Emulsion Based Cold Mixes

The standard pavement structure in Canada consists of layers of manufactured materials laid onto a heterogeneous roadbed soil. The upper portion of the roadbed soil is commonly exposed to frost action. Differential frost heaving of frost-susceptible roadbed soils may lead to distortion and subsequent cracking of the pavement upper layers. Complete excavation of frost-susceptible material is not economically viable in all cases. Emulsion mixes have been used successfully to follow roadbed soil movement without cracking. Laboratory and field observations indicate that emulsion mixes have membrane-like mechanical properties rather than the slab-like properties of standard hot bituminous mixtures. Furthermore, emulsion mixes are not as susceptible to thermal cracking as regular standard hot bituminous mixtures. The experience acquired over the last 25 years in Ontario has contributed to define the conditions under which these emulsion mixes may be produced and used.

Emulsion mixes are obtained by dispersing a cationic bitumen emulsion in an aggregate with the use of cold mix plants or mix-pavers. The aggregate gradation of these mixes may be open or dense. The coating of aggregate by the cationic bitumen emulsion may be selective or complete. Emulsion mixes are used as a base layer in a pavement structure as well as a surfacing course. The application rates vary from 50 mm to 200 mm in thickness.

The U.S. Federal Highway Administration initiated the development of Open Graded Emulsion Mixes (OGEM) in the mid-sixties. The development work was to result in a product that would favour the construction of low cost pavement structures and low maintenance roads. The product was to provide a high degree of flexibility to minimize cracking due to low temperature and subgrade movement.

In the mid-seventies, a stabilized granular material called Dense Graded Emulsion Mix (DGEM) was developed. The product was used as an alternative paving mixture to Hot Mix Asphalt (HMA) on low volume roads in remote locations. In order to provide an equivalent performance to HMA, DGEM needed to be sealed with surface treatment. Unfortunately, the necessary combination of DGEM with surface treatment did not provide considerable saving compared to HMA and the production of DGEM was slowly abandoned. During the same period in Europe a similar Emulsion Stabilized Granular Material (ESGM) was developed for road base applications. Contrary to the Ontario experience, the savings associated with the usage of this product as a road base product were substantial and the performance was excellent. Nowadays, it is a widely accepted and used product, precisely defined with respect to the manufacturing process and the conditions under which it must be used.

The standard Ontario mix design procedure for OGEM consist of mixing measured quantities of aggregates and emulsion for a predetermined length of time to determined the capacity of the emulsion to properly coat the aggregate. The mix design procedure generally indicated that between 5.0% and 6.0% of Cationic Medium Setting (CMS-2) is required to properly coat the selected aggregate. The selected aggregate is usually a clean coarse aggregate.

The Open Graded Emulsion Mix is generally produced and placed with a mix-paver that combines the functions of a cold mix plant and a laydown machine. The compaction is performed using a standard vibratory double drum steel roller in the static mode.

In order to remove the tackiness of the surface and to allow traffic to circulate, a cover aggregate is spread over the surface after the initial rolling. The cover aggregate also provides surface cohesion and choking of the surface of the OGEM. The aggregate used was sand and the rate of application was approximately 5.0 kg/m2.

The standard Ontario mix design procedure for ESGM consists of mixing measured quantities of aggregates and emulsion to determine various standard mix properties. The mix design procedure generally indicated that between 5.0% and 6.0% of Cationic Slow Setting (CSS-1) is required to provide an optimum mixture. The aggregate usually consist of a fine and coarse aggregate, generally less than 16 mm nominal size. The selected gradation is usually continuous.

The ESGM is produced using a mobile cold mix plant, the placement operation is carried out with a standard paver and the ESGM is compacted using both a vibratory double drum steel roller and a pneumatic roller.

Performance data collected during three studies indicates that the service life of OGEM under normal conditions is 10 to 15 years. Structural layer coefficient (AASHTO coefficient) values of 0.25 to 0.30 have been used. Even though layer coefficients may, in certain cases, be equivalent for OGEM and HMA, the two materials have different mechanical characteristics and different properties with respect to long-term durability. Comparing the two materials' structural properties solely on layer coefficient may not be appropriate.

