EFFECT

OF

SPECIMEN

TRANSMISSION

SIZE

THROUGH

P. P. Tsulukidze

ON

VELOCITY

OF

CONCRETE

ULTRASONIC UDC 666.972 : 620.179.16

and G. V. Sokhadze

The most effective method of testing concrete in situ is by measurement directly in the structures by the ultrasonic method. As is well known, when determining the strength of concrete by the ulatraonic method, use i s m a d e of the v e l o c i t y - s t r e n g t h correlation curve, constructed from the data obtained by subjecting control cubes of 10to 30-era size to ultrasonic and mechanical testing. Soviet and overseas specialists have experimentally determined the facts concerning the change in the v e l o city of ultrasonic transmission over different base lengths [1 to 5]. It has been established that the measurement of the travel time of an acoustic impulse over extremely short base lengths gives velocity values v which are too low. lnasmueh as the dynamic modulus of elasticky of materials is directly proportional to the square of the impulse velocity, the error incurred in its calculation leads to significantly reduced results. Some specialists [4] recommend that ultrasonic velocity should be determined over values of Z >-- 30 cm. In fact, if the size of the control specimen is not considered, the results may lead to definite errors. In order to establish the effect that concrete-specimen dimensions have on the velocity of ultrasonic transmission, 96 twin specimens of the same concrete composition were prepared and tested. Twelve cube specimens prepared from each of two batches of a concrete mix, the sets having the respective edge dimensions of 20, 15, 10, and 7.07 era. The specimens were stored in a building with a relative humidity of 95 to 100% and a temperature of 18 to 22"C. After curing for 28 days, the specimens were ultrasonically tested along the two hotizontal axes and the average velocity for each specimen was determined. Then the overall average velocity of ultrasonic transmission through all the specimens of each size was derived (Table 1). The ultrasonic transmission was carried out by a sound generator DUK-20. A tight acoustical contact between the gauges and the concrete surface was ensured by smearing the contiguous faces with grease and clamping the gauges to the concrete with a pressure of 5 to 6 k g / c m z. For the pressure elements, crystals of Rochelle salt (potassium sodium tartrate) were used; these are characterized by a vibration frequency of 150 kI-~. As the result of the investigations carried out by the authors, it was ascertained that for specimen cubes of 7 to 20-cm edges the velocity of ul~asonie transmission is increased by 2 to 11% (see Table and Fig. 1). On the other hand, it has been established by investigations carded out at the V. V. Kuibyshev Moscow Institute of Civil Engineers [6] and elsewhere, that the velocity of ultrasonic transmission over base lengths of 1-6 m is reduced by 1 to 12%. The reason for such variation of the velocity of ultrasonic transmission has so far not been discovered. During the propagation of ultrasonic waves in concrete - a heterogeneous elastic-plastic material, a complex process, comprising their reflection, refraction, diffraction, and dispersion, occurs [2, 7].

jZ

~jJ

f Fig. 1. Values of correction coefficient (abscissae) for various ratios of 20-cm edge to edges of other sizes of cubes (ordinates).

The complex acoustic impulse passing through the concrete breaks up into separate waves of different frequencies, which are propagated to different distances [1]. A change in the specimen d i m e n sions changes the propagation conditions of the ultrasonic transmission, their overall effect being either an increase or a decrease in the propagation velocity of the vibrations. Thus, on the one hand, an increase in the base length of ultrasonic transmission causes a reduction in the velocity of vibration penetration, because various obstacles, comprising zones with different acoustic resistance, occur along the way. The dimensions of such an obstacle exceed the wavelength of the high-frequency vibrations, or are commensurate therewith, and this leads to reflection, refraction, and dispersion of these waves. The high-frequency components of ultrasonic transmission in concrete are thus rapidly damped out, reducing the propagation velocity of the impulse.

Translated from Gidrotekhnicheskoe Stroitel'srvo, No. 2, pp. 11-12, February, 1972.

115

116

P, P. TSULU KIDZE AND G, V. SOKHADZE

TABLE 1. Velocity of Ultrasonic Transmission through Concrete Specimens 15X!SXI5 CITI

20X~X'20 c m

Batch ~r i

I Batch

4 800 4 720 4 940 4 800 4 850 4 940 4 840 4 720 4 800 4 800 4 750 4810

so, 2

Batch x, I

4 560 4 530 4 710 4610 4 650 4 720 4 660 4 530 4 600 4 610 4 530 4 570

4 707

4 720 4 760 4 690 4715 4 780 4 660 4 715 4 635 4 690 4 675 4 690 4 740

Batch ~, 9_ 4 480 4 550 4 490 4 480 4 535 4 450 4 520 4 400 4 380 4 480 4 490 4 480

