The strength and development of residual strain of normal strength mortars subjected to freeze-thaw cycles incorporating carbon nanotubes (CNTs) and carbon microfibers (CMF) were studied. The workability was influenced by the fiber type, the dispersion method, and the amount of fibers. The obtained results showed that the measured flexural strength increased only in the case of mortars incorporating CMFs. The compressive strength remained unchanged in the case of mortars containing CMFs and was slightly lower when CNTs were present. The residual strain due to freeze-thaw cycles was lowered in comparison with reference mortar only when incorporating CMFs. The obtained results confirmed that in order to utilize the outstanding mechanical properties of CNTs the binder matrix must be very homogenous to provide sufficient contact area for stress transfer. The used water to binder ratio was sufficiently low only for long CMFs, which were able to bridge numerous weak inclusions present on the binder matrix.
This study presents a new connection technique to retrofit existing RC frames by steel braced frames. The proposed connection which is called the “hybrid connection” plays two important structural roles. The first role is to provide connection between an existing RC frame and a steel braced frame. The second role is to increase shear strength and axial compression capacity of boundary RC columns of retrofitted frames. The hybrid connection consists of steel plates, high-strength bolts, and high-strength grout. Experimental investigations were conducted on four one-bay one-story RC frames and one one-bay two-story RC frame which were retrofitted by the proposed method. Moreover, one one-bay one-story RC frame was tested as a non-retrofitted benchmark frame. Experimental results of the specimens indicated that the proposed hybrid connection transferred relatively high direct shear forces between the RC frames and the installed steel braced frames to obtain lateral capacity of the steel braces. In addition, the proposed hybrid connection prevented possible shear failure of the boundary RC columns with the help of the utilized steel plates. Taking into consideration experimental results and observations, simplified design approaches are presented to estimate lateral strength of fundamental failure mechanisms of retrofitted RC frames.
Long-term durability against leaching is considered necessary and is currently treated as an important issue. Electrical treatment, which can accelerate deterioration due to ion migration, has been used to evaluate long-term durability against leaching. In the case of nuclear waste disposal and facilities, long-term performance for hundreds or thousands of years is expected. The purposes of this research were to investigate with electrical treatment 1) chemical alteration, such as decrease of Ca(OH)2 and Ca/Si ratio of C-S-H, 2) physical alteration, such as increase of cumulative pore volume and threshold pore, 3) the feasibility of electrical treatment for the evaluation of diffusion coefficient alteration due to leaching with void ratio, and 4) the proposition of a simple and feasible method to evaluate the continuous pore structure after electrical treatment with MIP (Mercury Intrusion Porosimeter). This study yielded important data for understanding leaching alteration by electrical treatment and indicating the feasibility of applying electrical treatment. Moreover, it was revealed that the continuous pore structure, concerning the durability of cementitious materials, can be resolved by hysteresis curves under increased and decreased pressures in MIP.
This investigation presents a comparative study on mechanical properties, electrical resistivity and microstructure of mortar under DC current, compared to mortar in rest (no current) conditions. Monitoring was performed from 24h after casting until 84 days of cement hydration. A current density level of 10 mA/m2 was chosen as a simulation regime. It was found that the DC current exerts microstructural changes in the bulk mortar matrix and thereby affects electrical properties and mechanical performance. Whereas the latter were slightly influenced, the former were modified to a more significant extent; the current flow was found to cause initial densification of the bulk matrix at earlier stages (until 14 days of age), whereas coarsening of the material was observed after 56 and until 84 days of cement hydration. Additionally, numerical simulation of the stray current distribution is performed, meant to serve as a basis for further elaborated modelling of the level of current density that could exert significant microstructural alterations in a bulk cement-based matrix.
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