OPTIMISING SPRAYED SEAL LIFE IN RESPONSE TO
GLOBAL CHALLENGES
John W H Oliver and Susannah Boer, ARRB Group, Australia
ABSTRACT
Increasing the life of sprayed seals may be an important response to future major rises in the
cost of bitumen and the need to reduce greenhouse gas emissions. A seal life model, based on
road trial data, was used to prepare seal life maps for Australia, South Africa and New Zealand.
These showed that Australia has the greatest range of calculated seal lives due to its extreme
climatic conditions. South African seal lives tend to be clustered towards the short life end of
the overall distribution, whereas New Zealand seal lives are towards the long life end. The
model was used to estimate possible increases in seal life due to measures such as improved
bitumen durability, the use of larger seal sizes and resealing based on need rather than age.
These measures have disadvantages such as increased initial cost, greater road noise or
increased risk of bleeding and, while unlikely to be adopted in the short term, may become
necessary in future.
1.
INTRODUCTION
1.1 Overview
For Australia, South Africa and New Zealand, sprayed seals are a key component of the
national road network. In Australia, spray sealed pavements account for more than 80% of allweather roads (Oliver 1999). Future performance and continuing use of this cost efficient
surfacing type will depend on an ability to adapt operations in order to respond to important
emerging issues, such as peak oil and greenhouse gases.
This paper looks at one important response: that of increasing the average life of sprayed seals.
This will reduce the whole of life cost, the requirement for (increasingly scarce) materials and
the generation of greenhouse gases associated with road construction operations.
In order to assess the likely increases in average seal life that might be achieved, a seal life
model is used. The model is based on field data obtained from road trials and from seals which
had reached their distress condition. The model is described in Section 2 and is used in Section
3 to construct seal life maps for Australia, South Africa and New Zealand.
A number of possible ways of increasing seal life are introduced in Section 4 and their likely
effect is determined for each of the three countries.
1.2 Peak oil
The concept of peak oil arose from a paper presented over 50 years ago by M King Hubbert, of
the Shell Development Company, to an American Petroleum Institute conference (Hubbert
1956). “Peak oil” refers to the point in time when crude oil production reaches a maximum and
then starts to decline. In the past, new discoveries have compensated for oil consumed but this
situation cannot continue indefinitely.
There are disparate views on the timing of peak oil. Some observers believe that the peak has
already passed while others consider it could be delayed for 25 years or longer. A recent
Australian paper (Hart 2000) summarises the case for a near-term peak in world oil production
and the reasons for the uncertainty in timing as:

Oil discovery is well short of consumption levels. While demand continues to increase, oil
discovery rates are on a declining trend which has continued for several decades despite
technological advances.

Stated OPEC oil reserves are not audited or verified. The latest assessment of two of Saudi
Arabia's largest fields lends considerable weight to the view that reserves in OPEC
countries are substantially over-estimated.

