Cement-based materials have complex multi-component, multiscale structures that first form through chemical reaction and then continue to change with time. As with most classes of materials, the porosity of cement paste strongly influences its properties, including strength, shrinkage, creep, permeability and diffusion. Pores in cement paste range in size from nanometers to millimeters, and numerous investigations and models have been reported in the literature. This paper reviews some key concepts and models related to our understanding of the pore system and surface area. A major reason for the complexity of cement-based materials is that the principal reaction product, calcium silicate hydrate (C-S-H), forms with a significant volume fraction of internal, nanometer-scale pores. This gel pore system contains water that is also adsorbed to the solid surfaces, blurring the distinction between the solid phase and pores. The gel pore system changes not only with the chemical composition and extent of reaction, but also with changes in relative humidity, temperature, and applied load. Pores can be characterized by their surface area, size, volume fraction, saturation, and connectivity, but precise quantitative models are still not available. A useful approach for characterizing the structure of cement paste is to document the influence of time and external factors on structural changes. Scientific progress will be facilitated by the development of models that accurately describe the structure and use that structure to predict properties. This is particularly important because the composition and chemistry of commercial concretes is changing more rapidly than laboratory experimentation can document long-term properties such as durability. Some of the possible models are discussed.
Aggregate accounts for a large volumetric rate within concrete and influences the properties of concrete. In this investigation, the effect of aggregate properties on drying shrinkage of concrete was examined. It had been thought before that aggregate plays a role in restraining the shrinkage of cement paste, and that the shrinkage of aggregate itself can be neglected. However, according to the results obtained by this experiment, the shrinkage of aggregate is not necessarily negligible and it is strongly related to the shrinkage of concrete. Lightweight aggregate with a lower modulus of elasticity offers less restraint on the potential shrinkage of cement paste, then the large shrinkage of concrete is expected, but the actual shrinkage of lightweight concrete is comparatively small. It was demonstrated by this experiment that because of the small shrinkage of lightweight aggregate, shrinkage of lightweight concrete is relatively small. The shrinkage of normal aggregate is generally larger than that of lightweight aggregate, and it is necessary to pay attention to the usage of normal aggregate.
The effect of early age shrinkage on the diagonal cracking strength of reinforced high-strength concrete beams is investigated, using thirty two reinforced concrete beams with a distance from the compressive fiber to the centroid of the reinforcing bars (effective depth) of 250 mm, 500 mm and 1000 mm, made of conventional high-shrinkage and low-shrinkage high-strength concretes with a water-to-binder ratio of 0.23. The loading test results show that the shear strength at diagonal cracking of reinforced conventional high-shrinkage high-strength concrete beams is 5% to 18% lower depending on the effective depth compared with that of reinforced low-shrinkage high-strength concrete beams, and the dependence of diagonal cracking strength on the effective depth is apparently different between both. The ultimate shear strength of the former is also 23% to 45% smaller than that of the latter on average. Moreover, a new concept based on the equivalent tension reinforcement ratio for the evaluation of the shrinkage effect on the shear strength at diagonal cracking, consisting of a tension reinforcement ratio modified taking into consideration the effect of the tension reinforcement strain produced by deformation of concrete at early ages, is proposed. This concept shows succesfully the linear relationship between the shear strength at diagonal cracking and the effective depth to -2/5 power independent of the magnitude of the early age deformation of concrete, and a design equation for the shear strength at diagonal cracking applicable to concrete compressive strengths from 90 to 130 N/mm2 is proposed.
This paper deals with the evaluation of shrinking aggregate based on its specific surface area. The drying shrinkage strains of concretes with various types of aggregates were measured and their influences on the fundamental properties of the different types of aggregates were investigated. Furthermore, the specific surface areas (SSAs) of aggregates were obtained by the BET one point method using both nitrogen (N2) and water vapor (H2O). The SSAs determined by using H2O exhibited higher values than those obtained by using N2. The drying shrinkage strains of concretes increased with increases of the aggregate SSAs with H2O. This finding will contribute the development of simplified testing method for the evaluation of shrinking aggregates.
