Selection of Concrete Mix Proportions

Concrete mix proportions is a process of optimization of several desirable characteristics based on the project requirements.

The characteristics to be optimized are economy, strength, durability, and placeability.

1. Economy

The primary reason for systematically determining mixture proportions is economy. The maximum economy can be achieved by minimizing the amount of cement used and where appropriate, by replacing portland cement with usually less expensive pozzolana or GGBF slag. Economy is also improved by using the largest nominal maximum size aggregate consistent with the dimensional requirements of the structures on the project, and available to the project.

2. Strength

Strength is an important characteristic of concrete but other characteristics such as durability, permeability, and wear resistance may be equally or more important. These may be related to strength in a general way but are also dependent on other factors.

For a given set of materials, strength is inversely proportional to the w/c.

Since the materials which make up concrete are complex and variable, an accurate prediction of strength cannot be based solely on the selected w/c but must be confirmed by tests of cylinders made from trial batches with the materials to be used on the project. Strength at the age of 28 days is frequently used as a parameter for structural design, concrete proportioning, and evaluation of concrete. When mass concrete is used, the design strength is generally required at an age greater than 28 days, generally 90 days, because mixtures are proportioned with relatively large quantities of pozzolana or GGBF slag to reduce internal heat generation. The early strength of mass concrete will be low compared to that of structural concrete; therefore, mass concrete should be proportioned for an adequate early strength as may be necessary for form removal and form anchorage. A compressive strength of 500 psi at 3 days age is typical of that necessary to meet form-removal and form anchorage requirements.

3. Durability

Concrete must resist deterioration by the environment to which it is exposed, including freezing and thawing, wetting and drying, chemical attack, and abrasion. Concrete must meet three requirements before it may be considered immune to frost action. It must be made with non-frost susceptible aggregates and a proper air-void system, and it must achieve an appropriate degree of maturity before repeated freezing and thawing is allowed to take place while the concrete is critically saturated. All exposed concrete placed by the Corps should be air entrained unless it is shown to be improper for a specific situation. The appropriate maturity exists when the concrete has a compressive strength of approximately 3,500 psi. Generally, durability is also improved by the use of a low w/c since this reduces permeability and the penetration of aggressive liquids.

4. Placeability

Placeability, including satisfactory finishing characteristics, encompasses traits described by the terms “workability” and “consistency.” Workability is that property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed, placed, consolidated, and finished. Consistency is the relative mobility or ability of freshly mixed concrete to flow.

Workability embodies such concepts as moldability, cohesiveness, and compactability and is affected by the grading, particle shape, and proportions of aggregate; the quantities and qualities of cementitious materials used; the presence or absence of entrained air and chemical admixtures; and the consistency of the mixture. The slump test, ASTM C 143 is the only test commonly available to measure any aspect of the several characteristics included in the term “placeability.”

Moldability, cohesiveness, compactability, and finishability are mostly evaluated by visual observation, and, therefore, the evaluations are somewhat subjective. Typically, the Contractor will evaluate these characteristics from a different perspective than the government personnel involved, and within the Contractor’s organization, the placing foreman may evaluate the placeability differently than the finishing foreman. In general, the Contractor would like a high-slump mixture, while the Government desires a closely controlled slump. The key consideration must be a carefully proportioned concrete mixture which is placeable by the conveying and placing equipment to be used on the project without the addition of water at the placement site. Simply adjusting the water content of a mixture that was proportioned for placement by crane and bucket will not assure that it is pumpable or that such an adjustment will result in concrete that meets strength and durability requirements. Mass concrete mixtures are particularly susceptible to placing problems if not correctly proportioned. Care must be exercised to assure that the mortar content of lean, mass concrete mixtures is sufficient to provide suitable placing and workability. Water-reducing admixtures should not be used to reduce the paste content and the resulting mortar content of these mixtures to a level which causes the mixture to be harsh and unworkable.