Relating lab properties of high percentage RAP mixtures to field performance – the NL-LAB program

Relating lab properties of high percentage RAP

mixtures to field performance – the NL-LAB program

S.M.J.G. Erkens,

Section of Road and Railway Engineering, Faculty of Civil Engineering & Geosciences, Delft University of Technology

D. van Vliet,

TNO (Dutch Institute for Applied Physics Research) Theme Build Environment, business line Infrastructure,

J. Stigter,

Boskalis Nederland

S.D. Mookhoek,

TNO (Dutch Institute for Applied Physics Research) Theme Build Environment, business line Infrastructure

B. Sluer, Boskalis Nederland R. Khedoe, Ooms Civiel

A.

Abstract— In 2008 Europe introduced the CEN standards for Asphalt Concrete. The Netherlands adopted the approach of functional requirements, rather than empirical, recipe based requirement. The experiences since 2008 showed that, although this approach allows for a better, more fundamental understanding of Asphalt Concrete, the current understanding is far from complete. Especially the effect of higher percentages (60-70%) of reclaimed asphalt was surprising, since it appeared to improve all functional requirements without the typical interrelation where an increase in stiffness corresponds with a decrease in fatigue resistance. These experiences led to the initiation of a program using the Dutch road network as a living laboratory (NL-LAB). In combination with actual laboratory research on mixes used in pavement construction projects, this program aims first of all to assess the effects of mixing and compaction on functional properties. Secondly, it tries to establish the predictive quality of lab determined functional properties for field performance. This paper describes the NL-LAB program and the results for the first four projects that are analysed. Considering the variation in the results, it is clear that more results are needed to arrive at any definite conclusions. However, the available data do show that the stiffness can be determined quite well and the values found are consistent. For the resistance against fatigue the results

are fairly reproducible, but the trends are not consistent over the different projects. The resistance to moisture damage appears to be predicted fairly well from the lab data, but the variation in the individual indirect tensile tests is such that that actual value of the indirect tensile strength ratio remains to be determined. Finally, the cyclic triaxial test results vary considerably over the first two projects, but they are very consistent over the second two projects. This could be related to the mixing and compaction used in the different projects.

Keywords: lab production, field production, performance, high RAP content

I. INTRODUCTION: REASONS FOR THE RESEARCH PROGRAM

In 2008 Europe introduced the harmonized CEN standards for Asphalt Concrete. In a series of standards the way to characterise several types of Asphalt mixtures (EN 13108-1 to 13108-7), Reclaimed Asphalt (EN 13108-8) and the requirements for testing the mixes (EN 13108-20) and ensuring production quality (EN 13108-21) are described.

Table I

:

Very Thin Asphalt Concrete EN 13108-2 Soft Asphalt EN 13108-3 Hot Rolled Asphalt EN 13108-4 Stone Mastic Asphalt EN 13108-5 Mastic asphalt EN 13108-6 Porous Asphalt EN 13108-7 Reclaimed Asphalt EN 13108-8

Type Testing EN 13108-20

Factory Production Control EN 13108-21

For Asphalt Concrete (13108-1) the standard offers the choice between a classical, recipe based characterisation or a functional characterisation based on more mechanical-type properties in combination with limited composition requirements. The Netherlands adopted the functional characterisation for AC mixtures, aiming at developing a more fundamental understanding of asphalt concrete response and thus providing the opportunity to develop well performing mixtures in a time of rapidly changing constituent materials. The functional characterisation consist of limited composition requirements in combination with requirements for four functional characteristics:

 Resistance to fatigue (EN 12697-24, four point bending in continuous, full sinusoidal strain control at 20oC and 30 Hz, aimed at determining the strain at which the material can take 1x106 load repetitions)  Stiffness (EN 12697-26, four point bending in

continuous, full sinusoidal strain control at 20oC and 8 Hz)

 Water sensitivity (EN 12697-12)

 Resistance to permanent deformation (EN 12697-25, cyclic triaxial compression, temperature and loading conditions dependent on the position of the material in the pavement, aimed at determining the minimum slope of the permanent deformation versus load repetition curve)

In the Netherlands there was elaborate experience with the resistance to fatigue and stiffness (determined using the four point bending test), while the triaxial test and the Indirect Tensile test Ratio (ITSR) as standardized by CEN were relatively new.

