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Other Uses of the Skid Tester
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Apart from it's key role in calculating Polished Stone Values in a laboratory environment, the Portable Skid Resistance Tester is a principal instrument for testing existing roads, and is an inexpensive alternative to special purpose vehicles. In many countries, the use of a Portable Skid Resistance Tester usually precedes the purchase and use of an Accelerated Polishing Machine.

Petrology and Polishing
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Extensive research has shown it is not possible to predict polishing qualities of natural roadstone from petrological data. However some indicators have emerged:

Rocks composed of minerals of widely different hardness, and rocks that wear by the pulling out of mineral grains from a relatively soft matrix, had relatively high resistance to polishing. Conversely rocks consisting of minerals having nearly the same hardness wore uniformly and tended to have a low resistance to polishing.


The gritstone group is excellent, with resistance to polishing being always high, whereas the lime stone and flint groups yield the lowest resistance. Other groups, basalt, granite and quartzite, yield intermediate results.


Resistance to polishing of samples from the basalt group show a wide range. Resistance is higher when minerals of different hardness are present, and when the ground-mass is foliated or fluxioned. The resistance is also influenced by the proportion and hardness of secondary minerals, softer minerals giving higher resistance.


In groups of igneous rocks the petrological characteristics which most readily affect resistance to polishing are variation in hardness between the minerals and the proportion of soft minerals. Rocks with cracks and fractured minerals are of higher resistance, whereas finer- grained allotriomorphic rocks tend to polish more readily.

Types of Polishing and Control Material
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Four types of material are used in equipment for calculating PSVs.

  • Emery Corn The first three hours of the polishing operation uses this material to remove high spots, and condition the surface of the specimen.
  • Emery Flour  The second three hours of the polishing operation uses this material to polish the samples.
  • Control Stone This stone is used in the polishing Machine to provide a comparison against which the results of the aggregate under test can be measured. 2 out of 14 samples in each test are from this material.
  • Criggion Stone Used with the Portable Skid Resistance Tester for calibration purposes.

The machine is based on the Izod principle. It has a pendulum consisting of a tubular arm rotating about a spindle attached to a vertical pillar. At the end of the tubular arm is a head of constant mass with a spring loaded rubber slider. The pendulum is released from a horizontal position so that it strikes the sample of aggregate with a constant velocity. The friction of the surface of the sample, which has undergone preparation by the Accelerated Polishing Machine, determines the distance the head travels after striking the sample.

 

The results shown by the Portable Skid Resistance Tester, as Polished-Stone Values are the coefficient of friction multiplied by 100.


The Portable Skid Resistance Tester is calibrated by the use of Criggion Stone, a material of exceptionally consistent characteristics, which comes from a quarry in North Wales.