With the many different types of fibrous materials available for use in concrete, it is important to understand that physical properties such as size, shape and surface area, as well as volume fraction (dosage) and distribution, may all have varying influences on the fresh and hardened properties of the concrete matrix. In general, it is well recognized that common, commercially available fibers for concrete do not have a detrimental impact on the strength and durability properties of concrete.
The proper addition and mixing of fibers invariably improve both fresh and hardened concrete properties. However, an apparent effect on concrete workability may be experienced and should be understood in order to ensure a successful project outcome. Batching, mixing, delivery and sampling of fiber-reinforced concrete (FRC) should always be performed in accordance with ASTM C1116/C1116M.
How Fibers Impact Fresh Concrete Properties
Fibers increase the total surface area of materials in the concrete mixture that are coated with paste. Therefore, if no adjustments are made, the visible impact of added fibers on fresh concrete is generally observed as a reduction of workability or slump. This impact can be minor or significant depending on the application, physical properties of the fiber, dosage and initial concrete mixture proportions.
At commonly used dosages, synthetic microfibers will generally not need any modifications to the mixture proportions and typically only alter the slump of concrete by minor amounts. On the other hand, macrofibers—either synthetic or steel—can have a larger impact on slump, especially when used at higher dosages. These impacts can be mitigated by adjustments to the concrete mixture proportions, such as additional fine material (cementitious materials or sand) to account for the additional surface area provided by the fiber, or through the use of chemical admixtures to increase workability. The fiber supplier’s specific recommendations regarding mixture adjustments should be followed to ensure adequate slump and workability for a given project.
A primary function of fibers when concrete is in the fresh or plastic state is to provide early-age tensile resistance to the formation of plastic shrinkage cracks. These cracks can be caused by rapid evaporation of bleed water from the concrete surface due to prevailing ambient conditions, such as temperature, relative humidity and wind speed. The fibers can bridge the cracks and, in some cases, provide enough resistance to prevent their formation by redistributing the induced tensile stresses.
Fibers may also delay the upward movement of bleed water in concrete slabs and other flatwork applications, depending again on the dosage rate. In addition to reducing bleed water channels and the potential for settlement cracking, this may have a beneficial impact on concrete as it will better maintain internal humidity and moisture of concrete during the setting period. Fibers also improve the cohesiveness of concrete and help to reduce rebound losses in shotcrete applications.
Fiber material type, architecture, dimensions and dosage, as well as the methods of placement, consolidation and finishing, may each affect the surface finish of FRC for a given project. In general, the addition of microfibers or less than 5 lb/yd3 (3 kg/m3) of a synthetic macrofiber will not require adjustments to the mixture proportions. However, higher dosages of synthetic and steel macrofibers may require mixture adjustments. A laser screed or vibrating screed is recommended for finishing of industrial, commercial and warehouse floors where high dosages of synthetic macrofibers and steel fibers are used. However, for a given project, the recommendations of the fiber supplier should be followed to ensure a satisfactory surface finish.
For more information, refer to FRCA’s FIP 2 – Batching Fiber-Reinforced Concrete and FIP 3 – Placing, Pumping & Finishing Fiber-Reinforced Concrete.
How Fibers Impact Hardened Concrete Properties
Fiber reinforcement provides hardened concrete with multiple benefits that are maximized when the concrete is properly mixed, placed, finished and cured. These benefits include:
- Reduced Cracking & Crack Width Reduction: Fibers modify the cracking mechanism from macro-cracking to micro-cracking. They also help to distribute the internal stresses caused by drying, thereby reducing the potential for cracking. In addition, fibers bridge cracks and, at an appropriate dosage, will hold a crack tight in the same capacity as conventional welded-wire reinforcement and light-gage rebar. Thus, fibers are used as an economical alternate to temperature and shrinkage reinforcement for a safer working environment.
- Increased Ductility & Toughness: This is particularly true with the use of steel or synthetic macrofibers, due to the interlocking network of fibers that hold concrete together when it cracks. Consequently, fibers increase load-carrying capacity of fully supported concrete elements after cracking, and boost the impact, fatigue and abrasion resistance of concrete.
- Potential for Concrete Thickness Reduction: Steel or synthetic macrofibers can increase moment capacity of fully supported slabs and, depending on dosage and increased toughness provided, may permit thickness reduction in a slab-on-ground or shotcrete application.
- Improved Explosive Spalling Resistance: Synthetic microfibers, of appropriate size, will melt in an extreme fire event and provide a relief system for steam and entrapped gases to escape from the concrete matrix, thus minimizing or preventing explosive spalling.
Depending on the material and dosage of fiber, other key aspects such as permeability, electrical and thermal conductivity, scaling resistance and corrosion resistance of concrete can all be improved. Resistance to freezing and thawing is not impacted by fibers and, as with non-FRC, it requires an adequate air-void system in the concrete matrix.
Because of all of these improvements in the hardened state of concrete, fiber reinforcement generally improves the long-term integrity and durability of concrete—which ultimately results in longer service life and less maintenance on concrete structures.
FRC will improve both the fresh and hardened properties of concrete when properly specified and used. Manufacturer recommendations should always be followed for proper selection, dosage, mixture adjustments, method of addition, mixing and finishing.
For more information, refer to FRCA’s Fibers in Practice (FIP) materials, ACI Committee 544 – Report on Fiber-Reinforced Concrete, and the National Concrete Pavement Technology Center (CP Tech Center).
- ACI 544.1 R-96 – Report on Fiber-Reinforced Concrete
- ACI 544.3 R-08 – Guide for Specifying, Proportioning & Production of Fiber-Reinforced Concrete
- ACI 544.4 R-18 – Guide to Design with Fiber-Reinforced Concrete
- ACI 544.5 R-10 – Report on the Physical Properties & Durability of Fiber-Reinforced Concrete
- ASTM C1116/C1116M – Standard Specification for Fiber-Reinforced Concrete
- Fiber-Reinforced Concrete for Pavement Overlays: Technical Overview, Final Report, National Concrete Pavement Technology Center (CP Tech Center), Ames, IA, April 2019
- Technical Brief: Fiber-Reinforced Concrete for Pavement Overlays, National Concrete Pavement Technology Center (CP Tech Center), Ames, IA, March 2019
- Webinar: Overview of Macrofiber Software & Guidelines for Concrete Overlay Design