H.P. WITT
TEMPERATURE CONDITIONS IN
BITUMINOUS SURFACINGS AT DARWIN
DURING A PERIOD OF ONE YEAR
ABSTRACT
TEST SITE
The temperatures at different depths in a 250 mm
deep asphaltic concrete (AC) layer and in the bitumen of two sprayed seals have been recorded over a
period of 16 months at a Darwin test site. The
recorded data were analysed for seasonal and daily
variations and, for one case, compared with those obtained for a Brisbane site previously reported. Darwin
is in a tropical coastal location with a summer monsoon climate and pavement temperatures are warm
to hot throughout the year. The maximum temperatures obtained in the seal bitumens were significantly less than those at the surface of the AC constructions and the AC construction temperature
regimes were higher when it was new than after the
surface had been exposed for a period. These
differences are explained by a significantly lower absorptivity for solar radiation by the seals than by the
AC and by a reduction in absorptivity of the AC when
the surface becomes weathered.
INTRODUCTION
Temperature conditions in asphaltic concrete (AC)
and sprayed seals over a year or more have been
reported for Melbourne, Sydney, Perth, Brisbane and
Canberra (Dickinson 1969a and b , 1971, 1975 and
1977) and the equipment used for the collection of
data has been described in Dunstan (1967).
The temperature of each thermocouple is
recorded continuously on a chart recorder (recorder
data) and the cumulative time that each thermocouple
has spent in a particular 6°C temperature range is
monitored on a set of counters (counter data).
Data have now been collected for a site at Darwin over a period of 16 months (December 1979 to
March 1981) and are presented in this paper. The
temperature measurements were supplemented by
measurements of the total (global) daily insolation
using a silicon solar cell radiation integrator (Whillier
1964).
Darwin (latitude 1 2.5°S) is a tropical coastal site
with a summer monsoon (wet) season from mid
December to late March.
30
The site consisted of three 3.5 m square sections:
one a 250 mm layer of AC and the other two sprayed
seals with different seal aggregates. All were placed
over a drained crushed rock base.
The aggregate for the crushed rock base, the AC
and one of the seals was a white quartz (Acacia
Creek quarry); the other was a pink syenite (Mt Bundy
quarry).
The temperature sensors (thermocouples) were
positioned at the centre of the sections. In the 250
mm AC layer they were placed at the top (6 mm below
the surface), 50, 100 and 200 mm deep and in the
seals they were placed in the bitumen layer before
the seal aggregate was applied. The 'surface' thermocouples in all three sections were joined to a heat
sink consisting of an open copper wire grid.
Continuous record and time integrated data for
the period 6 December 1979 to 23 March 1981 have
been analysed. The amplifier of the recorder had an
intermittent malfunction during the period 25 January
1980 to 21 March 1980 and 21 days of data were
lost. The integrating solarimeter was read every
working day at 9.4 h local time which is 2.5 h after
sunrise at the equinox (local standard time is 0.9 h
behind astronomical time).
ANALYSIS OF RESULTS
DAILY SOLAR RADIATION AT THE SITE
The solar radiation (insolation) received by the pavement during any one day is the controlling factor so
far as pavement temperature levels are concerned
and this is very dependent on the extent of cloud
cover. The maximum daily values of insolation
recorded at the site during each month of the observation period are compared in Table .1 with the
theoretical values (Spencer 1965). The theoretical
level is only attained in the winter months when clear
sky days occur.
The monthly averages of the measured daily insolation are compared with the theoretical values in
Fig. 1. These averages are particularly low in the wet
season when thick cloud cover is persistent. During
the passage of Cyclone Max over the site on 12
March 1981 the insolation for the day was only 1.48
kW.h/m2 and the surface temperature of the (inundated) sections remained at 22°C for a period of 27 h.
AUSTRALIAN ROAD RESEARCH, Vol 11, No. 4, December 1981
WITT -TEMPERATURES IN BITUMINOUS SURFACINGS AT DARWIN
8.0
IL THEORETICALlj
7. 0
z
0
2
6. 0
1979
O
LMEASURED
1980
5. 0
7
40
DEC
WET SEASON
JAN
FEB
MARCH APRIL MAY
JUNE JULY AUG SEPT OCT NOV DEC
Fig. 1 -Monthly average of daily insolation at Darwin (global radiation - horizontal surface)
TABLE I
MAXIMUM DAILY INSOLATION RECORDED
IN EACH MONTH OF THE OBSERVATION
PERIOD
Date
Daily Insolation (kW.h/m 2)
Max.
