DEVELOPMENT OF APPLICATION DIFFERENTIATED ULTRA-THIN ASPHALT FRICTION COURSES FOR SOUTHERN AFRICAN APPLICATION F.J. Pretorius1, J.C. Wise2 and M. Henderson3 1
Gibb Consultants PO Box 3965, Cape Town, 8000, South Africa. Tel: +27214699172. Fax: +27214245571. E-mail:
[email protected] 2 Zebra Bituminous Surfacings South Africa. E-mail
[email protected] 3 Pavement Engineering, Western Cape Provincial Administration E-mail:
[email protected]
ABSTRACT This paper describes the market research into and the development of a range of application differentiated, ultra-thin asphalt friction courses (UTFC’s), each tailor-designed for specialist southern African applications. It was found that these new type of surfacings offer high quality functional properties (skid resistance, road noise reduction, spray reduction, surface sealing) over the full design life at cost-effective rates when compared to conventional alternatives. The strict performance criteria developed for these specialised products as well as the binder and sealing membrane durability aspects are reported. The paper also includes a case study on the design and performance testing and product selection process applied on the N7 resurfacing contract between Cape Town and Malmesbury.
1. INTRODUCTION Ultra Thin Friction Courses (UTFC’s) are specialised asphalt wearing courses designed specifically for application as cost-effective thin surfacings or re-sealing layers with enhanced long-term friction/ skid resistance and sealing properties. Worldwide, these new thin surfacing types are typically developed as proprietary products designed for specialised application. The purpose of this study was to explore the market need for Ultra Thin Friction Courses in the South African road network context. It also reports on the technical development of engineering specifications which pavement engineers can use to ensure optimal utilisation and selection of available products for different product application areas. A mix selection and performance testing case study (N7, Cape Town) is also included in the paper.
2. THE NEED FOR UTFC’S IN SOUTHERN AFRICA The road user and the road authority have different needs and expectations concerning the performance of road surfacings, especially in developing and third world countries, where the road authority’s budget constraints are generally high. User requirements focus on safety and comfort while the road authority requires low cost and long-term durability. This translated into a complex series of engineering properties to be addressed by the road designer. With respect to the road surfacing, these engineering properties include adequate low speed and high speed skid resistance, low noise production from vehicle tyre interaction with the road surface, smooth ride, minimal vehicle generated spray, good conspicuity of road markings and low construction and maintenance costs.
Proceedings of the 8th Conference on Asphalt Pavements for Southern Africa (CAPSA'04) ISBN Number: 1-920-01718-6 Proceedings produced by: Document Transformation Technologies cc
12 – 16 September 2004 Sun City, South Africa
8th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICA
The concept of “separation of layer functions” argues that the load bearing capacity of the pavement can be entirely met by the structural layers, while the surfacing layer can be optimally designed to meet specific road engineering requirements. Application of this concept has given rise to a new class of thin and ultra thin surfacings. Due the developing country nature of the South Africa and broader Southern African economies these specialised products need to be designed and differentiated to cost-effectively fulfill specific layer requirements.
3. BACKGROUND AND FUNDAMENTALS OF ROAD SURFACINGS In order to provide the required road user safety and riding comfort, the following engineering properties of the surfacing layers are essential (summarised version extracted from ARR 325 Report; Oliver, 1998): •
Skid Resistance: Low speed skid resistance is determined mainly by the resistance to polishing under traffic of the coarse aggregate used in the surfacing. The Polished Stone Value of the stone normally indicates this. High-speed skid resistance is determined both by surface texture (a coarse texture, helped further by interconnected interlayer voids, allows water to escape from beneath vehicle tyres and so minimises the risk of hydroplaning) and the polish resistance of the coarse aggregate.
•
Spray Generation: Water on the road surface is atomised by the action of vehicle tyres and the droplets are thrown into the air. This results in considerable volumes of water being deposited on vehicle windscreens and as a general fog surrounding fast moving vehicles. Both these effects reduce driver visibility. Spray can be almost entirely eliminated if water is able to rapidly drain through the surfacing (interconnected voids) and is expelled at the sides. Some relief can also be obtained by having high surface voids i.e. good surface texture.
