Spall velocity of blast loaded concrete specimen

Martin Knzel*, Ondej Nmec*, and Jiri Pachman*

*Institute of Energetic Materials, Faculty of Chemical Technology,

University of Pardubice, Pardubice, CZ

kunzel.martin@gmail.com

Abstract:

The paper is focused on measurement of spall velocity of blast loaded concrete samples.

The tests were performed using small scale plain concrete tiles (300 300 34 mm). Small charges of Semtex 1A industrial plastic explosive (10 to 92 g) were used as shock

wave generators, being placed in contact with the samples. The spall velocity was meas-

ured on the soffit using simple photonic Doppler velocimetry setup while the whole process

of blast induced spall formation was recorded by high speed framing camera. The results

and instrumental methods tested during these experiments will be useful for future meas-

urements on real scale concrete slabs.

Keywords: concrete; spall; photonic Doppler velocimetry; plastic explosive; framing cam-

era

1 Introduction

Stability of engineering structures such as bridges, buildings or road constructions under

unforeseen terrorist attack using large explosive charges started to be a question for structural

engineers, architects and even public authorities. Some preliminary tests with large scale con-

crete slabs uncovered that the response of concrete to adjacent blast loading is more complex

than had been expected and a deeper examination is needed [1]. Behavior of a shock loaded

concrete on a small scale has become a research subject for the Institute of Energetic Materials

during recent year.

Photonic Doppler velocimetry (PDV) system and high speed framing camera were em-

ployed for measurements on concrete samples. The paper shows some results of the very first

experiments which were conducted in order to test the instrumentation and also to confirm

some basic assumptions on shock loading of concrete.

2 Experimental

The spall velocity was measured on commercial grade plain concrete tiles (300 300 34 mm) which were subjected to blast loading using explosive charges placed directly to the

surface. Industrial plastic explosive Semtex 1A (S1A) based on pentaerythritol tetranitrate was

used for testing in order to ensure the best contact with the specimen. The shape of charges was

hemispherical.

Measurements of spall velocities were performed using a single channel basic PDV with

a reference signal taken from the probe back reflection [2-4]. The measurement setup consisted

of a laser emitter, a circulator, a detector and a fast oscilloscope (4 GHz, 25 GS.s-1

). The laser

module was operating at 1550 nm and it was limited in its power output to 40 mW. The real

power output used for testing was 36 mW at the laser which corresponded to 22 mW at the

probe tip. Loses were caused by attenuation in cables, connections and in the circulator. Bare

fiber probes were fixed to a steel holder about 10 mm from the soffit of the test specimen in a

position perpendicular to the specimen surface.

The whole process of spall formation was captured using high speed photography. The ex-

periments were observed from the side using IVV UHSi 12/24 ultra-high speed framing cam-

era equipped with Sigma 70-300 mm f/4-5.6 lens. The lens aperture was fully opened to f/5.6

at 300 mm focal length while the camera gain was set to 64 %. The shutter speed was 2 s (ex-cept 0.5 s for the test 1) at the frame rate of 100 000 frames per second. A mirror was placed in front of the camera in order to allow indirect observation without the risk of camera damage

by fragments.

The UHSi camera recording was triggered using an ionization pin inserted in the explosive

charge in contact with the specimen surface. Twelve frames were captured in each test. Back

illumination of the test setup was ensured by light from the explosive charge reflected from

a sheet of white paper behind the specimen. The measurement setup is shown in figure 1.

In the test 1, the paper sheet was excluded and an electronic flash lamp was projected to

the background using another mirror as can be seen from figure 2. Moreover, a piece of alu-

minium tape was placed on the soffit in order to improve the PDV signal.

Figure 1: Scheme of the test setup

3 Results and discussion

The process of blast loading of concrete is described by following sequence of steps. The

detonation wave of the main charge generates a strong shock wave in the concrete specimen.

The shock wave partially reflects back from the specimen surface. The compressive stress at

the top surface of the specimen cause local crushing of concrete. The shock wave passes

through the specimen to the bottom side, where it partially reflects again, interfering with the

release part of itself. At this time, cracks develop on the soffit and the spall begins to form. The

shockwave again passes through the interface, creating a shock wave in air under the specimen.

The shock wave in the air is later followed by flying spall debris. Although the detonation

wave pressure in the explosive material is roughly the same regardless of the mass of the ex-

plosive, the overall impulse sent to the specimen is different as well as losses caused by crush-

ing and heating the specimen.

The spall velocity data from the PDV probe were treated using short-time Fourier trans-

form (STFT) to obtain velocity vs. time curves. It can be seen that the spall is accelerated with

short rise time and then it holds the velocity until it reaches the probe (figure 2).

Figure 2: Example of PDV velocity record (test 1)

Figure 3: Examples of UHSi frames (1 shock wave in air, 2 spall)

Figure 4: Example of camera record evaluation (test 1)

The UHSi camera records (figure 3) were spatially evaluated. Time-distance data were ex-

trapolated using least square fitting, giving initial time from the y-intercept (figure 4). Deriva-

tion of distance-time data in the x-intercept gave the initial spall velocity.

y = 0.0102x2 + 4.4766x + 12.188 R = 0.9982

y = 2.35x + 12.772 R = 0.9997

0

50

100

150

200

250

0 20 40 60 80 100

Tim

e (

s)

Distance to the soffit (mm)

Spall debris

Air shock wave

Test 1 was performed using bright back light source which led to visualization of both

shock waves in air and spall marked in figure 3 as 1 and 2 respectively. It can be seen from

figure 4, that spalling begins just on the shock wave arrival to the concrete-air boundary. As-

suming spall inception to take place immediately on the shock wave arrival to the soffit, the

average shock wave velocity in the specimen can be calculated.

The summary of the experiments and results is contained in table 1 where m is weight of

the explosive, v is the spall velocity, t is the shock wave transition time through the specimen

and U is the estimated shock wave velocity. For the test 4, the UHSi spall velocity is underes-

timated due to its fast attenuation in the air and no data availability for the very beginning of

spall formation. The consequent shock wave velocity is then overestimated evidently. The

comparison of spall velocity measurement using PDV system and the camera is also shown in

figure 5.

Table 1: Summary of experiments and measured velocities

Test no. m v (PDV) v (UHSi) t U

(g) (mm.s-1) (mm.s-1) (s) (mm.s-1)

1 10 0.20 0.22 12.2 2.8

2 20 0.31 0.32 10.2 3.3

3 40 0.64 0.78 9.1 3.7

4 92 1.02 0.92 4.9 6.9

Figure 5: Spall velocity dependence on the explosive charge weight

4 Conclusions

The spall velocity of blast loaded concrete samples was measured on the soffit using sim-

ple photonic Doppler velocimetry while the whole process of blast induced spall formation was

recorded by high speed framing camera. The results clearly show an agreement between PDV

measured velocities and those obtained from the spatial displacement on high speed photo-

graphs. Aluminium tape placed on the soffit proved to be useful for intensifying the PDV sig-

nal.

The times of the beginning of spall formation and corresponding shock wave velocities

were also estimated from the camera records.

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100

Sp

all

deb

ris

vel

oci

ty (

mm

.s-

1)

S1A charge weight (g)

PDV

UHSi

Acknowledgments

This paper was supported by the Grant Agency of the Czech Republic Grant Project No.

GAR13-30441S

References

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77, p. 083108, 2006.

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rd Annual PDV Conference, Albuquerque, USA, 2008.

[4] M. Knzel, O. Nmec, J. Pachman, Terminal Wall Velocities of Emulsion Explosive Charges, in proc. 39

th International Pyrotechnics Seminar, Valencia, 2013.