Comparison of Some NDT Methods for Determining Concrete Properties

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Rainer, J. H.; Marsh, P.; Hoogeveen, T. J.; Pernica, G.; Guan, W.; Hunaidi, O.

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National Research Conseil national

institute for lnstitut de Research In recherche en Construction construction

ining Concrete Properties

b . i :> I L ,L (". J ::; I ijt-:.~,) L.,NF<L

lit-,i-:ii\ i.vc?d cji-, :: 0)3

..

1.6-76 iiil:~.i-i.i.3,1 r.ep<-,i,t.

by Hains Rainer, Philip Marsh, Ted Hoogeveen, Gerry Pernica, Wei Guan and Osama Al-Hunaidi

Internal Report No. 720

Date of issue: August 1996

This is an internal report of the Institute for Research in Construction. Although not intended for general distribution, it may be cited as a reference in other publications.

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COMPARISON OF SOME NDT METHODS FOR DETERMINING

CONCRETE PROPERTIES

Hans Rainer, Philip Marsh, Ted Hoogeveen, Gerry Pernica, Wei Guan and Osama Al-Hunaidi

1. Introduction:

The development of non-destructive (NDT) test methods has made great strides in the last decade. Instruments have been developed that are user-friendly and new techniques have evolved to a point where repeatable results can be expected.

Numerous text books and articles describe the various methods and their applications. When these methods are applied to measure the properties of concrete, however, consistency of results and repeatability has not always been satisfactory. In particular, systematic comparisons among different methods have not been extensive.

The purpose of this series of tests is to compare the test results among four different methods, each based on wave propagation principles through solids. These methods are the ultrasonic-pulse-velocity (UPV) method, the impact-echo (IE) method,

the impulse-response (IR) method, and the Spectrum Analysis of Surface Waves (SASW) method. While each of these methods has it advantages and limitations, enough common characteristics are embodied in each so that some meaningful

comparisons of results should be possible. The theory for these methods can be found in "Handbook for Nondestructive Testing of Concrete", Malhotra, V.M. and Carino, N.J.,

pages 169 and 284.

2. Description of Concrete:

The following describes in detail the pertinent parameters of the concrete

samples:

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES

IRC/NRC Canada, Ottawa, August. 1996

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2.1

Mix

Design and Strength Properties:

I

Parameter

2.2 Placement:

The concrete was run through a chute into a bucket which was hoisted to the top of the 2.4 m high forms and the concrete was dropped into the form. Vibration was unnecessary but the concrete was tamped with a piece of 2 x 4 lumber.

2.3 Curing:

The concrete was left in the forms for 3 days. The forms were then stripped and the samples cured in the laboratory.

S11:

The concrete was left in the forms for 2 days before stripping. The

samples were then cured in the laboratoty for 1 week but subsequently a plastic enclave was placed around the samples and a humidifier installed in order to raise the humidity.

2.4 Freeze-Thaw Test:

S11:

After 28 days of curing, the prisms cast for this batch were placed in the

freeze-thaw cabinet and subjected to 300 cycles of freezing and thawing according to ASTM C666 Procedure A. The prisms passed the test, showing no significant

deterioration or length change after 300 cycles.

IRCINRC Csnada. Ottawa, August 1996

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3.

Ultrasonic Pulse Velocity and Impact Echo Methods:

3.1 Description of samples:

A plan view of samples WSI and S11 are shown in Figures 1 and 2.

Sample "WSI", located outside of Building M20 was tested on May 24, 1996; sample "S11" located inside the structures laboratory of Building M20 was tested on May 30,1996. The latter had been used as a test specimen for strength-enhancing injections at about 20 cm spacings. The test locations for this study were chosen to lie away from these injection points.

Figure 1

IRC/NRC Canada. Ottawa, August. 1996

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Saw-cut Surface Slab Thickness 0.205111

Figure 2

3.2 Ultrasonic Pulse Velocity

In this method, two ultrasonic transducers are attached to the sample, one acts as a transmitter and the other as a receiver. The travel time of an ultrasonic pulse between the sensors is determined by a processor, called "Pundit", which sends and receives the pulses via the transducers and outputs the travel time in psec.

Before each series of tests, the Pundit was calibrated to 0.1 psec against a standard aluminum bar (26.0 psec).

The transducers (54 kHz sensors, 50 mm diameter) were coupled to the surface of the slabs (slabs WS1 and Sf 1) with Gorptech Ultrasonic coupling gel. The gel was applied to the base of the transducers before each test. The surface of the slab where the tests were performed was scraped with a putty knife to remove major imperfections (i.e. bumps, etc.) to ensure the best possible contact for the transducers.

