ATC 115 Preliminary Cover

Roadmap for the use of high-strength reinforcement in reinforced concrete design

Funded by theCharles Pankow Foundation

ATC 115

Applied Technology Council

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ATC-115

Roadmap for the Use of High-Strength Reinforcement in Reinforced Concrete Design

95% Draft Report

by

APPLIED TECHNOLOGY COUNCIL 201 Redwood Shore Parkway, Suite 240

Redwood City, California 94065 www.ATCouncil.org

Funded by

CHARLES PANKOW FOUNDATION P.O. Box 820631

Vancouver, Washington 98682

CHARLES PANKOW FOUNDATION REPRESENTATIVE Mark J. Perniconi

ATC PROJECT MANAGER

Jon A. Heintz

PROJECT MANAGEMENT COMMITTEE Dominic J. Kelly (Project Technical Director) David Darwin David C. Fields Robert J. Frosch Andres Lepage Joseph C. Sanders Andrew Whittaker

PROJECT REVIEW PANEL Wassim Ghannoum S.K. Ghosh Ramon Gilsanz James O. Jirsa Mike Mota Thomas C. Schaeffer Loring A. Wyllie, Jr.

2014

ATC-115 Preface iii

Preface

In 2012, the Charles Pankow Foundation (CPF) began to investigate the feasibility of incorporating reinforcing steel in excess of 60ksi into the ACI 318 Building Code. This investigation was prompted by interest on the part of structural engineering practitioners, structural concrete constructors and key academic researchers who felt that the use of higher strength reinforcing bars could provide a significant benefit to the industry. The initial investigative efforts of CPF first included several informal meetings with an expert panel followed by the commissioning of several research projects that studied the technical feasibility of using higher strength reinforcing bar as well as developing a technical definition of the product. At the same time, CPF engaged the producers of steel reinforcing bars to evaluate the technical and financial feasibility of making high strength reinforcing bars commercially available.

By mid-2013, initial feasibility studies commissioned by CPF confirmed the technical feasibility of using high strength reinforcing bar in design. In addition, reinforcing bar producers verified that higher strength reinforcing bars could be manufactured and be made available through normal distribution channels. Also in 2013, an unrelated study funded by NIST (NIST GCR 14-917-30) and titled Use of High-Strength Reinforcement in Earthquake-Resistant Concrete Structures (also called the ATC 98 Project) was completed. This study confirmed the feasibility of using high strength reinforcing bar in seismic applications. The next task was to determine the supporting research necessary to effect an ACI 318 Building Code update to incorporate the general use of reinforcing bar in excess of 60ksi. The last comprehensive ACI 318 Code update related to rebar strength was done in 1971.

The vehicle chosen by CPF to define the steps necessary to effect ACI 318 Code changes was a roadmap that essentially reviewed every applicable section of the ACI 318-14 Building Code and identified new research or engineering studies necessary to support the required code changes and updates. The entity chosen by CPF to manage this process is the Applied Technology Council (ATC). CPF and ATC have a long history of successful collaboration on research projects. The name of the project is The Development of a Roadmap on Use of High-Strength Reinforcement in

iv Preface ATC-115

Reinforced Concrete Design, and the working title is the ATC 115 Project. This research project may be found on the CPF website (www.pankowfoundation.org) as Research Grant Agreement #05-13.

CPF wishes to acknowledge the hard work of Jon A. Heintz, Project Manager for ATC. CPF is indebted to the leadership of Dominic J. Kelly, Project Technical Director, and to the members of the Project Management Committee, consisting of Andres Lepage, David C. Fields, David Darwin, Joseph C. Sanders, Robert J. Frosch and Andrew Whittaker, for their contributions in developing this report and the resulting recommendations. The Project Review Panel, consisting of Wassim Ghannoum, James O. Jirsa, Ramon Gilsanz, Mike Mota, Thomas C. Schaeffer and Loring A. Wyllie, Jr., provided technical review and commentary at key developmental milestones during the project. CPF also wishes to acknowledge the contributions of Conrad Paulson and Jack Moehle as special consultants to CPF and Robert Risser, CEO of CRSI and its many producers and member who contributed to this effort. Finally, CPF wishes to thank its Board of Directors, Richard Kunnath, Ron Klemencic and Tim Murphy for their support and contributions to this effort.

The completion of the ATC 115 Project is an important next step in the process of adding high strength reinforcing bar into the ACI 318 Building Code. The research effort defined in the ATC 115 Report will be one of the largest research programs ever undertaken in the industry. To accomplish this goal, it will take the support of the entire reinforced concrete industry. The net result will be much more than just the addition of high strength reinforcing bar into the ACI Building Code. The process of adding high strength reinforcing bar into the ACI 318 Building Code will essentially bring most of the technical specifications in the ACI 318 Building Code into the 21st Century.

Mark J. Perniconi, P.E. Executive Director Charles Pankow Foundation

ATC-115 Table of Contents v

Table of Contents

Preface ........................................................................................................... iii

List of Figures ............................................................................................... ix

List of Tables ................................................................................................ xi

1. Introduction ...................................................................................... 1-1 1.1 Purpose and Scope of Work ..................................................... 1-1 1.2 Historical Perspective of High-Strength Reinforcement and

ACI 318 .................................................................................... 1-2 1.3 Prospects for Adoption of High-Strength Reinforcement into

ACI 318 .................................................................................... 1-3 1.4 Prospects for Adoption of High-Strength Reinforcement into

other U.S. Codes and Standards ............................................... 1-4 1.5 Key Issues with the Use of High-Strength Reinforcement ....... 1-5 1.6 Report Organization and Content ............................................. 1-5

2. Production, Fabrication, and Construction Issues ........................ 2-1

2.1 Reinforcement Production and Specification Issues ................ 2-1 2.1.1 Production of Deformed Steel Reinforcement ............ 2-1 2.1.2 Specification of High-Strength Reinforcement ........... 2-5

2.2 Fabrication Issues ..................................................................... 2-7 2.3 Constructability Issues and Construction Efficiencies ............. 2-9

3. Current State-of-Knowledge on Key Design Issues ...................... 3-1

3.1 Strength and Ductility Issues .................................................... 3-2 3.1.1 Flexural and Axial Load Strength ............................... 3-2

3.1.1.1 Strain Limits ................................................ 3-2 3.1.1.2 Beams .......................................................... 3-5 3.1.1.3 Columns ...................................................... 3-7

3.1.2 Shear Strength ........................................................... 3-10 3.1.2.1 Shear Strength of Members without Shear

Reinforcement ........................................... 3-10 3.1.2.2 Shear Strength of Members with Shear

Reinforcement ........................................... 3-13 3.1.2.3 Shear Friction ............................................ 3-15 3.1.2.4 Deep Beams .............................................. 3-16 3.1.2.5 Ordinary Structural Walls ......................... 3-17

3.1.3 Strut and Tie Modeling.............................................. 3-19 3.1.4 Other Strength Issues ................................................ 3-20

3.1.4.1 Concrete Strength ...................................... 3-20 3.1.4.2 Tension Regions of Shells and Folded

Plates ......................................................... 3-21 3.1.4.3 Bonded Reinforcement Ratios for Members

with Unbonded Post-Tensioning ............... 3-22

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3.1.4.4 High-Cycle Elastic Fatigue of Reinforcement ........................................... 3-22

3.2 Serviceability .......................................................................... 3-23 3.2.1 Deflections ................................................................. 3-23 3.2.2 Drift ........................................................................... 3-27 3.2.3 Crack Control ............................................................ 3-27

3.3 Reinforcement Limits ............................................................. 3-29 3.3.1 Beams ........................................................................ 3-29 3.3.2 Slabs and Footings ..................................................... 3-30 3.3.3 Columns ..................................................................... 3-31 3.3.4 Walls .......................................................................... 3-32

3.4 Detailing and Other Design Considerations ........................... 3-33 3.4.1 Bend Test Requirements and Minimum Bend

Diameters ................................................................... 3-33 3.4.2 Transverse Reinforcement Spacing ........................... 3-35

3.4.2.1 Spacing of Transverse Reinforcement in Members Resisting Gravity and Wind Loads or in Ordinary Seismic-Force-Resisting Systems ...................................................... 3-35

3.4.2.2 Transverse Reinforcement Spacing for Members of Intermediate and Special Seismic-Force-Resisting Systems ............. 3-36

3.4.3 Head Attachment of Headed Deformed Bars ............ 3-38 3.4.4 Development and Splice Lengths .............................. 3-39

