This paper introduces the recent state of research on creep and shrinkage of concrete in Japan, focusing on several unique advances: new prediction models in design code, development of database, autogenous shrinkage and prediction of cracks in structural members. New prediction models for creep and shrinkage of concrete, which can be applied to high-strength concrete, were adopted in the JSCE standard specification for concrete structures in 2002. A creep and shrinkage database was developed and made available for international use. Thus Japanese researchers can be said to have made important contributions to research on autogenous shrinkage of concrete in the last ten years.
Durability of concrete structures is seriously compromised by cracking in early age concrete, particularly in high-strength or massive concrete structures. Since early age cracking is influenced by various highly interrelated factors that affect the hydration process and stress/strain development, its behavior is highly complex and no rational methodologies for its control have yet been established. On the other hand, demands for high strength and massive concrete structures in modern cities are ever growing, regardless of the many durability problems. More comprehensive methodologies for the control are therefore essential to ensure sustainability of such structures. This report reviews state-of-the-art research on mechanisms that cause complex cracking phenomena and newly developed methodologies to control early age cracking.
Expansive additive is well known to be effective in compensating early-age shrinkage and the resultant induced stress in reinforced high-strength concrete (HSC) members. On the other hand, there have been few studies on numerical analysis methods for evaluating such early-age induced stress, which are vital to verify the risk of cracking. The present study formulates a 3-dimensional finite element method as well as a practical calculation method based on the beam theory, both of which consider the principle of superposition and linear stress-strain relationship of creep, in order to evaluate the early-age shrinkage/expansion-induced stress in reinforced members. The applicability of the proposed methods is evaluated by comparing computed values with experimental values on shrinkage/expansion-induced stress in RC beam specimens composed of various HSCs, using expansive additive and/or shrinkage reducing chemical agent and/or low-heat Portland cement. The results demonstrate that the proposed finite element method can accurately simulate induced stress in the reinforced concrete beams, even when expansive additive is used, and this indicates that the linear stress-strain relationship may be valid for expansive high strength concrete. Furthermore, there is a good agreement between the finite element method and a practical calculation method based on the beam theory, even in the case of RC beams with stirrups that cause a three-dimensional restraint condition in concrete.
The Broken-Off Specimens by Splitting - BOSS method was first formulated in 1985 by professor Shirayama of the University of Tsukuba. This new estimation method for concrete strength in structures has been studied subsequently by the authors in terms of practical applications. These included a simplified method and experiments under various conditions to ensure the accuracy of the test method. As a result, a standard test method with a regression equation for estimating the concrete strength in structures was proposed. Compiling these results, this paper deals with a comparative study of BOSS strength and core strength in a full-scale model structure, variance analysis of influences of sampling height and water- cement ratio of concrete on these compressive strength, and applicability of the BOSS method on the basis of these studies. High correlation was found between BOSS and core strength both in normal and high-strength concrete, and the proposed estimation equation was found to be applicable to the reliable evaluation of concrete strength in structures.
This paper presents the results of laboratory and field investigations on the influence of bleeding on minute properties and steel corrosion in concrete. Test methods such as minute compressive strength test, minute tensile strength test and minute diffusion test were performed in the laboratory to assess the effect of bleeding on minute properties of concrete. In addition, electrochemical investigations were conducted both in the laboratory and in the field to determine the influence of bleeding on the rate of steel corrosion in concrete. The various test results indicate a strong agreement between the laboratory experiment and the field investigations. The upper layer of concrete affected by bleeding exhibited weaker strength, higher permeability, lower concrete resistance, and higher oxygen permeability. Consequently, a higher macrocell corrosion rate than the microcell corrosion rate prevailed in both the vertical and horizontal steel bar and the corrosion rate was enhanced at elevated temperatures (20-40°C).
In this paper, the strengths under different curing conditions, heat evolution as well as resistance to sulphate have been discussed for High Belite Cement (HBC) in comparison with Ordinary Portland cement (OPC). In addition, R&D of high-performance concrete prepared by HBC as the main component of cementitious material with strength grades C50∼C80 (equivalent to minimum requirement for 28-day compressive strength 50MPa∼80MPa) was conducted. And the workability, physical mechanical properties, durability of HBC concrete have been discussed as compared to OPC concrete. The research results indicate that HBC possesses higher late strength after 28-day age though its early strength is relatively lower, lower hydration heat evolution and excellent resistance to sulphate. Moreover, HBC concrete exhibits excellent workability, physical mechanical properties and durability.
