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Slide 1:

5. Water/cement ratio . The selection of the appropriate w/c ratio may be governed not only by strength but also by durability requirements . (a) Strength: In the absence of strength vs. w\c ratio data for the specific materials, a conservative estimate can be made for the expected 28-day compressive strength from Table A.3. Of course , it is possible that the specifications may be based on a required strength at a time other than 28 days , or the design may require the use of high-early-strength (Type III) or low-heat cements (Type IV).

Slide 7:

The designer must develop his or her own data for these cases, or when the design is governed by a flexural strength requirement. It is always more desirable to develop the appropriate strength –time-w\c ratio relationships for the materials that are actually to be used on the job. In this way, the effects of admixtures can also be determined. (b) Durability : If there are severe exposure conditions, such as freezing and thawing, or exposure to sea water , or sulfates, more severe w/c ratio requirements of Table A.4 may govern.

Slide 8:

Calculation of cement content . Once the content and w/c ratio are determined , the amount of cement per unit volume of concrete is determined by simply by dividing the estimated water requirement by the w/c ratio. However , many specifications, in addition, require a minimum cement content. Such a requirement may be used to ensure satisfactory finishability , quality of vertical surfaces , or workability; it may also ensure against low strength due to increased water demands at the job site. In the absence of suitable test data, or when the w/c ratio-strength relationships are not available or are inadequate , the minimum cement contents such those in Table A.8 may be used, but only for concretes with a specified compressive strength less than 25 MPa .

Slide 10:

Estimation of coarse aggregate content . It has been found empirically that aggregates having the same maximum size and grading will yield workable mixes when used in concrete in the volumes (on a dry- rodded basis) shown in Table A.9. For the same workability, the volume of course aggregate depends only on its maximum size and on the fineness modulus of the fine aggregate. The OD weight of course aggregate required per cubic meter of concrete is simply equal to the value from Table A.9 multiplied by the dry- rodded unit weight of the aggregate in kg/m^3. To convert from OD to SSD weights, multiply by (1+ AC/100).

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