What is self compacted concrete |Mix design

Procedure

• Determine the desire air content.
• Determine the coarse aggregate volume.
• Determine the sand content. 
• Determine the paste composition. 
• Determine the optimum water to powder ratio and superplasticizer dosage in mortar. 
• Finally the concrete properties are assessed by standard test. 

Air content. 

Generally the air content may b assumed to be 2%. In case of freeze-thaw condition in  cold weather concreting higher percentbof air content may be specified.

Determination of coarse aggregate volume. 

It  is defined by bulk density. Generally coarse aggregate (D› 4. 75) should've between 50% to 60%. Optimum coarse aggregate content  depend on the following parameters. 

The lower the maximum aggregate size, the higher the proportion. 

The rounded aggregate can be used at higher percentage than crushed aggregates
. 

Determination of Sand content. 

Sand in the context of mix design procedure is defined as all particles bigger than 125 micron and smaller than 4.75 mm. 

Sand content is defined by bulk density. The optimum volume content of sand in the mortar varies between 40 to 50 % depending upon paste properties.

Design of paste composition. 

Initially the water/cement ratio for zero flow is determined in the paste , with chosen proportion of cement and addition. Flow cone test with water/powder ratio by volume are perfomed with the selected powder composition.

Determination of optimum volumetric water/powder ratio and super plasticizer dosage in mortar. 

Test with flow cone and V-funnel for mortar are performed at varying water/powder ration in the range of  0.8 to 0.9 zero flow and dosage if superplasticizers. The superplasticizers is used to balance the rheology of the paste .The target values of slump flow of 24 to 26 cm and V-Funnel time of 7 to 11 seconds. At target slump flow, where V-funnel time us lower than 7 seconds then decrease the water/powder ratio. For largest slump flow and V-funnel time excess 11 seconds water/powder ratio should be  increased.

Tests on concrete. 

The concrete composition is now determined and the superplasticizers dosage is finally selected on the base of test on concrete. 

Guideline for mix composition. 

• Coarse aggregate   ‹50%
• Water/powder ratio =0. 8 to 1. 0
• Total powder content =400-600kg/m3
• Sand content =‹40% of the mortar by volume
• Sand =‹50% paste volume. 
• Sand  /›50%  by weight of total aggregate. 
• Free water  ‹ 200litres. 
• Paste  › 40% of the volume of the mix. 

When the above parameters conduct rhe workability tests and see whether you get the follows results ..
1. Slump flow by Abrams cone.      650 to 800 mm

2. T50 cm slump flow.              2 to 5 secs. 

3. J-ring.                                      0-10mm

4. V-funnel at T5 minutes.        8 to 12 secs. 

5. L-box.                                      H2/HQ= 0. 8 to 1. 0 

6. U-box.                                      H2-H1=30mm max. 

7. Fill box.                                    90 to 100%

8. Screen stability .                     0 to 15 secs. 

9. Orimet.                                     0 to 5 secs. 

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