Workability Properties of Self Compacting Concrete Used as Ready-Mixed Concrete

Ready mix concrete poured from a heavy cement mixer If you are looking to repair cracking (spalling) or damaged concrete, you need to understand some of the chemical properties that make up advanced concrete finishing materials. This is a brief guide to some chemicals used in popular ready mix products.

Self compacting concretes (SCC) are distinguished by their special fresh concrete properties. However, self compacting properties can only be achieved by persistent optimization of the starting materials and of the production method. When SCC is used as ready-mixed concrete it is important that the concrete still exhibits the required fresh concrete properties at the time when it is placed.

The time span between end of mixing and start of placement can be more than 90 minutes. Unlike with the use of SCC in precast element plants this means that the changes in workability characteristics with time and as a function of temperature have to
be taken into account.

This requires an adequate understanding of the interactions between the powder materials used, the water content and the mode of operation of the plasticizer. The influence of the concrete starting materials and of the concrete composition on the initial flowability, the stiffening behavior and the duration of workability were determined by laboratory tests.

Check tests on concretes produced with the same starting materials and the same composition in a ready-mixed concrete plant under realistic conditions show that the laboratory results are also applicable to practical conditions. All the concretes investigated exhibited solid concrete properties which complied with the requirements.

Self compacting concrete (SCC) is a concrete which flows to a virtually uniform level under the influence of gravity without segregation, during which it de-aerates and completely fills the formwork and the spaces between the reinforcement. It is a high-performance concrete with the special property of the fresh concrete of “self compacting”.

As with other high-performance concretes (e.g. high strength concrete, acid-resistant concrete) the special properties of these concretes which differ from normal concretes are achieved only by systematic optimization both of the individual constituents and of the composition.

The flow and mix stability of the SCC are determined primarily by the interactions between the powder (cement and additions with a particle diameter), water and plasticizer. The gradation of the individual size groups in the overall grading curve also affects the property of the concrete of not being blocked by the reinforcement.

It has been found that, in contrast to vibrated concrete, the workability properties required for self compaction cannot be
maintained relatively easily over a fairly long period. Fluctuations in the workability of vibrated concrete can be largely offset by the intensity of vibration applied during placement, but this is not possible with SCC.

The effects of production and transport on the workability properties of SCC must therefore be taken into account in the initial testing.

SCC in a precast element plant, for example, is normally placed about 10 min to 30 min after mixing but with ready-mixed concrete this time span is often 30 min up to 120 min.

In addition, it is usually also necessary to provide a very large number of individual mixes with substantially the same workability properties under very diverse conditions on the building site for a cast-in-place concrete component.

The additional influencing factors are, among others, the sharply fluctuating weather conditions (temperature, rate of water evaporation) and the fact that the concrete already present in the component must have the same workability properties as the newly delivered concrete. Interruptions in the supply can therefore have worse consequences than with vibrated concrete.

Also crucial is the question of the means of correction available on the building site if the delivered SCC does not meet the requirements. Dependable production of SCC as ready-mixed concrete using the constituent materials and equipment available at the concrete plant is currently being investigated at the Research Institute of the Cement Industry in a joint research project with the FTB (Ready-mixed Concrete Research Association) which is financing the investigations.

This report deals with investigations into the following questions:

  • effects of small deviations in the water addition on the initial
    flow behaviour
  • stiffening behaviour as a function of the quantities of water and
    plasticizer added
  • corrections that can be carried out at the building site by addition of plasticizer or water while maintaining adequate mix stability
    The investigations carried out on laboratory concretes were checked by investigations on concretes produced in a ready-mixed
    concrete plant.

The rheological properties of the powder suspension, consisting of cement, concrete additions, mixing water and plasticizer, are determined by the water/solids ratio and the plasticizer content. The planned quantity of mixing water should be at least as high as the
saturation water content. This is the water content that is necessary to wet the surfaces of the solid particles and to fill the voids in the particulate mass of powder.

Read the full abstract of the properties of using self compacting concrete and ready mixed concrete.

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