tensile strengths of the fibers. It is evident that the ultimate
strength of PBO is the highest but the envelope of the results is significantly
large. On the contrary the small variability of the results
for G- FRCM is correlated to the lowest mechanical properties.
The fabric with the highest performance (PBO) provided the
best composite behavior in terms of ultimate strength even if the
scatter of the results was larger. The specimens made with PBO
fibers showed also the best bond properties. The push–pull double
lap tests and the pull-off tests showed a good adhesion between
the masonry substrate and the FRCM system. Slippage phenomena
can occur in samples made with carbon fibers in the third phase of
the tensile tests or before the collapse in push–pull shear tests.
The authors believe that the experimental set-up and the
results proposed in this paper could be of help to define guidelines
for the mechanical characterization of FRCM materials. In particular
the following aspect should be considered. The mechanical
properties of the composites are influenced by the presence of
small defects in the cementitious matrix, by the non-constant
dimensions of the section and by the localization of the first cracks
with respect to the extensometer. The literature review showed
different experimental set-ups used for tensile tests on FRCM
materials. The set-up here proposed is the most efficient to avoid
damage of the samples and slippage in the grips.
The guidelines shall indicate the method to identify the points of
transition between the three phases. If these points are not clearly
detected, it is possible to draw the tangent lines and find their intersection.
All these parameters must be calculated using the area of
the textile. In the un-cracked first phase, the elastic modulus and
the maximum stress could be calculated also using the minimum
area of the matrix, after a proper detailed geometrical measurement
of the thickness and width of the specimens, in order to compare
these values with the mechanical properties of the matrix. The
analysis of the cracks spacing and size showed a good correlation
between the geometry of the fabric and the location of the cracks.
This could be an important parameter in the design procedures.
It is suggested that the push–pull and the pull-off tests should
be proposed in the guidelines as tools to evaluate the adhesion of
the FRCM to the masonry substrate. Among different available test
set-ups for push–pull tests, the authors believe that the double lap,
as presented in this paper, is the more convenient both in terms of
costs and to avoid alignment problems.
A problem that was not considered in this paper refers to the
dynamic performance of the FRCM reinforcement [39]. This important
issue could be investigated in future steps of this research
project.