Pull-out Strength
The key feature of Helix® is the TWISTED profile. This drives the failure mechanism from a frictional mode to untwisting, setting Helix® apart from other fibers. Helix® behaves more like conventional steel reinforcement than other fibers because of this mechanism.
This 'untwisting' requires six times more pull out force compared with a non-twisted steel fiber of equivalent area. Furthermore Helix® maintains a constant resistance regardless of how far it has been untwisted. This differs from other fibers that fail when the cross sectional area is reduced as the fibers pull out of the concrete.
Furthermore the pull-out resistance of Helix® is not dependent on the inclination angle of the micro rebar with respect to a crack.
For more detailed technical information, go to the manufacturer's website: Helix Steel.
Physical Properties
- Material: cold drawn steel wire, zinc galvanized
- Length: 1" (25 mm)
- Configuration: rectangular cross section twisted five times along axis
- Cross sectional area: 0.0003 in2 (0.196 mm2)
- Weight: 0.00141 oz. (0.0401 gm.) per micro bar
- Steel tensile: 245 ksi (1,700 MPa)
- Micro bar tensile: 66.2 poundforce (294.5 N) each
- Modulus of Elasticity: 29,000,000
Stress Strain Curve
The graph to the right compares the stress strain curve of Helix® to conventional steel reinforcement. Note that with rebar the stress increases with minimal strain until the first dominant crack occurs (at about 120,000 kN). At this point there is a drop in stress, and a large increase in the strain (from 0% to approximately 5%), as the rebar engages and starts to resist the load.
Helix® begins to engage soon after loading as micro-cracks form in the concrete. This behavior may be attributed to the short development length of Helix®: 1/4” compared with 24” for 15M rebar. Therefore the strain in the Helix® reinforced concrete can be seen to increase gradually with increasing stress. This improves the performance of the concrete and prevents the formation of a single dominant crack.
Equivalence to Rebar
The bending moment capacity and shear strength of Helix® reinforced concrete is dependent on the dosage in the concrete. Dosage is measured in pounds per cubic yard or kilograms per cubic meter.
For example, consider a 6" thick concrete wall with #4 bar at 20" on center in both directions. Substituting a Helix® dosage of 20 lbs/yd3 will achieve:
- Bending moment capacity: 16.7 kip-in/ft. (16.7 kip-in/ft. for rebar)
- Shear strength: 8.5 kip/ft. (3.0 kip/ft. for rebar), a 186% improvement
- First crack strength: 512 psi (493 psi for rebar), a 4% improvement
- Durability, toughness: 122 lb.-in/ft. (99 lb.-in/ft. for rebar), a 23% improvement
