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Strain-rate effects in fibre-reinforced concrete subjected to impact and impulsive loading
Strain-rate effects in fibre-reinforced concrete subjected to impact and impulsive loading WIMAL SUARIS and SURENDRA P. SHAH
One of the most important properties of fibre-reinforced concrete is its considerably improved impact resistance compared to that of the unreinforced matrix. Although tests for determining the impact resistance can be tailored to suit practical situations, a more basic knowledge of the mechanical properties of the composite at the high strain-rates caused by impact would pave the way for more rational analysis and design of fibre-reinforced concrete structures subjected to impact. In this paper the tests conducted by various investigators to assess the impact resistance of fibre-reinforced concrete are reviewed, and some recent work conducted by the authors to obtain more basic material properties is presented. Key words: composite materials; fibre-reinforced concrete; impact testing; impact resistance; bonding The relatively low tensile strength and the fracture energy of concrete has led to the development and use of fibrereinforced cementitious composites where tensile and flexural stresses are to be sustained and where it is desirable to control cracking. A significant improvement in impact resistance of fibre-reinforced concrete over that of plain concrete has also been observed by many investigators. 1 For example, the inclusion of fibres (steel or polypropylene) has been observed to significantly reduce the spall velocity and the size of fragments in slabs subjected to explosive loading.Z, 3 Significant improvement in the cracking behaviour of fibre-reinforced concrete over that of plain concrete has also been demonstrated by subjecting stair treads made of both to impact loading. 4 This enhanced impact resistance of fibre-reinforced concrete has led to its use where impact loads are to be encountered. For instance, steel fibre-reinforced concrete is used in airfield pavements, industrial flooring, and hydraulic structures, and polypropylene fibres have also been used in the construction of pile shells, s As there is no acceptable standard method for determining the impact resistance of fibre-reinforced concrete, a variety of tests, including explosives and drop weight tests, have been carried out on a range of specimen sizes and shapes. 6-14 All these investigators have reported that the measured impact resistance of concrete increases with the inclusion of fibres. Although these tests are useful in determining the relative merits of different fibre/cement composites they do not yield basic material properties which can be used for design. The nature of these tests also does not render it possible to compare the impact behaviour of the composites with their static behaviour and no inference of strain-rate can be made. The usefulness of these tests lies in the possibility of tailoring test methods to situations in practice, is
For instance, energy absorption up to the first crack in a falling ball test can be used to evaluate the impact resistance of fibre concrete where relatively low impact values are to be experienced, such as road building, air hammering, e t c . Explosive tests, where the volume of material removed and longestcrack e t c are measured, can be used for situations where explosive loadings are likely to occur. Results from tests which involve firing projectiles at test panels, where the volume of material removed in the contact zone and scabbing is measured, can be used where fibre-reinforced concrete is intended to be used as a safety wall against flying objects such as ballistic or tornado generated missiles. However, more rational design of fibre-reinforced composites subjected to impact and impulsive loadings would be facilitated by the knowledge of the mechanical properties of the material at the high strain-rates caused by impact. As a more ambitious approach, attempts can also be made to predict the impact behaviour of the composites from the knowledge of the mechanical properties of the concrete matrix, fibres and the fibre/concrete interface over a wide range of strain rates. The mechanical properties of concrete subjected to different strain rates have been discussed by the authors in a recent paper. 16 The results of some recent impact and high strain-rate tests on fibrereinforced concrete will be discussed in this article. Some results on the properties of the fibre/matrix interface at different strain rates will also be presented.
A REVIEW OF EXPERIMENTAL RESULTS ON FIBRE-REINFORCED CONCRETE Explosive tests Exposive tests on fibre-reinforced concrete slabs have been carried out by Williamson. ~ He observed that the result of shock loading applied to plain concrete by explosives, was
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to completely disintegrate the slab specimens. A considerable reduction in spall velocity of the fragments was obtained by him when the matrix was reinforced with 1.75% nylon fibres. The tests conducted by Robins & Calderwod 3 also show that the inclusion of steel and polypropylene fibres significantly reduces the size and particle velocity of fragments caused by subjecting slabs to explosive loading. Drop weight tests
Nanda & Hannant 6 using a 'number of blows to no rebound' test, found that plain concrete failed after 5 blows while concrete reinforced with 5% steel fibres withstood up to 100 blows. Dixon & Mayfield 7 also recorded an increase in the number of blows to no rebound when concrete was reinforced with 1% by volume of steel fibres. Jamrozy & Swamy ~3 have published results of tests conducted to study the behaviour of fibre-reinforced concrete cubes (200 mm) subjected to repeated drop-weight impact loading applied by a 50 kg hammer falling through 300 mm. Three types of steel fibre were used: straight round fibres (0.25 x 15 mm and 0.25 x 25 mm), crimped fibres (0.25 x 25 ram) and hooked fibres (0.4 x 40 ram). A measure of the impact resistance was obtained by counting the number of blows up to the first crack. For straight, steel (0.25 x 25 mm) fibre-reinforced concrete, with 1% by volume of fibres, the first crack was found to appear after about 150 blows. Increasing fibre aspect ratio and the volume fraction was found to increase the number of blows up to first crack. They also found that the crimped and hooked fibres performed better than the smooth fibres under impact loading. The American Concrete Institute's committee on ~ c has also suggested the use of a drop hammer test to evaluate the impact resistance of concrete. 17 The tests are to be conducted by dropping a 4.5 kg hammer repeatedly from a height of ~ 460 mm onto a hardened steel ball resting on a cylindrical specimen of 152.4 mm diameter and 63.5 mm height. The number of blows required for the first visible crack to appear was considered to be an indication of the impact resistance of the material. Using this procedure, Ramakrislman e t al is recorded about 100 to 150 blows to first crack for concrete reinforced with hooked-end fibres. Bailey e t al 4 have reported results of tests conducted on fibre.reinforced concrete stair treads to assess their impact behaviour. The loading system consisted of a loosely pivoted loaded lever which was pin released from a preftxed height onto a steel plate resting on a stair tread (1180 x 360 x 65 mm). Each tread was subjected to lever blows with gradually increasing drop height. Three types of fibres were used for the tests: fibrillated polypropylene fibres (50 mm long), crimped steel fibres (50 mm long) and indented steel fibres (63 mm long). A 1% fibre volume fraction was used for all mixes. It was found that first cracking occurred at approximately the same height of fall irrespective of whether the tread contained fibres. However, the inclusion of fibres was found to reduce the severity of subsequent cracking behaviour, ie, minimizing crack widths. With the three types of fibre used, the effectiveness in controlling cracking was found to be greatest for sinusoidally crimped steel fibres, lowest for polypropylene fibres, with indented steel fibres showing an intermediate behaviour.
