The adhered mortars in recycled aggregates (RA) may lower the performance of the concrete, by for instance reducing its strength and durability, and by cracking. In the present study, the effect of nylon fiber (NF) on the permeability as well as on the mechanical properties of concrete incorporating 100% RA was experimentally investigated. Concrete was produced by adding 0, 0.6 and 1.2 kg/m³ of NF and then cured in water for a predetermined period. Measurements of compressive and split tensile strengths, ultrasonic pulse velocity and total charge passed through concrete were carried out, and the corresponding test results were compared to those of concrete incorporating crushed stone aggregate (CA). In addition, the microstructures of 28-day concretes were examined by using the FE-SEM technique. The test results indicated that recycled coarse aggregate concrete (RAC) showed a lower performance than crushed stone aggregate concrete (CAC) because of the adhered mortars in RA. However, it was obvious that the addition of NF in RAC mixes was much more effective in enhancing the performance of the concretes due to the crack bridging effect from NF. In particular, a high content of NF (1.2 kg/m³) led to a beneficial effect on concrete properties compared to a low content of NF (0.6 kg/m³) with respect to mechanical properties and permeability, especially for RAC mixes.

This paper describes the resistance of mortar specimens with two different recycled fine aggregates when they were exposed to sodium and magnesium sulfate solutions up to 15 months . The important difference between the recycled fine aggregates used in this study was the water absorption. Control mortar specimen made with only river sand was also tested to compare the degree of deterioration by sulfate attack. The tests include visual examination, compressive strength ratio, expansion and mass loss, as well as microstructural observations. The experimental data indicated that the use of 50% replacement level of recycled fine aggregates led to the increased or similar resistance to both sodium and magnesium sulfate attacks. At the 100% replacement level, however, mortar specimens containing recycled fine aggregate with higher water absorption suffered serious damage in mechanical properties, and the deterioration degree is likely to be more pronounced with further exposure. This work suggests that the abso...

This paper presents an experimental study on the durability of mortar specimens incorporating inorganic alkali-free accelerator (AFA) exposed to external sulfate attack. Visual examination, compressive strength, expansion and density of mortar specimens with or without AFA were monitored up to 360 days of exposure. Additionally, in order to identify the products formed by sulfate attack, mineralogical analysis based on x-ray diffraction (XRD) was also performed on paste samples. Test results confirmed that the mortar specimens with AFA suffered a serious deterioration by sulfate attack, especially in 10% sodium sulfate solution. Furthermore, XRD traces indicated that the formation of ettringite and/or thaumasite is primarily responsible for the sulfate deterioration of mortar specimens. Conclusively, it must be taken a special care when the shotcrete with AFA is applied under the sulfate-bearing environments.

An experimental study on the resistance of concrete containing air-cooled slag (AS) and water-cooled slag (WS) against freeze–thaw cycles was conducted. For comparison, the durability of ASTM Type I ordinary Portland cement (OPC) concrete exposed to the same freeze–thaw environment was examined. To evaluate the durability of concrete exposed to the freeze–thaw environment, an experiment was conducted according to ASTM C 666 procedure A. Furthermore, the relative dynamic modulus of elasticity, surface electrical resistivity, and compressive strength of concrete specimens were measured after exposing them to freeze–thaw cycles for a predetermined period, and the results were compared with those of OPC concrete. The relationship between the freeze and thaw resistances of concrete and the air-void system (spacing factor and specific surface area) was identified. Furthermore, the microstructure of concrete exposed to freeze–thaw cycles was observed using scanning electron microscopy to identify the interfacial transition zone, cracks, and micropores. Experimental results showed that the resistance of blended cement concrete containing WS and AS against freeze–thaw cycles was significantly higher than that of OPC concrete. The concrete in which 10% of OPC was replaced by AS exhibited a similar durability as that of the concrete in which 40% of OPC was replaced only by WS. Therefore, it is expected that blended cement concrete containing WS and AS based on an appropriate mix proportion design will exhibit excellent durability in regions experiencing freezing temperatures.

With the purpose of investigating the influence of both fineness levels and replacement ratios of limestone filler on sodium sulfate attack, cement mortar and paste specimens incorporating the material were exposed to 5% sodium sulfate solution for 1 year. The resistance of mortar specimens to sulfate attack was evaluated by visual appearance, expansion and compressive strength measurements. Additionally, microstructural observations such as XRD and SEM/EDS were also performed on paste samples stored in similar conditions of sulfate attack. Experimental results demonstrated that the worst performance was noted in the mortar specimens with high replacement ratio as well as high fineness level of limestone filler, showing extensive surface damages in addition to significant expansion and strength loss. Thus, it was observed that both high replacement ratio and high fineness level have potentially a negative effect in resisting sodium sulfate attack. This phenomenon is likely attributed to thaumasite formation as a result of sulfate attack rather than gypsum formation. The present study may suggest useful information on both reasonable replacement ratio and fineness level for the application of limestone filler in sulfate environments.