STRIPPING VALUE OF AGGREGATES.


STRIPPING VALUE OF AGGREGATES.
(IS: 6241 – 1959)

INTRODUCTION:

Bitumen and tar adhere well to all normal types of aggregates provided they are dry and are not exceptionally dusty. Largely the viscosity of the binder controls the process of binding. When the viscosity of the binder is high, coating of aggregates by the binder is slower. In the absence of water there is practically no adhesion problem in bituminous road construction. Two problems are observed due to the presence of water. First, if aggregate is wet and cool it is normally not possible to coat with a bituminous binder. This problem can be dealt with by removing the water film on aggregate by drying, and by increasing the mixing temperature. Second problem is stripping of coated binder from the aggregate due to the presence of water. This problem of stripping is experienced only with bituminous mixtures, which are permeable to water.

Object:

To determine the stripping value of aggregates by static immersion method.

Apparatus:

Thermostatically controlled water bath, beaker.

Procedure:

This method covers the procedure for determining the stripping value of aggregates by static immersion method, when bitumen and tar binder are used. 200gm of dry and clean aggregates passing 20mm IS sieve and retained on 12.5mm sieve are heated up to 1500C when these are to be mixed with bitumen and the aggregates are heated up to 1000C when these are to be mixed with tar. 5% by weight of bitumen / tar binder is heated to 1600C (1100C in the case of tar binder). The aggregate and binder are mixed thoroughly till they are completely coated and the mixture is transferred to a  beaker and allowed to cool 500ml at room temperature for about two hours. Distilled water is then added to immerse the coated aggregates. The beaker is covered and kept in a water bath maintained at 400C, taking care that the level of water in the water bath is at least half the height of the beaker. After 24 hours the beaker is taken out, cooled at room temperature and the extent of stripping is estimated visually while the specimen is still under water.

The stripping value is the ratio of the uncovered area observed visually to the total area of aggregates in each test, expressed as a percentage.

Limits: The maximum stripping value is 5 %( i.e., minimum retained coating is 95%).


















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