Page 297 - Forensic Structural Engineering Handbook

9.8 CAUSES OF FAILURES

to cracking, freeze-thaw damage, and chemical attack can all arise from poor mix design.
In addition, poor quality aggregates and impurities in the mix water can adversely affect
quality.
Many construction defects in cast-in-place concrete come from improper placement,
curing, and finishing. A common consequence of these defects is uncontrolled cracking,
particularly of slabs. While concrete cracking is inevitable due to the nature of the mater-
ial, it can be limited. In addition, crack locations can be controlled when the design is prop-
erly detailed and construction is monitored. Placement and finishing, adequate control
joints and expansion joints, proper curing, and adequate concrete cover and/or slab depth
are all required for prevention of unwanted cracks in concrete structures.
Additional problems often arise due to quality control during concrete placement, the
most infamous of which is the on-site addition of water to the concrete mix to increase work-
ability. Improper vibration or consolidation during placement can lead to honeycombing,
delamination, stratification, or spalling, as well as a reduction of structural capacity.
Most concrete defects can be prevented through proper design and construction moni-
toring. A comprehensive resource for design and construction of concrete structures is
ACI’s Manual of Concrete Practice. 7
Deterioration. Much of the deterioration found in concrete stems from either the defects
discussed above or the deleterious effects of water. Concrete is particularly susceptible
to chemical incompatibility, including alkali-silica reactions, alkali-carbonate reactions,
carbonation, and sulfate attack. 2,8 Water is the primary catalyst for concrete weathering,
scaling, erosion, bleeding, leaching, freeze-thaw damage, and chemical reactions. In some
instances, intrinsic mechanisms will not manifest distress without moisture (e.g., alkali-silica
reactions or corrosion of steel reinforcement).

Examples: Steel
Defects. Of the structural materials commonly used in construction, steel is manufactured
under the most controlled conditions, and consequently material defects are not particularly
common. Most defects in steel members and structures arise from either improper design
and detailing or errors in fabrication and erection. A dominant source of construction
defects (and subsequent structural distress) is the improper installation of connections,
especially poor quality welding. Misalignment of steel members, particularly at connec-
tions, is a common cause of unanticipated eccentric loads on steel structures. Failures in
steel structures also may arise from design errors, including undersized members and
improper detailing for fatigue resistance.
The cross-sectional shapes utilized in steel assemblages are particularly susceptible to
stability failures. These failures may be on the local level (e.g., web buckling), at the mem-
ber level (e.g., lateral-torsional buckling), or on a system level (e.g., story buckling). 9
System instability is typically catastrophic in nature. This phenomenon is not limited to
fully constructed facilities; in fact, it more often occurs during construction due to insuffi-
cient bracing.
Deterioration. Deterioration of steel at the material level stems primarily from incom-
patibility factors, such as chemical attack, that lead to corrosion. There are numerous ways
that steel can corrode; a partial list of corrosion types includes chloride-accelerated, con-
centration cell, crevice, deposit, electrochemical, electrolytic, galvanic, pin-point, and
2
thermo-galvanic. Section loss may occur due to corrosion, reducing the strength or stabil-
ity of a steel member or system. Proper selection of steel types for the particular exposure
environment, or corrosion-resistant coatings, can reduce or prevent corrosion.