The measurements through the slabs were done with the transmitter on the top

of the slab and repeated at the same locations with the transmitter on the bottom. At each location, the transmitter was moved around slightly to obtain the minimum time and thus to ensure the proper alignment of the two transducers. The results are shown in Table 1 a.

Also surface measurements of wave speed were carried out on sample "WS1" with transducer spacings measured from center to center of the transducers. The results are shown in Table 1 b.

IRCrmRC Canada, Ottawa. Augusf. 1996

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Thicknesses of the slab were measured with a tape measure as close as

possible to the locations where the measurements were taken, or from the cores taken at these locations.

From the cores taken at the tests locations, UPV and Impact Echo (to be described subsequently) measurements were carried out; the results are shown in Table lc.

3.3 Impact Echo:

In this method, an accelerometer is placed on the surface of the slab to capture the waves generated by the impact of a 112" steel ball adjacent to the accelerometer. The accelerometer was coupled to the surface using petroleum jelly.

The impact echo measurements were carried out at the same locations as the through transmission tests with the Pundit, except no measurement were taken at Location 6 on "WS1" because the 112" ball was too small for this depth of concrete.

Signals were captured with a single channel RlON signal analyzer, then downloaded to a computer and analyzed to obtain Fourier spectra. The results are presented in Figures 3 to 16.

Compression wave propagation speeds were calculated by multiplying the dominant resonant frequency by twice the thickness of the slab. The results are shown in Tables l a and 2a.

From the cores taken from the slab S11, Impact Echo tests were carried out as described before. The results are shown in Table 2c.

3.4 List of Equipment:

Ultrasonic Pulse Velocity:

"Pundit" C.N.S. Electronics PAM 1 19252

Ultrasonic Transducers #54/8582 & #54/8581 Gorptech Ultrasonic Coupling Gel

IRC/NRC Canada, Ottawa, ~ugust. 1996

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Im.oact Echo:

Accelerometer PCB 309A SIN 5157

5.34 mVlg

Natural Freq. 120

kHz

Power Supply PCB 480B SIN 742

Signal Analyzer RlON Model SA-77 SIN 0080039

Impactor 112" Steel Ball on Spring Handle Petroleum Jelly

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRONRC Canada, Ottawa. August 1996

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Table 1. Results from Tests on Slab WS1 Average, Tests 1 to 5 a. Through Slab: Location of Test 4 to 5 20 51.8 3861

Average. excluding 1 to 4 and 1 to 5 3883

b. Surface (UPV only):

c. Cores (UPV only):

1 21.1 50.8 4152 9062.5 3823 Distance between Sensors (cm) Location of Test 2 21.3 50.5 4208 3 21.1 50.3 4185 4 21.1 50.8 4154 5 21.3 50.6 421 5 Average 1 to 5

-

4179 1 t o 2 19.7 51.4 3833 1 to3 37.5 99.8 _,

_...

3758 _" 1 t o 4 55.2 174 3172 j 1 to 5 75.2 222.6 i '

...

3378

i

" 2 to 3 18 45.6 3947 3 to 4 17.5 43.6 401 4 Distance between Sensors (cm) Travel Time (~sec)

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES I R m R C Canada. Ottawa. August 1996

7 UPV Wave Speed (WS) Core # Travel Time ( V S ~ C ) Impact Echo 1 21.1 51 .O 4135 Travel Time (vsec) Distance between Sensors (cm) Wave Speed (Ws) Frequency of Reflection (Hz) Wave Speed (WS) Wave Speed (Ws)

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Table 2. Results from Tests on Slab S11 a. Through Slab: 3 21.1 52.2 4042 8750 3693 Average

-

41 27

-

3752 b. Cores: 1 21.1 51.7 4083 9125 3853 Location of Test Impact Echo 3 21.1 52.3 4034 8526 3598 Average 4125 3685 Distance between Sensors (cm) Frequency of Reflection (Hz) UPV

COMPARISON OF SOME NOT METHODS FOR DETERMINING CONCRETE PROPERTIES IRCMRC Canada. Ottawa, August, 1996

8 Wave Speed ( d s ) Travel Time (~sec) Wave Speed ( d s ) 1 21.1 51.4 41 07 8750 3694 2 21.2 50.1 4232 8875 3763 Core # Impact Echo Distance between Sensors (cm) Frequency of Reflection (HZ) . U PV Wave Speed ( d s ) Travel Time ( ~ s e c ) Wave Speed ( d s )

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lrnpoct Echo Test on the Bottom of Core #1 of Slob S l 1 2W Time Signoi 1 0 0 -100 Figure 3

Impact Echo Test on the Top of Core # I of Slob 811

I I Time Signol - FFT 30 25 20 1 5 . I 0 r--

.

