The use of combined non-destructive testing methods to determine the compressive strength of concrete P. Knaze, P. Beno Construction Engineeringand Testing Institute, Bratislava, Czechoslovakia.

1. INTRODUCTION One of the expected ways of increasing the results obtained by non-destructive testing methods is the application of combined methods, i.e. the use of two or several methods for testing the same structure. Another region where combined non-destructive testing methods are used is the determination of concrete compressive strength on structures when calibration sample s of the concrete making up the structure are not available. These testing methods are considered also for the determination of the compressive strength of concrete on structures with available information on concrete mix design, or in cases where a limited number of cores can subsequently be drilled from the structure. Mostly, the combination of the following two methods is used : -the hardness measurement method with the Schmidt's rebound hammer; - the ultrasonic pulse method. These methods are used in combination since the rebound hammer provides information on concrete properties in the surface layers, and the ultrasonic method enables the determination of concrete properties in the thickness of the structure.

For hardness measurements, a commonly used Schmidt rebound hammer (without registration device), model N, was applied.

4. E X P E R I M E N T A L WORK P R O C E D U R E

4. Test Samples

For the application of both the given non-destructive testing methods, 21 sets of test samples were produced, each set consisting of three cubes with 20 cm edges. During the preparation of separate test mixes, variations were made in different batches in water content and compaction time of the test samples. The following basic materials were used for test sample preparation: -SPC 325 cement, produced by the Stupava Cement Works, national enterprise (Stupavski cement/l refi, n.p.), Stupava; - aggregates-excavated Danube gravel sand, 0-4 mm and 8-16 mm size fractions; - water-from the urban water mains. The composition of test batches and the compaction time are given in Table I. After preparation, the test samples were stored for 28 days under standard conditions.

2. PRINCIPLES OF USING THE METHOD 4.2.

In compliance with the recommented R I L E M method, the experimental work was oriented towards drawing curves of equal strength from which it is then possible to determine the concrete compressive strength directly from the measured ultrasound propagation rate and the Schmidt's hammer rebound value.

3. DESCRIPTION OF THE TESTING ASSEMBLY In the experimental work, for the determination of the ultrasonic propagation rate the BT-2 ultrasonic apparatus with digital reading of the ultrasonic pulse transition time through the tested material was used. 0025-5432/84/03 207 04/$ 2.40/9 BORDAS-GAUTHIER-VILLARS

Experimental

work

After 28-32 days of test samples hardening, nondestructive tests were made with them by using the Schmidt rebound hammer as well as ultrasonic methods. Each cube was tested with the Schmidt rebound hammer on two places, normal to the direction of the cube compaction. During this test the cubes were loaded in the press by a force of approx. 10% of the anticipated bearing capacity. Ultrasonic tests were made on three places (normal to the compaction direction), the first of which was near the cube bottom, the second one half way up the cube, and the third one near the top of the cube (always, however, at least 4 cm from the cube edge). 207

Vol. 1 7 - N ~ 9 9 - M a t r r i a u x et C o n s t r u c t i o n s

Vaseline was used as coupling medium for ultrasonic tests. After carrying out non-destructive tests, each of the cubes was measured, weighted, and destructive tests were made in a hydraulic press to obtain comparative values of concrete compressive strength.

ta

T,,

,

,\

\ \

5. T E S T R E S U L T

The measured results of non-destructive tests, i.e. the Schmidt rebound hammer values R,, ultrasonic pulse rate v (m. s-~), as well as the concrete compressive strength R b (MPa) obtained by destructive tests, are given in table II.

