Control of Crack Widths

Shear Behaviour of Large, Lightly-Reinforced Lightly-Reinforced Concrete Beams and One-Way Slabs

CHAPTER 6:

Control of Crack Widths

CONTROL OF CRACK WIDTHS

“Surface cracking is inevitable but, with proper structural design and detailing, the cracks are very  narrow and barely perceptible.” -A.M. Neville

This chapter describes a series of experiments designed to investigate the effects of bar  diameter of skin reinforcing steel on crack widths in the webs of large flexural elements. This chapter also describes experiments to investigate the effects of skin reinforcement on the shear behaviour of flexural elements, and whether the shear strength is related to the vertical spacing between skin reinforcing bars. It is found that the diameter of skin reinforcing bars has a clear effect on crack widths, and that the ACI skin reinforcement   provisions should specify a minimum bar diameter. It is also found that the shear  strength of large members is not entirely related to the vertical spacing between skin reinforcement, and a modified method by which the SMCFT should calculate s x is recommended, based on the effective depth of the member.

6.1

General

The low tensile strength of concrete relative to its compressive strength means that most non-prestressed concrete in service is cracked to some degree. In zones of tension, the steel reinforcement is engaged primarily when a crack occurs, and design of reinforced concrete structures is carried out based on the fact that significant portions of the structure are cracked. cracked. However, the widths of these cracks must must be limited for  appearance, durability and structural integrity. It is important to limit limit crack width so as to ensure adequate shear behaviour. As crack  widths increase, their ability to transfer shear stresses by aggregate interlock decreases. Members in which there is insufficient reinforcement to control crack widths are at risk  of developing wide cracks that may result in a premature shear failure. This is of   particular concern for very thick members without stirrups, as cracks widths within the web can be considerably greater than those at the level level of the steel. It is an aspect that has typically not been addressed by previous studies on crack widths in reinforced 232