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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
3.1
Determining the coefficient of variation
of the probe penetration test
3.1.1 Description of the experimental program
The tests that were used to determine the coefficient of variation
of the probe penetration test were performed in shotcrete plates
measuring 0.60×0.60×0.15 m (width x length x height) with a com-
position identical to that used on-site. The dosage of the concrete
used in the projection of the plates and in the tunnel is highlighted
in Table 1. The plate mouldings were inclined at an angle of 60° (in
the horizontal plane) and shaped during the projection of the con-
crete in the tunnel. This measure aims not only to represent the
conditions observed on site but also to prevent the incorporation of
the material reflected and the consequent formation of low strength
concrete lenses.
For the probe penetration test, it was initially considered to mea-
sure the inclination angles and exposed lengths of the driven
pins. Nonetheless, due to the lighting conditions of the tunnel and
the high number of tests to be performed, the average length of the
exposed probe was determined by averaging two measurements
of the exposed length using a digital calliper, as shown in Figure 3.
It should also be noted that the increased roughness of the tunnel
finishing, and the surface finish of the concrete plates, are deter-
mining factors for the adoption of the measurement method used
in this study.
The probe penetration test was performed using controlled pro-
pulsion energy (kinetic energy). Such control was required when
observing the results of the early-age tests, which indicated that
the use of maximum propulsion energy resulted in the complete
penetration of the pin in most cases. This initial response is
likely associated with the low level of strength of the shotcrete
in the tunnel.
The reduction in the propulsion energy was obtained by increas-
ing the distance between the capsule where the powder load is
inserted and the pin, as illustrated in Figure 4a,b. The control of
the distance, d
p
=100 mm, from the probe to the powder load was
performed using a metal rod with a diameter similar to the probe
head and a backstop defined by a metal ring welded to the rod
(Figure 4b). This procedure was carried out throughout the experi-
mental program with the objective of reducing errors and, at the
same time, facilitating the test performance, as the operator need
than that specified in the project, i.e., f
ck
= 25.0 MPa.
To perform the probe penetration tests, the Walsywa gun -
DFG 40S Model was used. Furthermore, 22.0 mm cartridges and
55.0×6.35 mm (length × diameter) pins were used. The equipment
used in the test is illustrated in Figure 2. The Walsywa gun was
adopted to replace the Windsor gun because the latter was dif-
ficult to obtain and relatively costly. The Walsywa gun was first
used in Brazil to predict concrete properties, such as compressive
strength, in a study developed by Vieira, [13]. Currently, this meth-
od has been widely employed to investigate possible non-confor-
mities in reinforced concrete structures in studies such as those by
Evangelista, [14], and Pinto & Baggio, [9].
According to the literature, the probe penetration test is charac-
terised by a high variability, [1,2,14]. This variability results from
factors associated with errors caused by both the operator and the
equipment due to the heterogeneous character of concrete. Errors
caused by the operator, who should be duly qualified to perform
the tests, can be considered minimal, and the variability originates
mainly from the factors associated with the equipment, such as
varying the powder charge in the cartridge. The presence of coarse
aggregates in the concrete and the distribution of voids throughout
the mass greatly affect the test results and are considered to be
the main reason for the high variability of the test [2].
As a result of the above-mentioned factors, a greater number of
tests is necessary to detect significant variations in the concrete
strength when comparing, for example, the probe penetration test
with the rebound hammer test. Nevertheless, several published re-
sults, e.g., [15-17], indicate that the coefficient of variation of the
probe penetration test would not be as high, which would reduce
the total number of tests required to obtain reliable results when
investigating the strength of the finished concrete structure.
Because divergences exist, the coefficient of variation of the probe
penetration test was determined with the equipment used in this
study. Note that the determination of the coefficient of variation
of this test was performed prior to the completion of the stages
described earlier in this section.
It is important to mention that the results from the preliminary study,
which were used to determine the coefficient of variation of the
probe penetration test, are required to define the number of tests
that need to be performed in the following steps, namely, in stages
1 and 2. Thus, to facilitate the understanding of the experimental
program, the results of the preliminary steps were presented along
with a description.
Table 1 – Shotcrete mixture composition
used in the tunnel lining
Materials
Shotcrete composition
Cement CP IV 32 RS
3
400 Kg/m
Natural sand
3
720 Kg/m
Stone powder
3
120 Kg/m
Coarse aggregate
4.75/12.5mm
3
980 Kg/m
Accelerating admixture
40 l/m
3
Figure 3 – Reading of the exposed
length of the probe (pin)