Use of Fly Ash in Self Compacting Concrete (SCC)

Project Number 209

Self compacting concrete (SCC) is different to ordinary concrete, which needs to be vibrated, by is ability to fill every kind of formwork without any influence from outside. To reach this high performance the concrete needs to have special properties. On one hand, SCC should have an extremely high flowability, on the other hand the stability of the paste must be high enough to avoid segregation of coarse aggregates. The properties of the hardened SCC should differ as little as possible from those of ordinary vibrated concrete.

Because of his special performance SCC is expected to have an enormous technical and economical potential. For this reason intensiv research programs take place regarding the conditions of designing concrete structures with respect to German standard DIN 1045 which are technically and economically adequate. The prescriptions should be summarised in a guideline for SCC.

The enormous flowability of the SCC stipulates two changes in the composition of the SCC-mix. These concern the amount of powder and the admixtures. The amount of powder (particles with grain size 0,25 mm) rises up to 700 kg/m³ while the amount of coarse aggregate decreases. As additional powders quartz- and limestone or latent hydraulic (blast furnace slag) or puzzolanic additives (fly ash or trass) may be used. The content of water is equal to ordinary concrete. The high flowability is obtained by the addition of superplasticizer with an extremely high efficiency based on polycarboxylatether or with a combination of admixtures enhancing flowability, viscosity and stability. Since the extremely high flowability should remain as long as possible, the interaction of fillers and admixtures is very important for the performance of the SCC.

Within this project the influence of the mixtures consisting in different types of cement, fly ashes and plasticizers on the flowability and the properties of hardened concrete will be investigated. The investigations on the properties of hardened concrete consist in relevant properties for designing (e.g. compressive strength, bond), for durability (freeze-thaw-resistance, resistance to carbonation) as well as the suitability for fairfaced concrete. The literature about these items is only incomplete so far.