Cement solidification/stabilization is an effective way of suppressing the leaching of hazardous materials from municipal solid waste incinerator (MSWI) fly ash, though the solidification of the fly ash from fluidized bed incinerators has been difficult because it causes the evolution of hydrogen gas by the reaction of contained metallic aluminum and alkaline water and results in the formation of porous and brittle solid. We found that the evolution of hydrogen gas is possible to be suppressed by improving the mixing method of the ingredients and controlling the amount of CaCl2 in the mixture. Thorough mixing of the fly ash and cement powders with a drum-type mixer and subsequent addition of water at once scarcely induces the mechanical destruction of protective oxide layers surrounding metallic aluminum particles during the mixing process, so that the aluminum particles are protected from the attack of alkaline water. Water soluble CaCl2 externally added and/or contained in the fly ash helps not only the homogeneous penetration of water into the powder mixtures but also reduces the alkalinity of water, so that it suppresses the evolution of hydrogen gas.
In the Hokuriku region, alkali-silica reaction (ASR) caused severe degradations in many concrete bridges. However, criteria for repair and replacement works of these structures are still unclear. For the development of damage evalua-tion procedure, it is necessary to investigate the load-carrying capacity of prestressed concrete (PC) girders affected by ASR-induced deterioration in actuality. This study constructed two full-size PC specimens from the high-early-strength Portland cement and reactive aggregates, and then exposed them to outdoor environmental conditions. One of them was specially mixed with a controlled amount of fly ash. After one and a half years of outdoor exposure, destructive loading tests were carried out to investigate the difference in loading capacity of the girders. From results of the long-term exposure and the tests, the flexural strength and the rigidity of the specimen with fly ash were not degraded while ASR was also effectively suppressed. In addition, cylindrical concrete cores were taken at different positions of both girders to analyze the relationship between mechanical properties of concrete such as compressive strength, static elastic modulus, and ultrasonic wave propagation speed. Results of the coring test showed that the mechanical properties of concrete cores varied according to their collecting positions and directions.
Water permeation into concrete causes various types of deterioration in concrete structures such as rebar corrosion, frost damage, ASR, etc.; however, the evaluation of the water permeation rate is difficult. This paper proposes a simple equation to evaluate the water permeation rate into concrete, based on basic information such as the mix design, curing condition, and degree of drying. Concrete specimens are cast, cut into slices of different thicknesses, and the time taken for water penetration is measured. The permeation rate is calculated from the relationship between the obtained permeation time and the slice thickness. Additionally, a literature survey is conducted to collect the water permeation rate in various types of concrete. A prediction equation for the water permeation rate is proposed, based on the data. Although this data contains various types of concrete with/without the admixture material, different water-to-binder ratios, and different curing conditions, all the calculated values fall within a range of ±150% of the data from literature.
A nondestructive inspection method which is in type using backscattered neutrons to detect void and water in concrete with RIKEN Accelerator-driven Compact Neutron Source (RANS) is developed. In this method, it is not necessary to be interposed the sample between the radiation source and the detector as in the conventional method using neutron transmission. The position of water and void in the concrete are measured from the intensity change of the backscattered neutrons selected from the incident up to 1 ms. This study reveals that it is possible to observe the two dimensional distribution of water or void inside slabs up to 6 cm below the slab surface.
[This paper was presented at the 9th Symposium on Decks of Highway Bridges (JSCE, Japan Society of Civil Engi-neers) in November 2016.]
In this paper, the transformation of solid phase and expansion cracking behaviors due to sulfate attack were evaluated by using RBSM-Truss Network Model. This analysis was constructed by combining a hydration model, a diffusion-reaction model, and a crack propagation model in order to evaluate the influence of feedback system of ion transfer through cracks on transformation of solid phase and expansion cracking behavior due to sulfate attack. As a result, the proposed model can predict the change in phase assemblage and physical properties due to hydration process and the change in the transformation of solid phase and the distribution of sulfate ion due to sulfate attack. It was found that expansion crack can affect the diffusion process and expansion cracking behavior as the crack behaves as a buffer of sulfate ion.
This work reports on the synergetic effect of water-to-cement ratio, curing conditions, varying external environment and stray current on the microstructural (porosity and pore size), electrical (resistivity) and mechanical (compressive strength) properties of 28 days-cured cement-based materials. The influence of curing on porosity and pore size, in stray current conditions, was assessed by correlating the performance of 28 days cured mortar with that of fresh (24h-cured only) mortar specimens in identical environmental medium.
Three different levels of electrical current density (i.e. 10mA/m2, 100 mA/m2 and 1 A/m2) were applied to simulate stray current flow through hardened mortar specimens with water-to-cement ratio of 0.5 and 0.35. Different environmental conditions were employed i.e. sealed conditions, partly immersed, and fully submerged in water and calcium hydroxide medium. Microstructural, mechanical and electrical properties were monitored in the course of 140 days. The outcomes suggest a potentially positive effect of the stray current, where water and/or humidity exchange with the external environment is restricted. The potential for this positive effect was experimentally supported through the recorded matrix densification and increased compressive strength of mortar specimens, subjected to stray current and treated in calcium hydroxide and/or sealed conditions, compared to equally handled and treated control cases.
In contrast, for water submerged mortar specimens, subjected to stray current, coarsening of the bulk matrix and reduced compressive strength were observed. The outcomes were irrespective of w/c ratio and curing conditions. The effect of stray current was found to be predominantly determined by the current density level and increased at values > 100mA/m2. This would result in compromised mechanical properties and potentially reduced performance of cement-based materials within service life. Therefore, concrete curing and conditioning on site need to include considerations for the potential effect of stray currents.