Observed°
Theoretical
7.1
7.1
7.1
7.3
6.6
5.9
5.5
5.8
6.2
7.1
7.2
7.0
7.0
5.9
7.0
7.2
7.75
7.75
7.65
7.60
6.85
6.00
5.65
5.85
6.30
7.25
7.70
7.75
7.75
7.75
7.65
7.40
27 Dec. 1979
10 Jan. 1980
28 Feb. 1980
3 March 1980
8 April 1980
14 May 1980
3 June 1980
22 July 1980
11 Aug. 1980
22 Sept. 1980
21 Oct. 1980
24 Nov. 1980
23 Dec. 1980
5 Jan. 1981
25 Feb. 1981
16 March 1981
•
The solarimeter recorded insolation in steps of 0.08 kW.h/m2
Fig. 2 shows the variation of the daily maximum
surface temperature of the AC layer before and
through the 1980-81 wet season. The highest pavement temperatures at the site were observed in late
November or early December, just before the onset of
the wet season.
ABSOLUTE MAXIMUM AND MINIMUM PAVEMENT
TEMPERATURES
Data for the maximum temperatures on the hottest
day and minimum temperatures on the coolest day
are given in Table II. The maxima for the two seals are
about 9°C lower than the 'surface' temperature of the
(new) AC.
A comparison with the other sites investigated
can be made for the absolute maximum and minimum
temperatures at a depth of 50 mm in an AC layer and
the results are given in Table Ill. The generally
warmer winter conditions at Darwin are indicated.
SEASONAL VARIATION
At a tropical site seasonal variation is minimal. For
Darwin, the monthly average of the daily maximum air
(screen) temperature ranges from 29.8°C (July) to
33.4°C (November) (Bureau of Meteorology 1975).
65
60
55
TEMPERATUREI C)
50
45
40
35
30
CYCLONE
1.155
25
20
NOV 10
1980
DEC 1
JAN 1
1981
FEB I
MAR 1
VAR 29
Fig. 2 -Variation of daily maximum surface temperature of the (old) AC before and through the
wet season 1980-81
AUSTRALIAN ROAD RESEARCH, Vol. 11, No. 4, December 1981
31
WITT -TEMPERATURES IN BITUMINOUS SURFACINGS AT DARWIN
TABLE II
Two periods were selected for an analysis of
seasonal variation.
MAXIMUM TEMPERATURES ON HOTTEST
DAY AND MINIMUM TEMPERATURES ON
THE COOLEST DAY
Construc tion
Seal (White)
Seal (Pink)
AC top
50 mm Deep
100 mm Deep
200 mm Deep
(a) The cool season which was identified from the
counter data as extending from 18 June to 13
August 1980 (56 days).
Hottest Day
(Max.) (°C)
22.12.79
Coolest Day
(Min.) (°C)
21.6.80
(b) The wet season which was identified from the
recorder data as extending from 17 December
1980 to 18 March 1981 (91 days).
56.0
57.0
65.4
59.5
54.2
48.5
21.5
20.0
19.6
21.0
23.0
26.0
The temperature distributions for these two
periods for the different depths in the AC structure
and for the seals are given in Table IV.
In the cool season, surface temperatures ranged
from 18°C to 48°C for the seals and 18°C to 54°C for
TABLE III
ABSOLUTE MAXIMUM AND MINIMUM TEMPERATURES AT A
DEPTH OF 50 mm IN AN AC LAYER
Site Melbourne Sydney Perth Brisbane Canberra Darwin
(40 mm)
Maximum
(°C)
56
60
64
59
59
60
Minimum
(°C)
1
6
7
13
-1
21
TABLE IV
TEMPERATURE DISTRIBUTIONS FOR THE COOL AND WET SEASONS
(% OF PERIOD)
Temp.
Bracket (°C)
12-18
18-24
24-30
30-36
36-42
42-48
7.8
9.6
50.1
49.5
18.3
16.9
13.4
12.6
10.5
11.5
10.1
6.4
1.5
45.7
44.6
45.3
36.9
15.1
21.3
28.5
48.0
11.6
14.4
21.2
15.1
13.2
13.3
3.5
4.4
9.0
10.3
49.5
49.6
22.0
20.3
10.7
10.0
6.7
6.8
1.7
2.4
9.3
7.5
5.2
2.9
43.6
39.9
37.8
32.0
21.0
26.6
32.9
45.4
10.2
12.7
15.2
17.5
7.5
8.5
7.7
2.3
5.8
4.4
1.2
2.1
0.2
< 0.1
60-66
48-54
54-60
60-66
(a) Coo/ Season (56 days)
Seal (white)
Seal (pink)
AC (top)
50 mm deep
100 mm deep
200 mm deep
(b) Wet Season (91 days)
Seal (white)
Seal (pink)
AC (top)
50 mm deep
100 mm deep
200 mm deep
0.4
0.6
0.5
0.3
0.1
< 0.1
TABLE V
TEMPERATURE DISTRIBUTIONS OVER A YEAR
(% OF PERIOD)
Temp.
