Pavement Treatment Types
Pavement preservation is the proactive maintenance of roads to prevent them from getting to a condition where major rehabilitation or reconstruction is necessary. Preventative maintenance costs are much less than the ultimate repair cost. As depicted in the figure below, as pavement ages, the pavement condition declines to a poor condition after it has aged approximately 75% of its life (typically 20 years). As shown on the curve as the pavement condition worsens the treatment changes resulting in higher treatment costs.
By proactively maintaining the roads the pavement life is extended maximizing the value of each dollar spent on the roads. Another important benefit of utilizing the pavement preservation approach is that because the treatment costs to preserve the good roads is substantially less it enables the City to preserve more streets than if the was focused on fixing the bad roads first.
What? A fog seal is an application of a specially formulated asphalt emulsion (a thin liquid oil) to an existing asphalt pavement surface. A fog seal gets its name from its spray application, sometimes referred to as “fogging.”
Asphalt emulsions used in fog seal applications contain globules of paving asphalt, water, an “emulsifying agent” or surfactant, and sometimes a “rejuvenator.” Soap is a common form of a surfactant. In washing clothes or dishes, the surfactant helps remove the dirt and suspend the dirt particles in the wash water. Similarly, in asphalt emulsions, the surfactant keeps the paving asphalt globules in suspension until it is applied to the pavement surface when the water in the asphalt emulsion starts to evaporate. A “rejuvenator” is an asphalt additive which when applied to the existing pavement will slightly soften the pavement it is applied to creating a better bond.
Why? As asphalt pavement is subjected to traffic loads and ages, it oxidizes and cracks develop in the surface due in part to the pavement becoming more brittle. Oxidation is one of the reasons asphalt concrete pavement fades in color from the deep, rich black color everyone remembers from when the road was constructed or last resurfaced. Fog seal applications serve to seal narrow cracks, slightly restore lost flexibility to the pavement surface, provide a deep, rich black pavement surface color, and most importantly help preserve the underlying pavement structure.
When? On existing asphalt pavement, fog seals are typically applied on either an intermittent or a cyclical basis. Location, weather, traffic loading, and pavement conditions are factors used to determine if a fog seal application is appropriate. Roadways selected for fog seal treatment are commonly those that have minor cracking, faded color, and where a fog seal would help extend the pavement life until resurfacing becomes necessary. Roadways chosen for cyclical fog seal applications would typically be treated every three to five years.
How? Fog seals are applied by a distributor truck. The distributor truck slightly heats the asphalt emulsion before spraying it onto the pavement. Once applied the surface has the appearance similar to the pavement having been spray painted black.
What? A slurry seal is the application of a mixture of water, asphalt emulsion, aggregate (very small crushed rock), and additives to an existing asphalt pavement surface. A slurry seal is similar to a fog seal except the slurry seal has aggregates as part of the mixture. This combined mixture of the emulsion and aggregates represents “slurry.” Polymer is commonly added to the asphalt emulsion to provide better mixture properties. The placement of this mixture on existing pavement is the “seal” as it is intended to seal the pavement surface. Slurry seals are generally used on residential streets.
Why? Slurry seal is applied in order to help preserve and protect the underlying pavement structure and provide a new driving surface. Roads chosen for slurry seal applications generally have low to moderate distress and narrow crack width. Slurry seal applications serve to seal the cracks, restore lost flexibility to the pavement surface, provide a deep, rich black pavement surface color, and help preserve the underlying pavement structure.
When? Slurry seal is typically applied on either an intermittent or cyclical basis. Location, weather, traffic loading, and pavement conditions are factors used to determine if a slurry seal application is appropriate. Roadways selected for slurry seal treatment are commonly those which have slight to moderate distress, no rutting, and generally narrow crack widths, and where a slurry seal treatment would help extend the pavement life until resurfacing becomes necessary.
Roadways chosen for cyclical slurry seal applications would typically be treated every five to seven years.
How? The asphalt emulsion and the aggregates are mixed in, and applied using, a purposely built truck, generally referred to as a “slurry truck.”
Slurry trucks contain various compartments, which hold the aggregate (fine crushed rock), water, polymer modified emulsion, and other additives, which are mixed in the on-board mixer. The slurry mixture flows out of the rear of the truck and onto the pavement within the confines of a box attached to the rear of the truck. The box serves to distribute the slurry mixture over the pavement. Workers with squeegees follow behind and assist in spreading the mixture, correcting areas not properly covered, and keeping the mixture off concrete improvements such as gutters.
