Cracks In Concrete
Plastic Shrinkage Cracking
Shrinkage resulting in surface cracking that takes place before the set of cement, mortar, grout or concrete i.e., when the concrete is still in plastic state is referred to as plastic shrinkage cracking and the cracks themselves as plastic shrinkage cracks.
These cracks typically come between 1 – 4 hours after placement and in today’s context, primarily depending upon the early strength of cement paste to resist the tensile stresses induced by shrinkage. These cracks are typically seen on concreted members with large surface areas such as floors, large raft foundations or slabs.
Mixes with Retarding super plasticisers are more prone to plastic shrinkage as the cement paste is very weak (due to set retardation) to resist the tensile stresses. Typically, when we design mix for high retention aided with poor placing finishing practices, we tend to see larger amount of shrinkage cracking. Wind, Humidity, temperature, placing, finishing and curing practices are the other contributors significantly affecting the formation of cracking.
The appearance of these cracks is haphazard. These cracks may be varying in length, are very shallow and very small in width initially, but may become deep as the time progresses. During windy and or hot conditions, the surface of concrete dries rapidly and results in loss of water from concrete through evaporation.
The volume vacated by water, gets occupied by air, which further dries the surrounding concrete. The drying and loss of water volume results in further cracking. The concrete which is weak in resisting these tensile forces breaks apart very easily causing the cracks.
As we now see more and more projects using concrete designed for 3 – 4 hour retention as is the present day need, the IST (Initial Setting Time) of concrete usually is quite high, which results in concrete being very weak up to 8 – 10 hours from the addition of water. Concretes with initial setting times of 8 – 10 hours due to use of retarding superplasticiser may be more prone to these types of cracks, in absence of proper site practices, than a concrete which sets in 3 – 5 hours. Gypsum and C3A ratio/content in cement can also affect the setting properties already altered by set-retarding admixtures.
These cracks can be plugged by doing a late finishing, or final finishing just before the concrete starts setting. Use of Bull float, power float has been proven to reduce these cracks drastically. Use of surface hardeners have also shown good results to arrest shrinkage cracking by increasing the paste strength.
Impact of Temperature wind and humidity on cracking
The image on the right hand side is from IS 7861: Part 1, “Code of practice for hot weather concreting” provides the chart for estimating the rate of evaporation.