3.4.4.1 Straight Bar Development Lengths ........... 3-39 3.4.4.2 Hooked Bar Development Length ............. 3-41 3.4.4.3 Headed Deformed Bar Development

Length ........................................................ 3-42 3.4.4.4 Mechanical Splice ..................................... 3-43

3.4.5 Bar Extensions in Slabs ............................................. 3-44 3.4.5.1 Bar Extensions in One-Way Slabs ............ 3-44 3.4.5.2 Bar Extensions in Two-Way Slabs ............ 3-44

3.4.6 Horizontal Support of Offset Column Reinforcement ........................................................... 3-45

3.4.7 Cover for Fire Protection ........................................... 3-46 3.4.8 Beam-Column and Slab-Column Joints (Type 1) ...... 3-48

3.4.8.1 Interior Beam-Column Joints .................... 3-48 3.4.8.2 Exterior Beam-Column Joints ................... 3-50 3.4.8.3 Exterior Slab-Column Joints ..................... 3-51

3.5 General Considerations for Analysis ...................................... 3-51 3.5.1 Flexural Stiffness ....................................................... 3-51 3.5.2 Moment Redistribution .............................................. 3-53

3.6 Seismic-Force-Resisting Systems ........................................... 3-54 3.6.1 Shape of Stress Strain Relationship ........................... 3-55 3.6.2 Intermediate Moment Frames .................................... 3-58 3.6.3 Special Moment Frames ............................................ 3-59

3.6.3.1 Beams ........................................................ 3-60 3.6.3.2 Columns .................................................... 3-62 3.6.3.3 Shear Demand on Beams and Columns .... 3-65 3.6.3.4 Strong Column-Weak Beam Behavior ...... 3-66 3.6.3.5 Beam-Column Joints ................................. 3-67

3.6.4 Flexure-Critical Special Structural Walls .................. 3-70

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3.6.5 Shear-Critical walls ................................................... 3-73 3.6.6 Diaphragms ............................................................... 3-75

4. Research Studies............................................................................... 4-1

4.1 Objectives ................................................................................. 4-1 4.2 Overview .................................................................................. 4-1 4.3 Bar Production and Specification ............................................. 4-1

4.3.1 Mechanical Properties of Recent Heats of High Strength Reinforcement ............................................... 4-2

4.3.2 Detailed Mechanical Property Tests of Grade 100 and Grade 120 Reinforcement ........................................... 4-3

4.4 Strength of Members ................................................................ 4-4 4.4.1 Flexural Strength and Tensile Strain Limits ................ 4-4 4.4.2 Deflection Warning for Flexural Members Subjected

to Gravity Loads .......................................................... 4-5 4.4.3 Column Strength ......................................................... 4-7 4.4.4 Tension Regions of Shells and Folded Plates .............. 4-8 4.4.5 Shear Strength of Beams and Slabs without Shear

Reinforcement ............................................................. 4-9 4.4.5.1 One-Way Shear in Beams without Shear

Reinforcement ............................................. 4-9 4.4.5.2 Two-Way Shear in Slabs without Shear

Reinforcement ........................................... 4-10 4.4.6 Shear Strength of Beams with Shear

Reinforcement ........................................................... 4-11 4.4.7 Shear Friction ............................................................ 4-12 4.4.8 High-Cycle Elastic Fatigue of High-Strength

Reinforcing Bars ....................................................... 4-13 4.5 Serviceability .......................................................................... 4-14

4.5.1 Deflection of Flexural Members ............................... 4-14 4.5.2 Crack Control of Flexural Members ......................... 4-16

4.6 Reinforcing Limits ................................................................. 4-17 4.6.1 Minimum Reinforcement Ratio for Beams ............... 4-17 4.6.2 Minimum Reinforcement Ratio for Slabs and

Footings ..................................................................... 4-18 4.7 Detailing of Members ............................................................. 4-19

4.7.1 Development and Splice Lengths .............................. 4-19 4.7.2 Hooked Bar Development Length ............................. 4-20 4.7.3 Headed Bar Development Length ............................. 4-21

4.8 General Considerations for Analysis ...................................... 4-22 4.8.1 Flexural Stiffness ....................................................... 4-22 4.8.2 Effective Stiffness for Column Slenderness .............. 4-23 4.8.3 Moment Redistribution.............................................. 4-23

4.9 Earthquake-Resistant Structures ............................................. 4-24 4.9.1 Moment Curvature and Rotational Capacity ............. 4-24 4.9.2 Factor for Estimating Expected Flexural Strength .... 4-26 4.9.3 Cyclically Loaded Beams and Columns Initial

Tests and Analytical Studies ..................................... 4-27 4.9.4 Cyclically Loaded Beams, Columns, and Joints ....... 4-28

4.9.4.1 Cyclically Loaded Beams ......................... 4-28 4.9.4.2 Cyclically Loaded Columns ...................... 4-29 4.9.4.3 Cyclically Loaded Interior Joints .............. 4-30

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4.9.4.4 Cyclically Loaded Exterior Joints ............. 4-31 4.9.4.5 Two-Way Shear in Slab-Column

Intermediate Moment Frames.................... 4-33 4.9.5 Performance of Moment Frames Systems ................. 4-33 4.9.6 Multi-Bay, Multi-Story Frames ................................. 4-35 4.9.7 Ordinary Flexure-Critical Walls ................................ 4-35 4.9.8 Special and Ordinary Shear-Critical Walls ................ 4-37 4.9.9 Special Flexure-Critical Walls Initial Tests ............ 4-38 4.9.10 Special Flexure-Critical Walls ................................... 4-39 4.9.11 Performance of Flexure-Critical Wall Systems ......... 4-41

4.10 Engineering Design Studies .................................................... 4-42 4.10.1 Trial Engineering Designs for Use of Grade 80

Reinforcement in Special Seismic Systems ............... 4-42 4.10.2 Trial Engineering Designs for Use of Grade 100

Reinforcement in General Applications .................... 4-44 4.10.3 Trial Engineering Designs for Use of Grade 100

Reinforcement in Special Seismic Systems ............... 4-45 5. Program Recommendations ............................................................ 5-1

5.1 Summary of Program ................................................................ 5-1 5.2 Estimated Budget Requirements ............................................... 5-1

5.2.1 Budget Assumptions .................................................... 5-4 5.3 Priority and Schedule Recommendations ................................. 5-5

5.3.1 Priority Level 1 Revise ASTM A615 to Include Grade 100 Reinforcement ............................................ 5-6

5.3.2 Priority Level 2 Modify ACI 318 to Allow the use of ASTM A706 Grade 80 Reinforcement in Special Seismic Systems .......................................................... 5-6

5.3.3 Priority Level 3 Modify ACI 318 to Allow the use of ASTM A615 Grade 100 Reinforcement in General Applications (Gravity, Wind, and Ordinary Seismic Systems) ....................................................................... 5-7

5.3.4 Priority Level 4 Develop a new ASTM specification for Grade 100 Reinforcement for use in Special Seismic Systems .......................................................... 5-9

5.3.5 Priority Level 5 Modify ACI 318 to Allow the use of Grade 100 Reinforcement in Special Seismic Systems ........................................................................ 5-9

5.4 Other Conclusions and Recommendations ............................... 5-9 5.4.1 Potential Code Changes that can be Implemented

without Additional Research ..................................... 5-10 5.4.2 Issues Not Requiring Further Action or Code

Change ....................................................................... 5-11 5.4.3 Other Potential Studies .............................................. 5-11

5.5 Implementation Recommendations ........................................ 5-12 5.5.1 Key Collaborators ...................................................... 5-12 5.5.2 Technical Oversight ................................................... 5-13 5.5.3 Technical Synthesis ................................................... 5-13 5.5.4 Adoption into Codes and Standards .......................... 5-14

ATC-115 Table of Contents ix

References .................................................................................................. A-1 Project Participants ................................................................................... B-1

ATC-115 List of Figures ix

List of Figures

Figure 3-1 Strength reduction factor based on strain........................ 3-2 Figure 3-2 Types of stress-strain curves with distinct shapes ............. 3-4

Figure 3-3 Influence of reinforcement ratio on shear strength .......... 3-11

Figure 3-4 Ratios of tested to predicted punching shear capacity using ACI 318-05 ............................................................. 3-13

Figure 3-5 Developed bar stresses and concrete compressive strengths for development and splice tests on bars without confining transverse reinforcement .................... 3-40

Figure 3-6 Developed bar stresses and concrete compressive strengths for development and splice tests on bars with confining transverse reinforcement .................................. 3-40

