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Historic Preservation - Technical Procedures


Types Of Cracks In Concrete And Typical Causes

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Concrete Repair

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Types Of Cracks In Concrete And Typical Causes


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".

Cracks can be more specifically classified based on three factors:
1) direction, 2) width, and 3) depth of the crack.  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).  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

-    Checking:  Shallow openings, closely and irregularly spaced.

    Hairline Cracking:  Small cracks, randomly placed, in exposed

-    D-Cracking:  Fine cracks at close intervals in a progressive
    random pattern.

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.

                         END OF SECTION

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