South African
Seal Design &
Practice
Trevor Distin
Overview
• Introduction
– Use of seals
• Design
– Parameters used in
SA seal design
• Construction
– Pushing the
operational limits of
seals
Acknowledgement
• CAPSA07 papers on
consolidating best
practice in surfacing
seals
– Gerry Van Zyl
– Douglas Judd
• AAPA for sponsoring
my trip
Introduction…
Area (Sq km)
1,219,912
7,686,850
Population
44 million
20 million
Road Network
750,000km
810,000km
Surfaced Roads
150,000km
337,000km
Bitumen Consumed
310,000T
800,000T
• 80% of roads sealed with a surfacing seal
• Less than 2600km carry ADT > 10,000 in both directions
Introduction…
Road Pavements in South Africa
bituminous
surfacing
Seals > 80%
(3mm – 20mm)
kerb
side walk
Base (+90 % Granular)
Sub-base (granular or cemented)
pavement
structure
Selected (granular)
FILL OR IN SITU MATERIAL
Introduction…
Common seals
• Single seal
• Double seal
fog spray optional
stone
tack coat
existing substrate
fog spray optional
2nd layer - stone
penetration coat
1st layer - stone
tack coat
existing substrate
• 1 ½ seal
fog spray optional
2nd layer - stone
penetration coat
1st layer - stone
tack coat
existing substrate
• Cape seal
fine slurry (1 or 2 layers)
fog spray
stone
tack coat
existing substrate
• Sand seal
sand or grit
tack coat
existing substrate
Introduction…
Less common seals
stone
• Geofabric seal
penetration coat
thin layer of aggregate
tack coat
existing substrate
3rd layer - stone
penetration coat
• Split seal
2nd layer - stone
1st layer stone
tack coat
existing substrate
• Choke seal
2nd layer - stone
1st layer stone
tack coat
existing substrate
2nd layer - stone
• Inverted seal
penetration coat
1st layer stone
tack coat
existing substrate
• Graded seal (Otta)
sand
tack coat
graded aggregate
tack coat
existing substrate
Introduction…
Seal design process
•
•
•
•
•
Site investigation
Define uniform sections
Sample and test material
Select appropriate seal and binder type
Measurement and interpretation of design
input parameters
• Calculate binder application rates
• Determine aggregate spread rates
• Monitor conditions on site and early
performance and make adjustments
Design…
Principles for the determination of
the binder application rate
VOID LOSS DUE TO AGGREGATE WEAR
TOTAL VOIDS
TEXTURE FOR
SKID RESISTANCE
MAXIMUM
VOIDS TO BE
FILLED
MINIMUM
VOIDS TO BE
FILLED
VOID LOSS DUE TO
EMBEDMENT
Design and construction of surfacing seals TRH 3:2007
Design…
Design input parameters
• Basic Design Parameters
– Traffic volume in
ELVs/lane/day
– Corrected Ball
Penetration Value
– Preferred texture depth
• Adjustment Factors
– Existing texture
– Slow moving heavy
vehicles/ gradient
– Macro & Micro climates
– Aggregate Spread
• Conversion Factors
– Hot applied modified
binders
– Cold to hot application
Design…
Corrected ball penetration
Determine potential embedment
And void loss
Embedment…
Corrected ball penetration
TMH6 Method ST4
Temperature
isotherms
Pen T0 = Pen T1 – K (T1 – T0)
Where
ZIMBABWE
NAMIBIA
CHIPINGE
BEIRA
55.0
FRANCISTOWN
BOTSWANA
WINDHOEK
MOCAMBIQUE
MUSSINA
WALVIS BAY
50.0
NORTHERN PROV.
T0 = Design Surface temperature
T1 = Measured Surface temperature
K = Temperature-susceptibility based
on seal type
45.0
GABORONE
55.0
50.0
MPUMULANGA
45.0
JHB
KEETMANSHOOP
MAPUTO
NORTH WEST
40.0
40.0
FREE STATE
50.0
KWAZULU
NATAL
BLOEMFONTEIN
NORTHERN CAPE
LESOTHO
DURBAN
45.0
40.0
EASTERN CAPE
40.0
45.0 WESTERN CAPE
CAPE TOWN
45.0
45.0
45.0
50.0
EAST LONDON
PORT ELIZABETH
MOSSEL BAY
Embedment…
Result of ignoring embedment
Values above 3mm – Warning !