0 $ 0 ' 5 20 25 30 35 40 15 50 55 BD Figure 4

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRCMRC Canada. Mtawa. August. 1996

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I m p a c t Echo Tesf on the Bottom of Core # 2 of Slab S1 1 ZOO 300 -100 2 0 0 1 ( 5 Z 2.5 3 3.5 4 1 . 5 5 5 5 6 6 5 / 7.5 B 8.5 lmsecl

I

FFT Figure 5

I m p a c t Echo Test on the Top of Core # 2 of Slob S l l

i 1.5 2 2.5 J 3.5 4 1.5 5 5.5 6 6.5 1 7.5 8 6.5

[msec]

[kHz1

Figure 6

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRWNRC Canada. Ottawa, August 1996

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FFI

-

6"

Figure 7

I m p a c t Echo Test an the Top of Core # 3 of Slob 511

100 50 0 -M -100 0 1 2 3 1 5 5 1 B 9 10 1 1 72 13 1 1 1 3 [msecl 1. 39 10 B 20 2 5 50 60 Figure 8

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRGINRC Canada, Ottawa, August, 1896

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Impact Echo Test a t Locotion 1 on Slob 511

200

Figure 9

I r n ~ a c t Echo Test a t Location 2 on Slob 511

m Time Signol

I

o,="z

5

/

n

FFT

Figure 10

IRCNRC Canada. Ottawa, August, 1996

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lrnpoct Echo Test a t Locotion 3 o n Slob 511 200 _Time_Signoi i w - P -100 -200 -300 - 0.75 i 125 1.5 1.75 2 2.25 2.5 2.75 3 5.25 3.5 3.75 1 4.25 4.5 (i 5 10 15 20 25 30 35 40 15 50 55 60 • ,""a > 9 g 6 [kHz1 Figure 11

Test o n Slob Outside near M 2 0 , Locotion 1

[kHz1

Figure 12

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRCMRC Canada, Ottawa, August, 1996

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T e s t on Slab Ouiside near M 2 0 , Location 2

Figure 13

10

Test on Slob Outside near M 2 0 . Location 3

I [meec] 02 - FFT 0 15 - 0 , - - 10 25 10 45 50 55 50 Time sigma, " -20 ( 0 Figure 14 - 2 0 - Time Signal

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRC/NRC Canada, Ottawa, August, 1996

14

-

-20 -30 2 3 4 5 6 7 B 9 10 l i 12 1 3 14 15 0.75 1.25 1.5 175 2 2.25 2.5 2-75 3 3.25 3.5 3.75 1 4.25 4.5

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Figure 15

Test on Slab Outside near M20; Location 4

20

10 -

[kHz1

Figure 16

Test on Slab Outside near M 2 0 , Locotion 5

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRCMRC Canada, Mtawa, August, 1946

15 0 - L -10 -20 - 2 3 1 5 5 7 B 9 30 ii i Z i J 1 1 1 5 [msec] sm0 "I 0.175 0.15 0.125 0.7 0.075 0.05 0.025 15 20 25 30 35 50 55 80 [kHz1 ' 20 T i m e Signal Time Signol

I,

....

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4.

Impulse Response

and

SASW Tests:

4.1 lmpulse Response:

4.1.1 Method: This method is similar to the impact echo except that the transfer function, rather than only the spectrum of the response, is used to identify the

resonance frequency of the slab thickness. In this way, the input variation in the frequency domain is taken into consideration.

This method is carried out by applying an input at the same location as the

pickup of the response. The input to the test object is applied by using an instrumented hammer that has a force transducer to measure the input force. The response

acceleration is picked up by an accelerometer. The transfer function is defined as:

where H, is the transfer function, Xis the Fourier transfer of input force, Y is the Fourier

transfer of response acceleration, and 'represents the conjugate. The peak frequency,

f,,

in the transfer function indicates the time that the P-wave travels from the impact point to the bottom of the slab and back to the accelerometer at the surface of the slab. The compression velocity V, of the concrete slab is calculated as:

v,

= 2f$

where t is the thickness of the slab.

4 .

1 .

