Experimental and numerical studies of longitudinal crackcontrol for pre-stressed concrete sleepers

Experimental and numerical studies of longitudinal crack control

for pre-stressed concrete sleepers

F.Rezaie ?,M.R.Shiri,S.M.Farnam

Department of Civil Engineering,Bu-Ali Sina University,Hamedan,Iran

a r t i c l e i n f o Article history:Received 22March 2011Received in revised form 11July 2012Accepted 23July 2012Available online 31July 2012Keywords:Monobloc pre-stressed concrete sleeper Sleeper modelling Observational Katrak test Excessive cylindrical pressure

a b s t r a c t

In the network of Iranian railways,pre-stressed concrete sleepers are more popular than

the other types due to their high performance.One of the defects observed in these sleep-

ers is longitudinal cracks caused by irregular and excessive pressure before and during the

track operation.These cracks usually initiate from the rawlplug locations and propagate to

the middle or even to the whole lengths of the sleeper.This excessive and additional pres-

sure in rawlplug positions are due to the water freezing,existence of rock ?nes or ?ne

aggregates in the fastening holes,deformation of the rawlplug or a combination this

effects.This paper presents a test methodology named as Katrak test to exhibit failure

planes in non-strengthened and strengthened (transversely reinforced)sleepers.In this

study,the effects of the extra pressure in rawlplug positions are simulated by applying

cylindrical pressure inside the rawlplug holes using a physical and numerical (FEM)mod-

els.The results show that the Katrak test and simulation performed could be an appropri-

ate method for studying sleeper behaviour and formation of longitudinal cracks under

excessive pressure.

ó2012Elsevier Ltd.All rights reserved.1.Introduction

Railway sleepers are one of the most important elements railway track systems.In railway track structures,railway sleep-ers play a major role in distributing train axle loads from the rails to the underlying ballast supporting system [1].Pre-stressed concrete sleepers have been utilised in Iran railway industry for over 40years.They are the beams/ties laid under-neath the rails to support the track [2].Their function is transferring and distributing the transported rail loads to the ballast,transversely secures the rails to maintain the correct gauge-width,resist the cutting and abrading actions of the bearing plates and the ballast material [3].Sleepers also resist the transverse and the longitudinal movement of the rail system.The main components of a rail track system are:wheel,rail,rail pad and fastening,sleeper,ballast bed [4].

Railway sleeper (or railroad tie),which is an important component of railway tracks,is commonly made of the pre-stressed concrete [5].This type of sleepers has become widely accepted for railway sleeper usage especially in high speed lines.Their economic and technical advantages are the results of longer life cycles and lower maintenance costs.With their great weight,concrete sleepers assure optimal position permanence and stability even for high speeds traf?c.In fact,many pre-stressed concrete sleeper technologies have been developed and successfully tested.Among this,monobloc pre-stressed concrete sleepers are the most commonly used ones.Also,twin-bloc sleepers are gaining popularity too,due to their weight advantage.Twin-bloc sleepers are made up of two concrete parts supported by steel reinforcements.However,handling and 1350-6307/$-see front matter ó2012Elsevier Ltd.All rights reserved.https://www.wendangku.net/doc/9e9654766.html,/10.1016/j.engfailanal.2012.07.001

?Corresponding author.

E-mail address:frezaie@basu.ac.ir (F.Rezaie).

22 F.Rezaie et al./Engineering Failure Analysis26(2012)21–30

Fig.1.Geometrical features of B70sleeper used in Iranian railways.

Fig.2.Longitudinal cracks in B70sleepers before and during operations.

placing of twin-bloc sleepers can be dif?cult because of their tendency to be twisted when lifted.‘Ladder sleepers’are also another development[6].

Monobloc pre-stressed sleeper(type B70)is the most widely used railways sleepers in Iran,which is an integrated con-crete block,fabricated using pre-tensioning technology[7].To reduce the tensile forces,internal compressive forces are in-duced through pre-stressing steel.When loads are applied to the member,any tensile forces developed are counterbalanced by the internal compressive forces induced by the pre-stressing steel.By pre-stressing the concrete in this manner,the?nal tensile forces are typically within the tensile strength limits of plain concrete.Therefore,properly designed pre-stressed con-crete members do not develop?exure cracks under service loads.The geometrical features of the type B70monobloc sleeper are illustrated in Fig.1.The main materials of B70sleeper are concrete,wires,rawlplug,and small and big steel plaques.Con-crete is the most important material of sleeper with high class of compressive strength between50MPa and70MPa.The approximate weight of B70sleeper is350kg in which,8longitudinal wires of7mm diameter(ST160)are used to fabricate pre-tensioned member.

