Building Design and Construction Handbook - Merritt - Ventech!

9.40 SECTION NINE
9.39 JOINTS IN CONCRETE
Several types of joints may occur or be formed in concrete structures:
Construction joints are formed when fresh concrete is placed against hardened
concrete.
Expansion joints are provided in long components to relieve compressive
stresses that would otherwise result from a temperature rise.
Contraction joints (control joints) are provided to permit concrete to contract
during a drop in temperature and to permit drying shrinkage without resulting
uncontrolled random cracking.
Contraction joints should be located at places where concrete is likely to crack
because of temperature changes or shrinkage. The joints should be inserted where
there are thickness changes and offsets. Ordinarily, joints should be spaced 30 ft
on center or less in exposed structures, such as retaining walls.
To avoid unsightly cracks due to shrinkage, a dummy-type contraction joint is
frequently used (Fig. 9.8). When contraction takes place, a crack occurs at this
deliberately made plane of weakness. In this way, the crack is made to occur in a
straight line easily sealed.
Control joints may also consist of a 2- or 3-ft gap left in a long wall or slab,
with the reinforcement from both ends lapped in the gap. Several weeks after the
wall or slab has been concreted, the gap is filled with concrete. By that time, most
of the shrinkage has taken place.
In expansion joints, a filler is usually provided to separate the two parts of the
structure. This filler should be a compressive substance, such as corkboard or premolded
mastic. The filler should have properties such that it will not be squeezed
out of the joint, will not slump when heated by the sun, and will not stain the
surface of the concrete.
To be waterproof, a joint must be sealed. For this purpose, copper flashing may
be used. It is usually embedded in the concrete on both sides of the joint, and
folded into the joint so that the joint may open without rupturing the metal. The
flashing must be strong enough to hold its position when the concrete is cast.
Proprietary flexible water stops and polysulfide calking compounds may also be
used as sealers.
Open expansion joints are sometimes used for interior locations where the opening
is not objectionable. When exposed to water from above, as in parking decks,
open joints may be provided with a gutter below to drain away water.
FIGURE 9.8 Control joints for restraining temperature and shrinkage cracks: (a) vertical section
through a slab on grade; (b) horizontal section through a wall.