Presented at the XXIV Nordic Concrete Research Symposium in Stockholm, August 17-19, 2022.

1. Wireless Monitoring for Assessment of Concrete
Railway Bridges – Experiences from Field Tests.
Professor Dr. Björn Täljsten1
Dr. Cosmin Popescu1
Dr. Rasoul Nilforoush2
1Luleå University of Technology
2AFRY AB

2. Agenda
• Introduction
• Hypothesis
• Monitoring
• Evaluation
• Conclusions
The Abisko bridge

3. Luleå
Abisko
Stockholm
Narvik

4. The Abisko bridge is located in the North of Sweden, at the iron ore line. It’s a
three span prestressed trough bridge built 1978. Since construction the bridge
axle loads from the trains has increased from 140 kN to 325 kN and are now
aiming for 400 kN per axle.
Introduction
The iron ore line is an important part of the infrastructure for
transporting iron ore to Narvik (Norway) and to Luleå.

5. Abiskojåkka.
A train has 68 wagons, each with ca 100 ton iron ore
12 trains/day transport about 25 Mton/year
Maintenance cost ~ 45 k€/km, year

6. Introduction
The latest inspection (specific) showed cracks in the box girders, why it is a subject of interest to
investigate the cause and behaviour of the bridge in a greater extent. When the current axle load,
325 kN, was to be implemented a classification of the bridge was conducted. The classification
done in 2016, Bennitz (2016), showed that the bridge was able to handle the new axle load.
The cause of the cracks was at that point not completely understood.
Crack widths:
0.1 - 0.4mm unloaded

7. Hypotheses
Specific drivers for the Abisko bridge is:
o Cause of cracks
o Load carrying capacity (increased axle loads)
o Expected life
Hypotheses:
o The cracks are caused by temperature
o The cracks are caused early in the building stage due to normal forces of the prestressing cables
Methods:
o Monitoring
o NDT (Non destructive testing)
o Analysis/Modelling

8. Hypotheses:
o The cracks are caused by temperature
o The cracks are caused due to normal forces of the prestressing cables
Cracks
Cracks
Cracks
F
F
Hypotheses

9. Monitoring
A systems for wireless monitoring has been installed. It consists in general of a base
station, the hub, with an external antenna and nodes attached. The sensors are
connected to the nodes. The sensors in this project are
o Temperature
o Strain
o Deflection/crack opening
o Accelerometers
The base station is run on 220 V, but the nodes can either be run on battery or 12 V,
here we have chosen 12 V supply for the nodes and sensors.
In addition to the system above we have also installed a optical
monitoring systems for crack monitoring and are also investigating a fibre
optic system for crack sensing.
We have also used NDT methods to measure the placement of the
tendons and the depths of the cracks.

10. A general overview of the installed monitoring system is shown below. Sensors
have been installed in all three spans where cracks were found.
The following abbreviations have been used:
A: Accelerometer
N: Node
P: Power supply, node
S: Crack gauge
R: Antenna
T: Temperature sensors (thermo-element)
In addition we also have strain gauges, not shown in
the figures to the left
The nodes also have internal temperature sensors.
In total we have:
3 accelerometers:
10 LVDT (Crack gauges)
6 external temperature sensors
4 nodes
6 strain gauges
Monitoring

11. The base station and some of the sensors are shown in the figure below
Base station with 220 V power supply. Antenna placed outside. All data are
transferred from the nodes to the base-station. The data is then transferred
to the cloud via a 4g router. It is equipped with a back-up battery for 24
hours.
Crack sensor connected
to the node and
protected with a plastic
box.
Node to which the sensors are connected. The data are
also stored locally on the node.
Accelerometer
Strain gauge
Monitoring

12. Overview of the system in span 2.
The base station is in span 1.
Temperature sensors are placed in span 1 and through
the thickness of the webs. Typical results is shown
below.
We can notice some minor variation between west and
east side
Some results from temperature
Monitoring

13. Some results from LVDT (Span 2). Here we have several train passages over approximately 24
hours. The results indicate that the crack opens and closes during the passage. The maximum
opening is approximately 0,02 mm for LVDT5.
Corresponding strains over the crack is shown in the lower figure. These are measured in the same
crack as LVDT5 and LVDT 7 respectively. The strain levels are high, but they are not possible to
transfer into stress since they are placed over a crack.
Monitoring

14. Fibre optic sensing
ODiSI 6000 platform (Optical Distributed Sensor Interrogator)
Controller and interrogator Standoff cable and Remote module High-Definition fiber optic sensor
Monitoring

15. Non-Destructive Testing
The purpose with the NDT assessment is to reveal to actual placement of the steel
reinforcement and concrete cover and also, if possible, the prestressing cables. Here
also the strength of the concrete is measured. We have used:
o A smith hammer for strength
o Cover meter for steel reinforcement
o A GPR for ducts and tendons
o UPV for crack depth measurements
o FOS for detecting cracks
The results from the cover-meter shows that the concrete cover varies between 20 mm up to ca 70
mm. The mean distance of the reinforcement is 204 mm and the diameter 10 mm. In the figures
below the process for measurement and some results are shown.
The average result from the Smith hammer tests show that the concrete quality
varies between ca 50 MPa up to above 70 MPa.

16. It was possible to map the location of the ducts with GPR. We mapped
over a distance to find the horizontal location of the cable.
Section 17.5 m from east support
We also mapped the crack depth by UPV (ultrasound pulse velocity). The crack
depths varied between ca 30 mm up to ca 150 mm.
a a
2a 2a
Non-Destructive Testing

17. Modelling
The geometry of the bridge was drawn according to the drawings in BaTMan.
This was done using the format dwg which was later transformed to the IGES
format before being imported to the pre-processor GiD. Also, all the
reinforcement bars and tendons were drawn and transformed to the IGES
format before imported, approximately 14 500 reinforcement bars in total has
been incorporated in the model. However, we have not updated the model
with the actual data from the NDT and measurements.

18. Summary
o The wireless system works as intended but the amount of data created is very large. A
solution to this is to only record “important” data, i.e. crack widths over a given value.
o The battery back up did not work during the winter, the battery froze.
o Collecting data from NDT takes a long time due to hand held equipment. However, primary
data shows that the concrete cover varies between 30 mm up to 70 mm. GPR and UPV very
valuable to detect tendon location and crack depths.
o The FOS system used is very promising, we could detect cracks that can not be seen by the
eye. However, the system was very sensitive and we will replace the current fibres with a
more resistant one.
o Making the digital model has taken long time and we have just recently started to simulate
effects from temperature. A primary result is that temperature is not the cause of the cracks.
o The conclusion from the inspection/monitoring is that the bursting forces have created the
cracks. However, it is complicated to calculate actual forces since the prestressing force is
unknow.