High-Strength Concrete for Seismic Applications

An Innovation Task Group (ITG) is a specialized ACI committee with a limited mission and lifetime.1 Its role is to evaluate new technology and, if appropriate, produce ACI documents, including specifications, standards, recommended practices, or guides. Through this process, an ITG helps expeditiously

ITG-4

Over the past 3 years, ITG-4, High-Strength Concrete for Seismic Applications, has developed three documents that will soon be published. The documents are designed to allow normalweight concrete with a specified compressive strength of 6000 psi (40 MPa) or greater to be used in cast-in-place buildings constructed in regions where the seismic hazard is moderate to high. Although building codes do not specifically preclude the use of high-strength concrete in these regions, many building officials were reluctant to approve it without additional information regarding its performance in seismic applications.

The members of ITG-4 are Joseph M. Bracci, Michael A. Caldarone, S. K. Ghosh, D. Kirk Harman, Daniel C. Jansen, Adolfo Matamoros, and Andrew W. Taylor. Further, ACI members Dominic J. Kelly, Andres Lepage, and Henry G. Russell have provided review comments.

All are to be commended for their participation.

The documents, listed below, have been approved by ACI's Technical Activities Committee (TAC) and are now being prepared for publication:

* ITG-4.1, "Specification for High-Strength Concrete in Moderate to High Seismic Applications;"

* ITG-4.2, "Materials and Quality Considerations for High-Strength Concrete in Moderate to High Seismic Applications;" and

* ITG-4.3, "Structural Design and Detailing for High-Strength Concrete in Moderate to High Seismic Applications."

DOCUMENTATION

ITG-4.1

ITG-4.1 is written in mandatory language as a reference specification that can be cited in specifications for building projects that are:

* Located in Seismic Zones 2, 3, or 4 of the Uniform Building Code;2

* Assigned to Seismic Performance Categories C, D, or E of the BOCA/National Building Code (1993 or subsequent editions)3 or the Standard Building Code (1994 or subsequent editions);4 or

* Assigned to Seismic Design Categories C, D, E, or F of the International Building Code5 or of NFPA 5000?.6

The ITG-4.1 specification supplements ACI 301-05, "Specifications for Structural Concrete,"7 by providing more restrictive selected requirements when highstrength concrete is used in moderate to high seismic applications. Such requirements include limiting sand equivalent values for fine aggregate, washing aggregates at the aggregate production facility, limiting water additions after the concrete has been batched, and ensuring that curing commences within 15 minutes after final placement or finishing of the concrete. The requirements of ACI 301-05 apply unless otherwise specified in ITG-4.1.

Licensed design professionals will be able to supplement the provisions of ITG-4.1 by stipulating individual project requirements, and building officials will be able to rely on ITG-4.1 for qualification of high-strength concrete.

ITG-4.1 is now available for public comment at www.concrete.org/pubs/standaction.asp.

ITG-4.2

ITG-4.2 is a supporting document for ITG-4.1, written in nonmandatory language and providing commentary and recommendations on the topics of materials and quality considerations for high-strength concrete. Specific topics include plastic and hardened properties, raw materials, and mixture proportioning for high-strength concrete. Further, recommendations are made regarding submittals and preconstruction activities; production, delivery, and placement; and sampling and testing.

ITG-4.3

ITG-4.3 is also written in nonmandatory language and provides a review of the current literature covering structural design using high-strength concrete. The document is organized in sections addressing the design of columns, beams, beam-column joints, and structural walls.

Based on the findings cited in the literature, there are a number of recommended modifications to ACI 318-05. The recommendations include proposals for the modification of the equivalent rectangular stress block (Fig. 1) to replace the normally assumed concrete compressive stress of 0.85f'^sub c^ with a^sub 1^ f'^sub c^, where a^sub 1^ varies as shown in Fig. 2. For comparison, the values of a^sub 1^ used in the Canadian and New Zealand concrete design standards (CSA A23.3(8) and NZS 3101(9), respectively) are also shown in Fig. 2. Additional recommendations include modifications to the equations for calculating the axial strength of columns, requirements for column confinement, limits on the specified yield strength of the reinforcement, factors defining compression struts, and provisions for the development of straight bars and hooks.