TRANSPORT RESEARCH LABORATORY An Executive Agency of the Department of Transport

RESEARCH REPORT 358

THE PERFORMANCE OF MODIFIED DENSE BITUMEN MACADAM ROADBASES

by J Carswell and D R Gershkoff

Crown Copyright 1993. The views expressed in this publication are not necessarily those of the Department of Transport. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged. The work described in this paper forms part of a Highways Engineering Division, DOT funded research programme conducted by the Transport Research Laboratory.

Highways Resource Centre Transport Research Laboratory Crowthorne, Berkshire, RG11 6AU 1993

ISSN 0266-5247

The Transport Research Laboratory is no longer an Executive Agency of the Department of Transport as ownership was transferred to a subsidiary of the Transport Research Foundation on 1st April 1996.

This report has been reproduced by permission of the Controller of HMSO. The views expressed in this publication are not necessarily those of the Department of Transport.

CONTENTS

Page

Abstract 1

1. Introduction 1

2. Trial Materials 1

2.1 Mixing and laying 1

2.2 Material analysis and compliance 2

3. Binder properties 4

3.1 Sample preparation 4

3.2 Binder tests 4

3.2.1 Empirical tests 4

3.2.2 Dynamic modulus measurements 5

4. Material properties

4.1 Fatigue properties

4.2 Resistance to deformation

4.3 Load spreading ability

5. Design considerations

6. Conclusions

7. Acknowledgements

8. References

6

6

7

8

11

14

14

14

THE PERFORMANCE OF MODIFIED DENSE BITUMEN MACADAM ROADBASES

ABSTRACT

Eight trial lengths of dense bitumen macadam roadbase, seven with modifiers intended to improve performance, were laid at TRL. The modifications included polymer additives, harder grade bitumens and a metallic modifier. Tests carried out on the binders alone were used to predict their potential performance in a DBM roadbase for the majority of the modifications investigated. Measure- ments of the elastic modulus of each material were used in an analysis of structural properties, which confirmed that the polymer modified bitumens did not confer any significant improvement. The principal reasons for this are the low binder content of roadbase macadams and the fairly narrow temperature range experienced by the roadbase (compared with a wearing course hot rolled asphalt). The material with the harder binder and that with a metallic modifier offered about a 20 per cent reduction in thickness over the control material for an equivalent design life. However, the performance of the metallic modifier in other trials has been variable.

1. INTRODUCTION

The British Standard specifications for both hot rolled asphalt (HRA) and dense bitumen macadam (DBM) roadbase materials (BS 594, 1985 and BS 4987, 1988 respectively) are generally satisfactory for the majority of traffic conditions. However, with increasing traffic loading and volume there is a continuing need to improve the performance of these bituminous pavement layers.

For new construction work an allowance can be made for a greater depth of construction for increased durability under traffic, defined by the number of million 80 kN standard axles (msa) (Powell, Potter, Mayhew and Nunn, 1984). In the case of reconstruction work the available depth is usually restricted by existing height clearances and drainage levels. This reduces the design life of the reconstructed road if the thickness has to remain the same, while coping with a heavier traffic demand.

For a number of years various polymer modifiers have been tried and tested in the wearing course layer of the road, principally in order to improve resistance to defor- mation. Some of these modifiers have been shown to be beneficial in this regard, and to be cost effective. Logi- cally, it has been thought that by extending the use of these modifiers into the lower, and much thicker, roadbase layers that improvements in the load spreading ability could be gained without adversely affecting resistance to fatigue cracking and durability.

Binder modification is not without cost and thus any modification needs to show an improvement that can clearly be demonstrated before being more generally

specified. Polymer additives are about 5 to 10 times more expensive than bitumen and, though the proportion used is small, the cost of the modified bitumen is likely to be about 20 per cent higher than conventional bitumens. It is therefore imperative to devise an assessment procedure that is able to discriminate between modifications to roadbases likely to show a marked improvement and those whose effects are negligible. One such procedure has been proposed (Denning and Carswell, 1983).

It is not sufficient to assume that if a polymer improves the performance of a HRA wearing course that it will necessarily affect the roadbase layer to a similar degree. The principal reasons for this are:

a DBM material is more dependent upon aggregate interlock, and less on the properties of the binder matrix. Further, the binder content used is much lower reducing the overall effect of the modified binder.

ii. the temperature range experienced in the roadbase layer is narrower.

Early pilot-scale trials investigating the potential use of modifiers in both HRA and DBM roadbase materials were promising, although the effects were not as marked as for wearing course materials (Carswell, 1986). Further work carried out (Rant and Schoepe, 1989) was less encour- aging; it showed that the use of an ethylene vinyl acetate (EVA) co-polymer in roadbase macadams gave inconclu- sive results in terms of improved load spreading ability. In fact, changing the binder grade from 100 pen to 50 pen yielded a much better improvement in the performance of roadbase macadams (Nunn et al, 1987).

In the present work a number of modifiers and modifica- tions in DBM roadbase were assessed.

2. TRIAL SECT IONS

2.1 MIXING AND LAYING

Eight sections of DBM roadbase were specified to the mid-point of Table 1 of BS 4987 with the only variation being the type of modification. The exception to this was for the heavy duty macadam (HDM) roadbase material which was specified in accordance with the Department of Transport's Specification for Highway Works, clause 923A (Department of Transport, 1986). Table 1 shows the roadbase sections together with the type and degree of modification.

