Division of Structural and Fire Engineering – Structural Engineering Department of Civil, Environmental and Natural Resources Engineering

Luleå University of Technology SE-971 87 Luleå

Increased Axle Loads on Railway Bridges

Final report

Jonny Nilimaa1 and Thomas Blanksvärd1,2 1Luleå University of Technology

2Skanska Sverige AB

Preface This document is the final report for the research project denoted “Increased Axle Loads on Railway Bridges”. The project started in 2012 and has disseminated results in several scientific reports, journal papers and conference proceedings. The project was carried out in a collaboration, with partners from:

• Swedish Universities of the Built Environment (SBU) o LTU o Chalmers o KTH o LTH

• LKAB • Trafikverket • MAINLINE (European Commission financed project) • Skanska • NCC • SBUF

Project financing The project received financing by:

• Hjalmar Lundbohm Research Center (HLRC) • LKAB • Trafikverket • The European Commission • The construction industry's organisation for research and development (SBUF) • LTU

1. Project summary The main objective of the project was to investigate the possibilities and obstacles of increased traffic loads and -intensities on the existing railway infrastructure. Two types of bridge structures were investigated: (1) Concrete bridges and (2) Steel bridges. The scientific approach was to conduct full-scale tests to calibrate different (1) Assessment tools and procedures in form of digital finite element models, and (2) strengthening methods. 1.1 Concrete bridges Gruvvägsbron One of the concrete bridges investigated within this project was a post-tensioned bridge in Kiruna, known as Gruvvägsbron (Figure 1). Full-scale tests for comparative studies were carried out to demonstrate and evaluate an effective structural assessment procedure for prestressed concrete bridges. Gruvvägsbron was due for demolition as part of a city transformation project, necessitated by large ground deformations caused by the large excavation mine situated directly underneath the bridge. Thus, it was available for destructive experimental investigation within this project. The bridge, constructed 1959, had five continuous spans over 121.5 m and consisted of three parallel girders with a connecting slab at the top. Both the girders and slab were tested to failure to investigate their structural behaviour and load-carrying capacity. Non-destructive and destructive tests were also applied to determine the residual prestress forces in the bridge girders and investigate the in situ applicability of methods developed for this purpose. In situ failure tests of bridges are rarely used for development and examination of assessment methods, although such tests are essential to ensure the methods’ validity. This is not surprising because full-scale tests are challenging and require substantial amounts of resources. There are organisational and safety issues to address and there can be difficulties in terms of loading, measurements and sometimes limited accessibility to the structural elements that should be investigated. Moreover, such tests are expensive and may be constrained by various factors, for instance requirements to complete assessments within a tightly limited time frame. In this case, to investigate the girders’ shear capacity, the tests mainly focused on span 2 (see Figure 4.2). This was partly to reduce risks of interfering with traffic, and meet a stipulation of the Swedish Transport Administration that traffic had to be allowed to pass under span 5 during the tests. In addition, accessibility under span 2 was good.

Figure 1. Gruvvägsbron.

