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Research & Development Conference Papers

A.H. de Bondt, J.G.F. Schrader

JOINTLESSASPHALTPAVEMENTSATBRIDGEENDS

prepared for

3D Finite Element Modeling of Pavement Structures

Amsterdam, The Netherlands

2002

P.O. Box 1 Phone +31 229 5477001633 ZG Avenhorn Fax +31 229 547701The Netherlands www.ooms.nl/research


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JOINTLESS ASPHALT PAVEMENTS AT BRIDGE ENDS

A.H. de Bondt1* and J.G.F. Schrader2

ABSTRACT

Bridge decks expand and contract during a year due to temperature variations, as anyother non-restrained structure. The amplitude of this movement depends on the type ofbridge, its length and the climatic circumstances. The standard method of connecting thepavement of a bridge deck to the pavement on an embankment, is by making a joint, oftenconsisting of steel grips or a soft asphalt mixture. This has always been problematic withrespect to comfort (for drivers as well as people living in the neighbourhood) andperformance. Reasons for this are, that joints restrict runoff of rain and ice, create noise,cause bumps, show ruts and allow water penetration (and subsequently erosion ofsubbase/soil layers). The paper describes a specific project where extensive three-

dimensional finite element analyses, in combination with sufficient material testing, haveresulted in a cost-effective proposal for a jointless pavement structure; this includingtendering specifications.

Keywords: bridge ends, cracking, asphalt reinforcement, stress-relief, thermal loading

INTRODUCTION

Since the introduction of continuous pavement structures such as the ones built by meansof the elasto-viscoplastic material bitumen, the transition between the pavement on thebridge deck and the pavement laying on the natural soil has been a problem. If the asphaltpavement is simply paved without measures onto the bridge deck, one can expect after afew or even during one severe winter, that wide cracks become visible at the bridge end.This is due to the large strains which are generated in the asphalt concrete layers,especially at its bottom fibre. These wide cracks will not only allow water penetration intothe foundation layers, but will also cause ravelling to occur at the crack edges at thepavement surface. Also the faces of the crack itself will deteriorate by the repetitiveshearing actions of trucks, implying progressive crack widening in time, de Bondt (1999).

Given the problem described above, several types of joints have been developed and

applied over the past years. However, these have in common that their lifetime is shortand difficult to asses. This means that quite rapidly and often unexpectedly (costly)maintenance, in the form of replacement is needed, Maijenburg (2000). In the situationsuch as in the Netherlands, where the current motorway system is already loaded beyondits capacity, closing lanes for joint maintenance causes a lot of disturbance and isunacceptable from the user point of view.

It is clear that long lasting jointless asphalt pavements at bridge ends should bedeveloped. More specific, the wearing course layer should be a continuous layer with amaintenance interval for the aspect cracking, which is at least similar to the maintenanceinterval of the bridge itself (often planned every 50 years). This is a much more stringent

1Ooms Avenhorn Holding bv PO Box 1, 1633 ZG Avenhorn, the Netherlands, [email protected]

2Unihorn bv PO Box 58, 1633 ZH Avenhorn, the Netherlands, [email protected]


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requirement than simply demanding that the wearing course layer should be free of cracksas long as the wearing course layer itself lasts; the latter period is in most cases limited byravelling, rutting or lack of skid resistance and normally varies between about 8 and 15years. It is important to realize this aspect, because placing a new wearing course on anunderlying pavement, which shows (active!), wide cracks implies certainly within a veryshort period of time (a few or even one single summer/winter cycle), that a reflection of the

crack pattern becomes visible.

In the paper, a specific project will be described where based on three-dimensional finiteelement analyses, supported with asphalt mixture tensile testing, layer interface (bond)testing and reinforcement single-end tensile testing, tender specifications for a durablejointless pavement have been drawn-up. These specifications are of a so-called functionalnature (the asphalt mixture is defined in terms of mechanical properties instead of in termsof composition).