Concrete structural elements of bridges or tunnels may be in contact with water containing chloride during the winter. Furthermore, marine structures are permanently exposed to seawater containing chlorides. In all these cases, penetration of chlorides through the covercrete is a major risk and the service life of structures may be reduced considerably if permeability is too high. Early repair measures are both an economical and an ecological problem nowadays. By adding appropriate admixtures to the fresh concrete, it is possible to produce an internally water repellent material. It is shown that it is possible to substantially reduce chloride penetration through ECC (Engineered Cementitious Composites) in this way, thereby considerably extending the service life of new structures and of repair layers. The properties of the modified ECC are described in detail.
In order to predict the chemo-physical process of carbonation, a finite element based computational method is implemented based upon multi-phase/scale governing equations of moisture and flux of both heat and carbon dioxide. Influencing parameters of carbonation involving reaction rate, CO2 diffusivity and the reduction of porosity are discussed. It is found that such modeling can accurately show high nonlinearity among carbonation reaction, pore structure development and moisture distribution in micropore structures. By using the proposed assumptions, the reliability of the predictive method of the carbonation mechanism in cementitious materials under arbitrary environmental and curing conditions is examined by comparing available experimental results with theoretical ones. Through sensitivity analyses that focus on the nonlinearity of the moisture profile and local carbonation, it is clarified that different moisture distribution may bring the opposite trend of the carbonation depth under low and high CO2 concentrations.
The current Japanese seismic design code on railway structures is designed to verify whether the response calculated through dynamic analysis satisfies the seismic performance required for railway structures. The seismic performance of railway structures set forth in the code was determined through experimental studies on RC members using normal-strength materials (nominal concrete strength: 50 N/mm2 maximum; nominal yield strength of the reinforcement: 390 N/mm2 maximum). Therefore, when RC members are designed with high-strength reinforcement (nominal yield strength: higher than 390 N/mm2 ) and high-strength concrete (nominal compressive strength: higher than 50 N/mm2 ), it is necessary to evaluate the seismic performance of the members and reflect this in the design. For this purpose, cyclic loading tests were performed on RC members made of high-strength materials. This paper describes the results of studies performed to verify the applicability of the current Japanese railway structure design code to RC members using high-strength materials and to identify the key points in using high-strength materials.
As nonlinear post-peak mechanics is time dependent in nature and the transient process of collapse is influenced by loading rates, post-peak analyses have to be conducted considering softened structural concrete under varying rates of straining. To meet this challenge, a new time-dependent constitutive model, which encompasses both near- and post-peak regions in concrete compression, is proposed. Towards better evaluation of structural collapse under extreme loads, a coupled plastic-damaging law is presented. For the purpose of identifying evolution laws of plasticity and continuum damage, experimental investigation into rate-dependent nonlinearity was performed under different levels of lateral confinement. The plastic and damage evolutions were formulated with respect to paths of intrinsic stress intensity of damage continua and time. The combined law of short-term elasto-plastic and fracture successfully convey nonlinear creep deformation and rate dependent strength, as well as delayed creep rupture of material instability.
This study deals with the application of the strut-and-tie models in the analysis and design of non-prismatic reinforced concrete beams. Seven beams were designed, fabricated and tested to failure. Test results showed that the ultimate loads exceeded the design loads for all beams. Non-prismatic beams with a recess through the web performed satisfactorily, compared to beams with equivalent transverse rectangular openings. For non-prismatic beams with a recess at the bottom, an increase in the recess width resulted in a decrease in the stiffness and an increase in the beam deflection. Non-prismatic beams with a recess in the compression zone performed better with regards to cracking but not deflection, compared to beams with a recess in the tensile zone. Also, beams strengthened with carbon fibre-reinforced polymer (FRP) plates performed satisfactory with regard to strength; however, the deflection and crack widths increased rapidly thereafter, leading to a sudden and non-ductile failure of the beam.
Repetition of higher stresses apparently causes progressive damage, which is also strain/stress rate-dependent. This paper aims to separate time-dependent cumulative nolinearity and the effect of repetition of strain path on the overall damage evolution of concrete in compression. Cyclic uniaxial compression tests of concrete were conducted and time-dependent plasticity and stiffness degradation were subtracted from total stress-strain relations to purely extract the effect of repeated stress cycle. Coupling of time dependency and authentic cyclic accumulation of damage was examined by both material and structural experiments especially in post-peak regions, and its applicability was experimentally verified mainly under low cyclic (N<100) actions. The high cycle (N>100) fatigue life predicted by the proposed constitutive law was also verified in terms of the S-N curve.