While there are an infinite number of questions that can be asked, we compiled a list of those questions that have been directed to us the most.
These FAQs are categorized into subject areas listed in the contents dropdown box below.

We tried to keep both the questions and answers concise. Additional information is referenced where applicable for those seeking more in-depth information on a given subject.
For further information see the other engineering areas, Asphalt magazine and APA websites, Asphalt Institute online store and the links page for other information related to these topic area.

We also recommend that you attend our Asphalt Academy courses at sites throughout the country to obtain expert instruction on asphalt topics.

Maintenance & Rehabilitation FAQs Contents
DRIVEWAY CONSTRUCTION
Q. Should construction crews be allowed to pave in the rain?
A.

This common question can mean different things to different people because of the wide range of precipitation encompassed by the word “rain.” On one end, occasional light sprinkles should not be cause to shut down operations. However, a steady downpour, either light or heavy, should result in cessation of paving activities. To avoid waste, some states have verbiage in their specifications stating that trucks in route to the project when rain begins can be laid at the contractor’s risk.

Also keep in mind that the surface on which you are paving may influence your decision. Paving on a firm, stable, well-draining crushed aggregate base might be given more leeway than a thin asphalt overlay. Raining or not, new pavement must be placed on a firm, unyielding base.

Critical ideas to keep in mind when dealing with rain:

  • rain will cool the asphalt mix and could make obtaining proper compaction more difficult
  • the asphalt lifts must be able to properly bond together and moisture can be a hindrance to that bond
  • puddles overlaid with HMA turn to steam, which may cause stripping (separation of the asphalt binder from the aggregate) – never pave over puddles whether it is raining or not

If you temporarily suspend paving operations due to rain, don’t forget to:

  • keep all trucks tarped
  • construct a vertical-faced construction joint
  • properly dispose of all material left in the hopper
  • be careful not to track mud and dirt onto the project

Asphalt pavements are designed to last for many years, so don’t let a sense of urgency to get the job done quickly allow you to make decisions which could strip years away from the pavement life.

Q. What does the Asphalt Institute say regarding pavement preservation and maintenance, including timing, distresses, materials, crack sealing, patching, and the various surface treatments options?
A. The Asphalt Institute’s MS-16, Asphalt in Pavement Preservation and Maintenance, is our official publication on these topics.
Q. What do I need to know about building my asphalt driveway?
A. The documents below are good references for asphalt driveways and parking lots.  Regarding asphalt thicknesses for driveways, the first two documents are specifically about driveways, while the two documents on parking lots can also be referenced by assuming a low level of truck traffic for a driveway.

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SEAL-COATS
Q. What does the Asphalt Institute say regarding pavement preservation and maintenance, including timing, distresses, materials, crack sealing, patching, and the various surface treatments options.
A. The Asphalt Institute’s MS-16, Asphalt in Pavement Preservation and Maintenance, is our official publication on these topics.
Q. Should a newly paved driveway or parking lot be sealed (or seal-coated)?
A.

No. A well-designed and constructed low traffic volume pavement, such as a driveway or parking lot, should not require sealing for approximately 2 to 5 years – depending on severity of climate and quality of original work.

If a new pavement is porous, meaning it allows water into the pavement rather than shedding, or draining off, the rain, this pavement might benefit from a light application of a low viscosity asphalt emulsion. See Application Instructions below.

Q. When should a driveway or parking lot be sealed?
A. Sealing is effective to renew old asphalt surfaces that have become dry and brittle with age, to seal small surface cracks and surface voids, and to inhibit raveling (loss of surface aggregate). So, sealing should be done as soon as any of these distresses are noted.
Q. How should driveways and parking lots be sealed?
A.

A light application of a slow-setting asphalt emulsion diluted with water should be applied. In most cases, a dilution of one part emulsion to one part water is used. SS-1, SS-1h, CSS-1, or CSS-1h asphalt emulsions are typically used. Commercial sealers are also available. Those containing coal-tar compounds are not recommended. The diluted material is sprayed or squeegeed onto the surface in a thin, uniform coating. The total quantity of diluted sealant normally applied is 0.1 to 0.15 gallons per square yard. Exact quantities should be based on the surface texture, dryness, and degree of cracking or raveling.

Excess application must be avoided since this may result in pick-up on shoes or tires. Vehicles must be kept off the seal until it is absorbed into the existing surface.

