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Floor Slab Cracks

What causes random cracks in floor slabs. by Ron
Posted: 11 Dec 01

There are several conditions which promote "random" cracking in concrete slabs.  The most common are:
(1)Improper joint spacing
(2)Excess water in the mix
(3)Improper grade/thickness control
(4)Mix not proportioned to mitigate excessive shrinkage
(5)Sawcut control joints not done properly or improperly timed
(6)Poor finishing techniques
(7)Poor curing techniques
(8)Excessive load on the concrete

Drying shrinkage in concrete is inevitable.  The key to reducing so-called "random" cracks is to pay specific attention to each of the items noted.

Joints should be spaced as closely as practicable, preferably at about 30 times the concrete thickness (but never more than 20 feet (6.1 m)).  Further, the ratio of length-to-width of a jointed section should be no more than 1.2:1 .

Joints should be sawn into the concrete as soon as the sawing activity can proceed without causing aggregate raveling, but generally no more than 12 hours after placement.

The grade should be controlled to match the thickness tolerance of the concrete section.  For example, if the slab is to be 6 inches (150mm) thick, a typical tolerance would be -1/4 inch (-6.4 mm), +3/8 inch (9.5 mm).  This means that the subbase (subgrade)cannot vary more than 5/8-inch (15.9 mm) in 10 feet (3048 mm or 3.05 m).

The concrete mix should be proportioned with as large an aggregate as practicable and should have a water cement ratio that does not exceed 0.55 for common concrete, or 0.50 for concrete with abrasive activity or traffic.

Finishing should be done as soon as is practicable after placement and should proceed as a continuous operation.  Adding cement to dry the surface should not be done, and adding water to the surface to re-wet it should be discouraged, though it is sometimes necessary in localized conditions.

Curing should begin immediately after finishing and should include wet curing for the first few days (7 to 14 days preferred), with specific measures taken to prevent water loss in the first 30 days after placement.

Slabs should be designed for the intended loads, during and after initial curing/strength gain.  For ultimate loads, the use of the slab should be restricted until the design compressive or flexural strength is achieved.

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