University of Birmingham Effects of vertical and through holes on cyclic behaviour of railway concrete sleepers

Railway sleeper is the very important element in rail track structure. Nowadays, prestressed concrete is the most common type used in railway sleeper because of its good structural performance. The main duties of sleeper are to distribute the wheel load to the formation and keep the rail gauge. Most of current design codes for sleepers design techniques rely on allowable stresses and material strength. However, railway sleepers also experience high frequent cyclic load due to wheel-rail interaction. The cracking could occur in prestressed concrete sleepers due to cyclic behaviour and accumulation of fatigue could ﬁnally result in prestressed concrete sleepers’ failure. In addition, the concrete sleepers are often modiﬁed on construction sites to ﬁt in other systems such as cables, signalling gears, drainage pipes, etc. This paper presents the results of the extensive theoretical and analytical investigation aimed at predicted cyclic behaviour on prestressed concrete sleepers with holes. It highlights the eﬀects on cyclic behaviours of railway concrete sleepers. The insight of this paper can be used to evaluate the service performance and predict the cyclic behaviours of the concrete sleeper, as well providing design ﬂexibility and broadening the design principle. The outcome of this study may also improve the rail track maintenance and inspection criteria, in order to establish an appropriate track condition monitoring network in practice. This paper investigates eﬀects of vertical and through holes on cyclic behaviour of prestressed concrete sleeper. The CEB-ﬁp model and EN 1992-2 have been used as theoretical method to calculate the maximum applied number of cycles for constant stress. For concrete sleepers, the cracks are commonly considered as a failure criterion. The crack could occur due to excessive ﬂexural, shear or bond stress. However, the progressive of crack depends on the cyclic behaviour. The cyclic damage of prestressed concrete sleepers is mainly due to the cumulative accumulation caused by wheel-rail interaction. In order to evaluate the fatigue life of prestressed concrete, the dynamic/impact load and bending moment needs to be analysed. The fatigue characteristics of prestressed concrete sleepers including S-N curve should be studied. The material properties are also very important to inﬂuence cyclic behaviour of prestressed concrete sleeper. According to material characteristics and the load moments, the reasonable cyclic behaviour criteria are developed. The concrete is not a ho-mogenous material which means the material performance is very hard to deﬁne. Therefore, to obtain more accurate fatigue life prediction, the great deal of theoretical analysis, lab tests and ﬁeld information are still required. This paper reviewed the previous research on the main characteristics of fatigue failure and fatigue load of prestressed concrete, in the presence of the fatigue resistance of prestressed concrete. This article also presents a convenient fatigue life assessment method for prestressed concrete sleepers. The outcome of this study can be used to evaluate the service performance and predict the fatigue life of concrete sleepers, as well as to help to engineer and improve concrete sleeper design and maintenance.


Introduction
Nowadays railway transportation has been becoming the most popular transportation with its safe ride, high carry-capacity and high speed. Conventional track consists of rail, sleeper, rail pad, fastening system, ballast, sub-ballast, formation etc. railway sleeper is the very important component in track structure. The most important functions of railway sleeper are: a) Transfer and distribute vertical loads from superstructure to foundation, b) And restrain lateral, longitudinal and vertical movement of rails. The sleepers are usually manufactured by timber, steel, concrete, and any other engineered materials. Prestressed concrete are the most popular type used in sleeper around world [ -]. In many countries, the permissible stress method is used in concrete sleeper design [ , ]. The permissible stress of materials and load factor are used to consider dynamic effects. However, the permissible stress method can't take into account dynamic load accurately. Much research has been developed limit state method for concrete sleeper design [ -]. Fatigue limit state is the time-dependent limit state which the damage of concrete sleepers will be accumulated over years when it reached to failure.
Fatigue first found in steel construction was in . The concrete fatigue research started at the turn of century [ ]. Fatigue failure can be explained as the failure which occurs below the stress limit under cyclic load. The fatigue can be determined by: the magnitude of the stress range, the type and quality of the structure and the number of cycles. Cracking is considered ordinarily as the failure criterion to define fatigue state for concrete sleeper design. This paper reviews the main characteristics of cyclic behaviour and fatigue load of prestressed concrete sleepers. It also investigates the fatigue resistance. The outcome of this paper will improve prestressed concrete sleeper design and rail track maintenance system.

Fatigue load assessment
Cracks occur in concrete sleepers normally could cause by excessive flexural, shear or bond stresses [ ]. However, many cracks depend on fatigue load and the material properties. The fatigue load in concrete sleepers is from the wheel-rail interaction. It can be divided into two parts: dynamic load and impact load. Dynamic load is normal wheel-rail interaction. Impact load is wheel-rail interaction with irregular wheel or rail.

