Concrete MythBuster #2 – As Slab Thickness Increases So Must the Fiber Dosage

This myth deals with a common misconception regarding how you determine steel fiber dosage for temperature/shrinkage reinforcement. It is commonly thought that since the amount of mesh must increase as slab thickness increases, the same would be true with fibers – as the slab thickness increases so would the fiber dosage. But this is not the case, for the simple reason that fiber dosage is calculated using volume, and mesh is calculated using area. Perhaps it’s not intuitive, so let me show you.

ACI 360 lists two areas of steel to limit crack width. For temperature/shrinkage reinforcement it lists a value of .1% with joints not to exceed 15’ and early entry saws used to install contraction joints. Here is the formula –

%area = As/Tx12(100)

So for a 6” slab, the area of steel would be .072 sq in. For an 8” slab, the area of steel would be .096 in sq.
As = .1xTx12/100

As = .1x6x12/100
= .072 in sq

As the area of the slab section increases, the steel area must increase to maintain the .1% of the slab cross sectional area. Hence, an 8” slab requires an area of steel of .096 in sq.

But fibers are dosed so many pounds of fiber per cubic yard of concrete. With fibers we have to go from area to volume to get an equivalent dosage of fibers. Based on research and an article in ACI Materials Journal titled “Distribution and Orientation in Steel Fiber ReinforcedConcrete,” a formula has been developed to give a fiber dosage when the “As” is known. The formula is:

Fd= As x 2119/T

Using the above formula for a 6” slab, the fiber dosage would be 25 pounds per cubic yard. For an 8” slab, the fiber dosage would be 25 pounds per cubic yard.

How can this be? The areas have increased but the fiber dosage remains the same. Remember, area is in square inches while fiber dosage is in cubic yards. In reality we are increasing the fiber amount for a given square foot of slab because a cubic yard of concrete in a 6” slab covers 54sf while an 8” slab only covers 40sf. For mesh, you are increasing the area of steel by 25% to reflect the increased area of the slab. With fibers you are decreasing the area of concrete by 25% using the same fiber dosage. So for a given quantity of fiber, we have a higher concentration of fibers as the thickness increases.

So remember for temperature/shrinkage reinforcement using steel fibers and the ACI 360 recommendation of .1% area of steel, the fiber dosage will always be 25 pounds per cubic yard. Please note, this only applies to steel fibers. To my knowledge there has not been work done on the orientation of synthetic fibers.

While steel fibers and mesh may approach the design of temperature/shrinkage reinforcement from different directions, fibers get to the same solution. And in addition to this benefit, fibers don’t require chairs to position, they allow easier placement of the concrete, easier use of LaserScreeds, present no tripping hazard and they come at a lower cost, Therefore, the final conclusion you can draw from all of this is that CFS100-2 and CFS150-5 steel fibers should be your choice for temperature/shrinkage reinforcement on all of your projects.