Civil Engineering Reference
In-Depth Information
Curing should begin immediately after finishing
(Fig.
11-22). Curing is needed to ensure continued hydration of
the cement, strength gain of the concrete, and a minimum
of early drying shrinkage.
Special precautions are necessary when concrete work
continues during periods of adverse weather. In cold
weather, arrangements should be made in advance for
heating, covering, insulating, or enclosing the concrete.
Hot-weather work may require special precautions against
rapid evaporation and drying and high temperatures.
ularly true with high rates of placement. Also, the
encasing formwork prevents the escape of moisture from
the fresh concrete. While formwork provides adequate
curing as long as it remains in place, the top surface where
there is no encasing formwork may dry out too rapidly;
this may result in a weak porous layer unless protection
and curing are provided.
Preparing Hardened Concrete
When freshly mixed concrete is placed on recently hard-
ened concrete, certain precautions must be taken to secure
a well-bonded, watertight joint. The hardened concrete
must be clean, sound, and reasonably rough with some
coarse aggregate particles exposed. Any laitance, soft
mortar, dirt, wood chips, form oil, or other foreign mate-
rials must be removed since they could interfere with
proper bonding of the subsequent placement.
The surface of old concrete upon which fresh concrete
is to be placed must be thoroughly roughened and cleaned
of all dust, surface films, deposits, loose particles, grease,
oil, and other foreign material. In most cases it will be nec-
essary to remove the entire surface down to sound con-
crete. Roughening and cleaning with lightweight chipping
hammers, waterblasting, scarifiers, sandblasting (Fig.
11-23), shotblasting and hydrojetting are some of the satis-
factory methods for exposing sound concrete. Care must
be taken to avoid contamination of the clean surface before
a bonding grout and overlay concrete are placed.
Partially set or recently hardened concrete may only
require stiff-wire brushing. In some types of construction
Fig. 11-22. An excellent method of wet curing is to
completely cover the surface with wet burlap and keep it
continuously wet during the curing period. (69946)
PLACING ON HARDENED CONCRETE
Bonded Construction Joints
in Structural Concrete
A bonded construction joint is needed between two struc-
tural concrete placements. When freshly mixed concrete is
placed in contact with existing hardened concrete, a high-
quality bond and watertight joint are required. Poorly
bonded construction joints are usually the result of (1) lack
of bond between old and new concrete, or (2) a weak
porous layer in the hardened concrete at the joint. The
quality of a bonded joint therefore depends on the quality
of the hardened concrete and preparation of its surface.
In columns and walls, the concrete near the top sur-
face of a lift is often of inferior quality to the concrete
below. This may be due to poor consolidation or use of
badly proportioned or high-slump mixtures that cause
excessive laitance, bleeding, and segregation. Even in
well-proportioned and carefully consolidated mixtures,
some aggregate particle settlement and water gain
(bleeding) at the top surface is unavoidable; this is partic-
Fig. 11-23. Sandblasting can clean any size or shape sur-
face - horizontal, vertical or overhead. Consult local envi-
ronmental regulations regarding sandblasting. (55805)
