Asphalt Pavement Construction

An application of a low viscosity asphalt to a granular base in preparation for an asphalt surface course.

• 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 for construction equipment.
• To plug capillary voids in the base course surface to prevent migration of moisture.
• To provide adhesion between the base course and the succeeding course.

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:

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

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.

To ensure a bond between the succeeding layers of a pavement.

A slow-setting emulsion, either SS-1, CSS-1, SS-I h, or CSS-1 h, works well when diluted 50/50 with water.

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.

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.

No rule of thumb answers your question, but two issues should be considered:

1. Is the pavement structure (subgrade, subbase, base, and all asphalt layers) adequate to support the loads? You need to purchase our MS-23 Manual, Thickness Design of Asphalt Pavements for Heavy Wheel Loads.
2. Is the hot mix asphalt surface stiff enough to resist deformation (ruts or indentations)? This is dependent on many factors, such as stiffness of the original mixture, age of the mix (gets stiffer over time), temperature of the mix during loading, loading itself, duration of applied load, etc. While not usually a problem, when it occurs it can typically be resolved by placing some steel (or other rigid material) plates below the point load to distribute the load across a wider area.

Yes. However, since Superpave mixes tend to be coarser and contain modified binders than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used. Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low-void, low-permeability seam.

Generally speaking, there are no unique problems with using polymer modified mixes as RAP. Some individuals express environmental concerns about running millings containing ground tire rubber (GTR) through a drum plant. Florida uses a small percentage of GTR on most of their highway surface mixes. California and Arizona also use GTR frequently.

Mix temperature is dependent on the grade of asphalt used in the mix: Less viscous asphalt requires lower temperatures, while more viscous asphalt requiers higher temperatures. At the start of a mix design, target temperatures are specified for proper mixing and compaction. These temperatures should be adjusted for 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.

Mixes must be placed and compacted before they cool to 185^o F, so the minimum temperature will depend on the temperature of the layer upon which it is being placed as well as ambient conditions. Generally, agency specifications will spell out a minimum acceptable temperature for the mix. Some specifications will use 225^o F, and others may use 250^o F.

Conventional mixes should be impervious to water as long as the total in-place air void content is below 7 to 8%. Mixes with higher void contents can be pervious to air and water leading to premature aging and raveling.

The Asphalt Institute strongly endorses the use of RAP in asphalt mixtures. RAP has a history of positive performance. The specifying agency or owner will set the limit for RAP content. Almost all state highway departments now allow the use of RAP. A few restrict its use in wearing courses; even fewer (one or two) prohibit its use completely. Most agencies have developed a means of accomodating the stiffness of the reclaimed asphalt from the RAP by the selection of the particular grade of the virgin binder. The FHWA Asphalt Mixture Expert Task Group developed recommendations that are being considered by the Association of State Highway and Transportation Officials (AASHTO) to provide guidance in asphalt binder grade selection when using RAP. These recommendations are summarized below.

• When 15% or less RAP is used: “The binder grade for the mixture is selected for the environment and traffic conditions the same as for a virgin mix. No grade adjustment is made to compensate for the stiffness of the asphalt in the RAP”.
• When 16 to 25% RAP is used: “The selected binder grade for the new asphalt is one grade lower for both the high and low temperature stiffness than the binder grade required for a virgin asphalt. For example, if the specified binder grade for the virgin mix is a PG 64-22, the required grade for the recycled mix would be a PG 58-28”.
• When more than 25% RAP is used: “The binder grade for the new asphalt binder is selected using an appropriate blending chart for high and low temperature. The low temperature grade is one grade lower than the binder grade required for a virgin asphalt”.

Normally, the above guidelines would be applied to both new and existing pavements. If a warranty was applied to a project, a more conservative approach – such as the use of blending charts – might be taken.

It is suggested that you contact the local state highway agency and/or asphalt binder supplier for the prevailing local practices.

Rice (Gmm) is typically not run on material from cores as it is not the preferred method of material collection for this test. In fact, ASTM D5361, Standard Practice for Sampling Compacted Bituminous Mixtures for Laboratory Testing, does not include Rice testing in its Significance and Use section.

