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Light-Coloured Grey Asphalt Pavements (4)

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International Journal of Pavement Engineering
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Light-coloured grey asphalt pavements: from theory to
practice
ab
b
b
c
d
John J. Emery , Peijun Guo , Dieter F.E. Stolle , Jessica Hernandez & Lixin Zhang
a
Shiloh Canconstruct Limited, CaledonONCanada L7E 0P5
b
Department of Civil Engineering, McMaster University, HamiltonONCanadaL8S 4L7.
c
LVM Inc., TorontoONCanadaM9 W 5W8.
d
Terraprobe, BramptonONCanadaL6T 3Y3.
Published online: 28 Mar 2013.
To cite this article: John J. Emery, Peijun Guo, Dieter F.E. Stolle, Jessica Hernandez & Lixin Zhang (2014) Light-coloured
grey asphalt pavements: from theory to practice, International Journal of Pavement Engineering, 15:1, 23-35, DOI:
10.1080/10298436.2013.782402
To link to this article: http://dx.doi.org/10.1080/10298436.2013.782402
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International Journal of Pavement Engineering, 2014
Vol. 15, No. 1, 23–35, http://dx.doi.org/10.1080/10298436.2013.782402
Light-coloured grey asphalt pavements: from theory to practice
John J. Emerya,b, Peijun Guob*, Dieter F.E. Stolleb, Jessica Hernandezc and Lixin Zhangd
a
Shiloh Canconstruct Limited, Caledon, ON, Canada L7E 0P5; bDepartment of Civil Engineering, McMaster University, Hamilton, ON,
Canada L8S 4L7; cLVM Inc., Toronto, ON, Canada M9 W 5W8; dTerraprobe, Brampton, ON, Canada L6T 3Y3
(Received 15 May 2012; final version received 11 February 2013)
Downloaded by [John Emery] at 11:37 30 April 2014
This paper presents the technological development and application of hydrated lime in treating the surface of asphalt
concrete to develop light-coloured, grey asphalt pavements. When appropriately applied on the surface of fresh asphalt
concrete, hydrated lime makes the surface grey, significantly increases its albedo and effectively reduces the pavement’s
temperature caused by hot weather. Two application case studies are presented, focusing on how to ensure hydrated lime’s
long-term effectiveness on the surface of asphalt pavements and take into account the effect of the subsequent reduced
temperature on the resilient modulus of asphalt concrete in the design of long-life flexible pavements. The increased asphalt
concrete modulus, owing to lowered temperature, can reduce the design thickness of the asphalt concrete without sacrificing
pavement performance. This also has a positive influence on reduced pavement heat island effects. It is concluded that the
appropriate use of hydrated lime on asphalt pavement surfaces is an effective and economical method to produce lightcoloured, grey asphalt pavements.
Keywords: long-life flexible pavement; albedo; temperature effect; light-coloured asphalt pavement
1. Introduction
Hydrated lime has been used as a relatively inexpensive,
readily available, safe additive in hot mix asphalte (HMA)
for some time. When used as an additive in HMA mixes,
hydrated lime creates multiple benefits and contributes to
the improvement of a mix’s mechanical and rheological
properties (Little and Epps 2001) including a significant
reduction in stripping potential (moisture susceptibility),
the major current use; somewhat reduced design asphalt
binder content; improved toughness and resistance to low
temperature cracking; reduced age hardening (decreased
rate of oxidation) of the asphalt binder and increased
mixture stability, durability and dynamic modulus. As a
result, the use of hydrated lime has been shown to reduce
distresses and to extend the service life of asphalt
pavements.
The advantage of using hydrated lime on the black
asphalt concrete surface has not been investigated
systematically, even though it also has multiple advantages through improvements in pavement performance.
For some time, hydrated lime was applied on highperformance asphalt concrete surfaces to toughen the
pavement’s surface, accelerate the ‘curing’ process, and
hence prevent early tire scuffing (race track surfaces for
instance) (Emery 2007). This technology was used in the
construction of several car and motorcycle race tracks in
Canada, including Shannonville Motorsport Park, Toronto
Molson Indy Race Track and Calabogie Race Track
(Figure 1(a)). Unfortunately, the hydrated lime in these
projects did not stay very long on the surface of
*Corresponding author. Email: [email protected]
q 2014 Taylor & Francis
conventional asphalt concrete pavements because its
long-term effectiveness requires a ‘sticky’ surface such
as that a polymer-modified asphalt (PMA) binder
provides.
