Nondestructive Evaluation of

Concrete Dams

16 June 2020

Larry D. Olson, P.E.President and Chief Engineer

Larry.Olson@OlsonEngineering.com

NDT Methods for Estimating In-Place Concrete Strength

Rebound (Schmidt or Swiss) Hammer

Windsor Probe Penetration

Pin Penetration Resistance

Pull-out or Capo Tests

Ultrasonic Pulse Velocity

Maturity (hydration temperatures and time vs. strength)

American Concrete Institute Manual of Concrete Practice - ACI 228.1R-19 –

Report on Methods for Estimating In-Place Concrete Strength

NDT Methods for Evaluation of Concrete in Structures

Stress Waves for Structures and Pavements

◦ Ultrasonic Pulse Velocity

◦ Impact Echo

◦ Pulse Echo

◦ Spectral Analysis of Surface Waves

Stress Waves for Deep Foundations

◦ Sonic Echo

◦ Impulse Response

◦ Crosshole Sonic Logging

◦ Parallel Seismic

American Concrete Institute Manual of Concrete Practice ACI 228.2R-13 –

Report on Nondestructive Test Methods for Evaluation of Concrete in Structures

Magnetic Methods

Electrical Methods

Nuclear Methods

Penetrability Methods

Infrared Thermography

Ground Penetrating Radar

Sonic/Ultrasonic Velocity Tomography

Physics – Elastic Wave Propagation

(3 most used Wave Modes of Primary-compressional, Secondary-shear and Rayleigh-surface)

Stress Wave Basics - Primary Compressional Waves

The particle motion associated with compressional waves can be described as vibration parallel

to the direction of wave travel.

Stress Wave Basics – Secondary Shear Waves

The particle motion associated with shear waves can be described as vibration perpendicular to the

direction of wave travel.

Stress Wave Basics – Rayleigh Surface Wave

A Rayleigh wave moves across a surface. As it passes, a surface particle moves in a circle or

ellipse in the direction of propagation.

Stress Wave Basics Wave Velocity for Concrete at Poisson’s Ratio of 0.2

NDE of Concrete Dams Case Histories Outline

Impact Echo for Cracking, Thickness and Integrity

Spectral Analysis of Surface Waves for Freeze-thaw Cracking and Alkali-Silica Reaction Cracking and Concrete/Masonry Quality

Sonic Cross Dam Tomography for Imaging Internal Conditions of Concrete and Masonry Dams

Slab Impulse Response for Void/Subgrade Support Evaluation of Spillways and Conduits

Ground Penetrating Radar for Void/Subgrade Support Evaluation of Spillways and Conduits and Steel Reinforcement Mapping

Infrared Thermography for Shallow Delamination Mapping

Impact Echo Test

(ASTM C1338 and ACI 228.2R)

D = bVp/(2*f)

b= Shape Factor (0.96 for

slab/wall shape)

D = Thickness Echo Depth

Vp = Compressional

Wave Velocity

f = Resonant Echo Frequency

Impact Echo IE-1 Test Head

Displacement Transducer with

Automated Impactor for thickness

echoes from 8 to 60 cm – use

hammer for greater thickness

echoes to 180 cm

Waterproof Version for Underwater

Testing (with a Diver) has been used

on canal linings and bridge piers

Impact Echo Testing was done

on Grid on the Downstream

Face to Determine Extent of

Freeze-Thaw Cracking Damage

Impact Echo Testing of a Thin-Arch Concrete Dam

Impact Echo Testing on Face

Example Impact Echo Result Indicative of Cracking

IE Velocity Check and Cracking Confirmed by Coring

NDE can evaluate thicknesses & corrosion conditions of

concrete, steel, composite outflow works and pipes (any size)

Impact Echo and Ultrasonic Pulse Echo

NDE Evaluation of Dam Conduit Steel Lining and Concrete Conditions

IE Testing on Steel-Lined (Bonded) Concrete Dam Conduit

Testing on steel-lined

(bonded) concrete dam

conduit to check

thickness and integrity

IE Test Result

Result indicating

25.7 inch thick

sound concrete

behind steel-lined

dam conduit

Impact Echo Result through Steel Liner

Ch 3: Time Domain IE Data

Time (us)