A piece of burlap is often dragged behind the slurry truck with the goal of producing a smooth texture while spreading the slurry seal. The texture of a slurry seal is different from that of asphalt pavement. The texture may have faint lines and be somewhat coarse. In addition, when the slurry is first placed the surface is tender and can be marked up by sudden turning movements. These impressions on the pavement are temporary as seasonal weather and traffic smooth out these imperfections over time.
Once placed slurry seals need 4 to 6 hours to set or harden before the road is open for traffic.
What? Microsurfacing is similar to slurry seal. It consists of the application of a mixture of water, asphalt emulsion, aggregate (very small crushed rock), and chemical additives to an existing asphalt concrete pavement surface. Polymer is commonly added to the asphalt emulsion to provide better mixture properties. The major difference between slurry seal and microsurfacing is in how they “break” or harden. Slurry relies on evaporation of the water in the asphalt emulsion. The asphalt emulsion used in microsurfacing contains chemical additives that allow it to break without relying on the sun or heat for evaporation to occur. Thus, microsurfacing is an application that hardens quicker than slurry seals and can be used when conditions would not allow slurry seal to be successfully placed. Streets that have a lot of shade and streets that have a lot of traffic are good candidates for microsurfacing.
Why? Microsurfacing is applied in order to help preserve and protect the underlying pavement structure and provide a new driving surface. Roads chosen for microsurfacing application generally have low to moderate distress and narrow crack width.
When? Microsurfacing is typically applied on an intermittent, project-specific basis. Location, weather, traffic loading, and pavement conditions are factors used to determine if a microsurfacing application is appropriate. Roadways selected for microsurfacing treatment are commonly those which have slight to moderate distress, no rutting, and generally narrow crack widths, and in which a microsurfacing treatment would help extend the pavement life until resurfacing becomes necessary. Roadways chosen for cyclical microsurfacing applications would typically be treated every five to seven years.
How? Microsurfacing is commonly mixed in, and applied using, a purposely built truck, generally referred to as a “slurry truck.” If the roadway segments to be microsurfaced are long and do not contain sharp curves, a “continuous machine” is sometimes used. This machine is capable of constant or continuous mixing and application of microsurfacing due to its large storage compartments and capability to attach to aggregate delivery trucks and tankers containing microsurfacing emulsion. This continuous operation provides the benefit of having few if any seams in the roadway, which occur when operation stops.
Slurry trucks and continuous machines contain various compartments that hold the aggregate (fine crushed rock), water, polymer modified emulsion, and other additives, which are mixed in the on-board mixer. The microsurfacing mixture flows out of the rear and onto the pavement within the confines of a rear-mounted box referred to as a “micro box.” This box contains an auger, which turns to uniformly distribute the mixture across the width. Workers with squeegees follow behind and assist in spreading the mixture, correcting areas not properly covered, and keeping the mixture off concrete improvements such as gutters.
A piece of burlap is often dragged behind with the goal of producing a smooth texture while applying the microsurfacing material. The texture of microsurfacing is different from that of asphalt pavement. The texture may have faint lines and be somewhat coarse. In addition, when the microsurfacing is first placed the surface is tender and can be marked up by sudden turning movements. These impressions on the pavement are temporary as seasonal weather and traffic smooth out these imperfections over time.
What? A chip seal is a two-step process that includes first an application of asphalt emulsion and then a layer of crushed rock to an existing asphalt pavement surface. A chip seal gets its name from the “chips” or small crushed rock placed on the surface.
Asphalt emulsions used in chip seal applications contain globules of paving asphalt, water, an “emulsifying agent” or surfactant, polymer, and sometimes a “rejuvenator.” Soap is a common form of a surfactant. In washing clothes or dishes, the surfactant helps remove the dirt and suspend the dirt particles in the wash water. Similarly, in asphalt emulsions, the surfactant keeps the paving asphalt globules in suspension until it is applied to the pavement surface when the water in the asphalt emulsion starts to evaporate. The chips (small crushed rocks) are immediately applied after the asphalt emulsion is applied to the pavement surface. The polymer in the asphalt emulsion is a “hardener” which serves to improve the adhesion to the crushed rock and to the pavement surface. A “rejuvenator” is an asphalt or additive which when applied to the existing pavement will slightly soften the pavement it is applied to creating a better bond.
