Ecobond Road Design
For roads, Ecobond is typically used to construct the following products:
- Ecobond Bitumen stabilized base courses (BSMs) for South African Government projects (e.g. municipal, provincial, national roads).
- Ecobond Bound base courses and sub bases (innovative designs for private sector, mines and some SADC government clients).
- Ecobond Aggregate Modification for improving granular layers (improving shear strength through increased cohesion).
- Ecobond Wearing Courses (reducing gravel loss on gravel roads and maintenance on road shoulders).
- Ecobond SprayOn dust inhibition/erosion control (water resistant dust suppression for environmental erosion, gravel loss or dust prevention).
- These are also used in combination.
Ecobond BSM is a synthetic resin reinforced bitumen emulsion which is mixed into an aggregate such as natural gravel or crushed stone for the purpose of improving its engineering properties. Bitumen emulsion stabilization has been used very successfully in many projects worldwide, Ecobond just improves the elastic strength (to counter visco-elastic behaviour) and water resistance (to reduce moisture sensitivity) of bitumen stabilized aggregates potentially allowing a design engineer the option of achieving the desired properties at a lower binder content (price), or utilizing a more durable design at a similar price. Ecobond bitumen stabilization has shown its worth in scores of commercial applications and thousands of laboratory tests over the past 15 years in the SADC region, Europe and pilot projects in the Middle East and Australia.
Depending on what is required Ecobond can be applied as a non-continuously bound, partially bound or bound pavement layer, depending on the aggregate, binder content and ratio of resin to bitumen used.
Although other design methods can be used, Ecobond BSM pavement layers are currently designed according to the principles established in Asphalt Academy's Technical Guideline number 2 of 2009 (TG2, www.asphaltacademy.co.za/Documents/TG2May09.pdf), treating the resin as an active filler (the resin component usually is approx. 0.5% and does not exceed 1.5% of compacted aggregate weight). The resin component of the Ecobond formulation replaces the use of cement as “breaking medium” in a bitumen stabilized material. It forms a bond to the substrate and then breaks the bitumen emulsion at a controlled rate when cross linking takes place. The resin also bonds chemically with the bitumen resulting in a more elastic bitumen which is elastic rather than visco-elastic and more water resistant due to a more even dispersal of binder. Effectively this could be compared to stabilizing with emulsion but using a stiffer grade of base bitumen.
An Ecobond Bitumen Stabilized Material (BSM) improves the shear strength by increasing the cohesion of aggregates without significant reduction of the angle of internal friction.
It is important to understand the mix design method used for Ecobond (which is unchanged from TG2 2009.) Depending on project size the following guidelines are used.
The classification of BSMs includes three material classes:
- BSM1: This material has a high shear strength and is typically used as a base layer for design traffic applications of more than 6 million equivalent standard axles (MESA). For this class of material, the source material is typically a well graded crushed stone or reclaimed asphalt. This is mainly used in cold in situ recycling of existing roads (e.g. improve the long term stiffness of the base course without importing aggregate) but has applications in many other circumstances (e.g. mine haul roads, other pavements that can benefit from very high strength/stiffness upper layers or granular/crushed stone base courses that are moisture sensitive).
- BSM2: This material has a moderately high shear strength and would typically be used as a base layer for design traffic applications of less than 6 MESA. For this class of material the source material is typically a graded natural gravel or reclaimed asphalt. (e.g. Road recycling, new road construction when a crushed stone base is not appropriate or uneconomical or in-situ stabilization for upgrading gravel roads to surfaced roads).
- BSM3: This material is typically a soil-gravel and/or sand, stabilised with higher bitumen contents. As a base layer, the material is only suitable for design traffic applications of less than 1 MESA. Could also be considered as a sub base.
