High-performance Self-compacting Concrete

Memoria tratta da "The New Boundaries of Structural Concrete: Session B – Controlled-performance Concrete", ACI IC - Ancona 2011

Self-compacting concrete (SCC) is manufactured so that it can be placed safely and reliably, independently of the quality of workmanship during the placement of the concrete and the congested-reinforcement in the concrete structure. High-performance properties of these concretes include high compressive strength (at least 55 MPa at 28 days) and long-term durability with low watercement ratio (less than 0.42). All the concrete mixtures with about 1730 kg/m3 of natural aggregates (maximum size of 15 mm), 400 kg/m3 of portland cement, and 95 kg of ground calcite filler were mixed in the presence of a poly-carboxylate admixture (PCA) as a superplasticizer (3 % by the weight of cement). Neither bleeding nor segregation were observed in the SCC mixtures produced. In some concrete mixtures a shrinkage-reducing admixture (SRA) was used (4 kg/m3) to reduce the drying shrinkage by 30 % with respect to the control mixture without SRA, when the specimens were kept at room temperature with a relative humidity (RH) permanently kept at as low as 55%. A combination of a CaO-based expansive agent (30 kg/m3) with 4 kg/m3 of SRA was also used ia a SCC mixture. The restrained expansion was measured for this mixture, without wet curing in a dry environment (RH = 50%). After 2-3 months the restrained expansion was lost and cracks were not visible even though the conditions of humidity were very sever. A similar result was obtained by using a combination of PVA(?) fibres (15 mm long and 0.5 mm thick) with 4 kg/m3 of SRA: drying shrinkage was not reduced in the PVA-SRA concrete with respect to the SRA concrete. However, in the presence of PVA, both the number and the width of the cracks were noticeably reduced. The use of the expansive agent or the fibres combined with SRA made concrete more reliable from the durability point of view.

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