Levels of residual prestress forces are key parameters when assessing the structural behaviour of existing prestressed concrete bridges. However, these parameters are often unknown and not easy to determine. To explore them, two existing non-destructive and destructive approaches were further developed for practical application and demonstrated. Due to the pursuit of practical applications for existing bridges, the focus was on a non-destructive methodology, combining experimental data and finite element modelling to obtain the residual prestress forces. Assuming that the initial prestress force corresponded to 85% of the characteristic 0.2% proof strength of the reinforcing steel, estimated losses in investigated sections ranged between 5-70%. However, determined residual prestress forces were generally higher than theoretically based estimates accounting for friction and time-dependent losses in the prestressing system. The initial assessment of Gruvvägsbron, based on assumed material and structural properties indicated load-carrying capacities ranging between 25-78% of the tested failure load, depending on the resistance model applied. Due to the conservative results, and with the current failure mode clearly difficult to predict accurately, an improved analysis was suggested. Thus, the enhanced level of the assessment strategy was evaluated in conjunction with guidelines for the nonlinear finite element analysis (NLFEA) of concrete structures. An initial finite element (FE) model, based on the recommendations from the guidelines, yielded an overestimation of the load-carrying capacity by 18% and failed to predict the failure mode accurately. In an updated FE model, small changes were made based on the preloading of the tested bridge, additional findings in literature and a sensitivity study. The updated simulation of the test was able to predict the load-carrying capacity and the failure mode of the bridge accurately. Consequently, assessment at the enhanced level can be considered to be very accurate and of great benefit in the pursuit of optimised assessments of bridges, provided that the material parameters are known and boundary conditions are properly modelled. At the same time, the study showed the importance of using guidelines for NLFEA based on current knowledge in the field. One objective of the full-scale study was to evaluate the function of two separate carbon fibre reinforced polymer (CFRP) strengthening systems bonded to the concrete. Both systems were bonded, with a thixotropic epoxy adhesive as the bonding agent, at the soffit of the girders in span 2. Three near-surface-mounted (NSM) CFRP 10 × 10 mm2 rods were installed in sawn grooves on the central girder, three 1.4 × 80 mm2 prestressed CFRP laminates were used on the south girder and the north girder remained unstrengthened. A prestress force of 100 kN was applied using stressing devices at the end of the laminates, which also served as temporary mechanical anchors while the epoxy was curing. The bridge was monitored using linear displacement sensors, draw-wire sensors, strain gauges, photometric sensors, laser sensors, load cells and yard sticks. Corresponding load tests up to 6.0 MN were performed before and after strengthening, and the bridge was finally loaded to failure. Both the NSM- and laminate-strengthened girders showed reduced tensile strains in the steel reinforcement after strengthening. The NSM was utilized up to 85% at failure, and the failure was a combination of flexure and shear. The laminates debonded before the ultimate load was reached and the maximum CFRP utilization was 37% at debonding. An incomplete bond line induced the debonding. The prestressed laminates reduced the strain in the tensile steel reinforcement, particularly for lower loads. However, the differences between non-stressed and prestressed strengthening were small and the failure modes were similar. The results from Gruvvägsbron regarding assessment procedures are found in Bagge et al. (2014, 2017) and Bagge (2017). The results regarding strengthening are found in Nilimaa (2015) and Nilimaa et al. (2015, 2016a, 2017). The Haparanda Bridge The Haparanda Bridge (Figure 2), built in 1959, is a concrete double-trough bridge, located right by the railway yard in Haparanda. The trough bridge is a standard bridge type that has been the most common

Figure 2. The Haparanda Bridge was strengthened by horizontal post-tensioning.

solution for short to medium span bridges in Sweden since the 1950s. Because of upgrading of the load capacity of the Haparanda railway line, the maximum allowed axle loads were increased from 250 to 300 kN, and for this reason, the Haparanda Bridge required a higher transverse shear capacity of the slab. A novel strengthening system consisting of unbonded internal post-tensioning was implemented. Up to this field test, all trough bridges with insufficient shear capacity of the slab had been replaced with new bridges. The strengthening method was first tested in the laboratory with smaller trough specimen (scale 1:3) showing that post-tensioning can be an appropriate method for increasing the transverse shear and flexural capacities of reinforced concrete slabs. The prestressing force can be introduced by implementing an internal bonded or unbonded posttensioning solution consisting of prestressing bars or tendons transversely inserted through the slab. By choosing a low, vertical placement of the prestressing bar, the flexural capacity may be increased further. However, existing reinforcement layers in the slab may complicate or prevent this option. One advantage of post-tensioning is the minimal disturbance to existing traffic on the railway line during the strengthening process. All strengthening work can be performed below the topmost level of the bridge, which is ideal for safety reasons, and traffic may continue to use the bridge during the reinforcement process. The primary advantage of choosing an unbonded strengthening system is that the level of prestressing can be easily adjusted and single strands can be exchanged if necessary. The risks of this strengthening method include accidental cutting of the internal reinforcement during drilling. The focus of the project was to increase the shear resistance, but the flexural resistance was also affected, and the shear and flexural capacities increased by 25 and 13%, respectively, according to the design calculations. The bridge was tested before and after strengthening, and the results showed that the strains caused by a train with axle loads of 215 kN were completely counteracted by the prestressing.