tests conducted on steel and nylon fibre-reinforced concrete specimens. The nylon fibres used were 0.25 mm in diameter and the steel fibres 0.43 mm. The length of the fibres used was 25.4 mm and the fibre volume fraction used was 1%. Tests were conducted by striking the cylindrical specimens at one end with high velocity projectiles. The compression wave generated at the striking end was reflected as a tensile wave when it reached the far end of the cylindrical specimen, causing the specimen to spaU. Measuring the fly-off velocity of an impedance matched pellet placed at the far end, enabled the particle velocity to be determined. From this the stresses and strains induced in the specimen were calculated. For strain rates of about 30 s-1 , they recorded a 4 to 5 fold increase in fracture strains/stresses over the corresponding static values. Bhargava & Rehnstrom 2° used the 'split Hopkinson bar test' to study the dynamic behaviour of fibre-reinforced concrete. The specimens used in the tests were reinforced with 0.2% by volume of polypropylene fibres. Specimens were sandwiched between two very long (about 5 meters each) aluminium bars and the bars were used to measure the incident and tra~'.smitted pulses. The amplitude of the incident pulse was increased by increasing the impact velocity. The impact strength of the specimen was assumed to have been reached when the transmitted pulse showed no increase with increasing amplitude of the incident pulse. For observed pulse rise-times of about 50/as, the dynamic strength was found to be about 50% greater than the static tensile strength. Some results showing the increase in tensile strength with strain rate of plain concrete are shown in Fig. 1. From this it can be seen that the results cited above for fibre-reinforced concrete show a similar relative increase in tensile strength with increasing strain rate as for unreinforced concrete. This result is in accordance with the results of static tensile and flexural tests on fibre-reinforced concrete where it has been observed that the first cracking strain is not too different from that of the unteinforced matrix. 21 However, as the important contribution of the fibres is evident only after first cracking, 22 the entire stress/strain relationship should be obtained to fully quantify the improvement in the energy absorption capacity of fibre-reinforced concrete under impact loading. Charpy impact tests
For many materials, the most common method of obtaining
4-
2
I-
10 .7
Tensile tests
Mellinger & Birkimer 19 have reported results of tensile
154
J/Flexur~
2-
I 10 .5
I 10 -5
I 10 .4
I t 0 .3
I 10 ~2
I 10 ~
I 10 0
t 10 ~
( s e c -1) Fig. 1 Relative increase in s t r e n g t h vs s t r a i n r a t e for plain concrete in t e n s i o n , flexure and compression
C O M P O S I T E S . A P R I L 1982
impact resistance is by Charpy impact testing. In this test, a three point bend specimen is struck at the centre by a pendulum. By knowing the height of the pendulum before and after impact, and assuming that all the energy lost by the striker is absorbed in breaking the beam, one can calculate the energy absorbed due to impact loading. Using this test method, Johnston 8 observed that the impact resistance of steel fibre-reinforced mortar is 10 times greater than that of unreinforced mortar. Shah & Baehr 14 also observed a tenfold increase in Charpy impact resistance when concrete specimens were reinforced with glass fibres. Krenchel9 observed an increase in impact energy with increasing fibre aspect ratio and Edgington u found that the Charpy impact energy increased linearly with increasing fibre volume fraction. Instrumented impact tests
In the above mentioned Charpy impact tests, stresses induced in the specimen during impact are not monitored. Hibbert 23 has attempted to obtain more basic material properties during a Charpy impact test by instrumenting the striker (tup) which yielded the load/time history of the impacted specimen. A specially constructed pendulum impact machine was designed to test 100 x 100 x 500 mm beam specimens in the Charpy configuration. He observed that for all specimens (unreinforced as well as FRC) the peak load under impact loading (impact velocity ~ 2.85 ms -1 ) was about 10 times that under static loading (applied at a cross-head movement rate of 0.05 mm s-1 ). This increase is quite high when compared with only a two fold increase generally reported by other investigators for similar strain rates (Fig. 1). This high load recorded is not representative of true material properties but is a consequence of specimen inertia effects. The problem of inertial effects in the observed load/time response is particularly severe for concrete materials because of its inherent brittleness and low strength/weight ratio and may result in the material response being completely overshadowed by the inertial response of the specimen, as in this case. The pendulum energy loss during impact can be calculated by integrating the area under the load/time trace obtained from the instrumented tup. ~ Using this method he obtained a value of 57 Nm for the plain concrete beams tested. Restraining the broken specimen halves against helical springs, he also calculated the kinetic energy of the broken beam (~ 26 Nm). He then estimated the energy in failing the concrete matrix as the energy loss by the pendulum minus the kinetic energy of the beam. This procedure resulted in a value of 33 Nm for the plain concrete matrix, which was unreasonably high when compared with the energy absorption under static loading of only 2.8 Nm obtained by him. For the fibre-reinforced concrete beams he then calculated the energy absorption solely due to fibre debonding, pull-out and fracture by subtracting the kinetic energy of the specimen and the energy absorbed in failing the concrete matrix from the pendulum energy loss. But as the energy absorbed by failing the matrix under impact loading was overestimated this procedure resulted in lower than actual values for the energy absorption in fibre debonding, pull-out and fracture. Therefore, one can expect higher values in fibre debonding, pull-out and fracture at impact loading rates than at static loading rates, although similar values were reported by Hibbert. Hibbert's results with only the specimen kinetic energy being subtracted from the pendulum energy loss are presented in Table 1. Here, it can
COMPOSITES. APRIL 1982
be seen that fibre-reinforced concrete beams, on average, absorb about 75% more bending energy when tested under impact loading conditions than under static loading conditions. Radomski 2s has also attempted to obtain the load/time history of the specimen during impact. He used a rotating impact machine to study the dynamic behaviour of fibrereinforced concrete specimens (15 x 15 x 105 mm). The impact load was imposed on the specimen by means of a striker hitting the centre of the specimen. The striker was released from the flywheel of the machine when the required impact velocity was reached. The load/time signal was obtained by the use of piezo-electric gauges. However, the author did not report the load history; only the energy values calculated from the load/time trace were reported. But, he noticed that the energy measurement obtained from the rotating impact machine was, substantially different from that obtained by a Charpy impact machine using identical size specimens. This is probably due to the different material and geometrical properties of the striker which determines the energy absorption at the contact zone. This emphasizes the need. for the measurement of material properties which are, as far as possible, test system independent. The authors' attempt to obtain fundamental material properties is briefly described in a later section. Flexural tests at variable strain rates
The effect of the rate of load application on the flexural strength of fibre-reinforced concrete has been studied by Butler & Keating, 26 They used a hydraulic ram capable of moving at different speeds for load application and their specimens, (200 x 200 × 1500 mm in size), were tested in four-point bending. The fibres used in the tests were 0.5 mm in diameter and 50 mm in length, high-carbon duoform steel fibres. A fibre volume fraction of 1.2% was used and the MOR at the lowest stress rate was 7.4 MN m -2. They observed a 35% increase in flexural strength when the rate of stress application was increased from 0.017 to 170 MN m -2 s-~ . This increase is low when compared with a 75% increase observed for plain concrete when the stress rate was increased over the same range. Some results of the rate of load application on the load/ deflection behaviour of polyethylene fibre-reinforced concrete have also been published by Kobayashi & Cho. 2s An Instron testing machine with variable cross-head velocities was used by them to test 100 x 100 x 400 mm specimens in three-point bending. The results obtained for specimens reinforced with a 4% fibre volume fraction showed a 25% increase in the cracking load when the crosshead velocity was increased from 1 mm/min to 200 mm/min. Another effect of increasing the loading rate on the load/ deflection behaviour was also noticed; ie the increase of the post cracking slope of the load/deflection curve, with increasing loading rate. This increase in post cracking stiffness was attributed to the viscoelastic properties of the polyethylene fibres. SUMMA R Y OF A U THORS" RECEN T RESEA R CH
A high increase in peak load under impact as observed by Hibbert 22 was also observed by the authors in their preliminary impact tests on fibre-reinforced concrete beams, conducted by using a drop hammer impact machine, with an instrumented tup. Many investigators have recognized that during an initial period of time, the load measured by the
155
Table 1.
