Civil Engineering Reference
In-Depth Information
Impulse radar systems have gained the greatest acceptance for field use
and most commercially obtainable systems are of this type. The power
output of the transmitted radar signal is very low, and no special safety
precautions are needed. However, in the UK a Department of Trade and
Industry radiocomms licence is required to permit use of investigative radar
equipment.
Radar equipment
Impulse radar equipment comprises a pulse generator connected to a
transmitting antenna. This is commonly of a bow-tie configuration, which
is held in contact with the concrete and produces a divergent beam with a
degree of spatial polarisation. A centre frequency antenna of 1-1.5 GHz
is often used in the investigation of relatively small concrete elements, up
to 500 mm thick, while a 500 MHz antenna may be more appropriate for
deeper investigations. However, a lower frequency loses resolution of detail
despite the improved penetration.
An alternative to using surface-contact antennae is to use focused beam
horn antenna with an air gap of about 300 mm between the horn and the
concrete surface. These systems have been used in the USA and Canada to
survey bridge decks from a vehicle moving at speeds of up to 50 km/hr,
principally to detect corrosion-induced delamination of the concrete slab.
Operational details are provided in ASTM D4748 (ASTM, 2006).
Structural applications and limitations
In addition to the assessment of concrete bridge decks, radar has been used to
detect a variety of features buried within concrete, ranging from reinforcing
bars and voids to murder victims.
The range of principal reported structural applications is summarised
in
Table 1.3.
Interpretation of radar results to identify and evaluate the
dimensions of sub-surface features is not always straightforward. The
radar picture obtained often does not resemble the form of the embedded
features. Circular reflective sections such as metal pipes or reinforcing bars,
for example, present a complex hyperbolic pattern due to the diverging
nature of the beam. The use of signal processing can simplify the image but
interpretation is still complex. Evaluating the depth of a feature of interest
necessitates a foreknowledge of the speed at which radar waves will travel
through concrete. This is principally determined by the relative permittivity
of the concrete, which in turn is determined predominantly by the moisture
content.
Figure 1.27
shows radar in use on a concrete investigation.
Because of the difficulties of interpretation, surveys are normally conducted
by specialists who rely on practical experience and have a knowledge of the
limitations of the technique in practical situations. For example, features
such as voids can be particularly difficult to detect if located very deep
or beneath a layer of closely spaced reinforcing steel. Neural networks or
