In residential construction, the minimum thickness for hallways and patios is 4 inches, while garage slabs and driveways should be 5 to 6 inches thick if there are any heavy trucks. To reduce drying shrinkage and cracking, it is important to specify adequate compressive strength, minimize water content, space control joints properly, and cure concrete properly. Compressive strengths are usually determined by local building codes. For basement walls, a minimum of 2,500 psi of concrete is required, while floors range from 3,000 to 4,000 psi.
For flat work, the recommended drop is 3 to 5 inches, while for basement walls it is 5 to 7 inches. Once the concrete is ordered to a specific settlement, do not add more water to the site to speed up the pouring. Stress load cracking can occur when the weight of an object on a slab stresses concrete beyond its tensile strength. This type of cracking often happens when a heavy truck passes over a sidewalk designed only for pedestrian and light vehicle traffic. To avoid this issue, make sure the slab is built on a well-drained, evenly compacted subgrade and that it is thick enough to withstand the type of use it will receive.
Plastic shrinkage cracks can occur anywhere on a slab or wall, but they most often occur in reentrant corners (corners that point toward the slab) or with circular objects in the middle of a slab (pipes, plumbing fixtures, drains, and manholes). Since concrete cannot contract around a corner, stress will cause the concrete to crack from the point of that corner. These cracks are usually very narrow in width and barely visible. Although they are almost invisible, it is important to remember that plastic shrinkage cracks not only exist on the surface but extend throughout the thickness of the slab. Excessively wet mix is a contributing factor to shrinkage in concrete.
While water is an essential ingredient in every concrete mix, there is such a thing as too much water. When the mixture contains too much water, the tile will shrink more than if the correct amount of water were used. Hot weather is another big reason for plastic shrinkage cracks. The key point to understand in relation to cracking is that water is a certain percentage 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 square meters. Each set of specifications you look at and each set of architectural plans you see has pages dedicated to concrete cracking. During the setting and hardening steps, excess mixing water in the concrete evaporates, causing the concrete to dry from the surface inward. Even if it's your first concrete project, you can ensure a professional and long-lasting finish by avoiding some pitfalls and using the right repair products if necessary. Understand what your contractor is doing with respect to each of the items listed above and you will get a good concrete job.
Crusting cracking generally occurs during the concrete stamping process which is a way of adding texture or pattern to concrete surfaces. You can cut them into the concrete slab the day after pouring them with a circular saw equipped with a concrete blade. The American Concrete Institute addresses the issue of concrete cracking in its American Concrete Institute manual ACI 302. Can the concrete supplier provide a continuous supply of concrete without interruptions to avoid defects such as cold joints? Read here about methods to cure concrete and understand how your contractor will cure concrete. Other cracks that occur after hardening can be caused by lack of adequate reinforcement at corners, insufficient depth of concrete over bends in reinforcement, nesting of reinforcing steel in concrete, lack of expansion and contraction joints. Even though technology behind concrete slab construction has improved over time, one of the main challenges facing construction industry is how to build a “crack-free” slab. One of the most common mistakes made by DIYers who are new to concrete is adding too much water to dry mix for easier mixing which leads to weak concrete and high risk of cracking. However sometimes too much emphasis is placed on drying shrinkage of hardened concrete as criterion for susceptibility to cracking.
If you visit websites of American Concrete Institute or American Society of Concrete Contractors you will find lots of useful information on how to manage concrete cracks. Subsequently during night when concrete cools it contracts and since tensile strength of concrete is relatively low it can cause cracking. When properly installed, concrete is one of most durable and long-lasting products you can use in your home.
