Design and Control of Concrete Mixtures CHAPTER 10 Fibers Design and Control of Concrete Mixtures CHAPTER 10 Design and Control of Concrete Mixtures, 16th edition, Chapter 10 - Fibers

Overview Advantages and Disadvantages Types, Properties and Effects Multiple Fiber Systems (Hybrid Fibers) Fiber-Reinforced Polymers This module will discuss the use of fibers in concrete including their types, properties, and effects on concrete properties.

Fibers Fibers have been used in construction materials for centuries in ready mixed concrete, precast concrete, and shotcrete. The main factors that control the performance of the composite material are physical properties of fibers and matrix and strength of bond between fibers and matrix. Fibers are typically added to concrete in low volume dosages (often less than 1%), and have been effective in reducing plastic shrinkage cracking. Fiber concretes are best suited for thin section shapes where correct placement of conventional reinforcement would be extremely difficult.

Types of Fibers Microfiber Macrofiber ̶ equivalent diameter less than 0.3 mm (0.012 in.) Macrofiber ̶ equivalent diameter greater than or equal to 0.3 mm (0.012 in.) Fibers are generally described as micro or macro, depending on the equivalent diameter of the fiber. A microfiber is defined as a fiber with an equivalent diameter less than 0.3 mm (0.012 in.) for use in concrete and a macrofiber has an equivalent diameter greater than or equal to 0.3 mm (0.012 in.).

Types of Fibers Type I Steel Fiber-Reinforced Concrete – Contains stainless steel, alloy steel, or carbon steel fibers conforming to ASTM A820 Type II Glass Fiber-Reinforced Concrete – Contains alkali-resistant (AR) glass fibers conforming to ASTM C1666 Type III Synthetic Fiber-Reinforced Concrete – Contains synthetic fibers including polyolefin fibers. Polyolefin fibers shall conform to ASTM D7508 Type IV Natural Fiber-Reinforced Concrete – Contains natural fibers including cellulose fibers. Cellulose fibers shall conform to ASTM D7357 ASTM C1116, Standard Specification for Fiber Reinforced Concrete, classifies fiber-reinforced concrete into four different categories based on the material type of fiber used.

Properties of Selected Fibers Fibers made from steel, plastic, glass, and natural materials (such as wood cellulose) are available in a variety of shapes, sizes, and thicknesses. This table lists many commonly available types of fibers and their properties.

Steel Fibers Steel fibers are short, discrete lengths of steel with an aspect ratio (ratio of length to diameter) from about 20 to 100, and with a variety of cross sections and profiles. Some steel fibers have hooked ends to improve resistance to pullout from a cement-based matrix. Steel-fiber volumes used in concrete typically range from 0.25% to 2%.

Types of Steel Fibers ASTM A820-Standard Specification for Steel Fibers for Fiber-Reinforced Concrete Type I Cold-drawn wire fibers Type II Cut sheet fibers Type III Melt-extracted fibers Type IV Mill cut fibers Type V Modified cold-drawn wire fibers There are 5 types of steel fibers listed in ASTM A820. Type I: the most commercially available, manufactured from drawn steel wire Type II: manufactured as the name implies: steel fibers are laterally sheared off steel sheets Type III: manufactured with a relatively complicated technique where a rotating wheel is used to lift metal from a molten metal surface by capillary action. The extracted molten metal is then rapidly frozen into fibers and spun off the wheel. The resulting fibers have a crescen-shaped cross section. Type IV: manufactured by a high-speed rotor equipped with special cutting plates which comminute the sample material through special milling techniques. This results in fibers with a triangular longitudinal twist, rough surface, hooked end and a soft longitudinal torsion. Type V: made by cutting cold drawn wires into short fibers according to regulated length with a cutting knife, punch or rotary tool.

Steel Fibers Steel fibers are most commonly used in industrial floors. They have also been used in bridge decks, airport runway/taxi overlays, and highway pavements. They are protected from corrosion by the alkaline environment in the cement matrix.

Glass Fibers ASTM C1666- Standard Specification for Alkali Resistant (AR) Glass Fiber for GFRC and Fiber-Reinforced Concrete and Cement The standard for AR-glass fibers is ASTM C1666, Standard Specification for Alkali Resistant (AR) Glass Fiber for GFRC and Fiber-Reinforced Concrete and Cement. The single largest application of glass-fiber concrete has been the manufacture of exterior building façade panels.

Glass Fibers- Alkali Reactivity Calcium hydroxide penetration of fiber bundles Reduces fiber tensile strength Lowers compressive strength Inhibiting fiber pullout Alkali reactivity and cement hydration are the basis for the following two widely held theories explaining strength and ductility loss, particularly in exterior glass fiber concrete: Alkali attack on glass-fiber surfaces reduces fiber tensile strength, and subsequently, lowers compressive strength Ongoing cement hydration causes calcium hydroxide penetration of fiber bundles, thereby increasing fiber-to-matrix bond strength and embrittlement; the latter lowers tensile strength by inhibiting fiber pullout

Synthetic Fibers Synthetic fibers are man-made fibers including acrylic, aramid, carbon, nylon, polyester, polyethylene, and polypropylene. Synthetic fibers can reduce plastic shrinkage and subsidence cracking and may help strengthen concrete after it cracks. Synethic microfibers are typically either monofilaments or fibrillated in nature.

