A Guide to Curing Concrete
Concrete curing is an essential part of the overall concrete laying process. Curing is vital for both strength and durability, and it helps to maintain concrete hydration, ensuring that water and concrete can react together for the strongest setting.
In this guide, we'll cover the benefits of concrete curing as well as the most effective methods of curing concrete, including timescales and how to perform the procedure in different conditions.
What is concrete curing?
Concrete curing refers to maintaining adequate moisture in your concrete, at the correct temperature levels. This ensures proper hydration as the chemical reaction between concrete and water causes your concrete to set and harden. It often requires maintaining the presence of water in your cement during the early hardening period, reducing water loss from the surface of your concrete and supplying heat and additional moisture for more strength.
Curing using applied insulation can also help control damaging internal temperature differentials in large masses of concrete, and maintain an adequate temperature in the concrete during cold and frosty weather to fully hydrate the cementitious material present.
Why cure concrete?
In its early life, fresh concrete must be protected from the detrimental effects of hot sun, dry air, drying winds, and frost. Care must be given to curing to achieve any concrete's required durability and strength.
Curing is used primarily to assist strength development and improve the durability potential of freshly placed concrete. In some cases, curing also reduces the effects of thermal contraction and produces satisfactory surface finishes.
Why is curing concrete important for its durability and performance?
The development of strength and durability of any concrete mix, regardless of the type of cement used, depends on the maximum hydration of the cement since these properties depend upon the production of a dense matrix of low permeability that is resistant to the passage of water, carbon dioxide and oxygen. The chemical reaction between the cement and water must continue to its maximum level.
If concrete is allowed to dry prematurely, this reaction ceases, and the designed strength and durability properties are seriously reduced. This is a major concern with surface wear properties on concrete slabs. Insufficiently cured slabs can lose up to 50% of their surface durability, causing cracking, dusting and surface erosion within a short time frame.
Premature water loss must be prevented if the full benefits of cement hydration on the properties of hardened concrete are to be achieved.
What are the benefits of properly curing concrete?
Proper concrete curing provides a greater level of quality assurance for your concrete, including:
- Increased wear resistance
- Reduction of surface erosion
- Increased frost resistance
- Improved life span
- Optimum strength development. Reduction in cracking. For more information, please see our guide on avoiding cracked concrete.
- Increased resistance to thermal contraction
How should concrete be cured to ensure maximum strength?
Curing horizontal surfaces such as roads, pavements, and domestic and industrial floor slabs requires a proprietary spray-applied membrane (at least 90% efficiency recommended). The application timing is critical; it must be applied when the concrete has lost its surface sheen.
Early curing of slabs is vital to minimise the risk of plastic shrinkage cracking, especially in climatic conditions combining high temperatures with strong drying winds.
Spray-applied curing compounds are rated by their manufacturers according to the efficiency with which they provide a barrier to evaporation. This is measured as a percentage of total water retained in the concrete. Research has shown the effects of delay in applying curing compounds to flat slabs and formed vertical surfaces, by measuring the water loss from concrete specimens at various ages. The results of the tests on a trowelled slab, where one of the best resin-based curing compounds was used, clearly show the rapidity with which water can be lost if curing is delayed.
Although membranes formed by curing compounds normally degenerate after a period of time, their use is not recommended on surfaces, which are subsequently to receive an applied finish, such as a screed, because of the likelihood of reduced bond.
Water is the most effective and cheapest curing medium, but it is seldom used because of the practical problems associated with supply, containment and ultimate disposal. Materials such as wet hessian or wet sand are sometimes used but they must be kept continually wet and not be allowed to dry out.
In extreme weather conditions, special precautions are necessary to minimise the evaporation of water from hydration and to control temperature differentials.
How to cure concrete in hot weather
In the worst condition of hot, dry weather with high winds, to cope with the fast drying conditions:
- Provide wind shields to cut down air movement and minimise loss of water.
- Provide effective shading to minimise surface temperature variations.
- Be extra careful when applying a waterproof membrane. (Polythene or similar sheeting, laid in close contact with the fresh concrete, is extremely effective if applied quickly.)
How to cure concrete in cold weather
In a cold but dry atmosphere, particularly during frosty conditions, immediate application of combined curing and protective measures is necessary. This is not only to minimise water loss but also to maintain an adequate temperature and, in the case of thick sections, to control the surface temperature so that the temperature gradient between the core and the surface does not become excessive.
One of the most effective ways for slabs is to apply waterproof glass fibre or mineral wool insulation blankets directly on the freshly placed concrete. Alternatively, straw or other lightweight insulation material laid over polythene sheeting provides adequate protection provided the insulation material is kept dry. Under no circumstances should water be used in periods of cold weather.
For more advice on laying concrete in cold weather, see our cold weather placement technical information page.
Curing concrete on vertical surfaces
In temperate conditions, such as the UK, for most of the year, formwork (a mold used to shape concrete) left in place for two or three days is sufficient to protect the immature concrete from water loss by evaporation.
Depending on wind speed, air temperature, and relative humidity, the formwork may need to be left in place significantly longer in dry, windy, or arid conditions.
Where formwork has to be removed for use the day after placement, additional curing may be required. Plastic sheeting or insulating panels can be effective, provided they are applied immediately after the formwork is struck and are held in close contact with the concrete surface at all times throughout the process.
The use of spray-applied curing compounds on formed vertical surfaces is difficult and is unlikely to be effective unless the selected grade of the compound is applied immediately after the formwork is removed. Furthermore, coverage must be total, leaving no areas unprotected by the membrane.
Spray-applied curing membranes can be useful where slip form construction is used and the concrete emerges from the form within four to six hours after placement. Alternatively, wet hessian draped beneath the formwork can be used.
Curing compounds are not recommended on surfaces, which will receive an applied finish such as rendering, paints and other coatings. Using cold water can be hazardous, especially in hot climates, because of the risk of thermal shock leading to cracking and surface defects. Although expensive to apply, artificial fog spray can be the most effective curing method, since creating a high-humidity environment surrounding concrete surfaces virtually stops evaporation and the premature loss of water. Like any other curing process, however, it depends on its effectiveness on rapid application following formwork removal.
How long should concrete cure before it can support weight?
Curing periods required by most specifications range from approximately three to seven days, irrespective of weather conditions. Rates of moisture loss from laboratory-controlled specimens made with ordinary Portland Cement show that the evaporation rate decreases rapidly after the first 24 hours and reaches an almost insignificant value within three or four days. The first 24 hours are, therefore, the most critical.
CP 11013 recommends different curing periods depending on the type of cement used. Four days is recommended for ordinary Portland Cement concrete, and two days when rapid-hardening Portland Cement is used. In adverse conditions, i.e. hot or windy weather, full curing should continue for at least seven days with all Portland Cements.
Slightly longer curing periods may be necessary when Heidelberg Materials Regen has been incorporated into the concrete.
If applying a sealer to your concrete, you should wait until it is fully cured to do so. In accordance with the manufacturer's recommendations
For more helpful advice on concrete, visit our technical information page.