Bracket (°C)
Seal (white)
Seal (pink)
AC (top)
50 mm deep
100 mm deep
200 mm deep
32
12-18
18-24
24-30
30-36
36-42
42-48
48-54
54-60
0.1
0.2
3.5
4.1
35.9
37.0
30.3
28.4
14.1
13.3
11.8
11.5
4.2
5.1
0.2
0.5
0.1
0.1
< 0.1
3.9
2.9
1.5
0.7
33.3
27.6
22.6
15.1
26.9
31.8
37.6
45.7
12.8
16.9
22.7
31.1
9.8
12.3
12.8
7.4
8.9
7.2
3.2
< 0.1
3.8
1.3
0.5
AUSTRALIAN ROAD RESEARCH, Vol 11, No. 4, December 1981
WITT —TEMPERATURES IN BITUMINOUS SURFACINGS AT DARWIN
/0
70
Ir = 0.85)
OBSERVED
OBSERVED
••••
CALCULATED
60
60
— — CALCULATED
SURFACE
50
50
16 mm depth/
16 mm deep)
30
30
-0/
60
TEMPERA TUREI C)
20
TEMPERATURE IC)
SURFACE
40
40
60
50
50
40
40
30
30
50
50
40
200 mm depth
.01
200 mm Jepth
40
30
30
10 NOON 14
16
18
20
22
MIDN
2
8
10 NOON 14 16
18 20
22 MIDN 2
4
8
6
LOCAL STANDARD TIME
LOCAL STANDARD TIME
Fig. 4 —Observed and calculated temperatures at different
depths in the (old) AC layer —25 Nov. 1980
Fig. 3 —Observed and calculated temperatures at different
depths in the (new) AC layer 22 Dec. 1979
the AC, confirming the generally hotter conditions experienced at the AC surface. At a depth of 200 mm in
the AC layer the temperature variation was small (24
to 42°C). The surface of the AC was, however, above
54°C for a significant period.
In the wet season the surface temperature of the
AC ranged from 18°C to 60°C with minor periods
below and above these levels.
was 25 November 1980 after the AC had been exposed for a year.
New AC is known to have a better absorptivity
for solar radiation than old AC (Gebhart 1971)
because the highly absorbing bitumen films on the
pieces of aggregate protruding from the surface are
rapidly oxidised and weathered away to expose (uncoated) aggregate surfaces.
For both periods the seal with the pink (syenite)
aggregate experienced a slightly greater range of
temperature than the white (quartz) seal.
CALCULATED
40
TEMPERATURE DISTRIBUTIONS OVER A YEAR
Temperature distributions over a twelve-month
period at a particular site are of importance in relation to the service performance of bituminous structures at that site. These distributions for the different
depths in the AC structure and for the seals through
the period 26 March 1980 to 25 March 1981 are
given in Table V. The small difference between the
surface temperature ranges in the two seals and the
large difference between the seals and the AC surface was again indicated.
OBSERVED
50
SURFACE
30
I6 mm depth,
20
50
— 40
E
100 mm depth
30
20
DAILY VARIATION
50
The greatest daily temperature variation occur when
there are clear skies throughout the day and wind
speeds are low. These conditions are fulfilled during
the winter months at Darwin but a cloud-free day during summer is rare.
The daily variations at different depths in the AC
structure for two hot days and one cool day at the
site are shown in Figs 3 , 4 and 5. Two hot days are
considered because the first hot day selected was
22 December 1979 which was only two weeks after
the AC had been laid. The second hot day selected
40
30
200 mm depth
20
S
10 NOON 14 16 18 20 22 MIDN
2
LOCAL STANDARD TIME
Fig. 5 —Observed and calculated temperatures at different
depths in the (old) AC layer 22 June 1980
AUSTRALIAN ROAD RESEARCH, Vol. 11, No. 4, December 1981
33
WITT -TEMPERATURES IN BITUMINOUS SURFACINGS AT DARWIN
CALCULATION OF DAILY
VARIATION
The calculation method can also be applied to
the seals if it is assumed that the thermal conductivity
and diffusivity of the underlying crushed rock base is
the same as that of the AC. This assumption is known
to be approximately true from an analysis by Dickinson (1978a) of the daily temperature variations in AC
layers 100 mm thick overlying crushed rock bases
derived from the same mineral aggregate source.
