Page 434 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.25

between the surface and reinforcement, a hollower sound will be heard. Chain drags and
hammer tapping can be reliable ways to identify the extent of corrosion damage.
Another method of determining concrete quality is the Windsor Probe. Although listed as a
nondestructive test, the Windsor Probe does cause some damage to the concrete. ASTM C803
provides a standard for use of the Windsor Probe. Concrete strength is determined by measur-
ing the resistance of the concrete to penetration by a steel probe driven into the concrete. Note
that the Windsor Probe measures properties only of the concrete at the location where it is used.
As in the case of the rebound hammer, results of tests using the Windsor Probe will vary
depending on conditions. Surface conditions, type of aggregate, and other variables will influ-
ence results. For best results, the Windsor Probe can be calibrated by testing cores obtained
from the concrete being evaluated.
In recent years, many powerful techniques have been developed for nondestructive test-
ing. These developments include the following:

• Impact echo
• Impulse radar
• Impulse response
• Ultrasonic pulse velocity
• Modal analysis
• Cover meter
• Half-cell potential testing

Among the most useful of these systems are impulse radar, impact echo, impulse response,
and half-cell potential testing.
Impulse Radar. The impulse radar technique, illustrated in Fig. 12.22, employs high-
frequency electromagnetic energy for rapidly and continuously assessing a variety of charac-
teristics of concrete structures. Its principle of operation is based on reflection of electromagnetic
waves from dielectric constant boundaries in the material being probed.
A single contacting transducer (antenna) is used to transmit and receive radar signals.
High-frequency, short-pulse electromagnetic energy is transmitted into the concrete. The
transmitted pulse travels through the material and is partially reflected when it encounters a
change in dielectric constant. Reflected pulses are detected by the receiving section of the
transducer. The location of the dielectric constant boundary is evaluated by noting the transit
time from start of pulse to reception of reflected pulse. Boundary depth is proportional to tran-
sit time. Since concrete-to-metal, concrete-to-air, concrete-to-water, and concrete-to-backfill
interfaces are electronically detected by the instrument as dielectric constant boundaries, the
impulse radar method is capable of assessing a variety of reinforced-concrete and masonry
characteristics. The impulse radar technique can be used for the following applications:
• Evaluate presence and location of reinforcing steel, conduits, drain lines, and other
embedments in and beneath reinforced-concrete structures
• Assess thickness of reinforced-concrete structural elements
• Evaluate presence of voids beneath concrete structural elements
• Inspect concrete masonry construction for presence of reinforcement and grout in block
cavities

Impact Echo. The impact-echo technique is one of several nondestructive test methods
that specialists use for existing-condition assessment, material property evaluation, and
quality assurance testing. The impact-echo technique, illustrated in Fig. 12.23, employs