Civil Engineering Reference
In-Depth Information
Place concrete to level of broken line.
Allow to settle and strike off to bottom
of strip. Remove strip, clean and cure
joint as specified.
Tierod not over
150 mm (6")
above joint.
50 mm (2")
100 mm
(4") max.
25 mm
(1") strip
Lap over
hardened
concrete not
more than
25 mm (1")
16 mm (5/8") threaded bolt
greased for easy removal.
Bolt to hold forms tight against
hardened concrete
50 mm (2")
Fig. 11-24. A straight, horizontal construction joint can be built using this detail.
such as dams, the surface of each concrete lift is cut with a
high-velocity air-water jet to expose clean, sound concrete
just before final set. This is usually done 4 to 12 hours after
placing. The surface must then be protected and continu-
ously cured until concreting is resumed for the next lift.
For two-course floors, the top surface of the base slab
can be roughened just before it sets with a steel or stiff
fiber broom. The surface should be level, heavily scored,
and free of laitance; then it should be protected until it is
thoroughly cleaned just before the grout coat and topping
mix are placed. When placing a bonded topping on a floor
slab, the base slab should be cleaned of all laitance, dust,
debris, grease or other foreign substances by using one of
the following methods:
a. Wet- or dry-grit sandblasting
b. High-pressure water blasting
c. Mechanical removal by scabblers, or grinding wheels
d. Power brooming and vacuuming
Hardened concrete may be left dry or be moistened
before new concrete is placed on it; however, the surface
should not be wet or have any free-standing water. Labor-
atory studies indicate a slightly better bond is obtained on
a dry surface than on a damp surface; however, the
increased moisture level in the hardened concrete and in
the environment around the concrete reduces water loss
from the concrete mixture. This can be very beneficial,
especially on hot, dry days.
For making a horizontal construction joint in rein-
forced concrete wall construction, good results have been
obtained by constructing the forms to the level of the joint,
overfilling the forms a few centimeters (inches), and then
removing the excess concrete just before hardening occurs;
the top surface then can be manually roughened with stiff
brushes. The procedure is illustrated in Fig. 11-24.
In the case of vertical construction joints cast against a
bulkhead, the concrete surface generally is too smooth to
permit proper bonding. So, particular care should be given
to removal of the smooth surface finish before reerecting
the forms for placing freshly mixed concrete against the
joint. Stiff-wire brushing may be sufficient if the concrete is
less than three days old; otherwise, bushhammering or
sandblasting may be needed, followed by washing with
clean water to remove all dust and loose particles.
Bonding New to
Previously Hardened Concrete
Care must be used when making horizontal construction
joints in wall sections where freshly-mixed concrete is to
be placed on hardened concrete. A good bond can be
obtained by placing a rich concrete (higher cement and
sand content than normal) in the bottom 150 mm (6 in.) of
the new lift and thoroughly vibrating the joint interface.
Alternatively, a cement-sand grout can be scrubbed into a
clean surface immediately ahead of concreting.
A topping concrete mix for slabs can be bonded to
the previously prepared base slab by one of the following
procedures:
1.
Portland cement-sand grouting:
A 1 to 1 cement-sand
grout having a water-cement ratio of not greater than
0.45, mixed to a creamlike consistency, is scrubbed
into the prepared dry or damp (no free water) base
slab surface.
