With Option 2, we allow slabs to crack randomly but control crack widths with steel reinforcing bars or welded wire reinforcement. Typically, contraction joints are not installed with this option. Instead, cracking occurs randomly forming numerous, tightly held together cracks. Because of appearance, this crack control option should always be discussed with the owner.
Cutting reinforcement at joints
Use caution when using both crack control options in the same slab. If too much reinforcement passes through contraction joints, joints become too stiff and may not crack and open as designed. When contraction joints fail to activate (i.e., crack and open) because of reinforcement, out-of-joint or random cracking typically occurs. If both options are used, it is necessary to limit the amount of reinforcement passing through joints to ensure proper activation.
Some designers specify to cut all the reinforcement at contraction joints while others may specify to cut every other bar or wire. By cutting every other bar or wire, the remaining reinforcement will help provide load-transfer and minimize differential panel movements but not restrict joints from activating. If the specifications and construction drawings do not indicate what to do with temperature and shrinkage reinforcement at joints, contractors should submit a request for information. Many times contractors are inappropriately blamed for out-of-joint cracking associated with this design issue.
Location of reinforcement
Steel reinforcing bars and welded wire reinforcement should be positioned in the upper third of the slab thickness because shrinkage and temperature cracks originate at the surface of the slab. Cracks are wider at the surface and narrow with depth. So, crack-control reinforcement should never be positioned below the slab's mid-depth. Reinforcement should also be placed low enough so saw cutting does not cut the reinforcement. For welded wire reinforcement, the Wire Reinforcement Institute recommends steel placement 2 inches below the surface or within the upper third of the slab thickness, whichever is closer to the surface. Designers typically specify the reinforcement position by specifying concrete cover (1 1/2 to 2 inches) for the reinforcement.
Positioning a single layer of reinforcement in the center or at mid-depth of the slab is not recommended (except for 4-inch-thick slabs). This is an all-purpose location where the designer hopes to increase the load capacity of the slab in addition to provide crack-width control. However, positioning reinforcement in the middle of the slab will not effectively accomplish either objective.
Steel reinforcing and welded wire reinforcement should be supported and sufficiently tied together to minimize movements during concrete placing and finishing operations. Otherwise, reinforcement may not be properly located in the slab. Support reinforcement with chairs or precast-concrete bar supports. Chairs should have sand or base plates and bars should have at least a 4-inch square base to ensure they don't sink into the subbase. Use support spacings that ensure reinforcement does not sag between supports or is not pushed down by foot traffic or fresh concrete. Flexible reinforcement including welded wire reinforcement requires closer support spacing. In addition to specifying the type and amount of reinforcement, designers should specify the type and spacing of supports to ensure proper positioning of the reinforcement.
Welded wire reinforcement should never be placed on the ground and pulled into position after concrete placement. The "hook and pull" technique always results in improperly positioned reinforcement. How can workers uniformly "hook and pull" welded wire reinforcement into the specified location while standing on the reinforcement?
Vertical placement tolerance for reinforcement in slabs-on-ground is ± 3/4 inch from the specified location. For slab thicknesses 12 inches or less, the concrete cover tolerance is - 3/8 inch measured perpendicular to the concrete surface and the reduction in cover cannot exceed one-third of the specified cover. In many cases, the cover tolerance overrides the vertical placement tolerance. Properly placing and supporting reinforcement will help ensure compliance with these vertical placement tolerances.