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Determination of The Stripping Value of Aggregates on Bituminous Mix

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AIM OF THE EXPERIMENT:
To determine the stripping test of aggregates on bituminous mix and to ascertain the suitability of road aggregates for bituminous road construction

CODE OF REFERENCE:
• IS: 6241- 1971 Methods of Testing Tar and Bituminous Materials: Determination of Stripping value of aggregates, Bureau of Indian Standards.

APPARATUS USED:
• Water bath: It should be thermostatically controlled so as to exhibit the experiment accurately.
• IS Sieves: IS Sieves of 20 mm and 12.5 mm sizes should be used for the experiment.
• Beaker: A small beaker for heating of aggregates and mixing them with bitumen will be used.
• Casserole: A small casserole is used to mix the hot bitumen and aggregate.
• Thermometer: It should be accurate and should have graduations that are clearly visible. Any leaks should be inspected or if broken should be replaced immediately as slight impurities may vary test results.

THEORY

Good adhesion between aggregate surface andbitumen filmis mandatory for a durable bituminous pavement. Disbonding of bitumen film from the surface of the aggregate in the presence of water or moisture is usually known as stripping.Bitumen and tar adhere well to all normal types of aggregates provided that they are not exceptionally dusty and dry.The stripping value is defined as the ratio of the average uncovered or stripped area observed visually to the total area of aggregates in each test, expressed as percentage. This test engulfs the static immersion method for determining stripping value of aggregates used in road construction. A stripping test method is developed by Road Research Laboratory (RRL) England.In this test, the stripping value is measured by visual inspection. In this test, about 200 g of aggregates passing through 20 mm sieve and retained on 12.5 mm sieves are taken and about 5% by weight of the aggregates bitumen is taken. The bitumen and aggregate are preheated separately. Then they are mixed thoroughly and the mixture is transferred to a beaker and allowed to cool to room temperature for 2 hours. It is then kept in water bath at a temperature of 400 C for 24 hours. Afterwards, the beaker is taken out and allowed to cool to room temperature. The extent of stripping is estimated by visual examination when the mixture is immersed in water.

Stripping value = (Uncovered surface area of aggregate by visuale xamination)/(Total surface area of aggregates)

RELEVANCE OF THE EXPERIMENT:
• This test is conducted to determine the effects of moisture upon the adhesion of bituminous film to the surface particles of the aggregates.
• This test of significant value to ascertain the suitability of two material viz bitumen (binder) and aggregates because one particular aggregate may be satisfactory with one binder and unsatisfactory with another and the same being true for binders.
• The specifications of Ministry of Transport and Shipping recommend the determination of stripping value by the static immersion method in accordance with IS 6241-1971.
• This is used to ascertain the suitability of road aggregates for bituminous road construction.
• This test also provides information about penetration grade bitumen, flux bitumen and road tar.
• The problem of stripping is experienced when the pavement layer constructed of bituminous binder, is subjected to prolonged soaking under water due to rain or flood.
• In bituminous road construction, Indian Road Congress (IRC) specified maximum stripping value as 25% to be used in bituminous construction like bituminous macadam, surface dressing and bitumen mastic.

PROCEDURE
1. 200 g of aggregate is taken passing through 20 mm sieve and retained on 12.5 mm sieve and heated upto 1500C.
2. 5% of weight (10 g) of bitumen is taken and heated it upto 1600C.
3. Aggregate and bitumen are mixed thoroughly to coat aggregate surfaces completely with bitumen.
4. The mixture is transferred to a beaker of capacity 500 ml and allowed it to cool for 2 hours at room temperature.
5. Beaker is then filled with distilled water to immerse aggregate-bitumen mix.
6. The beaker is covered and then kept in water bath at temperature 400 C for 24 hours. Care is taken so that level of water in water bath is at least half of height of beaker.
7. Beaker is taken out of water bath after 24 hours and cooled it to room temperature.
8. The stripping value is estimated by visual examination.
9. Stripping value is expressed as uncovered surface area of aggregate by visual examination to total surface area.

OBSERVATIONS AND RESULTS
i. Percentage of area of aggregates uncoated by immersion in water =
ii. Therefore, stripping value =

DISCUSSION
• The mean of three results are reported as stripping value of the tested aggregates to thenearest whole number.
• The results of the stripping value test given is an indication regarding susceptibility of aggregates to the action of water or moisture.
• More the stripping value, the poorer are the aggregates from point of view of adhesion.
• The visual assessment of stripping value of aggregate is more of subjective type rather than experimental and may lead to poor reproducibility. But still this test remains an indicator as how a mixture of aggregates and binder may behave in the presence of water. Thus, an adhesion test such as the simple static immersion test or the stripping test would be more suitable to assess that whether the binder would adhere to the aggregate when it is immersed in water.
• Indian Road Congress (IRC) has specified the maximum value of 25% for aggregates to be used in bituminous road construction.
• Generally, the stripping value less than 5% is preferred for road aggregates.
• The maximum value suggested by IRC is 10% for aggregates used in open graded premix carpet.
• If the stripping value exceeds the specific range, use of anti-stripping agents may be used. Several anti-stripping agents are available which when used with the bituminous mix could reduce stripping.
• The stripping is found to be more in type of aggregates because of the fact that such aggregates have higher affinity towards water than with bitumen. The displacement of bituminous coating from the aggregates depends on the physio-chemical forces acting on the system.
• Also, bituminous binders are available that are either cationic or positive and anionic or negative and hence a suitable selection is made depending upon aggregates available.
• Apart from this alternative method can be used but this test is generally preferred keeping in mind that it is less time consuming, no skills needed, no expert supervision needed and it is economical to use.

PRECAUTIONS
• The hand gloves should be used while removing containers from oven after switching off the oven
• Safety shoes and aprons at the time of test must be worn for protection.
• Cleaning and drying before the commencement of test is recommended.
• Care shall be taken while mixing aggregates with bitumen so that no surface remains uncovered.
• If possible, distilled water should be used for the test in order to obtain accurate results.
• Before mixing with the binder, the aggregates should be thoroughly dried.

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