News Detail

Fiber in My Diet Concrete

8/1/2022

Dietitians and nutritionists recommend fiber in one’s diet to help maintain good health or treat health conditions. Likewise, fiber in concrete is good and that will be the subject this month—in line with my focus on expediting technology transfer. Fibers are transforming the industry and gone are the days when phrases like “it doesn’t hurt my concrete” were used to describe fibers. So, what’s going on in the “fiber world?”

Generally, fibers—micro or macro—do not change the behavior of uncracked concrete. However, fibers bridge cracks, carry tensile stresses, and provide post-crack residual strength in hardened concrete. If one were to survey design engineers and concrete professionals regarding fiber use, their responses would likely fall into one of these categories—to minimize plastic shrinkage cracking, to replace temperature and shrinkage reinforcement, to hold cracks tight, and to reduce rebound and sagging in shotcrete. Indeed, fiber reinforcement, specifically macrofibers, can easily be used where conventional steel reinforcement is accepted as secondary reinforcement. This is recognized in ACI documents, specifically, ACI 302.1R, “Guide to Concrete Floor and Slab Construction”; ACI 330.2R, “Guide for the Design and Construction of Concrete Site Paving for Industrial and Trucking Facilities”; ACI 332.1R, “Guide to Residential Concrete Construction”; ACI 360R, “Guide to Design of Slabs-on-Ground”; and ACI 544.1R, “Report on Fiber-Reinforced Concrete.” Increasingly though, fibers are being used in: jointless slabs-on-ground or to reduce saw-cut joints; residential walls in nonseismic areas; engineered cementitious composites (ECC); ultra-high-performance concrete (UHPC); and, specific to steel fibers, as shear reinforcement in beams in accordance with ACI 318-19 performance requirements. Fibers are also being used either as partial or complete replacement of steel reinforcement in utility precast concrete elements.

Fibers are produced from a wide variety of materials including steel, alkali-resistant glass, synthetic polymers such as polyolefin/polypropylene and nylon, polyvinyl alcohol (PVA), natural materials (jute, sisal, hemp, and cellulose fibers), and carbon. In addition, commercially available fibers vary in size, geometry, and other characteristics that influence their performance in concrete. Therefore, project specifications for fiber-reinforced concrete (FRC) should be based on the performance requirements needed for the application using design methodologies such as those provided in ACI 544.4R, “Design Guide for Fiber-Reinforced Concrete.” These design methodologies use parameters obtained from industry-accepted standard test methods that should be performed by independent accredited laboratories with demonstrated expertise in testing FRC. The most common and preferred tests for FRC in North America are ASTM C1609/C1609M, “Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading)” and, for shotcrete applications, ASTM C1550, “Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete (Using Centrally Loaded Round Panel).” Similar test methods exist in European standards.

Design engineers and concrete professionals should be particularly wary of nonstandard calculations (and test methods) used by some fiber suppliers in their bids to offer low fiber dosages for competitive reasons. Such practices are a disservice to design engineers, contractors, and the fiber industry, and underscore the need for effective technology transfer to ensure that concrete professionals gain pertinent knowledge regarding FRC. A wealth of information exists in ACI documents such as those from ACI Committee 544, Fiber-Reinforced Concrete, but they need to be synthesized into short, simple-to-read documents for the benefits of interested parties. The Fiber Reinforced Concrete Association (FRCA) based in North America and the Macro Synthetic Fibre Association, a global group of fiber suppliers headquartered in Switzerland, are committed to furthering the development, knowledge, and market acceptance of FRC. Technology transfer on FRC can also be accomplished through student activities such as ACI’s bowling ball and the American Society of Civil Engineers’ (ASCE) concrete canoe competitions. It’s always refreshing to attend these events and listen to the students describe how they used fibers.

Recent supply chain issues with steel reinforcement are leading to increased specification and approval of fibers for use in slabs-on-ground and residential walls by design engineers at the request of contractors. In addition to its use in closure placements with precast deck panels to expedite bridge projects, UHPC is making it possible for precast producers to optimize precast member sizes, produce aesthetically phenomenal architectural pieces and, in the case of precast pile producers, to manufacture prestressed concrete piles that compete effectively with steel H-piles. Truly, fibers are transforming the concrete industry!

Charles K. Nmai