Better Roads

July 2013

Better Roads Digital Magazine

Issue link: https://read.dmtmag.com/i/142598

Contents of this Issue

Navigation

Page 13 of 35

RoadScience by Tom Kuennen, Contributing Editor Loss of Cover Decoding the secrets of asphalt stripping and raveling R aveling describes the loss of aggregate from the driving surface of a bituminous concrete pavement. Loss of aggregate through raveling encourages pothole formation and cracking, which can introduce water into a pavement structure. Thus raveling can endanger a pavement's long-term performance. Raveling also puts motorist safety at risk, which makes it a public safety problem as well. As raveled areas collect rain or melt water, they pose hydroplaning risks not unlike those shallow surface "bird baths" that plague asphalt paving crews. The raveled area can degenerate into a pothole, which can damage tires, wheels, worsen pavement conditions and even cause accidents. Broken windshields, rough rides, and loss of stopping power in loose aggregate are fodder for road user complaints. Although external conditions can precipitate raveling – like snow plowing and studded tires – most raveling is the result of failure of the aggregate to bond fully with the asphalt binder. Thus to "build pavements not to fail," the best place to forestall raveling is right where the conditions favoring it begin: at the hot mix plant stage. Raveling is not limited to HMA pavements. Chip seals can ravel too. "The most common distresses in chip seals are flushing or bleeding, and aggregate loss from the top layer, i.e., raveling," say Raquel Moraes and Hussain Bahia, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, in their 2013 Transportation Research Board paper, Effects of Curing and Oxidative Aging on Raveling in Emulsion Chip Seals. "Raveling is not restricted to the early life of the chip seal, and may continue to occur throughout the service life." Testing results showed that the rate of in-service raveling is dependent on the curing process and subsequent in-service oxidative aging as the adhesive and cohesive forces at the emulsion-aggregate interface change, they write. "The initial bond maturation results in a decreased rate of chip loss, while the in-service oxidative aging of the asphalt emulsion leads to a higher aggregate loss as a result of the emulsion's strain intolerance and inability to relax stresses induced by traffic." Causes of Raveling Raveling begins at the light stage, in which surface fines are lost. This can segue into moderate raveling, in which fines are lost and larger aggregate is exposed. Severe raveling describes the loss of both fine and coarse aggregate. While there are different degrees of raveling, except for raveling caused by external damage, most raveling can be traced to moisture issues in mix design, or errors in compaction of the lifts. Whatever the initial condition, raveling usually is moisture-induced. Stripping describes the loss of the adhesive bond between asphalt and aggregate. When the adhesive bond between asphalt and aggregate is broken, usually in the presence of moisture, stripping ensues and raveling may begin. Stripping of asphalt binder from aggregate typically begins at the bottom of the HMA layer and progresses upward. But when stripping starts at the surface and progresses downward it results in raveling. Stripping can be tough to identify because its clues take different forms: rutting, shoving, corrugations, cracking or raveling. The only accurate way to determine if stripping is the cause of the observed distress is to core the pavement and examine the pavement structure cross-section. This moisture damage can come from an inadequate pavement drainage system; a mix with excessive air voids due to inadequate compaction and low asphalt content; a mix where the aggregate surface is incompatible with asphalt binder and the binder is displaced by water; excessive pore pressure caused by traffic; and even the spontaneous emulsification of binder by water, says the Federal Highway Administration. 12 July 2013 Better Roads RoadScience_BR0713.indd 12 6/26/13 3:22 PM

Articles in this issue

Archives of this issue

view archives of Better Roads - July 2013