TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport RESEARCH REPORT 280 Adverse weather conditions for laying rolled asphalt by J C Nicholls 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
Transcript
TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport
RESEARCH REPORT 280
Adverse weather conditions for laying rolled asphalt
by J C Nicholls
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 1 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. Meteorological data 1
3. Criteria defining adverse weather 1
3.1 Universal criterion 1
3.2 Minimum-temperature criteria 3
3.3 Wind-chill factor criteria 3
4. Probability of adverse weather 4
4.1 General 4
4.2 Variation with location 4
4.3 Variation with time of year 5
4.4 Variation with time of day 5
4.5 Variability 8
4.6 Mathematical models 10
5. Correlation between criteria 10
6. Forecasting adverse weather 13
6.1 Meteorological Office forecasts 13
6.2 Persistence of fine or adverse weather 13
7. Conclusions 15
8. Acknowledgements 15
9. References 15
ADVERSE WEATHER CONDITIONS FOR LAYING ROLLED ASPHALT
ABSTRACT
Previous work has shown that current specifications do not always correctly identify the weather conditions that are unsuitable for laying and compacting rolled asphalt wearing course materials. Using data from the Meteoro- logical Office for nine airfield sites, it is shown that the weather conditions in the UK can be considered adverse for over 90 per cent of the time for compacting a 40 mm thick rolled asphalt mat. On the other hand, the weather is rarely unsuitable for laying a 50 mm mat. Models are developed for predicting the probability of the weather being adverse for sites at given latitude and altitude by month and time of day.
1. INTRODUCTION
Advice on the laying and compaction of bituminous surfacings has traditionally referred to the temperature of the material and the ambient conditions; the former to reflect the stiffness of the material and the latter, the rate of cooling. This aspect of specifications has become more critical in recent years because chippings are being spread at higher rates to achieve good texture depth, the degree of embedment of the chippings being a critical factor in this process. Also it has been demonstrated that incomplete compaction, which has become more likely with the introduction of higher stability, design mixes, can lead to surfacings with poor durability.
Research on this topic by Jordan and Thomas (1976), Brown (1980), Daines (1985), Hunter (1986) and Hunter, Fordyce and McGuire (1986) focused on the rate of cooling of the layer. Brown (1980) identified the principal factors, in order of priority, as layer thickness and wind speed with ambient temperature being less important. Daines (1985) devised a method of calculating the time that is available for spreading the chippings and rolling the mat for various weather conditions. Using that method and assuming that a minimum of ten minutes is necessary for compaction, the climatic conditions can be checked to assess the suitability of weather conditions for a particular site at any time.
In order to assess the probability of climatic conditions being suitable for compaction, an analysis has been made of the prevailing conditions at a number of mete- orological stations in the United Kingdom. Various limiting conditions have been applied in order to assess the proportion of time when they would permit material to be laid.
The consequential costs of laying material in adverse weather conditions can be significant. Poor durability results in more frequent maintenance operations with the
associated traffic congestion. Further, the need to restrict working on certain primary routes to off-peak hours (including winter nights), and the use of lane-rental contracts, means that the conditions under which rolled asphalt is laid are more diverse and closer to the bounda- ries of acceptability than experienced previously. How- ever, changing from a criterion based solely on ambient temperature to one based on more parameters may increase the periods when materials cannot be laid; this would have economic consequences for Industry.
2. METEOROLOGICAL DATA
Data of hourly conditions at nine stations between 1969 and 1988 were obtained from the Meteorological Office. The nearest town or city to each of the stations, selected to cover the whole of the United Kingdom, is shown in Figure 1.
The data included the ambient temperature, wind speed at 10 m height and rainfall. Unfortunately, not all hourly records were complete. For the two primary variables, ambient temperature and wind speed, any missing values were regarded as rendering the data invalid, and that hour's data were ignored. For other parameters, it was assumed that any missing data were due to equipment malfunctions, and the values recorded for the previous hour were used. The extent of such substitutions and omissions was generally small, varying across the stations from 0.1 to 0.9 per cent and 0.01 to 1.6 per cent, respectively.
3. CRITERIA DEFINING ADVERSE WEATHER
It is not feasible to define a criterion which will correctly identify conditions that will have an adverse effect on the laying of the mat. Realistically, an adverse weather criterion can only define weather during which there is a heightened risk that the conditions will have a deleterious effect on the finished mat.
