TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 4

COOLING OF B I T U M I N O U S LAYERS A N D

T IME A V A I L A B L E FOR THEIR C O M P A C T I O N

by M E Daines

The views expressed in this Report are not necessarily those of the Department of Transport

Pavement Materials and Construction Division Highways and Structures Department Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1985

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

Page

Abstract 1

1. Introduction 1

2. Definitions 1

3. Computer simulation 1

4. Experimental verification 2

4.1 Laboratory tests 2

4.2 Road trials 2

5. Effect of various parameters on cooling time 3

5.1 Initial laying and final temperatures 3

5.2 Laid thickness 4

5.3 Solar radiation 4

5.4 Comparative effects of factors involved 6

5.5 Surface temperature 6

6. Prediction of compaction time 6

6.1 Use of predictive tables 6

6.1.1 At the planning stage 6

6.1.2 On the day of laying 8

6.1.3 Monitoring on site 8

7. Application to rolled asphalt wearing course 8

7.1 Specifications 8

7.1.1 Normal asphalt 9

7.1.2 Heavy-duty asphalt 9

7.1.3 Asphalt with modified binders 9

8. Conclusions 9

9. Acknowledgements 10

10. References 10

Appendix A: Beaufort Scale: specifications and equivalent speeds 11

©CROWN COPYRIGHT 1985 Extracts from the text may be reproduced, except for

commercial purposes, provided the source is acknowledged.

COOLING OF B I T U M I N O U S L A Y E R S A N D T I M E AVAILABLE FOR THEIR C O M P A C T I O N

M E Daines, Transport and Road Research Laboratory

ABSTRACT

A computer model has shown that the cooling time of a hot bituminous layer is very dependent upon the laid thickness, being proportional to the thickness to the power 1.8. The cooling time is also strongly dependent upon the initial laying and specified minimum compaction temperatures and on the ambient windspeed and temperature. A set of tables is presented enabling the compaction time to be predicted from a knowledge of the laying and environmental conditions. Recommendations are given to establish minimum laid thicknesses for various types of rolled asphalt, to achieve a reasonable compaction time.

1 INTRODUCTION

The structural properties and long-term durability of a bituminous material can be seriously reduced by inadequate compaction. Consequently in recent years considerable attention has been paid to modifying compaction practice with the aim of improving the compacted state.

Jordan and Thomas (1976) described a computer model to simulate the cooling behaviour of hot paving material after laying. The model showed that immediately after laying the greater rate of heat loss was through the bottom of the layer into the substrate, but that with increasing time the surface rate of heat loss became dominant. Powell and Leech (1976) applied the model to devise improved compaction regimes for road base layers. Brown (1980) studied the cooling effects of air temperature and wind speed on 40 mm thick rolled asphalt surfacings. He found that the rate of cooling was more dependent on wind speed than air temperature.

This report studies in detail the cooling of hot bituminous layers with special reference to the laid thicknesses primarily relevant to surfacing materials. A set of tables is presented enabling the time available for compaction to be predicted from specified laying conditions and environmental factors.

2 DEF IN IT IONS

The fol lowing definit ions are used in this report:

1. The initial laying temperature is the temperature of the hot-mixed material emerging from the paver screed.

2. The laid thickness is the specified compacted thickness of the material.

3. Temperatures of the laid material refer to mid-layer temperatures, (unless otherwise stated).

4. The cooling t ime is the time taken for the mid-layer temperature to cool from the initial laying temperature to a stated lower temperature.

5. The compaction time is the time taken to cool f rom the initial laying temperature, or a lower specified temperature to the specified minimum compaction temperature.

6. The (specified) minimum compact ion temperature is the temperature by which t ime rolling should have been completed. Al though some further compact ion may still be possible, effect ive compaction takes place above this minimum temperature.

3 C O M P U T E R S I M U L A T I O N

The computer model developed by Jordan and Thomas (1976) was used again in this work wi th the same assumed physical constants. However the program was modified to provide cooling curves showing the mid- layer temperature as a funct ion of t ime after laying. A number of these curves were produced by assigning different values to each of the fol lowing variables, wi th the substrate temperature assumed to be at ambient temperature initially. The thickness of the hot layers ranged from 20 mm to 80 ram, in steps of 5 ram; the initial temperatures of these layers were set at 160°C and 150°C and the values chosen for the ambient temperature were 0 °, 5 °, 10 °, 15 ° and 20°C, the values used for wind speeds were 0, 5, 10, 15, 20 and 40 km/h. Solar radiation was initially assumed to be zero. For each value of layer thickness and initial and ambient temperatures, a family of 6 cooling curves was

produced, one for each wind speed. To reduce the volume of data generated in this way, cooling times were deduced from these curves for layers with initial laying temperatures of 150 ° and 160°C cooling to 120 °, 110 °, 100 ° and 90°C. To study the effects of solar radiation, a range of incident energies were applied to the period of t ime during laying only, and to various periods of t ime preceding and during laying.