Why? Chip seals are placed under one of three scenarios, as a “wearing course” meaning vehicles drive directly upon it; as part of a “cape seal” meaning a chip seal is first placed, then covered by slurry seal or microsurfacing; or as an “interlayer” meaning a layer between underlying asphalt pavement and a new asphalt or rubberized asphalt surface.
As asphalt pavement is subjected to traffic loads and as it ages, it oxidizes, becomes more brittle and cracking develops in the pavement structure. A chip seal serves to seal the narrow cracks, help bind together cracked pavement, provide a wearing (driving) surface, and protect the underlying pavement structure. Chip seals used as interlayers serve to retard reflective cracking which develops from the bottom of the pavement structure towards the top.
When? Chip seals are typically applied on an intermittent or one-time basis. Location, weather, traffic loading, and pavement conditions are factors used to determine if a chip seal application is appropriate. Roadways selected for chip seal treatments are commonly those which have moderate block (or “alligator”) cracking which is not spalling (or “popping out”), no rutting, an acceptable ride quality, and in which a chip seal would help extend the pavement life until resurfacing or rehabilitation can be performed.
Roadways chosen for cyclical chip seal applications would typically be treated every seven to ten years.
How? A distributor truck first applies the asphalt emulsion to help seal the pavement surface. Using an asphalt emulsion is considered a “cold applied” chip seal even though the liquid temperature is typically between 110 and 150 degrees Fahrenheit. No smoke results from the application of emulsified asphalt. If modified paving asphalt is applied, it is considered a “hot applied” chip seal as the liquid application temperature is typically between 325 and 375 degrees Fahrenheit. Smoke will result from the application of modified paving asphalt but should not be of concern. Depending upon formulation, the distributor truck may be equipped with a smoke reduction device that reduces the level of blue smoke emitted. This is normal. These devices are subject to permitting and oversight by the jurisdictional air quality management district.
Immediately following application of the asphalt emulsion, a “chip spreader” follows and applies crushed rock to the top of the asphalt emulsion. A dump truck dumps crushed rock into the chip spreader as it travels. The dump truck appears to be going in reverse but the chip spreader actually is towing it backward.
Following chip application, rubber tired rollers follow closely behind. The purpose of the rollers is to push the chips down into the binder and enhance the bond between them. A steel drum roller commonly follows the rubber tired rollers on “hot applied” chip seals.
After rolling and once the binder and chips have properly adhered, power sweepers sweep the surface. The sweepers sweep the loose crushed rock, which has not adhered to the underlying asphalt emulsion. Loss of crushed rock due to sweeping is a normal part of the operations. A chip seal would not be successful if the result was areas of the existing pavement not covered by crushed rock. The rate of crushed rock application is intended to result in a thickness of “one rock.”
The final operation is placement of a “flush coat.” The flush coat consists of the application of a fog seal (see fog seal) followed by the application of very clean sand. The purpose of the flush coat is to help “lock down” the chips and absorb any excess asphalt emulsion. The result is a pavement surface that has a rough texture and a very deep, rich black in color.
What? Preparation work is the work that needs to be performed prior to the application of any treatment. It is important that areas of severe damage are isolated and repaired before any preventative or rehabilitative treatments are applied. Preparation work may also entail the removal of thermal plastic striping, street sweeping, or grinding.
Why? If isolated areas are not addressed before the application of a preventative maintenance, such as a slurry seal, the benefits of the preventative maintenance treatment may not be maximized. The unrepaired section will deteriorate sooner than other areas. In addition, if the pavement is not properly cleaned and swept, the new treatment may not bond properly.
When? When there are isolated areas of failure, prep work is necessary in order to maximize the benefits of any maintenance or rehabilitation treatment.
As pavement ages, it will inevitably begin to deteriorate due to weathering and traffic loading. The condition of the pavement may not be so severe as to warrant complete reconstruction. At this point in the pavement’s life is when rehabilitation techniques can be performed.
Rehabilitation is carried out on pavements that exhibit distresses beyond the effectiveness of maintenance and
What Rubberized asphalt is a hot-mixed asphalt pavement containing crumb rubber. The crumb rubber serves as a “modifier” of the liquid asphalt. Its addition gives the liquid asphalt greater viscosity (resistance to flow) and improves other properties, which resist reflective cracking and rutting, and prolong pavement life.
The crumb rubber utilized is generated from processing scrap tires. The tires are shredded and the steel reinforcement and fibers separated from the rubber. The steel reinforcement is commonly recycled into new reinforcing steel used in structure construction and the fibers commonly recycled into pads used underneath carpeting.
