1、DESIGN AND CONSTRUCTION PRACTICES TO MITIGATE CRACKING EDITOR: EDWARD G. NAW CO-EDITORS: FLOW G. BARTH ROBERT J. FROSCH Design and Construction Practices to Mitigate Cracking Editor Edward G. Nawy Co-Editors Florian G. Barth Robert J. Frosch o international SP-204 DISCUSSION of individual papers in
2、this symposium may be submitted in accordance with general requirements of the AC1 Publication Policy to AC1 headquarters at the address given below. Closing date for submission of discussion is December 2001. All discussion approved by the Technical Activities Committee along with closing remarks b
3、y the authors will be published in the MarcWApril 2002 issue of either AC1 Structural Journal or AC1 Materials I Journal depending on the subject emphasis of the individual paper. i The Institute is not responsible for the statements or opinions expressed in its publications. Institute publications
4、are not able to, nor intended to, supplant individual training, responsibility, or judgment of the user, or the supplier, of the information presented. The papers in this volume have been reviewed under Institute publication procedures by individuals expert in the subject areas of the papers. Copyri
5、ght O 2001 AMERICAN CONCRETE INSTITUTE P.O. Box 9094 Farmington Hills, Michigan 48333-9094 All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed or written or o
6、ral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. The photos on the front and back covers are courtesy of Edward G. Nawy, Distinguished Professor of Civil Engineering
7、at Rutgers University, New Brunswick, New Jersey. Printed in the United States of America Editorial production: Bonnie L. Gold Library of Congress catalog card number: 2001093041 ISBN: 0-8703 1-043-7 Correct design of concrete structural systems requires consideration of serviceability requirements,
8、 as determined by crack-control measures. Higher- strength reinforcement, higher-strength concrete, more slender concrete elements, use of a host of admixtures, and emerging construction techniques have increased the need for serious consideration of crack mitigation and crack control in concrete st
9、ructural systems. This volume is derived from a national symposium of the American Concrete Institute sponsored by AC1 Committee 224, Cracking. The majority of the papers were presented in two highly attended technical sessions in San Diego, CA, entitled “Design and Construction Practices to Mitigat
10、e Cracking.” The first session was chaired by Edward G. Nawy, and the second session was jointly chaired by Grant T. Halverson and Harvey H. Haynes. The topics in this volume encompass a wide range of subjects, including a detailed summary of worldwide provisions for crack control in reinforced and
11、prestressed concrete beams; two- way slabs and circular tanks, together with the latest Eurocode provisions, including design examples; early-age thermal cracking; diagonal cracking, including seismically induced diagonal cracks; crack mitigation effects of shrinkage reducing admixtures fibers; repa
12、ir of cracks; cracking in water- retaining structures; and an overview of the cracking developed in the 1999 earthquake in Turkey. This special publication also includes a list of references at the end of each paper, which can be helpful to design engineers and constructors. All papers presented in
13、this publication were reviewed by recognized experts in accordance with the AC1 review procedures. Each paper had a minimum of two reviewers. It is hoped that designers, constructors, and codifying bodies will be able to draw on the vast material presented in this volume along with the recently revi
14、sed and updated AC1 224 committee report “Control of Cracking in Concrete Structures,” in improving the long-term cracking behavior and performance of concrete-constructed facilities. Edward G. Nawy Editor and Founding Chairman, AC1 Committee 224 Florian G. Barth Co-editor and Current Chairman, AC1
15、Committee 224 Robert J. Frosch Co-editor and Secretary, AC1 Committee 224 . Preface 111 Design for Crack Control in Reinforced and Prestressed Concrete Beams, Two-way Slabs and Circular Tanks-A State-of-the-Art by E. G. Nawy . 1 Early-Age Thermal Cracking in Laser-Screeded Concrete Slabs by H. Hayne
16、s . 43 Crack Control Provisions in the New Eurocode for the Design of Concrete Structures by A. W. Beeby . 57 Diagonal Cracking and Diagonal Crack Control in Structural Concrete by P. Adebar 85 Positive Moment Cracking in Diaphragms of Simple-Span Prestressed Girders Made Continuous by A. Mirmiran,
