The successful production of high-strength concrete which meets the desired strength and durability is dependent on optimising its rheological (or flow) properties and reducing its loss of workability during the transportation, placing and compaction stages. The research presented in this thesis aimed to: 1. Determine whether mix stability and compactability can be adequately described by the two Bingham parameters of yield value and plastic viscosity. 2. Reduce the uncertainties in material selection with regards to the performance of superplasticizers and cement replacement materials. 3. Examine how the two Bingham parameters vary at different degrees of compaction by vibration. 4. Determine how these influence the strength development characteristics in the hardened state. 5. An additional aim was to carry out any modifications to the test apparatus and methods which experience makes necessary. Measurements with Tattersall's MH two-point workability test apparatus indicated that mix stability correlates more closely with the yield value than with plastic viscosity, whilst the opposite is true with respect to compactability under self-weight. The performance of conventional and new-generation superplasticizers (based on SMF, SNF, MLS, Vinyl and Acrylate polymers) was evaluated with different dosages, mixing procedures and cements. The SNF superplasticizer produced slightly lower initial workabilities than the Acrylate superplasticizer, but the longest workability retentions of the superplasticizers tested. Partial cement replacements by CSF in binary blends produced lower superplasticizer dosage demands, higher initial workabilities and longer workability retentions than those due to PFA and GGBS. When used in ternary blended cements, CSF enhanced the performance of mixes containing PFA or GGBS at w/b ratios of 0.30-0.22. A novel method developed to assess the vibration response of fresh concretes has, for the first time, demonstrated that both the yield value and plastic viscosity decrease during compaction. The method has also demonstrated that the maximum compressive strengths and densities of concretes compacted for different vibration durations/amplitudes coincide with the attainment of zero yield value. Continuous reductions in plastic viscosity during vibration mainly reduced the homogeneity of concrete compacted in short columns.