An estimated 10% of paving-grade asphalt in North America is used in the emulsified form.
The advantages of low temperature application, solventless, or low solvent content formulations and easy modification with water-based latex emulsions make asphalt emulsions the material of choice for sealers and binders in many pavement maintenance and construction applications. Although asphalt emulsions have been in use for almost a century, use in specific applications varies significantly throughout North America, with some applications in common use throughout Mexico, the United States, and Canada, and other applications used extensively in some areas and not in others. Since lack of awareness of and exposure to the broad range of construction and maintenance applications for emulsions can determine usage, this paper gives an overview of practical applications carried out with asphalt emulsions for construction,
maintenance, and preventive maintenance applications. The purpose of each treatment, the type of emulsion used, and the equipment involved is indicated.
The most common construction applications of asphalt emulsions include the following.
Tack Coat Purpose
Tack coats are used in construction applications to provide a good bond between the existing surface and a surface treatment (Figure 1).
Types of Emulsions Used
Emulsions used for tack coats can be anionic or cationic, slow-setting emulsions. Tack coats are intended to break quickly (usually within 15 min of application). They have very low application rates (around 0.1 gsy), usually at a dilution rate of one part emulsion to one part water. In order to be able to dilute tack coat emulsions to guard against over application and potential bleed through, these emulsions must be sufficiently stable and therefore are slow setting by category.
The very thin film application rate results in a fast break. The intent is to ensure a good bond between pavement layers by allaying any dust on the existing surface and wetting existing oxidized surfaces with a bond coat, without adding excess binder to the surface treatment.
Tack coats are applied through an asphalt distributor spray bar. Distributors are generally computer controlled to achieve a very accurate application rate.
FIGURE 1 Tack coat application
Emulsion chemistry is such that it enables excellent coating and adhesion of the residual asphalt cement to aggregate surfaces in both dense and open-graded mixes, base stabilization, and stockpiled patching mixes (Figures 2, 3, 4, and 5). Emulsion mixes are economical where small quantities of mix need to be produced at locations remote from hot plants and when ambient temperatures make the use of hot mix problematic.
Types of Emulsion
Slow- and medium-setting, mixing-grade anionic, and cationic emulsions, with and without solvent or adhesion agents, are used for both mixes and base stabilization. Polymer modifiers may be used to control the performance properties of the residual binder.
Emulsion mixes can be produced with a wide variety of equipment, either on-site, with in-place pulverizers, mixer/pavers, or at central plants. Both blade- and paver-laid mixes are commonly used. Mixing and laydown equipment is essentially the same as that for hot mix production.
Processes vary to accommodate the difference in binders including the following: pavers with a heated screed are not used; delayed rolling may be implemented to accommodate evaporation of excess moisture; lime stabilization and subsequent sanding of solvent containing mixes may be necessary prior to rolling; and rolling patterns may differ significantly from those used for hot mix.
FIGURE 2 Emulsion mix mixing table
FIGURE 3 Central plant-mixed (pugmill) emulsion mix
FIGURE 4 Dense-graded emulsion mix, paver laid
FIGURE 5 Windrow laydown, paver-laid emulsion mix
Traditionally penetrating primes were cutback asphalts. With increased knowledge of the chemistry of emulsions and control over the breaking and setting characteristics, solventless emulsions are successfully being used as penetrating primes on compacted base courses to seal and prime the surface prior to surface treatments to ensure adequate bond or as an application for dust abatement.
Types of Emulsions
Specially formulated penetrating grade, slow-setting cationic and anionic emulsions are used and are designated AEP (asphalt emulsion prime) or PEP (penetrating emulsion prime, generally dilute). Some emulsions for prime coats may still contain low levels of solvents.
Standard, computer-controlled asphalt distributor trucks are used to apply penetrating primes.
PAVEMENT MAINTENANCE APPLICATIONS
Scrub seals are an effective surface treatment for oxidized or distressed pavements (Figure 6).
Scrub seals function by sealing fine cracks prior to application of surface chips or by creating a mastic seal on distressed pavements.
Type of Emulsion
Emulsions for scrub seals may be anionic or cationic, with or without rejuvenating agents and polymer modifiers. Emulsions for scrub seals should be formulated to give adequate time and stability to enable scrubbing of the emulsion into the existing surface prior to chip placement and, when necessary, scrubbing of the chip and emulsion into a distressed pavement .