Why? Rubberized asphalt concrete provides a longer lasting, more durable surface course than asphalt paving which does not contain crumb rubber. Thus, the frequency of maintenance is greatly reduced due to the longer life cycle.
When? Rubberized asphalt is frequently used as the surface course material when roads are resurfaced or reconstructed. While it is impossible to predict actual pavement performance, there are roads surfaced with rubberized asphalt, which have not been resurfaced in over 15 years. Typical performance should be in the range of 14 to 18 years.
How? There are two common processes for adding crumb rubber to liquid asphalt. The first is referred to as the “field blend” or “wet process.” Under this scenario, the crumb rubber is added to the liquid asphalt at the asphalt plant before being mixed with the aggregate (crushed rock and sand). The other process is referred to as the “terminal blend” method since the crumb rubber is blended with the paving asphalt at a refinery. Under this scenario, the crumb rubber is dissolved into the liquid paving asphalt at a refinery before being shipped to the asphalt plant.
Rubberized asphalt is placed similar to other hot-mixed asphalt concrete pavements using a paving machine and rollers. After the rubberized asphalt is placed and before it is opened to traffic, a rock dust blotter is sometimes placed on the surface. The purpose of the rock dust blotter, which is gray in appearance, is to prevent vehicle tires from sticking to the pavement surface.
Cold In-Place Recycling (CIR)
What? Cold in-place recycling (CIR) process involves a milling machine with a paver mixer. The milling machine breaks and pulverizes a thin amount of the top layer of the old pavement. It can also break and pulverize all of the Hot Mix Asphalt (HMA) and some of the base. This material is crushed and screened to the proper size and recycling agents are mixed in to rejuvenate it and give it more life. If necessary, virgin aggregate can be added and spread on the existing surface. Recycling agents that are used include: emulsified asphalt, foamed asphalt, and chemical additives. The material is deposited into a windrow behind the machine and then picked up by the paver and spread. Then, it is compacted using steel-wheel, pneumatic-tire, or vibratory rollers. CIR material tends to have high voids so a HMA overlay or double surface treatment is applied as a wearing course to seal and preserve the recycled pavement layer.
Why? CIR is useful in treating a wide range of distresses and failures such as: raveling, potholes, bleeding, skid resistance, rutting, corrugation, shoving, fatigue, edge, and block cracking. It can improve the ride quality caused by bumps, swells, sags, and depressions. CIR can be performed relatively quickly, which reduces user delay. Brittleness of aged existing pavement is enhanced, and it can provide improved rutting resistance in the pavement life. Extensive laboratory research has shown that CIR is effective in improving performance of the pavement while also reducing overall cost.
When? CIR is limited to applications where underlying soil structures are adequate. It addresses issues restores old pavements to a like new condition while also being environmentally friendly.
Cold Central Plant Recycling
What? Asphalt pavements are often being cold planed or milled to maintain roadway geometries or removed prior to an asphalt overlay. This has created large quantities of stockpiled reclaimed asphalt pavement (RAP). RAP is very effective and useful for a variety of pavement maintenance applications. It is often used for the production of maintenance mixtures. This material is cold central plant recycled (CCPR) with an asphalt emulsion in order to rejuvenate it. Besides being very high quality and salvaging pavement materials, RAP mixes result in good cost savings of 25 to 50 percent compared to new mixes.
Why? CCPR provides better process control of recycled materials compared to CIR. Aggregate gradation can be better broken down and additives can be more precisely added to rejuvenate the recycled material. CCPR provides environmental benefits and reduced costs. The consumption of virgin aggregates from dwindling supplies is minimized by using RAP. The cost associated with disposal instead of reuse of RAP has saved tax payers $300 million annually. Additionally, mixtures that include RAP have shown to have equivalent or better long term performance as compared to virgin mixtures.
When? Cold Central Plant Recycling is selected as the strategy of choice when precise process control is required. It may also be required when there is incorporation of RAP from other projects or stock piles or if there is not adequate space at the job site for processing of the recycled materials.
What? Asphalt overlay is applies a new layer of asphalt over the current one. First, any potholes, alligatored asphalt, and depressions need to be repaired prior to installation of the new surface. Also, since the road will be slightly higher with an overlay, maintenance hole covers, water valves, monuments and other utility markers need to be adjusted to the new grade The existing asphalt is then cleaned of all debris and then a tack coat (thin coating of tar or asphalt) is applied to the old pavement to help with binding of the new pavement. Then the asphalt can be placed.
