Issues Of Cracks In Fresh Concrete And Importance Of Curing Concrete Structures

Do you know why such cracks appear after concrete casting?
Dr. R. Manikandan, Head - Customer Support Service (Technical) South region, ACC Limited

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Issues of such cracks formation on the surface of fresh concrete, more particularly on the horizontal surfaces, are essentially due to the high evaporation rates causing the surface to dry out fast. They are called “Plastic Shrinkage cracks, which are generally parallel to each other and are shallow in depth. Mostly plastic shrinkage cracks do not impair the strength of concrete members. However, they can act as an access point for movement of aggressive chemicals into concrete and steel and impact the durability of the reinforced concrete structures. Such cracks can be minimized by taking proper precautionary measures during placing and finishing operations in the concrete.

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Normally the cement in concrete reacts when it comes in contact with water and the process of setting and strength gaining in concrete takes place. During this process of chemical reaction, the cement liberates heat, which will promote the mix water into water vapour if the outside air temperature is also high. This liberation of heat is predominantly observed to be more in case of OPC cement than any blended cement. In addition to this, during the summer, the intensity of solar radiation and wind velocity, particularly in coastal areas, will further aggravate the evaporation of water in concrete. If such evaporation of water in concrete is more than the bleeding rate on the surface of concrete, then the chance of formation of plastic shrinkage cracks increases. Various research articles reveal that the bleeding rate for normal grade concrete is 1.0 kg/ m2 (ACI 305). If the evaporation rate in the concrete exceeds this bleeding rate, then the plastic shrinkage cracks will tend to form on the surface of concrete members. This issue of plastic shrinkage cracks is predominantly observed in slab member, which is due to a higher surface area to thickness. Traditionally for site mix concrete, mostly nominal mix proportion is being adopted. For building up to G+2 stories, for structural work, nominal mix of 1:1.5:3 or 1:2:3 are being adopted at the site. If we calculate the cement content per cubic metre for this nominal mix proportion, it is more than 400 kg/ m3, which is on a much higher side. Moreover, due to river sand non availability, mostly M. Sand or CRF are being extensively used for concrete purpose. M. Sand or CRF, which is available in the market, mostly have a fine particle of less than 150 microns, more than 35 to 45%. The presence of such fine particles further demands more water for a given workability and also reduces the bleeding rate in the concrete. Usage of M. Sand or CRF in nominal mix concrete on site, tends to reduce drastically the bleeding rate in concrete. In addition to these factors, during summer, the outside temperature increases significantly, i.e., up to 40°C and RH value in inland area are also less than 50%, which will further aggravate the rate of evaporation of water in the concrete. Also during night time, the wind velocity was observed to be more than 15 km/hr, which is also a factor for the increase in evaporation of moisture from the surface of the concrete (this is like drying clothes under the fan). This is the most critical issue for structures built near a coastal environment. Let us understand and arrive the rate of water evaporation using the below given nomogram. With the help of a weather app in smart phones, one can easily get the outside air temperature, wind velocity and Relative Humidity of the given site location. Normally the temperature of fresh concrete after 5 hrs of casting would be 36°C to 39°C which again depends on grade of concrete, cement content, cement type, water-cement ratio and ambient temperature.
Case 1: Suppose if the air temperature is 36°C, Relative Humidity at 50%, concrete temperature 38°C and wind velocity of 20 km/hr, then the rate of evaporation is 1.5 kg (m2/hr). This is a typical situation for inland areas. This combination of high solar radiation, wind velocity and lower RH, increases the rate of hydration of cement and rate of water evaporation on the concrete surface. Case 2: Suppose if the air temperature is 23°C, Relative Humidity at 95%, concrete temperature 35.5°C and wind velocity of 22 km/hr, then the rate of evaporation is 1.8 kg (m2/ hr). This is a typical situation during the winter in coastal areas. Wind velocity plays a major role on rate of evaporation. When evaporation rate exceeds 1kg(m2/hr), then the plastic shrinkage cracks starts appearing on the concrete surface if the proper curing methods are not adopted.
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Plastic shrinkage cracks in concrete are essentially due to these following reasons:
1. Increase in air temperature and lower Relative Humidity – Ideal temperature to avoid cracks in concrete is 27 ± 2°C
2. Wind velocity
3. High fines or dusty content in the fine aggregate (M.Sand) or more clay substances in river sand
4. High cement content by volume batch at site
5. Excessive vibration in concrete elements – leads to the accumulation of more cement paste on the top surface of concrete elements
6. Fineness of cement.
Curing
Curing is the process of protecting the freshly placed concrete, from the loss of moisture and maintains the concrete temperature within the range. Curing not only mitigate the plastic shrinkage cracks, but also greatly helps concrete to gain the strength by continued hydration of cement and also reduces the permeability in concrete. If you refer to this below strength development graph, suppose if the concrete is allowed to dry in the air, it will attain only 50% of strength of moist cured concrete. In fact the rate of strength development is almost stopped for air cured concrete after 7 days. However, if the moist curing period extended to more than 7 days, the rate of strength development is increased significantly. Hence, for better strength development and durability, curing of concrete elements should be of at least 14 days and it can be extended up to 28 days to attain maximum benefit if blended cement is used for making concrete.
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How to prevent these plastic shrinkage cracks?
The formation of Plastic Shrinkage cracks in concrete can be prevented by adopting the following proper curing methods.
1. Preventing the loss of moisture by sealing the surface -
curing methods include (i) applying membrane forming curing compounds, (ii) plastic sheet or tarpaulin covering (iii) leaving the formwork in place and (iv) saturated jute bags covering
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2. Maintaining mix water during early stage of concrete –
Curing methods include (i) water ponding or immersion, (ii) spraying and (iii) fogging. These methods of curing, in fact, cool the concrete.
Curing under different weather conditions:
For normal weather condition, variation of temperature is not a major problem. However, the key challenges will be the influence of wind speed. Suppose if the wind speed is more than 10 km/hr, it may be a cause of concern for rapid drying of concrete surface. This may be prevented by adopting appropriate methods such as installing wind barriers or sealing of concrete surface with curing compounds. For extreme climate such as hot weather conditions, the rate of evaporation of moisture is significant due to higher air temperature and rapid reaction of cement compounds. Keeping the surface of concrete under damp conditions continuously by water ponding or covering with wet gunny bags, will prevent such rapid loss of moisture. If air temperature is below 5°C, the rate of cement hydration will be slower and will impair the strength gain in concrete. This is a matter of concern for large surface area structural members like slab. However, for massive members like deep beams, the heat generated by cement hydration is sufficient for continuing the hydration process in concrete. For non massive members, the best method of curing would be steam curing or installing heating coil or insulating the members. The favorable temperature to maintain the hydration process active is in the range of 10 - 21ºC with the extended period of curing.
Curing period
The period of curing concrete members are essentially depends on grade of concrete, mix proportion, type of cement, ambient condition, member shape and curing methods. As mentioned above, the properties of concrete, particularly the strength of concrete, enhances over period of time under normal temperature. IS 456:2000 states in clause 13.5.1, the concrete made with OPC should be kept wet/damp for at least 7 days in case of normal weather and 10 days for hot weather condition. However, in case of blended cements (PPC and PSC) moist curing should be employed continuously at least for 14 days.