From experience it was known that stiffness and fatigue resistance are inter-related: if the if the stiffness increases, due to less or harder bitumen in the mix, the fatigue resistance decrease [1]. Based on some research projects [2] it was expected that this would also hold true for the other properties, with resistance to permanent deformation increasing and the resistance moisture damage decreasing (where decreasing resistance to moisture damage = increasing water sensitivity) with increasing stiffness. As such it was expected that although the CEN standard allowed more freedom in mixture composition, the general trends would hold and that this combination of properties would provide a reliable framework for mixture design.

In the period between 2008 and 2012 there were several developments that lead to a wish to evaluate the current functional tests and to try and establish the relation between these properties and the performance in the pavement. These developments included [3]:

 The stiffnesses that were reported appeared to be higher than those known from the past, this could be due to improvements in the test set-ups, the materials used, or testing protocols. It is important since the mix stiffness directly affects the structural design (pavement thickness) and thus structural safety  There were some issues regarding repeatability and

reproducibility of the tests, this may be a matter of experience but it can also be due to the relatively long time between Type Tests (1 per five years if the mixture composition stays the same) the variety in constituent materials, particular PR may play a role.  The relations based on past experience were valid for

large groups of mixes, will they also hold when predicting the performance of a single mixture based on its lab characterisation?

 Mixes with increasing (50%) RAP appear not to follow the past trend, all four functional characteristics improve with increasing RAP%. If this is true also in the pavement that is good news, but past experience showed that for more 50% RAP the mixtures field performance become very variable, probable due to increased sensitivity to production and construction conditions, this raises questions about the limits of the current functional tests for predicting pavement performance

For some, especially low temperature, mixtures laboratory production proved difficult. This raises the question how well lab conditions represented actual field conditions, which has a direct impact on the reliability of the performance predictions These questions led to a long term research program that uses the Dutch road system as a living laboratory, trough long term field monitoring. Previous projects like the program that introduced double layer Porous Asphalt wearing courses showed the advantages of such a systematic approach [4,5]. The aim of the current project is to get an up to date reference frame based on commonly used mixtures as well as a frame work for the evaluation and possibly improvement of the functional tests and the requirements based on them. In this paper some preliminary results from the first four construction projects are presented [6,7,8].

II. RESEARCH QUESTIONS

In the previous paragraph the experiences and developments that initiated the research program were described. These

experiences and developments were used to derive the research questions:

1. How (well) do the functional characteristics relate to field performance?

2. Is testing on laboratory mixed and compacted specimens the correct choice? 3. Are the current functional tests able to

distinguish “good” from “bad” mixtures?

III. RESEARCH PROGRAM A. Overall approach

The research program aims to use the Dutch road network as a living laboratory (NL-LAB = National Living LAB) to get the answers to the research questions. Although the Netherlands is a small country, the density of its road network provide ample opportunity for field testing (Fig. 1: Dutch road network 6th densest of the world with 331 km of road per 100 km2 land area, USA = 67 km/km2).

However, field tests alone won’t provide answers to the research questions. The program combines lab testing with field monitoring as follows:

I. Assess the effect of mixing and compaction on the lab determined properties

II. Follow the changes of lab determined properties over time

III. Monitor the pavement performance in time

The first step (I) is addressed by making specimens in three different ways:

1 Lab mixed and lab compacted 2 Plant mixed and lab compacted

3 Plant mixed and field compacted, specimens taken from the pavement

Fig. 1: The Dutch road network is one of the most dense in the world

Fig. 2: In step I of the program, specimens produced in three different ways are tested

This stage gives insight in the effects of mixing and compacting as well as providing a first indication of the relation between the predictive quality of lab mixed and compacted specimens for field properties.

Stage II consists of repeated tests on specimens taken from the pavement, showing the variation that occurs over time. Since damaging the pavement to take plates and repeatedly disrupting traffic discourages road agencies from participating, another method was chosen after the first project. Now, directly after construction plates are taken for the specimens for immediate testing as well as for testing after 2 and 6 years, respectively. Those plates are stored under controlled conditions. This way the effect of traffic is excluded and the changes in properties are solely related to changing material characteristics. Eventually, these changes can be related to aging indicators.

