TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 267

BITUMEN PERMITTIVITY AND SURFACE TEXTURE IN ROLLED ASPHALT

by M E Daines

Crown Copyright 1991. The views expressed in this Report 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.

Materials and Construction Division Highways Group Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1991

ISSN 0266-5247

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1. Introduction

2. Brief description of the 1982 trial

3. Measurements and observations

4. Simple correlations

4.1 Texture depth of unchipped areas

4.2 Skid Resistance Value (SRV) of unchipped areas

4.3 Mean Summer Scrim Coefficient (MSSC) of chipped sections

4.4 Comparison of development of texture depth in chipped sections and unchipped areas

4.5 Development of texture depth of chipped sections compared with permittivity

5. Traffic

5.1 Effect of traffic on texture depth in unchipped areas

5.2 Effect of traffic on the change in texture depth of chipped asphalt

5.3 Effect of traffic on Mean Summer Scrim Coefficient

5.4 Effect of traffic on Skid Resistance Value (SRV) of unchipped areas

6. Inspections

7. Discussion

8. Conclusions

9. Acknowledgements

10. References

Page

1

1

1

2

2

2

2

2

2

2

6

6

6

6

7

7

7

8

8

8

Appendix A:

A.1

A.2

A.3

A.4

A.5

A.6

Appendix B:

Tabulated results

Texture depths (sand patch) of unchipped areas in the wheel path

Texture depths (sand patch) of chipped asphalt in the wheel path

Annual mean sensor-measured texture depth (HSTM)

Texture depth (Mini Texture Meter)

Pendulum Skid Resistance Value (SRV) of unchipped areas

Sideway force coefficients

Deformation

Page

9

9

11

11

15

15

15

17

BITUMEN PERMITTIVITY AND SURFACE TEXTURE IN ROLLED ASPHALT

ABSTRACT

The performance of surfacings made with bitumens of differing permittivity is reported after 7 years trafficking. Texture depths generally increased in the heavily- chipped asphalts and in unchipped areas in the wheel paths. There was no significant correlation between permittivity and changes in texture depth occurring in the wheel paths of chipped asphalt but there was a highly significant correlation between permittivity and the development of texture depth in unchipped areas in the offside lane. Changes in texture depth and skidding resistance in the three traffic lanes correlate strongly with traffic levels. On this site, all the high-stability asphalt surfacings performed satisfactorily.

1. INTRODUCTION

In hot rolled asphalt wearing course it is necessary for the mortar to erode or 'weather' away in a controlled manner in order to counteract the embedment of the pre-coated chippings by traffic and maintain macrotexture (and therefore high-speed skidding resistance). Research showed that the weathering of the asphalt mortar was related to the permittivity (dielectric constant) of the bitumen; most of this work was carried out using a core- implant technique with asphalts that were not chipped (Green 1977).

A design method to determine the optimum binder content for rolled asphalt was first included in British Standards in BS 594:1973. This has resulted in the use of higher stability asphalts, more resistant to the embed- ment of chippings and a slight reduction in binder con- tent.

Rates of spread of coated chippings have increased resulting from the publication of specification amendment H16/76 (Department of the Environment, 1976). Since the introduction of a minimum texture depth of 1.5 mm about 70 per cent of the surface is now covered with chippings, with the result that less mortar is in contact with the vehicle tyre.

The rate of deposition of oil has been reduced very significantly by the phasing out of the constant loss lubrication systems commonly fitted to commercial vehicles in the 1970s. At one stage the oil deposition rate was estimated to be 4.5 x 106 litres per annum and the build-up of rubbery deposits, with oil contents up to about 40 per cent, during hot dry weather was quite common (Green 1974). However, changes in the chassis lubrica- tion of commercial vehicles removed the problem and there have been no cases reported of rubbery deposits (except in some tunnels), even during the hot summer of 1976.

Because of these changes in rolled asphalt technology, the relationship between the development of texture depth in modern-heavily-chipped asphaltsrequired investigation in order to study the importance of bitumen weathering to the maintenance of texture depth and skidding resistance.

In August 1982 a full-scale experiment was initiated to study the effect of the permittivity of bitumen on the development of texture depth in heavily-chipped, high- stability rolled asphalts (Daines 1983). Seven sections of asphalt to the design method of BS 594:1973 were laid on all three lanes of the M4 motorway in Wiltshire. The asphalts were nominally identical apart from the permittivities of the bitumens used. This report summa- rises the measurements made during the seven years since laying.

2. BRIEF DESCRIPTION OF THE 1982 TRIAL

The objective was to study the effect of bitumen permittiv- ity on the development of the texture depth and skidding resistance of heavily-chipped designed hot rolled asphalts. The only planned material variable was the level of bitumen permittivity with, as far as was practica- ble, all other factors being kept constant. Seven experi- mental bitumens were used with permittivity ranging from 2.62 to 2.68 at 25°C. Since softening point can affect the rate of chipping embedment and deformation, it was planned that this parameter should be similar for all the bitumens; the actual range was 51.0 to 53.5°C. The penetration of the binders ranged from 36 to'68, but because penetration was known not to correlate well with chipping embedment, this range of penetration was not expected to affect the results."

The rolled asphalts were laid 50 mm thick as an overlay and covering the three lanes and hard shoulder of about 800 m of the M4 eastbound carriageway, in Wiltshire. By monitoring the three lanes it was possible to study the effect of traffic on the development of texture depth and skidding resistance.

The asphalt contained 30 per cent of 14 mm basalt coarse aggregate; the mortar comprised a blend of two parts of Cromhall doleritic limestone and one part of Hilton sand. The mixture, designed according to BS 594:1973, had a stability of 6.9 kN (at the optimum binder content of 9.0 per cent on the mortar only) for a 50 pen bitumen with a softening point of 51°C. At the specified binder content, 7.6 per cent, the stability was about 6.1 kN on the mortar only, equivalent to about 9 kN on a 30 per cent stone-filled mix.

