Section 3: Traffic Volume Considerations for Seal Coat Treatment
Overview
Although a seal coat can be applied to high-traffic volume roadways, it is generally limited to low-traffic volume roadways. Facilities with average daily traffic in excess of 10,000 vehicles per day will be considered as high traffic for purposes of this manual. Problems that can occur on high-traffic volume roadways are sometimes related to the following:
- short-term aggregate loss
- vehicular damage from loose aggregate
- potential for flushing
- tire noise
- prolonged traffic control.
Scott Shuler1 (1990) offers several solutions to alleviate the impediments to using chip seals (seal coats) on high-volume facilities. Some of these solutions are discussed below.
Short-Term Aggregate Loss
Short-term aggregate loss refers to aggregate loss within
hours or days after construction. If loss occurs within a few days,
causes may be related to the following:
- binder too cold when aggregate applied
- inadequate binder quantity
- inadequate aggregate embedment
- existing pavement too cold when asphalt is applied
- cold weather immediately following construction.
Vehicular Damage
The potential liability due to vehicle damage from dislodged
aggregate may be the primary reason for not using seal coats on
high-traffic volume roadways. Damage can occur to windshields, headlights,
radiators, and vehicle paint.
While there is sometimes a tendency to apply excess aggregate
to avoid tracking by rollers, this excess aggregate can cause damage
to vehicles. In addition, when more than one aggregate thickness
is present, additional aggregate particles on the surface are pushed
into those below. This action dislodges aggregate in the first layer
causing loss of aggregate and changes in grading (due to aggregate
crushing). The correct aggregate quantity should produce a layer,
which is one-stone thick.
Allowing slow-moving traffic on a new seal coat after final
rolling and sweeping is one of the best means to reduce premature
aggregate loss. Slowly moving vehicles also seem to provide a level
of aggregate orientation not achievable by conventional pneumatic
rollers. One method to assure the traffic will move slowly is to
use pilot vehicles. This practice is often not followed because
of the inconvenience to motorists on high-volume facilities. To
alleviate this problem, seal coat operations can be performed when
traffic is at reduced levels.
The potential for vehicle damage can also be reduced by using
a small aggregate size (such as a Grade 4) or by using lightweight
aggregates which have a much lower specific gravity than conventional
mineral aggregates.
Tire Noise
The best seal coats are those that are effective sealing mechanisms
and those that provide a long-lasting, high-friction riding surface.
One of the ways to achieve these two objectives is by using large,
one-sized aggregates (1/2 inch or greater). The larger aggregates
require greater asphalt quantities to bind the aggregate that provides
greater sealing capabilities while providing necessary friction.
These larger aggregates often generate complaints by motorists because
the tire noise level is greater. While the larger aggregates can
increase the surface texture and improve friction, the key to friction
is in the skid properties of the aggregate.
Double application seal coats (two-course surface treatment)
using a smaller aggregate for the top layer will provide for less
tire noise. This results in a first layer with more voids in the
surface, while the second application of aggregate fills in the
voids.
Prolonged Traffic Control
Prolonged traffic control is primarily associated with emulsified
asphalt binders. Increased traffic control is often necessary until
the emulsion has had time to break and develop tensile strength
to hold the aggregate.
Modified binders may offer a higher level of adhesion than
corresponding conventional binders. Therefore, aggregate retention
is better during the early life of the seal coat, and often the
rigid levels of traffic control required for emulsions are not as
significant when polymer-modified binders are used.
1.
Shuler, T.S., 1990. “Chip Seals for High Traffic Pavements,”
Transportation Research Record No. 1259
, Transportation Research Board, National Research Council, Washington D.C.