Figure 3-7 Yield strength and tensile strength of reinforcing bars after heating and cooling .................................................. 3-47

Figure 3-8 Yield strength and tensile strength of reinforcing bars at high temperature .............................................................. 3-47

Figure 3-9 Permissible redistribution in accordance with ACI 318 ................................................................................... 3-53

Figure 3-10 Beam specimens used to study the effect of the tensile-strength-to-yield-strength ratio ........................................ 3-57

Figure 3-11 Load-deflection curves for beam specimens with: (a) yield ratio of 75% with splice; and (b) yield ratio of 90% without splice. ............................................................................... 3-57

Figure 3-12 Details for beam specimens with high-strength reinforcing bars ................................................................................... 3-61

Figure 3-13 Measured shear versus drift ratio in beam tests: (a) Specimen CC4-X, with Grade 60 reinforcing bars; and (b) Specimen UC4-X, with Grade 97 reinforcing bars .......................... 3-61

Figure 3-14 Hysteretic response of two circular columns based on Restrepo et al. (2006): Unit 1 including Grade 60 reinforcement, and Unit 2 including Grade 100 reinforcement ................................................................... 3-63

x List of Figures ATC-115

Figure 3-15 Details for column specimens with high-strength reinforcing ........................................................................ 3-64

Figure 3-16 Measured shear versus drift ratio in column tests: (a) Specimen CC-3.3-20, with Grade 60 reinforcement; and (b) Specimen UC-1.6-20, with Grade 120 reinforcement ................................................................... 3-64

ATC-115 List of Tables xi

List of Tables

Table 3-1 Minimum Percentage of Fracture Elongation for ASTM A615, A706, and A1035 Reinforcing Bars ...................... 3-24

Table 3-2 Minimum Bend Diameter of Hooks in ACI 318 and Bend Test Requirements for ASTM A615, A706, and A1035 Reinforcing Bars .............................................................. 3-24

Table 3-3 Stress-Strain Parameters for Reinforcing Steel Used to Create Moment-Curvature Relationships in NIST GCR 14-917-30 ......................................................................... 3-56

Table 4-1 Summary of Studies Related to Bar Production and Specification ...................................................................... 4-1

Table 4-2 Summary of Studies Related to Strength of Members ....... 4-4

Table 4-3 Summary of Studies Related to Serviceability ................ 4-14

Table 4-4 Summary of Studies Related to Reinforcing Limits ........ 4-17

Table 4-5 Summary of Studies Related to Detailing of Members ... 4-19

Table 4-6 Summary of Studies Related to General Considerations for Analysis ...................................................................... 4-22

Table 4-7 Summary of Studies Related to Earthquake-Resistant Structures ......................................................................... 4-25

Table 4-8 Summary of Studies Related to Trial Engineering Designs ............................................................................ 4-42

Table 5-1 Summary of Proposed Research and Engineering Studies ................................................................................ 5-2

Table 5-2 Estimated Budget Requirements by Program Area ........... 5-3

Table 5-3 Recommended Priorities Based on Implementation Objectives and Target Milestone Dates ............................. 5-5

Table 5-4 Priority Level 1 Revise ASTM A615 to Include Grade 100 Reinforcement ............................................................. 5-6

Table 5-5 Priority Level 2 Modify ACI 318 to Allow the use of ASTM A706 Grade 80 Reinforcement in Special Seismic Systems .............................................................................. 5-7

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Table 5-6 Priority Level 3 Modify ACI 318 to Allow the use of ASTM A615 Grade 100 Reinforcement in General Applications (Gravity, Wind, and Ordinary Seismic Systems) ............................................................................. 5-8

Table 5-7 Priority Level 4 Develop a new ASTM specification for Grade 100 Reinforcement for use in Special Seismic Systems .............................................................................. 5-9

Table 5-8 Priority Level 5 Modify ACI 318 to Allow the use of Grade 100 Reinforcement in Special Seismic Systems .... 5-10

Table 5-9 Potential Code Changes that can be Implemented without Additional Research ......................................................... 5-11

Table 5-10 Issues Not Requiring Further Action or Code Change ..... 5-11

Table 5-11 Other Potential Studies ..................................................... 5-12

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ATC-115 1: Introduction 1-1

Chapter 1

Introduction

Concrete reinforcement with yield strength greater than 75 ksi is becoming more available in the United States. In 2009, ASTM International included Grade 80 in specification A615, which is the most commonly referenced reinforcing bar specification. ASTM A1035 includes Grades 100 and 120, which are available from a single producer. A bar produced in Germany, SAS670, has a minimum yield strength of 97 ksi. ASTM A1035 Grade 100 and SAS670 have been approved by the New York City Building Department as column reinforcement. In the near future, many producers in the United States will be capable of reliably producing Grade 100 bars, and within 10 years, will likely be able to produce Grade 120 bars.

1.1 Purpose and Scope of Work

High-strength reinforcement refers to reinforcement with grades higher than Grade 60. The primary objective of the ATC-115 Project was to prepare a detailed Roadmap that specifically identifies the technical support required, whether it be the results of new research, engineering studies, or re-evaluation of existing research findings, to effect updates of ACI 318-14, Building Code Requirements for Structural Concrete and Commentary (ACI, 2014a), to allow the general use of steel reinforcement in excess of Grade 80 for gravity and wind load applications, and in excess of Grade 60 for special seismic systems. The project specifically considered the use of Grade 80, Grade 100, and Grade 120 high-strength reinforcement, and considered applications in members designed for gravity loads, wind loads, or seismic loads, as part of a gravity system, or as part of an ordinary, intermediate, or special seismic-force-resisting system.

The resulting Roadmap outlines the steps needed to:

further investigate the use of high-strength reinforcement in general reinforced concrete design and construction in building and non-building structural applications; and

develop code-change proposals and encourage adoption of high-strength reinforcement in the code development process.

Development of this Roadmap was based on the following activities:

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1-2 1: Introduction ATC-115

identification of production, fabrication, and design issues to be considered;

identification of constructability challenges; search for relevant research and other available information on high-

strength reinforcement in the published literature;

determination of the current state-of-knowledge regarding issues and challenges associated with high-strength reinforcement;

evaluation of the current-state-of knowledge and determination of when existing information was sufficient or when additional research or study was needed.

identification of additional experimental research and engineering studies needed; and

estimation of the approximate budget, schedule, and prioritization for a recommended program.

1.2 Historical Perspective of High-Strength Reinforcement and ACI 318

American Concrete Institute (ACI) Committee 318 has a history of revising provisions to include the use of higher strength reinforcement than was readily available in the past. As of the 1950s and early 1960s, Intermediate Grade (Grade 40) reinforcement and Hard Grade (Grade 50) reinforcement had been available for about 50 years, and was commonly used. In 1959, the American Society for Testing and Materials (ASTM) specifications A432 and A431 were published, which introduced Grade 60 and Grade 75 reinforcement, respectively. The 1963 version of ACI 318 allowed the use of steel bars with a yield strength of 60 ksi. ASTM A615, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement, which included Grades 40, 60 and 75, was introduced in 1968.

In the late 1950s and 1960s, the Portland Cement Association (PCA) conducted a series of tests reported in eight parts that examined beams, girders, and columns (Hognestad, 1961; Hognestad, 1962; Gaston and Hognestad, 1962; Kaar and Mattock, 1963; Pfister and Mattock, 1963; Pfister and Hognestad, 1964; Kaar and Hognestad, 1965; and Kaar, 1966). These tests covered flexural strength, control of flexural cracking, compression splices in columns, and fatigue. Reinforcement strengths ranged from 55 ksi to 120 ksi. At about the same time, Thomas and Sozen (1965) published the results of tests of beams reinforced with unstressed prestressing

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reinforcement with a yield strength of 230 ksi. These early tests were considered in changes to the 1971 edition of ACI 318 when the upper limit for yield strength was increased to 80 ksi, even though there were no ASTM specifications for reinforcement with yield strengths more than 75 ksi at the time.

In the 1971 edition of ACI 318, the maximum specified yield strength was restricted to 60 ksi for reinforcement in special seismic systems, and this limit is still in effect in ACI 318-14. The Structural Engineers Association of California (SEAOC) developed a specification for reinforcement with more restrictive tensile properties and chemistry controls, published as ASTM A706, Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement (ASTM, 1974). ASTM A706 was permitted in the 1977 edition of ACI 318. It became required for reinforcement in special seismic systems in 1983, but an exception was made for the use of A615 reinforcement if certain mechanical properties were met.

