CONSTRUCTION OF ASPHALT MIX 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 contractors 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, dont 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 dont 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. 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 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. 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 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, its mixed. 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 (Joy) mixed with water. The portions depend on the water with which it is mixed. Soft water wont 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. Q. What is the proper paver speed? A. 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. Q. Why does the paver pass has 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 paver pass 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 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. Q. Is there a limit on the percentage of RAP utilized in new installations. What about RAP use for resurfacing of old asphalt roads? Any limits? If there are limits on the use of RAP in new or resurfacing installations, who sets the limits? A. 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.
Posted on: Tue, 18 Mar 2014 06:13:02 +0000
Trending Topics
Recently Viewed Topics
© 2015