Impact behaviour of concrete beams - Springer Link

Materials and Structures/MatOriaux et Constructions, 1987.20, 293-302

Impact behaviour of concrete beams N. P. BANTHIA*, SIDNEY MINDESS*, and ARNON BENTUR? *Department of Civil Engineering, University of British Columbia, Vancouver BC V6T 1W5, Canada and "?Building Research Station, Department of Civil Engineering, Technion - Israel Institute of Technology, Technion City, Haifa 32000, Israel An instrumented impact machine was used to carry out impact tests on concrete beams, 100 x 125 mm in cross-section and 1400 mm long. The simply supported beams were struck at their midpoints by a 345 kg mass impact hammer, dropped from various heights. The instrumentation included strain gauges mounted on the striking end of the hammer, strain gauges mounted on one support anvil, and three accelerometers placed at various locations along the beam. The data were collected using a 5-channel data acquisition system. Normal strength, high strength, and fibre reinforced concrete beams were tested. In general, it was found that the properties of concrete under the high stress" rates associated with impact loading could not be predicted from conventional static tests. 1. INTRODUCTION There are many examples [1] of structures or structural elements subjected to high strain rate loading. A very rapid rise of the external load from zero to the peak load requires that the material in the structural element quickly develops the internal stresses necessary to balance the external load. Concrete, with its heterogeneous structure, is found to behave differently when it is subjected to high strain rate loadings, as compared to its behaviour under quasistatic loading [2-12]. Many studies have been conducted to assess the behaviour of plain concrete at different strain rates [7-9]. Since the development of fibre reinforced concretes, with their dramatically improved toughness compared to plain concrete, there has been a growing interest in understanding the performance of both plain concrete and fibre reinforced concrete [11-13] under impact loading. These efforts, including the use of fracture mechanics, have not yet provided a satisfactory way of predicting correctly the change in the behaviour of concrete upon increasing the strain rate. Moreover, because of the complex energy transfer mechanisms, impact cannot be looked upon simply as an extreme case of high stress rate application. Various techniques have been used to test concretes at high rates of straining, such as: (a) (b) (c) (d) (e) (f)

Free fall drop weight tests; Work of fracture tests; Explosive tests; Hopkinson's Split Bar tests; Charpy/Izod tests; and Fracture mechanics tests.

In all of the above test methods there is an attempt to quantify the energy required to achieve failure. However, because both the failure criteria and the physical processes by which failure occurs vary from test to test, comparisons between any of the above tests are very 0025-5432/87 9 RILEM

difficult. In this study, free fall drop weight tests were carried out on concrete beams to assess their impact performance.

2. EXPERIMENTAL PROCEDURES The instrumented drop weight impact machine used in this study has been described in full elsewhere [2]. Basically, the machine is capable of dropping a 345 kg mass hammer from heights of up to 2.4 m. Strain gauges mounted in the striking end of the hammer (called the 'tup') are used to record the contact load between the impacting hammer and the beam. Additional strain gauges are similarly mounted in one of the support anvils, to monitor the reactions. Three accelerometers are placed along the length of the beam. A 5-channel data acquisition system, based upon an IBM personal computer, is used to collect the data; the five channels can be read simultaneously at 200 microsecond intervals. The data thus acquired are stored on a magnetic disc and then transferred to an Amdahl computer for analysis.

3. SPECIMENS The details of the production of the concrete beams, with the dimensions (length x width x depth) of 1400 x 100 x 125 mm, have been described elsewhere [2]. The normal strength beams, made with CSA Type 10 normal Portland cement (equivalent to ASTM Type I), with a maximum aggregate size of 10 ram, had the following mix proportions: water:cement:fine aggregate:coarse aggregate = 0.5:1.0:2.0:3.5. This concrete developed an average strength of about 42 MPa at the time of testing. High strength concrete was made in the same way as normal strength concrete, but with the addition of 16%

294

Banthia, Mindess and Bentur

condensed silica fumet (by weight of cement). This resulted in a concrete with a compressive strength of 82 MPa. It had the following mix proportions: water: cement:silica fume:fine aggregate:coarse aggregate = 0.33:0.86:0.14:1.57:1.04. The steel fibre reinforced concrete contained, in addition, 1.5% by volume of steel fibres. These fibres were 50 mm long, 0.60 mm in diameter, with both ends hooked.t Polypropylene fibre reinforced concrete contained 0.5% by volume of 37 mm long fibrillated polypropylene fibres.$ The beams were stored in a moist room until they were tested, at test ages varying from 3-6 months.

