When you come across a crack in a concrete slab or wall, it's natural to assume something has gone wrong. However, this isn't always the case. In reality, cracks in concrete are very common and some are even unavoidable. In this article, we'll explain the six most common types of concrete cracks and why they occur.
Tight cracks are a common occurrence in concrete slabs. Generally speaking, if the crack is stable and doesn't leak water, it doesn't indicate a structural problem. Most of the time, these are shrinkage cracks that form when the concrete cures. It's understandable to be concerned about cracks in freshly poured concrete, but some of them are unavoidable due to the surface's structure. Let's take a closer look at why fresh concrete can crack.
As a rule of thumb, any concrete slab longer than nine feet will crack. Even if the concrete cures slowly, a large slab such as a patio or sidewalk can still crack due to shrinkage caused by changing temperatures and water consumption during the hydration process. Additionally, due to the natural movement of the ground below, concrete slabs will eventually crack. To anticipate and direct any future cracks, control joints are essential. These intentional weak spots are cut into the slab to about a quarter of its depth.
Most likely, there will be some cracking in these weaker parts. After nearly 20 years and thousands of projects, these three rules still apply: concrete cracks whether you want it or not. Engineers dedicate their lives to trying to get concrete slabs and walls to break where they want. Every set of specifications and architectural plans have pages dedicated to concrete cracking. The key point to understand is that water is an essential part of the concrete mix.
Any material that contains water will shrink as it dries and the water evaporates. A typical 4-inch slab will shrink at least ¼ inch per 100 m2. I usually see control joints being placed along the slab with cuts forming squares 10 by 10 or 15 by 15 feet wide. If the spacing between these control joints is greater, wide joints will form which become more difficult to fill and maintain over time. With traffic, wide joint edges break more easily creating safety problems. One of the first places where these stresses are relieved is in concrete. Here its strength is also its greatest weakness since it cannot flex and must crack instead.
These cracks come in a variety of widths and directions. I often see elevation variations where one side of the crack is higher than the other. Structural cracks can become a tripping hazard when the height variation is too high or the crack extends too far. If you're a decorative concrete contractor, remember that you did nothing wrong - cracks are a natural part of concrete curing and hardening. There are good repair options to prevent cracks from getting worse but there's no good method to make them go away. When it comes to how to handle cracks in concrete, preparing your customer in advance is best.
Cracks in ground slabs are relatively common and usually not structurally concerning unless they leak water which is usually caused by hydrostatic pressure or a high water table. Therefore, sealing the crack will redirect those problems elsewhere so it's recommended to fix the source beforehand. If a structural contractor drives heavy equipment loaded with wood on a 4-inch thick slab, they can break green (not fully cured) concrete. In addition, concrete poured in mid-summer will have more cracks than when it's colder.
Along with traditional curing methods, admixtures and curing compounds can help concrete cure faster and more evenly. It takes a full month for new concrete to fully settle so while shrinkage cracks can appear on the surface within hours of pouring, they must be repaired to prevent common problems such as moisture, insects and damage from leaking out. In general, cracks wider than a credit card and that go through the depth of the concrete are structural in nature and could be a sign of more serious problems. After curing has finished, you can use a sealing compound to improve appearance and reduce cracking.
You can cut control joints into the slab with a circular saw equipped with a concrete blade the day after pouring them. To help even out curing rates, cover the slab with an insulating plastic sheet or even straw.
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