Embedment…
Recommended adjustments
• Subdivision of representative areas
• Measurements:
– inside and outside wheel track
– record penetration for 1 and 2 blows
• Observe and record
– Main cause of ball penetration value
• Embedment
• Crushing
• Displacement
– Existing surface type
– Degree of dry/brittleness and fattiness
– Measure surface temperature (> 25 oC)
Embedment…
Recommended adjustments
Existing surface type and
degree of fattiness
Recommended K –factor (mm/°C)
Dry/ Brittleness
(TMH9)
Fattiness/ Bleeding
(TMH9)
(Degree
>=3)
(Degree
<3)
(Degree
3 - 4)
(Degree 5)
0,0
0,02
0,04
0,08
0,03
0,05
0,06
0,08
Cape Seals
0.03
0,06
0,07
0,08
Asphalt (Sand Mastic)
0,05
0,07
0,08
0,08
Asphalt (Stone mastic)
0,02
0,04
0,05
0,08
Single and multiple seals
Slurry
seals
seals
and
sand
Embedment…
Macro texture depth
• Function of vehicle speed to
– displace water
– improve skid resistance
Speed < 60km/h
Speed >60 < 100km/h
Speed > 100km/h
= min 0.5mm
= min 0.7mm
= min 1.0mm
Texture…
Adjustment for existing surface
texture
Max
• How much extra ?
Min
30%
Texture…
Concerns raised
• Test method
– Different methods
(Hand or box)
– Variation up to 30%
– Difference in and inbetween wheel tracks
• Could we ignore
existing texture
depth for specific
conditions ?
Texture…
Impact of 30% variation
Additional binder required (l/m2)
Existing Texture Adjustment
0.45
0.40
<2000 elv
0.35
3000 elv
0.30
4000 elv
0.25
5000 elv
0.20
>10000 elv
0.15
only modified binders
or split applications
0.10
texture
treatment
recommended
0.05
0.00
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Existing texture depth
Impact not significant
- Very small with high traffic
- Low volume (large envelope)
Texture…
When to ignore texture depth
Texture…
Best solution
• Designer on site
• Understand what is
measured
• Evaluate how the
new seal will fit into
the existing texture
• Decide if additional
binder is required
Texture…
Traffic/gradient
Adjustment for Gradient
0
2
4
6
8
0
Binder reduction (%)
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
Gradient (%)
Bleeding generally occurs when heavy
vehicle speed below 40km
10
Vehicle speed vs gradient
Aggregate spread rate
Shoulder to shoulder
Open matrix
Impacts on binder application rate
Pushing the operational limits
• Major performance benefits in using seals on high
volume roads
• Seals can compete with UTFC on high volume roads
• High volume roads = heavy truck traffic
• 1 heavy = 40 Equivalent Light Vehicle (ELV)
• TRH 3 covers up to 20,000 elv
• Good performance report up to 60,000 elv
Design aspects
• Traffic
– Concentrated in wheel paths
• Geometry
– Heavier trafficked roads generally higher
geometric standards
60% of lane width subject to ELV of 20-30% of design traffic
Design aspects - binder
• Use modified binders TG1:2007
– Bitumen rubber (S-R1)
– SBS (S-E2)
– SBR (S-E1)
R&B > 55 °C
R&B > 60 °C
R&B > 50 °C
• Performance characteristics required
– Good initial adhesion
– High binder application rate
– High temperature resistance to flow
– Low temperature adhesion
• No cutting back
Design aspects - aggregates
• Most control over
– Shape, size and hardness
• Aim for single sized with
low flakines index
– eg <10% on 13.2 mm
• Do seal design in reverse
to determine ALD to fit
traffic and binder
• Will result in higher
aggregate costs
• Consider alternative such
as steel slag
Construction aspects
• Accommodation of traffic
– Ability to deal with traffic influences seal
selection
– Can the lane be closed for 24 hours?
• Opening to traffic
– Open seal to traffic for 2 hours before
temperature drops <25°C
– Only open seal to traffic when temperature
15°C below softer point
Joints between sprays
• Position of
longitudinal joints
• Binder overlap
– String lines
– 100% coverage
Contractors equipment
• Adequate capacity and
good working order
• Sufficient equipment to
cover binder in 5 min
– Trucks
– Rollers
– Self propelled brooms
Project specifications
•
•
•
•
•
•
•
Restrictions with respect to climatic conditions
Aggregate requirements
Accommodation of Traffic
Opening to Traffic
Isolated application of additional binder
Joint positions
Equipment
– Rollers
– Rotary Brooms
Conclusions
• Selection based on performance and not
economics
• Best quality materials required
• Attention to detail during construction
• Design cannot be done from the office
• Adjustments on site if/ when required
• Recommendations to be considered for
minor adjustments to TRH3 and TMH6
Conclusions
Need to link between the countries to transfer
updating in concepts and practice
– Texture depth / speed
– Texture depth – spreader box vs sand
patch
– Aggregate spread rate – visual standard ?
– Ball penetration – corrections &
interpretation
– Heavy vehicle “high impact areas” – slow
travel zones
Thank you
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