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.
This paper describes an experimental study that investigated the influence of ultrafine fly ash (UFFA), 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 restrained mortar mixtures containing the typical fly ash (FA) (ASTM Type-F) or the ultrafine fly ash demonstrated lower levels of residual tensile stress than the control mixture. Also, both typical fly ash mortar and ultrafine fly ash mortars demonstrated a reduction in free shrinkage. In addition, an increase in the age of cracking and a decrease in stress relaxation (creep effect) were observed in the restrained mortar ring specimens containing FA or UFFA. However, at equal replacement of cement with either FA or UFFA, the delay in the age of cracking and the reduction in stress relaxation were higher for the UFFA modified mortar.
Aside from porosity, the degree of pore connectivity and tortuosity are important pore structure characteristics of cement-based materials to understand better the effect of microstructure on transport processes that influence the durability of these materials. Synchrotron X-ray computed microtomography is employed to provide information about the three-dimensional (3D) pore structure at submicron resolution (0.5 μm/voxel) using the SPring-8 facility in Hyogo, Japan. This paper presents a method to characterize the pore space of hardened cement pastes taken from different specimens of various ages (2, 7, and 28 days) of curing. By defining the pore threshold value on the basis of the transition point in the porosity-threshold dependency curve, the pores were distinguished from the solid matrix in the microtomographic images. Further, pore cluster multiple labeling 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 decreased and became more disconnected. Sensitivity analysis to pore threshold value was performed to evaluate the robustness of the method.
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.
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.
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 well with experimental findings.
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 numerical model provides reasonable results not only for the stress-strain characteristics under cyclic loading but also for the inapplicability of Miner's law under varying stress levels. The mechanism of the time-dependent failure of mortar was clarified by investigating the local stress-strain behaviors.
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.
The shear fatigue behavior of cracked concrete interface was experimentally investigated on a single crack plane to quantify the degree of deterioration per load cycle. A simple experimental setup was used, in which a finite lateral stiffness was provided to the crack interface by using unbonded steel bars. The effects of loading amplitude, loading pattern and water exposure were examined. Time-dependent behavior of shear transfer under sustained shear load was also investigated. The shear fatigue response of cracked concrete interface was found to be characterized by gradual increments of shear slip and dilation and majority of the incremental displacements to occur in the first few cycles. The degree of deterioration was found to be highly sensitive to the load amplitude level and loading pattern. The relative extent of deterioration in the case of reversed cyclic loading was much larger than that of single-sided fatigue loading. Worse yet, the downward flow of water through the crack interface was 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 intrinsic accumulated normalized slip with respect to crack opening.
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 in either concentric or eccentric compression with varying initial eccentricities. The RPC mix contained 2% (by volume) of 0.2 mm diameter by 13 mm long straight steel fibres with concrete strengths ranging from 140 to 155 MPa. The columns contained either 4% or 7% 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 without observing spalling of concrete cover or buckling of the longitudinal reinforcement to well beyond the peak load. The columns were further modelled using the finite element (FE) software DIANA, the results from which reasonably correlate to test data.