In order to minimise mix variation all the materials were mixed by Bardon Ltd (London) at their West Drayton asphalt mixing plant. The proprietary binders were

TABLE 1

Description of Materials

Material Material type Type and Degree of Modification Other information

DBM 100 Control 100 pen, 40 mm Dense Bitumen Macadam (BS4987)

DBM 50 DBM

HDM Heavy Duty Macadam

SBR DBM

SBS DBM

50 pen bitumen

50 pen bitumen, revised grading, added filler

5 per cent of styrene-butadiene-rubber (SBR) (as a proportion of 100 pen bitumen)

7 per cent blend of styrene-butadiene-styrene (SBS) block co-polymer in 100 pen bitumen

Non-proprietary

Proprietary- added in latex form

Proprietary- pre-blended in a compatible bitumen

EVA 1 . DBM 5 per cent blend of ethylene-vinyl-acetate Non-proprietary (EVA) grade 19/150 in 100 pen bitumen

5 per cent blend of ethylene vinyl acetate (EVA) grade 33/25 in 100 pen bitumen

EVA 2 DBM

METAL DBM 2 per cent blend of organo-metallic compound of manganese (plus copper and cobalt) in 100 pen bitumen

Non-proprietary

Proprietary modifier in a carrier solvent

supplied by the respective binder manufacturers, while the other modifiers were blended with the 100 pen bitumen prior to mixing. The SBR and METAL modifiers were added directly to the pug-mill when the material was being mixed.

Twenty tonnes of each material were mixed, and laid at TRL by Associated Asphalt within a prepared 3 metre strip on a crushed limestone base. The same paving and compaction procedure was used throughout. The nominal depth of the sections was 150 mm. While 20 tonne is a small quantity for a roadbase material, it was thought that it would be representative of normal produc- tion material, and be sufficient to provide a suitable length from which to extract samples to test in the laboratory.

None of the materials posed any problem in either the mixing or laying processes, although metered dosing pumps would need to be specified and incorporated into the mixing cycle for the SBR and METAL modifiers. Mixing time was slightly longer for the modified materials. This, coupled with generally higher mixing temperatures for the modified materials indicate an additional cost for mixing compared with the DBM 100 material. For the polymer modified materials there is also about a 20 per cent increase in cost over a penetration grade bitumen to cover the cost of the polymer and, where necessary, increased blending costs. Overall, it is estimated that

polymer modified DBM roadbases would cost between 15 and 20 per cent more than a conventional DBM.

Table 2 summarises the mixing and laying temperatures for each material. It was noted during the laying of the materials that the SBS and DBM 50, in particular, appeared to be variable in quality with areas of poorly coated coarse aggregate. This suggests that either the mixing time was too short for these materials or that the mixing temperature was too low.

2.2 MATERIAL ANALYSIS AND COMPLIANCE

The materials were analysed for compliance with specifi- cation, and these results are shown in Table 3; all were within specification.

Core samples were taken from the test site at TRL and measurements of density and percentage refusal density (PRD) made. These results, together with the calculated air voids in the mix (Vv) are shown in Table 4 for each material.

The PRD values are high, particularly for the DBM 100, the METAL and polymer modified materials, indicating a high level of compaction. The air voids (Vv) for the two EVA and the METAL modified materials are low, suggest- ing that these modifiers aid the compaction process.

2

TABLE 2

Mixing, Laying and Compaction Temperature Ranges

Material Mix Temperature (specified) Paver Temperature Rolling Temperature (oc) (oc) (oc)

DBM 100 130-145 125-130 115-125 DBM 50 135-150 120-125 115-120 HDM 140-145 120-130 120-125 SBR 140-150 135-140 115-125 SBS 135-145 130-140 115-120 EVA 1 135-150 125-135 110-125 EVA 2 140-150 115-125 110-115 METAL 150-170 145-160 110-130

TABLE 3

Compositional Analysis of Materials.

Material Sieve size 50 (mm) 37.5 (mm) 28 (mm) 14 (mm) 6.3(mm) 3.35 (mm) 300 (pm) 75 (~m)

Specification (per cent by mass passing)

100 95-100 70-94 56-76 44-60 32-46 7-21 2-9

Binder Content

(per cent)

2.9-4.1

DBM 100 100 100 90 64 46 39 8 5 3.5 DBM 50 100 100 94 70 57 42 11 6 3.7 SBR 100 100 86 67 47 37 10 6 3.4 SBS 100 97 85 70 48 36 12 8 3.3 EVA 1 100 100 84 66 45 33 11 7 3.1 EVA 2 100 100 80 69 50 40 11 6 3.6 METAL 100 100 85 64 49 37 11 6 3.4

Specification (per cent by mass passing)

100 95-100 70-94 56-76 44-66 32-42 7-21 7-11 2.9-4.1

HDM 100 100 93 73 52 42 13 9 3.5

TABLE 4

Density and Voids of Materials

Material Binder Density PRD Air Voids" content (Mg/m 3) (Vv)

(per cent) Mean Standard deviation (per cent) (per cent)

DBM 100 3.5 2.36 0.01 97.5 7.5 DBM 50 3.7 2.37 0.02 95.3 6.7 HDM 3.5 2.35 0.02 95.7 7.7 SBR 3.4 2.37 0.01 97.2 7.2 SBS 3.3 2.38 0.01 96.8 6.9 EVA 1 3.1 2.43 0.02 96.5 5.1 EVA 2 3.6 2.40 0.01 96.4 5.7 M ETAL 3.4 2.42 0.02 98.7 5.1

