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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 3
W. R. L. da Silva | L. R. Prudencio Jr
|
A. L. de Oliveira
results from the probe penetration test are listed in Table 3. It is im-
portant to mention that the results deemed spurious were removed
from the sample to produce the correlation curve.
The evaluation of the average coefficient of variation of the test, as
measured by the mean values and standard deviations listed in Table
3 as well as by equations (7) and (8), result in a coefficient of varia-
tion CV equal to 7.1%. This value is consistent with the value obtained
in the variability study of the test and is used to determine the number
of tests to be performed at this stage, as stated in section 2.1.1.
The test results of the compressive strength of the samples ex-
tracted from the plates are highlighted in Table 4. It should be not-
ed that the results of this test had to be corrected due to the varia-
tion in the height of the core samples and the possible presence of
micro-cracks caused by excessive vibration during the extraction
of the core samples. Corrections were performed according to the
recommendations presented in [21]. The values of the increased
average strength, which are highlighted in Table 4, were used in
the definition of the correlation curve.
The correlation curve of the probe penetration test, along with a
confidence interval of the individual and average values defined
by equations (3) and (4), is depicted in Figure 7. The lower limit
of the confidence interval outlined in Figure 7 is equivalent to the
concrete characteristic strength curve when a significance level of
5.0% was adopted.
As presented in Figure 7, the correlation curve does not cover in-
tervals over 25.0 MPa. Therefore, if the strength values observed
in the concrete lining of the tunnel are above the results obtained
in the samples, the correlation curve must be extrapolated. How-
ever, because the plates have been moulded during the projection
process, the existence of in situ values similar to those observed in
the concrete plates is probable, which presents evidence that the
concrete lining of the tunnel shows non-conforming results.
The expression of the correlation curve of the probe penetration
test is detailed in equation (9). The correlation coefficient R
2
ob-
tained in the definition of the curve was 0.807. Considering the
variability that is intrinsic of the probe penetration test in addition
to the variability associated with the characteristics of shotcrete,
the correlation coefficient of 0.807 can be considered accept-
able. It is important to note that although this value is acceptable,
complementary studies are required to obtain a better correlation
coefficient among the results. Such studies include, for instance,
increasing the sampling universe, thus enabling a more profound
statistical analyses.
Table 4 – Results of the compressive strength testing of samples extracted from the concrete plates
Plate
Core
sample
Compressive
strength,
[MPa]
h/d
Correction
factor
Corrected
compressive
strength,
[MPa]
Increased
compressive
strength,
[MPa]
Average
increase
compressive
strength,
[MPa]
Age,
[days]
1
1-1
18.3
2.13
1.00
18.3
20.1
20.6
13
2-1
19.2
2.12
1.00
19.2
21.1
13
3-1
20.3
1.87
0.99
20.1
22.1
24.3
36
4-1
25.0
1.58
0.97
24.1
26.5
36
2
1-2
8.7
1.39
0.95
8.2
9.0
9.4
13
2-2
9.2
1.44
0.95
8.8
9.7
13
3-2
15.4
1.42
0.95
14.7
16.1
17.8
36
4-2
18.8
1.37
0.94
17.7
19.5
36
Figure 7 – Correlation curve:
exposed length of the
pin × compressive strength