A: ”I don’t want to see any cracks on the surface!
B: ” Hey, com’on...are you kidding? No cracks? All types of concrete shrinks!”
Who is right? Well ”B” has a good point, isn’t it? Concrete shrinks. Yes indeed, and there are several forms of shrinkage. We have discussed about some of them in the posts ”Concrete shrinkage – part 1” and ”Concrete shrinkage – part 2”.
But ”A” is not totally out of this world in asking to see no cracks. In fact there are many ways to fight the natural tendency of concrete to shrink. That doesn’t mean that there will be no shrinkage at all, but at least that it is possible to have a ”crack free” concrete.
In this post we only want to highlight two ways to achieve that goal, namely: thwarted shrinkage and compensated shrinkage.
THWARTED SHRINKAGE
A little reminder to start with: Hydrated cement’s shape can be likened to a ”morning star mace” the medieval weapon, just picture it out with a way smaller core and a plenty of radial spikes all around it. These spikes are the hydrated parts of the cement, which links together with other hydrated cement particles creating a closely connected and bounded material (the cement paste). Among these ”spikes” there are voids filled up with the residual water, the water that remains after the hydration of the cement (see also my post ”Low permeability concrete”). Well, this water exerts surface tension pulling the ”spikes” to each other and that phenomena generates visible shrinkage.
Integrating the mix with SRA admixtures (Shrinkage Reduction Admixture) in proper dosage can help fight this phenomena because they reduce the water surface tension. To illustrate the effect here you are a ”funny picture”.
The two mules in the left picture represents the water surface tension which pulls the hydrated cement "spikes" one towards the other.
The green grass for the mules represent the SRA admixture which "calms down" the force of the water tension.
The effect turns out to be visible minimizing hydraulic shrinkage and reducing at the same time the curling effect on low thickness bodies.
So, no cracks? Well….not really. This is a good step towards perfection but it’s not the only one. We’ll come back to that in a while.
COMPENSATED SHRINKAGE
You know...I like to make examples...I try to explain things in an easy way. Yes, I know, maybe it is not all that ”scientifically accurate” but...helps to figure out, don’t you agree?
OK, so try to picture yourself attempting a pole vault. Let’s say that you are good enough to jump 4 meters and you can repeat this performance again and again. Now what do you think: if you have to attempt the same jump but running and landing onto a raised platform won’t you jump 4 meters anyway? But yet, if you measure the height of your jump from the ground level it looks like your jump is higher, isn’t it?
Now try to use this metaphor to figure out what happens to concrete when we ”compensate” its natural tendency to shrink by means of expansive admixture added to the concrete mix. Compensation here, means expansion of the concrete (a sort of yest for concrete) designed to equal the shrinkage in absolute value, with the final result of its nullification. The concrete will shrink exactly the same (4 meters jump) but, since in its initial stage it was ”inflated” so to say (the pole vault contest onto a raised platform) it will end to have (the jump landing) no cracks.
Another benefit of this technique is that while expanding, internal compression will occur since the concrete is confined in formworks and in presence of rebars (this phenomenon however is almost not visible when it comes about slabs with low rebars density) having therefore a sort of ”chemical prestress”.
NEVER UNDERESTIMATE THESE
So Mr. ”A” is satisfied? Maybe not completely. Why? Because you have to do something more to draw closer to perfection. Some details never to be underestimate:
Higher percentage of aggregates (and large diameters) always helps to minimize the shrinkage....you know, they don’t shrink...it’s just the cement paste which does that.
The lesser the water the better it will be, so use superplasticizers!
Fibers….they are super helpful!
Curing. The slower the hydration the greater the reduction in shrinkage.