2 :

The test was done on May 24,1996 on the concrete slab which was stacked in the campus of NRC, to the north of building M-20 (sample WSI), and on May 29, 1996 on the concrete slab in Structures Lab (sample S11). The

measurement setup for the test is illustrated in Figure 17.

PCB fnstnrmented PCB 309A Accelerometer, SM 5156

I I

PCB 480D06

concrete slab

I

Figure 17 Measurement Setup of Impulse Response Test

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES

IRCMRC Canada. Ottawa. August. 1996

16

ampufier amplfier

ON0 SOKKl

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The accelerometer was mounted to the concrete slab directly with Grand & Toy glue stick, at Location 3 as shown in Figure 1. For sample WSl, the impact point was in between Locations 2 and 3, about 1.5 cm from the accelerometer. For sample S11 (Figure 2), the impact points were at Locations 1,2, and 3. A total number of 100 spectra were averaged so that the variation between different impact was minimized. The results are shown in Figures 18a to 18c for test on sample WS1, and Figures 19a to 19c for test on sample S11. The thickness of the slab was measured to be 0.21 1 m for sample WSl at Location 3. For sample S11, the thickness of the slabs is 0.21 1 rn for Location 1, 0.212 m for Location 2, and 0.21 1 m for Location 3. The frequency range used was 0

-

20 kHz.

4.1.3 Results: For sample WS1, the transfer function between the input force and response acceleration is given in Figure 18a. The auto spectra of both force and acceleration are also given in Figure 18b and Figure 18c, respectively. The transfer function indicates that the peak happened at frequency 8900 Hz, thus

f,

= 8900 Hz; the compression wave velocity can then be obtained as:

<

= 2*f, *t = 2'8900'0.2105 = 3747 d s

For sample S11, the transfer functions between the input forces and their

response accelerations are given in Figures 19a, 20a and 21 a for Locations 1,2 and 3, respectively. The auto spectra of both force and acceleration are also given in

Figures 19b, 19c, 20b, 20c, 21 b, and 21c, for Locations 1, 2, and 3, respectively. The transfer functions indicate that the peaks happened at frequencies 8750 Hz, 8550 Hz, and 9200 Hz for Locations 1, 2, and 3, respectively. The compression wave velocities can then be obtained as:

Location 1:

v,

= 2'ct = 2*8750'0.211 = 3693 d s Location 2:

v,

= 2"ct = 2*8550R0.212 = 3625 d s Location 3:

v,

=

2'Yt = 2'9200*0.2111 = 3884 m/s

Average: 3734 m/s

IRC/NRC Canada, Oitaw. August 1996

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frequency (Hz)

Figure 18a Frequency Spectrum of Transfer Function, Sample WS1

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Figure 18b Auto Spectrum of Input, Force, Sample SW1

IRUNRC Canada. Ottawa. August. 1996

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9.00604 8.OOE-04 7.OOE-04

-

6.OOE-04 9s 5.OOE-04 41 ₌ 5 4.OOE-04 P

6

3.ooE-04 2.OOE-04 1 .OOE04 0.ooE+o0

0 2000 4WO 6000 Boo0 10000 12000 14000 16000 18000 2WOO frequency (Hz)

Figure 18c Auto Spectrum of Response, Acceleration, Sample SW1

Figure 19a Frequency Spectrum of Transfer Function, Sample S11, Location 1

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRCNRC Canada. Ottawa August 1996

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0 2W0 4WO 6000 BWO 10004 12WO 14WO 1M)W 18WO 2WW frequency ( M )

Figure 19b Auto Spectrum of Input, Force, Sample S11, Location 1

r .W&W Q.W&03 8.00&03 7.WE-03

-

_6.W&W L 8

-

₌ s.oo&m

-

4 . o o ~ m * 3.OOE-03 2.ooE-a3 I . w & m O.WE+OU

0 2030 4WO 6WO 8WO 1WW 12WO 14WO 1- lWW 20000

frequency (tk)

Figure 19c Auto Spectrum of Response, Acceleration, Sample S11, Location 1

IRCMRC Canada, Ottawa. August. 1996

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0 2000 4W0 6000 8WO 1WW 12WO 14003 16000 1BOW 20WO frequency (k)

Figure 20a Frequency Spectrum of Transfer Function, Sample S11, Location 2

Figure 20b Auto Spectrum of Input, Force, Sample S11, Location 2

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRCMRC Canada. Mtawa, August. 1996

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frequency (tk)