PROBLEM DESCRIPTION

The motorway A50 in the Netherlands, which runs from Eindhoven to Emmeloord, has amissing section between Eindhoven and Oss. At the moment construction has started tobuilt this part (opening is planned at the end of 2003). A number of special type of bridgeshave been designed for this stretch by the Engineering Office on Bridges and Tunnels ofthe Dutch Road Administration (Bouwdienst Rijkwaterstaat). Some of these bridges havealready been built, in order to ease the road construction process (transport of subbasematerial). The so-called integral bridges, varying in length between about 20 and 70 m,have in common, that they have a special type of support at the ends. More specifically,the (continuous) concrete bridge deck only rests on steel bearing piles. This method isattractive from several points of view (construction costs, esthetics, etc.), as summarizedby Maijenburg, (2000). It is clear that given the relatively low rotational stiffness of thesupports, as compared to the bridge power, a considerable thermal movement at thebridge ends needs to be taken into account, when designing the transition to the roadpavement. Figure 1 shows a photo (dated end of 1999) of the 70 m long bridge at Son(crossing the Wilhelmina-canal), in the phase that earth construction work of the road hasstill not been started. This bridge was finished in 1997.

Figure 1 Overview of Situation of the Bridge at Son at the End of 1999


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The area in which the road is being made, has good supporting conditions; at least fromthe Dutch point of view, since the soil consists of sand. However, given the fact that anembankment needs to be made, still so-called approach slabs will be utilized. The functionof an approach slab is to create a smooth vertical pavement surface profile in case ofsettlements; in other words to avoid sudden bumps when hitting the bridge deckpavement. For this reason, these slabs are placed on an angle (see figure 2). At the bridge

Son this angle is equal to 2.6 .

Existing Soil

Ground Level

Steel Bearing PilesEmbankment

Settlement

Steel BarsRoad Pavement

ApproachSlab

Canal

Integral Bridge Deck

Figure 2 Explanation of the Function of an Approach slab

The connection between the bridge deck itself and the approach slab is via steel bars,which are embedded in such a way that only rotations are possible (in case ofsettlements). This configuration implies that the approach slab will be subjected to thethermal expansion and contraction process of the bridge. Given the length of the bridge(70 m) and the length of the approach slabs on both sides (each side 5 m), it is clear thatthe amplitude of the thermal movements (summer/winter cycle) is extremely large.

From the foregoing it can be concluded that if the wearing course layer should be acontinuous layer with a maintenance interval for the criterion cracking (caused by thethermally induced bridge movement), which is at least similar to the maintenance interval

of the bridge itself (a period of 50 years), a complicated design problem would arise. TheResearch & Development department of Ooms Avenhorn Holding took this challenge atthe end of 1999. All in all, the challenge was defined as follows:

Design a cost-effective jointless pavement near a bridge end(including the preparation of tendering specifications), which can sustain 2 mmdaily movement (day/night) and 20 mm seasonal movement (summer/winter);

this for a period of 50 years (under Dutch climatic conditions)

It is obvious that this goal could not be achieved without finite element modelling, given thecomplex geometry. The development work started by preparing a three-dimensional finite

element mesh of a composition of the pavement structure which was at that time thoughtto be adequate. In the end, it became apparent that two major (and time consuming!)changes in the mesh configuration had been necessary.


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FINITE ELEMENT MESH

A sketch of the superelement configuration of the final three-dimensional mesh ispresented in figure 3 (note that the horizontal and vertical scale are different). The meshhas a width of 3500 mm and consists of 60 superelements, representing 40 differentmaterials or interfaces (encircled in figure 3); the latter simulating friction, adhesion or

bond. The superelements were eventually subdivided into 6515 elements: 5265 cubicelements and 1250 interface elements. The program CAPA-3D, Scarpas and Karsbergen(1999) was used for the analyses.

36

35

3

27

5

29

7

31

9

33

13

22

34

32

30

28

33

31

29

27

10

8

6

4

9

7

5

3

14 24

1

1

1

1

18

11

15

17

26

2 1

2 1

2 1

2 1

19 18

12 11

16

20

21

11

25

25

25

26

23

20017 mm

3650mm

Road

Pavement

Bridge

Pavement

Figure 3 Sketch of the Superelement Configuration (not on scale!)

The geometry is quite complicated. Table 1 gives an explanation of the material / interfacenumbers given in figure 3 (note that the reinforcement elements were not shown).

