Detailed step-by-step procedure for Concrete Mix Design? - teamlceted

MIX DESIGN PROCEDURE | DETAILED PROCEDURE FOR CONCRETE MIX DESIGN 

The general step-by-step procedure for proportioning concrete mixes is summarised below.

1. Required material information The tests for relevant material properties are: the sieve analysis of both fine and coarse aggregates, determination of unit weight, specific gravities, and absorption capacities of aggregates.

 

2. Choice of workability Workability is generally specified for a particular job. However, if it is not specified; an appropriate value of workability in terms of slump, compacting factor or Vee-Bee time  (as per specification) is selected as recommended by the code. As a general rule, the lowest slump that will permit adequate placement, compaction and finishing should be selected.

 

3. Maximum aggregate size The maximum nominal size of the aggregate, which is economically available, is selected as per the specified requirements. It is determined by sieve analysis and should conform to the limitations based on the minimum dimension of structural members, and the clearance between reinforcing bars and forms. These restrictions may limit maximum aggregate size to 40 mm, except in mass concrete applications. A reduced full aggregate size for a given w/c ratio can achieve higher strengths.

 

4. Estimation of mixing water and air content An estimation of the amount of water required for air-entrained and non-air-entrained concretes can be obtained from the code provisions. Air-entraining admixtures are used to make the concrete frost-resistant. Concrete is routinely air-entrained in colder regions.

 

5. Water/cement ratio This component is governed by target strength and durability requirements.

a) Target strength The mean target strength is estimated from the specified characteristic strength and the level of quality control. In case of non-availability of strength vs. w/c ratio data for a specific material, a conservative estimate can be made for the accepted 28-day compressive strength.

For the estimated mean target strength, water–cement ratio is selected from the curves or tables provided by the relevant code.

b) Durability The water–cement ratio so chosen is compared with that required for durability, the lower value is adopted. If there are severe exposure conditions, such as freezing and thawing,  exposure to seawater, or sulfates, the w/c ratio requirements may have to be adjusted.

 

6. Calculation of cement content Once the water content and the w/c ratio are determined, the cement per unit volume of the concrete is found by dividing the estimated water content by the  w/c ratio. The cement content so calculated is checked against the minimum cement required from durability considerations to ensure good finishability and workability.

 

7. Estimation of coarse or fine aggregate content The percentage of coarse or fine aggregate in concrete or total aggregate is determined from the characteristics of coarse and fine aggregates as recommended in the relevant code.

 

8. Estimation of fine or coarse aggregate content When the percentage of one aggregate is determined the other may be established by, the mass method and the volume method. The “volume” method is generally preferred, as it is a somewhat more exact procedure. When the percentage of coarse aggregate is known, the volume of fine aggregates is determined by subtracting the volumes of cement, water, air,  and coarse aggregate from the total concrete volume.

 

9. Adjustment for moisture in the aggregate Suitable adjustment in the water content for the concrete is made to account for the moisture content of the aggregate.

 

10. First trial batch Using the proportions developed in the preceding steps; the trial batch of concrete is mixed using only as much water as is needed to reach the desired slump (but not exceeding the permissible w/c ratio).

 

The fresh concrete should be tested for the slump, unit weight, yield, air content, its tendencies to segregate, bleed, and finishing characteristics. Also, hardened concrete samples for compressive and flexural strengths are cast as per standard codal procedures. After the required period of curing, the specimens are tested in wet condition for the compressive and flexural strengths of the mix.

 

11. Trial batches Based on the tests on the first trial batch, additional trial batches obtained by making suitable adjustments in water-cement ratio or aggregate–cement ratio or in proportions of cement, sand and aggregate, are tested till the final mix composition is arrived at.

 

12. Final proportions The final proportions are expressed either on a mass or volume basis.

Most of the available mix design methods are essentially based on the above procedure. The methods use empirical relationships, charts and tables developed from extensive experimental investigations.

Also read: ACI Method of Concrete Mix Design

Also read: Guidelines of IS Concrete Mix Proportioning

Also read: British DoE Concrete Mix Design | Mix design of Concrete