Fiber-reinforced concrete (FRC) is a composite material composed of cement or hydraulic cement, water, coarse and fine aggregates, and short and uniformly distributed discontinuous fibers. The fibers can be steel fibers, glass fibers, carbon fibers, polymer fibers, plant fibers, etc. The length usually varies from 3mm to 64mm, and the diameter can vary from a few microns to 1mm. The cross-sectional shape of the fiber can be circular, elliptical, polygonal, triangular, crescent, or square, which mainly depends on the raw materials used and the processing and manufacturing process. Fibers are mainly divided into two categories: coarse fibers and fine fibers. The diameter or equivalent diameter of fine fibers is usually less than 0.3mm, while the diameter or equivalent diameter of coarse fibers is ≥0.3mm. The so-called equivalent diameter is the circular diameter converted from the same cross-sectional area as the circular fiber, that is, (4A/π)0.5.
The volume percentage of fiber in concrete is usually 0.1% to 5%. The size of this volume percentage depends mainly on the ease of mixing of the mixture and the application scenario of the project. For example, secondary stresses caused by shrinkage and temperature changes in concrete are usually controlled and resolved by low dosages of fiber (0.1% to 0.3% by volume). When the fiber content exceeds 0.3%, the mechanical response of fiber concrete will be significantly different from that of ordinary concrete without fiber, mainly in its load-bearing capacity after cracking. The ability of fiber concrete to absorb energy after cracking is called "toughness". When higher dosages of fiber are added to concrete, in addition to toughness after cracking, fiber concrete also shows strain-strengthening characteristics. In other words, this composite material can withstand tensile stresses that exceed those of ordinary concrete itself. In these pseudo-ductile composites, multiple cracks and considerable energy absorption and energy dissipation characteristics are often seen.
Types of Fiber Reinforced Concrete
The American standard ASTM C116/C116M gives four types of fiber concrete: the first is steel fiber concrete (SFRC), which mainly includes stainless steel fiber, alloy steel fiber, and carbon steel fiber; the second is glass fiber concrete (GFRC), which is composed of alkali-resistant glass fiber; the third is synthetic fiber concrete (SynFRC), and the fourth is natural fiber concrete (NFRC).
As can be seen from the table above, the strength and elastic modulus of steel fiber are relatively high, and it is not easy to rust because it is in a highly alkaline environment. The bonding effect between it and the mixture can achieve more effective mechanical anchoring by enhancing the surface roughness and deformation.
Synthetic fibers are mainly non-metallic fibers produced by the development of the petrochemical and textile industries, including various forms of polymers. The following are some synthetic fibers commonly used in precast concrete:
Carbon Fiber
Compared with steel fiber, glass fiber, polypropylene fiber, etc., the advantage of carbon fiber lies in its characteristics, high modulus, heat resistance, chemical stability in alkaline environment, and other corrosive chemical environments; in addition, it has the characteristic of significantly improving mechanical properties.
Nylon Fiber/Polyamide Fiber
This type of fiber has good tensile strength, high toughness, elastic recovery, and good hydrophilicity, and is relatively stable in cement-based alkaline environments.
Polypropylene
This fiber has a low elastic modulus and a low melting point, so it is not suitable for precast concrete products under high-temperature autoclaving. However, due to its low melting point, it can be used to produce refractory materials or products with high fire resistance. There are two types of polypropylene fibers used for concrete reinforcement: monofilaments and fibrillated fibers (stretched fibers). These fibers are hydrophobic and have a large contact angle with water. Therefore, they have a poorer bond with concrete than hydrophilic fibers.
Polyvinyl Alcohol Fiber
This fiber is made of PVA resin through multiple processes of high stretching and has high stiffness and water resistance. The fiber distribution state in the concrete base can be changed through special surface treatment. Unfortunately, PVA fiber has a large thermal shrinkage coefficient, and its shrinkage rate is as high as 4% at 200°C. It has good resistance to alkaline environments and organic solvents, and has little strength loss under long-term ultraviolet radiation.