Scrub brooms are added to conventional chip seal equipment for scrub seal application (Figure 7).
Primary scrub brooms are designed to force emulsion (generally polymer modified) into
microcracks in the pavement surface. Secondary scrub brooms designed to force the emulsion–chip blend into fatigue cracks in a distressed pavement may be used also.
Technology in design of scrub brooms ensures filling of cracks and uniform application
of binder and chip across pavement cross-section without excess binder at the edges of pavement (Figure 8).
FIGURE 6 Scrub seal application
FIGURE 7 Fatigue cracks sealed with scrub broom before chipping
FIGURE 8 Redesigned hydraulic control scrub broom
eliminates excess emulsion at seal edge
Chip seals are an effective maintenance tool for restoring a wearing course to a pavement. As a preventive maintenance tool, chip sealing prevents ingress of moisture into a pavement or base course and can prevent deterioration due to oxidative aging of a pavement.
Type of Emulsion
Emulsions for chip sealing include anionic, cationic, rejuvenating, polymer-modified, and high float formulations. Chip seal binders do not generally contain solvents but some, such as high floats, have a provision for solvent content, as required, to wet chips and ensure adhesion.
Because many different geologic types and gradations of cover aggregate are used for
chip sealing (single size, graded, hard, porous) the emulsion should be selected to accommodate the aggregate in a performance system.
Chip sealing utilizes computer controlled asphalt distributor trucks, chip spreaders designed and calibrated to deliver accurate quantities of chip, pneumatic rollers, and brooms for removal of excess chip (Figures 9, 10, 11). Steel wheeled rollers may be used on multiple application chip seals to “rack-in” aggregate.
FIGURE 9 Chip spreader
FIGURE 10 Pneumatic roller
FIGURE 11 Vacuum broom
Slurry seals are fine aggregate emulsion mixes that can provide a smooth to moderately textured surface for low-speed, low-traffic volume streets and roads. Slurry seals cure quickly and have the advantage over chip seals of being water-based systems that produce no dust or loose chip during resurfacing. Slurry seals can produce smooth, aesthetically pleasing surface textures similar to hot mix in a very thin (<3/8 in.) application (Figure 12).
Type of Emulsion
Slurry seal technology has advanced over the past 40 years from slow-setting systems using anionic emulsions. These systems were dependent on atmospheric conditions for adequate curing by evaporation before the surface could be opened to traffic. Current slurry seal technology provides for very rapid cure systems based on the use cationic emulsions with and without polymer modification. Slurry technology derives its success from joint development of appropriate emulsions, equipment and additive systems, which combine with well-defined aggregate characteristics to ensure production of a controlled cure mix which resists raveling.
FIGURE 12 Type II slurry seal
Sophisticated slurry machines are on-grade traveling production plants that provide storage and mixing capability for aggregate, emulsion binder, water for consistency control and additives, which may be used for control of breaking and setting characteristics. Slurry machines include a pull-along laydown box for application of the slurry to the roadway (Figure 13). Laydown boxes are equipped with augers to ensure uniform mixing and application and to prevent premature breaking of the mix prior to placement.
Microsurfacing is state of the art resurfacing of pavements with a thin layer emulsion mix
designed to withstand heavy traffic. Like slurry sealing, microsurfacing produces no dust or loose chip. Microsurfacing design enables rapid cure of the surface regardless of the depth of placement and is therefore an excellent treatment for rutted or irregular surfaces.
Types of Emulsion
Microsurfacing emulsions are specially formulated cationic emulsions designed to work with a specific aggregate in a performance mix composition. The emulsion formulation is tailored to the aggregate characteristics to produce a mix which must comply with stringent specifications designed to ensure against rutting, raveling, bleeding, or premature aging. Microsurfacing emulsions are highly polymer modified.
Equipment Microsurfacing machines are sophisticated travel plants designed to store and mix the aggregate, emulsion, mix water and additives for breaking control. Microsurfacing machines generally have twin shaft mixing chambers specially designed to eliminate dead spots associated with a pugmill.