200 microseconds. This information was recorded on a magnetic disc by the host computer. The use of three accelerometers on the beam (Fig. la) provided the acceleration distribution along the length of the beam. This, in turn, could be used to generate the shape of the deformed beam undergoing impact. Results from the tests done on normal strength, high strength, and fibre reinforced concrete supported the assumption made by Bentur et al. [2] that the accelerations and displacements along the length of the beam (Fig. lb) were linearly distributed. Knowing the accelerations along the length of the beam, the distributed inertial forces can be replaced by a generalized inertial load Pi(t) acting at the centre [2],

4. M E T H O D O F T E S T I N G AND ANALYSIS O F RESULTS The beams made with normal strength, high strength, and fibre reinforced concrete were tested as described by Bentur et al. [2]. Briefly, the testing involved mounting the accelerometers on the beam, supporting the beam on the test anvils, raising the hammer to the required height and, finally, letting it drop on the beam. With the data acquisition system triggered by a photocell attached to the hammer itself, the strain gauges and the accelerometers simultaneously sent their relevant signals to the acquisition system at intervals of

~ HAMMER L

Pi(t) = g A tio(t) [e/3 +

(8/3)/(h3/?~2)1

(1)

where

Pi(t): Generalized inertial load; Q: Mass density of concrete; A: Area of cross-section of the beam; //o(t): Acceleration at the centre; e: Span of the test beam; h: Length of the overhang. Then, once the generalized inertial load Pi(t) at the centre is known, the beam can be modelled as a single degree of freedom system (SDOF) and the actual bending load can be evaluated using the equation of dynamic equilibrium, Pb(t) = Pt(t) - Pi(t)

ACCELEROMETERS

(2)

where

96cm

-~

Pb(t): Generalized bending load; Pt(t): Observed tup load; Pi(t): Generalized inertial load. '

Velocities (rio(t)) and deflections (uo(t)) at midspan were calculated by integration of the midspan acceleration (//o(t)) with respect to time:

(a)

t

ito(t) ffio(t ) dt =

(velocity)

(3)

(deflection)

(4)

0 t

ASSUMED"

'~"kl

NEARLY EXTRAPOLATED

u(t) = f ao(t) dt 0

(b) Fig. 1. Position of the accelerometers (a), and acceleration distribution along a concrete beam (b). t Provided by Pennsylvania.

Elkem

Chemicals,

Inc.,

Pittsburgh,

t Produced by Bekaert NV, Belgium + Produced by Forta Fibres Inc., Grove, Pennsylvania.

From these data, the bending load versus deflection relationships at midspan could be determined. The energy consumed by the beam in the process of deforming is then equal to the area under the bending load vs. central deflection, Pb(t) VS. Uo(t) (Equation 2). The support reaction obtained from the strain gauges in the support anvil was used only to confirm whether this technique to evaluate the inertial loading on the beam is valid. An account of this verification is presented below.