A method for calculating the daily temperature variation in pavement structures for clear sky days was
developed by Dickinson (1978a) which is based on
a knowledge of the insolation for the day (obtained
from Spencer's tables), the absorptivity of the surface for solar radiation (r) and a calculation starting
temperature (CST) which can be evaluated from the
monthly average of the daily maximum air screen temperature (MMAT) for the time of year being considered.
A value for the thermal conductivity and the thermal diffusivity of the AC has either to be assumed or
measured for the calculation and these physical properties were measured on a sample cut from the AC
construction. The thermal conductivity was found to
be 2.90 W/mK and the thermal diffusivity 1630 nm/s.
These values are abnormally high for AC but are explained by the very high content of quartz in the aggregate. (Quartz has a thermal conductivity of 5.5
W/mK which is much higher than the other common
constituents of rocks.)
The two seals at the site showed only minor
differences in daily temperature variation which have
been indicated from observations over the two
chosen seasons and the whole year (see Tables IV
and V).
A comparison of observed and calculated daily
variations for one of the seals for a hot and a cool day
is shown in Fig. 6. Data used for the calculations are
given in Table VI. Best agreement was obtained by
assigning an absorptivity of 0.65 for the seal which is
a significantly lower value than that of the old AC
(0.85).
OBSERVED
60
HOT DAY Dec. 22.1979
CALCULATED FOR
r = 0.65
Following measurement of the thermal conductivity of the AC used at the Darwin, Brisbane and Canberra sites, the original method of calculation
(Dickinson 1978a) has been slightly modified to obtain best overall fit for data for all three sites (Dickinson 1961). The lag between the maximum rate of
energy (solar radiation) input to the pavement and the
maximum rate of energy output has been changed to
1.5 h for summer and 0.5 h for winter. (For the coolest
day at Darwin a value of 1.0 h is used because the
temperature regime is relatively high (see Table VI).
Also, the 'adiabatic' boundary is now placed at a
depth of 300 mm.)
50
40
TEMPERATUR E IC)
30
20
so
COOL DAY June 22. 1980
40
30
The calculated daily variation for the three
chosen days is also shown in Figs 3, 4 and 5 (dotted
curves) and the data used for the calculations are
given in Table VI. The best agreement between observed and calculated values was obtained by
assigning an absorptivity of 0.95 for the new AC and
0.85 for the old AC.
20
8 10 NOON 14 16 18 20 22 MIDN 2 4 6 8
LOCAL STANDARD TIME
Fig. 6 -Observed and calculated temperatures in the surface
seals for hot and cool days (r = 0.65)
TABLE VI
CALCULATION INPUT DATA FOR TWO HOT DAYS AND ONE COOL DAY
Day
22 Dec. 1979
AC (New)
Seal
25 Nov. 1980*
AC (Old)
22 June 1980
AC (Old)
Seal
Sunrise
Time
(h)
Sunset
Time
(h)
Insolation
(kW.h/m 2)
Calculation
Start Temp.
(°C)
(observed)
Lag Between
Surface Input
and Output
(h)
Absorptivity
(r)
6.25
19.00
7.70
34.7
1.5
0.95
0.65
6.25
19.00
7.60
35.5
1.5
0.85
7.00
18.50
5.54
26.0
1.0
0.85
0.65
Slight Cloud 10.5 h and 11.5 h
34
AUSTRALIAN ROAD RESEARCH, Vol 11, No. 4, December 1981
WITT —TEMPERATURES IN BITUMINOUS SURFACINGS AT DARWIN
TEMPERATURE REGIMES IN THICK
AC LAYERS AT THE SITE AND
STRUCTURAL PERFORMANCE
the upper 200 mm of the layer (the equivalent temperature is defined as that temperature which the 200
mm layer would have if the heat in it were distributed
uniformly (Dickinson 1978b) ).
Thick AC structures become weaker with increase in
temperature. The (time of loading dependent) elastic
modulus decreases as the temperature rises and the
material becomes less resistant to rutting (permanent
deformation).
For the hottest days, the daily variation of the
heat content of the layer was roughly the same but,
for the coolest days, the heat content of the Darwin
layer was much bigger. This indicates that AC structures at a tropical coastal site like Darwin will be
much more likely to become distressed under heavy
vehicle loading than AC structures elsewhere in
Australia. Accordingly, the AC should be designed to
have a high modulus and good resistance to permanent deformation (low creep at high pavement
temperatures). A harder grade bitumen than the traditional Class 170 (Standard Association of Australia
1980) could be used with advantage.