•
Road Noise: Porous surfaces, originally developed for spray reduction, have been found to appreciably reduce tyre noise. Where a conventional porous (open graded) asphalt cannot be used then a surfacing where the upper surface of the stones is flat and texture is obtained by voids “intruding” into the surface is to be preferred (called negative surface texture; as obtained in UTFC’s and partially in SMA’s). Surfacings where “positive” surface texture is obtained by particles protruding upwards from the surface (such as sprayed seals) produce noisier surfacings. The level of noise tends to increase as the maximum size of the aggregate particle increases.
•
Conspicuity of Road Markings and Glare/reflection: The two factors of importance for good conspicuity are texture and color. If surface texture is too low a film of water may cover the markings during rain. Specular reflection from the water film surface can then totally obscure the markings.
•
Windscreen Breakage: Windscreen breakage is almost exclusively related to surface seals. So seriously is the problem regarded overseas that many countries restrict the use of seals to lightly trafficked roads limiting the size of aggregate used in seals to a maximum of 10 mm where possible.
•
Smooth Ride: The ride smoothness that can be achieved is controlled to a large extent by the shape of the underlying layers.
•
Low Construction and Maintenance Costs: The factors important in the construction cost of a surfacing are the price of the materials and the thickness of the layer. Maintenance costs are determined by the properties and durability of the surfacing. Internationally there is a trend in recent times to use more expensive materials that give improved surfacing performance, and to control costs by reducing layer thickness. Paper 012
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•
Avoidance of Delays during Construction and Maintenance: Emulsion seals can, under certain conditions, take many hours to build sufficient strength to avoid stone loss under traffic. Asphalt can generally be traveled on within an hour.
•
Fuel Consumption and Wear and Tear on Vehicle: The main factor affecting vehicle wear and tear is pavement roughness (texture and undulations).
4. COMPARISON OF SURFACING LAYER CHARACTERISTICS OF UTFC’S VERSUS CONVENTIONAL SURFACINGS In comparing the surfacing layer characteristics of conventional Open Graded Asphalt (OGA), Continuous Graded Asphalt (CGA), Stone Mastic Asphalt (SMA) and surfacing seals with that of the new thin and ultra thin surfacings, the following summarised comparison follows: Table 1. Comparison of functional properties of UTFC’s versus conventional surfacings. Prefered Functional properties
Required Fundamental mix properties
Typical values of various surfacing types(13mm size) OGA
UTFC
SMA
Seals
CGA
High skid resistance
High Surface texture
2- 3 mm
1.5 – 3 mm
1-2 mm
1 –3 mm
0.2 –0.5mm
High Intercon. Internal voids
18 – 25%
12 – 20%
0%
0%
0%
High Aggr. Polish resist.
Aggregate dependant
Aggregate dependant
Aggregate dependant
Aggregate dependant
Aggregate dependant
High Intercon. Internal voids
18 – 25%
12 – 20%
0%
0%
0%
High surface texture/voids
2- 3 mm
1.5 – 3 mm
1-2 mm
1 –3 mm
0.2 –0.5mm
High intercon. Voids
18 – 25%
12 – 20%
0%
0%
0%
High layer thickness
30 –40 mm
18 –20 mm
Not porous
Not porous
Not porous
Negative texture
Yes
Yes
Partially
No
No
Low max. aggregate size
13 mm
9.5 – 13 mm
13 mm
9.5 – 13 mm
13 mm
High road marking conspicuity and low glare
High surface texture
2- 3 mm
1.5 – 3 mm
1-2 mm
1 –3 mm
0.2–0.5 mm
Darker/Contrasting surface
High
High
High
High – medium
High
Low construction & mainte-nance cost and construction delays & vehicle damage
Low layer thickness
30 –40 mm
18 – 20 mm
30-40 mm
8 –12 mm
30 –40 mm
High durability (low maint.)