6. E V A L U A T I O N

OF TESTS

9 The choice of the most convenient calibration relation for the separate non-destructive test methods was made using a H P 9825 table-top computer standard program, with the aid of which the most convenient calibration equation form (from 15 possible ones) was determined by the method of least squares. For the Schmidt rebound hammer, the most convenient calibration relation proved to be: y=a+b.x+c.x

2

where

TABLE I COMPOSITION AND COMPACTION TIME OF THE TEST BATCHES

13 ,soo

9

~

3~$0

] 4000

~oSo

4100

:alSO ' ~'

Fig. 1. -

~200

Ultrason~c

4~SO fete

13~0

415o

(m 9 6 -1 )

Curves o f equal concrete strength.

y, concrete compressive strength (MPa), x, rebound value R a, a, b, c, coefficients, determined on the basis of experiments. The concrete form of the equation was: y = -46.426,0 + 2.044,9 x -0.0018,6 x 2. The correlation degree for this relation was r=0.732. The average deviation of the real concrete compressive strength from the concrete compressive strength determined by the Schmidt rebound hammer with the aid of the given calibration relation, was 10.37%. For the ultrasonic pulse method, the most convenient proved to be a relation of the same form - a second order parabola. On the basis of experimental work performed, for the given case a, calibration was derived in the form of: y = -267.524,9 + 11.273,5 x - 0 . 1 0 1 , 3 x 2

Test set i designation

Used basic materials

Water quantity for 1 m 3 finished concrete

Water- Compaction to-cement duration ratio (min)

(l)

C D E F H K M O p S T U

-SPC 325 Stupava works cement 380 kg. - Aggregates 364 kg 0-4 nun 1456 kg 8-16 size f~action western Slovakia Gravel Sand Works, national enterprise (ZK~ n.p.), Bratislava. - Danube gravel sand.

Z4 Z3 Z2 Z1 A

-SPC 325 Stupava works cement, 380 kg. - Aggregates variable: for filling up to l 1 m 3. -SPC 325 Stupava works cement, 380 kg. I - Aggregates, 1820 kg.

208

2 170

5.1%.

0.45 0.5 1.0 1.5 2.0 3.0

140 150 190

where y, concrete compressive strength (MPa), x, ultrasound pulse ratio (m. s -1 . 10-2). The degree of correlation for this relation was r=0.931. The average deviation of the real concrete compressive strength from the concrete compressive strength determined by the aid of this relation, was

2

IS0 160

0,37 0.39 0.50 0.47 0.42

200

0.53

2

Then the so-called curves of equal concrete compression strengths (the so-called isostrength curves) were determined, shown in figure 1. These curves determine the equal concrete compression strengths with changing ultrasonic rate and changing rebound values Ra of the Schmidt hammer. From these curves and from the known rebound values R, of the Schmidt hammer and the ultrasonic rate v, concrete compressive strength values that are given in Table II, are read off. The average deviation of the real concrete compressive strength from the concrete strength calculated by the method mentioned above, i. e. with the combination of two testing methods, was 5.8%.

P. K n a z e -

P. Beno

TABLE II RESULTS AND EVALUATION OF NON-DESTRUCTIVE MEASUREMENTS OF CONCR FTE COMPRESSIVE STRENGTH AS WELL AS DESTRUCTIVE TESTS BY THE COMBINED METHOD

Calculated concrete compression strength MPa according to:

Sample designation

Rebound value

R,

Ultrasonic rate t~

(m. s- 1)

Concrete compressive strength by destructive test R~ (MPa)