The criteria defining adverse weather in this study were divided into three categories, universal, minimum- temperature and wind-chill factor. The basic conditions to be met for the weather to be classified as "adverse" by each criterion are shown in Table 1.
3.1 UNIVERSAL CRITERION
There are certain conditions under which outdoor con- struction work is generally impracticable. In this report,
4
o ~ )
8~FAS~
7' •
E O
Fig.1 L o c a t i o n o f stat ions
TABLE 1
Criteria for defining adverse weather
C RITERION CODE CONDITIONS
Universal UN
Minimum- T3 temperature T5
T8
Wind-chill M40 factor M45
M50
ground frozen or snow lying; or heavy precipitation (>0.2 mm/hour); or ground wet or flooded; or thick fog/low visibility (<25 m); or gale force winds (>17 m/s at 10 m height).
ambient temperature < 3°C. ambient temperature < 5°C. ambient temperature < 8°C.
compaction time for 40 mm HRA w/c < 10 minutes. compaction time for 45 mm HRA w/c < 10 minutes. compaction time for 50 mm HRA w/c < 10 minutes.
specific conditions have been defined and they have been classified as universally adverse. When considering other criteria, the weather conditions are analysed only when they are not universally adverse.
3.2 MINIMUM-TEMPERATURE CRITERIA
Minimum-temperature criteria are typical of the require- ments commonly included in current specifications. No laying is permitted, subject to the Engineer's discretion, if the ambient temperature falls below a specified tempera- ture; a simple requirement to implement. Three tempera- tures, 3°C, 5°C and 8°C, are examined as the minima above which work is permitted to proceed.
3.3 WIND-CHILL FACTOR CRITERIA
The compaction time available, using the method accord- ing to Daines (1985), is found by taking account of the weather conditions, initial laying and compaction tem- peratures, the laid thickness and any solar radiation. In this study, it is assumed that a ten minute compaction time needs to be available when the initial laying and compaction temperatures are 140°C and 100°C, respec- tively. These values are judged typical for rolled asphalt wearing courses.
The initial temperature is taken as an estimate of the lowest paver-out temperature that will occur for a signifi- cant number of loads on a job carried out by a reason- able contractor. The minimum delivery temperature recommended in BS 594: Part 2 (British Standards Institution, 1985) is 130°C, but this was considered unreasonably low. The mean value is probably nearer 150°C (Daines, 1985), but variation will result in many loads being less than that.
The thicknesses investigated are 40 mm, 45 mm and 50 mm, with the nominal thicknesses being used in the analysis. As with the initial rolling temperature, the typical worst case of nominal minus the tolerance could have been used, but it was considered too severe to use a low value for both instances together. The difference in the criteria for a 5 mm thinner layer is similar to that for a 10°C drop in initial temperature.
Solar radiation is only a significant factor on relatively clear summer days when conditions are least likely to be adverse. In consequence, the effect of solar radiation was not considered in this study.
The values in Table 2 give maximum wind speeds at 5°C intervals of ambient temperature for which the ten-minute compaction time is expected to be available. Wind speeds greater than 17 m/s are not given because the universal criterion comes into operation at that level.
From Table 2 it is seen that for 50 mm thick surfacing, criterion M50 will effectively never be adverse unless the conditions are universally adverse. Therefore the occur- rence of criterion M50 was not investigated further.
4. P R O B A B I L I T Y OF A D V E R S E W E A T H E R
4.1 GENERAL
For each criterion, the probability of adverse weather was found by adding up the number of hours of adverse weather that had occurred at that time of day during that month over the 20 years for which records were analysed and dividing that number by the potential number of hours. This produces a separate probability value for each hour of each month for each site for each criterion, a total of 15552 values. This number is increased if the probability of two criteria being satisfied simultaneously is examined. Fortunately it is not difficult to condense this rather large data set for presentation.
4.2 VARIATION WITH LOCATION
The probabilities found for each hour of each month were averaged to give the mean probabilities for each site using each criterion for adverse weather, as shown in Table 3.
The probabilities are all higher than might have been expected, with the weather being universally adverse for 19 per cent of the time. However, these average values include winter and night data; and the meteorological data were obtained from airfield sites, which tend to be more open and exposed to the wind than typical highway construction sites.
