Crack-Free, High-Performance Concrete Structures

Contrary to some recent research, virtually crack-free, high-performance concrete structures are possible. This article describes how Montreal is enforcing specifications on detailing, placing and curing of high-performance concrete to ensure that structures are virtually crack-free. This goal involves coordination with all parties involved in the process. The designer must take into account the shape, size and restraint conditions of the particular element and allow volume changes to occur without creating tensile stresses that exceed the tensile strength of the concrete. Materials must be selected and proportioned to minimize volume change while producing a concrete that is workable during placing, and with the strength and durability needed to perform well under service conditions. The constructor must ensure that concrete is placed at a proper consistency and thoroughly consolidated to its maximum density to develop its maximum tensile strength. Uncontrolled autogenous shrinkage is the cause of much of the cracking of high-performance concrete. The curing subcontractor therefore must maintain a permanently humid surface and supply enough external water to fill the pores created by volumetric contraction during hydration to ensure that water never escapes from the capillaries near the surface. A highway reconstruction project in downtown Montreal is used to illustrate the practical application of these concepts.