Full scale test on prefabricated slabs for electrical supply by induction of urban transport systems

Transport Research Arena 2014, Paris

Full scale test on prefabricated slabs for electrical supply by induction

of urban transport systems

Mai-Lan Nguyena*

, Pierre Hornycha, Jean-Pierre Kerzrého

a, Sergio Perez

b

aLUNAM Université, IFSTTAR, Materials and Structures Department, Route de Bouaye, 44344 Bouguenais, France

bBombardier Transportation, Neustadter Straße 62, 68309 Mannheim, Germany

Abstract

Advanced and innovative technologies are increasingly used in transport infrastructures, especially in urban transportation. Recently, Bombardier Transport has developed a new system for supply by induction of electrical public transportation vehicles. The proposed solution consists in integrating the electrical supply cables, for creating an electromagnetic induction field, in a prefabricated concrete slab implemented in the road. A key aspect of the project is the design and resistance to heavy traffic of the proposed modular pavement. For this purpose, it was decided to carry out a full scale test on the accelerated pavement testing facility of IFSTTAR Nantes. Five different prototype slabs are tested, differing by the width of the slab, and the thickness of the concrete cover above the supply cables. The experiment consists of subjecting the slabs to one million passes with a dual wheel load of 65 kN and monitoring the mechanical behaviour of the slabs pavement by measuring strains and displacements in chosen points. The paper presents the design of the proposed prefabricated slabs pavement, the installation procedure of these slabs and first results of the current full scale test. Keywords: prefabricated road slab; full scale test; induction; electric vehicle supply; urban transport system.

Résumé

Les technologies avancées et innovantes sont de plus en plus utilisées dans les infrastructures de transport, en particulier dans les transports urbains. Bombardier Transport a récemment développé un nouveau système d'alimentation par induction des véhicules électriques. La solution proposée intègre dans une dalle en béton préfabriquée installée dans la chaussée, les câbles d'alimentation électrique permettant de créer un champ d'induction électromagnétique. Un aspect clé du projet est le concept et la résistance au trafic lourd du module proposé. A cet effet, il a été décidé de réaliser un essai en vraie grandeur avec le simulateur de trafic routier lourd, le manège de fatigue de l'IFSTTAR à Nantes. Cinq dalles prototypes ont été testées, différenciées par leur largeur et par l'épaisseur de la couverture en béton au-dessus des câbles. L’expérience consiste à soumettre ces dalles à un million de passages du jumelage de 65 kN et à suivre le comportement mécanique de la chaussée des dalles en mesurant les déformations et les déplacements en des points choisis. Cet article présente le concept de la chaussée expérimentale en dalles préfabriquées, la solution de mise en place et le suivi du comportement de ces dalles dans l’essai manège en cours.

Mots-clé: dalle chaussée préfabriquée ; essai en vraie grandeur ; induction ; alimentation de véhicule électrique ; système de transport urban

* Tel.: +33-2-4084-5715; fax: +33-2-4084-5994. E-mail address: [email protected].

Nguyen Mai-Lan / Transport Research Arena 2014, Paris 2

1. Introduction

Electric road becomes more realistic. Among the technologies under development, inductive charging represents

a promising technique and is being experimented in Germany and Belgium (Beeldens, 2013). In addition to the

advantages of clean solutions (no use of ordinary fuels, such as petrol or diesel, no local emissions), inductive

charging technology is practical to use and may contribute to minimizing different types of urban pollution (air,

visibility and noise pollutions). It also solves the problem of autonomy of electric vehicles (EV) and reduces

their weight by reducing the battery size, in particular for public transport vehicles.The development of this new

technology for transport infrastructures matches the ideas of current projects in France and Europe: the 5th

Generation Road, the European project Forever Open Road, and the Infravation programme.

Recently, Bombardier Transportation has developed a new system for supply by inductive charging of electric

buses. The idea is to implement the inductive charging system in the road at bus stops for recharging while

passengers get on and off. This implementation is based on the principle of modular construction consisting of

prefabricated elements ready to be installed simply and quickly. In particular, the component responsible for the

inductive power transfer is integrated in a prefabricated concrete slab which is implemented in the road. The slab

contains electrical supply cables, for creating a magnetic field. A power receiver system installed underneath the

EV, when the last one parks on the induction segment, turns this magnetic field into an electric current that feeds

the vehicle traction system.