Figure 20c Auto Spectrum of Response, Acceleration, Sample S11, Location 2

Figure 21 a Frequency Spectrum of Transfer Function, Sample S11, Location 3

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRCINRC Canada. Ottawa. August. 1996

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0 2000 4000 60M) 8000 1OWO 12000 1 W 16000 18000 2WM)

frequency ( M )

Figure 21 b Auto Spectrum of Input, Force, Sample S11, Location 3

Figure 21c Auto Spectrum of Response, Acceleration, Sample S11, Location 3

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRCMRC Canada. Ottawa, August. 1996

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4.2 Spectrum Analysis of Surface Waves (SASW) Test:

4.2.1 Method: The SASW can be used for evaluating the stiffness gradient of concrete slabs using the dispersion characteristics of surface waves. The SASW method involves the generation of surface Rayleigh waves over a wide range of frequencies usually using an impact device such as hammers or drop weights. Using the dispersion characteristics of surface Rayleigh waves, one can obtain information about the variation of stiffness with depth"].

At present, the method has several difficulties in producing reliable stiffness gradient. However, thickness and overall stiffness can be obtained accurately. 4.2.2 Test Procedure: The test was done on May 24, 1996 on the concrete slab sample WS1 stacked to the north of Building M-20 on the campus of NRC. The measurement setup for the test is illustrated in Figure 22.

c@ncrete slab

PCB 480D06

Figure 22 Measurement Setup of SASW Test

amplMer

amplser

For this setup a mechanical impact was applied at the surface of the concrete slab with a steel ball. The resulting vibrations were measured simultaneously at two surface points with accelerometers. Vibrations were then transformed to frequency and the relative phase spectrum was obtained. The thickness of the slab was then found based on the phase spectrum by fitting a straight line on the phase curve.

The accelerometers were mounted directly to the concrete slab with Grand &

Toy glue stick at Locations 3 and 4 as shown in Figure 1. The impact point was at Location 2. The distance between two accelerometers was 0.1 75 m.

Impact stell ball

PCB 309A Accelerometers. A: S/N 5156,B: SM 5157

I

-

4.2.3 Results: The phase spectrum between accelerometers A and B is given in Figure 23a, and the coherence in Figure 23b. From the phase curve and fitting

a

straight line it can be calculated that the thickness of the slab WS1 is 0.21 m. This agrees well with the measured thickness of the cores, given in Table lc.

ON0 SOKKl

FFT Analyzer

-

IRCINRC Canada. Ottawa. August. 1996

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-. .

Figure 23a Phase Spectrum between Accelerometers A and B

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRUNRC Canada. Ottawa, August 1996

25

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Figure 23b Coherence between Accelerometers A and B

1. M.O. Al-Hunaidi. Insights on the SASW Non-Destructive Testing Method.

Canadian Journal of Civil Engineering, Vol.

20,

No. 6, p.

940-950,

1993.

COMPARISON OF SOME NDT METHODS FOR DETERMINING CONCRETE PROPERTIES IRCMRC Canada. Ottawa, August 1996

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Table 3. Results of Impact Response Test

Sample Location Measured Resonant Freq. Computed Thickness (Hz) Velocity fm) I mlsl WS1 point 3 0.21 1 S11 point I 0.21 1 point 2 0.212 8550 3625

1

S11 point 3 0.21 1 9200 3884

5. Compression Tests on Cores:

Cores were taken at the locations where Pundit and lmpact Echo tests were conducted on the slabs. A 4" I.D. diamond-studded core bit mounted on a universal core drilling machine was used. Sample cores were wet cut at 350 rpm with an advance rate of 20

-

25 mrnlmin.

Core length and diameters were measured. After the appropriate NDT tests were carried out as described before, cores were then capped in accordance with ASTM Standard C617-94 with a lime sulphur capping compound.

Compression tests were performed on the cores in accordance with ASTM

Standard ~ 3 9 - 8 6 . A Riehle hydraulically operated test machine was used at a load rate of 0.25 MPa to load the core samples to failure. The results are shown in Table 4.

COMPARISON OF SOME NDT METHODS FOR OETEFiMlNlNG CONCRETE PROPERTIES

I R m R C Canada. Ottawa. August. 1996

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Table 4. Core Strengths for Slabs WS1 and S11

Average 19.42

6. Observations:

6.1 Consistency of Results:

6.1.1 UPV Throuah Slab; The wave speed determined through the slab at

predetermined locations and through the cores taken from the same locations shows great consistency. From Table l a and l c , a point-by-point comparison shows wave speed variations in Slab WS1 between 0.5 and 1.5% of each other. Similarly for slab S l l , Table 2a and 2b show variations between 0.15 and 0.2% among the 5 slab and core measurements.