The 2009 versions of ASTM A615 and A706 specifications (ASTM, 2009a; ASTM, 2009b) were the first to include requirements for Grade 80 reinforcement. ACI Committee 318 adopted these specifications without restriction in the main body of ACI 318-11 (ACI, 2011) because the use of Grade 80 reinforcement was already permitted. However, Grade 80 reinforcement is currently not permitted for use in special moment-resisting frames and special structural walls due to the perceived lack of test data for cyclically loaded members with Grade 80 reinforcement.

1.3 Prospects for Adoption of High-Strength Reinforcement into ACI 318

Based on the current availability of high-strength reinforcement, its use in Japan, research completed to date, and progress made towards codifying its use, the prospects for adoption of high-strength reinforcement into ACI 318 are good. High-strength reinforcement is available from some producers in the United States, and many other producers will soon be capable of rolling it. Significant research was completed in Japan as part of the New RC Project (Aoyama, 2001), which took place between 1988 and 1993. Research has continued in Japan, and is being performed in Taiwan, Korea, and the United States. Much of this research is identified and described in NIST GCR 14-917-30, Use of High-Strength Reinforcement in Earthquake-Resistant Concrete Structures (NIST, 2014).

High-strength reinforcement has been used in hundreds of high-rise buildings in Japan, and has been used in some buildings in New York City. Documents such as ACI ITG-6, Design Guide for the Use of ASTM

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1-4 1: Introduction ATC-115

A1035/A1035M Grade 100 Steel Bars for Structural Concrete (ACI, 2010a), and NCHRP Report 679, Design of Concrete Structures Using High-Strength Steel Reinforcement (Shahrooz et al., 2011), have made progress towards identifying how some code provisions in ACI 318 and the AASHTO Bridge Design Specifications should be changed.

Adopting the use of high-strength reinforcement into ACI 318 will require a substantial effort because many sections of the code will require new or revised provisions, and many questions and concerns will likely be raised by the members of ACI Committee 318. Research and engineering studies identified in this Roadmap are intended to develop the necessary code change proposals, and are also intended to comprehensively address the questions and concerns that are likely to be raised.

1.4 Prospects for Adoption of High-Strength Reinforcement into other U.S. Codes and Standards

The nuclear industry is keen on using high-strength reinforcement in new construction to reduce congestion, improve the quality of placed concrete, and speed construction time. Prospects for the adoption of high-strength reinforcement into ACI 349-13, Code Requirements for Nuclear Safety-Related Concrete Structures and Commentary (ACI, 2014b), are high, and its use has been debated at recent ACI committee meetings. The reduction in minimum elongation associated with the use of higher strength reinforcement is of lesser concern in the nuclear industry because component nonlinear deformation demands are much smaller than those in buildings and bridges at design level intensities of shaking.

The bridge industry is also interested in using high-strength reinforcement in new construction. Goals for the use of high-strength reinforcement are similar to those of the nuclear industry. The California Department of Transportation is currently working with North Carolina State University to provide strain information on ASTM A706 Grade 80 reinforcement. If adequate ductility is shown, A706 Grade 80 bars could be used in capacity-protected components, such as bent caps, where rebar congestion is the highest. For use in seismic members that are expected to yield (e.g., columns), however, rigorous testing of couplers and splices will be necessary. Other State Departments of Transportation (DOTs), and the Federal Highway Administration (FHWA), have also shown significant interest in high-strength reinforcement for general reinforced concrete design.

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1.5 Key Issues with the Use of High-Strength Reinforcement

Several challenges, questions, and concerns must be resolved before high-strength reinforcement can be used extensively in the construction of reinforced concrete structures. These stem from bar production and fabrication challenges, design requirements, and impacts on constructability, as described in Chapters 2 and 3 of this Roadmap.

1.6 Report Organization and Content

This Roadmap identifies key issues in production, fabrication, design, and construction related to the use of high-strength reinforcement, identifies the current state-of-knowledge, and outlines a recommended program of experimental research and engineering studies to investigate the use of high-strength reinforcement and support its adoption in building codes and standards for reinforced concrete design.

Chapter 2 identifies the current state-of-knowledge on issues related to production and fabrication of high-strength reinforcement, and construction using high-strength reinforcement.

Chapter 3 identifies the current state-of-knowledge on issues related to design using high-strength reinforcement.

Chapter 4 outlines recommended experimental research and engineering studies needed to investigate the use of high-strength reinforcement and support the development of code changes.

Chapter 5 provides a summary of the overall program, including estimated budget requirements, priority and schedule recommendations, and additional recommendations related to implementation of the program.

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ATC-115 2: Production, Fabrication, and Construction Issues 2-1

Chapter 2

Production, Fabrication, and Construction Issues

Challenges associated with the production and fabrication of high-strength steel for concrete reinforcement will be most affected by the required mechanical properties and the cost of production. This chapter describes production, fabrication, and construction issues associated with the use of high-strength reinforcement.

2.1 Reinforcement Production and Specification Issues

In general, higher yield strength is accompanied by reductions in the tensile-to-yield strength ratio, uniform elongation (elongation at tensile strength), and length of yield plateau. In some cases, the yield plateau is completely eliminated.

The processes used to produce high-strength reinforcement will likely depend on the mechanical properties of the reinforcement needed for safe and serviceable designs. These properties are addressed in sections of this report that deal with design issues. The requirements for of mechanical properties to be specified must also realistically account for the capabilities of producers and the economics of producing the bar.

The requirements for mechanical properties of high-strength steel bars and the means by which bars will be specified needs to be established as part of the program described in this report. Issues related to inventory and storage, identifications of bars on the site, production costs, and required properties for various design conditions will affect the criteria used for specifying high-strength reinforcement.

2.1.1 Production of Deformed Steel Reinforcement

Reinforcement must be rolled in a very malleable state so that bar deformations will form properly when it is rolled. Therefore, reinforcing steel is red hot when being rolled. Higher strength reinforcement is commonly achieved by micro-alloying. It can also be achieved by quenching and self-tempering or by work hardening, although work hardening is not commonly used to produce reinforcing bars in the United States.

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2-2 2: Production, Fabrication, and Construction Issues ATC-115

The means by which high-strength reinforcement will be produced may be somewhat dependent on the properties that need to be achieved for the reinforcement. Reinforcement used for resisting gravity loads (i.e., dead, live, ice, snow, and rain) requires reliable yield strength with less emphasis on ductility, whereas reinforcement used to resist earthquake forces requires more emphasis on the elongation and ratio of tensile-to-yield strength.

Where quenching is used to produce bars, it involves the use of water to rapidly cool the steel that has been heated to the austenite phase (the phase at which solid steel recrystallizes), while self-tempering is achieved by the gradual release of the heat that is trapped in the core of the quenched steel. Quenching results in a hard metal structure while self-tempering softens the steel and increases its toughness.

The quenching and self-tempering process for reinforcing steel involves spraying bars with water just after rolling, producing a layer of Martensite (hard phase), and then allowing the residual heat from the core of the bar to re-heat the quenched outer layer. Upon cooling, the core of the bar contracts and converts to Ferrite-Pearlite. This contraction develops compressive stresses in the cooler outer portion of the bar while the core develops tensile stresses. These stresses improve bendability and fatigue resistance of the bar. When the bar is subjected to tension, the residual stresses in the bar result in the core yielding before the outer portion leading to a stress-strain curve without a sharp yield point.

Increasing the yield strength of reinforcing steel by means of micro-alloying involves the addition of small quantities of certain alloying elements to the molten steel to induce grain refinement. Vanadium (V) is the most commonly used alloy due to its dependability and consistent results. It refines the grain size and increases the strength without a negative impact on weldability or notch toughness. Molybdenum (Mo), Niobium (Nb), Tantalum (Ta), and Titanium (Ti), to name a few, are also used for micro-alloying. The addition of Chromium (Cr) in higher than trace amounts is also effective in increasing yield strength and in combination with copper improves corrosion resistance. Generally, Grade 80 reinforcement produced in the United States is micro-alloyed, as is also commonly done for large Grade 60 bars (i.e., No. 9 and larger). Less grain refinement is achieved by rolling larger bars so more micro-alloying makes up the difference to achieve the required properties.

Generally, micro-alloyed bars have a higher ratio of tensile-to-yield strength than quenched and tempered bars, and a lower ratio of tensile-to-yield strength than plain carbon steel bars.

