Curing means hardening of concrete mix under controlled conditions. Temperature and moisture content of concrete are two important aspects of curing and depending upon these two factors, concrete achieve its strength. Efficient curing ensures that concrete is durable, resistive to corrosion and is resistant to wind and heat. Period of curing is different for different types of work and temperature conditions. In wet and freezing conditions conventional methods do not work.
Underwater or Underground Curing of Concrete
Underground construction always poses serious threats to curing of concrete. For example, underground curing of concrete in tunnels is always affected by seeping water. The surface always remains wet and setting of concrete takes time, rendering it weak and unstable. Similarly working on a basement or foundation also has moisture issues. Working in watery conditions requires waterproofing, which increases costs and secondly drying the area also takes plenty of time. As far as underwater tunnel construction is concerned, the immersed tube tunnel method is an effective way to reduce costs and construct a stable tunnel that is least affected by water or moisture seepage.
Similarly, curing of concrete in wet conditions needs special care and treatment; you cannot just place it and forget about it. Despite the fact that curing is not possible without water, too much water will definitely render the concrete weak and result in cracks on the surface. In regions with high rainfall, concrete curing in time becomes very crucial. Similarly in wet and waterlogged conditions, curing needs to be done carefully. If the surface is too wet, the first thing to do is to dry the surface by heating. It may not be possible to dry the surface completely and that is where the free water to cement ratio becomes crucial. This ratio must be set according to the moisture content of the surface because if the water cement ratio exceeds the desired limits, it will result in capillary pores on the set concrete, resulting in poor strength and durability. Additives can also be used to speed up the process of concrete setting.
Concrete Curing in Freezing Conditions
Construction in freezing areas is a difficult task; snow does not allow you and your concrete to settle down. Hydration of concrete, or rather the setting of concrete, is greatly affected if the temperature is below zero degrees Celsius. The first thing before to do before placing the concrete is to clean the surface. Putting concrete on an icy surface is a waste of money and resources straightaway. Use hand picking tools and scratching tools so that the surface allows a certain amount of friction for the concrete to hold onto the surface. Secondly, after the concrete is placed, using heat preserving techniques will help. Increasing the heat of hydration of concrete results in better curing and setting. Insulating sheets, straw-plastic, insulating blankets, and windbreaks can be used because they help in increasing or at least maintaining a constant heat of hydration. However, if the temperature is below 20 Degree F, simply skip the idea of placing concrete because it will lead you nowhere as hydration stops completely at such temperatures.
Here is in-depth description of things to do for curing concrete in below freezing temperatures.
- Stick to the design code, maintain proper water-cement ratio, and ensure a safe gap between the reinforcement bars. The water to cement ratio should not be more than 0.40 in wet and freezing conditions.
- In extremely cold regions, a propane heater and a polyethylene enclosure can be used to keep the temperature above the freezing point.
- Wet and freezing conditions mean moisture, which means corrosion when related to steel and concrete. Use Portland Type III cement as it helps in easy setting without degrading the quality of concrete.
- Use additives like fly ash, silica fume, and furnace slag as they help in controlling formation of chloride ions, which is one of the main reasons for steel corrosion in concrete.
- Concrete under water curing for flatwork applications becomes easy with previous concrete. Previous Concrete is all coarse aggregates and it contains a negligible percentage of fine aggregates, especially sand. Additives are mixed into it that do not allow water to penetrate inside the concrete surface. Previous concrete is suitable for constructing pavement as it does not soak in water but allows gallons of water pass through it without damaging concrete pavement and strength.
- Use concrete sealants so that water does not seep inside the concrete. Sealants not only increase the life of concrete but also help in preventing concrete curing failure. In extremely cold regions, only a breathable concrete sealant must be used, as it will allow the evaporation of water and moisture, helping in fast setting of the concrete.
Concrete Curing in Hot Conditions
Even in hot conditions, curing requires special attention. The problems in such conditions are opposite of those in cold conditions. In very hot and humid regions, heat loss and continuous evaporation results in lack of moisture from the surface of concrete and thus leads to cracking. Such weather conditions require a high water-cement ratio, which means the strength of concrete is going to be affected. In such conditions retarding agents can be used. Using Type II cement is also helpful as it generates less heat and helps in preserving moisture content. An adequate supply of water, and keeping the concrete wet after placement, is important so that concrete does not crack and strength is not compromised at all.
Cold Weather Concreting, Nrmca.org(pdf)
Cold Weather Concrete, Flboa.com(pdf)
Hot Weather Concreting, Nrmca.org(pdf)