CEMENT and CONCRETE RESEARCH. Vol. 5, pp. 239-248, 1975. Pergamon Press, Inc Printed in the United States.

HIGH-SPEED PHOTOGRAPHY FOR FRACTURE

STUDIES OF CONCRETE

Jitendra Bhargava Inst. for Structural Engineering & Bridge Building,

S-lO0 44 Stockholm 70, Sweden

Ake Rehnstr~m Inst. for Photography, KTH

S-IO0 44 Stockholm 70, Sweden

KTH

(Refereed) (Received Nov. 12, 1974; in final form March 12, 1975)

ABSTRACT The fracture of ordinary and polymer-impregnated concrete has been studied by high-speed photography. 20-40 mm thick concrete prisms were subjected to a dynamic loading by detonation of a high explo- sive in contact with the top surface of the specimen. A multiple Kerr-cell camera was used for recording the fracture. A descrip- tion of the Kerr-cell and the equipment used is given, and some typical fractograms taken at different intervals are shown. An esti- mation of the stresses transmitted showed that under dynamic loading the polymer-impregnated concrete did not show any significant in- crease in strength above the static strength.

Bruchvorg~nge in gew~hnlichem und polymerimpregniertem Beton wurden mit Hilfe der Kurzzeitfotografie untersucht. 20 bis 40 mm dicke Betonprismen wurden durch Detonation von Sprengk~rper an der oberen Begrenzungsfl~che der Proben einer dynamischen Belastung ausgesetzt. Zur Registrierung der Bruchvorg~nge wurde eine Multipel-Kerrzellen- kamera benutzt. Die Kerrzellenkamera sowie die Versuchsanordnung werden beschrieben und einige typische Bruchbilder bei verschiedenen Intervallen gezeigt. Eine Sch~tzung der fortgepflanzten Spannungen bei dynamischer Belastung zeigt keine signifikante Festigkeits- steigerung Uber der statischer Festigkeit in polymerimpregniertem Beton.

239

240 Vol. 5, No. 3 J. Bhargava, ~. Rehnstr~m

Introduction

Fracture of concrete has been studied by several techniques varying

from the visual observation of surface cracking I l l to the measurement of

internal cracking by radiography of sawn prisms under loading 121.

High-speed photography has been used during the recent years for

studying the dynamic characteristics and fracture behaviour of various

materials such as glass 131, rocks 141 and plastics 151.

The present paper describes some fractographic studies of different

types of concretes subjected to a dynamic loading produced by detonating

a high-explosive in contact with the top surface of the specimen. High-speed

photography using a multiple Kerr-cell camera was used for recording the

fracture of concrete.

Kerr-cell Camera

For recording the image of a concrete specimen subjected to a shock

loading i t is preferable to use a camera system with a shutter, permitting

intermittent exposures. A camera with a Kerr-cell shutter was chosen for

our studies, as i t permits exposures of very short duration (O.03-O.3us)

necessary for avoiding unsharpness due to crack propagation.

The construction of a Kerr-cell shutter is based on the property of

rotating the plane of polarization of transmitted l ight , acquired by

certain f luids under the influence of an electr ic f ie ld.

The Kerr-cell shutter consists of a glass vessel containing nitro-

benzene, which is placed between two polaroid f i l t e rs . With crossed polaroids

no l ight passes through the shutter. A short high-tension pulse applied

to the cell (through a pair of metal plates immersed in the dielectr ic)

results in the rotation of the plane of polarization of transmitted l ight ,

and the shutter is "opened" for the duration of the pulse. The image of

the specimen is recorded by the camera placed behind the second polaroid.

By connecting a number of Kerr-cell cameras, operating in sequence,

various phases of the fracture of a material can be studied. Multiple

Kerr-cell camera used in our studies was equipped with 4 such units.

.Experimental Details

Details of Specimens

Two types of concretes were studied:

Vol. 5, No. 3 241 HIGH-SPEED PHOTOGRAPHY, FRACTURE, POLYMER IMPREGNATED CONCRETE

I. Ordinary concrete, grade K 600 (nominal 28-days strength 60 MPa)

2. Polymer-impregnated concrete. These specimens were dried at 105°C and were

impregnated with styrene containing 3% by weight of benzoylperoxide as

in i t ia to r . Impregnated specimens were stored in an oven at 85°C for ther-

mal polymerization.