Materials and Structures

295

5. T H E S U P P O R T R E A C T I O N

6. R E S U L T S

As described earlier, one of the anvils on which the beam rested was also instrumented to permit the direct reading of the support reaction. This was used as a check to determine whether the technique used in this study to eliminate the inertial load from the total tup load was successful [13]. (Strictly speaking, the generalized bending load, Pb(t), obtained by the SDOF approximation of the beam (Equation 2), cannot be compared to the support reaction directly, but the error involved is not substantial.) On doubling the support reaction and plotting it along with the corrected bending load as in Fig. 2, it can be seen that the calculated bending load is very close to the true bending load on the beam. Thus the technique used in this study for handling the inertial problem appears to work successfully. However, although the total support reaction is very close to the calculated bending load, there appears to be a lag of about 0.4 msec. between the peaks of the two loads, with the support reaction lagging behind the bending load. This can be attributed to the finite time required for the propagation of the stress waves from the centre to the support. For concrete (E = 25 x 109 Nmm -2 and ,o = 2400 kgm -3) the velocity of the longitudinal stress waves, c, (given by c = X/E/9) is about 3300 msec -L. At this velocity, a stress wave takes about 0.150 msec. to travel from the centre to the support (a distance of 480 ram). Also, the sampling is done at intervals of 0.20 msec. The travel time for the stress waves, the discrete sampling interval, and the possible initial softness of the beam at the supports can, to some extent at least, explain this lag.

6.1 Normal strength concrete compared to high strength concrete at various drop heights Tables 1 and 2 and Figs 3-9 refer to the behaviour of normal strength concrete and high strength concrete under static and impact loading. Variation in the stress rate was achieved by varying the drop height of the hammer. Three drop heights were chosen: 0.15 m, 0.25 m and 0.50 m, corresponding to impact velocities of about 1.70, 2.20, and 3.10 ms -1, respectively. Tables i and 2 contain the maximum, the minimum and the mean values obtained for each set of beams tested under identical conditions. These tables also show the standard deviations. Because of the scatter in the results, which is inherent in impact tests on concrete, only the mean values obtained from half a dozen or more beams were used in plotting Figs 5-9. Figure 3 shows typical observed tup load vs. time plots for the normat strength concrete, for the three different heights. Similar curves are obtained for high strength concretes. Fig. 3 also shows the inertial loads on the beams as obtained from the accelerometer readings and subsequent evaluation using Equation 1. The different starting times of the pulses shown in Fig. 3 are due to the fact that since the triggering of the data acquisition system occurs at a fixed distance above the beam, a hammer travelling at a higher velocity (0.5 m drop) takes less time to reach the beam from the triggering height as compared to a lower velocity hammer (0.-15 m drop). It can be noted from Fig. 3 that a major

38 34

16

30 fSUPPORT

12

LOAD x 2

7 E3

i~l~ /~EVALUATED l/ BENDING LOAD

8

0 J

4 / P

0

z

22

~ o 'J

18

P 4

1 7:--f----. 8

,~---.-O. 15 m DROP (OBS.)

,0.5m DROP(INT.)

14

25m ] OP (INT,) I

fR

I0

/

0

-0.5m DROP(08S.)

26

6 -

i

_

I

| !

I I /O.15m

I 2

TIM E, reset.

Fig. 2. Experimental verification of the technique used in this study to assess the inertial loading: twice the support reaction is approximately equal to the bending load.

2 0

t,.'7",~ 0

8

16

, 24

32

(% 40

48

,

I 56

TI M E , msec. Fig. 3. Typical observed tup load (OBS.) vs. time and the inertial load (INT.) vs. time plots at various drop heights.

296


Table 1 Normal strength concrete Height of hammer drop (m) (Impact) 0.15 (6)*

0.25 (6)

0.50 (7)

Max. value

Min. value

s.t).

Max. value

Min. value

Mean

Max. observed tup load (N)

21309

18803 19776 963

29840

21666

Max. observed inertial load (N)

12957

10512 11306 632

Max. corrected bending load (N)

9440

Fracture energy (N-m) Energy at peak load (N-m)

30.9

3.50

7782

19.1

1.53

Mean

8 4 7 0 604

25.8

2.53

4.3

0.708

Max. value

Min. value

Mean

25386 3 1 2 1

37567

35810

36196

15401

11987 13203 1 3 1 4

20291

16868 19264 1278

14668

9 1 7 8 12183 2 4 0 1

59.6

26.5

3.73

42.0

2.74

3.005

s.o.