In addition to the computer simulation study, the thickness range was extended using data from other studies employing the same program (Jordan and Thomas 1976 and BACMI 1983). Cooling curves for initial laying temperatures in the range 180 ° to 100°C were obtained experimentally.

4 E X P E R I M E N T A L V E R I F I C A T I O N

As verif ication of the original computer model had been carried out by Jordan and Thomas (1976), only a limited amount of work was necessary to verify the modified program.

4.1 L A B O R A T O R Y TESTS The test method involved heating a slab of rolled asphalt, placing it on a thicker asphalt substrate and recording the temperature at the centre of the slab at its mid depth. The slab was an unchipped wearing course of 30 per cent stone content prepared 305 x 305 x 40 mm thick in a 20 mm thick wooden-surround mould that had

• a 3 mm thick aluminium base for support. A thermocouple was embedded in the centre of the slab at a depth of 20 mm. The wooden surround, having a thermal conduct iv i ty only one tenth of that of asphal t , effect ively eliminated heat loss from the edges, and the presence of the aluminium base, having a very high thermal conductivi ty, could be ignored. The slab was heated in an oven and placed on an 80 mm thick rolled asphalt substrate; this stood on 25 mm of concrete and was kept in a temperature-controlled room. Wind was provided by a simple multi-speed fan arrangement. Solar radiation was not simulated. A thin layer of fine sand

was used to provide good thermal contact between the aluminium base and asphalt substrate. The mid-slab temperature was recorded for a range of cooling conditions generated by varying the ambient temperature and wind speed. Very good agreement between the computer-predicted and experimental cooling times was obtained as shown by the examples in Table 1.

Some tests were also made using slabs of open-textured pervious macadam and friction course.

4.2 ROAD TRIALS The mid-layer temperature of rolled asphalt wearing course was recorded from the time that the asphalt emerged from the paver. Two sites were investigated, the first being on M4 at Almondsbury where 35 mm laid thickness asphalt was laid during summer and the second on A33 near Winchester where 50 mm thickness asphalt was laid during winter. Wind speed was monitored using a hand-held anemometer, and the air temperature measured using a mercury-in-glass thermometer. At both sites there was general agreement between predicted and measured cooling times, and in particular, the longer cooling times for thicker asphalt layers were demonstrated. However, a number of other significant observations was made.

1. on M4 site (35 mm laid thickness) cooling times were comparatively short, despite the season, being about 8 minutes for a temperature drop from 150 to 100°C, when the wind speed was 20 km/h. Under the most favourable conditions, (wind 5 km/h air 20°C) a 10 minute cooling time was only just achieved.

2. On A33, wooden edge boards, laid adjacent to the kerb to prevent chippings falling on the edge of the asphalt that formed the gully, had the effect of considerably slowing down the rate of cooling.

3. Shelter provided by overhanging trees, or by the sides of a cutting, made the measurement of wind speed unreliable. Gusty conditions were also difficult to assess. Traffic induced gusts up to 27 km/h have been measured on M6 Motorway hard shoulder, even when the wind speed away from the motorway was only 0.8 km/h (Staffs C.C. (1984)).

T A B L E 1

Experimental cooling times for rolled asphalt compared with theoretical )rediction

Ambient conditions Time to cool 150-100°C Experimental

Wind km/h Air °C Theoretical Experimental Theoretical

0 0

13.5 14.4

22 9.5

16.4 1.9

15.3 13.2 11.1 9.5

15.2 13.2 10.8 9.7

0.99 1.00 0.97 1.02

5 EFFECTS OF V A R I O U S PARAMETERS ON COOLING T IME

Because the cooling behaviour of a bituminous layer is complex, the cooling time cannot be represented by a simple equation. However it is possible to study the effects of individual parameters. Brown (1980) studied the effects of wind speed and air temperature, and in the fol lowing sections the effects of laid thickness, initial laying temperature, final temperature and solar radiation are examined. Empirical relationships have been derived that can be used to predict compaction times under a variety of laying conditions.

5.1 INITIAL LAYING A N D FINAL TEM PERATU RES

Figure 1 shows graphically a family of cooling curves for a range of laid thicknesses for poor environmental conditions (wind 40 k in /h , air temperature 0°C). Figure 2 shows a similar family for good condit ions (wind 0 km/h, air temperature 20°C). With mid-layer temperature plotted on a logarithmic scale, the lines become almost linear. Examples are shown in Figure 3, covering a wide range of environmental condit ions and laid thicknesses. The relationships can be described by the general equation:

Log T = Log To - m t . . . . 1

where T = mid-layer temperature at time t after laying To= initial laying temperature

and m is a constant, the value of which depends on the laid thickness and environmental conditions.

If the time taken for the temperature in a layer of given thickness, and for f ixed environmental condit ions to drop from 150°C to 100°C is taken as unity, then for other initial laying and final temperatures, cooling times, relative to unity, may be calculated. These 'relative cooling factors' are given in Table 2. Errors introduced by this linearisation are largely restricted to _+10 per cent

TABLE 2

Relative cooling factors for various initial and final temperatures

Initial Temp.