Stage III consists of monitoring pavement performance over time. This is straight forward for wearing courses, for binder and base courses it is more complicated. For those locations the monitoring is more indirect, based on the performance of the pavement structure as a whole.

B. Tests in the research program On asphalt concrete:

1. Provide standard CE marking information on Type Test (TT) results (mix composition, void percentage, bitumen content and functional characteristics) which is provided with the mixture in projects, requires no extra work

2. extra Type Test (TT) using the constituent materials used in the construction project, mixed and compacted in the lab

3. extra TT using the constituent materials used in the construction project, mixed at the plant and compacted in the lab

4. extra TT using the constituent materials used in the construction project, mixed and compacted in the lab on specimens taken from the pavement

(plant mixed and field compacted), the batch from which the specimens tested in 3) are takes should also be the one from which the pavement samples are taken

5. two years after construction, repeat TT on specimens taken from the pavement and stored under controlled conditions since construction

6. two years after construction, repeat TT on specimens taken from the pavement and stored under controlled conditions since construction

7. follow pavement performance in time C. on bitumen:

All bitumen samples are tested for penetration (NEN-EN 1426:2007), ring and ball temperature (NEN-EN 1427:2007), DSR stiffness (NEN-EN 14770:2012) and DSR fatigue (protocol RILEM TC 182 ATB, TG1 "Binders", 20) and gel permeation chromatography (GPC, Method TNO, version 3, 27-02-2012). The samples tested in time are:

1. test a sample from the lab batch used for the TT (from the can)

2. test reclaimed bitumen from specimens used for the 2nd step in the asphalt testing

3. test a sample from the binder from the silo at the plant

4. test a sample from reclaimed bitumen from

specimens used for the 3rd step in the asphalt testing

5. test a sample from reclaimed bitumen from

specimens used for step 4 in the asphalt testing 6. 5. test a sample from reclaimed bitumen from

specimens used for step 4 in the asphalt testing

7. test a sample from reclaimed bitumen from

specimens used for step 5 in the asphalt testing

8. test a sample from reclaimed bitumen from

specimens used for step 6 in the asphalt testing IV. RESULTS

A. Materials

For now, the program focusses on the testing of AC mixtures since those mixtures are functionally specified under the CEN standards. Test sections for two mixtures have been constructed in 2012 and 2013 each, the mixture characteristics are given in TABLE II. In 2014 a single test section will be constructed while the material from the first section in 2012 will be re-tested, the second section is unfortunately not available for repeated sampling and testing. The results from the testing 2014 are not available in time to include them in this paper.

The mixes used in the first four projects contain 60, 50, 65 and 65% reclaimed asphalt, respectively. The tests on asphalt concrete are performed by the contractor, the bitumen tests (except for project 3 (P3)) are performed by TNO, a research institute in the Netherlands.

B. Test results

The test results found in the four projects are shown and discussed in next few paragraphs. The results are plotted in three dimensional graphs, where the horizontal axis shows the project number (1 through 4), the depth axis shows the type of specimen production (1= lab mixing and compaction, 2 = plant mixing, lab compaction and 3= plant mixing, field compaction). The vertical axis gives the test result.

The differentiation in a and b on the horizontal (project) axis for the first two projects indicates that within these projects the contractors used two different methods of laboratory specimen production. In the first project (P1) the tests were carried out in two different laboratories, resulting in different mixing (Freundl (P1a) versus Bear (P1b) mixer) and compaction (plate compactor (P1a) versus mini roller compactor (P1b)) in type 1 specimen production and in different compaction for type 2. For type 3 the mixing and compaction was the same, but the specimens were still tested in two laboratories, giving to results. Because two labs were used, the only half the number of specimens (9 instead of 18) was tested for stiffness and fatigue, respectively. Moisture sensitivity and permanent deformation were tested on the usual number of specimens.

For the second project (P2), the mixing was the same and the tests were carried out in a single laboratory, but they compacted the specimens using both automated (2a) and manual (2b) plate compaction. As a result, there are two data sets for type 1 and type 2 of the specimen production. Since only a single lab was involved, there is a single result for type 3 specimen production.

1) Stiffness

The average stiffness values found in the four projects are shown in Fig. 3. Because the three dimensional plot can make it difficult to read the actual values, these are added as data labels. The number before the semi colon denotes the type of specimen production.

