By: Scott Shuler, P.E., Ph.D.
Many roads are located in areas of the world where high-quality construction aggregates are scarce.
Therefore, when high-quality aggregates are needed for pavement-preservation activities they must be transported to these locations. This transportation increases the cost of pavement construction and preservation. Increased costs often mean timely pavement preservation activities are postponed. This postponement leads to deterioration of the infrastructure and, ultimately, increased costs. However, many pavements requiring preservation are low-volume facilities. These low-volume roads may not require the high-quality aggregates necessary on higher-traffic-volume facilities. Therefore, if more economical local aggregates could be demonstrated to perform acceptably, pavement preservation could be accomplished within budget at appropriate intervals. This would save costs in both the short and long term.
Chip seals are used almost everywhere in the world for road construction and extending pavement life. Most chip seals are placed on low- to medium-traffic-volume roads. Although high-quality crushed aggregates offer the best chance for success in chip seals on any roadway, they may be overkill on low-volume roads. So, if acceptable chip-seal performance on low-volume roads could be demonstrated utilizing locally available and minimally processed aggregates, costs would be reduced and resources would be better utilized. Therefore, an experiment was designed to demonstrate the performance of chip seals constructed using two different aggregates on two low-volume state highways. One aggregate was a material routinely, and successfully, used for chip-seal construction in this area of the state. The second aggregate was a material that did not meet specifications for gradation or fracture but was locally available and less costly.
Construction of the test pavements was conducted by state maintenance forces in 2009. Evaluation sections within the two test pavements consisted of two 500-ft-long sections for each of the two aggregates, resulting in 2,000 lane-ft of test area for each roadway.
Condition surveys were performed to determine prechip-seal condition and then periodically for the next three years to track performance of the evaluation sections.
Objectives of the experiment:
- Construct chip-seal test and control sections using locally available and minimally processed aggregates and document the performance of these pavements for three consecutive years;
- Develop and/or adapt monitoring and documentation procedures for evaluating the performance of the test sections; and
- Develop or adapt a design procedure, aggregate specifications and construction guidelines for chip seals constructed with local, minimally processed aggregates on low-traffic-volume roadways.
Heavy emphasis on low volume
It has been well-documented that aggregate characteristics affect chip-seal performance, and especially on high-traffic-volume roadways. This experiment was designed to determine how much aggregate characteristics affect performance of chip seals on low-volume roads. To test this idea the performance of two aggregates was evaluated on two two-lane state highways.
One aggregate with a history of acceptable performance was the material routinely used for chip-seal construction by state maintenance forces. The second aggregate represented a locally available and marginal material not meeting state specifications and with unknown performance. These materials will be identified as Control and Experimental, respectively.
Test sections were constructed on SH 71 north of Snyder, Colo., and on SH 59 south of Sedgwick, Colo. Both of these pavements are rural, farm-to-market two-lane highways with 12-ft-wide driving lanes, no shoulders on SH 71 and 10-ft shoulders on SH 59. Traffic volumes are 360 average annual daily traffic (AADT) with 30 single-unit trucks and 120 combination trucks on SH 71 and between 160 and 470 AADT with 20 single-unit trucks and 20 combination trucks on SH 59.
Evaluation sections were established on each highway to measure performance over time for each aggregate being evaluated. Two 500-ft-long evaluation sections were established for each highway for each aggregate. This resulted in four 500-ft-long evaluation sections for each highway, or eight evaluation sections total.
Locations of the evaluation sections on each pavement are shown in Figures 2 and 3. Performance of the chip seals was evaluated by conducting visual condition surveys of the sites after winter and before fall each year. These condition surveys evaluated performance by measuring cracking, raveling and chip loss.
The following steps were used to conduct this experiment:
- Evaluate condition of the pavement in the area of the evaluation sections prior to application of the chip seals;
- Sample the materials used for construction and determine physical properties;
- Construct test sections and establish location of evaluation sections; and
- Evaluate condition of the evaluation sections after winter and before fall each year for three consecutive years.
Condition prior to construction of test sections
Prior to construction of the test sections, condition surveys were performed in the areas of the evaluation sections to determine prechip-seal condition. These surveys were conducted visually following the procedures outlined by the Strategic Highway Research Program (SHRP 2003). Results of these surveys can be found in the final research report and consisted primarily of longitudinal, transverse, alligator cracking and chip loss.
