DIFFERENT METHOD OF MIX DESIGN OF CONCRETE PROCEDURE

MIX DESIGN

The procedure of selecting suitable ingredients and determine their relative proportions with the objectives of producing concrete of having certain minimum workability, strength and durability as economically as possible is known as mix design.

The various factors to be consider in specifying a concrete mix are :

• Water cement ratio

• Cement content gradation

• Consistency of the fresh concrete

METHOD OF MIX DESIGN OF CONCRETE

• Trial and error method 
• American concrete institute method.
• Maximum density of aggregate method
• Surface area of aggregate method
• Fineness modulus method
• Void cement ratio and mortar void ratio method
• Road note method

TRIAL AND ERROR METHOD

Based on the relationship between the w/c ratio and compressive strength. the design of the mix can be made by following steps :

• Select the water cement ratio to suit the requirement of the strength and durability.

• With this water cement ratio prepare trial batches with different aggregate cement ratios. From this it is possible to determine the optimum proportion and amount of the aggregate that will produce a workable mix with a minimum of the pastes content for determining the fine aggregate. It can be said that in the correct amount of sand is the minimum which will produce enough mortar to fill the spaces between the coarse aggregate and slightly more for workability. Understanding  creates hardness and over standing results in less yield. with judgement and experience one ma arrive at the proportions of fine and coarse aggregate.

• Having fixed water cement ratio, aggregate cement ratio and the amount of fine aggregate in the total aggregate content, block of concrete can be prepared in the laboratory following the standard procedure. After the required period of curing the blocks can be tested for its compressive strength.

• mix can be slightly adjusted, if necessary, by changing the w/c ratio or the aggregate cement ratio suit the actual requirement of the job.

In the trial and error method, due consideration has to be given for the moisture content of the aggregates.

AMERICAN CONCRETE INSTITUTE METHOD (ACI METHOD)

This is the common method for design of concrete mix. the following are the steps : 

• Select the water cement ratio to concrete of the required durability and strength. if water cement ratio is determine by both methods and then the lower of the two values obtained should be used. 

• Select the lowest slump from ASI table.

• The maximum size of aggregate recommended for the types of construction is selected by ASI table. Within the limit shown, the larges permissible maximum size should be employed, except as dictated by available and economy. 

• Estimate from the test data, minimum proportion of fine to coarse aggregate and the lowest water content as  that will give the required degree of workability.

• The cement content is computed by dividing the total water content by the w/c ratio.

• The bulk volume of the dry rodded coarse aggregate per unit volume of concrete for the particular maximum size of the aggregate and fineness modulus of the sand is determine.

• Determine the trial mix proportions and make adjustment if necessary for the materials and water cement ratio.

MAXIMUM DENSITY OF AGGREGATE METHOD

It is also known as fullar and thomson method of concrete mix design, the basis for proportioning the aggregate is maximum density. For this method, the studies made by fullar and thomson, in which sieve analyses were made to obtain proportions aggregate giving maximum density of concrete, were used. But this method is not in much uses now.

Maximum density of this combined aggregate can be determine is follow. In a of known volume certain proportions of any rodded mixed aggregate are filled in. Sand is taken as 0, 20, 30, 40, 50, 60 and 100% of the total weight. After each filling of the mould with the combined aggregates, weight is taken. A curve is then drawn showing the percentage of sand in the aggregate by weight v/s unit weight of the total aggregate in the mould, from this percentage of sand at which unit weight is maximum can be determined.