FIBER: GOOD FOR THE CONCRETE DIET?

WILLIAM C. PANARESE

Inadequately tested for design criteria and methods or poised for a golden age after three decades of extensive research and development: The debate over fiber reinforcement of concrete continues. Fibers can control cracking and improve strength and ductility. But their individual properties and the lack of performance data currently restrict fiber-reinforced concrete to a narrow range of applications.

Research, product development and marketing steer the growth of fiber reinforcement, a rapidly evolving technology in concrete practice. Properly applied, current technology can yield a stronger, mechanically superior and more economically produced concrete, but deciding when to choose fiber-reinforced concrete (FRC) over conventional reinforcement can be difficult.

Like admixtures, the concrete industry's other high-geared product- driven quest, FRC spawns sharply conflicting views over performance assessment and value engineering. Few dispute the potential weight savings, ease of use, wider joint spacing, added ductility and strength, and greater toughness from various types of fiber reinforcement. But many question such issues as fiber dosages and the extent of performance characteristics. Almost everyone with a hand in buildingcodes and specifications agrees that each category of commercially viable fibers warrants additional field and lab testing.

Present codes and standards limit the engineering basis for fiber reinforcement to essentially nonstructural, non-load-bearing applications - notwithstanding, for example, slabs on grade and floor slabs. The limitations reflect a lack of design and material-performance criteria. ASTM's recently published Standard Specification for Fiber-Reinforced Concrete and Shotcrete classifies fiber type but, unlike specifications for conventional steel-reinforcing materials, does not define most individual fibers' mechanical or material properties.

Practical design guidelines from the American Concrete Institute (ACI) Committee 544 and the Precast/Prestressed concrete Institute cover some FRC applications. In the absence of ACI and ASTM specifications and test methods for individual fiber and FRCs, however, fiber reinforcements' structural values and performance characteristics cannot be fully acknowledged and exploited. Some answers and clarifications regarding FRC's structural and performance parameters could be forthcoming in ACI's 544's new State of the Art Report on Fiber Reinforced Concrete, the release of which has not been announced.

Regardless of new standards or design guidelines, fundamental differences in the engineering qualities of conventional and fiber-reinforcing systems remain:

• Fibers are distributed throughout a given cross section, while steel rebar and wire are placed where required.

• Fiber reinforcement bears a fundamental engineering inefficiency as many fibers are not positioned to resist tensile stresses from applied loads.

• In contrast to rebars' or wires' continuous placements, fibers are most often relatively short and closely spaced.

• Higher reinforcement ratios (area of reinforcement to area of concrete) are almost always more easily achieved when conventional techniques are used.

Fiber reinforcements principal advantage is in the fabrications process: components, pavements and non-load-bearing structures can be cast or sprayed without the need for labor-intensive bar or wire placement. Fibers best suit thin-section and irregular shapes where correct placement or conventional reinforcement is difficult. They also offer considerable potential weight savings, as fibre-reinforced sections can have strengths equivalent to thicker conventionally reinforced sections.

CIVIL ENGINEERING