Radiation technology such as X-ray computed tomography (X-ray CT) is a powerful tool for materials research. Such non-destructive imaging technology allows us to visualize in three dimensions (3D) the internal structure of materials without damaging the specimen. This paper thus presents our studies on the application of microfocus X-ray CT, synchrotron radiation X-ray CT, and the integrated computed microtomography and X-ray diffraction (CT-XRD) method to advance cement and concrete research. CT images coupled with 3D image analyses allow us to identify and measure the air voids, pore scale microstructure and crack geometry. Image-based computational simulations provide us to estimate transport parameters such as diffusion tortuosity and water permeability in the digitized pore space. With the introduction of novel experimental techniques, deterioration or microstructure changes in hydrated cement systems, for example, attributed to calcium leaching, freeze-thaw cycles, elevated temperature, and steel reinforcement under tension are further elucidated. This review paper then ends with speculation of the future directions of concrete durability research via X-ray CT.
The authors have reported from experimental investigation that the use of closed stirrups, U-shaped stirrups and rod-shaped reinforcements as shear reinforcements in reinforced concrete (RC) beams results in clear differences in internal crack patterns and maximum strength. In this study, simulations of the experimental beams were performed using a three-dimensional rigid-body spring model, demonstrating that the behavior observed in the experiments is a mechanical phenomenon that can be reproduced numerically. Using the stress distribution over the beam cross section obtained from the analysis, the beam and arch action components of shear resistance are decoupled and considered from the perspective of the shear resistance mechanism. Further, through analysis of additional cases, the role of the upper and lower horizontal legs of the shear reinforcement, the effect of end anchorages on rod-shaped reinforcements were clarified.
For better understanding of irreversible shrinkage, nine hardened cement paste (hcp) samples with three different cement types and three different water to cement ratio were prepared. Four different relative humidity conditioning histories containing the first desorption, re-humidification and the second desorption are investigated for all the specimens to obtain the length change and water sorption isotherms. The irreversible shrinkage strain was developed when the specimen was dried up to less than 80% relative humidity (RH), while other previous experiments in literatures showed that the shrinkage strain between 40% RH and 11% RH is reversible. It is concluded that the irreversible shrinkage strain is developed between 80% RH and 40% RH, which is also supported by the change in water vapor BET surface area of hardened cement paste after long-term drying.
This study aims to develop and propose a seismic retrofitting method for unreinforced brick masonry for improving the seismic safety of the 1st and 3rd headquarters buildings of Kyushu University. Five brick wall specimens representing parts of the walls that were adjacent to the openings in the headquarters buildings were prepared on a 3/4th scale of the actual structure and they reinforced on either one or sides of their surfaces with reinforced concrete (RC) walls. Horizontal loading experiment was conducted to confirm the reinforcing effect. Results indicated that the maximum load of the specimens (CS01, CS02) reinforced on one side with RC walls was 6.1 to 6.2 times higher than that of the unreinforced specimen. Similarly, the maximum load of the specimens (CD01, CD02) reinforced on both sides with RC walls was 12.6 to 14.2 times higher than that of the unreinforced specimen. The specimen CS02 reached its maximum load at a smaller deformation angle compared to the specimen CS01 because of the effect of twisting. Additionally, the authors also derived and verified a horizontal strength evaluation formula for brick wall specimens reinforced with RC walls. The values calculated using horizontal strength evaluation formulas were close to the experimental values.
It can be said that corrosion of reinforcing steel is a serious problem for concrete structures. However, the corrosion mechanism of steel is still unclear, and it is difficult to prevent corrosion perfectly. In this study, in order to make clear the corrosion process in alkaline environment, the corrosion products formed in solutions of various OH- and Cl- concentrations were evaluated by Raman spectroscopy. In addition, the influence of dissolved oxygen on corrosion products was investigated. Furthermore, the corrosion mechanism of steel in concrete was investigated based on field study. These investigations made it clear that the behavior of moisture in concrete has large influence on corrosion propagation. Therefore, the influence of W/C, chloride ions and cracks on the behavior of moisture in concrete was also investigated. Based on the the results obtained, the authors propose a maintenance scenario for concrete structures damaged by corrosion of the reinforcement.
