Normally bitumen adheres to aggregates when aggregates are free from dust, debris, foreign particles and moisture.
In absence of water, bitumen performs well adheres to aggregates. But in presence of water the adhesion with bitumen and aggregates start to decrease. Water decreases the bond between aggregates and bitumen. This is why water is called Enemy of road construction.
Adhesion is difficult when aggregates are wet and cold. So precautionary measures need to be taken to remove moisture from aggregates. Also water causes stripling of binder from coated aggregates.
Do determine the adhesion of bitumen to aggregates in presence of water, there are several test are conducted.
Static Immersion Test is one of simple test specified by IRC (Indian Road Commission).
To find out Bitumen adhesion between aggregates and bitumen Stripping value test is commonly used.
IRC has specified maximum stripping value of aggregates should not exceed 5%.
What is Stripping Value Test?
- The Stripping value is determined as ratio of uncovered area to total area of aggregates. This uncovered area are responsible to hold the water and causes breakage of adhesion between aggregates and bitumen.
- Thus, the Stripping value is nothing but the breakage of adhesion between aggregates and bitumen.
- The purpose of stripping value of aggregate to find out the presence of water in adhesion bond between aggregates and bitumen.
To determine the stripping value of aggregates
Apparatus for Test:
The apparatus required for this test are:
- Thermostatically controlled water bath.
- Oven to heat aggregate.
- Sieves of sizes 20 mm and 12.5 mm.
- Beaker of 500 ml capacity.
- Mixer to mix aggregate and bitumen.
- Take 200gm of dry and clean coarse aggregates (Passing on 20mm sieve and retained on 12.5mm sieve) and heated it up 150°C when to be mixed with bitumen
- Take bitumen binder 10gm (5% by weight of aggregates) and heated to 160°C.
- After heating process of both, in beaker mix aggregates and bitumen thoroughly till aggregates completely coated with bitumen. And allow it cool room temperature at least for 2 hours.
- Add distilled water to beaker till coated aggregates completely immerse in water.
- Cover the beaker and maintain the water bath at 40°C for 24 hours
- After 24 hours, take out the beaker at cool it to room temperature.
- And by visually see how much bitumen remains coated in percentage and note down the reading.
The percentage of bitumen remains coated after test (take 3 reading of 3 sample)
Stripping value of read aggregates = _________% (Mean of 3 reading)
As per IRC (Indian Road Congress) the maximum stripping value as 5 percent for aggregates to be used in bituminous construction like surface dressing penetration macadam, bituminous macadam and carpet.