Batch so, I

Batch ~ s

4 460 4515 4515 4 450 4 400 4 460 4515 4 480 4 425 4 460 4515 4 480

Batch ~r I

4 2O0 4 270 4 290 4 200 4 135 4210 4210 4210 4 240 4 165 4 325 4 250

Average velocity for given specimen size 4 591 ] 4 348

I

7,07X7,07X7,07 c m

10XIOXI0 c m

[ Batch x, 2

4 340 4 340 4 470 4 500 4 28O 4 340 4 360 4310 4 255 4 280 4 240 4 255

[

4 140 4 080 4 160 4415 4 08O 4415 4415 4 160 4 140 4 O8O 4 060 4 040

4 204

TABLE 2 Correction coef. for translating ul- Reduction of velocity of uRrasonic Average velocity of uluasonic transmission trasonic velocity to the basis o f a transmission for cubes of dimen20-cm cube, foi cubes of dimenthrough cubes of dimensions (cm) sions, qo sions (cm) 15X15• 10XIOXI0 7,07X7,07X7,07 15XISXI5I 10XlOXlO] 7,07x7oO7x7o07 20x20x20 [ ]SXlSXI5 I 10XI0XI0

[7,OTxT,OTx7,O7

4 707

I

4 591

4 348

i

4 204

I ,025

1,085

I, 125

2,2 1 703 1

10,7

On the other hand, an increase in the specimen dimensions also increases the transverse section of the w a v e guide medium, and this leads in turn to an increase in the propagation velocity of the ultrasonic transmission, as it is well known [1] that ultrasonic waves emitted by a source into any medium disperse with a certain expansion angle of the beam, which depends on the radio k / 13, D being the diameter of the emission source and k the wavelength. With an increase of the wave-guide medium, the degree of envelopment of the wave beam is also increased accordingly, and this creates conditions favorable for the transmission of long-wave vibrations through the concrete, i.e., passing through,and we q u o t e , " . . , the heterogeneous medium that concrete is, more rapidly than waves whose length is commensurate with the geometric measurements of the discontinuities. This is a consequence of the ability of long waves, in accordance with the laws of diffraction, to bend around obstacles whose dimensions are less than their l e n g t h . . . " [7]. When the increase in the velocity of ultrasonic transmission, depending on the cross-sectional dimensions of the wave-guide medium, becomes more significant than the effect of increasing the base length, then the overall effect will be an increase in the velocity. With an increase in specimen size, the relative increment in the velocity diminishes, and the correction coefficient approaches unity (Table 2). For certain specimen dimensions, the increase in propagation velocity, caused by an increase in the cross section, is cancelled by the effect of increasing the base length of the transmission. It follows from the above that, for ultrasonic transmission, there are optimum (limiting) dimensions of the m e dium (or wave guide). If the medium's dimensions are less than optimum, then the velocity of the transmission increases with increase in specimen dimensions; but i f they exceed the optimum values, then the velocity diminishes with increase in specimen dimensions. As is well known, " . . . a single picture of the process of wave motion, taking into account the phenomena of geometric dispersion, has not been obtained up m now . . . " [7]. Therefore, the subject touched upon undoubtedly deserves due attention and demands further investigation and development, with the aim of establishing accurate conversion factors which will enable the strength of concrete in structures to be determined.

VELOCITY OF ULTRASONIC TRANSMISSION T H R O U G H C O N C R E T E LITERATURE I. 2. 3. 4. 5. 6. 7. 8.

117

CITED

A.K. Tret'yakov and A. M. H1onidov, Ultrasonic Control of Concrete in Hydraulic Engineering Construction [in Russian], ~nergiya, Moscow (1964). N.A. Krylov and A. S. Durasov, Physical Methods for Controlling Concrete Quality [in Russian], Gosstroiizdat, Moscow (1959). O, ~. Pflaumer, Fundamentals and Practice of Ultrasonic Investigationsof Concrete [in Russian], Gosstroiizdat, Moscow (1962). V . M . Huslov and V. N. Morshikhin, "Reliabilityof the impulse method of testing concrete under industrial conditions," Stroite1'stvoi Arkhitektura Leningrada, No. i0 (1964). F. PoI', Nondestructive Methods of Testing Concrete [in Russian], Stroiizdat, Moscow (1967). A.M. Filonidov, Investigation of the Possibilityof Applying Ultrasonics for Controlling Concrete Quality in Hydraulic Engineering Construction, Author's Abstract of Candidate's Thesis, Moscow (1069). N.A. Krylov, V. A. Kalashnikov, and A. M. Polishchuk, Radio-Engineering Methods for Controlling Quality of Reinforced Concrete [in Russian], Stroiizdat, Moscow (1966). Instructions for Controlling Concrete Quality by Ultrasonic Device Type UKB-1 [in Russian], Gosstroi USSP,, Kiev, Nauchno-Issledovatel'skii Institut Stroitel'nykh Konstruktsu (1964).