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For the New Zealand market, both quenching and tempering and micro-alloying are used to produce ductile reinforcement with strength on the order of 75 ksi specifically for use in structures resisting earthquake effects. The use of these processes (or a combination of the two) has potential for producing even higher strength reinforcement.

Work hardening involves the plastic deformation of a metal at or near room temperature. Cold drawing plain and deformed wire reinforcement is a form of work hardening. In some countries, the strength of bars is increased by the process of cold twisting after hot rolling. Work hardening improves yield strength but eliminates a yield plateau, and reduces both the achievable elongation and the ratio of tensile-to-yield strength. Work hardening may be suitable to produce reinforcement for use in members where yielding is not expected. For current design practice in the United States, work hardening is likely not suitable to produce reinforcement for members resisting earthquake effects or where yielding is expected.

The processes by which producers will manufacture high-strength reinforcement will be driven by minimizing the costs relative to producing Grade 60 reinforcement. This will likely mean minimizing changes to current methods of producing reinforcing bars. For example, using the same bar deformations as those for Grade 60 bars and eliminating bar marks will allow the same rolls to be used. However, doing so may increase the likelihood of the wrong reinforcement being used. Also, some producers within the reinforcing steel industry oppose the elimination of bar marks because it is a deviation from long-established practice.

Some producers are interested in eliminating the marks from bars because bar marks must be chosen before the bars are produced and tested. Occasionally, bars do not meet the required mechanical properties. Because they have a mark that they do not comply with, these bars must be scrapped. One idea to address this is that the bar could be rolled without the marks and laser printing could be used to indicate the heat on the bar. The test results for the heats could then be placed on an accessible database. This approach would allow the producer to sell the reinforcement as complying with a different specification or grade. The downside of this approach is that inspectors and engineers checking bar placement in the field likely will not have access to the database on site, so they may not have access to the type and grade of reinforcement while on the site. There has also been an indication that color coding the ends of bars could be used, but this would not be effective for bars that are shop cut. Another idea that could be explored is to laser print the specification and grade that the bar meets after testing is complete; however, this would require additional bar handling. An added

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benefit of not rolling the marks into the bar is elimination of the known weakness that bars tend to fail in low and high cycle fatigue at these marks. Also, in epoxy coated bars, coating damage tends to occur at sharp edges of bar marks during delivery and handling in the field.

Reinforcing bars are most commonly produced in straight lengths at the mill. However, coiling smaller sized bars (i.e., No. 3 to No. 6), is becoming more common, and currently represents nearly 5% of the market for bars of all sizes. Bars are coiled soon after rolling and while hot, which traps heat in the coil. The trapped heat can lead to unintentional annealing. Annealing is the process of heating steel between 700 and 800C for several hours and allowing it to cool slowly. Annealing softens the steel and increases ductility and workability of steel. It leads to a lower yield strength and lower tensile strength with a rounded post-yield stress-strain curve. To overcome unintentional annealing, high-strength reinforcement that is coiled will likely require higher quantities of micro-alloying elements than for bundled straight bars, which have more opportunity to cool before being bundled.

In addition to the production methods affecting the tensile-to-yield strength ratio, they will also affect bar elongation. From an engineering perspective, reporting uniform elongation, i.e., the elongation that exists in the bar as the tensile strength is reached, is preferable to elongation at fracture, which is the increase in length over the 8- in. gage length including the necked-down region of the bar. U.S. producers are considering the impacts of reporting uniform elongation. They do not have a history of reporting uniform elongation, so they are currently trying to determine if and how uniform elongation is affected by chemistry and production processes. It is also recognized that uniform elongation is likely reduced with an increase in bar size. Although one or more producers are willing to roll all bar sizes, at least initially, producers may want to roll a limited number of bar sizes. Doing so will allow the producers to minimize extra storage space required with the availability of additional grades of bars. At least one producer is rolling No. 20 bars and has plans for rolling No. 24 bars.

Producers have expressed an interest in having Grade 100 included in the ASTM A615 specification, or equivalent, in the near future. If such a specification were to exist, producers believe there would be demand for the bar. Producers would use the production of this bar as an opportunity to explore means of producing more ductile Grade 100 bars. This will help determine realistic material properties to include in a future specification for Grade 100 reinforcement targeted for use in earthquake-resistant construction (i.e., special moment frames and special structural walls).

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As producers refine how they will be producing high-strength steel bars, an initial test program is recommended to define structurally acceptable properties of high-strength bars to assist with the development of ASTM specifications. In addition to tensile and bend tests, consideration should be given to performing less common tests such as low-cycle fatigue and bend-rebend tests. To evaluate the effects of bar bending on the ductile-to-brittle transition temperature of steel, careful consideration should be given to performing tests similar to those conducted by Hopkins and Poole (2005) on AS/NZ 500E reinforcement (AS/NZS 4671 2001). Data collected from Charpy V-notch toughness test are also relevant for assessing the expected performance of the bar if used in structures exposed to colder climates. In addition, the effects of strain aging need to be studied. An initial test program will account for most of these issues.

Once the higher grade reinforcement is being reliably produced, a second more detailed test program of reinforcement properties should be performed.

Recommendations for Studies of Bar Mechanical Properties

An initial test program as described in Section 4.3.1 is needed to explore issues related to the mechanical properties of high-strength bar. A detailed test program of reinforcement properties as described in Section 4.3.2 should be performed once the high-strength reinforcement is being reliably produced.

2.1.2 Specification of High-Strength Reinforcement

The development of specifications for high-strength reinforcement will likely involve the following actions:

Expand ASTM A615 (carbon steel bars) to cover higher grades for reinforcing bars, possibly from 80 to 120 ksi with 20-ksi increments. This specification would not apply for bars in members that are part of special seismic-force-resisting systems;

Create a new ASTM specification to cover all grades for reinforcing bars to be used in members that are part of special seismic-force-resisting systems;

Leave ASTM A706 (low-alloy steel bars) to cover only Grades 60 and 80.

The approach of having Grade 100 in ASTM A615 will give producers room to experiment with the production of more ductile Grade 100 bars for the future seismic specification with reduced risk of scrapping out entire heats.

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Specification overlap should be maximized. For example, 100% of the bars meeting the specification for seismic applications should be adequate for meeting the specification for ASTM A615, though not all ASTM A615 bars would be adequate for seismic applications. The specification overlap should be performed at all strength levels, not just higher grades. Taking this approach would allow producers and fabricators to minimize bar inventories and storage.

The chemical restrictions in ASTM A706 (weldable steel) make it difficult to obtain high-strength steel bars with tensile-to-yield strength ratios and uniform elongations similar in magnitude to those routinely obtained for Grades 60 and 80 bars. Imposing limits on carbon content (or carbon equivalency) restricts achievable strengths. Therefore, including grades higher than 80 in ASTM A706 is not recommended at this time.

Specifications for bars with specific ranges of chemical composition or for specific uses are likely to be maintained, but perhaps with some modifications. For example, Grades 100 and 120 bars are already produced to specification ASTM A1035. Changes are likely to be made to these specifications that would provide for increased acceptance and perhaps to meet requirements for use in earthquake-resistant structures. The producers of bars to these specifications may also be willing to consider using high relative rib area deformations to improve bond characteristics. However, potential reduced fatigue resistance of bars with high relative rib area should be explored.

Regardless of how the high-strength reinforcement is specified, yield strength will be reported as the 0.2% offset (Paulson, et al., 2013) required in ACI 318-14, ASTM A615, ASTM A706, and ASTM A1035. For elongation, consideration should be given to reporting uniform elongation at peak stress rather than total elongation (typically measured over a gage length that includes the necked-down region), as recommended in NIST GCR 14-917-30.

Producers are requesting to increase the bend test radii in the ASTM specification for Grade 100 bars to match the bend radii specified for standard hooks in ACI 318-14. The reason is to reduce the risk of having heats rejected (by failing the bend test) while they are still refining how high-strength steel bars should be produced. Increasing the radii for the bend test will increase the chance of bar fracture as it is shop bent. This issue should be resolved as higher grade bars are added to the ASTM A615 specification.

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There will be some applications in which bars will be used in structures exposed to cold weather. ASTM A706 bar is sometimes specified for these applications because of its more stringent ductility requirements. Because ASTM A706 will not include requirements for Grade 100 bar, supplemental requirements could be added to the future specification of ASTM A615 Grade 100 bars and to specifications of high-strength bars for use in earthquake-resistant structures.. The supplemental requirements would likely include passing Charpy V-notch tests conducted on the reinforcement at cold temperatures.