General calibration curves

Constructed calibration curves

for Roumanian standard concrete

by Czechoslovak State " Standard CSN 732411

for Italian concretes

6

7

8

39.7 38.7 37.9 37.1 37.5 37.6

4,072 4,086 4,175 4,158 4,184 4,189

27.75 26.25 27.25 26.00 27.75 28.25

27.0 26.3 27.8 26.6 27.7 27.9

29.5 27.5 29.0 27.0 28.5 28.5

38.5 38.0 39.0 38.5 38.5 38.5

31.5 31.0 32.0 31.0 31.5 32.0

E/1 E/2 E/3

c/1 c/2 c/3

35.9 37.6 39.6 37.4 38.7 37.9

4,255 4,210 4,313 4,205 4,167 4,338

29.25 29.25 30.25 28.00 28.75 30.50

27.6 28.4 33.1 28.1 28.4 32.0

28.0 29.0 33.5 29.0 29.5 32.0

39.0 39.0 41.5 38.5 39.0 41.0

31.5 32.0 36.5 32.0 32.5 35.5

F/1 F/2 F/3 M/1 M/2 M/3 H/I H/2 H/3

37.1 36.9 38.2 36.3 37.5 37.4 38.0 39.8 39.7

4,202 4,188

28.50 28.00 27.50 31.25 33.00 31.00 28.50 30.50 30.00

27.5 27.2 28.5 31.0 32.0 32.6 31.0 33.0 34.3

28.5 28.0 29.0 30.5 31.5 32.5 32.0 34.0 34.5

39.0 38.5 39.0 41.0 41.0 41.5 41.0 42.0 42.0

31.5 31.0 32.5 34.0 35.5 36.0 35.0 37.0 38.0

K/1 K/2 K/3

39.0 38.6 38.2 38.1 37.5 37.0

4,336 4,348 4,367 4,367

31.50 32.00 33.50 30.00 32.25 30. 50

32.6 31.0 32.3 32.5 32.3 31.8

33.0 31.5 34.0 32.5 32.5 32.5

41.5 31.5 41.0 41.5 41.0 40.5

36.0 34.5 36.0 36.0 35.0 35.0

P/2 t/3 u/1 u/2 u/3

37.6 36.8 36.3 38.9 38.1 38.5

4,329 4,292 4,298 4,307 4,329 4,348

31.00 30.25 29.50 27.25 28.75 32.00

31.5 29.6 29.3 32.2 32.0 32.8

31.5 30.5 30.5 33.0 31.5 33.0

41.0 40.0 39.5 41.0 41.0 41.0

35.0 33.5 33.0 36.0 35.5 37.0

S/l s/2 s/3 T/1 T/2 T/3

38.8 39.0 38.8 38.2 39.5 38.8

4,283 4~310 4,320 4,317 4,292 4,310

27.25 29.75 28.25 30. 50 29.75 28.25

31.6 32.4 32.5 31.8 32.5 32.2

32.5 33.0 27.5 32.0 33.0 32.5

40.5 41.5 41.0 41.0 41.0 41.0

36.0 36.5 36.0 35.5 37.0 36.5

Zl/l

36.5 26.7 35.4 33.5 35.1 34.8

4,036 4,000 3,984 3,945 3,937 3,992

23.50 24.00 22.50 20.25 19.25 18.00

23.2 22.5 20.7 17.7 19.3 20.2

24.5 23.5 21.5 20.0 20.5 22.0

36.5 36.5 36.0 34.0 35.0 35.5

27.5 27.0 25.0 22.5 24.0 25.5

38.1 37.9 38.9 40.8 37.3 38.5

4,386 4,357 4,376 4,396 4,427 4,427

29.50 33.00 32.00 30.75 35.50 34.25

33.3 32.5 34.1 36.2 32.2 35.0

33.5 32.0 33.5 36.0 34.0 34.5

42.0 41.0 42.0 43.5 41.5 42.5

37.0 36.0 38.0 41.0 36.5 38.0

36.7 30.3 34.6 33.4 35.3 35.2

4,040 4,064 4,044 4,000 4,000

4,069

21.00 19.00 21.25 19.00 21.25 20.00

23.4 17.0 21.2 28.8 21.0 22.4

24.5 18.5 22.5 21.0 22.0 23.5

37.0 33.5 36.0 35.0 36.0 36.0

28.0 21.5 26.0 23.5 25.5 27.0

36.1 35.2 34.7 35.6 36.0 36.0

4,016 3,918 4,024 3,861 3,976 3,957

21.00 19.25 19.00 18.50 20.25 20.00

22.2 18.9 20.8 17.8 21.1 20.7

23.5 20.0 22.0 20.0 23.0 22.5

36.5 34.5 35.5 34.0 36.0 35.5

36.5 23.5 25.5 23.5 26.0 25.0

A/1 A/2 A/3 D/1 D/2

1>/3

o/1 0/2 0/3 P/1

Zl/2 Z1/3 Z2/1

Z2/2 Z2/3 Z3/1 Z3/2 Z3/3 Z4/I

z4/2 z4/3 1/1 I/2 1/3 2/1 2/2 2/3

3/1 3/2 3/3 4/1 4/2 4/3

4,188 4,367 4,354 4,386 4,295 4,301 4,351 4,317

4,264

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- M a t a r i a u x et C o n s t r u c t i o n s

