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This paper aims to provide guidelines for selecting the correct type of carbon nanotube (CNT) to improve the mechanical properties of cementitious materials. Previous researchers have discussed the effect of CNT characteristics on their dispersion quality. However, the effect of these characteristics on the mechanical properties of CNT-reinforced cementitious materials is not fully understood. To clarify this, the study reported in this paper was conducted in two phases. In the first phase, a database was established from the literature to study the influences of three different parameters associated with CNTs (length, diameter and concentration based on the weight percent of cement powder (c-wt%)) on compressive and flexural strengths. The analyses revealed that short and small-diameter CNTs could be beneficial for increasing compressive strength. Conversely, relatively long and large-diameter CNTs were more effective in increasing flexural strength. In general, an average CNT length of 10–20 μm and an average diameter of 20–32·5 nm resulted in the highest overall mechanical performance. The optimal upper limit concentrations for flexural and compressive strengths were found to be 0·15 and 0·20 c-wt%, respectively. In the second phase of this study, the statistical analyses were experimentally verified using the CNT optimum length, two diameters and three levels of concentrations.

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This study investigates the impact of accelerated aging conditions on the long-term flexural behavior and ductility of reinforced concrete (RC) members with glass fiber-reinforced polymer (GFRP) bars (RC-GFRP specimen) and steel bars (RC-steel specimen). A total of thirty six specimens were designed with different amounts of reinforcement with three types of reinforcing bars (i.e., helically wrapped GFRP, sand-coated surface GFRP and steel). Eighteen specimens were subjected to sustained loads and accelerated aging conditions (i.e., 47 °C and 80% relative humidity) in a chamber. The flexural behavior of specimens under 300-day exposure was compared to that of the companion specimens without experiencing accelerated aging conditions. Results indicate that the accelerated aging conditions reduced flexural capacity in not only RC-steel, but also RC-GFRP specimens, with different rates of reduction. Different types of GFRP reinforcement exhibited different rates of degradation of the flexural capacity when embedded in concrete under the same exposure conditions. Several existing models were compared with experimental results for predicting the deflection and deformability index for specimens. Bischoff and Gross's model exhibited an excellent prediction of the time-dependent deflections. Except for the deformability index proposed by Jaeger, there was no general trend related to the aging duration. This study recommends the need for further investigation on the prediction of the deformability index.

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