For more information, view High-strength Concrete, a Historical Background
What is high-strength?
Defining “high-strength” in terms of a universally applicable numerical value is not possible, at least not with any sound degree of rationale. High-strength is a relative term that is dependent on many things. I suggest a range that most authorities might agree is a reasonable threshold for what would be considered HSC.
Strength is not an intrinsic property of concrete. It is a relative property that depends on numerous factors. Primary factors influencing the measured strength of concrete include specimen geometry, size, age, and curing history; testing equipment parameters, such as loading capacity; lateral and longitudinal stiffness; and the loading rate and uniformity of load distribution.
There also are geographic considerations. In regions where compressive strengths of 9000 psi is commercially produced routinely, concrete might not be considered high-strength until it attains a measured strength in the range of 10,000 or 12,000 psi. Conversely, in regions where the upper limit on commercially available concrete has been 4000 psi, concrete successfully meeting a design requirement for 6000 psi might be considered high-strength, and for good reason.
The reason for such diversity is two-fold: need and ability. Although realize that both are relative; need to the type of construction and the initiative of the designer, and ability to the commitment of the concrete producer and quality of the locally available materials.
Defining HSC by a specific strength value in essence establishes an arbitrarily selected line of demarcation that is neither practical nor warranted. The principal concern with arbitrarily chosen values defining high-strength is that concrete routinely produced in one market might not be considered a major achievement in another.
The definition is by no means static. Where HSC has been defined in terms of precise numerical value, its definition has changed over the years.
In the 1984 version of ACI Committee Report 363R-92, 6000 psi was selected as a lower limit for HSC. Although the report selected this as the lower limit, it was not intended to imply that any drastic change in material properties or production techniques occurs at this level of compressive strength.
In reality, all of the gradual changes that take place represent a process that starts with very modest strength levels and continues well into the realm of ultra-high-strength concrete.
In revising the 1992 version of State-of-the-Art of High-Strength Concrete report, Committee 363 defined HSC as having a specified compressive strength for design of 8000 psi. Committee 363 also recognized that the definition of HSC varies on a geographic basis.
In most industrialized countries, producers and users generally consider concrete to be high-strength when the specified compressive strength of the material is in the 6000 to 8000 psi range at acceptance ages of 28 days or later. Of course, if the industry's rate of advancement in materials technology continues, it might not be long before values of the magnitude become obsolete.