In most of the precasting factories, it is economical to use faster curing methods or artificial curing methods, which in turn will allow the elements to be demoulded much earlier permitting early re-use of the forms. Any of the following methods may be adopted:
a) By heating the aggregates and water before mixing the concrete — By heating of the aggregates as well as water to about C to C before making the concrete mix and
placing the same in the moulds, sufficiently high earlier strengths are developed to allow the elements to be stripped and transported.
b) Steam curing — Steam curing may be done under high pressure and high temperature in an autoclave. This technique is more suited to smaller elements. Alternatively, this could be done using low pressure steam having temperature around 80°C. This type of curing shall be done as specified in 10.5.2. For light weight concrete products when steam cured under high pressure, the drying shrinkage is reduced considerably. Due to this reason, high pressure steam curing in autoclave is specified for light weight low densities ranging from 300 to 1 000 kg/m³. For normal (heavy) concretes as well as light weight concretes of higher densities, low pressure steam curing
may be desirable as it does not involve using high pressures and temperatures requiring high investment in an autoclave (see also 10.5.2).
c) Steam injection during mixing of concrete — In this method low pressure saturated steam is injected into the mixer while the aggregates are being mixed. This enables the heating up of concrete to approximately C. Such a concrete after being placed in the moulds attains high early strength.
d) Heated air method — In this method, the concrete elements are kept in contact with hot air with a relative humidity not less than 80 percent. This method is specially useful for light weight concrete products using porous coarse aggregates.
e) Hot water method — In this method, the concrete elements are kept in a bath of hot water around C to C. The general principles of this type of curing are not much different from steam curing.
f) Electrical method — The passage of current through the concrete panels generates heat through its electro-resistivity and accelerates curing. In this method, the concrete is heated up by an alternating current ranging from 50 V for a plastic concrete and gradually increasing to 230 V for the set concrete. This method is normally used for massive concrete products.
10.4.1 After the accelerated hardening of the above products by any of the above accepted methods, the elements shall be cured further by normal curing methods to attain full final strength.
10.4.2 Accelerated hardening may also be achieved by the following techniques:
a) Construction chemicals — Suitable construction chemicals may be used.
b) Consolidation by spinning — Such a method is generally used in the centrifugal moulding of pipes and such units. The spinning motion removes excess water, effects consolidation and permits earlier demoulding.
c) Pressed concrete — This method is suitable for fabrication of small or large products at high speed of production. A 100 to 200 t press compresses the wet concrete in rigid moulds and expels water. Early handling and a dense wear resistant concrete is obtained.
d) Vacuum treatment — This method removes the surplus air and water from the newly placed concrete as in slabs and similar elements. A suction up to about 70 percent of an atmosphere is applied for 20 to 30 min/cm thickness of the units.
e) Consolidation by shock — This method is suitable for small concrete units dropped repeatedly from a height in strong moulds. The number of shocks required to remove excess water and air may vary from 6 to 20 and the height of lift may be up to as much as half the depth of the mould.
10.4.3 After the accelerated curing of the above products by any of the above accepted methods, the elements shall be cured further by normal curing methods to attain full final strength.