Increase in asphalt concrete performances has often focused on the use of modification polymers, for bitumen – PMB, What is often unconsidered is the importance of bitumen-aggregate adhesion which is independent from any polymer modification performed.
Perfect adhesion between the binder and the stone elements prevents water from filtering between the two materials, this prevent premature aging of the bituminous mixture and leading to the formation of instabilities that decrease the pavement’s performances.
Better adhesion between aggregates and bitumen can be ensured through the usage of anti-stripping agents.
One of the main triggers of pavement deterioration is the weakening or the detachment of the bitumen film adhering to the surface of the aggregate (stripping). In detail, the factors that affect stripping are:
- Characteristics of aggregates and bitumen used
- Type of Mixture whether dense graded, porous, SMA etc
- Production process such as Hot mix asphalt, warm asphalt, and use of wet aggregates.
- Also condition of end use such as loading, maintenance methods.
As soon as the bitumen film separates, the pavement immediately starts to deteriorate.
Asphalt mix without anti stripping agent after boiling test
Asphalt mix with anti stripping agent after boiling test
Every time there is a separation of the bitumen layer from the stone surface, there is the interposition of water and a direct reduction in performances of the asphalt concrete. Various studies have shown that water-proofing of pavement is the best solution. The situation is guaranteed both through the correct mix design of the laboratory mixtures and through the use of the necessary anti-stripping agents.
If it is not possible to alter the nature of the aggregate constituting the bituminous mixtures, the only controllable factor is the bitumen, through the use of anti-stripping agents. Anti-stripping agents are chemical mixtures containing active functional groups that improve the adhesion of the bitumen film on the surface of the aggregates. Mixed with the binder, they lower the surface tension of the bitumen. There are different types of anti-stripping (amino, polyphosphoric or silane).
Since the bitumen is acidic in nature, we have that:
- Polyphosphoric and the silane are used with granite, limestone and basalt (in general, with all the types of aggregates);
- Amine is used with basalt and granite (aggregates with medium and high silica content)
Some of the tests mostly used for the verification of the stripping are:
- thin-layer chromatography test;
- Patti test;
- ASTM stripping test (ASTM D1664-80 (1985);
- Method of test for film stripping (California Test 302 – 2014)
- Effect of Water on Bituminous-Coated Aggregate Using Boiling Water (ASTM 3625 – 96 (2005);
- boiling Test (EN 12697-11).