Note paragraph 3.1 from the standard reads: 3.1 Samples obtained in accordance with the procedure given in this practice may be used to measure pavement thickness, density, resilient or dynamic modulus, tensile strength, Marshall or Hveem stability, or for extraction testing, to determine asphalt content, asphalt properties and mix gradation. There are a couple of reasons for this. First, coring is naturally a destructive process which alters the gradation. The level to which the gradation shifts varies with the nature of the parent gradation and material. i.e., a half-inch SMA is likely to see a greater gradation shift then say a fine, dense-graded three-eighths mix. Secondly, and more importantly, by coring you are creating aggregate that is not coated with asphalt. This lack of coating can then allow for water absorption into these non-protected surfaces. Naturally, the more absorptive the aggregate the greater the potential issue with this situation. The AASHTO standard for Rice is T-209. It addresses absorption in part 15 of the standard entitled, “Supplemental Procedure for Mixtures Containing Porous Aggregate.” This is also known as the “dry-back procedure.” It is used on mixes produced with aggregate who’s water absorption is greater than 1.5%.However, while collection of Rice material via cores is not the preferred method, it is an acceptable method when more preferred alternatives (plant or lab produced samples) are not available. I am unaware of any state that does not allow for cores to be used for Gmm when no good alternative is an option. With the previous discussion in mind, one should do what they can to minimize any potential problems that may arise from field-cut specimens. What this leads to is a bigger is better mindset. A 6-inch core will have a smaller percentage of its aggregate affected by the coring than would a 4-inch core from the same road. Therefore, it is highly recommended that if alternative methods of producing materials for Rice are not an option, to use at least a 6-inch core. If a bigger specimen can be collected, such as saw-cutting, then it should be considered. Judgment, and locally acceptable practice, will certainly need to come into play.

Lift thickness governs aggregate size. 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.

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.

Information on fuel-resistant asphalt sealers can be found at www.aaptp.us with Report 05-02.

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.

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.

Mixes must be placed and compacted before they cool to 185^o 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 225^o F, and others may use 250^o F.

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 the new MS-22 and pages 162 and 163 of the old MS-22.

Minimum mixing times to meet the specified requirement should 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.

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 (Joy) 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.

Paver speed should be geared to mix production and delivery. 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 and width of paver pass. Thicker lift – slower speed; thinner lift – faster speed. Wider pass – slower speed; narrower pass – faster speed. 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.

The paver screed has too much lead crown in it.

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 very 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.

The Asphalt Institute strongly endorses the use of RAP in asphalt mixtures. RAP has a history of positive performance. Regarding limiting the RAP content, that is the decision of the specifying agency or owner. Almost all of the state highway departments now allow the use of RAP. A few restrict its use in wearing courses; even fewer (one or two) do not allow its use at all. Most agencies have developed a means of accomodating the stiffness of the reclaimed asphalt from the RAP by the selection of the particular grade of the virgin binder. The FHWA Asphalt Mixture Expert Task Group developed recommendations that are being considered by the Association of State Highway and Transportation Officials (AASHTO) to provide guidance in asphalt binder grade selection when using RAP. These recommendations are summarized below.

• When 15% or less RAP is used: “The binder grade for the mixture is selected for the environment and traffic conditions the same as for a virgin mix. No grade adjustment is made to compensate for the stiffness of the asphalt in the RAP”.
• When 16 to 25% RAP is used: “The selected binder grade for the new asphalt is one grade lower for both the high and low temperature stiffness than the binder grade required for a virgin asphalt. For example, if the specified binder grade for the virgin mix is a PG 64-22, the required grade for the recycled mix would be a PG 58-28”.
• When more than 25% RAP is used: “The binder grade for the new asphalt binder is selected using an appropriate blending chart for high and low temperature. The low temperature grade is one grade lower than the binder grade required for a virgin asphalt”.

Normally, the above guidelines would be applied to both new and existing pavements. If a warranty was applied to a project, a more conservative approach – such as the use of blending charts – might be taken.

It is suggested that you contact the local state highway agency and/or asphalt binder supplier for the prevailing local practices.