Med-high
Med-high
High
Med –low
High
Long functional/design life
8-12 years
8 –12 years
10 –12 years
6 years
Low **
Early trafficking
Yes
Yes
Yes
Not always possible
Yes
Low windscreen breakages
Low
Low
None
Med –high
None
Low spray generation
Low tyre road noise
Note:*Based on values reported in studies by TRL (Nicholls, 1998 a &b and Nicholls et al, 1995) in UK in1995 to 1997; VicRoad (Yeo et al,1997) Australia in 1994 to 1997, various European research studies and reports, as well as studies done in RSA by Pretorius and Wise (results included in this report).** Functional life is low or non-existing relating to spray reduction and wet weather friction. Paper 012
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In addition to these fundamental comparative properties, various performance testing with friction testing equipment, noise measuring equipment and spray measuring equipment were also performed in the above studies. A summary of the results follows in Table 2 below: Table 2. Performance results of TRL & VICROAD surface trials. Initial values obtained (values in brackets after 6 years) CGA Property
Vic Road
TRL
Vic Road
TRL
Vic Road
TRL
Vic Road
-
0.6* (0.5)
0.55** (0.47)
0.8* (0.6)
0.5** (0.55)
0.7* (0.6)
0.65** (0.5)
0.62*(0 .6)
Pavement Friction Tester (Mean Skid No.100 km / h)
32
-
-
-
30
-
40
-
Texture Depth (Sand Patch)
0.3
0.3
-
2.0
2.2
1.0
2.6
1.2
0.3** (0.4)
0.3
1.7** (0.4)
0.7
0.9** (0.8)
0.5
1.2** (0.9)
0.6
87.3
-
-
-
84.2
-
85.4
-
Decibel @ 80 km / h for cars (value in brackets after 2 years)
-
71.7 (72.8)
-
74.7 (76.3)
70.6 (73.2)
70.6 (73.2)
-
68.5 (72.2)
Hydraulic Conductivity (inverse of porousness)
0.01
-
N/T
-
0.03
-
0.07
-
600
-
350
-
350
-
Texture Depth (Laser)
Noise generation*** Spray generation
UTFC Product
SMA
TRL SCRIM (MSSC@ 50km/h)
Skid resistance
Seal
Testing apparatus
Decibel @ 90 km / h (heavy vehicle)
Mean Spray reading (mV), (light back scattered fastl.)
330
*** Traffic noise level reduction of 3.7dB (versus seals) and 1.5dB (versus CGA) recorded in Australia tests (Wonson, 1997) ** Initial value after opening; values in brackets after 6 years * Initial values after opening; values in brackets after 3 years Note: 14mm Max Aggr. Mixes used in TRL trials; 10 mm mixes used in Vic Road trials
While it is clear that although not all requirements can be simultaneously met in a surfacing, an optimum solution can be found Thus, while emphasis should be placed on road user needs, restrictions imposed by the engineering properties of the surfacings (such as strength) must be taken into account. The simultaneous requirement for defined surface properties, appropriate engineering properties and ease and rapidity of construction has led to the development of the thin and ultra-thin surfacings that are the subject of this report.
5. TECHNICAL DEFINITIONS AND COMPARITIVE CHARACTERISTICS OF UTFC LAYERS Ultra-thin friction courses (UTFC’s) are technically defines as a special group of open to gap-graded asphalt mixes which generally have less than 22 to 28% of aggregates passing the 2.36 mm sieve and the remainder of the aggregate consisting of a single size stone (between 6.7 mm and 13.2 mm). Figure 1 below illustrates the grading comparison with conventional SMA, CGA, and OGA’s. The gap in the grading between 2.36 mm and 4.75 mm is very important in providing adequate Voids-in-Mineral-Aggregate (VMA), i.e. approximately 25%, and to ensure that high Voids-in-Mix (VIM), i.e.12 – 18%, exist after the binder (approx. 5% by mass or 11% by volume) has been added. It is generally paved in a layer thickness of 12 mm to 22 mm and a special modified tack “hot-spray” process is used to ensure optimum adhesion to the “base” and durable waterproofing of the interface.
