Specification:Types of cracks in concrete and typical causes

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Cracks can be broadly classified as either active or dormant. If they are active, they show some movement in direction, width or depth over a measured period of time. If the cracks are dormant, they remain unchanged. Some dormant cracks are of no danger, but if left unrepaired, cracks provide channels for moisture penetration, which can lead to future damage. For guidance on patching dormant cracks, see 03732-01-R "Repairing Cracks in Concrete by Injecting Epoxy Resin".

Classification
Cracks can be more specifically classified based on three factors: They may be longitudinal, transverse, vertical, diagonal or random. They may range in size from less than 1 mm (fine) to between 1 and 2 mm (medium) to over 2 mm (wide).
 * 1) direction
 * 2) width
 * 3) depth of the crack

The following are some crack classifications and a brief description:
 * Pattern Cracking: Fine openings in regular pattern usually due to inconsistent volume of concrete which is lower near the surface.
 * Checking: Shallow openings, closely and irregularly spaced.
 * Hairline Cracking: Small cracks, randomly placed, in exposed areas.
 * D-Cracking: Fine cracks at close intervals in a progressive random pattern.

Conditions
Cracks can occur in hardened or unhardened concrete and may be caused by some of the following conditions:
 * Shrinkage cracking: A crack that occurs only in unhardened concrete.  It is often seen as relatively straight lines running parallel with the span of the floor.
 * Plastic cracking: A type of shrinkage crack that also only occurs in unhardened concrete.  It is seen as diagonal lines in the top of a slab.  It is often caused by rapid drying of the surface due to delays in applying the curing membrane.
 * Settlement cracking: Caused by local restraining of unhardened concrete around reinforcement or some other obstruction.
 * Structural cracking: Usually a result of corrosion of the reinforcing steel or structural over stressing.
 * Tension cracking: Only occurs in reinforced concrete and is caused by elongation of the reinforcement in tension zones.  It is sometimes seen around columns in flat slabs and on beam soffits near the middle of a span.
 * Rust cracking: The most common and most serious cause of structural cracking caused by inadequate reinforcement cover.  It gradually develops at varying rates over time depending upon the degree of protection offered by the concrete cover.
 * Thermally-induced cracking: Results from stresses produced by temperature changes.