WHEN TO SAFELY BROOM OR REMOVE TRAFFIC CONTROL ON FRESH EMULSIFIED ASPHALT CHIP SEALS

Transcription

1 1 WHEN TO SAFELY BROOM OR REMOVE TRAFFIC CONTROL ON FRESH EMULSIFIED ASPHALT CHIP SEALS Scott Shuler, Ph. D., P. E. Colorado State University Ft. Collins, CO : telephone : fax Paper submitted for presentation and publication at the Transportation Research Board 90 th Annual Meeting January Washington, D.C. Re-Submission Date: OCTOBER 11, 2010 Word Count: 2841 Tables: 3 x 250 = 750 Figures: 9 x 250 = 2250 Total: 5841

2 2 ABSTRACT The optimal time to allow brooms to sweep a fresh chip seal is usually determined by experienced field personnel. Factors that affect this decision include aggregate type, aggregate moisture content, emulsion type and grade, application rates of both emulsion and aggregate, weather conditions at the site, including humidity, air and pavement temperature, wind and number of rollers. Given all of these factors, and that changes in one can have a significant effect on how an emulsion gains strength and, thus, can resist the forces of brooms and traffic, the decision to allow brooms and traffic on fresh chip seals becomes difficult, at best to predict in advance. The results of this research suggest that moisture content of the chip seal system is directly related to the strength of an asphalt emulsion residue. Laboratory and field testing confirms that the strength of an asphalt emulsion residue as measured by a modification to the ASTM D7000 sweep test can be used to predict the moisture content at which a chip seal develops enough strength to resist damage by brooms and vehicular traffic. Three full-scale test pavements were constructed in differing climates and the results of moisture content testing in the field compared with modified sweep test results in the laboratory. Results indicate the three field tests were capable of resisting brooming and traffic damage when moisture content of the chip seal system ranged between 15 and 25 percent. This correlated well with results of laboratory testing using the modified sweep test on the materials from field tests as well as experimental laboratory materials.

3 3 INTRODUCTION Determining when the first brooming can be accomplished to remove excess chips or when to open a fresh chip seal to traffic is one of the most subjective decisions that must be made during chip seal construction. Releasing traffic without adequate pilot cars too soon can lead to vehicle damage due to flying aggregate particles. Brooming too soon can destroy the chip seal by sweeping off chips not adequately cemented in the unbroken emulsion. Releasing traffic too late can lead to delays, congestion and complaints. And, if early brooming results in damage to the chip seal the chip seal is often left unbroomed until binder strength increases. However, allowing uncontrolled traffic on the fresh, unswept, or partially swept chip seal can lead to flying chips, potential damage, unprotected residual asphalt due to chip loss and friction loss. The modified sweep test (Shuler and Lord 2009) which measures the relative adhesive strength of emulsions and emulsion residues in the laboratory was used to evaluate materials from three full-scale chip seal projects in different climates. The idea of this research was to determine if the results found in this laboratory experiment could be correlated to real chip seal construction. Literature Review There has not been very much research published regarding objective measurements for determining when to release traffic onto fresh chip seals. In fact, the Austroads organization, which has done a considerable amount of research in chip seal technology, identified this as one of the fertile areas for current and future development (Austroads 2006). Because the aggregate layer in a freshly placed chip seal is often fragile for several hours after the completion of rolling and sweeping, high speed vehicular traffic may dislodge aggregates during the first few hours after the placement of the seal coat. Therefore, reduced speeds are needed to avoid flying chips and, have been shown, to aid in the embedment of the chips in the new seal (Shuler, 1998). However, active speed enforcement will be necessary to ensure that traffic adheres to the speed limitations (Croteau, et al., 2005) as traffic volumes increase or damage to vehicles and the fresh seal may result. After chipping, pilot cars should be used for between 2 and 24 hours to ensure that traffic speed is limited to less than 20 mph (30 kph) (Caltrans Division of Maintenance, 2003). The primary purpose of the pilot car is to control the speed of the traffic through the project as high speed, uncontrolled traffic can be detrimental to the chip seal resulting in excess aggregate loss and dislodged chips. In addition, the pilot car can move traffic back and forth across the roads to prevent traveling in the same wheel paths. This traffic will supply some additional pneumatic tired rolling and helps embed the aggregate further (Washington State Department of Transportation, 2003, Gransberg, 2005). FULL-SCALE FIELD TESTS Three full-scale chip seal projects were included in this research. Test pavements were located on County Road 11 near Frederick, Colorado, approximately 30 miles north of Denver, Colorado; the Main Entrance Road in Arches National Park, Utah, approximately 15 miles north of Moab, Utah; and US101 near Forks, Washington, on the western edge of Olympic National Park.

