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On the strength and toughness properties of SFRC under

static-dynamic compression

Z.L. Wang a,b,⇑, Z.M. Shi a, J.G. Wang c

a Department of Geotechnical Engineering, Tongji University, Shanghai 200092, Chinab Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai 200092, Chinac School of Mechanical and Chemical Engineering, The University of Western Australia, Crawley, WA 6009, Australia

a r t i c l e i n f o

 Article history:

Received 14 October 2010Received in revised form 5 December 2010Accepted 23 January 2011Available online 1 February 2011

Keywords:

B. StrengthB. Impact behaviorD. Mechanical testingSteel fiber-reinforced concrete

a b s t r a c t

The addition of steel fibers into concrete mix can significantly improve the engineering properties of con-crete. This paper experimentally studies the mechanical behaviors of steel fiber-reinforced concrete(SFRC) through both static and dynamic compression tests. Cylindrical specimens with three differentpercentages of short and fine fibers 0%, 1.5% and 3% by volume of concrete are firstly fabricated. Thesespecimens are then tested by MTS for static compression and split Hopkinson pressure bar (SHPB) fordynamic impact. It is revealed that thefailure mode of concrete considerably changes from fragile to duc-tile with the increase of steel fibers. The plain concrete may fail under low strain-rate single impactwhereas the fibrous concrete can resist high strain-rate repeated impact. Stain-rate exerts great influenceon concrete strength. Besides, toughness energy is proportional to the fiber content in both static anddynamic compressions.

 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Steel fiber-reinforced concrete has become a practical alterna-tive construction material in various structures. The steel fiberscan be used externally to improve the compressive strength, flex-ure and shear capacities of beams, slabs and wall made by conven-tional concrete effectively. Also, it can be used internally asreinforcement replacing common steel reinforcing bars in struc-tures due to its advantages [1,2]. These randomly distributed fibersmay bridge microcracks and restrain their widening, thus delayingthe cracks further propagation. Such reinforcements may largelyimprove the post-peak ductility and energy absorption capacityof concrete [3].

Well designed members should be able to avoid catastrophicfailure of a structure [4]. Concrete structures are usually exposedto various load environments in their service periods. A key designissue is to fully understand the responses of these structures toboth static and dynamic loads. In conventional concrete members,crack widths are restrained by the use of steel reinforcing bars.However, the thicker steel bar has different heat expansioncompared to the surrounding concrete. This may produce micro-cracks on the interface between bar and concrete. How to maintainthe concrete structure with less or no propagation of microcracks isa notable issue. Numerous publications can be found in both

experimental studies and in situ applications of steel fibers in con-crete frame buildings [5–8].Strength and toughness have been recognized as two important

characteristics of steel fiber-reinforced concrete [8,9]. In general,toughness or energy absorption capacity (hereafter called toughnessenergy) is determined from the area encompassed by the stress–strain curve in compression. This expresses the total energy ab-sorbed by the specimen prior to its complete damage or failure.The additional load-bearing capacity is in direct proportion to thetoughness that the steel fibers impart to the concrete. Theimprovement of residual strength of concrete also reflects thecapability to carry more loads even after cracking [10].

Over the past several decades, several attempts have been madein both numerical and experimental methods to understand themechanical responses of SFRC. Test methods like servo-controlledmaterial testing system (MTS) [11], drop-weight tester [8] and splitHopkinson pressure bar (SHPB) technique [12] have been adoptedso far. The MTS device is typically used for quasi-static tests. For ahigher strain-rate like dozens, even hundreds per second, drop-weight tester or pneumatic SHPB are usually employed. In partic-ular, the SHPB is a very popular experimental apparatus for thestudy of the dynamic responses of materials. It has been used bynumerous investigators to elucidate the dynamic mechanical prop-erties of solid media [2,13].

This study will experimentally investigate the mechanical re-sponses of the concrete cylindrical specimens by using MTS andSHPB techniques. The emphasis is on the comparison of strengthand toughness properties between the plain concrete and the steel

1359-8368/$ - see front matter  2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.compositesb.2011.01.027

⇑ Corresponding author at: Department of Geotechnical Engineering, TongjiUniversity, Shanghai 200092, China. Fax: +86 21 65985210.

E-mail address: cvewzL@tongji.edu.cn (Z.L. Wang).

