Reinforcing of concrete structures is essential to ensure their strength and durability. For common slabs on grade, contractors have a few options, including conventional rebar, welded wire reinforcing (WWR), and fiber reinforced concrete (FRC). While each approach has unique benefits and limitations, understanding their differences is key to selecting the right solution for a project.
WWR is made from high tensile strength steel wires welded into a grid pattern and is used to carry tensile loading in concrete slabs. This reinforcement option allows for precise calculations of concrete section capacity using well-established design equations based on the known cross-sectional area and position of the steel. Specifiers and designers must define placement tolerances and ensure proper support (chairs) spacing to address challenges associated with small-diameter WWR, which can be flexible and difficult to position without proper supports. When installed correctly, WWR provides crack control, durability enhancement, and flexural continuity, offering a reliable reinforcement solution.
FRC incorporates fibers made of plastic, glass, or steel into the concrete mix. The primary purpose of FRC is to enhance crack resistance, with its effectiveness depending on the uniform distribution of fibers within the mix. However, this uniformity can be challenging to achieve, as fibers tend to clump, and measures must be taken to prevent this during both the mixing and placement stages. An experienced crew is required since the finishing process can be labor-intensive, as fibers can protrude from the surface, and the crew needs to pay special attention to the timing of the saw-cutting of contraction joints to prevent the fibers from being pulled up.
The design of FRC slabs is performance-based and reliant on project-specific test data, typically derived from ASTM C1609 testing of the concrete. Without such data, it is impossible to calculate the capacity of an FRC slab or design it reliably. An optimal design with FRC would also include WWR or rebar to carry the tensile loading in the concrete slab, which is not directly provided with FRC on its own.
Inspections, on behalf of the designer or owner, can confirm fiber addition at the batch plant or on-site to ensure compliance and these measures introduce time and cost implications for the designer to adequately approve of the FRC capacity. Additionally, many design codes lack published strength-reduction factors for FRC, leaving designers to rely on their discretion and judgment. While manufacturers maintain records of past test results, variability between documented and actual project conditions is inevitable, further complicating FRC's application.
Both WWR and FRC are passive reinforcement types, activated after cracks develop. However, WWR provides enhanced crack control as well as flexural continuity beyond first cracking, while FRC primarily controls crack formation. Improper mixing of fibers is comparable to poorly placed WWR or rebar mats. Therefore, the effectiveness of both methods relies on proper installation and adherence to project specifications.
While FRC and WWR can achieve similar performance in certain scenarios, FRC is not a straightforward replacement for conventional reinforcement. It demands unique design considerations, inspection of fiber dosages, and additional performance-based testing that may lead to unexpected costs. On the other hand, WWR provides a more predictable and easily calculable reinforcement solution but requires careful placement tolerances and supports during installation.
