International Concrete Abstracts Portal

The rubble from construction is generally used in landfill or thrown to natural areas at the end of their service life. Because continuously increasing production of concrete consumption, recycling of concrete waste materials will provide environmental protection and economical benefits. In this study, effects of fine and coarse recycled concrete aggregates on Hot-Mix Asphalt (HMA) performance were investigated. In performed experiments, Marshall Mixtures were prepared by using recycled concrete aggregates in the proportion of 10%, 20%, and 30% by mixture weight for the replacement of limestone in suitable gradation. Six different bitumen ratios were added to each mixture, respectively. Void %, flow and stability values were examined on 54 specimens. Furthermore, indirect tensile strength experiments were examined on the specimens having optimum 4.5% bitumen content and 30% recycled concrete aggregates. The results indicate that waste concrete can be used in HMA as aggregate to obtain the required Marshall stability and indirect tensile strength of the mixtures. However, the void percent of the mixture are not desirable due to the dense gradation of aggregate. Hence, gradation change is needed to Marshall Design criteria.

Among the harmful substances carried into reinforced concrete by water, none is potentially more damaging than chloride ions, which destroy the passivity layer on steel in alkaline environment leading to corrosion processes of reinforcement and loss of durability. In this work we present direct potetiometric measurements of chloride ions in the pore water of mortar samples by using Ag/AgCl indicator electrodes. The long-term stability of electrodes in highly alkaline environment of cementitious materials and selectivity to chloride ions were improved with the deposition of polymer membrane consisting of polyvinyl chloride matrix and chloride ionophore. The chloride ingress experiments were performed on cylindrical mortar samples with over 50 sensors embedded into the solid body of the sample. The ingress of dissolved chloride ions into the mortar sample was monitored for more than six months. The concentration profiles of chlorides at different places in the sample were determined and used for the evaluation of diffusion processes of chlorides through the pores of the microporous matrix.

This study evaluated the viability of electric heating sheets for the curing of concrete housing foundations in severe, low-temperature environment. The heating sheets tested use PTC (positive temperature coefficient resistor) ceramic elements. The heating temperature is automatically adjusted by these elements in response to adjacent air temperature. The experiment was carried out in two series. The first phase was carried out at -10°C in a thermostatic chamber. The second phase involved on-site testing. Under severe, low-temperature conditions, of up to -11°C outside air temperature, it was found that curing was possible at about +15°C through the use of heating sheets. Compressive strength was found to be sufficient. The heating sheet is considered to be extremely useful for curing concrete in cold regions.

This paper presents an experimental study of combined use of steel fibers and stirrups as shear reinforcement in reinforced concrete beams as a more practical solution than using any one of them alone. For this purpose twelve T-shaped beams were tested. The studied variables were; volume percentage of fibers and transverse reinforcement ratio. To give an economical design, only low percentages of stirrups were used in the test beams. The test results showed that steel fibers are more efficient in increasing ultimate shear strength and ductility when used with small amounts of stirrups, and this efficiency will decrease when using large amounts of stirrups . The tests also demonstrated that the contributions of steel fibers and stirrups are not cumulative and there are limits for an optimum combination between them, which lead to higher shear strength and ductility. A pre-existing equation for estimating shear strength for steel fiber reinforced concrete beams without stirrups was further extended to include the effect of stirrups in low ratios.

Recycled aggregates manufactured on a trial basis with newly developed mechanical grinding equipment was evaluated in terms of quality, cost and carbon dioxide gas emission when applied to a construction site. For practical reasons, recycled aggregate processing rate was assumed to be 30 ton per hour. The recycled coarse aggregate met the quality requirements for structural concrete specified in JIS A 5308, while recycled fine aggregate satisfied a tempered specification. The proposed recycling system resulted in a 1.6 times higher estimated cost than the conventional systems, while further curtailment is possible with a more precise analysis. Estimated carbon dioxide emission was reduced 10 percent compared with conventional systems, contributing to reduction of environmental burdens.