However, recently the use of
controlled permeability formwork (CPF) liner has proved very helpful in enhancing
concrete curing. CPF aids concrete curing in two ways. In the critical period between
pouring and stripping of the formwork, the water-rich CPF liner acts like a curing
membrane. Upon stripping, the reduced porosity of the surface means that there is
no rapid moisture loss, as can occur with conventionally cast surfaces [22]. The
benefits of CPF are also addressed in Sect. 7.5.2.
Curing Compound: While using membrane curing, the concrete is coated with a
sealing compound, which is usually done by spraying. The curing membrane serves
as a physical barrier to prevent loss of moisture from the concrete to be cured.
Membrane curing may not assure full hydration like moist curing, but is adequate
and particularly suitable for concrete members in contact with soil. The coating
should be applied at a uniform rate. Usually two applications at right angles to each
other are suggested for complete coverage. The coating must be protected against
damage.
The curing compound shall preferably be water-based membrane forming and of
a type approved by the engineer and be applied as directed by the manufacturer.
When curing compound is used, the exposed concrete shall be thoroughly sealed
immediately after the free water has left the surface. Formed surfaces shall be sealed
immediately after the forms are removed and necessary finishing has been com-
pleted. The curing compound shall be applied by power-operated atomising spray
equipment in one or two separate applications. Hand-operated sprayers may be used
for coating small areas.
Curing compound solutions containing pigment shall be thoroughly mixed prior
to use and agitated during application. If the solution is applied in two applications,
the second application shall follow the first within 30 min. Satisfactory equipment
shall be provided, together with means to properly control and assure the direct
application of the curing solution on the concrete surface so as to result in a uniform
coverage rate of at least 0.27 L/m2. If rain falls on the newly coated concrete before
the film has dried sufficiently to resist damage, or if the film is damaged in any other
manner during the curing period, a new coat of solution shall be applied to the
affected portions equal in curing value to that specified above.
It has been observed [33] that if the water/cement ratio is greater than about 0.42,
membrane curing is adequate. However, if water/cement ratio is smaller than 0.42,
autogenous shrinkage will develop rapidly even if the curing membrane has been
applied. In fact, a membrane will prevent evaporation of water from the concrete but
will not allow ingress of water to cover the water loss by self-desiccation. When the
membrane ceases to be effective, drying shrinkage will also develop. It is only at
water/cement ratio in excess of about 0.5 that membrane curing is fully
satisfactory.
