Types of Cracks in Concrete
Concrete expands and contracts as other construction materials do whenever there is change in temperature or moisture. Concrete also deflects due to imposed or self-load and support conditions. Cracks in concrete appear when these movements are restricted and no provisions are made to accommodate these movements. Types of cracks are as follows:
- Plastic shrinkage cracks
- Cracks due to improper jointing
- Cracks due to continuous external restraint
- Cracks due to lack of isolation joint
- Cracks due to freezing and thawing
- Craze cracks or crazing
- Settlement cracks
Use the following table in to see types of cracks, their common locations, primary and secondary causes and time of appearance.
Cracks that appear in early age of concrete are not very critical. These cracks don’t affect structural integrity, strength or service life of the concrete structure. Cracks that appear in later stages are critical and may affect structural integrity, strength or service life of the concrete structure. These cracks include cracks due to freeze and thaw cycles and due to settlement etcetera. These cracks lead to deterioration of the concrete structure.
Why concrete surfaces crack?
Cracks in concrete occurs mostly due to inappropriate design and construction practices. These include:
- Improper jointing practices and not including isolation and construction joints.
- Improper compaction of earth or subgrade preparation.
- Excessive addition of water in concrete or using high slump.
- Improper finishing of concrete.
- Inadequate, improper or no curing of concrete.
How to prevent or minimize cracking?
It is not possible to prevent cracks completely in concrete. However cracks can be minimized by adapting good concrete practices. Some of guidelines to prevent or minimize cracking are as follows:
Subgrade and Formwork
In preparation of subgrade, remove all topsoil and soft spots as deep as possible. The soil below the slab on ground (SOG) should be well compacted soil or granular fill. The soil should be well compacted by rolling, vibrating or tamping usually in layers of not more than 6 inches I thickness. The slab on grade should be properly sloped for drainage. In winter, remove ice and snow prior to placing concrete.
All formwork for concrete must be built and braced so that it can tolerate the high pressure of the concrete without movement. Vapor retarders which are used directly under a concrete slab increase bleeding and significantly increase the cracking potential, especially with concrete high in slump. If a vapor retarder is used, it should be covered with 3 to 4 inches of a compactible granular fill to reduce bleeding of concrete. In critical drying conditions, it is good practice to dampen up the formwork and reinforcement to avoid cracks.
To avoid cracks in concrete, avoid using high slump concrete. Concrete of moderate slump (less than 5 inches) results in less cracking. High slump concrete can only be used if it is controlled by concrete admixtures or designed such that segregation and bleeding of concrete is controlled. Concrete admixtures such as water reducers or super plasticizers can be used to increase slump while reducing addition of water content. Air entertained concrete can also be produced to increase slump especially when concrete is to be subjected to freeze and thaw cycles.
Finishing Concrete Surface
Finishing operations are very important in concrete. Finishing operations must not started until bleeding of concrete is completed or there is water present on concrete surface. Overworking concrete surface finish also results in cracking of concrete surface. Concrete should be bull floated or broom finished if adhesion is required with other surface. In hot weather, arrangements should be made to avoid rapid drying of concrete. These arrangements can include covering the concrete with polythene sheet or burlap. Fog sprays or wind breaks may also be used for the same purpose. Rapid drying results in plastic shrinkage cracks on concrete surface.
Curing of Concrete
Curing of concrete is very important in controlling concrete cracks. Curing of concrete must be started as soon as possible. There are many ways to cure the concrete. Concrete can be cured by spraying the surface with curing compound or covering it with moist burlap ho hessian cloth. Curing of concrete should be done for at least 7 days or as directed by the engineer.
Joints in Concrete
Changes in concrete volume due to temperature or moisture should be controlled by providing contraction joints in concrete slabs. Contraction joint can be made by providing a grove of about ¼ to 1/3 of the thickness of slab. Spacing between contraction joints should be about 24 to 36 times the thickness of concrete slab with maximum spacing of 15 feet.
Isolation joints are another type of joints which are used when movement of slab is restricted by some other element such as columns, walls or footings. These joints are constructed by making full depth groove across thickness of the slab and then adding some bond preventing material to avoid bond of concrete slab with other elements.
Cover over Reinforcement
Cracks in concrete can also form due to expansion caused by rusting of steel embedded in concrete. In reinforced concrete, corrosion cause formation of rust around the steel bars which is as big as two to four times the original volume of steel. To avoid this, provide concrete cover of at least 2 inches to keep moisture and salt from contacting the steel.
Cracks in concrete can be reduced by ensuring the following:
- Concrete structure should be designed properly for all type of anticipated loads
- Provide contraction and isolation joints properly
- Proper sub grade should be made for slab on grade
- Concrete should be placed and finished properly
- Concrete should be cured properly
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