Precast Arches – Lightweight

The Challenge

A U.S. company wanted to pursue the storm water retention market in response to increased requirements for storm water  management and treatment mandated by the EPA. To be competitive in this market, they determined a new ultra lightweight precast arch design was needed – less than half the normal concrete thickness and no costly rebar or mesh as the added fabrication was cost prohibitive. Unsuccessful testing of several precast designs and various methods to reinforce the precast arches, including using steel fibers, lead them to try Helix Micro-Rebar. The minimum Department of Transport specifications called for 6.8 tonnes of load, but the company decided on a 27.2 tonnes acceptance level.

Helix Alternative Design for Precast

Because Helix Micro-Rebar is not a steel fibre, but Twisted Steel Micro Rebar (TSMR), the strengths provided by the alternative design easily met the clients stringent criteria. This is because it relies on a torsional mechanical bond that, much like a cork screw, tightly grips unto the concrete, rather than rely on a pull-out resistance, like trying to pull a nail out of a piece of timber.

Helix was tested at several higher dosages, but the company settled on 47.5kg/m3 for their precast arches. Their tests results yielded loads of 57.6 tonnes and even then the failures were in no way catastrophic (small cracks formed and load capacity was reduced, but the structure did not collapsed at failure). Helix is better, faster and cheaper, but it can also do things which aren’t possible with rebar or mesh in precast applications.

In Australia, concrete cover over conventional reinforcement, governed by the AS3600 standard, does not allow for such thin sections to be constructed without meeting other stringent criteria. Using Helix Micro-Rebar, which is a discontinuous reinforcement system, overcomes this challenge. It allows for the construction of thinner sections whilst in many cases increasing its shear strength.

Other Benefits for Precast

Using Helix Micro-Rebar instead of mesh and rebar, also reduces the wastage often incurred when transporting such thin precast elements.

To discuss how Helix can help you solve the problem: Contact Us.