As can be seen from the results, there is some variation, but it is fairly limited. In project 1 the mini roller gives a higher stiffness for both type 1 and type 2 specimens, for project 2 the same is true for the manual compaction. Overall, the relation between lab (type 1) and field (type 3) stiffness is quite good. The plant (type 2) specimens show a less consistent relation with lab and field results.

The stiffness values in project 3 seem very consistent, but they are rather low compared to the other results. The numerical results are shown in TABLE III, as can be seen there are some inconsistencies in the number of tests for project 2b and 3. For 2b this was because these were additional tests, for project 3 there was a misunderstanding because of the limited tests in project 1 (two labs with each half the number of tests).

TABLE II: Mix characteristics for projects (P) 1 through 4

Fig. 3: Average stiffnesses for 4 projects, 3 types of production

CONSTRUCTION PROJECT: A4 N345 A28, HRL  157.700‐156.100 km Bennebroekerweg te Hoofddorp MIX INDENTIFICATION P1 P2 P3 P4

mixture type (EN13108‐1) AC 22 Base AC 22 Base 35/50, 60% pr AC 22 Base/Bind AC 22 base 40/60 (60% PR)

mixture code: 251 167163/267163 27774 A252

date type test report: 23‐11‐2012 9‐9‐2011 november 2013 21‐12‐2011 Rapportnummer typetest: K FEC 2.0 APRR Platen 035‐11 FEC 2.0_fase A 11806364 A CONSTITUENT MATERIALS "IN" 100%mass % "IN" 100%mass % "IN" 100%mass % "IN" 100%mass Bestone 8/11 Norwegian  Granite 8/16 7,2 14,92 stone Scotish Granite  16/22 10,8 Scotish Granite  16/22 8 Bestone 16/22 Norwegian  Granite  16/22 9,6 8,93 Scotish Granite 8/16 13,8 ECO‐gravel  10,0

course sand ECO‐sand 20,3 13,17 sand river sand 20 washed sand 12 baghouse dust baghouse du 1,0 1,22 filler wigras 40k 2,6

crushed DAC 0/20 crushed DAC  25,0 57,36 reclaimemilled AC 0/16 32,5 Frees 0/20 32,5 milled PA 25,0  asphalt milled PA 0/16 32,5 Gebroken frees 0/20 32,5

70/100 70/100 1,9 1,76 bitumen160/220 1,6 70/100 1,2

from RAC from RAC 2,4 2,64 from RAC 2,4 from RAC 3,1

COMPOSITION (% through sieve) C22.4 C22.4 100,0 100 C22.4 99,0 C22.4 97,0 C16  C16  94,9 91 C16  87,0 C16  90,0 C11.2  C11.2  80,5 84 C11.2  80,0 C11.2  80,0 C8  C8  64,9 71 C8  60,0 C8  65,0 C5.6  C5.6  55,0 58 C5.6  52,0 C5.6  55,0 2 mm 2 mm 39,9 47 2 mm 43,0 2 mm 44,0 63 μm 63 μm 5,6 6,9 63 μm 8,0 63 μm 6,6

filler filler 5,6 6,6 filler 8,0 filler 6,6

bitumen (in 100% mass) bitumen (in 1 4,3 4,3 bitumen (in 100% ma 4,5 bitumen (in 100% ma 4,3 FUNCTIONAL CHARACTERISTICS

void% void% 4,0 3,3% [%] void% 3,4 void% 3,0

maximum density maximum  density 2480 2473 [kg/m 3 ] maximum density 2477 maximum density 2458 bulk density bulk density 2388 2375 [kg/m3 ] bulk density 2393 bulk density 2384 water sensitivity water 

sensitivity 87 86 [%] water sensitivity 96 water sensitivity 80 stiffness stiffness 9592 9363 [MPa] stiffness 8266 stiffness 9353 restistance to permanent def restistance  to  0,16 0,05 μm/m/s restistance to  permanent def 0,04 restistance to  permanent def 0,01 resistance to fatigue resistance to  108 101 [μm/m] resistance to fatigue 115 resistance to fatigue 120

2) ITT – ITSR

The average indirect tensile test (ITT) results are shown in TABLE IV and Fig. 4. The table and figure show the dry ITT strength, the wet (or after retaining) strength and the ratio of the two. The latter is the property which is used to characterise the resistance to moisture damage in the CEN standards. TABLE III: Numerical results for the stiffness, average, number of tests and standard deviation

TABLE IV: Average indirect tensile results, dry and wet strength and the ratio of both

For this test the results are not very consistent, for projects 1a, 2a and 4a the dry strengths from type 2 and 3 specimen production (plant-lab and plant-field) are fairly similar, but the type 1 results (lab-lab) are lower for 1a and 4a, while for 2a they are higher. For projects 1a and (to a lesser extend) this trend is also present in the wet strength.

