Chip Seals for Asphalt Concrete Pavements: A Proposed Emulsion Residue Specification and Existing Pavement Texture Evaluation

Chip seals are a pavement surface treatment used for maintaining asphalt concrete pavements. National Cooperative Highway Research Program (NCHRP) Project 14-17 was performed to produce a national Chip Seal Manual which would consolidate the best chip seal engineering practices. A subcontract to NCHRP Project 14-17 performed at Texas A&M University was the basis for this thesis. It included the following tasks: investigation of a testing and grading system for grading asphalt binder residues from chip seal emulsions; and investigation of texture measurement methods for assessing existing pavement macrotexture before a chip seal is placed.

The performance graded (PG) asphalt binder specification, which was developed to characterize asphalt binder properties related to the performance of hot mix asphalt concrete in pavements, cannot be directly applied to asphalt binders or emulsion residues for use in chip seals. Therefore, the surface performance graded (SPG) specification was developed using the same equipment as the PG system but with some procedural modifications and different limiting values for the test parameters. NCHRP Project 14-17 utilized the PG and SPG systems to grade base asphalt binders and recovered emulsion residues. Two emulsion residue recovery methods were compared: hot oven evaporation with nitrogen blanket and stirred can with nitrogen purge. The PG and SPG grades were found to be similar for the two emulsion residue recovery methods but slightly different from the base asphalt binder. A strawman specification for emulsion residues in chip seals was recommended for use with the stirred can recovery method.

In chip seal construction, macrotexture of the existing pavement affects the rate at which chip seal emulsion must be applied. In this project, existing pavement macrotextures were measured at three chip seal projects immediately before construction using both the sand patch test and the circular track meter, CT Meter. The CT Meter was found to quickly and effectively measure pavement macrotexture. The CT Meter measurements correlated well with the sand patch test measurements.

Finally, this project investigated the utility of measuring pavement macrotexture in the laboratory using the aggregate imaging system (AIMS) on pavement cores and on small samples cut from fabricated slabs. Statistical analyses showed good correlation between the mean profile depth, MPD, calculated from AIMS measurements on pavement cores and small samples, based on analysis using 50 mm (2 inch) segment lengths, and the MPD measured on the pavement or on the large fabricated slabs with the CTMeter. These results supported the use of AIMS to measure pavement macrotexture using small samples in the laboratory.