Major concrete roads are usually sprayed with a curing membrane by a machine that is part of the paving train. This is not discussed in this publication. For smaller paved areas where semi-manual methods of construction are used, a curing membrane can be applied using a hand-operated spray of the garden type.
A white pigmented or aluminized super grade of compound should be used, taking particular care to ensure an even coat is applied, especially in strong winds.
Although curing membranes can be used for small areas of external concrete, it may be more convenient, and just as good, to use polythene sheeting. The covering should be kept in place for at least seven days, paying particular care at the edges of sheets, as there is a tendency for the concrete to dry out here due to wind tunnelling effects.
Slabs to receive a screed
A curing compound should not be used when a slab will later receive a cement-sand levelling screed, a wearing screed or any other bonded covering, as the bond may be affected.
Polythene sheeting will usually be the most convenient method of curing. Covering with damp hessian is not recommended because of the difficulty of keeping it moist in dry weather, especially overnight and at weekends. The concrete should be covered as soon as possible after finishing, especially in a drying wind.
Direct-finished concrete and wearing screeds
Power-floated finishes and wearing screeds have to be hard- wearing and abrasion-resistant, and particular attention to curing is essential.
After the final trowelling (either by hand or by power trowel) the surface should be firm enough for immediately covering with plastic sheeting or similar, or applying a curing compound.
The surface should not be allowed to dry out before curing. Polythene sheeting should be kept in place for at least seven days. Some loss of moisture may occur at the edges and at laps and it may be necessary every other day to turn back the sheeting and spray with water, and then replace the sheeting.
Resin-based hardeners also act as curing agents.
Cement-sand screeds
Curing compounds are not recommended because of the need for floor covering materials to bond to the screed. After laying, a cement-sand screed should be kept continuously damp for at least seven days, preferably by covering with polythene sheeting.
Vertical surfaces
The curing of columns, walls and beam sides is more difficult than horizontal surfaces. Curing membranes can sometimes be used but are not generally suitable when any subsequent treatment or rendering is to be applied.
While in position, formwork protects the concrete against loss of moisture and it is only after striking that further curing may be necessary.
When formwork has been kept in position for four days there is usually no need for further curing of concrete made with CEM I, even in drying conditions. Concretes containing pfa or ggbs may require a longer moist-curing period.
For concrete surfaces not exposed to the weather, which are to receive an applied decorative treatment such as rendering, plaster or paint, or will be tooled or abrasive blasted, no further curing of CEM I concrete is usually necessary, however soon the formwork is struck.
In general, further curing of vertical surfaces of concrete is required in temperate climates only when the formwork is struck within four days of placing the concrete and either:
n The surface will be permanently exposed to the weather; or
n The surfaces have to be uniform in colour, e.g. a number of similar columns or series of pours making up a wall.
Exposed concrete
All vertical surfaces of concrete, including white and coloured, which will be permanently exposed to the weather need extra care with curing. A well-cured surface will be more impermeable and better able to withstand the action of freezing and thawing and wetting and drying; surface crazing will also be reduced. Good curing will also help the long-term appearance of the concrete by reducing dirt collection.
All concrete surfaces that will be permanently exposed to the weather, including those to be abrasive blasted or tooled, should be cured for at least seven days. Although polythene sheeting can be used for this, it will usually be more convenient to use a sprayed-on curing compound.
Uniformity of colour
The colour of concrete can be affected by the age at which formwork is removed and by the weather, both at the time of striking and subsequently.
Where uniformity of colour is important, for example with as struck 'fair-faced' and board-marked surfaces, either:
n The formwork should be left in position for four days, in which case no further curing is usually necessary; or
n When the formwork is removed in less than four days the concrete should be covered or wrapped in polythene sheeting for at least another three days. Alternatively, but only if the concrete will be permanently exposed to the weather, a curing compound may be applied.
Damp hessian is not recommended because it contains a dye, which can stain the surface. It is also ineffective if it dries out.
White and coloured concrete
It is not advisable to use a curing compound on white or coloured concrete if there is a risk of discolouration.
Polythene sheeting is the preferred material because it cannot stain the concrete. Sheeting that is firmly fixed and left in place also protects the surface from dust caused by other site work; subsequent removal of dirt and stains is both time-consuming and expensive.