Skidding accidents on wet pavements is a critical safety issue for highway and aviation agencies. Aggregates play a major role in providing friction and texture, and consequently, in skidding accidents. Dry pavements usually provide good tire-pavement friction, whereas wet pavements reduce skid resistance. Wet friction depends on the hardness of aggregate minerals exposed on the surface and their micro-texture* roughness. Also, for speeds over about 40 mph, pavement macro-texture* is necessary to prevent hydroplaning by helping water escape from the tire-pavement interface.

For Portland Cement Concrete pavement, finishing and texturing equipment (floats, drags, tines or grooving) provides the initial texture. But soon the cement paste wears away, exposing fine aggregate surfaces. It is the fine aggregate exposed at the PCC surface that controls micro-texture frictional properties. Later, some coarse aggregate may be exposed as the macro-texture wears in high traffic areas and in cold climates where studded tires, chains, and abrasives are common.

Many states limit softer, more polishing limestone manufactured fine aggregates in PCC surface layers, or they require a blend with harder silica minerals. Some states use the acid insoluble residue test (ASTM D 3042) to assure that surface aggregates are not essentially all carbonate minerals (limestone, dolomite) that dissolve in the acid. These minerals wear and polish more rapidly under traffic. Polishing is slower if the acid-insoluble residue represents a significant percentage of harder, plus No. 200 size, sandy mineral particles in the aggregate.

In Hot Mix Asphalt pavement surfaces, or chip seals, it is the coarse aggregate that is exposed at the surface. Initially, after HMA laydown and rolling, the aggregate will have a layer of asphalt on top; but with traffic wear, aggregate will be exposed. The micro-texture and polishing properties of the aggregate surfaces influence wet friction. And the grading and texture of the aggregates exposed at the surface influence pavement macro-texture and hydroplaning properties. Some states use ASTM D 3319 for Accelerated Polishing of Aggregates Using the British Wheel to assure good friction. The Federal Highway Administration ran this polish value test on a variety of coarse aggregate types — a high value shows good friction and a low value poor friction after polishing.

Also, several laboratory circular track methods are used to evaluate the polishing tendencies of aggregates in laboratory mixtures. Most agencies regularly evaluate highway pavement friction in the field using a skid trailer (ASTM E 274, AASHTO T 242) or evaluate airfield runways using the Mu-Meter side-force friction device (ASTM E 670). These results can help categorize aggregate types and sources into high, medium or low friction and to point out pavements needing immediate attention to correct low-friction conditions. Not all cases of low friction are directly attributable to aggregate type. Things such as over-asphalted HMA that causes flushing of the asphalt to the surface or a hard-troweled finish on PCC can both be slippery when wet.

Selecting the proper surface aggregates will help assure friction needs in wet weather. Minerals that are hard (Mohs hardness 6 or greater, such as feldspar and quartz) and aggregates with good micro-texture, such as sandstone and slag, or those having rough grains or a mix of minerals with different texture will resist polishing and maintain good wet-weather frictional properties. When potentially polishing limestones or natural polished gravels are to be used in pavement surfaces, improve frictional properties by blending in aggregates with hard minerals and desirable texture.

Richard Meininger, PE has been active in developing aggregate standards for decades. He is past vice president of engineering for both the former National Aggregates Association and the National Ready Mixed Concrete Association.

Aggregate Sources	Polish Values
Expanded Clay Lightweight	45
Blast Furnace Slag	35
Limestone (Sandy)	31
Granites	29, 28, 27
Gravels	27, 26, 25
Limestones	23, 19
*Micro-texture: Microscopic roughness of aggregate surfaces (texture relief less than 0.5-mm) Macro-texture: Larger, visible texture due to projections, depressions, and grooves (more than 0.5-mm texture relief), such as that seen in an open graded friction course or a grooved surface.
(ASTM Terminology E 867 defines micro- and macro-texture relating to pavement systems.)