Volume 6, No. 1 Special Issue: Time Dependent Behavior of Concrete and Concrete Stuctures edited by Prof. H. Mihashi Characterization and Modeling of Pores and Surfaces in Cement Paste: Correlations to Processing and Properties (Invited paper) Hamlin Jennings, Jeffrey W. Bullard, Jeffrey J. Thomas, Jose E. Andrade, Jeffrey J. Chen and George W. Scherer Journal of Advanced Concrete Technology, 6(1) 5-29, 2008 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 concretes is changing more rapidly than laboratory experimentation can document long-term properties such as durability. Some of the possible models are discussed. A new concept for the early age shrinkage effect on diagonal cracking strength of reinforced HSC beams (Invited paper) Ryoichi Sato and Hajime Kawakane Journal of Advanced Concrete Technology, 6(1) 45-67, 2008 The effect of early age shrinkage on diagonal cracking strength of reinforced high strength concrete beams is investigated, in which thirty two reinforced concrete beams with the distance from compressive fiber to centroid of reinforcing bars(effective depth) of 250mm, 500mm and 1000mm are prepared, made of conventional high shrinkage- and low shrinkage- high strength concretes with water to binder ratio of 0.23, respectively. Loading test results show that the shear strength at diagonal cracking of reinforced conventional high shrinkage-high strength concretes beams is 5-18% lower depending on the effective depth compared with that of reinforced low shrinkage- high strength concretes beams, and the dependence of diagonal cracking strength on the effective depth is apparently different between both. Ultimate shear strength of the former is also 23-45% smaller than that of the latter on the average. Moreover, a new concept based on the equivalent tension reinforcement ratio for the evaluation of shrinkage effect on the shear strength at diagonal cracking is proposed, which is tension reinforcement ratio modified by considering the effect of tension reinforcement strain produced by deformation of concrete at early ages. The 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 strength from 90 to 130 N/mm2 is proposed. Simplified Evaluation of Shrinking Aggregate Based on BET Surface Area using Water Vapor Kei-ichi Imamoto and Masanao Arai Journal of Advanced Concrete Technology, 6(1) 69-75, 2008 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 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. Our results suggest that the SSA determined by using H2O is an effective index for evaluating the influence of the aggregate type on the drying shrinkage of concrete. Based on this finding, the authors propose a practical and simplified evaluation method for the evaluation of shrinking aggregate by moisture sorption. nvestigation into Volumetric Stability of Aggregates and Shrinkage of Concrete as a Composites Shingo Asamoto, Tetsuya Ishida and Koichi Maekawa Journal of Advanced Concrete Technology, 6(1) 77-90, 2008 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 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. Multiscale Model for Creep of Shotcrete- From Logarithmic-Type Viscous Behavior of CSH at the É m-Scale to Macroscopic Tunnel Analysis Christian Pichler, Roman Lackner and Herbert A. Mang Journal of Advanced Concrete Technology, 6(1) 91-110, 2008 A recently presented multiscale model for early-age cement-based materials [Pichler et al. A multiscale micromechanics model for the autogenous-shrinkage deformation of earlyage cement-based materials. Engineering Fracture Mechanics 2007;74:3-58] is extended towards upscaling of viscoelastic properties. The obtained model links macroscopic behavior, i.e., creep compliance of concrete, to the composition of concrete at finer scales and the material properties of distinct phases at these scales. Whereas finer-scale composition (and its history) is accessible through so-called multiphase hydration models for the main clinker phases in ordinary Portland cement (OPC), viscous properties of thecreep-active constituent at finer scales, i.e., calcium-silicate-hydrates (CSH) are identified from macroscopic creep tests using the proposed multiscale model and assessed by nanoindentation experiments. 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. Simplified Prediction of Drying Shrinkage Stress in Reinforced Concrete Building Walls Kei-ichi Imamoto Journal of Advanced Concrete Technology, 6(1) 111-120, 2008 This paper proposes a simplified prediction model for drying shrinkage stress in a concrete wall externally restrained by beams. This prediction model is a uniaxial model with an effective YoungÅfs 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, dummy concrete walls and beams were also constructed in order to measure the free shrinkage strains. Based on these mechanical properties, the shrinkage stresses in the walls were well simulated by the proposed model. The author proposes this model to predict the drying shrinkage-induced stress in reinforced concrete building wall. Stochastic Approach to Shrinkage Cracking Control for Reinforced Concrete Structural Elements Tetsushi Kanda, Haruki Momose, Kei-ichi Imamoto and Hirozo Mihashi Journal of Advanced Concrete Technology, 6(1) 121-133, 2008 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. Artificial Neural Network for Predicting Creep and Shrinkage of High Performance Concrete Jayakumar Karthikeyan, Akhil Upadhyay and Navaratan M. Bhandari Journal of Advanced Concrete Technology, 6(1) 135-142, 2008 Concrete undergoes time-dependent deformations that must be considered in the design of reinforced/prestressed HPC bridge girders. In this research, experiments on creep and shrinkage properties of HPC mix were conducted for 500 days. This 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 in Indian environment. However in a far zone some deviation is observed and to get a better model the experimental data base is used along with CEB-90 model data base to train the neural network. The developed ANN model will serve as a more rational as well as computationally efficient model in the Indian environment. Simulation of Chloride Diffusivity of Cracked Concrete Based on RBSM and Truss Network Model Licheng Wang, Mitsutaka Soda and Tamon Ueda Journal of Advanced Concrete Technology, 6(1) 143-155, 2008 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 permeability of the cracked concrete. On the basis of the statistics analysis of diffusion coefficients of concrete and mortar determined experimentally, the permeability of ITZ is analytically clarified combined with the RBSM and truss network model. 