normspanners
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depth of reinforcement mesh varies, when we did the scrap yard surface the mesh went in the top section to minimise open cracks and some was done by laying it on the surface and 'walking' it in, because of time constraints , but with my garage base as most of the weight was on the walls I had to to thicken the slab as a 'footing ' and put the mesh in the top 1/3 was what building regs said IIRC
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.
When mesh is used:
One of the problems of placing steel mesh within concrete is keeping it in the correct position within the slab so that it carries out its job properly. It's not uncommon for a contractor to complain that the ground floor slab has cracked because the concrete is at fault, only for a subsequent investigation (generally done by coring or using a cover-meter) to reveal that the crack control mesh is sitting just a few millimetres from the base of the slab. When mesh is used, it should always be placed near the surface of a slab with a minimum 50mm cover if it is to minimise plastic shrinkage cracking.
Most slabs-on-ground are unreinforced or nominally reinforced for crack-width control. When positioned in the upper or top portion of the slab thickness, steel reinforcement limits the widths of random cracks that may occur because of concrete shrinkage and temperature restraints, subbase settlement, applied loads or other issues. This type of reinforcement is commonly called shrinkage and temperature reinforcement.
Shrinkage and temperature reinforcement is different than structural reinforcement. Structural reinforcement is typically placed in the bottom portion of the slab thickness for the purpose of increasing the slab's load capacity. Most structural slabs-on-ground have both top and bottom layers of reinforcement for controlling crack-widths and increasing load capacities. Because of constructability issues and costs associated with two layers of reinforcement, structural slabs-on-ground are not as common as nonstructural slabs.
While there are several reinforcing options for nonstructural slabs-on-ground, this article focuses on steel reinforcing bars and welded wire reinforcement for crack width control.
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.