The current design approach for pre-stressed concrete sleepers is based on permissible stress principle taking into ac-count only the static and quasi-static loads,which are unrealistic for the actual dynamic loads on railway tracks[8].Cracks in concrete are precluded as a prescribed serviceability of the pre-stressed concrete sleepers[9].As a result,the concrete sleepers would be replaced simply once instead of visible cracks which appeared;in result of excessive track maintenance. It is important to ascertain forces applied to the railway track,for the purpose of understand more the manner in which track components respond to those forces,and to clarify the processes whereby concrete sleepers in particular carry those actions [10].Ultimate limit states concept,then becomes more logical entity for use as the design approach for pre-stressed concrete sleepers;in a similar manner to the Australian Standard AS3600[11].In order to devise a new limit states design concept, comprehensive studies of the loading conditions are required.In Iranian railway system,if there is no problem during the manufacturing or operation for B70concrete sleeper,they can survive for50years.One of the de?ciencies observed before and during track operations is longitudinal cracks seen in the sleeper surface initiating from rawlplug position to the middle and the ends of the sleeper(Fig.2).

The tension clamp in Vossloh fastening system is pushed on the rail foot and tightened with torque of about250Nm. Direct sunlight and inappropriate deposits can cause the rawlplugs to have tilting and twisting,which change the mechan-ical properties such as fragility.This paper aims at investigating the effect of irregular and excessive pressures from inside the rawlplug holes to the concrete around rawlplug in creating longitudinal cracks.This pressure can be as a result of water freezing remaining in the rawlplug,existence of rock?nes or particulate aggregates at the time of close of screw into the

Fig.3.Modelling of rawlplug hole in B70sleeper for investigation the effect of excessive cylindrical pressure.

rawlplug,plastic deformation and twisting of screws or rawlplugs,etc.These effects have been approximated and modelled by cylindrical pressure loading(as radial)in the circular hole of rawlplug positions.Then,control of the longitudinal cracks by Katrak test using different types of transverse reinforcing bars is investigated.

2.Numerical studies under the cylindrical pressure

To analyze the B70pre-stressed concrete sleeper under excessive cylindrical pressure loading of rawlplug hole,as radial pressure to the concrete around rawlplug,?nite element method was used by ANSYS software.A simple model is usually used in sleeper modelling and rawlplug hole,which is the place of sleeper rail connection,is not often modelled[12].In this research,the rawlplug hole locations are entered in the mesh and the effect of increasing cylindrical pressure on the internal wall of the hole on rawlplug position is investigated(Fig.3).Regarding the lack of uniformity for the sleeper cross section in its length that varies along the sleeper(Fig.9),pyramid elements are used.

Modelling is performed with the minimum difference from real geometry.Due to symmetry,only half of the sleeper is modelled by applying boundary conditions compatible with symmetry plane[13].Solid65element and Link8element were used to model concrete and steel,respectively.Solid65element is a cubic3D element with8nodes and3translational free-dom degrees in x,y and z directions,which is able to model nonlinear behaviour,plastic deformations and cracks in addition to revealing cracking paths[14].Modelling of longitudinal wires is performed through Link8elements with mechanical char-acteristics of ST160steel and38.5mm2cross section.Link8is a3D element with2nodes and3translational freedom de-grees in x,y and z directions,which is able to model plastic deformation and pre-stressing.Based on the principals of

Fig.4.Placement of the longitudinal wires[15].

be 26,000to obtain a suitable accuracy and speed.FE model is also made by SAP2000software for validation of global behav-iour like load–de?ection.

The mechanical properties of materials in B70sleeper used in this research are shown in Table 1.The high strength con-crete material is used to cast the pre-stressed concrete sleepers,with average compressive strength at 28days of 55MPa.For pre-stressing wires,bilinear elasto-plastic model is used [16].The yielding stress of 1430MPa,initial elasticity module of 210,000MPa and module of plastic zone of 2000MPa are applied.The pre-stress force in each of the wires is 4.5ton [7].

At the ?rst step,only the pre-stressing forces are applied to B70sleeper.Based on the results of analysis and the contours of stress,signi?cant tensile stresses in the direction perpendicular to the longitudinal axis of sleeper are produced around rawlplug hole.In this case,these tensile stresses do not exceed the tensile strength of concrete.But as mentioned,if the addi-tional effects due to freezing of water in the rawlplug,the presence of rock ?nes or particulate aggregates at the time of

turn

number of elements and its effect on the response of maximum transverse tensile B70sleeper used in this research.

Elastic modulus (MPa)Compressive strength (MPa)3710055210000–210000–Fig.6.Stress contour around rawlplug hole in top surface of B70sleeper.

