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.
Construction joints should be designed and located if possible at sections of¬†minimum shear. These sections will usually be at the center of beams and slabs,¬†where the bending moment is highest. They should be located where it is most¬†convenient to stop work. The construction joint is often keyed for shearing strength.
If it is not possible to concrete an entire floor in one operation, vertical joints¬†preferably should be located in the center of a span. Horizontal joints are usually¬†provided between columns and floor; columns are concreted first, then the entire¬†floor system.
Various types of construction joints¬†are shown in Fig. 9.9. The numbers on ¬†each section refer to the sequence of¬†placing concrete.
If the joint is horizontal as in Fig.¬†9.9a, water may be trapped in the key¬†of the joint. If the joint is vertical, the¬†key is easily formed by nailing a wood¬†strip to the inside of the forms. A raised¬†key, as in Fig. 9.9b, makes formwork¬†difficult for horizontal joints.
In the horizontal joint in Fig. 9.9c,¬†the key is made by setting precastconcrete¬†blocks into the concrete at intermittent¬†intervals. The key in Fig. 9.9d
is good if the shear acts in the directions¬†shown.
The V-shaped key in Fig. 9.9e can be¬†made manually in the wet concrete for¬†horizontal joints.
The key is eliminated in Fig. 9.9∆í,¬†reliance being placed on friction on the roughened surface. This method may be¬†used if the shear forces are small, or if there are large compressive forces or sufficient
reinforcement across the joint.
See also Arts. 9.32 to 9.34.