Unconventional oil resources are volumetrically large but limited in their sustainable
production rates. Objective reports cast doubt on high-end expectations.
In 2005, the US Department of Energy published what became known as the “Hirsch Report”
(Hirsch 2005) which suggested that, as peaking approaches, fuel prices and price volatility
would increase dramatically and, without timely mitigation, the economic, social, and political
costs would be unprecedented.
A large increase in the price of fuel will affect State Road Authority operations through changes
such as vehicle miles travelled, the modal split between public and private transport and likely
increases in vehicle mass limits to maximise freight vehicle efficiency, to name but a few. This
paper focuses on sprayed seals where the major effect will be on the price and availability of
bitumen.
1.3 Greenhouse gases
Global warming – the increase in the average temperature of the Earth's near-surface air and
oceans in recent decades and its projected continuation – has been a controversial topic.
However, following the Intergovernmental Panel on Climate Change (IPCC) report for
policymakers (Intergovernmental Panel on Climate Change 2007) and numerous scientific
papers on the subject, most observers now accept that global temperatures have been
increasing and that efforts must be made to prevent further increases.
Global warming is believed to be the result of increased concentrations of greenhouse gases in
the atmosphere. Greenhouse gases are those which absorb infrared radiation and thus
contribute to heating. The most important are water vapour, carbon dioxide and ozone.
Based on an extensive review of the scientific literature, the Intergovernmental Panel on Climate
Change (Intergovernmental Panel on Climate Change 2007) concluded that most of the
observed increase in globally averaged temperatures since the mid-20th century is very likely
due to the observed increase in anthropogenic (caused by mankind) greenhouse gas
concentrations. Carbon dioxide is regarded as the most important anthropogenic greenhouse
gas.
Road Authorities are likely to devote increasing attention to measures which will help reduce
carbon dioxide production. This will involve a review of their construction operations in order to
increase efficiency and to minimise the use of processes which result in carbon dioxide
production either directly, or through secondary processes such as the use of electricity
generated by coal fired power stations.
One approach is to use life cycle analysis (LCA) whereby the full environmental impact of a
process is taken into account. ISO standards in the 1440 series (ISO 1997, 1998, 2000a,
2000b) describe the collection, management and interpretation of LCA information. Although
some initial work on road construction has been reported (Jullien et al., 2004, 2006), an LCA
study can be very complex, and road authorities may choose to wait until more reliable base
data becomes available before considering such a detailed approach. However, this need not
delay the adoption of measures to reduce greenhouse gases associated with road construction
and maintenance operations. Industry has made a start in this regard with the introduction of
warm asphalt technology which reduces the energy and carbon dioxide emissions associated
with the production and placement of hot asphalt.
2.
SEAL LIFE
2.1 Seal failure mechanisms
The life of a properly designed and constructed sprayed seal is limited to the time when it no
longer provides a continuous waterproof surface to the underlying structure (becomes
permanently cracked) and allows surface water to enter the road base, or it starts to lose cover
aggregate by traffic attrition (fretting). Another less likely reason for retirement can be loss of
texture leading to an undesirable, smooth surface.
Cracking and fretting are associated with the bitumen films becoming so hard that, under
stressing by traffic, diurnal temperature change or seasonal moisture movement in the road
base, they start to crack. Conditions for crack propagation of the micro cracks first formed are
complex since hot conditions soon after their formation can lead to crack healing.
The main cause of the bitumen films in a seal becoming hard is chemical attack by atmospheric
oxygen. This is a slow chemical reaction and the rate at which it proceeds at a particular site
depends on:

the temperature of the bitumen (which can be estimated from meteorological records for the
site area),

the reactivity of the bitumen (indicated by the bitumen’s durability test result – see below),
and