A comparison of the hottest and coolest conditions observed in a 250 mm layer of AC at Brisbane
and Darwin is shown in Fig. 7 (Brisbane was chosen
for the comparison because it was the hottest of the
other five sites investigated). Fig. 7 shows the variation of the equivalent temperature through a day in
HOT DAYS
55
BRISBANE
/
EQUIVALENT TEMPERATUREO F 200 mm L A YER IC)
-- DARWIN
/
50
/
/
/
40
/—N
/
•
•
\
BRISBANE DEC 12 1972
DARWIN NOV 25 1980
/ COOLEST
DAYS
/
30
/
DARWIN JUNE 22 1980
BRISBANE JUNE 10 1973
20
CONCLUSIONS
Bituminous pavement temperature regimes at
Darwin are uniformly warm to hot throughout the
year.
The higher temperatures which might be expected in the summer months are tempered by
the cloudy conditions of the monsoon season.
Maximum temperatures in the bitumen of the
sprayed seals were significantly less than at the
surface of the AC. This is explained by the seals
having a lower absorptivity for solar radiation
than the AC surface.
The absorptivity of the new AC surface was also
found to be significantly higher than when it had
been weathered by exposure.
10
MIDN 2 4
6
8 10 NOON 14 16 18 20 22 MIDN
TIME OF DAY (local time)
Fig. 7 — Daily variation of the heat content of a 200 mm AC layer
(Brisbane and Darwin)
REFERENCES
Thick AC pavement structures placed at sites
with climates similar to Darwin will be very subject to distress by heavy vehicle loading
because of the prevailing high temperature conditions.
BUREAU OF METEOROLOGY (1975). Climatic averages of Australia.
DICKINSON, E.J. (1969a ). Temperature conditions in bituminous concrete surfacings at a
site near Melbourne during a period of three years. Aust. Rd Res. 3(9), pp. 35-41.
(1969b ). Temperature conditions in bituminous surfacings at a site near Sydney during a period of one year. Aust. Rd Res. 3(9), pp. 42-8.
—
(1971). Temperature conditions in bituminous surfacings at a site near Perth during a
period of one year. Aust. Rd Res. 4(7), pp. 9-15.
(1975). Temperature conditions in bituminous concrete pavements at a site near
Brisbane during a period of one year. Aust. Rd Res. 5(8), pp. 9-15.
(1977). Temperature conditions in bituminous concrete pavements at a site near Canberra during a period of eleven months. Aust. Rd Res. 7(4), pp. 17-20.
(1978a ). A method for calculating the temperature gradients in AC pavement structures based on climatic data. Aust. Rd Res. 8(4), pp. 16-34.
(1978b ). The cooling of asphalt layers during the compaction operation. Proc. of 9th
ARRB Conf. 9(4), pp. 247-59.
-- (1981). Pavement temperature regimes in Australia: their effect on the performance of
bituminous conditions and their relationship to average climate indicators. Australian
Road Research Board. Special Report, SR No. 23.
DUNSTAN, D.G. (1967). Temperature variation in a bituminous concrete surfacing at a site
near Melbourne. Aust. Rd Res. 3(3), pp. 3-11.
GEBHART, B. (1971). Heat Transfer. (2nd Ed.). (McGraw Hill: New York.) (Appendix Table
A-10d, p. 577).
SPENCER, J.W. (1965). Solar position and radiation tables for Darwin (Latitude 12.5°S).
CSIRO, Div. Build. Res., Tech. Paper No. 19.
STANDARDS ASSOCIATION OF AUSTRALIA (1980). Residual bitumen for pavements. AS
2008.
WHILLIER, A. (1964). A silicon solar cell radiation integrator. Solar Energy8(4), pp. 134-36.
AUSTRALIAN ROAD RESEARCH, Vol. 11, No. 4, December 1981
35
WITT —TEMPERATURES IN BITUMINOUS SURFACINGS AT DARWIN
Peter Witt graduated in Physics from
Monash University and, after a short
period with the Bureau of
Meteorology. joined the staff of the
Bituminous Materials research group
at ARRB in 1966. He has been working
on the rheological properties of
bituminous materials and is interested in the practical aspects of
measuring temperature.
H.P. WITT,
B.Sc.
ACKNOWLEDGEMENTS
36
The author wishes to thank members of staff of the Roads Division of the Northern Territory
Department of Transport and Works for their co-operation in the installation and supervision
of the test site. He also wishes to thank the Building Research Laboratory of the Colonial
Sugar Refinery Company for the determination of the thermal conductivity of AC specimens
cut from the site.
AUSTRALIAN ROAD RESEARCH, Vol 11, No. 4, December 1981