Crack control in concrete ground bearing slabs

Crack Control of Ground bearing Concrete Slabs

In the past welded wire steel reinforcement provided crack control in non-structural ground bearing slabs. Now Sirch BM offer an easy to use Basalt Fibre Reinforced Polymer (BFRP) mesh reinforcement from Orlitech.

Most ground bearing slabs are reinforced for crack-width control using welded steel mesh which is located in the top half of the concrete slab. During curing, the concrete may shrink creating random cracking throughout the slab, the mesh does not prevent cracking but limits the width of the crack to prevent further degradation of the slab. Steel mesh used for this purpose is referred to as shrinkage and temperature reinforcement.

Welded steel mesh is generally supplied in 4.8M x 2.4M (11.52M2) sheets and is difficult to transport and handle on site. A high speed cut off saw such as a Stihl saw is required with the associated health and safety risks.

Orlitech mesh is a basalt fibre mesh which is four times lighter and more than twice as strong as steel. The 3mm 100mm x 100mm mesh comes on a 0.75M x 30M roll or for smaller jobs as a 0.75M x 3M roll. Orlitech mesh does not have welded connections, each BFRP wire is continuous and jointed with a patented connection nodule. Orlitech mesh lies completely flat once taken off the roll and can be easily cut with a pair of pliers.

Shrinkage and temperature reinforcement is different than structural reinforcement. Structural reinforcement mesh from Orlitech consists of 6mm – 12mm mesh and can be used to increase the load bearing capacity of the slab. Most ground bearing structural slabs have both top and bottom layers of reinforcement for controlling crack-widths and increasing load capacities. In this case the larger diameter mesh would be used for the bottom layer and the 3mm mesh for crack control for the top layer.

Due to the increased tensile strength of BFRP a smaller diameter mesh can be used as opposed to steel. For load bearing slabs we recommend that we provide you with a structural calculation to ensure the best alternative is used.

As in the case of steel reinforcing bars, and welded wire reinforcement, Orlitech mesh will not prevent cracking. Both forms of reinforcement are inactive until such a time that the concrete cracks. If the concrete cracks the reinforcement becomes active and restricts the width of the crack.

In an un-reinforced slab as a crack forms and opens to 1mm the effectiveness transfer of loads by the concrete is reduced and differential movement within the slab can occur. In this scenario the edges of the crack can be broken leading to further problems and degradation of the slab and widening of the initial crack.

For control of shrinkage and temperature cracking Orlitech mesh reinforcement 3mm x 100mm x 100mm should be situated in the upper third of the slab because shrinkage and temperature cracks initiate at the top of the concrete slab. Crack formation results in cracks wider at the top so any crack-control reinforcement should not be placed in the lower section of the slab.

Orlitech reinforcement should be supported and tied together to reduce movement during pouring of the concrete to maintain correct placement within the concrete.