What is Curing? Methods of curing

What is curing?

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.

1.Moisture 
2.Heat

3.Time

Method of Concrete

The most common methods of site curing of concrete are:

1. form work retention
2. suspension of covering above the surface before the concrete has set (horizontal surfaces)
3. spraying with water
4. ponding with water
5. covering with wet sand, earth, sawdust, straw, or periodically whetted hessian or cotton mats, or the use of an absorbent covering with access to water
6. application of a curing membrane
7. waterproof reinforced paper or plastic sheeting
8. tenting or other shelter against drying winds
9. sun shields
10. covering with an insulating layer or heated enclosure.

No specific measures are needed when the ambient conditions of moisture, humidity and temperature are sufficiently favorable to provide adequate curing by themselves But great caution should be exercised in making such a judgement – general assumptions about climate can be dangerous, particularly in temperate climates such as the UK. For example, the annual average relative humidity, which may be approximately 80 per cent,is of absolutely no relevance on a hot dry summer day when the relative humidity may be as low as 50 per cent or less.

Retention of form work

Where the constraints of the construction programme allow, curing can be provided by retention of the form work. Enhanced curing may be obtained by loosening the form work,once the concrete has hardened sufficiently to allow this (the next day, for example), to allow water to be introduced into the gap between the form work and the concrete surface.Care must be taken to avoid the risk of thermal shock from using water that is too cold and to ensure the concrete is kept moist.

Impermeable coverings

Impermeable material in contact with a concrete surface will prevent loss of moisture. In the form of thin sheet materials such as polythene, it has the advantage of being very lightweight and flexible so it can be applied in most locations and on complicated shape elements. Sheeting on formed surfaces (i.e. cast against form work) should be in intimate contact with the surface, and securely fixed at the edges to form an effective moisture seal. This can, however, result in a mottled surface appearance and thus should not be used where appearance is a critical factor. Sheeting on fresh unformed (horizontal) surfaces should preferably be suspended above but sealed at the sides and ends to prevent through passage of air, and to form an effective high relative humidity curing chamber. Thus in  this application a small amount of evaporation from the surface is inevitable but should be small provided the enclosure is effectively sealed. In practice, suspension of sheeting above the fresh concrete surface is often considered impractical and, instead, it is placed directly on the concrete. In this case it is essential that the surface is allowed to stiffen sufficiently to prevent damage from contact with the cover. White sheeting will help control surface temperature of the concrete by reflecting the sun’s rays; black will have the opposite effect and should be avoided for outdoor applications in warm conditions. Clear sheeting will have little effect on heat absorption.The surface of the concrete should be moistened by spraying with water prior to application of the covering to minimize the effect of any initial loss of water into the air space between the concrete and the covering.

Absorptive coverings

Absorptive materials, such as hessian or sand, will keep the surface of the concrete damp for as long as they are in place and are themselves kept wet. Use of such materials may not be appropriate in large sections where it is necessary to insulate the outer surfaces to prevent the risk of cracking through excessive temperature across the section.Materials should not contain harmful amounts of substances, such as sugars or nitrate fertilizers, that could damage the surface of the concrete. Where appearance is important,these materials should also be free of substances that could stain or discolor the surface.At the end of the required period of curing, subsequent drying of the concrete will be beneficially slowed if the absorptive material is allowed to dry thoroughly before its removal.

Water addition

Ponding is easily achieved on flat surfaces by building a small bund around the perimeter and keeping the enclosed area flooded. Where spraying or misting is to be used there is a need to consider factors such as continuity of water supply, clogging of nozzles, wind direction and strength, and drainage paths for the surplus water. The risk of thermal shock and excessive temperature differences across sections should also be considered. ACI 308-92 (1992) recommends that the curing water should not be more than about 11°C cooler than the concrete as rapid cooling and contraction of the surface, restrained by the warmer layer below, could result in the development of sufficient stress to cause cracking.The use of free water should also be avoided where the possibility of freezing of the concrete surface exists during curing.Water used for curing should generally be of the same quality as that used as mix water. In particular, if the concrete is reinforced, or if it contains any other embedded metal, the water should not contain significant levels of chloride that could enter the concrete and increase the risk of premature corrosion of reinforcement. Where the appearance of the concrete surface is important, the water should not contain harmful amounts of any substances that could stain, attack or discolor the surface.

Curing membranes

Curing membranes are hand- or spray-applied sealing compounds applied in liquid form after free water has disappeared from the concrete surface (for horizontal surfaces) or upon removal of form work (for formed surfaces). Such materials typically include natural and synthetic resins, waxes and solvents that are highly volatile at normal ambient temperatures. They work by greatly reducing the rate of evaporation of water from the surface. When the rate of evaporation exceeds the rate of bleed, it is advisable to wet the surface of the concrete and wait for bleeding to cease before applying a curing membrane.If bleeding continues below the membrane a layer of water may form below the upper layer of matrix which has adhered to the membrane; this layer will be very prone to later damage in service. Map-cracking of the membrane, requiring reapplication, is another possible result.

spraying with water

spraying method

Curing by spraying of water or fogging is one of the conventional concrete curing methods in the construction industry. It can be an efficient method for curing by supplying additional moisture during hot weather. It helps to reduce the temperature of the concrete. Spraying of water may be done manually. 

Pounding with water

pounding

Pond curing is another great way to cure concrete, and the process is just as it sounds: You form temporary berms around a new concrete slab, and then flood the area inside them with one foot of water.