Concrete is a
time-proven building product, however it is the inherent nature of
the product to crack. Design and construction professionals
make allowances for this normal occurrence by using crack control
measures prior to and during the construction process. Cracks
that occur during the finishing process are a result of:
- Surface Shrinkage : Caused by rapid water loss, or by
- Settlement while the concrete is still in plastic or
has yet to gain strength.
Cracks, which occur after
hardening, are the result of drying-shrinkage, curling, and/or other
thermal expansion/contraction. Using well-established concrete
construction practices will minimize and control cracking at the
jobsite. Quality workmanship and "know how" can prevent normal
cracking from becoming a cracking "problem".
QUALITY CONCRETE STARTS
WITH A SOLID SUB-BASE
- A compacted, uniform sub-base is essential to avoid settlement
cracking.
- Concrete thickness must be equal throughout the placed section for
control joins to function properly.
- Exterior concrete should have grades established to promote good
drainage away from dwellings.
ESTABLISH A JOINT PLAN
PRIOR TO THE POUR
- Tooled or sawed
control joints should be at least 1/4 the depth of the slab.
- Joint spacing should be no more than ten feet apart for 4-inch
thick concrete (Spacing = 30 times thickness in inches to a maximum
of 15 feet, regardless of thickness.)
- Joint placement should be completed within 24 hours after
placement and finishing of the concrete surface.
- Isolation joins are used to separate dissimilar construction and
mist be full depth.
PLACE CONCRETE AT A
REASONABLE SLUMP
- Use pumps, chutes, and wheelbarrows to avoid segregation.
- The correct slump for residential requirements is 3 to 5 inches.
- Excessive water will delay set time and reduce strength.
- Excessive water also contributes to cracking, dusting, and
scaling.
- Use producer approved admixtures, like super plasticizers, to
improve concrete flow.
SHRINKAGE CRACKING CAN BE
MINIMIZED OR AVOIDED
- Direct sun, wind and low humidity cause rapid
moisture loss from the surface, resulting in shrinkage cracks
occurring before concrete hardens.
- Freshly placed concrete
must be protected from the environment. Use windbreaks,
sunscreens, and fogging when necessary.
- Dampening sub-grade
prior to placement and immediate use of curing compounds minimize
rapid water loss.
- Environmental conditions cannot be
ignored.
CURING IS MANDATORY TO
MINIMIZE CRACKING
- Controlling the rate of water loss
from the concrete after placement reduces the potential for cracking
and surface defects.
- Constant water curing is preferred.
The early application of a curing compound is a practical
alternative to water curing.
- Pigmented curing compounds aid in
even application and results.
AVOID RE-ENTRANT CORNERS
WITH PROPER PREPARATION
- Cracks running out from the corners of columns
or other rigid fixtures, which extend through the floor slab, are
re-entrant corners.
- Use expansion joint material to create an
isolation joint around these fixtures or block them out.
- Incorporate rigid structures into your joint plan to avoid cracking
between slab corners and the fixture.
AVOID CRAZING BY PROPER
CURING
- Shallow
hairline cracks, commonly called "map cracking" are unsightly but do
not affect the slabs' durability.
- Troweling bleedwater into
the surface will promote excessive surface shrinkage (crazing).
- Starting the the curing process as early as possible can help
minimize the occurrence of crazing.
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