The asphalts were mixed at Cromhall Quarry and laid during August 1982.20 mm Craig-yr-Hesg precoated chippings, with a polished stone value (PSV) of 68-70, were applied at a rate averaging 13.0 kg/m 2, equivalent to a 66 per cent coverage.

For each section and lane, three 300 mm square areas of asphalt in the wheel paths were left unchipped in order to study the development of texture depth of the asphalt mortar.

Details of the mixing and laying operations have been published elsewhere (Daines 1983).

3. MEASUREMENTS

The following measurements were carried out at regular intervals during the period 1982 to 1987, with some routine measurements continued until 1989.

(i) Texture depth. This has been measured by a number of means during a period of developing technology.

(a) Texture depth (sand patch method) of unchipped areas in the wheel paths.

(b) Texture depth (sand patch method) of chipped areas in the wheel paths.

(c) Texture depth (High Speed Texture Meter, HSTM).

(d) Texture depth (Mini Texture Meter, MTM) (Hosking Roe and Tubey 1987)

(ii) Sideway force coefficient using SCRIM.

(iii) Skid resistance value (SRV) of unchipped areas using a pendulum skid-resistance tester (HMSO 1969).

The measurements are described in detail in Appendix A.

4. S I M P L E C O R R E L A T I O N S

Table 1 shows selected simple correlation coefficients between various aspects of performance, for each traffic lane separately. Correlation coefficients are significant at the 5 per cent level of probability if r>0.75, significant at the 1 per cent level of probability if r>0.87, and significant at the 0.1 per cent level if greater than 0.95, for n=7 points. In Table 1 correlation coefficients that are signifi- cant at the 5 per cent level or better are shown in bold type. For overall results for 3 lanes where n=21, the significant values of r are 0.37, 0.44 and 0.51 respectively (Young 1962). Caution must be exercised in interpreting correlation coefficients that are statistically significant. A sound scientific basis for a causal link must be estab- lished and the following comments on apparently significant and insignificant correlations offer further interpretation.

4.1 TEXTURE DEPTH OF UNCHIPPED AREAS

A highly significant correlation exists between permittivity and the development of texture depth after 5 years for the unchipped patches in the relatively untrafficked offside lane, where natural weathering appears to predominate (see Figure 1). A significant correlation was not found for the data from the centre lane, and those from the near- side lane gave a negative correlation which appears to be the result of the high permittivity bitumen in Section 1 developing only a comparatively low texture depth; this effect was also seen to a lesser extent in the centre and offside lanes. The reason for the relative lack of weather- ing of this binder is not known, but it may be due to the method of refining this bitumen or because oil deposits in the centre and nearside lanes may have modified the natural weathering effect. In general, for the offside lane there is a factor of two increase in texture depth for a permittivity increase from 2.62 to 2.68.

4.2 SKID RESISTANCE VALUE (SRV) OF UNCHIPPED AREAS

Skid resistance (pendulum values) correlated significantly with permittivity in the offside lane, where natural weath- ering was greatest, and also for the centre lane for the 1984 results (see Figure 2). Increasing traffic reduced both the level of SRV and the correlation with permittivity.

4.3 MEAN SUMMER SCRIM COEFFICIENT (MSSC) OF CHIPPED SECTIONS

The results for 1989 correlated significantly with permit- tivity in the nearside lane but not significantly in the other lanes. However, the range was small, only about +0.02 in MSSC, for an increase in permittivity from 2.62 to 2.68.

4.4 COMPARISON OF DEVELOPMENT OF TEXTURE DEPTH IN CHIPPED SECTIONS AND UNCHIPPED AREAS

For each lane, there did not appear to be any significant correlation between the development of texture depth in unchipped areas compared with changes of texture depth (measured by any method) of the heavily-chipped asphalts.

4.5 DEVELOPMENT OF TEXTURE DEPTH OF CHIPPED SECTIONS COMPARED WITH PERMITTIVITY

Correlations of permittivity with the changes occurring in the texture depth of chipped sections, as measured by sand patch, HSTM or MTM, show no significance after 5 years. Additional results up to 1989 using the HSTM also show no significance. Thus bitumen permittivity within the range studied (2.62 to 2.68) is an insignificant factor contributing to the development of texture depth of heavily-chipped high stability asphalt.

2

T A B L E 1

Simple Correlation Coefficients

Parameter Delivery Tanker

Permittivity of bitumen

Storage Tank

Recovered

Unchipped areas

Texture depth (sand patch) unchipped areas (1987)

ns lane c lane os lane

Skid resistance value (SRV) unchipped areas (1987)

ns lane c lane os lane

Chipped sections

Mean Summer SCRIM Coefficient, MSSC (1989) ns lane c lane os lane

Change in Texture Depth of chipped asphalt (sand patch) (1982-1987)

ns lane cs lane os lane

Change in SMTD (1983-1989) (High Speed Texture Meter)

ns lane c lane os lane

Change in SMTD (1982-1987) (Mini Texture Meter)

as lane c lane os lane

-0.61 +0.22 +0.93

-0.11 +0.38 +0.81

+0.96 +0.43 +0.54

+0.06 +0.01 +0.37

-0.69 -0.57

+0.08

+0.02 +0.03 +0.28

+0.93

+0.78

+0.96

-0.64

+0.51

+0.37

0.8

EE 0.6

-o

0.4 I,-

0.2

Offside lane

I I I I I I I I

0.8 Centre lane

E v

{z oa

p-

0.6

0.4

0.2

• R = +0.22

u - • •

I I I I I I I I

E

t~

0.8

0.6

0.4

0.2

2.61

Nearside lane

1 I I I I I I I

2.62 2.63 2.64 2.65 2.66 2.67 2.68 2.69

Permit t iv i ty 25 ° C (delivery tanker)

E f f e c t o f p e r m i t t i v i t y on t e x t u r e dep th o f unch ipped areas, a f te r 5 years Fig.1

4

70

60

50

i O f f s i d e l a n e A 3 9 B 4 P, 1984 Lr~ ~ V 1987 I" ~ ~rv-

? 1987

I I 1 I I I I l

r~ £/)

8 c

=.