12.4

0.429

1 7 7 2 7 16452 16932

100.5

9.07

87.8

2.21

90.1

s.D. 677

428

6.5

6.416 2.51

Avg. disp. at peak load (ram)

0.335

0.423

0.431

Avg. deformation rate (ms -1)

0.168

0.302

0.539

* Number of specimens tested Static* (3) Max. value

Min. value

Mean

s.o.

Peak bending load (N)

6776

6000

6344

306

Fracture energy (N-m)

6.50

2.90

5.5

1.5

Energy at peak load (N-m)

1.098

0.885

1.00

0.079

Avg. disp. at peak load (mm) Deformation rate (ms -t )

0.307 0.417 • 10-6

* Crosshead speed of 4.17 x 10-7 ms -1.

portion of the load recorded by the tup is used up in accelerating the beam from the position of rest, and the actual bending load on the beam is only a fraction of the observed tup load. Fig. 4 shows the calculated bending load vs. deflection plots for the three different drop heights. As described by Bentur et al. [21, the true bending load, as obtained from the tup load after applying the inertial correction, and the deflections as calculated from the accelerometer readings, were used

for plotting Fig. 4. Again, the plots for high strength concrete look similar to those of normal strength concrete. Figures 5-9 present the comparisons between normal strength (NS) concrete and high strength (HS) concrete. Fig. 5 shows the observed tup loads for NS concrete and HS concrete at various heights of h a m m e r drop. The observed tup loads vary almost linearly with the drop height. Moreover, the observed tup loads in


297

Table 2 High strength concrete Height of hammer drop (m) (Impact) 0.15 (6)*

0.25 (6)

Max. value

Min. value


24172

17011 19588 2 7 1 5


12456

8606

9682 1604

11777


11694

8388

9906 1183

1 8 5 7 9 10573 13371 2 9 9 1

1 9 2 0 6 18314 18760


33.5

43.7

100.7


2.92

20.8

1.79

Mean

25.1

2.37

S.D.

5.0

Max. value

0.50 (7) Min. value

Mean

S.D.

Max. value

Min. value

Mean

2 8 7 8 7 22384

24144

2497

39320

35110

36652 1725

9 4 8 0 10773

925

0.54

31.0

2.96

35.0

1.86

2.55

4.7

0.376

S.D.

19025 16760 17892 1132

5.41

57.4

3.80

74.9

4.64

Avg. disp. at peak load (mm)

0.245

0.299

0.427

Avg. deformation rate (ms -I )

0.123

0.249

0.711

446

18.6

0.659

* Number of specimens tested.

Static* (3) Max. value

Min. value

Mean

S.D.


12806

8184

9720

1809


3.4

2.0

2.80

0.547


3.58

1.96

2.50

0.652

Avg. disp. at peak load (mm)

0.500

Deformation rate

0.417 x 10-6

(ms-') * Crosshead speed of 4.17 x 10-7 ms-I.

the case of NS concrete can be as high as 38 000 N ( = 8500 lb). Fig. 6 shows the peak inertial loads for NS and HS concretes at various drop heights. Most of the load picked up by the tup (Fig. 5) is inertial in nature. Upon applying the inertial correction to the loads in Fig. 5, the actual bending loads can be evaluated. Fig. 7 shows the variation of the peak bending load with hammer drop height. This can be considered as the 'impact strength' at various heights of hammer drop. As is clear

from Fig. 7, the apparent impact strength for both the NS and HS concrete goes up with an increase in the hammer drop height, i.e. with an increase in the stressing rate. Also, HS concrete seems to be stronger in impact than NS concrete. On calculating the areas under the load vs. deflection plots of Fig. 4, the fracture energy absorbed by the beam during the impact can be evaluated [2]. Fig. 8 shows the fracture energies required to cause fracture in NS and HS concretes at

298

Banthia, Mindess and Bentur 40

A

B

32 B

Z -=24

12

d

B

ol8 _1

H$

/

~

I

I

I

_ B

"

R',/\

0

I

0

I

0.15

HEIGHT

0.3

I

I

I

I

0 . 4 5 0.6

OF HAMMER:

0.75

DROP, m

Fig. 7. Effect of drop height on the maximum corrected bending load for both normal strength (NS) and high strength (HS) concrete. 0

""~'"

0

....