Table 3 shows that the conventional wearing course mat of 40 mm thickness, when using the Daines' criterion of ten minutes available for compaction, is the most de- manding with paving being permitted for only seven per cent of the time. The length of time deemed necessary for compaction and the associated temperature range
may be considered severe constraints and the meteoro- logical data may have been obtained from relatively exposed (airfield) sites, but the results highlight the problems and the risk. As such, the wide spread and generally successful laying of 40 mm mats could be regarded as implying that the additional risks identified by this criterion are small in many instances, and can be overcome by competent personnel carrying out the work. Nevertheless, the most severe criterion in general use at present, the ambient temperature not being less than 8°C, will lead to conditions being classified as acceptable for laying on average for not more than 50 per cent of the time.
The paver-out temperature of 140°C was chosen as the lower end of the typical values that can be expected to occur in practice. However, if the mean value is 150°C (Daines, 1985), then the probability of adverse weather for a 40 mm mat will be approximately that given here for criterion M45, whilst that for a 45 mm thick mat will be given by criterion M50.
The reduction in the probability of encountering adverse weather when the mat thickness is increased from 40 mm to 45 mm is significant, changing from 93 per cent to 32 per cent; or put another way, increasing the potential working time from 7 to 68 per cent. The probability for a mat thickness of 45 mm according to the wind-chill factor criterion is similar to that for the 5°C minimum-tempera- ture requirement, although the two criteria will not necessarily be met concurrently.
The stations were ranked for each criteria according to the probability of adverse weather and re-ordered in Table 4 according to their average ranking. This table indicates some correlation between latitude and the extent of adverse weather, with Plymouth having the lowest probability while Northallerton and Glasgow have the highest probabilities. The altitude of the stations appears to be of secondary importance.
TABLE 3
Mean probability of adverse weather at each station
The probability of adverse weather occurring during each month of the year is the average of values obtained at each site for each hour of the day. Rolled asphalt is generally laid during daylight hours, so the day is as- sumed to include only hours between sunrise and sunset on the 15 th of the month for the majority of the locations. The hours counted as day for data collected are given in Table 5.
Hence, the records deemed as falling within a day shift in, say, January are 10.00, 11.00, 12.00, 13.00, 14.00, 15.00 and 16.00 GMT, with the remaining hours being treated as constituting the night shift.
It is becoming necessary to lay asphalt at night because that is the only time when possession of major roads and airfields is acceptable because of traffic congestion. Therefore, the probabilities were calculated for night shifts as well as for day shifts. The average values across sites for both shifts are presented in Table 6.
The minimum-temperature criteria have maxima in February and minima in July for adverse weather, whilst the universal and M45 criteria have their minima at the same time but their maxima in January and the M40 criterion has a maximum probability in March and a minimum probability in August.
Plots of the mean values of the probability of laying being permitted according to the various criteria, after allowing for universally adverse weather, are given in Figures 2 and 3. The graphs show the extent of the seasonal effect with the differences between winter and summer being most marked for the minimum-temperature criteria, as expected.
4.4 VARIATION WITH TIME OF DAY
The probability of adverse weather occurring at each hour of the day is the average of those values obtained for each month of the year. The average values across sites are presented in Table 7.
The probability of adverse weather according to the universal criterion is not very time dependent, but does have a peak at about 07.00 to 08.00 and a trough at about 12.00 to 15.00 GMT. The probabilities for the minimum-temperature criteria (conditional on the weather not being universally adverse) have definite maxima at 05.00 and minima at 14.00 GMT. However, the probabili- ties for the wind-chill factor criteria (again conditional on the weather not being universally adverse) have less well defined maxima at 13.00 to 14.00 and minima between 20.00 and 00.00 GMT, varying less with the time of day than the universal criterion.
TABLE 5
Daylight hours for each month (GMT)
MONTH JAN FEB MARCH APRIL MAY JUNE JULY AUG SEPT OCT NOV DEC
F ig .2 Overa l l mean p r o b a b i l i t y o f acceptab le weather fo r lay ing asphalt dur ing the
day f o r t i m e o f year
Dec
1.0 1.0
0.9
0.8
0.7
IUN & M50 I = 0.6 2 t~
_~ 0.5 J3 n~
0.4 o
2~
0
0.3
0.2
0.1
0
~ :'"'--./
-;
I \ \
B •
// .¢ ..." .
/ o J
J
lp
/ /
$
• ~ I~
I
I
t ~ -
I
I g
I I , ' \ \
• ~ -. \
I "
I
I
I I ~ -
~ • f - .
