We get questioned all the time...
Should I use fibers instead of Welded Wire Reinforcement?
First of all, Welded Wire Reinforcement (WWR) for reinforced concrete has performed well for as long as historical records tell us its been out there. The first patent rights for the product were issued in 1901 in the U.S. First, there was triangular wire reinforcing (before there were welders) up to 1911, and then WWR when welders were invented. There are over 100 years of WWR performance on record - interesting, and it is still being produced in greater quantities than ever before. But more on that later. We have no reports on poor performance of steel reinforcement; on the contrary, we get many reports where people were convinced to leave the steel reinforcement out and substitute some type and some quantity of fiber in the mix, and then unhappy they did not have steel reinforced concrete. We'll leave the failure modes to your research and investigations. Needless to say, the reports state that the fibers did not do what they were promised they would do....????

Fibers are not concrete reinforcement.
Welded Wire Reinforcement (WWR) is concrete reinforcement.
Watch what the fiber people say about fibers - they say it protects concrete from early plastic shrinkage cracking, they say it increases impact resistance of the surface, and they say it improves the durability of the concrete. They may call it fibrous or fiber-reinforced concrete - BUT do not get it confused with concrete reinforcement in the form of welded wire reinforcement (WWR).

Here is what the WWR does in concrete.
First of all, steel reinforcement is primary, not secondary, and engineers know it. There are no secondary stresses in structures. Steel reinforcement is a structural element in concrete that handles stresses due to shrinkage, and we might add, tensile strength for expansion in the case of shrinkage compensating concrete. It also restrains the concrete to help prevent wide and displaced cracking. See reference: WRI's TF 705, "Innovative Ways to Reinforce Slabs on Ground", by Bob Anderson, consultant.

Where and how should I place WWR?
We hear that some contractors have placed it on the ground and they are satisfied with the performance and will not change their methods. More power to them. WRI prefers, and has said for a long time, that WWR needs to be supported like rebars. Many contractors know where steel reinforcement needs to be placed and their project superintendents oversee labor forces to insure the proper placement. Engineers must specify where the reinforcing should be placed and type/s of supports to be used. Engineers should discuss problems and corrections with the contractors as the job progresses. When discussions include the placement of welded wire reinforcement, it needs to be emphasized that welded wire is supported just like supported rebars. Welded wire with wide spacings and larger wire sizes has been used successfully for years to support construction loads without displacing it. See reference guide, WRI's TF 702-R2, "Supporting WWR". This document will answer questions about types of supports, methods to keep the WWR positioned, and how far to space the supports.

If the steel reinforcing is placed close to the bottom (assuming single layer applications), the cracks would be wider at the top—but the steel reinforcing will still work to control cracking and crack widths much better than plain or fibrous concrete slabs. That is why we either suggest another layer or a single layer of reinforcement be positioned within 1/3 the thickness of the slab from the top, or a minimum of 2" - that is to protect from wide or displaced cracking and curling if it does occur - a kind of insurance policy. When the steel reinforcing is properly positioned there is a history of quality projects that prove there is better crack width control and little to no displacement compared to slabs without steel reinforcement. Another item to note- There is no ACI Code or guide that says that steel reinforcement can be substituted with fibers. In fact, ACI 302-1R-96 (latest) Guide for Concrete Floor and Slab Construction, Section 5.8.3, does reflect on the subject. It states "fibers should not be used to replace temperature and shrinkage reinforcement because they have little impact on the behavior of concrete after it hardens".

How is the fiber market affecting WWR?
There has been great growth in specifying and the purchase of WWR. Tonnage figures show that the light building styles are continuing to increase - up 6% over 1980 numbers and structural tonnage numbers are 300% over 1980 totals. Rather than compare WWR with fibers, we prefer to compare WWR with the rebar market. There is approximately a 5 million ton per year market for #3, #4, #5, & #6 rebar sizes (the metric call-outs are #10, #13, #16 & #19). WWR styles of reinforcement can be compared with those rebar sizes and spacings. Even some larger areas of rebars can be compared with WWR styles, if the engineers will allow increased strengths of WWR and/ or reduced spacings.

WRI wishes to thank the owners, contractors, and engineers that have had the confidence and are satisfied with WWR performance to not just continue to specify and place WWR, but have doubled the WWR concrete reinforcement market in recent years.

Summary
It would be interesting to see how many people have realized that fibers and steel reinforcement do different things in concrete and have specified and placed both materials in their projects. With all the talk of high-performance concrete and improved quality of concrete construction, one wonders if people are considering supplements and not alternatives for welded wire reinforcement.

Conclusion
A final question can be pondered? What crack width and how much displacement or curling, and what maintenance costs would occur if WWR steel reinforcing were not present!!

NOTE:
The Tech Facts (TF's) referenced above may be downloaded from the WRI Publications page on this web site. Two additional articles, written by Roy H. Reiterman, may be viewed/downloaded (PDF Files: if you do not have Acrobat Reader Click here to download it.) by clicking the links below.