Nondestructive testing methods are increasingly important for assessing aging infrastructure in a cost-effective manner. Common nondestructive testing techniques for concrete include rebound hammer testing, ultrasonic pulse velocity testing, impact-echo testing, and ground penetrating radar. Each method has advantages and limitations depending on the type of defect being detected and material properties. Operator experience is important for proper use and accurate interpretation of nondestructive testing results.

1. Nondestructive Testing
Kerry Hall
Department of Civil and Environmental Engineering
Adapted from L. J. Struble and J. S. Popovics
Motivation
US infrastructure is deteriorating: 2009 ASCE Report card for
American infrastructure gave an overall grade of “D” – estimated $2.2
trillion investment needed for improvements
Infrastructure agencies are shifting efforts from building new
structures to assessing and rehabilitating existing structures
Minneapolis I-35
bridge collapse

2. Momentum and Collisions Rebound Hammer Test
Elastic collisions, both
momentum and kinetic energy
are conserved
Inelastic collision, momentum
is conserved, but energy is
absorbed
Object impacts solid, energy
absorbed, thus V2<V1
adapted from J.S. Popovics
Low strength material absorbs
more energy, thus lower
rebound height

3. Rebound Hammer - ASTM C 805
Rebound Hammer - ASTM C 805
Sometimes called the
Measures surface hardness Schmidt Hammer or Swiss
Related to Modulus of Elasticity Hammer
Light weight, portable, hand
Affected by varied conditions
operated
form material and type of finish
Spring loaded steel hammer
moisture content
impacts plunger – hammer
aggregate type and proportion rebound is measured
surface smoothness
A test is the average rebound
temperature number of ten determinations
direction of impact made in a small area.
depth of carbonation of the surface
Penetrating Probe – ASTM C 803 Penetrating Probe – ASTM C 803
Drives steel probe or pin into concrete A steel probe driven
Measures toughness of the concrete, into the concrete
the ability to resist fracturing. surface using a powder
Related to the tensile strength actuated gun
Affected by varied conditions
Hazardous – wear
Nature of formed surface
protective equipment
Coarse aggregate type, size and hardness
Moisture content
Penetrating Probe – ASTM C 803 Correlation with strength
Portable and hand A correlation of the nondestructive test
operated parameter to strength for each type of concrete
May require a license to be tested is necessary to determine in-place
to operate strength.
Three probes shot into A statistical evaluation of the correlated data is
the concrete and the necessary
average penetration ACI 228.1R, In-Place Methods to Estimate
determined. Concrete Strength presents methods for
correlation and statistical analysis.

4. What are waves? Mechanical body waves in
Propagation of a disturbance through a medium; solids: P-waves
mass is not transported in propagation direction also: Longitudinal (L-) Waves, Compression Waves
The time dependant disturbance is
1.5
T
usually expressed in harmonic form
1
The period (T) is the time required for Excitation
Dis place me nt
0.5
<T. Voigt >
0
wave motion to complete a round trip
-0.5
(measured in seconds)
-1 The frequency (f) is the inverse of T
phase delay
-1.5 (measured in 1/seconds or “Hertz”) In
0 20 40 60 80 100
audible sound, frequency is interpreted
Time
as the pitch Direction of Travel Direction of Particle Motion
Frequency-wavelength relation for all harmonic waves Wave Velocity: Governing Parameters
Propagation velocity V V =λ f Wavelength λ in Young’s Modulus E
in units of distance units of distance E(1 − ν )
per time ω= 2πf is “circular frequency” and vP = Poisson’s Ratio v
k = 1/λ is “wave number” ρ (1 + ν )(1 − 2ν )
Density ρ
Mechanical body waves in
Guided wave modes
solids: S-waves Shear Waves
also: Transverse (T-) Waves,
<T. Voigt >
Excitation
<www.lamit.ro/earthquake-early-warning-system.htm>
Rayleigh surface wave travels along free surface, slightly slower than S-wave
Direction of Travel Direction of Particle Motion
Wave Velocity in solids: a propagating “resonance”, must be set up over distance or time
Governing Parameters
propagation
G Shear Modulus G E
vS = v BAR =
ρ Density ρ ρ
no propagation in liquids or gases !
VP > VS in all known solids 1-D bar wave travels along long cylinder or prism, slightly slower than P-wave
Reflection and refraction –
Guided waves in plates
Impact echo frequency normal incidence
When an incident wave
Lamb wave are set up in large plates encounters the boundary with
another material, part of the
Multiple (infinite) modes of
incident energy is reflected, and
propagation, with varying motion
character and propagation velocity
the rest is transmitted (refracted)
Can be visualized as a propagating pr/po is the reflection coefficient
resonance (r); pt/po is the transmission
Increasing frequency or plate thickness
coefficient (t)
r is maximized and t minimized
when Z1>>Z2 or Z1<<Z2.
<Krautkramer and Krautkramer>
Acoustic impedance : Z = V ρ
<N. Ryden>