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CHIP SEALS
Q. What structural value does a chip seal add to a pavement?
A. None
Q. When is the optimum time or season for chip seals?
A. Most agencies have a beginning and cut-off date which varies with their climatic regions. Chip seals are ideally placed when the pavement surface temperature is 70o F or more.This temperature range must be maintained for the following 3-5 days for proper setting and curing. Humidity also affects the curing process. A good policy is to seal in the summer months, June, July and August.
Q. Can chip seals be of multiple applications?
A. Yes – Chips seals are commonly placed as a double or triple treatment on new construction with single applications used as maintenance or re-sealing.
Q. What type of equipment is needed for this application?
A. A distributor, a chip spreader, self-propelled pneumatic tired roller (at least one, in larger projects up to three), haul trucks, and a power broom for cleaning the surface and sweeping the loose chips from the roadway within 24 hours of application.
Q. What are the benefits of a chip seal?
A. Chips seals water-proof the roadway surface, provide uniform surface texture and skid resistance, and seals minor cracks, which prolong pavement life.
Q. What is the average life of chip seal?
A. Five to eight years, depending on the traffic and environmental conditions.
Q. Why do the chips not adhere to the asphalt?
A. Several factors could be involved: The most common mistake in chips seals is – too much aggregate and not enough asphalt. For aggregate to adhere, it must be adequately embedded into the asphalt. Other factors are:

  • Weather too coolChips rolled too lateChips were dirtyChips were excessively wet
  • Traffic was not properly controlled for the following 24 hours after application
  • Rain occurred before the asphalt fully setAsphalt was incompatible with aggregateDemulsibility on emulsion was too lowMaterial failed to break and set adequately
  • Highly absorptive aggregates.
Q. Do the chips need to be clean?
A. Yes – AASHTO T-11 Dust ratio should be less than 0.75.
Q. What is the best size?
A. Single size chips are preferred; however, good seals can be obtained with well graded aggregates. Common chip sizes range from 1/4 to 3/4 inches, depending on the purpose of the seal.
Q. How soon should the chips be spread after the asphalt application?
A. Immediately
Q. What is a Cape seal?
A. This is a slurry seal placed over a chip seal. The slurry seal can be placed anytime after the chip seal has set and cured.
Q. What type asphalt should be used?
A. A liquid asphalt, such as a Rapid Setting Emulsion (RS-1,2 or CRS-1,2 includes modified), Cutback asphalts in some areas depending on EPA regulations which would include RC-250, 800 or 3000, are normally used. Highly skilled crews could also use an AC-5 or 10.
Q. How much asphalt should be applied for the aggregate? (chips)
A. The amount of asphalt applied depends on three factors:

  1. The existing surface condition, The amount of traffic, and
  2. The average particle size of the chips. Allowance should be made for surface conditions – dry, pocked, badly cracked, flushed, bleeding, etc. Lower traffic volumes require higher asphalt applications than higher traffic. The average particle size should be embedded 60-75% into the asphalt. Higher traffic should be closer to the 60% and lower traffic should be closer to the 75% embedment factor. The average particle size is the average size of chip in the gradation, the 50% passing size can be used for this number.
Q. What causes roadway streaking in chip seals?
A. Several factors can lead to this appearance; improper distributor nozzle sizes, pump pressure, spray bar height, angle of nozzle, and cold asphalt.
Q. Is it acceptable to apply asphalt full width and chips half width?
A. No, the uncovered asphalt will cool below the acceptable application temperature and the chips will not adhere to the asphalt.
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CRACK SEALING
Q. What is the make-up of a crack sealant.
A.

All hot-pour sealants have an asphalt cement base.  Manufacturers of sealants modify them with rubber and other materials.

An asphalt emulsion sealant (which is not heated) is an asphalt cement which has been liquified using water and an emulsifying agent to allow the water and asphalt to mix.

Q. Can crack sealing precede a seal coat or hot mix overlay? My concern is that the crack seal material could have a negative reaction with the new surface material.
A. Crack Sealing prior to rehabilitation is a good idea. The majority of complaints concerning crack sealer problems arise when an excess of material is left on the surface of the pavement either due to overfilling or expansion of the sealant. The best method to treat cracks is to route a vessel 5/8″ x 5/8″ or 3/4″ x 3/4″ and use a modified joint sealer that meets ASTM 3405 or ASTM 3405-modified specifications, and be careful not to overfill the joint. The top of the sealant should be left about 3 to 6 mm (1/8″ to 1/4″) below the top of the crack. For more details on proper crack sealing procedures and all maintenance methods, please refer to MS-16, Asphalt in Pavement Maintenance.
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LIFT THICKNESS
Q. Should construction crews be allowed to pave in the rain?
A.

