Page 415 - Forensic Structural Engineering Handbook

12.6 MATERIAL-SPECIFIC FORENSIC ANALYSES

investigation should include analysis to determine whether lubricants or other contami-
nants are present on the surface of the strand.
Other indicators of possible bond failure include splitting of stems and other narrow
parts of precast members, excessive deflection at low loads, and open flexural cracks at low
loads.

Corrosion Properties

Most deformed bars are hot-rolled and conform to ASTM designation A615 or A706 (Refs. 4
and 5, respectively). Neither of these steels resists corrosion when exposed to the open air.
However, the alkaline conditions provided by concrete surrounding the bars give excellent
corrosion protection for reinforcement.
Corrosion protection can be destroyed in the presence of chloride ions or if natural car-
bonation that occurs at the surface of concrete works its way to the level of the reinforcing
bar. Both conditions cause the loss of an alkaline environment and thereby destroy corro-
sion protection. Details of this phenomenon are discussed in Ref. 6.
Where cracks occur to the depth of reinforcing bars, both carbonation and attack by
chloride ions may accelerate even more. When corrosion is indicated, investigations should
be done to determine the depth of carbonation, amount of chloride ion, and chloride profile
with depth.
Several methods are available to protect reinforcement against corrosion. A discussion
of these methods is provided in Ref. 7. In general, using high-quality concrete, providing
epoxy or galvanized coatings on bars, and preventing chlorides from getting to the level of
the reinforcing bars are effective.
Once corrosion has started because of carbonation or the presence of chlorides, several
methods are available to minimize its effects. Cathodic protection has been successful in
some cases. However, cathodic protection requires continued monitoring of the system to
be sure it is working. Surveys have shown that many cathodic protections are ineffective
because of lack of maintenance. 8
Barriers have been used to stop the intrusion of chloride and moisture and to control cor-
rosion. In general, drying of the concrete is most effective inside buildings where humidity
can be controlled. Drying in combination with periodic maintenance, which includes chip-
ping out damaged concrete, cleaning off reinforcing bars, coating the bars, and replacing
concrete that has been removed, has been proved to extend the life of buildings and bridges
substantially.

CODES AND STANDARDS

In North America, most buildings constructed after 1910 have been designed following rec-
2
ommendations of ACI 318, Building Code Requirements for Reinforced Concrete, and its
predecessors. Concrete bridges have been designed according to the AASHTO Standard
9
Specifications for Highway Bridges. Both documents have evolved with similar, though
not identical, design requirements.
Historical Perspective. A history of the development of North American design require-
ments for reinforced-concrete buildings is given in Ref. 10. That document notes that the
first building code for reinforced concrete was adopted in February 1910. The code was 12
pages long and covered reinforced concrete as listed in Table 12.1. A comparison with
more recent requirements is shown in the third column of Table 12.1.