(°C)

180 170 160 150 140 130 120 110 100

Final temperature (°C)

130 120 110 100 90 80 70

0.80 1.00 1.21 1.50 1.90 2.40 3.00 0.66 0.86 1.07 1.31 1.70 2.20 2.75 0.51 0.71 0.92 1.16 1.50 1.95 2.50 0.35 0.55 0.76 1.00 1.30 1.70 2.25 0.18 0.38 0.59 0.83 1.09 1.50 2.05

-- 0.20 0.41 0.65 0.91 1.30 1.80 -- -- 0.21 0.45 0.71 1.00 1.50 -- -- -- 0.24 0.49 0.79 1.25 . . . . 0.26 0.55 1.00

160 Initial laying temperature 150°C Wind speed 40km/h Air temperature 0°C 150

1 ' °

' °F l \ \ \ \ \ \ \ x

t 2025 30 35 40 45 50 55 60 Thickness in millimetres

80

70 I I t I I 0 4 8 16 20 12

Time (min) 24

Fig. 1 Theoretical cooling of bituminous layers of various thicknesses for poor environmental conditions

o .=

>=

20

80

25 30 35 40 Thickness in millimetres

701 I I I 1 0 4 8 12 16

Time (rain) 20 24

Fig. 2 Theoretical cooling of bituminous layers of various thicknesses for good environmental conditions

except at the lowest f inal temperatures where the relat ionship in equat ion 1 progressively underestimates the cool ing t ime. Consequently, in some cases, cool ing factors derived from experimental data have been used in Table 2.

For example, i f the condi t ions are such that the actual cool ing t ime is 10 minutes for the temperature to fall f rom 150°C to 100°C, then for a temperature drop from 160 ° to 110°C the cooling time is 1 0 x 0 . 9 2 = 9 . 2 minutes. The impl icat ions and practical use of these relative cool ing factors are discussed in Sections 6 and 7.

5.2 L A I D T H I C K N E S S Plots of laid thickness versus cool ing time, on logar- i thmic scales, are shown in Figure 4, for a range of envi ronmental condit ions and temperature drops, includ- ing data f rom other sources (Jordan and Thomas (1976), BACMI (1983)). These plots show the relationship between thickness and cool ing t ime to be linear on a logar i thmic basis. For various environmental condit ions, initial and f inal temperatures, the gradients are virtual ly identical, that is the effect of laid thickness on cool ing t ime is independent of environmental condit ions and reference temperatures. The plots fo l low closely the form:

t = kd 1.s . . . . (2)

where t is the cooling t ime from the initial laying temperature to the final temperature;

d = laid thickness,

and k = a constant , the value of wh ich depends on envi ronmenta l condit ions, initial laying and final temperatures.

Thus, for any specified condi t ions, doubl ing the laid thickness increases the cool ing t ime by a factor of 21.8= 3.5 t imes. Alternat ively, for a 50 per cent increase in cool ing t ime an increase in laid thickness of only 25 per cent is required. For example Figure 1 shows that a 40 mm thick layer takes 8.1 minutes to cool f rom 150°C to 100°C when the w ind s~)eed is 40 km/h and the ambient temperature is 0°C, but a 50 mm th ick layer takes 12.1 minutes under similar condit ions. Equation 2 enables relative thickness factors to be calculated, and Table 3 gives such factors compared wi th a cool ing t ime of un i ty for 40 mm laid thickness.

The usefulness of the thickness factors in est imating compact ion t ime is considered in Sect ion 6.

5.3 S O L A R R A D I A T I O N The bulk of the work in this report assumes that solar radiation is zero. Under winter conditions in the UK the effect of solar radiation is negligible and these are the working condit ions wi th which this report is primarily concerned. Even in summer, during overcast conditions the total incident solar radiation is comparatively small and its effects can be ignored. Measurement of solar f lux on site is not convenient and, in the case of varying cloud cover, the effects are diff icult to assess. However, it may under some circumstances be useful to consider solar radiation, which has two effects:

(a) Short-term effect. This is the effect of incident radiation on a (hot) newly-laid surfacing during the compaction period. It is unlikely that this effect will increase compaction time by more than about 20 per cent even under extreme conditions: Such an increase would arise, for example, when solar radiation was init ial ly negligible, the substrate temperature was equal to the air temperature of 20°C, the wind speed was zero, and when, immediately laying began, the sun came out giving a radiation level of 800 Js - lm -2.

