Page 418 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.9

Flexural damage to a concrete member is generally characterized by large deflections,
large cracks parallel to the direction of loading, and eventual crushing of the concrete, as
shown in Fig. 12.6.
Shear failures in reinforced concrete may be sudden and explosive and may occur with
little warning. Beams that display large cracks inclined at an angle of more than 45° to the
vertical may be near failure and should be carefully investigated for the potential of a shear
failure. Any evidence that a shear failure may occur should be handled with caution,
because such failures can be sudden and catastrophic. Figure 12.7 shows a photograph of a
beam with a large diagonal crack that resulted in a shear failure. When shear strength is
found to be low, external stirrups or straps can provide an effective repair.

FIGURE 12.7 Shear failure of reinforced concrete beam. (Courtesy of CTLGroup.)

Corrosion of reinforcement caused by the penetration of chlorides and/or carbonation
of concrete is one of the most prevalent causes of damage to reinforced-concrete structures.
If carbonation reaches the depth of the reinforcement or if chloride ion reaches a concen-
tration exceeding about 0.15 percent by weight of cement, corrosion can occur. In
uncracked concrete, penetration of chlorides can take many years. However, if chlorides
are present at a crack, they may reach the depth of the reinforcement virtually immediately
after chloride-laden water is applied to the concrete surface.
Remedial actions for corrosion include periodic maintenance that removes damaged
concrete, cleans rusty bars, and replaces damaged concrete with new materials. Drying the
concrete and using cathodic protection also have proved successful. As indicated in Ref. 8,
penetrating sealers have also been used to slow the rate at which chlorides enter concrete.
Forensic investigations to determine the cause of corrosion should include determination
of the depth of carbonation, the profile of chloride ion concentration, and whether or not
sealers have been applied. Where carbonation or high levels of chloride ion have reached the
depth of reinforcement, corrosion is expected to continue unless the concrete is dried below
the critical humidity, the concrete is replaced, or other corrosion protection is provided.

Prestressed Concrete
Prestressed concrete is a form of reinforced concrete. This mode of construction takes
advantage of very low-cost, but high-strength, wire and strand to make an extremely efficient
structural element. By applying a permanent pre-compression to concrete with high-strength