Glass Fiber
Glass fiber used in concrete must contain a minimum of 16% zirconium dioxide for alkali resistance; other types of glass fiber, such as alkali-free fiber, are not recommended for use in concrete. Glass fiber has a high modulus and high strength, and has a good bond with concrete. The difference between glass fiber reinforced concrete and other fiber reinforced concrete is the fiber content; the former has a fiber volume percentage of 4% to 6%, while the latter, or other fiber volume percentage is about 0.1% to 1%. To achieve a high content of glass fiber, the concrete composition needs a high content of cement, fine aggregate, and almost no coarse aggregate.
The Role of Fiber in Concrete
Quasi-Static Loading and Impact Response
Fibers can effectively improve mechanical properties. Impact drop hammer tests show that the impact strength of polypropylene fiber concrete with a volume content of 0.1% to 0.2% is higher than that of ordinary concrete in both the initial cracking stage and the final fracture stage. There is currently no unified standard test method to determine the compressive strength of fiber concrete, but relevant studies have shown that the axial compressive strength of fiber concrete is 85% to 100% higher than that of ordinary concrete; further studies have shown that under impact loads, fiber concrete does not have obvious peak ductility in the late compression period, which is mainly because the concrete fragments are not bonded to the fibers. Although the test results show that the impact coefficient of steel fiber concrete is polymer fiber concrete is no different from ordinary concrete, with an impact coefficient of about 1.5. In addition, the results show that three-dimensional deformed steel fibers have a more obvious dynamic impact coefficient than two-dimensional deformed steel fibers; however, the tensile strength under dynamic loads and the residual bending strength after cracking have been significantly improved.
The performance of fibers in concrete under impact loads depends largely on the bonding between the fibers and the concrete under displacements with high crack development rates. Studies have shown that with increasing loading rates, steel fiber concrete has a high resistance to crack development, compared to some concrete specimens with polypropylene fibers, but the latter can quickly catch up with the former; it is speculated that this is mainly because polypropylene fibers themselves are more sensitive to strain rates than steel fibers.
Control of Shrinkage Cracks
It is well known that fibers can significantly affect the free shrinkage and other related early-age properties of cement-based composites. Studies have shown that the use of polyethylene fibers with a volume percentage of about 1% can reduce the free plastic shrinkage of concrete by as much as 30%. In addition to free shrinkage, various techniques are also being used to study the effects of fibers on the constrained shrinkage of concrete. The addition of fibers is mainly used to change the width and length of shrinkage cracks in concrete under a constrained environment. The relevant research conclusions are roughly as follows.
1. Fiber material and type have a great influence on shrinkage cracks. For the same volume of fiber content, glass fiber is the most effective in inhibiting crack growth, followed by synthetic fiber.
2. For a given fiber volume fraction and fiber type, longer, smaller-diameter fibers are more effective than shorter, thicker fibers; fibers with a greater degree of geometric deformation on the surface are more effective than undeformed fibers.
3. As for plant fibers, coated or uncoated fibers are only effective when the volume percentage is above 0.3%.
Waterproof and Durable
Precast concrete components are prone to degradation due to sulfuric acid attack, thaw-freeze cycles, alkali-silica reactions, and corrosion of steel bars. In all of these cases, water penetration plays a crucial role. The durability of precast concrete products depends primarily on the rate of water intrusion/penetration. Results show that water permeability, in turn, depends on cracks in the concrete, and an increase in the width of concrete cracks will result in higher water permeability. Fiber reinforcement improves concrete cracking resistance, increases crack surface roughness, and promotes the development of multiple cracks, which significantly reduces concrete permeability. As for stress and stress-induced concrete cracking, results have shown that cracks in ordinary concrete significantly increase its permeability, while the permeability of fiber-reinforced concrete is significantly lower than that of ordinary concrete. As for how fibers improve water resistance, studies have shown that the micropores in ordinary concrete are changed to nanopores due to the addition of fibers.
Rebar corrosion in precast concrete is a significant problem. Chloride contamination in concrete is a major factor, and the mechanisms and processes by which it corrodes the steel are well understood. Unfortunately, cracks in the concrete allow chloride ions and other corrosive chemicals to enter more easily, thus promoting further corrosion. Chloride ions diffuse primarily through capillary water penetration, while chloride diffusion is primarily dependent on water permeability.
