What We Should Know about Cracking in Reinforced Concrete Structures

When cracking occurs in reinforced concrete structures, clients became skeptical about the competence of Engineers, they imagine that something very serious is happening. Is this the case? According to CIRIA C660, cracking in reinforced concrete is not a defect. This view was drawn from the stipulation of Clause 7.3.1 of EN 1992-1-1 which states that cracking is normal in reinforced concrete structures subject to bending, shear, torsion, and tension resulting from direct loading or restraint to imposed deformations.

Generally, the basis of reinforced concrete design is to control cracks in concrete. The reason is that concrete has low tensile strength. Thus, adequate reinforcement is required to be provided to ensure that crack is handled. In a situation where adequate reinforcement is not provided, cracks of sufficient magnitude may appear which may render the structure wholly or partially unserviceable. This then becomes a problem.

Note: two types of cracking are normally encountered in reinforced concrete design:

1. Cracking due to restraints which occur as a result of early-age thermal effects, autogenous shrinkage, and drying shrinkage in concrete
2. Cracking due to loading occurs as a result of flexure and axial tension in the concrete.

Cracking is significant if;

1. It makes the structure unappealing in aesthetics.
2. It makes the structure unserviceable such as cracks causing leaks in structures, allowing transfer of sound/radiation, and causing damage to finishes.
3. It affects the strength and durability of the structure.

Cracks can be prevented at the design stage as well as the construction stage of a structure. At the design stage, the limiting crack width (see Table below for limiting crack width) should be applied.

Application of limiting crack width requires providing minimum reinforcement (A_s,min) that satisfies the applicable limiting crack width. If this is done properly, cracking would not be expected. It chiefly involves serviceability limit checks of cracking.

At the construction stage, the application of certain principles to concrete mix and construction would help to control cracking.

At the Mix Stage

1. The use of an angular aggregate shape instead of a rounded shape would help to achieve better strain capacity by reduction of active cement content.
2. The use of low aggregate modulus would help to achieve a higher f_ctm/E_cm ratio and yield a higher strain capacity.
3. An aggregate type with low α_c is preferable.
4. The use of binder additions such as fly ash, and GGBS would help to reduce the effect of cement binder on cracking. However, this should be subject to limiting binder content.
5. The use of water reducers and superplasticizers would also help to reduce cement content and check durability requirements.

At the Construction Stage, the following measures could be adopted to reduce the risk of cracking

1. The use of low placing temperatures by cooling concrete constituents using chilled water, ice, or liquid nitrogen.
2. Control of ambient temperature to low by concreting in the night.
3. Insitu cooling using pipes and a surface spray of water for sections under 500 mm thick.
4. Using GRP or steel instead of plywood formworks for sections under 500 mm thick permits rapid heat loss and reduces T₁ which is the difference between the peak temperature of concrete during hydration and ambient temperature.
5. The use of plywood, insulated materials as formwork for members 500 mm and above of thickness helps to minimize thermal gradients and keep the upper surface insulated.
6. The use of thermal blankets for thick sections such as rafts.
7. Application of sequential construction or short infill bays instead of the alternate bay between lifts if full movement joints would not be used in the construction.
8. Reducing the period between successive lifts is also helpful.
9. Application of full-movement joints using dowels and sealants.
10. Application of prestressing at the base.
11. Reduction of the surface area of steel by using small diameter bars at close spacing instead of large diameter bars at wide spacing.

Prevention of cracking should be of the utmost interest to the structural engineer. However, Suppose cracking has occurred in reinforced concrete, maybe due to age or other shortcomings not captured at the design and construction stage. In that case, appropriate measures should be adopted by sealing the cracks or any other suitable methods to repair the cracks.