This common question can mean different things to different people because of the wide range of precipitation encompassed by the word “rain.” On one end, occasional light sprinkles should not be cause to shut down operations. However, a steady downpour, either light or heavy, should result in cessation of paving activities. To avoid waste, some states have verbiage in their specifications stating that trucks in route to the project when rain begins can be laid at the contractor’s risk.

Also keep in mind that the surface on which you are paving may influence your decision. Paving on a firm, stable, well-draining crushed aggregate base might be given more leeway than a thin asphalt overlay. Raining or not, new pavement must be placed on a firm, unyielding base.

Critical ideas to keep in mind when dealing with rain:

  • rain will cool the asphalt mix and could make obtaining proper compaction more difficult
  • the asphalt lifts must be able to properly bond together and moisture can be a hindrance to that bond
  • puddles overlaid with HMA turn to steam, which may cause stripping (separation of the asphalt binder from the aggregate) – never pave over puddles whether it is raining or not

If you temporarily suspend paving operations due to rain, don’t forget to:

  • keep all trucks tarped
  • construct a vertical-faced construction joint
  • properly dispose of all material left in the hopper
  • be careful not to track mud and dirt onto the project

Asphalt pavements are designed to last for many years, so don’t let a sense of urgency to get the job done quickly allow you to make decisions which could strip years away from the pavement life.

Q. What is the proper thickness of lift that should be used?
A.

Minimum lift thickness should be at least 3 times the nominal max. aggregate size to ensure aggregate can align themselves during compaction to achieve required density and also to ensure mix is impermeable. The maximum lift thickness is dependent also upon the type of compaction equipment that is being used. When static steel-wheeled rollers are used, the maximum lift thickness that can be properly compacted is three (3) inches. When pneumatic or vibratory rollers are used, the maximum thickness of lift that can be compacted is almost unlimited. Generally, lift thicknesses are limited to 6 or 8 inches. Proper placement becomes a problem in lifts thicker than 8 or 8 inches.

For open-graded mixes, compaction is not an issue since it is intended that these types of mixes remain very open. Therefore, the maximum size aggregate can be as much as 80 percent of the lift thickness.

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PRIME COATS
Q. What is a prime coat?
A. An application of a low viscosity asphalt to a granular base in preparation for an asphalt surface course.
Q. What is the purpose of a prime coat?
A.
  • To coat and bond loose material particles on the surface of the base.
  • To harden or toughen the base surface to provide a work platform forconstruction equipment
  • To protect the base from moisture.
  • To provide adhesion between the base course and the succeeding course.
Q. What asphalt materials should be used for prime coats?
A.

For a prime coat to be effective it must be able to penetrate into the base course. Usually a light grade of medium curing cutback such as an MC-30 will work well. However, in a lot of areas air quality is of concern and the EPA has restricted or eliminated the use of cutbacks. In such areas the use of an emulsified asphalt is necessary.

There are several ways to accomplish a prime when using an emulsion:

First: Most emulsion manufacturers make proprietary products, one of which is an emulsion specifically designed for use in prime coats.

Second: If the granular base material has a gradation that is somewhat porous, placing a prime coat can often be affected by placing a slow-setting emulsion (SS-1, SS-1 h, CSS-1, CSS-1 h) diluted 5 parts water to 1 part emulsion. By applying several (4 or 5) light applications (0.10 gal/sy), a waterproof surface can be obtained on the base course.

Third: Incorporate an emulsion into the compaction water while placing the last 2 to 3 inches of the base course. Use a dilution and application rate which will provide 0.1 to 0.3 gallon per square yard (3:1 dilution; 4 applications; 0.15 gal/sy rate).

Fourth: Complete placement of the base course material, then scarify up about 3/4 inch. Apply about 0.20 gal/sy 2 of straight emulsion (undiluted) and blade mix it with the scarified material. Then relay the mixed material and compact.

Q. Is a prime coat necessary?
A. At one time it was thought that a prime coat was an essential element of good pavement construction. However, in recent years some engineers have eliminated the use of a prime, especially when asphalt layer(s) (surface and/or base) is 4 inches or more in thickness. In many instances, prime coats have not been used even when surface thickness have been as thin as 2 inches. Over the past 20 years, few, if any, pavement failures can be attributed to the lack of prime coat.
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TACK COATS
Q. Why is a tack coat needed?
A. To ensure a bond between the succeeding layers of a pavement.
Q. What material should be used for a tack coat?
A. A slow-setting emulsion, either SS-1, CSS-1, SS-I h, or CSS-1 h, works well when diluted 50/50 with water.
Q. What application rate should be used?
A.

