The strength of concrete is affected by a number of factors, one of which is the length of time for which it is kept moist. Curing of concrete entails providing adequate moisture, temperature, and time to allow the concrete to achieve the desired properties for its intended use. The process takes place immediately after concrete placing and finishing. Properly cured concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability, resistance to freezing and thawing, and abrasion and scaling resistance.
The length of adequate curing is dependent on a number of factors among them mixture proportion, specified strength, weather conditions and future exposure conditions.
Concrete curing methods fall into three broad catagories: those that minimise moisture loss from the concrete, for example by covering it with a relatively impermeable membrane, those that prevent moisture loss by continuously wetting the exposed surface of the concrete and those that keep the surface moist and, at the same time, raise the temperature of the concrete, thereby increasing the rate of strength gain.
Formwork. Leaving formwork in place is often an efficient and cost-effective method of curing concrete, particularly during its early stages. In very hot dry weather, it may be desirable to moisten timber formwork, to prevent it drying out during the curing period, thereby increasing the length of time for which it remains effective. It is desirable that any exposed surfaces of the concrete like the tops of beams be covered with plastic sheeting or kept moist by other means. It should be noted that, when vertical formwork is eased from a surface its effectiveness as a curing system is significantly reduced.
Plastic sheeting. Plastic sheets, or other similar material, form an effective barrier against water loss, provided they are kept securely in place and are protected from damage. Their effectiveness is very much reduced if they are not kept securely in place. The movement of forced draughts under the sheeting must be prevented. They should be placed over the exposed surfaces of the concrete as soon as it is possible to do so without marring the finish. On flat surfaces, such as pavements, they should extend beyond the edges of the slab for some distance, eg or at least twice the thickness of the slab, or be turned down over the edge of the slab and sealed.
For flat work, sheeting should be placed on the surface of the concrete and, as far as practical, all wrinkles smoothed out to minimize the mottling effects (hydration staining), due to uneven curing, which might otherwise occur. Flooding the surface of the slab under the sheet can be a useful way to prevent mottling. Strips of wood, or windrows of sand or earth, should be placed across all edges and joints in the sheeting to prevent wind from lifting it, and also to seal in moisture and minimise drying. For decorative finishes or where colour uniformity of the surface is required sheeting may need to be supported clear of the surface if hydration staining is of concern. This can be achieved with wooden battens or even scaffolding components, provided that a complete seal can be achieved and maintained. For vertical work, the member should be wrapped with sheeting and taped to limit moisture loss. As with flatwork, where colour of the finished surface is a consideration, the plastic sheeting should be kept clear of the surface to avoid hydration staining. Care must also be taken to prevent the sheeting being torn or otherwise damaged during use. A minimum thickness is required to ensure adequate strength in the sheet; ASTM C 171 Sheet Materials for Curing Concrete specifies 0.01 mm. Plastic sheeting may be clear or coloured. Care must be taken that the colour is appropriate for the ambient conditions. For example, white or lightly coloured sheets reflect the rays of the sun and, hence, help to keep concrete relatively cool during hot weather. Black plastic, on the other hand, absorbs heat to a marked extent and may cause unacceptably high concrete temperatures. Its use should be avoided in hot weather, although in cold weather its use may be beneficial in accelerating the rate at which the concrete gains strength. Clear plastic sheeting tends to be more neutral in its effect on temperature but tends to be less durable than the coloured sheets, thereby reducing its potential for re-use.
Membrane-forming curing compounds Curing compounds are liquids which are usually sprayed directly onto concrete surfaces and which then dry to form a relatively impermeable membrane that retards the loss of moisture from the concrete. Their properties and use are described in AS 3799 Liquid Membrane-forming Curing Compounds for Concrete. They are an efficient and cost-effective means of curing concrete and may be applied to freshly placed concrete or that which has been partially cured by some other means. However, they may affect the bond between concrete and subsequent surface treatments. Special care in the choice of a suitable compound needs to be exercised in such circumstances. The residue from some products may prevent the adhesion of flooring products and tiles onto the concrete surface.
It is extremely important to check the subsequent floor finish. Most curing compounds must be removed before the application of any applied floor finishes such as direct stick carpet and vinyl, epoxy or polyurethane coatings and ceramic tile adhesives. Finally, it should be noted when using curing compounds that are solvent-based adequate ventilation must always be provided in enclosed spaces and other necessary safety precautions taken. Manufacturer’s recommendations should always be followed.