Composites: Part B 42 (2011) 1285–1290

Contents lists available at   ScienceDirect

Composites: Part B

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / c o m p o s i t e s b

http://dx.doi.org/10.1016/j.compositesb.2011.01.027mailto:cvewzL@tongji.edu.cnhttp://dx.doi.org/10.1016/j.compositesb.2011.01.027http://www.sciencedirect.com/science/journal/13598368http://www.elsevier.com/locate/compositesbhttp://www.elsevier.com/locate/compositesbhttp://www.sciencedirect.com/science/journal/13598368http://dx.doi.org/10.1016/j.compositesb.2011.01.027mailto:cvewzL@tongji.edu.cnhttp://dx.doi.org/10.1016/j.compositesb.2011.01.027

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fiber-reinforced concrete with volume fractions of 1.5% and 3.0%,respectively. The mechanical responses of these specimens areevaluated in terms of strain-rate effect, the fiber content by volumeand the failure patterns of the tested specimens.

2. Experiment program

 2.1. Materials

Ordinary Portland cement was used as the cementitious mate-rial. Dry non-compacted silica fume was provided by  Zhongxing Technology Company of China. The coarse aggregate was crushedlimestone with a maximum size of 10 mm. The fine aggregate

was river sand with a fineness modulus of 2.6. Short and straightsteel fibers were added in concrete mixes at different volume frac-tions. The super plasticizer (SP) is a liquor of phenolic aldehydewhich was added to the mix with 1.5% dosage of the cement vol-ume. Fiber shapes are shown in   Fig. 1, and their specificationsare listed in Table 1. The fibers were added to each series of mixesat 0.0%, 1.5% and 3.0%, by volume of concrete. These values corre-

spond to the steel fiber weight of 0, 117 and 234 kg per cubic meterof concrete. Their volume fraction is denoted by a symbol  V  f .

 2.2. Specimen preparation

The ingredients of concrete mixes are presented in Table 2. Inthe process of specimen-making, steel fibers, cement, crushedstone, sand, and silica fume were firstly mixed for about 5 min.Water and super plasticizer were then added. The mixture wasmixed until uniform concrete was obtained. From each mix, twosizes of cylinders (U70mm 35 mm and   U50 mm 100 mm)were cast in steel moulds. The moulds were oiled and placed ona vibration table vibrating at low speed to ensure good compactionwhile the concrete was poured. The cylindrical specimens were

demoulded 24 h later and cured in lime-saturated water for28 days at room temperature. The ends of all cylinders are carefullyground in order to assure the parallelism of the end surfaces.

 2.3. Experimental set-up

Quasi-static compression was performed in closed-loop servo-controlled material testing machine (MTS) with a capacity of 1000 kN. A complete uniaxial stress–strain response can be ob-tained through its measurement system. This experimental processused displacement control. Two loading rates ( _e ¼ 105 and 102)were tested. An inbuilt electronic data acquisition system was usedto record the axial displacements and corresponding loads. Thisdata acquisition can measure the total axial displacement overFig. 1.   Photograph of steel fibers tested.

 Table 1

Properties of steel fibers tested.

Type Density (kg/m3) Length (mm) Diameter (mm) Tensile strength (MPa) Appearance

WSF1 7800.0 13.0 0.2 2500.0 Bright/straight

 Table 2

Mix proportions (kg/m3).

V  f    w/(c + sf) Cement Water Silica fume Coarse aggregate River sand Steel fiber

0.0% 0.35 440 171.5 50 850 900 01.5% 0.35 440 171.5 50 811 900 1173.0% 0.35 440 171.5 50 770 900 234

Fig. 2.  Split Hopkinson pressure bar device.

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the whole length of the specimen instead of the middle half-length.It was expected that this would lead to better displacement mea-surements. The compressive strength and the stress–strain curveof each specimen were thus determined. Remarkably, the shapeof the uniaxial stress–strain curves is strongly affected by testingconditions such as the stiffness of the testing machine, the size

and shape of specimens [14], the loading rate, the concrete charac-teristics (like the water/cement ratio, aggregate type). Particularcare was exercised to avoid variation in the experimental process,thus minimizing the influences of testing conditions.

