Issue 18

G. Ferro et alii, Frattura ed Integrità Strutturale, 18 (2011) 34-44; DOI: 10.3221/IGF-ESIS.18.04 35 composite properties over traditional reinforcing materials, more recent results have been much more promising, showing significant improvements in fracture toughness, hardness and strength in both ceramic and polymer matrices. Key factors that have contributed to these improvements include the process of distributing the CNT in the matrix material and the degree of bonding between the reinforcement and the matrix. Traditional reinforcing mechanisms such as crack bridging, fiber pull out and crack deflection have been identified in ceramic matrices, with additional, nanoscale reinforcement mechanisms also being seen [2]. Concrete is the most widely used construction material (more than 11 billion metric tons are consumed every year all over the world and the cement industry is responsible for approximately 5–8% worldwide of all anthropogenic emissions of carbon dioxide [3]) and has experienced the developing stages of normal strength concrete, high strength concrete and high performance concrete. Nowadays, the investigation of the properties of this material in the nanometric and micrometric scale range constitutes a major focus of the research, as the comprehension of these characteristics enables the understanding and the control of its macroscopic properties [4]. On the other hand, carbon nanotubes have many advantageous mechanical and electrical properties such as high strength, high conductivity and, therefore, are attractive for producing fiber-reinforced concrete. It is expected that carbon nanotubes, when added to concrete, will increase compression strength beyond 200 MPa, thus allowing the construction of mile-high skyscrapers. [5]. Moreover, the incorporation of fibers at the nanoscale will allow the control of the matrix cracks at the nanoscale level, owing, thus to increase its toughness. High performance concrete also allows a reduction of raw materials consumption, because it is then possible to achieve a specific strength with less material, and, as a consequence, to reduce energy consumption and to minimize CO 2 emissions (CO 2 emission scales with L 3 (volume), while structural strength scales with section S (columns) and S 1.5 (beams)). Generally, since aggregates occupy about 60–70% or more of the volume of a concrete mix, the possibility to use aggregates from construction demolition is also beneficial for reducing CO 2 emissions. Concrete production from recycled aggregates is mandatory to solve the problem of waste land-filling too: in Italy, in 2004 , wastes from construction demolition were equal to 800 kg per person, which signals a need for reuse to protect the environment. In particular, recycling of concrete blocks, which make up to 37% of construction waste, is an important issue to be promoted [6]. However, the incorporation of 30% of recycled aggregates into a new concrete leads to a significant reduction of the mechanical properties, thus CNTs additions can compensate for this strength loss. To conclude, nanotechnology is able to help in producing a sustainable concrete. Mechanically, CNTs show elastic behavior, with a Young’s Modulus of approximately 1 TPa and a density of about 1.33 g/cm 3 . Single walled CNTs have yield stresses between 20 and 60 GPa, with measured yield strains of up to 10% [2]. Moreover, carbon nanotubes can bear torsion and bending without breaking. Since CNTs exhibit great mechanical properties along with extremely high aspect ratios (length-to-diameter ratio) ranging from 30 to more than many thousands, they are expected to produce significantly stronger and tougher cement composites than traditional reinforcing materials (e.g. glass fibers or carbon fibers). In fact, because of their size (ranging from 1 nm to tens of nm) and aspect ratios, CNTs can be distributed in a much finer scale than common fibers, giving as a result a more efficient crack bridging at the very preliminary stage of crack propagation within composites [2] (Fig. 2). Figure 2 : Crack bridging effect in cement/CNTs composites [2].