Civil Engineering Reference
In-Depth Information
In the single-tube test, the radial extent of the zone of concrete examined increases
as the separation between the transmitter and receiver is increased. The resolution
decreases however, and the limiting size of a detectable defect increases. With a spac-
ing between the transmitter and receiver of 30 mm, Kennet (1971) quotes a vertical
resolution of 50mm. A number of traverses can be made at different separations to
increase the volume of concrete included in the inspection, enabling finer detail to
be detected in the zone immediately around the access hole. There is, however, some
doubt as to the actual volume of material effectively scanned in the single sonde test,
and the method is not in common use.
8.2.1.2 Double-hole test
This test is essentially similar to the single-hole test, except that the transmitter and
receiver are traversed in individual holes, whilst being held level with each other,
or obliquely. Defects which are present between the two tubes are well defined, but
there is no radial scanning as with the single-tube method. If the transducers are held
obliquely, additional information may be obtained, which can assist in the delineation
of the extent of a zone of anomalous response.
For this test, holes are usually preformed with steel or plastic tubes, as it is unlikely
that a pair of holes could be drilled sufficiently parallel to each other over the length
of a pile. The test arrangement is shown in Figure 8.3. Some errors can occur at the
base of the pile if the holes are not of the same depth, if the cables are not precisely
Figure 8.3
General arrangement of equipment for twin-tube acoustic test. Redrawn from material
provided by Testconsult Ltd.
