Dynamic tensile behaviour of fibre reinforced concrete with spiral fibres

Dynamic tensile behaviour of ?bre reinforced concrete with spiral ?bres

Z.Xu a ,b ,?,H.Hao a ,1,H.N.Li b

a School of Civil and Resource Engineering,The University of Western Australia,Crawley,WA 6009,Australia

b

The State Key Laboratory of Coastal and Offshore Engineering,Dalian University of Technology,Dalian,Liaoning 116023,China

a r t i c l e i n f o Article history:

Received 16November 2011Accepted 23May 2012

Available online 5June 2012Keywords:

A.Fibre reinforced concrete

B.Spiral steel ?bre

E.Dynamic split tensile test

a b s t r a c t

This paper performs drop-weight splitting tests to study the dynamic tensile properties of ?bre rein-forced concrete (FRC)materials with different steel ?bres.A renovated splitting tensile testing method was developed to ensure a more quali?ed experimental process.The splitting tensile impact tests were conducted with an instrumented drop-weight impact system consisting of a hard steel drop weight,a fast-response load cell,a high-speed video camera and a high-frequency data acquisition system.The quasi-static compressive and splitting tests were also conducted to obtain the static properties of the FRC materials.The commonly used hooked-end steel ?bre and a new spiral shaped steel ?bre were tested in this study.The high-speed video camera was used to capture the detailed failure process,deformation and cracking process of the tested specimens.Average strain rates and the cracking extension displace-ment and velocity under impact loading were estimated by analysing the recorded high-speed images.The strains were also measured by the strain gages on the specimen surface.The dynamic stress–strain and stress–COD (cracking opening displacement)relations,the rate sensitivity of tensile strength and the corresponding energy absorption capacity of plain concrete and FRC with different ?bres were obtained,compared and discussed.The advantage and effectiveness of the new spiral ?bre in increasing the perfor-mance of FRC under dynamic tensile loading were examined.The results show that FRC with spiral ?bres outperforms that with hooked-end ?bres,and is a promising construction material in resisting dynamic loadings.

ó2012Elsevier Ltd.All rights reserved.

1.Introduction

Desire has never been more ambitious for more ductile and tougher cementitious materials to enhance the performance of civil engineering structures under high rate loading,such as earth-quake,impact and blast loading.Fibre reinforced concrete is deemed to be a promising class of material for such purposes.FRC composites have been proved to signi?cantly improve load-carrying capacity,energy absorption capacity,abrasion resistance,stability and ?exibility of buildings,bridges,pavements,tunnels and other infrastructures.Their performance under dynamic load-ing has drawn intense interests of many researchers.

Attempts have been made to predict the impact response of concrete and FRC components by various experimental tests over a wide range of strain rates.As reviewed in [1],before 1982,com-pressive,tensile,and ?exural impact tests had been conducted to investigate the in?uences of ?bres with different shapes and mate-rial properties.By summarising the results up to 1982from many

researchers,the authors concluded that the ?exural strength of smooth steel FRC was slightly more rate sensitive than the plain mortar because of multiple cracking of the matrix.The bond be-tween smooth ?bres and the concrete matrix was not signi?cantly affected by the strain rate.As there was no pullout test result avail-able,the authors predicted the bond strength between deformed ?bres and concrete matrix would increase with strain rate and result in a higher proportion increase in energy absorption with deformed ?bres than smooth ?bres.A similar conclusion was also drawn in [2],in which the behaviour of straight ?bres was found to be insensitive to loading rate.Not until 1991had pullout resistance of deformed ?bres embedded in cement based matrices been stud-ied at high rates of pullout [3].A Charpy type pendulum based im-pact tester with modi?ed specimen support system was used.As a benchmark,static pullout tests were also carried out such that load versus pullout distance curves were obtained to investigate the pullout energy.Three steel ?bre types including hooked,undulated and ?attened shape ?bres were tested and it was reported that changing the loading rate would result in changing the maximum pullout load and pullout energy along with failure modes.Several factors were considered to primarily affect the pullout behaviours,which included matrix strength,material characteristics of ?bre steel and especially ?bre–matrix inter-facial strength which gov-erns adhesive bond strength,frictional resistance to slipping and

0261-3069/$-see front matter ó2012Elsevier Ltd.All rights reserved.https://www.360docs.net/doc/e616652957.html,/10.1016/j.matdes.2012.05.047

?Corresponding author at:School of Civil and Resource Engineering,The

University of Western Australia,Crawley,WA 6009,Australia.Tel.:+61864883974.

E-mail addresses:zxu@https://www.360docs.net/doc/e616652957.html,.au (Z.Xu),hao@https://www.360docs.net/doc/e616652957.html,.au (H.Hao).1

Tel.:+61864881825.

?bre anchorage capability.As an extended research,improvements in the impact resistance of cement matrices reinforced with high volume fractions of carbon,steel,and polypropylene micro?bres were investigated through the same impact instrument in[4].Con-siderable sensitivities were found in peak stress and fracture en-ergy.Also,the FRC composites were observed to be stronger and tougher under impact and this improvement was even more pro-nounced at higher?bre volume dosages.Moreover,single?bre pullout test was conducted to investigate the loading rate effect on single-?bre pullout behaviours[5].A type of proposed high strength twisted?bre was tested together with the hooked-end ?bre.It was revealed that high strength hooked-end?bres did not show rate sensitivity under pullout for the various matrices tested and the pullout energy of twisted?bres increased with ma-trix strength and could be up to?ve times that of hooked?bres.

