360R-06 Design of Slabs-on-Ground
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particle angularity. Another load-transfer mechanism is enhanced aggregate interlock. Enhanced aggregate interlock depends on a combination of the effect of a small amount of deformed reinforcement continued through the joint and the irregular face of the cracked concrete at joints for load transfer. The continuation of a small percentage of deformed reinforce- ment (0.1%) through sawcut contraction joints, in combina- tion with joint spacings shown in Fig. 5.6 , has been used successfully by some designers to provide load-transfer capability without using dowels. A slab design that uses this small amount of deformed reinforcement to enhance aggre- gate interlock at the joints should conform to the following requirements: 1. Joints should be spaced per Fig. 5.6 ; 2. The reinforcement should be placed above mid-depth but low enough that the sawcut will not cut the reinforcement; 3. A construction or a smooth doweled sawcut contraction joint should be placed at a maximum of 125 ft (38 m). This will force activation at these joints if the other joints with the deformed reinforcement do not activate; 4. An early-entry saw should be used to cut all sawcut contraction joints; and 5. The slab should be a uniform thickness. As a general rule, the continuation of larger percentages of deformed reinforcing bars should not be used across sawcut contraction joints or construction joints because they restrain joints from opening as the slab shrinks during drying, and this will increase the probability of out-of-joint random cracking. The restraint provided by the reinforcement varies with the quantity of reinforcement in the slab, expressed as a percentage of the cross-sectional area of the slab. Park and Paulay (1975) offer a method of calculating the reduction in unrestrained internal shrinkage strain that can be attributed to the presence of reinforcement. Table 5.3 provides the calculated reduction in strain that can be attributed to the presence of various percentages of reinforcement located at midheight of a slab using the following values: E s = modulus of elasticity of steel: 29,000,000 psi (2,000,000 MPa); E c = modulus of elasticity of concrete: 2,900,000 psi (20,000 MPa); C t = creep coefficient: 2.0; and e sh = unrestrained shrinkage strain: 0.000500. This table suggests that the reduction in strain that could be anticipated from 0.1% reinforcement at midheight of the Fig. 5.10—Rectangular load plate basket assembly. Fig. 5.11—Diamond-shaped load plate at construction joint. Fig. 5.13—Diamond-shaped load plates at slab corner. Fig. 5.12—Doweled joint detail for movement parallel and perpendicular to the joint. |