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B. From fresh concrete to hardened concrete 1. Composition of Concrete Importance of the water/cement(w/c)ratio A central characteristics of concrete, and one that Concrete is a composite material that consists largely determines its performance, is the water/ essentially of fine and coarse aggregates, glued cement ratio,or w/c ratio(Fig 1.16). together by the cement paste. Aggregates occupy 60-75% of the concrete (measured by weight or by Low water/ High water/ volume, as Fig 1.15 and they are important cement ratio cement ratio constituents from a technical and economical point of view. Aggregates play a central role in concrete strength and durability. high low ent ' io,N high ttane stone and sand low high water ent ut water air sand stone segrergated 7-15 1419 2-! 22-32 30-48 9-18 6-9 23-35 33-65 93-95 46 0,5-1 wate weak strong Percent by volume Percent by weight Percent by internal surface fresh concrete Fig 1.15- Composition of Concrete no scaling scaling But the picture looks a bit different when we consider the so-called internal surface area, that is, the combined surfaces of all the particles in concrete. Measured in this way, the dominant darker lighter component in concrete is clearly cement and the surface surface cement paste is fundamental in defining many concrete characteristics. Concrete mixing In proportioning the constituents of concrete, or Fig 1.16-Influence of the w/c ratio on concrete properties determining the so-called concrete mix or mix design, the producer is primarily concerned with The relationships between the w/c ratio and optimizing concrete's: required characteristics of concrete are well known • Workability in practice. Thus, the designing engineer usually • Strength specifies the w/c ratio when he specifies the type of • Production cost concrete. • Durability