Principles for proportioning concrete to achieve a prescribed compressive strength after a given age under standard curing are simple.
1. The strength of a hardened concrete mix depends on the water-cementitious materials ratio (ratio of water to cementitious materials, by weight). The water and cementitious materials form a paste. If the paste is made with more water, it becomes weaker (Fig. 9.2).
2. The ideal minimum amount of paste is that which will coat all aggregate particles and fill all voids.
3. For practical purposes, fresh concrete must possess workability sufficient for the placement conditions. For a given strength and with given materials, the cost of the mix increases as the workability increases. Additional workability is provided by more fine aggregate and more water, but more cementitious materials must also be added to keep the same water-cementitious materials ratio.
Because of the variations in material constituents, temperature, and workability required at jobsites, theoretical approaches for determining ideal mix proportions usually do not give satisfactory results on the jobsite. Most concrete therefore is proportioned empirically, in accordance with results from trial batches made with the materials to be used on the jobsite. Small adjustments in the initial basic mix may be made as a project progresses; the frequency of such adjustments usually depends on the degree of quality control.
When new materials or exceptional quality control will be employed, the trialbatch method is the most reliable and efficient procedure for establishing proportions.
In determination of a concrete mix, past field experience or a series of trial batches is used to establish a curve relating the water-cementitious materials ratio to the strength and ingredient proportions of concrete, including admixtures if specified, for the range of desired strengths and workability (slump). Each point on the curve should represent the average test results on at least three specimens, and the curve should be determined by at least three points. Depending on anticipated quality control, a demonstrated or expected coefficient of variation or standard deviation is assumed for determination of minimum average strength of test specimens (Art. 9.10). Mix proportions are selected from the curve to produce this average strength.
For any large project, significant savings can be made through use of quality control to reduce the overdesign otherwise required by a building code (law). When the owners project specifications include a minimum content of cementitious materials, however, much of the economic incentive for the use of quality control is lost. See Fig. 9.3 for typical water-cementitious materials ratios.
Note that separate procedures are required for selecting proportions when lightweight aggregates are used, because their water-absorption properties differ from those of normal-weight aggregates.
(Building Code Requirements for Structural Concrete, ACI 318 Standard Specifications for Structural Concrete, ACI 301; Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete, ACI 211.1; Standard Practice for Selecting Proportions for Structural Lightweight Concrete, ACI 211.2; Recommended Practice for Evaluation of Strength Test Results of Concrete, ACI 214, American Concrete Institute,38800 Country Club Drive
Farmington Hills, MI 48331 Design and Control of Concrete Mixtures, EB001TC, Portland Cement Association, 5420 Old Orchard Road, Skokie, IL 60077.)