PAVEMENT MAINTENANCE: Double covered

WSDOT tests enhanced chip seal on S.R. 20

Maintenance Article April 05, 2012
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A chip seal constructed on an existing flushed roadway has the potential to result in bleeding or flushing of the new chip seal.


The excess binder, if not properly accounted for during design and construction, will migrate to the surface of the chip seal and fill the aggregate void spaces, leaving a flushed surface. Limited information is available regarding construction of a new chip seal on a flushed existing surface. A review of the literature indicated that both an inverted double chip seal and a sandwich seal are capable of correcting a flushed existing pavement, but construction details or performance data for these types of chip seals were not included. Although the chip seals reported in the literature are not conventional double chip seals, they both involve two applications of aggregate and one or two applications of binder. This led to the proposal to construct a double chip seal as a method to address a severely flushing pavement on S.R. 20 north of Spokane in Washington state.
A single chip seal consists of one application of aggregate over one application of asphalt binder. A double chip seal is essentially two single chip seals, one placed on top of the other. The aggregate on top is usually a finer gradation than that placed on the bottom in a standard double chip seal. An inverted double chip seal is the opposite of a conventional double chip seal with the finer aggregate gradation placed on the bottom during the first application of aggregate and binder. Double chip seals are more durable and seal the roadway against water better than a single chip seal, leading to their use in locations where there is high truck traffic or on steep grades. The disadvantage of a double chip seal is higher cost due to two applications of binder and aggregate.


A double chip seal is a departure from the normal practice used by the Washington State Department of Transportation (WSDOT). WSDOT uses single chip seals almost exclusively to preserve low-volume highways. If it can successfully address the flushing pavement on S.R. 20, a double chip seal will provide an economical method of addressing flushing on chip seal roadways in the future. This article documents the design and construction of the double chip seal on S.R. 20. WSDOT will monitor the double chip seal for a period of five years at which time a final report will be prepared documenting its performance.


Fixing the flushing
The goal of the pavement design on S.R. 20 was to find an economical solution to the flushing pavement. In 2000, WSDOT placed hot-mix asphalt (HMA) on this section of S.R. 20 to improve the structure, but this is a low-traffic-volume highway, and it is WSDOT’s intent to maintain it as a chip seal route in the future. Milling and replacing the flushing HMA would provide a reliable method of eliminating the flushing pavement but would be much more costly than using a chip seal to correct the flushing.


The successful use of a double chip seal to correct a flushing pavement in Thurston County, Wash., influenced WSDOT’s design. The double chip seal used in Thurston County consisted of two applications of CRS-2P binder and two applications of 1?2 in. to No. 4 aggregate with a reduced application rate of the CRS-2P for the first application. To simplify design and use available materials, WSDOT selected a double chip seal similar to the Thurston County design consisting of two applications of CRS-2P binder and two applications of 3?8 in. to No. 4 aggregate (Table 1).


An application of No. 4 to 0 choke (Table 1) placed after the second application of aggregate would fill surface voids and lock in the second application of 3?8 in. to No. 4 aggregate. The double chip seal on S.R. 20 was part of a larger project to place over 300 lane-miles of single chip seal. The double chip seal used the same 3?8 in. to No. 4 gradation as the single chip seal, which eliminated the need to produce a relatively small quantity of aggregate of a different gradation. The design called for reducing the first application of binder to account for the flushing pavement and not placing a fog seal over the finished surface. WSDOT’s Eastern Region’s standard practice is to place choke stone and a fog seal on chip seals, but the fog seal was eliminated to reduce the possibility of flushing. WSDOT constructed test sections in 2008 to evaluate the double chip seal’s effectiveness at correcting the flushing and to assist in developing application rates for the CRS-2P and the aggregate.


The lower, the better
Construction of the test sections occurred in July 2008 in both lanes of a 1?2-mile section of flushing pavement on S.R. 20. WSDOT selected the location because of its relatively high rate of flushing and because its geometry was representative of most of the remainder of the flushed roadway.


The plan was to use four different application rates of aggregate and CRS-2P for the test sections. Actual application rates varied from the planned application rates, resulting in the first application for Test Section 1 having application rates very similar to Test Section 2 and the first application for Test Sections 3 and 4 having the same application rates. Table 2 shows the actual application rates for the first and second application of binder and aggregate.


Checking embedment after the first applications of aggregate gave an indication of the effect the flushing pavement would have on the chip seal (Table 3). Binder application rates should be adjusted so that embedment is between 50% and 70%. Higher embedment rates indicate too much binder and could result in flushing. As expected, the embedment measurements for the first application were higher than they would be if the existing pavement surface was not flushing. There also were some indications of bleeding during construction especially in the sections with higher binder application rates.


