Recently bond splitting failure prior to the yielding of stirrups has attracted more attention for reinforced concrete (RC) structures located in seismic areas due to the increased popularity of high strength steel. However, bond splitting failure is complicated, particularly in RC beams with multiple layers of reinforcement with different cutoffs. Pullout tests indicate the bond strengths of bars in the second (inner) layer of RC beams are weaker than those in the first (outer) layer. In contrast, test results of RC beams indicate that the bond strengths of cutoff bars in the second layer are larger than those in the first layer. To examine this contradiction, previous studies of pullout test in which deformed bars were embedded in concrete were reviewed. In contrast to the conventional method of evaluating the surface bond strength of each bar, a new method for evaluating bond resistance is developed, in which a discussion point is focused on the shear strength at a potential failure plane below the reinforcement layer. The proposed method shows good results when compared with test results of RC beams and columns that failed in bond splitting prior to yielding of longitudinal reinforcement, with an average ratio of measured-to-predicted failure stress of 1.23 and a coefficient of variation of 14%. In contrast, ACI 318-19 shear equation slightly underestimated some of the test results of single-layered beams, with an average ratio of measured-to-predicted failure stress of 0.98 and a coefficient of variation of 15%. These findings suggest that side splitting failure of RC beams and columns under seismic action can be treated as shear failure.
Problems due to the corrosion of steel reinforcement in concrete structures can be efficiently and economically solved by using fiber-reinforced polymer (FRP) composites. This paper presents the results of experimental tests and numerical simulations of large-scale concrete slab strips symmetrically reinforced with steel and glass fiber-reinforced polymer (GFRP) bars subjected to axial tension. The results showed that due to their lower modulus of elasticity and bond strength, the slabs reinforced with GFRP bars exhibit wider early-age cracks compared to similar slabs reinforced with conventional steel bars. In order to limit the crack width to a certain value, approximately twice the reinforcement ratio of the GFRP bars as in the case of the steel bars is required. This can particularly be a problem for watertight concrete structures of underground facilities and water tanks.
The purpose of this study is to examine means of creating light and very strong prestressed concrete slabs made of lightweight aggregate concrete mixed with short fibers. The experiments reported in this paper demonstrated that the ratio of short fibers in the mix must be kept to 0.5 vol% or less, in order to obtain the same strength as that demonstrated by lightweight aggregate concrete designed to a standard strength of 50 N/mm2. In addition, the results of load testing various slabs confirmed that lightweight PC slabs mixed with 0.5 vol% of short PVA fibers had the same punching shear capacity as PC slabs made from normal weight concrete. The results led to the conclusion that the increase in the shear capacity of lightweight PC slabs mixed with short fibers was caused by the suppression of the progress of cracks in the RC direction and an accompanying tide arch formation in the PC direction.
This paper is the English translation of the authors’ previous work [Kitano, Y., Ito, H. and Suzuki, S., (2020). “Study on lightweight and high strength prestressed concrete slab by using lightweight concrete mixed with short fibers.” Journal of Japan Society of Civil Engineers, Ser. E2 (Materials and Concrete Structures), 76(3), 239-254. (in Japanese)].
The purpose of this paper is to clarify the mechanism of shear strength reduction for reinforced concrete deep beams with varying stirrup ratios, due to the beam size increase. Three Dimensional Rigid-Body-Spring-Method was the applied numerical tool. By using the simulation results of reinforced concrete deep beams with the effective depth smaller than 1500 mm, the size dependence of each shear resistance component such as arch, beam and truss actions were investigated, along with the size dependence of localized information such as crack pattern, diagonal crack width and compressive stress. It was revealed that the size effect on shear strength results from the size dependence of the arch action, and the change of stirrup ratio does not show any inhibiting effects on it. The crucial reason for the size effect on the arch action is the size dependence of concrete compressive resultant on beam cross sections, and the critical diagonal crack width, which increases with beam size, is a key factor for the deterioration of concrete compressive strength and the capacity of compressive stress in struts. Moreover, it was found that the critical diagonal crack width not only affects the normalized shear strength, but also has influence on the post-peak behavior of deep beam.
The paper aims to clarify the influence of fiber properties on the working performance and mechanical properties of 3D printing concrete (3DPC). This paper investigates the effects of polypropylene fiber content and length on the working performance and mechanical anisotropy of 3DPC. The research results show that with the increase of polypropylene fiber content, the static yield stress, and the dynamic yield stress of 3DPC increase, the extrudability decreases. The buildability first increases and then decreases. As the length of polypropylene fiber increases, the static yield stress and dynamic yield stress increase, and the extrudability decreases. The buildability first increases and then decreases. The printability of 3DPC with 9 mm length fiber is better than that with 6 mm and 12 mm. Through the compressive strength and flexural strength tests of 3DPC, it was found that both the compressive strength and flexural strength have obvious anisotropy. The addition of fiber has a positive effect on the improvement of the compressive strength in three directions, and the flexural strength in the vertical and transverse directions is improved. Through work performance and mechanical anisotropy testing, it can provide a reference for optimizing the use of fibers in 3DPC.
