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The strength of no-slump concrete is mainly achieved by a high particle packing density (ppd), which causes a strong interlocking of the coarse aggregates. Also inert fillers like fly ash are enhancing the strength by help of interlocking of micro-sized particles (Bornemann 2005). Both the compressive strength in the green and hardened state is enhanced. However, additional water or liquifier is needed to keep a sufficient workability. Accordingly there’s often a lack of water for the cement hydration process, that is either retained by the filler or is replaced by high superplasticizer content. Anyway there is a discrepancy to be noted between high green strength (little water needed) and high compression strength of hardened concrete (more water needed). The stickiness of a filler-optimized mixture causes hindrances in manufacturing, yet another problem. Another theory, wherein green strength is explained by capillary forces between micro-sized particles (Wierig 1972) seems to fail. Capillary forces have a maximum effect on particles with diameters from 0.1-1 mm, while friction forces are dominant when particle diameters exceed 1 mm (Bornemann 2005). |