Material / Interface Number Description

1/2/11/12/15/16/18/19 Asphalt Concrete Layers

3/4/5/6/7/8/9/10/27/28/29/30/31/32/33/34/35/36 Pavement Layer Interfaces (Bond)

13/14/20 Stress-Relieving System

17 Unbound Granular Base Course

21 Approach Slab (PCC)

22 Dry Friction Simulation

23 Cement Stabilized Sand

24/25 Air (Simulation of No Contact)

26 Sand Subbase Course

37/38/39/40 Asphalt Reinforcement

Table 1 Explanation of Superelements


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The solution which was finally adopted, can be described as follows: on a gravel asphalt

ne of the advantages of this solution is, that it fits well with the road pavement (left side in

Figure 4

concrete layer (which acts as working platform) and the PCC-approach slab, a stress-relieving system, consisting of 3 mm polymer modified bitumen (PMB) is sprayed. Afterthis, an extremely ductile (but still stable enough) 30 mm thick special type of asphaltconcrete (called Thermifalt) is layed down. On top of this, a stiff and strong asphaltreinforcement is attached. Then two more of these special asphalt layers are placed, each

with reinforcement put on their surface. On top of this, three standard asphalt mixtures ofeach 60 mm thickness are layed (including the use of a high quality PMB). Again,reinforcement is placed on their surface, except for the top layer of these three. Thismeans that all in all, five layers of asphalt reinforcement are planned (note that the modelhas four reinforcement layers). Finally, a 70 mm thick porous asphalt wearing course willbe paved (also on the road pavement).

Ofigure 3) and the bridge deck pavement (right side in figure 3). Figure 4 shows the entiresubdivided mesh utilized for the final analyses (note that the angle of the approach slab isclearly visible).

Mesh used for the Final Analyses

he main purpose of using the finite element method was the investigation if crackingwould occur (and to what extent) in the cross-section at the end of the PCC-approach slab(see figure 5 for a detail of the mesh at the centre location).

T


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Figure 5 Detail Mesh Focussed on the Important Location

OADING

wo major loading cases have been discerned: an extreme daily temperature drop and anasonal temperature drop. The extreme daily one has been represented by a

hange in temperature from 0 to 15 C; for the seasonal drop these values were +35 and

h slab (at the location of the connection with theridge itself, where the steel bars are). This approach saves a lot of elements (reduces the

LTextreme sec15 C. Based on analyses performed by Maijenburg (2000), it can be deduced that the

bridge movements (at the end of the approach slab!) are: 2 mm for the extreme daily caseand 20 mm for the extreme seasonal case. Note that because of the time needed to warm-up the bridge, these displacements are not related to the temperature interval (15,respectively 50 degrees Celsius) in the same way. Only in case of the seasonal case (thelong period), the bridge movement can be directly related to the length (70 m), by simply

using the well-known equation T L.

The simulation of the bridge movement has been carried out via setting prescribeddisplacements at the end of the approacbmesh size) and is allowed from the reliability point of view, if the displacements along theapproach slab itself do not vary. Given the difference in length between the bridge itselfand the approach slab the latter is no problem.


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Note that this approach is not allowed for the analysis of reflective cracking in bituminuoussurfacings which are laying on slabs in pavements. Inputting the actual temperature drops,the individual coefficients of thermal contraction of each layer and modelling half the slab

quality subgrade materialand) and the fact that only standard truck traffic passes.

linear elastic approach has been followed. Since some of the material and interfaceture and displacement (strain) rate dependent, it meant that

anual interations had to be carried out, in order to have representative input parameters

d

roach Slab and Cement Stabilized Sandven by CUR (1998). These rules

ere transformed in such a way that they were applicable for a linear elastic approach.(N/mm)/mm2for the

ch was carried out ate Research & Development Laboratory of Ooms Avenhorn Holding. These cores were

were made in a routine way by means of standard spraying

seasonalmperature drop. These stiffness values are based on a temperature of 15 C and a rate

(using symmetry) is then necessary, see de Bondt (1999, 2000).

