It is by the forces of nature that concrete, no matter how experienced the mason is, can crack. Even with the best of care, the most skillful laborer, and the most leveled ground, concrete can and will crack. When concrete is prevented from moving and is exposed to a force greater that its ability to resist (forces of nature), cracking will occur. There are three ways that concrete can be prevented from moving. One way is when the concrete slab/structure is physically affixed to a given point or position such as a walkway slab attached to the house’s foundation. A second way that concrete is prevented from moving is when it is restrained by friction between the concrete and the sub-base over which it is placed, meaning it is not allowed to expand or contract. And the last way is the result when a change happens in the applied load or volume of the concrete, such as adding a structure on top or installing a hot tub without proper footing. Now that you understand the basics of concrete, we can consider the how. As the concrete slab begins to mature, it transitions from a plastic state to a hardened mass which allows hydration and drying to occur at the same time. This drying process continues for months to years. While the concrete is drying, it is losing moisture, which means it is also losing volume. The slab reacts to this lose of volume and the resistance to movement by developing cracks. When the concrete is resistant to movement friction builds between the concrete and the sub-base, without prevention the slab will crack. Normally, a crack will vary from about 8 feet to 20 feet, but when an expansion/control joint is put into place, this can prevent or reduce the length of cracking. The joint is an intentional weakness created in the slab allowing the slab to crack in specified areas. The placement and timing of placement of these joints are critical in preventing or reducing the cracking. If the cutting of joints is done too late in the drying process, the concrete will be predisposed to crack in places other than the specified cuts, which can occur within a few hours after setting. The joints should be cut as soon as the concrete has set enough to allow a clean cut without crumbling. Even though a mason designs the slab to prevent random cracks, the bottom line is that due to the nature of concrete, while going through its drying process, will still be prone to cracking.
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