The dynamic compression tests are performed by using a SHPBtechnique (see Fig. 2). This device consists of main body, energysource and measurement system. Main body mainly contains alaunch tube, a projectile (or striker bar), an incident bar, a trans-mission bar and energy-absorbing parts. The energy source systemhas air compressor and pressure vessel. The measurement systemconsists of velocity and dynamic strain indicator. The projectile,incident and transmission bars have the Young’s modulus of 210.0 GPa and the wave velocity of 5190.0 m/s. The compressivelongitudinal incident wave is initiated by the impact of the strikerbar on the incident bar. The incident wave travels along the bar andis recorded at the strain gauge A. When the stress wave reaches theinterface between the incident bar and the specimen, it is partiallyreflected by this interface and partially transmitted into the con-crete specimen. The reflected wave can be also recorded by thestrain gauge  A  on the incident bar. At the interface between thespecimen and the transmitted bar, the stress wave is again par-

tially reflected and partially transmitted. The strain gauge   B   re-cords the transmitted stress wave in the transmitted bar.

The average strain and stress of the specimen are calculated by[2,12]:

es  ¼c 0

ls

Z   t 

0

ðeI ðt Þ eRðt Þ eT ðt ÞÞdt    ð1Þ

r ¼  A

2 AsE ðeI ðt Þ þ eRðt Þ þ eT ðt ÞÞ ð2Þ

where A  and E  are the cross-section area and the Young’s modulusof the elastic incident and transmitted bars, respectively;  c 0 is thewave velocity in the bars.  As  and ls  are instantaneous cross-section

0 0.005 0.01 0.015 0.02 0.025 0.030

20

40

60

80

100

120

ε

     σ    (

   M   P  a   )

Vf =0.0%

Vf =1.5%

Vf =3.0%

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.040

20

40

60

80

100

120

ε

     σ    (

   M   P  a   )

Vf =0.0%

Vf =1.5%

Vf =3.0%

Fig. 3.   Stress–strain curves of SFRC under quasi-static compression.

Fig. 4.   Comparison of toughness under quasi-static compression (_e ¼ 102).

(a) V f =0.0% (b) V f =1.5% (c) V f =3.0%

Fig. 5.  Comparison of failure patterns under quasi-static compression.

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Secondly, the dynamic response of the SFRC is sensitive to strainrate. It is observed that the peak stress and corresponding strainin-crease obviously with the increase of strain rate. Also, the additionof fibers increases peak strain as well as compressive strength. Thatis, both ascending and descending branches of the stress–straincurves are affected by the addition of steel fibers, but the effectin the descending branch is more obvious [2]. Thirdly, steel fibershave an excellent performance on the toughness of SFRC. The

toughness energy also increases with the increase of strain rate[17,18]. In addition, the volume fraction of fibers has vital impactson the failure patterns of concrete.  Fig. 7 gives the photographs of post-test specimens after single impact. The plain concrete breaksinto pieces, but for the specimen with the V  f  = 1.5% still remains theoverall outline although small segments peel off. The specimenwith fiber volume fraction of 3.0% keeps essential integrity.

 3.3. Evaluation of strain-rate effect 

These static and dynamic test results show that the effect of strain-rate on concrete strength is distinct. Fig. 8 illustrates the in-creases of the compression strength with the logarithm of strain-rate in the experiments. This figure shows that the strain-rate ef-fect exists in both quasi-static and dynamic tests. This effect ismore obvious for dynamic impact than for quasi-static compres-sion. Besides, larger fiber content would imply higher compressivestrength.

The stress–strain curves of the various specimens under singleimpact (six independent impacts in all) have been presented inFig. 6. It is found that, with increasing the fiber content, higherstrain-rate loading is required to completely fail the specimen.

