When we observe a crack in the concrete slab or wall, our first guess is usually that the material is faulty – but this isn’t always the case. Concrete cracks are very common, and some are even unavoidable.
Here are 6 common concrete cracks that are worth taking note of.
1. Plastic shrinkage concrete cracks
When the concrete is still wet – before hardening – the water content of the concrete stays pretty high. As the concrete hardens and the water evaporates from the slab, it forms huge pores between the solid particles. The concrete becomes weaker and more prone to breaking as a result of these empty pores. This kind of cracking is known as “plastic shrinkage cracking” and occurs frequently
Plastic shrinkage cracks may appear anywhere in a slab or wall, but they usually always occur at re entrant corners (corners that point into the slab) or in the centre of a slab containing circular objects (pipes, plumbing fixtures, drains, and manholes). Because concrete cannot shrink around a corner, tension will cause it to crack at the corner’s point.
Plastic shrinkage cracks are usually extremely small and hardly noticeable. While plastic shrinkage cracks are virtually undetectable, it’s essential to note that they not only reside on the surface but also run across the length of the slab.
Concrete shrinkage may be exacerbated by an overly wet mix. While water is an important component of every concrete mix, there is such a thing as too much of it. The slab will shrink more if there is too much water in the mix than if the proper quantity of water is utilised. Plastic shrinkage cracks are also caused by hot temperatures.
To avoid shrinkage cracking, control joints can be included in the slab. As the concrete slab becomes smaller, the joints will open up.
2. Concrete cracks that happen from expansion
Concrete expands in the same way as a balloon does when it is heated. Concrete grows and pushes against everything that gets in its path (a brick wall or adjacent slab for example). When neither can flex, the increasing force is strong enough to cause concrete to crack.
Expansion joints serve as a separation (or isolation) point between two static surfaces. Expansion joints, which are often constructed of a compressible material such as asphalt, rubber, or timber, must serve as shock absorbers to alleviate the stress that concrete expansion places on it and avoid cracking.
3. Heaving concrete cracks
When the ground freezes, it can rise several inches before thawing and returning to its original position. The freezing and thawing cycle causes significant ground movement, which contributes to concrete cracking. The slab will fracture if it is not allowed to move with the earth.
On a slab, large tree roots may have the same effect. The spreading roots of a tree planted too near to a slab may raise and fracture the concrete. When laying a slab, keep this in mind.
4. Filling up gaps in concrete
Ground settling under a concrete slab, on the other hand, can create cracks.
Settling cracks are most common when a gap is formed in the earth underneath the concrete surface.
Consider what happens when a big tree is cut down nearby and the roots begin to decay, or when a utility company digs a trench for its lines, pipes, and other equipment and doesn’t compress the soil while filling it–these are examples of situations where settling cracks are likely to occur.
5. Overloading the slab causes concrete cracks
Concrete is an extremely robust construction material, yet it has limitations. Putting too much weight on top of a concrete slab may cause it to crack. When you hear that a concrete mix has a PSI of 2000, 3000, 4000, or 5000+, it refers to the pounds per square inch required to crush that concrete slab.
Overloading of the concrete slab itself is uncommon in residential concrete slabs. Excessive stress on the ground underneath the slab is more likely to occur.
Excessive weight on the slab after a strong rain or snowmelt, when the earth underneath is soft and moist, may push the concrete down and cause fractures. This kind of cracking is more likely to appear in driveways where big recreational vehicles or dumpsters are parked.
6. Concrete fractures as a result of early-drying
There are two kinds of cracks that are often caused by early drying.
Surface cracks that resemble spider webs or broken glass are known as crazing cracks. Crazing cracks appear when the top of a concrete slab loses moisture too fast. Crazing cracks, although unattractive, are not a structural issue.
Crusting cracks are most often seen during the concrete stamping process, which adds texture and design to concrete surfaces. The top of the concrete surface may get crusty on bright or windy days when the top of the slab dries out faster than the bottom.
When embedded, the stamp pushes the surface apart at the stamped joints, causing tiny fractures around the “stones” edges. While crusting cracks are less attractive, they are not a structural problem to be concerned about.
It’s not always easy to figure out what produced a specific crack. Proper site preparation, a good mix, and excellent concrete finishing techniques may all help to reduce the appearance of cracks and provide a more visually appealing concrete project.
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