Why? Rather than tearing up an old asphalt surface entirely, an asphalt overlay project will use the existing layers as a base for the new asphalt pavement.
When? The existing pavement needs to be in stable condition with no repairs or replacement needed.
Mill and Fill
What? Mill and Fill is a structural pavement treatment that involves the removal of the existing surface layer and in some cases the entire asphalt pavement thickness - with a milling machine and the replacement of the milled location with new asphalt.
Why? Mill & Fill restores and strengthens a road’s surface layer by restoring the pavement to a “like new” ondition. This process is used when a surface has deteriorated to poor condition and surrounding grades (i.e. curbs, driveways) must be met. This is advantageous to an asphalt overlay when asphalt surfaces have severe damage like rutting, potholes, large cracks, and expansions.
When? When a road has deteriorated to poor condition, pavement preservation treatments no longer provide long-term improvements and structural reconstruction is required.
All pavement structures degrade due to wear and tear caused by traffic and the environment. Typically, most pavement maintenance programs overlay or rehabilitate pavements in order to restore its ride quality and structural capacity. Eventually, the roadways deteriorate to a point where rehabilitation becomes too expensive and cannot perform well due to the condition of the underlying pavement conditions. It is at this point when reconstruction is specified.
Full-Depth Reclamation (FDR)
What? In today's environment, the practice of remove and replace has become impractical from the standpoint of cost and environmental impact. A very popular reconstruction alternative is known as Full-Depth Reclamation (FDR). FDR is an in-place recycling method for reconstruction of existing flexible pavements using the existing pavement section material as the base for the new roadway-wearing surface. This process can include adding chemicals to the base layer in order to increase its strength capacity. The treatment of the base layer and recycled asphalt provides a stronger foundation for present and future traffic. This process effectively produces a cost-effective solution that maximizes limited budgets.
The new chemically treated base section provides engineering benefits that perform as a foundation for the new wearing surface. These benefits include higher unconfined compressive strength than the previous unbound aggregate base material, and lower permeability of the treated base, which reduces the influence of water, the main reason for premature pavement failures. Stabilization of the reclaimed pavement can be done by mechanical, chemical, or bituminous means.
Mechanical stabilization methods include the addition of:
- Virgin aggregate
- Reclaimed Asphalt Pavement (RAP)
- Crushed Portland Cement Concrete (PCC)
Chemical stabilization is achieved with the addition of:
- Portland Cement
- Fly ash
- Cement kiln dust
- Calcium/magnesium chloride
- Other proprietary chemical products
Bituminous stabilization can be accomplished with the use of:
- Liquid asphalt
- Asphalt emulsion
- Foamed Asphalt
For increased stabilization requirements, combinations of all three can also be used.
Why? The advantages of FDR are considerable compared to remove and replace reconstruction. It provides the benefit of being equal or better in performance while also minimizing the consumption of fuel and natural resources. FDR treats all types of failures to the highest severity. It eliminates ruts, rough areas, and potholes. It also eliminates alligator, transverse, longitudinal, and reflection cracking. In addition, it FDR restores the grade contours to allow for better surface drainage.
When? FDR can be performed in place of traditional remove and replace reconstruction. FDR takes cold in-place recycling to the next level by grinding up the old pavement and using it as a stronger foundation for the new roadway. This is beneficial in cases where reconstruction is necessary in order to increase the structural capacity of the roadway due to increased vehicle traffic.
FDR has been defined as Cement Stabilized Pulverized Base (CSPB) or Lime Stabilized Pulverized Base (LSPB) where all of the asphalt pavement section and a predetermined amount of underlying materials are treated to produce a stabilized base course.
What? Lime is an additive that is mixed into asphalt pavements in order to enhance durability.
Why? The addition of lime in Hot Mix Asphalt (HMA) mixtures has significant benefits. It has proven to be a very effective additive in laboratory testing and field results. By including mixtures with lime, it provides several advantages:
- Improved resistance to moisture damage
- Acts as a mineral filler which stiffness the asphalt binder and HMA
- Improves resistance to fracture growth caused by low temperatures
- Alters plastic properties of clay fines to improve moisture stability and durability
- Reduces deleterious effects therefore reducing oxidation and rate of aging
- Increases stability and durability of mixtures
When? Lime can be mixed into new HMA mixtures or into pulverized base layers resulting from FDR processes.