You want to accomplish a very uniform application of about 0.03 to 0.05 gal/sy of residual asphalt on the layer to be tacked (a paint job, so to speak). Slow-setting emulsions generally have a residual asphalt content of about 2/3. Therefore, an application rate of 0.10 to 0.15 gals/sy of the diluted material will give you the 0.03 to 0.05 gals/sy.Caution #1: Once the tack coat is applied, time must be allowed for emulsion to break (turn from brown to black) prior to placing hot mix on it. The length of time required for this to happen will depend on the weather. In good paving weather, it will take only a few minutes. In marginal weather it may take several minutes.Caution #2: Never apply an emulsion tack coat to a cold pavement (below the freezing point). The emulsion will break, but the water and emulsifying agents will freeze and remain in the layer that has been tack coated.

If either of these cautions is violated, there is a good chance that upper layer will not bond to the under layer and a slip plane will develop.

Q. When is a tack coat necessary?
A. Almost always! On rare occasions when a pavement is being constructed which is not being used by traveling public and each succeeding lift is placed in rapid succesion, a tack coat may not be necessary. However, a good cheap insurance policy is to always use tack coats.
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MIX RELEASE AGENTS
Q. What should be used as a mix release agent for truck beds and rollers?
A.

Far too often we still see diesel fuel used as a mix release agent. Diesel fuel is a solvent. Any excess amount will dissolve the asphalt films on the aggregate particles, thus contaminating the mix. Commercial mix release agents are readily available and should be used. They generally are soap or emulsified wax or other stick-resistant materials that do not contaminate the mix. A couple of suggestions are a bag of hydrated lime mixed with 1000 gallons of water or a bottle of dish soap mixed with water. The portions depend on the water with which it is mixed. Soft water won’t need nearly as much as hard water.

It has been our experience that a special release agent is required for modified asphalts. Contact your local State Department of Transportation for a list of approaved release agents.

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PLACEMENT
Q. Should construction crews be allowed to pave in the rain?
A.

This common question can mean different things to different people because of the wide range of precipitation encompassed by the word “rain.” On one end, occasional light sprinkles should not be cause to shut down operations. However, a steady downpour, either light or heavy, should result in cessation of paving activities. To avoid waste, some states have verbiage in their specifications stating that trucks in route to the project when rain begins can be laid at the contractor’s risk.

Also keep in mind that the surface on which you are paving may influence your decision. Paving on a firm, stable, well-draining crushed aggregate base might be given more leeway than a thin asphalt overlay. Raining or not, new pavement must be placed on a firm, unyielding base.

Critical ideas to keep in mind when dealing with rain:

  • rain will cool the asphalt mix and could make obtaining proper compaction more difficult
  • the asphalt lifts must be able to properly bond together and moisture can be a hindrance to that bond
  • puddles overlaid with HMA turn to steam, which may cause stripping (separation of the asphalt binder from the aggregate) – never pave over puddles whether it is raining or not

If you temporarily suspend paving operations due to rain, don’t forget to:

  • keep all trucks tarped
  • construct a vertical-faced construction joint
  • properly dispose of all material left in the hopper
  • be careful not to track mud and dirt onto the project

Asphalt pavements are designed to last for many years, so don’t let a sense of urgency to get the job done quickly allow you to make decisions which could strip years away from the pavement life.

Q. How do I determine how much asphalt is required for a project?
A.

Here’s the process for a 10’x25′ area and a 4″ mat: 1. Calculate the number of cubic feet to be paved. (Remember to convert the thickness to feet – by dividing by 12 inches per 1 foot). 10′ x 25′ x (4/12)’ = 83.3 cubic feet of HMA 2. Asphalt Mixture typically weighs from 142 to 148 pounds per cubic foot (PCF) in-place. Use 148 PCF. 3. Calculate the tonnage needed. (remember to convert from pounds to tons; 2000 pounds per ton).