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The UTFC product has a similar or somewhat better stone-on-stone grading matrix than SMA layers but differs by being significantly more open-graded (less mastic or more voids; VIM of 12 to 18% versus 4 to 6% of SMA). It is applied in a much thinner layer (10 to 20 mm versus 30 to 40 mm for SMA’s). Unlike SMA and CGA wearing courses, it also has a combination of excellent macro-texture and interconnected internal voids that enhances skid resistance and effectively eliminates water spraying. UTFC’s differ from conventional open-graded asphalt (OGA) mixes by being less open-graded, 12-19% VIM’s (22-30% Field Voids) versus 20-30% VIM’s (28-35% Field Voids) of OGA, with more mastic and filler (approximately 25/75 crusher dust & filler to stone ratio instead of a 10/90 ratio for OGA’s). It therefore has enhanced “structural value”, long-term durability and design life when compared to OGA’s . If properly designed it can provide friction and spray elimination properties similar tp those of conventional OGA mixes, though with cost (and thickness) reductions of approximately 40% and with much higher durability (binder ageing resistance, stone loss prevention and especially waterproofing of existing aged surfacing/base layers). UTFC’s offer enhanced functional properties (Oliver, 1998) and functional design lives of up to 200% compared to the properties of conventional, continuously graded wearing courses, i.e. friction/ skid resistance, spray elimination, hydroplane formation prevention) and compared to surfacing seals (with reference to, noise reduction, bleeding prevention and surface ravelling). Significant road noise reductions of 2 to 3 Decibels (or approximately 50 and 100%) less than conventional continuous graded wearing courses and seals respectively, can be obtained in thicker (20 mm) applications.
Figure 1. Grading curve comparison of UTFC versus conventional OGA, CGA and SMA.
6. SPECIAL DESIGN ASPECTS OF UTFC LAYERS A very effective method tack waterproofing, directly in front of the mix paving operation render these thin friction course layers both effective as sealing layers as well as specialist open graded functional layers. From the literature review (Oliver, 1998; Bellanger 1999 and Gordillo, 1996 as well as various other European and Australian workshops and research notes) it was identified that the following fundamental design aspects and criteria are essential: Paper 012
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6.1 Optimal Functional Design To be a true wearing course UTFC’s must be designed to have the following safety and riding comfort properties: 6.1.1 For skid resistance and spray elimination • Good micro-texture through rough and polish resistant aggregate. •
Open-graded mix of adequate thickness to obtain good (i) surface macro-texture and (ii) inter-connected field voids and (iii) drainage reservoir for effective water displacement and below-surface run-off. Minimum of 12% VIM (20% field voids), minimum layer thickness 12 mm, and layer thickness at > 1.5 times maximum aggregate is essential. Higher speed road surfaces require more field voids and thicker layers.
•
Also cubical aggregate and distinct gap in grading to ensure inter-connected voids/open grading.
6.1.2 To ensure adequate waterproofing of pavement/base layers • The Tack (sealing membrane) must provide long-term waterproofing between open-graded friction course and pavement structure. The thickness and type of tack and sealing membrane are critical for the long-term sealing properties of the tack seal. 6.1.3 To obtain road noise reduction • An open graded mix with an adequate sound-absorbing (acoustic) reservoir (directly related to thickness of layer and field voids) and also, •
A low sound generating interaction with tyres – the smaller the maximum-aggregate size the lower the noise generation.
6.1.4 To ensure long-term protection of functional properties and durability • Mix durability is required through aggregate stability, binder ageing, stripping resistance and designed resistance against traffic closing-up are essential. •
Binder ageing protection is essential in these open-graded mixes, both during production heating process (30-50% of ageing caused) and also by selecting durable binder coatings and mix/binder additives that enhance in-site durability.
•
Aggregate durability: Aggregate crushing resistance and polishing resistance to be designed to suit traffic conditions, also maximum aggregate size to suit traffic loading
6.1.5 For cost effectiveness • Layer thickness must be design to obtain desired properties; 12 – 22 mm thickness typically effective. A maximum layer thickness of 35 mm should not be exceeded on 13.2mm mixes (30mm on 9.5mm aggregate mixes).
6.2 Layer Durability Binder durability is essential for all open-graded mixes. With >7% VIM most mixes (binders) age at high rates, whether it’s a poorly compacted continuous graded mix (7-10% field voids), or a UTFC (20-30% field voids), or a conventional Open Graded mix (30-35% field voids). However, water-damage (binder stripping) is most likely to appear in intermediate semi-open mixes (8 – 15% field voids) where confined pumping (rather than free-flow) through the micro openings caused binder loss and accelerated mix ageing and therefore aggregate loss or bonding disintegration. Special binder anti-ageing and anti-stripping durability enhancers and polymer modifiers are required to ensure long term durability.
