PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 236

The materials engineer is responsible for assuring that the concrete is properly
proportioned, mixed, placed, and cured so to have the properties specified by
the designer.
The proportioning of the concrete mix affects its properties in both the
plastic and solid states. During the plastic state, the materials engineer is
concerned with the workability and finishing characteristics of the concrete.
Properties of the hardened concrete important to the materials engineer are
the strength, modulus of elasticity, durability, and porosity. Strength is gen-
erally the controlling design factor. Unless otherwise specified, concrete
strength refers to the average compressive strength of three tests. Each
test is the average result of two

f¿ c

0.15-m * 0.30-m 1 6-in. * 12-in. 2 cylinders

tested in compression after curing for 28 days.
The PCA specifies three qualities required of properly proportioned
concrete mixtures (Kosmatka et al. 2002):

1. acceptable workability of freshly mixed concrete
2. durability, strength, and uniform appearance of hardened concrete
3. economy

In order to achieve these characteristics, the materials engineer must de-
termine the proportions of cement, water, fine and coarse aggregates, and the
use of admixtures. Several mix design methods have been developed over
the years, ranging from an arbitrary volume method (1:2:3 cement: sand:
coarse aggregate) to the weight and absolute volume methods prescribed by
the American Concrete Institute's Committee 211. The weight method pro-
vides relatively simple techniques for estimating mix proportions, using an
assumed or known unit weight of concrete. The absolute volume method
uses the specific gravity of each ingredient to calculate the unit volume each
will occupy in a unit volume of concrete. The absolute volume method is
more accurate than the weight method. The mix design process for the
weight and absolute volume methods differs only in how the amount of fine
aggregates is determined.

■ 7.1.1 Basic Steps for Weight and Absolute

Volume Methods
The basic steps required for determining mix design proportions for both
weight and absolute volume methods are as follows (Kosmatka et al. 2002):

1. Evaluate strength requirements.
2. Determine the water-cementitious materials ratio required.
3. Evaluate coarse aggregate requirements.
■ maximum aggregate size of the coarse aggregate
■ quantity of the coarse aggregate
4. Determine air entrainment requirements.
5. Evaluate workability requirements of the plastic concrete.
6. Estimate the water content requirements of the mix.
7. Determine cementing materials content and type needed.