fastening of bridges

8/9/2019 Fastening of Bridges

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Investigation of fastening failure in tram track structureon a bridge over the Vistula River

JanuszHOOWATYAssistant Professor

West Pomeranian Universityof Technology in Szczecin,Poland

[email protected]

Janusz Hoowaty, born 1956,graduated in civil engineering fromthe Technical University of Szczecin,

Poland (now WPUT). PhD fromWrocaw University of Technology,Poland in 1995. His main area ofresearch is the testing and design of

bridges.

Summary

The paper presents an initial investigation undertaken when partial disintegration of rail supporting

material occurred on a bridge with a total length of over 500 m. Due to traffic problems, the bridgehad been designated for public transport and the emergency services only. The traditional tramwaytrack structure was replaced with an embedded rail structure in 2009, which allowed two furtherlanes for buses and other public transport services to be added. Modified polyurethane resin wasused as rail fastening material. Soon after completion, problems with the fastening material integrityappeared, including delamination, deformation and cracking in the polyurethane resins.

This paper describes the site investigation and laboratory tests undertaken to assess the basicparameters of the fastening material. The composition of the material was assessed using DMAtests. The fastening material was found to have lost its adhesion and integrity due to design faultsand errors in the execution of the works. The investigations were able to explain the causes of thetramway track structure degradation and to give recommendations for repairs.

Keywords:urban bridge; assessment; track structure; railway systems; track failure.

1. Introduction

For cities to develop, efficient public transport is a necessity. To avoid delays, low speed and poorreliability in mixed traffic, public transport requires priority over private vehicles on streets and alsoon bridges. Modern tramway track technology allows tram lanes to be shared by buses and other

passenger services [2, 4, 5]. Embedded rail systems on streets also allows the use of tram lanes bypassenger cars to increase street capacities. Plastics with good adhesive properties are used to fastenthe grooved rails into the road surfaces without the need for anchors. However, the properties ofsuch plastics are highly vulnerable to site and weather conditions during construction. On bridge

decks other problems arise, and the track structures usual for roads need careful adaptation, takinginto account the typically different bases and drainage systems of bridge decks.The paper presents an investigation undertaken when partial disintegration of rail supportingmaterial occurred on a bridge with a total length of 527 m. The bridge is located over the VistulaRiver in Warsaw. Due to traffic problems in the city, the bridge had been designated for publictransport and the emergency services only. The traditional tramway track structure was replacedwith an embedded rail structure in 2009, which allowed the addition of two lanes for buses andother public transport services. The regular track system was adjusted in the existing bridge deck.Modified polyurethane resin was used as rail fastening material. Soon after completion, problemswith the fastening material integrity appeared, including cracking, delamination and deformation inthe polyurethane resins. The damage quickly spread over the whole length of the bridge, with thedisintegration of the fastening material worsened by an insufficient drainage system. On theapproaching streets the same system was used, but in this case the condition of the embedded railstructures remains good and no damage has been noted.The paper presents the damage to the track structure and the results of the investigation undertaken


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by the author in 2012. Samples of fastening material were taken from the track structure on thebridge. The replacement track structure design and the construction photo database were studied.Laboratory tests allowed the existing parameters of the material after three years in service to bechecked. Dynamic-Mechanical-Thermal Analysis (DMA) allowed the composition of the materialto be assessed [1]. The basic mechanical properties of the material were evaluated, which enabledits stiffness to be calculated. The fastening material was assessed to have lost its adhesion and

integrity. Following studies of the design and archive photos, design faults and errors in theexecution of the works were discovered. The site investigation and laboratory test results are

presented in the paper. Investigations were undertaken to explain the causes of the track structuredegradation and to give recommendations for repairs to the track because the repairs which had

been carried out had also failed.

2. The bridge and tramway track structures

The bridge was constructed in 1864 as the first permanent steel bridge over the Vistula river inWarsaw. It also had a horse-powered tramway line. After its steel spans were destroyed at the end ofthe Second World War, it received a new steel structure in 1949 and since then has had sixcontinuous plate-girder spans. During reconstruction, the bridge piers which had surviveddemolition were used. The new superstructure has two main girders of variable depth and steeldecking with through plates filled with concrete. The main girders are a riveted structure and thedecking is of rolled or welded sections with rivet connections to the main girders. The total lengthof the bridge is 527 m. A general view of the bridge is shown in Figure 1. The width of carriagewayis 10,5 m, including four running lanes with a double track tramway line. The sidewalks are of 3,5m width each. Under the carriageway there is a flooring of stingers and transverse beams. Thesidewalks are on welded steel cantilevers. The steel decking is not composite with the main girders.