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6.3 Tack and Sealing Membrane Aspects The “tack and sealing membrane” have essentially two functions, i.e. to effectively waterproof pavement structure and to durably bond the friction course onto the pavement. The following design aspects followed from the product development study: •
0.5–0.6 l/m2 of net binder required for adequate sealing characteristics.
•
Polymer modification of tack binder required for adequate tacking (adhesion to pavement/substrata) of this very thin layer – various studies on efficiency of tack products (see RILEM 2003) confirm that both polymer modification and tack thickness of application significantly enhance the “grip” and the anti-ageing durability of the bonding to the pavement/substrata.
•
Polymer modification also significantly enhances the flexibility of the seal membrane and the resistance to crack reflection.
•
For old surfaces with brittle-ageing cracking or low degree structural cracking, polymer modifier content of the tack binder should be increased.
•
It is also essential that a proportion of the binder penetrate into the bottom part of the friction course to (i) enhance the durability of the layer (thicker binder coating with better quality “un-aged” binder; enhanced aggregate bonding/embededness) and (ii) “fuse” the friction course to the pavement/substrata.
•
Dry, aged and/or polished surfacings require a higher application due to absorption into the layer. Application on bleeding surfaces must be reduced.
•
Course textured surfaces (>1.5 – 2 mm macro texture) or surfaces with brittle-ageing cracking or low degree cracking require a higher application.
7. UTFC LAYER DESIGN ASPECTS 7.1 Mix Design Procedures The following mix design process and fundamentals were investigated from the literature study (Korsgaard et al, 2002; APRG, 1994 and various other European, USA and Australian asphalt workshop notes) and further developed with detailed laboratory and field testing experiments.
7.2 Aggregate Grading, Composition and Quality Specifications 7.2.1
Aggregate grading and composition
Grading curves are proprietary with aggregate compositions typically as follows: •
25% ±5% of Crusher dust and filler
•
75% ±5% of Single size stone
Single size stone type = 6.7 mm, 9.5 mm or 13.2 mm (selected to suit application) Aggregate quality properties: •
Standard mixes:
•
High performance mixes (HPM): FI 48, ACV 1.5
>0.5
LCS(initially)
> 12%
>16%
> 10%
LCS (100k MMLS,40 C)
>8
>12
>5
SCRIM Friction Coefficient(50km/h)
>0.55
>0.65
>0.5
Grip Tester Friction (GN @ 80km/h)
>0. 5
>0.6
>0.5
Pendulum Friction Value (initial and after 100K in MMLS 40°C)
>65
>70
>60
Mini Texture Meter Interconnected Voids Skid Resistance
>0.5
7.4 Layer Thickness (Incl. Max Aggregate Size) Design Guidelines Layer thickness (Wonson, 1999; Gordillo, 1996) needs to be optimised for specific mix application. The following criteria and conditions will determine the layer thickness: •
Constructability and layer stability necessitates layer thickness to be between 1.5 and 2.5 times the max. aggregate size;
•
For higher traffic volumes, a thicker application (with larger aggregate size, i.e. 9.5 mm or 13.2 mm) is needed to prevent closing-up of mix; Paper 012
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•
For higher design speeds, a larger maximum aggregate, i.e. 9.5mm or 13.2mm, (and therefor a thicker layer application by implication) is required to provide adequate macro-texture for effective spray/splashing elimination and to prevent hydroplane skidding under severe down-pours;
•
For effective noise reduction a thick noise absorbing layer (20 mm preferably) is required with intermediate maximum aggregate size (6.75 mm or 9.5 mm preferably) to reduce noise generation;
•
For residential application where smooth surfaces and/or thinner layers are required the smaller aggregate size mixes (6.7 mm standard mix) are applicable;
•
For cost-efficiency reasons the thinnest possible layer, which still satisfy the above functional and structural layer property requirements (and construction limitations), is recommended.