"Voids calculated in accordance with BS 598: Part 104:1989

3

3. B I N D E R P R O P E R T I E S

3.1 SAMPLE PREPARATION

The addition of polymers to bitumens can dramatically change their rheological behaviour. In order to assess the effects of the polymer additives used in this trial, tests were undertaken on the binders both as supplied and after the Rolling Thin-Film Oven Test (RTFOT) (ASTM, 1974). The RTFOT is a simulative ageing test which exposes the binder to a controlled heated air flow at 163°C for 175 minutes. This method of obtaining repre- sentative binder samples was used in preference to binder recovery because of the difficulties and uncertain- ties associated with the chemical recovery of polymer- modified bitumens. Although the conditions in the test are not identical to those found in practice, research has shown that the amount of hardening in the RTFOT correlates reasonably well with that observed in a conventional batch mixer (Whiteoak, 1990).

The METAL modified binder cannot easily be assessed by binder only testing as the binder undergoes a pro- nounced curing phase. This is difficult to reproduce, although the standard penetration test can yield some information. Work carried out using the metallic modifier (Daines et al, 1985) found that the drop in penetration during the mixing and laying processes resembled that of the base bitumen. However, the recovered penetration after 16 months fell to 12 pen, indicating an extremely hard bitumen. It would thus appear that the rheological effect of the METAL modifier is to accelerate the harden- ing of the binder.

3.2 BINDER TESTS

For the two unmodified bitumens and the SBS binder, tests were performed on samples as supplied to the mixing plant. The other polymer modified binders, namely the SBR, EVA 1 and EVA 2, were blended using a high-

shear mixer in the laboratory, prior to testing. Tests undertaken on the binders fall into two categories:-

(a) empirical tests

(b) dynamic modulus measurements.

3.2.1 Empirical Tests

The empirical tests consisted of the standard penetration and softening point determinations as generally used for specifying bitumen consistency in the UK, and the Fraass breaking point test. The Fraass test (Institute of Petro- leum, 1992) provides an indication of the binder's resistance to brittle fracture at low temperatures and has been used by other researchers to assess the effects of modification at these temperatures. A common way of expressing the change in viscosity with temperature is to calculate the penetration index (PI) of the bitumen (Pfeiffer and Van Doormaal, 1936). For conventional bitumens, the PI gives an indication of bitumen type and a measure of temperature susceptibility. The test results along with PI values, calculated from the penetration and softening point results, are listed in Table 5.

Considering the penetration and softening point results for the binders prior to ageing, the 50 and 100 pen bitumens had typical values for their grades, complying with the limits set in BS3690. The SBS binder (manufac- tured using a nominal 100 pen grade bitumen) had a higher penetration and a markedly higher softening point than the 100 pen bitumen resulting in a calculated PI of 7.3. Of the binders manufactured in the laboratory, EVA 1 showed the most change from the 100 pen bitumen with a significantly lower penetration and increased softening point. EVA 2 showed the next greatest degree of modifi- cation, with the SBR binder showing the least effect. The Fraass breaking point results also show improvement on the 100 pen results, the SBS binder again providing the best result followed by EVA 2, EVA 1 and SBR.

TABLE 5

Empirical test results for Binders before and after the Rolling Thin-Film Oven Test

Binder Pre-RTFOT

Penetration Softening Fraass at 25°C point point (mm/10) (°C) (°C)

Post-RTFOT Penetration Penetration Softening Fraass Penetration

index at 25°C point point index (mm/10) (°C) (°C)

100 PEN

50 PEN

SBR

SBS

EVA 1

EVA 2

101 42.6 -15

50 51.4 -14

89 46.0 -17

107 82.6 -23

64 58.8 -20

75 51.6 -22

-1.1 52 52.0 -11 -0.3

-0.5 29 58.4 -12 -0.2

-0.4 50 55.8 -14 -0.5

7.3 73 87.6 -19 6.6

1.7 37 64.4 -15 1.4

0.6 43 61.0 -16 1.1

The results after the RTFOT show reduced penetrations, increased softening points and increased Fraass break- ing points for all the binders tested. As before, the same trends were found with the SBS binder showing the greatest effects of modification followed by EVA 1, EVA 2 and SBR. However, it was noted that the 100 pen bitumen had aged more than would have been expected with its penetration dropping by nearly 50 per-cent. The Fraass results indicated the same trend as before. The PIs of all but the two most heavily modified binders increased as would be expected; unusually, the SBS and EVA 1 showed a small decrease in PI.

3.2.2 Dynamic Modulus Measurements

Research has shown that the elastic modulus of a bituminous mix is strongly dependent upon the dynamic modulus of the binder used (Bonnaure et al, 1977, Brown and Brunton, 1988). For conventional bitumens, Van der Poel's nomograph can be used to predict the (tensile) stiffness modulus at any temperature or loading time to within a factor of two simply from the penetration and softening point test results. This predicted tensile modu- lus is three times greater than the shear modulus (Van der Poel, 1954). The accuracy of the nomograph is generally accepted to be adequate for most design purposes. However, where polymers or other modifiers have been added to the binder, the relationships on which the nomograph are based are not necessarily valid (Vonk and Van Gooswilligen, 1989 and Gershkoff, 1991). Thus where high accuracy is needed, or when modified binders are used, it is necessary to measure directly the dynamic modulus of the binder.