A simple and accurate method is developed to model the effective elastic properties of early age cement pastes. It uses available data of media of which the microstructures are close to that of the predicting medium. Such technique allows us to model accurately the effect of the hydration degree on the effective elastic properties. On the other hand, the hydration degree of a hydrating cement paste is predicted from the acquisition of ultrasonic velocity. The simulated results are perfectly validated against a large variety of experimental data. This simple concept provides a great potential of application in practice.
Addition of sulphoaluminate expansive agent could be one useful method to reduce the shrinkage crack destruction of shotcrete. While the setting and hardening behavior and early age mechanical strength of cement pastes containing liquid setting accelerators may be influenced by the introduction of sulphoaluminate expansive agent (SEA). Therefore, in this work, the early age hydration behavior of different accelerated cement pastes with SEA was investigated via setting times measurement, early age mechanical strength test, isothermal calorimeter, mineralogical composition, thermal gravimetric analysis (TGA) and scanning electron microscope with EDS detection. Results showed that the ability of liquid setting accelerators on shortening the setting times of cement pastes could be enhanced by the addition of SEA, especially for alkali-free type accelerator due to the prompted formation of massive rod-like ettringite phase. In addition, the further hydration extent of C3S phase in accelerated cement pastes was advanced by more dissipative portlandite phase to form such massive rod-like ettringite, thus improving the initial mechanical strength.
This paper presents a catalogue of possible radiation-induced damage of rock aggregates, which was compiled using the relevant literature and confirmed numerically. The catalogue describes two common and six specific cases of rock aggregate damage. Additionally, the catalogue is supported by a validated numerical model, which is based on Rigid-Body Spring Model. The detailed numerical analysis and the result description of all the cases shown in the catalogue are presented in this paper. The main dependencies are also discussed and the related conclusions are drawn, of which the most important are that the damage of the rock can be delayed and increased rapidly after the delay; even a small amount (1%) of a highly expansive mineral leads to a significant reduction of mechanical properties of the rock; and a partial recovery of the elastic modulus of the rocks is possible even for significantly damaged rocks. It is believed that this paper will help to predict the radiation-induced degradation of rock aggregates as well as support the future development of related analytical models.
This study investigates the effect of relative humidity (RH) on the carbonation of hydration products. The samples of ordinary Portland cement (OPC) and OPC/blast furnace slag paste were exposed to accelerated carbonation under 3% CO2 concentration at different RHs from 11% to 85%. Thermogravimetric/differential thermal analysis and X-ray diffraction/Rietveld analysis were performed to quantify the amount of portlandite and calcium carbonate polymorphs. The Ca/Si ratio of the carbonated C-S-H phase was determined using a phase equilibrium calculation. To evaluate the microstructural changes in carbonated sample, water vapor sorption and nitrogen sorption measurements were conducted before and after carbonation. Results show that the carbonation shrinkage progressively increased at intermediate RHs (43% and 66% RH). The relation between carbonation shrinkage and the Ca/Si ratio of C-S-H agrees with the change of C-S-H mean chain length. There were significant changes in the carbonation rates of C-S-H and portlandite at different RHs. The coarsening of the meso scale pore likely derives from the polymerization and aggregation of C-S-H due to decalcification. This paper is an extended and enhanced version of an earlier study [Suda, Y., Tomiyama, J., Saito, T. and Saeki, T., (2020). “Impact of relative humidity on carbonation shrinkage and microstructure of hardened cement paste.” In: Proc. 6th International Conference on Construction Materials (ConMat'20), Fukuoka, Japan 27-29 August 2020. Tokyo: Japan Concrete Institute].
