Unintended restraint ? risk of unintended flexural cracks

Although precast elements are normally designed as simply supported to aid fast erection, there is normally a difference between the conceptual idea of the connection behaviour and the real conditions in the completed connection, a typical example is shown in Fig. 26. In design a simple mechanical model has been used assuming ?pinned? conditions at the supports. However, for several reasons, such as the need for structural interaction between walls and floors, and need for structural integrity and robustness, the connections are completed with small quantities of reinforced concrete sufficient to cause a so-called ?unintended? restraint. The elements can also be ?clamped? (serrer) in the connection between other elements. Restraint forces can also occur due to intrinsic effects, such as shrinkage, creep and thermal strains, where there would be no stress if the deformation of the precast element develops freely. Even if the unintended restraint is ignored in the design of the elements themselves, the consequences must be evaluated and appropriate measures should be taken to avoid possible problems.

Fig. 26. Differences between the mechanical model used in the design (top)
and the real condition in the completed connection (bottom).

In many cases the precast elements are strong enough to resist the stresses arising from unintended restraint. In other cases the tensile strength may be reached and cracks develop as a consequence. Often limited cracking can be accepted, but in certain cases cracking can be dangerous with regard to the load-bearing capacity of the elements. For example, when the connection zone is not provided with reinforcement in the upper part of the section, flexural cracks starting from the top can open up considerably and significantly reduce the shear capacity of the section. Especially dangerous are cracks in locations away from the support, since the bottom concrete cover might spall off when the shear force is resisted mainly by dowel action of the bottom reinforcement (Fig. 27a). Such crack locations are therefore considered as unfavourable in relation to crack positions at the edge of the support (Fig. 27b). In the latter case the shear transfer by dowel action of the bottom reinforcement can be balanced by the support pressure.

Fig. 27. Various locations of a flexural restraint crack:
a) unfavourable location where shear transfer by dowel action of the bottom reinforcement might spall off the bottom concrete cover,
b) preferred location where shear can be transferred by dowel action balanced by the support pressure

Restraint stresses can occur due to load-imposed deformations, for instance when the support rotation due to the service load cannot develop freely but a negative moment arises (Fig. 28). The bond between the joint face of the precast element and the joint fill can be significant. It has been observed that the bond in joint interfaces can be of the same magnitude as the tensile strength of the joint filling material, grout or concrete. Since the bond across a joint interface is normally a brittle and unreliable (non fiable) parameter, it cannot be utilized in design. Consequently, the joints are assumed to be cracked. However, in practise they can often remain uncracked and transfer considerable tensile stresses that were not accounted for in the design.

Fig. 28. A negative moment at the support may develop when the element is subjected to service load because the support rotation is fully or partly prevented

Date: 2016-06-12; view: 5