Cracks in Structures

5.1. Cracks:

It is fundamental that hardened reinforced concrete cracks in the tensile zone when subjected to externally impose structural loads. By means of appropriate design and detailing techniques, these cracks can be limited to acceptable levels in terms of structural integrity and aesthetics.

5.2. Causes of development:

The four primary causes of cracking that the designer can help to prevent are:

5.2.1. Flexural cracking:

Flexural cracking is a well known phenomenon. As a structure deflects to support the applied design loads, tension forces are induced in one face of the concrete which leads to the formation of cracks.

5.2.2. Early thermal contraction cracking:

Early thermal contraction cracking is caused due to temperature rise which results from heat of hydration of cementitious materials. As the interior concrete increases in temperature and expands, the surface concrete may be cooling and contracting. This causes tensile stresses that may result in thermal cracks at the surface if the temperature differential between the surface and the center is too great.

5.2.3. Long term drying shrinkage cracking:

Long term drying is caused as the concrete matures; the excess water used in the initial mix is lost through a variety of means, such as evaporation and the drying effects of wind. The concrete mix design has a large part in defining the degree to which the concrete is affected by long term drying shrinkage. Principle factors such as ambient humidity, surface area, and water/cement ratio affect the drying shrinkage.

5.2.4. Seasonal thermal contraction cracking

Seasonal thermal contraction/expansion is due to concrete volume changes with temperature. It is important to note that the temperature of the concrete is relatively insensitive to rapid changes in the surrounding air temperature. This is true where concrete is not subject to direct solar gain, is subject to internal or partially internal environments or the concrete is insulated.

5.3. Types of cracks on the basis of concrete hardening:

5.3.1. After hardening:

Physical (shrinkage aggregates, drying shrinkage, crazing)

Chemical (corrosion of reinforcement, alkali aggregate reactions, cement carbonation)

Thermal (freeze/thaw cycles, external seasonal temperature variations, early thermal contraction)

Structural (accidental overload, creep, design loads)

5.3.2. Before hardening:

Early frost damage

Plastic (plastic shrinkage, plastic settlement)

Constructional movements (form-work movement, sub-grade movement)

5.4. Types based on shape of crack:

Depending on the shape of cracks we have 4 types

5.4.1. Flexure cracks:

Flexure cracks are those that start near the mid span from the tension face and propagate perpendicular to axis of the member. If these cracks are wide, it leads to corrosion of the reinforcing bars and pre-stressed tendons. Also these cracks tend to widen under sustained or cyclic load.

5.4.2. Shear cracks:

Shear cracks are those that occur in the shear zone. The shear force is maximum near the supports, cracks due to shear occurs near the supports.

5.4.3. Flexure cum shear cracks:

Flexures cum shear cracks are those cracks formed at the bottom due to flexure and propagate due to both flexure and shear.

5.4.4. Horizontal cracks

Horizontal cracks occurs in the ends of pretension prestressed concrete girders. These are observed in I-Beams and in inverted tee and rectangular sections also. These cracks occur near the centroidal axis and close to the junction of the web and the lower flange in case of I- and inverted tee girders. If vertical stirrups reinforcement is not provided, the horizontal crack widens and spread along the girder from one end to the other.