The strain rates at failure are approximately 20/s, 80/s and 90/sfor the concrete specimens containing 0.0%, 1.5% and 3.0% volumefraction of fibers. Clearly, the fibrous concrete specimens fail onlyunder higher strain-rate loading. In order to compare the mechan-ical response of the specimen with three fiber volume fractions, theV  f  = 1.5% and V  f  = 3.0% SFRC specimens are specially tested underlow-velocity impact ( _e 20/s). The stress–strain curves of fiber-

reinforced concrete specimens under the same striker velocityand pulse shaper are shown in Fig. 9, where the strain rate in eachimpact is calculated and denoted in legend. Owing to the weakcapability of impact resistance, the plain concrete specimen iscompletely broken into small pieces after the first impact (seeFigs. 6aand7a). At the same strain-rate loading, the specimen withV  f  = 1.5% fails only at the fifth impact. The specimen containing vol-ume fraction of 3.0% can resist the ninth impact. Besides, thedecreasing rate of curve peak is well controlled by increasing fibervolume fraction. For example, in the V  f  = 3.0% case, the curve peaksof the 2nd–5th impacts are even higher than the peak of the firstimpact. This phenomenon is attributed to the fact that thehardening effect of strain-rate prevails over the effect of damagesoftening. It also exhibits the excellence of fibrous concrete inthe resistance against repeated impact. Due to the damage

(a) V f =0.0% (b) V f =1.5% (c) V f =3.0%

Fig. 7.  Comparison of failure patterns under dynamic compression.

-6 -5 -4 -3 -2 -1 0 1 2 360

70

80

90

100

110

120

130

Logarithm of strain rate

   S   t  r  e  n  g   t   h   (   M   P  a   )

Vf =0.0%

Vf =1.5%

Vf =3.0%

Fig. 8.   Effect of strain-rate on concrete strength.

0 0.002 0.004 0.006 0.008 0.01 0.012 0.0140

10

20

30

40

50

60

70

80

90

ε

     σ    (

   M   P  a   )

16.7/s

21.4/s

23.7/s

30.0/s

33.6/s

 1st

 2nd

3rd

 4th

 5th

(a) V f =1.5%

0 1 2 3 4 5 6 7 8

x 10-3

0

10

20

30

40

50

60

70

80

90

100

ε

     σ    (

   M   P  a   )

17.3/s

16.4/s

17.0/s

18.5/s

17.9/s

19.9/s

19.1/s

25.4/s29.1/s

1st

2nd

3rd

4th

5th

6th

7th

8th

9th

(b) V f =3.0%

Fig. 9.   Stress–strain curves for SFRC under low strain-rate repeated impact.

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accumulation, the strain rate increases with the increase of impact(see Fig. 9).

Fig. 10 shows the toughness energy of the fibrous specimens

accumulated during the impact process. For the V  f  = 1.5% specimen,the values corresponding to five impacts are 0.152, 0.244, 0.310,0.279 and 0.301 MPa, respectively. The V  f  = 3.0% specimen has ninetoughness values in all: 0.171, 0.150, 0.137, 0.168, 0.148, 0.176,0.212, 0.258 and 0.322 MPa, respectively. However, the plain con-crete can only experience single impact. Its toughness energy is0.355 MPa. Their total toughness values are 0.355 MPa, 1.286 MPaand 1.742 MPa, respectively. These experimental results suggestthat the resistance of concrete against repeated impact is signifi-cantly improved by fiber reinforcement.

4. Conclusions

It is important to investigate how SFRC behaves under static-

dynamic compression. Through this experimental study, the fol-lowing conclusions may be drawn:

(1) Uniaxial compression tests show that the failure mode of fiber-reinforced concrete considerably changes from fragileto ductile. Due to bridging effect of fibers, the fibrous speci-mens remain essentially intact and only have some spallingon the sides up to end of the test.

(2) The inclusion of steel fibers in concrete mix definitelyimproves the mechanical behaviors of concrete. Under thesame loading rate, the plain concrete probably fails by singleimpact whereas the fibrous concrete can resist repeatedimpact.

(3) The impact properties of SFRC, including dynamic strength

and toughness energy, exhibit strong strain-rate depen-dency. The hardening effect of strain-rate in concrete withhigh fiber content can match or even exceed the effect of damage softening to a certain extent.

 Acknowledgements

This study was supported by NSFC_the Civil Aeronautics JointResearch Foundation (60776821), the Program for New Century

Excellent Talents in University (NCET-08-0525), the SpecializedResearch Fund for the Doctoral Program of Higher Education(20070358073), the Shanghai Leading Academic Discipline Project

(B308), the Kwang-Hua Fund for College of Civil Engineering(Tongji University) and the Talent Introduction Fund of TongjiUniversity (0200144054; 0200144056).

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Fig. 10.  Comparison of concrete toughness under each dynamic impact.

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