High Performance Fiber Reinforced Composites
Development of High-Performance Fiber Reinforced Asphalt and Concrete
Stronger and more durable construction materials improve safety/serviceability and extend service life of our infrastructure with less maintenance efforts. Cement and asphalt concrete are particle reinforced composites, and a proper use of fiber and/or filler with thorough understanding of their reinforcing mechanisms is expected to bring significant improvements in structural performance.
Superior reinforcing effects of randomly oriented discrete fibers in Portland cement concrete have been widely investigated, and the current state-of-the-art of fiber reinforced concrete (FRC) is in the pilot field application stage. As a part of the project sponsored by the Arizona DOT, I have conducted a series of tension, compression, and bending tests to verify a model predicting the flexural behavior from uniaxial constitutive behavior of FRC.
In the case of asphalt concrete, fibers and fillers have been used as stabilizers, but their effects on improving structural performance are controversial among active investigators. My recently published paper on fiber reinforced asphalt mixtures (FRA) suggested a new hypothesis that the fiber-aggregate interlocking provides a stronger reinforcing effect than the fiber-binder bonding. Based on the hypothesis, an idea of a novel fiber-particle composite that can replace traditional cement/asphalt concrete was developed. The improvements in indirect tensile strength (62.5%) and toughness (895%) at cold temperature shown in the paper are the highest among the reported investigations on FRA.
Superior reinforcing effects of randomly oriented discrete fibers in Portland cement concrete have been widely investigated, and the current state-of-the-art of fiber reinforced concrete (FRC) is in the pilot field application stage. As a part of the project sponsored by the Arizona DOT, I have conducted a series of tension, compression, and bending tests to verify a model predicting the flexural behavior from uniaxial constitutive behavior of FRC.
In the case of asphalt concrete, fibers and fillers have been used as stabilizers, but their effects on improving structural performance are controversial among active investigators. My recently published paper on fiber reinforced asphalt mixtures (FRA) suggested a new hypothesis that the fiber-aggregate interlocking provides a stronger reinforcing effect than the fiber-binder bonding. Based on the hypothesis, an idea of a novel fiber-particle composite that can replace traditional cement/asphalt concrete was developed. The improvements in indirect tensile strength (62.5%) and toughness (895%) at cold temperature shown in the paper are the highest among the reported investigations on FRA.
Fiber Reinforcing Mechanism
Composites are multiphase materials designed to have more desirable material properties than a single phase material. Traditional concept classifies the components of composites into continuous phase (matrix or binder) and dispersed phases (particles or fibers). The failure strength of particle reinforced composites (cement concrete or asphalt concrete) depends primarily on the tensile resistance of the continuous phase because, even under compressive loads, the failure is caused by tensile and/or shear stress developed from the compression (split tension or shear band).
In case of densely packed particles with a compliant binder (e.g., asphalt concrete, treated base/subbase, and wet soil), the coarse aggregate particles form a skeleton structure that carries the compressive load through the stone-on-stone contact. On the other hand, the load carrying mechanism through the skeleton structure will not occur if a stiff binder is used with a relatively large volume in the composite, such as cement concrete. When fibers are added in this stiff binder composite (e.g., fiber reinforced concrete), the reinforcing effect of fibers depends on the adhesion and friction between the stiff binder and fiber. This fiber reinforcing mechanism in the stiff matrix is explained with a single fiber pull-out model (Wang et al., 1988; Naaman et al., 1991a; 1991b; Curtin, 1991; Li et al., 2002; Manoharan et al., 2009). On the other hand, when fibers exist near a stone-on-stone interface in a compliant binder, the fibers and aggregates are interlocked under compressive loading. Then the split tension, which causes cracking, is resisted both by the bond strength of the binder, and by the friction between the particle and fiber. As the compressive stress increases, the interlock and the corresponding friction becomes stronger. In this circumstance, fracture of the fiber-particle reinforced composite is accompanied by the breaking of the fiber-aggregate-binder interlock mechanism, resulting in pulverization of the surrounding particles (aggregate) as the fiber pulls-out. This means that well-dispersed fibers interlocked between densely packed particles can provide additional resistance to split tension and formation of shear band beyond the adhesion of the binder, so that the fiber-particle composite (with or without compliant binder) will have high compressive strength. To activate and maximize the reinforcing mechanism of fiber-particle interlocking, the following conditions will be needed:
In case of densely packed particles with a compliant binder (e.g., asphalt concrete, treated base/subbase, and wet soil), the coarse aggregate particles form a skeleton structure that carries the compressive load through the stone-on-stone contact. On the other hand, the load carrying mechanism through the skeleton structure will not occur if a stiff binder is used with a relatively large volume in the composite, such as cement concrete. When fibers are added in this stiff binder composite (e.g., fiber reinforced concrete), the reinforcing effect of fibers depends on the adhesion and friction between the stiff binder and fiber. This fiber reinforcing mechanism in the stiff matrix is explained with a single fiber pull-out model (Wang et al., 1988; Naaman et al., 1991a; 1991b; Curtin, 1991; Li et al., 2002; Manoharan et al., 2009). On the other hand, when fibers exist near a stone-on-stone interface in a compliant binder, the fibers and aggregates are interlocked under compressive loading. Then the split tension, which causes cracking, is resisted both by the bond strength of the binder, and by the friction between the particle and fiber. As the compressive stress increases, the interlock and the corresponding friction becomes stronger. In this circumstance, fracture of the fiber-particle reinforced composite is accompanied by the breaking of the fiber-aggregate-binder interlock mechanism, resulting in pulverization of the surrounding particles (aggregate) as the fiber pulls-out. This means that well-dispersed fibers interlocked between densely packed particles can provide additional resistance to split tension and formation of shear band beyond the adhesion of the binder, so that the fiber-particle composite (with or without compliant binder) will have high compressive strength. To activate and maximize the reinforcing mechanism of fiber-particle interlocking, the following conditions will be needed:
- Particles are densely packed and in contact with each other to form a skeleton structure.
- Binder is compliant enough to ensure direct interaction between fiber and particle.
- Fibers have a proper length and thickness to be interlocked between the particles forming the skeleton structure, and have sufficient strength, stiffness, and friction to resist tensile deformation.
- Since fiber-particle interlocking is strengthened by the compressive loads, the primary load has to be compressive.
Performance Improvement of Bridge Asphalt Plug Joint
An asphalt plug joint (APJ) is a type of bridge expansion joint that is becoming popular with some State Departments of Transportation in the United States. It is made of flexible asphalt concrete usually comprising 20% asphalt and 80% aggregates by weight. The APJ material is placed into a prepared space between pavements permitting a smooth ride across the joint while accommodating thermal movements of the bridge deck at the same time. The typical size of an APJ is 500 mm wide and 100 mm deep and its allowable movement without cracking at the lowest operating design temperature is ±20 mm.
By providing a smooth transition across an expansion joint, an APJ offers better bridge surface flatness than other types of joints. Simplicity and low cost of its installation are additional important advantages of APJs. On the other hand, APJs frequently suffer from premature failure, sometimes as early as 6 months after installation, even though they are generally expected to have a life of about 6-7 years. The overall cost of replacing a damaged APJ can exceed the cost of a new APJ installation in the United Kingdom. According to a survey by Bramel et al. (1999), 41 states in the US have installed APJs. Of those, 23 states still use APJs for either new construction or retrofit, without geographic preference. Premature failure is one of the important problems hindering the widespread use of APJs in the US.
Through detailed finite element analysis employing nonlinear viscoelastic material model, the behavior and vulnerabilities of conventional APJ are thoroughly investigated. Consequently, an improved and practically feasible APJ design that mitigates stress and strain concentrations under operating thermal and traffic loads is proposed. Based on the information obtained from the parametric studies, the superiority of the proposed APJ design over the traditional design is demonstrated.
By providing a smooth transition across an expansion joint, an APJ offers better bridge surface flatness than other types of joints. Simplicity and low cost of its installation are additional important advantages of APJs. On the other hand, APJs frequently suffer from premature failure, sometimes as early as 6 months after installation, even though they are generally expected to have a life of about 6-7 years. The overall cost of replacing a damaged APJ can exceed the cost of a new APJ installation in the United Kingdom. According to a survey by Bramel et al. (1999), 41 states in the US have installed APJs. Of those, 23 states still use APJs for either new construction or retrofit, without geographic preference. Premature failure is one of the important problems hindering the widespread use of APJs in the US.
Through detailed finite element analysis employing nonlinear viscoelastic material model, the behavior and vulnerabilities of conventional APJ are thoroughly investigated. Consequently, an improved and practically feasible APJ design that mitigates stress and strain concentrations under operating thermal and traffic loads is proposed. Based on the information obtained from the parametric studies, the superiority of the proposed APJ design over the traditional design is demonstrated.