I0

20

DEFLECTION,

I00

30 mm

Fig. 4. Typical corrected bending load vs. deflection plots at various drop heights.

E

z

>:

-

32

~:

NS

80

-

60

-

40

-

20

-

r

"' Z ILl ILl

40

m

I-U

"~ n-

Z24

/"~"--

d ol8 _1

14.

HS

0

I 0

-

I 0.15

I

I 0.3

I

I 0.45

I

I 0.6

I 0.75

m

HEIGHT

OF HAMMER

DROP,

m

B

Fig. 8. Effect of drop height on the total fracture energy for both normal strength (NS) and high strength (HS) concrete.

m

0

I

1 I

I

I

I

I

I

I

0.3 0 . 4 5 0.6 0.75 HEIGHT OF HAMMER DROP, m

0

0.15

Fig. 5. Effect of drop height on maximum observed tup load

for normal strength (NS) and high strength (HS) concrete. 40 32

z24

-

d

-

ol8 ._1

_

$//~...~

HS

m

0

1

I

I

I

I

I

I

I

1

0.15 0.3 0.45 0.6 0.75 0 HEIGHT OF HAMMER DROP, m

Fig. 6. Effect of drop height on the inertial load for normal strength (NS) and high strength (HS) concrete.

various drop heights; both NS and HS concretes require higher fracture energies at higher stressing rates. Also, HS concrete seems to be more brittle than NS concrete. The fracture energy in impact loading was about 17% lower for the HS concrete, suggesting a somewhat more brittle response. The difference between the two types of concrete was much larger in static loading, amounting to about 50%. However, it should be noted that the machine used for the static testing was not particularly rigid, and as a result the post-peak load curves may be underestimated, giving lower energy values. The effect of deflection rate on the relative maximum load (load bearing capacity in impact relative to the static maximum load) is shown in Fig. 9. The curves for both concretes show a mild increase as the deflection rate is increased from about 0.42 • 10 -6 ms -1 to 0.15 ms - l , and then a sharp increase by about a factor of 2 when the deflection rate increases to about 0.7 ms -1. A similar trend was reported by Reinhardt [14]. Upon examining the fractured surfaces of the broken beams it was seen that the HS concrete had more


299

4

3

/

E

0

I

-8

l -7

I I -6 -5 -4 -3 -2 -1 0 log DEFORMATION RATE ,uo,m/sec. [

1

F

[

aggregate failures than the NS concrete, and that the cracks had propagated in relatively straighter paths through both the paste and the aggregates in the HS concrete (Fig. lOa). In NS concrete, aggregate failure was not as common, and the crack followed a more tortuous path resulting in an uneven fracture surface (Fig. lOb).

6.2 Plain concrete c o m p a r e d to fibre reinforced concrete (frc) Figures 11 and 12 show the effect of adding polypropylene fibres and steel fibres, respectively, to the plain

I I

2

Fig. 9 Effect of deformation rate on the ratio of load bearing capacity in impact relative to the static maximum load.

concrete matrix. The impact strength and the fracture energy were found to have increased due to the addition of the fibres, as shown in Table 3. On average, the i m p r o v e m e n t in fracture energy over plain concrete was about 300% in the case of steel fibre reinforced concrete, while the corresponding improvement in the case of polypropylene fibre reinforced concrete was only about 30%. The smaller i m p r o v e m e n t obtained with polypropylene fibres may be the result of their lower volume content or modulus of elasticity. The fracture surfaces for frc were different from those of plain concrete. The steel frc (Fig. 10c) had a fracture surface that was much rougher and more uneven than that for plain concrete.

Fig. 10. Photographs of the fracture surfaces obtained in the cases of (a) high strength concrete, (b) normal strength concrete and (c) fibre reinforced concrete.

300


Table 3 Fibre reinforced concrete Impact (0.5 m drop) Polypropylene frc (6)*

Steel frc (6)

Max. value

Min. value

Me an

s.D.

