The authors have proposed a new method, epoxy-coating on MIP (Mercury Intrusion Porosimetry) sample, to measure threshold pore radius of concrete to obtain an indicator of pore structure which has correlation with air and water permeability. In this paper, first, the validity of the above method was studied through observation on splitting surface of samples after MIP analysis and comparison with obtained threshold pore radius and permeability. Results showed that the proposed method is suitable to extract threshold pore radius, and it showed good correlation with water permeability and, if concrete is enough dried, air permeability. Good correlation was found even on samples prepared with overseas concrete material and core samples taken from existing structures overseas. The above results indicate that pore structure governs both air and water permeability of concrete and that threshold pore radius can be an indicator of the permeability of concrete.
In order to study tensile failure in fiber-reinforced concrete, split-tensile tests were conducted. Disc-shaped samples of mortar, plain concrete, steel-fiber reinforced concrete (SFRC) and polyvinyl-alcohol-fiber reinforced concrete (PVAFRC) were tested. Fracture mechanisms at the meso-scale are identified by applying SiGMA (Simplified Green’s functions for Moment tensor Analysis) procedure of acoustic emission (AE). In fiber-reinforced concrete, a self-healing draws an attention in concrete engineering. The effect could bridge opened (tensile) cracks due to hydration of unreacted cement. Since the crack width of the tensile crack is a key issue, kinematical information of the crack formation process is investigated in the split-tensile test prior to experimental confirmation on the self-healing effect. It is found that sliding motions on the crack surface are more dominantly observed in PVAFRC than those in mortar, plain concrete and SFRC, prior to reaching the final stage, i. e. possibly under the service load. This suggests that self-healing products, which could be generated at the cracked surface, could play the more effective role in PVAFRC for bridging meso-scale cracks due to tight spaces (gaps) across fibers.
This paper presents an analytical investigation of the double-counting problem in finite element analyses in which bond link elements, discrete reinforcement elements, and tension-stiffening models are used simultaneously. Engineers and researchers often use the bond link elements in the FEA to express bond slip between reinforcing bar and concrete. On the other hand, in case of smeared crack modeling, bond behavior is indirectly expressed by a tension-stiffening model of concrete. If the bond link elements and tension-stiffening models are used simultaneously, there is some concern that the effect of the bond is double counted. Hence, example uniaxial tension specimens are analyzed by three different analysis methods to investigate the double-counting effect. (1) Smeared crack analyses present no double-counting effect while (2) discrete crack analyses cause the double counting of tensile concrete stresses and result in over cracking. (3) Discrete crack analyses with delayed cracking also cause double counting although over cracking does not occur but the tension-stiffening relationships are overestimated.