Here at the Concrete Raising Company, we love cracking concrete. We love it in the sense that cracking and sinking concrete is our livelihood. Yet we know how frustrating it can be to homeowners. We have found that some homeowners are less stressed about their concrete problems when they understand the science behind them.
In previous posts we have talked about what causes sinking concrete. This post will deal with cracking. Though the two are often related in real-world settings, they do not have to be. A perfectly level piece of concrete can still crack. Likewise, a concrete slab can sink without cracking.
Compressive Strength vs. Tensile Strength
Concrete is a composite material consisting of fine aggregates, coarse aggregates, binder, and water. A concrete manufacturer might start with sand as the fine aggregate. Gravel is added as the coarse aggregate while Portland cement is the binder. Everything is mixed together with water to create the finished product.
As a composite, concrete is fairly strong. But there is a rub. There are two types of strength we are concerned about when it comes to things like concrete slabs, staircases, and larger structures. The first is compressive strength. The second is tensile strength. Concrete has a very high compressive strength but a comparably low tensile strength.
Compressive strength is the ability to withstand compression, or dead weight if you will. Tensile strength is the ability to withstand the force of being pulled in one direction or another. In the industry, we call this force ‘load’. Tensile strength is measured by putting a material under load and seeing how much force can be applied before it breaks.
More often than not, concrete cracks because its tensile strength is exceeded. Forces pull it in one direction or another until cracks appear. Sometimes, a force that seems to be pushing on the concrete (rather than pulling) results in failure.
Thermal Expansion
Concrete exhibits a comparatively low coefficient of thermal expansion. This is to say that it does not expand or contract a great deal when exposed to environmental conditions. However, expansion and contraction still do occur to some degree. If accommodations are not made to counter it, repeated cycles of expansion and contraction will lead to cracks.
The thermal expansion issue explains why concrete driveways and patio slabs in northern environments crack after just a year or two. The repeated freeze-thaw cycles of winter weather put too much stress on concrete slabs.
Concrete Shrinkage
Finally, all concrete shrinks as it matures. The most shrinkage occurs when concrete is still new and fresh. As it matures, the rate of shrinkage declines. All this is to say that concrete never stops shrinking altogether. Over time, shrinkage can lead to cracking.
The extent to which shrinkage cracking takes place is influenced by a number of factors including the tensile strength of a concrete recipe, the amount of water the concrete retains, the amount of tension it is subject to, and the amount of creep taking place within the slab or structure.
So many influences dictate that is nearly impossible to prevent cracking fully. Even the best concrete jobs are subject to cracking over time. So if you have cracking concrete on your property, don’t sweat it. Cracks are to be expected. Nearly all of them can be fixed without harming the integrity of the structure.
If you are dealing with cracked concrete that is also sinking, feel welcome to contact us. We are Salt Lake City’s specialists in slab repair. We can raise that concrete to its original level and fill in the cracks while we are at it.