The most common method of creating high-strength concrete is to design and control the combination of cementitious material, aggregates, water, and admixtures. Cementitious materials include portland cement, fly ash, silica fume, ground granulated blast furnace slag, or natural pozzolans. Typically, fly ash or ground granulated blast furnace slag substituted for some of the portland cement is an effective method to increase the long-term strength of a mixture. Low water/cementitious materials ratios likewise increase concrete strength (up to a limit), in accordance with Abram’s Law. Strong, cubical aggregates are also important in high-strength mixes.

Several specific parameters are considered in the production of high-strength concrete.

• The higher the compressive strength desired, the smaller the maximum size of the coarse aggregate should be.
• Fine aggregate coarseness should increase proportionally with the compressive strength and with the amount of cement used. A fineness modulus of between 2.70 and 3.00 is preferred.
• High-strength concrete, in the absence of pozzolanic materials, would have a substantial problem with slump loss. The use of fly ash or blast furnace slag reduces this effect and reduces the production costs of the concrete. The optimum substitution level is usually controlled by a minimum value or by the acceptable loss in 12 hour or 24 hour strength (pozzolanic materials produce lower short-term strengths).
• For target strengths exceeding 100 Mpa, silica fume is a required ingredient in the mixture.

Figures 4.1 and 4.2 (Nawy)

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