Introduction

The design of concrete with specified properties for an application is not a new science, but it has taken on a new meaning with the wide use of high performance concretes (HPC). Recently, an ACI task group (subcommittee of the Technical Activities Committee on HPC, THPC) published a new definition of HPC [1]. This definition states: "HPC is a concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing". They continue by citing some of the properties that are "critical for an application." The following properties are related to fresh concrete: "ease of placement" and "compaction without segregation." The term, ease of placement, covers various other properties of fresh concrete that are called: workability, flowability, compactibility, stability, finishability, pumpability, and/or consistency. These words are often used interchangeably without a definition based on fundamental measurements of properties. Typical qualitative definitions are: 1) "the ease with which concrete can be mixed, placed, compacted and finished [2]; 2) "consistency is the ability of freshly mixed concrete to flow." [3]

Several attempts were made to better relate fresh concrete properties with measurable quantities. Ritchie [4] attempted to define the flow of concrete by linking it to various effects such as bleeding, sedimentation, and density. He distinguished three properties: stability, compactibility and mobility. Stability is linked to bleeding and segregation. Compactibility is equivalent to density, while mobility is linked to internal friction angle, bonding force and viscosity. These descriptions, although subjective, at least link commonly used words with physical factors that can be measured. Other researchers [5, 6, 7] treated fresh concrete as a fluid and used the fluid rheology methods to describe concrete flow. This approach, the most fundamental one, is reviewed in this paper.

The main topics that will be addressed are:

• Review the fundamental definitions of quantities used to uniquely describe the flow of concrete
• An overview of the tests that are commonly used to measure the rheology of fresh concrete.
• Methods to predict the flow of concrete from either composition or laboratory tests
• The main parameters that affect the flow of fresh concrete, other than composition
• Two special applications of rheology: pumpable concrete and self-compacting concrete