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.

Information on fuel-resistant asphalt sealers can be found at www.aaptp.us with Report 05-02.

Here’s the process:

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

Yes. However, since Superpave mixes do tend to be coarser and contain modified binders than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used.Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low void, low permeability seam.

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.

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.

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.

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.

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 should be 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.

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.

Air voids are 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 Page 7-17 to 7-21 of the new MS-22 and Page 241 of the old MS-22.

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

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.

There is not a predictable value or ‘rule-of-thumb number’ for the difference in air void content of original and reheated samples. The general trend would be for the reheated samples to have higher air voids than the original, compacted specimens. Absorption and hardening or stiffening of the asphalt binder in the reheated samples likely causes this difference. Reheated samples can be utilized to give an overall check of the original sample results. Before any significant precision is attributed to reheated sample results, a correlation should be developed for reheated sample air voids and original sample air voids by performing a series of comparative tests.

Without knowing what the surface cracking looks like, it is hard for us to identify the problem. Could the “surface cracking” be checking cracking from the rolling operation? It is 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 on the mat too hot and/or too tender of a mix. You can reference page 6-9 of the new MS-22 and page 219-220 of the old MS-22 manual if you are not sure what checking is.

Yes. However, since Superpave mixes do tend to be coarser and contain modified binders than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used.Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low void, low permeability seam.

There are several ways to establish density targets. Some of the more common approaches include:

• Specifying a percentage of the unit weight from the laboratory mix design. Example: 96% of the Marshall unit weight
• Establishing a value based on results achieved on a project-site test strip. Example: 98% of test strip density.
• Specifying a percentage of the maximum unit weight. Example: 94% of the maximum unit weight.

Specifying some minimum percent of the maximum unit weight has gained acceptance with many specifying agencies. The maximum unit weight is sometimes called the “solid density”. This value is based on the asphalt mixture’s maximum specific gravity – also known as the Rice value or G mm in Superpave. The maximum unit weight is determined by multiplying the Rice value by 62.4 pounds per cubic foot (PCF). For example, 2.500 is a typical Rice value. 2.500 X 62.4 = 156.0 PCF. Then, if 95% compaction is specified, the minimum acceptable unit weight is: 0.95 X 156.0 = 148.2 PCF. If 93% of solid is specified, or a maximum of 7% air voids are allowed in the compacted mat, then the minimum target value would be 145.1 PCF (0.93 X 156.0).

The thickness of the course being compacted does influence its compactability. Too thin a mat does not have sufficient workability, and too thick a mat may be unstable. In order to be compacted, the mixture must have controlled workability. Typically, for dense-graded mixes, a lift thickness of 3 to 4 times the nominal maximum size (NMS) of the aggregate is needed. For example, a mix containing ½-inch NMS stone should be placed at a compacted depth of at least 1-½ to 2 inches. If a ½ -inch top-size mix is placed at 1 inch compacted depth, the mat may pull and tear and the stones may be broken by the rollers. Thus, the “depth of paving” does influence the ability to obtain proper compaction. The target value for compaction, based on a materials property – the maximum specific gravity – does not change but the likelihood of meeting the target density is changed.

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.

Yes. However, since Superpave mixes do tend to be coarser and contain modified binders than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used. Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low void, low permeability seam.

Minimum lift thickness should be at least 3 times the nominal maximum aggregate size to ensure aggregates can align themselves during compaction to achieve required density and also to ensure mix is impermeable. The maximum lift thickness is also dependent on the type of compaction equipment being used. When static steel-wheeled rollers are used, the maximum lift thickness that can be properly compacted is 3 inches. When pneumatic or vibratory roller is used, the maximum lift thickness 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 6 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.

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.

It is not advisable 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 achieving adequate density.

Generally speaking, there are no unique problems with using polymer modified mixes as RAP. Some individuals express environmental concerns about running millings containing ground tire rubber (GTR) through a drum plant. Florida uses a small percentage of GTR on most of their highway surface mixes. California and Arizona also use GTR frequently.

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, where the producer mixes 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 the new MS-22 and pages 162 and 163 of the old 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.

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.