In addition to increasing the resistance of an asphalt
concrete surface course to early tire scuffing, hydrated
lime on the black asphalt concrete surface makes its colour
lighter to develop a grey asphalt pavement, significantly
increasing its albedo (fraction of solar radiation energy
reflected from the road surface). The increased albedo can
effectively reduce the surface temperature of the
pavement, which in turn somewhat enhances its frictional
resistance owing to the increased stiffness of both asphalt
concrete and tired rubber (Luo 2003; Baran 2011) and
significantly reduces any runoff-water temperature (Van
Buren et al. 2000) and/or heat island effects. As
anticipated from asphalt binder rheology, the resilient
modulus (Mr) of the asphalt concrete increases as a result
of the lowered pavement temperature, which in turn
improves the rutting resistance and fatigue endurance of
the pavement (ARA 2004; NCHRP 2004; El-Basyouny
and Witczak 2005; Kawakami and Kubo 2008). An early
example of using hydrated lime on an airside asphalt
concrete pavement surface to decrease its hot weather
temperature was reported in 2003 (Emery 2003, 2007).
The objective of this paper is to demonstrate the
temperature reduction potential of hydrated lime application (or equivalent light-coloured materials, such as
agricultural lime and fly-ash) to the surface of new asphalt
concrete and incorporating the increased asphalt concrete
24
J.J. Emery et al.
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Figure 1. Lime-treated pavements of the Calabogie race track
(photo courtesy of Calabogie Race Track).
Mr in flexible pavement designs. A brief review of the
reduction of hot weather pavement temperatures using
hydrated lime is presented first. Two application case
studies are provided thereafter, focusing on how to
effectively apply hydrated lime to the surface of asphalt
concrete pavements and then incorporate the enhanced
asphalt concrete Mr in the American Association of State
Highway and Transportation Officials (AASHTO)
mechanistic-empirical (M-E) flexible pavement design
method. The increased Mr allows a reduction in asphalt
concrete layer thickness without sacrificing pavement lifecycle performance.
2. Reduction of hot weather pavement temperatures
in grey asphalt pavement
Using hydrated lime to reduce black body solar energy
absorption is not new. For example, limewash has long
been used to paint objects (e.g., tree trunks, pipes) white to
help keep them cool. Dr Steven Chu, the US Secretary of
Figure 2.
Energy and a Nobel prize-winning scientist, has even
suggested making all roofs and pavements white or light
coloured to help reduce global warming by both
conserving energy and reflecting sunlight back into
space. According to him, it would be the equivalent of
taking all the cars in the world off the road for 11 years
(Connor 2009). Dr Chu’s statement is in line with the data
of Akbari (2011), who estimated that the world-wide CO2
offset of cool roofs and pavements is in the range of 44– 78
GT CO2, which would offset emissions from all cars for
18 –32 years (Akbari and Matthews 2010; Akbari 2011).
As illustrated in Figure 2, which shows a schematic
pavement cross-section and the heat-related processes that
affect the pavement structure, the pavement temperature
greatly depends on how solar energy heats the pavement
and how the pavement influences the air above it (Sun et al.
2006). In general, the temperature of the pavement is
affected by many factors, such as solar energy, solar
reflectance, material heat capacities, surface roughness,
heat transfer rates, thermal emittance, wind and permeability (Ting et al. 2011). The albedo of new black
asphalt pavements is as low as 0.05, which increases to
0.1 –0.15 for aged asphalt concrete (Bolz and Tuve 1973;
Levinson and Akbari 2001). Comparing this with 0.35–
0.40 for new Portland cement concrete (PCC) and 0.2– 0.3
for aged PCC (Bolz and Tuve 1973; Levinson and Akbari
2001), one may expect that if asphalt pavements are made
off-white grey in surface colour using hydrated lime (or
equivalent such as limestone dust or cement kiln dust), the
pavement temperature will significantly reduce. It should
be noted that the change in albedo for all exposed surfaces
is very important to climate change, particularly for cold
regions (light, fresh snow compared to dark, bare soil for
instance).
The effectiveness of applying hydrated lime on asphalt
concrete surfaces to reduce pavement temperatures has
Heat-related characteristics and processes in a pavement. Source: Adapted from Sun et al. (2006).
International Journal of Pavement Engineering
25
(a)
Surface temperature (˚C)
70
60
Black
20g/m2
35g/m2
50g/m2
75g/m2
100g/m2
125g/m2
150g/m2
50
40
30
20
0
2
4
6
8
Time (hours)
(b)
Figure 4. Structure of heat-shield pavements. Source: Adapted
from Kubo et al. (2006).
Surface temperature (˚C)
50
40
Black
50g/m2
100g/m2
30
150g/m2
20
0
2
4
6
Time (hours)
(c)
Pavement temperature (˚c)
40
50
60
70
0
20
Depth (mm)
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60
40
60
80
100 g/m2
Black
100
Figure 3. Change in surface temperature between black
unmodified asphalt concrete and those modified with different
hydrated lime concentrations in (a) laboratory tests, (b) in situ
tests and (c) temperature profiles of laboratory HMA specimens
with different surface hydrated lime concentrations at peak
surface temperature. Source: Adapted from Abu-Halimeh (2007).
Table 1.
been confirmed in laboratory and in situ tests reported by
Abu-Halimeh (2007) and Abu-Halimeh et al. (2008).
