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. Top Contents 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. Top Contents 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. 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 Top Contents 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 bon
Post time: Jun-19-2017