Producers are the ones who should take the lead in determining how bars are specified and how they will be produced. As the challenges of manufacturing bars to meet specified properties will be resolved by individual producers, and the producers will be most affected by how high-strength bars are specified, they should have the most input on how to resolve these issues. They will need input from the engineering and research communities on the requirements to include in the specifications assuming current design procedures are continued. The requirements must be achievable by the producers. If not, design requirements could be revised so that acceptable designs can be obtained using the bars with achievable properties. Although funded research is not required to resolve these issues, resolving them is on the critical path so that bars can be produced for the research that is necessary for accepting the general use of high-strength steel bars as concrete reinforcement. Producers need to develop a database for measured mechanical properties associated with different manufacturing processes and conduct a cost-benefit analysis based on the cost of process modifications to improve key mechanical properties.

Recommendations for Specification Development

The reinforcing bar producers need to take the lead in developing specifications containing the minimum acceptable properties of high-strength reinforcement. The specifications are likely to include an expansion of ASTM A615 to incorporate Grades 100 and 120. In addition, a new ASTM specification should be developed for high-strength reinforcement with controlled mechanical properties for use in earthquake-resistant structures.

2.2 Fabrication Issues

Issues with fabrication of high-strength reinforcement are related to the following two general categories:

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The introduction of multiple grades of reinforcing bars that need to be scheduled, received, and stored at a fabricating facility prior to their use; and

The actual fabricating processes required for high-strength bars. Fabrication facilities will need more space to store multiple bar sizes of multiple grades prior to, and during, their fabrication. This problem may be reduced by standardization around the use of fewer bar sizes in higher strengths.

The actual fabrication processes of shearing the bars and bending the reinforcement will be impacted when using higher strength reinforcement. High-strength reinforcement requires greater force to shear and bend the same size bar, and involves increased rebound at bends after the bending force is removed. This leads to issues and concerns regarding:

wear and tear on bending equipment; safety of workers when bending bar; and complying with fabrication tolerances for bars. Fabricating bends of high-strength reinforcement requires greater force to make the bends and causes more wear and tear on equipment used to make the bends. Bending high-strength reinforcing bars imparts higher force on the central pin that holds the disc about which the bar is bent. Some fabrication shops have reported that these pins break more frequently and release more energy upon breaking when high-strength reinforcement is fabricated. Safety concerns are heightened with this increase in breakage. In cases where bars have defects that cause them to fracture when bending, the release of energy is also greater, which creates a safety concern. Extra precautions may be necessary to maintain a safe work environment. Worker protection cages are more often required, which leads to lower efficiency in the shop.

Bending higher strength bars will require larger equipment that is capable of imparting larger forces on the bars. Larger and stronger equipment will likely require more floor space in a shop.

ACI 117 (ACI, 2010b) provides specific tolerances for the tails of hooks and the resulting angle between straight portions of bent bars. Increased rebound of bars at bends makes it harder to control the final bar configuration in higher strength reinforcement. Consideration needs to be given to whether the tolerances should be adjusted for high-strength reinforcement.

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Issues related to fabrication and concerns for wear and tear on bending equipment, safety of workers when bending bars, and compliance with fabrication tolerances for bars are best addressed by fabrication shops. Concerns of wear and tear on the equipment and worker safety may require modifications to bending equipment.

Issues related to bar fracture and tolerances may be resolved by increasing the inside bend diameter. Engineers and researchers can assist with determining whether changes to tolerances are acceptable, and if larger diameter bends are necessary, engineers and researchers can determine the impacts on the design requirements. However, doing so will require additional research to determine the effects on development length of standard hooks.

The difficulty in bending bars may lead to a greater use of headed bars involving the threading of bars or otherwise attaching the head to the end of the bar.

In general, the fabricating challenges associated with the use of higher strength bars will be lessened with greater acceptance and use of the bars. Adaptation of equipment to deal with high-strength steel bars will be a natural consequence of the increased use of the bars.

Recommendations for Resolving Fabrication Issues

Fabricators should determine whether bend diameters and tolerances at bends for high-strength reinforcement need to be adjusted in relation to conventional strength reinforcement. Any changes required for detailing of high-strength reinforcement will need a close examination of their impact on current design and construction practices.

2.3 Constructability Issues and Construction Efficiencies

From a construction and cost effectiveness perspective, the benefit of higher strength reinforcement is that it can allow the use of a lower volume of reinforcing steel to accomplish the same design goals as those obtained with conventional reinforcement. This will generally result in lower construction time and lower cost for concrete structures. The challenge for the design community will be to integrate the use of high-strength reinforcement into concrete structures in ways that optimize and utilize the higher yield strength of the steel bars. As there will be some cost premium for the higher strength steel on a per pound basis, the reduction in overall reinforcing steel weight within a structure will need to be sufficiently large to offset the premium in cost per pound. Minimum spacing, minimum reinforcement ratios, and other

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non-strength related requirements in ACI 318-14 will have to be evaluated to determine their impact on obtaining the full benefit of high-strength bars. The use of higher strength concrete may be necessary to completely utilize the strength of the bars.

If substituting Grades 80, 100, or 120 for Grade 60, while maintaining the member cross section, the use of higher strength reinforcement can be implemented in two ways:

Using bars of the same size as Grade 60, but spacing them at greater intervals; or

Using smaller size bars but maintaining spacing similar to that of Grade 60.

Using bars spaced at greater intervals means that construction and cost efficiencies are achieved through lower placement costs, less congestion and better consolidation potential for concrete placement. Fewer bars also mean fewer lap splices or mechanical splices required. An unfavorable aspect of reinforcement cages built with bars at a greater spacing is that they are less stable when in a free-standing condition. Also, higher strength bars are more difficult to bend, couple, and terminate.

Using smaller size bars at a spacing similar to that required of Grade 60 bars means that, although bar weight will still be reduced, there will be less placement cost savings. Smaller diameter bars are easier to bend, terminate, and couple as compared to larger diameter bars. Cages built with a greater number of smaller diameter bars will likely be more stable in a free-standing condition during construction.

Higher strength bars will require longer splice and development lengths than Grade 60; this will impact construction and cost efficiencies. If greater bar length is needed to develop higher strength bars, this will negate some of the benefit of reduced steel volume. Splice and development length issues may be reduced by exploring higher relative rib area designs for the bars to increase their force transfer to the concrete.

If higher strength is used in a slab, this may lead to a greater concern for slab deflections. Larger slab deflections impact such things as exterior wall attachments and floor levelness. Post-tensioning of slabs might be a way to overcome this concern.

Although the use of high-strength reinforcement is generally associated with reducing congestion, increased congestion is possible. Designers may be tempted to use smaller concrete member dimensions in which case

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congestion will be similar to what occurs in members reinforced with Grade 60 bars. Also, the spacing of transverse reinforcement may need to be reduced to better resist longitudinal bar buckling.

During construction, contractors frequently have to order additional bars on short notice from fabricators. In the near term, as plants are still ramping up production capabilities, the availability of reinforcement on short notice could be a problem. If the bars needed are high strength and the fabricator has not stored extra bars of this grade, construction delays could ensue. More likely, Grade 60 bars will be substituted for the higher strength bars, which could increase congestion, especially if member sizes have been reduced based on the assumed use of higher strength reinforcement.

If multiple grades of the same size bars are used on a construction project, the contractor may unintentionally install bars of the incorrect grade. This is more likely to occur if the bars are straight and of similar lengths. Bar markings or other identifiers will have to be clear to prevent this from occurring.

In the early stages of a transition to higher strength reinforcement, the use of all bar sizes for multiple grades of reinforcement will lead to supply chain, inventory, and inspection complexities for producers, fabricators, and installers. These types of issues are best considered early in the design process by focusing on using a limited quantity of bar sizes for the higher strength bars.

The unit material cost of high-strength reinforcement has been high in recent years because demand has been low. With increased demand, prices will decrease. In addition, cost savings in labor costs associated with placing lower amounts of steel are likely to exceed the difference in material costs.

A cost study performed for NIST GCR 14-917-30 determined that cost savings associated with the use of Grade 80 reinforcement (in lieu of Grade 60) was on the order of 4% of the cost of the concrete structure. The study by Price, et al., (2014) evaluated the potential for high-strength reinforcement to reduce the volume of reinforcement in typical concrete buildings and reduce construction time, ultimately leading to a reduction in the total cost of building construction. Price et al. concluded that if the ACI Code limits on steel yield strength are ignored, high-strength reinforcement can be effectively used to further reduce steel volumes for reinforced concrete building components. The study suggested future research activities to support ACI Code changes and enable the full potential of high-strength reinforcement.