For dry strength in project 3a there is a fairly good similarity between type 1 and type 3 specimens, while the type 2 results differ and for project 1b the type 1 and 2 results correspond, but the field result (type 3) veers off. Per project, the relation between the three types of specimen production is more or less consistent, but overall there is no consistent trend.

Fig. 4: Results for the dry and retained (wet) ITS and the ratio of both (ITSR) However, except for project 1a, where the differences between all three types of specimens are small, in all cases the field value (type 3) of the ratio is higher than the lab value (type 1). As such, the lab value of the ratio appears to be a safe indicator for the field value, even though the variations in the actual values are considerable.

project  number  /ID sub  number  /ID type of  specimen  production stiffness  [MPa] number  of tests stiffness  (st dev) 1 a 1 9397 8 221 1 b 1 10002 9 447 2 a 1 9363 18 359 2 b 1 10359 12 635 3 a 1 8266 9 881 4 a 1 9353 18 310 1 a 2 8604 9 389 1 b 2 9592 9 374 2 a 2 11245 18 383 2 b 2 12205 6 257 3 a 2 7750 10 678 4 a 2 10649 18 311 1 a 3 9781 9 253 1 b 3 9850 9 438 2 a 3 10161 18 223 3 a 3 7405 10 832 4 a 3 9810 18 541 project  number  /ID sub  number  /ID type of  specimen  production ITSR [%] ITS dry  [MPa] ITS wet  [MPa] stdev  ITSdry   [MPa] stdev  ITSwet   [MPa] 1 a 1 96 2,33 2,24 0,31 0,21 1 b 1 78 2,51 1,96 0,28 0,23 2 a 1 86 3,46 2,97 0,21 0,12 2 b 1 3 a 1 96 2,69 2,60 0,05 0,21 4 a 1 80 2,80 2,24 0,28 0,04 1 a 2 93 2,62 2,44 0,19 0,10 1 b 2 87 2,61 2,28 0,14 0,13 2 a 2 97 2,91 2,81 0,39 0,06 2 b 2 3 a 2 94 3,22 3,02 0,04 0,09 4 a 2 89 3,08 2,75 0,19 0,10 1 a 3 93 2,60 2,41 0,24 0,18 1 b 3 106 3,01 3,19 0,14 0,11 2 a 3 91 3,00 2,73 0,07 0,06 3 a 3 97 2,02 1,96 0,22 0,12 4 a 3 97 2,98 2,90 0,08 0,06

3) Resistance to permanent deformation

In the cyclic triaxial test, also a lot of variation is found. In this test, the values found are very small. As a result, even with limited standard deviation the relative variation (coefficient of variation) is considerable (TABLE V). What is also striking, is that in the first two projects the variation between the types of specimen production is large, but the trend in the differences is not consistent (Fig. 5). Contrarily, for the third and fourth project, the results between the three types of specimen production are very consistent. The researchers need to consult with the contractors and laboratories involved in order to try and find out the possible causes for this extreme difference.

TABLE V: Numerical results for the resistance to permanent deformation

Fig. 5: Results for resistance to permanent deformation

When looking closely at project 1a and b, which deal with the same mixture but produced and tested in two different

labs, it is striking to see that the tests on field cores (type 3 specimens) produce comparable results (0,17 versus 0,16 [(m/m)/N] ), while the type 2 and especially the type 1 tests are quite different. This indicates that the differences are not caused by differences in testing protocols or equipment, but by differences in mixing and compacting of the specimens.