(b) Accumulat ive effect. This is the effect of longer term exposure of the road surface to sunlight prior to laying the surfacing. It has the effect of raising the substrate temperature relative to the air

No. Thickness Wind Air (ram) (km/h) (°C)

1 20 2 25 3 30 4 35 5 40 6 45 7 50 8 65

40 10 10 20 15 15 40 40

0 0

10 20 15 10 0 5

2°°I I

;,001- I

i ~ 1 I I I I P t I I 50

0 2 4 6 8 10 12 14 16

Time after laying (min)

Fig. 3 Theoretical variation of mid-layer temperature with time for a range of environmental conditions and laid thicknesses

Laid thickness (mm) 20 25 30

Factor 0.29 0.43 0.60

TABLE 3

Relative thickness factors

35 4O 45 5O 55

0.79 1.00 1.24 1.49 1.77

60 65 70 75 80 100 150

2.07 2.40 2.74 3.10 3.48 5.2 10.8

No. Temp. drop (°C)

1 160--90 2 160--100 3 150--90 4 150--100 5 160--110 6 150--110 7 160--120 8 150--120 9* 116--100 10"* 180--170

Wind (kin/h)

0 0 15 10 10 5

20 40 25 40

Air (°C)

20 20 10 5 0 0 0 0 0 0

.E E

== -5 O O

40

20

10

8

6

4

* Jordan and Thomas (1976) ** BACMI (1983)

2 -

1 - - 10 20 30 40 60

Thickness(mm) logscale 80 100 140

Fig. 4 Theoretical effect of thickness on cooling t ime for various environmental conditions and temperature drops

temperature. Consequently the compaction time is increased. The overall effect is the combination of the short-term and accumulative effects.

Figure 5 shows the relationship between compaction time and the excess of road surface (&T) over air temperature, computed for a range of solar radiation levels for varying times and laid thicknesses. The various solar radiation levels used in compiling this figure are assumed to be continuous for the period stated, which is prior to laying, and continuing throughout the compaction period. The excess of road surface temperature over air temperature (AT) depends not only on the level and period of solar radiation, but it is also reduced markedly by higher wind speeds and increased, to a lesser extent, by higher air temperatures. In Figure 5 the lower envelope represents the effect for an air

temperature of 0°C whereas the upper envelope is representative of an air temperature of 20°C al though there also appear to be minor variations attr ibutable to different laid thicknesses. Because high excess road surface temperatures are unlikely to be associated wi th low air temperatures the relationship is more likely to fo l low the upper envelope for high values of AT. Whether the road surface temperature is raised rapidly by a high rate of solar radiation for a short period or less rapidly by a lower rate for a longer period does not appear to be important: the small differences in the resulting substrate temperature gradients at the t ime of laying have only a negligible effect on compact ion time. The initial excess of road surface temperature over air temperature thus gives a reasonably good measure of the residual accumulative effect of solar radiation, namely the accumulative warming effect of solar radiation, modif ied by wind speed and air temperature. If the effects of solar radiation are simplif ied by using only the solid line in Figure 5, the errors in compact ion t ime factors introduced by this simplif ication are l imited to about _+5%. The application of these factors is discussed in Section 6.

Solar radiation

Symbol Intensity (J s -I m-2)

o 200 500

o 800 • 800

• 300

Time (h)

4 4 8 8 4

Laid thickness

(mm)

40 40 40 20 60

-& O9

LL

s

E o

7

1 . 6 -

1.5 -

, 4 _

,,S i , 4 ,.,_

1.0 P i i I I 0 5 10 1 5 20 25

AT, road surface temperature minus air temperature (°C)

Fig. 5 Theoret ical ef fect of accumulat ive total solar radiat ion on road surface temperature and compact ion t ime

5.4 C O M P A R A T I V E EFFECTS OF F A C T O R S I N V O L V E D

The relative importance of various factors on compaction t ime is summarised in the table below for typical ranges.

TABLE 4

Effect of various factors on corn

Factor

Wind speed Air temperature Accumulat ive net solar radiation Laid thickness Initial laying temp* Min imum compaction temperature**

Range

0 to 20 km/h 0 ° to 20°C

+ 10°Ct

35 to 45 mm 140 ° to 160°C 100 ° to 90°C

3action time

% change in compaction time over stated range

- 2 6 +19 +15

+ 57 +40 +30

texcess of road surface temperature over air temperature * cooling to 100°C ** initially at 150°C

For the typical ranges shown, the wind effect is greater than the air temperature effect, but both these uncontrollable factors are smaller than the controllable factors of laid thickness and initial laying temperature. To increase the compaction t ime the greatest benefit will be obtained by increasing the laid thickness. The initial laying temperature can also be raised but, in the case of rolled asphalt, there may be an increased risk of slippage occurring. Further benefit would be obtained if the specified minimum compaction temperature could be" lowered. These possibilities are discussed in Section 7.

5.5 S U R F A C E T E M P E R A T U R E The near-surface temperature of hot rolled asphalt is of importance to the embedment of coated chippings during compaction. Relative to the mid-layer temperature the fol lowing points should be noted.

1. The surface temperature equals the mid-layer temperature at the instant of laying and also when the surfacing has cooled to ambient temperature.

2. A t any other t ime during the cooling period the surface temperature is lower than the mid-layer temperature.

3. A thicker layer has a higher surface temperature than a thinner one for equivalent weather conditions at the same instant in t ime.