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Thoughtful consideration of the construction and cost efficiencies of the aspects of the reinforcing steel design identified in this section will be necessary to maximize the benefit from using higher strength reinforcing bars.

Recommendations for Addressing Construction Issues and Efficiencies

No studies are necessary to address construction issues.

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ATC-115 3: Key Design Issues 3-1

Chapter 3

Key Design Issues

Design issues associated with the use of high-strength reinforcement could potentially affect many design provisions in ACI 318, as well as other codes and standards for reinforced concrete construction. Issues related to design for gravity and wind forces acting on the structure can broadly be grouped as affecting provisions related to:

Design strength and ductility issues Serviceability Reinforcement Limits Detailing Analysis Resolution of issues or concerns related to the use of high-strength reinforcement first requires an understanding of the effects of high-strength reinforcement on structural behavior. Once the effects of high-strength reinforcement on behavior are established, revisions to design provisions can be considered.

Serviceability is more likely to control the design of members with high-strength reinforcement than with Grade 60 reinforcement. Some serviceability provisions in ACI 318 explicitly considered that Grade 80 reinforcement could be used, and likely require modification only for Grade 100 or 120 bars. For example, the provision for minimum beam side-face reinforcement is based on research that considered the use of Grade 80 reinforcement for the beam. In contrast, recommendations for effective moment of inertia for determination of lateral drift are based on the opinion of designers that were using Grade 60 reinforcement for design of structures.

Use of high-strength reinforcement in beams, columns, and walls of seismic-force-resisting systems has a high potential to take full advantage of the increase in bar strength because strength provisions generally control over serviceability issues in these members. However, detailing requirements may lessen the advantage of using high-strength reinforcement, as closer spacing of transverse reinforcement may be required.

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3-2 3: Key Design Issues ATC-115

This chapter identifies the current state-of-knowledge on key design issues associated with the use of high-strength reinforcement.

3.1 Strength and Ductility Issues

Design provisions for computing flexural strength, axial load capacity, and shear strength may require adjustments for members reinforced with high-strength reinforcement. Other miscellaneous provisions and approaches to strength computation may also be affected.

3.1.1 Flexural and Axial Load Strength

Accurately determining the strength of members under flexural or combined flexural and axial loading is key to the design of reinforced concrete members because these loads generally establish the size of the members. Design for other load effects, such as shear and torsion, necessarily follow. Thus, changes in member behavior that result from the use of high-strength reinforcing steel must be well understood to allow for the safe and economical use of this new material.

3.1.1.1 Strain Limits

In determining the design strength of a member, it is necessary to reduce the nominal strength through the use of a strength reduction factor, . The strength reduction factor for flexure, axial load, or both, is computed based on the strain conditions of the cross-section at nominal strength and in particular, the extreme tensile strain, t, as shown in Figure 3-1. This approach to computing has been in the main body of ACI 318 since 2002, and is based on the Unified Design Provisions for Reinforced and Prestressed Concrete Flexural and Compression Members recommended by Mast (1992).

Figure 3-1 Strength reduction factor based on strain (ACI 318-14).

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As evident in Figure 3-1, is dependent on the region in which the computed strain lies, and two limits are used to control the value, the compression controlled limit (ty) and the tension controlled limit (0.005). In considering the use of high-strength reinforcement, it is important to address whether these two limits remain appropriate.

Compression-controlled strain limit

For the compression-controlled strain limit, ACI 318 defines ty as fy/Es which is considered the yield strain of the reinforcement. The use of the yield strain to define this limit is appropriate regardless of reinforcement strength, and this computation is correct for use with reinforcement that remains linear-elastic up to the yield plateau. However, for stress-strain curves with a gradual reduction in stiffness and rounded post yield strength that becomes nonlinear before reaching the yield strength, the definition of the yield strain as fy/Es may not be appropriate especially if fy is being defined through the offset method or other alternate procedure. Therefore, changes to ACI 318-14 may be required in the definition depending on the variation of the actual yield strain to the computed value using the elastic computation. Alternately, specific values of ty for a given reinforcement grade may be needed similar to that provided for prestressed reinforcement.

Tension-controlled strain limit

For the tension-controlled strain limit, a fixed value of 0.005 is used. This value, which is designed to provide ductility, is approximately 2.5 times the yield strain of about 0.002 for A615 Grade 60 reinforcement, and is higher than required in ACI 318 prior to 2002. According to Mast (1992), the value of 0.005 was selected because:

the net tensile strain is measured at dt instead of d (or dp), and the limit should be slightly higher when more than one layer of tension steel is used; and

use of 0.005 produces interaction diagrams that look reasonable. For high-strength reinforcement, the yield strain will be greater; therefore, it appears that the tension controlled limit should be increased to provide levels of ductility similar to that designed with A615 Grade 60 reinforcement. Consequently, it may be appropriate to provide a limit that is a function of the reinforcement yield strain.

The shape of the stress-strain curve may affect the strain that should be used for this limit. Similar to the discussion for the compression-controlled limit, the use of fy/Es to define the yield strain, which may be used to define the

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tension-controlled limit, may not always be appropriate. ACI ITG-6 (2010) points out the tensile strain corresponding to a tension-controlled member for ASTM A1035 bars is 0.0066, based on realistic stress-strain curves, and 0.009, based on a simplified representation of the highly nonlinear ASTM A1035 stress-strain curve idealized as elastic-plastic.

Stress-strain curves for high-strength reinforcement will likely exhibit greater variation in shape than those of ASTM A615 Grade 60 reinforcement. As illustrated in Figure 3-2, three distinct shapes for the stress-strain curves of bars likely to be available are: (1) linear-elastic behavior up to the yield strength, with a sharp bend representing the yield strength, a relatively flat yield plateau, and a rounded strain hardening region (curves designated S3 in the figure); (2) a gradual reduction in stiffness with a rounded post-yield shape that becomes nonlinear before reaching the yield strength, which is defined by the 0.2% offset method, followed by gradual softening up to the tensile strength (curve designated S1 in the figure); and (3) a nearly bi-linear curve in which linear-elastic behavior occurs almost to the yield strength, followed by linear strain-hardening behavior in the inelastic range until the tensile strength is reached (curve designated S2 in the figure).

Figure 3-2 Types of stress-strain curves with distinct shapes.

Maximum strain limit

With the use of high-strength reinforcement, there is likely to be a reduction in the elongation capacity of the reinforcement relative to that of Grade 60. While the current code does not include a limit on the tensile strain, there is the potential that a limit will need to be established so that fracture of the reinforcement does not occur prior to crushing of the concrete in

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compression. Studies should be conducted to evaluate if a maximum strain limit is required.

Recommendations for Strain Limits

It is recommended that the strain limits defining the boundaries between compression-controlled, transition, and tension controlled sections for the selection of factor values be examined. Considering that the current limit is partially based on the appearance of interaction diagrams, this study should not only investigate flexural response, but also evaluate the interaction between flexure and axial loads. In addition, it should be examined if a maximum strain limit is required. The proposed study is outlined in 4.4.1.

3.1.1.2 Beams

Flexure strength provisions for tension-controlled members, which include most slabs and beams, are intended to provide an acceptable level of reliability that the computed design strength will be achieved at a reasonable deflection. These provisions are also intended to result in designs that provide ample warning of a potential failure if the member is inadvertently overloaded. The warning includes substantial cracking and large deflections forming prior to complete failure. The provisions were developed assuming reinforcing steel has a yield plateau and based on confirmation of adequate performance of beam specimens tested in laboratories.

In determining whether existing flexural strength design provisions are adequate or require change for members reinforced with high-strength reinforcement, the following should be considered:

The shape of the stress-strain curve for the reinforcement affects the deflection at which the design strength is developed and potentially the spread of plasticity as the member is loaded monotonically to failure.

A minimum extreme tensile strain t of 0.004 is required by ACI 318 for flexural members.

The yield strain of compression reinforcement may exceed the assumed maximum concrete strain of 0.003.

Stress-strain curve

The shape of the load-deflection curve of tension-controlled members constructed with high-strength reinforcement will be affected by the shape of the stress-strain curve. Current provisions for computing flexural strength are based on the assumption that the stress-strain curve for the bar has a yield plateau. It is not yet clear whether the flexural strength provisions need to

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explicitly account for the shape of the stress-strain curve if the bar does not have a yield plateau. The effects of the stress-strain curve shape should be explored to determine whether changes to the flexural design provisions are required, which could be accomplished using moment-curvature studies and tests to validate the analyses.