binder film thickness.
2.2 The Durability Test
The hardening of bitumen which occurs slowly in the field can be accelerated in a laboratory test
by (a) increasing the temperature, (b) reducing the film thickness of the exposed bitumen, and
(c) increasing the oxygen pressure.
A combination of methods (a) and (b) is used in the ARRB Durability test. The test temperature
is set at 100C because this temperature is considered to be sufficiently high to produce a test
result in a reasonable period of time but not so high that the character of the laboratory and field
oxidation reactions will be markedly different. Maximum surface temperatures of about 70C
are encountered in pavement surfacings (Dickinson 1981b).
The ARRB Durability Test measures the intrinsic resistance of a bitumen to thermal oxidation
hardening. In the test, a 20 m film of bitumen is deposited onto the walls of glass bottles and
these are exposed in a special oven at 100C. Bottles are withdrawn periodically, the bitumen is
removed, and its viscosity measured at 45C. The durability of the bitumen is the time in days
for it to reach an apparent viscosity of 5.7 log Pa.s (i.e. the higher the number of days, the more
durable the bitumen). This viscosity level was adopted since it was associated with distress in
seals in Victoria at the time the test was developed.
The test has been used in Australia for over 30 years and some Australian State Road
Authorities (SRAs) have a minimum durability requirement of around 9 days for their sealing
grade bitumen. In Australia, bitumen durabilities between 5 and 18 days have been recorded.
Full details of the test are given in an Australian Standard (Standards Australia 1997).
2.3 Road Trial Data
In order to determine whether field hardening correlated with the Durability Test result, full scale
road trials were laid with bitumens covering a range of durabilities at various sites around
Australia. The first of these sprayed seal trials was placed in 1969 and a further series of trials
subsequently laid at sites covering a wide range of climatic conditions.
The trials were followed for periods of up to 15 years and a correlation between the Durability
Test result and bitumen hardening in the road trials was established (Dickinson 1981a). The
road trial data was subsequently analysed by Oliver (1987) and used to develop a bitumen
hardening model.
2.4 Bitumen Hardening Model
A simple mathematical model, describing the rate at which the bitumen binder in a sprayed seal
hardens in different areas of Australia, was developed by Oliver (1987) based on information
from 10 specially arranged full scale road trials and 13 non-trial sites. The non-trial sites were
normal seals and reseals which had not been placed as road trials but which had been sampled
and the binder viscosity tested at some stage of their life. In all, a total of 124 data points
covering 45 different bitumens were used to construct the bitumen hardening model.
Inputs into the model were seal age at the time the seal was sampled, temperature at the seal
site and the durability of the bitumen in the seal. The model was subsequently updated and
expanded to include binder film thickness by including seal size as an input variable (Oliver
2004). The current bitumen hardening model is described by equation 1.
log  = .0498 T Y
0.5
- .0216 D Y
0.5
2
- .000381 S Y
0.5
+ 3.65
(1)
Where :
 = the viscosity of bitumen recovered from the sprayed seal (Pa.s at 45ºC and 5 x 10-3 s-1),
T = the average temperature of the site (ºC), calculated from equation (2),
D = the ARRB Durability Test result (days),
S = nominal seal size (mm),
Y = the number of years since the seal was constructed,
T = (TMAX + TMIN)/2
(2)
where
TMAX = the yearly mean of the daily maximum air temperature (ºC), and
TMIN = the yearly mean of the daily minimum air temperature (ºC).
The variables TMAX and TMIN were obtained from published tables of climatic data. The
2
square of the Pearson correlation coefficient (R ) was 0.879. The standard error of estimate of
log  was 0.20 log Pa.s.
2.5 Distress Viscosity Model
The viscosity level at which distress occurs in a cool climate area, such as Tasmania, is likely to
be lower than the distress viscosity level in a warmer climate area, such as Darwin. A binder
which hardens during service to a viscosity of, say, 6.0 log Pa.s (measured at 45ºC) would
become brittle and crack due to the low temperatures experienced during a Tasmanian winter.
However, a binder of the same viscosity would remain at a relatively high temperature all year
round in Darwin and would not crack until it had hardened to a higher viscosity level. Thus the
viscosity level at which a seal shows distress will depend on climate as well as some other
factors.
In order to develop a distress viscosity model, State Road Authorities (SRAs) were requested to
provide samples of seals which were just starting to show distress due to aging. Twenty seven
samples were obtained and these were used to develop a model (Oliver 1990) based on the
daily minimum temperature (obtained from meteorological records) at the seal site. This model
was subsequently updated (Oliver 2006a) to include a risk factor
The risk factor is an indicator of the level of risk an asset manager is prepared to accept by
delaying resealing at a site past the indication of distress. Risk factors were developed for the
sites in the distress viscosity database and a correlation exercise carried out to develop a new
distress viscosity model.
log  = 0.158 TMIN – 0.107 RISK + 4.49
(3)
where

-3
-1
= viscosity of bitumen recovered from the distressed seal (Pa.s at 45ºC and 5 x 10 s )
TMIN = the yearly mean of the daily minimum air temperature (ºC )
RISK = risk factor with a scale of 1 to 10 as described below.
Risk = 1 - equivalent to resealing very late since there is considered to be a low risk of
subsequent pavement damage.
Risk = 10 - equivalent to resealing very early since there a high risk of pavement damage if
sealing is delayed or delays to traffic because of repairs cannot be tolerated.
Starting points for assigning a risk factor to a particular site are:
Risk = 3 appropriate for typical local government roads
Risk = 6 appropriate for typical SRA roads.
Factors which would tend to reduce the risk factor are:

low consequences if cracking or stone loss develops further

dry area

low traffic

good (moisture insensitive) subgrade and pavement materials.
Factors which would tend to increase the risk factor are:

high consequences if cracking or stone loss develops further

high traffic

strategic route

truck route

high rainfall or irrigated area

moisture sensitive subgrade and pavement materials.
2.6 Combined Model
A combination of the binder hardening and distress viscosity models (see equation 4) permits
prediction of seal life in different climatic areas, where distress is related to bitumen ageing. An
example of the use of the model is given in Figure 1 which shows predicted binder lives in
Mildura (Victoria, Southern Australia) and Longreach (Queensland, Northern Australia). The
Mildura plots are shown as continuous lines and the Longreach plots as dashed lines. As
indicated in the figure, binder hardens more rapidly in Longreach than in Mildura but this is
compensated for, to some extent, by the distress viscosity level in Longreach being higher than
in Mildura. This is because Longreach has hotter summers and warmer winters than Mildura.
 0.158 *TMIN  0.107 * RISK  0.84 
Y 