8.1.2 UPV Surface: Table 1 b shows that the wave speeds for Locations 1 to 4 and 1 to 5 are substantially below the rest of the results. In view of the consistency of the rest of the results these 2 values are not considered reliable. They are associated with the larger distances between the two sensors, which would lead one to deduce that distances beyond about 40 cm do not give consistent results.

6.1.3 l m ~ a c t Echo Throuah Slab: Point-by-point comparisons of results between the slab S11 and the cores taken therefrom, Table 2, show variations between 1.7 and 5%. No comparable results are available for slab WSl and its cores.

For Slab WS1 the variation of IE wave speed among the 5 locations is 0.7%.

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5.1.4 Core Strengths: The strengths of the 5 cores for slab WS1 show great consistency, varying 4% from the highest to the lowest value. For Slab S11 the variation among the 3 samples is substantially larger at about 25%.

6.2 Comparisons Between UPV and Impulse Echo:

A comparison of wave speeds between the impact echo through the slab and UPV, Table la, shows that the impulse echo values are consistently smaller than those from UPV at the same locations. This is also the case for Table 2a and Table 2b. On the other hand, the surface UPV measurements are comparable to those of the impact echo results through the slab. The reasons for this phenomenon have been ascribed to difference in wave lengths for the various methods*.

6.3

Comparison Between Impact Echo and Impulse Responses:

The wave speed of 3747 m/s from the impulse response is comparable to the values from the impact echo, presented in Table la. One weak point of the impulse

response method is the limited frequency content of the impact hammer, as shown in Figures 4b, 5b and 6b. This frequency content should be improved, especially for thinner slabs or denser materials, where the frequency peak is expected to be even higher than those investigated here.

6.4 Comparison With SASW Method:

The development of the SASW for applications to slabs is rather new. The SASW thickness determination of 0.21 m being very close to those measured is very promising indeed. Further development of this technique pertaining to concrete slabs is indicated.

6.5 Core Strength Vs.

UPV

and IE:

The correlation between core strengths and NDT methods of wave speed is carried out with respect to the surface UPV and the IE only since these two methods give mutually consistent results. As can be seen from Table 1 and Table 4, the

average concrete strength for Slab WS1 of 19.24 MPa can be correlated with an UPV surface wave speed of 3883 m/s or impact echo speed through the slab of 3820 mls, or overall average of 3851 m/s. For Slab S11, while the average strength of 3 samples is slightly higher than for the average of WS1, the variability of the strengths is greater, indicating that Slab S11 is of much less uniform concrete than WSI

.

S11 will therefore not be used to establish a reference value for correlating the strength and wave speed.

IRCMRC Canada. Ottawa. August. 1996

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7. Discussion:

The results of wave speed determinations on slab WS1 indicate that consistent results are obtainable within each of the UPV and IE methods. Since a rather uniform concrete member was being examined, this consistency of results indicates that the method itself gives consistent results. In contrast, Slab S11 was more non-uniform in its strength properties since it had previously been used as a test bed for a strength enhancing injection method that was applied on a grid of drilled holes approximately at 20 x 20 cm. However, this injection material had not penetrated deeply from the drilled holes and thus it was felt that some useful results for this comparison study could still be obtained. But clearly, the sample S11 is much more variable than WS1 and the various wave speed determinations and the strength results confirm this. From the comparisons of results from these two test slabs it is clear that measured variations of wave speed within any given method are the result of real material variations and not a peculiarity or variability of the test method.

7.1 Reference:

2. Olson, L.D., Sack, D.A., and Phelps, G.C. Sonic NDE of Bridges and Other Concrete Structures. Proceedings, Conference on Nondestructive Evaluation of Civil Structures and Materials, Boulder, Colorado, May 11- 13, 1992, p. 279-288.

8. Conclusions:

These NDT tests have demonstrated that consistent wave speeds can be

obtained by the UPV and IE methods in uniform samples of concrete. One limitation for surface UPV measurements that needs to be taken into account is the maximum

distance between the two probes. The limitation of this distance needs to be checked by way of calibration exercises and subsequently heeded. There is good correlation between UPV surface measurements, IE through the slab measurements and impulse response measurements. These values, however, are consistently lower than the UPV through the slab or through the core measurements.

IRCMRC Canada. Ottawa, August 1996