Abstract
Following the evolution of currently enforced Performance Based Design standards of reinforced concrete (RC) structures for durability, the designer, rather than complying with given prescriptive limits, may instead specify a cementitious mix design that is proven to exhibit a code prescribed resistance level (class) to a given exposure environment. Such compliance will lead to the protection of the steel reinforcement from corrosion and the cementitious mortar from degradation, during the design lifespan of the structure, under aggressive environmental exposure conditions such as, marine or deicing salts and carbonation. In this context, the enhancement of the physical and durability properties of common cement-based mortars under chloride exposure are experimentally investigated herein. In particular, the experimental program reported herein aims to evaluate the influence of incorporating multi-walled carbon nanotubes on the physical and mechanical properties of reinforced mortars against chloride ions. Furthermore, the anticorrosion protection of cementitious composites prepared with nanomaterials at 0.2% w/w is further investigated, by comparing all test results against reference specimens prepared without any additive. Electrochemical (Half-cell potential, corrosion current) and mass loss of reinforcement steel measurements were performed, while the porosity, capillary absorption and flexural strength were measured to evaluate the mechanical and durability characteristics of the mortars, following a period of exposure of eleven months; SEM images coupled with EDX analysis were further recorded and used for microstructure observation. The test results indicate that the inclusion of the nanomaterials in the mix improved the durability of the mortar specimens, while the nano-modified composites exhibited higher chloride penetration resistance and flexural strength than the corresponding values of the reference mortars. The test results and the comparison between nanomodified and reference mortars showed that the use of CNTs as addition led to protection of steel reinforcing bars against pitting corrosion and a significant improvement in flexural strength and porosity of the mortars.
Highlights
Fundamental research undertaken in recent years focuses on the design and development of modified cementitious materials with improved mechanical and durability properties
The results of the porosity measurements performed on cement mortars using vacuum saturation at 7, 28, 120 and 240 days are shown in Figure 5, where it can be seen that both examined groups exhibit a gradual reduction in their porosity from 7 to 240 days
From the results it is evident that the initial capillary porosity of the nano modified composites is lower than that of the REF mortars without any addition, in agreement with test results reported for different mortar mixes and carbon nanotubes (CNTs) concentrations by other investigators
Summary
Fundamental research undertaken in recent years focuses on the design and development of modified cementitious materials with improved mechanical and durability properties. Apart from the excellent mechanical properties, carbon nanotubes have the ability to fill the gap between the hydration products and the unhardened cement grains, reducing the porosity at the micro- and nano-scales, as well as the permeability of the mortar. These factors above lead to an increase in the density of the structural material and a corresponding reduction in the corrosion of the reinforcement [16]. The main objective of the present experimental investigation is the study of the utilization of multi-walled carbon nanotubes (MWCNTs) for the production of high-performance cementitious mortars with improved physicalmechanical properties and chloride penetration resistance
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