Fig.7.Displacements in sleeper only under pre-stressing forces.

Fig.8.Maximum transverse tensile stress around rawlplug hole.

of screw into the rawlplug,tilting and twisting of screw or rawlplug,etc.,cause the tensile stress to increase which helps to exceed the limit of tensile strength of the concrete and the longitudinal cracks happen.The value of tensile stress around rawlplug hole reaches3.2MPa under the pre-stressing load which is a remarkable load about80%tensile strength(Fig.6).

The displacements in sleeper only under pre-stressing force are shown in Fig.7.The reduction of dimension causing the pre-stressing effects in longitudinal direction of sleeper reaches0.27mm.In this case,the maximum height displacement in the middle of sleeper reaches0.51mm.

To investigate the effect of excessive pressures,rawlplug hole loading starts from20MPa in terms of cylindrical pressure. Then,the nonlinear analysis is repeated statically and the different values of tensile stresses on the sleeper are recorded in each loading.The value of20MPa is the cylindrical load value,which does not have much in?uence on the sleeper.

After increasing cylindrical pressure,the maximum tensile stress of concrete at the direction perpendicular to the line connecting of two rawlplugs of fastening system is recorded in each loading stage(Fig.8).Finally,these transverse tensile stresses exceed the tensile strength,the concrete cracks at the direction of the line of connecting along two rawlplugs of each fastening system(Fig.2).The length and opening of these cracks increase with the increase of cylindrical pressure.The accu-mulation of these cracks in a region leads to major longitudinal crack and concrete failure(ascending region,Fig.8).The numerical results are in accordance with longitudinal cracks in the?eld,which con?rm the right selection of destructive and cracking parameters of sleeper.

3.Katrak test and manufacturing pre-stressed concrete sleeper strengthened with transverse bars

In this investigation,the test specimens are typical full-scale pre-stressed concrete sleepers commonly used in Iran(type B70).They were provided by an Iranian manufacturer,under a collaborative research project.In this stage,the B70sleepers

were strengthened using additional transverse reinforcing bars (type AIII),12mm diameter in six different meshes for better control of longitudinal cracking (Fig.9).Of course,the diameter smaller than N12was size suitable,but it was out of stock.

Longitudinal bars were used only to ?x the transverse reinforcing bars and to facilitate installation (Fig.10).The mesh of reinforcing bars was connected to upper pre-stressing wires (bottom of mold).

To exhibit fracture planes under excessive expansion within the rawlplug hole conspicuously,the Katrak test was sched-uled.Then the Katrak test with Katrak expanding grout was carried out.The Katrak test is a visual observation experiment.The Katrak non-explosive demolition agent is the predictable safe and silent demolition solution.Katrak is the chemical demolition agents work for projects ranging from building demolition to concrete demolition,from rock wall demolition to rock trenching and many more.The longer Katrak sets the more it will expand.The core temperature,water temperature,or ambient conditions may effect cracking time.The expansive pressure of Katrak develops in a progressive manner and is proportional to the time elapsed since loading.Power increases as time passes.Although fragmentation takes place within 12–24h,the progressive action of Katrak expanding grout continues for 4days in summer and 8days in winter reaching a pressure of over 70MPa (rock and concrete break at between 15and 30MPa).The rate of Katrak and water is 1.33l of water and 5kg Katrak.At the ?rst,the four rawlplugs of each sleeper were pulled out and then certain amount of Katrak expanding grout was used inside of the whole rawlplug holes on non-strengthened and greater amount of Katrak expanding grout on strengthened sleepers.The expansive pressure was not quanti?ed.

The results of observational Katrak test in this study on non-strengthened and strengthened sleepers show that:

(a)The Katrak test will lead to better understand the global behaviour and the fracture modes in strengthened and non-

strengthened sleepers (Fig.11).In future study,it is also important to determine the magnitude of forces applied to the sleepers and to understand more clearly how the sleepers respond to these forces to ef?ciently design the pre-stressed concrete sleepers.

(b)The strengthened sleepers versus non-strengthened sleepers possess better resistance against longitudinal cracking

due to using the greater amount of Katrak expanding grout for major cracking failure.It is found that using transverse reinforcing bars causes the adequate tensile strength of sleeper.The mode of fracture will also change from longitu-dinal cracking to other modes.

(c)Sleeper presents greater resistance against longitudinal cracking using better mesh type,so that the increasing of

transverse reinforcing bars density around rawlplug hole,make the sleeper be strengthened transversely and sustain more inducing pressure.The direction of cracks and cracking planes change (Fig.12

).

Fig.9.Additional transverse reinforcing bars in six different mesh types and cross-section of sleepers.

