Keru Wu

MECHANICAL PROPERTIES OF HYBRID FIBER-REINFORCED CONCRETE AT LOW FIBER VOLUME FRACTION

Abstract Concretes containing different types of hybrid fibers at the same volume fraction (0.5%) were compared in terms of compressive, splitting tensile, and flexural properties. Three types of hybrid composites were constructed using fiber combinations of polypropylene (PP) and carbon, carbon and steel, and steel and PP fibers. Test results showed that the fibers, when used in a hybrid form, could result in superior composite performance compared to their individual fiber-reinforced concretes. Among the three types of hybrids, the carbon–steel combination gave concrete of the highest strength and flexural toughness because of the similar modulus and the synergistic interaction between the two reinforcing fibers.

Conductivity of carbon fiber reinforced cement-based composites

The conductive behavior of carbon fiber cement-based composites is presented. The influence of carbon fiber volume, size, cement-based matrix, relative humidity and curing age on the characteristic of system were studied. The relationship between conductivity and volume fraction of carbon fiber indicated that the statistical percolation theory is suitable and applicable for the change rule of conductivity of system with the volume of carbon fiber. Based on the classic percolation theory, the percolation threshold of carbon fiber cement-based composites was determined as φ=φc2 and the conductive mechanism changes from electron tunneling conduction to ohmic contacting conduction. The studies have offered basic theory for smart cement-based composites.

Effect of coarse aggregate type on mechanical properties of high-performance concrete

Tests were carried out to study the effect of the coarse aggregate type on the compressive strength, splitting tensile strength, fracture energy, characteristic length, and elastic modulus of concrete produced at different strength levels with 28-day target compressive strengths of 30, 60, and 90 MPa, respectively. Concretes considered in this paper were produced using crushed quartzite, crushed granite, limestone, and marble coarse aggregate. The results show that the strength, stiffness, and fracture energy of concrete for a given water/cement ratio (W/C) depend on the type of aggregate, especially for high-strength concrete. It is suggested that high-strength concrete with lower brittleness can be made by selecting high-strength aggregate with low brittleness.

Fracture process zone of notched three-point-bending concrete beams

Abstract By measuring and analyzing the load-deflection curve and acoustic emission characteristics of notched three-point-bending concrete beams with different notch depths, the length of the fracture process zone (FPZ) was calculated from the difference between the equivalent crack lengths obtained by different kinds of equivalence. Then, the evolution of the FPZ was quantitatively described. It was found that length of the FPZ is not a material parameter; it is greatly influenced by the specimen size. But in a relative sense, the influence of the specimen size can be eliminated. Also, the evolution of the FPZ in specimens of different sizes is almost the same. When the crack length is small, the FPZ increases linearly with the crack extension. As the crack extends to half of the ligament, the FPZ reaches its maximum size. Thereafter, the FPZ moves ahead and shrinks, but the ratio of its length to the length of the residual ligament remains constant, approximately equal to 0.77.

2-2 Piezoelectric cement matrix composite: Part II. Actuator effect

In this paper, experimental results of the actuator effect of a 2-2 piezoelectric cement matrix composite are presented. A desirable actuator effect was observed in the composite. In the first part of this study, the actuator effect of the composite under free conditions was studied. It was found that the amplitude of the response increases, but the phase angle of the response decreases, both almost linearly with the frequency of the actuator effect. The second part of the study focused on the behavior of the actuator in a structure. Distinct differences were observed in the behaviors of the composite under free conditions and in a structural system (simulated by precompression in the frame of an MTS machine). Clearly, the behavior of the actuator in a structure is influenced by factors including the properties of other components of the structure and the interaction between the actuator and other components, which was represented here by the precompression level in the experiment.

Study of the influence of aggregate size distribution on mechanical properties of concrete by acoustic emission technique

Acoustic emission (AE) has been widely used in concrete research as a dynamic nondestructive test method. In this study, the AE characteristics of concrete with different aggregate size distributions under uniaxial compression and three-point-bending were studied. Parameters such as maximum aggregate size, compressive strain and fracture energies of different concretes were also measured and discussed. The test results show that the characteristics of AE signals from concrete can illustrate the failure process in both compression and three-point-bending. The fracture energy increases with increment of the maximum aggregate size. Moreover, there is a good relationship between AE hits and fracture energy.

Study on the AE characteristics of fracture process of mortar, concrete and steel-fiber-reinforced concrete beams

The acoustic emission (AE) signals from mortar, concrete and steel fiber reinforced concrete beams during the entire fracture process were recorded and analyzed. Different filters were set on the AE signal duration based on the characteristic of amplitude distribution. From the value of AE signal amplitude, which corresponds to the occurrence of the peak for AE hits, the AE signals from mortar, concrete and steel reinforced concrete were divided into five, seven and nine sections, respectively. The relationship between the AE signal section and the failure mechanism of these materials, analyzed on the meso-structure level was determined. Based on the experiments, the AE characteristics of each failure mechanism are given. The results show that the AE technique is a valuable tool to study the failure mechanism of concrete.

A quantitative study on the surface crack pattern of concrete with high content of steel fiber

The mechanical properties and digital image analysis of slurry-infiltrated fiber reinforced concrete (SIFCON) were investigated experimentally. Fractal dimension is used as a parameter to characterize the crack pattern on the surface of SIFCON. It is found that there exists fractal phenomenon for different fiber contents of SIFCON, fractal dimension can be a parameter to characterize crack pattern on the surface of SIFCON quantitatively, and there exists a good correlation between mechanical properties and fractal dimension.

Preparation and solar reflectance spectra of chameleon-type building coatings ☆

Abstract In this paper, the preparation of chameleon-type building coatings was investigated. The reversible thermochromic properties of the chameleon-type building coatings at normal temperatures were measured, and their solar reflectance spectra were measured. The results showed that the colors of the chameleon-type building coatings could be changed reversibly between red, violet etc. below 18°C and white above 18°C. The solar reflectance spectra of the coatings showed that they could absorb more solar energy below 18°C than above 18°C, which indicated that the coatings had transformed between light-absorbing and light-reflecting at normal temperatures. The characteristics of the coatings could be used to create a thermally comfortable building environment.

Effect of fracture path on the fracture energy of high-strength concrete

Abstract The paper indicates that there may not exist a direct relationship between fracture energy and compressive strength. The fractal theory is used to quantitatively study the fracture surface and the relationship between fracture energy and fracture characteristics. The test results show that fractal dimension increases with the increase of maximum aggregate size. For the concrete with higher water–binder ratio, fractal dimension increases more rapidly than that for the concrete with lower water–binder ratio. For the same water–binder ratio, fracture energy of concrete increases with the increase of fractal dimension. Fracture energy for lower water–binder ratio increases with fractal dimension more rapidly than that for higher water–binder ratio. The ductility index is used to assess the brittleness of concrete. There exists a good lineal correlation between ductility index and fractal dimension for all test series. Thus, fractal dimension can be used as a parameter to characterize the brittleness of concrete.