Can Fiber Reinforcing Replace Welded Wire Fabric in Concrete Slabs?

By:  David Simpson, P.E., SECB

Opinions differ when considering if synthetic fibers can replace welded wire fabric. What first must be understood is the function of synthetic fibers and welded wire fabric.

When concrete is first placed, moisture will evaporate from the surface faster than within the mass of the concrete. As the surface begins to cure faster than the concrete below the surface, tensile stresses build up and shallow microscopic cracks develop in random directions.

If allowed to develop without restraint, these cracks can grow wider and lengthen to become major aesthetic and functional problems.

Subgrade_problems

CRACK DUE TO SUBGRADE PROBLEMS:  FIBER ALONE WON’T HELP HERE

curing_stresses

CRACKS DUE TO CURING STRESSES:  FIBER WOULD HAVE LIMITED THEM GREATLY

Cracks also occur due to excessive flexural stresses brought on by bending when spanning distances. These cracks are considered structural failure cracks and are wider and predictable (not random). These cracks rarely occur in slabs-on-grade but will always occur on elevated slabs if not reinforced with rebar or welded wire fabric. If flexural cracks do develop in slabs-on-grade, there is a problem with the subgrade.

Fiber reinforcing is very good in restricting the initial shrinkage cracking that occurs in the initial stages. Our office, Allegheny Design Services, has examples on exterior slabs-on-grade of over 20 years old, with only fiber reinforcing that still exhibits no cracks. The use of fiber does not have a major effect on the compressive strength of the concrete.

Several types of fiber exist. They include steel, glass, synthetic and natural fibers. Their selection depends on the usage and environment of the finished slab.  Consult with the design engineer in selecting the correct type. In all cases, the fiber is considered another admixture and should be submitted with the concrete mix submittal for approval.

Fiber reinforcing has no impact on the air content of concrete compared to the same sample of non-fiber concrete, but the impact on slump is significant. The use of fiber generally will decrease the same sample by 2 inches. In some cases, the use of a high-range water-reducing admixture (HRWRA) may be needed to improve workability.

concrete_cracks

FIBER MATRIX IN CURED CONCRETE

Getting back to the initial question of whether fiber can replace mesh, we typically say yes–with exceptions. The exceptions are as follows:

1.)   Elevated cast-in-place slabs

2.)   Elevated slabs on metal deck

3.)   Structural slabs-on-grade (mats)

4.)   Where poor subgrade conditions are expected

Keep in mind that if fiber is only used, the subgrade must be properly compacted and prepared in accordance with the geotechnical engineer’s recommendations. Once any differential settlement occurs, the fiber will not prevent cracks from developing and increasing in width.

The dosage will vary based on the type of fiber and end usage of the slab.  In all cases, follow the ACI recommendations for spacing of control joints.  If sawn, they should be done between 8 and 24 hours after placement.

These are general recommendations. Make sure you consult with a structural engineer for your specific conditions before proceeding.

To learn more about Allegheny Design Services (ADS) and structural engineering, visit this link!

Thanks for reading,

David

Written by David Simpson, P.E., SECB, MBA, President, Principal Engineer

dave-simpson2David Simpson’s experience includes over 30 years in structural design and project management for industrial, commercial, institutional and nuclear/chemical facilities utilizing steel, concrete, masonry and wood. His accomplishments include design and construction administration of health care facilities, hotels, schools, shopping centers, aircraft hangars, numerous retail facilities and several forensic engineering assignments. He has professional registrations in D.C., Maryland, Ohio, Pennsylvania, South Carolina, Virginia and West Virginia. Simpson graduated from the West Virginia Institute of Technology with a bachelor’s degree in civil engineering and an MBA from West Virginia University.