83.3 cubic feet x 148 PCF = 12328 pounds of mix = 12328 / 2000 tons = 6.1 tons

Q. What is the proper paver speed?
A. Paver speed should be geared to mix production, delivery and compaction; with emphasis placed on compaction. Every effort should be made to maintain a constant paver speed. Several factors effect that constant speed.  With a consistent production and delivery flow, the speed of the paver will vary with lift thickness (thicker/slower; thinner/faster) and width of paver pass wider/slower; narrow/faster). Most equipment manufacturers will give a suggested maximum speed for their paver. A lot of agency specifications will specify a maximum speed, such as 30 or 40 feet per minute. Most compaction manufacturers recommend a maximum roller speed of 3 mph and most often more than one roller pass is needed to get compaction. Therefore, the number and type of rollers being used is very important.
Q. Is it ok to cool down the laid mat immediately using water for early traffic?
A. We do not recommend spraying water on freshly laid hot mix asphalt (HMA) in order to cool the mat faster and open to traffic sooner.  First, spraying water on the hot mat is not very effective since the water should drain properly on a new surface and only cools the crust temporarily, with the internal HMA temperature not being affected much.  In addition, there is a concern that the water could cause a foaming effect with the hot asphalt binder, making the HMA less stable under traffic.  We believe it is best to let the hot mat cool naturally.
Q. What is acceptable in terms of standing water or “ponding” on parking lots and other asphalt pavements?
A. Standing water should be avoided, thus the Asphalt Institute recommends a transverse slope of between 1.5 to 3.0% on all pavement surfaces, and an even steeper slope of 3 to 6% on shoulders.  Maintaining a slope of at least 1.5% on parking lots will ensure proper surface drainage (no ponding or birdbaths) and minimize infiltration, hydroplaning and the detrimental effects of water.
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COMPACTION
Q. Should construction crews be allowed to pave in the rain?
A.

This common question can mean different things to different people because of the wide range of precipitation encompassed by the word “rain.” On one end, occasional light sprinkles should not be cause to shut down operations. However, a steady downpour, either light or heavy, should result in cessation of paving activities. To avoid waste, some states have verbiage in their specifications stating that trucks in route to the project when rain begins can be laid at the contractor’s risk.

Also keep in mind that the surface on which you are paving may influence your decision. Paving on a firm, stable, well-draining crushed aggregate base might be given more leeway than a thin asphalt overlay. Raining or not, new pavement must be placed on a firm, unyielding base.

Critical ideas to keep in mind when dealing with rain:

  • rain will cool the asphalt mix and could make obtaining proper compaction more difficult
  • the asphalt lifts must be able to properly bond together and moisture can be a hindrance to that bond
  • puddles overlaid with HMA turn to steam, which may cause stripping (separation of the asphalt binder from the aggregate) – never pave over puddles whether it is raining or not

If you temporarily suspend paving operations due to rain, don’t forget to:

  • keep all trucks tarped
  • construct a vertical-faced construction joint
  • properly dispose of all material left in the hopper
  • be careful not to track mud and dirt onto the project

Asphalt pavements are designed to last for many years, so don’t let a sense of urgency to get the job done quickly allow you to make decisions which could strip years away from the pavement life.

Q. How many rollers are required?
A.

Contrary to popular belief, the number of rollers required for proper compaction is based on the square yardage placed rather than the production or delivery tonnage. Roller speed is typically limited to 3 mph. With this speed and the width of the roller, the coverage rate can be calculated. The width of paver pass and speed can give you the square yardage placed. The number of required coverages will then tell you the total area in square yards the roller must be able to cover. On very small jobs, one roller may be adequate. On very large projects, six or eight rollers may be needed. A lot of projects are compacted with three rollers: a breakdown roller, a compaction roller, and a finish roller. On most average projects, two rollers are used – a vibratory steel-wheeled roller for breakdown and compaction, and a heavy static steel wheel for finish rolling.

Occasionally, agency specifications will require a light (65 to 75 psi contact pressure) pneumatic roller to be used to knead or seal the surface prior to the finish rolling.

Q. How is air void content controlled?
A.

Air voids is a reverse proportion of the density of the compacted mix. By specifying a density requirement, the voids are inversely controlled. Keep in mind that density is a relative term, compared to a target density of either lab compacted mix, a maximum theoretical density, or a control strip density.

Procedures for using the three methods are spelled out on 7-17 to 7-21 of the new MS-22 and Page 241 of the old MS-22.

Q. What is the recommended air void content for compaction of asphalt pavements?
A. Efforts should be made to control compacted air voids between 7% and 3%. At 8% or higher, interconnected voids which allow air and moisture to permeate the pavement, reducing its durability. On the other hand, if air voids fall below 3%, there will be inadequate room for expansion of the asphalt binder in hot weather. When the void content drops to 2% or less, the mix becomes plastic and unstable.
Q. What should compaction requirements be?
A.