E

-1

£1.

70

60

C e n t r e l a n e

O 1984 • 1987

[]

[]

[ ] [ ]

" i - - •

o

1 9 8 4 r = +0.78

o D

• r = + 0 . 3 8 1 9 8 7

50 I I I I I 1 I I ,-

70

60

- Nearside lane

O 1984 • 1987

i li

t

50 I I I

2.61 2.62 2.63 2.64

O

- _ 4

1

2.65

r = +0.24 1984

O r = -0 .11 1987

1 I I

2.66 2.67 2.68

Permi t t iv i ty 25°C (del ivery tanker)

F i g . : ) E f f e c t o f p e r m i t t i v i t y on p e n d u l u m s k i d res i s tance va l ue ( S R V ) ( u n c h i p p e d areas)

I

2.69

5. TRAFFIC

Table 2 shows the results of traffic counts.

TABLE 2

Traffic Flow (Eastbound Carriageway)

Lane

Traffic flow (commercial vehicle per day*)

> 1.5 t onne

9.9.82 20.6.84

>6. l m

14.7.88

Nearside 3676 4076 5178 (3647) (3771) (7117)

Centre 759 936 1502 (6693) (7091) (11962)

Offside 10 190 25 (1603) (2399) (4944)

* Basis: 16h count x 1.06; bracketed values are cars and other light vehicles

5.1 EFFECT OF TRAFFIC ON TEXTURE DEPTH IN UNCHIPPED AREAS

Correlating the estimated traffic intensity (1987) with the texture depth as determined in September 1987 (TD (mm)) yielded the following statistically significant relationship for all sections and lanes (r = +0.60 for n=21)

TD(mm) = 0.45 + 4.1 x 10 -s x cvd (i)

where cvd = number of commercial vehicles (>1.5 tonne) per lane per day. A multiple regression analysis showed that traffic was the only significant variable, the other variables included were binder content, permittivity and softening point, which were all insignificant. Regarding permittivity, the only significant correlation with texture depth was obtained for the offside lane (see Section 4.1 ); traffic intensity swamps the permittivity effect for the other two lanes.

5.2 EFFECT OF TRAFFIC ON THE CHANGE IN TEXTURE DEPTH OF CHIPPED ASPHALT

By correlating the change in texture depth (sand patch) over 5 years from August 1982 to September 1987

(STD(mm)) with the estimated traffic level for 1987 the following statistically significant relationship was obtained (r = -0.54 for n=21 )

5TD(mm) = 0.54 - 6.8 x 10 .5 x cvd (2)

The average changes over the 5 year period were +0.03 mm, +0.21 mm and +0.41 mm for the nearside, centre and offside lanes respectively. On average, the changes in the nearside lane are very small, even under the heavy traffic experienced. This may be atypical, and is probably explained by the use of a high stability asphalt (6.8 kN for the mortar only, approximately equivalent to a 9 kN stability on a 30 per cent stone-filled mix).

The same analysis was applied to changes in texture depth as measured using the Mini Texture Meter from 1982 to 1987 and correlating with the measured commer- cial traffic level in 1988 produced a significant result (r = - 0.62 for n=21):

5TD (mm rms) = 0.41 - 4.7 x 10-Sx cvd (3)

Both equations (2) and (3) indicate that the texture depth increased with time and that the change in texture depth was less at higher traffic levels. For a zero change in texture depth, a traffic intensity of about 8,000 cvd is indicated from equations (2) and (3).

5.3 EFFECT OF TRAFFIC ON MEAN SUMMER SCRIM COEFFICIENT

Table 3 gives the average values of MSSC for the three lanes (averages were used because the variations along the lanes were very small) and compares them with the predicted value of MSSC calculated according to the following formula of Szatkowski and Hosking (1972):

MSSC = 0.033 + 0.98 x 10 -2 x PSV- 0.664 x 10 -4 x cvd (4)

where PSV is the polished stone value of the precoated chippings.

Table 3 shows marked deviations of MSSC from the predicted value according to the 1972 formula. A better representation of the results may be obtained by correlat- ing MSSC against traffic (both for 1988); the following highly significant relationship was obtained (r = -0.92 for n=21 ).

MSSC = 0.5991 - 1.113 x 10 ~ x cvd (5)

Taking the PSV of the coated chippings as 69 the equation can be expressed in the same form as equation 4:

MSSC = -0.09 + 1.0 x 10-2 x PSV- 0.11 x 10-4 x cvd (6)

This apparently shows reduced traffic dependence compared with the 1972 equation. A possible explanation is that a greater area of chipping surface remained in contact with the type, as embedment of chippings was minimal in these heavily chipped high stability asphalts.

TABLE 3

Skidding Resistance

Lane

Overall Average Predicted Equilibrium MSSC for Sections MSSC SCRIM

1 to 7 Coefficient

1984 1988 1984 1988 1986-89

Nearside 0.56 0.54 Centre 0.59 0.57 Offside 0.62 0.61

0.44 0.37 0.53 0.65 0.61 0.57 0.70 0.71 0.60

5.4 EFFECT OF TRAFFIC ON SKID RESISTANCE VALUE (SRV) OF UNCHIPPED AREAS

Overall, SRV measured in 1987 correlated strongly with the 1988 traffic level (r = -0.84 for n=21) whereas permit- tivity did not correlate significantly (r = +0.13 for n=21). Correlating both traffic and permittivity with SRV gave the following equation (r = +0.86 for n=21):

SRV = 62 - 1.5 x 10 .3 x cvd + 25 (E-2.65) (7)

Equation 7 suggests that a reduction in permittivity of 0.01 leads to a reduction of 0.25 in SRV; this is appar- ently greater than the effect of permittivity on MSSC for ~the chipped sections, and may be relevant to asphalts with lower rates of spread of chippings, where the PSV of the coarse aggregate contained within the asphalt will contribute to the resistance to skidding of the surfacing, once weathering has exposed it. This is the subject of another study (Jacobs 1985).