Max. value

Min. value

Mean

s.D.


40431

36008

38318

1584

43281

37286

39999

2345


23000

19804

21018

1267

17094

13819

15993

1262


18488

16203

17300

821

26800

22786

24006

1629


130.0

112.0

119.4

248.0

229.0

237.6

7.5

8.1

* Number of specimens tested. Static*

(3)

Polypropylene frc (3)

Steel frc (3)

Max. value

Min. value

Mean

s.o.

Max. value

Min. value

Me an

s.o.


7436

7201

7302

99

12436

10902

11500

670


20.20

9.90

14.00

4.44

46.32

42.00

44.8

1.98

* Crosshead speed of 4.17

x

10-7 m

s- l .

7. D I S C U S S I O N On the basis of the results presented here, it is clear that concrete is a very strain rate sensitive material. Normal strength plain concrete and high strength plain concrete both showed higher impact strengths and also higher fracture energies at higher stressing rates (Figs 7, 8 and 9). Several explanations can be suggested to account for these trends. One explanation may be based on fracture mechanics concepts [15]. The p h e n o m e n o n of strain rate sensitivity can be explained by combining the classical Griffith theory with the concept of sub-critical crack growth. According to the Griffith theory, failure in brittle materials occurs when a flaw exceeds the critical flaw size for a given stress. According to the concept of sub-critical crack growth, under sustained load a subcritical flaw may grow until it reaches the critical flaw size and failure will then occur. Thus if the load is applied very slowly, the sub-critical flaws have time to grow and thus the failure occurs at a lower value of load. However, if the load is applied at a very high rate, there is little or no time available for the growth of the sub-critical flaws, and a higher load can be reached by the structural element before failure occurs. John and Shah [16] reported that pre-peak crack growth is reduced at high rates of loading.

An alternative explanation of the observed trends may be given on the basis of non-linear fracture mechanics. It has been recognized that immediately ahead of a moving crack is a zone of microcracking called the process zone. Reinhardt [17] has suggested that the size of this zone of microcracking depends upon the velocity of the crack; a faster crack has a larger zone of microcracking ahead of it. At a higher stress rate the crack propagates faster, and therefore the process zone will be bigger. This increased microcracking m a y explain the higher fracture energy requirements at higher stress rates. This argument, may, at first glance, seem to contradict with the argument presented above on the basis of sub-critical crack growth, which predicts less microcracking in high stress rate loading situations. H o w e v e r , these two phenomena occur on the opposite sides of the peak load. The concept of sub-critical crack growth is applicable prior to the peak load; the concept of a larger process zone applies in the post-peak load region, where the unstable crack propagation commences. It can be noted from Tables 1 and 2 that the energy consumed by the beam up to the peak load is only a small fraction of the total fracture energy. This suggests that almost all of the energy is expended in advancing the crack in the post-peak load region.


301

For both NS and HS concretes, there were increases in the impact strength and the fracture energy with an increase in the stress rate. However, over the entire range of stress rate, the HS concrete showed higher strengths but lower fracture energies. The more brittle response of HS concrete has been reported by several investigators, and may be attributed to the better bonding achieved in these systems. This is reflected in the mode of failure of the high strength concrete, where the crack propagates through the aggregate particles and not around them (Fig. 10). Concrete is inherently brittle. That is why it has always been used in conjunction with ductile materials, such as steel. The inherent brittleness of concrete can, to some extent, be overcome by adding fibres. The major advantage of adding fibres to the matrix is not so much in the increased impact strength, but in the increased ductility and the increased impact resistance (Figs 11 and 12). When a plain concrete beam is impacted, the matrix cracks and the beam fails suddenly. However, in the case of fibre reinforced concrete, even after the matrix cracks, the beam can still carry some load owing to the fibres that bridge the crack. It should be noted that fibres can fail either by pullout or by fracture. In the case of the polypropylene fibres, the mode of failure was predominantly fracture of the fibres, in static as well as in impact loading. With the steel fibres, pullout of the fibres was the dominant failure mode in static loading, while fibre fracture became more common in impact loading. The change in failure mode may be associated with the fact that under static loading the 18 16

