Civil Engineering Reference
In-Depth Information
factors, such as: (1) the amount of aggregate used; (2) prop-
erties of the aggregate; (3) size and shape of the concrete
element; (4) relative humidity and temperature of the
ambient air; (5) method of curing; (6) degree of hydration;
and (7) time.
Two basic causes of cracks in concrete are: (1) stress
due to applied loads and (2) stress due to drying shrinkage
or temperature changes when concrete is restrained.
Drying shrinkage is an inherent, unavoidable property
of concrete; therefore, properly positioned reinforcing steel
is used to reduce crack widths, or joints are used to prede-
termine and control the location of cracks. Thermal stress
due to fluctuations in ambient temperature also can cause
cracking, particularly at an early age.
Concrete shrinkage cracks can occur because of
restraint. When drying shrinkage occurs and there is no
restraint, the concrete does not crack. Restraint comes
from several sources. Drying shrinkage is always greater
near the surface of concrete; the moist inner portions
restrain the concrete near the surface, which can cause
cracking. Other sources of restraint are reinforcing steel
embedded in concrete, the interconnected parts of a
concrete structure, and the friction of the subgrade on
which concrete is placed.
Joints.
Joints are the most effective method of controlling
unsightly cracking. If a sizable expanse of concrete (a wall,
slab, or pavement) is not provided with properly spaced
joints to accommodate drying shrinkage and temperature
contraction, the concrete will crack in a random manner.
Contraction (shrinkage control) joints are grooved,
formed, or sawed into sidewalks, driveways, pavements,
floors, and walls so that cracking will occur in these joints
rather than in a random manner. Contraction joints permit
movement in the plane of a slab or wall. They extend to a
depth of approximately one-quarter the concrete thickness.
Isolation joints separate a concrete placement from
other parts of a structure and permit horizontal and verti-
cal movements. They should be used at the junction of
floors with walls, columns, footings, and other points
where restraint can occur. They extend the full depth of
slabs and include a premolded joint filler.
Construction joints occur where concrete work is
concluded for the day; they separate areas of concrete
placed at different times. In slabs-on-ground, construction
joints usually align with, and function as, control or isola-
tion joints. They may require dowels for load transfer.
of those ingredients, interactions between the ingredients,
and placing and curing practices determine the ultimate
durability and life of the concrete.
Resistance to Freezing and Thawing
Concrete used in structures and pavements is expected to
have long life and low maintenance. It must have good
durability to resist anticipated exposure conditions. The
most potentially destructive weathering factor is freezing
and thawing while the concrete is wet, particularly in the
presence of deicing chemicals. Deterioration is caused by
the freezing of water and subsequent expansion in the
paste, the aggregate particles, or both.
With air entrainment, concrete is highly resistant to this
type of deterioration as shown in Fig. 1-25. During freezing,
the water displaced by ice formation in the paste is accom-
modated so that it is not disruptive; the microscopic air
bubbles in the paste provide chambers for the water to enter
and thus relieve the hydraulic pressure generated.
When freezing occurs in concrete containing saturated
aggregate, disruptive hydraulic pressures can also be gener-
ated within the aggregate. Water displaced from the aggre-
gate particles during the formation of ice cannot escape fast
enough to the surrounding paste to relieve pressure.
However, under most exposure conditions, a good-quality
paste (low water-cement ratio) will prevent most aggregate
particles from becoming saturated. Also, if the paste is air-
entrained, it will accommodate the small amounts of excess
water that may be expelled from aggregates, thus protecting
the concrete from freeze-thaw damage.
Fig. 1-26 illustrates, for a range of water-cement ratios,
that (1) air-entrained concrete is much more resistant to
freeze-thaw cycles than non-air-entrained concrete, (2) con-
crete with a low water-cement ratio is more durable than
DURABILITY
The durability of concrete may be defined as the ability of
concrete to resist weathering action, chemical attack, and
abrasion while maintaining its desired engineering proper-
ties. Different concretes require different degrees of dura-
bility depending on the exposure environment and the
properties desired. The concrete ingredients, proportioning
Fig. 1-25. Air-entrained concrete (bottom bar) is highly
resistant to repeated freeze-thaw cycles. (P25542)
