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4-68. Concrete structures are subjected to internal and external stresses caused by volume shrinkage during hydration, differential movement due to temperature changes, and differential movement caused by different loading conditions. These stresses can cause cracking, and scaling of concrete surfaces and in extreme cases result in failure of the concrete structure. These stresses can be controlled by the proper placement of joints in the structure. Three basic types of joints will be addressed--the isolation joint, the control joint, and the construction joint.


4-69. Isolation joints are used to separate or isolate adjacent structural members. An example would be the joint that separates the floor slab from a column (to allow for differential movement in the vertical plane due to loading conditions or uneven settlement). Isolation joints are sometimes referred to as expansion or contraction joints to allow for differential movement as a result of temperature changes (as in two adjacent slabs). All isolation joints extend completely through the member and do not have any load transfer devices built into them. See Figures 4-19, 4-20, and 4-21 below.


Figure 4-19. Typical isolation and control joints



Figure 4-20. Joints at columns and walls



Figure 4-21. Expansion/contraction joint for a bridge


4-70. Movement in the plane of a concrete slab is caused by drying shrinkage and thermal contraction. Some shrinkage is expected and can be tolerated depending on the design and exposure of the particular structural elements. In a slab the shrinkage occurs more rapidly at the exposed surfaces and causes upward curling at the edges. If the slab is restrained from curling, cracking will occur wherever the restraint imposes stress greater than the tensile strength. Control joints (see Figure 4-22 below) are cut into the concrete slab to create a plane of weakness that forces cracking (if it happens) to occur at a designated place rather than randomly. These joints run in both directions at right angles to each other. Control joints in interior slabs are typically cut 1/3 to 1/4, of the slab thickness and then filled with lead or joint filler, see Table 4-13 for suggested control joint spacing. Temperature steel (welded wire fabric) may be used to restrict crack width for sidewalks, driveways, and tooled joints spaced at intervals equal to the width of the slab but not more than 20 ft (6 meters) apart. Surface irregularities along the plane of the cracks are usually sufficient to transfer loads across the joint in slabs on grade, the joint should be 3/4 to 1-in deep.


Figure 4-22. Control joint

Table 4-13. Spacing of Control joints

Slab Thickness, in Inches Less Than 3/4 inch Aggregate Spacing, in Feet Larger Than 3/4 inch Aggregate Spacing, in Feet Slump Less Than 4 Inches Spacing, in Feet
5 10* 13 15
6 12 15 18
7 14 18 21
8 16 20 24
9 18 23 27
10 20 25 30
NOTE: Spacing also applies to the distances from the control joints to the parallel isolation joints or to the parallel construction joints.


4-71. Construction joints (see Figures 4-23 through 4-26 below) are made where the concrete placement operations end for the day or where one structural element will be cast against previously placed concrete. These joints allow some load to be transferred from one structural element to another through the use of keys or (for some slabs and pavement) dowels. Note that the construction joint extends entirely through the concrete element.


Figure 4-23. Keyed, wall construction joint (perspective view)



Figure 4-24. Keyed, wall construction joint (plan view)



Figure 4-25. Construction joint between wall and footing showing keyway



Figure 4-26. Types of construction joints


4-72. The anchor bolt (see Figure 4-27 below) is used to anchor either machinery or structural steel. The diameter of the sleeve should be at least 1 inch larger than the bolt. The sleeve allows the bolt to shift to compensate for any small positioning error. The pipe sleeves are packed with grease to keep concrete out during paving operations. Do not use the sleeve if machinery mounts are on a raised base that is 4 or more inches above the floor. Instead set the anchor bolts in the floor so they extend above the base. Place the machine--blocked up on scrap steel and leveled--and tighten the anchor bolts. Finally pour the base by packing the concrete into place from the sides.


Figure 4-27. Anchor bolt with pipe sleeve


4-73. The hooked anchor bolt (see Figure 4-28) and the suspended anchor bolt (see Figure 4-29 below) are both used to fasten a wood sill to either a concrete or masonry wall. Be careful not to disturb suspended bolt alignment when placing the concrete. Make the bolt holes in the board 1/16 inches larger than the bolt to permit adjustment.


Figure 4-28. Hooked anchor bolt



Figure 4-29. Suspended anchor bolt


4-74. (See Figure 4-30). A templet is used to hold anchor bolts in place while placing concrete. When using sleeves in a templet adjust them so that the top of the sleeve is level with the top of the finished concrete.


Figure 4-30. Anchor bolts held in place by a templet

David L. Heiserman, Editor

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Revised: June 06, 2015