The dynamic shear modulus of the artificially aged binders was measured at frequencies from 0.1 to 10 Hz

over the temperature range 0°C to +30°C using a controlled stress rheometer. Small strain amplitudes were used, typically 1 per cent, to ensure that the response was kept in the linear visco-elastic range. Van der Poel's nomograph has been used to calculate predicted stiffnesses for all the binders at 5 Hz at 10°C increments over the temperature range studied. The results for the SBS binder had to be obtained by extrapolation of the nomograph because of its very high PI value. These predicted stiffnesses (divided by three to give equivalent shear modulus) along with the measured dynamic shear moduli are listed in Table 6.

For the two unmodified bitumens, the nomograph tended to overestimate stiffnesses, but all the results were well within the acceptable factor of two of the measured modulus. With the modified binders, the nomograph produced very variable results. The SBR, EVA 1 and EVA 2 predictions were within a factor of two of the measured values. However, the nomograph underesti- mated stiffness below 10°C and overestimated them above 10°C implying less sensitivity to temperature than was observed. For the SBS, the most heavily modified binder, predictions were all far too low with only the 30°C result falling within a factor of two of the measured modulus.

The modulus values of each binder at a loading fre- quency of 5 Hz relative to the 100 pen control bitumen are shown in Figure 1. Below 15°C, none of the modified binders are stiffer than the control. Indeed, the SBR and SBS binders are less stiff than the control over the entire temperature range studied. EVA 2 does show slight improvement above 25°C, but the most promising modified binder is EVA 1 which although slightly less stiff below 15°C, increases in relative stiffness to be twice as

TABLE 6

Dynamic shear modulus results and stiffnesses predicted from the Van der Poel nomograph at 5 Hz

100 Pen Bitumen 50 Pen Bitumen SBS

Temperature Measured SvJ3 Ratio of Measured SvJ3 Ratio of Measured SvJ3 (°C) Modulus Predicted to Modulus Predicted to Modulus

(MPa) (MPa) Measured (MPa) (MPa) Measured (MPa) (MPa)

Ratio of Predicted to Measured

0 93 100 1.08 91 130 1.43 42 3.0 10 27 27 1.00 32 50 1.56 11 1.3 20 5.6 10 1.79 8.9 13 1.46 2.0 0.67 30 0.79 1.3 1.65 1.8 3.3 1.83 0.36 0.23

0.07 0.12 0.34 0.64

SBR EVA 1 EVA 2

Temperature Measured SvJ3 Ratio of Measured SvJ3 Ratio of Measured Swp/3 (°C) Modulus Predicted to Modulus Predicted to Modulus

(MPa) (MPa) Measured (MPa) (MPa) Measured (MPa) (MPa)

Ratio of Predicted to Measured

0 79 50 0.63 91 50 0.55 71 50 10 20 23 1.15 30 23 0.77 19 20 20 4.0 6.7 1.68 7.8 lO 1.28 4.4 6.7 30 0.69 1.3 1.88 1.5 2.3 1.53 1.1 1.7

0.70 1 .o5 1.53 1.55

5

Frequency = 5 Hertz

O 50 Pen bitumen

• EVA 1

D EVA 2

v SBR

• SBS

== "10 O E Q)

t r

0.9 --

0.8

0.6 I V

0.5

0 . 3 1 I I I 0 5 10 15

V

20 25 30

Temperature (°C)

Figure 1. Modulus of aged binders relative to 100 pen bitumen

stiff as the control at 30°C. However, EVA 1 is still not as stiff as the 50 pen bitumen which has the greatest modulus from 5°C upwards.

These results indicate that, of the modified binders tested, only EVA 1 has the potential for giving better load spreading ability than the control 100 pen at 20°C.

If it were possible to formulate modified binders with better properties over the entire in-service temperature range, binder modulus testing could be used to assess the likely merits of different bitumen-modifier blends. Equally, binder modulus testing could be used to elimi- nate unsuitable binders without the need to produce aggregate mixes.

4. MATERIAL P R O P E R T I E S

Cores were taken from each trial length and laboratory testing of fatigue, creep and elastic modulus carried out

on prepared samples. Creep tests were performed 3 months after laying, and for the DBM 100 and METAL materials again after 15 months. This was to examine the effect of age-hardening of the binder (DBM 100) and accelerated binder curing (METAL). It was assumed that the other materials would be similar to the DBM 100 in terms of age-hardening. Modulus and fatigue measure- ments were carried out 15 months after laying. Modulus measurements were also made on the METAL-modified material after 3 months to provide information on the material's initial state (before curing).

4.1 FATIGUE PROPERTIES

Uniaxial fatigue tests were carried out on five beams (300 x 100 x 100 mm) of each material. Constant stress amplitudes were applied to each beam and the initial strain induced (measured after 1500 load cycles) and the load repetitions to failure recorded.