4 4 Moisture Tests Moisture in a chip seal comes from two sources: the chips and the asphalt emulsion, and on some projects additional moisture may be present in the roadway. If the amount of moisture in the chips and the emulsion is known at the time the chip seal is constructed, the amount of moisture that evaporates after emulsion and chip application can be measured. The objective of this part of the research was to measure the moisture loss in the three chip seal projects and develop a relationship to chip adhesion. The amount of moisture remaining in each chip seal was measured and compared with the relative strength of the residue on a scale of 1 (no strength) to 10 (ready for traffic), judged by pulling three chips out of the fresh seal and qualitatively judging dislodgement potential. This qualitative evaluation was conducted after rolling. Moisture remaining in the emulsion was determined by placing plywood pads covered with aluminum foil measuring 24 by 24 inches in front of the asphalt distributor prior to spraying with emulsion. The pads were weighed before and after spraying and chipping and the loss in weight was determined periodically during the day until approximately 95 percent of the water had evaporated. Figure 1 shows the setup used to measure the tare weight of the apparatus prior to spraying and chipping. Figure 1. Moisture Test Pads Prior to Spraying/Chipping

5 5 The tared pad was placed in front of the asphalt distributor and chip spreader before chip seal operations began. After the emulsion and chips were applied to the pavement and tared pad the pad was removed from the pavement and re-weighed. As moisture evaporated from the pad the weight was recorded and the strength of the emulsion residue was evaluated using the 1 to 10 scale. The resulting relationship between emulsion strength and moisture loss was developed. Field Moisture Tests The results of this experiment indicate that chip adhesion reaches the point where significant force is required to dislodge the chip at approximately 75 to 85 percent moisture loss. At that time sweeping can commence and traffic can be allowed to travel on the new surface. Figures 2, 3, and 4 show the relationship between chip seal binder strength and moisture loss for each test pavement. The chip seal binder strength was judged subjectively by pulling three chips out of the emulsion and rating the relative strength with respect to how difficult the chips were to pull out of the emulsion residue on a scale of 1 (no strength) to 10 (ready for traffic. This qualitative rating was made after rolling. When the chip seal was judged to have sufficient strength to resist brooming and traffic, the final moisture content was recorded. This is the last value shown in Figure 2 through 4. Arches, Entrance Road Residue Strength (1-10 scale) Location 1 Location 2 Location Field Moisture Loss, % Figure 2. Residue Strength vs Emulsion Moisture at Arches NP, UT

6 6 Frederick, CR Residue Strength (1-10 scale) Location 1 Location 2 Location Field Moisture Loss, % Figure 3. Residue Strength vs Emulsion Moisture for CR 11, Frederick, CO Forks, US Residue Strength (1-10 Scale) Loc ation 1 Loc ation Field Moistue Loss, % Figure 4. Residue Strength vs Emulsion Moisture at US101, Forks, WA

