Effect Of ACI Concrete Mix Design Parameters On Mix Proportion & Strength
The effect of different mix design parameters in predicting mix proportions by ACI method of concrete mix design and their strength attainment has been studied. The parametric study revealed that ACI method fails in rational proportioning of fine aggregate content where coarse aggregate of lower unit weight is to be used.
In such events, the overestimation of fine aggregate content with increased surface area, together with underestimation of coarse aggregate content, brings a total failure in realistic prediction of mix proportions. The situation deteriorates further in the cases of designing low slump mixes; using fine aggregate of higher specific gravity or greater fineness modulus. The mix design computations and subsequent trial mix castings on different material sets were found to conform to these findings. This indicates the necessity of some modifications in ACI normal concrete mix design method to make it usable for wider needs.
Concrete is a composite material which essentially consists of cement, coarse aggregate (CA), fine aggregate (FA) and water. Coarse aggregate gives the volume to the concrete and fine aggregate makes the concrete denser by filling the voids of coarse aggregate. Water hydrates and sets the cement which thus acts as a binder for all the ingredient particles of concrete. The ultimate properties of concrete in terms of its strength, durability and economy depend not only on the various properties of its ingredients but also on the concrete mix design standards, method of preparation, handling and curing conditions.
The possibility of the use of locally available ingredients as aggregates to produce concrete of a particular design strength makes concrete exclusive among the major construction materials.
Due to the dependence on the natural sources, the engineering properties of the aggregates are found to vary widely.
But to attain a particular design strength, the engineers generally follow different mix design methods that involve selection of suitable ingredients of concrete and determination of their relative quantities for producing concrete in an economic way not withstanding the workability, strength and durability requirements. The process of mix design is also important in having an approximate idea about the required material quantity and subsequent cost estimation.
CONCLUSION
The ACI method of mix design for normal mixes cannot rationally design the mixes for the cases where coarse aggregates of lower unit weights are to be used. In such cases, the design suggests for higher value of fine aggregate/coarse aggregate ratio, which in turn increases the total aggregate surface area to a great extent. The situation worsens further when the designer goes for designing low slump mixes; uses fine aggregate of higher specific gravity or greater fineness modulus, since the ACI mix design method, in such cases gives lower cement/total aggregate ratio. The present study indicates the need for further research with a view to incorporating some modifications into ACI method in predicting coarse and fine aggregate contents. Such efforts will evidently enable the ACI method to address a wider user community.
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