4. Brown (1980) showed that the near-surface temperature of a 40 mm laid thickness (4 mm below the surface) is related to the wind speed and is relatively independent of the air temperature; it may be predicted to within _ 1.5°C from a knowledge of the mid-layer temperature and wind speed. The surface temperature is less than 20°C lower than the

mid-layer temperature for wind speeds below 25 km/h. However for very high wind speeds surface temperatures can be very much lower than the mid-layer temperatures.

6. PREDICTION OF C O M P A C T I O N TIME

Using the relationships established in Section 5 tables have been devised to enable the compaction time to be predicted for a range of environmental conditions and laying specifications. These are brought together in Table 5.

The compaction time is derived from three factors, one being selected from each of three tables. The first, the 'Climate Factor', depends on the combination of wind speed and air temperature, and is the compaction time (in minutes) for the 'standardised conditions' of a 40 mm thick layer cooling from 150°C to 100°C. The values given in the table were obtained from outputs of the computer program. The second factor is selected according to the initial laying and specified minimum compaction temperatures, the values in Table 5 being derived from Table 2. The third factor, 'Thickness', is related to the laid thickness, values being obtained from Table 3. The estimated compaction time, for zero solar radiation, is the product of the three factors, the result being in minutes and generally accurate to within _ 10 per cent when compared with the values predicted directly by the computer. An optional fourth factor gives an adjustment for the accumulative effect of solar radiation on the road surface temperature; these values were derived from Figure 5. For some materials, for example chipped rolled asphalt, when the roller cannot roll the material immediately after laying, it is more appropriate to consider the compaction time as starting from a temperature below the initial laying temperature. To obtain the compaction time in these cases, the time to cool from the initial laying temperature to the temperature at which compaction can begin is subtracted from the time to cool from the initial laying temperature to the minimum compaction temperature. Table 5 applies to all dense bituminous construction of average thermal conductivity and capacity, and laid on similar bases. Pervious macadam and friction course cool more rapidly; a correction factor of 0.83, obtained experimentally, is appropriate for these materials.

6.1 USE OF PREDICTIVE TABLES 6.1.1 AT THE P L A N N I N G STAGE Having quantified the effects of the variables on compaction time it is now possible to determine both the practicability of laying and also the thickness and initial temperature of the material required to give adequate compaction time. Brown (1980) has suggested that a minimum compaction time of 10 minutes, from the time of laying, is appropriate for rolled asphalt. The

6

T A B L E 5

Tables to predict compaction time

1. Select the appropriate climate factor, F~, for the relevant wind speed and air temperature.

2. Select the appropriate initial laying temperature and specified minimum compaction temperature factor, Ft.

CLIMATE FACTORS (Fc)

Wind speed

km/h knots 2 m height 10 m height

0 5

10 15 20 40

0 3 7

10 14 28

Air temperature (°C)

0 5 10 15 20

12.1 12.6 13.3 13.9 14.8 10.6 11.1 11.6 12.1 12.8 9.9 10.3 10.8 11.3 11.8 9.4 9.8 10.3 10.8 11.3 9.1 9.4 9.8 10.3 10.7 8.2 8.5 8.8 9.3 9.7

INITIAL LAYING AND COMPACTION TEMPERATURE FACTORS (Ft)

T o Initial Laying temp

(°C)

180 0.80 170 0.66 160 0.51 150 0.35 140 0.18 130 120 110 100

Note

T~ compaction temperature (°C)

130 120 110 100 90 80 70

1.00 1.21 1.50 1.90 2.40 3.00 0.86 1.07 1.31 1.70 2.20 2.75 0.71 0.92 1.16 1.50 1.95 2.50 0.55 0.76 1.00 1.30 1.70 2.25 0.38 0.59 0.83 1.09 1.50 2.05 0.20 0.41 0.65 0.91 1.30 1.80

- - 0.21 0.451 0.71 1.00 1.50 - - -- 0.24 0.49 0.79 1.25 - - -- -- 0.26 0.55 1.00

1. The m~xing temperature should be at least 20°C higher than the initial laying temperature, subject to the specified maximum for the grade of bitumen concerned.

3. Select the appropriate laid thickness factor (Fd).

LAID THICKNESS FACTORS (Fd)

Thickness (mm) 20 25 30 35 40 45 50 55 60

Factor 0.29 0.43 0.60 0.79 1.00 1 .24 1 .49 1.77 2.07

65 70 75 80 100

2.40 2.74 3.10 3.48 5.2

150

10..8

4. COMPACTION TIME = Fc x F, x Fa (minutes) Note 2. To calculate the compaction time starting from

a temperature Tot, considerably lower than To, subtract the time to cool from To to Td from the time to cool from To to the minimum compaction temperature.

CORRECTION FACTOR FOR ACCUMULATIVE SOLAR RADIATION

Measure the road surface temperature, TRS and subtract the air temperature TA. Select the appropriate factor, FSR, from the table below.