 0.0498 *T  0.0216 * D  0.000381* S 2 
2
(4)
where Y is years to reach critical viscosity (distress), and the other variables are as described
for equations 1, 2 and 3.
Viscosity at 45ºC (Log Pa.s)
7
distress viscosity
Longreach
distress viscosity Mildura
6
5
4
hardening
Longreach
hardening
Mildura
seal life
Longreach
seal life
Mildura
3
0
5
10
15
20
Years since construction
Figure 1 Comparison of seal life estimates for sites in Longreach and Mildura
It should be noted that the seal life determined using the model is only an estimate, and there
will be a distribution of possible seal lives around the single value calculated, due to effects not
taken into account by the model. These effects include stone colour, shading by trees,
variations in binder and aggregate application rates, differences in interpretation of possible
signs of distress, etc. This is considered in more detail when survivor curves are introduced in
section 4.2.
3.
COMPARISON OF SEAL LIVES IN THREE COUNTRIES
3.1 Climatic zone seal life map of Australia
The combined model was use to calculate seal lives in different climatic regions in Australia. In
order to isolate the effects of climatic region, the input variables of durability, seal size and risk
were held constant. The value of durability was set at 9.5 days since a study of the durability of
Australian bitumens (Oliver 2007) indicated that this was the mean value for Class 170
bitumens manufactured between 1978 and 2004. Seal size was set at 10 mm which is the most
common Australian seal size (Oliver 1999). A risk of 6 was assigned as being appropriate to a
State Road Authority maintained road (Oliver 2006a).
The range of temperature inputs covered by the model (in its various stage of development) are
shown in Table 1. It was considered that applying the model to a temperature range extended
by  10% would have little effect on accuracy but would present a more useful picture of seal
lives. This extended range is shown in Table 1.
Table 1: Temperature variable ranges covered by the model
and used to prepare climatic maps
Temperature
variable
Range covered
by model (°C)
Extended range
(°C)
TMAX
16.2 to 30.1
14.8 to 31.5
TMIN
8.2 to 20.2
7.0 to 21.4
TMAX and TMIN data, together with latitude and longitude information, were provided by the
Australian Bureau of Meteorology for 1,446 weather stations. Of these, 1,068 were within the
extended temperature range and were used for seal life calculation.
Table 2: Calculated lives for Australian seals assigned to bands
(durability = 9.5 d, seal size = 10 mm, risk factor = 6)
Band name
Range covered
(years)
Number of
points
≤8
≤ 8.5
199
9
8.6 - 9.5
259
10
9.6 - 10.5
153
11
10.6 - 11.5
137
12
11.6 - 12.5
100
13
12.6 - 13.5
65
14
13.6 - 14.5
53
15
14.6 - 15.5
≥16
≥ 15.6
36
66
Arcview software was used to plot seal lives onto a map of Australia. Calculated seal lives were
assigned to the bands shown in Table 2. The extended temperature ranges discussed above
only permitted the calculation of lives greater than 6.1 y. Regions of Australia where
temperatures were higher than the range covered by the model would be expected to have
shorter seal lives. Accordingly, these regions were assigned to the shortest seal life band (≤ 8).
At the other end of the temperature range, a similar situation existed for seal lives greater than
21.6 y. These regions were assigned to the longest life band ( ≥ 16 y).
While it might be desirable to have a greater number of bands, this presents difficulties for the
reader in distinguishing between different colours. Furthermore, it was considered important to
use the same colour scheme for the maps of South Africa and New Zealand presented later.
The range of seal lives in these countries is different to Australia, and the bands selected are
considered to provide the best compromise between detail and readability for all the maps
presented.
A map of estimated Australian seal lives is presented in Fig. 2. Large areas of the country are
shown with seal lives of 8 y or less but these are generally high temperature areas where the
population density is low and there are comparatively few roads. Because of the small number
of weather stations in the area, it was sometimes difficult to decide on the boundaries for the
region. In some areas in the south east portion of the continent, seal lives are greater than 16 y.
The average Australian seal life for the assigned conditions was 10.8 y. This was determined by
dividing the sum of the calculated seal lives for all sites within the extended temperature range
described in Table 1, by the total number of sites. There is generally a higher density of weather
stations in highly populated areas compared to lower population areas. Since there are more
roads in high population areas, the calculated average would seem to be a reasonable indicator
of average seal life throughout the country.
≤8
9
10
11
12
13
14
15
≥16
Figure 2 Estimated seal lives in different temperature regions in Australia
(durability 9.5 days, seal size 10 mm, risk factor 6)
3.2 New Zealand
Temperature data for 223 sites in New Zealand was obtained from the National Institute of
Water and Atmospheric Research. Of these, 130 sites were within the extended temperature
range shown in Table1 and were used to calculate seal lives on the same basis as for Australian
seals (durability = 9.5 d, seal size = 10 mm, risk factor = 6). While some factors may be
different in New Zealand (e.g. bitumen durability), it was considered useful to carry out a direct
comparison.
Table 3: Calculated lives for New Zealand seals assigned to bands
(durability = 9.5 d, seal size = 10 mm, risk factor = 6)
Band name
Range
covered (y)
Number of
points
≤8
≤ 8.5
0
9
8.6 - 9.5
0
10
9.6 - 10.5
0
11
10.6 - 11.5
6
12
11.6 - 12.5
21
13
12.6 - 13.5
16
14
13.6 - 14.5
22
15
14.6 - 15.5
14
≥16
≥ 15.6
51
The results are shown in Table 3. It was possible to calculate seal lives up to 21.7 y using the
model and it would have been desirable to have extra bands above 16 y. However, this would