Internal curing compounds These are incorporated into the concrete as an admixture hence known as internal curing compounds. They inhibit moisture loss and thereby improve long term strength and reduce drying shrinkage. Internal curing compounds are relatively new and care should be taken when utilised. They have been used in tunnel linings and underground mines to provide at least partial curing when traditional methods are difficult or even impossible to employ.
General Water curing is carried out by supplying water to the surface of concrete in a way that ensures that it is kept continuously moist. The water used for this purpose should not be more than about 5°C cooler than the concrete surface. Spraying warm concrete with cold water may give rise to ‘thermal shock’ that may cause or contribute to cracking. Alternate wetting and drying of the concrete must also be avoided as this causes volume changes that may also contribute to surface crazing and cracking.
Ponding Flat or near-flat surfaces such as floors, pavements, flat roofs and the like may be cured by ponding. A ‘dam’ or ‘dike’ is erected around the edge of the slab and water is then added to create a shallow ‘pond’. Care must be taken to ensure the pond does not empty due to evaporation or leaks. Ponding is a quick, inexpensive and effective form of curing when there is a ready supply of good ‘dam’ material, a supply of water, and the ‘pond’ does not interfere with subsequent building operations. It has the added advantage of helping to maintain a uniform temperature on the surface of the slab. There is thus less likelihood of early age thermal cracking in slabs that are cured by water ponding.
Sprinkling or fog curing Using a fine spray or fog of water can be an efficient method of supplying additional moisture for curing and, during hot weather, helps to reduce the temperature of the concrete. As with other methods of moist curing, it is important that the sprinklers keep the concrete permanently wet. However, the sprinklers do not have to be on permanently; they may be on an intermittent timer. Sprinklers require a major water supply, can be wasteful of water and may need a drainage system to handle run-off. The alternative is to have a ‘closed’ system where the water is collected and recycled. Sprinkler systems may be affected by windy conditions and supervision is required to see that all of the concrete is being kept moist and that no part of it is being subjected to alternated wetting and drying. This is not easy to achieve.
Wet coverings Fabrics such as hessian, or materials such as sand, can be used like a ‘mulch’ to maintain water on the surface of the concrete. On flat areas, fabrics may need to be weighed down. Also, it is important to see that the whole area is covered. Wet coverings should be placed as soon as the concrete has hardened sufficiently to prevent surface damage. They should not be allowed to dry out as they can act as a wick and effectively draw water out of the concrete. Fabrics may be particularly useful on vertical surfaces since they help distribute water evenly over the surface and even where not in contact with it, will reduce the rate of surface evaporation. Care should be taken however, that the surface of the concrete is not stained, perhaps by impurities in the water, or by the covering material. New fabrics can leach fabric stains, pre‑washing should be essential. Prior to placement of any fabric – pre moisten to avoid wicking of moisture from the concrete that can result in the fabric texture negatively absorbed into the concrete surface.
With several brands flocking the market, a buyer is at the risk of falling for substandard materials. Mr. Andrea Di Iorio, East Africa Sales Manager at CONTROLS GROUP advices clients and suppliers to invest in quality. He say the materials, the equipment used as well as the personnel involved should be of high quality. Mr. Andrea suggests that the industry should be well equipped with the latest technology as opposed to use of obsolete standards and techniques. CONTROLS is the global leader in the production of Testing Equipment for the construction industry with seven branches and more than 4.000 products in three main business areas (Concrete, Soil and Road) , the group has a consolidated presence in Africa for over three decades supplying the main contractors, universities, public and private laboratories.
Tom Schuler, President and CEO of Solidia Technologies, says it is advisable to work with customers on the adoption of new technology and incorporate their feedback and guidance into making the systems easy to adopt as well as making the end-products equal or superior to traditional concrete products. He says customers are pivotal in highlighting the challenges encountered in rising energy and inventory. Solidia Technologies is a cement and concrete technology company that has developed patented processes to ease production and improve performance of cement and concrete, while reducing the carbon footprint of products up to 70% and water consumption 60-80% during manufacturing. Starting with sustainable Solidia Cement™, Solidia Concrete™ is cured with CO₂ instead of water, all using the same raw materials, formulations and equipment with only slight modifications. Saving manufacturers time, energy and money, Solidia Concrete products cure within 24 hours, compared to the 28 days required for traditional concrete to reach full strength. Solidia Technologies has partnered with global industry leader LarfargeHolcim to work on research and development, as well as marketing for their cement and concrete systems.