Civil Engineering Reference
In-Depth Information
cementitious matrix reduced, they also trigger the modification of the CSH
gel towards longer silicate chains. as a result, silica nanoparticles appear
to greatly delay the degradation rate of the calcium leaching process in
high-performance concretes, while the mechanical properties are enhanced
(Gaitero et al. 2008).
All the above-mentioned topics present excellent examples of nanoscale
additions that optimize the processing, modify the nanostructure of the
matrix and develop nano-engineered materials with improved macroscopic
properties and desired performance.
20.2 Nano-modification of cement-based
materials
CNTs and CNFs are leading the way into nanotechnology. The concept of
creating multi-phase, high-performance nanocomposites, is currently under
development in a wide variety of matrices, with the emphasis to date being
on polymers.
Cement-based materials, such as concrete, are typically characterized
as quasi-brittle materials that exhibit low tensile strength and low strain
capacity. typical reinforcement of cementitious materials is usually done at
the millimeter scale and/or at the microscale using macrofibers and microfibers,
respectively. However, while microfibers delay the development of formed
microcracks, they do not stop their initiation. The incorporation of fibers
at the nanoscale will allow the control of the matrix flows and cracks at
the nanoscale level and essentially create a new generation of a 'crack-free
material' (Shah et al. 2009; Konsta-Gdoutos et al. 2010a).
CNTs are considered one of the most beneficial nanomaterials for nano-
reinforcement. Cnts are long, hollow cylindrical structures with the length
extending from few tens of nanometers to several micrometers and the outer
diameters ranging from 2.5 to 30 nm. Cnts can be roughly divided into two
categories: the single-walled Cnts (SWCnts), and the multi-walled Cnts
(MWCnts). the SWCnt is a single graphite sheet that has been rolled into
a hollow cylinder. MWCnts consist of many concentric graphite layers, held
in place by van der Waals forces. Their unique mechanical, electrical and
chemical properties make them a revolutionary candidate for reinforcement of
composite materials. the Young's modulus of an individual nanotube should
be around 1 tPa and its density is about 1.33 g/cm
3
(Salvetat et al. 1999).
Molecular mechanic simulations suggested that Cnt fracture strains were
between 10% and 15%, with corresponding tensile stresses on the order of
65-93 GPa (Belytschko et al. 2002). their aspect ratios are generally beyond
1000.
