Basalt Rebar Design
How to design your building with Basalt Rebar.
Basalt Rebar is FRP Rebar
Basalt Rebar is a type of Fiber Reinforced Rebar (FRP).
The American Concrete Institute has produced a spec to recommend how to use FRP rebar. The spec is ACI 440.1R-06 .
The only reason ACI 440.IR-06 does not specify basalt rebar as an example of FRP rebar is that ACI 440.1R-06 was written a while ago before basalt rebar was invented.
A later written ACI spec (ACI 440R-07) does list basalt rebar as an example of allowed FRP rebar. It says... "Fibers commonly used to make FRP bars are glass, carbon, and aramid. Recently, continuous Basalt fibers have become commercially available as an alternative to glass fibers." It talks about Basalt FRP rebar all through the document and includes it in its various tables, but the key point is that it is classed as FRP. The term BFRP is often used instead of saying Basalt Rebar.
Basalt FRP rebar is approved as "natural fiberglass", meeting the certification specifications of ACI 440.6-08 and signed off the same as fiberglass FRP rebar. In doing so, the job will simply be overbuilt because the physicals of Basalt rebar are higher than fiberglass, falling between fiberglass and carbon fiber.
Basalt rebar can be placed to meet code requirements by using the
calculations and installation guidelines for fiberglass reinforcement of
concrete as defined in ACI 440.6-08. Recommendations for maximum
deflection and shear of concrete elements reinforced with fiber
reinforced polymer (FRP) rebar's are presented in ACI 440.1R-06 (2006)
"Guide for the Design and Construction of Structural Concrete Reinforced
with FRP Bars".
Basalt rebar has been tested at various universities and approved by the American Concrete Institute under ACI 440-10.
Basalt rebar is used according to ACI 440.1R-06.
The construction use is dictated by code 440.6-08.
It is specified by 440.5-08.
The conclusion from all the documentation is that Basalt rebar is FRP rebar and can be treated and approved as FRP rebar.
The spec for the basalt rebar that I use is...
Tensile strength: 130,000 psi
(900 MPa) (Worst case 116,030 psi)
Shear strength: 29,000 psi (200 MPa) (Worst case 26,100 psi)
Tensile modulus: 7,000,000 psi (48 GPa) (Worst case 6,526,698 psi)
Elongation at break: 2.3% (Worst case 1.6%)
Density: 1.9 - 2.1 g/cm3
To confirm that the basalt rebar from my Chinese manufacturer met the required tensile strength I had it tested by an ASTM accredited test house.
The important take-aways from ACI 440.1R-06 are...
1) The minimum bend radius (measured on the inside of the bend) is 3 times the diameter of the basalt rebar bar used to make the stirrup.
2) The minimum tail length (measured from the center of the bend radius) is recommended to be 12 times the diameter of the basalt rebar used to make the stirrup.
Minimum Inside Bend Radius
Here are the relevant snippets from ACI 440.1R-06...
There's a useful article at http://www.mmsonline.com/columns/frp-rebar-shear-reinforcement-and-detailing .
The minimum radius requirement is not just due to not wanting to excessively bend the basalt fibers. Tight bends also induce stress in the concrete. Here are some snippets from the web...
It would be nicer to use a tight bend because that would more accurately hold the longitudinal rebar in the correct location and also would allow the longitudinal rebar to be closer to the concrete edge (and still maintain the same concrete cover), but unfortunately it is necessary to abide by the 3d rule. Something like the following would be good for constraining the longitudinal rebar but is not allowed...
Compare this with the following stirrup design that does meet the rules. The #5 rebar longitudinal bars are not as well constrained and have had to move inwards by 1/4".
The 12 times diameter recommendation in ACI 440.1R-06 is primarily referring to tails for a 90 degree bend. If the bend is 135 degrees or 180 degrees then other research has shown that a tail of 6 times diameter and not less than 70mm has been found to be sufficient. Using 6d is also consistent with the guidelines and common practice for steel stirrups (that are 4d or 6d).
For a small stirrup designed to fit within an 8" cavity concrete wall it is not practical to use 12d, so 6d or 70mm is used (provided the hook bend angle is 135 degrees or greater). Where there is room to do it then it is good to use a 12d tail. When considering if there is enough room for a longer tail it is also important to think about whether a longer tail will impede the flow of wet concrete into the column.
For 10mm stirrups where space is limited, use a tail of 70mm or greater, and where space is not limited use 120mm.
For 13mm stirrups where space is limited use a tail of 80mm or greater, and where space is not limited use 160mm.
In some situations it is not possible to use even a 6d tail. It is ok to make the tail a little shorter as long as it is recognized that the stirrup will have a slightly reduced earthquakes resistance. The tail length used must not make it impossible to fit the stirrup after the longitudinal rebar has been approximately added (but not yet tied).
Where possible it is best to use a 180 degree bend on the end. This is not always possible because you need to ensure that it is possible to fit a stirrup after the longitudinal rebar has been approximately added (but not yet tied).
The majority of my stirrups are designed to provide 1.5" concrete coverage, ie no edge of basalt rebar closer to 1.5" from the edge. Sometimes the actual distance is an additional 1/16" or 1/8" from the edge.
Having 1.5" of concrete coverage is plenty as the concrete is not exposed to the weather and basalt rebar is not subject to corrosion. The figure could be smaller, but I wanted to ensure the concrete has room to flow around the rebar.
There are a few exceptions (eg floor beam stirrups) where the concrete coverage has to be less, but that is acceptable because basalt rebar does not corrode so doesn't actually need any concrete coverage.
Offset from standard vertical positions
When using Carnation Walling, the standard center of rebar vertical rebar positions measured from the outside of the wall concrete are 2-1/4", 4-1/2", and 5-3/4". For 12" wide concrete the numbers are 2-1/4", 4-1/2", 5-3/4", 8", and 9-3/4".
Because of the stirrup minimum bend radius it is necessary for vertical rebar (in horizontally placed rectangular stirrups) in the outside positions to move towards the center of the wall by 1/4". This is within the flex capabilities of basalt rebar.
Offset from standard horizontal positions
When using Carnation Walling, the standard center of rebar horizontal positions measured from the outside of the wall concrete for 8" wide concrete are 1-3/4", 4", and/or 6-1/4". For 12" wide concrete the numbers are 1-3/4", 4", 6-1/4", 8-1/2", and/or 10-1/4".
Because of the stirrup minimum bend radius it is necessary for horizontal rebar (in inverted stirrups and cow heads) in the outside positions to move towards the center of the wall by 3/4" and upwards by 1/4". This is within the flex capabilities of basalt rebar.
Carnation Basalt Rebar Stirrups and standard lengths
These are the standard rebar parts that I get manufactured. As well as the stirrups there are also drawings for the standard lengths of straight rebar that I use. If you need stirrups or lengths of basalt rebar then I am happy to supply to you. Send me email for the prices on the different standard items.