POLISHED STONE VALUE
DETERMINATION OF POLISHED STONE VALUE
BS 812: Part 114
The polished stone value (PSV) gives a measure of the resistance of road stone to the polishing action of vehicle tyres under conditions similar to those occurring on the surface of a road. This test is conducted in two stages. In the first stage the sample of stone aggregates placed in a mould is subjected to a polishing action in an accelerated polishing machine, under standard set of test conditions. In the second stage, the polished sample is subjected to friction test using a pendulum type skid resistance tester to determine the coefficient of friction expressed as percentage or the „Polished Stone Value?.
(i) Accelerated Polishing Machine (With a road wheel with a flat periphery to fix 14 specimens of stone aggregates, 2 solid rubber tyred wheels 200 mm dia, and two feeding mechanism for abrading sand and emery powder) rigidly mounted on a level, firm, concrete base.
(ii) IS test sieves of size 10, 8, 0.425, 0.300, 0.212 and 0.150 mm.
(iii) Mould of size 90.6 mm x 44.5 mm for the preparation of test specimen of stone aggregate.
(iv) Release agent or liquid car polish.
(v) Abrading sand passing 425 micron, 85-100% passing 300 micron, 20-50% passing 212 micron and 0-5% passing 150 micron IS sieve.
(vi) Emery powder 100% passing 60 micron sieve and more than 70% passing 2 micron IS sieve
(vii) Polyester resin and hardener.
(viii) Clean flexible plastic sheets for cutting strips to fix the samples to the polishing machine.
(ix) Pendulum type friction tester.
3. Preparation of test specimen
a) About 3 kg of clean stone aggregate sample, all passing through 10 mm and retained on 8 mm IS test sieves which are not flaky or elongated is collected for the preparation of the test specimen.
b) Prepare a stiff paste of cement mortar with equal proportion of cement and sand of similar grading as the abrading sand.
c) Place a thin layer of this mortar inside the specimen mould.
d) Carefully place the selected particles of the aggregate sample over the mortar in a single layer, as closely as possible and to cover an area of 90.6 x 44.5 mm, with flat surface of the aggregates lying on the bottom of the mould.
e) Fill the space between the particles up to 3 quarters of its depth with fine sand (Passing 0.212 mm sieve) and level it with fine haired brush.
f) The exposed internal faces and top edges of the metal moulds shall be lightly coated with release agent, using a fine haired brush.
g) Mix the hardener with resin and fill the mould to over flowing with mixed resin. The excess resin can be trimmed off using a knife after 5 to 10 minutes
h) After the resin has completely set (About 30 minutes from mixing), remove the specimen from the mould, brush off the loose sand and subject to polishing after a further 30 minutes.
i) Clean the moulds, tools etc with solvent.
j) The mould as well as the prepared sample is flat across the width 44.6 mm but these are curved along the length 90.5 mm as an arc of a circle diameter 406 mm.
k) A minimum of two such test specimens are prepared using each sample of stone aggregates.
4. Accelerated polishing test procedure
The rim of the road wheel can accommodate 14 test specimens which can be simultaneously subjected to accelerating polishing. Therefore it is possible to conduct polishing test simultaneously on different aggregate samples.
a) The test specimens are clamped around the rim of the road wheel with the help of strips of polythene sheet beneath and in between the adjoining test specimens.
b) The rubber tyred test wheel is lowered until it rests on the surface of the test specimens fixed around the road wheel.
c) The required weight is added at the end of the lever so that a total effective load of 40 kg is applied on the road wheel.
d) The motor is switched on and the road wheel rotated to a speed of 320±5 rpm.
e) Abrading sand and water are released at a rate of 27±7 gm/min and these are uniformly spread over the surface of the test specimen and the tyre of the test wheel where they are in contact.
f) The machine is operated for a further period of three hours after releasing the emery powder and water at the specified rate.
g) The test is interrupted after 1 and 2 hours to remove any excess sand collected at the base.
h) After the three hours period, the machine is stopped and the test specimens and the machine are cleaned by washing.
i) The whole procedure is then repeated with emery powder as abrading agent added at a rate of 3±1g/min. Feed water at twice the rate of emery powder
j) After the three hours of polishing with emery powder, the machine is stopped and the test specimens and the machine are cleaned by washing.
k) The polished set of specimens is now ready for determination of the friction coefficient/skid number or the polished stone value.
5. Measurement of friction of polished specimen
Pendulum type friction tester is used to determine the coefficient of friction or the skid resistance value of the test specimen.
a) Keep the friction test apparatus and slider in a room where the temperature is controlled at 20±2°C for at least 2 hours before the test begins and for duration of the test.
b) Keep the friction tester on a firm level surface and adjust the leveling screws such that the column is vertical.
c) The hinge of the pendulum unit is raised and fixed on the column such that the rubber slider does not touch the ground surface and the pendulum can swing freely.
d) The pendulum arm is released from the horizontal rest position by pressing the button and the pendulum is allowed to swing freely, moving the dead pointer along.
e) If the pointer reads zero of the graduated scale, the calibration is with zero error. If not, the friction adjustment screw head at the hinge is adjusted by tightening or loosening as required, until repeat trial of free swing of the pendulum results in pointer showing zero reading on the graduated scale.
f) One of the specimens of aggregate which was subjected to accelerated polishing is rigidly fixed in the slot provided with its longer side lying in the track of the pendulum swing.
g) The hinge of the pendulum is adjusted such that the sliding length of the rubber shoe along the specimen is 75 mm. The hinge of the pendulum unit is fixed in this position by tightening the fixing head.
h) The surfaces of the specimen and the rubber shoe are copiously wetted with clean water.
i) The pendulum and the pointer are released from the horizontal position by pressing the button. The pointer reading from the graduated scale is recorded to the nearest whole number and is noted as the „Skid Number? or the Polished Stone Value.
j) Perform this operation 5 times, each time wetting the specimen. Record the mean of the last three readings to the nearest 0.1.
k) After this test, the sample is removed from the slot and the process repeated with the second sample is fixed in position. If the mean value of the two specimens of the same material differs by more than 3%, the result is discarded and test is repeated with fresh specimens. This procedure may be repeated a number of times using new specimens until two values are within this limit.
l) The samples are tested in the order 13, 1, 10, 3, 5, 12, 8, 7, 11, 6, 4, 9, 12, 14. 6. Report The mean of the two values of the skid number or coefficient of friction expressed as percentage, is reported as the Polished Stone Value (PSV) of the stone aggregate to the nearest whole number.