0 5000 10000 15000 20000 25000 30000 35000 40000

-2

-1

0

1

Avg Frequency Spectrum, T = 68.6 in V = 144000 in/sec

Frequency (Hz)

0 5000 10000 15000 20000 25000

0

0.005

0.01

0.015

0.02

0.025

Result shows ringing void

Ultrasonic Thickness/Pulse Echo Testing

Testing found steel lining

thicknesses of ~0.4 inch

(ranged from 0.34 to 0.52

inch)

Spectral Analysis of Surface Waves (ACI 228.2R)

Velocity = Frequency x Wavelength

NDE Data Acquisition Platforms and SASW Systems

Freedom Data PC Platform

Windows 10 Field Ruggedized

NDE-360 Platform

Touch Screen w/ Compact Flash

SASW-S Bar for 6 to 80 cm spacings

SASW-S Accelerometers for variable

spacings

Example Test Results SASW

Typical Time Domain

Records for the Two

Receivers used in SASW

Testing, R1-R2 = 30 cm

for 2.62ft SASW Bar spacing = 1 wavelength (-360o phase), Velocity=frequency x wavelength=2.62 x 2628Hz=6900 ft/s

SASW Phase Plot from Sound Area

2628 Hz at -360o

+180o

Wrapped Phase

0o

-180o

Signal 1Coherence

0

0 Frequency (Hz) 13000

V =6900 ft/s Surface Wave Velocity for Sound Concrete

0 Wavelength (ft) 5

Surface Wave Velocity vs. Wavelength

1.0

Signal Coherence

0

8,000Surface Wave Velocity (ft/s)

0

SASW on Outflow Pipe Wall for:

SASW Testing on Steel-Lined (Bonded) Concrete Dam

Voids behind steel pipe wall

Concrete thickness and

integrity from IE and concrete

quality from SASW as it

shows velocity vs. depth

SASW Results

Sound concrete

behind steel lining

with velocity of

~7,300 ft/s

SASW Result from Void/Intermittent Location

Note slow Surface Wave Velocities of 3000 to 5000 ft/s

SASW Data Collection – Concrete Dams

SASW Testing on a Concrete Dam:

Investigating interior degradation

due to freeze/thaw cycles

Cracking due to alkali-silica or

alkali-aggregate reaction (ASR

or AAR)

Typical SASW Data on a Dam Face – Freeze/Thaw Cracking

Location with Freeze/Thaw Cracking to about 3 ft

deep behind Surface Patch where surface wave

velocity increases (Visually LOOKS Sound…)

Sound Concrete to 4 feet deep – surface wae

velocity ~6,500 ft/s

SASW Crack Damage Depth Plot

This plot shows the depth of damage (scale on the right) from freeze-thaw versus location on the downstream face of a thin-arch concrete dam

Evaluation of freeze-thaw cracking damage depth behind 3 year old failed,

debonded shotcrete repair at Rogers Dam in Big Rapids, Michigan – ICRI

Award Winner in 1996

Debonded Shotcrete on Pier by Tainter Gate

Debonded Shotcrete

SASW test with

Accelerometers

SASW Dispersion Curve From East Abutment Wall

SASW Velocity Plot From East Abutment Wall

Cracked & Sound Impact Echo Results

Ultrasonic/Sonic Pulse Velocity Tests for Tomography

UPV Applications

ASTM Standard C597-16 Standard Test Method for Pulse Velocity Through Concrete

Used to locate voids, honeycomb, cracks, discontinuities or poor quality concrete

Best used on structures with 2 accessible sides◦ Beams◦ Columns◦ Elevated Slabs

Sonic Pulse Velocity (SPV) used on large structures like bridge piers and dams UPV Testing on a Column

Below 7,000 (2,100)Very Poor

7,000-10,000 (2,100 – 3,000)Poor

10,000-12,000 (3,000 – 3,600)Questionable

12,000-15,000 (3,600 – 4,500)Good

Above 15,000 (4,500)Excellent

Ultrasonic Pulse Velocity

ft/s (m/s)