. Dynamic Load
Dynamic load is usually related to the train speed and it is used for permissible stress design. The dynamic wheel load can be expressed by: Where P d is the dynamic wheel load, ∅ is the dynamic wheel load factor (∅ > ), P s is the static wheel load. The dynamic factor ∅ depends on train speed and it can be calculated: Where V is train speed.

. Impact Load
The impact factor (k i ) can be determined by: The bending moments of railway sleepers are caused by dynamic load on the railseat (railseat load) which affected by wheel load, impact factor and distribution factor. Railseat load can be determined by: Where P is the static load, DF is distribution factor, IF is impact factor

Material properties . Concrete
Concete fatigue is a progressive process that microcracks occur intially then the crack will become larger forward to the failure point. The mechanical properties of concrete will change under repeated cyclic loading, such as permanently increasing strain on the concrete, causing the stiffness to decrease. Cyclic loading may also cause a concentration of stress at the prestressed wires' surface, which can lead to sudden  The characteristic fatigue strength function for prestressed steel is shown in Figure . The parameters of prestressed steel are shown in Table . The failure cycles of the prestressing steel under a constant amplitude cyclic load can be given by: Where ∆σ is the stress range in the prestressing steel and ∆σ N* is the stress range at N* cycles which shown in

Section and material properties
The detail of railway sleeper is shown at Figure and Table . a) b) c)

Results
To investigate effects of vertical and through holes on cyclic behaviour of railway concrete sleepers, cases have been analysed including vertical holes with mm, mm, mm diameter and horizontal holes with mm, mm, mm, mm, mm diameters. The corresponding S-N curve is used to determine the maximum applied number of cycles for single stress amplitude as theoretical method (Figure ). According to formula ( ) and ( ), the full cross-section failure cycles of the prestressing steel under constant amplitude cyclic load is . Figure shows the effects of vertical and through holes on cyclic behaviour of railway prestressed concrete sleepers. From the graph, the vertical holes have significant influence on cyclic behaviour which mm dia with . % change, mm dia with . % change and mm dia with . % change. The horizontal holes have less influence than vertical holes which they are mm dia with . % change, mm dia with . % change, mm dia with . % change, mm dia with . % change and mm dia with . % change. In theoretical calculation, both of vertical or horizontal holes impact concrete stress at bottom and first cracking moment, therefore the tension stress range can be influenced. In the other hand, holes in railway sleepers change the cross-section area which results in loss of prestress due to time-dependent behaviour like creep and shrinkage increase. It also influences the performance of cyclic behaviour of railway prestressed concrete sleepers [ ].

Conclusion
This paper investigates effects of vertical and through holes on cyclic behaviour of prestressed concrete sleeper. The CEB-fip model and EN -have been used as theoretical method to calculate the maximum applied number of cycles for constant stress. For concrete sleepers, the cracks are commonly considered as a failure criterion. The crack could occur due to excessive flexural, shear or bond stress. However, the progressive of crack depends on the cyclic behaviour. The cyclic damage of prestressed concrete sleepers is mainly due to the cumulative accumulation caused by wheel-rail interaction. In order to evaluate the fatigue life of prestressed concrete, the dynamic/impact load and bending moment needs to be analysed. The fatigue characteristics of prestressed concrete sleepers including S-N curve should be studied. The material properties are also very important to influence cyclic behaviour of prestressed concrete sleeper. According to material characteristics and the load moments, the reasonable cyclic behaviour criteria are developed. The concrete is not a homogenous material which means the material performance is very hard to define. Therefore, to obtain more accurate fatigue life prediction, the great deal of theoretical analysis, lab tests and field information are still required. This paper reviewed the previous research on the main characteristics of fatigue failure and fatigue load of prestressed concrete, in the presence of the fatigue resistance of prestressed concrete. This article also presents a convenient fatigue life assessment method for prestressed concrete sleepers. The outcome of this study can be used to evaluate the service performance and predict the fatigue life of concrete sleepers, as well as to help to engineer and improve concrete sleeper design and maintenance.
[ ] Kaewunruen, S.: Experimental and numerical studies for evaluating dynamic behaviour of prestressed concrete sleepers subject to severe impact loading, .
[ ] EN -. Eurocode -Design of concrete structures. Part : Concrete bridges -Design and detailing rules Brussels European Commitiee for Standardization, .