This paper presents a numerical investigation of the mechanical performance and ductility of concrete beams reinforced by both fiber-reinforced polymer (FRP) and steel tension reinforcement. Three-dimensional (3D) finite element (FE) analysis of beams with both FRP and steel reinforcement was conducted by first using ANSYS to verify the reliability of the FE analysis. The FE analysis can predict the mechanical behavior of the tested beams. To evaluate the ductility of the FRP-steel reinforced concrete (RC) beams considering various factors, such as the effects of the FRP on the steel reinforcement ratio, the location of the FRP reinforcement, the FRP type and the concrete compressive strength, on the mechanical performance of the beams, a parametric study was used to complement the FE analysis. Based on the parametric study, the conditions of the ratio of FRP to steel reinforcement, the location of the FRP reinforcement and the type of FRP reinforcement required to obtain reasonable ductility in practical use were presented.

This paper presents the results of research concerning the potential ability of ultra-high performance cementitious composites to self-heal the cracks that appear at various curing stages, and the effect of additional hydrothermal treatment on this ability. Test specimens were fractured in a 3-point bending test. The preload was induced during the initial curing period, i.e. 24 hours after the mix was made, and also in matured materials: concrete that was cured for 28 days in water or subjected to low-pressure steaming or autoclaving. Flexural strength and total fracture energy tests were carried out on reference specimens and also on specimens with a crack healed by immersion in water and by the additional use of steaming at 90℃ and autoclaving at 250℃. The results of studies performed with respect to mechanical properties and microstructure observations show that material that was cracked at early curing stages can be most effectively healed (i.e. its strength restored) by the autoclaving process. The main reason for this is the crystallisation of xonotlite and tobermorite whiskers in the healed crack. The self-healing effect tested in matured composites where binder hydration was determined to be of around 40% turned out to be virtually negligible.

In order to enhancement accuracy of shear design of reinforced concrete (RC) beams, detail understanding of the shear resistance mechanism is required. This study evaluated the shear resistance mechanism of RC beams based on arch and beam actions by using three dimensional Rigid-Body-Spring-Method (3-D RBSM). Firstly, RC deep and slender beams with and without shear reinforcement failed in shear were tested to measure local behavior. Then, the validity of local behaviors obtained from 3-D RBSM was confirmed by comparing with the test results and the applicability of decoupling of shear resistance mechanism using simulated stress distribution was presented. Moreover, the contributions of arch and beam actions in RC beams until failure stage were investigated numerically by changing the shear reinforcement ratio and shear span to depth ratio and was compared with the current shear design recommendations in JSCE Standard Specification. As a significant finding, the numerical results upon the quantitatively evaluation of shear resistance mechanisms that the shear strength of RC beam could be evaluated without classification of deep beams and slender beams was presented.

Fluoride ions decrease the fluidity of cement paste containing superplasticizer. This paper discusses the fluidity decrease from the influence of fluoride on the solid and liquid phases in cement paste, and investigates the difference between allylether-type and methacrylate-type polycarboxylate-based superplasticizers in cement paste containing fluoride ions. Regarding the solid phase, addition of fluoride ions generates fine particles, resulting in adsorption of superplasticizer on the fine particles. Regarding the liquid phase, addition of fluoride ions decreased Ca²⁺ concentration and increased SO₄^2- concentration; i.e., the adsorption hindrance. Allylether-type superplasticizers preferentially adsorb on the fine particles even with the adsorption hindrance; i.e., they are susceptible to the preferential adsorption. Methacrylate-type superplasticizers were less susceptible, but adsorption was hindered by the adsorption hindrance. When water-cement ratio increased, Ca²⁺ concentration increased and SO₄^2- was diluted, resulting in weak adsorption hindrance. Because of these concentration changes, methacrylate-type superplasticizers showed the preferential adsorption at high water-cement ratio.