General Concrete

Condition

Concrete Compressional Wave Pulse Velocity and Quality

Note: Surface Wave Velocity is 0.56 of Compressional

Pulse Velocity for Concrete with a Poisson’s ratio of 0.2 (After Leslie and Cheeseman, 1949)

UPV/SPV Test

Using 2 transducers - source and receiver

Measure signal time and signal amplitude between the source and receiver (transmission test)

Calculate concrete compression wave velocity (Vp)

SPV uses an impact source rather than piezotransducer

Pulse Velocity = Vp = Distance/Wave Travel Time

Strength Correlations per ACI 228.1R-89f’c ~ E2 ~ V4 – must calibrate with cores or cylinders

UPV Test Procedure: Calibration on Standard Test Bar

SPV (Sonic Pulse Velocity) Testing

SPV is a low-frequency/high energy version of UPV.

Requires an instrumented hammer or a steel hammer and sensor

as a source

Can be done using two UPV transducers and a hammer

Example Sonic Pulse Velocity Data

tDVelocity /=

Sonic Pulse Velocity = Distance/Travel Time = 8 ft / (2566 ms – 1990 ms) = 13,889 ft/s

Figure 5: Example SPV results for the North Pier – File NL7 at height = 15 feet, Path 2 to 6. Sound Concrete

Condtion Rating with SPV = 13,455 ft/s Receiver Response at 8 ft from impact point Impulse Hammer impact response

Acoustic Sonic Dam Velocity Tomography

Acoustic method – measures the compression (or shear) wave velocity

of sonic waves traveling on a mesh of paths through the dam interior

Allows 2-D slices and 3-D images of the velocity profile of the dam to be

created

Velocity of concrete is related to strength and condition – low velocity

zones are areas of weak, cracked, or degraded concrete

The testing is EASIEST done on dams with a full pool to allow

hydrophones to be used on the upstream face

Wave source can be from climbers or remote operated impactor

Sonic Dam Tomography: Fieldwork

Small or Large Concrete Dams

Remote Access for Source Impacts

Instrumented Electrical Solenoid Impact Source on smooth concrete

Sonic Dam Tomography Testing by Climber

3-lb Impulse Hammer

impacts on downstream

face for testing on rough

surfaces and thick dams

Crossing Velocity Tomography Ray Paths: Straight vs. Curved

Rays - Infinitesimally narrow path perpendicular to the spherically spreading seismic wave front.

Straight Rays

◦ “Travel” from Source Location to the Receiver Location in the most direct path.

Curved Rays (AKA Bending Rays)

◦ Seismic Waves and therefore their associated rays can bend within a volume if there are changes in the material properties (I.e. density)

◦ These rays are initially estimated as Straight Rays and then iteratively perturbed until the residuals are minimized.

◦ More appropriate for mediums containing strong velocity contrasts.

Raypath Diagram

Note Hydrophone

Receivers are typically 3

ft apart underwater on

upstream and automated

impacts or 3 lb impulse

hammer impacts with

climber downstream

Sonic Dam Tomography - Method

Sonic Cross Dam Tomography

Sonic Cross Dam

Tomography conducted

for safety evaluation on

24 2-D cross-sections

over 480 ft dam length in

5 field days

Velocity Tomography Remotely Operated Impact Source

Solenoid Source in use on Dam Downstream Face

Hydrophone Receiver String and Solenoid Impactor Source

Freedom PC Data Acquisition Equipment and

Solenoid Driver

These plots show the typical raypath mesh

resulting from tomographic data collection of

sonic pulse veloctity travel times along the

test paths (scales in ft) R denotes a

hydrophone receiver upstream and S denotes

downstream solenoid source impact point

Raypath Density Plots

Example Sonic Pulse Velocity Result for CDT

Velocity Tomogram with Anomalous Areas and Debonded Joint

Imaged DebondedJoint – confirmed by Divers that the small green band at 38 feet is a degraded copper waterstop embedded in the concrete

