Page 295 - Forensic Structural Engineering Handbook

9.6 CAUSES OF FAILURES

insufficient concrete cover are common sources of distress in parking structures.
Fabrication errors, such as misaligned bolt holes, are common defects in steel construction.
With proper preparation and review of shop drawings, and field monitoring by the design
team, many of these types of defects can be prevented.

Deterioration
Unlike defects that are typically present at the beginning of the service life of a structure,
deterioration of a material or system is time-dependent. While some forms of deterioration
may develop early in the service life of a structure, others will not manifest until later.
Water as Catalyst. Deterioration is often initiated or accelerated by the presence of a
defect or the introduction of a catalyst. Water is the most significant catalyst for deteriora-
tion. From this author’s research, at least half of all failure mechanisms identified in con-
2
struction involve water, including some subtle interactions. For example, overstress may
not seem to be dependent on water, but the strength and stiffness of wood can be adversely
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affected by high moisture content, in addition to supporting decay that further decreases
the wood’s strength. Water-induced mechanisms include (but are not limited to) rot, corro-
sion, freeze-thaw damage, and soil erosion. The deleterious presence of water is not lim-
ited to naturally occurring events, such as rainfall, but may be attributed to infiltration of
water due to other construction defects, such as leaking pipes.
Degradation Factors. Several other deterioration catalysts or “degradation factors” that
initiate or accelerate the deterioration of building materials and components have been clas-
4
sified in ASTM E632, Standard Practice for Developing Accelerated Tests to Aid
Prediction of the Service Life of Building Components and Materials, as follows:
Weathering factors: Deterioration from exposure to water, temperature, wind, radiation,
air, and air contaminants.
Biological factors: Attack of a material by a living organism, such as bacteria or insect
infestation.
Stress factors: Loads on a system, either sustained or periodic, such as gravity loads,
thermal loads, shrinkage, swelling, or settlement.
Incompatibility factors: Chemical reactions, such as chloride attack of steel; or physi-
cal interactions between materials, such as abrasion.
Use factors: Wear and tear associated with construction and service; or application of
loads that exceed the strength of the system.
In many instances, degradation factors work in tandem. For example, corrosion of steel
is a chemical attack (incompatibility factor), but usually occurs in the presence of water (a
weathering factor).
Time-dependence is a fundamental property of deterioration mechanisms (this discus-
sion would also apply to non-deterioration mechanisms that develop over time, such as
spontaneous glass breakage from nickel-sulfide inclusions). In general, a mechanism may
develop at a rate that increases, decreases, or remains roughly constant over time.
Decreasing-rate mechanisms, such as shrinkage-cracking of concrete, will diminish in rate
of deterioration over time, even though the cumulative deterioration increases in magnitude.
Increasing-rate mechanisms will develop faster over time (even exponentially), typically
because the deterioration “feeds” on itself; for example, the erosion of the concrete shown
in Fig. 9.4 facilitates ponding of additional water, thereby accelerating the erosion.