TRs--TA (°C)

FSR

0 5 10 15 20 25

1.00 1.07 1.15 1.24 1.36 1.55

Corrected Compaction Time = FSR X compaction time (minutes) Note 3. This table applies to dense bituminous

surfacings of average thermal conductivity and capacity.

Note 4. For pervious macadam and friction course multiply the compaction time by 0.83.

Meteorological Off ice* is able to provide analyses of hourly mean air temperature and wind speed which enable the likely cl imate factor (Fc) to be assessed for a particular location and t ime of year. These analyses, which are on a monthly basis, can relate to the whole or part of a daytime period, eg 0700-1700 GMT. Long- period data are available from some 40 places, mostly in lowland Britain. It should be noted that the wind speeds used are in knots (1 kno t= 1.853 km/h) and normally have been measured at a height of 10 m above ground in an open, level situation. (Wind speed at 2 m height is approximately 0.78 t imes the wind speed at 10 m height). Appendix A gives a comparison of wind speed scales. It is probably best to assume at this stage that solar radiation will be zero, and so aim to provide a compaction t ime of at least 10 minutes wi thout the positive contribution of solar radiation.

To specify the mixing temperature, an allowance should be made for the temperature drop between the time when the material is discharged from the lorry to the t ime when it emerges from the paver, and also for the small drop during haulage from the mixing plant. An addition of at least 20°C to the initial laying temperature is reasonable, depending on environmental conditions and haulage distance, and, subject to the mixing temperature not exceeding the specified maximum for the grade of bitumen concerned.

6.1.2 ON THE DAY OF LAYING The decision whether or not to lay material may be based on weather forecasts obtained from the Regional Weather Centre** and the predictive tables used as a check to ensure that a workable compaction t ime is still available for the material in question. Adjustments to the mixing temperature may then be made if necessary.

6.1.3 MONITORING ON SITE Once the material has arrived on site there is little that can be done, apart from ensuring that undue delays in the paving operation and, in particular, rolling do not occur, and that the specified thickness is maintained. Monitoring wind speed and air temperature may be useful if conditions deteriorate rapidly; the accumulative solar radiation effect, obtained from the measurement of road surface temperature, may also be taken into account. In order to take the appropriate action when condit ions deteriorate it may be preferable to use information from the nearest Meteorological Office Station rather than rely on site measurements.

*Meteorological Office Climatological Services (Met 03) London Road Bracknell Berkshire RG12 2SZ Tel: 0344-420242 Ext 2278/9

** Regional Weather Centres are located at London, Bristol, Southampton, Cardiff, Manchester, Nottingham, Newcastle and Glasgow.

Wind speed can be monitored on site using a hand-held 10-second integrating anemometer* at a height of 2 m in a position well away from, and up-wind of, obstructions; several determinations should be made and averaged. Air temperature is easily measured using a mercury-in- glass or electronic thermometer, but up-wind of any source of heat such as a paving machine or freshly laid surfacing. Measurement of road surface temperature may be made with an electronic thermometer having a surface measurement probe of low heat capacity.

7. A P P L I C A T I O N TO ROLLED A S P H A L T W E A R I N G COURSE

The cooling predictions in this report apply ideally to dense bituminous surfacings that are rolled without coated chippings. The chippings are usually applied immediately after laying the asphalt and prior to compaction and so there is a delay of a few minutes before compaction can commence. The application of coated chippings has a complex effect on cooling; when first laid and resting on the surface they tend to shield the surfacing from wind and relatively little heat is transferred into the chippings. Once the chippings are rolled in, heat is transferred more quickly, reducing the temperature rapidly in the top part of the asphalt layer. For a 40 mm thick layer, it is estimated that the average asphalt temperature is reduced by 15°C when chippings are rolled in at a rate of 13 kg/m 2, but this effect is greater in the top of the layer. The embedment of chippings also increases the layer thickness by about 5 mm and the increased thickness and reduced surface temperature will tend to slow down the subsequent overall cooling rate. Water on the roller applied to the surfacing also causes cooling by evaporation; overall it is estimated that the average temperature of 40 mm thick asphalt is reduced by 4°C, but the reduction will of course be greater at the surface. A minimum amount of water should therefore be used.

7.1 SPECIFICATIONS The minimum compaction temperature for rolled asphalt containing 50 pen grade bitumen is at present specified as 100°C by BS 594:1973. For the newer heavy-duty asphalts using a 40 pen HD bitumen grade Department of Transport Standard HD/2/79 specifies the same temperature as for 35 pen grade bitumen in BS 594, ie 120°C. Figure 6 shows the effects of laid thickness on compaction time for initial laying temperature to minimum compaction temperatue ranges of 150°-120°C., 150°-100°C and 150°-90°C, both for good and poor environmental conditions. The implications for three types of rolled asphalt are discussed below. In these examples it should be remembered that the compaction times commence from the time the asphalt emerges from the paver. Allowing for the chipping operation, practical compaction times are therefore shorter by 2-3 minutes.