Different amounts of hydrated lime were placed uniformly
on the surface of asphalt concrete with the concentration
varying between 20 and 150 g/m2 to investigate dosage
influences on hot weather asphalt concrete temperatures.
Thermal couples were installed in the asphalt concrete at
different depths, and the surface temperature was
measured using an infrared camera. As shown in Figures
3(a) and (b), in both laboratory and in situ tests, the surface
temperatures of asphalt concrete with the surfaces treated
using different amounts of hydrated lime were much lower
than the untreated ones. Higher reductions in the surface
temperature took place as more hydrated lime was used to
produce whiter surfaces, with as high as a 148C
temperature reduction being achieved in approximately
4 h at higher hydrated lime concentrations of 100–
150 g/m2. The asphalt concrete temperature profile as
shown in Figure 3(c) demonstrates a decrease of pavement
temperature with depth when a surface is treated using
hydrated lime. It should be noted that the decrease of
pavement temperature in Figure 3(c) is generally
consistent with the Bell’s equation and measured data in
the literature (e.g., Lukanen et al. 2000).
The hydrated lime treated asphalt concrete surface is
almost as effective in decreasing the absorption of solar
radiation and reducing pavement temperature as proprietary heat-shield asphalt pavements based on current
Japanese research (Kubo et al. 2006; Kawakami and Kubo
2008). As illustrated in Figure 4, the heat-shield asphalt
pavement is coated by a layer of special paint containing
Comparison of the heat shield material and hydrated lime.
Temperature decrease
Albedo of treated HMA surface
Cost
Heat shield material
Hydrated lime (concentration 100– 150 g/m2)
Approximately 158C
0.4 – 0.5
$1.7– 3.2 (estimated)
Up to 148C
0.3 – 0.4
$0.15– 0.20
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26
Figure 5.
J.J. Emery et al.
Hydrated lime-treated asphalt pavements: the taxiway of Pearson International Airport. Source: Adapted from Emery (2007).
hollow ceramic particles and special pigment. It reduces
the absorption of solar radiation and energy from the
atmosphere, and hence reduces accumulated heat in
pavements. Compared to conventional asphalt concrete,
heat-shield pavements have surface temperatures approximately 158C cooler. However, the cost of heat-shield
asphalt pavements is very high owing to the expensive
coating materials. The experimental data in Figure 3 shows
that appropriate application of hydrated lime on an asphalt
pavement surface would have temperature reductions
similar to the relatively expensive heat-shield pavements.
Table 1 provides a brief comparison of the heat shield
material and the hydrated lime.
It should be noted that other approaches have been
used in engineering practice to increase the albedo of
asphalt pavements (US Environmental Protection Agency
2008; Tran et al. 2009). Typical methods include chip seals
with light-coloured aggregate, surface gritting using lightcoloured aggregate, sand/shot-blasting and abrading
pavement surface and using light-coloured aggregates in
the asphalt mix. It has been reported that using a lightly
coloured aggregate can raise the albedo of new asphalt
concrete to 0.15 – 0.20. The measured albedo of new
limestone chip seals is in the range of 0.2– 0.3, which then
declines with age. Shot blasting, which can remove the
asphalt coatings from a new asphalt pavement surface and
expose the natural colour of light-coloured aggregates
(such as limestone aggregates) used in the asphalt mixture
to improve the pavement surface reflectivity, can raise the
albedo of new asphalt concrete to 0.2 (Tran et al. 2009).
This value is the same as the typical albedo of a worndown asphalt concrete pavement with the aggregates being
revealed. It should be recalled that the albedo of unbound
limestone aggregate could be as high as 0.3– 0.45 (Bretz
et al. 1992; Tran et al. 2009). It should be emphasised that
the use of hydrated lime is intended to effectively reduce
the early stage albedo of new asphalt pavements, while in
the long term, the albedo of the hydrated lime-treated
asphalt pavements is expected to be similar to the worndown pavements that have exposed aggregates.
3.
Case studies
3.1 Toronto Pearson International Airport Taxiways
Taxiways A and H (originally PCC with cement-stabilised
base, CSB) at the Toronto Pearson International Airport
were upgraded in 2001/2002 by placing 125 mm of HMA
on top of the existing PCC pavements. After being
upgraded for less than 2 years, the composite pavement
(mainly Taxiway A) developed asphalt concrete distresses
of concern with potential loss of functional serviceability.
Various types of distress, including asphalt concrete
rutting, shearing and extensive progressive shoving in
heavy aircraft, slow, taxi wheel paths, were identified due
to an unfortunate combination of factors: a prolonged hot
(near record) weather period and slow moving heavy
aircraft on fresh HMA, including Antonov AN 124 air
freighters carrying imported steel and interface slippage
on a geogrid. The repair programme included removal of
the HMA overlay down to the concrete surface for the
27
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International Journal of Pavement Engineering
Figure 6. (a) Hourly spectra of single axle/dual tire loadings and (b) single axle load distribution measured on an expressway adjacent to
the Da’an to Jiliao Expressway.
middle width of the taxiway, transversely grinding the
exposed concrete surface as well as cleaning the surface
and application of polymer-modified surface course HMA.