A key aspect of the project is the design and resistance to heavy traffic of the proposed modular pavement. For

this purpose, it was decided to carry out a full scale test on the accelerated pavement testing (APT) facility of

IFSTTAR Nantes (fig. 1). It is a heavy road traffic simulator, dedicated to full-scale pavement experiments.

Since 2000s, the APT facility has demonstrated its capacity for testing of urban infrastructure solutions (de

Larrard et al., 2013; Fort, 2013).

Fig. 1. The IFSTTAR’s APT facility

Five different slab prototypes are tested, differing by the width of the slab, and the thickness of the concrete

cover above the supply cables. The slabs are instrumented using strain gages, displacement transducers, and

geophones, to verify their strains and displacements under loading. Electrical measurements are also made to

verify the performance of the cables throughout the experiment. The experiment consists of subjecting the slabs

to one million passes with a dual wheel load of 65 kN (corresponding to half of the French standard axle load of

130 kN, which is the highest in the world). From February 1st to April 4th 2013, a first period of loading was

performed with approximately 260 000 traffic passages. The paper presents the design and construction of the

proposed prefabricated slab pavement, results of the first period of the current full scale test, and their

perspectives, in terms of deployment of this technology.

2. Design of the full scale test

The proposed solution, to ensure the implementation of the inductive charging system in the road, consists of

prefabricated concrete slabs, containing the electrical supply cables. These slabs, reinforced with glass fibre

reinforced polymer (GFRP) rebars, have a length of 5 m, a thickness of 0.25 m, and are intended to be

incorporated into a pavement structure. The power supply of each slab is independent and is activated only by

passing of an applicable EV when it is detected by an antenna system.

Nguyen Mai-Lan / Transport Research Arena 2014, Paris

IFSTTAR has been asked to work on the design of the pavement structure to receive these prefabricated slabs

and then test them on the APT facility. The solution that has been proposed is to provide a bituminous

foundation layer of 8 cm thickness, then place on it the prefabricated slabs using aluminium supports of

adjustable height, placed at the corners of the slabs to control precisely the final level of the slabs. A high

performance, non-shrinking, cement slurry is placed in the space remaining below the slab (about 2 cm) to

ensure a good contact between the slab and the foundation support. After the placement of the slabs, the space

around the slabs was filled with cement concrete. Dowels made of glass fibres are used to connect the slabs with

the surrounding concrete pavement in the longitudinal direction. A pre-cracking in the transverse direction,

extending the joints between the slabs and the concrete fill, was made in the concrete fill. For that purpose, the

concrete has been sawn, one day after its casting, to a third of its depth, to avoid the appearance of uncontrolled

cracks. A transverse profile of the pavement structure, without concrete fill around the slab, is shown on fig. 2.

8 cm bituminous base

Platform 150 ÷ 200 MPa

2 cm cement

slurry

25 cm

aluminium

supports

Prefab.

slab

Component responsible for

inductive power transfer

Fig. 2. Transverse profile of the tested pavement.

The experimentation of inductive charging system integrated on prefabricated slabs is performed on a half-ring

of the circular test track (fig. 3). A classical bituminous pavement is placed on the other half-ring for proper

rotation of the traffic simulator. Five slab prototypes, including three standard slabs of 5 m 2.4 m and two

narrow slabs of 5 m 1.25 m, are being tested. The narrow size is intended for optimizing precast and transport

processes. These two narrow slabs have eight GFRP rebars coming out of the slabs on the longitudinal sides.

Pre-cracking was created, in the longitudinal direction on both sides of the narrow slabs at the end of these

rebars. Both standard and narrow slabs include the same module containing the electrical supply cables, which is

positioned in the central part of the slab. On the test track, the wheel loads run at a fixed radius, of 19 m. The

slabs have been placed at different transversal positions on the test track in order to apply the loads either on the

centre or on the edge of the slab. The thickness of the concrete cover above the supply cables is also an

important parameter, for the resistance of the slabs. On the standard slabs, two levels of thickness have been

tested: 4 cm on the two first slabs, and 5 cm on the third one. On the two narrow slabs, the thickness of the top

concrete layer is 3.5 cm and 4 cm respectively. Table 1 summarises the testing configuration of these slabs.