* A suitable anemometer is manufactured by R W Munro Ltd, Clive Road, Bounds Green, London, Nl l 2LY. It is listed as Model HA/6A/151 and a version calibrated in km/h is available to special order.

11 f 20

10 ---18

9 16

8

~ 7 ~ 2 2

.~6

E

2

1

0 0

24

22

20

• 1

g .~ 1

E

compaction time 150-- 120°C

! Good conditions (wind zero air 20°C)

~-~_~ /

f Poor conditions (wind 40km/h, air 0°C)

Minimum practical compaction time 150--100°C

Minimum practical compaction time 150-90°C

I I I I 20 30 40 50 60 70 80

Thickness (mm)

Fig. 6 Theoretical effect of laid thickness on compaction time

7.1.1 NORMAL A S P H A L T The ordinate scale in Figure 6 for 150 ° to 100°C may be considered as typical for normal asphalt using 50 pen grade bitumen, laid at 150°C and having a minimum compaction temperature of 100°C. Even under good environmental conditions an 'easy' compaction time of 15 minutes is achieved only with a laid thickness greater than 40 ram. Under poor environmental conditions only 8.2 minutes are available at 40 mm laid thickness. If, as suggested by Brown (1980), 10 minutes is taken as the minimum time required for compaction, then reference to Table 5 demonstrates that with an air temperature as high as 20°C, and wind speeds approaching 40 kin/h, laying is inadvisable. Similarly for an air temperature of 15°C with a wind speed of only 20 km/h, laying can just be accommodated. Such conditions can occur even during summer months in UK. These problems are made worse if the asphalt is laid only 35 mm thick (so that a compacted thickness of 40 mm is obtained after applying precoated chippings). From Figure 6, for 35 mm laid thickness, under good conditions only 11.4 minutes are available, and under poor conditions this falls to an impracticable 6.2 minutes. Clearly, laying at 35 mm thickness will present difficulties in achieving adequate compaction on most occasions. For very thin asphalt layers of less than 35 ram, compaction times are very short. These problems are alleviated if the laid thickness is increased; even in poor conditions, 10 minutes are available for compaction for a thickness of 45 mm and for 50 mm the compaction time is 12.2 minutes.

7.1.2 HEAVY-DUTY A S P H A L T Asphalt made with HD40 grade or 35 pen grade bitumen has a specified minimum compaction temperature of 120°C (see British Standards Institution (1973) and Department of Transport (1979)). From Figure 6, using the scale for 150°C to 120°C, it can be seen that the cooling times are much shorter than those for normal asphalt. All the problems associated with normal asphalt are thus accentuated with heavy-duty asphalt. For 40 mm laid thickness, even under good conditions, only 8.2 minutes are available for compaction. For poor conditions this falls to 4.4 minutes. Clearly, it is not possible even under good conditions to obtain a minimum of 10 minutes, unless there is an appreciable solar radiation effect equal to an excess road surface temperature over air temperature of + 15°C. Such conditions can occur only during the warmer months. Some improvement in compaction t ime may be achieved by raising the initial laying temperature to 160°C (but this may be diff icult in inclement conditions) giving an improved compaction time of 10.6 minutes for good conditions but still only 5.7 minutes under poor conditions. The other solution is to increase the laid thickness. From Figure 6 a laid thickness of 65 mm is necessary to give a compaction time of more than 10 minutes under poor conditions.

7.1.3 A S P H A L T WITH M O D I F I E D BINDERS

It has been shown (Denning and Carswell, (1981 a and b)) that additives such as ethylene-vinyl acetate (EVA) or sulphur can produce improvements in the resistance to permanent deformation of rolled asphalt equal to or greater than that of heavy duty bitumens, but wi thout adversely affecting the laying characteristics of the asphalt. Some EVA-bitumens should even enable the minimum compaction temperature to be reduced relative to that of both heavy duty and 50 pen grade bitumens. If, for example, the minimum compaction temperature could be reduced to 90°C, by using a modif ied or softer bitumen such as 70 pen, Table 5 indicates that a useful increase of 30 per cent in compaction t ime is obtained compared with 50 pen bitumen. Under poor conditions just 10 minutes are available for a 40 mm laid thickness, and at 50 mm 15 minutes are available. For good conditions 15 minutes can be achieved with only 34 mm laid thickness. Thus it would be possible to lay asphalt that can be compacted satisfactorily at temperatures down to 90°C, under poor conditions if 45 mm thickness (unchipped) is laid.

8. CONCLUSIONS 1. The time that a hot bituminous layer takes to cool

from one temperature to another is very dependent upon the laid thickness, being proportional to the laid thickness to the power of 1.8; an increase of only 25 per cent in laid thickness increases the t ime by 50 per cent. The t ime available for compacting

hot bituminous layers is therefore correspondingly increased.

2. Compaction time is also strongly dependent upon the initial laying and specified minimum compaction temperatures; a general relation is presented. Relative to a temperature range of 150 ° to 100°C, raising the initial laying temperature by 10°C increases the compaction time by 16 per cent and lowering the minimum compaction temperature by 10°C increases the compaction time, by 30 per cent.

