Guidelines on the Use of Thin Surface Mixes for Pavement Preservation

Thin surface mixes (TSMs) are high-performance overlays designed to be placed at a thickness of between 0.5 and 1 inch and are used primarily as a pavement preservation surfacing. These mixes are comprised of high quality aggregates and polymer-modified binders with minimum binder contents of 6.0%. No recycled materials are allowed. These mixes also must pass rutting and cracking requirements. The mixture design and performance test requirements result in a surfacing that is more flexible, durable, and skid resistant than conventional dense-graded overlays. The objective of this paper is to present recommended guidelines on the use of these mixes based on the experiences of Texas Department of Transportation's (TxDOT’s) Austin District. TSMs are best used on structurally sound pavements beginning to show signs of age-associated distress: block cracking, longitudinal cracking in the wheel path, shallow rutting, raveling, oxidation, and/or loss of friction. One of the key construction issues with TSMs is ensuring a good bond to the existing surface. Either a non-tracking tack or spray-applied underseal membrane is recommended to ensure a good bond. Another key construction issue is achieving good compaction. Because these surfaces are so thin and there is no density control, it is critical to select a good rolling pattern. The costs of TSMs are generally more (per ton) than conventional dense-graded mixes; however, due to their thin application, they cost less per square yard. The Austin District has realized a 30% cost savings per square yard over typical 2-inch conventional, dense-graded mixes.