7 7 Environmental conditions at the Frederick and Arches sites were similar with ambient temperatures of 85 to 90F at Frederick and F at Arches with pavement temperatures of 110 to 120F at Arches and 100 to 110F at Frederick. Both of the Arches and Frederick projects were constructed in full sun with relative humidity reported at 30 percent at Arches and 45 percent at Frederick.. Environmental conditions at Forks were considerably different. Ambient temperatures ranged from 60 to 75F with pavement temperatures from 80 to 90F. Overcast to broken clouds resulted in little to no sunlight during construction. Relative humidity was reported at 95 percent. As a result of these varying environmental conditions the time required for the chip seals to reach the point where brooming and then traffic could commence also varied widely. The time required for the chip seals to reach 75 to 80 percent moisture loss at Arches and Frederick was approximately two hours and two and one-half hours, respectively. However, the time required for the chip seal at Forks to reach this moisture condition was twelve hours. Predicting Chip Seal Emulsion Residue Strength in Advance of Construction The sweep test described by ASTM D7000, Standard Test Method for Sweep Test of Bituminous Surface Treatment Samples, appeared to be a reasonable approach to simulating the forces which dislodge aggregate chips from chip seals. This procedure is relatively effective at evaluating differences in adhesive abilities of different emulsions with a single aggregate. This test utilizes a template for specific aggregate gradations to establish the emulsion application rate. While a single emulsion application rate is suitable for relative comparison between emulsions, when aggregate sizes differ the embedment percentage changes which affects chip retention. In addition, the test describes a procedure of hand casting the aggregates onto the emulsion prior to testing. Attempts to repeatedly place precise amounts of aggregate on test samples during this research proved difficult to replicate. Therefore, the test apparatus was modified so the exact amount of chips was placed on the test pad each time and the following variables controlled: 40 percent initial embedment of the aggregate chips 40 and 80 percent emulsion moisture loss To determine if the modified test procedure would be useful to evaluate the adhesive ability of different emulsions and different aggregate chips under varying moisture conditions a controlled laboratory experiment was conducted. A detailed description of the test method can be found in NCHRP Report 680, Appendices (Shuler, et al, 2010) and is briefly described below for convenience: Asphalt emulsion is applied to a 15 pound per square yard roofing felt substrate in a circle by means of a steel template, with 11-inch diameter cut-out. Emulsified asphalt is screeded level with the template by means of a strike-off rod shown in Figure 5. Aggregate is then placed mechanically using a dropping apparatus as shown in Figure 6. The aggregate is then set in place, one stone thick, by means of a compactor as shown in Figure 7. The specimen is then placed in a 160F oven to allow the emulsified asphalt to cure to 40% moisture loss or 80% moisture loss after which the specimen is removed from the oven. It is then cooled, and any

8 8 loose particles are removed. The specimen is then swept under the action of a weighted brush which is spun by a planetary motion mixer for one minute as shown in Figure 8. The specimen is then removed from the machine, brushed by hand to remove all particles that were mechanically dislodged from the specimen surface and the mass loss is determined; expressed as percent loss of the original aggregate mass. Figure 5. Emulsion Strike-Off Apparatus

9 9 Figure 6. Dropping Apparatus Placing Aggregate on Test Pad Figure 7. Compactor Setting Aggregates on Test Pad

10 10 Figure 8. Modified Sweep Test Mixer A variety of emulsions were selected to represent the range available for construction. These included conventional and polymer modified anionic (RS-2 and RS-2P), high float (HFRS-2P) and cationic types (CRS-2 and CRS-2P). Properties of the emulsions are shown in Table 1. Table 1. Emulsion Properties Emulsion Tests RS-2P RS-2 CRS-2 CRS-2P HFRS-2P Viscosity, SFS 122F Storage Stability, 1 day, % Sieve Test, % Demulsibility, 35 ml Residue, by evaporation, % Residue Tests Penetration, 77F, 100g, 5s Ductility, 77F, 5cm/min Float, 140F, s na na na na 1290 A variety of aggregates was used to determine if the modified sweep test could discriminate between different mineralogy, shape, and texture. These were a limestone (LSTN), granite (GRNT), basalt (BSLT) and an alluvial source (ALLV). The properties of these materials are presented in Table 2.

