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South African Seal Design & Practice

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

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South African Seal Design & Practice

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  1. South African Seal Design & Practice Trevor Distin

  2. Overview • Introduction • Use of seals • Design • Parameters used in SA seal design • Construction • Pushing the operational limits of seals

  3. Acknowledgement • CAPSA07 papers on consolidating best practice in surfacing seals • Gerry Van Zyl • Douglas Judd • AAPA for sponsoring my trip Introduction…

  4. 80% of roads sealed with a surfacing seal • Less than 2600km carry ADT > 10,000 in both directions Introduction…

  5. Road Pavements in South Africa bituminous surfacing Seals > 80% (3mm – 20mm) kerb side walk Base (+90 % Granular) pavement structure Sub-base (granular or cemented) Selected (granular) FILL OR IN SITU MATERIAL Introduction…

  6. Single seal Double seal 1 ½ seal Cape seal Sand seal Common seals Introduction…

  7. Geofabric seal Split seal Choke seal Inverted seal Graded seal (Otta) Less common seals Introduction…

  8. 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…

  9. 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 ALD 100 % MINIMUM VOIDS TO BE FILLED 30 % VOID LOSS DUE TO EMBEDMENT Design and construction of surfacing seals TRH 3:2007 Design…

  10. 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…

  11. Corrected ball penetration Determine potential embedment And void loss Embedment…

  12. 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 Corrected ball penetration Embedment…

  13. Result of ignoring embedment Values above 3mm – Warning ! Embedment…

  14. 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…

  15. Recommended adjustments Embedment…

  16. Macro texture depth • Function of vehicle speed to • displace water • improve skid resistance Speed < 60km/h = min 0.5mm Speed >60 < 100km/h = min 0.7mm Speed > 100km/h = min 1.0mm Texture…

  17. Adjustment for existing surface texture • How much extra ? Max Min 30% Texture…

  18. 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…

  19. Impact of 30% variation Additional binder required (l/m2) • Impact not significant • - Very small with high traffic • - Low volume (large envelope) Texture…

  20. When to ignore texture depth Texture…

  21. 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…

  22. Traffic/gradient Bleeding generally occurs when heavy vehicle speed below 40km

  23. Vehicle speed vs gradient

  24. Shoulder to shoulder Open matrix Aggregate spread rate Impacts on binder application rate

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. Joints between sprays • Position of longitudinal joints • Binder overlap • String lines • 100% coverage

  31. Contractors equipment • Adequate capacity and good working order • Sufficient equipment to cover binder in 5 min • Trucks • Rollers • Self propelled brooms

  32. 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

  33. 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

  34. 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

  35. Thank you

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