Cracking of Concrete Members in Direct Tension

Description

Because concrete is relatively weak and brittle intension, cracking is expected when significant tensile stress is induced in a member. Mild reinforcement and/orprestressing steel can be used to provide the necessary tensile strength of a tension member. However, a number of factors must be considered in both design and con-struction to insure proper control of cracking that may occur.

A separate report by ACI Committee 224 (ACI 224R)covers control of cracking in concrete members in gen-eral, but contains only a brief reference to tensioncracking. This report deals specifically with cracking inmembers subjected to direct tension.Chapter 2 reviews the primary causes of direct tensioncracking, applied loads, and restraint of volume change .Chapter 3 discusses crack mechanisms in tension mem-bers and presents methods for predicting crack spacingand width. The effect of cracking on axial stiffness isdiscussed in Chapter 4.

As cracks develop, a progressivereduction in axial stiffness takes place. Methods forestimating the reduced stiffness in the post-crackingrange are presented for both one-dimensional membersand more complex systems. Chapter 5 reviews measuresthat should be taken in both design and construction tocontrol cracking in direct tension members.

Concrete members and structures that transmit loadsprimarily by direct tension rather than bending includebins and silos, tanks, shells, ties of arches, roof andbridge trusses, and braced frames and towers. Memberssuch as floor and roof slabs, walls, and tunnel linings mayalso be subjected to direct tension as a result of therestraint of volume change. In many instances, crackingmay be attributed to a combination of stresses due toapplied load and restraint of volume change. In the fol-lowing sections, the effects of applied loads and restraintof volume change are discussed in relation to the for-mation of direct tension cracks.