Mechanical performance of novel cement-based composites prepared with nano-fibres, and hybrid nano- and micro- fibres

Use of hybrid fibre composites that exploit the synergistic effect of nano- and micro-additives can potentially lead to significant improvements in the toughness and mechanical properties of fibre reinforced cementitious materials. In this study, the mechanical properties of two types of novel cementitious composite (Carbon Nano-Fibre (CNF) Composites, and Hybrid-Fibre Composites) at various curing ages have been evaluated, along with their microstructure. Experimental results show a positive impact of nano-fibres on the mechanical performance of the cementitious composites: improvements of 40% in flexural strength, 45% in tensile strength, and 85% in toughness were observed when a low mass % (0.025%) of CNFs was combined with steel fibres. SEM observations revealed that reinforcement at the nanoscale prevented nano-crack development within the composites, with a greater amount of energy required to initiate and propagate cracks and cause material failure

Effect of Undensified Silica Fume on the Dispersion of Carbon Nanotubes within a Cementitious Composite

The synergistic effect of multi-walled carbon nanotubes (MWCNTs) and Undensified Silica Fume (USF) on the microstructure of cementitious composites has been studied. In the current work, USF was used to enhance the dispersion of nanotubes throughout the composite and prevent the re-agglomeration of nanotubes by providing a physical barrier of particles of small size. Ultrasonication was employed to disperse MWCNTs in water in the presence of polycarboxylate-based superplasticizer (PCE) as a dispersion agent. The results indicate that incorporation of USF considerably improves the dispersion of nanotubes in the composites, with subsequent enhancement of composite packing density. This enhancement can be attributed to the synergistic effect of MWCNTs and USF in reducing the volume of pores through the cementitious composites

Reactive powder concrete reinforced with steel fibres exposed to high temperatures

An experimental investigation was carried out to assess the mechanical properties of reactive powder concrete (RPC) reinforced with steel fibres (2% in vol.) when exposed to high temperatures. The compressive, flexural and tensile strength, modulus of elasticity and postcracking behaviour were assessed after specimens’ exposure to different high temperatures ranging from 400 to 700ºC. The mechanical properties of the RPC were assessed for specimens dried for 24 hours at 60 ºC and 100 ºC. Partially dried specimens (60 ºC) exhibited explosive spalling at nearby 450 ºC, while fully dried RPC specimens (100 ºC) maintained their integrity after heating exposure. In general, the mechanical properties of RPC significantly decreased with the increase of the temperature exposure. The rate of decrease with temperature of the compressive, tensile and flexural strengths, as well the corresponding post-cracking residual stresses was higher for exposure temperatures above the 400 ºC.The authors would like to acknowledge the Zhejiang Boen Company and MAPEI Company for providing gratuitously, respectively, the steel fibers and micro silica fume. The first author would also like to acknowledge the grant obtained under the scope of the Erasmus Mundus - Marhaba project. The third author wishes to acknowledge the grant SFRH/BSAB/114302/2016 provided by FCT.info:eu-repo/semantics/publishedVersio

Effect of high-intensity sonication on the dispersion of carbon-based nanofilaments in cementitious composites, and its impact on mechanical performance

Carbon-based nanofilaments are promising materials for improving the mechanical performance of cementitious composites. To date, the main challenge in their effective use has been controlling the dispersion of these additives in water and in the resulting mixed composites due to their strong van der Waals self-attraction and hydrophobic surfaces. This study uses high-intensity sonication to disperse different nanofilament types in water, and assesses their resulting reinforcing efficiency in cementitious composites. The proportion of nanofilaments used (in this case, multiwall carbon nanotubes MWCNTs, functionalized multiwall carbon nanotubes F-MWCNTs, and carbon nanofibres CNFs) was 0.025% by weight of cement. Aqueous dispersions were examined using transmission electron microscopy (TEM) and optical microscopy, and ultraviolet-visible (UV–vis) spectroscopy. Compressive, flexural and splitting tensile strengths tests, and porosity and density measurements, were used to evaluate the mechanical properties of the composites. High-intensity sonication over short durations significantly improved the dispersion, and reinforcing and filling effects, of carbon-based nanofilaments in cementitious composites, with increases in compressive strength of 24–32%, splitting tensile strength of 45–50%, and flexural toughness factor of 30–40%, observed after 28 days curing. A 17–26% reduction in the porosity of the composite materials was also recorded

Effect of undensified silica fume on the dispersion of carbon nanotubes within a cementitious composite

The synergistic effect of multi-walled carbon nanotubes (MWCNTs) and Undensified Silica Fume (USF) on the microstructure of cementitious composites has been studied. In the current work, USF was used to enhance the dispersion of nanotubes throughout the composite and prevent the re-agglomeration of nanotubes by providing a physical barrier of particles of small size. Ultrasonication was employed to disperse MWCNTs in water in the presence of polycarboxylate-based superplasticizer (PCE) as a dispersion agent. The results indicate that incorporation of USF considerably improves the dispersion of nanotubes in the composites, with subsequent enhancement of composite packing density. This enhancement can be attributed to the synergistic effect of MWCNTs and USF in reducing the volume of pores through the cementitious composites