All the materials showed a linear relationship between the logarithm of initial tensile strain and logarithm of load cycles to failure. Figure 2 shows the fatigue lives for each

e -

10 4 ='~ " ' i ~

-% - eta,

N EVA 1 ~ ~ ~ DBM 100

~ ~ DBM 50 ~ HDM

EVA 2

SBS

104 10 s

\ \

~ SBR 106

Number of cycles

Figure 2. Fatigue results at 25°C, 25Hz

material. No definite trend can be established from these lines with all the materials showing strains of the same order of magnitude. Therefore, none of the modifications investigated would appear to have an improved fatigue performance.

In practice, the road experiences rest periods, of varying duration, between repetitions of traffic load. For the more elastic modifiers (such as the SBS) there is the thought that these short rest periods could allow autogenic healing of the road to occur, due to the elastic nature of the modifier. This effect, if demonstrated in a revised fatigue test method incorporating rest periods, would improve the fatigue life of this type of modified material above that of a more viscous material, where recovery does not occur.

4.2 RESISTANCE TO DEFORMATION

Most deformation takes place within the wearing course layer of the road structure where higher temperatures are experienced. However, any internal deformation of the

lower layers will affect the surface deformation, so it is important that these layers also have good deformation resistance.

Uniaxial creep tests were performed on all the materials 3 months after laying, and additionally for the DBM 100 and METAL materials 15 months after laying. Table 7 gives the mean permanent strain of four creep tests carried out on each material after a loading time of 104 seconds (applied stress 100 kPa) at a constant temperature of 30°C (British Standards Institution, 1990).

Both the control and the modified macadams show a relatively high resistance to internal deformation, al- though the scatter in some of the results is rather high. However, with the exception of the DBM 50 and SBS, all indicate better creep resistance than the control DBM 100, with the modifiers HDM, the two EVAs and the METAL (after 15 months) offering a marked improve- ment. The result for the DBM 50 is surprising, given the harder penetration grade of bitumen used. It may be that material variability has masked what was expected to be an improved material.

7

TABLE 7

Uniaxial Creep Test Results at 30°C (tested 3 months after laying)

Material Permanent axial strain after104 seconds (per cent)

Mean Standard deviation

Potential deformation relative to DBM100

DBM 100 0.6 0.2 1.0

DBM 50 0.6 0.2 1.0

HDM 0.35 0.08 0.6

SBR 0.55 0.04 0.9

SBS 0.6 0.2 1.0

EVA 1 0.25 0.03 0.4

EVA 2 0.3 0.05 0.5

METAL 0.5 0.2 0.8

METAL (15 months) 0.2 0.05 0.3

The METAL modified material has nearly trebled its internal resistance to flow in a 12 month period, indicating a marked curing effect. Further creep tests carried out on the DBM 100 material after 15 months showed that its stiffness increased by just over 12 per cent, though the value was still within the range of the earlier (3 month) results. It can therefore be assumed that, for the METAL modified DBM, its threefold increase in stiffness is principally due to the curing process, and not due to age- hardening of the bitumen component.

4.3 LOAD SPREADING ABILITY

The load spreading ability of a bituminous layer is directly related to its elastic modulus. A high elastic modulus will help reduce the stresses induced in the lower sub-base layers. The elastic modulus of a bituminous layer is dependent on both the frequency of traffic loading and temperature. If the decrease in elastic modulus with temperature can be reduced such that the material has a relatively higher modulus at the higher in-service tem- peratures, then it will have a better performance. At the lower pavement temperatures (less than 10°C) the material should not be prone to brittle fracture.

The binder tests discussed earlier have shown that thermoplastic polymers cause changes in the rheological properties of a binder that can alter the temperature and frequency dependence over the typical road temperature range. To determine whether these effects are repro- duced in the mixed material the elastic modulus of each material was measured, in the laboratory, over a range of temperature from 0 to 30°C and frequency from 0.3 to 30 Hz. The load amplitudes applied were such that testing was carried out within the linear visco-elastic range.

For the DBM 100 material six samples (300 x 100 x 100 mm) were tested in order to produce a datum against which to compare the other materials. Two or three samples of the other macadams were tested in order to

make an initial assessment of material variability. The results obtained suggested the material variability would be unlikely to be worse than the DBM 100; in fact, for the material types DBM 50, HDM and EVA 1 this has been demonstrated in full-scale road trials (Nunn et al, 1987, and Rant and Schoepe, 1989).

In representing the elastic modulus results it can be misleading to quote the values at one temperature/ frequency condition. This is because modified materials exhibit different temperature and time effects, which would result in a different ranking order should a different temperature/frequency condition be chosen.

A method of representing the elastic modulus of a material over a wide frequency range at a chosen temperature has been proposed by Ferry (1961). The method enables the individual modulus curves, obtained at each temperature, to be shifted to a chosen tempera- ture (within the temperature range examined) to produce a 'master' curve. The master curves thus produced enable the materials to be directly compared with each other at the chosen reference temperature over a wide range of loading time. However, the temperature shifting curves are different for each material, being dependent on the temperature susceptibility of the binder.

Figure 3 shows the elastic modulus master curves at 20°C. The results were such that it was possible to combine the DBM 50, HDM and EVA 1 into one group and the SBR, SBS and EVA 2 into a second group in Figure 3. The master curve for the METAL modified material tested 3 months after laying (not shown) was nearly identical to the DBM 100 in shape, and slightly less stiff over the whole frequency range.

