To achieve the required durability and strength, care must be given to curing. Fresh concrete must be protected whilst in its early life from the detrimental effects of hot sun, dry air, drying winds, and frost.
The main reasons for curing are to assist with strength development and improve the long-term durability of freshly placed concrete. In some cases, curing also reduces the effects of thermal contraction while enabling the production of satisfactory surface finishes.
Effective curing prevents the evaporation of water from a new concrete surface. Curing utilising applied insulation can also help control damaging internal temperature differentials in large masses of concrete.
Additionally, it will help maintain an adequate temperature in the concrete during cold and frosty weather to fully hydrate the cementitious material present. To ensure that the full benefits of curing are achieved, all parties involved in concrete production – designers, engineers, operatives – must clearly understand why and how a particular curing process is to be used.
In this guide, we will explain what curing of concrete is and gives practical recommendations for its application, including:
- Propriety Spray Applied Membrane
- Influence of Hydration on the Properties in Question
The development of strength and durability in any concrete mix, regardless of the type of cement, depends on maximum hydration of the cement since these properties rely on the production of a dense matrix of low permeability that’s resistant to the passage of water, carbon dioxide and oxygen. The chemical reaction between the cement and the water therefore must be allowed to continue to its maximum level.
If concrete is allowed to dry prematurely, the reaction ceases and the desired strength and durability properties are seriously reduced. This is a major concern with surface wear properties on concrete slabs. Slabs that are insufficiently cured can lose up to 50% of their durability, eventually causing cracking, dusting, and surface erosion.
Premature loss of water must be prevented if the full benefits of cement hydration on the properties of hardened concrete are to be achieved.
These benefits include:
• Improved Life Span
• Increased Frost Resistance
• Increased Resistance to Thermal Contraction
• Increased Wear Resistance
• Optimum Strength Development
• Reduction in Cracking
• Reduction of Surface Erosion
Horizontal Surfaces
The materials that are commonly used for curing horizontal surfaces, such as roads, pavements, domestic, and industrial floor slabs include:
- Proprietary Spray Applied Membrane
- Impervious Sheeting Laid in Close Contact with the Concrete Surface
Early curing of slabs is vital to minimise the risk of plastic shrinkage cracking, especially in climatic conditions that experience a combination of 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 the application of curing compounds to flat slabs and formed vertical surfaces through the process of measuring 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 shows 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. This includes use 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 its supply, containment, and ultimate disposal. Materials like wet hessian or wet sand are sometimes utilised but they must be kept continually wet and not be allowed to dry out.
In extreme weather conditions, special precautions are necessary to minimise evaporation of hydration water and to control temperature differentials.
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Special Precautions in Hot Weather
In the worst condition of hot dry weather with high winds, to cope with the fast drying conditions, take the following steps:
- Be Extra Careful with the Early Application of Waterproof Membrane
- Provide Effective Shading to Minimise Surface Temperature Variations
- Provide Wind Shields to Cut Down Air Movement and Minimise Loss of Water
Special Precautions in Cold Weather
In a cold but dry atmosphere – particularly during frosty conditions – immediate application of combined curing and protective measures is necessary. This not only minimises loss of water but is necessary to maintain an adequate temperature. In the case of thick sections, it helps 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 methods for slabs is to apply waterproof glass fibre or mineral wool insulation blankets directly on the freshly placed concrete. Alternatively, straw or other types of lightweight insulation material laid over polythene sheeting may provide adequate protection.
It’s important to ensure that all insulation materials are kept dry. Under no circumstances should water be used in periods of cold weather. For more information on cold weather placement, please view our cold weather placement guide.
Vertical Surfaces
Special Precautions in Cold Weather
Duration and Effectiveness of Curing
Curing periods required by most specifications range from approximately 3 to 7 days, irrespective of the weather conditions. Rates of moisture loss from laboratory specimens made with ordinary Portland Cement show that the rate of evaporation decreases rapidly after the first 24 hours.
It will reach an almost insignificant value within 3 or 4 days, so the first 24 hours are the most critical. CP 11013 recommends different curing periods, depending on the type of cement that’s being used.
Four days is recommended for ordinary Portland Cement concrete, and two days when rapid-hardening Portland Cement is used. In adverse conditions, including hot or windy weather, full curing should continue for a minimum of 7 days with all Portland Cements.
Please note that slightly longer periods of curing may be necessary when Hanson Regen has been incorporated in the concrete.