The coefficient of friction on dry highway surfaces regardless of stone composition and texture has in most cases been at least 0.6 or above. However, some of these same surfaces when lubricated by a small amount of water have given test results dangerously lower. Some interesting theoretical aspects of this situation are presented here along with results from a laboratory study of the fundamental factors affecting tractive friction.

A machine is described for measuring the coefficient of friction between the plane surfaces of four-inch diameter stone specimens and a rubber annulus of slightly smaller diameter. Measurements were made both wet and dry on finely polished surfaces and on surfaces ground with 80 and 150 grit Carborundum. Tests were conducted under varying loads and speeds. A 60-degree reflectometer was used to evaluate texture and roughness of the plane surfaces. Reflectivity (gloss) values correlated significantly with wet friction values in the highly polished ranges. Tests were conducted on representative samples of four limestones and two sandstones.

Coefficient of friction values of 0.01 and lower were measured on finely polished wet limestone surfaces. Sandstones subjected to the same polishing action averaged about 0.22 when wet. In another series of testing, the specimens were abraded with a coarse Carborundum grit, and the wet friction values were consistently between 0.6 and 0.7 for both limestones and sandstones. For further comparison a piece of plate glass was abraded with this same material, and it too measured within the above limits. Dry friction values remained fairly constant regardless of type of stone or texture.

Test results reveal the tendency for fine grained particles bound in a matrix of similar hardness to polish more readily and to a greater extent than hard particles such as quartz bound in a soft matrix. Limestones, being typical of the former condition, polished easier than sands tones.

Report Date


Report Number

No. 124

Digital Object Identifier



A report prepared for presentation to the 37th Annual Meeting of the Highway Research Board (January, 1958).