Research Confirms Engineering Properties of Real-World High-Strength Concrete

High-strength or high-performance concrete revolutionized building and transportation construction in the 1980's. The material behind these interchangeably used terms stands as one of the decade's major contributions to both architecture and infrastructure.

The American Concrete Institute (ACI) defines it as concrete with a compressive strength in excess of 6000 psi (41 MPa). However, the high-strength or high-performance designation is most often associated with mixes formulated for compressive strengths of 8000 psi (55 MPa) or more. Easily placed concrete having a compressive strength approaching 20,000 psi (138 MPa) is now commercially available.

The material's most common applications to date are in building columns, bridge superstructures and decks, and parking decks. In structures engineers specify high-strength concrete chiefly for its compressive values. The term high performance is perhaps a better tag for deck and specialty pavement applications, where the mix design might be similar to that for a structural component, but is selected more for its low permeability characteristics. The density of high-performance concrete provides a composite with permeability lower than normal-strength concrete and more durable in the presence of corrosion-inducing deicing chemicals.

The industry did not invent high-strength or high-performance concrete during the 1980s, but rather perfected the processes necessary for its commercial availability. In that time, development, standardization, and marketing of new chemical and mineral admixtures helped eliminate long-standing production obstacles that had made high-strength concrete impractical for normal ready mix delivery and unworkable for jobsite placement.

The 1980s advent of high-strength concrete - evidenced by highly successful installations in major structures and millions of square feet of deck placements - saw advances in materials technology outpace confirmation of engineering properties. The resulting data void left some specifiers and engineers reluctant to capitalize on this new technology, and challenged the industry to document not only engineering properties, but confirm the applicability of existing conventional concrete test methods to high-strength specimens as well.

Conventional Rules Apply

Normal-strength concretes have traditionally been specified with the assurance they possess certain engineering properties that can be verified with standard laboratory equipment and procedures. The measure of those properties is more than remote data: It influences codes, standards, and specifications that ultimately define concrete's acceptance for structural and other applications.

To address areas of concern specific to high-strength concrete, a three-year research program ensued in 1989. Nine cosponsors participated with the Portland Cement Association in this study:

Canada Centre for Mineral and Energy Technology (CANMET);

Conoco; Elkem Materials; Gifford-Hill & Company; W.R. Grace & Co. -Conn.; Master Builders, Inc.; Mobil Research and Development; and Chicago-based ready mix producers, Material Service Corporation, and Prairie Group.

Based on three years' testing and evaluation, researchers contend that many of the rules for normal-strength concrete practice apply to high-strength concrete as well. A benchmark report, Engineering Properties of Commercially Available High-Strength Concretes (RD104T), by R. G. Burg and B.W. Ost of Construction Technology

Laboratories, Inc., summarizes the findings and brings documentation in line with industry needs.

The program's guiding principle has been replicating common practice. Researchers have tested six commercially produced mixes designed with varying cement and water contents; low-to-high dosages of silica fume, fly ash, and high-range water reducer (superplasticizer); and targeted compressive strengths ranging from 10,000 psi to 20,000 psi (69 MPa to 138 MPa). Cast in 4x8-in. (100x200-mm) and 6x12-in. (150x300-mm) cylinders and 4-ft (1220-mm) cube specimens, the mixes were moist cured, air cured, or insulated to allow through comparison of likely jobsite construction conditions. Air curing, unfortunately, mimics the most commonly encountered jobsite practice. Moist curing, the optimal process for concrete maximizes potential compressive strength development. Insulating specimens maintains an ambient temperature range approximately equivalent to that of concrete within a structural member.

The 4x8-in. cylinder size, which has been used for some commercial high-strength concretes due to testing equipment limitations, was included as a means of comparing compressive strength measurements from these smaller specimens to those of conventional 6x12-in. cylinders. The 4-ft cubes were designed as realistically scaled structural members from which heat generation could be recorded and representative core specimens drilled.