The effect of passing traffic has not been taken into account, given the considerable

thickness of the entire pavement structure (340 mm), the good(s

MATERIAL PROPERTIES

Acharacteristics are temperamfor the analyses. For the first run, this meant that interface displacements (slip values) hadto be assessed from the given bridge movements. From table 1 it is clear that an extensive

list of material data was inputted. The most important data is discussed in detail below.

Approach Slab (PCC)The Youngs modulus of this material was set to 35000 MPa.

Cement Stabilized SanA value of 4500 MPa was used for the Youngs modulus.

Dry Friction between AppThe input data has been based on dry friction rules, as giwShear stiffnesses used for the interface elements were equal to 0.025daily drop and 0.0025 (N/mm)/mm

2for the seasonal drop. The so-called normal stiffnesses

of the interface (perpendicular to its plane) were 4500 (N/mm)/mm2.

Stress-Relieving SystemThe input data originated from interface shear testing on cores whithdrilled from sections whichtrucks. Figure 6 shows typical data from this testing (4 specimens). It can be seen that theinterface shows a ductile behaviour, even at this low temperature (-10 C). It is mentioned

that in this situation the end of the test is caused by limitations of the current (standardavailable) test equipment; when the test ends the two opposite faces of the stress-relievingsystem are still sticking together; this even at a slip of nearly 8 mm. Different series of testswere performed. Using the whole data set, a mastercurve was derived which enables todeduce (extrapolate) input data for the extremely slow movements which occur during thesummer/winter cycle. Details can be found in de Bondt and van Rooijen (2002).

The inputted shear stiffnesses for the interface elements were 0.3 (N/mm)/mm2 for the

extreme daily temperature drop and 0.1 (N/mm)/mm2 for the extremeteof displacement (slip) of 1 mm in 6 hours (daily) or 10 mm in 2000 hours (seasonal). The

normal stiffnesses of the interface were equal to 1350/300 (N/mm)/mm2

for the daily drop,respectively 80/15 (N/mm)/mm

2for the seasonal drop.


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Interface Shear Testing - Overview Series i01Shear Rate 0.0001 mm/s, Temperature -10 C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 1 2 3 4 5 6 7 8 9

Slip (mm)

ShearStres

s(MPa)

Figure 6 Example of Shear Test Results on Stress-Relieving System

iven the typical nature of bitumen, mechanical properties of asphaltic mixtures highlyepend on temperature and strain rate. Within this project, stiffness values for several

binations were obtained by means of a so-called frequency

ase of the daily temperaturerop. These values were 70, respectively 15 MPa for the seasonal temperature drop.

theinforcement was placed (embedded): 0.6 (N/mm)/mm

2for the extreme daily temperature

eme seasonal temperature drop. Note that this value

with a specificxial stiffness, which is located (embedded) between two interface elements. The axial

al terminology product stiffness, depends on the stiffness of the

Asphalt Concrete LayersGdtemperature/frequency comsweep procedure; this for three different asphaltic mixtures. In addition, stiffness data was

obtained from slow monotonic uniaxial tensile tests, which were carried out at severaltemperatures and strain rates. The latter experiments also provided information on criticalstrain value (strain at maximum load) and fracture energy. By making use of the wholedata set, mastercurves could be derived, which enable extrapolation to the slow strainrates which are typical for seasonal thermal cycles. It is noted that this process isexplained in more detail in van Rooijen and de Bondt (2003).

The stiffness values used for the design process were equal to 900 MPa (for the 60 mmPMA-layer) and 300 MPa (for the 30 mm Thermifalt layer) in cd

Pavement Layer Interfaces (Bond)The following values were used to decribe the pavement layer interfaces whereredrop and 0.2 (N/mm)/mm2for the extrwas used for each interface element (above and below the reinforcement).

Asphalt ReinforcementIn CAPA-3D, reinforcement is modelled as a continuous, equivalent sheetastiffness, or in more practicreinforcing material and the cross-sectional area of the strands per unit width. Thisparameter can easily be obtained from single-end tensile testing, de Bondt (2000). Therequired product stiffness value (EA)eq,rfwas in this project found to be 4000 N/mm.