1) Fatigue resistance

For the fatigue tests the variation is less large (TABLE VI and Fig. 6), although there is variation over the types of production. For the first two projects the lab value is lower than the both the field and the pant value, making it a safe indicator of field response. For these projects the plant performance is closer to field performance. For projects 3 and 4, however, the field values are the lowest, which means that for these projects both the lab (type 1), but especially the plant (type 2) specimens overestimate the field (type 3) response. TABLE VI: Numerical test results for the resistance to fatigue

For P1a and P1b, the differences are again small in the field (type 3) specimens and larger in the other two. This indicates that the differences found are not due to variation in the test protocol or set-up. They are the effect of difference in mixing and compaction. Whether this also explains the differences between the first and second set of projects, remains to be seen. project  number  /ID sub  number  /ID type of  specimen 

production fc #tests stdev(fc)

1 a 1 0,10 4 0,024 1 b 1 0,35 4 0,076 2 a 1 0,05 5 0,029 2 b 1 0 3 a 1 0,04 3 0,007 4 a 1 0,01 4 0,003 1 a 2 0,19 4 0,029 1 b 2 0,16 4 0,014 2 a 2 0,08 5 0,009 2 b 2 0 3 a 2 0,04 4 0,012 4 a 2 0,01 4 0,002 1 a 3 0,17 4 0,014 1 b 3 0,16 4 0,020 2 a 3 0,27 5 0,019 3 a 3 0,04 4 0,009 4 a 3 0,01 4 0,003 project  number  /ID sub  number  /ID type of  specimen  production strain  m/m@  N=10^6 # tests slope  fatigue  line 1 a 1 91 8 ‐5,17 1 b 1 69 9 ‐4,06 2 a 1 101 18 ‐4,73 2 b 1 115 12 ‐5,29 3 a 1 115 9 ‐7,31 4 a 1 120 14 ‐4,75 1 a 2 97 9 ‐5,80 1 b 2 108 9 ‐7,00 2 a 2 106 18 ‐4,92 2 b 2 119 6 ‐4,89 3 a 2 129 10 ‐3,53 4 a 2 117 15 ‐4,82 1 a 3 111 9 ‐4,97 1 b 3 107 9 ‐5,23 2 a 3 109 18 ‐4,69 3 a 3 109 10 ‐3,04 4 a 3 92 13 ‐3,85

Fig. 6: Results for the resistance to fatigue

V. CONCLUSIONS AND RECOMMENDATIONS A. Conclusions

This testing program addresses the relation between functional characteristics and field performance, whether laboratory mixing and compaction are representative for field mixing and compaction and whether the current European functional tests effectively distinguish “good” from “bad” mixtures. The current data set is limited to four materials and construction projects. Considering the variation in the results, it is clear that more results are needed to arrive at any definite conclusions. However, the available data do lead to some preliminary conclusions:

 stiffness values show limited variation and the relation between the lab (type 1) and field (type 3) specimens is fairly good

 for the ITS ratio the lab (type 1) results also seem to give a fair and safe indication of the field (type 3) property, but it must be noted that the variation in both the dry and wet (retained) strength values is such, that the usefulness of the ratio is questionable  the cyclic triaxial data for the first two projects are

very variable, with no consistent trend in the data. The field values for the first project are quite similar for two different labs, but the type 1 (lab) and 2 (plant) specimens are very different. This indicates that mixing and compaction have considerable influence on this property. For the third and fourth project, the results between all three types of specimen production are very consistent, the cause of this large difference between the first two and the second two projects in not yet clear.

 For the fatigue resistance the variation is again limited, but the trends are not consistent. For the first two projects, the type 1 and 2 results are safe indicators (lower than those) for the field (type 3) results, but for project 3 and 4 the field cores give the lowest result. The results for the two labs involved in project 1 show that the differences are due to differences in mixing and compaction, not in testing. Whether or not this can also explain the different

trends between the first and second set of projects, remains to be determined

B. Recommendations

The large differences in the trends between the data for the first set and second set of projects need to be investigated in more detail. First of all, the mixing and compaction equipment used in the labs for the second set of projects need to be determined and compared to those used in the first set. Secondly, more projects should be carried out, increasing the data set and enabling the development of more a more elaborate data-analysis. Finally, it would be interesting to initiation similar initiatives in other countries, to exchange results and information on both the effect of mixing and compaction on mix properties and on the predictive quality of various functional test for field performance.

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