تاريخ : یکشنبه ۱ اسفند ۱۳۸۹
منبع : ايران سازه
Control of Cracking in Concrete State of the Art
Size: 1.99 MB | Format: PDF | Publisher: TRANSPORTATION RESEARCH BOARD | Year: 2006 | pages: 56
http://www.filesonic.com/file/95425561/ec107.pdf
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Size: 1.99 MB | Format: PDF | Publisher: TRANSPORTATION RESEARCH BOARD | Year: 2006 | pages: 56
Causes of
Cracking oncrete structures do not frequently fail due to lack of
strength, rather due to inadequate durability or due to improper
maintenance techniques. The most common cause of premature deterioration
is attributed to the development of cracks (Mehta, 1992; Hobbs, 1999).
Cracking can occur in concrete pavements and structures for several
reasons that can primarily be grouped into either mechanical loading or
environmental effects. It should also be noted that for most practical
structures, reinforcement is used to bridge and hold cracks together
when they develop, thereby assuring load transfer while adding ductility
to a relatively brittle material. Therefore not all cracking causes
concern. Reinforced concrete elements are frequently designed on the
assumption that cracking should take place under standard loading
conditions (Nilsson and Winter, 1985; Nawy, 2000). For example
continuously reinforced concrete pavements (CRCP) are designed with
longitudinal steel in an amount adequate to hold shrinkage cracks tight,
while joints exist only at locations of construction transitions and
on-grade structures. In this pavement type wherein shrinkage cracks
develop over time and stabilize over the first 3 to 4 years, cracking in
the transverse direction in specific patterns is not detrimental to the
structure as long as the cracks remain tight and retain good load
transfer. Therefore, cause of cracking should be carefully identified to
determine which cracks are common and acceptable and which cracks merit
repair or further investigation. Several guides currently exist to
assist in determining the cause of cracking including the American
Concrete Institute (ACI) committee reports “Guide for Making and
Condition Survey of Concrete in Service” (ACI 201-92) and “Causes,
Evaluation and Repair of Cracks in Concrete Structures” (ACI 224-R93).
Mechanical loads induce strains that can exceed the strain capacity (or
strength capacity) of concrete, thereby causing cracking. Concrete may
be particularly susceptible to cracking that occurs at early-ages when
concrete has a low tensile capacity (Kasai, 1972). If the loads are
applied repeatedly or over a long period of time, fatigue and creep can
affect the strain (or strength) development that can lead to failure
(Bazant and Celodin, 1991) or reduce stresses (Shah et al., 1998).
Although numerous factors influence whether concrete would be expected
to crack due to environmental effects, it can be simply stated that
cracking will occur if the stress that develops in response to internal
expansion or the restraint of a volumetric contraction that results in
stress development exceeds the strength (or fracture resistance) of the
material. Internal expansion is primarily caused by chemical attack or
freezing of the pore water while volumetric contraction is typically
attributed to moisture changes, chemical reactions, and thermal changes
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