Use of Chemical Admixtures in Roller-Compacted Concrete for Pavements

Use of roller compacted concrete (RCC) for pavement applications is growing in the United States. This material offers great technical and economic benefits, however there is insufficient research done to understand it better. The drier consistency and lack of adequate paste in RCC makes its fresh behavior very different from other types of concretes. This also leads to challenges in characterizing its properties adequately to be translated to practice. The use of chemical admixture in RCC has not been studied in detail before and hence there is an apprehension in using them. What further aggravates the problem is the use of multiple mixing technologies used in producing RCC. This research attempts to resolve some of these problems. The workability of concrete is considered to be constituted by the cohesion, compactibility, and segregation resistance, retention of workability, water reduction and consistency. Each of these properties was characterized using a test method. These include the use of vibrated slump test, direct shear test as used in soils, and gyratory compaction test as used in asphalt industry. Furthermore, ten most widely used chemical admixtures were tested in a typical RCC mixture. These include water reducers, retarders, air entraining agents and dry cast industry products. For each of these product types, different chemical formulations were selected to evaluate the comparative performances. It is observed that individually each admixture offers distinct benefits and improves different properties of fresh RCC including changing the setting behavior and finishibility. Moreover, for a given mixture, the improvement in workability is a composite function of its components viz. cohesion, compactibility, consistency, water reduction, admixture type and dosage. Finally, a set of recommendations are offered along with some precautions to be taken in using these admixtures individually. It is anticipated that this work will lead to the better characterization of different properties of RCC and use of chemical admixtures with greater confidence