•
A maximum rutting of 17mm and 13 mm can safely be eliminated respectively with the 13.2mm and 6.7mm mixes
7.5 Tack Type and Application Specifications 7.5.1 Tack type A special modified binder tack coating is applied by the paver directly in front of the asphalt mix to ensure penetration, interface fusion, optimum layer adhesion and surface sealing. The tack type is also modified to provide a maximum level of interface adhesion and fine- crack reflection resistance. It typically consists of a 65% cationic emulsion modified with selected polymers (adhesion, flexing and sealing enhancers) and durability enhancers. 7.5.2 Typical tack applications A laboratory test series was undertaken to test the overseas literature recommendations on tack application thickness. A set of shear friction tests, done at 25 degrees were done at different tack application rates on different surface types to identify optimum tack application rates that ensure maximum interlayer adhesion. Permeability tests, inside a Marshall briquette mould which simulated the field two-layer conditions, were done on various permeable lower surface areas with various thickness of tack coatings to study the required level of tack sealing which ensure impermeability. Tack applications of 0.4 l/m2, 0.6 l/m2 and 0.8 l/m2 were evaluated. From this study it was confirmed that at least 0.55 l/m2 of net binder is required for the tack to ensure water permeability of less than 1 l/h on a 100 mm diameter sample area (equivalent Marvell apparatus type) on relatively open lower layers. On new, relatively impermeable surfaces (new asphalt layers), a minimum of 0.45 l/m2 of net binder is required to ensure adequate adhesion and impermeability. The following application criteria were derived and are based on the test results and the literature research: (i) Tack application for resealing of existing bituminous surfacings: •
Minimum Emulsion Application: 0.8 l/m2 (on rich and/or fine textured surfaces)
•
Maximum Emulsion Application: 1.2 l/m2 (on dry, opened texture surfaces)
(ii) Tack application for surfacing of new asphalt bases: •
Emulsion Application: 0.65-0.75 l/m2
Note: All emulsion tack applications are based on a 65% net bitumen content.
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8. PRODUCT DIFFERENTIATION AND APPLICATIONS UTFC’s functional characteristics depend on a combination of mix volumetric properties, compositional properties, binder and tack properties and aggregate properties. These fundamental UTFC mix and layer requirements, as discussed in section 4 and 7 above, were combined to formulate optimum mix types for specific road application areas. The identified application areas where specialised UTFC mixes can cost-effectively be used, together with their major functional advantages are: •
Metropolitan/urban freeways and main roads - low noise and spray generation, high friction
•
Rural highways and main roads – high friction, low spray generation, cost effective resealing
•
Residential distributor roads – cost-effective (thin) resurfacing alternative, low noise, high friction
•
Airport runways – high friction, low de-rubberisation and repainting maintenance cost
Note: Trial sections are also currently being tested to study the potential to use these mixes directly on crushed stone and natural gravel base layers; thick sealing membranes in excess of 1.2 l/m2 may be required for essential base waterproofing. Table 5 gives a summary and detailed layout of the differentiated UTFC’s mixes developed for specialised application in the South African road industry. A cost-benefit comparison with the conventional surfacing products is also given.
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Table 5. Specialised application areas and product differentiation.
Specialised Application areas
Rural highways and main roads
Urban freeways (incl. metro distributors.)
Residential roads
Re-sealing and long term friction enhancing of major rural freeways and national and provincial roads
For urban and semi-urban freeways: Low noise asphalt seal and friction course; for new surfaces, resurfacing or rehabilitated structure surfacing
Sealing and preventative overlaying of residential distributor and main access roads
Optimum mix type
Main benefits
Traffic: 1-3 million E80s;
Special High Performance Mixes for > 3 million E80’s
9.5 mm –SM, 16 mm thick for resealing;
9.5 mm HPM, 18 to 20 mm thick layer for Freeways
13.2 mm –SM, 18 to 20 mm thick for “rehab” application (> 10mm ruts) and/or > 3 million E80 traffic
Traffic: 1 – 10 million E80’s
Traffic: 2.0 mm)
•
High end surface texture after 10 years (> 1.5mm)
•
High interconnected field voids (> 16 initially, > 12 at end of life)
•
High durability (film thickness> 10.5 micron m; Cantabro < 20) for 10 years life
•
High aggregate polish resistance (PSV > 50)
Three mixes were evaluated at two binder contents each (see Table 6) Table 6. UTFC mix characteristics recorded on the N7 mix selection exercise.