Energy absorption of fibre reinforced concrete beams under impact loading 23 Bending energy absorption up to 10 mm deflection (Nm)
Fibre type
Fibre size (mm) diameter x length
Vf(%)
Age at test Static
-
-
2 mths 2½ yrs
Dynamic (% increase)
2.8
33 (1080)
2.1
30 (1330)
Steel, hooked ends Steel, hooked ends
0.5 x 50 0.5 x 50
1.2 0.6
2 mths 2 mths
85 57
145 (71) 94 (65)
Steel, crimped high C high C high C
0.5 x 50 0.5 x 50 0.5 x 50
1.2 1.2 0.6
2 mths 2½ yrs 2 mths
66 95 53
139 (111) 143 (51) 97 (83)
Steel, crimped low C
0 . 5 x 50
1.2
2½ yrs
80
132 (65)
Steel duoform Steel duoform Steel duoform Stainless steel
0.38 x 38 0.38 x 38 0.64 x 60
1.2 1.2 1.2
2 mths 2½ yrs 2 mths
30 24 -
48 (60) 56 (133) 148
melt extract
0.7 x 25
1.2
2 mths
47
84 (79)
Polypropylene
1200 demer x 75 mm long 1200 demer x 75 mm 1200 demer x 75 mm 1200 demer x 35 mm 1200 demer x 35mm 1200 demer x 35 mm
1.2
2 mths
43
77 (79)
1.2
2½ yrs
42
69 (64)
0.6
2 mths
1.2
2 mths
30
45 (50)
1.2
2½ yrs
28
44 (57)
0.6
2 mths
Polypropylene Polypropylene Polypropylene Polypropylene Polypropylene
tup and that resisted by the beam are not the same, due to specimen inertia effects. These inertial effects are manifest as oscillations in the tup-load versus time signal. Server, Wullaert, and Sheckhard 2s have recommended that, to obtain reliable measurements from the instrumented impact test, the time to fracture of the specimen should be greater than three times the period of these inertial oscillations. For metallic and polymeric materials, the time to fracture is generally larger than the period of these inertial oscillations and as a result it is possible to satisfy the guidelines suggested by Server et al. 2s However, for brittle materials such as concrete, the time to fracture is quite low even for small impact velocities and it may not be possible to avoid fracture even during the first oscillation, as observed both by the authors and Hibbert. 2a In subsequent impact tests conducted by the authors, it was found that it is possible to reduce the amplitude of the inertial oscillations substantially.by introducing a rubber pad between the tup and specimen and thereby get reliable data. 29 A two degree of freedom analytical model was also developed to better understand the nature of inertial effects and to predict such effects. The equations of motion of the modelled system given by:
[m°
156
61
50
where xl and x2 are the displacement of the hammer and the specimen from their equilibrium configuration; mh is the weight of the harmner; ke the stiffness of the contact zone; m s the weight of the specimen and ks the stiffness of the beam specimen. Solving these equations with proper initial conditions will yield the values of the tup load pw(t) and the beam load pB(t) histories from the relations: pT(t) = ke [xl(t) - x2(t)] + mhg pB(t) = ks[x=(t)] + mhg These two values are plotted for a particular set of parameters in Fig. 2. Fig. 2a is for a test conducted without any padding material while Fig. 2b is for a test conducted with a rubber pad. It can be seen that the introduction of the rubber pad should substantially reduce the difference between the measured tup load and the actual beam load. These predictions were confirmed by test results on asbestos cement specimens. 29 In addition to selecting the specimen dimensions according to the analytical model, it was also found necessary to measure the anvil loads and measure the strains on the bottom fibres of the beam to assure that the beam fails in a flexural mode. For instance, load/time and strain/time
COMPOSITES. APRI L 1982
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15
a
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5
v
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0.4 T i m e (_m s )
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®
/ /
Tup - load ~ _
/
/J_
× -20( c
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IQ
®
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iT
T"~ 2 5 m m ~
-300
I"
Bending load
/
//
- 400
I 2.0
1.0
/ 4
/
/ the kinetic energy of the beam.
/
/
k¢ 0
/
Using these experimental techniques, tests are now being conducted by the authors on concrete beam specimens reinforced with smooth and hooked-end steel fibres, glass fibres and polypropylene fibres. Some results from this continuing investigation are shown in Figs 4 and 5. In Fig. 4 the relative values of modulus of rupture at different strain rates are shown for both concrete and smooth steel fibre-
/
S p e c i m e n size = 2 5 x 5 0 * 2 5 0 m m Impact velocity = 1 m/s p = 2000 kg/m3 E = 1 7 . 2 3 × 103 MP(]
/•'/// I 0.5
Time
5.0
I 4.0
Fig. 3 Typical curves for FRC beam specimen: a -- load vs time; b -- strain vs time
/
"o o o J
I 30 T~me ( m s)
I 1.0 (m s )
t 1.5
o S t e e l f , b r e - r e , nforced m o r t a r
1.75
MOR ( s t a t , c ) = 9 4 4 k N m "2
z~ Plain m o r t a r , Beam
Fig. 2 Analytical tup-load and beam-bending load vs time. Curves for tests conducted: a) w i t h o u t a rubber pad (k e = 8 7 . 5 M N m -1 } and; b) with a rubber pad (k e = 5 . 8 4 M N m -1 )
3-
Mob(static)=731 kNm2
d i m e n s i o n : 3 8 1~ 7 6 2 ,
point
381 m m
loading
20
o o
curves obtained for a ~]~c specimen are shown in Fig. 3. It can be seen that, up to matrix cracking, the strains calculated, assuming elastic bending behaviour, agree closely with the measured values, assuring that the beam failed in flexure. The energy dissipated by the falling hammer can be calculated by integrating the area under the tup-load versus time trace ~ ; estimates of the bending energy absorbed by the beam specimen could then be made by subtracting the elastic energy of the contact zone and the kinetic energy of the beam from it. However, in the tests conducted by the authors, the direct measurement of the load-point deflection using AC-LVDT was found to be a more reliable way of obtaining the energy absorption, as it eliminated the need for estimating the energy absorbed in the contact zone and
COMPOSITES. APRIL 1982
v
~
Fibre - reinforced
1.6
c
~ 14
/
/
106
I 105
Mortor
1.2
10
~ 10 .7
I 10-4
I 10 3
I 10-2
[ 10 ~
2--_. 10 °
101
(sec') Fig. 4 Relative increase in flexural strength vs strain rate for smooth steel fibre-reinforcad concrete and plain concrete
157
~ 626 6 0 2 2 ~
mortar (12: 0 5~1%)
"
Steel - re~nforced From fibre Suaris ond Shah I = 2 5 4 r a m . d = 0 2 6 r a m brass tooted Mo~ (static)= kN rn 2 O c ~ o (statt¢) = 6Z 5 6 kN m 2
z 4440
254508762
640
~z _~~ 4o5
3.
The energy absorbed by steel fibre-reinforced concrete under impact appears to be about 20-100 times that absorbed by plain concrete. The authors' results as well as those of Hibbert, indicate that the bending energy absorption of straight-fibre-reinforced concrete beam specimens increases significantly when the strain rate is increased. (Fig. 5, Table l)
4.
The bond between smooth fibres and the concrete matrix does not appear to be significantly influenced by the strain rate. However, the bond strength between deformed fibres and the concrete matrix may be expected to increase with strain rate resulting in a higher percentage increase in energy absorption by deformed fibres than by smooth fibres.
944
016127
~0
/
~
o
io
o
:~ ~
270 C
~/joO~ f~r~-r~1o,ced ,
010"7
.....
