Why Does Concrete Crack?

Shrinkage is one of the main causes of cracking in concrete. As concrete hardens and dries, it shrinks due to the evaporation of excess mixing water. The wetter or thicker the concrete mix, the greater the shrinkage. The Concrete Structures Council (CSC) was formed in 1958 by merging three premixed companies.

Although concrete is a very strong building material, it has its limits. Placing excessive amounts of weight on a concrete slab can cause cracks. When you hear that a concrete mix has a strength of 2000, 3000, 4000, or more than 5000 PSI, it means the pounds per square inch it would take to crush that concrete slab. This category covers the performance of concrete whose shape can no longer be altered without damage.

It includes cracks caused by drying, shrinkage, as well as those resulting from temperature movements that take place in all materials exposed to the elements. Unless the structure in question allows movements of its limbs without developing excessive stresses, extensive cracks can often occur. Cracking cracks after hardening can result from excessive flotation that tends to attract water and cement to the surface, which then comprises weak concrete subject to high shrinkage stresses. More often, they are the result of poor curing. Other cracks that occur after hardening can be caused by the lack of proper reinforcement in the corners, insufficient depth of concrete over the bends in the reinforcement, the nesting of reinforcing steel in concrete, the lack of expansion and contraction joints.

Cracks in concrete are common and develop when stresses in concrete exceed its strength. Cracks are often caused by normal shrinkage of concrete as it hardens and dries. Cracks in concrete can range from being non-structural and unsightly, to being detrimental to the structural integrity and safety of a building. Plastic Shrinkage Cracks are probably the most common reason for early cracks in concrete. When concrete is still in its plastic state (before hardening), it is filled with water.

This water takes up space and makes the slab a certain size. As the slab loses moisture during curing, it becomes a little smaller. Because concrete is a very rigid material, this shrinkage creates stress in the concrete slab. As concrete shrinks, it crawls through its granular sub-base. This impediment to their free movement creates a tension that can literally separate the slab.

When the stress becomes too great for the now hardened concrete, the slab cracks to relieve stress. Especially in hot weather, shrinkage cracks can occur as early as a few hours after the slab has been poured and finished. Chemical reactions in concrete are due both to the materials used in the mixture and to those with which it may have been in contact. The cause of cracking is expansive reactions between aggregate and alkalis in cement paste. The chemical reaction occurs between active silica and alkalis, producing an alkali-silica gel as a by-product. Alkaline silica gel forms around the surface of aggregate, which increases its volume and puts pressure on surrounding concrete.

This increase in pressure can cause tensile stresses to increase beyond tensile strength of concrete. When this happens, concrete cracks to relieve pressure. Early heat shrinkage cracks are common in cantilever walls that are often used in reservoirs, dams, concrete tanks, retaining walls, bridge pillars, and basements. An example of an external constraint is when concrete is poured onto a previously hardened base or adjacent to similar elements without provision of an expansion joint. The key point to understand in relation to cracking is that water is a certain percentage of concrete mix. While early cracks may initially be considered as minor deficiency, it is recommended that building owners and building maintenance managers appoint qualified repair contractors to properly repair these cracks sooner rather than later to help extend life of concrete structures.

Active curing is always best practice that improves overall integrity of concrete and helps prevent other surface defects such as scale. If you use hand tool to join concrete, make sure edge is not worn and provide all necessary depth. However, it is important to keep close eye on these cracks as they can form weakness in concrete. Selection and use of suitable materials with good construction practices reduces occurrence of shrinkage and resulting cracks which improves service life and reduces life cycle cost of concrete structures. He said “David there are only three things you can guarantee to client when you work with concrete” Such objectives include: (a) restoring or increasing strength; (b) restoring or increasing rigidity; (c) improving functional performance; (d) providing watertightness; (e) improving appearance of surface; (f) improving durability; (g) preventing development of corrosive environment for reinforcement. Alkali-silica (ASR) reactions in concrete take place between highly alkaline cement paste and non-crystalline silicon dioxide which is present in some aggregates typically used in US Control joints are shrink joints because they open as concrete shrinks or gets smaller. We discussed 7 most common types of cracks in concrete why they occur their impact and how to fix them. When homeowner sees crack in his slab or wall especially if concrete is relatively new he automatically assumes something is wrong.

Jack Brown
Jack Brown

Freelance beer maven. Hipster-friendly web ninja. Freelance pop culture maven. Award-winning music junkie. Amateur food evangelist.