(scales in ft on dam cross-

section and kilo ft/s) and

Hydrophones (R) upstream-

Source (S) Impacts downstream

Impact Source Options

Tomography on Rough Concrete: Direct Impacting Using Climber

Above Velocities in 1000’s of ft/s

Low velocity zones at downstream face

(left side) correspond to degraded,

cracked concrete due to freeze-thaw

Velocity Tomography Test Results

Note blue colors indicate slower

velocity concrete due to freeze-

thaw cracking damage

Sonic Dam Velocity Tomography:

Multiple 2D Results

Sonic Cross Dam Tomography Advantages & Disadvantages

Image internal flaws in 2-D and now 3-D fashion with angled and direct tests

A picture is worth a thousand words and velocity tomograms provide an image of internal void, cracking and honeycomb and other concrete damage/defect conditions

Slab Impulse Response (SIR) & Ground Penetrating

Radar for Spillway Subgrade Support Evaluation

Used to Locate and Define Voids Under

Spillways, Roadways, Building Slabs, Tunnel

Liners, Pipe Walls, and Rigid Concrete

Pavements

Slab Impulse (SIR) Response Method

ASTM C1740 - 16 Standard

Practice for Evaluating the

Condition of Concrete Plates

Using the Impulse-Response

Method

Slab Impulse Response Method & Equipment

Detects thin voids less than 1/16 inch

Compliments GPR and improves accuracy of void detection greatly

Test slab thicknesses up to ~18-24 inches

Olson Instruments Freedom Data PC with Slab IR system (SIR-1)

3-lb instrumented hammer impacts and geophone records time domain data◦ Wilcoxson velocity transducer for non-flat

structural elements such as conduits, tunnel liners, walls and other structural elements

Data converted to frequency domain via FFT

Measure velocity response of slabTime domain and Velocity Spectrum of Geophone

Time Domain Velocity

Receiver

Spectral Domain

Velocity as a function

of frequency

FF

T

Measure Impulse Hammer force in time and spectra

2

ms

3000 lbf

Time of

Impact

Delay ~

10% of Total

Time

Total Time =

20.48 ms

2000 Hz

FF

T

Transfer Function: Mobility (Velocity/Force Spectra)

=

Δf

𝐹(𝑓) ∗𝑉(𝑓)

𝐹(𝑓)= 𝑉(𝑓)

Flexibility Transfer Function Flexibility (inches/lbf) which is inverse of Stiffness - lbf/inch)

)()(

)(*)( fD

fF

fDfF =

MEASURED

OUTPUT

MEASURED

INPUTFLEXIBILITY

TRANSFER

FUNCTION

Dam Spillway Case History

NDE methods of Slab Impulse

Response, Ground Penetrating Radar

and Video Borescope

Field Project background

Survey design and data collection

procedures

Example results

Combined NDE results - data

presentation and interpretation for

subgrade void evaluation

Corehole Ground-truthing

and conclusion

Spillway Project Background

Spillway Characteristics

◦ Alpine reservoir dam concrete spillway in

Colorado at 3000 m above sea level

◦ Reservoir capacity ~ 800 acre-ft serves as

water source for nearby town

◦ Dimensions are 45.7 m long x 15.8m wide

at top, tapering to 9.7m wide at bottom

◦ 180-350mm (7-14 inches) thick concrete,

reinforced with one mat at nominally

304mm (12 inches)

Reasons for NDT&E

◦ Observed water seepage at joints and

concrete spalling

◦ Prior hammer sounding was hollow

◦ Drilling investigation showed subgrade void

NDE Methods

Ground Penetrating Radar (GPR)

◦ Electromagnetic wave reflection

– 400 MHz Antenna

Slab Impulse Response (Slab IR)

◦ Acoustic modal vibration method

– 3 lb impulse hammer and

geophone (velocity transducer)

Complementary tools for

determining areas of poor

subgrade support or voids

Slab IR Field Survey

Data collected at a 4 x 4 ft grid

3 hammer impact (records) collected at each point and averaged

428 data points total

Centerline of the spillway, running longitudinally for more than

47mdownstream named ‘C’