6. I N S P E C T I O N S

Visual inspections were carried out in 1982 and 1985 by an inspection panel, and by individuals in 1983, 84 and 87 (twice). All sections were rated VG (very good) up to May 1985 and G (good) thereafter, except for section 2 which declined to G/FG (good/fairly good). Apart from minor differences, the inspections revealed no systematic differences between the performance of sections that might have influenced the result of the trial.

7. D I S C U S S I O N

All the experimental sections of hot rolled asphalt have performed well. Variations of permittivity within the range 2.62 to 2.68 had no significant effect on the retention of skidding resistance or texture depth, except in unchipped areas of asphalt. However, previous work (Green 1977) showed that for very low bitumen permittivities weather- ing can be minimal and smooth surfaces can result.

Traffic level had a significant effect on skidding resist- ance. Texture depth levels have generally increased and even in the heavily trafficked nearside lane levels have been maintained.

Some possible reasons for the reduced influence of permittivity are discussed below.

1. Heavy rate of spread of pre-coated chippings. Not only does a heavy rate of spread cover more of the asphalt mortar, but resistance to embedment is also greater. The resulting greater texture depth further restricts contact between vehicle tyres and the mortar. Thus changes in mortar surface properties are less likely to affect performance.

2. High stability asphalt. This will restrict the effect of traffic on the embedment of chippings and, coupled with a high rate of spread of chippings, appears to be the principal reason for the maintenance of high levels of texture depth, even under heavy traffic. ~

3. Narrow range of bitumen permittivity. In previous work (Green, 1977), most bitumens had average 'weathering' performance in the range tested here (2.62 to 2.68) whilst the bitumens with a significantly poorer performance had permittivities below 2.60 and those with the highest 'weathering' ability had permittivities greater than 2.70. The narrow range, coupled with a relatively low number of bitumens (7) is probably the reason why no statistically significant correlation with permittivity was obtained for the unchipped asphalt areas in the centre and nearside lanes.

4. Selected bitumens. Previous work (Green 1977) showed that weathering was not uniquely related to permittivity. Some bitumens consistently weathered to a greater or lesser extent than their permittivity value suggested. This appears to be true for at least one bitumen in the present trial. The experimental bitumen in Section 1 had a penetration of 68 and a Softening Point (IP method) of 52.5°C yielding a Penetration Index of +0.5. Such a bitumen can be prepared by blending an oxidised grade with a penetration grade and adjusting the penetration by adding an aromatic oil. This would pro- duce an artificially high permittivity value that would not necessarily be reflected in the weathering of the bitumen. This bitumen weathered less than some bitumens with a lower permittivity (see Figure 1).

7

The bitumen in Section 1 conformed more closely to a 70 penetration grade whereas that in Section 7 conformed to a 35 pen grade, having the highest softening point (Daines 1983). These rheological differences might have marginally affected chipping embedment and consequen- tial changes in texture depth, tending to cancel out changes due to permittivity.

5. Oil deposition. This can significantlyalter the natural weathering of bitumens. Low permittivity bitumens are more compatible with oil resulting in softening and reducing the weathering effect. In high permittivity bitumens, oil deposition can cause the colloidal structure to breakdown, thus promoting weathering (Green 1977). All the circumstantial evidence shows that considerably less oil is now deposited on the road network (see Section 2.1), thus reducing the permittivity effect.

6. Other variables. The target binder content was 7.6 per cent. Measured binder contents varied from 7.1 to 8.0 per cent, but the correlation with texture depth of the unchipped areas was statistically insignificant. Neither was the correlation with softening point significant.

The present work provides no supPort for the use of low permittivity bitumens outside the present tested range, or for the lower rate of spread of coated chippings at present allowed in BS 594 where a minimum texture depth of 1.5mm is not required. Based on the results of this trial, Appendix 'L' in the Department of Transport Specification for Highway Works was modified to allow the use of bitumen having a minimum permittivity of 2.630 for 35, 40 HD, 50, 70 and 100 penetration grades (Department of Transport 1988), as was BS 3690: Part 1: 1989 (British Standards Institution, 1989). Since the permittivity limit has been relaxed to 2.630 and investiga- tory levels of skidding resistance have been promulgated (Department of Transport 1987) consideration should be given to phasing out the lower rate of spread of 60% cover at present allowed in BS594. A single rate of spread of 65% cover, similar to that used in this trial, replacing both the 60% and 70% rates could also be considered.

8. CONCLUSIONS

1. The introduction of designed asphalts having high stability and used with high rates of spread of coated chippings coupled with a reduction in oil deposition on the road have combined to make the development of texture depth and skid resistance independent of bitumen permittivity in the range 2.62 to 2.68.

2. In the present trial traffic level had an overriding effect on the development of texture depth and resistance to skidding. A relationship between traffic level, MSSC and Polished Stone Value (PSV) showed that on these heavily-chipped high stability asphalts, traffic had a lower effect on skidding resistance compared with an earlier study.

9. ACKNOWLEDGEMENTS

The cooperation and assistance of the County Surveyor of Wiltshire and his staff, the Regional Controller South West, the Refined Bitumen Association and colleagues at TRRL are gratefully acknowledged. The laying work was carried out by ARC, Frome, Somerset.

This report was prepared in the Materials and Construc- tion Division (Division Head: Mr D M Colwill) of the Highways Group.

10. REFERENCES

Daines M E (1983). Bitumen permittivity and texture depth of rolled asphalt: an experiment on Motorway M4. Department of the Environment, Department of Trans- port, TRRL Supplementary Report SR816 Crowthcrne (Transport and Road Research Laboratory).

