FAQs Archive - Page 2 of 37 - American Shotcrete Association

Q: : I am looking for information on where, when and why welded wire mesh (WWF) would be recommended in lieu of structural steel reinforcement, or with steel reinforcement, or guidance on how to determine which option or combination is the preferred one.

WWF should only be used when the area of reinforcing provides the required reinforcement area. It may serve as supplemental reinforcement to bars, or entirely replace reinforcing bars as long as the required area of reinforcing steel is provided in the concrete section. Where multiple sheets (more than 2 at say a corner) overlap you may need to cutout one or more of the layers to allow good encasement. Don’t use steel fibers with WWF as they tend to bunch up where the wires cross. WWF can be used to provide reinforcing in double curved sections where it would be hard to bend and place reinforcing bars. Also, it may be used where you have a thick concrete cover or unreinforced thickness of a repair area that you wanted to get some reinforcing in place to control depth of any surface or shrinkage cracking. If the mesh is just being provided to help support shotcrete being shot overhead you may not worry about the area of steel, just adding it in addition to the original design reinforcing. Then a lighter mesh might be used since you aren’t really worried about its ability to be reinforcing.

Q: Our pool company just completed the shotcrete today and temperatures are expected to reach near freezing tonight. Should the shotcrete still cure properly? Should we be concerned?

You say near freezing, so we assume it did not reach freezing. Though the newly place concrete will not gain strength quickly it should not be damaged. As temperatures rise during the day the warmer temperatures should allow more strength gain. Concrete has a chemical reaction to build strength, that generates internal heat. If freezing weather was expected, we would recommend use of a protective blanket on the concrete surface to help hold the heat in and promote quicker strength gain. Generally, in concrete we like to see the concrete surface temperature at 50 °F (10 °C) or higher in cold weather to encourage strength gain.

Q: I’m interested in any information you can provide about recommended expansion/contraction joint spacing for lazy river concrete flumes.

Shotcrete is a placement method for concrete. As the lazy river is a long concrete trough that is intended to be functionally watertight, the best guidance is ACI, 350-06 Code Requirements for Environmental Engineering Concrete Structures . As the predominate stresses in the horizontal direction result from shrinkage and temperature, Section 7.12.2.1 provides guidance on joint spacing based on the provided reinforcement ratio in the concrete section. The closer the movement joints, the less reinforcement required. Chapter 5 of ACI 350.4R-04, Design Considerations for Environmental Engineering Concrete Structures , has guidance on design of joints for water containing structures to help maintain water tightness.