Testing should be done on a random sampling basis with a minimum of five tests per lot (agency requirements define a “lot” as “A day’s or full day’s production”). The average of the five density determinations should be equal to or greater than: 1) 96% of lab density with no test less than 94% 2) 92% of maximum theoretical with no test less than 90%

3) 99% of the control strip density

Q. What is the best way to check density?
A.

Nuclear gauges are generally used for density testing because of the ease and speed with which the testing can be done. This allows for many more tests – more than the five minimum for a better statistical result.

Caution : The nuclear density gauge needs to be correlated to core densities that are taken from the same location as was nuclear gauge tested. This should be done for each different mix that might be used.

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PLANT OPERATIONS
Q. How can you tell that a mix is properly mixed?
A.

When all the aggregate particles are coated with asphalt. The large aggregate particles are always the last to be coated. If the large aggregate particles are completely coated, the mix is properly mixed. Generally we see mixing problems only with batch plants. The producer is trying to mix each batch as quickly as possible (probably in about 30 seconds) which may or may not be adequate mixing time. Typical specifications set minimum coated particle percentages at 90 to 95 percent. The Ross Count procedure for determining these percentages (ASTM-D2489 or AASHTO T195) is outlined on pages 4-41 to 4-44 of MS-22. Minimum mixing times to meet the specified requirement should be carefully adhered to in order to avoid excess oxidation of the asphalt films on the aggregate particles as it is exposed to air (oxygen) during the mixing process. As a general rule we do not see this problem with drum mixes. The mix remains in the mixing portion of the drum for much longer periods of time (maybe 2 to 3 minutes) than in the pugmill of a batch plant, so the aggregate particles get very well coated. Keep in mind that we are not as concerned about oxidation in drum mixes as the mixing portion of the drum mixer is essentially an oxygen-free atmosphere.

Another way to look at it is this: In a 6000 lb. batch of mix, there are about 5600 lbs. of aggregate and about 400 lbs. of asphalt. Dense-graded aggregate has about 35 sq. ft. of surface area per pound, or 196,000 sq. ft/6000 lb. batch; 400 pounds of asphalt is about 48 gallons. The mixing process has to take 48 gallons of asphalt and paint about 3.8 football fields. When the aggregate particles are coated, it’s mixed.

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ASPHALT MIXTURES
Q. What is the proper mix temperature?
A.

Mix temperature will be dependent on the grade of asphalt used in the mix. The less viscous the asphalt, the lower the temperatures should be. The more viscous the asphalt, the higher the temperature can be. During mix design temperatures are specified for proper mixing and for compaction. These are good targets with which to start a project. However, they will have to be adjusted for the project conditions (weather, haul distances, etc.). If at all possible, avoid discrepancies from the mix design temperature of more than 25 degrees.

Note: When working with modified binder, the binder supplier should provide mix temperature recommendations.

Q. What is a minimum temperature for asphalt mixes?
A.

Mixes must be placed and compacted before they cool to 185o F, so the minimum temperature will depend on the temperature of the layer upon which it is being placed as well as ambient conditions. Temperature session charts are shown on Page 6-6, Fig 6.03 of the new MS-22 and Page 234 of the old MS-22.

Generally, agency specifications will spell out a minimum acceptable temperature for the mix. Some specifications will use 225o F, and others may use 250o F.

The advent of various Wam Mix Asphalt technologies have lowered temperatures at which mixtures remain workable. Therefore consultation with technology manufacturers is recommended when warm mix is used.

Q. How do I ensure HMA is impervious to water?
A. Conventional mixes should be impervious to water as long as the total in-place air void content is below 7 to 8%. Mixes with void contents higher than this can be pervious to air and water leading to premature aging and raveling.
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INTERSECTIONS
Q. How do you design a good quality asphalt instersection?
A.

The tools now exist to gain improved performance from HMA intersections. Well-designed, properly constructed HMA intersections provide an economical, long-lasting pavement with minimal disruption to traffic.

In order to achieve these benefits, we must recognize that intersection pavements are subject to extreme stresses. Ordinary materials and techniques may not be sufficient. There must be adequate pavement structure, select materials, appropriate construction techniques, and careful attention to detail in the process.

To learn more about how to design and build high performance HMA intersections see the following series of ASPHALT magazine articles.