The lower and the upper course HMAs incorporated PG
64-28 and PG 70-28 (polymer-modified) with air voids of
3 –5%, respectively. In order to reduce both pavement
temperatures during the summer and the black body
absorption of radiation from the sun and aircraft engines,
hydrated lime was applied on the finished HMA upper
surface course.
It is important to appropriately apply hydrated lime on
the finished HMA surface so that the hydrated lime sticks
on the surface for a long time to maintain a uniform offwhite surface condition. A special provision was
developed describing the procedures to place hydrated
lime on the new HMA surface (see Appendix). More
specifically, immediately after paving and completion of
compaction, when the asphalt concrete was still warm
following conventional finish rolling, a light application of
hydrated lime (100 g/m2) was applied to the asphalt
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28
J.J. Emery et al.
pavement surface using a small rotary spreader. The
applied hydrated lime was then rolled into the pavement
surface with multiple passes of a light, unballasted, rubbertire roller. The hydrated lime and rolling process was
repeated, as necessary, to achieve a uniform off-white,
grey surface colour condition. Care was taken to avoid
creating a dusty condition when spreading the dry
hydrated lime. Rather fortuitously, it was found in this
case study that the hydrated lime were essentially bound as
a long-term surface coating, which was attributed to the
surface course HMA incorporating PMA. This confirmed
that the use of PMA and the correct procedures for the
application of dry hydrated lime was the two key factors
for the success of this project. The PMA provided a very
sticky surface to hold the dry hydrated lime and to
effectively make the pavement surface grey.
The effectiveness of the procedures developed in this
airport pavement project and the durability of the lightcoloured asphalt pavement surface have been clearly
demonstrated through monitoring of the project with
Google Earth and during flight operations. As shown in
Figure 5, after almost 7 years, the surface colour of the
asphalt pavement area treated using hydrated lime is close
to that of adjacent PCC pavements and much lighter than
that of conventional asphalt pavements. For the marked
areas in Figure 5, the greyscale (0 for black and 1 for
white) of pavements is 0.034 (Patch of HMA pavement,
Area 1), 0.375 –0.445 (Areas 2 –5, hydrated lime-treated
HMA pavements), 0.526– 0.533 (Area 6, PCC pavements)
and 0.159 –0.242 (Areas 7 –8, aged HMA pavements
without surface lime treatments). Even though the HMA in
this project used a light-coloured aggregate (crushed
limestone), after 7 years when the natural colour of the
light-coloured aggregate was exposed, the area without
hydrated lime surface treatment was found to still be
darker than the treated area, and the greyscale values of
lime-treated asphalt concrete had almost the same level of
greyscale as the PCC pavements.
Testing by the airport maintenance department
reported no difference in the surface skid resistance
between the pavement areas with or without hydrated lime
treatment. In addition, the pavements have no significant
distresses after three successive hot summers of 2005–
2007.
3.2 Da’an to Jiliao Expressway
With the successful use of hydrated lime for the surface
treatment of asphalt concrete pavements to produce light
coloured asphalt as demonstrated in the Toronto Pearson
International Airport Project, this technology was then
implemented in the design and construction of the Chinese
Da’an to Jiliao Expressway long-life flexible pavements.
This Project is a 26.78-km section in Ruyang County,
Henan Province, China, of the new Erlianhaote to
Guangzhou Expressway. The four-lane tolled expressway
has a truck design speed of 100 km/h (cars 120 km/h) and
includes twin tunnels (12.5 m in width and approximately
2.1 km in length), many culverts/small bridges and two
large, low-level bridges.
To demonstrate the benefits in terms of enhancing the
quality and long-term performance of expressways, as well
as to promote implementation of Superpave HMA and
long-life flexible pavements in China, based on North
American technology transfer, this Project incorporated
Superpave HMA and was designed as a 30-year long-life
flexible pavement structure. In addition to the Superpave
and M-E design methodology, the Project design adopted
several new concepts, including high performance granular
base/subbase, light-coloured asphalt to deal with the high
summer temperatures and the typical severe overload of
Chinese heavy trucks, pavement structure quality monitoring that made use of deflection testing (Benkelman beam
and FWD) together with Dynamic Cone Penetration testing
of the high-performance granular base/subbase layers.
Overview details of the long-life flexible pavement
analysis and structural design, including traffic data and
analysis, climate and pavement temperature information
and materials characterisation within the framework of the
M-E design method, are summarised below.