Available experimental and analytical studies suggested that there was direct relation between pullout and tensile stress–strain response.It was believed that general?bre pullout integrates the contribution of each?bre to the corresponding tensile resistance. On the basis of[5],the tensile behaviour of the hooked and twisted ?bres were further studied by dynamic direct tensile tests in[6]. The test results indicated that the tensile behaviour of matrix with twisted?bres showed rate sensitivity while that using hooked?-bre shows no rate sensitivity,the same as the?nding in?bre pull-out tests conducted in[5].It was concluded that the rate sensitivity of?bre pullout response played a role in the sensitivity of the com-posite to strain rate.In addition,experimental study of concrete reinforced with steel and polyvinyl-alcohol?bres subjected to dy-namic tensile loading were conducted in[7].The tests were carried out by a Modi?ed Hopkinson Pressure Bar apparatus on notched FRC specimens.Two ways of static uniaxial tensile tests and bend-ing tests were also performed for comparison reason.The dynamic stress–COD curves were derived to investigate the rate effect on the post-peak energy.It was found that the steel?bres provided much signi?cant toughness than the PVA?bres in the static tests. Under dynamic loading,enhancements of tensile strengths were found for both FRC types.However,post-peak strength and ductil-ity were found to decrease with increasing strain rates.

As reviewed above,?bre shape has a signi?cant effect on FRC dynamic material properties and hence on its performance in resisting dynamic loads.Recently,the authors proposed a spiral shaped steel?bres[8].Impact tests performed in[8]have demon-strated that FRC specimens reinforced with this new spiral shaped ?bres display the largest ultimate compressive strength,largest post-failure strength and the largest energy-absorption capacity amongst FRC specimens reinforced with seven types of?bres tested in the study.It demonstrated that the rate sensitivity and material properties of FRC under compressive impact loading are dependent on the material types and shapes of?bres.Better anchorage bond and mechanical deformation capability of steel?-bres lead to more signi?cant rate sensitivity for both strength and toughness[8].

To investigate the dynamic tensile properties of FRC specimens reinforced with this new spiral shaped?bres,static and drop weight splitting tests have been conducted.For comparison,spec-imens reinforced with the commonly used hooked-end steel?bres were also prepared and tested.A similar impact system has also been used in[8].Fig.1shows the two types of?bres used in this study.A drop-weight impact system was used to investigate the dynamic tensile strength increase,fracture and post-peak energy absorptions and crack opening displacements and velocities of the plain concrete together with hooked-end and spiral FRC https://www.360docs.net/doc/e616652957.html,parison of the test results demonstrates that the FRC specimens with spiral?bres perform better in terms of ductility, rate sensitivity and crack control capability than those with hooked-end?bres.

2.Experimental programme

2.1.Materials,mixing and specimens

The cement used in the study is ordinary Portland cement (OPC).The concrete mixture and slump tests were conducted according to Australian standard AS1012.3.1-1998[9]to ensure the consistency of concrete for all of concrete and FRC specimens. The maximum coarse aggregate size used in the concrete mixture is10mm.The mix portion detail is given in Table1.

In order to minimise the number of cracks and better control the potential crack locations within the traditional circular speci-men under impact loading(suggested by Australian standard AS 1012.10-2000[10]),the test specimens were made with a notch as shown in Fig.2to better control the crack initiation and propa-gation.It was proven that using this specimen shape,splitting of specimens into two pieces with a reasonably clear splitting surface was achieved,whereas if the circular specimen was used,speci-mens often break into several pieces as shown in Fig.3,which

(a) Hooked-end fibre (b) Spiral fibre

Fig.1.Two employed?bres.

Table1

Mix proportions of concrete.

Component Mass(kg/m3)Quantity(kg)

Water205 2.316

Cement426 4.814

10mm Aggregate444 5.017

7mm Aggregate306 3.458

Minus4aggregate130 1.469

Sand8439.526

Totals235426.6 Z.Xu et al./Materials and Design42(2012)72–8873

makes the reliable extraction of the dynamic tensile properties of the tested specimens dif?cult.The dimension of the test specimen is100mm by100mm cylinder with an18mm triangular notch on This dimension was chosen in order to maintain dimension about three times as large as the?bre length.coarse aggregates,cement,and sand were?rst mixed before add-ing water.Mixing process continued for another3min before add-ing?bres.The?bres were manually dispersed and added to the mixer in order to avoid?bre clump.After being cured in the humid room for28days,the specimens were demolded and then both surfaces were properly levelled,sanded,polished,cleaned and dried to attain smooth surfaces before placing the strain gages.

The mechanical properties of the commonly used hooked-end steel?bre and the new spiral steel?bre suggested in[8]are listed in Table2.

2.2.Experimental test setup

In this study,45specimens were tested including six specimens for static compressive tests,eight specimens for static splitting tests and31specimens for dynamic splitting tests.

2.2.1.Quasi-static compressive and splitting tensile tests

Quasi-static compressive tests were conducted on a Baldwin Hydraulic Testing Machine in the UWA Structures lab with the standard procedures conforming with AS1012.9-1999[11].A Dig-iDaq logging system was used to record the load and displacement versus time.Strain gages were placed at the middle height of the specimen allowing the axial compressive strain histories of the specimens to be recorded[12].In this study,strain gages were placed at the two symmetric positions on each concrete cylinder surface and the strain histories were obtained by averaging the re-sults.The static compressive tests setup is shown in Fig.4a.

Quasi-static splitting tensile tests were also conducted on the Baldwin Hydraulic Testing Machine,as shown in Fig.4b.As shown, a high-strength steel adaptor was used between the platen and specimen notch to ensure that the loading was added properly to the notch and hereby the occurrence of vertical crack from the notch tip to the specimen bottom(Fig.4b).Two80mm strain gages were placed in parallel on the front surface of each tested specimen to measure the tensile strain history up to fracture,as shown in Fig.5a.Furthermore,a clip gage was placed horizontally across the left and right end of the tested specimen to measure the

Notched specimen:18mm triangular

Possible failure mode of circular specimen without notch under

?bres(Radmix).

