Consider This: Using Fibers in Wastewater Structures

Wastewater tank manufacturers are always looking for an edge over competing products. As such, the prospect of replacing some conventional reinforcing with fibers – which may save considerable production time – is enticing to many producers. However, the decision to make such a change must be approached with caution, as hasty transitions can lead to problems in the field and a stain on your company’s reputation.

Fiber technology has evolved significantly, with steel, synthetic, glass and many other types of fibers available for use. Most fibers are still used to complement primary conventional reinforcing and to help reduce early age cracking. However, structural fibers are now being incorporated as primary reinforcing. Steel fibers and macro-synthetic fibers are leading the charge in this area.

ASTM C1765-16, “Standard Specification for Steel Fiber Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe,” was recently published for concrete pipe reinforced solely with steel fiber. ASTM C1818-15 “Standard Specification for Synthetic Fiber Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe” has also been recently published. Manhole riser sections are also being reinforced with fiber alone, where permitted by local code. Some producers of wastewater structures are following suit and using fibers to replace primary reinforcing in septic tanks and grease interceptors.

Despite the continued increase of fibers in precast concrete products, there are many important factors to consider:

• If you are bound by the limitations of ASTM C1227, “Standard Specification for Precast Concrete Septic Tanks,” or ASTM C1613, “Standard Specification for Precast Concrete Grease Interceptor Tanks,” only Type III fibers conforming to ASTM C1116, “Standard Specification for Fiber-Reinforced Concrete,” can be used, and only as a secondary reinforcing material. For the purposes of this specification, secondary reinforcing material is used only to resist temperature and shrinkage effects.
• Currently, no design methodology is available for fiber-reinforced concrete except for Twisted Steel Micro Rebar which has Uniform Evaluation Report (UES) 279 which guides users using TSMR as alternative reinforcing for structural concrete constructed as per ACI 318 or ACI 360. Otherwise, the only way to verify that the volume and type of fibers used is sufficient is to conduct performance testing – in other words, proof of load testing with an engineer present. If abiding by ASTM C1227, the load applied must be 1.5 times the anticipated actual loads for septic tanks.
• When considering the use of fiber for a tank’s lid and base, exercise caution. According to Ron Thornton, consulting engineer and member of NPCA’s Technical Committee, “Fiber reinforced concrete does not provide the same ductility as steel reinforced concrete. Ductility allows for a member to fail by yielding as opposed to an abrupt collapse. For this reason, top and bottom slabs should always contain steel reinforcing.”
• In most cases, fibers will weigh less than your conventional reinforcing. This will come into play when designing for buoyancy in high-water-table environments.
• Be aware of the effects fibers will have on the concrete’s workability and ability to be placed. A slump loss may require additional measures to ensure the concrete flows throughout the form.

Fibers are an exciting technology that can provide significant savings in manufacturing while simultaneously raising the quality and durability of precast structures. As with any other change in design or manufacturing, the effects of its use must be carefully examined to avoid issues. NPCA is here to help and has a fibers white paper available.  There is also a recorded webinar from June 13, 2107, available for purchase.