Synthetic Fibers Polypropylene fibers, the most popular of the synthetics, are chemically inert, hydrophobic, and lightweight. They are produced as continuous cylindrical monofilaments that can be cut to specified lengths or cut as films and tapes and formed into fine fibrils of rectangular cross section. Used at a rate of at least 0.1 percent by volume of concrete, polypropylene fibers reduce plastic shrinkage cracking and subsidence cracking over steel reinforcement. Polypropylene fibers can help reduce spalling of high-strength, low-permeability concrete exposed to fire in a moist condition.

Synthetic Macrofiber Synthetic macrofiber dosages of 3 to 4.5 kg/m3 (5 to 7.5 lb/yd3) have been used to either extend joints, without any intermediate sawcut contraction joints, or to produce jointless slabs-on-ground. Due to their efficiency in reducing crack width, synthetic macrofibers, and also steel fibers, have been used to extend joint spacings beyond the typical recommendations.

Synthetic Fibers Acrylic fibers – asbestos replacement Aramid fibers – 2.5x stronger than E-glass, 5x stronger than steel, good temperature stability Carbon fibers – high strength and elastic modulus Nylon fibers – good tenacity, toughness, elastic recovery Synthetic fibers also used in stucco and mortar Acrylic fibers are used in cement board and roof-shingle production, where fiber volumes of up to 3% can produce a composite with mechanical properties similar to that of an asbestos-cement composite. Aramid fibers have high tensile strength and a high tensile modulus. Aramid fibers are two and a half times as strong as E-glass fibers and five times as strong as steel fibers. In addition to excellent strength characteristics, aramid fibers have excellent strength retention up to 160°C (320°F), dimensional stability up to 200°C (392°F), static and dynamic fatigue resistance, and creep resistance. Carbon fibers were developed primarily for their high strength and elastic modulus and stiffness properties. Compared with most other synthetic fibers, the manufacture of carbon fibers is expensive and this has limited their commercial development. Only two types of nylon fiber are currently marketed for use in concrete, nylon 6 and nylon 66. Nylon fibers exhibit good tenacity, toughness, and elastic recovery.

Natural Fibers Unprocessed natural fibers Processed– wood fibers ASTM D7375-Standard Specification for Cellulose Fibers for Fiber-Reinforced Concrete ASTM D6942- Standard Test Method for Stability of Cellulose Fibers in Alkaline Environments Unprocessed natural fibers Processed– wood fibers Mud bricks reinforced with straw and mortars reinforced with horsehair are just a few examples of how natural fibers were used historically as a form of reinforcement. Relevant ASTM standards include ASTM D7357, Standard Specification for Cellulose Fibers for Fiber-Reinforced Concrete, and D6942, Standard Test Method for Stability of Cellulose Fibers in Alkaline Environments. Products made using portland cement and unprocessed natural fibers, such as coconut coir, sisal, bamboo, jute, wood, and vegetable fibers, have good mechanical properties. However, they have some deficiencies in durability. Processed natural fibers are typically wood fibers produced during the kraft process and have relatively good mechanical properties compared to many manmade fibers such as polypropylene, polyethylene, polyester, and acrylic.

Multiple Fiber Systems Hybrid-fiber concrete Macro- and microsteel fibers Steel and polypropylene For a multiple fiber system, two or more fibers are blended into one system. The hybrid-fiber concrete combines macro- and microfibers, or blends of steel macrofibers and synthetic microfibers. Hybrid fibers provide fibers of varying length that result in a closer fiber-to-fiber spacing, which reduces microcracking and increases tensile strength. A blend of steel and polypropylene fibers has also been used for some applications. This system is purported to combine the toughness and impact-resistance of steel fiber concrete with the reduced plastic cracking of polypropylene fiber concrete.

Multiple Fiber Systems (Hybrid Fibers) Hybrid fibers combine the toughness and increased post-crack flexural toughess provided by macrofibers with the reduced plastic cracking benefit obtained from microfibers. As is typical with fibers, the concrete with blended fibers had a lower slump compared to plain concrete, but had enhanced elastic and post-elastic strength. Work by Ostertag and others has shown that a hybrid fiber reinforced concrete composite (called HyFRC) had significantly improved flexural performance and damage resistance over plain concrete and reinforced concrete with the same steel reinforcement ratio. Damage resistance of HyFRC panel after being subjected to a steel projectile with velocity of 167 m/s (shown on left). Damage of a plain concrete panel after being subjected to a steel projectile with velocity of 127 m/s (shown on right).

Flexural Performance of Fiber Types The graph shows the flexural performance of hybrid fiber reinforced concrete composite (HyFRC), reinforced HyFRC (reinforced with conventional steel reinforcing bar), compared to plain concrete and reinforced concrete with the same steel reinforcement ratio. Developed at the University of California, Berkeley, by Ostertag and others, HyFRC includes coarse aggregate and three fibers with a total fiber volume fraction of 1.3% (0.2% PVA microfibers, 0.5% steel macrofibers 30 mm in length, and 0.8% steel macrofibers 60 mm in length). Ostertag and others 2015

Fiber-Reinforced Polymers Fiber-reinforced polymers (FRP) are composite materials that typically consist of strong fibers embedded in a resin matrix, with the fibers providing strength and stiffness and carrying the most of the loads, while the matrix acts to bond and protect the fibers, and to provide shear resistance within the material. FRP can be manufactured in shapes that mimic steel rebar. ACI Committee440

Summary Types, Properties and Effects Multiple Fiber Systems (Hybrid Fibers) Fiber-Reinforced Polymers

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