Emulsion Treated Bases are commonly made from in-situ or reclaimed gravels (often of insufficient quality for use as a base course) treated with a small percentage of bitumen emulsion22 to increase the strength of the base
course. Usually, less than 2.5% of emulsion is added to the material but again this depends on the quality of the parent material.
5.6.5.1 Advantages of ETB
The addition of bitumen emulsion to in-situ or reclaimed gravels offers the following advantages:
• The use of local material obviates the need to import suitable base course material from quarries located far away from the construction site. Adding small percentages of emulsion to locally available soils can be an effective alternative solution from the point of view of costs.
• ETBs are particularly suitable for labour-based road works, not only because the technique lends itself to the construction of a quality base using labour and light equipment, but the completed base course can be opened to traffic for an extended period without untoward damage to the surface. This is an advantage as pavement works using labour-based methods normally proceed more slowly than is the case with conventional methods often resulting in the need to manage traffic away from partially finished road sections for longer periods of time. This is also a common feature for bases stabilised with cement or lime.
• Emulsion reduces the internal friction of the gravel materials during construction, thus improving its compacted density and workability.
• It reduces water susceptibility and improves cohesion by binding fine aggregate,
• The addition of a fairly small amount of emulsion can significantly increase the CBR and Unconfined Compressive Strength (UCS) of the material,
• The strength and stiffness of ETBs are similar to those of lightly cemented materials,
• Rutting of the base course can be reduced significantly when opened to traffic prior to sealing,
• The cohesiveness of an ETB limits the development of potholes in the base when the surfacing is damaged, • By enriching the upper layer of the ETB during construction, usually eliminates the need for priming,
• When using crushed stone aggregate as a base, the risk of segregation of the aggregate matrix is reduced during construction,
• It allows the layer to be trafficked sooner than most other base layers – as soon as the emulsion has broken and no pick-up is evident under traffic,
• The use of emulsions results in savings in fuel consumption required for heating as emulsions are applied at ambient temperatures (cold mix),
• Gravels do not need to be dried, as emulsions adhere readily to damp aggregate,
• It is usually possible to core ETBs and to recover complete cores, which facilitates easy and speedy testing for quality control purposes.
5.6.5.2 Material components
As a guideline the following materials may be considered for use in an ETB: Cement
A small percentage of cement (1 - 1.5%) is added to assist with the breaking of the emulsion as it acts as a catalyst. Cement increases early strength of the ETB when the layer needs to be opened to traffic.
Lime
If the soils have a high plasticity (P.I > 6), it is advisable to add lime to the material to reduce the P.I. The effect of the lime on the material must be established, with emulsion added.
22 Bitumen emulsion is made up of minute droplets of bitumen suspended in water. The bitumen, in emulsified form, can be easily mixed with
Emulsion
The emulsion to be used must be an anionic23 stable grade 60% emulsion. Normally the emulsion will be supplied
in 210 litre drums. Natural gravel
ETBs can be produced from a variety of natural gravels, including: • Decomposed granites (may require lime)
• Decomposed dolerites (may require lime) • Decomposed basalts • Quartzitic gravels • Laterite/ferricrete gravels • Chert gravels • Sandstone gravels • Calcretes
Sometimes only fine grained sands are available. By adding 15% to 20% of 6.7 or 9.5mm crushed aggregate to this fine material, substantial improvement of bearing qualities can be obtained.
The following sections describe the preparatory work, materials, plant and equipment and construction process for the construction of an emulsion stabilised base course.
It comprises:
• Emulsion treated base course (ETB), 100mm thick • Composite gravel and ETB base course, 100mm thick Design Process
The design process for materials treated with small percentages of emulsion (less than 3%) is based on the following procedure:
• Characterisation of materials to be used in ETBs; • Determination of the optimum fluid24 content;
• Preparation of test samples, in which the amount of residual bitumen varies from 0 - 2% in increments of 0.5%, but where the optimum fluid content (hydroscopic water, emulsion and compaction water) remains unchanged;
• Compaction in accordance with the standard modified AASHTO method at room temperature;
• Curing of samples (24 hours at ambient temperature followed by 48 hours’ oven curing at 40°C if the optimum moisture content is less than 8%, or 45 hours at 60°C if the optimum moisture content is greater than 8%); • Determination of CBR and UCS after 4 days and 6 hours soaking, respectively; and
• Determination of optimum residual bitumen content.
For some materials such as dolerites with a P.I of, say, 3 to 4, but which may contain clays, it is recommended that lime be added to the material. In order to quantify the presence of such clays, the P.I test should be conducted on material passing the 0.075mm sieve.
Note: Before any local soil materials are used, it is recommended that a reputable materials laboratory carries out tests based on standard methods for testing ETB.25
Chart 5-4 below can also be used to check if a 100mm Emulsion Treated Base (ETB) will be adequate as a base layer on a well-shaped and compacted subgrade. From Chart 5-4, the in-situ field CBR of the subgrade/sub-base must not be less than 40 to be able to place the 100mm ETB on a shaped subgrade/sub-base, otherwise the appropriate imported soil cover layer as per the chart must be provided.
23 An anionic emulsion is more appropriate and very absorptive for dusty aggregates as compared to cationic emulsions.
24 Fluid content is the total quantity of fluid in the mix, including hygroscopic moisture, the bitumen and water within the emulsion and moisture
added for compaction. The optimum fluid content for a specific compactive effort is the fluid content at which the maximum density is obtained.