Concrete is one of the hardest, strongest and most durable of all contemporary building materials. For that reason, many people make the mistake of assuming that concrete is naturally resistant to the stresses of expansion and contraction. In reality, concrete is actually quite vulnerable to such forces.
To avoid unwanted cracking, spalling and potholing caused by the expansive nature of concrete, it is common practice to cut special joints into the surface using heavy duty concrete saws. Here's a look at two of the most commonly utilized concrete joints.
One of the most common problems affecting freshly poured concrete is plastic shrinkage cracks. These cracks tend to form as the concrete slab dries out. The evaporating water causes a loss of volume within the concrete - leading to the formation of networks of fine cracks as the concrete contracts. In other words, it literally pulls apart from itself.
The good news about plastic shrinkage cracks is that they seldom affect the overall integrity of the concrete - at least not from a structural point of view. However, these cracks have a marked effect on the overall appearance of the slab. If they are serious enough, they can also provide areas in which water can pool up. This will lead to a hastened rate of degradation as time goes on.
Contraction joints are used to reduce the likelihood of plastic shrinkage cracks. The regularly spaced joints in a sidewalk are a prime example of contraction joints. The idea is that such joints allow the concrete to safely shrink as it dries. Contraction joints are cut while the concrete is still relatively wet, using special concrete saws that are equipped with diamond-tipped blades.
A contraction joint must be of an appropriate depth in order to do its job correctly. Generally speaking, the depth of the joint should be no less than a quarter of the thickness of the slab. Likewise, it is important that the joints be spaced at a proper distance from one another. This spacing is also dictated by the thickness of the slab, with the distance between joints being 24 - 30 times that of the concrete's depth.
As discussed above, contraction joints are used to prevent cracks from forming as the concrete cures. However, concrete is no less susceptible to forming cracks once it has dried. Once again, the culprit involved is water. In this case, the water causes cracks by penetrating into the highly porous material - not by evaporating out of the concrete.
When concrete absorbs latent moisture from the air, it undergoes a slight - but often catastrophic - change in volume. Essentially, that water causes the concrete to swell outward. If the concrete is directly abutting some another fixed structural element - whether it's a support beam, wall or even another concrete slab - the expansive force of that water will cause cracks to form
Expansion joints allow the concrete to increase in volume without damaging itself or other nearby structures. They essentially consist of a deep groove between the concrete and the other structural element. These groove are generally then filled in with a compressible material such as asphalt-saturated felt.
Expansion joints are usually topped off with a layer of water-tight sealant. This acts to prevent water from penetrating down into the crack. If this barrier wasn't there, the water would end up causing a whole host of other problems - more or less undermining the protective aspects of the joint entirely.
Nobody expects you to install your own concrete joints - that's a job that should be left to the experts. However, it is important to understand the basics of how and why concrete joints are implemented. This will help you appreciate what it takes to get the longest-possible life span out of your concrete.
As concrete experts, the professionals at Capital City Concrete Cutters LLC can help you will all your residential and commercial concrete cutting needs.