have made the maps difficult to read and it was considered more important to show detail at the
low seal life end of the range since this is the critical area for improvement. As was the case
with Australian seals, regions of the country where seal lives were almost certainly shorter or
longer than the range given in Table 3 were assigned to the ≤ 8 y and ≥16 y bands, respectively.
≤8
9
10
11
12
13
14
15
≥16
(!
Nelson
(!
(!
(!
(!
Auckland
Wellington
(!
Christchurch
Dunedin
Invercargill
Figure 3 Estimated seal lives in different temperature regions in New Zealand
(durability 9.5 days, seal size 10 mm, risk factor 6)
3.3 South Africa
A request was made for temperature data for South African weather stations on a similar basis
to that for Australian and New Zealand stations. However, the cost of such data was 300 times
that of the Australian data. Since data covering 30 sites was publicly available, it was decided
to use this. Although the sensitivity of the seal life map would be reduced this was considered
acceptable in an initial study comparing different countries.
Of the 30 sites available, only one was outside the extended temperature range. Data from the
remaining 29 sites was used to calculate seal lives on the same basis as for Australian and New
Zealand sites, and the results are shown in Table 4.
Table 4: Calculated lives for South African seals assigned to bands
(durability = 9.5 d, seal size = 10 mm, risk factor = 6)
Band name
Range
covered (y)
Number of
points
≤8
≤ 8.5
12
9
8.6 - 9.5
7
10
9.6 - 10.5
3
11
10.6 - 11.5
1
12
11.6 - 12.5
2
13
12.6 - 13.5
2
14
13.6 - 14.5
1
15
14.6 - 15.5
1
≥16
≥ 15.6
0
≤8
9
10
11
12
13
14
15
≥16
Figure 4 Estimated seal lives in different temperature regions in South Africa
(durability 9.5 days, seal size 10 mm, risk factor 6)
A map was prepared and this is shown in Fig. 4. As before, regions with seal lives almost
certainly shorter or longer than the range given in Table 4 were assigned to the ≤ 8 y and ≥16 y
bands, respectively. While coarser than the other maps, Fig. 4 provides enough information for
comparison purposes.
3.4 Discussion
Fig. 5 is a bar chart of the percentage of seals in each country plotted against calculated seal
life. About 26% of sites in Australia were outside the extended model range, and an
examination indicated that about half would be expected to have long seal lives and about half
short seal lives. Of the 42% of New Zealand sites which were excluded, all would likely have
long seal lives. Only one South African site (short seal life) was excluded.
This information, together with the data in Figs 2 to 5 clearly shows that Australia has the widest
range of seal lives. South African seal lives are clustered towards the short life end whereas
New Zealand sites are clustered towards the long life end.
35
Australia
New Zealand
South Africa
30
Percentage
25
20
15
10
5
0
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Seal life (y)
Figure 5 Distribution of seal lives calculated using the model
(durability 9.5 days, seal size 10 mm, risk factor 6)
The model was derived from field data on the performance of single coat Australian seals, and
may not be applicable to situations where construction and maintenance practices are very
different. However, in all three countries seals are designed using a similar method, and the
level of skill and expertise in seal construction and maintenance is probably similar. Multi-coat
seals might be expected to provide longer lives than predicted by the model but responses to a
questionnaire on Australian and New Zealand seal indicated that two application and single
application seals gave the same average life (Oliver 1999). Surface enrichment (see Section
4.6) would be expected to extend the seal life predicted by the model.
Traffic level will have little effect on the bitumen hardening model (eqn 1) but can influence the
viscosity at which distress will occur (eqn 3). Traffic can be accounted for by adjustment of the
risk factor as described in Section 2.5. This is a subjective process requiring consideration of all
the variables that may influence the risk which an asset manager assigns to a particular road
section.
The bitumen hardening model is likely to apply equally well in all three countries. The distress
viscosity model may have to be tailored to New Zealand and South African conditions since the
trigger for resealing (due to binder ageing) may be different. This can be taken into account by
altering the risk factor, and the use of a different value for the risk factor could be a way of
calibrating the model for different countries.
In general, it appears that seal lives are likely to be shorter in Australia and South Africa than
they are in New Zealand. However, this is only the case where resealing is based on binder
ageing. In the 2002/03 season in New Zealand the most common reasons for resealing State
Highways in two NZ networks (totalling around 1,500 km) were (Robertson 2002) flushing (30%
of reseals) and SCRIM value (15% of reseals). Thus close to 50% of reseals were placed to
overcome skid resistance problems and not because the binder in the seals had hardened
causing cracking or stone loss.
Where a reseal is carried out over a binder which has not substantially age hardened then the
soft binder underneath can contribute to aggregate embedment in the new seal and thus loss of
texture. This, in turn, can lead to skid resistance problems and a need for a further reseal.
Repeated resealing in this manner can lead to surfacing instability. This subject is discussed in
more detail in a paper by Oliver (2003).
New Zealand and South African bitumens are likely to be less durable than those in Australia
where a durability requirement is imposed by some State Road Authorities. The effect of
durability on seal life is considered in the next section.
4.
INCREASING SEAL LIFE
4.1 Introduction
Assuming seals are placed on a sound pavement, there are a number of ways by which seal life
might be increased. Among the most important are:

