| High-Strength Concrete
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.
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
most common use of high-strength concrete is for construction of
high-rise buildings. At 969 ft (295 m), Chicago's 311 South Wacker
Drive uses concrete with compressive strengths up to 12,000 psi
(83 MPa) and is the tallest concrete building in the United States.