Use of Phosphoric Acid as a Modifier for Hot-Mix Asphalt

The use of phosphoric acid to stiffen paving asphalt has become popular. Several premature paving failures have been attributed to its use, largely based on incomplete information. This has created uncertainty about the hidden pitfalls surrounding the use of this additive. The primary concerns are the effect of different grades of phosphoric acid, reaction with asphalt from different sources, accelerated aging of binders, increased moisture sensitivity and use with antistrip additives. This research seeks to address these issues. Data will be presented to show how the moisture sensitivity increases with increasing levels of acid modification. This is presented in terms of moisture being absorbed into binder and mastic samples as well as the leaching of phosphoric acid from gyratory specimens, a potential source of groundwater pollution particularly at high levels of acid modification. Preconceived notions about the use of phosphoric acid with alkaline antistrip additives and the effect on increased binder oxidation rates are also discussed. Phosphoric acid was found to not increase asphalt oxidation rates. Use with antistrip additives is perfectly feasible if the correct screening tests are conducted before use.