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WEATHER
Q. Should construction crews be allowed to pave in the rain?
A.

This common question can mean different things to different people because of the wide range of precipitation encompassed by the word “rain.” On one end, occasional light sprinkles should not be cause to shut down operations. However, a steady downpour, either light or heavy, should result in cessation of paving activities. To avoid waste, some states have verbiage in their specifications stating that trucks in route to the project when rain begins can be laid at the contractor’s risk.

Also keep in mind that the surface on which you are paving may influence your decision. Paving on a firm, stable, well-draining crushed aggregate base might be given more leeway than a thin asphalt overlay. Raining or not, new pavement must be placed on a firm, unyielding base.

Critical ideas to keep in mind when dealing with rain:

  • rain will cool the asphalt mix and could make obtaining proper compaction more difficult
  • the asphalt lifts must be able to properly bond together and moisture can be a hindrance to that bond
  • puddles overlaid with HMA turn to steam, which may cause stripping (separation of the asphalt binder from the aggregate) – never pave over puddles whether it is raining or not

If you temporarily suspend paving operations due to rain, don’t forget to:

  • keep all trucks tarped
  • construct a vertical-faced construction joint
  • properly dispose of all material left in the hopper
  • be careful not to track mud and dirt onto the project

Asphalt pavements are designed to last for many years, so don’t let a sense of urgency to get the job done quickly allow you to make decisions which could strip years away from the pavement life.

Q. Can asphalt be applied in the rain (light drizzle)?
A. It is not avisable to start paving if it is raining. If rain starts after paving has begun, the work can continue as long as there is no standing water and the rain is not too hard. The primary concern is achieving adequate compaction, as the mix will cool much faster due to evaporative cooling if laid on a wet surface or rain falls on an uncompacted mat. Additional compactive effort will be needed and monitoring temperatures is key to acheiving adequate density.
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DE-ICING CHEMICALS
Q. Do de-icing salts or calcium magnesium acetate (CMA) have a detrimental affect on asphalt?
A.

Research has proven that well-designed and well-constructed asphalt pavements are not damaged by sodium and calcium chloride salts used for ice and snow control. The same is true for calcium magnesium acetate (CMA), which is often used as an alternative to road salt. References include: Performance of Asphalt Pavements Subjected to De-Icing Salts , B.F. Kallas, Highway Research Record #24, 1963. Effects of Calcium Magnesium Acetate on Pavements and Motor Vehicles , D.S. Slick, Transportation Research Record #1157, 1988.

Colorado DOT Research Report 99-2
AAPT Report 05-03

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TROUBLESHOOTING
Q. Should construction crews be allowed to pave in the rain?
A.

This common question can mean different things to different people because of the wide range of precipitation encompassed by the word “rain.” On one end, occasional light sprinkles should not be cause to shut down operations. However, a steady downpour, either light or heavy, should result in cessation of paving activities. To avoid waste, some states have verbiage in their specifications stating that trucks in route to the project when rain begins can be laid at the contractor’s risk.

Also keep in mind that the surface on which you are paving may influence your decision. Paving on a firm, stable, well-draining crushed aggregate base might be given more leeway than a thin asphalt overlay. Raining or not, new pavement must be placed on a firm, unyielding base.

Critical ideas to keep in mind when dealing with rain:

  • rain will cool the asphalt mix and could make obtaining proper compaction more difficult
  • the asphalt lifts must be able to properly bond together and moisture can be a hindrance to that bond
  • puddles overlaid with HMA turn to steam, which may cause stripping (separation of the asphalt binder from the aggregate) – never pave over puddles whether it is raining or not

If you temporarily suspend paving operations due to rain, don’t forget to:

  • keep all trucks tarped
  • construct a vertical-faced construction joint
  • properly dispose of all material left in the hopper
  • be careful not to track mud and dirt onto the project

Asphalt pavements are designed to last for many years, so don’t let a sense of urgency to get the job done quickly allow you to make decisions which could strip years away from the pavement life.

Q. What causes tire scuffing and what should be done about it?
A. See the following pdf document: Position Paper – Tire Scuffing & Indentations by the Ontario Hot Mix Producers Association
Q. Will an oil leak from a car hurt an asphalt surface?
A.

The “glue” in an asphalt pavement mixture is referred to as asphalt cement.  Since asphalt cement is a petroleum product, it can be dissolved by many other petroleum-derived products, such as gasoline and car oil.  Therefore, in localized areas where extensive fuel spillage is likely, it may be advisable to seal the pavement with a commercially-available proprietary sealer that is impervious to petroleum solvents.

