Page 296 - Forensic Structural Engineering Handbook

DEFECTS, DETERIORATION, AND DURABILITY 9.7

FIGURE 9.4 Erosion of concrete, an increasing-rate failure mechanism due to the self-feeding
nature of the process.

Constant-rate mechanisms, such as carbonation of concrete, generally proceed linearly
over time.
Obviously, it is easiest to predict the future behavior of systems affected by constant-
rate mechanisms. Also, decreasing-rate mechanisms tend to be fairly straightforward to
deal with in forensic investigations, because at any time that the structure is studied the rate
of future deterioration will be diminishing, by definition. Increasing-rate mechanisms pose
the hardest problem in predicting future behavior; for instance, once freeze-thaw damage
becomes evident, there may be little time remaining to arrest it before complete disintegra-
tion occurs.
The rate that a deterioration mechanism changes over time (linear, decreasing, or
increasing) can be used not only for predicting future behavior but also as a diagnostic tool.
Many mechanisms have a “temporal fingerprint” that differentiates them from other
mechanisms; Reviewing the evidence of the development of previous deterioration can be
used to determine which mechanism is occurring. For instance, the mechanisms affecting
concrete discussed above (shrinkage cracking, carbonation, and erosion) have distinct
temporal fingerprints, so that a plot of data collected over time would indicate which of
these mechanisms was operative.
Below are examples of defects and deterioration specific to four common building
materials: concrete, steel, wood, and masonry. More comprehensive catalogs of defects and
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deterioration mechanisms are provided by SEI/ASCE 11-99 and Nicastro, as well as in
other chapters of this book.
Examples: Concrete

Defects. Concrete is a composite material; the interaction of its components leads to
many of the design and construction defects common to the material. One of the principal
design defects that cause distress in concrete members is improper detailing of reinforce-
ment. The lack of proper detailing in concrete can lead to minor distress such as spalling
and as catastrophic as collapse. At one point, the American Concrete Institute (ACI) esti-
mated that repair and rehabilitation accounted for approximately 70 percent of the U.S.
construction market. 6
Other sources of concrete defects stem from errors in concrete mix design based on
numerous possible combinations of admixtures, aggregates, and cement types. Susceptibility