5. Reflection and refraction, mode
Beam divergence
conversion wave encounters the boundary with
When an obliquely incident The principles of wave interference and superposition control the
directivity of the generated pressure field. A given transducer may
another material, reflection and refraction become dependant on ϑi
primarily generate P-wave energy in some directivity field, although
(Snell’s law). Conversion to other wave modes also occurs. some S-wave and Rayleigh wave energy, may also generated in solid
media
The beam divergence
angle α of a given
transducer can be
estimated:
α
1.2λ
sin α =
<www.ndt-ed.org> D
<Gibson 2005> <www.ndt-ed.org>
Beam divergence: point source Scattering
of waves The reflection of ultrasonic energy away from the original direction of
Point sources of waves have poor directivity and generate P-waves, S- propagation; caused by reflection, refraction and mode conversion from internal
waves and Rayleigh waves inclusions. Causes signal loss, signal dispersion and scattering “noise”
Solid material
Detected back-
<Richard et al. 1970> scattered signal
Snapshot of wave fields (stress) in material owing to transient point load <Oelze 2007>
at some time “t” after wave excitation
Implications: transducer contact
Absorption and attenuation
Wave absorption is the conversion Wave attenuation is the overall loss
(coupling)
To eliminate significant wave reflection at the
of ultrasonic wave energy to other of wave energy with propagation, transducer-test material interface, must use a
forms of energy (heat). A caused by substance to displace air and ensure good
significant source of wave energy contact: oil, gel, grease, solid
loss for asphalt concrete * beam divergence (geometric)
* scattering Problematic for rough or uneven surfaces
* absorption <www.ndt-ed.org>
Dry point contact transducers
obviate the need for couplant
material. Each point transducer
needs vertical pressure to
ensure wave energy transfer
<http://www.greerindustries.com> <www.ndt-ed.org> <M. Schickert and MSIA Spectrum>

6. Implications: Detection of Implications: lateral defect
defects resolution
Ultrasonic waves show large reflection at interfaces between high (concrete) and low The ability to resolve side by side reflectors is improved by reducing α
(air-filled defects) acoustic impedance
time
voltage
General Rule: D
Echo height size
of defects (but shape Ultrasound can resolve
dependant!) defects of size x if x is
Simplified A-scan the same size or larger
than the wavelength λ
of wave pulse. α
voltage
Solution: use small λ (large f) 1.2λ
sin α =
<www.ndt-ed.org> D
time Solution: use small λ (large f)
Application: Ultrasonic pulse UPV application: concrete strength?
velocity (UPV) Effects
Parameter on UPV on concrete strength
w/cm ratio
Measurement of very first wave
arrival (P-wave) through a
age
specific wave path. Requires
good coupling to surface
moisture content
Standard method in many Agg type and content n/a useful for
countries (ASTM C597) relative
n/a
Proximity of steel measurements
Frequencies between 20kHz to within
Presence of defects
100 kHz typically used a single
structure
However, UPV cannot be used to measure in place
<Naik, Carino and Popovics 2005>
strength absolutely in most cases!
UPV application: defect UPV applications: Modulus
detection? determination 60
concrete specimens
υ d = 0.20
50
E u measured from UPV, GPa
40
void
crack Paste Specimens
30 υd = 0.25
Limestone
River Gravel
20
Air-Entrained
High Strength
10 PC, w/c = 0.34
PC, w/c = 0.45
Loss of transmission or Loss of transmission or Little to no effect
0
apparent lower velocity apparent lower velocity 0 10 20 30 40 50 60
E d measured from resonant frequency, GPa
Ed is directly related to VP by wave theory. However, measurements obtained
Defects cause wave path to deviate, thus lowering the apparent velocity from wave velocity (UPV) do not agree with those obtained by vibration!
in most cases. However, UPV cannot be used to fully characterize Wave propagation over predicts Ed for concrete samples, assuming median
defects (shape, depth, location, etc.) values of Poisson’s ratio