Length(mm)Diameter(mm)

Fig.4.Quasi-static test setup on the Baldwin Hydraulic Testing Machine:(a)compression tests and(b)splitting tests.

Design42(2012)72–88

measure the https://www.360docs.net/doc/e616652957.html,yout of the two measuring devices is shown in https://www.360docs.net/doc/e616652957.html,parison of the results of the two methods will be dis-cussed in later sections.

2.2.2.Dynamic splitting tensile tests

2.2.2.1.Test layout.The layout of the instrumented drop-weight impact system used in this study is shown in Fig.7.The maximum drop height in this test rig is about 4m,which gives a theoretical drop velocity up to 8.85m/s.The impactor is made of a solid cylin-drical steel head,weighing 12.5kg.A plastic guide tube is used to ensure the impactor to be falling vertically to the target specimen.A number of circular holes were evenly made on the guide in order that the air could be squeezed out of the tube without dwindling the impactor velocity during the impact process.The test specimen was placed right beneath the guide tube with a load cell at the bot-tom of the specimen.The middle line of the impactor,guide tube,steel adapter,specimen and the load cell were aligned in a vertical line.

2.2.2.2.Data acquisition system and measuring devices.Two 80mm strain gages were placed on the front surface of each specimen to measure the strain history,which were in the same positions as those in the static tensile test setup shown in Fig.5a.The strain gages and load cells were connected to a high frequency data acquisition system composed of a fast-response A/D conversion card,a fast-response ampli?er and a graphical programming Lab-

view package.The same acquisition system was also used in the dynamic compressive tests in [8].

As described in the last section,the crack opening displacement was measured by the clip gage and the laser LVDTs in the static splitting test.However,neither of the two methods is directly applicable for the dynamic tests.The result of the clip gage under dynamic testing can be signi?cantly disturbed by the stress wave propagating from the notch tip,whereas the maximum sampling rate of the LVDT has a limitation of 700Hz (1.4ms per sampling point)which is not fast enough for the present impact test of which the loading process occurs within 1ms.The logging system devel-oped in this study has a maximum sampling rate of 2MHz for each channel,which is ideal to measure the deformation and impact loading of test specimens.However,since concrete is a brittle material,strain gages placed on concrete surface can only record strain histories before concrete tensile fracture occurs.In order to record concrete deformation after tensile splitting occurs,in the present study,a 10mm strain gage attached on a thin brass shim strip with a thickness of 0.02mm was made and used to measure the deformation after concrete cracking.This is termed as ‘brass gage’in this paper.Brass is an alloy of copper and zinc which has signi?cant ductility such that the strain gage on the brass can re-cord further deformation after concrete fracture under tensile loading.The crack opening displacement could then been derived by:

d ?

e ál

e1T

(a)Strain gages on the front surface (b) Brass gage on the back surface

Fig.5.Strain gage setup.

(a) LVDT (b) Clip gage

Clip gage

A

B

Fig.6.Two ways of measuring crack opening displacement in static splitting tests.

Z.Xu et al./Materials and Design 42(2012)72–8875

where d denotes crack opening displacement,e denotes the mea-sured strain value from brass gage and l denotes the length of strain grid.

As shown in Fig.5b,the brass strip was glued on the back sur-face at the middle height of the specimen with its middle section unglued so that the strain gage attached to the middle section of the brass strip can bridge across the predictable cracking area.It is of importance to point out that as exposing in the atmo-sphere,the brass shim has oxide ?lm layer covering on its surface which undoubtedly has a negative in?uence on the gluing quality between brass shim and concrete surface.The brass shim can be easily peeled off the specimen surface during the impact hence

the recorded brass strain is much lower than it ought to be.To min-imise this in?uence,in this study,each brass shim strip was metic-ulously and evenly deoxidized,including surface sanding,polishing and cleaning,in order to properly remove the oxide ?lm before being glued to the concrete surface so that the brass strip on each side of the brass gage can be closely clung to the left and right part of the test specimen,respectively.Deformation of the brass gage,which is mainly from the middle part across the concrete ‘cracking zone’,initiates when the concrete fracture occurs.Exam-ples of the measured strain results from the strain gages and the brass gage under the impact loading after deoxidizing are shown in Fig.9.It can be found that the stress wave propagates through

Crane

Guide tube

Trigger

Specimen and strain gage

Loadcell

High speed camera

Drop weight

SIGNAL FROM STRAIN GAGES AND LOAD CELL

Fig.7.Drop-weight High speed camera

1500w halogen

lights

Fig.8.Drop-weight splitting test setup.

76Z.Xu et al./Materials and Design 42(2012)72–88

Fig.10.A typical progressive failure process for plain concrete specimen under2m drop-weight impact at1000frame/s.

Table3

Quasi-static compressive test results.

Type of specimen Average

compressive

strength(MPa)Young’s

modulus

(GPa)

Strain gage Baldwin Hydraulic Testing Machine

Max.strain Toughness

(MJ/m3)

Fracture energy

(MJ/m3)

Post-peak energy

(MJ/m3)

Total toughness

(MJ/m3)

Plain concrete51.0930.70.00280.0973979080.0767458360.0032751280.0800210 Hooked-end FRC49.7633.40.0020830.0624012630.0522586670.0607428650.1130015

Spiral FRC53.334.10.00337170.137383280.0963030390.0687859660.165089005

Z.Xu et al./Materials and Design42(2012)72–8877

maximum strain at peak was measured by the strain gages placed on specimen surface.The average strain histories were also derived from the displacement measurement in the Baldwin Hydraulic Testing Machine.Accordingly,the fracture energy is obtained by both the strain gage results and the derived strain values,and is given in Table3.As can be noticed the fracture energies calculated by the two methods are comparable.

