Ultrasonic Wave Reflection Approach to Evaluation of Fresh Concrete Friction
Yannick Vanhove, Chafika Djelal and Thierry Chartier Journal of Advanced Concrete Technology, 6(2) 253-260, 2008
The friction of fresh concrete and granular materials against a metallic plate was investigated using ultrasonic wave reflection. Two complementary experimental devices, one a global device using a tribometer, the other a device based on an ultrasonic reflectometer giving a local approach at the formwork/fresh concrete interface. This approach is based on the evaluation of the density variation of the interface between the metallic wall and the material during the casting process. The authors propose a method to maintain a high impedance contrast by using a water layer interleaved between the transducer and the studied material. Good agreement between the ultrasonic parameter and the friction parameter was observed..
Early Age Stress Development, Relaxation, and Cracking in Restrained Low W/B Ultrafine Fly Ash Mortars
Akhter B. Hossain, Anushka Fonseka and Herb Bullock Journal of Advanced Concrete Technology, 6(2) 261-271, 2008
This paper describes an experimental study that investigated the influence of ultrafine fly ash, a relatively new pozzolanic admixture, on the early age stress development, stress relaxation, and cracking in restrained low water-to-binder ratio (w/b) mortars due to shrinkage. The restrained ring test was used to assess the early age residual stress development in mortar ring specimens. In addition, free shrinkage strain, elastic modulus, and splitting tensile strength measurements were performed to assess the stress relaxation (creep effect) and cracking potential. It was found that the mortar mixtures containing ultrafine fly ash demonstrated lower levels of residual tensile stress than the control mixture. In addition, a reduction in the shrinkage, an increase in the age of restrained shrinkage cracking, and a decrease in stress relaxation (creep effect) were observed in the ultrafine fly ash mortars.
Characterizing 3D Pore Structure of Hardened Cement Paste with Synchrotron Microtomography Michael Angelo B. Promentilla, Takafumi Sugiyama, Takashi Hitomi and Nobufumi Takeda Journal of Advanced Concrete Technology, 6(2) 273-286, 2008
Aside from porosity, degree of pore connectivity and tortuosity are important variables to characterize the pore structure of cement-based materials to understand better the transport processes that influence the durability performance of these materials. Synchrotron X-ray computed microtomography is employed to provide information about the three dimensional (3D) pore structure at sub-micron resolution. This paper presents a method to quantitatively characterize the pore space of hardened cement pastes taken from specimen of different ages (2, 7, and 28 days) of curing. Pore segmentation allowed us to extract the pores from the microtomographic images. Further, cluster multiple labeling of the pores was performed to gain information on the pore connectivity and the associated effective porosity. 3D random walk simulation in the largest percolating pore cluster was then conducted to evaluate the diffusion tortuosity. The results from this study provide indications of increasing diffusion tortuosity as pores that can be resolved by the X-ray imaging system become more disconnected in the course of cement paste aging. Sensitivity analyses were also performed to evaluate the robustness of the method.
Chloride Binding Capacity of Mortars Made with Various Portland Cements and Mineral Admixtures
Tetsuya Ishida, Shigeyoshi Miyahara and Tsuyoshi Maruya Journal of Advanced Concrete Technology, 6(2) 287-301, 2008
The authors experimentally studied the chloride binding capacity of mortar specimens made with various combinations of Portland cement, blast furnace slag, and pozzolans. In the experiment, a pore liquid extraction method, chloride titration test, a quantitative analysis of Friedel's salt based on the XRD method, and a mercury intrusion porosimetry test were conducted in order to measure chloride ions, adsorbed chlorides on the pore wall, and solid-phase chlorides (Friedel's salt), separately. It was clearly shown that the amount of Friedel's salt strongly depends on the type of binder used, whereas adsorbed chlorides is controlled by the micropore structure and the characteristics of the hydrated products.
Modeling of Carbonation Based on Thermo-Hygro Physics with Strong Coupling of Mass Transport and Equilibrium in Micro-pore Structure of Concrete
Tetsuya Ishida and Chun-He Li Journal of Advanced Concrete Technology, 6(2) 303-316, 2008
A carbonation model based on thermo-hygro physics is presented in this paper. Reaction of C-S-H gel was newly added to the existing model as well as calcium hydroxide reaction, and a micro-pore structure model for carbonated concrete was improved by considering volume change and surface-area increase of hydrated products. The proposed model coupled with moisture equilibrium/transport gives reasonable predictions for carbonation progresses under low and high CO2 concentrations in a unified manner. In addition, temperature dependent parameters were installed in the system. Although detailed mechanisms should be further investigated, the proposed methodology is able to simulate carbonation phenomena under various temperature conditions.
