Civil Engineering Reference
In-Depth Information
strains. A possible explanation could be that the bonding between the CNFs
and the matrix is enhanced due to the unusual outer surface texture of the
carbon nanofibers. The CNFs used in this study exhibit graphite planes
which extend beyond the diameter of the nanofiber and are present along
the circumference of the fiber. These edges probably help anchor the fiber
in the matrix, preventing interfacial slip and enabling more sufficient load
transfer across nanocracks and pores.
the Young's modulus of the samples reinforced with either MWCnts or
CNFs at the age of 3, 7 and 28 days is illustrated in Fig. 20.7. Similar to the
flexural strength results, samples reinforced with nanofibers clearly exhibit
improved Young's modulus over OPC specimens. Specifically, an increase
of the Young's modulus of 44% to 50% over plain cement specimens is
achieved with the use of MWCNTs. In the case of the samples with CNFs,
an increase of at least 50% is observed.
20.5 Mechanical properties at the nanoscale
The nanomechanical properties of the MWCNT/ CNF cementitious composites
were investigated using a triboindenter. the triboindenter is a special type of
nanoindenter that combines nanoindentation to determine the local properties
of the material at the nanoscale, with high-resolution
in situ
scanning probe
microscopy (SPM) imaging that allows pre- and post-test observation of the
sample (Fig. 20.8). A Berkovich tip with a total included angle of 142.3
degrees was used for indentation and SPM imaging. Multiple cycles of partial
loading and unloading were used to make each indention, eliminating creep
and size effects (nemecek 2009). the oliver and Pharr method was used
to determine the mechanical properties, where the indentation modulus is
calculated from the final unloading curve (Oliver and Pharr 1992). Prismatic
14
12
10
8
Plain OPC (CP)
CP + CNFs 0.048 wt%
CP + MWCNTs 0.048 wt%
6
4
0
5
10
15
20
25
30
Age (days)
20.7
Young's modulus of plain cement paste (W/C = 0.5) and cement
paste reinforced with 0.048% by weight of cement MWCNTs or CNFs.
