High-speed photography for fracture studies of concrete

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  • 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 105C 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 85C 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

    l , T.C. Hansen, Causes, Mechanism, and Control of Cracking in Concrete, Amer. Conc. Inst. pub. SP-20, 43(1968).

    .

    3.

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

    S-I. Hyodo, and H. Nakatsuka, Proc. Int. Conf. on Fracture, 1865 (1966).

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

    .

    .

    .

    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)

    . W. Johnson, Impact Strength of Materials, p. 6. Edward Arnold Ltd., London (1972).

    . J. K. Bhargava, Polymers in Concrete, Amer. Conc. Inst. pub. SP-40, 205 (1973).

    lO.

    I I .

    12.

    B. P. Hughes and R. Gregory, Mag. Conc. Res. 24, 25 (1972).

    W. Manns, Proc. Materials Conf.: Structure, Solid Mechanics and Engg. Design, l , 667 (1971).

    D. G. Manning, and B. B. Hope, Cem. and Conc. Res. l , 631 (1971).