Comparison of the average compressive strengths and the max-imum strain for the three types of specimens is shown in Fig.11.As shown the spiral FRC has a higher strength than the hooked-end FRC and plain concrete.This observation is consistent with the tests results presented in[8].The maximum strain capacity of spir-al FRC is also found to be the largest in the three types of tested specimens,indicating that the spiral FRC has the largest fracture energy under compressive loading.

Fig.12shows the fracture energy,post-peak energy and total toughness of the three types of specimens.As shown,the plain concrete specimen has a total energy of0.08MJ/m3,a large propor-tion of which is fracture energy.Both the FRC specimens,however, have much higher post-peak energy and therefore higher total toughness,which indicates that FRC specimens have better energy absorption capacity than plain concrete specimen under compres-sive loading.It is also apparent in Fig.12that the spiral FRC has higher fracture energy,higher post-peak energy and hence higher total toughness than the hooked-end FRC and plain concrete under static compressive loading.Failure modes of the three types of specimens under static compressive loading are shown in Fig.13 where brittle shear failure occurs in plain concrete specimen, whereas?bres still bridge the cracks well after failure occurs in both of the FRC specimens,indicating better crack controllability and ductility.

3.1.2.Split tensile tests

3.1.2.1.Method of analysis.According to[14,15],the splitting ten-sile strength in a classic splitting test is calculated by:

T?

2P

p LDe2Twhere T is the splitting tensile strength,P is the maximum applied force indicated by the testing machine,L is the specimen length and D is the specimen cross-section diameter,and p LD/2can be consid-ered as an equivalent area for estimating the tensile strength of the specimen.

However,this equation cannot be applied to the present study because of the notch in the current test specimen,which leads to a smaller vertical force P.To determine the tensile strength of the current specimen,a modi?ed relation is derived in the present study as described below.In the present quasi-static splitting ten-sile tests,the splitting tensile strength can be derived by:

t?E ee3Twhere t is the splitting tensile strength,E is the Young’s modulus obtained from the static compressive tests,e is the maximum ten-sile strain measured by the strain gage.

By comparing the measured load and splitting tensile stress in static splitting tests,an‘equivalent area’A0can be derived based on P=tA0and used for splitting tensile strength estimation in both the static and dynamic splitting test.Table4shows the average equivalent areas for plain concrete,hooked-end FRC and spiral FRC specimen,respectively.As shown,the equivalent area some-what varies with various material types which can be due to the

C

R

F

l a r i p S

)c(

C

R

F

d n e

d e k o o

H

)b(

n i a l P

)a(

Fig.13.Failure modes under static compression tests.

78Z.Xu et al./Materials and Design42(2012)72–88

variety of the?bre’s bridging effect to the concrete matrix.Accord-ingly,the splitting strength can be calculated by:

r splitting ?aá

2P

p LDe4T

where r splitting is the splitting tensile strength,P is the maximum applied force indicated by the testing machine,L is the specimen length and D is the specimen cross-section diameter.Eq.(4)indi-cates that with the same tensile strength,the specimen without notch has a larger load capacity at failure than the current test spec-imen.The coef?cient a equals to2.13,2.5and2.67for the plain con-crete,hooked-end FRC and spiral FRC tested in this study, respectively.It should be noted that this is only an approximate relation of the splitting strength of specimens and the vertical force P based on equivalent tensile area.With the notch on the specimen, the splitting force the specimen can take is less than half of that without notch.However,the a coef?cients derived are only applica-ble to the specimen and testing conditions in this study.Changing such conditions,e.g.,use a different notch size,will change these

a coef?cients.

3.1.2.2.Failure modes under static splitting tests.The failure modes of the three types of specimens under static splitting tests are shown in Fig.14,where all specimens failed with one major crack in a direction almost parallel to the loading axis.It is also shown that the major crack split the plain concrete specimen all the way from top to bottom whereas the?bres still hold the two parts of FRC specimen together.Due to the spiral shape and therefore better bonding with concrete matrix,the spiral FRC displays larger elongation,higher residual strength together with higher energy absorption under static splitting tests.This observation will be fur-ther discussed in the next sections.

3.1.2.3.Stress–COD curves and stress–train relations.As mentioned above,two sets of sensors were used to measure the crack opening displacement,i.e.,LVDT and clip https://www.360docs.net/doc/e616652957.html,parisons of the mea-sured COD results are shown in Fig.15.As shown the results from the LVDT and the clip gage for all the specimen types are compara-ble although the result of the clip gage appears slightly larger than the LVDT.The difference was caused by the fact that the?xed LVDT are not always targeting at the same middle points of the specimen lateral surfaces during the loading and deformation process,as points A and B indicated in Fig.5.The targeting points would change when the two parts of specimen start to move apart and ro-tate around the bottom.

Fig.16shows the typical stress–strain relations up to fracture. The corresponding strain in the?gure was measured by the middle strain gage placed on specimen surface.The ultimate strain at frac-ture is summarised in Table5.It can be seen that the ultimate strains at fracture of the two FRC specimens are more than seven times larger than that of the plain concrete specimen,indicating larger deformation before fracture and therefore better ductility. The areas under the stress–strain relations of spiral and hooked-end FRC are very similar indicating similar energy absorption capacity before fracture(de?ned as fracture energy).Fig.17com-pares the typical stress–COD curves of hooked-end and spiral FRC.Since Fig.15a has shown that the plain concrete specimen has negligible COD as compared to the two types of FRC specimens, the stress–COD curve for plain concrete is not included in Fig.17. More clearly,the average residual strengths of two types of FRCs measured at COD of4mm and8mm are plotted in Fig.18.It is shown in the?gures that although the spiral FRC has a slightly lower tensile strength as compared to the hook-end FRC,it has higher ultimate displacement,higher residual strength at the same level of COD and larger area under the stress–strain curve,which implies higher post-peak energy capacity.

