Chapter 7 of the EA3 TRAMAN discusses the properties that comprise a good quality concrete and introduces the use of concrete as a construction material. In Part 1, chapter 13, of this TRAMAN, you learned about the different types of portland cement, the methods used to identify cement, and the purpose and effect of various admixtures that are often used in the production of concrete. You also studied the physical requirements for water and aggregates used in concrete and the various tests used to determine the suitability of water and aggregates as ingredients in a concrete mixture. The discussion of concrete in this chapter is directed towards the design of concrete mixtures. This discussion presupposes that you are well versed in the previous topics. If you are not, then it is strongly recommended that you review the aforementioned chapters before you begin the study of this chapter. Also covered in this chapter is bituminous mixture design. Once again, it is strongly recommended that you first review chapter 13 of this TRAMAN to refresh your knowledge of bituminous pavement materials and the test-ing methods used in the control of bituminous mixtures.

From your previous studies, you know that the basic ingredients used in the production of concrete are cement (usually portland cement), water, and both fine and coarse aggregates. You also know that certain admixtures are used occasionally to meet special requirements. Design of a concrete mixture consists of determining the correct amount of each ingredient needed to produce a concrete that has the necessary consistency or workability in the freshly mixed condition and that has desired strength and durability characteristics in the hardened condition.

Two methods of proportioning concrete mixtures are discussed in this chapter. One methodthe trial batch method is based on an estimated weight of concrete per unit volume. The other method, based on calculations of the absolute volume occupied by the ingredients used in the concrete mixture, is called the absolute volume method. Our discussion of these methods is only intended to provide you with a basic understanding of mixture design. For a thorough discussion, you should refer to the most recent edition of Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass(ACI 211.1), published by the American Concrete Institute (ACI).

The water-cement ratio is determined by the strength, durability, and 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 17-1 to select a suitable

Table 17-1.Maximum Permissible Water-Cement Ratio for Concrete in Severe Exposures

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water-cement ratio for normal weight concrete that will meet anticipated exposure conditions. Note that the water-cement ratios in table 17-1 are based on concrete strength under certain exposure conditions. If possible, perform 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 17-2 as a guide. Enter table 17-2 at the desired fc (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 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 workability of a mix. In general, aggregate that does not have a large grading gap or an excess of any size, but gives a smooth grading curve, produces the best mix. Fineness modulus and fine aggregate grading are discussed in Part 1, chapter 13, of this TRAMAN. Use the largest practical size of coarse aggregate in the mix. The maximum size of coarse aggregate that produces concrete of maximum strength for a given cement content depends upon the aggregate source as well as the aggregate shape and grading. The larger the maximum size of the coarse aggregate, the less paste (water and cement) required for a given concrete quality. The maximum size of aggregate should never exceed one fifth of the narrowest dimension between side forms, one third of the depth of slabs, or three fourths of the distance between reinforcing bars