Page 439 - Forensic Structural Engineering Handbook

12.30 MATERIAL-SPECIFIC FORENSIC ANALYSES

building codes require that this possibility be checked, it is a condition sometimes missed
by inexperienced designers.

Corrosion
One of the most ubiquitous causes of poor performance of concrete structures is corrosion
of reinforcement. Chlorides as deicers were not commonly used prior to about the middle
of the 20th century. Once they were in common use, corrosion began to occur in parking
decks, bridge decks, balconies of buildings, and other places where deicers were used.
While complete replacement using the best of current technology for corrosion protection
is a solution that will minimize the possibility of future damage, this procedure also is usu-
ally the most expensive.
Less expensive maintenance programs that include periodic removal of damaged con-
crete and replacement with good concrete often are more economical alternatives. Use of
coatings such as silane has also been proved to slow down the potential for corrosion.

Post-tensioning
Post-tensioned concrete is a form of prestressed concrete. In post-tensioned concrete,
unbonded reinforcement is placed through openings in the hardened concrete. After the
concrete has gained adequate strength, the unbonded tendons are pulled to a predetermined
force and anchored to apply stress to the structure.
One common cause of failure in post-tensioned concrete is overload in the anchorage
zone. Overload can cause excessive bursting stresses to develop under the anchorage plate.
If the bursting stresses exceed the tensile strength of the concrete, failure can occur. For
some commonly used prestressing systems, reinforcement is needed to resist bursting
stresses when anchorages are placed close together.
Another cause of failure in post-tensioned concrete is corrosion of the tendons. In older
systems, corrosion inhibitors were sometimes not adequate to protect tendons under highly
corrosive conditions. Water that works its way into the tendon duct can cause corrosion that
eventually leads to fracture of the tendon. After a tendon fractures, it is often possible to
replace the broken strand with a smaller-diameter tendon.
For grouted post-tensioning systems, voids in the grout may also lead to corrosion dam-
age. Voids may be caused by air pockets or water pockets left in the post-tensioning duct
at the time grout is pumped through the opening. Corrosion caused by voids in ducts often
will lead to surface cracking before tendons fracture. Horizontal cracks following the ducts
often are the warning that repairs are needed.
Another potential problem results from shortening of beams or girders due to post-ten-
sioning, which induces horizontal forces into already constructed columns. If the columns
are very stiff, this creates high diagonal tension shear in them that is manifested by diagonal
cracking across the column.

Misplaced Reinforcing Steel
Strength of reinforced concrete depends heavily upon proper placing of reinforcement. In
flexural members, the most important dimension is the distance between the centroid of the
reinforcement and the extreme fiber of the compression zone of the concrete. Where shear
reinforcement is required, excessively wide spacing of stirrup reinforcement can lead to a
potential for shear failure.
Where earthquake is not a major consideration in the design of a structure, effective depths
of reinforcement greater than that called for in the plans may be acceptable. However, where