3.1.2.

4.Energy absorption capacity.Fig.19a shows the fracture energy,post-peak energy and the total toughness capacity of the three types of specimens,respectively.The post-peak energy is cal-culated by integrating the area under the stress–COD curve shown in Fig.17,whereas the fracture energy is obtained by the area un-der the stress–strain curves illustrated in Fig.16.In order to keep unit consistency,the strain histories in Fig.16are converted to specimen elongation by Eq.(1).

As shown in these?gures,although the hooked-end FRC has higher fracture energy capacity than the spiral FRC,the contribu-tion of fracture energy to the total toughness is trivial.It also dem-onstrates that the total toughness of spiral FRC is almost twice as high as the hooked-end FRC,indicating better ductility of spiral FRC under static splitting tests.This is because the spiral?bre has better bond than the hooked-end?bre at?bre-concrete interfaces.

In general,during?bre pullout process,adhesion and frictional bond are triggered in the?rst place which is followed by activation of the anchorage bond(mechanical component of bond caused by the deformation of the?bre such as the hooks in hooked-end?bre), as shown in Fig.20.Hooked-end?bres have relatively weaker adhesive and frictional bond due to its straight shape and weaker anchorage bond than the spiral?bres since their anchorage is only localised near the hooked-end areas.Moreover,both the frictional and anchorage bond of spiral?bres can be greatly enhanced by the fully deformed spiral shape comparing to the hooked-end?bres

Table4

Average equivalent area of the splitting tensile test.

Plain concrete Hooked-end FRC Spiral FRC

Equivalent area(mm2)7382.36142.015886.06

(a)Plain(b)Hooked end FRC(c)Spiral FRC

Fig.14.Failure modes under static splitting tests.

Z.Xu et al./Materials and Design42(2012)72–8879

(Fig.20).In particular,the frictional bond on the spiral?bre can be potentially strengthened by the radial?bre–matrix extrusion dur-ing?bre pullout.Based on the spiral?bres’three-dimensional characteristics,the spiral?bre also has a torsional resistance dur-ing untwisting procedure.All of the above potentially lead to sig-ni?cant energy dissipation during?bre pullout process.Similar discussions regarding the bond-slip mechanisms of the hooked-end,fully deformed and smooth?bres under static pull-out tests were also made in[16]where the hooked-end and deformed?bre (undulated?bre)were found to have higher resistance and the pull-out energy than the smooth?bres due to the mechanical con-tributions(end hooks and surface deformations,respectively).The study did not compare the hooked-end?bre with the deformed ?bre.

3.2.Dynamic split tensile test results

3.2.1.Time lag effect

In this study,the impact split tensile tests were conducted with two drop heights:2m and3.8m.The impact force was measured

80Z.Xu et al./Materials and Design42(2012)72–88

by a fast response load cell placed under the test specimen and the fracture strain and the COD at the middle height of specimens were measured and derived by the strain gages and the brass gages.As shown in Fig.9,there are time lags between the top strain,middle strain,and load histories,indicating stress wave propagation dur-ing the impact process.The time lag between the top and middle strain is trivial since the two strain gages were placed very close to each other.

In Fig.9a,it can be seen that both the top and middle strain gage readings reach their maximum values of about 7000microstrain and then rocket up to the truncated level immediately,indicating the strain gage has fractured and failed together with concrete.On the other hand,the brass strain reading which bridged across the ‘potential cracking zone’initiates when fracture happens and

Table 5

Quasi-static split tensile test results.Type of specimen Average tensile strength (MPa)Ultimate strain at fracture Residual strength (MPa)Fracture energy (MJ/m 3)

Post-peak energy (MJ/m 3)Total toughness (MJ/m 3)

Plain concrete 4.4579927670.0005830

0.0022800510

0.002280051Hooked-end FRC 6.8716997990.004020.5193199380.022*******.224070990.246515652Spiral FRC

6.306781069

0.00384

1.99381762

0.019840019

0.472220478

0.492060498

e

r b i f l a r i p S )b (e r b i f d n e -d e k o o H )a (Anchorage bond

Adhesive and frictional bond

Matrix

Adhesive and frictional bond

Anchorage bond Matrix

Fig.20.Anchorage bond bond between ?bres and matrix.

Z.Xu et al./Materials and Design 42(2012)72–88

81

ascends all the way to about17,000microstrain and then to the truncated level,this result demonstrates the ductility of the brass material and the effectiveness of the brass gage for measuring the post-peak deformation.

Fig.21shows the relationship between strain rates and the cor-responding measurable maximum strain capacities from both mid-dle strain gages and brass gages.The maximum strain rates of specimens are obtained from the recorded strain histories,which are de?ned as the strain rate value in this study.The same de?ni-tion was also used in[8].It is shown in the?gure that in spite of some variability in test results,the measurable maximum brass strains are about three times larger than those recorded by the middle strain gages,indicating that as a more ductile material,brass gage is able to capture more deformation than the strain gage under the same impact loadings.As shown strains measured by both the normal and brass strain gages illustrate obvious rate sen-sitivity under impact loading.The rate effects on the fracture strain capacities from the both measurements are,as expected,very similar.

https://www.360docs.net/doc/e616652957.html,parison of impact loadings

The split tensile stress histories up to8000l s for different spec-imen types measured in this study are presented in Fig.22.As shown,under the2m and3.8m drop weight impact,the tensile stresses of plain concrete specimens vary from4.5MPa to9MPa with the complete failure occurred at about600l s.Brittle tensile failure due to splitting of the specimen along its centreline was ob-served under the both drop heights as shown in Fig.23a.The hooked-end FRC shows a tensile stress level up to22MPa.The dy-namic stresses?uctuate with time after reaching the peak value, which is accompanied by?bre pullout.These processes would con-tinue until about2600l s when the specimen was completely damaged after all?bres had been pulled out and residual stress values dropped to zero.The corresponding failure modes of hooked-end FRC are shown in Fig.23b.

