Structural Concrete - Hassoun

244 Chapter 6 Deflection and Control of Cracking
due to the self-weight of the member and any other dead loads. There are three types of secondary
cracks: Shrinkage, flexural, and corrosion.
Shrinkage Cracks. Shrinkage cracks are important cracks, because they affect the pattern of
cracking that is produced by loads in flexural members. When they develop, they form a weak path
in the concrete. When load is applied, cracks start to appear at the weakest sections, such as along
the reinforcing bars. The number of cracks formed is limited by the amount of shrinkage in concrete
and the presence of restraints. Shrinkage cracks are difficult to control.
Secondary Flexural Cracks. Usually secondary flexural cracks are widely spaced, and one crack
does not influence the formation of others [8]. They are expected to occur under low loads, such
as dead loads. When a load is applied gradually on a simple beam, tensile stress develops at the
bottom fibers, and when it exceeds the flexural tensile stress of concrete, cracks start to develop.
They widen gradually and extend toward the neutral axis. It is difficult to predict the sections at
which secondary cracks start because concrete is not a homogeneous, isotropic material.
Salinger [9] and Billing [10] estimated the steel stress just before cracking to be from about
6000 to 7000 psi (42 to 49 MPa). An initial crack width of the order of 0.001 in. (0.025 mm) is
expected at the extreme concrete tensile fibers. Once cracks are formed, the tensile stress of concrete
at the cracked section decreases to zero, and the steel bars take all the tensile force. At this
moment, some slip occurs between the steel bars and the concrete due to the differential elongation
of concrete and steel and extends to a section where the concrete and steel strains are equal.
Figure 6.6 shows the typical stress distribution between cracks in a member under axial tension.
Corrosion Secondary Cracks. Corrosion secondary cracks form when moisture containing
deleterious agents such as sodium chloride, carbon dioxide, and dissolved oxygen penetrates
Figure 6.6
Typical stress distribution between cracks.