Further results from Haparanda and the previous laboratory tests are found in Nilimaa (2013, 2015) and Nilimaa et al. (2012, 2014, 2015). 1.2 Steel bridges The Åby Bridge The Åby Bridge (Figure 3) consisted of an unballasted open steel truss bridge from 1955. The superstructure was replaced with a new bridge and track in 2012. Before replacement, measurements were performed on the bridge to evaluate the current performance while it was still in service. When the Åby Bridge was replaced, the old steel truss superstructure was placed on the riverside to undergo further testing where it was finally loaded to failure in a static load test 2013. The monitoring comprehended strain gauges, LVDT’s (Linear Voltage Displacement Transducers), temperature sensors as well as photogrammetry and accelerometers. One reason for the particular interest in the Åby Bridge was that there is an identical “twin bridge” in the Rautasjokk Bridge, along the Ore line. Due to the transportation of iron ore, this railway line is exposed to higher traffic loads. The ambition was to use the assessment results of the Åby Bridge in order to understand the behaviour and keep the Rautasjokk Bridge in service for a longer time without affecting the safety. A fatigue-monitoring system was installed on the Rautasjokk Bridge in 2015, with the objective of assessing critical details and thereby verifying the findings from the Åby Bridge. The Åby Bridge was statically tested with and without the rails. It was observed that the rails have a significant influence on the behaviour of the stringer beams, as they distribute the load and create a composite action together with the stringers. The measured response in the stringers beams indicated a complex behaviour where torsion and out-of-plane bending are assumed to have a major influence on the state of stress. These effects are assumed to be displacement induced and therefore less prominent in the ULS (Ultimate Limit State). However, for the structure’s fatigue response, these effects will give rise to significantly higher stresses at some points. Results indicated that the Åby Bride was able to carry loads, in a magnitude of more than four times the maximally allowed axle load (STAX) along the line. The governing failure mode of the Åby Bridge

Figure 3. The Åby Bridge

was a ductile lateral buckling of the top chord, as expected by the numerical simulations. Furthermore, no signs of failure in the riveted or welded connections were observed. The Rautasjokk Bridge has been concluded to have sufficient capacity in the ULS, but there are still doubts related to the fatigue limit state, where the bridge indicates insufficient capacity with regard to code‐based controls, however no cracks or damage have been detected. The results in this project were used to make a deeper fatigue analysis, out of the scope of existing codes. This analysis showed that the bridge still had fatigue capacity left but had some details in need of monitoring. By doing the analysis, the technical life span of the bridge could be prolonged. Thus, also creating time for better and more effective planning for an effective exchange of the bridge in a near future. Results regarding the Åby and Rautasjokk Bridges are presented in Häggström (2014, 2016) and Häggström et al. (2017).

2. Project results 2.1 Can probabilistic LCCA methods for assessing the lifetime of existing assets be used in the management process in order estimate the time to remedial actions and the size of action at hand (maintenance, repair, upgrading and replacement)? Tools for Life cycle cost assessments (LCCA) were investigated in the Mainline-project and the results were discussed in Nilimaa et al. (2016b). Growth in demand for rail transportation across Europe is predicted to continue. Much of this growth will have to be accommodated on existing track and infrastructure. This demand will increase both the rate of deterioration of these elderly assets which eventually increases the need for shorter line closures for maintenance or renewal interventions. For such interventions infrastructure managers and decision makers within the railway infrastructure in general strive for economic and environmental viable solutions. Thus, there is a great interest from these stakeholders for the development of tools capable of evaluating different alternative solutions by considering the economic and environmental consequences of different intervention measures being considered, since such tools are not readily available at present time. The overall aim of WP5 in Mainline was to create a Life Cycle Assessment Tool (LCAT) that can compare different maintenance and replacement strategies for track and infrastructure based on a life cycle evaluation. The life cycle evaluation shall quantify direct economic costs and indirect costs like availability costs (for example service disruptions) and environmental impacts (costs). Moreover, the tool can also consider the target safety levels in the optimization process and relate such target safety levels to the actual condition of the asset. Information about the actual condition of the asset should be based on inspections and/or monitoring. Potentially such data could be used as input for asset-specific degradation models being a part of the LCAT. Such degradation models are useful when optimizing plans for future interventions, i.e. M&R plans. A number of software programmes for asset management within the railway sector exist. These programmes cover LCC or LCA analyses, whereas only a limited number of the identified programmes consider LCC and LCA at the same time. Some programmes concern deterministic as well as probabilistic evaluation techniques. Existing programmes can be used for planning of future interventions. However, for most of the programmes the module used for this planning is based on experience from previous projects and not actual lifetime performance models (incl. degradation) for the assets.

