Association of Asphalt Paving Technologists A Study of Cement Modified Bitumen Emulsion Mixtures

Earlier studies have demonstrated the beneficial effects of adding Ordinary Portland Cement (OPC) to bitumen emulsion mixtures. These effects were confirmed for some dense graded mixtures based on recipes currently used for hot mix in the UK and being considered for use in cold mix. Laboratory tests included stiffness modulus, resistance to permanent deformation and resistance to fatigue cracking. In order to obtain a clearer understanding of the mechanisms causing the improvements in properties, some basic studies were carried out. These included measurement of the rate at which coalescence of bitumen droplets developed and adhered to the aggregate particles, since this is the initial mechanism by which mechanical properties of the mixture are developed. The study was then extended to obtain an understanding of the properties of emulsion blended with OPC, hydrated lime or limestone filler. This was done since it was thought that a contribution to 'binding' of the aggregate in mixtures came from the hydration of cement as well as from the coalesced bitumen. Dynamic Shear Rheometer tests were used on various blends demonstrating the stiffening effects of both OPC and hydrated lime and that filler had little influence. The emulsifying process was also shown to have no effect on the characteristics of the base bitumen. Electron microscopy was used to study the crystalline structures of fully cured mixtures with and without OPC addition. Expert interpretation considered that some of the characteristics of cement hydration effects were present in those mixtures incorporating OPC. The study concluded that the improvements to key properties of cold mix by the addition of OPC can be explained by a range of mechanisms, including improved rate of emulsion coalescence after compaction, cement hydration and enhancement of binder viscosity.