For paving contractors, the word "density" is probably as familiar as that first cup of coffee in the morning. Followed by two other key words that will guide him or her through the day: "bonus" and "penalty."
Unlike any other contractor in the world of construction, the paving contractor probably faces more stressful situations on a day-to-day basis than any other. There's nothing worse than a paver screed failure or a plugged roller spray nozzle with a load of hot-mix asphalt in the hopper. Or a road surface that's paved, rolled and striped that fails to impress the inspector. Along with a host of other on-site, plant- or weather-related disruptions comes the dreaded penalty for failure to deliver on time or failure to deliver consistent density across the length and width of the mat.
The flip side of this story is "bonus," a tantalizing incentive for any paving contractor worth his bitumen. To enjoy bonuses, as many contractors do these days, the paving team must morph to an orchestra of sorts, with all instruments tuned and ready to go; the music well written and practiced and the orchestra itself confident in the success of the group. Without this orchestration, success - and thus bonus - may be difficult to attain.
Along with the introduction of Superpave and other contemporary mix designs came some nasty roadblocks to bonus land. Like the need to make density before the tender "zone," a four-letter word connected to the actual temperature of the asphalt mat. Prior to these new super mixes, contractors spent little time analyzing the precise temperature of the mat, and in most cases didn't own the equipment to run such tests. Never mind temperature guns--an experienced hand placed briefly on the mat would tell the contractor whether the mix was hot or not. Not so today. Knowledge of the exact temperature is a must throughout the rolling process.
Tender, loving care
Typical of all asphalt problems of the past, the paving contractors and
manufacturers were faced with the task of finding solutions to these new problems. Initially, both worked out new rolling patterns and equipment
utilization that primarily focused on adding vibratory rollers prior to and pneumatic rollers following the obstinate tender zone. These were tentative fixes at best that forced the contractors to invest in additional machines and manpower to get the job done. Forget about bonus.
The need was there for the manufacturers to find a more permanent solution to the tender zone problem. A couple of R & D-minded manufacturers jumped into the lead with equipment and instrumentation designed to get the job done faster.
Double-drum vibratory asphalt rollers have been the norm since the mid-1970s when states began the change from method-based to end-result specifications. Contractors of that era discovered that they could replace roller trains of six, eight, 12 or even 20 or more compactors and their operators with a couple of vibratories, saving a ton of money in the process.
Unlike the old static roller, the ground speed of the vibratory was limited by the impact spacing required to meet smoothness and density specs.
Roll too fast and the impacts were spread too far apart. Roll too slow and you sacrificed time and thus money. A minimum impact spacing of 10 to 12 per ft (or about an impact an in.) was, and still is, the rule of thumb required to meet specs. Most all vibratory rollers of that era were designed with eccentric weights that rotated at 2,400 vpm. This meant that the roller operator could run at a maximum ground speed of 2.73 mph and maintain the desired impact spacing. With conventional mixes of the day, this posed no immediate problem. Enter Superpave and we have a different story.
For more on the story, read the August issue of ROADS & BRIDGES.