106
O' -L --L- -'-- .-'.- .-' ,
10-5
10 4
10-3
,
102
101
10o
101
(sec'D
Fig. 5 Variation with strain rate of the bending energy absorption of a smooth fibre-reinforced beam
A CKNOWL EDGEMEN TS reinforced concrete. Fig. 5 shows the energy absorption of smooth steel fibre-reinforced concrete beams at different strain rates. The load/deflection curves at the two extreme strain rates are also presented. BOND PROPERTIES Tests to determine the bond strength at different strain rates have been conducted by Gokoz & NaamanJ ° A hollow cored cylinder with a solid bottom and with 20 fibres embedded along its periphery were used in their tests. The impact load was applied by an instrumented tup hitting the bottom of the specimen and causing the fibres to pull out of the matrix. For the steel, polypropylene and glass fibres they tested, no significant effect of strain rate on the bond strength was observed. Although no pull-out tests have been reported for deformed fibres, tests have been conducted by Vos & Reinhardt 31 to study the bond strength between the reinforcement and concrete under impact loading. Their tests were conducted by using the 'split Hopkinson bar technique'; the results indicated that the bond stresses of strands and plain bars were not influenced by the loading rate while the bond stress of the deformed bars was significantly influenced by it. This rate sensitivity of deformed bars is believed to occur as a result of the bearing of the ribs on concrete producing splitting of the concrete, which is rate dependent. Such behaviour can be expected of crimped fibres and indented fibres. If true, this means that deformed fibre-reinforced concrete would be more efficient than smooth fibrereinforced concrete in absorbing energy under impact loading. In I-libbert's results, presented in Table 1, it is noticeable that crimped steel fibres showed higher percentage increases in energy absorption under impact loading than the other fibre types tested. CONCLUSIONS
1.
2.
158
It has been well established that fibre-reinforced concrete exhibits considerably improved impact resistance over plain concrete. Although impact tests such as drop hammer tests and explosive tests are useful in predicting its impact resistance, the knowledge of basic material parameters would facilitate more rational design of fibre-reinforced concrete structures subjected to impact. The flexural strength of smooth steel fibre-reinforced mortar appears to be slightly more strain rate sensitive than plain mortar. This is likely to be due to the multiple cracking of the matrix. (Fig. 4)
This paper was prepared while the authors were working on a grant from the US Army Research Office, Metallurgy and Materials Division. REFERENCES 1
2 3 4 5 6 7 8 9 10
11 12 13 14 15
16
Hoff, G.C. 'Selected bibliography on fibre-reinforced cement and conciete, Supplement No 2' Miscellaneous Paper C-75-6 (US Army WaterwaysExperiment Station, Vicksburg, MS, USA, 1979) WiUiamson,G.R. 'Response of fibrous reinforced concrete to explosive loading' Technical Report 2-48 (US ArmyCorps of Engineers, Ohio, USA - River Division Laboratories, Jan 1966'. Robins,P.J. and Caldet~vod, R.W. 'Explosivetesting of fibrereinforced concrete' Concrete (January 1978) pp 76-78 Bailey, J.H., Bentley, S., Mayfield, B. and PeU, P.S. 'Impact testing of fibre reinforced concrete stair treads'Mag of Concrete Res 27 No 92 (September 1975) pp 167-170 Hannant,D.J. Fibre Cements and Fibre Concretes (A WileyInterscience Publication, 1978) Nanda,V.K. and l-lannant, DJ. 'Fibre reinforced concrete' Concrete Bldg and Concrete Prods XLIV No 10 (October 1969) pp 179-181 Dixon,J. and Mayfield, B. 'Concrete reinforced with fibrous wire' Concrete (March 1971) pp 73-76 Johnston, C.D. 'Steel fibre reinforced concrete: a review of mechanical properties, fibre reinforced concrete' A CI Publication SP-44 (1974) pp 127-142 Krenchel,H. 'Fibre reinforced brittle matrix materials, fibre reinforced concrete' A CI Publication SP--44 (1974) pp 45-77 All, M.A., Majumdar, AJ. and Singh, B. 'Properties of glassfibre cement, the effect of fibre length and content' Building Research Establishment, Current Paper CP 94/75 (October 1975) Edgington,J. 'Steel fibre reinforced concrete' Ph D Thesis (University of Surrey, 1973) Raouf,Z.A., AI Hassani, S.T.S. and Simpson, J.W. 'Explosive testing of fibre reinforced cement composites' Concrete (April 1976) pp 28-30 Jamrozy, Z. and Swamy, R.N. 'Use of steel fibre reinforcement for impact resistance and machinery foundations' Int J Cement Composites 1 No 2 (July 1979) pp 65-76 Shah,S.P. and Baehr, D. 'Properties of glass fibre reinforced gypsum sheets' J Structural Div ASCE (January 1977) pp 23-33 Verhagen, A.IL 'Impact testing of fibre reinforced concrete: reflection on possible test methods' RILEM Symp on Testing and Test Methods of Fibre Cement Composites, edited by R.N. Swamy(RILEM Symposium, 1978) Suaris,W. and Shah, S.P. 'Mechanicalproperties of materials under impact and impulsive loading' Introductory Report for the Interassociation Symposium on Impact Loading of Concrete Structures (West Berlin, June 1982)
17 18
ACI Committee 544 Report 'Measurement of properties of fibre reinforced concrete' A C I J (July 1978) pp 283-289 Ramakrishnan,V., Brandshaug, I., Coyle, W.V. and Sehrader, E.K. 'A comparative evaluation of concrete reinforced with straight steel fibres with deformed ends glued together into bundles'ACIJProc 77 No 3 (May-June 1980) pp 135-143
COMPOSITES. APR I L 1982
19
20 21 22 23
24
25 26
Mellinger,F.M. and Birkimer, D.L. 'Measurement of stress and strain on cylindrical test specimens of rock and concrete under impact loading' Tech Report No 4-46 (Ohio River Division Laboratories, Corps of Engineers, April 1966) Bhargava,J. and Rehnstrom, A. 'Dynamic strength of polymer modified and fibre-reinforced concretes' Cement and Concrete Res 7 (1977) pp 199-208 Shah, S.P. and Rangan, B.V. 'Fibre reinforced concrete properties' A C I J 68 No 2 (February 1971) pp 126-135 Shah, S.P. and Naaman, A.E. 'Mechanical properties of glass and steel fibre reinforced mortar' A CI J (January 1976) pp 50-53 I-Iibbert,A.P. 'Impact resistance of fibre concrete' Report From the Construction Materials Research Group, Department of Civil Engineering, University of Surrey (University of Surrey, Guildford, UK, July 1979) Ireland, D.R. 'Procedures and problems associated with reliable control of the instrumented impact test' Instrumented Impact Testing, A S T M STP 563 (American Society for Testing and Materials, 1974) pp 3-29 Radomski, W. 'Application of the rotating impact machine for testing fibre-reinforced concrete' In t J o f Cement Composites and Lightweight Concrete 3 No 1 (February 1981) Butler, J.E. and Keating, J. 'Preliminary data derived using a flexural cyclic loading machine to test plain and fibrous concrete' Mater and Structures 14 No 79 (June 1981) pp 25-33
C O M P O S I T E S . A P R I L 1982
27 28
29 30
31
Kobayashi, K. and Cho, R. 'Flexural behaviour of polyethylene fibre reinforced concrete' In t J o f Cement Composites and Lightweight Concrete 3 No 1 (February 1981) pp 19-25 Server,W.L., Wullaert, R.A. and Sheekhard, LW. 'Evaluation of current procedures for dynamic fracture-toughness testing' Flaw Growth and Fracture. A S T M STP 631 (American Society for Testing and Materials, 1977) pp 446-461 Suaris, W. and Shah, S.P. 'Inertial effects in the instrumented impact testing of cementitious composites' A S T M J Cemen t, Concrete and Aggregates (March 1982) Gokoz, U. and Naaman, A.E. 'Effect of strain rate on the pullout behaviour of fibres in mortar' Progress Report Submitted to the US Army Research Office, Grant No DAA G 29- 79-C0162 (January 1981) Vos. E. and Reinhardt, H.W. 'Bond resistance of deformed bars, plain bars and strands under impact-loading' Report No 5-80-6 (Department of Civil Engineering, Delft University of Technology, The Netherlands, September 1980)
AUTHORS The authors are with the Northwestern University's Department of Civil Engineering. Inquiries should be addressed to: Professor Surendra P. Shah, Department of Civil Engineering, Northwestern University, Evanston, Illinois 60201, USA.
