Civil Engineering Reference
In-Depth Information
paired in length, or if the tubes are of significantly larger diameter than the transmitter
or receiver units.
8.2.1.3 3D testing
Representing the extent and form of a defect in 3D is a significant improvement on
2D outputs from other forms of sonic logging and can be achieved by employing four
tubes within a pile shaft and producing six 2D profiles for the planes that exist between
the tubes. By combining the data a 3D image can be achieved with a good idea of its
extent and location across a pile shaft. The system has been developed in the United
Kingdom by Tesconsult Ltd.
8.2.2 Radiometric tests
These test methods employ nuclear radiation techniques for the detection of variations
in density of the pile material, and hence the location of apparent defects.
In many respects, radiometric test methods are similar to those employed in acoustic
tests. Both generally require pre-selection, as access tubes require to be cast in the piles
for test. As a probe traverses the pile, radiation is received and measured, either from
a source mounted on the probe in the same tube, or from a source and detector in
separate tubes. The method can however be employed in fresh concrete. At the present
time, radiometric methods are not commonly used in the United Kingdom.
8.2.2.1 Backscatter test (gamma-ray)
When a single tube is used (or a number of individual tubes in the same pile), the probe
is constructed as shown in Figure 8.4. A probe of this type is described by Preiss and
Caiserman (1975); overall dimensions of 48mm diameter by 600mm long are quoted.
A 54mm-diameter access hole is required. Such a hole could be drilled in a pile by
rotary percussion; or more usually preformed with a steel tube.
The radioactive source produces gamma rays which travel into the concrete of the
pile. A proportion of the rays emitted are scattered in the pile material and reflected
back towards a detector which is separated from the source by a lead shield within the
probe. The proportion of backscattered particles counted by the detector varies with
the density of the pile material. Low-density zones, such as may occur at an inclusion
of soil, increase the count rate, as less radiation is adsorbed. A density profile with
depth can be produced in this way. The probe is lowered on a cable, and count rate
observed and recorded at the surface. With a probe of the dimensions described, a zone
around the access tube of approximately 100mm radial extent is examined. In order to
satisfactorily examine the majority of the concrete in a large-diameter pile, a number
of parallel access holes would be required.
8.2.2.2 Neutron test method
A neutron source is employed for checking between pairs of tubes in a pile, as a gamma-
ray source capable of being used in this way could pose a radiation hazard. Neutron
transmission varies with the moisture content of the material through which it passes.