As to the spiral FRC specimens,the strength level is close to the hooked-end FRC,ranging from12to22MPa.However,the stress histories displayed signi?cant multi-peaks and therefore substan-tial residual strengths after the?rst peak.This is because of the better frictional and anchorage bond between spiral?bres and the concrete matrix.For hooked-end?bres,only the anchorage bond mainly contributes to the tensile resistance[3,5].After anchorage bond is overcome(Fig.20a),i.e.,the hooked ends are straightened or partly straightened,the frictional bond would also

82Z.Xu et al./Materials and Design42(2012)72–88

be overcome before ?bres are fully pulled out,which,nevertheless,has been found only accounts for a minority proportion of the total bond for hooked-end steel ?bres [3].On the other hand,the ?bre pullout process for spiral ?bres is signi?cantly more complex.The spiral ?bre is fully curved along its length in concrete matrix such that the anchorage bond,which acts radially on the ?bre body (Fig.20b),would accompany the ?bre pullout process all the way until the ?bre end is fully pulled out of the matrix.The frictional bond acting tangentially is also signi?cantly enhanced because of the radial bond force from the concrete matrix (Fig.20b).During the pull-out process,the mechanical deformation along the spiral ?bres displaces the ‘concrete tunnel’encasing them.When the concrete is crushed,the splitting load decreases.However,the loads rise up again when the deformation enters other ‘concrete tunnels’.This can be further observed in the stress histories of spir-al FRC in which the ?bre pullout processes lasted for longer than the hooked-end ?bres,indicating that more energy is needed to fully pull out the spiral ?bres.Typical failure modes for spiral FRC under impact tests are shown in Fig.23 c.

2m

4m

(a)Plain concrete

(b)Hooked-end FRC

(c)Spiral FRC

Fig.23.Failure modes under drop weight impact tests.

0.5

1.5

2.5

3.5

4.5

5.5

6.5D I F

Concrete Hooked-end FRC

Spiral FRC

0.5

1.5

2.5

3.5

4.5

5.5

0.0001

0.011

100

10000

0.0001

0.01110010000

D I F

Maximum strain rate Average strain rate

(a)

(b)

Z.Xu et al./Materials and Design 42(2012)72–8883

3.2.3.Rate effect on dynamic tensile strength and strain capacity

The effect of strain rate on the dynamic tensile strength is illus-trated in Fig.24a.The dynamic increase factors(DIF)are obtained by normalising the dynamic tensile strengths by the corresponding static tensile strength.As shown,the three types of specimens are all rate sensitive with respect to the dynamic tensile strength.It also shows that the dynamic increase factors for both the FRC materials are larger than that for plain concrete material,indicat-ing that FRC materials with?bre dosage of1%tested in this study are more rate sensitive than the concrete material without?bres. As also shown in the?gure the DIF for spiral FRC increases faster than the hooked-end FRC and plain concrete material,indicating the?bre shapes have an effect on rate sensitivity.However,it should be noted that the difference in rate sensitivities between the hooked-end FRC and spiral FRC are not as prominent as that be-tween the hooked-end FRC and plain concrete,implying the?bre content is more critical in the tensile strength DIF than the?bre shapes.It should be pointed out that the de?nition of strain rate plays an essential role in the concrete tensile DIF values.The inher-ent time-variable attribute of strain rate for concrete brings dif?-culty and controversy when investigating materials’dynamic properties.In this study,the strain rate is assumed as the maxi-mum strain rate which results in a smaller DIF than some of the experimental results summarised in[17]where the strain rates were conservatively de?ned as the average value.A comparison of the in?uence of strain rate assumption on the DIF is shown in Fig.24b,in which it is shown that by using the average strain rate the DIFs are signi?cantly larger than DIFs calculated by the maxi-mum strain rate values.

Plots of the fracture strain capacities versus strain rate of the three tested materials are compared in Fig.25,in which it is obvi-ous that the maximum measurable strain capacities are also rate sensitive.This observation is consistent with the experimental re-sults in[18].Similar to the trend curves in Fig.24,spiral FRC dis-plays the most signi?cant rate effect on the fracture strain capacity among the three types of specimens.This observation im-plies that the spiral?bres have a better capability of controlling microcracks than the hooked-end?bres so that under the same drop weight impact,i.e.,the same energy input,the spiral FRC de-forms in a more uniform and ductile way than the hooked-end FRC and plain concrete specimen before fracture.

3.2.

https://www.360docs.net/doc/e616652957.html,parison of stress–strain relations,stress–COD relations and rate effect on energy absorption

Some typical stress–strain and stress–COD relations of the investigated three material types are illustrated in Figs.26and 27,respectively,in which the stress histories are derived by Eq.

(4),while the strain histories are obtained through the middle strain gages placed on specimen surfaces.The stress–strain curves are plotted up to the measurable fracture strains from strain gages whereas stress–COD curves are plotted up to the fracture strains from the brass gages,as indicated in Fig.9b.As stated in Section 2.2.2,the crack opening displacement is obtained by Eq.(1).The rate effect on the corresponding fracture energy,post-peak energy and total energy for all the tested specimens are listed in Table6 and plotted in Fig.28.