With the reconstruction of the piers andassembly of the new steel spans in 1949,the bridge became part of the east-west(W-Z) route across the Vistula River

between the Praga district on the eastbank of the river and the city centre, and

the newly-constructed residential districts.Over the following decades, the steelworkwas repainted on a regular basis and

pavement maintenance repairs werecarried out; however, the volume oftraffic, both vehicular and rail, wasgrowing steadily. From 1992-1993, the

bridge underwent major repairs. Thetramway track structure was constructed,as was usual for that period, with groovedrails and precast concrete slabs. In 2007,

a tram lane separation from vehicle traffic was introduced on the bridge to avoid delays to the trams.

Buses and emergency vehicles have shared the tramway lanes since then. The development of bothmotor vehicle and tram transport in the city resulted in a conflict of these means of transport overthe bridge. However, two new road bridges were later constructed over the Vistula River and the

bridge was then designated for bus and tram traffic only.In 2009, a refurbishment project for the W-Z route was conducted, which included replacement ofthe tramway track structure on the bridge. A slab track structure with embedded groove rails wasintroduced, and asphalt pavement laid on the tram lanes. A cross section of the tramway tracks onthe bridge is shown in Figure 2. The contract was a design & build project. Because of restrictedconstruction time, the track works went ahead of the design. The introduction of the slab trackstructure with asphalt pavement allowed a reduction in vibration and noise as well as an increase inspeed and axle load of traffic. Replacement of the track structure on the bridge followed theadministrative procedure for repair works. The design for rebuilding the track structure was drawnup with limited changes to the vertical layout of the track structure. The design focused oncomplying with the required load resistance and asphalt pavement along with future lowmaintenance costs. This all resulted in an individual solution for the structure.

Fig. 1: General view of the bridge


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Fig. 2: Cross sections of the tramway track structure on the bridge:before (left) and afterreplacement (right)

The design for rebuilding the tram track structure on the bridge adapted a typical solution forrailway track structures [2, 3]. A continuously embedded rail system with rails poured in-situ wasdeveloped and adjusted to the existing layout of the bridge deck as well as to contractual

requirements. As a main requirement, the wearing course of the tram lanes was designed as agussasphalt layer of nominal depth 45 mm (Fig. 2). The axial spacing of the tram tracks wasenlarged to 3,5 m, which enables two-directional bus traffic on the tramway lanes. This adaptedsystem is known as an Embedded Rail Structure (ERS), and involves continuous support for therails by composite fastening material. A cross-sectional scheme for the design fastening system isshown in Figure 3. The basic elements are:

1.

the channel in the reinforced concrete slab2.

levelling shim under the rail3.

continuous resilient base strip glued to the rail foot4.

grooved rail, type 60R25.

polyurethane filler blocks and wedges6.

poured elastic fastening material

In this system, the usual steel anchors or clamps are not used at all. The fixation of the rail isrealized by the bond (adhesive contact) of the fastening (embedded) material with the rail and theconcrete channel surfaces. The fastening material is a polymer composite made of rubber granulate,with polyurethane resin as a bonding agent. Recycled rubber material was used. To reduce thevolume of fastening material, filler blocks were designed. The filler blocks and fastening materialare almost waterproof with low water absorption. The geometric layout of the rails was to beregulated by levering shims and wedges. The main element of the poured embedded track system isa liquid poured polymer-based composite material in the channels of the RC slab, that polymerasesin situ. The installation of the track structure may be influenced by environmental (low or hightemperature, moisture) and construction (dirt, rust) conditions.

A clean and dry surface is a necessary conditionfor adhesion but it is not a sufficient condition

for bond durability. Polyurethane resin is astructural adhesive that forms chemical bonds

between the adherent surface atoms and thecompounds constituting the chemical link.Cleaning and treatment of the surfaces isimportant in order to remove contaminants.The design for the tram track structure evolvedover the construction period. During the initialstage, three design concepts with varying levelsof noise mitigation were drawn up to evaluatenoise emissions. During the construction phase,these were evaluated by the existing site layout

and eventually the solution presented inFigure 3 was the only one that was possible torealise.