\ \
\
0.9
0.6
0.7
rUN & M50J
0.6
F~ 0.5
0.4
0.3
0.2
F~
0.1
°
Jan Feb Mar ApJ May Jun Jul Aug Sep Ocl Nov
Fig.3 Overall mean probabil i ty of acceptable weather for laying asphalt during the night for time of year
I ]ec
TABLE 6
Mean probability of adverse conditions each month
CRITERION UN T3 T5 T8 M40 M45
DAY SHIFT
JANUARY 0.41 0.12 0.34 0.70 0.98 0.40 FEBRUARY 0.35 0.17 0.39 0.75 0.99 0.37 MARCH 0.20 0.07 0.22 0.60 0.99 0.37 APRIL 0.11 0.02 0.08 0.34 0.96 0.21 MAY 0.09 0.00 0.01 0.09 0.93 0.08 JUNE 0.08 0.00 0.00 0.02 0.85 0.02 JULY 0.06 0.00 0.00 0.00 0.79 0.01 AUGUST 0.08 0.00 0.00 0.00 0.77 0.01 SEPTEMBER 0.08 0.00 0.00 0.02 0.87 0.03 OCTOBER 0.12 0.01 0.02 0.10 0.93 0.09 NOVEMBER 0.21 0.04 0.16 0.45 0.98 0.24 DECEMBER 0.30 0.09 0.27 0.59 0.97 0.31
MEAN 0.17 0.04 0.12 0.30 0.92 0.18
NIGHT SHIFT
JANUARY 0.44 0.24 0.47 0.78 0.97 0.37 FEBRUARY 0.39 0.32 0.58 0.87 0.97 0.32 MARCH 0.24 0.26 0.53 0.86 0.98 0.29 APRIL 0.12 0.15 0.34 0.72 0.96 0.15 MAY 0.09 0.03 0.10 0.35 0.93 0.06 J U N E 0.08 0.00 0.01 0.09 0.83 0.01 JULY 0.07 0.00 0.00 0.01 0.75 0.01 AUGUST 0.08 0.00 0.00 0.02 0.74 0.01 SEPTEMBER 0.10 0.01 0.02 0.11 0.84 0.03 OCTOBER 0.13 0.04 0.10 0.32 0.93 0.07 NOVEMBER 0.24 0.14 0.32 0.61 0.98 0.21 DECEMBER 0.32 0.17 0.37 0.66 0.97 0.29
MEAN 0.19 0.11 0.24 0.45 0.90 0.15
Plots of the mean values of the probability of laying being permitted according to the various criteria, after allowing for universally adverse weather, are given in Figure 4. The graph shows the probability of the universal criterion as not very time dependent, but still sufficient to mask the shape of the curve for criterion M45.
4.5 VARIABILITY
The variability in the weather is assumed to be the variation in different years of the probability of adverse weather occurring at the same site, same time of day and same time of year. To assess the variability of the weather, the standard deviations of the relative frequency (or probability) for each criterion at each station for each hour of each month were derived using the values obtained from the 28 to 31 days in that month for each year. Coefficients of variation were also derived.
The average values of standard deviation and coefficient of variation are given in Table 8, together with the mean values.
The values represent the considerable variation in the weather in different years. Criterion M40 has the lowest coefficient of variation, but this is to be expected because of the consistently high probability of the criterion occur- ring. For the other criteria, their coefficients are similar with M45 being the greatest. The practical implication of this finding is that the uncertainty in any calculated probability of adverse weather will not be markedly affected by changing from a minimum-temperature criterion to a wind-chill factor criterion. Therefore, al- though the expected down time may be different, the year to year variation in the extent of that down time should not differ according to the type of criterion used.
1.0 1.0
0 . 9 P
0.8
.7 "°°- / ~ " % - " "
. *%.. . / : ".. • ~ . . . . . . . . °°°° : ",,
/
0.5
0.4
0.3
0.2
0.1
..Q
0
/ /
/ /
/ /
/
%%.