Taking into account the average deviation of the real concrete strength from the strength, determined by non-destructive test methods, the most precise one in this experiment appears to be the ultrasonic pulse method. The Schmidt rebound hammer method was substantially less precise. Using a combination of both these methods, it was not possible to reach a higher precision, contrarily, only a somewhat lower precision than by the ultrasonic pulse method. It appears that by using two non-destructive test methods, it is not possible to gain an increase of precision in results, if one of the methods used is substantially less precise (in this case the rebound hammer method) than the other (in this case the ultrasonic pulse method). The precision with which the concrete compressive strength was determined by the combination of methods, is very good for current purposes in practice. The determination of equal strength curves, however, is tedious and experimental work must be carried out deliberately before the tests on the structures. Therefore, for comparison purposes, the results were evaluated according to generally valid curves (i. e. not those, determined experimentally for the given basic materials). The test results were evaluated according to the generally valid curves given in the appendix to the Czechoslovak State Standard ~ S N 732411 " N o n Destructive Tests of Concrete Structures", further curves were used, derived by Facaoaru [1, 2] for concrete produced in Northern Italy as well as for Rumanian standard concrete. The average deviation according to the standard (2SN 73 241,1, was 47.2~ (with the Czechoslovak State Standard higher results of concrete strength were always obtained), according to the curves for Italian concrete, on the other hand, 18.7~, and according to the curves for the Roumanian standard concrete, the deviation was only 7.1~. In the course of experiments carried out in the second half of 1981 in the framework of a joint program of CMEA member countries, the application of combined

210

methods was also verified. Using the combination of two methods, the Schmidt rebound hammer and the ultrasonic pulse procedure, it was possible to obtain (with tests on 75 samples) a certain reduction in the average deviation. Using the ultrasonic pulse method, the average deviation was 13.3~o, using the Schmidt rebound hammer method, 13.7~, using the HPS hammer, 14.8~0, using the Waitzman hammer, 16.0~, and using the ultrasonic method with the "frequency of the first input of signar'-parameter, 16.7~. If two ultrasonic parameters (rate and frequency) were combined, the average deviation was reduced to 13.4~, and using the combination of the ultrasonic pulse method with the Schimdt's rebound hammer one, the average deviation was reduced to ll.0~o.

7. CONCLUSIONS

According to the measurement by two nondestructive testing methods and their mutual combination, it is not possible to obtain an overall higher precision, if one of the used methods is considerably more inaccurate. Therefore it is justified to use a combination of two non-destructive methods only in those cases, when both of the methods are of approximately equal precision. When using generally walid calibration relations for the combination of two non-destructive testing methods, it is necessary to proceed with caution in evaluating the results since concrete strength results obtained in such a way, can be distorted by considerable errors. 8. BIBLIOGRAPHY

[1] FACAOARU,J. - ln-situ concrete strength determination by combined ultrasonic pulse velocity. Rebound index method.

1st RILEM TC 43 Meeting, Paris, 1978. [2] CIANFRONE,F., FACAOARU,J. -- Study on the introduction into Italy of the combined non-destructive method for the determination of in-situ concrete strength. 1st RILEM

TC 43 Meeting, Paris, 1978.