MIX-DESIGN TIPS FOR SELF COMPACTING CONCRETE

Self-compacting or self-consolidating concrete (SCC) is a type of high-performance concrete with the ability to settle and compact inside a mould, under its own weight, without the need of externally applied vibration. This type of concrete has been developed initially to enhance concrete casting in high-rise building, but has since found many more applications, as it minimizes the need for labor during casting and significantly speeds up concrete placement.

Of course, it would have been impossible to design such concrete without development of high-performance superplasticizers, suitable for this task. There have been examples of SCC even before the last two decades of the 20th century, with the previous generations of superplasticizers, namely naphthalene-based ones. But, following introduction of PCE superplasticizers in the early years of 21st century, combined with viscosity-modifying agents, design and use of SCC have been really revolutionized, leading to realization of even the most demanding projects in terms of concrete application and performance.

So, what do we need to design a successful self-compacting concrete? My basic tips are the following:

1.????? Binder content: the total amount of cement and cementitious materials should be at least 350-400 kg/m3, provided we have a good quality sand available that will contribute another 150-200 kg/m3 of clean limestone silt (below 100 microns). Otherwise, addition of limestone filler is highly recommended. Generally, a good SCC requires 550-600 kg of binder paste for optimum performance and minimum risk of segregation.

2.????? Water/binder ratio: we are talking about water/binder instead of water/cement ratio, as our main interest is the total binder paste. We should target for a W/B below 0.35. This will make the mix cohesive enough to minimize risk of segregation.

3.????? Superplasticizer: the first task is to find the most suitable superplasticizer for our mix. Of course, it will be a PCE (polycarboxylate), but there are numerous available choices, mostly mixes of PCE with different dosages of VMA (viscosity modifiers), which are suitable for different types of cement and aggregates, so usually lab trials are essential for our final selection. The dosage used is most of the times the maximum recommended by the supplier, typically around 2% of binder dosage.

4.????? Maximum aggregate size: it is pretty much the same as for normal concrete, although use of finer gravel is advantageous for SCC, as it reduces danger of segregation and allows easier flow around thick rebar.

5.????? Workability: instead of slump, we are using flow to determine workability of SCC. For a concrete to be classified as SCC, it needs to have a flow of at least 65 cm, with no visible signs of bleeding or segregation. Additional flow tests performed specifically for SCC are V-funnel, J-ring, L-box and T-50, measuring speed of flow, viscosity of the mix and ability to move around rebar with no bleeding or segregation.

6.????? Strength: based on binder content and W/C ratio, we can expect a C35/45 as minimum compressive strength for SCC.

7.????? Setting time: based on the high dosage of superplasticizer used, we should expect a setting time of at least 3 hours since loading the truck.

8.????? Pumpability: SCC is typically a very pumpable concrete mix, but attention must be paid to the pressure applied and the speed of pumping, as it may lead to segregation and blockages.

9.????? Formwork pressure: due to its fluid state and long setting time, SCC will exert almost hydrostatic pressure on the formwork used at the jobsite, a behavior completely different than normal concrete. Therefore, it is important, when we use formwork exceeding 3 m in height, to support it in a much more robust way, ensuring also the watertightness of the formwork, otherwise significant loss of cement paste may occur. This becomes particularly important when used for fair-face facades, where the finishing result is the major goal.

10.?? Filling the formwork: depending on the height of the formwork, in order to avoid any chance of segregation and achieve optimum compaction, there are different methods used to introduce SCC in the formwork. One option is to use tremie tubes at the end of the pump pipeline, to better control the flow and avoid a big drop. The other option is to use inlets at the bottom of the formwork and let concrete rise within the formwork like a fluid. The right choice will depend on the mix itself, its flow speed and viscosity, so an application trial is mandatory before deciding.