Direction of rotation

Pre-cracking

Concrete pavement

Slab A

Slab E

Slab D Slab C

Slab B

Old concrete

slab pavement

Bituminous

pavement

Other

components

R 19 m

Fig. 3. Plan of the circular test track


Table 1. Testing configuration of slab prototypes

Slab Slab dimensions (m)

Lateral position of traffic along the slab

Thickness of the concrete cover (cm)

Slab A 52.40.25 middle 5.0

Slab B 52.40.25 edge 5.0

Slab C 52.40.25 middle 4.0

Slab D 51.250.25 middle 3.5

Slab E 51.250.25 edge 4.0

3. Construction of the full scale test and solution for installation of the prefabricated slabs

The construction of the test track required several phases. After removing the existing pavement to the level of

the subgrade platform, it has been adjusted at the right level for installation of the prefabricated slabs. The

bearing capacity of the existing platform was then measured at different positions by means of the dynamic plate

test, which gave values between 150 and 200 MPa.

To test the mechanical behaviour and resistance of the prefabricated slabs under heavy traffic, stabilization of

these slabs is very important. For the construction of the proposed structure (fig. 1), the following procedure was

applied for the first four slabs (A, B, C and D). For slab E, in order to simulate better actual conditions on

construction sites in urban areas, i.e. limited space and time, a slightly different procedure was tested.

3.1. Procedure for installation of the slabs A, B, C and D

Realization of an 8 cm thick bituminous subbase layer. This step was needed on the APT site. However, on an

actual site, it could be another support, e.g. milling of the existing pavement.

Fine milling of this base layer to obtain a first calibration of the final level of the prefabricated slab.

Positioning of aluminium supports at the four corners of each slab. At each corner, several support plates of

different thicknesses were used, firstly a thick plate of 15 mm, then thinner plates (of 1 mm minimum). The

level of these supports was verified (taking into account the slab thickness) by leveling measurements, in

order to reach precisely the final level of the slab.

Placement of the slab on the aluminium supports using a crane to ensure the final level of the slab.

Construction of a formwork around the slab for receiving the cement slurry.

Lifting of the slab and filling of the formwork with the cement slurry; before replacing the slab on top of the

cement slurry.

To ensure proper filling of the cement slurry, several blowholes have been created in the slabs in the

prefabrication, to allow entrapped air to escape and the cement slurry to be well distributed during installation of

the slabs. These blowholes were clogged at the end of the process.

To validate this procedure and check the stability of the slab under moving of a heavy vehicle, a feasibility test

was performed on slab A (fig. 4). The test was carried out 18 days after placement of the slab. It consisted of

moving slowly the half axle of a heavy vehicle of about 20 tons on the slab and measuring deflection of the slab

at different positions: at the four corners and in the middle of each lateral edge. Three load levels of the half axle

were applied: 66 kN, 76 kN and 96 kN. The measured maximum deflections, which increased proportionally

with the load level, were 0.10 mm, 0.11 mm and 0.16 mm respectively. These deflection levels remained small

and were consistent with estimations made using the pavement design software Alizé-Lcpc.

After installation of the slabs, the remaining space around the slabs was filled using a cement concrete of high

performance to form a continuous pavement.


Fig. 4. Feasibility test with a truck.

3.2. Procedure for installation of slab E

The same principle of slab stabilization using cement slurry was used, but two main points were modified:

To approach the actual conditions of insertion of the prefabricated slab into an existing pavement with limited

space, the concrete fill, next to the upstream end of the slab, was placed before installation of the slab. Several

grooves were made in the concrete fill to allow insertion of the dowels.

In order to accelerate the installation of the slab, a faster procedure of stabilization of the slabs was applied.

The slab E was placed on supports, the level of which could be adjusted using three screws. The cement slurry

was then injected under pressure (fig. 5) directly under the slab. This procedure avoided lifting the slab again

before filling of the cement slurry, which reduces the use of the crane on the construction site.