Protection of Hot Mix Asphalt (HMA) pavements from damage due to fuel spills or oil leaks has long been recognized as an important component of any airport pavement maintenance plan. Aircraft fuels, hydraulic fluids and most lubricating oils are produced by refining crude oil. Asphalt cement used in the construction of HMA pavements is also a product of the crude oil refining process. As such, jet fuel, oil, and asphalt are chemically compatible and readily mix with each other. This can cause a softening of the asphalt binder that can result in a degradation of the HMA pavement surface.

AAPTP Report 05-02, Fuel Resistant Sealers and Binders for HMA Airfield Pavements , provides valuable information that addresses this issue.  It can be found at www.aaptp.us

Q. Why does the mat have a rich shiny strip down the middle with dull, torn-looking edge strips?
A. The paver screed has too much lead crown in it.
Q. What causes the mat to have rich shiny strips on each side and a dull, torn look in the middle?
A.

The paver screed does not have enough lead crown in it.

Note : Paver screeds should have slightly more crown in the leading edge than in the trailing edge – usually about 1/8 inch. This may vary with equipment manufacturer and/or width of paver pass. Even if the trailing edge of the screed is to place a flat or straight grade, the leading edge must still have the increased crown.

Q. Why do potholes occur and what is the best way to fix them?
A. A very good reference on causes of potholes and the best way to fix them is our MS-16 manual, Asphalt In Pavement Maintenance. This manual discusses all the various asphalt pavement distress types and the proper repair methods. See also Pavement Distress and Repair Chart.
Q. What might cause surface cracking on newly placed asphalt concrete? The cracking occurred during the breakdown rolling and finish rolling.
A. Without knowing what the surface cracking looks like, it is hard for us to identify the problem. Could the “surface cracking” be check cracking from the rolling operation? “Checking” is the development of shallow hairline surface cracks spaced an inch or two apart from each other and running transverse to the direction of rolling. The cause is rolling when the mat too hot and/or the mix is too tender. You can reference our page 6-6 of the new MS-22 manual Construction of Quality Hot Mix Asphalt Pavements and page 219 & 220 of the old MS-22 if you are not sure what check cracking is.
Q. How do I ensure HMA is impervious to water?
A.

Conventional (Marshall or Hveem) mixes should be impervious to water as long as the total in-place air void content is below 8%. Conventional mixes with void contents higher than 8% can be pervious to air and water leading to premature aging and raveling.

There is a growing body of evidence that coarse-graded Superpave mixes do not become impervious to water until the total in-place air void content is lower than the 8% rule-of-thumb for conventional mixes. Additional research is being conducted on this subject to more adequately define the in-place density requirements to attain an impermeable mix using coarse-graded Superpave mixes.

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SPECIAL APPLICATIONS
Q. Does AI have any recommendations of an asphaltic concrete sealer?
A. Information on fuel-resistant asphalt sealers can be found at www.aaptp.us with Report 05-02.
Q. Is there a way to color an asphalt pavement other than shades of black and grey?
A. While not widely used, there are ways to color an asphalt pavement other than the common blacks and greys. The second and third options are considered specialty products and more information can be obtained by contacting individual manufacturers.

  • Use a naturally colored aggregate. As the asphalt binder wears way from the surface with traffic, the color of the aggregate is exposed.
  • Use an additive in the asphalt binder. Various iron compounds can impart a red, green, yellow or orange tint to a pavement, while other colors can be achieved using different metal additives. A special “synthetic” binder that contains no asphaltenes has been used because it takes color more readily. This method of tinting the mix allows color to permeate the entire depth of the material, so there are no surface wear-off concerns.
  • Coat the surface with a material that penetrates the voids and bonds well to asphalt pavement, such as an epoxy-fortified acrylic emulsion. Many colors are available. Care should be taken to ensure that surface friction is not compromised, especially if the pavement is used for vehicular traffic.  One possible disadvantage of this method is that the surface may wear off with time and need to be renewed.
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Railroads
Q. Does AI have any information about asphalt and its use in railroad beds?
A. Railroad information can be found in the Construction, Thickness and Design, Maintenance and Rehabilitation, and Pavement Performance Documents pages.
You can also visit a web page on the University of Kentucky website where you can download papers, PowerPoints and also the computer program called KENTRACK, which is computer program for hot mix asphalt and conventional ballast railway trackbeds.
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