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SHORT OVERVIEW OF COMPUTATIONAL RESULTS

An extensive series of computations on all kind of different alternatives has been carriedut, de Bondt and Schrader (2001); this during a period of almost two years. In this

Figure 7

osection, a short overview of this work will be given; mainly focussed on the finally adoptedsolution at the bridge Son. First of all, a better understanding of the mechanisms behind

the analyzed phenomenon has been obtained by plotting the deformed mesh fromdifferent perspectives. Figures 7 and 8 show in an exaggerated way (10 times enlarged),the deformations due to the seasonal temperature drop of the entire mesh, respectively ofa detail at the critical location (at the left end of the approach slab!)

Overview of Deformations of Entire Mesh (Exaggerated)


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Figure 8 Deformations around Critical Location (Exaggerated)

is clearly visible how the approach slab slides over the cement stabilized sand and thate asphalt is brought under tension.

rameters such as layer interface slip, asphalt tensiletrain and reinforcement force will be given; this for two computational cases where the

ItthIn the figures below, distributions of pas

seasonal temperature drop has been applied (characterized by 20 and 26). It is mentionedthat in the cases shown, the composition of the asphalt cross-section, etc. was similar. Theonly difference is that computation 20 simulates the behaviour of the final solution in itsvirgin phase, whereas in case of computation 26, the situation is represented that thebottom asphalt layer (one of the three layers with the special type of asphalt) has cracked.


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First of all, figures 9 and 10 present the computed slip values at the location (depth) of the

Figure 9

stress-relieving system. This at the road side of the approach slab (left of the criticallocation), respectively on the approach slab (right of the critical location); this for thementioned cases.

Layer Interface Slip along Stress-Relieving SystemLeft of Critical Location

0

1

2

3

4

5

6

7

-15000 -13500 -12000 -10500 -9000 -7500 -6000 -4500 -3000 -1500 0

Distance to Critical Location (mm)

Slip(mm)

Computation 20

Computation 26

Slip along Stress-Relieving System (Left of Critical Location)

Figure 10

Layer Interface Slip along Stress-Relieving System

Right of Critical Location

-7

-6

-5

-4

-3

-2

-1

0

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000


Slip(mm) Computation 20

Computation 26

Slip along Stress-Relieving System (Right of Critical Location)

can be seen that the plots of the slip distributions are quite similar left and right of thecritical location. Furthermore, it can be observed that, as expected, cracking of the bottomlayer implies that a lower degree of slip along the interface occurs.

It


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Figure 11 gives the strains in the asphalt layers along the pavement cross-section.

Tensile Strain Distribution in Asphaltat Critical Cross-Section

0

50

100

150

200

250

300

350

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Tensile Strain (%)

Depth(mm)

Computation 20

Computation 26

Figure 11 Computed Asphalt Strains for Several Cases

also be concluded thate cracking of the bottom layer hardly influences the strain distribution. The computedrce in the deepest layer of reinforcement (on top of the first layer of 30 mm special

Figure 12

The transition between each asphalt layer is clearly visible. It canthfo

asphalt, which means 310 mm under the pavement surface) is given in figure 12. It can beconcluded that the required anchorage length is several meters. This is due to the largeamplitude of the (slow) movements. It can also be seen that cracking of the bottom layercauses that this reinforcement is more activated.

Distribution of Reinforcement ForceReinforcement at Depth 310 mm

0

10

20

30

40

50

60

70

80

90

-15000 -12500 -10000 -7500 -5000 -2500 0 2500 5000


Force(N/m

m)

Computation 20Computation 26

Computed Force in Reinforcement for Several Cases


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EVALU C

is obvious that each alternative solution had to be evaluated whether it would meet the

ith respect to the stress-relieving system, the ultimate slip was used as a criterion, de

ll in all, it became clear that the summer/winter case was more damaging than the

Figure 13

ATION OF OMPUTATIONAL OUTPUT

It50 year lifetime requrement. This meant for the asphalt concrete layers a check on thecritical strain; given the severe demands for this bridge, this parameter (which is strain ratedependent) was chosen as criterion and not the (less severe) fracture energy. It was found

that after an extremely strong winter, the bottom asphalt layer would crack. However, thestrain in the asphalt layers directly on top of this layer, is low enough to expect no furtherprogress of cracking (see also figure 11).