3.2.1
Traffic data and information
The axle load spectra of different axle types required for
the design of long-life flexible pavements were established
from bus and truck traffic monitoring for eight days (24 h
per day) at a comparable adjacent expressway and toll
station with weigh-in-motion scales. Figure 6 presents the
hourly spectra of single axle/dual tire loadings and the
corresponding axle load distribution. Similar data for
single axle/single tire loadings were also developed. The
analysis of the axle load spectra provided the traffic
volume of 13,900/day with 42% trucks in which 40%
exceeded the Chinese standard axle load of 100 kN. The
design traffic data were determined by considering three
loading scenarios: 100th, 98th and 95th percentile of the
traffic loadings. When determining the truck load on the
design lane, a 50:50 directional split was used with the
percentage of heavy vehicles in the design lane assumed to
be 80%. A composite growth factor of 3.8% was used to
calculate the design equivalent single axle loads (ESALs).
3.2.2
Climate and pavement temperature information
The hourly mean air temperature data in the area of the
Project was collected from two nearby weather stations
(Zhengzhou and Ruyang) for five days every month
(12th – 16th) between 2004 and 2005. The recorded
International Journal of Pavement Engineering
Table 2.
Season
No. weeks
No. days
%
29
Number of days per year in various temperature ranges.
1
, 258C
2
25 –358C
3
35 – 408C
4
40 – 458C
5
.458C
Total
25
175
48
6
42
12
7
49
13
11
77
21
3
21
6
52
365
100
calculated using the climate information from Zhengzhou
(Year 2004 and 2005), and several methods (Dickinson
1971; Wahhab 1994; Bosscher et al. 1998) were used to
convert the air temperature data to pavement temperatures.
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monthly mean high-air temperatures of this region during
July to September were 30.1– 31.58C, with the monthly
maximum being 39.5 –39.98C. The pavement temperatures, which are higher than the air temperature, were then
Figure 7.
Variation of deformation with the number of loading cycles for different asphalt concrete samples in APA.
Without lime
47.5
45.0
38.0
32.0
30.0
41.0
38.5
32.5
28.5
26.5
With lime
Without lime
45.0
42.5
35.5
29.5
27.5
37.0
34.5
30.5
24.5
22.5
40.0
37.5
30.5
24.5
22.5
30.5
28.0
23.0
17.0
15.0
With lime
Without lime
With lime
32.5
30.0
23.0
17.0
15.0
Without lime
With lime
21.5
19.0
20.0
15.0
15.0
22.5
20.0
20.0
15.0
15.0
Without lime
,258C
An algorithm was developed to determine the pavement
temperature profiles with depth based on measured data at
different locations in China. In order to incorporate the
influence of temperature for the pavement performance
analyses and simultaneously simplify the design process,
the pavement surface temperatures were grouped into five
‘seasons’ of similar temperatures (, 25, 25– 35, 35– 40,
40 –45 and . 458C), with the number of weeks for each
season being calculated as presented in Table 2. The
pavement surface temperatures in each of the five seasons
were then used as the input to determine the pavement
temperature profiles with depth for each season. It should
be noted that the pavement temperatures could be divided
into more groups to get somewhat improved results.
To deal with high summer temperatures and the
severely overloaded heavy trucks reflected by the data
shown in Figure 6, the long-life flexible pavement designs
included an alternate with hydrated lime coating of the
upper (surface) course Superpave HMA (. 30 million
ESALs mix design) to reduce pavement temperatures in
the hot seasons and to enhance the stiffness of asphalt
concrete. Even though the reduction of the pavement
surface temperatures could be as high as 148C at the air
temperature of 34.3– 36.68C (Figure 3), it was assumed for
this Project that, for the scenario with hydrated lime, a 18C
(cool weather season) – 58C (hot weather season) reduction
in the pavement surface temperatures (with a similar
reduction in the temperature profile with depth) could be
readily achieved. This assumption was considered to be
conservative based on Figure 3. Table 3 presents the
estimated variation of pavement temperatures with depth
for the two scenarios with and without hydrated lime.
3.2.3
0 cm, 8C
2 cm, 8C
7 cm, 8C
15 cm, 8C
25 cm, 8C
.458C
35 – 408C
25 – 358C
40 – 458C
5
4
3
2
1
Depth (cm)
Pavement temperature variation with depth, with and without hydrated lime surface coating.
Table 3.
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42.5
40.0
35.0
30.0
29.0
J.J. Emery et al.
With lime
30
Mechanistic characteristics of HMA
AC-13/SP 12.5, AC-20/SP 19 and AC-25/SP 25 were
selected as upper (surface), middle and lower course
HMA, respectively, using Marshall and Superpave asphalt
mix design methods jointly, meeting the JTG D50/JTG
F40 (Marshall) and AASHTO R35-04 (Superpave)
requirements. The final HMA Marshall/Superpave mix
designs are summarised in Table 4. To deal with the high,
hot weather, pavement temperatures and the heavy truck
loadings in the Project, two asphalt binders, namely A70
(PGAC 64-22) and the polymer-modified SBS 1-D (PGAC
70-22), were selected to get high rutting resistance of the
surface and middle course hot-mix asphalt. The mechanistic characteristics of the three asphalt concretes were
determined at the mix design stage.