Daines M E (1985). Cooling of bituminous layers and time available for their compaction. Department of Transport TRRL Research Report RR4 Crowthorne (Transport and Road Research Laboratory).

Department Of The Environment (1976). Specification requirements for aggregate properties and texture depth for bituminous surfacings to new roads. Engineering Intelligence Division. Technical Memorandum H16/76 London (Department of the Environment).

Department Of Transport (1987). Skidding resistance of in-service trunk roads. Departmental Standard HD 15/87, London, (Department of Transport).

Department Of Transport (1988). Specification for Highway Works Part 7(ii) Appendix L (March 1988). Alterations 8 and 9 to Part 3, 1986. London (HMSO).

Green E H (1974). Black deposits on motorways. Interim report to December 1973. Department of the Environ- ment TRRL Supplementary Report SR741, Crowthorne, (Transport and Road Research Laboratory).

Green E H (1977). An acceptance test for bitumen for rolled asphalt wearing course. Department of the Envi- ronment, Department of Transport, TRRL Report LR777, Crowthorne (Transport and Road Research Laboratory).

HMSO (1969). Instructions for using the portable skid- resistance tester. Ministry of Transport Road Research Laboratory Road Note 27 London (HMSO).

Hosking J R, P G Roe and L W Tubey (1987). Measure- ment of the macro-texture of roads Part 2: a study of the TRRL mini texture meter. Department of Transport TRRL Research Report RR120 Crowthorne (Transport and Road Research Laboratory).

Jacobs F A (1985). An experiment to investigate rolled- asphalt wearing-courses with different coarse aggregates

on A30 Staines by-pass. Department of Transport. TRRL Research Report RR12 Crowthorne (Transport and Road Research Laboratory).

Szatkowski W S and J R Hosking (1972). The effect of traffic and aggregate on the skidding resistance of bituminous surfacings. Department of the Environment, TRRL Report LR504, Crowthorne (Transport and Road Research Laboratory).

Young (1962). Statistical Treatment of Experimental Data. (McGraw-Hill).

A P P E N D I X A: T A B U L A T E D RESULTS

A.1 TEXTURE DEPTHS (SAND PATCH) OF UNCHIPPED AREAS IN THE WHEEL PATH

Table A1 presents the results over 5 years. Each value is the mean of three areas (except for Section 1, all lanes, where there were only 2 areas as the section had been shortened by resurfacing work, and Section 2, centre lane, which was a short section including a length of non- experimental asphalt). The results are presented in Figure A1. Textures increased in every section and in all

TABLE A1

Summary Of Mean Texture Depths (Unchipped Areas) In Wheel Path

Section Bitumen*

Permittivity

Lane Texture Depth (sand patch) (mm) (month/year)

8/82 9/83 6/84 10/84 5/85 10/85 6/86 5/87 9/87

1 2.684

2 2.646

3 2.659

4 2.648

5 2.641

6 2.626

7 2.619

ns <0.i 0.17 0.21 0.26 0.30 0.30 0.38 c " 0.25 0.23 0.37 0.37 0.43

os " 0.20 0.29 0.42 0 .38 0.45

ns " 0.24 0.35 0.47 0.47 0.51 0.55 c " 0.22 0.21 - 0.39 0.34 0.50

os " 0.16 0.23 - 0.34 0.28 0.39

ns " 0.23 0.34 0.38 0.44 0.41 0.51 c " 0.25 0.25 0.38 0.43 0.40

os " 0.24 0.28 0.37 0.36 0.42

ns " 0.22 0.30 0.34 0.40 0.36 0.44 c " 0.22 0.29 0.41 0.37 0.42

os " 0.18 0.20 0.27 0.27 0.32

ns " 0.22 0.30 0.37 0.40 0.40 0.48 c " 0.23 0.28 0.39 0.39 0.42

os " 0.17 0.18 0.25 0.30 0.25

ns " 0.23 0.27 0.35 0.40 0.46 0.51 c " 0.22 0.25 0.42 0.35 0.39

os " 0.14 0.18 0.26 0.23 0.24

ns " 0.22 0.32 0.40 0.38 0.47 0.53 c " 0.17 0.20 - 0.29 0.25 0.29

os " 0.09 0.16 0.25 0.21 0.23

0.38 0.50 0.55

0.58 0.57 0.39

0.54 0.47 0.46

0.46 0.48 0.34

0.43 0.49 0.29

0.49 0.45 0.28

0.56 0.31 0.27

0.43 0.52 0.61

0.74 0.66 0.42

0.68 0.53, 0.56.

0.57 0.54 0.38

0.72 0.63 0.33

0.61 0.57 0.32

0.68 0.37 0.30

* ex delivery tanker ns = nearside c = centre os = offside

9

Section 1

10

Section 3

F

Section 2

0 Nearside lane V Centre lane Z~ Offside lane

I I I

20 30 40 50 60 70

I I

E E

A J ~

=~

" O

X

0.8

0.4

0

0.8

0.4

0

0.8

0.4

0

Section 4

Section 5

A

i I

I I

_ % -

I L

0.8

0.4

0

f Section 6

0.8

0.4

0 0

0.8

0.4

0

J I

0.8

0.4

0

Section 7

10

Fig.A1

20 30 40 50 60

Months after laying

Tex tu re depths o f unchipped areas in wheel paths (sand-patch)

J

70

10

three lanes. Generally the highest increases occurred in the nearside lane and the lowest in the offside lane; the exception was Section 1 where the bitumen with the highest permittivity had not weathered as much in the centre and nearside lanes. For all sections there appears to have been a significant increase in texture depths between May and September 1987, about 5 years after laying.

A.2 TEXTURE DEPTHS (SAND PATCH) OF CHIPPED ASPHALT IN THE WHEEL PATH

Table A2 presents the results after 5 years. Each value is the section mean of 10 patches, except for Section 2, centre lane, where 8 patches were measured. Figure A2 presents the results. Apart from some slight reductions in texture depth during the first year, probably reflecting some initial embedment of chippings, textures have

generally increased slightly since then for the centre and offside lanes, whereas those for the nearside lane have generally changed little. There was some evidence of an accelerating rate of increase in some sections during 1987.

