EMULSION-bASED SEALS

Due to the challenges with hot bitumen seals as indicated above, focus has been shifted to use of bitumen emulsion34 _{based sealing techniques. The reasons for using emulsion-based seals are that they provide:}

• Cold processes that save energy

• Easier handling and storage (low viscosity) • Safe and environmentally friendly

• Low-cost on-site and in-place techniques

New techniques using emulsion based seals have also been developed for low volume roads. Several options are now available for use on labour based road works projects, all of which are based on bitumen emulsion, viz: • Sand Seal

• Modified Otta Seal or Penetration Seal • Cold Mix Asphalt

Other types of labour-friendly emulsion based seals not covered in this guideline are slurry seals, cape seals, and slurry-bound macadam seals.

Un-even spreading of aggregate

33 _{The “Chippy” is an un-motorised plant custom-designed in South Africa for uniform labour application of aggregates} 34 _{Bitumen Emulsions are mixtures of two immiscible liquids, such as oil and water, stabilized by an Emulsifier.}

8.9.1 Types of emulsion binders

The following types of emulsion binders with characteristics as described in Table 8-7 can be used for low-volume sealing:

Table 8-7: Characteristics of anionic and cationic bitumen emulsion binders

Anionic Emulsion (60% bitumen and 40% water) Cationic Emulsion (65% bitumen and 35% water) a) Have negative charge, adheres to positively

charged particles like limestone, alkaline in nature.

b) Works better in low humidity climates c) The viscosity is lower than that of cationic

emulsion and the rate of application for a tack coat should be reduced to approximately 0.7 to 0.8litres/m2, otherwise the binder tends to flow even at minimum gradients.

d) More appropriate for dusty aggregates (Possibly very absorptive e.g. crusher dust from basic rock origin.) Wetting down stockpile may also help. e)

f) Hard non-absorptive stones

i. Have positive charge; adheres to negatively charged particles like silica, is acidic in nature, used with wet aggregates and in cold weather ii. Cationic stable breaks35 _{slower depending on the}

evaporation rate. Ideal in high humidity climates iii. Slow curing depending on evaporation rate. iv. May be used at ambient temperatures with

aggregates, which need not be completely dry. v. Lower costs are incurred due to a considerable

saving on fuel for heating purposes.

vi. Operating at lower temperatures for cold mixing gives a greater margin of safety.

vii. Enables cold mix to be stockpiled for long periods, or packaged in small containers and stored. This facilitates the treatment of very small areas quickly, cleanly and economically.

viii. Ideal for most naturally occurring aggregate e.g. river sand are negatively-charged in aqueous media, and have a capacity to absorb cationic emulsifiers.

ix. In the absence of Cationic Emulsion (65% bitumen and 35% water) Cationic (60% bitumen and 40% water) can be used but increasing the emulsion content by 1% (i.e. 65 divided by 60) Bitumen emulsions are classified by the different setting times (time taken for the bitumen droplets to coalesce after application and the water to evaporate) and the stability of the emulsion (ability of the droplets to stay in suspension). Both Anionic and Cationic emulsions can be:

• Rapid Setting (RS) • Medium Setting (MS) • Slow Setting (SS)

The grades of bitumen emulsions are determined by the amount of emulsifier being used in the manufacturing process. The emulsion grades are:

• Spray grade • Pre-mix grade • Stable grade

Normally a prime coat is required on the prepared base layer the application of the emulsion binder in the sealing process

Prime Coats and Tack Coats

A prime coat entails spray applications of low viscosity asphalt on a granular base in preparation for placing an asphalt mixture. A prime coat performs several important functions:

• Coats and bonds loose mineral particles on the surface of the base • Hardens or toughens the surface of the base

• Waterproofs the surface of the base by plugging capillary or interconnected voids • Provides adhesion or bond between the base and the asphalt mixture

Emulsions that can be used for priming are Slow Setting SS-1, SS-1h36_{, CSS-1 and CSS-1h and almost always}

require dilution with water. The dilution rates normally ranges from 1:1 to 10:1 (water to emulsion) dependent upon the base material characteristics and method of treatment. The application rates can vary for a 1:1 diluted emulsion from as low as 2.3 l/m2 for high fines and tight bases and up to 6.8 l/m2 for loose sands and very porous surfaces. In very dense material, it may be necessary to use a higher dilution and make multiple applications at lower rates. This is done to improve penetration and prevent runoff and pudding of the emulsion.

On the other hand, a tack coat is a very light spray application of diluted asphalt emulsion. It is used to create a bond between an existing surface and an asphalt latex (cold mix) being placed.

Asphalt emulsions commonly used for tack coats are diluted Slow Setting SS-1, SS-1h, CSS-1 and CSS-1h. The emulsion is diluted by adding an equal amount of water. To prevent premature breaking, the water must be added to the emulsion and not the emulsion to the water. Warm water is preferable for dilution and the diluted material is typically applied at a rate of 0.25 - 0.70 l/m2. A tack coat should be applied only to an area that can be covered by the same day’s paving. The best results are obtained when the tack coat is applied while the pavement surface is dry and the surface temperature is above 25°C. The surface to be tack coated must be clean and free of loose material so it will adhere. A good tack coat results in a very thin but uniform coating of residual asphalt on the surface when the emulsion has broken.

After spraying the tack coat, time must be allowed before the overlay is placed for the complete breaking of the diluted emulsion (brown to black colour). Traffic should be kept off the tacked area.

The procedures and equipment for the application of a prime are exactly the same as described in the following sections for tack coats and will not be covered separately.

These emulsion based techniques eliminates most of the problems faced by the contractors in the application of labour based methods with hot bitumen applications in that:

• The sealing can follow the pace of the base construction and thereby eliminates the need for long stretches of base to be constructed and being exposed to traffic and weather before sealing can take place. It affords daily ‘seal as you go’ process as in labour-based graveling operations;

• The prime and seal serve as a curing membrane for stabilized base layers thus solving the problem of achieving optimum curing of the base.

• Increases labour-intensity and daily production outputs without compromising on the quality of the finished product;

• Does not require high technical supervision as compared to the hot bitumen options; hence the technology is suitable for emerging (in-experienced) contractors.

• Affords easy transportation of emulsion to site in 210 litre drums and eliminates all bottlenecks associated with the use of heavy motorised bituminous distributors.

• Minimises handling and health hazards to the workforce.

• Requires less equipment since the tankers and the heating equipment are no longer needed.

36 _{The “SS” means slow setting, and the trailing number “1” denotes low viscosity. Usually the higher the number the higher the viscosity (higher}

bitumen content). The letter “h” denotes the hardness whilst an “s”, denotes a soft (penetration grade) of the base bitumen used for the emulsion.,

Slurry Seals and Cape seals also use emulsions and are labour-friendly but not covered in this guideline.

Other proprietary emulsion based mixes which have been tried with good results but not discussed in this document are:

• Labour based sealing (LBS) • Tar Fix

• Coulgar Cold mix

8.10 PROCEDURE FOR APPLyING bINDER