159
One of the most important properties of fibre-reinforced concrete is its considerably improved impact resistance compared to that of the unreinforced matrix. Although tests for determining the impact resistance can be tailored to suit practical situations, a more basic knowledge of the mechanical properties of the composite at the high strain-rates caused by impact would pave the way for more rational analysis and design of fibre-reinforced concrete structures subjected to impact. In this paper the tests conducted by various investigators to assess the impact resistance of fibre-reinforced concrete are reviewed, and some recent work conducted by the authors to obtain more basic material properties is presented. Key words: composite materials; fibre-reinforced concrete; impact testing; impact resistance; bonding The relatively low tensile strength and the fracture energy of concrete has led to the development and use of fibrereinforced cementitious composites where tensile and flexural stresses are to be sustained and where it is desirable to control cracking. A significant improvement in impact resistance of fibre-reinforced concrete over that of plain concrete has also been observed by many investigators. 1 For example, the inclusion of fibres (steel or polypropylene) has been observed to significantly reduce the spall velocity and the size of fragments in slabs subjected to explosive loading.Z, 3 Significant improvement in the cracking behaviour of fibre-reinforced concrete over that of plain concrete has also been demonstrated by subjecting stair treads made of both to impact loading. 4 This enhanced impact resistance of fibre-reinforced concrete has led to its use where impact loads are to be encountered. For instance, steel fibre-reinforced concrete is used in airfield pavements, industrial flooring, and hydraulic structures, and polypropylene fibres have also been used in the construction of pile shells, s As there is no acceptable standard method for determining the impact resistance of fibre-reinforced concrete, a variety of tests, including explosives and drop weight tests, have been carried out on a range of specimen sizes and shapes. 6-14 All these investigators have reported that the measured impact resistance of concrete increases with the inclusion of fibres. Although these tests are useful in determining the relative merits of different fibre/cement composites they do not yield basic material properties which can be used for design. The nature of these tests also does not render it possible to compare the impact behaviour of the composites with their static behaviour and no inference of strain-rate can be made. The usefulness of these tests lies in the possibility of tailoring test methods to situations in practice, is
For instance, energy absorption up to the first crack in a falling ball test can be used to evaluate the impact resistance of fibre concrete where relatively low impact values are to be experienced, such as road building, air hammering, e t c . Explosive tests, where the volume of material removed and longestcrack e t c are measured, can be used for situations where explosive loadings are likely to occur. Results from tests which involve firing projectiles at test panels, where the volume of material removed in the contact zone and scabbing is measured, can be used where fibre-reinforced concrete is intended to be used as a safety wall against flying objects such as ballistic or tornado generated missiles. However, more rational design of fibre-reinforced composites subjected to impact and impulsive loadings would be facilitated by the knowledge of the mechanical properties of the material at the high strain-rates caused by impact. As a more ambitious approach, attempts can also be made to predict the impact behaviour of the composites from the knowledge of the mechanical properties of the concrete matrix, fibres and the fibre/concrete interface over a wide range of strain rates. The mechanical properties of concrete subjected to different strain rates have been discussed by the authors in a recent paper. 16 The results of some recent impact and high strain-rate tests on fibrereinforced concrete will be discussed in this article. Some results on the properties of the fibre/matrix interface at different strain rates will also be presented.
A REVIEW OF EXPERIMENTAL RESULTS ON FIBRE-REINFORCED CONCRETE Explosive tests Exposive tests on fibre-reinforced concrete slabs have been carried out by Williamson. ~ He observed that the result of shock loading applied to plain concrete by explosives, was
0010-4361/82/020153-07 $03.00 © 1982 Butterworth & Co (Publishers) Ltd COMPOSITES. APRIL 1982 COMP
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E
153
to completely disintegrate the slab specimens. A considerable reduction in spall velocity of the fragments was obtained by him when the matrix was reinforced with 1.75% nylon fibres. The tests conducted by Robins & Calderwod 3 also show that the inclusion of steel and polypropylene fibres significantly reduces the size and particle velocity of fragments caused by subjecting slabs to explosive loading. Drop weight tests
Nanda & Hannant 6 using a 'number of blows to no rebound' test, found that plain concrete failed after 5 blows while concrete reinforced with 5% steel fibres withstood up to 100 blows. Dixon & Mayfield 7 also recorded an increase in the number of blows to no rebound when concrete was reinforced with 1% by volume of steel fibres. Jamrozy & Swamy ~3 have published results of tests conducted to study the behaviour of fibre-reinforced concrete cubes (200 mm) subjected to repeated drop-weight impact loading applied by a 50 kg hammer falling through 300 mm. Three types of steel fibre were used: straight round fibres (0.25 x 15 mm and 0.25 x 25 mm), crimped fibres (0.25 x 25 ram) and hooked fibres (0.4 x 40 ram). A measure of the impact resistance was obtained by counting the number of blows up to the first crack. For straight, steel (0.25 x 25 mm) fibre-reinforced concrete, with 1% by volume of fibres, the first crack was found to appear after about 150 blows. Increasing fibre aspect ratio and the volume fraction was found to increase the number of blows up to first crack. They also found that the crimped and hooked fibres performed better than the smooth fibres under impact loading. The American Concrete Institute's committee on ~ c has also suggested the use of a drop hammer test to evaluate the impact resistance of concrete. 17 The tests are to be conducted by dropping a 4.5 kg hammer repeatedly from a height of ~ 460 mm onto a hardened steel ball resting on a cylindrical specimen of 152.4 mm diameter and 63.5 mm height. The number of blows required for the first visible crack to appear was considered to be an indication of the impact resistance of the material. Using this procedure, Ramakrislman e t al is recorded about 100 to 150 blows to first crack for concrete reinforced with hooked-end fibres. Bailey e t al 4 have reported results of tests conducted on fibre.reinforced concrete stair treads to assess their impact behaviour. The loading system consisted of a loosely pivoted loaded lever which was pin released from a preftxed height onto a steel plate resting on a stair tread (1180 x 360 x 65 mm). Each tread was subjected to lever blows with gradually increasing drop height. Three types of fibres were used for the tests: fibrillated polypropylene fibres (50 mm long), crimped steel fibres (50 mm long) and indented steel fibres (63 mm long). A 1% fibre volume fraction was used for all mixes. It was found that first cracking occurred at approximately the same height of fall irrespective of whether the tread contained fibres. However, the inclusion of fibres was found to reduce the severity of subsequent cracking behaviour, ie, minimizing crack widths. With the three types of fibre used, the effectiveness in controlling cracking was found to be greatest for sinusoidally crimped steel fibres, lowest for polypropylene fibres, with indented steel fibres showing an intermediate behaviour.