7. Ultrasonic Pulse Velocity –
Ultrasonic pulse velocity
ASTM C 597 v >v >v
solids liquids gases
Calculate Young’s modulus
Young’
More sophisticated from UPV (theoretical):
electronic equipment 1 −ν E
Portable, may require V=
electrical power
(1 +ν )(1 − 2ν ) ρ
Usually requires two Calculate strength from
or more people for Young’s modulus (empirical):
Young’
testing E = 0.04 3 ρ 1.5 σ
where V is velocity (m/s)
E is Young’s m odulus (MPa)
ν is Poisson’s Ratio
ρ is bulk density (kg/m 3)
σ is compressive strength (MPa)
Serway and Faughn
Dynamic (vibration) methods Resonance Frequency Analysis
NDE Technique - shallow Impact-echo
Impact-echo (ASTM C 1383) Analysis Reflection from slab bottom
Phenomena The resonant
Propagating waves frequency (at the
generated by impact peak) is related to
event. Multiply-reflected distance to reflector
waves are detected by (d) and wave velocity
surface sensor. (V):
f = V/(2 d)
Reflected waves set
up a resonance Thus,
condition having a
characteristic frequency d = V/(2 f)
Reflection from delamination

8. Impact-echo Impact-echo
Analysis (cont’d) Advantages
Relatively simple test to perform; commercially
available test equipment. Effective for detecting
• Strong wave reflectors delamination and slab depth.
more readily detected.
Disadvantages
Operator experience needed for data interpretation.
• Reflections Not as effective slabs over very stiff subgrade. Not
from embedded rebar effective for rebar detection.
and at the interface of a Application
slab and a stiff subgrade
are weak. Slab depth and delamination detection for most
slab systems.
NDE Technique - shallow GPR
Ground Penetrating RADAR (ASTM D 4748) Analysis
Phenomena Many time domain signals stacked together to
Wave pulses are reflected
form an image
antenna at interfaces having Scan direction
air: εr = 1
a difference in
electrical properties (εr )
concrete: εr = 6 to 11 Slab
Reflected pulses (time depth
and amplitude) are
soil: εr = 2 to 10
monitored in the
time domain signal
(water: εr = 80; metal εr = infinite)
GPR
Analysis (cont’d) GPR
Large wave reflection from metallic objects and moist areas.
Less reflection from slab-subgrade interfaces and air-filled cracks • Physical contact
Slab surface between antenna and
Scan direction
slab not needed
antenna • Allows for rapid
non-contact
scanning
Rebar reflections

9. GPR
Advantages
NDT of Steel
Very rapid data collection (non-contact technique).
Sensitive to presence of embedded rebar and moisture. Liquid Dye Penetrant
Eddy Currents
Disadvantages
Very involved data interpretation; operator experience Ultrasound
needed. Testing limited to 750mm depth. Not sensitive to
delaminations. Not effective beyond congested reinforcement.
X-ray
Application
Rapid scanning of slabs for depth or rebar location.
Defects in Steel Liquid (Dye) Penetrants
Observe visually
Enhance with penetrating dye
Clean surface and apply penetrant
Allow liquid to penetrate
then remove excess from surface
Apply developer (draws penetrant
out of defects)
No indication of crack depth
No indication of subsurface
defects
Not for porous/rough materials
Liquid (Dye) Penetrants Eddy currents
Magnetic fields setup electrical currents in
a conductive material (eddies)
They in turn generate a secondary
magnetic field that counteracts the first
This change in the field can be detected
by original coil or a pick-up coil

10. Eddy currents Ultrasonic Wave Reflection
incident
reflected
wave
wave
θi θr
Medium 1
Medium 2 θt
transmitted
wave
Reflected angle equals the incident angle
Amplitude of reflected wave depends on the properties of the two media
If media have large differences in stiffness and density,
most energy is reflected (flaws!)
If media have similar stiffness and density, most energy is transmitted
Angle-Beam Transducer
Angle-
Ultrasound: Steel vs. Concrete
Inspection
Ultrasonic pulse echo not effective in concrete Why?
Wave frequency 1-10 MHz
1-
Angle beams allow lateral Aggregate scattering!
detection of flaws in and f = V/λ
around welded areas If aggregate size (D) is 1” and we need λ > D
1”
Vertical cracks are not V = 4000m/s f < 150 kHz
detectable by normal beam f was in MHz for steel!
incidence
Low frequency pulse echo is problematic
Reduce extra echoes with
angle beam Difficult to manufacture transducers
Low f leads to large beam divergence (poor lateral
Hole Crack
Hole
resolution)
Crack,
no Crack
Crack detection
no detection
Transducer face must be very large
X-ray Radiography NDT Lab Today
Concrete tests
Bright is low x-ray
x- Schmidt rebound hammer: Surface hardness,
intensity due to strength
high absorption Ultrasonic Pulse Velocity: thickness, strength,
modulus
Dark is high x-ray
x-
Ultrasonic Resonance Frequency: modulus
intensity due to
low density Steel Tests
Dye Penetration: surface defects
Ultrasonic wave reflection: thickness, defects
X-ray: surface/internal defects
Eddy Currents: surface defects