Stabilisation Using Chemical Binders

Many products have been tried and evaluated as chemical binders. Some have been proved ineffective while others such as petroleum products, if used excessively, may have adverse environmental effects.

Chemical stabilisation binders act as surface stabilisers providing stability to otherwise unstable surface materials. The benefits of chemical stabilisation of unsealed wearing courses are:  prevention of particles becoming airborne

 resistance to traffic wear

 retention in pavement, i.e. not lost through evaporation or leaching  resistance to ageing

 environmental compatibility

 easily applied with common road maintenance equipment  workable and responsive to maintenance

 cost competitive.

The primary function of chemical stabilisation is to bind the fine fractions such that they hold the aggregate fractions in place for a longer period of time. This is enhanced by the binder providing bonding and waterproofing of the fines to maintain the dry strength of the fine material.

6.4.1 Types of Chemical Stabilisation Binders

The categories of the mainstream chemical binders used in unsealed roads, and their reaction with subgrade soils and pavement materials, may be categorised as follows:

 synthetic polymers  natural polymers  ionic compounds  salts.

Synthetic polymers may be grouped into water soluble and water insoluble. Most synthetic

polymers in Australia and New Zealand are sold in a dry powdered format (commonly termed DPP, i.e. dry powder polymers).

Insoluble Dry Powdered Synthetic Polymers

A water insoluble dry powdered synthetic polymer is a manufactured material that is thermally bound to a very fine carrier such as fly ash. Typically it is classified as a stabilising binder rather than a dust suppressant. The fine powdered product, when mixed with hydrated lime, has the effect of flocculating and coating clay particles within the pavement material. The fly ash, which is encapsulated by the polymer, is effectively inert and does not react chemically in the stabilisation process. Its only function is to facilitate the distribution of the polymer throughout the pavement material. This polymer is used only in the powdered format and remains in a powder form during the pavement material mixing process.

Figure 6.2 illustrates the action of an insoluble dry powdered synthetic polymer (IDPSP) coated with a fly ash carrier surrounding soil particles to induce lower permeability and hence retard the loss of strength with wetting.

Three IDPSP blends are commercially available and spread at a rate typically 1% to 2% by dry mass of pavement material:

 a synthetic polymer thermally bonded to a fine powder carrier (i.e. fly ash)

 a blend of 2:1 synthetic polymer-coated fly ash/ hydrated lime for medium plasticity materials (PI < 12)

 a blend of 1:1 synthetic polymer-coated fly ash/ hydrated lime for higher plasticity materials (12 < PI < 20).

Source: Polymix Industries

Figure 6.2: Schematic of insoluble polymer encapsulating soil particles

Synthetic Soluble Polymers

These products are manufactured in granulated or liquid form and added to the compaction water to form the polymer chain which is an acrylimide or urethane copolymer. They encapsulate soil particles with a thin film of polymer and, upon drying, bonding and water insolubility is achieved. Figure 6.3 illustrates an acrylimide copolymer coating soil particles to induce bonding and low permeability and hence retard the loss of strength when the moisture content is greater than OMC.

Source: Biocentral Laboratories

Figure 6.3: Electron micrograph of acrylimide copolymer coating soil particles

Natural Polymers

These products include tall oil pitch, sulphonated lignin and di-limonene which bind fine particles to interlock with larger aggregates. In addition, they often have surfactant properties enhancing compaction by dilation of fine material when compacted with a vibrating roller. Their success is dependent upon both plasticity and particle size distribution. Cement or lime can be added as a secondary binder for increased stiffness.

These products are mostly obtained as resin by-products from the pulping industry. They are often highly acidic in addition to remaining soluble and subject to leaching over time. Their use should be strictly supervised in terms of worksite safety. Due consideration should also be taken with respect to any environmental impact associated with leaching.

Ionic Compounds

These products are generally produced by the petroleum industry. They produce an ionising action in water which induces cation (+ ions, e.g. Ca++, Na+, K+, Mg++, H+) exchange at the surface of negatively-charged clay particles. By the process of ionic exchange, water that would normally be electrostatically bound to the clay particles is replaced by ions, allowing much of this water to be expelled as free water. Other processes occur including coagulation and flocculation of clay particles after compaction and some cementing action through formation of insoluble salts. Salts

The most commonly used salt is water-attracting (hydroscopic) magnesium chloride; other salts include sodium chloride and calcium chloride. They require moisture (humidity) to be effective. They also require frequent re-application following rainfall. The consideration of salt leaching effects on the roadside environment must again be considered.

