PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 247

per m 3

increases about 25 mm (1in.) for each 6 kg of water added (10 lb per

cubic yard) of concrete.

6. Water Content Requirements The water content required for a given slump
depends on the maximum size and shape of the aggregates and whether an
air entrainer is used. Table 7.8 gives the approximate mixing water require-
ments for angular coarse aggregates (crushed stone). The recommendations
in Table 7.8 are reduced for other aggregate shape as shown in this table.

Reduction in
Water Content,

kg/m 3 (lb/yd 3 ) Aggregate Shape

Subangular 12 (20)

Gravel with
crushed particles 21 (35)

Round gravel 27 (45)

These recommendations are approximate and should be verified with
trial batches for local materials.

7. Cementing Materials Content Requirements With the water-cementitious
materials ratio and the required amount of water estimated, the amount of
cementing materials required for the mix is determined by dividing the
weight of the water by the water-cementitious materials ratio. PCA recom-
mends a minimum cement content of for concrete
exposed to severe freeze-thaw, deicers, and sulfate exposures, and not less
than for concrete placed under water. In addition,
Table 7.9 shows the minimum cement requirements for proper placing, fin-
ishing, abrasion resistance, and durability in flatwork, such as slabs.

334 kg/m 3 564 lb/yd 3

1 2

385 kg/m 3 650 lb/yd 3 1 2

8. Admixture Requirements If one or more admixtures are used to add a spe-
cific quality in the concrete (as discussed in Chapter 6), their quantities
should be considered in the mix proportioning. Admixture manufacturers
provide specific information on the quantity of admixture required to achieve
the desired results.

9. Fine Aggregate Requirements At this point, water, cement, and coarse ag-
gregate weights per cubic meter (cubic yard) are known and the volume of air
is estimated. The only remaining factor is the amount of fine aggregates needed.
The weight mix design method uses Table 7.10 to estimate the total weight of
a “typical” freshly mixed concrete for different maximum aggregate sizes. The
weight of the fine aggregates is determined by subtracting the weight of the
other ingredients from the total weight. Since Table 7.10 is based on a “typical”
mix, the weight-based mix design method is only approximate.