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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 3
Probe penetration test applied for evaluating shotcrete compressive strength
(9)
.
fc
=4,221
e ,
.
0,0475
C
em
where f
c
corresponds to the predicted compressive strength, in
[MPa], of the concrete for an average exposed length of c
em
,
in [mm].
After defining the correlation curve between the exposed length of
the probe and the compressive strength of the concrete (9), it is
possible to estimate the compressive strength at various points in
the projected concrete layer along the tunnel, as discussed in the
following section.
4.2 Stage 2: The probe penetration test along
the shotcrete layer of the tunnel
Based on the correlation curve and on the respective confidence
interval illustrated in Figure 7, the exposed length of the probe
should be at least 44.3 mm (individual values), such that the con-
crete characteristic strength could be considered at 25.0 MPa,
or 39.8 mm (mean values) to the extent that the average strength
of the section has a characteristic value of 25.0 MPa.
The use of the different limits specified above depends on the
analysis performed. Both values were considered in the present
study because the analysis of the results is sometimes related to
the individual results of the probe penetration test performed in
each of the regions of the tunnel sections (individual values) and,
at other times, it is related to the average of the test results in
each section of the tunnel, i.e., the mean values.
Subsequent
to the initial analysis of the limits of the exposed length of the probe
specified above, the characteristic strength value of the different
sections of the tunnel were evaluated by statistical analysis, as
described throughout this section.
The individual results obtained from the probe penetration test on
each of the evaluated sections are illustrated in Figure 8. The aver-
age results for each region of the tunnel are illustrated in Figure 9.
The probe penetration values obtained along the tunnel (Figure 8)
were similar to the penetration values obtained on the projected
plates. Therefore, this is an excellent condition for the comparison
of the populations, rendering the extrapolation of the results dis-
cussed in section 4.1 unnecessary.
Based on the results presented in Figure 8 and considering the
limit of 44.3 mm specified for the individual values, it can be stated
that the compressive strength of shotcrete does not meet the de-
sign specifications in any location of the tunnel.
The results of Figure 8 indicate that there is considerable variation
of the exposed length of the pin and, consequently, of the cor-
responding estimated compressive strength along the tunnel. The
results are more dispersed in the left-side region of the tunnel,
whereas the top has less dispersion. The high dispersion of the
results is due not only to the nature of the material but also to the
possible lack of homogeneity resulting from shotcrete, i.e., the lack
of control of the projection technique and the amount of concrete.
Considering Figure 9 and the limit of 39.8 mm specified for the
average values, it is possible to conclude yet again that the com-
pressive strength of the shotcrete is not compliant. Hence, it is
not possible to affirm that at 95% confidence the concrete aver-
age strength exceeds 25.0 MPa in all sections along the length
of the tunnel.
To provide more accurate information, which would thus enable
an analysis of the future stability of the structure, the characteristic
strength of the shotcrete was determined in different parts of the
tunnel to allow for differentiated solutions to structurally reinforce
each stretch. Therefore, the tunnel is divided into seven portions,
as illustrated in Figure 10.
The individual results of the compressive strength at each site pro-
vided by the probe penetration test and equation (9) were grouped,
and the mean
f
cj
and standard deviation
s
d
of each group were cal-
culated. With these data, the characteristic strength of the concrete
f
ck
was calculated through equation (10). A summary of all of the
results of this analysis is presented in Table 5. The values of the
concrete characteristic strength defined in each of the sections are
depicted in Figure 10.
(10)
.
645 ,1
d
cj
ck
s
f
f
×
- =
From Table 5, it can be noted that the strength values are less than
the characteristic value specified in the project. The top and right-
side sections presented, in most cases, the best results, whereas
the left-side section displayed the worst results, primarily in the
final stretch of the tunnel.
Finally, it is possible to observe in Figure 10 that the compres-
sive strength of the concrete generally decreases along the length
of the tunnel. Such trend was expected once the shotcrete at the
entrance of the tunnel is older than that projected onto the final
stretch of the tunnel.
5. Final considerations
In the present work, the probe penetration test was used as the
basis of a methodology assessing the shotcrete compressive
strength. In light of the observations made during the tests and the
results presented here, the following conclusions were obtained.
Regarding the tests, the low-cost solution proposed here shows
promising results. In this case, the Windsor gun, commonly used
in the probe penetration test, was replaced by a low-cost gun avail-
able on the market that can be used to fix steel pins into concrete.
Moreover, during the tests, the propulsion energy control proved to
be an important factor to ensure that the test shows sufficient sensi-
tivity to detect different levels of strength. For practical reasons, only
one level of energy was used to investigate the structure in ques-
tion. Nonetheless, the assessment of the influence of the propulsion
energy in concrete with different levels of strength is recommended
to optimise the test methodology proposed in this study.
The analysis of the structure shows that the compressive strength
of the tunnel did not meet the project specification of f
ck
=25,0 MPa.
However, the results indicate that in general, the characteristic
strength of the concrete is greater than 15.0 MPa, except for in the
last 100.0 m of the left-side region of the tunnel. With the exception
of the left stretch, f
ck
values between 15.0 MPa and 20.0 MPa could
be used as a reference in assessing the load capacity of the struc-
ture in its current state and the consequent definition of the struc-
tural reinforcements. The values of the characteristic compressive