Tests were carried out to determine the rutting
resistance using the Asphalt Pavement Analyzer (APA)
and fatigue endurance and Mr using the Nottingham
Asphalt Tester (NAT), generally following relevant
AASHTO designations.
International Journal of Pavement Engineering
Table 4.
31
HMA mix designs.
(a) AC 13/SP 12.5 FC2 HMA mix design for surface course: asphalt binder SBS 1-D (PG 70-22)
Aggregate (basalt) gradation percent passing (mm)
Asphalt binder
content (%)
Marshall
Superpave
Marshall
5.5
5.2
4.9
16
13.2
9.5
4.75
2.36
1.18
0.6
0.3
0.15
0.08
100
94
84.8
49.4
29.9
20.6
14.5
10.2
7.3
5.5
100
97.7
79
49.4
37.6
27
17
10.4
8.3
6.9
(b) AC 20/SP 19.0 HMA mix designs for intermediate course: asphalt binder SBS 1-D (PG 70-22)
Aggregate (limestone) gradation percent passing (mm)
Asphalt binder
content (%)
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Marshall
Superpave
4.2
4.3
Note
26.5
19
16
13.2
9.5
4.75
2.36
1.18
100
100
98.5
99.7
89.7
90.7
76.6
77.2
55.8
55.3
37
36.5
24.6
23.4
18.1
17
(c) AC-25/SP 25.0 HMA mix designs for lower course: asphalt binder A70 (PG64-22)
Aggregate (limestone) gradation percent passing (mm)
Asphalt binder
content (%)
Marshall
Superpave
3.8
3.8
37.5
26.5
19.0
16.0
13.2
9.5
4.75
2.36
1.18
0.60
0.30
0.15
0.075
100
100
98.2
98.1
86.6
85.8
77.1
76.2
67.8
66.8
54.4
53.4
35.9
35.0
24.8
23.8
20.1
19.1
15.3
14.3
10.0
9.0
7.4
6.4
5.4
4.5
Rutting resistance from APA tests. The APA allows for an
accelerated evaluation of rutting potential after volumetric
design. APA tests on various HMA samples were performed
at different high Superpave temperatures (58, 64 and 708C).
For clarity, Figure 7 presents the APA test results at
temperatures of 588C and 708C only. All of the asphalt
concretes are seen to meet the recommended deformation
resistance requirements for the Project at 588C and to exhibit
good rutting resistance at up to 708C, with the frictional
surface course AC-13/SP 12.5 (basalt aggregates and
incorporating the polymer modified SBS 1-D) having
satisfactory rutting resistance. The difference in the APA test
results is attributed to the aggregate structures, percent air
voids and the type of asphalt binder. Use of the polymermodified SBS 1-D, which has high softening point
temperature of 758C, is particularly important for the rutting
resistance of the surface and middle course hot-mix asphalt.
The APA test results for submerged asphalt concrete
samples were also carried out, and the results all indicated
a good resistance to stripping, noting the AC-13/SP 12.5
incorporated 1% hydrated lime. The overall laboratory
APA testing of the asphalt concrete samples confirmed
that these HMAs met the requirements of the Project.
Mr and fatigue endurance of HMA from NAT. The Mr of
different HMA samples at different air voids (AV ¼ 4.0%
and 7.0%) and asphalt binder contents (AC ¼ 3.8%, 4.0%
and 5.5%) was determined using the NAT. Figure 8(a)
summarises the NAT test results for the asphalt concrete
samples (for clarity, only the results for samples of
AV ¼ 4.0% are presented). In general, the Mrs of all HMA
samples decrease with an increase in temperature,
following log Mr ¼ a1 2 a2T with the coefficient a2 in
the range of 0.038– 0.05. These Mrs are favourable, being
at the high end of typical Mr values for quality asphalt
concretes (see, e.g. Croney and Croney 1997). However,
the exponential decreases of Mr with increasing temperature result in a factor 2.4– 3.2 for a 108C increase in
temperature for a2 ¼ 0.038 –0.05.
The fatigue endurance of the AC-25/SP 25 lower HMA
course was also determined in the NAT for HMA of
different air voids and asphalt binder contents, as shown in
Figure 8(b). The number of load repetitions to failure
decreases with the increase of tensile strain using the
transfer function N f ¼ c1 ð1t Þ2f 2 with f2 ¼ 6.109 and 6.875
for AV ¼ 7.0% and 5.0%, respectively. It should be noted
that practical experience has indicated the typical field
fatigue endurance to be approximately 100 times greater
than the laboratory-determined fatigue endurance for
quality asphalt concretes (Croney and Croney 1997).
3.2.4 Characterisation of granular base/subbase and
subgrade/select subgrade materials
The characterisation of the subgrade and granular
base/subbase materials was performed by comprehensive
laboratory tests, including modified Proctor compaction,
California Bearing Ratio tests and Mr testing following
relevant AASHTO designations (AASHTO T180/T99,
Downloaded by [John Emery] at 11:37 30 April 2014
32
J.J. Emery et al.
