What is Curing of Concrete? A Study of the Various Curing Methods
Cement, sand, and water in the right proportion forms a composite material which is known as concrete. The chemical interaction which takes place between cement and water helps in binding the aggregate. Fresh concrete acts like plastic and then therefore be molded into any desired shape and can be compacted to form a dense mass.
It is important to place concrete in the desired position before it starts losing its plasticity. Final setting time of concrete is the time during which the concrete completely loses its plasticity and becomes hard.
Curing of Concrete
Curing plays an extremely important role when it comes to concrete strength development and durability. When water is added to the concrete mix which is the mixture of cement, sand and aggregate an exothermic reaction (hydration) takes place, this helps the concrete harden. Hardening of concrete is not a quick process and takes a long period of time which leads to more requirement of water for the hydration process.
So, the process of keeping the concrete moist until the hydration reaction is completed is called the curing of concrete. It can also be described as the process by which the concrete is kept moist so as to protect it from loss of moisture due to atmospheric temperature and hydration reaction. It is the process of controlling the rate and extent to which moisture is lost from concrete during cement hydration.
Why is curing of concrete important?
After water is added to the concrete mix, the hydration process starts, making the concrete dry out quickly as the exothermic reaction releases heat. For the completion of the hydration process, concrete must be kept moist so as to attain the maximum strength soon.
What is the procedure for curing of concrete?
By draining water over the concrete, you can cure the concrete. Water which is cooler than 50C is not suitable for curing of concrete. As heat is released by concrete during the hydration reaction using water which is less than 50C can lead to the concrete cracking and failing. By alternative drying and wetting the concrete surface changes in volume which again results in cracking.
What is the time taken for curing of concrete?
It takes 28 days to complete the curing of concrete process to attain maximum strength.
In the first 3-7 days concrete attains 50% of its design strength. In 14 days, the compressive strength reaches 75%. By 28 days the concrete design strength reaches 90%. With the increase in time the concrete strength also increases.
Minimum curing time required for cement concrete
The initial strength of concrete is what determines the ultimate strength of concrete. Curing must be done properly keeping in mind environmental conditions, types of structural members, atmospheric temperature. It is extremely important to maintain the right temperature in concrete; it should not be below 50C. Concrete must be kept moist for 28 days. Due to lack of time and modern techniques curing is achieved in 14-20 days. It is best to keep the concrete moist for at least 14 days.
According to IS 456-2000 concrete must not be cured for less than 7 days for ordinary Portland cement and for a mixture of concrete with mineral admixtures or blended cement it must be for at least 10 days. If the weather is hot and the temperature is high then the curing must not be done in less than 10 days for OPC and 14 days for concrete with blended cement or mineral admixtures.
What are the factors on which curing of concrete time depends?
- Specified strength of concrete
- Grades of concrete
- Atmospheric temperature
During summer the water required for the hydration process is more as 50% of the water is evaporated during the sunny days.
Size and shape of concrete members also affect the curing time.
Methods for concrete curing:
1. Ponding: This method is adopted for the making of floor slabs. The concrete surface is divided into small ponds and they are filled with water continuously for 14 days.
2. Wet coverings: This method is used for columns, footings and the bottom surface of slabs where the ponding technique is not possible. Coverings which are impermeable are required such as gunny bags or hessian. These membranes are then sprayed with water to keep the concrete moist.
3. Membrane curing of concrete: Places where atmospheric temperature is high ponding is not suitable as water gets evaporated due to the extreme heat. By membrane curing loss of water content can be avoided. It helps to seal off by the formation of an impermeable layer on the surface of the concrete which leads to resistance in evaporation. This procedure is performed by brushing or spraying the curing compound on the concrete surface.
Curing compounds which helps in achieving the membrane curing:
- Synthetic resin curing compound: It is a compound which forms as an impermeable membrane on the surface of concrete by which it can resist the evaporation of water from concrete.
- The synthetic resin compound can easily be removed by just spraying hot water on the surface of the concrete. Therefore, it is suitable for areas where such treatments can be applied to the concrete.
- Acrylic curing compound: It is a polymer-based compound which is obtained from the polymers of Acrylic acid. The reason why acrylic curing compound is favorable is because there is no need to remove it for plastering. Acrylic rather helps to achieve adhesion to plastering.
- Wax curing compound: The properties of wax curing compound is similar to synthetic resin. Wax must not be used on the surfaces to be painted or tiled as wax hampers the adhesion between surface and plastering or tiling.
- Chlorinated rubber curing compound: It forms a thick membrane on the concrete surface when it is laid. By using chlorinated rubber-based curing compound you can seal the concrete very well leaving no pores behind. What is to be kept in mind is that chlorinated rubber’s age is very less and thus it cannot stay for a long time.
- Steam curing of concrete: At a precast concrete plant this process is adopted where there is a mass production of concrete membrane. Heat moisture is present in steam and when it is sprayed on the concrete surface it helps to keep the moisture and also increase the temperature of the concrete which results in quickening the pace of hardening the concrete.
- Infrared radiation for curing of concrete: In cold climatic region this method is used. By the application of infrared radiation to the concrete the initial temperature is increased which results in increase of strength of the concrete. This method is even more effective than steam curing, as by raising the initial temperature the ultimate strength of concrete does not decrease. In hollow concrete members this technique is used where heater is placed in concrete members which emit the temperature of 900.
- Electric current for concrete curing: By passing alternative current through the concrete the curing is done in this process. By placing two plates, one on top and the other at the bottom of the concrete the plates act as an electrode through which alternative current is passed. Among these electrodes it is important to maintain a potential difference of 30V to 60V. By performing this method, the curing of concrete process can be completed within 3 days which usually takes 28 days to complete.