0057 A Study on Mechanical Behavior of Kevlar Fiber Reinforced Concrete under Static and High-strain Rate Loading
Yeou-Fong Li, Yan-Ru Huang, Chih-Hong Huang, Ying-Kuan Tsai and Pei-Yao Hsu
Reinforced concrete structures sometimes are deteriorated and damaged by seismic and blast wave loadings. And the resistance of fiber-reinforced concrete was tested under of high-strain rate loading. Therefore, the concrete structures were need to improve the dynamic load resistance and energy absorption capabilities. In civil engineering, fiber was incorporated into concrete and used in strengthening structures to increase its durability and resist impact and blast wave loads. However, the mechanical performance of fiber-reinforced concrete (FRC) varied by using different weight ratios, volume fractions, fiber types and length of fibers. In this study, the quasi-static and dynamic mechanical behaviors of different weight proportions Kevlar fiber-reinforced concrete (KFRC) was studied by compressive strength test and split Hopkinson pressure bar test.
Aramid fiber is an organic fiber and it can be divided into para-aramid and meta-aramid, and DuPont's Kevlar fiber is one of the para-aramid fibers. Kevlar fiber was manufactured and invented by DuPont in 1965. The Kevlar fiber has excellent characteristics of non-conductivity, light weight, high specific tensile strength, good chemical stability and good weather resistance. Under the condition of the same weight, Kevlar fiber is 5 times as strong as steel wire, 2.5 times as strong as Grade E glass fiber, and 10 times as strong as aluminum. The elongation of Kevlar fiber is longer than carbon fiber. The concrete specimen’s water to cement ratio was fixed at 0.6; and the weight ratio of cement to fine aggregate to coarse aggregate was fixed at 1:1.05:2. Compressive test of concrete cylindrical specimens was done according to ASTM C39/C 39M – 01 standards. The dimension of the cylindrical specimen is ϕ 10 cm x 20 cm. The SHPB is a pneumatic impact testing machine. The SHPB test in this study was carried out in the materials laboratory of the Environmental Information and Engineering Department of Chung Cheng Institute of Technology, National Defense University. The dimension of the cylindrical specimen is ϕ 5 cm x 5 cm. The diameter of the incident and transmitted bars also is 5 cm.
In terms of fiber length, the quasi-static compressive strength of KFRC with 24 mm Kevlar fiber is slightly better than KFRC with 12 mm Kevlar fiber. In terms of fiber weight ratio, the compressive strength of KFRC with fiber weight ratio of 0.5 % is the best. However, the compressive strength of KFRC with fiber weight ratio of 1.0 % and 1.5 % is similar. Figure 1 shows the bar chart of quasi-static compressive strength for KFRC and benchmark specimens. The energy accumulated in a specimen owing to deformation is defined as strain energy which can be obtained by integrating the area of the stress-strain curve and multiplying the volume of the specimen. Figure 2 shows the bar chart of the strain energy of the benchmark specimen and KFRC specimens under quasi-static and dynamic loadings; it can be seen that the strain energies of KFRC specimens with the 24 mm Kevlar fiber are higher than the energies of the KFRC specimens with the 12 mm Kevlar fiber. In the SHPB test, the peak value of compressive stress is dynamic compressive strength, and the ratio of dynamic compressive strength to quasi-static compressive strength is defined as the dynamic increase factor (〖DIF〗_(f^' c)). 〖DIF〗_1 was calculated from quasi-static and dynamic compressive strength of the specimens, both the concrete proportions are the same; and 〖DIF〗_2 was calculated from the quasi-static of benchmark specimen and the dynamic compressive strength of KFRC specimen. Figure 3 show the comparison of 〖DIF〗_1 and 〖DIF〗_2 with the test data obtained from CEB-fib model
The quasi-static and dynamic compressive strengths test results show that KFRC with 0.5% Kevlar fiber has the best mechanical properties; but there is no significant difference between KFRC specimens with the KFRC specimens with 12 mm and 24 mm Kevlar fiber. However, it can be obviously seen that the strain energies of the KFRC specimen with 24 mm Kevlar fiber are higher than the energies of the KFRC specimens with the 12 mm Kevlar fiber. The comparative analysis results of the experimental data of this research and the dynamic increase factor (DIFf’c) curve of CEB-fib model show that the DIFf’c of KFRC are all on the curve, which shows that the KFRC strain rate sensitivity of DIFf’c is higher than that of the materials used in CEB.