After installation of the slabs, the remaining space around the slab was filled using a high performance cement

concrete as realized previously.

Fig. 5. Installation of prefabricated slab with injection of cement slurry.

4. Test conditions and parameters measured during the test

4.1. Test conditions

The test program consists in applying one million loads, with dual wheels, loaded at 50 kN for the first 100 000

passages and 65 kN for the remaining of the test. This represents a traffic of about 150 buses per day for 20

years, and corresponds to 250 000 rotations of the four arms carousel. Loading is applied at the radius of 19 m

and thus the total perimeter of the test track circuit is approximately 120 m. A reduced lateral wandering of 0.8

m wide of the load is applied in order to simulate the channelized traffic of buses.

The speed of rotation of the carousel applied under normal conditions is 5 rounds/minute, which corresponds to a

tangential velocity of about 35.8 km/h at a radius of 19 m. Lower speeds have also been applied for specific

Displacement transducer


measurements. However, it was decided not to run at higher speeds, in order to avoid dynamic effects in the

loading modules of the traffic simulator, which could occur due to the less even surface of the concrete slab

pavement compared to a conventional bituminous pavement.

4.2. Parameters measured during the test

The instrumentation of the test comprises mainly surface measurements close to the joints between the slabs and

the concrete fill. These measurements monitor the movements of the slabs under moving loads. Three

measurement systems are used:

A Benkelman beam with two LVDTs (Linear Variable Differential Transducer) measures the deflection on

both sides of each joint, compared to fixed supports located about 2 m away from the measured points. This

system can make measurements only at slow speed (1 round/minute). It can be moved to follow in turn the

deflections at different joints.

A LVDT, installed next to each joint, measures the relative movement, i.e. the faulting effect, between the

slab and the concrete fill. This equipment can operate regardless of the speed of the traffic simulator.

To measure the deflections at high speeds, four geophones were installed on slab C. These geophones, fixed

on both sides of each joint of the slab C, measure the speed of the vertical displacement of the measured

points under moving loads, which allows determining, by integration, the deflections at these points.

Periodic FWD (Falling Weight Deflectometer) measurements will be performed to evaluate the response of the

prefabricated slabs at different stages of the experiment.

Due to the difficulty of instrumenting on site, only two strain gauges were glued on the lower surface of the slab

A, near the middle of each lateral edge, in the longitudinal direction. These gauges allow measuring the

deformation of the slab at these positions.

Regarding the subgrade, three strain gauges were installed vertically at the top of the subgrade layer, under each

of the three slabs B, C and D, to follow the evolution of vertical deformations of the subgrade.

Two temperature profiles, one in slab B and the other one in the concrete fill located between slabs B and C, are

being recorded using thermocouple sensors placed at different depth levels, i.e. 0, - 12, - 24, - 29, - 34 and - 44

cm. Two other thermocouples are used to measure the air temperature in sunlight and in the shade.

The functioning of the integrated electric cables is also controlled at different stages of the test.

At the end of the test, it is planned to carry out detailed investigations, to core specimens to check different

aspects such as: contact conditions under the slabs or in the areas containing electric cables, possible

deterioration or cracking of the slabs, mechanical properties of concrete material and cement slurry.

5. First results and interpretation

Results presented in this paper concern the first period of the test with approximately 260 000 loads.

Measurements by different sensors were recorded at 10 000, 50 000, 100 000 and 260 000 loads. During this first

period of investigation, temperatures measured on the surface of the concrete slabs ranged between -3 °C and 22

°C. This period was also marked by a high rainfall level. Visual surveys were carried out regularly. Up to now,

no degradation has been observed on the surface of the slabs.

Evolution of deflections and vertical strains measured respectively on the upper surface of the slabs and in the

platform under the slabs, as well as an example of horizontal strain of a slab measured on its lower surface are

shown below.

5.1. Deflections of concrete slabs pavement measured at joints

Deflections measured with the double LVDT beam on both sides of the joints between the slabs and the concrete

fill, at 100 000 loads (the beginning of application of the standard load of 65 kN) and then at 260 000 loads are


summarised in fig.6. These measurements were carried out successively at all joints, by moving the beam on

each joint, over a period of about two hours.