Values at Various Binder Contents 9.5 mm HPMUTFC 2
9.5 mm HPM-UTFC 1
Property
Spec.
13.2 mm HPMUTFC 3
4.7% binder
5.1% binder
5.6% Bit.Rub
4.7% binder
5.1% binder
4.7% binder
5.1% binder
VMA
25 % target
28%
24%
22%
24%
22%
28%
26%
Marshall VIM (2 x 50blows)
15 – 19%
19%
14%
12%
18%
14%
19
15
Film thickness (micron m)
> 10.5
9.5
10.2
11.5
8.6
9.0
10.2
11
Cantabro (300 reps) durability
< 20
18
10
8
30
15
30
20
FI (SS-Stone)
50
49
49
49
49
49
49
49
% of SS-Stone Passing sieve size < nominal
< 20
24
22
22
13
13
15
15
15
% LCS-Interconnected voids, 100k
> 10
% LCS-Interconnected voids, 150k
2
0.65l Tack
1l/m Tack
5.1% Binder (Ave. for 0.65 & 1l/m2 Tack)
13.2 MM HPM 4.7% BC
0.65l Tack
Layer Thickness
Production mix (Field tested)
9.5 mm HPM, #1 (cubical stone)
0.65 Tack
1l/m2 Tack
5.1% Binder (Ave for 0.65 & 1l/m2 Tack)
3.6
2.5
2.0
Test error
4
11
6
7.5
19
N/T
N/T
19
N/T
25
22
Impermeable
Impermeable
Impermeable
Impermeable
Impermeable
Impermeable
Impermeable
Core Field voids, 0k (%)
> 22
24
23
22
28
21
20
23
Core Field voids, 150k (%)
> 12
15
17
15
23
15
14
13
SP-Texture depth, 0k (mm)
> 2 mm
2.3
2
2
2.6
2
2.5
3.3
SP-Texture depth, 100k (mm)
>1.5 mm
1.8
1.5
N/T
1.7
2.3
1.8
1.7
SP-Texture depth, 150k (mm)
> 1.5 mm
1.3
1.4
1.2
1.2
1.3
1.6
1.6
Visual Texture depth, 150k (mm)
> 1.5 mm
1.5
1.2
1.0
1.5
1.3
1.9
1.7
Pendulum friction factor*, 0k
> 70 (very good)
75
85
85
75
75
70
80
Pendulum friction factor, 100k
> 70 (very good)
75
75
75
80
78
70
85
Pendulum friction factor, 150k
> 60 (good)
78
75
75
80
80
75
85
Grip Tester (GN@ 50/80KM/H)
-
-
-
-
0.64/ 0.56
-
0.67/0.64
-
Tecture Depth (mm)
> 2 mm
-
-
-
1.6
-
2.5
-
% LCS Interconnected voids
> 15 %
-
-
-
18
-
12
-
NOTE:*BTB/CGA SURFACING VALUES = 45 – 55,SMA’S AND SEALS=55-75; 45 POOR(VALUES AS REPORTED BY SRT LABORATORIES ON RECENT TEST SECTIONS); # SHOVING HEIGHT DESCRIBE THE VERTICAL MOUNT FORMED OUTSIDE OF THE LOADING TRACK DUE TO HORISONTAL SHOVING. ALSO NOTE THAT VALUES NOT REPORTED FOR TWO DIFFERENT TACK APPLICATIONS REPRESENT AVERAGES FOR BOTH TACK SECTIONS.
