RESEARCH PAYS OFF Chip Seals for Improved …

1 TR NEWS 282 SEPTEMBER OCTOBER 201241 Corley-Lay is StatePavement ManagementEngineer, and Wofford isState PavementPreservation Engineer,North CarolinaDepartment ofTransportation, Seals are used routinely in the mainte-nance and preservation of roadways. In theUnited States, emulsion-based chip Seals , withemulsified asphalt binders and natural mineral aggre-gate chips, are commonly used. The chip seal is constructed by spraying theasphalt emulsion onto the asphalt pavement, thenspreading chips of aggregate into the emulsion,embedding the chips with pneumatic and rubber-tired rollers, and finally sweeping to remove theexcess chips.

2 Sometimes, the process is repeated adouble seal, with emulsion sprayed again and a sec-ond layer of chips added. The process Seals finecracks in the underlying pavement surface, reducingpavement deterioration by preventing water fromintruding into the base and subgrade. ProblemDespite the apparent benefits and widespread use ofchip Seals as a preservation treatment, some statedepartments of transportation (DOTs) have beenreluctant to adopt the technology because of limitedfamiliarity with chip seal practices.

3 North CarolinaDOT maintains an extensive network of low-volumeroads roads traveled by less than 3,000 vehiclesper day; for these roads, chip seal is a logical preser-vation option and has been employed for many tightening budgets, however, the departmentsought ways to improve chip seal performance byoptimizing the material selection, mix design, androlling equipment and pattern. SolutionDocumentation and ResearchNorth carolina DOT personnel began to documentchip seal practices and performance, to increasefamiliarity with the preservation treatment, to under-stand its performance and benefits, and to supporttargeted RESEARCH to enhance its application.

4 In par-ticular, NCHRP Synthesis 342, Chip Seal Best Prac-tices (1), helped department staff increase familiaritywith the technology. In addition to encouraging theuse of chip seal as a preservation treatment, the syn-thesis led North carolina DOT to sponsor a series ofresearch projects to optimize the chip seal processand increase its benefits. North carolina DOT s in-house maintenancecrews perform most of the state s chip sealing. Road-ways are selected for treatment at the division orcounty level. The NCHRP synthesis report notedthat every aspect of the chip seal process can beimproved and optimized; North carolina DOT there-fore sponsored a series of RESEARCH projects at NorthCarolina State University (NCSU) to evaluate andimprove the various aspects of chip seal design andconstruction.

5 A committee that included pavementmanagement engineers, road maintenance engineers,and bituminous supervisors was appointed to over-see each project. Projects and FindingsOne project aimed at optimizing chip investigated the aggregate retention andfrictional characteristics of lightweight and graniteChip Seals for ImprovedPavement PreservationNorth carolina s Approach J U D I T H C O R L E Y- L AY A N D D E N N I S W O F F O R D R E S E A R C H PAY S O F FRolling the chip-sealsurface : NORTHCAROLINADOTTR NEWS 282 SEPTEMBER OCTOBER 201242aggregates.

6 The project demonstrated that light-weight aggregate with a 5/16-inch, or (mm), nominal maximum aggregate size, a morecubical shape, and uniform gradation provided bet-ter aggregate retention than the fine-graded graniteaggregate ranging from to mm in size. Forboth aggregate types, uniform gradation was mostcritical in minimizing aggregate loss. The researchrecommended using only material larger than , with the amount of fines or material smallerthan mm not to exceed project undertook a performance-basedanalysis of polymer-modified emulsions.

7 Theresearch evaluated the performance of single, double,and triple Seals with unmodified and polymer-mod-ified emulsions. The polymer modification wasfound to enhance rutting resistance especially athigh temperatures as well as aggregate retention. Alife-cycle cost analysis concluded that to be cost-effective, polymer-modified chip Seals would need tolast at least two years longer than unmodified chipseals that is, for seven third project evaluated rolling methods todetermine the optimal equipment, number of cover-ages, and rolling pattern (see photo, page 41).

8 According to the findings, the best results wereachieved when the rolling began with a pneumatictire roller and finished with a combination roller;therefore, use of both the pneumatic roller and thecombination roller was recommended. Three coverages were found optimal, consideringboth the time for rolling and the aggregate multiple-layer chip Seals , another key findingwas that rolling the layer immediately below the toplayer Improved the aggregate retention in the projects developed a mix design for chipseals with modified polymer and lightweight aggre-gate, a field test to predict the performance ofrecently placed chip seal, and methods for con-structing samples in the field (see photo, left)

9 Thatcould be removed and tested in the laboratory (seephoto, below, left). In addition, the most promisinglaboratory tests for chip seal performance were iden-tified. An ongoing project is examining chip sealapplication on roadways with traffic volumes of morethan 5,000 vehicles per collaboration between NCSU and North Car-olina DOT has led to implementation of several sig-nificant findings. For example, polymer-modifiedemulsions are now in use, and the department plansto acquire combination rollers when equipment isdue for replacement.

10 In addition, maintenance crewsnow have a better understanding of what makes achip seal perform well and are testing newapproaches to improve performance 14 divisions of North carolina DOT haveadopted the recommended chip gradation and areusing lightweight aggregate and polymer-modifiedemulsions to some extent. All divisions have adoptedthe recommended number of coverages, and mostare using either a pneumatic tire or combinationroller. BenefitsThe program of RESEARCH on surface treatments hasproduced obvious benefits.