• Intersection Strategy 1: Developing a Strategy for Better Performing Intersection Pavements
• Intersection Strategy 2: Ensuring Structural Adequacy - A Key Step to Intersection Strategies
• Intersection Strategy 3: Materials and Construction Concerns for Improved Intersection Performance
• Intersection Strategy 4: Three Examples of Implementing the Plan
• World's Strongest Intersection

No rule of thumb answers your question. There are two issues:

1. Is the pavement structure (subgrade, subbase, base, and all asphalt layers) adequate to support the loads? You need to purchase our MS-23 Manual, Thickness Design of Asphalt Pavements for Heavy Wheel Loads.
2. Is the hot mix asphalt surface stiff enough to resist deformation (ruts or indentations)? This is dependent on many factors, such as stiffness of the original mixture, age of the mix (gets stiffer over time), temperature of the mix during loading, loading itself, duration of applied load, etc. This is generally not a problem, but if it is, can typically be resolved by placing some steel (or other rigid material) plates below the point load to distribute the load across a wider area.

There is not a predictable value or “rule-of-thumb number” for the difference in air void content of original and reheated samples. The general trend would be for the reheated samples to have higher air voids than the original, compacted specimens. Absorption and hardening or stiffening of the asphalt binder in the reheated samples likely causes this difference.

Reheated samples can be utilized to give an overall check of the original sample results. Before any significant precision is attributed to reheated sample results, a correlation should be developed for reheated sample air voids and original sample air voids by performing a series of comparative tests.

Generally speaking, there should be no unique problems with using polymer modified mixes as RAP. There have been some individuals express environmental concerns about running millings containing ground tire rubber (GTR) through a drum plant. Florida uses a small percentage of GTR on most of their highway surface mixes. California and Arizona also use GTR frequently.

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.

Information on fuel-resistant asphalt sealers can be found at www.aaptp.us with Report 05-02.

A liquid asphalt, such as a Rapid Setting Emulsion (RS-1,2 or CRS-1,2 includes modified) 1 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.

The amount of asphalt applied depends on three factors:

1. The existing surface condition
2. The amount of traffic
3. 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.

Yes – AASHTO T-11 Dust ratio should be less than 0.75

Several factors can lead to this appearance; improper distributor nozzle sizes, pump pressure, spray bar height, angle of nozzle, and cold asphalt.

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. 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 approved release agents.

Information on fuel-resistant asphalt sealers can be found at www.aaptp.us with Report 05-02.

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.

No rule of thumb answers to your question. There are two issues:

1. Is the pavement structure (subgrade, subbase, base, and all asphalt layers) adequate to support the loads? You need to purchase our MS-23 Manual, Thickness Design of Asphalt Pavements for Heavy Wheel Loads.
2. Is the hot mix asphalt surface stiff enough to resist deformation (ruts or indentations)? This is dependent on many factors, such as stiffness of the original mixture, age of the mix (gets stiffer over time), temperature of the mix during loading, loading itself, duration of applied load, etc. This is generally not a problem, but if it is, can typically be resolved by placing some steel (or other rigid material) plates below the point load to distribute the load across a wider area.

There is not a predictable value or “rule-of-thumb number” for the difference in air void content of original and reheated samples. The general trend would be for the reheated samples to have higher air voids than the original, compacted specimens. Absorption and hardening or stiffening of the asphalt binder in the reheated samples likely causes this difference.

Reheated samples can be utilized to give an overall check of the original sample results. Before any significant precision is attributed to reheated sample results, a correlation should be developed for reheated sample air voids and original sample air voids by performing a series of comparative tests.

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.

Information on fuel-resistant asphalt sealers can be found at www.aaptp.us with Report 05-02.

See the following pdf document: Position Paper – Tire Scuffing & Indentations by the Ontario Hot Mix Producers Association

The paver screed has too much lead crown in it.

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 very 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.

Yes. However, since Superpave mixes do tend to be coarser and contain modified binders more often than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used. Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low void, low permeability seam.

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 and page 219 & 220 of the old MS-22 if you are not sure what check cracking is.

Railroad information can be found in our Engineering section.

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.