The authors aim to quantitatively understand the influence of various aggregate properties on concrete shrinkage behavior based on both experimental and numerical approaches. The multi-scale constitutive model reveals that differences in aggregate Young's modulus cannot be solely responsible for the observed significant difference in corresponding shrinkage behavior of concrete. Hence, shrinkage of the aggregate itself is considered as a possibility and an aggregate shrinkage model that takes into account the surface area and the degree of aggregate saturation is proposed on the basis of earlier experimental results. The proposed model reasonably simulates the greatly varying different shrinkage behavior of concretes with various types of aggregates.
A previously published multiscale model for early-age cement-based materials [Pichler et al. 2007. “A multiscale micromechanics model for the autogenous-shrinkage deformation of early-age cement-based materials.” Engineering Fracture Mechanics, 74, 34-58] is extended towards upscaling of viscoelastic properties. The obtained model links macroscopic behavior, i.e., creep compliance of concrete samples, to the composition of concrete at finer scales and the (supposedly) intrinsic material properties of distinct phases at these scales. Whereas finer-scale composition (and its history) is accessible through recently developed hydration models for the main clinker phases in ordinary Portland cement (OPC), viscous properties of the creep active constituent at finer scales, i.e., calcium-silicate-hydrates (CSH) are identified from macroscopic creep tests using the proposed multiscale model. The proposed multiscale model is assessed by different concrete creep tests reported in the open literature. Moreover, the model prediction is compared to empirical creep models, such as the so-called B3 model. Finally, the developed multiscale model is incorporated in the macroscopic analysis of shotcrete tunnel linings. Hereby, the early-age properties of shotcrete are specified by the presented multiscale model, taking mix design, cement characteristics, and on-site conditions into account.
This paper proposes a simplified prediction model for drying shrinkage stress in a concrete building wall externally re-strained by beams. This prediction model is a uniaxial model with an effective Young's modulus method based on the following three assumptions: (1) the equilibrium condition of forces between a wall and restraining beams, (2) equal restraining effect between the upper and lower beams, and (3) Bernoulli-Euler hypothesis for the strains in the entire cross-sectional areas of the members. The accuracy of this model was verified using five types of reinforced concrete walls constructed using three kinds of coarse aggregates, namely, normal coarse aggregate and recycled coarse aggregate of grade 1 and grade 3. The elastic modulus, drying shrinkage strains and creep coefficients of concrete specimens were measured. Furthermore, control concrete walls and beams were also constructed in order to measure the free shrinkage strains of concrete elements. Based on these mechanical properties, the shrinkage stresses in the walls were well simulated by the proposed model.
This study aims at evaluating shrinkage cracking risk in reinforced concrete structures, which has not been established in the past studies. To achieve this goal, analytical scheme capable of calculating the probability of shrinkage cracking was proposed. In this scheme, the variation of shrinkage restrained stress, that of concrete cracking strength, and safety factor are to be determined. The first two were determined with simple analytical simulation of structural elements, and the last factor was based on the comparison between cracking record of actual RC member and analysis results. Finally, this scheme was applied to actual construction project, and its validity was confirmed during the construction process.
Concrete undergoes time-dependent deformations that must be considered in the design of reinforced/prestressed high-performance concrete (HPC) bridge girders. In this research, experiments on the creep and shrinkage properties of a HPC mix were conducted for 500 days. The test results obtained from this research were compared to different models to determine which model was the better one. The CEB-90 model was found better in predicting time-dependent strains and deformations for the above HPC mix. However, in a far zone, some deviation was observed, and to get a better model, the experimental data base was used along with the CEB-90 model database to train the neural network. The developed Artificial Neural Network (ANN) model will serve as a more rational as well as computationally efficient model in predicting creep coefficient and shrinkage strain.
