Cracking or shrinkage affect all types of concrete. Small or big cracks are normally visible almost in every concrete structure. But while cracking is so common, the reasons behind it are quite different. Just to summarize and simplify we can divide the causes of cracking in just four big categories: thermic shrinkage, autogenous shrinkage, plastic shrinkage, hydraulic shrinkage. In this post we will briefly discuss about the first two causes. Please read the post “Concrete shrinkage part 2” for the rest.
As the majority knows, the cement hydration is an exothermic process. If you ever noticed while dismantling the formworks after only 24 hours, maybe when the weather is not that warm, you´ve surely seen the concrete "smoking", and touching it you have found it warm. So, the thermal shock due to the lower temperature of the air in comparison with the concrete temperature can create cracks; that is because the concrete surface will rapidly cool while the concrete core is still warm. This thermal delta (difference in temperature) will cause cracks because the core is still expanding while the surface is shrinking.
It is useful to know that the highest concrete temperature is reached during the first days (one or two weeks) and it depend also on the thickness of the casted structure, the thicker the structure, the warmer the concrete and for longer time.
While some countermeasures can be taken with a specific mix-design, others can be set in place at the building site. If it is wintertime it is always a good idea to cast the concrete during the warmest part of the day and cover the structures with some insulating panels and/or fabric. These protections, together with the formwork, should remain in place as long as possible, even some weeks. In case of summertime casting it is better to choose the coolest part of the day and, while keeping the formwork in place some days is always advisable, the structure should be kept damp with sprayed water and/or damp fabric as a cover (more on that topic in my post “Concrete curing”).
Usually, I give a simple example to explain how important it is to keep the concrete covered and humid: after jogging your body temperature is quite high and you sweat, if you take off your jacket to find some relief while stopping for a little break, you risk getting a cold and/or to have cramps to your muscles because you tend to cool too quickly while the core of your muscles are still warm. Something similar happens to the concrete. Never “undress” it too early!
How to distinguish cracks for thermal shrinkage? They normally appear as “horizontal brackets” one after the other.
This specific type of shrinkage appears quite early while the concrete is still “plastic” and can develop in deep cracks which seldom runs throughout the entire section of the structure. Fortunately, this serious damage is not going to happen in the vast majority of commonly used concrete mixes. It is a possible side effect for concrete with a very low w/c ratio. It is named “autogenous” because it doesn´t depend on external factors, it develops because of a water migration into the inner porosity.
The manufacturer should pay a special care in designing, mixing and delivering these sensitive mixes but concrete cure is essential and have to be applied as soon as possible since the first hours are crucial.