The quantity of coarse aggregate in high-strength concrete should be the maximum consistent with required workability.
Because of the high percentage of cementitious material in high-strength concrete, an increase in coarse-aggregate content beyond values recommended in standards for normal-strength mixtures is necessary and allowable.
In high-rise buildings and in bridges, the stiffness of the structure is of interest to structural designers.
On certain projects a minimum static modulus of elasticity has been specified as a means of increasing the stiffness of a structure.
The two Union Square building in Seattle used concrete with a designed compressive strength of 131 Mpa (19,000 psi) in its steel tube and concrete composite columns. High-strength concrete was used to meet a design criteria of 41 GPa (6 million psi) modulus of elasticity.
The modulus of elasticity is not necessarily proportional to the compressive strength of a concrete. There are code formulas for normal-strength concrete and suggested formulas for high-strength concrete.
The modulus achievable is affected significantly by the properties of the aggregate and also by the mixture proportions.
If an aggregate has the ability to produce a high modulus, then the optimum modulus in concrete can be obtained by using as much of this aggregate as practical, while still meeting workability and cohesiveness requirements.
If the coarse aggregate being used is a crushed rock, and manufactured fine aggregate of good quality is available from the same source, then a combination of the two can be used to obtain the highest possible modulus.
Due to the high amount of cementitious material in high-strength concrete, the role of the fine aggregate (sand) in providing workability and good finishing characteristics is not as crucial as in conventional strength mixes.
Sand with a fineness modulus (FM) of about 3.0—considered a coarse sand—has been found to be satisfactory for producing good workability and high compressive strength.
For specified strengths of 70 Mpa (10,000 psi) or greater, FM should be between 2.8 and 3.2 and not vary by more than 0.10 from the FM selected for the duration of the project.
Finer sand, say with a FM of between 2.5 and 2.7, may produce lower-strength, sticky mixtures.
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