resealing only when necessary, and the avoidance of early failures

using a more durable bitumen

switching to a larger aggregate size (and thus greater bitumen film thickness)

sealing with a softer grade of bitumen

surface enrichment.
These are discussed in the following sections.
4.2 Utilising the full potential life of a seal
While average seal lives are often quoted there will, in fact, be a distribution of seal lives in a
particular area, reflecting the effect of a large number of variables on seal life. Dickinson (1965)
carried out a survey of the performance of single seals laid on the five main highways in Victoria
during the period 1938 – 1959, and the survivor curves for three sizes are shown in Fig. 6. This
information might be considered outdated but the principal author of the current report recently
carried out an unpublished study for a State Road Authority and obtained very similar results to
those of Dickinson.
There are probably two main areas where improvements can be made. The first is that of early
retiring seals. These are usually associated with design or construction problems and, in the
experience of the principal author, are often underreported. Staff training is probably the key to
improving the survival rates of short life seals.
The second, and more important, way of increasing average seal life is to ensure that seals are
not retired prematurely. This involves visual inspection of all candidate reseals and the
application of local knowledge to ensure the correct balance between resealing too early and
losing potential life, and resealing too late and possibly compromising the integrity of the
structural layers.
The seal model can be used to estimate the effect of altering the reseal trigger point by altering
the risk factor. A risk factor of 1 equates to sealing very late because there is a low risk of
pavement damage, while a risk factor of 10 equates to sealing early because the consequences
of distress or failure are severe (e.g. for a sealed motoway/freeway). Risk factors of 3 and 6 are
typical of the values used for local government roads and SRA roads, respectively.
Percentage surviving x years
100
80
average life
15.9 mm
12.7 mm
6.4 mm
60
average life
40
average life
20
0
0
2
4
6
8
10
12
Age x (years)
14
16
18
20
Fig. 6 Survivor curves for three seal sizes (Dickinson 1965)
The results are shown in Table 5. The durability and seal size values used in Section 3 were
retained
Table 5: Average seal lives in three countries for four risk factors
(durability = 9.5 d, seal size = 10 mm)
Risk factor
10
6
3
1
Average seal life (y)
Australia
6.5
10.8
14.7
17.7
South Africa
6.0
9.6
12.9
15.4
New Zealand
8.5
15.0
21.1
25.7
Quite commonly a decision to reseal is based on the age of a seal and not on seal condition.
As indicated by the results in Table 5 and the survivor curves in Fig. 6, this can result in the loss
of considerable proportion of a seal’s life.
Some seals with durable bitumens can survive for very long periods (up to 30 y). This can
occur where a strong, interlocking aggregate mosaic has been formed and there is little
pavement deflection under the prevailing traffic. Where the binder is around ⅔ of the way up
the stone, the stone can be held in place like a diamond in a ring clasp. In this case the binder
has been exposed for so long, and is so hard, that it acts not as an adhesive but as mortar
holding the stone in place mechanically. Such very long life seals were not included in the
database used to construct the seal life model, and are therefore not predicted by the model.
4.3 More durable bitumen
The intrinsic resistance of a bitumen to hardening, as indicated by its ARRB durability test result,
can have an important effect on seal life. The seal life model was used to estimate average
seal lives for three countries using three durability values: 9.5 days (the average Australian
value between 1978 and 2004), 7 days and 11 days. These latter two values are well within the
range of recent Australian durability values. The seal size and risk factor values used in Section
3 were retained. The results are shown in Table 6.
Table 6: Average seal lives in three countries for three levels of bitumen durability
(seal size = 10 mm, risk factor = 6)
Durability (days)
7.0
9.5
12.0
Average seal life (y)
Australia
9.0
10.8
13.2
South Africa
8.2
9.6
11.4