Key parameters for LCC analyses were identified as part of the literature review in Nilimaa et al. (2016b). The key parameters identified were:

• General − Discount rate − Time period studied

• Evaluation Methodology − NPV and possibly other economical evaluation methodologies

• Asset Information − Asset and element type/classification − Asset condition − Life span − Deterioration profile − Asset inventory database

• Interventions information − Interventions options − Agency costs − User costs − Environmental costs − Other costs

• Output − Forecast LCC incl. LCA costs − Deterministic vs. probabilistic evaluation − Future interventions and whole life profiles − Graphical outputs

• Others − Integration of LCC and LCA − Uncertainties in LCC − RAMS − Legislation − System platform

The methodology for the LCAT developed in Mainline, suggests that LCC and LCA analyses are carried out separately and results from LCA analyses are transformed into monetary units using pre-defined conversion functions. The input for such functions, e.g. the cost per ton CO2 emission, is politically decided. The input for the LCAT covers general information about the assets, asset condition, deterioration models, economic and environmental implications of interventions etc. Most input data is entered by the IMs and the LCAT must be able to facilitate the framework and underlying models for the LCC and LCA analyses. The output from LCAT covers the actual costs as well as the cost-drivers (economical as well as environmental) and an Operation, Maintenance & Restoration plan based on the need for future interventions. These results may be presented for all alternative solutions graphically as well as in tables. The tool can be found in Mainlines project portal: www.mainline-project.eu

2.2 How can new and existing methods for the assessment of capacity and life length of bridges be improved (toolbox) and how can monitoring in the service limit state be extrapolated to the load carrying capacity in the ultimate limit state? Structural analysis has a crucial role in bridge assessment. To support rational improvements of this analysis, a multi-level strategy was developed in Bagge (2017). By gradually increasing the complexity of the assessment, the structural responses and load-carrying capacity can be more accurately estimated. This methodology is an extension of the strategy developed for bridge deck slabs by Plos et al. (2016). The extended strategy is intended to be applicable to more complex concrete superstructures, composed of systems of beams and slabs. Four levels, representing different complexities of the analysis, are defined based on the types of failures that can be verified implicitly by the structural analysis. At the initial level (Level 1), no failures are covered by the structural analysis and the action effects are verified using local resistance models estimating the cross-sectional capacity. Examples of local resistance models are American (ACI 318, 2014), Canadian (CSA A23.3, 2014) and European (SS-EN 1992-1-1, 2005) design codes, Model Code 2010 (fib, 2013) or other national regulations. At the subsequent levels (Levels 2 – 4), called enhanced levels, flexural, shear-related and anchorage failures are integrated stepwise into the structural analysis. This leads to a one-step verification procedure at Level 4 that captures the main failure modes that can be expected. Thus, the initial level (Level 1) is similar to the traditional, commonly used approaches for structural analysis. At the enhanced levels (Levels 2 – 4), nonlinearities are taken into account using nonlinear FE analyses with different complexities, depending on the level of approximation. A framework adapted to the multi-level strategy for successive improvement and increasing complexity of safety verification is presented in Figure 4. The framework for safety verification is based on the safety concepts of partial safety factor (PSF), global resistance safety factor (GRSF) and full probabilistic methods. Structural analysis at the initial level (Level 1) is combined with the PSF method or full probabilistic analysis, which are well-established for this type of analysis. At enhanced levels of structural analysis (Levels 2 – 4), the purpose is to analyse the structural behaviour more accurately and, thus, estimate the load-carrying capacity more precisely. To do this, average material properties should be used for nonlinear FE analysis at these levels.

