Aci Concrete Mix Design

¨The most common method used in North America is established by American Concrete Institute.

Economy–

The material cost are most important in determining the relative costs of different mixes.

Since cement is more expensive than aggregate it is clear that cement should be minimized.

Workability

A good mix design must be capable of being placed & compacted with minimal bleeding & segregation & be finishable.

Strength & Durability

¨In general the minimum compressive strength & a range of W/C ratios are given for given concrete mix.

¨Possible requirements for resistance to freeze-thaw and chemical attack must be considered.

Required material information

Sieve analyses of both fine and coarse aggregates, unit weight, specific gravities, and absorption capacities of aggregates.

Choice of slump —

Generally specified for a particular job. However, if not given, an appropriate value may be chosen from Table 1. As a general rule, the lowest slump that will permit adequate placement should be selected.¨Once the water content and the w/c ratio is determined, the amount of cement per unit volume of the concrete is found by dividing the estimated water content by the w/c ratio.

¨weight of cement = weight of water /(w /c)

¨However, a minimum cement content is required to ensure good finishability, workability, and strength.

To establish the fine aggregate content by “volume” method.

ØVfa = [1- (Vcement +Vwater +Vcoarse agg.)]

Adjustment for moisture in the aggregate.

¨Decrease/Increase the amount of water required by surface moisture content of the weight of the fine aggregate

¨ Increase/decrease the amount of aggregate by the amount equal to the surface moisture

Trial batch

The fresh concrete should be tested for slump, unit weight, yield, air content, and its tendencies to segregate, bleed, and finishing characteristics. Also, hardened samples should be tested for compressive and flexural strength.