tests conducted on steel and nylon fibre-reinforced concrete specimens. The nylon fibres used were 0.25 mm in diameter and the steel fibres 0.43 mm. The length of the fibres used was 25.4 mm and the fibre volume fraction used was 1%. Tests were conducted by striking the cylindrical specimens at one end with high velocity projectiles. The compression wave generated at the striking end was reflected as a tensile wave when it reached the far end of the cylindrical specimen, causing the specimen to spaU. Measuring the fly-off velocity of an impedance matched pellet placed at the far end, enabled the particle velocity to be determined. From this the stresses and strains induced in the specimen were calculated. For strain rates of about 30 s-1 , they recorded a 4 to 5 fold increase in fracture strains/stresses over the corresponding static values. Bhargava & Rehnstrom 2° used the 'split Hopkinson bar test' to study the dynamic behaviour of fibre-reinforced concrete. The specimens used in the tests were reinforced with 0.2% by volume of polypropylene fibres. Specimens were sandwiched between two very long (about 5 meters each) aluminium bars and the bars were used to measure the incident and tra~'.smitted pulses. The amplitude of the incident pulse was increased by increasing the impact velocity. The impact strength of the specimen was assumed to have been reached when the transmitted pulse showed no increase with increasing amplitude of the incident pulse. For observed pulse rise-times of about 50/as, the dynamic strength was found to be about 50% greater than the static tensile strength. Some results showing the increase in tensile strength with strain rate of plain concrete are shown in Fig. 1. From this it can be seen that the results cited above for fibre-reinforced concrete show a similar relative increase in tensile strength with increasing strain rate as for unreinforced concrete. This result is in accordance with the results of static tensile and flexural tests on fibre-reinforced concrete where it has been observed that the first cracking strain is not too different from that of the unteinforced matrix. 21 However, as the important contribution of the fibres is evident only after first cracking, 22 the entire stress/strain relationship should be obtained to fully quantify the improvement in the energy absorption capacity of fibre-reinforced concrete under impact loading. Charpy impact tests
For many materials, the most common method of obtaining
4-
2
I-
10 .7
Tensile tests
Mellinger & Birkimer 19 have reported results of tensile
154
J/Flexur~
2-
I 10 .5
I 10 -5
I 10 .4
I t 0 .3
I 10 ~2
I 10 ~
I 10 0
t 10 ~
( s e c -1) Fig. 1 Relative increase in s t r e n g t h vs s t r a i n r a t e for plain concrete in t e n s i o n , flexure and compression
C O M P O S I T E S . A P R I L 1982
impact resistance is by Charpy impact testing. In this test, a three point bend specimen is struck at the centre by a pendulum. By knowing the height of the pendulum before and after impact, and assuming that all the energy lost by the striker is absorbed in breaking the beam, one can calculate the energy absorbed due to impact loading. Using this test method, Johnston 8 observed that the impact resistance of steel fibre-reinforced mortar is 10 times greater than that of unreinforced mortar. Shah & Baehr 14 also observed a tenfold increase in Charpy impact resistance when concrete specimens were reinforced with glass fibres. Krenchel9 observed an increase in impact energy with increasing fibre aspect ratio and Edgington u found that the Charpy impact energy increased linearly with increasing fibre volume fraction. Instrumented impact tests
In the above mentioned Charpy impact tests, stresses induced in the specimen during impact are not monitored. Hibbert 23 has attempted to obtain more basic material properties during a Charpy impact test by instrumenting the striker (tup) which yielded the load/time history of the impacted specimen. A specially constructed pendulum impact machine was designed to test 100 x 100 x 500 mm beam specimens in the Charpy configuration. He observed that for all specimens (unreinforced as well as FRC) the peak load under impact loading (impact velocity ~ 2.85 ms -1 ) was about 10 times that under static loading (applied at a cross-head movement rate of 0.05 mm s-1 ). This increase is quite high when compared with only a two fold increase generally reported by other investigators for similar strain rates (Fig. 1). This high load recorded is not representative of true material properties but is a consequence of specimen inertia effects. The problem of inertial effects in the observed load/time response is particularly severe for concrete materials because of its inherent brittleness and low strength/weight ratio and may result in the material response being completely overshadowed by the inertial response of the specimen, as in this case. The pendulum energy loss during impact can be calculated by integrating the area under the load/time trace obtained from the instrumented tup. ~ Using this method he obtained a value of 57 Nm for the plain concrete beams tested. Restraining the broken specimen halves against helical springs, he also calculated the kinetic energy of the broken beam (~ 26 Nm). He then estimated the energy in failing the concrete matrix as the energy loss by the pendulum minus the kinetic energy of the beam. This procedure resulted in a value of 33 Nm for the plain concrete matrix, which was unreasonably high when compared with the energy absorption under static loading of only 2.8 Nm obtained by him. For the fibre-reinforced concrete beams he then calculated the energy absorption solely due to fibre debonding, pull-out and fracture by subtracting the kinetic energy of the specimen and the energy absorbed in failing the concrete matrix from the pendulum energy loss. But as the energy absorbed by failing the matrix under impact loading was overestimated this procedure resulted in lower than actual values for the energy absorption in fibre debonding, pull-out and fracture. Therefore, one can expect higher values in fibre debonding, pull-out and fracture at impact loading rates than at static loading rates, although similar values were reported by Hibbert. Hibbert's results with only the specimen kinetic energy being subtracted from the pendulum energy loss are presented in Table 1. Here, it can
COMPOSITES. APRIL 1982
be seen that fibre-reinforced concrete beams, on average, absorb about 75% more bending energy when tested under impact loading conditions than under static loading conditions. Radomski 2s has also attempted to obtain the load/time history of the specimen during impact. He used a rotating impact machine to study the dynamic behaviour of fibrereinforced concrete specimens (15 x 15 x 105 mm). The impact load was imposed on the specimen by means of a striker hitting the centre of the specimen. The striker was released from the flywheel of the machine when the required impact velocity was reached. The load/time signal was obtained by the use of piezo-electric gauges. However, the author did not report the load history; only the energy values calculated from the load/time trace were reported. But, he noticed that the energy measurement obtained from the rotating impact machine was, substantially different from that obtained by a Charpy impact machine using identical size specimens. This is probably due to the different material and geometrical properties of the striker which determines the energy absorption at the contact zone. This emphasizes the need. for the measurement of material properties which are, as far as possible, test system independent. The authors' attempt to obtain fundamental material properties is briefly described in a later section. Flexural tests at variable strain rates
The effect of the rate of load application on the flexural strength of fibre-reinforced concrete has been studied by Butler & Keating, 26 They used a hydraulic ram capable of moving at different speeds for load application and their specimens, (200 x 200 × 1500 mm in size), were tested in four-point bending. The fibres used in the tests were 0.5 mm in diameter and 50 mm in length, high-carbon duoform steel fibres. A fibre volume fraction of 1.2% was used and the MOR at the lowest stress rate was 7.4 MN m -2. They observed a 35% increase in flexural strength when the rate of stress application was increased from 0.017 to 170 MN m -2 s-~ . This increase is low when compared with a 75% increase observed for plain concrete when the stress rate was increased over the same range. Some results of the rate of load application on the load/ deflection behaviour of polyethylene fibre-reinforced concrete have also been published by Kobayashi & Cho. 2s An Instron testing machine with variable cross-head velocities was used by them to test 100 x 100 x 400 mm specimens in three-point bending. The results obtained for specimens reinforced with a 4% fibre volume fraction showed a 25% increase in the cracking load when the crosshead velocity was increased from 1 mm/min to 200 mm/min. Another effect of increasing the loading rate on the load/ deflection behaviour was also noticed; ie the increase of the post cracking slope of the load/deflection curve, with increasing loading rate. This increase in post cracking stiffness was attributed to the viscoelastic properties of the polyethylene fibres. SUMMA R Y OF A U THORS" RECEN T RESEA R CH
A high increase in peak load under impact as observed by Hibbert 22 was also observed by the authors in their preliminary impact tests on fibre-reinforced concrete beams, conducted by using a drop hammer impact machine, with an instrumented tup. Many investigators have recognized that during an initial period of time, the load measured by the
155
Table 1.