Figure 4. Framework for successively improved safety verification

(Bagge, 2017).

Specifically, because of this, the GRSF method has been developed, since the PSF method involves reductions of material properties. However, the GRSF method is not as well-established and thoroughly tested as the PSF method. Therefore, the PSF method could be useful even at the enhanced levels. Moreover, due to the difficulties of evaluations of nonlinear structural analyses, the fib Model Code 2010 (fib, 2013) recommends verification of the structural safety using methods from at least two of the groups shown in Figure 4. Improving the structural analysis using nonlinear FE analysis increased the permitted axle loads on the Kiruna Bridge to 5.6 to 6.5 times those given by traditional and standardised assessment methods, depending on the concept used for safety verification. The model uncertainty was crucial for the verification of the structural safety and has to be properly taken into account. 2.3 How safe are new and existing methods to strengthen existing railway bridges as e.g. use of prestressing and use of Carbon Fibre Reinforced Polymers (CFRP), and can these methods be used to prolong the fatigue life of concrete structures? Different strengthening methods were studied in Nilimaa (2013, 2015) and Nilimaa et al. (2015, 2016a, 2016b). Numerous existing bridges are in need of some type of health intervention in order to function adequately and meet current and anticipated demands. For example, about two-thirds of the railway bridge stock in Europe is over 50 years old (Bell, 2004), and the old structures were not designed for the high stresses caused by modern rail traffic. Similar trends are also affecting highway bridges. Furthermore, several studies have shown that the actual service life of a bridge, until demolition, is often much shorter than the design life. For example, a study of the bridges demolished due to deterioration in Sweden between 1990 and 2005 showed they had an average ultimate life of 68 years rather than 100 years (Mattsson et al., 2008). Bridges can potentially be upgraded to permit carriage of heavier modern traffic flows and extend their service life by refined design calculations or strengthening. In such cases the refined design calculations comprise numerical or analytical capacity assessments using actual material properties, updated design procedures and current load models. However, some structures cannot be saved simply by design refinements and can therefore only be upgraded by strengthening or (in worst case scenarios) replacement. The most common type of railway bridge in Sweden is the concrete trough bridge, the name arising from its trough-like appearance, and similar ballasted girder slab bridges are common in many other countries. A previous full-scale, ultimate load assessment, reported in Puurula et al. (2014), showed that the slab is the weakest element of trough bridges, and that it must be strengthened in the transverse direction between the main beams to increase allowable loads. For other types of concrete bridges, the load capacity may be governed by factors other than the slab capacity, different structural elements for example. The slab of the Haparanda Bridge was strengthened by post-tensioning, see Nilimaa (2013, 2015) and Nilimaa et al. (2012, 2014, 2015). In addition to increasing the cross-sections, there are four other life-extending strengthening approaches, as illustrated in Figure 4:

a) External unbonded reinforcement; b) External bonded reinforcement; c) Near-surface mounted (NSM) reinforcement; d) Internal bonded or unbonded reinforcement.