The desire for large deflections to occur prior to failure, which would warn building occupants of the potential for failure of gravity loaded flexural members, is considered in the provisions of ACI 318. The ability to develop these large deflections is dependent in part on the mechanical properties of the reinforcement. In particular, bars in flexural tension with a yield plateau and a large value of the tensile-to-yield strength ratio will result in members that deflect more prior to failure. About 95% of ASTM A615 Grade 60 bars currently produced have a yield plateau (Paulson, et al., 2013). Based on the study of reinforcing bar data by Bournonville et al. (2004), the average tensile-to-yield strength ratio is about 1.5 with the average ratio for larger bars being as low as 1.4. Although ASTM A615 does not set minimum acceptable values for this ratio, the reinforcement produced has acceptable values for this ratio in most cases. An experimental program should be performed to confirm that slabs and beams reinforced with Grade 100 bars with low tensile-to-yield ratios, and low elongations, will deflect sufficiently to provide adequate warning prior to failure.

Minimum tensile strain

According to ACI 318, a minimum tensile strain is required to ensure a minimum level of ductility. The current limit of 0.004 is approximately twice the yield strain of about 0.002 for A615 Grade 60 reinforcement. Although this limit was not part of the recommendations in Mast (1992), this limit was included in ACI 318-02 to provide consistency with past practice, which included a maximum longitudinal reinforcement ratio of 0.75b. This reinforcement ratio produces a net tensile strain ty of 0.00376 for Grade 60 reinforcement, which is the reason ACI 318 adopted the slightly more conservative value of 0.004. Considering that this ratio was selected based on Grade 60 reinforcement, an adjustment may be required for high-strength reinforcement. Another possibility is to delete this requirement and address only the tension-controlled limit, as was originally recommended by Mast (1992).

Compression reinforcement

Compression reinforcement is commonly used in beams. In some instances, compression reinforcement is provided in beams with smaller cross sections

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to allow the use of more tension reinforcement. For high-strength reinforcement used in compression, the yield strain in the reinforcement is likely greater than the assumed maximum concrete strain of 0.003. Therefore, the design stress in compression reinforcement will be automatically limited by the strain profile of the cross-section, or by a prescribed limit in the maximum design stress, such the current 80 ksi limit in ACI 318. Considering that a strain of 0.003 would result in a steel stress of 87 ksi, it would seem that no change is needed in ACI 318 to address this issue. However, for practical considerations, the stress in the compression reinforcement will be limited by the assumed maximum concrete compressive strain, and the current 80 ksi limit on the stress in the reinforcing steel could be removed without complication, if desired.

Recommendations for Flexural Strength of Slabs and Beams

An analytical study should be performed to evaluate the impact of the roundhouse stress-strain curve on flexural strength. It is recommended that a moment-curvature analysis be performed that incorporates various stress-strain relationships for high-strength reinforcement. These computed flexural strengths should be compared with code-predicted nominal strengths to determine if any changes are needed to the code calculation procedure. This study is outlined in Section 4.4.1.

In addition, a series of tests should be performed to confirm that beams reinforced with Grade 100 reinforcement, but with lower tensile-to-yield strength ratios, and the low end of elongations that can realistically be produced, will deflect an adequate amount prior to failure. This study is presented in Section 4.4.2.

3.1.1.3 Columns

The yield strength of compression reinforcement is limited to 80 ksi in ACI 318-14. This limit is imposed because bars with yield strengths much above 80 ksi will not contribute to higher column capacity considering that higher yield strength can only be achieved at strains above 0.003 (the strain assumed for concrete crushing). At an assumed maximum strain of 0.003, the maximum usable stress is 87 ksi, considering linear-elastic behavior of the reinforcement.

Richart and Brown (1934) reported the results of a test program in which 564 concentrically loaded columns were tested. Eight of the tests were of spirally-reinforced columns that were longitudinally reinforced with bars having a yield strength of 96 ksi, tensile strength of 133 ksi, and fracture elongation of 11% (8-in. gauge length). The stress-strain curves for the

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longitudinal bars had well-defined yield plateaus. Richart and Brown (1934) concluded that the high-strength longitudinal bars were fully effective in producing the column strength. All columns had 4% vertical reinforcement and 1.2% spiral (volumetric reinforcement ratio) spaced at one-sixth of the core diameter. The spiral had a yield strength of 64 ksi and tensile strength of 100 ksi.

Pfister and Mattock (1963) reported the results of 16 concentrically-loaded circular and rectangular columns reinforced with high-strength steel bars with and without splices. The circular columns (12-in. diameter) were spirally reinforced and the rectangular columns (1012 in.) were tied. The longitudinal reinforcement consisted of six No. 8 bars, for a reinforcement ratio of about 4%, with yield strengths between 82 and 93 ksi. The yield strength of the transverse reinforcement was 65 ksi for the spiral and 59 ksi for the ties. The volumetric ratio of spiral reinforcement provided in the circular columns was 1.3%, while the area ratio of lateral ties in the rectangular columns was 0.1% (very low but compliant with the minimum permitted by ACI 318-63 (ACI, 1963)). Based on their test results, the following conclusion was reached, If the specified yield point of longitudinal reinforcement in tied columns is to be developed at ultimate strength of the columns, then it is necessary that the yield point be reached at or before a strain of 0.003 in./in. This condition will normally be more readily complied with by bars having a clearly defined yield point and a nearly linear stress-strain curve up to yield than by bars having a gradually curving stress-strain curve with no clearly defined yield point. The report also states, In future usage of 90 ksi column reinforcement, lapped splices will probably be impractical even with spiral reinforcement.

Todeschini et al. (1964) tested eccentrically loaded tied columns reinforced with high-strength steel with a specified yield strength of 75 ksi. Reinforcing bars both with and without yield plateaus were used. Reinforcement with a rounded stress-strain curve reached 90 ksi at a strain of 0.006. They found that the shape of the steel stress-strain curve affected the stress that can be utilized in the reinforcement. For reinforcement with a rounded stress-strain curve, the stress in the reinforcement that can be utilized was 70 to 80 ksi. For reinforcement with a relatively flat yield plateau, stresses up to 90 ksi could be developed.

References to more recent test programs and findings are presented in NIST GCR 14-917-30, with emphasis on members reinforced with high-strength steel bars and subjected to reverse cyclic loading. The test results indicate that replacing Grade 60 longitudinal reinforcement with reduced amounts of high-strength reinforcement (reduced in proportion to the yield strength of

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reinforcement) leads to comparable flexural strength and deformation capacity. The test specimens included reinforcement with limited variations in the shapes of the steel stress-strain curve.

Considering that the maximum usable concrete strain is limited to 0.003, and considering strain compatibility, the maximum design stress of the reinforcement is limited to 87 ksi. Therefore, it does not seem necessary to conduct further research in this area. The current limit of 80 ksi can be maintained, or this limit could be removed to allow stresses in the reinforcing steel consistent with the maximum strain limit. It should be realized that the maximum stress of 87 ksi is only realized under pure axial load conditions. Strain gradients will decrease the strain in the compression reinforcement.

Most of the discussion up to this point has concentrated on pure axial load. However, most practical columns include an interaction between axial load and flexure. Overall, the computation of the P-M interaction curve is controlled by the maximum concrete strain and the strain gradient. The reinforcement is directly accounted for through strain compatibility; therefore, it does not appear that high-strength reinforcement will cause any additional issues. The more important issue, however, will be the selection of the compression-controlled and tension-controlled strain limits as these significantly impact the design strength curves. As previously noted in Section 3.1.1.1, the selection of the tension-controlled strain limit was influenced by the appearance of the interaction diagrams.

Recommendations for Columns

It is recommended that interaction diagrams be considered as part of the selection of the tension-controlled strain limit as discussed in 3.1.1.1. A study investigating this topic is outlined in 4.4.1.

A code change is not considered required, but it appears that the maximum stress limit of 80 ksi could be removed. This minor change would result in the maximum strain limit of concrete controlling design. The stress of the reinforcement can then be computed directly from the stress-strain relationship of the reinforcement. If the stress-strains relationship of the high-strength reinforcement is linear up to 87 ksi, then the maximum usable stress of the reinforcement would be 87 ksi. If the reinforcement has a stress-strain curve with a significantly rounded shape and nonlinear behavior before the yield strength is reached, then the usable stress would be reduced. A study investigating the elimination of the 80 ksi limit is outlined in 4.4.3.

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3.1.2 Shear Strength

The proper calculation of shear strength is essential for the design and safety of structural co