Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/142598
RoadScience undercompacted mats if they stiffen beyond workability, also putting the pavement at risk for raveling. Photo courtesy of National Center for Pavement Preservation Compacting Cooled Mats Raveling describes the loss of aggregate from the driving surface of a bituminous concrete pavement. While much is being said about warm mix asphalt or cold bituminous mixes, the bulk of asphaltic concrete being placed still is conventional hot mix asphalt. There, to forestall raveling, mix temperature should be at a minimum of 290 degrees F in the middle of the mat behind the screed, says Granite Construction's Todd Munson. "The ideal mix temperature will vary slightly depending on the binder type used in the mix design, lift thickness, the time available for compaction, and ambient conditions," he says. "The most efficient breakdown rolling should always occur at temperatures above 290 degrees F. When breakdown rolling begins at temperatures lower than 290 degrees F, the risk of not achieving sufficient compaction is greatly increased." Lift thickness plays a critical role in preventing raveling, as it relates to compaction, Munson says. "A minimum lift thickness of two times the maximum aggregate size should always be maintained. This allows enough room for particle reorientation and proper compaction to occur. If lifts are thinner than twice the maximum aggregate size, there is insufficient room for the aggregate to reorient itself in a dense configuration that is impermeable to water." Also, if the lift thickness is less than 2:1, aggregate fracture often occurs in the rolling process, leaving uncoated aggregate surfaces in the mix that will not bond, he says. Placement of asphalt pavement in wet weather introduces water into the mix, promoting raveling. There, moisture may lodge on the coated aggregate particles, precluding a strong bond. If a mix is too cool to drive off moisture it encounters, risk of raveling is enhanced. If pavements are placed in wet weather, they also may cool more quickly, which can lead to A new compaction technology has shown to be of value in compacting cooled mats. Oscillation compaction, which compacts with a lateral drum movement, rather than an upand-down vibration, can attain required density on cooled mats, users say, precluding raveling and widening the paving season. In the oscillation drum, two eccentric masses turning in the same direction cause a movement around the drum axle. The movement changes its direction of effect during one turn so it generates an oscillating or rocking movement of the drum. Horizontal forces are transmitted from the drum into the pavement. The result can be compaction in fewer passes, with less vibration-related wear and tear on operators and surroundings. "Washington State has a spec that you cannot vibrate your mix below 175 degrees F," says Cliff Schroeder, president and general manager, Watson Asphalt Paving, Redmond, Wash., in a 2009 interview. "They feel it will be too cold, the liquid too brittle, and that the bond between the oil and rock will be broken." Static rolling of cooler mixes is permitted, Schroeder says, but with the ambient temperatures often encountered, that 175 degrees F point can be reached rather quickly. "At that point, there will not be enough energy left in the mat to permit the mat to be compacted, pushing the air out of it. You could be in trouble with a deduction on the mix for failure to compact." But oscillation will get density on those cooled mats, he says. Watson owns two oscillation rollers, a Hamm HD+ 120 VO, and an older HD O90V. Schroeder was able to actually see and compare the movement of aggregate particles by vibration and oscillation. "In our yard we have a ramp where a loader dumps excess stockpile material," Schroeder says. "It's unconsolidated, so to test performance we ran a vibratory roller up it, followed by an oscillation roller. I physically could see the metrics change in the way the aggregate was assembled together. Vibratory by itself just drives the particles straight down, leaving them not touching; but oscillation consolidates them and locks them down tighter." On a fall parking lot reconstruction over a poorly constructed base, saturated after heavy rain, as many as six passes with a vibratory roller failed to get density, the contractor says. "They were hitting it hard, but still not getting it," Schroeder says. "It was bringing water straight up into the mat, chilling it. I told them to go static and oscillation only. The result was a string of 93s and no water. Oscillation is yet another tool for us to use to meet our client's needs." 14 July 2013 Better Roads RoadScience_BR0713.indd 14 6/26/13 3:26 PM