The mix design of BSMs involves three levels of testing, which depend on the design traffic level. ITS testing in dry and soaked (24 hour soak) states is used for Level 1 (up to 3m ESAs) and level 2 testing involves ITS testing of sample at equilibrium moisture content (+-50% of moulding moisture content) and after 24 hour soak (typically used for pavements up to 6m ESAs). For Level 3 (design traffic exceeding 6 MESA), triaxial testing is recommended. A simple triaxial test was developed by SABITA members (and others) to facilitate such testing in standard laboratories. A method for testing the moisture sensitivity of BSMs in the triaxial test has also been developed, which utilises the MIST equipment for saturating the triaxial specimens to determine the effect of water on the cohesion and angle of internal friction. The structural design of BSMs utilises the Pavement Number (PN) design method. The PN method is recommended for design traffic between 1 and 30 MESA and for Category A and B roads. For design traffic less than 1 MESA, a catalogue of typical designs is available. In some cases a Mechanistic Empirical approach is more accurate.
Bound Ecobond stabilized aggregates (higher binder content and resin to bitumen ratio) are used as cemented base courses and have attained up to 13.5 MPa UCS (unconfined compressive strength), but are typically utilized in the range of 1.5MPa-3.5MPa UCS and 250kpa ITS (Indirect Tensile Strength) and have historically been designed according to previously established cemented base course designs. Typically the higher the resin to bitumen ratio is, the higher the stiffness of the binder (bound system typically utilizes 50% resin and 50% gross SS60). The major benefit of this system is to avoid or reduce stabilization and reflective related cracking, while still improving water resistance and elastic strength. Aggregates with a grading modulus (GM) of between 0.9 and 2.5 and plasticity index (PI) of Non-plastic (NP) to 18 have been successfully stabilized, but typically is used in the range of G4-G7 aggregates. A durable, water resistant base course can increase the performance of a bituminous surfacing or seal when used in a traffic appropriate design. This can be illustrated through the use of a bitumen slurry seal on an Ecobond base course at the Duvha Power station, which carried approximately 20 million ESA (Equivalent Standard Axle Loads) over 10 years, the road is still in use today, although the seal has been upgraded, and has carried over 28 million ESAs without base course maintenance.
Ecobond modification of aggregates is utilized to reduce moisture sensitivity of crushed stone and natural gravel aggregates in granular base courses (e.g. on a cement stabilized sub base) or improve sub standard sub bases/layers (e.g. non cohesive or low PI sandy aggregates) by increasing the cohesion of the aggregate (low binder contents have little to no effect on angle of internal frication) so that base course density and stiffness can be attained. The effect typically being determined through triaxial or shear testing (CBR tests complicate the curing of stabilized or modified aggregates due to the moulds and are better suited to untreated aggregates.) These can also be re-modified and compacted in future if it has been trafficked beyond service life/accumulated too much stiffness reduction.
Ecobond Wearing Courses stabilize the top layer to about C3 or C2 UCS/ITS and A19/Sil.Carbide aggregate loss standards to provide a durable water resistant wearing course that reduces aggregate loss that occurs due to traffic and environmental conditions. Wearing can be predicted based on comparing abrasion and erosion tests to untreated samples, structural layer durability should be determined as per established cemented base course systems. This can also be used in labour intensive erosion control for example on road shoulders to reduce water ingress into the pavement structures and to reduce edge breaks/aggregate loss. Labour intensive construction makes relatively thin layers possible to construct (e.g. 30mm-75mm).
Ecobond Spray on erosion control is similar to bitumen emulsion dust inhibition sprays but has been modified to be less temperature sensitive and the resin component penetrates into aggregate to form a miniature base/support layer for the modified bitumen. On un-trafficked areas these have historically lasted up to 5 years.
Ecobond combinations - Ecobond Soil Modification approach to improve the cohesion of the aggregate (Mohr Coulomb Parameters) which is also maintained by sprays ons. Modified aggregate can be re-graded easier than bounded cemented layers and could possibly be used as part of a future pavement structure (depending on aggregate stiffness reduction and age).
In Hungary Ecobond has been certified for cold in-situ or in plant recycling of asphalt base courses as well as use as an aggregate stabilizer. Cold stabilized reclaimed asphalt mixtures for Base Courses typically have similar marshal characteristics to standard asphalt base courses and around 13-14MPa wet UCS strengths. Depending on individual project details this recycled base course is then surfaced with a microsurfacing, seal or asphalt.
There are many other technical aspects that can be discussed, please
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