Control of cracking
Reinforced concrete will generally crack. It is only when cracks form that the reinforcement starts to carry any appreciable load; prior to that the concrete is effectively behaving as unreinforced. When considering the effects of movement on a structure, there are two approaches that can be adopted. The first is to avoid cracking, by limiting the stresses induced in the concrete by restricting the amount of movement and/or the level of restraint. The second is to assume that cracking will occur and to control crack widths by providing sufficient, correctly detailed reinforcement. The former approach will generally not be practical and hence sufficient reinforcement must be provided in critical regions of the structure.
Any number of unforeseen events can conspire to cause sufficient contraction or sufficient restraint (or both) for the tensile stress to exceed the tensile strength so that the concrete will crack. If the load capacity reinforcement is stronger than the force which causes the crack, any cracks will be controlled so that they will usually not be unsightly nor leak excessively. Also, the more reinforcement that is provided the finer the cracks will be. The provision of minimum percentages of reinforcement to control early age cracking is generally well understood and guidance is given in documents such as CIRIA guide C766, Early-age thermal crack control in concrete (2018) which supersedes CIRIA Report C660 (2007) and CIRIA R91 (1992). Also Eurocode 2 Part 3, Liquid retaining and containment structures which supersedes BS 8007 Code of practice for design of concrete structures for retaining aqueous liquids. However, this deals with the problem in the immature concrete (say about 3 days old); a higher minimum percentage may be required for the mature concrete (at say 2 years), for example to control cracking due to long-term shrinkage.
As explained in Concrete Society Technical Report 67, Movement, restraint and cracking in concrete structures, the minimum percentage will depend on the actual concrete strength and not the specified value, leading to a higher percentage. However, long-term loading tends to lower the tensile strength of concrete. The net result is:
minimum percentage = fctm/fyk
where fctm is the mean tensile strength of the concrete (from Table 3.1 of Eurocode 2) and fyk is the characteristic strength of the reinforcement. This will be around 1.5 times the percentage required in the immature case.