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. Top Contents 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
Post time: Jun-19-2017