\
0.9
IUN & M50 I
0.8
0.5
0.4
0 . 3 ,
0.2
0.1
0 0 0 0 0 0 0 0 0 0 0 0 0 o o o. o o. o o o. o. o o. o. o. 0 0 0 0 0 ~ ~ ~ ~ ~ ~ 04 0
Time (GMT)
Fig,4 Overa l l mean p r o b a b i l i t y o f accep tab le w e a t h e r f o r l a y i n g aspha l t f o r t i m e o f day 9
Multiple linear regression analyses were carried out using two probabilities in turn as the dependent variable for each criterion. The first was the probability of the weather being adverse according to the chosen criterion when the weather was not universally adverse (p~) while the second was probability of the weather being classified as adverse according to that criterion, or universally ad- verse, or both (P2). The independent variables used were the time of year (ty), the time of day (td), the latitude (lat, in minutes greater than 50°N) and the altitude (alt, in metres above sea level). The times of year and day were represented by the following arithmetic functions:
ty = cos( ~ (mth - 1.5)/6)
where mth is the month
which is a maximum of +1 between January and Febru- ary and a minimum of -1 between July and August; and
t d = cos(/~ (hr - kl)/k2)
where hr is the time in hours
k~ = 5 k 2 = 15 for hr ~< 5
kl = 5 k 2 = 9 for 5 < hr < 14
k 1 = -1 1% = 15 for hr/> 14
which is a maximum of +1 at 05.00 GMT and a minimum of -1 at 14.00 GMT.
The relationships obtained for the probability of the weather being adverse for a criterion are of the general form:
pl = k~ ty + 1%t~ + I% lat + k 4 alt + k 5
P2 = ksty + k 7t d + k 8 lat + k 9 alt + klo
for which the regression coefficients are given in Table 9.
The models fit the data well, and can therefore be used to predict probabilities.
5. C O R R E L A T I O N B E T W E E N C R I T E R I A
Values of the joint probability of both a minimum-tem- perature criterion and a wind-chill factor criterion defining adverse weather at the same time (i.e. their intersection) when the weather was not universally adverse are given in Table 10.
Regression coefficients for probabilities of adverse weather
CRITERION
EXCLUDING UNIVERSALL Y ADVERSE WEATHER
T3 T5 T8 M40 M45
k 1 ( x ty) 0.120 0.251 0.430 0.102 0.192 k 2 ( x td) 0.050 0.086 0.122 -0.0094 -0.0079 k 3 ( X lat) 0.00009 0.00018 0.00025 0.00005 0.00004 k 4 ( x alt) 0.00019 0.00041 0.00068 k s (constant) 0.058 0.133 0.297 0.901 0.153
R 2 0.70 0.81 0.90 0.66 0.86
CRITERION
INCLUDING UNIVERSALL Y ADVERSE WEATHER
T3 T5 T8 M40 M45 M50
k 6 ( x ty) 0.231 0.315 0.431 0.098 0.278 0.155 k 7 ( x td) 0.067 0.090 0.116 -0.0070 0.0183 0.0340 k 8 ( x lat) 0.00031 0.00033 0.00032 0.00006 0.00025 0.00028 k 9 ( x alt) 0.00049 0.00059 0.00071 0.00007 0.00034 0.00040 klo (constant) 0.155 0.218 0.357 0.904 0.233 0.108
R 2 0.80 0.86 0.92 0.69 0.87 0.69
TABLE 10
Mean joint probability
CRITERION T3 I~ M40 T5 CI M40 T8 N M40 T3 I'~ M45 T5 N M45 T8 I~ M45
By combining these data with the probabilities of single criteria being met, the probabilities both of one criterion being met while the other is not met and of neither criteria being met can be derived from the following standard relationships:
p(A (3 B ' )=p (A ) -p (A (3 B)
P(A' (3 B') = 1 - p(A (3 B) + p(A (3 B') + p(A' (3 B)
The average values across the sites are given in Table 11, where satisfactory implies that the criterion would permit laying and adverse that it would classify the weather as adverse.
From these values, it can be seen that the probabilities of the two criteria types are neither independent (in which case the ratio of probabilities for satisfactory and adverse to one criterion would be similar for both satisfactory and adverse conditions according to the other) nor nearly dependent (in which case at least one of the probabilities of the weather being different according to the different criteria would be approaching zero). Hence, as would be expected, the two types of criteria are correlated posi- tively but imperfectly.
6. F O R E C A S T I N G A D V E R S E W E A T H E R
readings for days with both measured and forecast values are shown in Table 12.
Based on this limited comparison from a single site and despite the general nature of the wind speed on the following day being able to be ascertained from these forecasts, the forecast speeds were not sufficiently precise for use in calculations. As such, the forecasts cannot be used with confidence in the wind-chill criterion to assess the expected time that will be available for compaction on the following day. However, wind speed forecasts, whilst not being perfect, are beneficial to operational planning.