Like ESGM, the surface of OGEM is also relatively fragile when compared to the surface of HMA. Even though OGEM is generally a fully coated material, the surface of OGEM is not as rich in fines as HMA, resulting in less cohesion at the surface, which makes unsealed OGEM also prone to raveling. The best performance of OGEM is obtained when the surface is sealed.

The unique residual bitumen distribution in the mineral skeleton of ESGM provides this bound material with several distinctive properties. Even if a hot bituminous mixture were to be made up of the same granular material and the same bitumen, at an equivalent residual binder, the properties of ESGM would still be quite distinctive. The residual bitumen on an ESGM selectively adheres to the smaller particles of the granular material, forming a rich mastic. This bitumen-rich mastic binds the larger particles of the ESGM mineral skeleton.

The mastic provides unique properties to a ESGM layer. On one hand, the binding mastic is very stiff under rapidly applied loading such as traffic, resulting in slab-like behaviour by the ESGM layer. On the other hand, under slowly applied loading such as subgrade movement, the ESGM layer tends to behave like a membrane. In this latter case, the ESGM does not creep but the layer of stabilized material has the ability to deform in such a manner that the layer follows the underlying material movement without cracking.

The blend of fluid (aggregate moisture and emulsion) also provides unique properties to ESGM. Compaction of ESGM is performed at optimum fluid content just like any other granular material. Consequently, the granular matrix of an ESGM has comparable internal friction properties when compared to the same granular material compacted at optimum moisture content without the emulsion treatment. The residual binder selectively coats the smaller particles without altering the friction between the larger aggregate particles. Because of the binder distribution within the mineral matrix, Emulsion Stabilized Granular Material may be more accurately described as Granular Material with tensile strength.

The surface of ESGM is relatively fragile when compared to the surface of HMA. Emulsion Stabilized Granular Material is not a fully coated material like HMA and the surface is prone to raveling. Emulsion Stabilized Granular Material is best used as pavement structure material.

Materials with more voids and lower stiffness characteristics perform very well with respect to resistance to thermal and fatigue cracking. Both materials, ESGM and OGEM, have high void content: in-place voids for ESGM range between 10% and 15% and in the case of OGEM the void contents may reach as much as 25%. Typical HMA in-place void contents are below 8.0%. The stiffness of both materials, ESGM and OGEM, is also relatively low when compared to HMA.

Oxidation of the binder also influences the performance of bitumen stabilized materials by reducing the ductility of the binder. As it oxidizes, a bituminous binder becomes harder and more brittle, which leads to thermal cracking and accelerates fatigue cracking. Oxidation of hot or cold treated bituminous materials occurs differently. For HMA, the process of oxidation occurs in two distinctive periods: during manufacture of HMA and during service life. In the case of emulsion mixes, only service life oxidation is of concern. Unlike HMA, the manufacturing of emulsion mixes does not require heat. Consequently, premature oxidation associated with the hot mixing of aggregates and bitumen does not occur. The ductility of bitumen used in the cold mix process is preserved and the long-term negative effects associated with oxidation are reduced.

Emulsion Mixes are precisely defined with respect to the manufacturing process and the conditions under which they must be used. Emulsions Mixes have distinctive properties when compared with hot mix technology. The binder within an Emulsion Stabilized Granular Material selectively coats smaller particles. The stiffness of emulsion mixtures is lower than Hot Mix Asphalt. The voids content ranges between 12% and 15% for the Emulsion Stabilized Granular Material and between 20% and 30% for Open Grade Emulsion Mixes. Finally, unlike Hot Mix Asphalt, the manufacturing of emulsion mixes does not require heat.

The surface of emulsion mixes is relatively fragile when compared to the surface of Hot Mix Asphalt. Unsealed emulsion mixes do not provide waterproofing and the cohesion at the surface may not be sufficient to withstand surface tangent stresses. By sealing the surface emulsion mixes, waterproofing is achieved and the inherent performance of emulsion mixes is not compromised by premature stripping, oxidation and raveling.

• Ontario
• Quebec
• Eastern Canada