New Zealand
11.9
15.0
19.4
The results indicate that increasing the durability of bitumen by 2.5 days can increase estimated
seal life by between 17% and 22% in Australia and South Africa, and by an even greater
amount in New Zealand.
Manufacturing a durable bitumen requires specialist knowledge on the part of a refiner. Only a
small number of crude oils are suitable. Where air blowing is used to increase the viscosity of
vacuum tower residue, a lower blowing temperature produces a more durable bitumen
(Dickinson 1974, Oliver 2007). However, it is more economic to operate blowing towers at
higher blowing temperatures.
In future years it is possible that the issue may not be how to increase bitumen durability but
how to prevent it from being reduced. Refiners adjust plant conditions to favour the production
of high value products, such as transport fuels and other lighter fractions. The saturates content
of Australian bitumens appears to have reduced over the period 1956 to 2004 (Oliver 2006b).
While this is unlikely to have significantly affected bitumen performance, it does suggest that, as
refineries are upgraded so they can convert the heavy residue of sour (high sulphur content)
crudes to lighter fractions, there may be continuing changes in bitumen composition. The heavy
residue from sour crudes is a major source of bitumen production.
For many years Australia had a stable bitumen supply situation, with mainly Middle East crudes
being refined in Australia to produce sealing and asphalt grade bitumens. Recently, however,
overseas manufactured bitumens have been imported into Australia, and the proportion of
imported bitumen is expected to increase in future. As a result, the composition of sealing
grade bitumens may be different to those used in the past.
4.4 Use of larger sized seals
The long term hardening of bitumen in a seal, caused by reaction with oxygen from the air, is
controlled by the rate of diffusion of oxygen into the bitumen film (Dickinson 2000). Thus an
increase in bitumen film thickness reduces the rate of bitumen hardening. Since bitumen film
thickness is related to the size of aggregate used in a seal, an increase in seal size results in a
reduction in bitumen hardening rate. Fig. 6 shows that this can have a substantial effect on seal
life.
Seal size has been incorporated into the seal life model. The model was used to estimate the
effect of different seal sizes on the average life of seals in Australia, New Zealand and South
Africa. Bitumen durability was held at 9.5 days and the risk factor at 6. The results of the
calculation are shown in Table 7.
Substantial increases in seal lives can be obtained by using larger sized aggregates, such as
14 mm and 19 mm. However, such seals have the disadvantages of increased tyre noise and
an increased risk of windscreen breakage during early life though use of multiple seals (e.g.
14/5) can reduce this problem. The initial cost is also higher.
Table 7: Average seal lives in three countries for four seal sizes
(durability = 9.5 d, risk factor = 6)
Seal size (mm)
7
10
14
19
Average seal life (y)
4.5
Australia
10.1
10.8
12.3
15.9
South Africa
9.0
9.6
10.8
13.4
New Zealand
13.7
15
17.8
25
Soft grade of sealing bitumen
A soft grade bitumen (Class 80, Class 50 or 180/200 pen) has been used in the past to
successfully seal low volume roads. Currently, Class 170 bitumen is very widely used, and
Class 80 bitumen is not often available. However, a soft grade of bitumen such as Class 80 can
have good durability (Oliver 2007) and, having a low starting viscosity, must field harden more
before distress viscosity is reached.
Successful sealing trials of a soft grade of bitumen were carried out on low volume roads in the
Alice Springs area some years ago but these have not been reported. Practitioners are
reluctant to use softer grade bitumens since they may exacerbate existing flushing problems.
However, if used judiciously on selected types of road they may provide considerably extended
seal life.
4.6 Surface enrichment
Surface enrichment is a treatment involving the application of a very thin layer of bituminous
binder to a seal without the application of a cover aggregate. Commonly a diluted bitumen
emulsion is used although heavily cutback bitumen is another possibility. The treatment is
sometimes called a fog spray.