6.2 PERSISTENCE OF FINE OR ADVERSE WEATHER
Another aspect of planning is the chance of the situation changing, where, say, the weather is not adverse at the start of a shift but may change significantly, becoming adverse during the shift (or vice versa). For this purpose, a shift waslaken to be either the day or the night, as defined in section 4.4. Continuing adverse weather was defined as when more than one further hour of adverse weather occurred in a shift after the first hour was adverse, whilst continuing fine weather was when there was no adverse weather in a shift. Changes from adverse and fine weather are their corollaries. The average results across the sites are given in Table 13 for both the day and night shifts according to the five criteria.
6.1 METEOROLOGICAL OFFICE FORECASTS
Unfortunately, it was not possible to obtain past weather forecasts from the Meteorological Office in terms of the parameters used in this study. However, Frank Graham & Partners (1989) gave details of the forecast wind speed and that measured on site during a Department of Transport specification trial on adverse weather working. The forecasts for the following day were obtained specifically for the purpose from the local weather centre, arriving between 15.00 and 16.00 GMT; records of the wind speeds on site were made available for days when laying had been planned, although the number of read- ings varied depending on the actual work carried out. The
The values show that the continuity from the first hour is fairly good. For the minimum-temperature criteria_, for which one would expect the coldest weather first thing for the day shift, adverse weather continues in about 30 per cent of the shifts and fine weather in about 40 per cent, with the majority of the rest being a change to fine weather. For the wind-chill factor criterion with a 40 mm mat, the incidence of adverse weather is so extensive that continuity is almost assured except for that of fine weather. For criterion M45, the continuity was similar to that for the minimum-temperature criteria, indicating that adverse weather by that criterion is not significantly more variable throughout the day than for the simple minimum- temperature criteria. Hence, the probability of changes in the weather are no greater with wind-chill factor criteria than with minimum-temperature criteria.
1 .The average probability of the weather not allowing 10 minutes for compaction whilst the temperature of the mat is between 140°C and 100°C is 93 per cent with a 40 mm thick rolled asphalt wearing course for the sites exam- ined. The value for a 50 mm thick rolled asphalt wearing course is less than 20 per cent. Even after allowing for seasonal and diurnal variations and for the data having been obtained from exposed airfield sites, this shows that a 50 mm wearing course offers considerable operational flexibility and the promise of more consistent and reliable performance.
2.The effect of increasing the temperature range within which effective compaction is assumed to occur from 140°C - 100°C to 150°C - 100°C is equivalent to increas- ing the thickness of the mat by 5 mm.
3.The correlation between existing minimum-temperature criteria and criteria incorporating wind-chill factors for adverse weather is poor. Criteria incorporating wind-chill factors are inherently more reliable than a minimum- temperature criterion, which cannot accurately define the weather conditions that will increase the risk of poor compaction when laying rolled asphalt.
4.The difficulty in forecasting adverse weather is not significantly different when adverse weather is defined by the wind-chill factor criterion than when defined by a minimum-temperature criterion. The assessment can be based either on Meteorological Office local forecasts or on the weather during a preceding hour.
DAINES, M E (1985). Cooling of bituminous layers and time available for their compaction. Department of Transport TRRL Report RR 4: Transport and Road Research Laboratory, Crowthorne.
FRANK GRAHAM & PARTNERS (1989). Private commu- nication.
HUNTER, R N (1986). The cooling of bituminous materi- als during laying. Journal of the Institute of Asphalt Technology, No 38, September, p 19-26.
HUNTER, R N, D FORDYCE and G R McGUIRE (1986). The cooling of bituminous layers. Highways and Trans- portation, May, p 2-8.
JORDAN, P G and M E THOMAS (1976). Prediction of cooling curves for hot-mix paving materials by a com- puter program. Department of the Environment TRRL Report LR729. Transport and Road Research Labora- tory, Crowthorne.
8. ACKNOWLEDGEMENTS
This report was prepared in the Materials and Construc- tion Division (Division Head, Mr D M Colwill) of the Highways Group. The meteorological data were provided by the Meteorological Office, Bracknell.
9. REFERENCES
BRITISH STANDARDS INSTITUTION (1985). British Standard 594: Part 2, Hot rolled asphalts for roads and other paved areas: Specification for the transport, laying and compaction of rolled asphalt. British Standards Institution, London.
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.
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