Longitudinal lines designated at 4 ft intervals from right of center

eastward as R1 through R6 and at 4 ft intervals from left of center

westward as L1 through L6 looking downstream from the spillway crest

Survey began 2 ft downstream of the reservoir shore edge through line

40 at 4 ft intervals

Slab IR Data Processing Time domain data of hammer input and receiver output shown below that are converted

to frequency domain by FFT operations on 2-3 hammer blows

Transfer function normalizes receiver spectrum in velocity units by hammer spectrum in

force units (Mobility = M = V(f)/F(f) on next slide

Ch 1: Time Domain SlabIR Data

Time (us)

0 50000 100000 150000 200000 250000 300000

-6

-4

-2

0

2

4

6

Ch 2: Time Domain SlabIR Data

Time (us)

0 50000 100000 150000 200000 250000 300000

0

2000

4000

6000

Hammer Signal (lbf)

Receiver Signal (in/s)

Slab IR Results

Subgrade support condition

evaluation parameters

◦ Mean mobility (in/sec/lbf)

◦ Shape of the mobility plot

◦ Initial slope of the mobility plot

gives the low-strain flexibility

(in/lbf)

Interpretation Pitfalls

◦ Slab thickness

◦ Local reinforcement

◦ Local joints/seams

Avg. Coherence

Frequency (Hz)

0 100 200 300 400 500 600 700 800 900 1000

0

0.2

0.4

0.6

0.8

1

Avg. Mobility from F1 = 100 to F2 = 800 : 1.23748e-004

Frequency (Hz)

0 100 200 300 400 500 600 700 800 900 1000

0

0.0005

0.001

0.0015

M

ob

ilit

y (

in /

s /

lb

f)

Avg. Coherence

Frequency (Hz)

0 100 200 300 400 500 600 700 800 900 1000

0

0.2

0.4

0.6

0.8

1

Avg. Mobility from F1 = 100 to F2 = 800 : 7.21102e-004

Frequency (Hz)

0 100 200 300 400 500 600 700 800 900 1000

0

0.0005

0.001

0.0015

Mo

bil

ity

(in

/ s

/ l

bf)

Good subgrade

support – low,

smooth mobility in

bottom plot and

good data

coherence in top

plot

Poor subgrade

support – high,

irregular mobility

in bottom plot –

much more

flexible/less stiff

slab

Ground Penetrating Radar (GPR) Method

EM (radio) wave reflection

Reflection occurs when

encountering a change in

electrical impedance or

material relative dielectric,

termed e

Concrete to steel, concrete to

air (void), concrete to soil

Amplitude of reflection, R,

defined by:

R = (erU0.5 – erL

0.5 ) /(erU0.5 + erL

0.5 )

Tim

e (n

s)

Ground Penetrating Radar: Physics & Theory for a Signal

The collected raw data is in the form of echo amplitude versus time.

By inputting the dielectric constant, which defines the material velocity, and by estimating the signal zero point, the echo time data can be converted to echo depth.

VEM = c / er0.5 Equation 1

D = (VEM * T) / 2 Equation 2

Where

◦ VEM = material electromagnetic wave velocity

◦ c = speed of light (in air)

◦ er= material relative dielectric constant to air (1.0 in air at speed of light

◦ D = depth of GPR reflection

◦ T = the two-way radar pulse travel time (shown for slab top and bottom)

◦ Typical concrete dielectric constant - er = 6

The round-trip transit time of the pulse emitted by the GPR and its reflection

provide range information on the target

GPR Working Principle

GPR Physical Principle – Corroded vs. Non-Corroded Rebar

GPR Test and Relative Dielectric Constants

Locations with dielectric contrast

between the two materials (see table)

Large concrete cracks/voids (air filled)

Smaller gap/void filled with water very

well – larger dialectic contrast

Smaller amplitude of the

reinforcement reflector indicates

possible reinforcement corrosion due

to diffraction by rust byproducts

Typical Spillway GPR Field Survey - Sloping Spillway

400 MHz antenna, 106 pulses

per meter

Survey wheel records distance

Data collected in 3-D fashion

with unidirectional parallel lines

at 1.2m intervals

Spillway scanned in lines from

top to bottom or side to side

GPR Field Survey

400 MHz antenna, 106 pulses per meter

Survey wheel records distance

Data collected in 3-D fashion with unidirectional

parallel lines at 1.2m intervals

Spillway split into upper and lower portions

Scanning from spillway crest to bottom for each

portion

Treacherous footing because of moss. Felt-

bottom shoes worn

Nor

thin

g, D

owns

tream

(ft)