11 11 Table 2. Aggregate Properties Sieve No. (in.) Passing, % Sieve Size (mm) LSTN GRNT BSLT ALLV 3/4" /2" /8" /16" /4" Bulk specific gravity Loose unit weight, lbs/cf L.A Loss, % Flakiness Index Results of this experiment indicated the test could discriminate between 40 and 80 percent moisture loss and between dry and SSD aggregates at the statistically significant α = 0.05 level. Predicting Residue Adhesive Strength The laboratory sweep test described above was conducted for aggregates and emulsions obtained from the three field test pavements. The idea for conducting the laboratory test was to determine if the test could help predict when the emulsion residues would gain enough strength to resist brooming and traffic forces. Aggregates were tested using two moisture contents and a range of moisture loss percentages. Results are presented in Table 3 and the relationship between moisture loss and chip loss is shown in Figure 9. At approximately 85 percent moisture loss, residue strength increased to the point where chips could not be dislodged during the test. This suggests that a relationship exists between the laboratory sweep test and actual residue strength in the field as a function of moisture content of the chip seal system. Results indicate a strong relationship between chip seal moisture loss and chip loss. Therefore, the moisture content of the chip seal system (i.e., the moisture of the emulsion and the moisture of the chips) could be used to determine when the chip seal has developed enough adhesive strength to resist the stresses of sweeping and uncontrolled traffic. The regression equation for both dry and SSD aggregates were similar with a better relationship for the SSD condition. Also, the chip seal locations had little effect.

12 12 Table 3. Chip Loss for Test Pavement Materials Site Aggregate Moisture Chip Seal Moisture Loss, % Avg. Sweep Test Chip Loss, % Arches Dry Arches Dry Frederick Dry Frederick Dry Forks Dry Forks Dry Arches SSD Arches SSD Frederick SSD Frederick SSD Forks SSD Forks SSD Sweep Test Chip Loss, % Dry Aggregate Chip Loss, % = (Moisture Loss, %) R 2 = SSD Aggregate Chip Loss, % = (Moisture Loss, %) R 2 = Emulsion Moisture Loss, % Figure 9. Sweep Test Chip Loss for Field Test Site Aggregates and Emulsions

13 13 CONCLUSIONS A new laboratory test that simulates the sweeping action of rotary brooms during chip seal construction was developed during this research. This test simulates the shear forces applied by brooms and uncontrolled traffic to fresh chip seals, and can be used to predict the time required before brooms or uncontrolled traffic can be allowed on the surface of the chip seal in terms of the moisture content of the chip seal. The test indicated the following: The moisture content at which 90 percent of the aggregate chips are retained during the sweep test is the critical moisture content corresponding to very high residue adhesive strength at which uncontrolled traffic could be allowed onto the chip seal field test sections. For equal residue strength more moisture loss in the chip seal was required when dry aggregates were used than when saturated surface dry aggregates were used, confirming the belief that moist aggregates provide higher early strength than dry aggregates when building chip seals. A subjective strength index was used in this research to judge when the field test sections could withstand brooming and uncontrolled traffic. Research is needed to develop a quantitative measure for evaluating chip seal binder adhesive strength. Although three full-scale chip seal pavements were utilized in this research and correlated to laboratory sweep tests many other combinations of pavements, climates, and materials should be evaluated to verify these results. REFERENCES ASTM D Standard Test Method for Sweep Test of Bituminous Emulsion Surface Treatment Samples. ASTM International, West Conshohocken, PA, Update of the Austroads Sprayed Seal Design Method, Austroads Project No. TT1132, Austroads Publication No. AP T68/06, Prepared by Allan Alderson, ARRB Group Published by Austroads Incorporated, ISBN X Caltrans, Chip Seals: Chapter 5, Caltrans Division of Maintenance. Oct Croteau, J., Linton, P., Davidson, K., & Houston, G. (2005). Seal Coat Systems in Canada: performances and practice, 2005 Annual Conference of the Transportation Association of Canada, Gransberg, D. and James, D.M. B., Chip Seal Best Practices, A Synthesis of Highway Practice, NCHRP Synthesis 342, Transportation Research Board, Washington, D. C., Shuler, S., Epps-Martin, A., Lord, T., and Hoyt, D., NCHRP Report 680: Manual for Emulsion- Based Chip Seals for Pavement Preservation (Project 14-17), Transportation Research Board, National Research Council, 2010.

14 14 Shuler, S. and Lord, A., 2009, A New Laboratory Test for Predicting Very Early Chip Seal Performance, Journal of the Association of Asphalt Paving Technologists, Vol. 78. Washington State Department of Transportation. (2003, March). Asphalt Seal, Technology Transfer, 1-30.