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Placement Requirements For Welded Wire Fabric

Q: What is the correct placement requirements for WWF in a concrete slab-on-ground?

A: Somewhere in any presentation on Fiber Reinforced Concrete the speaker will show a picture of WWF laying on the subgrade rusting. The speaker will declare that there is no way the laborer charged with the responsibility of elevating the wire mesh to the correct height in the concrete cross-section will ever succeed. The speaker goes on; as long as the laborer is standing on the wire mesh there is no way it will ever be raised to the correct height.

For years, the Wire Reinforcement Institute has declared in their publications that the WWF must be at the optimum height in the concrete slab-on-ground to perform as temperature-shrinkage reinforcement. Never have they stated that the need for supports was mandatory to achieve this goal. Seldom in their specifications do structural engineers spell out the requirement for support for the wire mesh. It should also be noted that ACI 318 Chapter 7 there is no statement regarding the need for support for the wire mesh.

It is obvious that the use of supports would be detrimental to the marketing of wire mesh as temperature-shrinkage reinforcement. The cost of the supports coupled with the cost of the time to place these supports would increase the considerable burden already associated with the use of wire mesh knowing the expensive placement costs before including supports.

Enter the code bodies. These leaders in standards of construction have finally recognized the need to require the contractors to place the wire mesh at the correct height in the concrete slabs-on-ground. The International Code Council (ICC) with construction codes for commercial and residential structures used throughout the U.S. and other countries has included in the 2009 International Residential Code the following verbiage in Chapter 5, Floors:

R506.2.4 Reinforcement Support. Where provided in slabs on ground, reinforcement shall be supported to remain in place from the center to upper one-third of the slab for the duration of the concrete placement.

The State of Florida in their amendments to the ICC Code for floors includes the use of microsynthetic fiber at 1.0 to 1.5 lbs/CY as an equal to 6×6 W1.4 x W1.4 WWF as temperature-shrinkage reinforcement in slabs-on-ground. The code also permits the use of microsynthetic fiber at 0.5 to 1.0 lb/cy as plastic shrinkage reinforcement.

The cost of wire mesh has become indefensible when compared to Fiber Reinforced Concrete particularly when including the 3-dimensional, quantifiable reinforcement benefits exhibited by the synthetic fibers. Reduced plastic shrinkage cracking, drying shrinkage cracking and permeability coupled with increased impact resistance and surface abrasion resistance definitely increase the value of the microsynthetic fibers.

ABC Polymer Industries (ABC) can provide the correct microsynthetic fiber or macrosynthetic fiber to meet the project specifications/requirements. ABC’s microsynthetic fibers, both the fibrillated and monofilament polypropylene are included in ICC ES ESR-1699 as plastic shrinkage crack reinforcement and temperature-shrinkage crack reinforcement. Furthermore, we can provide the engineering support required to obtain product approvals.

R.C. Zellers, PE/PLS

Director, Engineering Services