Self Consolidating Concrete Mix Design

The flowability of a concrete mix is a complex interaction of the inter-particle friction in the aggregate phase, and the fluidity of the paste phase. The water-to-powder ratio and admixtures control the fluidity of the paste phase. If the aggregate particles have too much friction due to poor grading or shape, the paste will have to be very fluid to compensate and achieve the desired concrete flowability. If the paste is too fluid, segregation will result.

Thus, the general approach is to select the best grading and shaped aggregate economically possible, to use high-paste contents to increase space between the aggregate particles, finally controlling the rheology of the mix by adjusting the water-to-powder ratio and using appropriate admixtures.

Although the definition of Self Consolidating Concrete varies with the particular application, these are some general parameters of mixes that our experience shows are required to produce trial mixes for quality Self Consolidating Concrete:

1. Coarse aggregate content – For normal-density aggregates this typically results in a specific volume that is 28%–32% of the concrete volume, with the balance (68–72%) being mortar.

2. Paste fraction – Approximately 35%–37% of the mix. For rounded well-graded fine aggregate this will be lower, for poor grades or manufactured fine aggregate this will be higher.

3. Powder (cement, supplementary cementitious materials and inert powder materials with particle sizes passing the 150 mm (No. 100) sieve) – Powder contents will generally be in the 295–365 kg/m3 (650–800 lbs/yd3) range.

4. Fine aggregate – Some people track fine aggregate to total aggregate ratio – which usually turns out to be approximately 45%–55%, with 50% being typical. (Fine aggregate is that which passes the #8 sieve.).

5. Water content – For first mix, as needed to get 25–75 mm (1–3 in.) slump in concrete without SCC admixtures. This would include water-reducing admixture or retarders for set control.

6. Water per Cement ratio – As needed for durability. Generally, powder content requirements for Self Consolidating Concrete properties will mean that W/C is low enough, and resultant strength high enough for most applications; however this must be confirmed.

7. Air – Air content should be as needed for durability. Air can improve the viscosity of a mix and increase the paste volume, but may adversely affect paste density.

There are many industry organizations at work developing specifications, test methods, and practices for Self Consolidating Concrete. As new information becomes available, this document will be updated accordingly.