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 permeability 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 permeability of cracked concrete are examined. Results indicate that the chloride ions permeability 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 permeability of cracked concrete is achieved. Mechanical Properties of Newly Developed Heat-Resisting FRP Bars Atsushi Sumida and Hiroshi Mutsuyoshi Journal of Advanced Concrete Technology, 6(1) 157-170, 2008 This paper presents a basic research of a new type of heat-resisting FRP bars. It was cleared that the heat resistance of commercially available FRP bars was low because of low heat resistance of applied matrix resin such as epoxy (EP), unsaturated polyester (USPE) or vinyl ester (VE). New heat-resisting matrix resins that would be suitable for the production of FRP bars were studied, and resol type phenolic (PH) and M type cross-linked polyester-amide (CP) resins were selected. Six different types of FRP bars consisting of carbon fiber (CF) or aramid fiber (AF) impregnated and cured with PH, CP or EP were prepared. The heat resistance of these bars was tested through tensile tests after heating and while heating. An alkaline resistance test was also tried as a measure of durability. Finally, pull-out tests and bending tests of concrete beams reinforced with the newly developed FRP bars and those with the standard steel bars were also implemented at normal temperature and at high temperatures. It was determined that the heat resistance of FRP bars made with CF and PH was close to that of steel bars. On-site Concrete Segregation Estimation Using Image Analysis Pierre Breul, Jean-Marie Geoffray and Younes Haddani Journal of Advanced Concrete Technology, 6(1) 171-180, 2008 Segregation remains one of the major concretes problems that they are traditional or self-compacting. Consequences of this pathology are numerous and can affect in the long term the structures properties. To ensure the concretes expected characteristics, it is essential to be able to control their homogeneity. If some tests allow the control of fresh concrete at the concrete mixing plant, there is at the present time no method to evaluate the material segregation on site. The development of a quick and low disturbance method allowing to quantify segregation automatically within structures constitutes a advance in the pathologies detection. Method offered here rests on the use of the geoendoscopy and the automatic image processing. After a short presentation of the tools and of the auscultation methodology, the image processing methods developed in order to measure the concrete homogeneity and to control the concrete particle size distribution are exposed. Results obtained with this methodology on laboratory experimentations are then compared with those obtained from the traditional technology by video counting. Finally, the last part is devoted to the application of this method on a real self-compacting concrete structure. Fiber-bridging Constitutive Law of Engineered Cementitious Composites En-Hua Yang, Shuxin Wang, Yingzi Yang and Victor C. Li Journal of Advanced Concrete Technology, 6(1) 181-193, 2008 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 which is the key for structural long-term durability and composite tensile properties which is important for structural safety at ultimate limit state. Modeling of Reinforcement Buckling in RC Columns Confined with FRP Yuichi Sato and Hunebum Ko Journal of Advanced Concrete Technology, 6(1) 195-204, 2008 A model of the buckling behavior of the longitudinal reinforcement in RC columns confined with fiber reinforced polymers (FRP) was proposed. The model includes three significant aspects: (i) the model takes into account the lateral stiffness of FRP to estimate the buckling length and critical stress; (ii) the asymptotic compressive stress of the buckled longitudinal bar under the cyclic load was modified considering the restraining effect of the FRP; and (iii) it was assumed that at least one of the lateral reinforcing hoops (made of mild steel) should have yielded to allow the concerned longitudinal bars to buckle. The model was implemented in 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 to the test results of four RC columns. valuation of Punching Shear Strength of Reinforced Concrete Slabs Based on Database Sumio Hamada, Qiuning Yang and Mingjie Mao Journal of Advanced Concrete Technology, 6(1) 205-214, 2008 Although various studies related to the punching shear strength of slabs have been published, there are a small databases related to the punching shear strength. A database with 313 specimens has been structured through the present study, whereas Kakuta collected 114 data and Gardner collected 138 data. In the present study six equations for punching shear strength prescribed in specifications are evaluated based on the database. This paper includes discussion for parameters of punching shear strength in JSCE and AIJ specifications, member factors forpunching shear strength equation are determined based on the present data base. A simplified strength equation is also proposed from the present database. Shear Fatigue Simulation of RC Beams Subjected to Fixed Pulsating and Moving Loads Esayas Gebreyouhannes, Nobuhiro Chijiiwa, Chikako Fujiyama and Koichi Maekawa Journal of Advanced Concrete Technology, 6(1) 215-226, 2008 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. Path-Dependent High Cycle Fatigue Modeling of Joint Interfaces in Structural Concrete Koichi Maekawa, Naoyuki Fukuura and Masoud Soltani Journal of Advanced Concrete Technology, 6(1) 227-242, 2008 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 pre-cast slabs with localized discrete joints and in monolithically constructed reinforced concrete, where dispersed cracking develops. Coatings for Concrete Protection against Aggressive Environments Jose B. Aguiar, Aires F. Camoes and Pedro M. Moreira Journal of Advanced Concrete Technology, 6(1) 243-250, 2008 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.