Energy absorption of fibre reinforced concrete beams under impact loading 23 Bending energy absorption up to 10 mm deflection (Nm)
Fibre type
Fibre size (mm) diameter x length
Vf(%)
Age at test Static
-
-
2 mths 2½ yrs
Dynamic (% increase)
2.8
33 (1080)
2.1
30 (1330)
Steel, hooked ends Steel, hooked ends
0.5 x 50 0.5 x 50
1.2 0.6
2 mths 2 mths
85 57
145 (71) 94 (65)
Steel, crimped high C high C high C
0.5 x 50 0.5 x 50 0.5 x 50
1.2 1.2 0.6
2 mths 2½ yrs 2 mths
66 95 53
139 (111) 143 (51) 97 (83)
Steel, crimped low C
0 . 5 x 50
1.2
2½ yrs
80
132 (65)
Steel duoform Steel duoform Steel duoform Stainless steel
0.38 x 38 0.38 x 38 0.64 x 60
1.2 1.2 1.2
2 mths 2½ yrs 2 mths
30 24 -
48 (60) 56 (133) 148
melt extract
0.7 x 25
1.2
2 mths
47
84 (79)
Polypropylene
1200 demer x 75 mm long 1200 demer x 75 mm 1200 demer x 75 mm 1200 demer x 35 mm 1200 demer x 35mm 1200 demer x 35 mm
1.2
2 mths
43
77 (79)
1.2
2½ yrs
42
69 (64)
0.6
2 mths
1.2
2 mths
30
45 (50)
1.2
2½ yrs
28
44 (57)
0.6
2 mths
Polypropylene Polypropylene Polypropylene Polypropylene Polypropylene
tup and that resisted by the beam are not the same, due to specimen inertia effects. These inertial effects are manifest as oscillations in the tup-load versus time signal. Server, Wullaert, and Sheckhard 2s have recommended that, to obtain reliable measurements from the instrumented impact test, the time to fracture of the specimen should be greater than three times the period of these inertial oscillations. For metallic and polymeric materials, the time to fracture is generally larger than the period of these inertial oscillations and as a result it is possible to satisfy the guidelines suggested by Server et al. 2s However, for brittle materials such as concrete, the time to fracture is quite low even for small impact velocities and it may not be possible to avoid fracture even during the first oscillation, as observed both by the authors and Hibbert. 2a In subsequent impact tests conducted by the authors, it was found that it is possible to reduce the amplitude of the inertial oscillations substantially.by introducing a rubber pad between the tup and specimen and thereby get reliable data. 29 A two degree of freedom analytical model was also developed to better understand the nature of inertial effects and to predict such effects. The equations of motion of the modelled system given by:
[m°
156
61
50
where xl and x2 are the displacement of the hammer and the specimen from their equilibrium configuration; mh is the weight of the harmner; ke the stiffness of the contact zone; m s the weight of the specimen and ks the stiffness of the beam specimen. Solving these equations with proper initial conditions will yield the values of the tup load pw(t) and the beam load pB(t) histories from the relations: pT(t) = ke [xl(t) - x2(t)] + mhg pB(t) = ks[x=(t)] + mhg These two values are plotted for a particular set of parameters in Fig. 2. Fig. 2a is for a test conducted without any padding material while Fig. 2b is for a test conducted with a rubber pad. It can be seen that the introduction of the rubber pad should substantially reduce the difference between the measured tup load and the actual beam load. These predictions were confirmed by test results on asbestos cement specimens. 29 In addition to selecting the specimen dimensions according to the analytical model, it was also found necessary to measure the anvil loads and measure the strains on the bottom fibres of the beam to assure that the beam fails in a flexural mode. For instance, load/time and strain/time
COMPOSITES. APRI L 1982
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15
a
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v
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-2
1
0.2
I
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0.6
0.7
'
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®
/ /
Tup - load ~ _
/
/J_
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IQ
®
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iT
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-300
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Bending load
/
//
- 400
I 2.0
1.0
/ 4
/
/ the kinetic energy of the beam.
/
/
k¢ 0
/
Using these experimental techniques, tests are now being conducted by the authors on concrete beam specimens reinforced with smooth and hooked-end steel fibres, glass fibres and polypropylene fibres. Some results from this continuing investigation are shown in Figs 4 and 5. In Fig. 4 the relative values of modulus of rupture at different strain rates are shown for both concrete and smooth steel fibre-
/
S p e c i m e n size = 2 5 x 5 0 * 2 5 0 m m Impact velocity = 1 m/s p = 2000 kg/m3 E = 1 7 . 2 3 × 103 MP(]
/•'/// I 0.5
Time
5.0
I 4.0
Fig. 3 Typical curves for FRC beam specimen: a -- load vs time; b -- strain vs time
/
"o o o J
I 30 T~me ( m s)
I 1.0 (m s )
t 1.5
o S t e e l f , b r e - r e , nforced m o r t a r
1.75
MOR ( s t a t , c ) = 9 4 4 k N m "2
z~ Plain m o r t a r , Beam
Fig. 2 Analytical tup-load and beam-bending load vs time. Curves for tests conducted: a) w i t h o u t a rubber pad (k e = 8 7 . 5 M N m -1 } and; b) with a rubber pad (k e = 5 . 8 4 M N m -1 )
3-
Mob(static)=731 kNm2
d i m e n s i o n : 3 8 1~ 7 6 2 ,
point
381 m m
loading
20
o o
curves obtained for a ~]~c specimen are shown in Fig. 3. It can be seen that, up to matrix cracking, the strains calculated, assuming elastic bending behaviour, agree closely with the measured values, assuring that the beam failed in flexure. The energy dissipated by the falling hammer can be calculated by integrating the area under the tup-load versus time trace ~ ; estimates of the bending energy absorbed by the beam specimen could then be made by subtracting the elastic energy of the contact zone and the kinetic energy of the beam from it. However, in the tests conducted by the authors, the direct measurement of the load-point deflection using AC-LVDT was found to be a more reliable way of obtaining the energy absorption, as it eliminated the need for estimating the energy absorbed in the contact zone and
COMPOSITES. APRIL 1982
v
~
Fibre - reinforced
1.6
c
~ 14
/
/
106
I 105
Mortor
1.2
10
~ 10 .7
I 10-4
I 10 3
I 10-2
[ 10 ~
2--_. 10 °
101
(sec') Fig. 4 Relative increase in flexural strength vs strain rate for smooth steel fibre-reinforcad concrete and plain concrete
157
~ 626 6 0 2 2 ~
mortar (12: 0 5~1%)
"
Steel - re~nforced From fibre Suaris ond Shah I = 2 5 4 r a m . d = 0 2 6 r a m brass tooted Mo~ (static)= kN rn 2 O c ~ o (statt¢) = 6Z 5 6 kN m 2
z 4440
254508762
640
~z _~~ 4o5
3.
The energy absorbed by steel fibre-reinforced concrete under impact appears to be about 20-100 times that absorbed by plain concrete. The authors' results as well as those of Hibbert, indicate that the bending energy absorption of straight-fibre-reinforced concrete beam specimens increases significantly when the strain rate is increased. (Fig. 5, Table l)
4.
The bond between smooth fibres and the concrete matrix does not appear to be significantly influenced by the strain rate. However, the bond strength between deformed fibres and the concrete matrix may be expected to increase with strain rate resulting in a higher percentage increase in energy absorption by deformed fibres than by smooth fibres.
944
016127
~0
/
~
o
io
o
:~ ~
270 C
~/joO~ f~r~-r~1o,ced ,
010"7
.....
