17.4
Influenced Factors of Cracking
1) Raw
material and mix proportion of concrete i.e. Aggregate, Cement, Water and
Admixture.
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Aggregate i.e. stone, sand, mineral
constituents, irregularity of surface and use proportion are all significant
influences in mix design. Despite of this, coefficient of thermal conduction,
drying shrinkage, stiffness, creep and concrete strength ex. Clay mixed aggregate
will give larger shrinkage comparing with cement, which undoubtedly it is one
of the cause that results cracks in concrete.
-
Cement: In general, use of excessive
amount of cement or highly-contained silica fume of that fine particles cement
i.e. Portland cement type 3, will give considerably high tendency for cracks to
occur.
-
Water is the most essential component in
mixing concrete and as equally important, it is also necessary not to add too
much water in as this will give an upward tendency of having cracks together
with reduction in compressive strength of concrete.
-
Admixture: Even some admixtures can ease
initiation of cracks i.e. accelerator but some can hold back cracking problem
i.e. retarder.
2) Concrete
Placing: apparently, rate of concrete placing and working condition can
significantly affect the initiation of cracks and quite often found that
bleeding is one of the causes as well. In addition to bleeding, water that
emerged toward the top will spare the bottom with voids, in which the volume of
void is proportional to depth/height of considering structure. As the result,
interior crack and separation of concrete will inevitably occur. Notwithstanding
bleeding, atmospheric temperature, asymmetrical settlement of that supporting
formwork or soil beneath concrete are also found to be essential in forming
cracks.
3) Working
Condition
-
Temperature: in generic case, both
compressive and tensile strength of concrete are direct proportional to
temperature, however the more obvious effect can be observed when concrete is
cool down, especially in mass production and concrete that subject to high
temperature. Moreover, reduction in number of cracks is generally attributed to
placing of concrete in low temperature rather than high, which this could arise
to any type of structures with no exceptions. Hence, concrete pouring might be often
found to implement at night.
-
Exposure: a direct contact to surrounded atmosphere
in various conditions is the most likely cause to influence on cracking.
Immense fluctuation of ambient temperature and humidity in daytime can result tremendous
amount of internal restraint within concrete due to internal strains have
varied or changed differently from what occurs at the surface, in which the
unbalanced stress will induce cracks in concrete.
4) Curing:
regardless before or after curing, moisture in concrete is vital and shall
be sustained among suitable attributes in making concrete. In particular,
according the concrete slab, evaporation at the surface will be more rapidly
react than the rate of bleeding. This will haste the process of surface
shrinkage, which eventually induces cracks. To avoid such problem, it is useful
to undertake: use of water-soaked formwork, placing concrete at night,
immediate curing (if possible) and protecting concrete from evaporation (wind
& sunlight) while placing.
5) Restraint:
concrete that subject to complete restraint from footing or adjacent structures
can adversely result cracks. In addition, vertical cracks at structure base are
found to be commonly occurred, in which it is deemed to be acceptable on the condition
that existing propagation should not be developed by further extension. Also often
found that long-span wall or slab with no joints separate will partially crack
but one that jointly integrated with structure will be damaged by cracks in
both horizontal and vertical direction.
In general,
crack width that induced from restraint or over-reinforcement of so-called
temperature steel will be similarly narrow. Correspondingly,
use of high-yield-point steel will also give the result of scattered
minor-cracks, which comparatively greater than structural-grade-steel when
used.
Presence of
internal restraint will merely be found when use of various mix design e.g.
different ratio in cement & aggregate use, is carried out within the same
structure.
In accordance
with above mention, it seems that cause of cracking can not be derived from just
one possible condition but rather appear severally and simultaneously.
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