In general,as clearly shown in Figs.26–28,both FRC specimen types achieve higher fracture energy,post-peak energy and total toughness than the plain concrete material,indicating the FRC materials investigated in this study have better energy absorption capacities than plain concrete under impact loading.Furthermore, it is apparent that the two types of FRC specimens have very sim-ilar level of fracture energy capacity and the corresponding rate sensitivity.The primary difference of the two FRC types is

their 84Z.Xu et al./Materials and Design42(2012)72–88

post-peak energy capacity and therefore the total toughness.As shown in Fig.27,in spite of some variabilities in impact test re-sults,the spiral FRC specimens generally show split-hardening behaviours whereas their counterparts with hooked-end?bres mainly display split-softening behaviours,indicating the higher post-peak ductility of spiral FRC.This is also shown in Fig.28b and c in which both the energy absorption and rate sensitivity of spiral FRC are more signi?cant than the hooked-end FRC material.

The different performances of the two types of FRC specimens are due to the different?bre shapes.As discussed above,the spiral FRC has better frictional bonding and anchorage bonding(Fig.20) so that it has higher energy absorption capacity under splitting tensile tests.On the basis of these reasoning,it is not dif?cult to ex-plain the above observations.For FRC specimens with the two?bre types,the anchorage bond plays an important role in the rate sen-sitivity of energy absorption capacity during the?bre pullout at high loading rate,as also found in[3,5,8].The hooked-end?bre FRC displays less rate sensitivity when pulled out from concrete matrix primarily because the anchorage bond only localised in vicinity of the hooks,whereas the spiral?bre provides a3D anchorage bond distributed along the entire embedded?bre length.Similar results have also been reached in[5]where the high-strength hooked?bres showed no appreciable rate sensitivity in high rate pullout tests.

Pullout tests from other researchers have also revealed that straight?bres shows no appreciable rate sensitivity([2]),indicat-ing the frictional component of bond is not rate sensitive for straight?bres.On the other hand,the frictional bond from the spir-al?bres takes much larger proportions of the total bond because it is increased by the extrusion between the matrix and?bres during the?bre pullout.Since concrete is a rate sensitive material,the?c-tional component of bond for spiral FRC can also be rate sensitive. Moreover,the untwisted moment resistance of the spiral?bre is also rate sensitive since they are fabricated by high strength steel, which itself is a rate sensitive material.Therefore,the spiral?bre has a greater in?uence on the rate sensitivity of energy absorption.

3.3.Image analysis

With the high speed video camera,the crack opening process of specimens can be recorded in detail.In this study,the drop weight split tensile tests were recorded at10,000frames/s.The crack opening processes of plain concrete and the two types of FRC spec-imens are investigated by analysing the images through a Matlab programme using image processing toolbox.The average strain rate estimated by image analysis is comparable to the strain rates obtained from the middle strain gages,as shown in Fig.29.The similar approach for deformation analysis was also employed in [8].This observation demonstrates that image analysis is applica-ble to accurately estimate the maximum strain rate of specimens in dynamic splitting tests,which brings convenience to future study.

Typical plots of crack opening displacements up to2500l s of the tested specimen under2m drop weight impact are shown

in Z.Xu et al./Materials and Design42(2012)72–8885

Table6

Dynamic split test results.

Specimen designation Fracture

strain

Max.brass

strain

Maximum strain

rate(1/s)

Dynamic tensile

strength(MPa)

DIF Fracture energy

(MJ/m3)

Post-peak energy

(MJ/m3)

Total energy

(MJ/m3)

Plain1a5831.6956017112.3412.34 2.80.02980.020.0498 Plain2a3552.21613541.22586.414989.55 2.10.005240.0470.05224 Plain3a6455.310,53237.3928.18 1.80.0140.020.034 Plain5a6097.516,599137.2318.72 4.20.04380.0780.1218 Plain33a5737–a101.2214.52 3.30.0462––Plain7b6593.811176.7114.916.71 3.70.0410.0240.065 Plain8b4968.516335.7106.08413.12 2.90.02640.0770.1034 Plain9b403213848.697.81614.62 3.30.010.0730.083 Plain91b367611,82684.1515.19 3.40.010.0690.079 Plain4a5610642157.1214.92 3.30.01950.0320.0515 Hooked-end1a530710,85398–––––Hooked-end12a44874877.7679.218.97 2.80.020.0450.065 Hooked-end11a5206.617874.59817.92 2.60.080.130.21 Hooked-end2a5211.96596267.528.65 4.20.1350.0590.194 Hooked-end3a5670.8718200.38874.88225.77 3.80.0770.07740.1544 Hooked-end52a407896377023.63 3.40.0710.07320.1442 Hooked-end51a5264670657.315.16 2.20.0730.0670.14 Hooked-end7b32058394.712832.28 4.70.1410.1230.264 Hooked-end8b202218,70960.3724.13 3.50.010.0950.105 Hooked-end11b881217,040107.33439.94 5.80.1170.190.307 Spiral13a6109.310,80278.3221.58 3.40.0940.280.374 Spiral10a9669848289.924.17 3.80.1230.180.303 Spiral11a4486.311917.69776.822.14 3.50.040.0980.138 Spiral3a660716,00067.45516.26 2.60.0480.150.198 Spiral5a4143.46581.377.916.61 2.60.02060.070.0906 Spiral6a822118,06885.27325.95 4.10.07440.190.2644 Spiral4a5315889772.628.19 4.50.050.0250.075 Spiral7b506917,56975.84327.07 4.30.140.270.41 Spiral8b787615,985112.9828.68 4.60.080.1740.254 Spiral4b462310,56677.630.88 4.90.05380.160.2138 Spiral9b4504.7937010033.36 2.80.1120.170.282

a Missed data.