20-40 mm thick prisms used for these studies were sawn from 15 cm cubes

and ~ lO x 20 cm cylinders. In case of polymer-impregnated concrete all the

aggregates visible on the sawn surface were painted white, to improve the

contrast in photographs. A network was drawn on the surface of some specimens

to fac i l i ta te a study of the relative movement of aggregates during fracture.

All specimens were photographed prior to loading.

Experimental Set-up

The experimental set-up used is shown in FIG. I . The surface of the

concrete prisms was illuminated by two electronic flash units. The charge

consisting of a capsule of high explosive was placed on the top surface of

the prism and was well covered with plastic clay. Multiple Kerr-cell camera

was placed behind a protective glass window.

The sequence of explosive detonation, flash ignit ion and Kerr-cell

exposures was programmed on an electronic unit so that different events

occurred successively after predetermined intervals. This allowed a study

of the different stages of fracture process. Suitable delay in triggering

of the flashes was necessary to ensure uniform illumination of the specimen.

FILM KERR-CELL ~ IGLASS '~ SHUTTER / / ~ I ' ~ ' " WINDOW

I " 4 I

. i ~ . . . . CHARGE

.- / ,Y j . J -g . . . . ~ / \ . Y f / /

J " F L A S H CONCRETE PELLETS

FIG. 1

Experimental set-up for Kerr-cell photography

242 o Vol. 5, No. 3 J. Bhargava, A. Rehnstr~m

The veloci ty of wave propagation in concrete was estimated by Goldsmith

et al 161 to be of the order of 3.6 mm/us. The wave can thus be anticipated

to traverse a 150 mm specimen in about 40 us. However there is also a certain

"incubation time", which is the time lag between detonation and the i n i t i a t i o n

of f racture J7 I. Hence the suitable intervals for exposures had to be obtained

by t r i a l . Two typical sequences used are given below:

I . Ordinary concrete

Flash 1 Flash 2

Detonation ~ - - ~ ~ us

KC 1 KC 2 KC 3 KC 4

4 exposures: I00, 150, 250 and 300 us af ter detonation

2. Polymer-impregnated concrete

Flash 1 Flash 2

Detonat i on ~ - - ~ ~ - -

KC 1 KC 2 KC 3 KC 4

~S

4 exposures: 50, I00, 150 and 200 us af ter detonation

Results and Discussions

Some of the fractograms showing the typical f racture character is t ics

of the two types of concrete are given in FIG:s 2 and 3.

FIG. 2a shows a fractogram taken 150 us af ter detonation. I t can be seen

that the i n i t i a l cracks were formed along the vert ical mortar-aggregate

interface as in case of s ta t i c loading.

FIG. 2b shows a photograph of the same specimen taken 300 us af ter de-

tonation. At th is stage some horizontal cracks, as well as some cracks

along the morphological grain boundaries of aggregates have also been

formed by the tensi le pulse.

The veloci ty of b r i t t l e crack propagation was estimated to be 180 m/s.

Large local variat ions of th is were also observed, which were due to

the aisturbance produced by f racture. For instance i t is well known

that forking ef fect ive ly l imi ts the veloci ty of propagation of cracks.

V01. 5, No. 3 243 HIGH-SPEED PHOTOGRAPHY, FRACTURE, POLYMER IMPREGNATED CONCRETE

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Vol. 5, No. 3 245 HIGH-SPEED PHOTOGRAPHY, FRACTURE, POLYMER IMPREGNATED CONCRETE

Study of aggregate positions in successive fractograms showed that the

displacement was mostly horizontal, indicating a fai lure by cleavage.

FIG. 3a shows the fracture behaviour of polymer-impregnated concrete;

this photograph was taken 50 ~s after detonation. I t can be seen that

in contradistinction to plain concrete, the fracture took place more

across the aggregates in polymer-impregnated concrete. Crack formation

through the aggregates indicates an improved paste-aggregate bond.

FIG. 3b is another fractogram in the same series taken 150 ~s after

detonation and shows that the cracks have propagated through several

aggregates. The velocity of crack propagation in this case was about

800 m/s.

I t is also interesting to note that both in ordinary and polymer-

-impregnated concrete cracking had already started during the f i r s t

passage of the compression wave. The cracks were formed in the vertical

direction, primarily due to the tensile stresses in the transverse di-

rection.

