FRP VS. STEEL STRESS STRAIN CURVE- THE ELASTIC BEHAVIOR OF FRP REBAR

So how does FRP rebar differ from steel rebar? In the previous video we mentioned that FRP rebar is five times less stiff than steel, or FRP has a modulus of elasticity that is five times less than that of steel. This being said, we have to remember that steel and FRP are different materials, therefore act differently under stress or load. Let’s take a look at how this looks on a graph, or stress strain curve.

Stress Strain CurveStress Strain Curve

Note that this chart compares #3 BFRP GatorBar to #4 steel rebar (Grade 40) so the units on the Y axis have been converted to pounds, instead of the typical pounds per square inch. This is to account for the difference in bar diameter.

On the X axis we have strain, or percent deformation. This chart will show us how much each material will give, or elongate, at certain stress or load amounts.

FRP rebar is linear elastic, therefore does not have a yield point.

Grade 40 steel rebar yields, or begins deforming plastically at 7,800 pounds. At this point and beyond, all deformation is permanent.

The key takeaway from this graph is the elastic behavior of FRP rebar. Until 16,000 pounds, GatorBar BFRP reinforcement will bounce back to its original form when the load is removed. To learn about how this can be beneficial in slab on grade applications, download a whitepaper here.

So what does elastic deformation look like in real life? The following video is from an actual test we performed at Michigan Technological University for a precaster in Houston Texas.

This precast concrete step was 2 ¼ inches thick reinforced with three pieces of #3 Gatorbar. It was 500 pounds to first crack, 1800 pounds and one inch deflection before strain took over when the stress flat-lined. When the stress is removed, the concrete makes a full, elastic recovery. This is a lot different than what you're used to, I'm sure!

Again, FRP rebar and steel rebar have different material properties, therefore they react differently under stress. Download the GatorBar spec sheet here. In other words, designing with FRP is a different ball game than designing with steel. 

Hopefully your wheels are spinning and you can already think of a few applications where this elastic behavior can be beneficial. Let us know if you have questions or have topics you’d like us to cover in future videos. 

Josh Kimbal

AllPros.biz

The bars work great!! Our prototype testing went good in the field. We are adjusting the designs and trying other applications.