Enrichment can extend seal life by several years by softening oxidised bitumen, providing
additional support where there is a lack of binder round the stones, and sealing fine cracks.
Because of the necessary interruption to traffic, enrichment is most appropriate for lightly
trafficked roads but it can be used effectively on sealed shoulders in more highly trafficked
areas. The treatment will extend seal life beyond that predicted by the model.
4.7
Discussion
The above sections have outlined measures which can be taken to increase seal life. However,
virtually all measures involve an increased initial cost or have disadvantages, such as greater
tyre noise or increased risk of bleeding. They are thus unlikely to be widely adopted in the short
term, particularly if road funding is limited.
In the future there is likely to be pressure on road managers to minimise greenhouse gas
emissions in road construction and maintenance processes. Major increases in the price of
bitumen are almost certain to occur. In these circumstances, priorities will change and road
authorities will need to consider suitable responses. This paper has presented some possible
options and it is expected that these and other suggestions will be debated as the road industry
prepares for future change.
5.
CONCLUSIONS
1
Increasing the average life of sprayed seals may assist in meeting the challenges posed
by the onset of peak oil and a need to reduce greenhouse gas emissions.
2
A seal life model, based on road trial data, was used to prepare seal life maps for
Australia, South Africa and New Zealand.
3
The model showed that Australia has the greatest range of seal lives due to its more
extreme climatic conditions. South African seal lives tend to be clustered towards the
short life end whereas New Zealand seal lives are towards the long life end.
4
The seal life estimation model indicated that substantial increases in seal life would be
obtained through the use of more durable bitumens, larger stone sizes and resealing
only when necessary, rather than age-based resealing.
5
Using a softer grade of bitumen is also expected to increase seal life, as will surface
enrichment, but the extent of the increase could not be determined in these two cases.
6
New Zealand has less need for the proposed seal life extension measures due to its
slower bitumen hardening rate. However, this can lead to problems when resealing is
carried out to maintain skid resistance, and the underlying seal binder is still soft. In
such circumstances stone embedment and loss of texture can occur, necessitating
further reseals and possibly resulting in an unstable surfacing.
7
The measures described have disadvantages such as increased initial cost, greater
road noise or increased risk of bleeding and so are unlikely to be adopted immediately.
However, they may form an important part of road construction agencies’ responses to
impending global challenges.
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AUTHOR BIOGRAPHY
Dr. Oliver graduated from Strathclyde University in Scotland with a B. Sc. (Hons) and a Ph. D. in
Applied Chemistry. He joined ARRB Group Limited in 1969 and is now a Chief Research
Scientist. He is the author of over 150 papers and reports, and has wide experience in binder
characterisation, bitumen durability and anti-oxidants, scrap rubber and polymer modified
binders, asphalt and sprayed seal design procedures, and skid resistance. He has presented
papers and lectured extensively both in Australia and overseas, serving for a short period as a
visiting professor at the Universiti Teknologi Malaysia.
Dr. Oliver received the inaugural Eldon J Yoder award from the U.S. Transportation Research
Board (TRB) and was runner-up for the W. J. Emmons award from the U.S. Association of
Asphalt Paving Technologists. Recently he was awarded the John Shaw Medal by the
Australian Road Federation. He has served on expert advisory panels for the U.S. Strategic
Highway Research Program (SHRP) projects and was the Australian SHRP Co-ordinator. Dr.
Oliver is a member of a U.S. TRB bitumen committee and chairs the Australian Standards
committee on bitumen, as well as serving on many other Standards and Austroads committees.
Susannah Boer is in her third year at Monash University studying for B.E (civil) and B.A.
degrees. She is working for ARRB Group during her vacation.