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

pavement structure

side walk

FILL OR IN SITU MATERIAL

kerb

bituminous

surfacing

Base (+90 % Granular)

Sub-base (granular or cemented)

Selected (granular)

Seals > 80%(3mm – 20mm)

Introduction…

fog spray optional

stone

tack coat

existing substrate

fog spray optional

stone

tack coat

existing substrate

fog spray optional

tack coat

existing substrate

1st layer - stone

2nd layer - stone

penetration coat

fog spray optional

tack coat

existing substrate

1st layer - stone

2nd layer - stone

penetration coat

fog spray optional

tack coat

existing substrate

1st layer - stone

2nd layer - stone

penetration coat

fog spray optional

tack coat

existing substrate

1st layer - stone

2nd layer - stone

penetration coat

fog spray optional

tack coat

existing substrate

1st layer - stone

2nd layer - stone

penetration coat

tack coat

existing substrate

1st layer - stone

2nd layer - stone

penetration coat

tack coat

existing substrate

stone

fine slurry (1 or 2 layers)

fog spray

tack coat

existing substrate

stone

fine slurry (1 or 2 layers)

fog spray

tack coat

existing substrate

sand or grit

tack coat

existing substrate

sand or grit

Common seals

• Single seal

• Double seal

• 1 ½ seal

• Cape seal

• Sand seal

Introduction…

Less common seals

• Geofabric seal

• Split seal

• Choke seal

• Inverted seal

• Graded seal (Otta)

stone

tack coat

existing substrate

thin layer of aggregate

penetration coat

stone

tack coat

existing substrate

thin layer of aggregate

penetration coat

tack coat

existing substrate

1st layer stone

penetration coat

2nd layer - stone

3rd layer - stone

tack coat

existing substrate

1st layer stone

penetration coat

2nd layer - stone

3rd layer - stone

tack coat

existing substrate

1st layer stone

2nd layer - stone

tack coat

existing substrate

1st layer stone

2nd layer - stone

tack coat

existing substrate

1st layer stone

2nd layer - stone

penetration coat

tack coat

existing substrate

1st layer stone

2nd layer - stone

penetration coat

tack coat

existing substrate

graded aggregate

sand

tack coat

tack coat

existing substrate

graded aggregate

sand

tack coat

Introduction…

Seal design process

• Site investigation • Define uniform sections• Sample and test material• Select appropriate seal and binder type• Measurement and interpretation of Measurement and interpretation of

design input parametersdesign input parameters• Calculate binder application rates• Determine aggregate spread rates• Monitor conditions on site and early

performance and make adjustmentsDesign…

Principles for the determination of the binder application rate

VOID LOSS DUE TO AGGREGATE WEAR

TEXTURE FORSKID RESISTANCE

MAXIMUM VOIDS TO BEFILLED

VOID LOSS DUE TOEMBEDMENT

TOTAL VOIDS

MINIMUMVOIDS TO BE FILLED 3

0 %

10

0 %

AL

D

Design…

Design and construction of surfacing seals TRH 3:2007

Design input parameters• Basic Design Parameters

– Traffic volume in Traffic volume in ELVs/lane/dayELVs/lane/day

– Corrected Ball Corrected Ball Penetration ValuePenetration Value

– Preferred texture depthPreferred texture depth• Adjustment Factors

– Existing textureExisting texture– Slow moving heavy Slow moving heavy

vehicles/ gradientvehicles/ gradient– Macro & Micro climates– Aggregate SpreadAggregate 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

Pen T0 = Pen T1 – K (T1 – T0)

Where

T0 = Design Surface temperature

T1 = Measured Surface temperature

K = Temperature-susceptibility based on seal type

Temperature isotherms

Embedment…

NORTHERN PROV.

MPUMULANGAJHB

NAMIBIA ZIMBABWE

BLOEMFONTEIN

CAPE TOWN

MUSSINA

55.0

50.0

40.0

40.0

45.0

CHIPINGE

FRANCISTOWN

KEETMANSHOOP

LESOTHO

EASTERN CAPE

WESTERN CAPE

NORTHERN CAPE

FREE STATE

NORTH WEST

KWAZULUNATAL

NAMIBIA

BOTSWANA

MOCAMBIQUE

DURBAN

GABORONE

WINDHOEK

PORT ELIZABETHMOSSEL BAY

55.0

MAPUTO

WALVIS BAY

BEIRA

EAST LONDON

50.0

50.0

50.0

45.0

45.0

45.0

45.0

45.0

40.0

40.0

45.0

NORTHERN PROV.

MPUMULANGAJHB

NAMIBIA ZIMBABWE

BLOEMFONTEIN

CAPE TOWN

MUSSINA

55.0

50.0

40.0

40.0

45.0

CHIPINGE

FRANCISTOWN

KEETMANSHOOP

LESOTHO

EASTERN CAPE

WESTERN CAPE

NORTHERN CAPE

FREE STATE

NORTH WEST

KWAZULUNATAL

NAMIBIA

BOTSWANA

MOCAMBIQUE

DURBAN

GABORONE

WINDHOEK

PORT ELIZABETHMOSSEL BAY

55.0

MAPUTO

WALVIS BAY

BEIRA

EAST LONDON

50.0

50.0

50.0

45.0

45.0

45.0

45.0

45.0

40.0

40.0

45.0

Result of ignoring embedment

Embedment…Values above 3mm – Warning !

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 adjustmentsExisting 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)

Single and multiple seals

0,0 0,02 0,04 0,08

Slurry seals and sand seals

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

Embedment…

Macro texture depth

• Function of vehicle speed to– displace water– improve skid resistance

Speed < 60km/h = min 0.5mmSpeed >60 < 100km/h = min 0.7mmSpeed > 100km/h = min 1.0mm

Texture…

Adjustment for existing surface texture

• How much extra ?30%

Min

Max

Texture…

Concerns raised

• Test method – Different methods

(Hand or box)– Variation up to 30%– Difference in and in-

between wheel tracks

• Could we ignore existing texture depth for specific conditions ?

Texture…

Impact of 30% variation Existing Texture Adjustment

Existing texture depth

0.20

0.30

0.40

0.10

0.00

0.05

0.15

0.25

0.35

0.45

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

<2000 elv

3000 elv

4000 elv

>10000 elv

5000 elv

texturetreatment

recommended

only modified bindersor split applications

Existing Texture Adjustment

Existing texture depth

0.20

0.30

0.40

0.10

0.00

0.05

0.15

0.25

0.35

0.45

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

<2000 elv

3000 elv

4000 elv

>10000 elv

5000 elv

texturetreatment

recommended

only modified bindersor split applications

0.20

0.30

0.40

0.10

0.00

0.05

0.15

0.25

0.35

0.45

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

<2000 elv<2000 elv

3000 elv3000 elv

4000 elv4000 elv

>10000 elv>10000 elv

5000 elv5000 elv

texturetreatment

recommended

only modified bindersor split applications

Ad

dit

ion

al

bin

de

r re

qu

ire

d (

l/m

2)

Texture…

Impact not significant- Very small with high traffic

- Low volume (large envelope)

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

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

00 2 4 6 8 10

Gradient (%)

Bin

de

r re

du

cti

on

(%

)

Adjustment for Gradient

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

00 2 4 6 8 10

Gradient (%)

Bin

de

r re

du

cti

on

(%

)

Bleeding generally occurs when heavy vehicle speed below 40km

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) R&B > 55 °C– SBS (S-E2) R&B > 60 °C– SBR (S-E1) 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