ACI Compilations are a series of special publications that combine material previously published by the Institute to provide compact and ready reference on specific topics. The fifteen papers in this compilation on High-Performance Concrete have been selected by a Task Group of the TAC Subcommittee on High-Performance Concrete (THPC) with a specific objective of providing technical information to readers who would find it useful in practical applications. For readers who may wish to pursue the subject further, a recommended reading list is also provided.
High-performance concrete (HPC) has been broadly defined by THPC as concrete that meets special performance and uniformity requirements that cannot always be achieved routinely by using only conventional materials and normal mixing, placing, and curing practices. The requirements may involve enhancements in ease of placement, compaction without segregation, long-term mechanical properties, early-age strength, volume stability, or service life in severe environments. Concretes possessing many of these characteristics often achieve higher strength. Therefore, high-performance concrete is often of high strength, but high-strength concrete may not necessarily be of high performance.
Two papers in this compilation are for general reading. In his paper, Forster discusses the definition of HPC and the basic considerations of when to use HPC. Factors such as workability, placing, and curing are discussed. The paper by Aitcin and Neville points out that HPC is not that different from ordinary concrete; but, through the application of special know-how and the judicious use of cementitious materials, chemical admixtures, and optimum water-cement ratio, properties of concrete can be enhanced and applications of concrete expanded. Changes in the composition, pore structure, and bond of the matrix with the aggregate are described as the result of using this know-how in selecting materials and proportions.
To produce HPC, it is essential to have good quality coarse aggregates. Aitcin and Mehta point out in their paper the importance of evaluating the characteristics of coarse aggregates to be used in very high-strength concrete. Test results of three Canadian and four Californian aggregates producing concrete strengths ranging from 85 to 105 MPa (12 to 15 ksi) are summarized. It is shown that coarse aggregate mineralogy, strength, and/or bonding with the cement paste play an important role in concrete strenth and elastic modulus. Collins discusses proportioning high-strength concrete to control creep and shrinkage. Several different high-strength concrete mixtures having compressive strengths ranging from 56 to 65 MPa (8 to 9.3 ksi) are compared. They show satisfactory creep and drying shrinkage behavior. The use of a high-range water reducer does not have a significant effect on deformations, and the factors that reduce deformation in normal strength concrete apply also to high-strength concrete.
Economics drives the use of high-strength concrete, since its load-carrying capacity increases more than its cost. Commercial production of high-strength concrete has had a long history. Moreno documents in his paper the development of 41 MPa (6 ksi) concrete in 1962 to 96 MPa (13.7 ksi) concrete in 1982 and demonstrates the commercial capability of producing 117 MPa (16.7 ksi) concrete. To translate laboratory trial mixtures of HPC for use in field applications is not a simple task. In the paper by Schemmel et al., lessons learned from field trials with high-early-strength, rapid slump loss pavement patching mixtures designed to produce 35 MPa (5 ksi) concrete in 24 hr are discussed. Very practical advice and guidelines are given by the authors.
For HPC, large amounts of cementitious materials are usually used, thus generating more heat of hydration and producing larger thermal gradients in newly cast large-sized HPC members. These thermal effects are of serious concern. Cook et al. in one paper and Miao et al. in another present their results of laboratory and field studies on large HPC columns. Analytical predictions are confirmed by thermocouple measurements. It is demonstrated that HPC is less susceptible to thermal cracking than normal strength concrete because of the higher tensile strength of HPC.
Two papers deal with testing of high-strength concrete. The paper by Lassard et al. examines the problems associated with testing high-strength concrete with standard size cylinders. Comparisons are made with the smaller size (100 x 200 mm [4 x 8 in.]) cylinders. The capping method is also compared with grinding and the effect of the bearing block size evaluated. The paper by Yuan et al. evaluates and compares test results for cylinders and cores from a large column with high-strength concrete. Standard cylinders were both field- and moist-cured. The effect of core location in large columns and the difference between cores and moist-cured and field-cured cylinders are determined. Temperature gradient and its effect on large columns are also evaluated.
Long-term characteristics of a very high-strength concrete are discussed by Aitcin et al. Their paper presents an evaluation of an experimental concrete mock column made from 85 MPa (12 ksi) concrete. Cores taken from the structure 2 and 4 years after casting showed strength equal to the 28-day cylinder strength and durability of 300 coulombs in the rapid chloride permeability test. Strain gages showed no internal strains after cooling within 4 days after casting and only surface shrinkage was observed. A microstructural examination also confirmed an absence of internal microcracks. In the paper by Li et al., freezing and thawing durability of high-strength pastes, mortar, and concretes with and without silica fume is discussed. All test samples were non-air-entrained, and the air void spacings in the concretes were between 0.75 and 1.3 mm, well above the 0.2 mm normally required for durable concrete. In all cases, samples with w/c = 0.24 showed very good freeze-thaw durability (to 1000 cycles) despite not being air-entrained. With w/c = 0.27 or higher, the samples did not perform well. The test results showed that freeze-thaw performance is independent of silica fume content.
Finally, three papers are included in this compilation as illustrations of several applications of HPC. Ozyildirim describes the experience of Virginia DOT in producing very low-permeability concretes for repairs of pavement and bridge decks. He also discusses their experience in producing concretes of very high-early-strength and very low permeability for fast-track operations. Test results of both laboratory and field concrete are presented. The economic advantage of using HPC for columns of high-rise buildings and several design approaches to achieve structural ductility for such applications are discussed by Webb in his paper, which covers the continuing development of HPC in Australia. In the area of long-span bridges, Malier et al. describe the design and construction of a three-span externally post-tensioned bridge using 60 MPa (8.7 ksi) concrete. A comparison with a similar bridge designed for 35 MPa (5 ksi) concrete revealed a 30 per cent reduction in concrete volume. The bridge is instrumented for long-term monitoring of its performance.