Figure 8. Variation of (a) resilient modulus and (b) fatigue endurance of different asphalt concrete samples tested using the Nottingham
Asphalt Tester.
AASHTO T193 and AASHTO T307-99). It should be
noted that the use of the Mrs of all materials is an essential
component of flexible pavement design using a M-E
method such as AASHTO M-E. In order to quantify the
influence of moisture content on the Mrs of these materials,
the Mr tests were carried out at moisture contents in the
range of wopt ^ 2%. In addition, the results of material
testing were used to select the aggregate structure of
granular base/subbbase materials to achieve the best
performance (highest resilient modulus) possible by
appropriate gradation adjustment.
3.2.5
Pavement structural design
For this project, the pavement structural design included
various analyses to access the likelihood that the critical
pavement responses may exceed predefined thresholds.
PerRoad 3.2 (Timm 2004) was used to determine the
fatigue life of the asphalt concrete, and KENPAVE
(Huang 2003) was used to determine the stresses and
strains at critical locations in the pavement structure. Both
PerRoad and KENPAVE require Mrs and Poisson’s ratio
of all material layers. The Mr of asphalt concrete is a
function of pavement temperature, which varies with depth
and whether hydrated lime is used for surface treatment or
not. The effect of lime is simulated using the Mrs of the
HMA concrete at reduced pavement temperature levels.
The recommended 30-year pavement structure design
for the flexible asphalt pavements was 30 cm of high
quality Superpave HMA, 20 cm of crushed rock base,
40 cm of crushed rock subbase and 80 cm of select
subgrade material. Details about the materials character-
International Journal of Pavement Engineering
Table 5.
PerRoad pavement design results (AADT ¼ 13,900).
Traffic (%)
100
98
95
Percent below critical (%)
With lime
Without lime
90.1
91.1
92.0
91.7
92.2
93.0
satisfactory long-life flexible pavement design for the
Project traffic and climate conditions analysed. When the
surface is treated with hydrated lime, the percentage below
the 70-m1 critical strain level is at least 1% lower than that
for conventional asphalt concrete.
3.2.7
Downloaded by [John Emery] at 11:37 30 April 2014
isation and the selection of Mrs for pavement structural
design have been previously presented (Emery 2007).
3.2.6 Likelihood of excessive strain responses
PerRoad, a mechanistic-based pavement design and analysis
programme that utilises layered elastic analysis with a
statistical modelling procedure (Monte Carlo simulation) to
estimate stresses and strains within asphalt pavements
(Timm 2004), was used to assess the likelihood that critical
pavement responses may exceed predefined thresholds: the
horizontal tensile strain of 70 m1 at the bottom of the asphalt
concrete to control fatigue cracking and the vertical
compressive strain of 200 m1 at the top of the select
subgrade material to control structural rutting. For high truck
traffic volume pavements, the threshold strains should not be
exceeded more than 5–10% of the time.
The likelihood analysis for excessive strain responses
was performed for different scenarios when considering
30 cm of Superpave HMA for 100th, 98th and 95th
percentile traffic loadings with and without hydrated lime
surface coating. The results showed that the vertical strain at
the top of the select subgrade material was below the limit of
200 m1 in all loading cases, and the horizontal strain at the
bottom of the asphalt layer was below the 70-m1 level for
about 90.1 –93.2% of the loadings for all scenarios
examined. Table 5 presents the summary of horizontal
tensile strains at the bottom of HMA with AADT ¼ 13,900.
Therefore, the 30 cm of HMA was considered to be a
Figure 9.
33
Analysis for critical stresses and strains
The KENPAVE program was used to compute the
horizontal tensile strains at the bottom of the asphalt
concrete layer and the vertical compressive strains at the top
of the select subgrade material for the two critical hot
seasons (T ¼ 40–458C and T . 458C), with the axle load of
70, 90, 110 and 140 kN for the single axle/single tire and 160,
200, and 240 kN for the single axle/dual tire configurations.
Both scenarios with and without hydrated lime were again
evaluated, with the results summarised in Figure 9. For all
loading scenarios with different pavement temperatures, the
vertical strains at the top of the subgrade and select subgrade
materials are seen to be consistently smaller than the critical
value of 200 m1. The tensile strains at the bottom of HMA
may exceed the critical value of 70 m1 under heavy truck
loadings but not exceed the fatigue endurance.
3.2.8
Benefit of surface hydrated lime coating
To demonstrate the benefit of using hydrated lime on
asphalt concrete surface, parametric studies were carried
out to examine the performance of asphalt pavements
(without lime treatment) by adjusting the Mr of the
granular base layer. When the Mr of the granular base layer
is increased from 350 to 400 MPa, the tensile strain at the
bottom of HMA is reduced by 5 – 6%. Using a hydrated
lime coating on the Superpave HMA surface course to
reduce pavement temperature and increase the stiffness of
the asphalt concrete is found to also reduce the tensile
strains by approximately the same amount. When the
KENPAVE pavement analysis results: tensile strain at the bottom of HMA (pavement surface temperature of 40 – 458C.