a)

b) c)

d)

a) External unbonded b) External bonded c) NSM d) Internal bonded/unbonded

Figure 4. Different types of strengthening. However, the main difference between possible options from a bridge designer’s perspective is whether the reinforcement is introduced in a slack form or subjected to some kind of prestress (pre- or post-tensioning). The aim of using prestressed reinforcement is reducing tensile strains in the concrete, increasing the utilization of the high-strength reinforcement, delaying crack initiation, reducing crack widths, and reducing structural deformations (Collins and Mitchell, 1997). Traditionally steel was almost invariably used to reinforce concrete structures, but today diverse composite reinforcement materials with varying properties are available, such as Fiber Reinforced Polymers (FRP). This diversity provides abundant scope for choosing reinforcement materials to meet most types of structural, environmental and aesthetic demands. FRPs are today widely accepted and often the preferred choice for repairing, strengthening and retrofitting concrete structures. The advantages of FRPs, compared to more conventional construction materials, include high strength-to-weight ratios, corrosion resistance, electromagnetic neutrality, durability and formability. Composite materials can be formed into almost any shape and are therefore suitable for applying to complex structural components. Using FRPs often provides strengthening more quickly and cheaply, with lower maintenance costs, than other options. However, FRPs have relatively low axial stiffness and transverse strength, making prestressed tendons difficult to anchor mechanically. Another concern for designers regarding FRPs is their lack of ductility and plastic deformation, which can lead to sudden and violent failures without warning, especially in prestressed concrete members. Fortunately, this can be avoided by over-reinforcing so the concrete will crush before the tensile FRP ruptures. Another way to increase the ductility of FRP-reinforced concrete structures is to induce a pseudo-ductile failure mode by using a reinforcing system consisting of multiple bar types or reinforcement layers at different strain levels (Noël, 2013). Mahal (2015) also demonstrated that CFRP strengthening may improve the ductility of concrete beams suffering from fatigue induced failures. It should be noted that long-term costs of FRP reinforced structures can be very low, relative to other options, from a life cycle cost perspective (Grace et al., 2012) although FRP materials are often regarded as expensive. Several general guidelines for FRP strengthening have been presented (e.g. ACI, 2014; FIB, 2001; FIB, 2006; Täljsten, 2006; Täljsten et al., 2011).

2.4 What new and existing monitoring techniques are most efficient in the assessment of railway bridges? A recent benchmark performed in the European research consortium In2Rail (www.in2rail.eu) identified 28 promising monitoring techniques, described in Technology Notecards in Nilimaa et al. (2017). The secondary phase benchmarked 17 technologies for application to railways and 6 methods were studied more thoroughly: Electrical Resistivity Tomography (ERT): an imaging technique that provides cross-sectional images with the help of penetrating waves. It may be a suitable technology for detecting and imaging the amount of pipe clogging due to mineral scale deposits in tunnel drainage; Fatigue assessment systems: integration of a monitoring system with a theoretical model for service life prediction. It is intended mainly for determining fatigue deterioration of steel bridges, however the idea may also be appropriate for other materials; Ground Penetrating Radar (GPR): uses pulses of electromagnetic radiation to penetrate the surface of the ground to reveal any anomalies in soil or other materials. May be used to evaluate the subsurface properties and condition of tunnels, bridges, foundations and geological layers affecting these structures; Image analysis: combination of digital image acquisition and processing for Structural Health Monitoring (SHM). This visual non-destructive assessment method can provide information on the condition of a structure, with minimal disruption to normal services. Possible applications may be detection of damage, strain and displacement as well as motion and vibration detection from images captured by remote control and robotic vehicle; Muon tomography: a muon (from the Greek letter mu, μ) is an elementary particle similar to the electron. Muon tomography is an imaging technique which uses naturally occurring showers of muons to scan the interior of large-scale geological structures. This may be particularly applicable in tunnels, for early identification of changes in geology below the surface; Terrestrial Microwave Interferometry (MI): a radar technology to monitor displacements and movements of structures. The principle is based on the measurement of amplitude and phase of the electromagnetic wave transmitted by the radar device. It may be used in the assessment of bridges.

