High-Strength Concrete (ACI 363R)


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Title: High-Strength Concrete (ACI 363R)

Author(s): ACI Committee 363

Publication: Special Publication

Volume: 228


Appears on pages(s): 79-80

Keywords: applications; concrete; design; economics; high-strength; materials; production; properties; proportions

Date: 6/1/2005

Although highstrength concrete is often times still considered a relatively new material, its development has been gradual and continual over the last 50 years. During this period, many notable changes have occurred and continue to occur in the area of high-strength concrete technology, including the definition of high-strength concrete itself. With the increased knowledge that has been gained with respect to material availability, design methodology, and construction techniques, the feasible realm of high-strength concrete applications has grown dramatically. One of the primary objectives of ACI Committee 363 during the last few years has been to update and republish document 363R, High-Strength Concrete. This synopsis is based on a full report on high-strength concrete to be published by ACI Committee 363 (High-Strength Concrete). Estimated publication time for the new document is early 2006. The objective of the document is to present state-of-the-art information on concrete with strengths in excess of about 55 MPa (8000 psi), but not including concrete made using exotic materials or techniques. In the 1950s, concrete with a compressive strength of 34 MPa (5000 psi) was considered high strength. Today, high-strength concrete is defined as concrete with a specified compressive strength of 55 MPa (8000 psi) or higher. In many markets today, concrete having a specified compressive strength in excess of 69 MPa (10,000 psi) is routinely produced on a daily basis. Although 55 MPa (8000 psi) was selected as the current lower limit, it is not intended to imply that any drastic change in material properties or in production techniques occurs at this compressive strength. In reality, all changes that take place at or above 55 MPa (8000 psi) represent a process which starts with the lower strength concretes and continues into the highstrength realm. Items to be considered in selecting materials include characteristics of cement and supplementary cementitious materials, aggregate properties, and the effects of chemical admixtures, particularly with respect to their water reduction and hydration controlling capabilities. To ensure that required concrete strengths and other desired properties would be obtained, trial mixtures are an essential part of the process. Depending on the appropriate application, mix proportions for high-strength concrete generally have been based on achieving a required compressive strength at a specified age, many times beyond the traditional 28 days. Factors included in selecting concrete mix proportions have included availability of constituent materials, desired workability, and effects of temperature rise. Research data have indicated that the measured modulus of elasticity of high-strength concrete can vary significantly from calculated values based on unit weight and concrete compressive strength. High-strength concrete has shown a higher rate of strength gain at early ages as compared to lower-strength concrete, but at later ages the relative difference is not as significant. Information on creep and shrinkage of high-strength concrete has indicated that the shrinkage is similar to that for lower-strength concrete. However, specific creep is much less for high-strength concretes than for lower-strength concretes. The use of high-strength concrete can have significant impacts on structural design, though changes in structural behavior generally occur gradually as concrete strength is increased. Modifications to standard design equations developed for lower-strength concretes are necessary for determining the strength of axially-loaded columns, axial and flexural strength of eccentrically-loaded columns, loss of prestress in prestressed concrete beams, and minimum reinforcement requirements for flexure, shear, and torsion in reinforced concrete beams. Proposed modifications in each of these areas have been summarized in the 363R document. Significant research has been completed but consensus design e