34
J.J. Emery et al.
Downloaded by [John Emery] at 11:37 30 April 2014
Figure 10. Effect of hydrated lime and the resilient modulus of
granular base on tensile strains at the bottom of HMA (pavement
surface temperature ¼ 40 – 458C).
pavement surface temperatures are in the range of 40 –
458C, the combination of increasing the granular base Mr
by 50 MPa and using hydrated lime surface treatment can
reduce the tensile strains at the bottom of the HMA by up
to 10 – 12%, as seen in Figure 10.
The benefit of applying hydrated lime on the surface of
fresh asphalt concrete pavements to reduce the hot weather
pavement temperatures is demonstrated by the tensile
strain distributions at the bottom of the asphalt concrete for
different temperature reduction scenarios, which is found
to have the same effect on tensile strain reduction as an
increase in granular base M r. For the relatively
conservative assumption about the reduced pavement
temperatures adapted (by only 1 – 58C), the surface
hydrated lime coating can reduce the tensile strains at
the bottom of the HMA by 5– 6%, which is equivalent to
increasing the Mr of granular base materials by 50 MPa
(from 350 to 400 MPa). Additional simulations showed
that the effect of applying hydrated lime was equivalent to
a 12.5-mm increase in the HMA thickness in terms of the
tensile strains at the bottom of the HMA for this Project.
By taking into account the additional cost of PMA for the
HMA surface course and hydrated lime, the reduced
asphalt concrete thickness implies significant savings.
4. Concluding remarks
Hydrated lime, when appropriately applied on the surface of
fresh asphalt concrete, makes the asphalt pavement surface
grey, which in turn significantly increases its albedo and
effectively reduces the hot weather pavement temperatures.
The use of polymer-modified surface course HMA and the
correct procedures for the application of dry hydrated lime
are two essential factors to ensure the long-term effectiveness of the surface hydrated lime coating. The reduction in
hot weather pavement temperatures effectively enhances the
stiffness of asphalt concrete, resulting in significant
improvement in its rutting resistance and fatigue endurance.
When implemented in the M-E design of the asphalt
pavement structures, the surface hydrated lime coating is
equivalent to reducing the HMA thickness or increasing the
Mrs of granular base layer materials when maintaining the
same pavement performance. While not discussed here, as
an effective and economical method to make light-coloured,
grey asphalt pavements, this technology also has the
potential to reduce urban heat island effects and to enhance
environmental conditions.
It should be noted that instead of hydrated lime, other
equivalent materials, such as limestone dust, cement dust
and fly-ash, may be used to make grey (light-coloured)
asphalt using the same technology. However, hydrated lime
is favoured in terms of its availability, reasonable cost, ease
of application and multiple functions in enhancing the
properties of surface course asphalt concrete. A laboratory
study is being carried out to investigate the effectiveness of
using these alternative by-product materials and develop
improved application techniques to generate light-coloured
grey asphalt concrete surfaces. In situ tests will also be
performed with the albedo of asphalt concrete surfaces
treated using different concentrations of the various
materials measured using a pyranometer, a device for
determining the albedo of a surface. The durability of the
alternative by-product materials to maintain a light-coloured
surface will be specifically examined.
Acknowledgements
Partial funding provided by the Natural Sciences and
Engineering Research Council of Canada is gratefully acknowledged. Support from Shengzhen Kang and Xiaozhong Li, Henan
Expressway Development Limited, Zhengzhou, Henan, China, is
also appreciated.
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Appendix: Special Provision – application of hydrated
lime
This Special Provision describes the procedures to be followed to
apply hydrated lime to the surface of the hot-mix asphalt surface
course immediately after paving and completion of compaction.
All MSDS and safety requirements must be observed throughout.
The procedures described should be modified in the field, as
necessary, to meet the intended purpose of toughening the
surface and providing a uniform, light-coloured surface.
(1) The surface course asphalt paving work is to be carried
out in conformance with the overall Project
requirements.
(2) A light application (‘dusting’) of hydrated lime shall be
applied to the asphalt pavement surface subsequent to
the conventional finish rolling (compaction). The
applied hydrated lime shall then be consistently rolled
onto the asphalt pavement surface with multiple passes
of a light, unballasted, rubber-tired roller. The hydrated
lime application and rolling process shall be repeated, as
necessary, to achieve a uniform ‘off-white’ surface
colour condition.
Guidance note:
The hydrated lime can be applied dry using a common garden
fertiliser spreader(s) (a rotary spreader is recommended, not a
broadcast spreader to keep the hydrated lime as close to the
asphalt pavement surface as possible). The gate opening on the
spreader should be sufficient to apply a consistently uniform,
light application. Care must be taken to avoid creation of ‘dusty’
conditions.
Alternatively, the hydrated lime can be mixed with water and
applied as a light slurry; however, the slurry will then have to be
permitted to dry before the rolling-in operation.
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