Mix design of self consolidating concretes Exercise 8

Self consolidating concretes a.k.a self compacting concretes Self consolidating concrete (SCC) is a flowing concrete mixture that is able to consolidate under its own weight. The highly fluid nature of SCC makes it suitable for placing in difficult conditions and in sections with dense reinforcement. SCC is ideal for concrete parts with complicated shapes and elements with high quality visible concrete.

Comparison of mix proportioning between self consolidating concrete and conventional concrete.

The materials used in SCC are the same as in conventional concrete except that an excess of fine material and chemical admixtures are used. Also a viscosity-modifying agent (VMA) may be used. The powdered materials used are for example fly ash, silica fume, limestone powder and quartz powder.

According to Okamura and Ozawa (1995) self- compactability can be achieved using the following methods: Limited aggregate content Low water-powder ratio Use of superplasticizer

SCC can be manufactured in all strength classes as ´normal´ concretes. However because of the high compactability, strengths of 40 MPa and over are more common.

Test methods for self compatibility Conventional workability tests, devised for normal ranges of concrete mixtures are not suitable for self-compacting concrete. because they are not sensitive enough to detect the tendency to segregation. Therefore test equipment has been made for judging the following characteristics of SCCs: Self-compatibility Deformability Viscosity Filling ability

Many different methods have been developed to characterise the properties of SCC. In Finland the consistency of SCCs is described usually with Slump flow (650…800 mm) T 50 slump flow (2…10 s) Also used for testing for example the passing ability are L-box J-ring

The method for achieving self-compactability involves: high deformability of paste or mortar resistance to segregation between coarse aggregate and mortar when the concrete flows through the confined zone of reinforcing bars.

Rheology is the science of the deformation and flow of materials. It is used to understand the workability characteristics of SCC. Rheology of SCCs

Yield stress: Yield stress: the measure of the amount of energy required to make SCC flow. To be considered self compacting, concrete must flow easily under its own weight, so its yield stress must be very low. The two most important properties of SCC’s rheology are:

Plastic viscosity Plastic viscosity: the measure of the resistance of SCC to flow due to internal friction. SCC must have a high viscosity in order to suspend aggregate particles in a homogenous manner within the concrete matrix without segregation, excessive bleeding, excessive air migration, or paste separation. In summary, SCC must have low yield stress and high viscosity!

SCC requires a higher level of quality control than “normal” concrete. Higher powder contents bleed less than conventional concrete: SCC tends to dry faster than conventional concrete →Initial curing should therefore be commenced as soon as practicable after placing in order to minimise the risk of shrinkage cracking.

Mix design guide for SCC developed at TKK/HUT

VariableMinimumMaximum Slump flow [mm] T 50 time [s]215 Amount of cement [kg/m 3 ] Amount of fly ash [kg/m 3 ]0200 Amount of limestone filler [kg/m 3 ]0200 Amount of superplasticizer [kg/m 3 ]210 Amount of water [kg/m 3 ] Limit values

Basic rules