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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 3
Probe penetration test applied for evaluating shotcrete compressive strength
where s
m
is the average standard deviation s
j
of each of the plates
for different ages, which is calculated by using equation (8); x
m
is
the mean of average values obtained in each of the plates for dif-
ferent ages; and CV
m
corresponds to the coefficient of variation.
(8)
,
1
1
2
å
=
×
=
N
j
j
m
s
N
s
where N is the number of sample groups; N = 4.
3.3
Stage 2: The probe penetration test along
the shotcrete layer of the tunnel
This stage aimed to evaluate whether the strength of the shotcrete
layer along the tunnel would meet the value specified in the proj-
ect. Nevertheless, performing probe penetration tests along the entire
surface of the lining layer would be impractical, mainly due to difficul-
ties in reaching all of the regions corresponding to the tunnel’s vault.
Therefore, three points for each pre-defined cross section were
evaluated. The points where the tests were performed correspond
to the sides and superior regions of the tunnel section, as depicted in
Figure 6. The tests were carried out in the tunnel over a period of 40
to 50 days after the projection of the last 100.0 m of concrete lining.
The tunnel sections were evaluated in different directions because of the
possibility of variation in the concrete compaction as a function of the pro-
jection angle and the distance. In ideal conditions, the concrete should be
projected perpendicularly to the structure. Nonetheless, in stretches such
as the roof of the tunnel, there are technical difficulties that can result in
variation in the projected angle. However, for a single projected mixture,
the experimental results indicate that such variation only affects the con-
crete strength and not the relationshipwith the probe penetration test [20].
Thus, the use of a 60º angle in the projection of the concrete plates used
to define the test’s correlation curve, which is discussed in section 3.1.1,
has no significant influence on the correlation of the test.
In general, the sections were marked every 5.0 m or 10.0 m along
the length of the tunnel. For each region, three tests were performed,
resulting in a total of nine rounds per section. In total, 610 tests were
conducted over the 450m length of the tunnel. More specifically, a
total of 235 tests were conducted in each of the sides and 140 tests
on the top. The reduced number of tests conducted on the top of the
tunnel is due to difficulties encountered in reaching the top of the
tunnel and performing the reading of the exposed length of the pin.
4. Results and discussion of results
4. 1
Stage 1: Determination of the correlation curve
of the probe penetration test
At this stage in the research, two shotcrete plates were tested. The
Figure 6 – Location of the points of the probe
penetration tests in each of the cross
sections measured in the tunnel
Table 3 – Probe penetration test results in the concrete plates
Plate 1
Plate 2
Age
13 days
36 days
13 days
36 days
Probe (pin)
C [mm]
em,
C [mm]
em,
C [mm]
em,
C [mm]
em,
1
34.21
35.05
18.23
19.30*
2
37.64
37.82
20.27
29.67
3
40.05*
33.89
21.82
24.32
4
37.37
36.91
16.59
25.29
5
33.59
32.93
22.65
26.35
6
36.14
35.76
28.68*
27.19
Average, [mm]
35.79
35.39
19.91
26.56
Standard deviation, [mm]
1.83
1.83
2.51
2.05
*Outlier: excluded from the analysis and not considered to compute average and standard deviation values;