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The need for a laboratory variable-speed friction tester was recognized several years ago when emphasis was placed on laboratory test procedures to predict pavement skid resistance in the field. A tester was needed that could provide speed gradients on small samples or speed gradients that are comparable to those obtained by using the locked wheel skid trailer. The variable-speed friction tester measures pavement or pavement sample friction at various speeds and simulates the capability of the ASTM locked wheel skid trailer with which an excellent field correlation has been obtained. The variable-speed friction tester is reasonably portable and may be used to measure friction values on ramps, curves, city streets, and short bridges where towed-type testers usually cannot be used.

Aggregates in stockpiles are collections of various sizes of irregularly shaped particles often with rough or convoluted surfaces, sometimes with pores and wrinkles or crevices that hold water. Because the particles do not fit together nicely, spaces called voids are left between them and the whole pile contains water in the pores and on the surface of each particle, even when located on the edge of the African desert. In order to use these aggregates as the largest part of a concrete mixture, five factors must be determined: (1) the volume of voids in order to fill them with paste and to float the particles apart with an excess of paste, (2) the volume that solids will contribute to the concrete mix, (3) the specific gravity in order to convert volume to weight, since most concrete is batched by weight, (4) the absorption because it affects the specific gravity that is used, and (5) the surface moisture because mixing water must be adjusted to compensate for it.

The purpose of the research reported in this paper is to provide a rapid means for determining the entrained air content of hardened concrete by using scanning electron microscope photographs of fractured faces of samples from hardened concrete. The method that has been developed and verified at a reasonable level of confidence consists of an array of low magnification scanning electron microscope photographs of fractured faces of concretes of known air contents. The air contents range from 0 to 20% in increments of approximately 1%. At a magnification of ×80, air-entrained bubbles interrupted by the fracture face are visible as portions of hollow spheres. To determine the air content of a concrete a random series of photographs is made of fractured faces, and these are compared as a group to the array of known air content photographs until a best match is made. With untrained personnel, 1 σ precision, where σ is the standard deviation, of less than 1.5% has been obtained in selecting the correct air content for a concrete with an unknown air content.