10.3 manifestation of shrinkage effects

A piece of concrete which is entirely free to expand and contract will do so without any risk of forming cracks. It is only when restraint is provided that stress may build up sufficiently to exceed the tensile capacity of the concrete. Restraint to volume changes may arise as:

(i) External End Restraint - For example restraint of slab ends by surrounding structure.

(ii) External Continuous Restraint - For example restraint acting along a contact surface between a member or previously placed concrete, e.g. placing a topping over a previous slab.

(iii) Internal Restraint - arising from non-uniform volume changes. For example placing of differing concrete adjacent to each other so that drying occurs at different rates

(iv) Internal Restraint - from embedded steel.

If any of these restraints is sufficient to suppress a potential strain of 100 to 200 x 10 -6 a crack can be expected to form. As further volume changes occur, such cracks will widen. It is acceptable practice to take steps which stop cracks opening in an uncontrolled manner, and to confine them to a tolerable width. The condition required to achieve this is to ensure that sufficient reinforcement is placed within the concrete to ensure that the force required to widen an existing crack is greater than the force required to form a new crack.