Fig.10.Test specimens with additional transverse reinforcing bars(type

Fig.11.Katrak test on non-strengthened sleepers.

macro-cracks in strengthened sleepers with transverse reinforcing bars and

4.Numerical modelling of strengthened sleeper

In order to investigate the performance of strengthened sleepers,the maximum transverse tensile stresses are compared in non-strengthened and strengthened sleepers by adding transverse reinforcing bars element to the mesh of FEM model and

Maximum transverse tensile stress,around rawlplug in non-strengthened and strengthened

Maximum transverse displacement,around rawlplug in non-strengthened and strengthened

Fig.15.Tensile stress distribution in transverse reinforcing bars.

repeating the loading stages(Fig.13).The results show that applying pre-stress and cylindrical pressure loading on strength-ened sleeper,the transverse tensile stress is reduced in the upper part of sleeper.The remarkable tensile stress in reinforced bars through increasing cylindrical pressure con?rms that there is a desirable performance in transverse direction of sleep-

F.Rezaie et al./Engineering Failure Analysis26(2012)21–3029

ers.On the contrary,in the lower part of sleepers,the tensile stress appears and starts to increase which causes some cracks when cylindrical pressure increases.As,the strengthening is performed in the upper half and in a layer adjacent to upper surface of sleeper(Fig.9),it is evident that there is the possibility of longitudinal cracking in the lower part of sleeper which is con?rmed by lab results of Katrak test(Fig.12).On the other hand,using reinforced bars improve the cylindrical pressure value required to reach tensile strength of concrete about13%.

The maximum transverse displacement of upper part of sleeper around rawlplug,which is a criterion for investigation of width of longitudinal crack,is compared in non-strengthened and strengthened sleepers(Fig.14).In strengthened sleeper, the maximum transverse displacement is reduced versus the non-strengthened case which shows the reduction of crack width.The maximum transverse displacement in cylindrical pressure of55MPa is reduced to0.0734mm and0.054mm in non-strengthened and strengthened sleepers,respectively,which indicates30%of displacement reduction and reduction of longitudinal crack width of sleeper in strengthened sleeper remarkably and creation of tensile stress in transverse rein-forcing bars.

The distribution of tensile stress in transverse reinforcing bars is shown in Fig.15.The transverse reinforcing bars around of rawlplugs are subject to more stress and play more roles.The tensile stress that appears in the rest is much lower(Fig.15). Therefore,to optimise the geometric mesh of the transverse reinforcing bars in strengthened sleepers,the density around hole would be increased and their density in other areas decreased.

5.Conclusion

The challenge of railway track and sleeper design engineers is to optimise the reinforced and pre-stressed cross-sections to ful?ll the tensile strength requirements.In this research,numerical and experimental(Katrak test)studies of the non-strengthened and strengthened sleepers are performed in order to investigate the effects of excessive pressures within the rawlplug holes.It is found that only pretension forces in pre-stressed concrete sleeper induce remarkable tensile stress around rawlplug hole in transverse direction.The additional effects such as water freezing within rawlplugs,the existence of ?ne rocks during tying the screws and tilting or twisting of screws or rawlplugs,lead to increase the transverse tensile stress and therefore longitudinal cracks incur in the pre-stressed concrete sleepers.These effects are simulated using cylindrical pressure within rawlplug holes.It is found that the maximum tensile stress due to excessive pressure within rawlplug holes is occurred between two rawlplugs of each fastening system.

The present paper proposes the use of Katrak test that presents a new manner for visual observation of fracture planes under excessive pressure in strengthened using different types of transverse reinforcing bars and non-strengthened sleepers. It is found that the strengthened sleepers versus non-strengthened sleepers possess better resistance against longitudinal cracking due to using the greater amount of Katrak expanding grout for major cracking failure.It is also found that using transverse reinforcing bars causes the adequate tensile strength of sleepers and so the longitudinal cracks are controlled. Acknowledgements

The authors are grateful to the Iranian company of producing of pre-stressed concrete sleeper for the?nancial support throughout this study.The authors would like to thank the computing support engineers of Tehran Municipality,in partic-ular S.Razaghi,for their advice regarding ANSYS software.

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__________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ ______________________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ _ __________________ ______________ ______________ ________________ _______________ __________________ ______________ ______________ ________________ _______________ Hello Hello Hello Hello Hello Hi hi hi hi hi Goodbye goodbye goodbye goodbye Bye bye bye bye bye I ’m I ’m I ’m I ’m I ’m I I I I I Am am am am am Fine fine fine fine fine Good good good good good Morning morning morning morning what ’s what ’s what ’s what ’s what ’s What what what what what Is is is is is My my my my my

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