Reactive-Aggregate - Aggregate containing certain silica or carbonate compounds that are capable of reacting with alkalis in portland cement, in some cases producing damaging expansion of concrete.

Reinforced Concrete - Concrete containing adequate reinforcement (prestressed or not prestressed) and designed on the assumption that the two materials act together in resisting forces.

Retardants: Admixtures that increase the setting time in concrete by slowing down hydration.