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In the early 1970s, experts predicted that the practical limit
of ready-mixed concrete would be unlikely to exceed a compressive strength
greater than 11,000 psi (76 MPa). Over the past two decades, the development of
high-strength concrete has enabled builders to easily meet and surpass this
estimate. Two buildings in Seattle, Washington, contain concrete with a
compressive strength of 19,000 psi (131 MPa).
The primary difference between high-strength concrete and
normal-strength concrete relates to the compressive strength that refers to the
maximum resistance of a concrete sample to applied pressure. Although there is
no precise point of separation between high-strength concrete and
normal-strength concrete, the American Concrete Institute defines high-strength concrete as
concrete with a compressive strength greater than 6000 psi (41 MPa).
Manufacture of high-strength concrete involves making optimal use
of the basic ingredients that constitute normal-strength concrete. Producers of
high-strength concrete know what factors affect compressive strength and know
how to manipulate those factors to achieve the required strength. In addition to
selecting a high-quality portland cement, producers optimize aggregates, then
optimize the combination of materials by varying the proportions of cement,
water, aggregates, and admixtures.
When selecting aggregates for high-strength concrete, producers
consider the strength of the aggregate, the optimum size of the aggregate, the
bond between the cement paste and the aggregate, and the surface characteristics
of the aggregate. Any of these properties could limit the ultimate strength of
high-strength concrete.
Admixtures
Pozzolans, such as fly ash and silica fume, are the most commonly used mineral admixtures in high-strength concrete. These materials impart additional strength to the concrete by reacting with portland cement hydration products to create additional C-S-H gel, the part of the paste responsible for concrete strength.
It would be difficult to produce high-strength concrete mixtures
without using chemical admixtures. A common practice is to use a
superplasticizer in combination with a water-reducing retarder. The
superplasticizer gives the concrete adequate workability at low water-cement
ratios, leading to concrete with greater strength. The water-reducing retarder
slows the hydration of the cement and allows workers more time to place the
concrete.
High-strength concrete is specified where reduced weight is
important or where architectural considerations call for small support elements.
By carrying loads more efficiently than normal-strength concrete, high-strength
concrete also reduces the total amount of material placed and lowers the overall
cost of the structure.
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Flexural strength testing |
Compressive strength testing |