Materials
One aggregate in the study is representative of what is typically used in eastern Colorado for chip-seal construction on low-volume roads. This is the control aggregate. The other aggregate has a finer gradation and with fewer crushed faces. This aggregate is the experimental aggregate. The gradations, percent of fractured faces and soundness loss measured for each of these aggregates, is shown in Table 1 and compared with the CDOT 703-6 specification for chip-seal aggregate and the Nebraska Department of Roads (NDOR) Section 1033 specification for what is termed “armour coat” by NDOR. Armour coat is a chip seal constructed with minimally processed aggregates for use on low-volume roads.
Asphalt emulsion used on the project had the properties shown in Table 2.
Construction
Construction of the test sections was conducted by CDOT Region 4 maintenance forces in summer 2009. Equipment utilized consisted of a conventional asphalt distributor, self-propelled aggregate spreader and two pneumatic-tire rollers. Traffic control consisted of diverting traffic on each of the two-lane pavements around the chip-seal operations until the strength of the emulsion was high enough to resist chip dislodgement.
Material application rates for SH 71 evaluation sections were 28 lb/sq yd for the control and 26 lb/sq yd for the experimental aggregate. Emulsion was applied at 0.28 gal/sq yd for both control and experimental sections. Chips on SH 59 were applied at 28 lb/sq yd for both control and experimental aggregates and at 0.29 gal/sq yd for the emulsion. Design application rates were estimated using the Texas chip-seal design procedure. Results of this design are shown in Table 3.
This design procedure uses a 1-sq-yd board to estimate the quantity of chips required to cover the surface one stone thick. The asphalt quantity is estimated by calculating the amount of asphalt to fill the voids between the chips to a specific embedment depth.
Construction proceeded with no difficulties for either test pavement. Aggregate embedment was achieved after approximately four passes of the pneumatic-tire rollers, and vehicular traffic was allowed back onto the fresh chip seals after approximately two hours from the time of application.
The environmental conditions at the time of construction are summarized in Table 4.
Evaluation sections were monitored to measure performance from spring 2010 until fall 2012. Methods used to evaluate performance were visual condition surveys conducted by walking along the shoulders of the pavements and observing condition according to the methods described by SHRP for the cracking and flushing and Epps, et al., for the chip loss.
Distress in both pavements is limited to a return of transverse and longitudinal cracks to prechip-seal conditions after approximately 2.5 years. Alligator cracking, which was only present in the control sections prior to treatment, has returned to approximately 22-35% of that present prior to treatment. Chip loss is practically nonexistent and ranges from 0.35% to approximately 0.50% of the area of the evaluation sections. Some areas of the pavements also contain longitudinal flushing streaks where distributor nozzles may not have been adjusted correctly and higher quantities of asphalt were applied. The cause of this is not related to either type of chip.
Closer a reality
The results of the condition surveys after three years of service were analyzed using conventional analysis of variance techniques (ANOVA) to determine whether any significant differences exist in performance for any of the evaluation sections. The dependent variable analyzed to determine differences in performance was the percent of the original distress observed for each evaluation section at the end of the performance period in fall 2012. For example, Table 5 indicates that the control sections on SH 71 had an average of 99% of the original longitudinal cracking returning during the fall 2012 condition survey, while the experimental sections had an average of 107%. Statistical analysis indicates there is no statistical difference between these values as shown in the table.
The results of the performance analysis for SH 71 indicate there were no significant differences between the control and the experimental chips for any of the performance criteria except flushing. Flushing occurred in the control sections over 1.3% of the area, while the experimental sections had flushing on 1% of the area. While this result is statistically significant, it has no practical significance. Results of the analysis for SH 59 indicate no significant differences in performance for any of the performance criteria.
Locally available, minimally processed aggregates can be successfully applied as chip-seal aggregate on low-volume roadways. After three years of service, two experimental pavements provided the same performance with respect to cracking, chip loss and flushing for both control and experimental aggregate chips.
The design procedure used to estimate aggregate-chip application quantity and emulsion spray rates matched the actual quantities placed reasonably well, and these quantities resulted in acceptable performance for three years.
The design procedure reported herein provided good estimates of the chip and emulsion quantities actually utilized during construction and resulted in good performance of the chip seals during the analysis period of three years.
Longitudinal streaking of the emulsion occurred on both pavements and led to flushing. This overapplication could have been caused by plugged nozzles in parts of the spray bar, spray-bar height or nozzles not adjusted to the same angle. In addition, although chip loss was minimal, much of the loss occurred at or near the roadway centerline. An edge nozzle designed to provide half the fan of a full nozzle can be used to reduce this potential loss of chips.
In some cases, tandem dump trucks delivering chips to the aggregate spreader overfilled the spreader hopper, and excess chips were applied to the surface. These excess chips also may have been a source of some of the flushing observed. R&B
About The Author: Shuler is an associate professor at Colorado State University.