In general, all tested mixes performed exceptionally well confirming the principles applied in the mix design methodology; only the 9.5mm bitumen-rubber (10% rubber by mass) modified mix showed some fattening and texture closure. Also the lighter tack application (0.65 l/m2 versus the higher 1 l/m2) seems to cause a higher degree of horizontal shear on the base interface. However the tack variation did not show any significant effect on deformation, texture depth or interconnected voids (results not attached - no significant difference noted between adjacent high and low tack areas on these aspects). The higher binder contents didn’t result in any higher deformation of the UTFC layer; to the contrary it appear to decrease the profiled rutting with approximately 20%. The surface texture at the end of the 150 000 MMLS repetitions (50 degrees C) did show a marginally higher texture depth closure at the higher binder contents of all three mixes. The texture depths and interconnected voids of the 13 mm mixes were consistently higher over the full loading spectrum and the visual end condition of the 13 mm mixes, especially on the higher binder contents, confirmed its superior characteristics. The 13mm mixes also showed up to 40% less rutting than the best of the 9.5 mm mixes. Friction tests with the pendulum test confirmed the excellent friction properties of the aggregate and mixes. The 13 mm mixes however showed somewhat lower Cantabro durability, but it does allow a better film thickness at optimum binder contents, especially if compared to the cubical 9.5 mm mix. Based on the main selection criteria - surface texture and interconnected voids to be optimal over the full functional life – the 13.2mm HPM mix at a 5.0 % binder content was identified as the preferred mix. All mixes satisfied the rutting resistance criteria. The 9.5mm HPM mix no.1, with the more flaky aggregate, at an optimal binder content of 4.9%, was the second best mix due its higher durability and film thickness if compared to the other 9.5 mm mix. However, this mix will only be recommended in areas of relative lower traffic loading (lower than 7 million E80’s, as applicable to the northern section of the contract).
10. CONCLUSIONS These new thin functional layers (UTFC’s) have specific application areas where they are costeffective and can provide durable surfacing layer solutions which satisfy most of the essential functional surfacing layer requirements. They offer better all-round and long-term functional properties when compared to conventional CGA’s, SMA’s and surfacing seals and are comparable to, and in some cases less expensive than the conventional alternatives. It is further concluded that a diversified range of specially designed products is required to ensure optimum application. A broad range of product design criteria and specifications are proposed for different application areas to ensure durability and optimal functional properties. The results of a detailed mix design and performance evaluation case study, done on the N7 in Cape Town, confirmed the reported design and performance criteria developed from the literature study, laboratory studies and trial projects. The following important aspects followed from the performance simulation testing: •
The coarser (13.2mm) mixes are generally more rut resistant and have better texture depth and interconnected voids over the full functional life. However they may be a bit less durable (aggregate loss) and less quiet and are not recommended for urban environments.
•
More cubical aggregate (and more single size stone) mixes do not necessarily produce higher performance mixes; although it may produce slightly better texture depths it may be less durable. Aggregate sources and mixes must be assessed in relation to all performance properties, including durability.
11. REFERENCES APRG, 1994. Ultra-Thin Asphalt Surfacings. APRG Technical Notes 3 and 8. ARRB Transport Research. Australia. Bellanger, J., Brosseaud, Y. and Gourdan, J L., 1999. Thinner and Thinner Asphalt Layers for Maintenance of French Roads. Transport Research Record 1334. France. Gordillo, J., Bardesi, A. and Ruiz, A., 1996. Hot bituminous mixes for thin layer applications. Spanish Experience. Proceedings of the Euroasphalt and Eurobitumen Congress. Korsgaard, HC and Bunner, HH., 2002. Very Thin Asphalt as Runway Wearing Course. Proceedings of International Symposium on Asphalt Pavements. Copenhagen. Nicholls, JC., 1998. Trials of high-friction surfaces for highways. TRL Report 125. Transport Research Laboratory. UK. Nicholls, JC., 1995. Road trials of thin wearing courses materials. Project Report PR79. Transport Research Laborotory. UK. Oliver R., 1998. Thin bituminous surfacings and desirable road user performance. ARRB Research Report ARR325. AARB Transport Research. Australia. Pretorius, F J., 2004. UTFC design experiments and mix selection tests for the N7 resurfacing between Marmesbury and Cape Town. Research Report for PAWC Government by Arcus Gibb, SRT and CSIR. RSA. Wonson, K., 1997. Novachip – the ultra thin asphalt surface. Asphalt Review, Vol 16, No.1. Australian Asphalt Pavement Association. Yeo, Rey., 1997. Bituminous and concrete surfacing trial: report on performance monitoring. Proceedings. of 10th AAPA International Flexible Pavements Conference, Vol 1, paper 25. Perth.
