After concrete is placed, a satisfactory moisture content and temperature (between 50°F (10°C) and 75°F (24°C)) must be maintained, through a process called curing. Adequate curing is vital to quality concrete.
Many other definitions exist which include references to hydration, durability and cost but there are three basic elements to consider:
Why cure concrete?
According to the British Standard for the structural use of concrete, BS 8110 (1997), the intention of curing is to protect concrete against:
A typical definition of curing (BS 8110, 1997) is ‘the process of preventing the loss of moisture from the concrete whilst maintaining a satisfactory temperature regime’. This particular definition adds that the curing regime should prevent the development of high temperature gradients within the concrete.
Many other definitions exist which include references to hydration, durability and cost but there are three basic elements to consider:
- Moisture
- Heat
- Time
Why cure concrete?
According to the British Standard for the structural use of concrete, BS 8110 (1997), the intention of curing is to protect concrete against:
- premature drying out, particularly by solar radiation and wind (plastic shrinkage)
- leaching out by rain and flowing water
- rapid cooling during the first few days after placing
- high internal thermal gradients
- low temperature or frost
- vibration and impact which may disrupt the concrete and interfere with bond to reinforcement
Adequate curing will facilitate, but not necessarily ensure, the optimal development of the surface zone of fresh, newly cast concrete into strong, impermeable, crack-free and durable-hardened concrete. The objective is to keep the concrete saturated, or as near saturated as possible, for sufficient time for the original water-filled space to become filled to the desired extent by cement hydration products. According to the research reviewed in CIRIA (1997), the depth of the surface zone directly affected by curing can be up to 20 mm in temperate climatic conditions, and up to 50 mm in more extreme arid conditions. Properties of the concrete beyond this zone are unlikely to be affected significantly by normal curing.
It is, however, this surface zone that is often relied upon to provide many of the essential requisites of a concrete structure or element such as abrasion and chemical resistance and protection of embedded reinforcement. Figure below shows the relative depths from the surface at which various properties will be affected by inadequate curing. The importance of appropriate curing must therefore not be overlooked.
It is, however, this surface zone that is often relied upon to provide many of the essential requisites of a concrete structure or element such as abrasion and chemical resistance and protection of embedded reinforcement. Figure below shows the relative depths from the surface at which various properties will be affected by inadequate curing. The importance of appropriate curing must therefore not be overlooked.
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| Concrete Deterioration Mechanisms |
The rate of evaporation of water from the surface, taking into account the combined influences of the ambient temperature and relative humidity, the concrete temperature, and the wind velocity can be estimated from Figure below taken from ACI 308 (1992). This standard requires that curing measures are taken if the predicted rate of evaporation exceeds 1.0 kg/m2/h, to prevent plastic shrinkage cracking, but also recommends that such measures may be needed if the rate exceeds 0.5 kg/m2/h.
In the absence of any deliberate curing measures the rate of water loss during the first few hours is similar to that of any wet surface. Nevertheless, once the concrete has set, the rate of evaporation of water from the near surface of concrete in temperate climates is relatively slow (depending upon the concrete composition, actual weather conditions, etc.) and hydration of concrete in the outer zone will be able to proceed to a certain degree.
How can curing be achieved in practice?
The most common methods of site curing of in-situ concrete are:
The most common methods of site curing of in-situ concrete are:
- formwork retention
- suspension of covering above the surface before the concrete has set (horizontal surfaces)
- spraying with water
- ponding with water
- covering with wet sand, earth, sawdust, straw, or periodically wetted hessian or cotton mats, or the use of an absorbent covering with access to water
- application of a curing membrane
- waterproof reinforced paper or plastic sheeting
- tenting or other shelter against drying winds
- sunshields
- covering with an insulating layer or heated enclosure
Duration of Curing
This depends upon:
- the reason for curing (plastic shrinkage, temperature control, strength, durability, etc.)
- the size of the element
- the type of concrete (especially rate of hardening)
- the ambient conditions during curing
- the exposure conditions to be expected after curing
- the requirements of the specification
The following circumstances warrant particular consideration of curing needs:
- horizontal surfaces
- dry, hot, windy conditions (one or more of these)
- wear-resistant floors
- high-strength concrete (initial curing is especially important)
Effects of Improper Curing
Concrete allowed to freeze before a certain minimum degree of hardening has been achieved will be permanently damaged by the disruption from the expansion of the water within the concrete as it freezes. This will result in irretrievable strength loss. Excessive evaporation from an exposed horizontal surface within the first approximately 24 hours after casting will result in plastic shrinkage cracking and a weak, dusty surface. An excessive temperature difference through the cross-section of an element will result in early thermal cracking due to restraint to contraction of the cooling outer layers from the warmer inner concrete. Inadequate curing will result in the properties of the surface layer of concrete, up to 30–50 mm, not meeting the intentions of the designer in terms of durability, strength and abrasion resistance.