Steel Fiber Reinforced Concrete

What is steel fiber reinforced concrete (SFRC)?

SFRC is a composite material made of hydraulic cements, water, fine and coarse aggregate, and a dispersion of discon­tinuous, small fibers.  It may also contain pozzolans and admixtures commonly used with conventional concrete.  All admixtures meeting ASTM specifications for use in con­crete are suitable for use in SFRC.  Calcium chloride and chlorides from other sources should be limited to amounts permitted to be added to conventional structural concrete as shown in ACI 318.

What is the function of steel fibers in concrete?

Unlike welded wire reinforcement or rebar, which is specifically located in a sin­gle plane, steel fibers are distributed uni­formly throughout the concrete matrix.  The primary function of steel fibers is to modify micro and macro cracking.  By intercepting cracks at their origin, the steel fibers inhibit crack growth. For this reason, SFRC can be used to replace welded wire reinforcement or rebar which is used to control temperature or shrinkage cracks.

What is the function of micro synthetic fibers?

Synthetic fibers have been shown to reduce the formation of plastic-shrinkage and settlements cracks at the surface by increas­ing the tensile strain capacity of the plastic concrete. These fibers should not be used to replace temperature and shrinkage reinforce­ment because they have little impact on the behavior of concrete after it hardens.

Where can steel fibers be added to the concrete?

Steel fibers can be introduced into the concrete at the batch plant or jobsite. Consult Fibercon Fact – Mixing, Placing and Finishing for more details on the proper methods of adding fibers to concrete.

Can SFRC be pumped?

Pumping has been used to transport SFRC on many projects.  In general a mixture that will pump satisfacto­rily without fibers will pump with fibers.  In pumping SFRC, the chute from the ready-mix truck should be 12” above the grate on the hopper. This will prevent the fiber from bridging the gaps in the grate and ensure a steady flow of concrete to the pump.  The grate should never be removed from the hop­per.

Can shake-on-toppings or liquid hardeners be applied over a SFRC floor?

Both can be used on a SFRC floor. Shake-on­toppings can act as a fiber suppressor and limit the number of fibers on the surface.  If liquid hardeners are used the floor should be moist cured for 7 days and allowed to air dry per manufacturer’s recommendations before installing the liquid hardener.

What depth should saw cuts be in a SFRC floor?

Contraction joints can be installed using either a wet saw or an early entry saw. For wet saws the depth should be ¼ the depth for slabs less than 6” thick. For slabs over 6” thick or slabs with over 40 pounds of steel fiber per cubic yard of concrete, the saw cuts should be ⅓ the depth. For early entry saws, the manufacturer’s recommendation should be followed.