Cracks in Fresh Concrete
There are two main reasons why concrete cracks in its early age. Mechanical movement due to either premature stripping or premature loading (like trucks on a suspended slab before the concrete has sufficiently gained strength). Shrinkage is the other cause. The following covers the issue of shrinkage.
All ready mixed concrete (and the stuff you make by the shovel-full) shrinks. Just as common mud shrinks when it dries, so does concrete. But there are more things to be said.
The shrinkage in the case of concrete comes from several sources, but it is all related to water in the mix.
Some of the water added to the mix evaporates or is absorbed by the underlying material early on after the pour. This causes a net reduction in the volume of the concrete. Windy or hot days can cause excessive evaporation on the day of the pour and the next day, leading to an array of short, parallel cracks known as “early age” cracks. One way to reduce such effects is to apply what is quaintly known as “aliphatic alcohol” to the surface of the concrete, which provides a short term block to the evaporation. (“Aliphatic” simply refers to straight chain molecules. Typically C16 length alcohols such as the fatty/waxy cetyl alchol are used dissolved in a shorter chain-length hydrocarbon.)
Ideally, a curing compound is applied. Such materials are typically waxy substances, or paints such as chlorinated rubber, or acrylic materials.
Some of that water chemically combines with the cementitious materials in the concrete. in doing so, the volume of the cement paste decreases.
Some slabs are designed to be unreinforced. Those rely on low shrinkage concrete, coupled with jointing at well defined spacing.
The addition of water to a truck full of concrete may be necessary, to make it workable, but that water has to go somewhere. If it ends up evaporating, that causes excessive drying shrinkage.
Reinforcement detailing is important in reinforced concrete elements. The reinforcement is usually expected to distribute the cracks so that they are not noticeable, rather than to prevent cracking all together.
Saw cut joints are designed to reduce the thickness of a slab to around 60 to 70% of its original thickness at the location of the intended joint. Where that occurs, the slab will crack at the base of the saw cut, and will not appear objectionable.
Saw cutting of joints where specified (or where they should have been) needs to be done as soon as possible after the concrete stiffens sufficiently to walk on. Some think that early cutting promotes tearing out of the coarse aggregate particles. Whilst that is possible, a good contractor will be able to overcome that. In any case, the cutting should happen typically a matter of hours after the pour. Not days.
The practice of having reinforcement running across sawn joints is potentially problematical. Steel is placed into concrete to give tensile strength. The sawn joint is done to reduce the strength along the line of the crack. The steel may overcome the benefit of the saw cut.
The type of coarse aggregate used can have a very significant effect on the drying shrinkage of the concrete.
The mix design (ie the engineered ratio of cement : sand : coarse aggregate coupled with water/cement ratio) can have a huge effect on drying shrinkage. The goal is to have smaller pieces of aggregate, and then sand filling the voids between the respective sized particles. At the lower end, we use cement paste to fill in the smallest gaps. Given that ti is largely the water/cement paste that shrinks, it makes sense to minimize the amount of cement paste. And the water. That might also mean having the concrete a lot less workable than the soup that the concretors would ideally like, but such is life.
The drying shrinkage of concrete is tested in the laboratory by making bars some 75mm square by 280mm long, with a stainless steel stud at both ends. They are left in water for 7 days, and then taken out and measured to the nearest micron. They are then placed on a rack in a shrinkage room at 23 C and 50% RH and a certain wind speed. Measurements are then taken after 7, 14, 21, 28 and 56 days of drying (ie add 7 days to get actual age). The results are calculated to give results in microstrain. Typical house slab concrete might give a drying shrinkage of 800 to 1000 microstrain. Really good concrete might give a value of 300 or 350 microstrain. We currently have over 200 sets of bars in our shrinkage room.