A.3 ANNUAL MEAN SENSOR- MEASURED TEXTURE DEPTH (HSTM)

Table A3 presents the results up to 1989. The high speed texture meter (HSTM) was not available on some occa- sions, and the annual means are therefore based on the number of runs that were accomplished in each season. The Table shows the changes that occurred from 1983 to 1989. These show an increase in texture depth for all sections and lanes during this period. The results are presented in Figure A3. The use of annual means removes within-season variations and should enable

TABLE A2

Summary Of Mean Texture Depths, Chipped Sections, (Nearside Wheel Path) 1982-87

Section Lane Bitumen

Permittivity

Texture Depth (Sand Patch) (mm)

8/82 9/83 6/84 10/84 5/85 10/85

(month/year) Change (mm)

8/82 to 6/86 5/87 9/8 7 9/87

1 2.684

2 2.646

3 2.659

4 2.648

5 2.641

6 2.626

7 2.619

ns 1.60 c 1.57

os 1.55

ns 1.80 c 2.14

os 1.59

ns 1.49 c 1.73

os 1.76

ns 2.15 c 1.73

os 1.56

ns 1.71 c 1.82

os 1.39

ns 2.13 c 1.79

os 1.74

ns 1.78 c 1.85

os 1.32

1.58 1.50 1.61 1.59 1.50 1.43 1.51 1.64 1.88 1.66 1.72 1.80 1.76

1.78 1.91 1.80 1.82 1.84 1.78 1.86 1.94 2.24 1.55 1.71 1.68 1.85

1.85 1.87 1.98 1.86 1.95 1.49 1.65 1.91 1.91 1.66 1.87 1.71 1.85

1.88 1.90 2.07 1.99 1.91 1.90 1.90 2.02 2.18 1.47 1.67 1.66 1.67

1.73 1.92 1.83 1.83 1.74 1.63 1.76 1.84 2.06 1.60 1.62 1.64 1.92

1.76 2.02 2.06 2.21 1.94 1.82 1.95 2.01 2.06 1.77 1.88 1.75 1.94

1.94 2.13 1.80 1.80 1.98 1.75 1.65 2.08 2.03 1.29 1.45 1.50 1.54

1.53 1.47 1.62 +0.02 1.66 1.93 1.76 +0.19 2.10 1.97 2.00 +0.45

1.76 1.69 1.85 +0.05 2.16 2.03 2.01 -0.13 1.93 2.02 1.99 +0.40

2.02 1.68 1.98 +0.49 1.76 1.88 1.95 +0.22 2.00 1.97 2.14 +0.38

1.89 1.68 1.78 -0.37 2.20 1.92 2.14 +0.41 1.90 1.73 2.09 +0.53

1.71 1.80 1.83 +0.12 1.95 2.01 2.24 +0.42 1.84 1.87 2.10 +0.71

1.90 1.89 2.02 +0.11 2.06 2.05 2.06 +0.27 1.98 1.98 1.88 +0.14

2.03 1.98 2.00 +0.22 1.99 1.90 1.95 +0.11 1.38 1.45 1.61 +0.29

ns = nearside c = centre os = offside

11

0 Nearside lane V Centre lane Z~ Offside lane

2.0 ~ Section 1

L

1.6 ~

1.2

0 10 20 30 40 50 60

2.0

1.6

1.2

70

! - Section 3

2.0

1.6

1.2

,~- Section 4 ~

2.0

1 . 2 1 ~ . ~ 3---

1.6

" 0

I - r - Section 5

2.0

Section 7

2.0

1.6

1.2

Section 6

2.0

1.6

1.2

0 10 20 30 40 50 60

Months after laying

F i g . A 2 M e a n t e x t u r e d e p t h s on ch ipped asphalt, (sand-patch) wheel path

70

12

TABLE A3

Annual Mean Sensor-Measured Texture Depth (HSTM)

Section Lane Bitumen

Permittivity

Sensor measured texture depth SMTD (mm rms)

1982 1983"* " 1984"** 1985"* 1988" 1987" 1988"* 1989"**

Change 1983- 1989

(mm fins)

ns (0.83) 0.68 0.80 (0.89) 0.86 0.90 0.97 0.99 0.99 1 2.684 c (0.73) 0.76 0.92 0.95 1.03 1.17 1.05 1.05

os (0.94) 1.01 1.29 1.23 1.24 1.51 1.11 1.34

ns (1.06) 0.78 0.94 (0.97) 1.01 1.02 1.06 1.10 1.13 2 2.646 c (0.85) 0.97 1.12 1.20 1.23 1.36 1.28 1.34

os (0.91) 0.95 1.13 1.13 1.13 1.36 1.13 1.22

ns (1.08) 0.88 1.03 (1.07) 1.13 1.18 1.15 1.19 1.21 3 2.659 c (0.79) 0.77 0.95 1.00 1.04 1.15 1.07 1.13

os (0.95) 0.93 1.17 1.15 1.22 1.41 1.07 1.23

ns (1.08) 0.81 0.94 (1.07) 1.02 1.02 1.03 1.10 1.12 4 2.648 c (0.76) 0.84 1.05 1.06 1.09 1.35 1.15 1.16

os (0.85) 0.83 1.04 1.07 1.16 1.31 1.07 1.20

ns (0.92) 0.69 0.86 (1.08) 0.93 0.99 0.98 1.04 1.05 5 2.641 c (0.68) 0.77 0.96 1.04 1.11 1.15 1.08 1.08

os (0.82) 0.77 1.02 1.08 1.10 1.45 1.03 1.16

ns (1.04) 0.75 0.89 (0.93) 1.02 1.00 1.09 1.11 1.14 6 2.626 c (0.79) 0.88 1.01 1.15 1.22 1.38 1.24 1.23