10 20 30 40 50

Easting (ft)

Upp

er S

pillw

ay

Low

er

Spill

way

GPR Example Results Slab bottom/subsurface amplitude reflection = bright spot analysis

R = (erU0.5 – erL

0.5 ) /(erU0.5 + erL

0.5 )

Material er R

Concrete 9.8 ---

Void (air) 1.0 0.52

Soil 4.0 0.22

Concrete/soil Concrete/void

Combined NDE Results

Ground-Truthing

Coring locations recommendations based on NDE results

Video borescope probe for motion and still pictures

Excellent correlation – extensive subgrade voids found in all coreholes

Corehole 9L1 - East - File 030821AXCorehole 9L1 - East - File 030821AX

GPR Applications

GPR Field Survey: Routine Spillway Inspection & Dam LinersLocating void below concrete spillways and soil-cement liners

400 MHz antenna on soil cement liner checking for void over 2.5 miles of liner

4 scans per inch (48 scans/ft)

Survey wheel to record distance

Can record with GPS

Data collected along lines (top to bottom or side to side, or both)

Fig. 2 GPR Testing with the 400 MHz Antenna on the Reservoir LinerFig. 2 GPR Testing with the 400 MHz Antenna on the Reservoir Liner

Core # 9 Location

Void Area

Small Void

Slab Top

GPR Results: Voids beneath Soil – Cement Embankment Liner

Verify Visual and Destructive Test Results with NDT Extend details beyond the known conditions*

Coring

Drilling

Chipping

Borescope

Visual Inspection

Standard Methods:

Ground-Truthing Results

Coring locations recommendations were based on NDE results

Cores Confirm Large Cavities with high GPR accuracy

Infrared Thermography

for Shallow Delaminations

ASTM D4788 - 03(2013) Standard Test Method for Detecting

Delaminations in Bridge Decks Using Infrared Thermography

Physical Principle of Infrared Thermography

Infrared Thermography of Cyclopean Masonry Dam Spillway

Infrared Thermography (IRT) Delaminations Spillway Crest

Picture 1. Captured at: Spillway Crest Sections 11-7

Date & Time: 11/7/2012 10:38:53 AM

46.2

57.7 °F

50

55

Comment:

Although the hottest areas are the most delaminated of the crest patch, the yellow-

orange areas are also debonded patches from the underlying concrete.

Recommendation:

IRT Delaminations Outlined in Black on SpillwayPicture 1. Captured at: DSF Sections 11-7

Date & Time: 11/7/2012 10:59:59 AM

48.3

60.6 °F

50

55

60

Comment:

Recommendation:

Summary for the Infrared Thermography Technique

Simple and easy to train

Good to detect shallow anomalies of 2 to 3 inches deep

Difficult to detect deeper anomalies

Both actual photograph accompanied by the thermographic image of the

same location are required in the interpretation

The test results are dependent on the weather condition and the time of the

day of the testing – solar heating generally required

NDE of Dam Facilities - General Application Guide Stress Wave Methods for Material Integrity and Deterioration of Concrete,

Masonry and Steel Materials

◦ Impact Echo

◦ Spectral Analysis of Surface Waves

◦ Ultrasonic and Sonic Pulse Velocity

◦ Sonic Velocity Cross Dam Tomography

◦ Ultrasonic Pulse Echo/Impact Echo for corrosion and bolt integrity/length

Electromagnetic and Modal Vibration Methods for steel mapping and Spillway/Conduit Subgrade Support

◦ Ground Penetrating Radar for subgrade void ¼+ inch and steel mapping

◦ Slab Impulse Response for even thinner voids and soft subgrade evaluation

Infrared Thermography (IRT) for shallow concrete delamination mapping

Thank You