For concrete structures exposed to salt environment, the microstructure and cracks play a crucial role in the ingress of chloride ions into concrete. In this study, concrete is simulated on the meso scale as a three-phase composite, i.e., aggregate particles, mortar and the interfacial transition zone (ITZ). Because of the advantages in predicting cracks behavior in concrete, Rigid Body Spring Model (RBSM) is employed to carry out the mechanical analysis to simulate the distribution and width of microcracks. And then, the truss network model is adopted to evaluate the chloride diffusivity of the cracked concrete. On the basis of the statistics analysis of diffusion coefficients of concrete and mortar determined experimentally, the diffusivity of ITZ is analytically clarified. The range of diffusion coefficient of ITZ estimated in this paper is approximately 3-16 times of that of mortar depending on the different assumed thickness, which agrees well with that of the previous experimental results. With the aim to validate the effect of microcracks on the diffusivity of concrete, a series of the chloride ions penetrating analysis is numerically carried out on the concrete specimen under different stress levels. The axial compressive and tensile loading conditions are investigated respectively and the effects of stress level on chloride diffusivity of cracked concrete are examined. Results indicate that the chloride diffusivity is significantly dependent on the stress level, but only considering the effect of cracks predicted by RBSM is not sufficient. So an empirical equation which can account for the microstructure variation of concrete under loading is proposed. With it, a reasonable estimation for chloride diffusivity of cracked concrete is achieved.
This paper presents a basic research into newly developed heat-resistant fiber reinforced polymer (FRP) bars. The heat resistance of commercially available FRP bars is low because of the low heat resistance of the resins used for the matrix such as epoxy (EP), unsaturated polyester (USPE) and vinyl ester (VE). The authors investigated new heat-resistant resins suitable for the production of FRP bars, and resol type phenolic (PH) and M type cross-linked polyester-amide (CP) resins were selected for bar fabrication and testing. Six different types of FRP bars made with carbon fiber or aramid fiber and PH, CP or EP matrix resin were prepared. The heat resistance of each bar was evaluated by tensile tests during and after heating. To assert the durability of the bars, an alkaline resistance test was performed. Pull-out tests and flexural tests of concrete members reinforced with the newly developed FRP bars and those with steel bars were also performed at normal temperature (20°C) and high temperatures. The test results indicate that the heat resistance of the FRP bar specimens made with PH or CP matrix resin was higher than that of the specimens made with EP matrix resin, and that the heat resistance of reinforcing fiber was essential for improvement of the heat resistance of the matrix resin. Of particular note is the fact that the heat resistance of FRP bars made with carbon fiber and PH matrix resin was found to be almost the same as that of steel bars.
Segregation remains one of the major problems for traditional and self-compacting concrete. The consequences of this pathology are numerous and may affect the long-term properties of the structures. In order to ensure the expected characteristics of the concrete, it is essential to be able to check its homogeneity. Some tests allow the checking of the fresh concrete properties at the concrete mixing plant, but there is at the present time no method to assess concrete segregation on site. The development of a quick and low disturbance method allowing quantification of the segregation phenomenon automatically within structures constitutes an advance in the pathology detection area. The method presented here relies on the use of geoendoscopy and automatic image processing techniques. After a short presentation of the tools and the auscultation methodology, the image processing techniques developed in order to measure the concrete homogeneity and to control the concrete particle size distribution are exposed. Results obtained with this methodology in laboratory experiments are then compared with those obtained with the traditional video counting technology. Finally, the last part is devoted to the application of this method to a real self-compacting concrete structure.
This paper is on modeling and measuring fiber-bridging constitutive law of Engineered Cementitious Composites (ECC), a high performance fiber-reinforced cementitious composite featuring high tensile ductility. Fiber-bridging constitutive law plays an important role in the multiple cracking behavior of ECC. Therefore, proper control of fiber-bridging behavior through tailoring material microstructure is the key to successfully designing tensile strain-hardening ECC. In this paper, an analytical fiber-bridging model of ECC which connects material constituent parameters and composite properties, built on a previous simplified version, was proposed. To improve accuracy of crack opening prediction, new mechanisms of fiber/matrix interactions, specifically fiber two-way debonding and pull-out, matrix micro-spalling, and Cook-Gordon effects were included. This revised model was compared with experimental measurement of fiber-bridging behavior and the validity of the model was confirmed. It is expected that this model will greatly improve ECC design technology in terms of steady-state crack width control, key for structural long-term durability, and in terms of composite tensile properties important for structural safety at ultimate limit state.
