Polished Stone Value (PSV)
The Polished Stone Value (PSV) test is an important evaluation method for the skid resistance of road surfaces. The test measures how quickly a sample of aggregate becomes polished or smooth after repeated passes of tyres. The rough texture of the road surface layer is essential for providing skid resistance to traffic. The higher the PSV, the longer the aggregate can maintain its rough texture in road surfacing, providing better skid resistance.
To perform the PSV test, an accelerated polishing machine is used, which simulates the effect of tyres on a road. Samples of aggregate are fixed into small moulds with resin according to a specified method. A control sample of known PSV is also included. Each sample is then subjected to polishing in the accelerated polishing machine, in the prescribed order.
After polishing, the friction test is conducted first on the control samples to calibrate the apparatus. Then, each test sample is tested in turn. PSV results are expressed as categories such as PSV68, PSV62, or PSV56, with the number representing the minimum value for the aggregate. For road surfacing in critical high grip locations, a value of at least 62 is often required to ensure adequate skid resistance.
The Polished Stone Value test is a crucial method for evaluating the skid resistance of road surfaces. The higher the PSV, the longer the aggregate can maintain its rough texture in road surfacing, providing better skid resistance to traffic.
Test Method
The Polished Stone Value (PSV) Test is a method used to determine the resistance of an aggregate to polishing and its ability to provide skid resistance on road surfaces. The test method is outlined in detail in BS 812-114, and EN 1097-8, and a copy of the document is necessary to perform the test correctly.
To carry out the PSV test, four curved test specimens are prepared from each sample to be tested. These specimens consist of at least 36 chippings of a specific size, typically a 6.3/10mm (de-flaked), which are carefully controlled and supported in a rigid matrix. Fourteen specimens, including two Control specimens, are clamped around the periphery of a "road wheel" and subjected to two phases of polishing using wheels with rubber tyres. The first phase involves abrasion by a corn emery for three hours, followed by three hours of polishing with an emery flour.
After the polishing phase, the degree of polish on the specimens is measured using a portable skid resistance tester. A special narrow slider, shorter test length, and supplementary scale are used under carefully controlled conditions. Control specimens are used to condition and check the slider before the test, and a pair of control specimens is included in each test run of fourteen specimens to check the entire procedure and to adjust the result for minor variations in the polishing and friction testing.
The results of the PSV test are expressed as "polished stone values" (PSVs), which represent the mean of the four test specimens of each aggregate. The PSV value indicates the aggregate's ability to retain its rough texture, which provides skid resistance to road surfaces. Higher PSV values indicate that the aggregate will hold its resistant texture for a longer period, making it suitable for road surfacing in critical locations where skid resistance is of utmost importance.
Related test
The Aggregate Abrasion Value (AAV)
Petrology and PSV
The petrological properties of natural roadstone do not provide a reliable means to predict its polishing qualities, but some indicators have been identified through extensive research.
Rocks that have minerals with varying hardness and those that wear by pulling out mineral grains from a relatively soft matrix tend to have higher resistance to polishing. On the other hand, rocks consisting of minerals with nearly the same hardness tend to wear uniformly and have lower resistance to polishing.
The gritstone group is excellent in terms of resistance to polishing, while the limestone and flint groups yield the lowest resistance. The basalt, granite, and quartzite groups yield intermediate results.
Samples from the basalt group show a wide range of resistance to polishing, which is influenced by the proportion and hardness of secondary minerals. The presence of minerals with different hardness, foliated or fluxioned ground-mass, and softer minerals tend to increase resistance.
In the case of igneous rocks, the petrological characteristics that most significantly affect resistance to polishing are the variation in hardness between minerals and the proportion of soft minerals. Rocks with cracks and fractured minerals tend to have higher resistance, while finer-grained allotriomorphic rocks tend to polish more readily.