3. Publications The project has contributed to one Licentiate- and two PhD disseminations. Furthermore, results are presented in several Journal Papers, Conference Papers, Technical Reports, Oral presentations and one university course (Maintenance, repair and upgrading at LTU). The peer-reviewed and published theses, reports and journal papers are listed below: Bagge, N. (2017). Structural assessment procedures for existing concrete bridges : Experiences from failure tests of the Kiruna Bridge. Doctoral Thesis. Luleå: Luleå University of Technology, Division of Structural Engineering, ISBN 978-91-7583-879-3 (pdf). Bagge, N., Nilimaa, J., Blanksvärd, T. & Elfgren, L. (2014). Instrumentation and Full-Scale Test of a Post-Tensioned Concrete Bridge. Nordic Concrete Research, 51, pp. 63-83. Bagge, N., Nilimaa, J. & Elfgren, L. (2017). In-situ methods to determine residual prestress forces in concrete bridges. Engineering Structures, 135, 41–52. https://doi.org/10.1016/j.engstruct.2016.12.059 Elfgren, L., Bagge N., Nilimaa, J., Blanksvärd, Th., Täljsten, B., Shu, J., Plos, M., Larsson, O. & Sundquist, H. (2015). Brottbelastning av en 55 år gammal spännbetongbro i Kiruna - Kalibrering av modeller för tillståndsbedömning. Slutrapport till BBT 2015-10-25. Elfgren, L., Bell, B., Nilimaa, J., Häggström, J., Tu, Y., Lundgren, K., … Casas, J. R. (2015). New technologies to extend the life of elderly rail infrastructure : Deliverable 1.3 in MAINLINE - a project within the EC 7th Framework Programme. http://www.mainline-project.eu/ Häggström, J. (2014). Bärighetsberäkning : Bro över södra Rautasjokk KM 1432+883. Technical Report. Luleå: Luleå University of Technology, Division of Structural Engineering. Häggström, J. (2016). Evaluation of the Load Carrying Capacity of a Steel Truss Railway Bridge : Testing, Theory and Evaluation. Licantiate Thesis. Luleå: Luleå University of Technology, Division of Structural Engineering, ISBN 978-91-7583-739-0, 139 pp. Häggström, J., Blanksvärd, T., & Täljsten, B. (2017). Bridge over Åby River : Evaluation of full scale testing. Technical Report. Luleå: Luleå University of Technology, Division of Structural Engineering. Nilimaa, J. (2013). Upgrading Concrete Bridges: Post-Tensioning for Higher Loads. Licantiate Thesis. Luleå: Luleå University of Technology, Division of Structural Engineering, ISBN 978-91-7439-546-4, 300 pp. Nilimaa, J. (2015). Concrete Bridges. Improved Load Capacity. Doctoral Thesis. Luleå: Luleå University of Technology, Division of Structural Engineering, ISBN 978-91-7583-344-3 (pdf), 180 pp. Nilimaa, J., Blanksvärd, T., Elfgren, L., & Täljsten, B. (2012). Transversal post tensioning of RC trough bridges : laboratory tests. Nordic Concrete Research, 46(2), 57–74. Nilimaa, J., Blanksvärd, T., Täljsten, B., & Elfgren, L. (2014). Unbonded Transverse Posttensioning of a Railway Bridge in Haparanda, Sweden. Journal of Bridge Engineering, 19(3). https://doi.org/10.1061/(ASCE)BE.1943-5592.0000527

Nilimaa, J., Blanksvärd, T., & Täljsten, B. (2015). Assessment of concrete double-trough bridges. Journal of Civil Structural Health Monitoring, 5(1), 29–36. https://doi.org/10.1007/s13349-015-0102-2 Nilimaa, J., Blanksvärd, T., Al-Emrani, M., Haghani, R., & Kliger, R. (2015). Innovativa metoder för förstärkning av befintliga konstruktioner med förspänt kompositmaterial : Förstärkning och brottbelastning av bro i full skala - SBUF 12849 Slutrapport. Luleå: Luleå University of Technology. Nilimaa, J., Bagge, N., Blanksvärd, T., & Täljsten, B. (2016a). NSM CFRP Strengthening and Failure Loading of a Posttensioned Concrete Bridge. Journal of Composites for Construction, 20(3). https://doi.org/10.1061/(ASCE)CC.1943-5614.0000635 Nilimaa, J., Blanksvärd, T., & Elfgren, L. (2016b). Kunskapsåterföring av erfarenheter från MAINLINE : Verktyg och metoder för att förbättra den svenska järnvägsinfrastrukturen - SBUF 13139 Slutrapport. Luleå: Luleå University of Technology. Nilimaa, J., & Elfgren, L. (2017). In2Rail D4.2 - Benchmark, Evaluation and Initial Test of Inspection Technologies to Facilitate Proactive Maintenance of Bridges and Tunnels. EU Horizon 2020 Project: In2Rail, Innovative intelligent Rail, Grant No. 635900, 114 pp. http://www.in2rail.eu/home.aspx Shu, J., Bagge, N., Plos, M., Johansson, M., & Zandi, K. (2018). Shear Capacity of a RC Bridge Deck Slab : Comparison between Multilevel Assessment and Field Test. Journal of Structural Engineering, 144(7). https://doi.org/10.1061/(ASCE)ST.1943-541X.0002076