A model of the buckling behavior of the longitudinal reinforcement in RC columns confined with fiber reinforced polymers (FRP) is proposed. The model includes three significant aspects: (i) the model takes into account the flexural stiffness of cracked cover concrete jacketed with FRP to estimate the buckling length and critical stress; (ii) the asymptotic compressive stress of the buckled longitudinal bars under a cyclic load is modified considering the restraining effect of the FRP; and (iii) it is assumed that at least one of the lateral reinforcing bars (made of mild steel) yield to allow the concerned longitudinal bars to buckle. The model was implemented in the form of a two-dimensional finite element algorithm to compute the hysteric response of the RC columns. The finite element analysis conducted herein considers spalling of the cover concrete due to the buckling along the longitudinal reinforcement. The analysis compared well overall with the test results of four RC columns.
Although various studies related to the punching shear strength of slabs have been published, databases related to punching shear strength are small in size. A database of 313 specimens has been structured through the present study, compared to the 114-data compiled by Kakuta et al. (1974) and the 138 data compiled by Gardner et al. (1996). In this study, six equations for the punching shear strength prescribed in specifications were evaluated based on the database. This paper includes a discussion of the parameters of punching shear strength in JSCE and AIJ specifications. A simplified strength equation is also proposed from the database.
The fatigue behavior of RC beams subjected to moving loads is experimentally investigated. Analytical scrutiny is made on the shear fatigue behavior of RC beams subjected to moving loads based on strain path and time dependent fatigue constitutive models rooted in the multi-scale fixed four-way crack modeling of concrete. Moving load is found to cause dramatic reduction in fatigue life of RC beams as compared to that of the fixed pulsating load both in the experiment and analysis. The mechanism for the reduced fatigue life under moving loads in RC beams is discussed in contrast to that of RC slabs. A simplified relation for the prediction of fatigue life under moving load is proposed for practical use on the basis of standard shear fatigue design equation of JSCE code, used for fixed fatigue loading. The effect of randomness in the position of loading is examined and its implication for the reliability of current fatigue life assessment method of RC members is put forward. The applicability of the multi-scale computational platform is verified for the fatigue investigation of RC beams subjected to moving loads.
A high-cycle fatigue constitutive model for concrete joint interfaces is proposed and the direct path-integral scheme for RC-PC structures with junction planes is presented. Both cyclic pullout and the associated dowel action of reinforcing bars are formulated at a crack/joint section in terms of the relative displacement derivatives of a pair of joint planes. The proposed differential formula is verified by high cycle fatigue experiments of dowel bars and pullout of reinforcement crossing a joint in structural concrete. In conducting the direct path integral of the constitutive equations, a logarithmic time integration method is adopted so as to achieve highly accelerated computation with reasonable accuracy. The scheme is applied to the assembly of pre-cast pre-stressed concrete members with reinforced concrete joints for the purpose of life-cycle assessment. A mechanics-based discussion is presented of the different fatigue life observed in precast slabs with localized discrete joints and in monolithically constructed reinforced concrete, where dispersed cracking develops.
The effect of polymeric coatings on concrete protection against chemically aggressive environments was evaluated. Two polymers - acrylic and epoxy - were applied on different concretes. The protection was measured by tests related with chemical resistance. The chlorides penetration, sulphates, acids and bases attack tests were used. Surface treatments act as a barrier between the environment and the concrete. This work intends to contribute to a better understanding of the performance of coated concrete in chemically aggressive environments, by presenting results of ion diffusion and resistance to aggressive solutions of several coatings used to protect concrete. The performance of the used coated concretes against chemically aggressive environments was generally better than the performance of the unprotected concretes. The results indicate that the overall performance of the used epoxy resin was better than that of other used coatings.
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