Curing 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.

Vertical Surfaces

In temperate conditions – such as the UK – for most of the year, formwork left in place for two or three days is sufficient to protect the immature concrete from loss of water by evaporation. In dry, windy or arid conditions, the formwork may need to be left in place significantly longer, depending on wind speed, air temperature, and relative humidity.

Where formwork has to be removed for use the day after placing, extra curing may be needed. The use of plastic sheeting or insulating panels can be effective, as long as they are applied immediately after the formwork is struck and are held in close contact with the concrete surface at all times.

The use of spray applied curing compounds on formed vertical surfaces is difficult and is unlikely to be effective unless the selected grade of 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 where the concrete emerges from the form within 4 to 6 hours after placing. Alternatively, wet hessian draped beneath the formwork can be used instead.

Curing compounds are not recommended on surfaces that will subsequently receive an applied finish, such as rendering, paints and other coatings. The use of cold water can be hazardous, especially in hot climates because of the risk of thermal shock leading to cracking and defects.
Although it’s somewhat expensive to apply, artificial fog spray can be a highly effective curing method, since the creation of a high humidity environment surrounding concrete surfaces virtually stops evaporation and the premature loss of water. Like any other curing process, though, its effectiveness depends on rapid application following formwork removal.

Special Precautions in Cold Weather

When formed concrete is placed during cold weather, formwork should be left in place for at least 7 days and insulation should be provided over exposed concrete surfaces. Additionally, the use of insulating materials laid over formwork helps limit the effects of changes in ambient temperature.

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.