Of the chlorides used as a chemical binder, calcium, sodium and magnesium are used, with calcium and magnesium being deliquescent substances and sodium hygroscopic. The deliquescent substances absorb moisture from the atmosphere and liquefy. Hygroscopic substances, on the other hand, depend on exposed surfaces to absorb moisture. Salts such as those mentioned above control dust by keeping road surfaces damp, but have little or no

Roads treated with calcium chloride should only be graded after rainfall, and then only lightly from the edges to the centre, then reversing the operation feathering the material to the road edge. Limiting the sections to be treated will enable compaction before the surface dries, allowing bonding of the surface. Maintenance of the crown is essential with all treatments to ensure adequate drainage.

6.4.2 Applications

The main applications of chemicals in stabilisation are either as compaction aids and stabilisation binders which are mixed into a pavement layer or surface treatments for dust suppression, i.e.  chemical binders used in stabilisation:

— synthetic polymers — natural polymers — ionic compounds

 chemical binders used to improve compaction: — wetting agents, soaps

— synthetic polymer — natural polymers

 chemical binders used for dust suppression: — wetting agents, soaps

— hygroscopic salts (e.g. calcium, magnesium or sodium chloride) — natural polymers (e.g. ligno-sulphonate, molasses, tannin extracts)

— synthetic polymer emulsions (e.g. polyvinyl acetate (PVA), polyvinyl chlorate (PVC), polyacrylamide copolymers (PAM)

— modified waxes — petroleum resins.

A study on dust control techniques, including a performance evaluation of numerous chemical dust suppressants (Foley, Cropley and Giummarra 1996), concluded that dust control methods

available fell into three main categories:

 good construction and maintenance practice

 use of mechanical stabilisation to form a good wearing course that forms a hard surface crust  use of chemical binders as an adjunct (not replacement) to the above methods.

The sequence of remedies should follow the order given above, with possibly all methods being used to reduce dust emissions to a satisfactory level. It is considered of little value to use a chemical dust suppressant if some of the basic roads’ building requirements are not first addressed.

Short of sealing a road, there are no known ways to eliminate dust emissions effectively on a long term basis by using a single process or just one application of a chemical binder (Foley et al. 1996). However, on a life cycle basis, they can lead to lower maintenance costs through less frequent patrol grading and longer sheeting life.

Benefits from chemical stabilisation include extended periods between resurfacing, lower levels of surface roughness and hence vehicle operating costs, a reduction in accidents, higher quality primary produce and an improved amenity for nearby residents.

Prior to using a chemical binder, unstable areas and poorly-graded material should be removed and replaced with selected material at optimum moisture content. Adequate drainage is the single most important characteristic for long-term results because it ensures the subgrade does not become saturated and therefore weakened. Surface gravel should be added and a proper crown formed to facilitate surface water runoff.

6.4.3 Product Selection and Mix Design

The selection of the type of chemical binder should be made bearing in mind the quantity of fines in the surface material or the subgrade (if there is no surfacing structure), climatic conditions and traffic volumes and construction logistics (e.g. transportation of stabilisation binder).

The consideration of proprietary chemical binders is generally based on the determination of their suitability to the parent material rather than the determination of the required application rates. Basic information is generally available from product literature together with field examples. In some cases quantitative measurement of performance or attribute improvement is available. Chemical binders are generally separated into either dust palliatives or stabilisers and the following performance properties need to be considered:

 resistance to abrasion (effect of traffic and wind on treated surfaces)  resistance to erosion

 resistance to leaching

 increased shear strength (all weather trafficability)  long-term durability.

The large variety of proprietary products available and classified as chemical or polymer binders, coupled with varying degrees of quality performance data, make them less definitive in their selection compared to cement, cementitious, lime or bituminous binders.

It is suggested in Part 4D of the Guide to Pavement Technology – Stabilised Materials (Austroads 2006b) that a simple capillary rise or vertical saturation test is the most appropriate (and

economical) way to evaluate the suitability of a material in the laboratory. These two tests are conducted on material screened on a 2.36 mm sieve since the chemical binder is associated with the fine fractions.

Figure 6.4 shows the vertical saturation test in which a compacted specimen is prepared with and without binder, allowed to cure and dry and then subjected to saturation from dripping. The annular mass is used to induce collapse. As can be seen, the use of a binder in this particular case has shown that it could be of some value for field trialling or adoption.

Source: ARRB Group

Figure 6.4: Vertical saturation test