WBondt and van Rooijen (2002). The reinforcement was checked via its tensile strengthvalue (100 kN/m per layer).

Aday/night case. In order to illustrate the mechanisms which occur, an evaluation of theforces which are acting on the approach slab and the asphalt is given in figure 13. Values

are given per meter width of the bridge (note that these are rounded off).

460kN/m

Approach

Slab

Asphalt

55kN/m

250kN/m

250kN/m

55kN/m

55kN/m

210kN/m

250 kN / m 45 kN / m

11

1

1

1

1

1

11

Sketch of Equilibrium of Forces (Free Body Diagram)

It can be seen that with the current configuration (and input data) roughly half the restraintforce is developed by the jointless asphalt pavement and roughly half the restraint force byfriction between the approach slab and the cement stabilized sand underneath. Aninteresting aspect is that the generated force in the steel bars which connect the approachslab and the bridge was higher than expected by the bridge engineers. This had led to

some design changes.


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Figure 14 presents a sketch of the forces along the critical cross-section in the asphalt.

Figure 14

Detailed Sketch of Forces along Critical Cross-Section

It can be deduced that the asphalt takes 50 % of the generated force in the cross-sectionand the reinforcement 50 %, in the phase represented by computation 20; after the bottomasphalt layer has cracked (computation 26) these values are 40, respectively 60 %.

Computation 20

35 kN / m1

10 kN / m1

60 kN / m1

35 kN / m1

N/m

1

15 kN / m1

20 kN / m1

20 kN / m1

25 kN / m1

30 kN / m1

460k

Computation 26

40 kN / m1

20 kN / m1

20 kN / m1

20 kN / m1

25 kN / m1

30 kN / m1

10 kN / m1

75 kN / m

1

0 kN / m1

450kN

/m

1


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CONCLUSIONS AND RECOMMENDATIONS

Based on the work described above, it can be concluded that via adequately detailedthree-dimensional finite elements modelling, in combination with sufficient material testingand the use of high quality materials, it is possible to develop durable (long-lasting)jointless asphalt pavement structures even for bridge ends which move 20 mm during a

summer/winter cycle. With respect to the design process itself, it has to be mentioned thatwith the current finite element software, it is time consuming to change the model (e.g.adding or removing asphalt concrete layers), because of new insights (e.g. ease ofconstruction, costs, etc.).

It is strongly recommended to monitor the behaviour of the jointless pavement which hasbeen developed; this during several (different) summer/winter cycles. The monitoringprogramme should include measurements of movements (strains) at a wide range oflocations as well as of the force which is transferred at the connection between the bridgeand the approach slab (via the steel bars).

ACKNOWLEDGEMENT

For their stimulating discussions during the whole project and their willingness to innovate,ir. W.A. de Bruijn and ing. F.A.M. van Gestel of the Engineering Office on Bridges andTunnels of the Dutch Road Administration (Bouwdienst Rijkwaterstaat) are highlyacknowledged.

REFERENCES

de Bondt, A.H. (1999). Anti-Reflective Cracking Design of (Reinforced) Asphaltic Overlays.Ph.D.-Thesis, Delft University of Technology.Maijenburg, A.T.G. (2000). Integral Bridges (in Dutch). Dutch Road Administration / DelftUniversity of Technology.Scarpas, A. and Kasbergen, C. (1999). CAPA-3D User's Manual.de Bondt, A.H. (2000). Effect of Reinforcement Properties. 4thRILEM Conference onReflective Cracking, Ottawa.CUR (1998). FLOOR 1.0 - Concrete Floor Design Background Report (in Dutch), Gouda.de Bondt, A.H. and van Rooijen, R. (2002). Layer Interface Shear Testing, Internal Report.van Rooijen, R. and de Bondt, A.H. (2003). Development and Use of Functional Asphalt

Tender Specifications, Paper prepared for RILEM - PTEBM, Zurich.de Bondt, A.H. and Schrader, J.G.F. (2001). Overview of Finite Element Analyses onJointless Asphalt Alternatives for Bridge Son (in Dutch), Period July 1999 April 2001.

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