86Z.Xu et al./Materials and Design42(2012)72–88

Fig.30.As shown,both the FRC specimens display slower crack opening rate than the plain concrete,and the spiral FRC has the slowest crack opening rate,indicating the spiral ?bres provided better control over the crack growth than the hooked-end ?bres.The corresponding crack opening velocity,which is taken as the average slope of the crack opening displacement time history is shown in Fig.31.As shown,the crack opening velocity of the spiral FRC specimen is the smallest,while that of the plain concrete is the

fastest.It also shows that,as expected,the crack growth rates are higher under higher drop height because of the higher loading rate.The difference of the crack opening rates between the hooked-end FRC and spiral FRC specimens decreases when the drop height in-creases,which implies the ?bre dosage could have more signi?cant in?uence on the crack opening than the ?bre shape at high loading rates.

4.Conclusion

As an extension of a study presented in [8],this study con-ducted both quasi-static and dynamic splitting tests to investigate dynamic tensile properties of FRC materials.The FRC specimens with the newly proposed spiral shaped ?bres and the conventional hooked-end ?bres,as well as specimens made of plain concrete material were tested.Due to the well-known uncontrollability of impact tests,notch shaped specimens were made in this study to better control the splitting surface of the specimens in drop-weight tests.

According to the test results,FRC specimens with the two ?bre types showed better performance than plain concrete material un-der both static and dynamic splitting tests in terms of tensile strength,fracture energy,post-peak energy,DIF,maximum mea-surable strain capacity,rate sensitivity and crack growing velocity.Although the FRC specimen with hooked-end ?bres,to some ex-tend,showed higher tensile strength and fracture energy under both static and dynamic conditions,it had lower measurable strain capacity,total toughness,crack controllability and therefore lower ductility under the tested strain rate range than the FRC specimens with spiral ?bres.This is because its weak adhesive and frictional bond and limited anchorage bond merely localises near the hooks.The spiral FRC also showed higher rate sensitivity to both strength and energy absorption because of the rate sensitivity of its fric-tional bond,anchorage bond,untwist resistance along with the inherent rate sensitive characteristic of concrete and steel material.As presented in [8]and the present study,the spiral ?bre FRC has comparable compressive strength and tensile strength under both static and dynamic loading as hooked-end FRC.However,it outper-forms the hooked-end FRC in terms of ductility,toughness,crack controllability and more signi?cant rate sensitivity because of its better bond between concrete matrixes owing to its three-dimen-sional twisted ?bres shape.

The test results demonstrated the effectiveness of the dynamic load resistance capacity of the spiral ?bre FRC.Acknowledgements

The authors would like to thank Australian Research Council for the ?nancial support under Grant No.DP1096439,and China Na-tional Nature Science Foundation under Grant No.51078094for conducting the research work.

References

[1]Suaris W,Shah SP.Strain-rate effects in ?ber-reinforced concrete subjected to

impact and impulsive https://www.360docs.net/doc/e616652957.html,posites 1982;13:153–9.

[2]Gokoz UN,Naaman AE.Effect of strain-rate on the pull-out behaviour of ?bres

in mortar.Int J Cem Compos Lightweight Concr 1981;3:187–202.

[3]Banthia N,Trottier J-F.Deformed steel ?ber-cementitious matrix bond under

impact.Cem Concr Res 1991;21:158–68.

[4]Banthia N,Chokri K,Ohama Y,Mindess S.Fiber-reinforced cement based

composites under tensile impact.Adv Cem Based Mater 1994;1:131–41.

[5]Kim DJ,EI-Tawil S,Naaman AE.Loading rate effect on pullout behavior of

deformed steel ?bres.ACI Mater J 2008.

[6]Kim DJ,El-Tawil S,Naaman A.Rate-dependent tensile behavior of high

performance ?ber reinforced cementitious composites.Mater Struct 2009;42:399–414.

[7]Cadoni E,Meda A,Plizzari G.Tensile behaviour of FRC under high strain-rate.

Mater Struct

2009;42:1283–94.

Z.Xu et al./Materials and Design 42(2012)72–8887

[8]Xu Z,Hao H,Li HN.Experimental study of dynamic compressive properties of

?bre reinforced concrete material with different?bres.Mater Des 2012;33:42–55.

[9]AS101231-1998.Methods of testing concrete–determination of properties

related to the consistency of concrete–slump test.Sydney:Australian Standard.

[10]AS101210-2000.Methods of testing concrete–determination of indirect

tensile strength of concrete cylinders(Brasil or splitting test).Sydney: Australian Standard.

[11]Swamy RN,Jojagha AH.Impact resistance of steel?bre reinforced lightweight

concrete.Int J Cem Compos Lightweight Concr1982;4:209–20.

[12]Bischoff PH,Perry https://www.360docs.net/doc/e616652957.html,pressive behaviour of concrete at high strain rates.

Mater Constr1991;24:425–50.[13]Zhai Y,Ma G,Zhao J,Hu https://www.360docs.net/doc/e616652957.html,parison of dynamic capabilities of granite and

concrete under uniaxial impact compressive loading.Yanshilixue Yu Gongcheng Xuebao/Chin J Rock Mech Eng2007;26:762–8.

[14]Timoshenko S,Goodier J.Theory of elasticity.3rd ed.London:McGraw-Hill

Book Company;1970.

[15]Wang ZL,Liu YS,Shen RF.Stress–strain relationship of steel?ber-reinforced

concrete under dynamic compression.Constr Build Mater2008;22:811–9. [16]Naaman AE,Najm H.Bond-slip mechanisms of steel?bers in concrete.ACI

Mater J1991;88:135–45.

[17]Malvar LJ,Ross CA.Review of stain rate effects for concrete in tension.ACI

Mater J1998;95:735–9.

[18]K?rmeling HA,Reinhardt HW.Strain rate effects on steel?bre concrete in

uniaxial tension.Int J Cem Compos Lightweight Concr1987;9:197–204.

88Z.Xu et al./Materials and Design42(2012)72–88