os (0.93) 0.89 1.13 1.13 1.32 1.46 1.12 1.27

ns (0.96) 0.80 0.95 (1.04) 0.97 1.07 1.15 1.12 1.14 7 2.619 c (0.77) 0.83 0.98 1.05 1.13 1.25 1.12 1.16

os (0.75) 0.78 0.90 0.94 1.02 1.12 0.91 1.03

+0.31 +0.25 +0.33

+0.35 +0.37 +0.27

+0.33 +0.36 +0.30

+0.31 +0.32 +0.37

+0.36 +0.31 +0.39

+0.39 +0.35 +0.38

+0.34 +0.33 +0.25

ns = nearside * 1 test c = centre ** 2 tests os = offside *** 3 tests

bracketed values: single tests in Nov '82 and March '84 using high-speed road monitor

13

0 Nearside lane V Centre lane Z~ Offside lane 14f °t°n1

0.6 I I I I I

I- Section 2 1,4

0.6 / I I I I I I I

i~= Section 3 1,4 ~ ~ O

1.0 _-- =,, ..,, : ~.

0 .6 I I I I I I I

A E s.con, i 1.4

0.6 / I I I I I I I

F-

1.4 I Section 5

1.0

f_- O.6 I I I I i

r- Section 6

1.0 ~ "

0.6 | I I ! i i i J

14 ~ Section7

~.o ~__= " " ;=_.s

0.6 ~= - I I I I I I I

1982 83 84 85 86 87 88 89

1982 resul ts: h igh speed road m o n i t o r Year

Fig.A3 Annual mean sensor-measured texture depths on chipped asphalt ( h i g h s p e e d t e x t u r e m e t e r ) , w h e e l path

14

underlying trends to be more easily established (compare Figures A2 and A3). Some relatively high values were recorded in 1987, but only one set of measurements was made in January, outside the normal monitoring period. Examination of individual runs show that generally textures tended to decrease marginally during the summer and increase during the winter, as expected.

A.4 TEXTURE DEPTH (MINI TEXTURE METER)

A limited number of tests were made on the same occasions as sand-patch measurements were made. These are shown in Table A4. In 1982 an early version of the meter was used and since then different analytical programs have been developed. The results for 1985 onwards were obtained on the same machine using the "texture: other surfaces" program. The results are presented in Figure A4. For all sections and lanes the texture depth increased from 1982 to 1987.

A.5 PENDULUM SKID RESISTANCE VALUE (SRV) OF UNCHIPPED AREAS

Table A5 presents the results obtained in June 1984 and May 1987. Each result is the mean of 3 unchipped areas (2 areas in all lanes of Sections 1, and Section 2, centre lane). The results are presented graphically as a function of permittivity in Figure 2. A traffic dependence emerges, heavy traffic depressing the SRV value. A relationship between SRV and permittivity was apparent in the offside lane and to a lesser extent in the centre lane. Values of SRV for 1987 are generally lower than those for 1984. Values of SRV for chipped areas are in every case higher than the unchipped areas for the same section and lane.

A.6 SIDEWAY FORCE COEFFICIENTS

Individual sfc runs for 1982-83 and the MSSC (Mean Summer Scrim Coefficient) for 1984 to 1989 are

TABLE A4

Texture Depth (Mini Texture Meter)

Section Bitumen

Permittivity

Lane

8/82

Texture depth (mm rms)(month/year) Change 1982to

1987

9/83 5/85 5/87 9/87 (mm rms)

2

2.684

2.646

2.659

2.648

2.641

2.626

7 2.619

ns 0.83 (0.71) 0.92 0.95 1.01 0.98 +0.15 c 0.79 (0.71) 1.06 1.02 1.14 1.11. +0.32

os 0.85 (0.80) 1.27 1.35 1.37 1.34 +0.49

ns 0.84 (1.01) 1.15 1.09 1.12 1.15 +0.31 c 0.92 (0.92) 1.28 1.31 1.34 1.37 +0.45

os 0.86 (0.82) 1.21 1.23 1.42 1.34 +0.46

ns 0.86 (1.03) 1.19 1.25 1.23 1.24 +0.38 c 0.96 (1.02) 1.14 1.08 1.15 1.13 +0.17

os 0.88 (1.07) 1.33 1.27 1.48 1.44 +0.56

ns 1.06 (0.93) 1.16 1.10 1.01 1.07 +0.01 c 0.93 (0.93) 1.23 1.19 1.20 1.20 +0.27

os 0.89 (0.76) 1.19 1.18 1.25 1.25 +0.36

ns 0.83 (0.74) 1.10 1.06 1.08 1.06 +0.23 c 0.89 (0.91) 1.18 1.14 1.20 1.17 +0.28

os 0.86 (0.85) 1.15 1.16 1.34 1.37 +0.51

ns 0.94 (0.98) 1.20 1.18 1.13 1.11 +0.17 c 0.90 (0.93) - 1.25 1.30 1.30 +0.40

os 0.90 (0.86) 1.40 1.45 1.51 +0.61

ns 1.02 (-) c 1.01 (1.10)

os 0.79 (0.83)

1.16 1.13 1.23 1.23 +0.21 1.14 1.2 11.19 +0.18 1.00 1.11 1.05 +0.26

Bracketed results are for 50 m diagonals

15

r Section 1

1.4 r J ~

' - ~ ~.~....~-'- - o

0 . 6 e

0 10 2 0

1 . 4 ~ S e c t i o n 2

1.0 ,.. ,,. ~ ..-

0.6

O Nearside lane V Centre lane Z~ Offside lane

I 30 40

I I 50 60

O

I I

I 70

r Section 3

1.4 ~- ~_

A

r Section 4

E 1.4 I__ ._ __,~,,~. ~ E

~" 0 6 ~ '

.~ 1.o

.