This paper presents an experimental study investigating the actual mechanical performance of a full-scale reinforced concrete beam that served for 60 years in an industrial environment. The primary objective is to verify early damage detection performance using acoustic emission (AE) and digital image correlation (DIC) methods based on the four-point bending test. Parameter analysis of AE provides an accurate indication of the crack opening process, especially for the early damage stage, while RA (rise time divided by amplitude) against AF (Acoustic counts divided by durable time) and b-value analyses based on AE parameters also provide specific damage propagation results for different load stages. Furthermore, owing to the good full-field measurement resolution of DIC, the strain results in each load stage more clearly present the crack pattern and distribution before the visible crack is found, and the results of the displacement derived from DIC images also capture the global and local deformation of the beam with satisfactory accuracy compared with conventional methods. A brief summary of the advantages and disadvantages of using AE and DIC for early damage detection of the full-scale beam is also provided. It is concluded that AE and DIC both exhibit good feasibility for early damage detection from the respective interior and exterior aspects. Robust signal and image processing algorithms can be integrated with these two methods for further quantitative analysis.
This study investigates the time-dependent mechanical properties of concrete deteriorated by the alkali–silica reaction (ASR). Previous analytical and experimental studies have indicated the positive impact of ASR gel in the cracks against mechanical damage in concrete. To study the effects of ASR gel on cracked concrete, groups of cylinder specimens with different expansion levels were prepared and tested at different material ages. The compression test results showed that the deteriorated elastic modulus of the specimens could be recovered over time. Mechanical property data from the other ASR studies were collected and assessed to observe similar trends across the literature. It was observed that the recovery of the elastic modulus also occurred in previously reported experiments. The recovery of the elastic modulus is assumed to be due to the time-dependent chemical and physical properties of ASR gel, which fills the cracks. Moreover, the data indicated that parameters other than material age and expansion could be attributed to the time-dependent mechanical properties of concrete affected by ASR.
Ordinary “loop joint” applied to precast prestressed concrete (PC) deck slab tends to increase its thickness. By decreas-ing this thickness, the dead load of the deck slab can be reduced. Hence, we have developed an “improved (inclined) loop joint” that reduces the deck slab thickness. This study presents the results of static bending test of slab specimens to confirm the load-carrying behavior and wheel moving load test to examine the fatigue durability of precast PC deck slabs with improved loop joints.
The bending load-carrying behavior satisfied the requirements for highways in Japan. In addition, in the wheel moving load test, no sudden increase in vertical deflection and joint opening was confirmed at a load step of 250 kN × 100 000 times equivalent to 100 years on an actual bridge. Moreover, there was no water leakage at the bottom surface of the deck slab during the water-filling test. Based on the test results, it was inferred that the required load-carrying behavior and fatigue durability could be retained for 100 years in the improved loop joint.
To decommission of the Fukushima-Daiichi nuclear power plant after the reactor accident, it is important to estimate the distribution of radionuclide contamination in the concrete for key elements such as Cs and Sr. A reaction transport model will be developed for these calculations. However, for a realistic model, the behaviors of Cs and Sr penetration in concrete must be experimentally investigated. A part of the results of a MEXT project called “The Analysis of Radionuclide Contamination Mechanisms of Concrete and the Estimation of Contamination Distribution at the Fukushima Daiichi Nuclear Power Station” are presented in this technical report. From our penetration analyses, the behaviors of neither Cs nor Sr were affected by each other. Additionally, the apparent diffusion coefficients of Cs and Sr were not significantly affected by the concentration or the presence of clay in the mortars. The penetration depth of Sr was smaller than that of Cs, and fly ash blended cement increased the resistance to penetration compared with ordinary Portland cement. Carbonation in the mortar samples increased the adsorption of Cs especially. Sr interacted with cement hydrates more than with clays. In oven-dried mortars, under the condition of water suction, the presence of clay retarded Cs penetration but had no effect on Sr. When the mortars were carbonated and oven-dried, the interactions between the solid phase and Cs or Sr took hours at least to complete.