"Realcrete," not "Labcrete"

The program used specimens that were designed, fabricated, and cured by methods closer to actual practice than previous laboratory work had attempted, in effect testing concrete from the real world -"realcrete" - as opposed to "labcrete".

The research builds on data from multiple sources referenced in ACI Committee 363's State-of-the-Art Report on High-Strength Concrete. Many observations from RD104T parallel those in the ACI 363 document. However, the R&D bulletin contains data gathered strictly from commercially available mixes tested in an all-encompassing, single source research effort.

Researchers behind Engineering Properties have followed applicable American Society for Testing and Materials standards. Observing mixes' early, intermediate, and long-term characteristics, they concluded:

Cylinder and Core Strengths. Compressive strengths of 4x8 in. (100x200-mm) cylinders measure within about 1 % of the compressive strengths of 6x12-in. (150x300-mm) cylinders. Core concrete strengths increase with age for all mixes and measured at 91 days, are 83% to 94% of companion moist cured 6x12-in. cylinder strengths. Care in handling and preparation of higher-strength specimens is critical in obtaining accurate measurements.

Drying Shrinkage and Creep. Drying shrinkage, a reflection of water content, is slightly lower on high-strength specimens than normal-strength concrete. Creep, the measure of a concrete's deformation under loading, is lower on high strength than on normal-strength concrete specimens.

Freezing and Thawing, Thermal Expansion. High-strength concretes having no intentionally entrained air can deteriorate or fail in freezing and thawing tests. However, the program's highest compressive strength mix, 19,500 psi (134 MPa) at 9 months, with both high cement and silica fume contents, showed no distress, not even after an inordinate 1800 cycles of freezing and thawing. Thermal expansion coefficients obtained from high-strength specimens were comparable to those of normal-strength concretes.

MOE, MOR, and Tensile Strength.

•As with normal-strength concretes, compressive strength can be used to estimate modualus of elasticity, modulus of rupture, and tensile strength of high-strength concrete. Each of the values is estimated by

multiplying the square root of the concrete's compressive strength by a constant. Based on the test results obtained, researchers have developed new constants for estimating these properties.

Permeability, Corrosion Resistance, Water Absorption. Even in moderate dosages, silica fume lowers a concrete's permeability to chloride ions - thus protecting reinforcing steel from corrosion. To a lesser extent, fly ash has the same effect. High-strength concretes can be produced with chloride ion permeability levels comparable to those of latex-modified concrete. Corrosion rates were gauged by the specimens' internal resistivity to AC impedance. They indicated that reinforcing steel would be passive to corrosion. Water absorption rates were all low and reflected trends similar to those of chloride permeability.

Heat generation. Temperatures of high-cement-content mixes-greater than 800 Ib per cubic yard (475 kg/m3)-rose from 10 to 12.5 deg F for each 100 Ib of cement per cubic yard of concrete (9.4 to 11.7 deg C for each 100 kg of cement per cubic meter). This rise was observed in the five mixes designed for strength measurement, as well as a sixth mix designed for low-heat generation. The latter mix had a maximum total temperature rise of 21 to 31 deg F (15 to 21 deg C) less than the five other mixes.

Mineral and Chemical Admixtures. For higher-strength mixes of 10,000 to 20,000 psi (69 to 138 MPa), superplasticizers in relatively large dosages of 290 to 520 oz per cubic yard (11.2 to 20.1 liters per cubic meter) permit an extremely low water-to-cementitious materials ration range of 0.22 to 0.32. Mixes of 8-to 10-in. (200- to 250-mm) slumps can be produced with sufficient superplasticizer, while still achieving very low water-to-cementitious materials ratios.

Results of this joint-industry study of high-strength concrete lay to rest some testing concerns while developing new data on its engineering properties and performance - data that will ultimately broaden applications as designers and specifiers gain confidence in high-strength mixes.

Copies of Engineering Properties of Commercially Available High-Strength Concrete (RD104T) are available for purchase in the United States from the Portland Cement Association, Order Processing, P.O. Box 726, Skokie, IL 60076-0726, telephone 708/966-6200 ext. 564.