scope:

INTRODUCTION

Historical background

Although high-strength concrete is often considered a relatively new material, its development has been gradual over many years. As the development has continued, the definition of high-strength concrete has changed. In the 1950s, concrete with a compressive strength of 5000 psi (34 MPa) was considered high strength. In the 1960s, concrete with 6000 and 7500 psi (41 and 52 MPa) compressive strengths were used commercially. In the early 1970s, 9000 psi (62 MPa) concrete was being produced. More recently, compressive strengths approaching 20,000 psi (138 MPa) have been used in cast-in-place buildings.

For many years, concrete with compressive strength in excess of 6000 psi (41 MPa) was available at only a few locations. However, in recent years, the applications of high-strength concrete have increased, and high-strength concrete has now been used in many parts of the world. The growth has been possible as a result of recent developments in material technology and a demand for higher-strength concrete. The construction of Chicago's Water Tower Place and 311 South Wacker Drive concrete buildings would not have been possible without the development of high-strength concrete. The use of concrete superstructures in long span cable-stayed bridges such as East Huntington, W.V., bridge over the Ohio River would not have taken place without the availability of high-strength concrete.

Committee objectives

Since the definition of high-strength concrete has changed over the years, the committee needed to define an applicable range of concrete strengths for its activities. The following working definition was adopted: "The immediate concern of Committee 363 shall be concretes have specified compressive strengths for design of 6000 psi (41 MPa) or greater, but for the present time, considerations shall not include concrete made using exotic materials or techniques."

The word exotic was included in the definition so that the committee would not be concerned with concretes such as polymer-impregnated concrete, epoxy concrete, or concrete with artificial normal and heavy-weight aggregates.

Although 6000 psi (41 MPa) was selected as the lower limit, it is not intended to imply that there is a drastic change in material properties or in production techniques that occur at this compressive strength.

In reality, all changes that take place above 6000 psi (41 MPa) represent a process which starts with the lower-strength concretes and continues into high-strength concretes. Many empirical equations used to predict properties of concrete or to design structural members are based on tests using concrete with compressive strengths less than about 6000 psi (41 MPa). The availability of data for higher-strength concretes requires a reassessment of the equations to determine their applicability with higher-strength concretes. Consequently, caution should be exercised in extrapolating data from lowerstrength to high-strength concretes. If necessary, tests should then be made to develop data for the materials or applications in question.

The committee also recognized that the definition of high-strength concrete varies on a geographical basis. In regions where concrete with a compressive strength of 9000 psi (62 MPa) is already being produced commercially, high-strength concrete might be in the range of 12,000 to 15,000 psi (83 to 103 MPa) compressive strength. However, in regions where the upper limit on commercially available material is currently 5000 psi (34 MPa) concrete, 9000 psi (62 MPa) concrete is considered high strength. The committee recognized that material selection, concrete mix proportioning, batching, mixing, transporting, placing, and control procedures are applicable across a wide range of concrete strengths. However, the committee felt that material properties and structural design considerations given in this report should be concerned with concretes having the highest compressive strengths. The committee has tried to cover both aspects in compiling this state-of-the-art report.