3. Compaction time is also influenced by ambient windspeed and temperature; an increase in the windspeed from 0 to 20 km/h reduces the time available by 26 per cent and a fall in ambient temperature from 20 ° to 0°C shortens the period by 19 per cent.

4. A set of tables has been devised to calculate the compaction time for a range of initial laying and minimum compaction temperatures and laid thicknesses: a knowledge of wind speed and air temperature is required.

5. There are significant benefits in terms of compaction time available, to be obtained in developing asphalts that can be compacted at temperatures below the present limit of 100°C in British Standard 594:1973.

9. ACKNOWLEDGEMENTS

This report was prepared in the Pavement Materials and Construction Division of the Highways and Structures Department (Division Head: Mr G F Salt). Advice from Mr M J Prior, Meteorological Office, Bracknell and the assistance of Mr P G Roe in the computing work are gratefully acknowledged.

10. REFERENCES

BRITISH AGGREGATE CONSTRUCTION MATERIALS INDUSTRIES (BACMI) (1983). Private communication.

BROWN J R (1980). The cooling effects of temperature and wind on rolled asphalt surfacings. Department of the Environment Department of Transport TRRL Report SR 624: Transport and Road Research Laboratory, Crowthorne.

BRITISH STANDARDS INSTITUTION (1973). British Standard 594:1973. Rolled asphalt (hot process) for roads and other paved areas. British Standards Institution, London.

DENNING J H and J CARSWELL (1981a). Improvements in rolled asphalt surfacings by the addition of sulphur. Department of the Environment Department of Transport TRRL Report LR 963: Transport and Road Research Laboratory, Crowthorne.

DENNING J H and J CARSWELL (1981b). Improvements in rolled asphalt surfacings by the addition of organic polymers. Department of the Environment Department of Transport TRRL Report LR 989: Transport and Road Research Laboratory, Crowthorne.

DEPARTMENT OF TRANSPORT (1979). Specification for road and bridge works: Rolled asphalt wearing course. Departmental Standard HD/2/79. Department of Transport, London.

JORDAN P G and M E THOMAS (1976). Prediction of cooling curves for hot-mix paving materials by a computer program. Department of the Environment TRRL Report LR 729. Transport and Road Research Laboratory, Crowthorne.

POWELL W D and D LEECH (1976). Rolling requirements to improve compaction of dense roadbase and basecourse.macadam. Department of the Environment TRRL Report LR 727. Transport and Road Research Laboratory, Crowthorne.

STAFFORDSHIRE COUNTY COUNCIL. (1984). Private Communication.

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A P P E N D I X A

BEAUFORT SCALE: SPECIFICATIONS AND EQUIVALENT SPEEDS*

O L L

7

8

10

11

12

Description

Calm.

Light air

Light breeze

Gentle breeze

Moderate breeze

Fresh breeze

Strong breeze

Near gale

Gale

Strong gale

Storm

Violent storm

Hurricane.

Specifications for use on land

Calm: smoke rises vertically.

Direction of wind shown by smoke drift, but not by wind vanes.

Wind felt on face; leaves rustle; ordinary vanes moved by wind.

Leaves and small twigs in constant motion; wind extends light flag.

Raises dust and loose paper; small branches are moved.

Small trees in leaf begin to sway; crested wavelets form on inland waters.

Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty.

Whole trees in motion; inconvenience felt when walking against wind.

Breaks twigs off trees; generally impedes progress.

Slight structural damage occurs (chimney-pots and slates removed).

Seldom experienced inland; trees uprooted; considerable structural damage occurs.

Very rarely experienced; accompanied by widespread damage.

Equivalent speed at 10 m above ground**

Knots Miles per hour

Mean Limits Mean Limits

0 <1 0 <1

2 1-3 2 1-3

Metres per second

Mean Limits

0.0 0.0-0.2

0.8 0.3-1.5

5 4 -6 5 4 -7 2.4 1.6-3.3

9 7-10 10 8-12 4.3 3.4-5.4

13 11-16 15 13-18 6.7 5.5-7.9

19 17-21 21 19-24 9.3 8.0-10.7

24 22-27 28 25-31 12.3 10.8-13.8

30 28-33 35 32-38 15.5 13.9-17.1

37 34-40 42 39-46 18.9 17.2-20.7

44 41-47 50 47-54 22.6 20 .8-24 .4

52 48-55 59 55-63 26.4 24.5-28.4

60 56-63 68 64-72 30.5 28.5-32.6

- - > ~ - ~ 3 - - ~ 2 . 7

* Observers Handbook, Meteorological Office, 1982 ** Wind speed at 2 m height is approximately 0.78 times the wind speed at 10 m height above ground.

Speed conversion factors: 1 knot= 1.853 km/h=0.5148 m/s= 1.152 Mile/h = 101.3 ft/min

Printed in the UK for HMSO (790) Dd8222651 7/85 H P Ltd So'ton G3371

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