Page 436 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.27

transient stress waves and their reflection to rapidly detect, locate, and classify flaws within
hardened concrete.
The impact-echo technique can serve as a very effective tool when there is a need to
• Evaluate the thickness of a concrete member
• Locate poor consolidation and voiding in reinforced concrete
• Detect areas of delamination in concrete
• Detect debonding of an overlay
• Detect degree of grouting in post-tensioning ducts
The impact-echo nondestructive technique employs low-frequency mechanical energy
to rapidly detect, locate, and classify discontinuities within hardened concrete; detect voids
and delaminations; and measure the thickness of concrete elements. The test method is
based on physical laws of elastic stress wave propagation in solids.
A mechanical impactor source and an electromechanical receiving transducer/ accelerom-
eter are positioned on the same face of the test object. The impactor generates a broadband
stress pulse. Waves of mechanical energy propagating through the concrete are reflected
from the opposite boundary of the test object, and the reflected energy is detected by the
transducer. The time-voltage responses of the receiving transducer are averaged for two to
three impacts with fast Fourier transform (FFT) frequency analysis algorithms by a
dynamic signal analyzer. Reflections or “echoes” are indicated by frequency peaks in resul-
tant spectral plots of displacement amplitude versus frequency.
Since amplitude, phase, and direction of mechanical energy are modified by interfaces
between materials of different density and stiffness, the location and characterization of
internal discontinuities such as defects in concrete are possible. Evaluation of reflected sig-
nal strength and shape allows characterization and classification of flaws.

Impulse Response. The impulse response test, illustrated in Fig. 12.24, has several advan-
tages over other nondestructive testing methods, including the robust nature of the apparatus,
which can be used to test relatively rough concrete surfaces; its fast output, with a test rate of
a point every minute in ideal access conditions; and the repeatability of test results. Structures
with very difficult access such as chimneys, silos, tunnels, and dams have been economically
tested using the impulse response method as the principal evaluation tool.
The method uses a low-strain impact to send a stress wave through the tested element.
The impactor is usually a 1-kg sledgehammer with a built-in load cell in the hammer head.
The maximum compressive stress at the impact point in concrete is directly related to the
elastic properties of the hammer tip. Typical stress levels range from 5 MPa for hard rub-
ber tips to more than 50 MPa for aluminum tips. The response to the input stress is normally
measured by a velocity transducer (geophone). This receiver is preferred because of its sta-
bility at low frequencies and its robust performance in practice. Both the hammer and the
geophone are linked to a portable field computer for data acquisition and storage.
Both the time records for the hammer force and the geophone velocity response are
processed in the field computer using the FFT algorithm, and the resulting velocity spec-
trum is divided by the force spectrum to obtain a transfer function, referred to as the mobil-
ity of the element under test. The test graph of mobility plotted versus frequency from 0 to
1 kHz contains information on the condition and integrity of the concrete in the tested ele-
ments, obtained from the measured parameters of dynamic stiffness, mobility and damping,
and the peak/mean mobility ratio.
Half-Cell Potential. Steel embedded in concrete is normally protected against corrosion
by the natural alkalinity of the concrete. Concrete typically has a pH greater than 12.5, and