4. Additional references Plos, M., Shu, J., Zandi, K. Z., & Lundgren, K. (2016). A multi-level structural assessment strategy for reinforced concrete bridge deck slabs. Structure and Infrastructure Engineering, 1-19. ACI 318 (2014). Building code requirements for structural concrete and commentary. American Concrete Institute (ACI), Farmington Hills, MI, United States, 520. Bell, B. (2004). D1.2 European Railway Bridge Demography. Brussels: Sustainable Bridges – Assessment for Future Traffic Demands and Longer Lives, 10 pp. Collins, M.P., Mitchell, D. (1997). Prestressed Concrete Structures. Response Publications, 766 pp. CSA A23.3 (2014). Design of concrete structures. Canadian Standards Association (CSA), Rexdale, ON, Canada, 297. fib (2001). Externally Bonded FRP Reinforcement for RC Structures: FIB Bulletin 14. Lausanne: International Federation for Structural Concrete fib, 138 pp. fib (2006). Retrofitting of Concrete Structures Bonded by FRPs with Emphasis on Seismic Applications: FIB Bulletin 35. Lausanne: International Federation for Structural Concrete fib, 220 pp.

fib (2013). fib Model Code for concrete structures 2010. Lausanne: International Federation for Structural Concrete fib, 402 pp. Grace, N.F., Jensen, E.A., Eamon, C.D., Shi, X. (2012). Life-Cycle Cost Analysis of Carbon Fiber-Reinforced Polymer Reinforced Concrete Bridges. ACI Structural Journal, 109(5), 697-704. Mahal, M. (2015). Fatigue Behaviour of RC Beams Strengthened with CFRP: Analytical and Experimental Investigations. Doctoral Thesis. Luleå: Luleå University of Technology, Division of Structural Engineering, ISBN: 978-91-7583-234-0, 138 pp. Mattsson, H.Å., Sundquist, H., Stenbeck, T. (2008). Bridge Demolition and Construction Rates: Inspection Data-Based Indicators, Bridge Structures - Assessment, Design and Construction, 4(1), 33-47. Noël, M. (2013). Behavior of Post-Tensioned Slab Bridges with FRP Reinforcement Under Monotonic and Fatigue Loading. Waterloo: University of Waterloo, 383 pp. Puurula, A., Enochsson, O., Sas, G., Blanksvärd, T., Ohlsson, U., Bernspång, L., Täljsten, B., Elfgren, L. (2014). Loading to Failure and 3D Nonlinear FE Modelling of a Strengthened RC Bridge. Structure & Infrastructure Engineering, 10(12):1606-1619. SS-EN 1992-1-1 (2005). Eurocode 2: Design of concrete structures – Part 1-1: General rules and rules for buildings. European Committee for Standardization (CEN), Brussels, Belgium, 236. Täljsten, B. (2006). FRP Strengthening of Existing Concrete Structures: Design Guideline (4th edition). Luleå: Luleå University of Technology, 228 pp. Täljsten, B., Blanksvärd, T., Sas, G. (2011). Handbok för dimensionering och utförande i samband med förstärkning av betongkonstruktioner med pålimmade fiberkompositer. Luleå: Luleå University of Technology, 184 pp.