Porosity-free concrete (PFC) is a newly developed ultra-high-strength concrete with a compressive strength of 400 MPa. PFC reduces the weight of bridge superstructures, protects against collisions with flying objects, increases seismic-resistant capacity, and improves long-term durability. At present, basic material properties of PFC, such as compressive strength, tensile strength, and tensile toughness have been revealed. However, impact resistance has not been examined. In this study, in order to investigate the impact-resistant behavior of steel-fiber-reinforced PFC, falling-weight impact-loading tests were conducted on a PFC beam, considering the mixing ratio of steel fiber and the height of falling-weight as variables. To investigate the effects of compressive strength on the impact-resistant behavior of the concrete beam, tests using high-strength concrete (HC) with a compressive strength of 100 MPa were also conducted as general high-strength concrete. From this experimental study, the following results were obtained: 1) impact resistance capacity of the PFC beam can be more drastically improved by mixing 2 vol.% of steel fiber compared to the HC beam; 2) bonding resistance between PFC and steel fiber could play an important role in upgrading the impact resistance. PFC mixed with 2 vol.% steel fiber could be used as an effective reinforcement material for impact protection structures.
When designing blast-resistant concrete members that are subject to contact detonation, it is necessary to reduce the spall damage due to tensile stress waves reflected from the back sides of the members. In a previous study, the authors confirmed the good spall-reducing performance under contact detonation of slurry-infiltrated fiber concrete (SIFCON), which is manufactured by first placing fibers into an empty mold and then infiltrating them with grout. In this study, to obtain SIFCON with further improved spall reduction, experimental investigations were conducted regarding the effects of different fiber types on the blast resistance of SIFCON slabs against contact detonation. Five types of steel fibers and four types of synthetic fibers were employed as reinforcing fibers. The thickness of the SIFCON slabs was fixed at 80 mm, and contact detonation tests were conducted using two amounts of SEP explosives. All of the SIFCON samples investigated reduced the spall damage due to contact detonation more effectively than normal concrete and other conventional fiber reinforced cementitious composites. Furthermore, by using SIFCON with fine straight steel fibers, the spall-limit thickness of the slab could be reduced by 54% or more.
Self-healing technology based on microbial induced carbonate precipitation can achieve cracks-healing of concrete. As concrete cracks appear, dormant bacterial spores introduced into concrete are activated, and calcium lactate is used as substrate to form calcium carbonate for healing crack. Due to the harsh environment in concrete, bacterial spores directly introduced become inactivated. Therefore, the introduction of good protective carrier is critical, and the mechanical properties and economy of the carriers are also the keys to improving self-healing technology. In this study, recycled aggregate was used as a protective carrier for Bacillus pasteurii to enhance the self-healing capacity. The effects of this technique were investigated by comparing with three other incorporation techniques, i.e., direct introduction of bacteria, diatomaceous earth-immobilized bacteria, and expanded perlite-immobilized bacteria. The healed crack width value of specimens incorporated with recycled aggregate-immobilized bacteria was close to that of specimens incorporated with expanded perlite-immobilized bacteria (the healed widths were 0.28 mm and 0.32 mm, respectively), which was larger than that of specimens incorporated with diatomaceous earth-immobilized bacteria (the healed width was 0.14 mm) and specimens directly introduced bacteria. Scanning electron microscope and electronic data switching analysis conﬁrmed that precipitation formed at cracks was calcium carbonate.
The objective of this research is to propose a simple and accurate prediction method for the shear capacity of reinforced concrete beams with steel fiber (RSF beams). Steel fiber reinforced concrete (SFRC) is being widely used nowadays, with the steel fibers added to the concrete to improve the tensile resistance. First, this research aims to investigate the material properties of SFRC in various concrete compressive strengths, shapes of steel fiber and volume fraction of steel fiber. In order to evaluate the shear capacity, several material properties, such as tension softening curves and fracture energy were investigated in the material tests. Second, four-point bending tests of RSF beams were conducted in order to investigate the shear capacity and diagonal crack pattern. Eight RSF beams with various concrete compressive strengths, volume fraction of steel fiber and stirrup ratios were fabricated and tested. The results revealed that the concrete compressive strength and steel fiber significantly affect the shear capacity of RSF beams. Finally, the shear equation for the RSF beams failing in a diagonal tension failure mode has been proposed by focusing on the residual tensile stress perpendicular to the critical diagonal crack at the failure condition.
To investigate the restraint effect of reinforcing bar on the expansion induced by alkali-silica reaction (ASR), and assess the mechanical behavior of reinforced concrete (RC) structure damaged by ASR, pullout tests of specimens with different ASR expansion were conducted. Accelerated ASR tests of specimens with diameters of 12 mm, 16 mm, and 20mm were conducted to qualify the restraint effect of reinforcing bar. Pullout tests were used to investigate the relationship between nominal bond strength and ASR expansion. Test results show that ASR expansion decreases with the increase of the rebar diameter. Nominal bond strength of specimens increased initially to attain their peak at 14 days, approximately with the expansion of 0.035%. After that, the nominal bond strength decreased near-linearly with the increment of the expansion. A simplified ASR expansion model integrated with the poro-mechanical model was adopted to analyze the restraint effect. The verification of the proposed method was conducted by comparing the analytically predicted results with the test data. The results show that the proposed method could accurately predict both the ASR expansion and bond strength.
Shear failure after flexural yielding is a typical failure mode for a reinforced concrete (RC) member subject to cyclic load, and it is caused by the phenomenon that the initial shear strength is decreased with the increasing plastic ductility until the shear capacity degrades to be lower than the flexural strength. In this study, the flexure-shear behaviors of a number of RC columns with a relatively wide range of crucial structural factors involving tensile reinforcement ratio, shear reinforcement ratio, shear span-depth ratio (covering deep and slender columns) and axial compression load were simulated by the Three Dimensional Rigid-Body-Spring-Method, and the degradation behaviors of shear strength of all specimens were quantitatively evaluated to comprehend the effect of the structural factors. Moreover, the degradation behaviors of shear strength were decoupled into the degradation behaviors of shear resistance components (beam action, arch action, truss action, concrete contribution for beam action) to clarify the mechanism of shear failure after flexural yielding for the RC columns at various crucial structural factors. Consequently, the effect of crucial structural factors on the degradation behaviors of shear strength and shear resistance components were understood and it was concluded that the stepwise degradation of arch action was the dominant mechanism for the investigated structural factors for the shear failure after flexural yielding of RC column. This paper is an extended version in English from the authors’ previous publication [Nakamura, H., Furuhashi, H., Yamamoto, Y. and Miura, T., (2015). “Evaluation of shear strength degradation of RC member subjected to cyclic loading.” Journal of JSCE E2, 71(1), 48-57. (in Japanese)].