Trends are apparent, with the modified materials gener- ally showing less change in modulus with loading time. At the lower temperatures (below 10°C), and corresponding to the high frequency end of the master curves shown in

8

(3.. (.9

O E

G~ UJ

10 ~

100

HDM

DBM 50

EVA 1

Metal DBM 100

SBS

EVA 2

SBR

1 0 -4 10 -2

I I 10-~ 100 10 ~ 102 103

Frequency (Hz)

I I I 1 0 4

Figure 3. Modulus master curves re ferenced to 20°C

10 s

Figure 4 shows the temperature shift curves for the DBM 100 and METAL modified materials to demonstrate the marked difference that the temperature susceptibility of the binder can have. It is clear from this figure that the METAL modified macadam is significantly more tempera- ture susceptible than the DBM 100. The shift curves associated with the other materials are similar to the DBM 100 over the temperature range 0 to 30°C.

For simplicity, plots were constructed showing the elastic modulus for each macadam relative to the DBM 100 at 5 Hz over the 0 to 30°C temperature range. The mean results are shown in Figure 5. Significantly, the ranking of

- . Meta__,

the materials follows closely the ranking of the binders (Figure 1 ).

It is also obvious from this work that the three polymer modified materials SBR, SBS and EVA 2 have produced no beneficial effect in terms of increased modulus, confirming the prediction from the binder work. Indeed, if these particular modifiers were to have a place in the roadbase layer, then either the base bitumen would need to be of a harder grade and/or the dosage level of polymer would need to be increased. Either route would incur an additional cost without a corresponding assur- ance that the resultant modulus would be significantly better than DBM 100.

Over the normal temperature range experienced in the roadbase, the EVA 1 material is no better than the harder penetration grade DBM materials (DBM 50 and HDM). An alternative, from a research viewpoint, would be to blend EVA 1 in a 70 pen bitumen to give an improved elastic modulus. If good compaction of the harder grade macadams is more difficult to achieve, then EVA 1 may prove to be a useful alternative.

6

5

4

a

,:I::::

~ e d

1

o

- 2 I o

Figure 3, the materials are all assymptoting to their elastic limit, with the METAL modified material approach- ing an exceptionally high modulus value. The group containing SBR, SBS and EVA 2 have modulus values lower than, or similar to, the DBM 100 over the whole range. As the temperature is increased the group con- taining DBM 50, HDM and EVA 1 show improved modu- lus over the DBM 100 (corresponding to the low fre- quency end of Figure 3).

I I I I 5 10 15 20

Temperature (°C)

Figure 4. Temperature shift curves for DBM 100 and Metal

I 25 30

10

Frequency = 5 Hertz 5

O

E

rr

1 0.9 0.8

0.6'

0.5

0.4 [--

0.3 t

0.2 0

m

r

z~ Metal • SBS

V HDM • EVA2

O DBM50 [] SBR • EVA 1

I I I I I 5 10 15 20 25 30

Temperature (°C)

Figure 5. Modulus of roadbase mixes relative to DBM 100

The only modifier to produce a marked effect is METAL in its cured state,_where.at 20°C_the material is over three times stiffer than the DBM 100. However, a review carried out by Nicholls on the metallic modifier (1990) showed that when used as a roadbase material it cannot be guaranteed that curing will take place, owing to the reduced presence of oxygen at this depth in the pave- ment. In this pilot-scale trial the materials were not covered by any other bituminous layer and the METAL modified material benefitted from full exposure to the atmosphere, yielding results that might not be repeated when used in-more typical roadbase conditions.

5 . D E S I G N C O N S I D E R A T I O N S

For roadbase design purposes the elastic modulus of a material is regarded as the most important parameter. Based on the work by Powell et al (1984) it is a relatively simple matter to calculate the required thickness of a roadbase layer to survive a given traffic loading. The

elastic modulus of the test material relative to a standard DBM. 1_00 of. modulus 3.1_ GPa, is used to calculate the required thickness of the test material necessary to give the same design life as the standard DBM for equivalent msa. In this work the DBM 100 had an elastic modulus of 4.4 GPa. For any modification to the roadbase layer, the potential reduction in thickness for equivalent life can be quantified, and the cost-effectiveness of the modified material assessed.

Many factors can influence the stiffness of a dense bitumen macadam,, including f i l ler and binde~ contents, degree-of compaction (PRD and voids) and type of binder. While the pilot-scale trial sought to eliminate all the variables except for the type of modification, it was nevertheless evident that differences in the other param- eters affected the measured modulus values. Various equations have been developed in order to take account of material variability in terms of filler content and recov- ered penetration of the binder (for example, Nunn, 1987). More recent work has included the effect of PRD on the measured elastic modulus (Goddard, 1991).

11

In this present work, a relationship was found between the logarithm of elastic modulus and the filler content (per cent), the binder penetration (after the RTFOT) and the PRD (per cent). The equation takes the form:

Log~oE = -4.752 +0.0574(PRD) -0.00906(P) +0.0317(F) (1)

(with a R 2 = 0.77)

where, E = measured elastic modulus (GPa)(20°C, 5 Hz)

PRD = percentage refusal density (per cent)

P = binder penetration (after RTFOT) (1/10 mm)

F = filler content (per cent)

The relationship is shown in Figure 6, where for the METAL modified material a nominal recovered penetra- tion of 12 has been assumed, as the curing process

cannot be simply reproduced by the RTFOT. Normally, the RTFOT represents the hardening undergone by the binder in the mixing and laying process and, as has already been stated in the binder section, agrees well with recovered penetration values.