0058 A Constitutive Model for Cylindrical Concrete Confined with Aramid Fiber Reinforced Plastics
Yeou-Fong Li, Bo-Yu Chen and Pei-Yao Hsu
The deterioration of concrete structures was usually happened inadequate maintenance, especially in some high seismic regions. It has been an important issue to increase the strength of the buildings. The existing reinforcement methods for concrete structures in Taiwan are mainly divided into two types: steel plate reinforcement and fiber composite patch reinforcement. Compared with steel reinforcement, FRP reinforcement is light weight, corrosion resistance, easy to use, less working space requirements, and high strength. Generally, the CFRP is a major composite material to confine the RC structural members due to its high strength and elongations. But the elongations of aramid fiber were higher than the carbon fiber and correspondingly to the high strength. This study aims to build a constitutive model for aramid fiber-reinforced confined concrete specimens with circular and square cross-sections. The compressive stress-strain relationships of AFRP confined concrete were used to obtain the parameters of the proposed constitutive model, and then compared with other researches to illustrate the accuracy of the proposed constitutive model.
A totally of 156 concrete specimens were tested in this study. The concrete specimens have different cross sections as 120 cylindrical concrete specimens (ϕ10×20 and ϕ15×30). In this study, five different strengths of concrete are obtained by using different water-cement ratios and the concrete mix-ratio (cement, sand and aggregate) was 1:1.81:4.52. The Kevlar fiber reinforced plastics (KFRP) were used to confined cylindrical and square cross-sectional concrete specimens. Kevlar fiber is aramid fiber, and it was obtained from DuPont Company. The concrete specimens are wrapped with KFRP sheets by hand lay-up on the 18th day. The epoxy resin was applied on the surface of the specimen to adhesive and saturate the Kevlar fiber sheet. The length of overlay is more than 10 cm for each layer of KFRP confinement. Applying the next layer of KFRP need to wait for epoxy resin curing (usually one day, at 25°C); and then repeat the above steps until the required number of layers. Finally, the Kevlar confined concrete with epoxy was cured for more than 7 days in ambient temperature. Two strain gauges were attached to the KFRP confined specimens to measure the circumferential strain. The KFRP confined concrete specimens were tested in universal testing machine with a load cell. According to ASTM C39/C 39M-01 standard, the loading rate of the actuator is 0.21 MPa/sec. And the loading process was stopped when the axial load began to decrease to 70 percent of maximum compressive strength.
The KFRP enhanced the mechanical capacity of concrete specimens. For example, the increasing percentages of compressive strength for different cross-sectional concrete specimens confined with different layers of KFRP (C10W65, C15W65) were 89%~271% and 42%~157%, respectively; and the compressive strains (C10W65) were increased by 221%~473%. The KFRP confinement also significantly enhance the strain energy capacity of concrete specimens, and the strain energies (C10W65) were increased by 614%~2,176%.
In order to predict the compressive strength of KFRP confined concrete specimens, a constitutive model was proposed, which was adopted from the Mohr-Coulomb failure criterion. The compressive strength of cylindrical concrete specimens confined with one and two layers of KFRP were used to determine the internal friction angle parameters of the proposed constitutive model. Then, the compressive strength of the concrete specimens confined with three layers of KFRP were predicted to verify the accuracy. The proposed constitutive models can accurately predict the compressive strength of cylindrical concrete specimens confined with KFRP confinement. The average absolute error was about 4.93% and the coefficient of determination (R2) was about 0.908.
The KFRP confinement can significantly enhance the strain energy capacity of concrete specimens. For C10W70L3, the strain energy capacity was increased up to 2,185%. Otherwise, the proposed constitutive models can accurately predict the compressive strength of concrete specimens confined with KFRP confinement. It shows a good relationship between proposed model and experimental compressive strength of concrete specimens.
Jiunn-Shyang Chiou and Yi-Wun Lee
JIUNN-SHYANG CHIOU and TZU-CHIEH LEE
黃 偉祥, 汪 祐毅, 黃 俊學 and 洪 汶宜