I -

1.4 ~ Sec t i on5

1.0 ~~.~.~ ~ ~ ~

0.6

r j

I I

I I

a- o--o

I I

Section 6

1.4 ~ - - - ~'. 7

I I 0.6 I

r Sect ion 7

, 0 - = . . . . - - - - - . . . . . . . . .

0.6 / I I I I I 0 10 20 30 40 50 60

Months after laying

F i g . A 4 M i n i t e x t u r e m e t e r resul ts o n c h i p p e d aspha l t (whee l pa th )

I 70

16

TABLE A5

Skid Resistance, SRV (Pendulum)

Month/Year

Section Lane 6/84 5/87

Bitumen Permittivity U* U* 'C*

1 2.684 ns 56 55 67

c 65 58 os 75 67 78

2 2.646 ns 58 53 73

c 6 257 os 65 63 70

3 2.659 ns 56 54 67

c 65 58 os 68 62 74

4 2.648 ns 56 53 65

c 64 59 os 64 64 71

5 2.641 ns 58 58 69

c 61 57 os 63 63 67

6 2.626 ns 56 56 70

c 63 56 os 65 62 73

7 2.619 ns 54 54 72

c 60 58 os 64 62 74

ns = nearside * U = Unchipped areas c = centre C = chipped asphalt os = offside

presented in Table A6. The coefficients are highest in the offside lane and lowest in the nearside lane, showing traffic dependency. Results since 1986 have been virtually identical and there has been little reduction in MSSC since this measurement was introduced in 1984, indicating that equilibrium conditions were reached in about 1986.

The levels and differences between sections and lanes are too small to be significant. The results in Table B.1 indicate the profile after 5 years. Overall the deformations were small; for the nearside lane, assuming the average value represents deformation, the rate was about 0.3 mm/annum. The actual stability was about 9 kN.

The surfacings contained 30 per cent of coarse aggre- gate and were laid 50 mm thick. This combination, which does not conform with BS594, was not more prone to deformation than the standard 40 mm thickness. Increas- ing the laid thickness from 40 mm to 50 mm increases the time available for compaction by almost 50 per cent (Daines 1985), thus enabling hot rolled asphalts to be laid in adverse weather conditions. 50 mm thick asphalts normally have a 40 per cent coarse aggregate content. In this case, where the asphalt contained crushed rock fines, the stability was high with 30 per cent of coarse aggregate, and the stability could possibly have been excessive at 40 per cent coarse aggregate content, with consequential workability problems. For asphalts using crushed rock fines the laid thickness limits for 30 per cent coarse aggregate content in BS 594 could be widened to include 50 mm thickness with resulting benefit in world- ability time, and without loss of resistance to deformation.

APPENDIX B: DEFORMATION

Rut depths were measured using a 2m straightedge, using reference points 1.5m apart, bridging the nearside wheelpath. In this test the rut depth is defined as the maximum peak-valley value, averaged over the 5 sampling points per section. The repeatability of the mean result is estimated to be 1.2 mm. Table B1 gives the results obtained after 5 years trafficking.

1 7

TABLE A6

Sideway Force Coefficients And MSSC At 50 km/h

Section Lane

Bitumen Permittivity 9/82*

Sideway Force Coefficient, sfc

month/year MSSC**

11/82 7/83 9/83 1984 1985 1986 1987 1988 1989

1 2.684

2 2.646

3 2.659

4 2.648

5 2.641

6 2.626

7 2.619

ns 0.64 c

o s

ns 0.64 C

OS

ns 0.63 C

OS

ns 0.62 c

OS

as

C

OS

n s

c

o s

ns

C

OS

0.53 0.50 0.57 0.56 0.53 0.53 0.54 0.51 0.66 0.60 0.54 0.58 0.57

- 0.51 0.65 0.63 0.58 0.61 0.61

0.54 0.48 0.56 0.55 0.53 0.53 0.53 - 0.49 0.62 0.60 0.55 0.58 0.57

0.51 0.66 0.62 0.57 0.61 0.59

0.55 0.48 0.55 0.56 0.53 0.54 0.54 0.48 0.60 0.59 0.54 0.57 0.56 0.51 0.65 0.62 0.57 0.59 0.59

0.54 0.47 0.55 0.56 0.52 0.52 0.53 0.49 0.59 0.59 0.54 0.57 0.56 0.49 0.65 0.61 0.57 0.59 0.59

0.63 0.55

0.61 0.53

0.60 0.55

0.55 0.58 0.62

0.55 0.58 0.61

0.55 0.56 0.60

0.54 0.57 0.60

0.53 0.58 0.61

0.52 0.58 0.61

0.52 0.56 0.59

0.52 0.56 0.59

0.47 0.55 0.56 0.53 0.54 0.53 0.54 0.52 0.49 0.58 0.59 0.55 0.58 0.57 0.57 0.57 0.49 0.66 0.62 0.57 0.60 0.59 0.61 0.60

0.47 0.54 0.55 0.52 0.53 0.52 0.54 0.51 0.49 0.59 0.59 0.56 0.58 0.57 0.57 0.57 0.49 0.66 0.61 0.58 0.60 0.59 0.61 0.60

0.48 0.54 0.55 0.52 0.54 0.53 0.54 0.51 0.49 0.58 0.59 0.56 0.57 0.57 0.58 0.56 0.50 0.65 0.61 0.57 0.59 0.59 0.59 0.58

*One week after opening to traffic **Mean Summer SCRIM Coefficient ns = nearside c = centre os = offside

18

TABLE B1

Rut Depths Using 2m Straightedge (mm) (1987)

Section Lane 1 Lane 2

1 1.3 1.0 2 1.0 1.4 3 1.5 0.7 4 1.8 O.9 5 1.3 1.0 6 1.5 1.2 7 2.2 1.5

Average 1.5 1.1 1 to7

Printed in the United Kingdom for HMSO DdK50120 10/91 C5 G2516 10170

19