Equation 1 was used to eliminate the effect of PRD on the modulus results. The normalization was carried out on the modulus results obtained at 20°C and 5 Hz so that the materials could be directly compared with the DBM 100. Table 8 shows the normalized elastic modulus results for each material.

These normalized modulus values were then used to calculate layer thicknesses to limit the vertical strain on the sub-grade and the tensile strain induced at the bottom of the roadbase layer. Two design cases were considered, one for traffic up to 80 msa, and the other for traffic over 80 msa. In both cases the pavement was

12

Q_

" o o E

(0 (1)

10

2

0 0

z~

, o . /

I I 2 4 6 8 10 12

Predicted modulus (GPa)

Figure 6. Comparison of predicted and measured values of elastic modulus

12

TABLE 8

Normalised elastic modulus results (20°C, 5 Hz)

Material PRD (per cent) Measured Normalized Normalized Modulus Modulus (GPa) Modulus • (GPa) relative to DBM 100

DBM 100 97.5 4.4 4.4 1.0

DBM 50 95.3 5.6 7.6 1.7

HDM 95.7 5.7 7.3 1.7

SBR 97.2 2.6 2.7 0.6

SBS 96.8 3.0 3.3 0.8

EVA 1 96.5 4.5 5.2 1.2

EVA 2 96.4 2.8 3.3 0.8

METAL 98.7 11.2 9.5 2.2

• normalized to 97.5 per cent PRD

constructed over a sub-grade of CBR 5 per cent on a 225 mm thick granular sub-base. For the heavier traffic design the lower layer of the pavement was 125 mm of HRA. For both design cases, the pavement was finished with a 40 mm HRA wearing course.

The required material thicknesses were calculated over a range of traffic levels. The designs showed that the sub- grade stress criterion required the greater thickness of roadbase material for all the materials, with the exception of the SBR material where the strain induced at the

bottom of the roadbase layer reached the critical condi- tion up to 20 msa. The thickness of each material for equivalent life for each expected traffic level is shown in Table 9, together with the approximate percentage reduction in thickness for the modified macadams relative to the DBM 100.

The percentage reductions obtained with the DBM 50 and HDM materials are similar to those found in other work (Nunn et al, 1987), and therefore similar reductions in costs of construction can be assumed. Although it was

TABLE 9

Design Thicknesses for Equivalent Traffic Loading

msa Material thickness (mm)

Design for normal traffic

DBM 100 DBM 50 HDM SBR SBS EVA 1 EVA 2 METAL

5 188 160 162 226 206 179 206 149

10 215 181 183 256 236 204 236 169

20 244 204 206 290 268 231 268 190

50 287 239 242 340 317 271 317 221

REDUCTION IN THICKNESS (per cent)

16 15 (-19) (-10) 6 (-10) 21

Design for heavy traffic

80 200 176 178 237 215 192 215 165

200 252 219 221 290 273 242 273 206

REDUCTION IN THICKNESS (per cent)

13 12 (-15) (-8) 4 (-8) 18

13

expected that the HDM would give the greater reduction in thickness, the atypical results are attributed to material variability.

The calculations for the EVA 1 modified material show a reduced thickness, but the amount is too small to be taken as significant.

The METAL modified material shows the greatest reduction in thickness relative to DBM 100, but it is not much different from the DBM 50 and HDM, and, given doubts over its curing, appears not to offer any cost effective advantage over these more proven materials.

For the rest of the modified materials, namely the SBR, SBS and EVA 2, their elastic modulus values at the design condition are such that the roadbase thickness would need to be increased by between 10 and 20 per cent, clearly making them non-viable.

The relatively disappointing performance of the polymer modified roadbases may ,in part, be explained by the nature of dense bitumen macadam. For DBM roadbase, the role of the binder is to bind the aggregates firmly together to achieve a mechanical interlock. The binder content of 3.7 per cent in DBM roadbase is generally sufficient to produce this effect. In a HRA wearing course (with a typical binder content of about 8 per cent) the mortar (binder, sand and filler) governs the resulting mechanical properties of the material.

Further, the modifiers used in HRA wearing course, generally speaking, come into their own at the higher road temperatures (over 30°C), where the changed visco- elastic properties of the binders can give substantial improvements in deformation resistance (Carswell, 1987). For conditions in the UK, over the temperature range normally experienced in the roadbase, the modified binder modulus values are similar to unmodified bitumens.

6. CONCLUSIONS

The pilot-scale trials with modified roadbase macadams have shown that none of the polymer additives studied are likely to significantly improve the performance of the roadbase layer over conventional penetration grade bitumens. While the metal modified bitumen showed an improved performance in this trial, its performance in other trials has been variable and thus cannot be guaran- teed in a roadbase layer.

This work confirms the improvements in roadbase performance from using harder grade bitumens, where reductions in roadbase thickness of around 20 per cent are possible without adversely affecting the life of the structure. However, these materials require higher mixing and rolling temperatures to ensure adequate coating and compaction.

Measurements of dynamic shear modulus of the binder alone are able to identify binders capable of improving

roadbase performance. At the same time it is possible to check that increases in shear modulus over the normal roadbase temperature range will not lead to greatly increased viscosities during normal mixing and laying operations.

The dynamic shear modulus of a binder could also be an effective measure in screening modifiers for use in wearing course layers.