The absorption and desorption curve of superabsorbent polymers (SAP), which reflects key information of the maximum absorbency and retention capacity, is crucial for the mix designs of internally cured concretes. In this paper, the effect of amount of Ca2+ on the absorption and desorption curve of SAP is studied. Results show that the proportion between amount of Ca2+ and SAP mass has a notable influence on the absorption and desorption curve of SAP, whether in tap water or mimicked pore solution of cement paste. Considering the important effect of amount of Ca2+, it is suggested that researchers need to determine the relationship between the retention capacity and the ratio of amount of Ca2+ to SAP mass in advance before formal measurement using classic tea bag method or filtration method. Furthermore, from the view of the ion exchange equilibrium between SAP and Ca2+, we discussed the mechanism that amount of Ca2+, concentration of Ca2+, SAP type, alkalinity, etc. have decisive effect on the absorption and desorption curve.
Large quantities of aqueous secondary waste are generated from the processing of contaminated water after the nu-clear accident in Japan. Cementation of these wastes is challenging because their significant radioactivity may cause the radiolysis of water contents in cement, posing a risk of hydrogen gas generation. The application of calcium aluminate cement modified with phosphates (CAP), as an alternative cementing system, is interesting because this system is based on acid-base reaction, and its water content can be reduced by mild heating once the system is mixed. The present study focused in the use of Secar 71, a calcium aluminate cement with a high alumina and low silica compositions, and its effects of on the production of CAP system at elevated temperatures. The modification of Secar 71 with phosphates was successful, and the reduction of water content by about 35% was achieved in the CAP system containing SrCl2 after curing the system at 80°C for 7 days. The micro cracks, typically observed in the CAP system cured at lower temperature, was significantly reduced by curing at 80°C. The obtained results show a potential of Secar 71 to prepare CAP for cementation of aqueous secondary wastes.
Cementitious materials used in geological disposal repositories are expected to have various functions for construction, operation and closure of the high-level radioactive waste (HLW)/TRans-Uranic (TRU) waste repositories and they also have functions for safety. In the long term after closure of the repositories, cementitious materials are expected to reduce the release of radionuclides from the waste. However, the expected performance of cementitious materials may decrease in the long term because of their gradual dissolution/alteration. In addition, there is a concern that the high pH groundwater due to alkaline ions leached from cementitious materials may degrade the safety functions of other components (buffer, backfill, host rock) of the repositories. Therefore, in order to understand how the expected safety functions of the cementitious materials and other components can be achieved in the post-closure period, NUMO carried out the analytical evaluation of the evolution of each component. The results showed that most of the cementitious materials and other components will remain during a long-term post-closure period. At present, we are aiming to improve the reliability of the analytical model and to develop a more realistic nuclide migration model that reflects the effect of cementitious materials on reducing mass transfer.
The influences of CaF2 and TiO2 as a mineraliser on the sintering temperature, phase formation, setting times, and the mechanical and hydration properties of calcium sulfoaluminate (C4A3S) were investigated. The results showed that the addition of CaF2 and TiO2 by 0.5 and 0.2 wt.% can reduce the sintering temperature of C4A3S from 1300 to 1100°C. The addition of appropriate amounts of CaF2 and TiO2 promoted hydration and significantly improved the compressive strength of C4A3S cement (CSC). The compressive strength and hydration speed of CSC containing 0.5 wt.% CaF2 increased with an increase in the TiO2 contents from 0 to 0.2 wt.% at all curing ages. However, when the TiO2 content was increased to 0.25 wt.%, a significant reduction in the compressive strength of CSC was observed. The initial and final setting times of CSC containing 0.5 wt.% CaF2 both decreased with increasing TiO2 content. Compared with the control paste, the 15-h and 28-d compressive strength of CSC containing 0.5 wt.% CaF2 and 0.2 wt.% TiO2 increased from 30.4 and 79.2 MPa to 46.2 and 88.3 MPa, respectively. The hydration speed of CSC containing 0.5 wt.% CaF2 and 0.2 wt.% TiO2 was almost three times that of CSC without CaF2 and TiO2. Therefore, the optimum amounts of CaF2 and TiO2 that should be added to C4A3S as a mineraliser are 0.5 and 0.2 wt.%, respectively.