Fig. 3: Cross section of the adapted continuously

embedded rail system


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3. Execution of track works

The track works were conducted from March to August 2009. Demolition of the existing precastslab track structure commenced immediately after the contract was signed. The construction phaseswere checked according to the contractors photo database. After levelling the existing reinforcedconcrete deck slab, the new bridge waterproofing system was applied under the tramway track

structure. Then the reinforced concrete slab was cast in-situ with channels for grooved rails. Themain elements of the construction technology for the track works were:

installation and levelling of the tram rails in channels between sections of the RC slab installation of filler blocks and horizontal alignment of the rails pouring in the lower part of the fastening material laying of the asphalt wearing course (gussasphalt) cutting of strips in gussasphalt over the channels near the rail heads (Fig. 4) pouring in the upper part of the fastening material near the rail heads

Fig. 4: General view of track works on the bridge: rails ready for installation (left) and cutting ofstrips in asphalt for pouring upper part of fastening material near rail heads (right)

The track system developed in these works hassome flexibility in the individual components;however, the existing layout of the concrete

bridge deck was unknown at thecommencement of the demolition works.A deck was discovered and levelled withrepair mortar and new waterproofing for twocomponents was added. The waterproofingthickness is 16 mm. On bitumen protective

boards strip drainage units were installed, butoutlet tubes were not installed. After theconcrete works (Fig. 4), rail levelling wasnecessary at a range of 0 mm up to 120 mm(levelling shims). The asphalt pavement waslaid down in sections between the rails andwas then cut to make sharp edges for the

pouring of the upper part of the fasteningmaterial. As shown in Figure 4, there was aconsiderable amount of dust and debris as the

asphalt was cut. The finished pavement for mixed traffic, both trams and buses, is shown in

Figure 5. The track works were completed in August in good weather conditions. Furthermore, atthat time, the final version of the tram track structure design was brought to completion.

Fig. 5: View of the pavement just after completion


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4. Damage to track structures

Shortly after completion of the work the first damage to the embedding material appeared, as thepoured upper part of the fastening material detached from the surfaces of the adherents. The bonddurability appeared to be very low and resulted in the splitting and loosening of the upper fasteningelements. After losing its bond, the fastening material deformed, twisted and settled down. The

damage quickly increased to cover the total length of the track structure on the bridge. Cracks andholes also appeared in the asphalt pavement.The delaminated part of the fastening material was easy to damage. The parts which had come offwere replaced with asphalt mass but they also quickly lost durability and became damaged.The degradation of the fastening material was assessed to be the result of a loss in bonding betweenthe parts of the track structure. The mechanism for the fastening material failure is shown inFigure 6. The bonding failure made cracks and crevices in the longitudinal joints between thefastening material and steel or concrete surfaces which allowed rainwater into the track structure,accelerating the damage.

Fig. 6: Fastening material failure: cracks and crevices in longitudinal joints (left) and deformationand twisting (right)

5. Investigation process

Following an initial field visit to the bridge and an official appointment, an initial investigation wasundertaken by the author, during which the entire track bridge structure was photographed. The firstvisit was on a rainy day (Fig. 6), so the insufficient subsurfacing drainage system could be seenimmediately. Contaminated rainwater was filling the voids left following damage to the embeddingmaterial.A total of fifteen samples of loose embedding material were taken from the track structure (Fig. 7).They were taken on a dry night, but the samples were wet or damp. The sample surfaces werelocally covered with dirt or rust. Clean sections were only received as cuttings. The structure of thefastening material was assessed at cutting to be homogenous, without cracks or splitting. The

samples were taken to assess the composition and mechanical properties of the fastening material.


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Fig. 7: View of the fastening material: after taking a sample (left) and dirty surface of a sample(right)

According to requirements for a failure investigation, the basic parameters for the polymercomposite used as the fastening material should be assessed. The material was tested in tension,compression and tear. Hardness was also measured. The schemes for specimens used in the tests areshown in Figure 8. Additionally, the composition of the fastening material was verified byDynamic-Mechanical-Thermal Analysis (DMA) at temperature range from -120C to +100C. The

primary focus was on static tensile tests. Five samples were chosen at random; from each sample,five standard specimens were prepared for tensile tests.

Fig. 8: Schemes of specimens used in compression, tension and tear tests

6.

Test results

The samples of fastening material are of dark grey colour with black inclusions (Fig. 7). Thematerial is a composite, of polyurethane and granulated rubber of different granulation. On cuttingthe surfaces of the samples, granulation grains are clearly visible. No other inclusion was found.After the tensile tests, the specimen surfaces are uneven at the break and the location of thegranulated rubber grains is non-homogeneous. Some air-holes are also visible.

