The general comments contained in ACI 211.1 are pertinent to the procedures discussed in this guide. The description ofthe constituent materials of concrete, the differences in proportioning the ingredients, and the need for knowledge ofthe physical properties of the aggregate and cementitious materials apply equally to this guide. The level of overdesignindicated in ACI 301 and ACI 318/318R should be appliedto the compressive strength used for proportioning.
Workability is the property of concrete that determines theease with which it can be mixed, placed, consolidated, andfinished. No single test is available that will measure this property in quantitative terms. It is usually expedient to usesome type of consistency measurement as an index to work-ability. Consistency may be defined as the relative ability offreshly mixed concrete to flow. The slump test is the mostfamiliar test method for consistency and is the basis forthe measurement of consistency under ACI 211.1.
No-slump concrete will have poor workability if consoli-dated by hand-rodding. If vibration is used, however, suchconcrete might be considered to have adequate workability.The range of workable mixtures can therefore be widened byconsolidation techniques that impart greater energy into themass to be consolidated. The Vebe apparatus,1,2 the compactingfactor apparatus,3 the modified compaction test, and theThaulow drop table4 are laboratory devices that can provide auseful measure of consistency for concrete mixtures withless than 25 mm (1 in.) slump. Of the three consistencymeasurements, the Vebe apparatus is frequently used today inroller-compacted concrete and will be referred to in this guide.
The Vebe test is described in Appendix 2. If none of thesemethods are available, consolidation of the trial mixtureunder actual placing conditions in the field or laboratorywill, of necessity, serve as a means for determiningwhether the consistency and workability are adequate.Suitable workability is often based on visual judgement formachine-made precast concrete products.