PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 241

T ABLE 7.2 Maximum Permissible Water-Cementitious Materials
Ratios for Concrete when Strength Data from Field Experience or Trial
Mixtures are not Available*

Specified 28-day Water-Cementitious Materials Ratio by Weight

compressive Strength,

f œ ,

MPa (psi)

Non-Air-Entrained Air-Entrained

Concrete Concrete

17 (2500) 0.67 0.54

21 (3000) 0.58 0.46

24 (3500) 0.51 0.40

28 (4000) 0.44 0.35

31 (4500) 0.38 **

35 (5000) ** **

*American Concrete Institute (ACI 318), 1999.
**For strength above 31.0 MPa (4500 psi) (non-air-entrained concrete) and 27.6
MPa (4000 psi) (air-entrained concrete), concrete proportions shall be
established from field data or trial mixtures.

For small projects of noncritical applications, Table 7.2 can be used in
lieu of trial mixes, with the permission of the project engineer. Table 7.2 is
conservative with respect to the strength versus water-cementitious materi-
als ratio relationship. This results in higher cement factors and greater aver-
age strengths than would be required if a mix design is performed. This table
is not intended for use in designing trial batches; use Table 7.1 for trial batch
design.
The water-cementitious materials ratio required for strength is checked
against the maximum allowable water-cementitious materials ratio for the
exposure conditions. Tables 7.3 and 7.4 provide guidance on the maximum
allowable water-cementitious materials ratio and the minimum design com-
pressive strength for exposure conditions. Generally, more severe exposure
conditions require lower water-cementitious materials ratios. The minimum
of the water-cementitious materials ratio for strength and exposure is select-
ed for proportioning the concrete.
If a pozzolan is used in the concrete, the water-cementitious materials
plus pozzolan ratio by weight may be used instead of the traditional water-
cementitious materials ratio. In other words, the weight of the water is di-
vided by the sum of the weights of cement plus pozzolan.

3. Coarse Aggregate Requirements The next step is to determine the suitable
aggregate characteristics for the project. In general, large dense graded aggre-
gates provide the most economical mix. Large aggregates minimize the amount
of water required and, therefore, reduce the amount of cement required per cubic