Monitoring the test sections for two years after construction revealed wheel-path flushing had occurred in many locations. Test Sections 1 and 2, which had higher binder application rates during the first application than Test Sections 3 and 4, appeared to have the more severe flushing. Despite the overall lower flushing severity, Test Sections 3 and 4 still had many severely flushed areas presumably where the flushing of the underlying pavement was more severe. By 2009 the embedment was 100% in the flushed areas of lanes 1 and 2 but was reported to be in the 70% range in Test Sections 3 and 4, which had the lower binder application rates for the first application. The test sections showed that at the lower application rates for the first application of CRS-2P a double chip seal reduced the flushing and could produce an acceptable pavement. The test sections also showed that the application rates for the first application of CRS-2P would need to be varied depending on the flushing present on the existing surface.


Getting a good rate
The application rates used for Test Sections 3 and 4 were the basis for the final design included in the contract documents (Table 4). The goal was to achieve an initial embedment of about 50%. The CRS-2P application rate for the first application of 0.20 gal/sq yd was about one half of the application rate typically used by WSDOT on a single chip seal. The remaining application rates for the CRS-2P and aggregate for both applications were within the normal range for WSDOT’s typical single chip seal.
The first application of aggregate and binder for the double chip seal occurred on July 27, 2010, and the second application for the double chip seal and placement of the areas to receive the single chip seal occurred the next day. The weather for the most part was ideal for chip seal placement, with clear skies and high temperatures in the upper 90s. The maximum surface temperature measured during placement was 116°F. The contractor, Central Washington Asphalt (CWA) used conventional chip seal equipment and placement procedures.


Overall the construction of the double chip seal went well. Two issues that may affect performance were that the aggregate gradations were outside of specification limits and that the application rates of the No. 4 to 0 choke were inconsistent and lower than specified.


The intent was that the application rates in the contract documents would be a starting point and field personnel would adjust the application rates during chip seal placement to account for field conditions.


Prior to placing the double chip seal, WSDOT field personnel gave each section of the roadway a 1 to 4 rating based on the extent of flushing visible with 1 being no flushing and 4 being severe flushing (Table 5). Adjustments to the first application of CRS-2P and aggregate were based on the rating for each section being chip sealed. Sections with a rating of 1 did not have significant flushing and did not receive an application of binder or aggregate during the first application. Instead these areas received a single chip seal using the same application rates as the second application of binder and aggregate for the double chip seal.


Changes in application rate for the first application of CRS-2P were marked with lath placed at the beginning of each section. The application rate written on the lath was entered into the computerized application rate-control system on the distributor truck to ensure the proper application of CRS-2P. WSDOT field personnel verified application rates by computing the yield based on the area chip sealed and the gallons of CRS-2P used.


Tables 6 and 7 show the actual application rates for each section along with the percent embedment. The embedment of most sections was around 50% but some were as low as 30%. The low embedment in some sections may have been due to minimal traffic allowed on the sections prior to checking the embedment.
Placement of the second application of the double chip seal and the single chip seal areas occurred at the same time as a continuous operation. The second application of the double chip seal consisted of 0.34-0.39 gal/sq yd of CRS-2P and 24 lb/sq yd of 3?8 in. to No. 4 aggregate with 2.3 lb/sq yd of No. 4 to 0 choke on all areas regardless of the condition rating. Application rates for the single chip seal areas were the same as the application rates for the second application of the double seal.


Testing of samples from the stockpiles revealed that the gradation of the 3?8 in. to No. 4 aggregate was outside specification requirements for the percentage passing the No. 4 sieve and that the gradation of the No. 4 to 0 choke was outside the specification for the percentage passing the No. 200 sieve. WSDOT used a specification for statistical acceptance of aggregates on this project. Although the aggregate gradations were outside specification requirements for individual sieves, the overall quality level was such that the specification allowed the aggregate to remain in place with a price reduction. The percentages passing the remaining sieves were all within specification, and the higher percentages passing the two sieves should not affect the performance of the double chip seal.


Double the effect
After three months in service the double chip seal appeared to be performing well with only a few areas in the wheel path where the embedment appears to be near 100%. WSDOT tested the friction of the double chip seal after construction with a ribbed tire using a locked-wheel friction tester meeting ASTM E-274 requirements. As would be expected for a chip seal, the friction numbers were good with an average friction number of 62.1 and ranging between 57.5 and 68.


WSDOT will monitor the pavement condition, rutting and ride of the double chip seal during annual pavement surveys and report the results in the final report.
WSDOT designed and constructed a double chip seal to correct the flushing of an existing HMA pavement on S.R. 20. Design of the double chip seal was straightforward, and construction was accomplished using conventional chip seal equipment and methods. Initial indications are that the double chip seal was able to correct the flushing and improve friction. R&B
 

About the author: 
Russell is state pavement design engineer in the State Materials Lab of WSDOT. Littleton is eastern region materials engineer in the Eastern Region Materials Lab of WSDOT. Uhlmeyer is state pavement engineer in the State Materials Lab of WSDOT.
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