Fracture Energy of Steel Fiber-Reinforced Concrete
Steel fiber-reinforced concrete (SFRC) is a cementitious material reinforced with discrete fibers. The energy absorption capacity is the main material property benefited by fiber reinforcement. Closed-loop servo-controlled equipment should be used to evaluate this property. The tests should be carried out using displacement control in order to obtain the postpeak force- displacement relationship (tensile strain-softening branch). To assess the fracture energy of SFRC, three-point bending tests were carried out using displacement control. Series of notched beams reinforced with 30, 60, and 90 kg/m3 of hooked-end steel fibers were tested. Besides the energy dissipated in fracturing the concrete, the energy determined from the force-displacement relationship can also include the energy absorbed during nonlinear behavior of concrete in compression. Ductile materials, such as concrete reinforced with a high content of fibers, develop large deflections before exhausting their energy absorption capacity. In these cases, the "fixed" points of the bar supporting the displacement transducer may not remain fixed, adding an extra deflection into the control displacement transducer and thus leading to incorrect evaluation of the fracture energy. These factors are analyzed in the present work in order to assess the suitability of the specimen dimensions and the test procedures for evaluating the fracture energy of SFRC.
1 NEAR SURFACE MOUNTED CFRP LAMINATES FOR SHEAR STRENGTHENING OF CONCRETE 1 BEAMS 2 3
NEAR SURFACE MOUNTED CFRP LAMINATES FOR SHEAR STRENGTHENING OF CONCRETE 1 BEAMS 2 3 J. A. O. Barros and S. J. E. Dias2 4 1 Associate Prof., Dep. of Civil Eng., School of Eng., Univ. of Minho, Azurém, 4810-058 Guimarães, Portugal 5 2 PhD Student, Dep. of Civil Eng., School of Eng., Univ. of Minho, Azurém, 4810-058 Guimarães, Portugal 6 7 ABSTRACT 8 A Near Surface Mounted (NSM) strengthening technique was developed to increase the shear resistance of concrete 9 beams. The NSM technique is based on fixing, by epoxy adhesive, Carbon Fiber Reinforced Polymer (CFRP) 10 laminate strips into pre-cut slits opened in the concrete cover of lateral surfaces of the beams. To assess the efficacy 11 of this technique, an experimental program of four-point bending tests was carried out with reinforced concrete 12 beams failing in shear. Each of the four tested series was composed of five beams: without any shear reinforcement; 13 reinforced with steel stirrups; strengthened with strips of wet lay-up CFRP sheets; and two beams strengthened with 14 NSM precured laminate strips of CFRP, one of them with laminates positioned at 90o and the other with laminates 15 positioned at 45o in relation to the beam axis. Influences of the laminate strip inclination, beam depth and 16 longitudinal tensile steel reinforcement ratio on the efficacy of the strengthening techniques were analyzed. 17 Amongst the CFRP strengthening techniques, the NSM with laminate strips at 45o was the most effective, not only 18 in terms of increasing beam shear resistance but also in assuring larger deformation capacity at beam failure. This 19 technique was also faster and easier to apply than the externally bonded one. The performance of the ACI and fib 20 analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP 21 systems predicted by the analytical formulations was slightly larger than the values registered experimentally. 22 Performance of the formulation by Nanni et al. for NSM strengthening technique was also appraised. Using bond 23 stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system, the 24 formulation by Nanni et al. predicted a contribution of this CFRP system for the beam shear resistance of 72% the 25 experimentally recorded values. 26 27
Experimental study on bond behavior between high-strength grout and deformed steel bars
Abstract Engineers use high-strength grout to connect precast concrete members for its low shrinkage and good fluidity. This paper designed 87 central pull-out specimens using the control variable method to investigate the influence of factors on the rebar-grout bond property such as compressive strength, steel grade, bar diameter, transverse stirrups, and bond length. The test result showed that improving grout compressive strength (85.8 MPa–122.7 MPa) cannot improve the bond strength, which was different from the previous conclusions of concrete pull-out tests. Simplified formulas were presented for calculating the characteristic values of bond strength, residual strength, peak slip, and relative slip of load-holding platform based on parameter analysis. Then, a constitutive model was proposed to predict the full-range bond stress-slip curve of high-strength grout and steel bar specimens from bearing to failure. Finally, FEA models were established to evaluate the applicability and accuracy of the constitutive model.