At 100 000 loads, it is observed that:

Deflection signals are very similar at all joints, and there is very little difference between the measurements

on the concrete fill side and on the prefabricated slab side, indicating good continuity of the pavement.

Deflection levels are small, between 0.06 and 0.12 mm. These values are in agreement with calculations using

the Alizé-LCPC pavement design software (which assumes a continuous, jointless pavement), which gives a

deflection of 0.11 mm.

However, we can note slightly higher deflections at the upstream joints of slabs B and D (in the direction of

rotation of the loads).

At 260 000 loads, measured deflections increase clearly, indicating possibly a decrease of rigidity of the support

at the joints:

At most joints, deflections vary between 0.10 and 0.16 mm, and have the same level on both sides of the joint.

However, on the upstream joints of slabs B and D, the differences between the measurements on the concrete

fill and on the slabs increase significantly. This indicates higher slab faulting, confirming the observations

done at 100 000 loads. Deflections measured on the slab side reach 0.25 mm for slab B and 0.20 mm for slab

D. It can be seen that the faulting increases only at the upstream joints (of slabs B and D for the moment).

This phenomenon can be explained by the fact that when the load reaches the upstream joint of the slab, it

will create an impact (dynamic overload) due to the unevenness of the joint. The subgrade under the upstream

part of the slab thus deteriorates more rapidly, with increasing number of loads. When the load leaves the

slab, the load variation is more gradual, and therefore, there is no dynamic impact on the downstream part of

the slab.

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0 5 10 15 20 25 30 35 40 45 50

Def

lect

ion

(m

m)

Distance (m)

Measurement on the concrete fill boder

Measurement on the slab border

A B C D E

Measurement 14/03/2013, 100 000 loads

65 kN; 7.2 km/h; T(surface) = 4.5 oC

Direction of rotation

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0 5 10 15 20 25 30 35 40 45 50

Def

lect

ion

(m

m)

Distance (m)

Measurement on the concrete fill boder

Measurement on the slab border

A B C D E



Fig. 6. Deflection measured by means of two LVDTs mounted on a Benkelman beam, on both sides of each slab joint: (a) at 100 000 loads; (b) at 260 000 loads


5.2. Vertical subgrade strains under the prefabricated slabs

Fig. 7 presents vertical strains of the subgrade under the three slabs B, C and D, respectively at 100 000 loads

and then at 260 000 loads. These measurements were recorded during only one rotation of the load.

At 100 000 loads:

At this stage, the vertical strains under each slab are still more or less similar.

Vertical strains under slab B (where the loads pass near the edge) are larger than the ones under slab C (where

the loads pass in the middle). This result indicates that passing of traffic near the edge of the prefabricated

modular slab, which will be the normal condition of bus operation, leads to higher strains (and settlements) of

the subgrade beneath the slab.

Even though the narrow slab D (with loads passing the middle) has eight GFRP rebars used to transfer the

load to the surrounding concrete fill, vertical strains under this slab are more visible than under the slab C.

At 260 000 loads:

Vertical strains under each slab increase mostly at both ends of the slab (indicating that the faulting of the

slabs increases).

However, under the upstream end of slabs B and D, the maximum vertical strain is significantly higher than

under the downstream end of the slabs. This confirms the results obtained for the deflection measurements,

which indicate a more pronounced loss of stiffness of the subgrade near the upstream end of the slabs.

-500

-400

-300

-200

-100

0

100

10 15 20 25 30 35

Ver

tica

l st

rain

(m

m/m

)

Distance (m)

ZB1

ZB2

ZB3

ZC4

ZC5

ZC6

ZD7

ZD8

ZD9



B C D

-500

-400

-300

-200

-100

0

100

10 15 20 25 30 35

Ver

tica

l st

rain

(m

m/m

)

Distance (m)

ZB1

ZB2

ZB3

ZC4

ZC5

ZC6

ZD7

ZD8

ZD9



B C D

Fig. 7. Vertical strain measurements by means of strain gauges installed in the subgrade under each slab: (a) at 100 000 loads; (b) at 260 000 loads

