will take 6 parts per cubic foot of material. For example:

Cement 1 x 6 6 cubic feet

Sand 2 x 6 12 cubic feet

Gravel 4 x 6 24 cubic feet

Total of dry mix 42 cubic feet = 1 cubic yard produced

NOTE: 1 bag of cement equals 1 cubic foot of cement.

In addition to the carpenter's mix, there are other popular rule-of-thumb mixes:

1:1:2 - a very rich mix. Use when great strength is required.

1:2:5 - a medium mix. Use in large slabs and walls.

1:35 - a lean mix. Use in large foundations or as a backing for masonry.

1:4:8 - a very lean mix. Use only in mass placing.

To achieve more control over the proportional quantities of cement, water, and aggregate for a concrete mix, you can use one of three methods (book, trial batch, or absolute volume). These three methods of proportioning concrete mixtures will be briefly covered in this section. First, the BOOK METHOD is a theoretical procedure establishing data to determine mix proportions. Second, the TRIAL BATCH METHOD is based on an estimated weight of concrete per unit volume, and the third method is based on calculations of the ABSOLUTE VOLUME occupied by the ingredients used in the concrete mixture. For a more thorough discussion, you should refer to the most recent edit ion of Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1), published by the American Concrete Institute (ACI), and the Engineering Aid Intermediate/Advanced.

The BOOK METHOD is a theoretical procedure in which established data is used to determine mix proportions. Because of the variation of the materials (aggregates) used, mixes arrived at by the book method require adjustment in the field following the mixing of trial batches and testing. Concrete mixtures should be designed to give the most economical and practical combination of the materials that will produce the necessary workability in the fresh concrete and the qualities in the hardened concrete.

Certain information must be known before a concrete mixture can be proportioned. The size and shape of structural members, the concrete strength required, and the exposure conditions must be determined. The water-cement ratio, the aggregate characteristics, the amount of entrained air, and the slump are significant factors in the selection of the appropriate concrete mixture.

The water-cement ratio is determined by the strength, the durability, and the watertightness requirements of the hardened concrete. The ratio is usually specified by the structural design engineer, but you can arrive at tentative mix proportions from knowledge of a prior job. Always remember that a change in the water-cement ratio changes the characteristics of the hardened concrete. Use table 3-10 to select a suitable water-cement ratio for normal weight concrete that will meet anticipated exposure conditions. Note that the water-cement ratios in table 3-10 are based on concrete strength under certain exposure conditions. If possible, perform the tests using job materials to determine the relationship between the water-cement ratio you select and the strength of the finished concrete. If you cannot obtain laboratory test data or experience records for the relationship, use table 3-11 as a guide. Enter table 3-11 at the desired f'c (specified compressive strength of the concrete in pounds per square inch, psi) and read across to determine the maximum water-cement ratio. You can interpolate between the values. When both exposure conditions and strength must be considered, use the lower of the two indicated water-cement ratios. If flexural strength, rather than compressive strength, is the basis of design, such as a pavement, perform the tests to determine the relationship between the water-cement ratio and the flexural strength. An approximate relationship between flexural strength and compressive strength is as follows:

Use fine aggregate to fill the spaces between the coarse aggregate particles and to increase the

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