Updated: Nov 11, 2020
Back to business!
What are plastic shrinkage and hydraulic shrinkage? How to minimize them? Let´s give an answer to these questions left from the previous post “Concrete shrinkage part 1”.
But before that it is necessary to lay a foundation: everybody knows that concrete is a conglomerate and that aggregates will never suffer any kind of shrinkage. Therefore, shrinkage is a phenomenon that happens to the cement paste. Why? Well, while cement is hydrating, changes its shape on a microscopic level, shifting from a spheroidal shape into a series of intricate small sticks which, linking together, create the bond that makes the cement past so hard to break under compression. But hydration implies the creation of connected voids as well. These voids are called “capillary porousness”, which is pivotal in concrete permeability (please read my post “Concrete permeability”), but this is another story. Anyway, this porousness is filled up with the water which didn´t participate in cement hydration. This water fills up voids and exert tension on the porousness, keeping the mass in shape. The destiny of this water is to evaporate, and this will lead to have empty spaces and no internal tension. The result is shrinkage and consequent cracking.
All the above happens immediately to a certain extent, so while the concrete is still plastic (which means up to the first hours after the casting), it starts to suffer from the first evaporation that mostly happens on the exposed surface. No need to underline that the more surface exposed, the more chance to have diffused cracking. In real life, slabs and floors are the most vulnerable structures to plastic shrinkage.
It is quite common to see slabs surfaces covered in thin cracks randomly disposed. They are not deep, seldom over 1 mm. and exert no structural influence but only aesthetic.
That is a serious problem, though, when it comes about concrete flooring (please read more in my post “Concrete flooring” and for more insight download the free booklet “Concrete Flooring Tips and Tricks” available for all the registered members) and can have an influence in durability of concrete and structure (read my post “Concrete durability”).
The way to protect the structures from plastic shrinkage is always the same Concrete curing (please read my post). A good allied is integration of synthetic fibers, read more about them in my post “Fiber-reinforced concrete”.
What we´ve mentioned at the inception is applicable also to this phenomenon. It happens with the same dynamic but later in time and affects not only the surface by even the core of the concrete. This time, since the shrinkage starts from inside, the final effect will be more devastating and deep cracks will appear on the surface. They can be dangerous both from a structural and durability standpoint.
It is possible to reduce the hydraulic shrinkage choosing specific mix-design as per “thwarted shrinkage” or “compensated shrinkage” (more about these in my post “Concrete shrinkage reduced”). Even the use of metallic fibers can help minimizing this phenomenon.
But always provide with concrete curing, never miss it! It is the first step to the best quality possible.