Flexural Behaviors of Corroded RC Members with Patch Repair- Method Experiments & Simulation
Raktipong Sahamitmongkol, Siam Suwathanangkul, Porntep Phoothong and Yoshitaka Kato Journal of Advanced Concrete Technology, 6(2) 317-336, 2008
Structural performance of RC repaired by patching method was experimentally investigated and compared with non-corroded as well as corroded RC. Two repair materials; namely, polymer-modified mortar and epoxy based repair material was applied for the repair work. The mechanical properties as well as the bonding characteristics of these two repair materials are different. It was found that the polymer-modified mortar can partially restore the structural performance while the more ductile epoxy based repair material strengthen corroded RC structure so that its ultimate load carrying capacity is beyond that of non-corroded RC.
The numerical analysis was conducted to reproduce the structural performance of repair RC observed by incorporating experimentally measured properties of repair materials. The bonding characteristic between repair materials and base concrete is represented by basic Coulomb friction model. The proposed concept of analysis shows satisfactorily accurate results which match very well with experimental findings.
Mesoscopic Analysis of Mortar under High-Stress Creep and Low-Cycle Fatigue Loading
Koji Matsumoto, Yasuhiko Sato, Tamon Ueda and Licheng Wang Journal of Advanced Concrete Technology, 6(2) 337-352, 2008
Mesoscopic analyses of mortar failure under high-stress creep and low-cycle fatigue loading are presented using a newly developed time-dependent constitutive model for Rigid Body Spring Model, which is a discrete analysis method. The failure process over time was successfully expressed by adopting a four-component combined mechanical model as the time-dependent model of connected springs, and by developing a new method for determining the failure state for load-controlled analysis. The analytical model provides reasonable results not only for the stress-strain characteristics under cyclic loading but also for the inapplicability of MinerŐs law under inconstant stress levels. The mechanism of the time-dependent failure of mortar was clarified by investigating the local stress-strain behaviors.
Simplified Inverse Method for Determining the Tensile Properties of Strain Hardening Cementitious Composites (SHCC)
Shunzhi Qian and Victor, C. Li Journal of Advanced Concrete Technology, 6(2) 353-363, 2008
As an emerging advanced construction material, strain hardening cementitious composite (SHCC) has seen increasing field applications in recent years. Reliable data on tensile properties, including tensile strength and tensile strain capacity, are needed for structural design and for quality control. However, existing uniaxial tensile tests are relatively complicated and sometime difficult to implement, particularly for quality control purpose in the field. A simple inverse method based on beam bending test was presented by the authors (Qian and Li, 2007) for indirect determination of tensile strain capacity, aimed at quality control of SHCC in field applications. This paper extends this method to also determining the tensile strength based on beam bending test data. This proposed method (UM method) has been validated with uniaxial tensile test results with reasonable agreement. In addition, this proposed method is also compared with the Japan Concrete Institute (JCI) method. Comparable accuracy is found, yet the present method is characterized by much simpler experiment setup requirement and data interpretation procedure. Therefore, it is expected that this proposed method can greatly simplify the quality control of SHCCs both in execution and interpretation phases, contributing to the wider acceptance of this type of new material in field applications.
Shear Fatigue Response of Cracked Concrete Interface
Esayas Gebreyouhannes, Toshiharu Kishi and Koichi Maekawa Journal of Advanced Concrete Technology, 6(2) 365-376, 2008
The shear fatigue behavior of cracked concrete interface is experimentally investigated, on a single crack plane, to quantify the degree of deterioration per cycling of load. A simple experimental setup is used, in which a finite lateral stiffness is provided to the crack interface by using unbonded steel bars. The effects of loading amplitude, loading pattern and water exposure are examined. Time-dependent behavior of shear transfer under sustained shear load is also investigated. It is observed that, the shear fatigue response of cracked concrete interface is characterized by gradual increment of shear slip and dilation and majority of the incremental displacements occur in the first few cycles. The degree of deterioration is found to be highly sensitive to the load amplitude level and loading pattern. The relative extent of deterioration, in case of reversed cyclic loading is exceedingly large than that of single sided fatigue loading. Worse yet, downward flow of water through the crack interface is found to accelerate the shear fatigue degradation. The experimental results are summarized and a simplified phenomenological model is proposed to quantify the degeneration of shear stiffness in terms of the intrinsic accumulated normalized slip with respect to crack opening..
Behaviour of Reactive Powder Concrete Columns without Steel Ties
Adnan R. Malik and Stephen J. Foster Journal of Advanced Concrete Technology, 6(2) 377-386, 2008
In this paper, an experimental and numerical investigation of six steel fibre reinforced reactive powder concrete (RPC) columns with 150 mm square cross sections is reported. The columns were tested to failure under load at various loading eccentricities. The RPC mix contained 2 percent (by volume) of 0.2 mm diameter by 13 mm long straight steel fibres with a concrete strength of 150 MPa. The columns contained either four or seven percent of longitudinal reinforcement but no tie reinforcement in the test region. Experimental data on the axial load and lateral and axial deformations was obtained for each test, together with the failure mode. All the columns failed in a controlled manner and no buckling of the longitudinal steel or cover spalling was observed, even well beyond the peak load. In addition to the experimental works, the columns were modelled using the finite element (FE) software DIANA. A reasonable correlation was observed between the FE model results and the test data.
