Volume 2, Issue 8 (September 2005)
New Chemical Antistripping Additives for Bituminous Mixtures
The main objective of this study was to develop new, low cost, chemical antistripping additives that have low negative effects on Hot-Mix Asphalt (HMA) properties such as stability and low temperature cracking potential. To achieve this objective, HMA specimens were prepared using limestone aggregate, asphalt with 80/100 penetration, and different types of chemical antistripping compounds. Eighteen different low cost chemical compounds were prepared and added to the HMA at different percentages. The effect of 28 different combinations of additive types and amounts on the HMA stripping was evaluated using the modified Texas Boiling Test. The results indicated that out of the 28 evaluated combinations of additive type and amount, five combinations caused a negative or no impact on stripping resistance. On the other hand, twelve additive combinations show relatively small improvements in stripping resistance. Another 11 additive combinations showed a positive effect on stripping resistance. Five of these 11 additives showed a high positive improvement in the stripping resistance (increase in coating areas was greater than 20%). However, of these five additives, four caused a high reduction in the asphalt ductility and penetration, while the fifth one (Aluminum mono-stearate) caused a small reduction in ductility and an insignificant change in penetration. This suggests that this additive is suitable for use as an antistripping agent.
The effects of antistripping additives in improving stripping resistance may be explained by the formation of strong links between the metal surfactant and with both the aggregate and asphalt. The stripping resistance of metal surfactants derived from the saponification reaction between the metal hydroxide [Ca(OH)2 and Al(OH)3] and the triglyceride of stearic acid was less than those derived from pure stearic acid (by neutralizing the stearic acid with the metal hydroxides). This may be explained by the presence of glycerol in the metal surfactants derived from the saponification reaction. Since glycerol contains three hydroxyl groups, it makes strong hydrogen bonding with water and appreciable interaction with the metal surfactant due to the presence of dipole-dipole forces. Accordingly, some of the metal surfactant and its attached asphalt are leached out during the boiling test producing a lower anti stripping effect.