'

14 Z

20 z

16

r,

12

+ STEEL • # PLAIN / fFIBRES

0 "

8

4 \yPLAN 0 0

~l" f.... r r i 8 24 40 56 DEFLECTION,

mm

Fig. 12. Corrected bending load vs. deflection plot for plain

concrete compared to steel fibre reinforced concrete.

fracture energy in the steel fibre reinforced beams was greater by a factor of about 8 than that of the unreinforced beam, whereas under impact loading the difference between the two decreased to a factor of about 3. This value, which is small compared to results reported by others [18], may be the result of the influence of the testing conditions. For instance, in the present work, the ratio of the hammer weight to the beam weight was about 8, which is higher than that commonly used. One important observation in the case of the steel fibre reinforced concrete is the multiple peaks in the load vs. deflection plot (Fig. 12). However, no multiple peaks were observed in the case of polypropylene fibres (Fig. 11). The first peak in Figure 12 corresponds to the matrix failure. The second peak refers to the maximum load the fibres can carry before the pullout begins. The subsequent peaks correspond to the crack arrest phenomena.

12 I

I0

+PP.

~PLAIN

\ f

8. CONCLUSIONS

FIBRES

o .J

24

8 6 4 2

-

0

8

16

24

32

DEFLECTION,

mm

40

Fig. 11. Corrected bending load vs. deflection plot for plain

concrete compared to polypropylene fibre reinforced concrete.

1. Accelerometers can be used successfully to predict the inertial loading effects in instrumented impact tests of concrete beams. 2. Concrete, either normal strength or high strength, is strain rate sensitive. Prediction of its behaviour under impact, on the basis of static testing, is not possible. 3. High strength concrete has a higher impact strength than does normal strength concrete. But, high strength concrete seems to be more brittle than normal strength concrete. This suggests that care must be taken in the use of high strength concrete in dynamic situations. 4. The higher fracture energy obtained in the case of concrete at a higher stress rate is probably due to the increased internal microcracking. 5. Fibre reinforced concrete is a better material than

302 plain concrete in dynamic situations because of its ductility and increased impact resistance. Steel fibres seem to behave much better than do polypropylene fibres.

ACKNOWLEDGEMENTS The authors wish to thank Mr Glenn D. Jolly and Mr R o d B. N u s s b a u m e r for designing and constructing the data acquisition system. This research was supported in part by a grant from the Natural Science and Engineering Research Council of Canada, and in part by E l k e m Chemicals Inc., Pittsburgh, Pennsylvania.


7.

8.

9.

10.

11.

REFERENCES 1. Struck, W. and Voggenreiter, W., 'Examples of impact and impulsive loading in the field of civil engineering', Mat. Struct. 8 (1975) 81-87. 2. Bentur, A., Mindess, S. and Banthia, N., 'The behaviour of concrete under impact loading: experimental procedures and method of analysis', Mat. Struct. 19 (1986) 371-378. 3. Mindess, S., Banthia, N. and Bentur, A., 'The impact behaviour of concrete in bending', Proc. International Conference on Fracture Mechanics of Concrete, Lausanne, 1985 (in press). 4. Mindess, S., Banthia, N., Ritter, A. and Skalny, J. P., 'Crack development in cementitious materials under impact loading in cement based composites', in "Strain Rate Effects on Fracture" (Eds S. Mindess and S. P. Shah), Materials Research Society Symposium Proceedings, Vol. 64 (Materials Research Society, Pittsburgh, 1986). 5. Bentur, A., Mindess, S. and Banthia, N., 'The fracture of reinforced concrete under impact loading', in "Strain Rate Effects on Fracture" (Eds S. Mindess and S. P. Shah), Material Research Society Symposium Proceedings, Vol. 64 (Materials Research Society, Pittsburgh, 1986). 6. Mindess, S., Banthia, N. and Bentur, A., 'The response o f reinforced concrete beams with a fibrous concrete

12.

13.

14.

15.

16. 17.

18.

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