7. ACKNOWLEDGEMENTS

The work described in this report was carried out in the Highways Resource Centre (Resource Centre Manager: Mr P G Jordan) of TRL. The authors wish to acknowledge the contribution made to this work by Bardon Ltd (Lon- don) and Associated Asphalt in the mixing and laying of the trial materials.

8. REFERENCES

AMERICAN SOCIETY FOR TESTING AND MATERIALS (1974). Standard method of test for: effect of heat and air on a moving film of asphalt (rolling thin-film oven test). ASTM D 2872-70. American Society for Testing and Materials, Philadelphia.

BONNAURE F, GEST G, GRAVOIS G and UGE P (1977). A new method of predicting the stiffness of asphalt paving mixtures. Volume 46 of the Proceedings of the Association of Asphalt Paving Technologists, 1977.

BROWN S F and BRUNTON J M (1988). An introduction to the analytical design of bituminous pavements. Third Edition. University of Nottingham.

BRITISH STANDARDS INSTITUTION (1985). Hot rolled asphalt for roads and other paved areas. Part 1. Specifi- cation for constituent materials and asphalt mixtures. British Standard BS 594:1985. British Standards Institu- tion, London.

BRITISH STANDARDS INSTITUTION (1988). Coated macadam for roads and other paved areas. Part 1. Specification for constituent materials and asphalt mixtures. British Standard BS 4987:1988. British Stand- ards Institution, London.

BRITISH STANDARDS INSTITUTION (1990). Method for determination of creep stiffness of bitumen aggregate mixtures subject to unconfined uniaxial loading. British Standard Draft for Development DD 185:1990. British Standards Institution, London.

CARSWELL J (1986). An assessment of bituminous basecourse and roadbase materials containing EVA and sulphur. Department of Transport TRRL Report RR92: Transport and Road Research Laboratory, Crowthorne.

14

CARSWELL J (1987). The effect of EVA-modified bitumens on rolled asphalts containing different fine aggregates. Department of Transport TRRL Report RR122t. Transport and Road Research Laboratory, Crowthorne.

DAINES M E, CARSWELL J and COLWlLL D M (1985). Assessment of 'Chem-crete' as an additive for binders for wearing courses and roadbases. Department of Trans- port TRRL Report RR54: Transport and Road Research Laboratory, Crowthorne.

DENNING J H and CARSWELL J (1983). Assessment of 'Novophalt' as a binder for rolled asphalt wearing course. Department of the Environment, Department of Transport TRRL Report LR1101: Transport and Road Research Laboratory, Crowthorne.

DEPARTMENT OF TRANSPORT (1986). Specification for highway works, Part 3. HMSO, London.

FERRY J D (1961). Viscoelastic properties of polymers. John Wiley & Sons, New York.

GERSHKOFF D R (1991). A study of the rheological behaviour of some surface dressing binders. MSc dissertation, University of Nottingham.

GODDARD R T N (1991). Private communication. TRL

INSTITUTE OF PETROLEUM (1992). Standard test methods for analysis and testing of petroleum and related products: breaking point of bitumen Fraass method. Institute of Petroleum IP 80/87. Institute of Petroleum, London.

NICHOLLS J C (1990). A review of the effects of using Chemcrete in bituminous materials. Department of Transport TRRL Report TR271: Transport and Road Research Laboratory, Crowthorne.

NUNN M E, RANT C J and SCHOEPE B (1987). Im- proved roadbase macadams: road trials and design considerations. Department of Transport TRRL Report RR132. Transport and Road Research Laboratory, Crowthorne.

PFEIFFER J Ph and VAN DOORMAAL P M (1936). The rheological properties of asphaltic bitumens. Journal of the Institute of Petroleum, Vo122, 1936.

POWELL W D, POTTER J F, MAYHEW H C and NUNN M E (1984). The structural design of bituminous roads. Department of Transport TRRL Report LR1132.. Trans- port and Road Research Laboratory, Crowthorne.

RANT C J and SCHOEPE B (1989). Trials of roadbase macadam containing EVA. Department of Transport TRRL Report RR196: Transport and Road Research Laboratory, Crowthorne.

VAN der POEL C (1954). A general system describing the visco-elastic properties of bitumen and its relation to routine test data. Journal of Applied Chemistry, Vol 4, May 1954.

VONK W C and VAN GOOSWlLLIGEN G (1989). Improvement of paving grade bitumens with SBS poly- mers. Fourth Eurobitume Symposium, Vol 1, 1989.

WHITEOAK D (1990). The Shell bitumen handbook. Shell Bitumen U.K, Chertsey.

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M O R E I N F © R M A T Z © N F R O M TRL T R L has pub l i shed the fo l lowing other reports on this area o f research:

L R 1132 The structural des ign of b i t uminous roads, W D Powell et al, Price £8

R R 5 4 A s s e s s m e n t o f 'Chemcrete ' as an addi t ive for binders for wearing courses and roadbases, M E Daines, J Carswel l and D M Colwil l , Price £3

R R 9 2 An assessment o f b i tuminous basecourse and roadbase materials containing E V A and sulphur, J Carswel l , Price Code B

R R 1 3 2 I m p r o v e d roadbase macadams : road trials and design considerations, M E Nunn, C J Rant and B Schoepe , Price C o d e B

R R 1 9 6 Trials o f roadbase m a c a d a m conta in ing EVA, C J Rant and B Schoepe, Price Code A

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