Figure 7-30.-Welded wire mesh fabric.

braces, walers, ties, and shebolts should be properly secured before placing concrete.

Splicing Reinforcing Bar

Because rebar is available only in certain lengths, it must be spliced together for longer runs. Where splices are not dimensioned on the drawings, the bars should be lapped not less than 30 times the bar diameter, or not less than 12 inches.

The stress in a tension bar can be transmitted through the concrete and into another adjoining bar by a lap splice of proper length. The lap is expressed as the number of bar diameters. If the bar is No. 2, make the lap at least 12 inches. Tie the bars together with a snap tie (figure 7-31).

CONCRETE CONSTRUCTION JOINTS

LEARNING OBJECTIVE: Upon completing this section, you should be able to determine the location of construction joints.

Concrete structures are subjected to a variety of stresses. These stresses are the result of shrinkage and differential movement. Shrinkage occurs during hydration, and differential movement is caused by temperature changes and different loading conditions. These stresses can cause cracking, spalling, and scaling of concrete surfaces and, in extreme cases, can result in failure of the structure.

TYPES OF JOINTS

Stresses in concrete can be controlled by the proper placement of joints in the structure. We'll discuss three basic types of joints: isolation joints, control joints, and construction joints.

Isolation Joints

Isolation joints are used to separate (isolate) adjacent structural members. An example is the joint that separates the floor slab from a column. An isolation joint allows for differential movement in the vertical plane due to loading conditions or uneven settlement. Isolation joints are sometimes called expansion or contraction joints. In this context, they allow for differential movement as a result of temperature changes (as in two adjacent slabs). All isolation joints (expansion or contraction) extend completely through the member and have no load

Figure 7-31.-Bars spliced by lapping.