Theoretical Analysis of ComPressive Pulse Propagation

Two aluminium pellets were attached l ight ly to the bottom face of the

prisms. When the induced compressive pulse is reflected from the free face as

a tensile wave, the pellets are propelled from the specimen at a velocity

equal to twice the particle velocity (this is s t r i c t l y true for a wave with

a square front). The velocity of pellets can be estimated from the successive

fractograms.

The relation between the intensity of stress propagated %, and

the particle velocity V ° is given by the following expression 181:

o o = p • c L • v o ( I )

For an isotropic material, the elastic longitudinal wave velocity C L is given by

C L : ~ (2) p

where E = modulus of e last ic i ty and p = mass density.

Goldsmith et al 161 had observed that this relation was approximately

valid for concrete also. By measurements at successive points on a long

concrete rod subjected to a central longitudinal impact, they had obtained

a value of 3600 m/s for C L. Average value of dynamic modulus of e last ic i ty

in their tests was 29 GPa. Mass density was 2170 kg/m 3. Substituting these

246 Vol. 5, No. 3 J. Bhargava, ~. Rehnstr~m

values in eq.2, we get C L value.

= 3655 m/s, which is fa i r ly close to the observed

(1), we get

o o = V o CE.p (3)

Substituting for C L from eq (2) in eq

We can now estimate the intensity of stress propagated from the velocity

induced to the aluminium pellet. For polymer-impregnated concrete (PIC), pellet

velocity was 16 ± 0.5 m/s (average of 3 values), and the particle velocity can

be estimated as 8 m/s.

The dynamic modulus of e last ic i ty of the PIC, determined by fundamental

frequency measurements was 45 GPa and p was 2400 kg/m 3. Substituting these

values in eq.2, we get C L = 4330 m/s, a value which lies between that of

glass and concrete. The value is higher for PIC because many microflaws

in concrete are eliminated by impregnation 19,121 and the concrete becomes

more homogeneousand br i t t le . Substituting the values for Vo, E and p in

eq (3), we get for PIC, o o = 84 MPa.

The fracture strength of this concrete determined by static tests was

88 ± 4 MPa. Making allowance for the error in determining the pellet velocity

because of sl ight adhesion, and the shape of the propagated wave, i t can be

concluded that there was no significant difference between the static strength

of PIC and the stress transmitted under transient loading. This result may at

f i r s t glance appear to be at divergence with the earlier results of testing

of concretes at high rates of loading, where i t had been reported that the

strength of concrete increases with an increase in the rate of loading llOI.

But this can be explained from the fracture mechanic considerations.

The concrete is a viscoelastic material, in the sense that i t undergoes

both instantaneous and delayed deformations when subjected to a load. Several

models of the structure of concrete have been proposed during the recent years

I l l I, using springs and dashpots to explain the instantaneous and time-dependent

deformations respectively. When a force is applied to such a material for

only a very short time the shear stresses bui l t up do not have time to produce

an appreciable amount of strain, and the material consequently can withstand

stresses of much greater magnitude than i ts static strength, as observed by

Hughes et al llOI. In the model, the strain is delayed because of strain rate

dependency of the dashpots.

By polymer impregnation the pores of cemen~ paste are f i l led and dis-

continuities at the paste aggregate interface are substantially reduced. The

Vol. 5, No. 3 247 HIGH-SPEED PHOTOGRAPHY, FRACTURE, POLYMER IMPREGNATED CONCRETE

new composite can be stressed to a higher level, but the "viscous forces"

(resulting from local microcracking and crushing) producing the lag between

the stress and the strain cannot be mobilized. This can be seen from the

stress-strain diagram of polymer-impregnated concrete I121, which shows

that such concrete undergoes very l i t t l e inelastic deformation prior to

fracture.

Acknowledgements

The authors would l ike to thank the Swedish Board for Technical Development

for the financial support, and Mr Algot Persson, Swedish Detonic Research

Foundation, for his help in Kerr-cell photography.

References

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3.

J. K. Bhargava, Materiaux et Constructions, 4, 231 (1971).

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4. A. Persson, Proc. 9th Int. Cong. on High-speed Photography 337 (1970).

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J. E. Field, and A. Ladegaard-Pedersen, Int. J. Rock Mech. Min. Sci., 8, 213 (Ig71).

W. Goldsmith, M. Polivka, and T. Yang, Experimental Mechanics, 6, 65 (1966).

H. C. van Elst, Proc. Int. Conf. on Dynamic Crack Propagation, 315 (1972)

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