106
O' -L --L- -'-- .-'.- .-' ,
10-5
10 4
10-3
,
102
101
10o
101
(sec'D
Fig. 5 Variation with strain rate of the bending energy absorption of a smooth fibre-reinforced beam
A CKNOWL EDGEMEN TS reinforced concrete. Fig. 5 shows the energy absorption of smooth steel fibre-reinforced concrete beams at different strain rates. The load/deflection curves at the two extreme strain rates are also presented. BOND PROPERTIES Tests to determine the bond strength at different strain rates have been conducted by Gokoz & NaamanJ ° A hollow cored cylinder with a solid bottom and with 20 fibres embedded along its periphery were used in their tests. The impact load was applied by an instrumented tup hitting the bottom of the specimen and causing the fibres to pull out of the matrix. For the steel, polypropylene and glass fibres they tested, no significant effect of strain rate on the bond strength was observed. Although no pull-out tests have been reported for deformed fibres, tests have been conducted by Vos & Reinhardt 31 to study the bond strength between the reinforcement and concrete under impact loading. Their tests were conducted by using the 'split Hopkinson bar technique'; the results indicated that the bond stresses of strands and plain bars were not influenced by the loading rate while the bond stress of the deformed bars was significantly influenced by it. This rate sensitivity of deformed bars is believed to occur as a result of the bearing of the ribs on concrete producing splitting of the concrete, which is rate dependent. Such behaviour can be expected of crimped fibres and indented fibres. If true, this means that deformed fibre-reinforced concrete would be more efficient than smooth fibrereinforced concrete in absorbing energy under impact loading. In I-libbert's results, presented in Table 1, it is noticeable that crimped steel fibres showed higher percentage increases in energy absorption under impact loading than the other fibre types tested. CONCLUSIONS
1.
2.
158
It has been well established that fibre-reinforced concrete exhibits considerably improved impact resistance over plain concrete. Although impact tests such as drop hammer tests and explosive tests are useful in predicting its impact resistance, the knowledge of basic material parameters would facilitate more rational design of fibre-reinforced concrete structures subjected to impact. The flexural strength of smooth steel fibre-reinforced mortar appears to be slightly more strain rate sensitive than plain mortar. This is likely to be due to the multiple cracking of the matrix. (Fig. 4)
This paper was prepared while the authors were working on a grant from the US Army Research Office, Metallurgy and Materials Division. REFERENCES 1
2 3 4 5 6 7 8 9 10
11 12 13 14 15
16
Hoff, G.C. 'Selected bibliography on fibre-reinforced cement and conciete, Supplement No 2' Miscellaneous Paper C-75-6 (US Army WaterwaysExperiment Station, Vicksburg, MS, USA, 1979) WiUiamson,G.R. 'Response of fibrous reinforced concrete to explosive loading' Technical Report 2-48 (US ArmyCorps of Engineers, Ohio, USA - River Division Laboratories, Jan 1966'. Robins,P.J. and Caldet~vod, R.W. 'Explosivetesting of fibrereinforced concrete' Concrete (January 1978) pp 76-78 Bailey, J.H., Bentley, S., Mayfield, B. and PeU, P.S. 'Impact testing of fibre reinforced concrete stair treads'Mag of Concrete Res 27 No 92 (September 1975) pp 167-170 Hannant,D.J. Fibre Cements and Fibre Concretes (A WileyInterscience Publication, 1978) Nanda,V.K. and l-lannant, DJ. 'Fibre reinforced concrete' Concrete Bldg and Concrete Prods XLIV No 10 (October 1969) pp 179-181 Dixon,J. and Mayfield, B. 'Concrete reinforced with fibrous wire' Concrete (March 1971) pp 73-76 Johnston, C.D. 'Steel fibre reinforced concrete: a review of mechanical properties, fibre reinforced concrete' A CI Publication SP-44 (1974) pp 127-142 Krenchel,H. 'Fibre reinforced brittle matrix materials, fibre reinforced concrete' A CI Publication SP--44 (1974) pp 45-77 All, M.A., Majumdar, AJ. and Singh, B. 'Properties of glassfibre cement, the effect of fibre length and content' Building Research Establishment, Current Paper CP 94/75 (October 1975) Edgington,J. 'Steel fibre reinforced concrete' Ph D Thesis (University of Surrey, 1973) Raouf,Z.A., AI Hassani, S.T.S. and Simpson, J.W. 'Explosive testing of fibre reinforced cement composites' Concrete (April 1976) pp 28-30 Jamrozy, Z. and Swamy, R.N. 'Use of steel fibre reinforcement for impact resistance and machinery foundations' Int J Cement Composites 1 No 2 (July 1979) pp 65-76 Shah,S.P. and Baehr, D. 'Properties of glass fibre reinforced gypsum sheets' J Structural Div ASCE (January 1977) pp 23-33 Verhagen, A.IL 'Impact testing of fibre reinforced concrete: reflection on possible test methods' RILEM Symp on Testing and Test Methods of Fibre Cement Composites, edited by R.N. Swamy(RILEM Symposium, 1978) Suaris,W. and Shah, S.P. 'Mechanicalproperties of materials under impact and impulsive loading' Introductory Report for the Interassociation Symposium on Impact Loading of Concrete Structures (West Berlin, June 1982)
17 18
ACI Committee 544 Report 'Measurement of properties of fibre reinforced concrete' A C I J (July 1978) pp 283-289 Ramakrishnan,V., Brandshaug, I., Coyle, W.V. and Sehrader, E.K. 'A comparative evaluation of concrete reinforced with straight steel fibres with deformed ends glued together into bundles'ACIJProc 77 No 3 (May-June 1980) pp 135-143
COMPOSITES. APR I L 1982
19
20 21 22 23
24
25 26
Mellinger,F.M. and Birkimer, D.L. 'Measurement of stress and strain on cylindrical test specimens of rock and concrete under impact loading' Tech Report No 4-46 (Ohio River Division Laboratories, Corps of Engineers, April 1966) Bhargava,J. and Rehnstrom, A. 'Dynamic strength of polymer modified and fibre-reinforced concretes' Cement and Concrete Res 7 (1977) pp 199-208 Shah, S.P. and Rangan, B.V. 'Fibre reinforced concrete properties' A C I J 68 No 2 (February 1971) pp 126-135 Shah, S.P. and Naaman, A.E. 'Mechanical properties of glass and steel fibre reinforced mortar' A CI J (January 1976) pp 50-53 I-Iibbert,A.P. 'Impact resistance of fibre concrete' Report From the Construction Materials Research Group, Department of Civil Engineering, University of Surrey (University of Surrey, Guildford, UK, July 1979) Ireland, D.R. 'Procedures and problems associated with reliable control of the instrumented impact test' Instrumented Impact Testing, A S T M STP 563 (American Society for Testing and Materials, 1974) pp 3-29 Radomski, W. 'Application of the rotating impact machine for testing fibre-reinforced concrete' In t J o f Cement Composites and Lightweight Concrete 3 No 1 (February 1981) Butler, J.E. and Keating, J. 'Preliminary data derived using a flexural cyclic loading machine to test plain and fibrous concrete' Mater and Structures 14 No 79 (June 1981) pp 25-33
C O M P O S I T E S . A P R I L 1982
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Kobayashi, K. and Cho, R. 'Flexural behaviour of polyethylene fibre reinforced concrete' In t J o f Cement Composites and Lightweight Concrete 3 No 1 (February 1981) pp 19-25 Server,W.L., Wullaert, R.A. and Sheekhard, LW. 'Evaluation of current procedures for dynamic fracture-toughness testing' Flaw Growth and Fracture. A S T M STP 631 (American Society for Testing and Materials, 1977) pp 446-461 Suaris, W. and Shah, S.P. 'Inertial effects in the instrumented impact testing of cementitious composites' A S T M J Cemen t, Concrete and Aggregates (March 1982) Gokoz, U. and Naaman, A.E. 'Effect of strain rate on the pullout behaviour of fibres in mortar' Progress Report Submitted to the US Army Research Office, Grant No DAA G 29- 79-C0162 (January 1981) Vos. E. and Reinhardt, H.W. 'Bond resistance of deformed bars, plain bars and strands under impact-loading' Report No 5-80-6 (Department of Civil Engineering, Delft University of Technology, The Netherlands, September 1980)
AUTHORS The authors are with the Northwestern University's Department of Civil Engineering. Inquiries should be addressed to: Professor Surendra P. Shah, Department of Civil Engineering, Northwestern University, Evanston, Illinois 60201, USA.
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