The hardening of concrete, the development of strength and impermeability depend on the presence of water.
At the time the concrete is placed there is always an adequate quantity of water present for full hydration but it is necessary to ensure that this water is retained so that the chemical reaction continues until the concrete has developed the necessary degree of impermeability and strength.
At the time the concrete is placed there is always an adequate quantity of water present for full hydration but it is necessary to ensure that this water is retained so that the chemical reaction continues until the concrete has developed the necessary degree of impermeability and strength.
The areas most affected by poor curing are the surface zones, and these are critical with respect to durability. In particular, the protection of reinforcement against corrosion depends on the quality of the concrete in the cover. Similarly, abrasion resistance depends on concrete quality in the top few millimetres. If the curing is inadequate the concrete may not be durable nor provide adequate protection to the reinforcement despite conforming to specification in all other aspects.
Curing should be carried out until the capillary voids are discontinuous, but at present it is not possible to establish the precise times when this occurs. The figures in Table 1 7 (based on Table 6.5 of BS 8110) provide a useful guide but are minimum values and may need to be exceeded. As indicated by the minimum periods in Table 1 7, when some of the Portland cement in a concrete is replaced by ggbs or pfa, curing times are increased by about 50% to compensate for the lower rate of strength development.
The minimum time required for preventing loss of moisture from the surface of the concrete depends on a number of factors including:
n The type of cement or combination
n The cement content and the free water/cement ratio
n The temperature of the surface layer
n The ambient conditions
n The intended use of the concrete.
In addition to these factors, ambient conditions after casting will vary during the period of curing, so it is recommended that curing, either by preventing evaporation or by keeping the surface of the concrete continually damp is carried out for at least:
n 7 days for horizontal surfaces
n 3 days for vertical surfaces.
Specific recommendations for particular elements are given on the following pages.
Concrete also needs to be kept at a favorable temperature - the lower the temperature, the slower is the rate at which concrete hardens and develops strength. If the temperature of the plastic concrete falls below freezing point before adequate strength has developed, the freezing and resulting expansion of the water may cause permanent damage. Refer to pages 38 - 40 for further information relating to cold and hot weather conditions.
Curing increases resistance to abrasion, so effective curing is essential for floors and other surfaces subject to wear.
Continuous curing from the time the concrete is placed helps to ensure a hard, dense surface which reduces the risk of crazing and dusting, increases impermeability and improves weathering characteristics. This is of vital importance for the concrete cover to reinforcement.
Table : Minimum periods of curing and protection.
Type of cement
|
Curing conditions after casting
|
Minimum periods
of curing and
protection (days)
|
||
Average surface
temperature of concrete
|
||||
5°C to 10°C
|
Above 10° C
|
t
(any temperature
between 5°C and 25 C)
|
||
CEM I 42,5N SRPC
CEM I 42,5R
CEM I 52,5
|
Average
|
4
|
3
|
60
t+10
|
Poor
|
6
|
4
|
80
t+10
|
|
CEM I I and CII CEM IIIA and CIIIA
CEM IV and CIV
|
Average
|
|||
Poor
|
10
|
7
|
140
t+10
|
|
All cement types
|
Good
|
No special requirements
|
||
NOTE
Curing conditions are defined
as:
Good - damp and protected (relative humidity greater than 80%, protected from sun and wind) Average - intermediate between good and poor
Poor - dry or unprotected (relative humidity less than 50%, not protected from sun and wind)
|
||||
Early curing will reduce evaporation of water from the surface of fresh concrete in drying conditions; unless evaporation is checked, 'plastic' cracks may appear while the concrete is still setting. This was discussed on page 19 under Plastic cracking.
Methods of curing
Methods of curing can conveniently be considered in two groups:
n Those which keep water or moisture in close contact with the surface of the concrete, such as pounding, spraying/sprinkling, damp sand and damp hessian
n Those which prevent the loss of moisture from the concrete, such as plastic sheeting, building paper, leaving the formwork in place, and sprayed-on curing membranes.
Although tests have shown that the methods in the first group are the most efficient and may be appropriate for some work, they tend to suffer from the practical disadvantages of being expensive in both materials and labour and, perhaps more importantly, it is difficult to ensure that they are done properly; damp hessian, for example, is seldom kept continuously damp.
Curing membranes
Curing membranes are liquids sprayed onto either fresh or hardened concrete surfaces, which dry leaving a thin film of resin to seal the surface and reduce the loss of moisture (Figure 30). They can be used on both horizontal areas of fresh concrete and vertical surfaces after the removal of formwork. Most sprayed-on curing compounds should be applied immediately after the water sheen which results from bleeding has evaporated.
The resin film remains intact for about four weeks, but then becomes brittle and peels off under the action of sun and weather. Curing membranes were developed for roads and airfield pavements which are difficult and impractical to cure satisfactorily by any other means, and although they are now used extensively for curing structural concrete there are some occasions when they may not be suitable.
disappears after application and will not stain the surface provided that it is not applied to a dry concrete surface. Special non-toxic compounds are available for use on concrete that is to contain drinking water. Pigmented and aluminized varieties should be agitated frequently to ensure uniform dispersion of the solids.
Generally, and certainly in the UK, the pigmented higher-efficiency grades should be used for external paved areas, and the non- pigmented lower efficiency grades on structural concrete. In tropical climates, the higher-efficiency grades should be used for all applications.
On any job it is therefore essential to make sure that the right type of curing membrane is used. Further information is available in the BCA publication Concrete on site - 6: Curing.
Horizontal surfaces
It is essential for most horizontal surfaces to be well cured. This is particularly important where the concrete will be trafficked, either by foot or by vehicles.
Curing should always start as soon as possible after the concrete has been compacted and finished, generally within 30 minutes of the water sheen (bleed water) disappearing.