Factors affecting crack width | Serviceability

A beam will not fail if there is a good anchorage bond between steel and concrete. In such case, the beam will not be affected even bond get damaged along the entire span. If a valuable resistance is provided against the slipping of the beam along the entire span than it leads to smaller cracks with little wider. There are four main factors that affect the width of crack very much. These are;

  1. Shape of bar
  2. Stresses in reinforcement
  3. Concrete cover
  4. Steel/ reinforcement distribution

Shape Of Bar

Flexural beams with smooth rounded bars will show a comparatively small number of cracks during the service. Similarly, if the beam is comprised of well slip resistance than it will show a large number of very fine cracks i.e., a hairline cracks that are invisible to the casual eye. Now in common practice bars are always provided after surface deformations. ASTM has specified the spacing and minimum height criteria’s in A 615, A 706, and A 996 codes.

Stress in Reinforcement

Stress in the reinforcement is the second main factor that was studied by Gergely and Lutz. They have proved that the width of the crack is directly related to . Here;  is the stress in steel and n is a defined exponent whose value varies from about 1.0 to 1.4. Before computing stresses in steel one should know about elastic analysis of cracked section. ACI has also defined  criteria.   can also be taken as 60% of fy (yield strength).

Concrete Cover

Spacing and width of cracks depends on the distance    of concrete cover. Concrete cover is measured from the face of the concrete to the center of the inner bar diameter. As the concrete covers increases, crack width and crack spacing also increases.

Reinforcement distribution

As concrete is weak in tension, therefore cracks width depends on the steel distribution at the tension face of the beam. Cracking can be controlled by providing a larger number of smaller diameter bars instead of larger diameter and a small number of bars in the required area of steel. Spacing between reinforcing bar should be equal.


steel distribution

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