Laydown boxes also have augers for uniform mixing and application. Special boxes have been designed for rut-filling prior to full-width resurfacing (Figures 13, 14, 15, and 16).
FIGURE 15 Rut-filling box on microsurfacing machine
FIGURE 16 Rut-filling application of microsurfacing
MULTIPLE APPLICATION SURFACE TREATMENTS
Double and triple chip seals are commonly used for new construction sealing in rural areas. In addition to these multiple seals, the following multiple application seals are also frequently used.
Cape seals consist of an application of slurry seal or microsurfacing placed over a chip sealed surface. The chip seal is placed to ensure sealing and waterproofing of the existing surface.
The slurry or microseal is placed over the chip seal to eliminate the risks associated with loose chips as well as to establish the desired surface texture.
Types of Emulsion
Emulsions for the chip seal portion of cape seals are the same as for regular chip seals. Slurry or microemulsion are most always cationic and generally polymer modified.
Equipment for both chip and slurry or micro portions of a Cape Seal are the same as for the individual treatments.
Cape Scrub Seals
Cape scrub seals are cape seals where the polymer-modified emulsion has been scrubbed into the existing distressed surface prior to chip and slurry or microsealing (Figure 17).
Types of Emulsion
Emulsions for each of the applications are the same varieties as those used for the individual applications.
The equipment used for cape scrub seals is the same as that for cape seals with the addition of the emulsion scrub broom used before applying chip.
FIGURE 17 Type II slurry being applied on a cape scrub seal
Ultrathin Bonded Wearing Course
An ultrathin bonded wearing course is an application of an asphalt emulsion immediately covered by a thin layer of hot mix (Figure 18). The emulsion is effectively drawn up into the hot mix to form a strong cohesive bond with both the pavement and the hot mix.
Types of Emulsion
Emulsions for ultrathin bonded wearing courses are mostly cationic polymer-modified emulsion formulations.
Equipment for this surface treatment is a specially modified paver equipped with an emulsion spray system which places the emulsion on the pavement surface just before the hot mix screed.
FIGURE 21 Lightly sanded fog seal
Recycled Asphalt Pavement Mixes
Both new construction and reconstruction are possible utilizing recycled asphalt pavement (RAP) mix technology. Mixes composed of graded RAP millings and state of the art emulsion binders can produce mix properties equal to those of hot mix in dense, gap, and open-graded mixes. These mixes can be produced more economically than hot mix.
Types of Emulsions
Mixing grade emulsions, both anionic and cationic, with and without solvent, are used in RAP mixes. Emulsions may contain rejuvenating agents or polymer modifiers and adhesion agents.
Emulsion chemistry is responsible for immediate coating and subsequent cohesive
strength development. Residual binder properties determine long term performance.
Equipment for processing and placement of RAP mixes is essentially the same as that for producing emulsion mixes from virgin materials. A wide variety of crushing, sizing, mixing, laydown, and compaction equipment is used.
New construction mixes (placed on compacted base) are generally on-site mixed and
hauled to grade for paver laying (Figures 22 and 23).
Reconstruction of existing pavements is generally more economical using cold-in-place recycling trains consisting of sizing, crushing, mixing, placement, and compaction equipment on grade (Figure 24).
FIGURE 24 Cold in-place recycling train
RAP Millings in Slurry Surfacing
RAP millings, when properly screened and graded, provide a cost effective aggregate for slurry surfacing (Figure 25).
Types of Emulsion
Primarily cationic mixing grade emulsions are used in RAP slurry mixes. These emulsion slurry mixes are designed for compliance with all standard slurry mix performance requirements.
RAP slurry mixes are placed with standard slurry mixing and application equipment.
Asphalts in their emulsified form are used in a wide range of applications for construction, maintenance and preventive maintenance treatments. Development of technologies utilizing asphalt emulsions are a product of cooperation among emulsion manufacturers, equipment manufacturers and contracting companies.
The advantages of low-temperature, low-volatile emission application without the safety
issues of hot asphalt make emulsions the preferred material in many applications.
Asphalt emulsion usage will certainly increase for the above reasons as well as due to efficiencies in use, which will increase with rising fuel and energy costs.
1. Scofield, L. FHWA Emulsified Sealer/Binder Study.
PEGGY L. SIMPSON