The DMA tests were carried out on two specimens taken from one sample but from differentlocations. The results for the two specimens are different (Fig. 9). For one sample, two amorphous

phases were discovered because two glass transition temperatures were measured (-74,0C and -31,2C). For the second specimen, the one glass transition temperature was -30,5C. DMA testsconfirmed that the continuous matrix of the fastening material is a polyurethane resin. The secondglass transition temperature indicates the second matrix presence and its lack in the secondspecimen shows the non-homogeneity of the composite material.


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Fig. 9: Change in properties at glass transition temperature for one sample: two temperatures inone specimen (left) and one temperature in the second specimen (right)

Determination of tensile properties was carried out according to EN ISO 527 on so-called smallpaddle specimens with an overall length of 150 mm. The results are shown in Figure 10. Theaverage values of the tensile properties are: tensile strength 0,92 MPa, tensile modulus 5,34 MPa

and elongation at break 57,9%.Determination of the compressive

properties was carried out according to ENISO 604 on four cube specimens withdimensions of 505025 mm. The curvesmeasured were almost linear. The averagecompressive modulus is 3,94 MPa.Determination of tear strength was carriedout according to ISO 34 on five rectangularspecimens with dimensions of 10015 mm.The average tear strength is 2,82 N/mm.Determination of hardness was carried out

according to ISO 868 on 20 points onfastening samples. The average hardness is51 Sh (A).The test parameters were evaluated for theactual condition of the fastening material.A small decrease in tensile properties wasfound compared with the test resultsconducted at the time of construction(3 years ago).The composite fastening materialconsisting of a polyurethane resin andgranulated rubber was developed in the

tram track structure on the bridge over theVistula River. Under test conditions, itsmaterial properties are quite good but

partial damage has occurred in the upperparts of the track structure. The elastomericpolyurethanes bind well to both the cleaned

rail surfaces and the concrete trough surfaces. They are expensive materials and also difficult for in-track construction; their handling and application should therefore be undertaken with care byqualified workers. Design and construction errors led to the premature failure of the fasteningmaterial.

7. Conclusions

The usual tram slab track structure, using precast slabs for pavement, has been standard for manyyears. Development of a non-ballasted rail system on railway lines has allowed their adoption in

Fig. 10: Stains stress curves for fastening material


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tramway systems. The continuously embedded rails are optimal systems for urban areas as theyreduce noise and maintenance costs. However, to achieve an optimal tram rail system for shared usewith street vehicles, the road surface needs to be taken into account. Using such systems on bridgesrequires the additional precise detailing of the bridge drainage system, which should be combinedwith tramway track drainage.

A modernisation programme for the urban tram network was undertaken several years ago in manyPolish cities. In many projects, the embedded rail system has been used successfully to reconstructtramways and has increased the capacities of tram lanes for sharing with buses. This paper has

presented part of the project for improving the tram network in Warsaw, which involved thereplacement of the tram track structure on a bridge over the Vistula river.

Because of degradation in the existing tram track structure on the bridge, the track structure wasreplaced by a new embedded rail system with an asphalt wearing course for shared used with buses.Shortly after going into service, a failure in the fastening material occurred. As the damage quicklyspread over the bridge deck and the repairs undertaken proved unsuccessful, an investigation intothe failure was launched. A common source of damage to the fastening material may have beena loss of bonding to the surrounding materials and elements. The mechanical properties andcomposition of the fastening material were assessed, along with the design and the construction

technology. This investigation highlighted faults both with the design and the execution of the trackworks that prevented the track system from functioning on the bridge. The failure of the fasteningmaterial was most likely caused by the combined effects of poor design detailing, poorworkmanship and neglecting the requirements for plastic composite materials poured in-situ. Theintroduction of an asphalt wearing course onto an embedded rail system was not sufficientlydetailed and considered in the track works adopted.

References

[1] HOLLOWAY L.C., Advanced fibre polymer composite materials and their properties forbridge engineering, in: ICE manual of bridge engineering, Thomas Telford, London 2008,pp. 485-501.

[2] HOOWATY J., New tramway track structure on steel deck of The Long Bridge in

Szczecin (in Polish), in: VI Seminar on Contemporary Methods for Strengthening andRebuilding Bridges,Pozna, 1996, pp. 61-65.[3] HOOWATY J., Removing Weak Spots from Polish Railways. Structural Engineering

International, SEI Volume 23, Number 1, February 2013, pp. 85-88.[4]

QCITY. Quiet city transport, Report on mitigation measures for wheel/rail noise, 2006,p. 137.

[5]

TCRP Report 155, Track design handbook for light rail, 2ndedition, Transportation ResearchBoard, Washington, D.C. 2012, p. 695.

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