5.3. Horizontal strains at bottom of the prefabricated slab

An example of typical signals measured by means of a strain gauge glued on the lower surface of slab A is

plotted in fig. 8. In this measurement, the four load modules (all loaded at 65 kN) of the traffic simulator passed

at four different lateral positions of the slab (corresponding to radii of 18.5, 18.7, 18.9 and 19.1 m), and rotated at

a speed of 5 rounds/minute. The strain gauge was placed close to the inner lateral edge of the slab, which

corresponds to a radius of 17.85 m. Thus, the closer the load approaches the position of the gauge, the higher the


strain level. However, the strain levels are very small (the maximum value is 9 μm/m when the load passed at the

radius of 18.5 m). These low strain levels indicate no fatigue due to tensile strains at the bottom of the slabs.

-4

-2

0

2

4

6

8

10

20 40 60 80 100 120 140

Hori

zon

tal

stra

in (m

m/m

)

Distance (m)

R=19.1 mR=18.9 m

R=18.7 m

R=18.5 m

Fig. 8. Horizontal strain measured on the lower surface of the slab A when the four loads (65 kN each) passed at four different lateral positions of the slab at 260 000 loads.

The results of these different measurements give a good idea of the mechanical response of the slabs. This will

enable comparisons with finite element calculations, using the Cesar-Lcpc software (Humbert et al., 2005) to

accurately reproduce the behavior of the prefabricated concrete slabs, the key element of the inductive charging

solution for EVs in urban areas, and to evaluate their durability.

6. Conclusions and perspectives

This experiment confirms the interest of the IFSTTAR's APT facility for testing an innovative solution of

electrified road, which represents a very promising solution for power supply of EVs in urban areas. A solution

of pavement support to receive the prefabricated slabs containing the charging system for electric vehicles by

induction was designed and tested on the heavy traffic simulator. A practical procedure for the implementation

of these slabs in urban areas has also been studied and validated by a feasibility test.

The full scale experiment, which aims to test these prefabricated slabs with a traffic of one million dual wheel

half axles loaded at 65 kN, started in February 2013. At this stage, 260 000 loads have been realized.

Measurements taken during this first period show satisfactory results:

The deflection of the slabs and the faulting level at joints remain small and consistent with simulations. Only

a slight increase of faulting of two slab ends with the evolution of traffic is observed.

The power supply systems of the five tested slabs continue to function properly, without any damage, after

260 000 loads.

At this stage, no deterioration was observed on the surface of the slabs.

The test will be continued up to one million loads. From 500 000 loads, it is planned to install on the test track a

new precast slab with a different design. A detailed assessment of the slabs behaviour will be conducted at the

end of the experiment, and will allow validation and improvement of this particular Bombardier’s solution for

inductive EVs recharging system integrated on a specific designed prefabricated slab.

Under development since 2008, the inductive charging technology is firstly in passenger operation in 2013 in

Braunschweig (Germany). This next generation of electric mobility solutions will be provided for all forms of

electric vehicles, including trams, buses, commercial vehicles, taxis and cars operating in urban areas.

References

Beeldens A. (2013). Inductive Charging: projet de charge par induction de véhicules électriques. Revue Générale

des Routes et des Aménagements, n°910.

de Larrard F., Sedran T., Balay J.M. (2012). Removable Urban Pavements: An innovative, sustainable

technology. International Journal of Pavement Engineering, Vol. 14, Issue 1, 1 - 11.


Fort T. (2013). Evaluation sous trafic lourds des systèmes d’alimentation des tramways par le sol – Essai

Alstom, Revue Générale des Routes et des Aménagements, n°914.

Humbert, P., Dubouchet, A., Fezans, G., & Remaud, D. (2005). CESAR-LCPC, un progiciel de calcul dédié au

génie civil. Bulletin de liaison des Laboratoires des Ponts et Chaussées, Numéro spécial CESAR-LCPC, (256-

257), 7-37.

The IFSTTAR’s Alizé-LCPC software for the thickness design of pavement according to the French rational

method: http://www.lcpc.fr/english/products/lcpc-products-alize-lcpc-routes/

http://www.lcpc.fr/english/products/lcpc-products-alize-lcpc-routes/

Science

Full scale test on prefabricated slabs for electrical supply by induction of urban transport systems