17、S. Kulkami, R. Miller, M. Hastak, B. Shahrooz, and R. Castrodale 117 Flexural Crack Control in Reinforced Concrete by R. J. Frosch . 135 Crack Mitigation Effects of Shrinkage Reducing Admixtures by A. Bentur, N. S. Berke, M. P. Dallaire, and T. A. Duming . 155 Use of Fibers for Plastic Shrinkage Cra
18、ck Reduction in Concrete by P. Balaguru . 171 Mitigation of Seismic Induced Diagonal Cracks in Concrete Columns by External Prestressing by M. Saatcioglu . 195 Improving Watertightness of Reinforced Concrete Structures with Shrinkage-Reducing Admixtures by J .K .Buffenbarger, C. K. Nmai, and M. A. M
19、iltenberger . 217 What is the Crack Width in Concrete Structures to Prevent Leakage? by L. G. Mrazek 237 V Cracks-Concrete Repairs Life Threatening Wounds by A. M.Vaysburd, R. W. Poston, and J. E. McDonald . 249 Cracking in Concrete Structures During the August 17, 1999 Earthquake in Turkey by M. Sa
20、atcioglu . 261 VI SP 204-1 Design for Crack Control in Reinforced and Prestressed Concrete Beams, Two-way Slabs and Circular Tanks - A State-of-t he- Art by E. G. Nawy Syiiopsis: This paper presents the state-of-the art in the evaluation of the flexural crack width development and crack control of f
21、lexural cracks in reinforced and prestressed concrete structures. It is based on extensive research over the past five decades in the United States and overseas in the area of macro-cracking in reinforced and prestressed concrete elements. Mitigation and control of cracking has become essential in o
22、rder to maintain the integrity and aesthetics of concrete structures and their long-term durability performance. The trend is stronger than ever towards better utilization of concrete strength, use of higher strength concretes in the range of 12,000-20,000 psi and higher compressive strength, more p
23、restressed concretes and increased uses of limit failure theories - all these trends require closer control of serviceability requirements of cracking and deflection behavior. The paper discusses and presents common expressions for the mitigation and control of cracking in reinforced concrete beams
24、and thick one-way slabs, prestressed, pretensioned and post-tensioned flanged beams, reinforced concrete two-way action structural floor dahs and plates, and large diameter circular tanks. In addition, recommendations are given for the maximum tolerable flexural crack widths in concrete elements bas
25、ed on the cumulative experience of many investigators over the past five decades. The expressions include the AC1 318-99 crack control provisions in reinforced concrete beams and one-way slabs, and the Concrete Euro Code 1999 for the design of concrete buildings. Kevwords: beams; concrete; concrete
26、strength; crack control; cracking; crack width; environment; equations for reinforced and prestressed beams; Eurocode; flexural crack width; long-term cracking; tanks; tolerable crack widths; two-way action structural slabs 1 2 Nawy Edward G. Nawy, FACI, is Professor of Civil Engineering, Rutgers Un
27、iversity, and holds the distinguished professor rank. Active in AC1 since 1949, Professor Nawy is the founding chairman and a current member of AC1 Committee 224 on Cracking; past chairman of AC1 Committee 435 on Deflection of Concrete Building Structures; member of AC1 Committee 340, Design Aids fo
28、r AC1 318 Building Code; member of Joint ASCE-AC1 Committee 421 on Design of Reinforced Concrete Slabs. Professor Nawy has published in excess of 160 papers and is the author four major textbooks and one handbook: SIMPLIFIED REINFORCED CONCRETE ( 1987); REINFORCED CONCREZE -A fbndamental Approach (4
29、h Ed., 2000) and translated into several languages; PRESTRESSED CONCRETE - A Fundamental Approach (3d Ed., 2000); FUNDAMENTALS OF HIGH PERFORMANCE CONCRETE (2“ Ed., 2001); and CONCRETE CONSUCTION ENGINEERING HAWBOOK (i998), as well as chapters in several handbooks. He holds several honors including
30、the AC1 Chapter Activities Award, the Henry L. Kennedy Award and the Concrete Research Councils Robert Philleo Award, was mce president of the ACI New Jersey Chapter, served two term on the Rutgers Univers the effective concrete area in tension, and the center-to-center spacing of reinforcement, inc
31、luding bonded tendons, is limited to 8 in. (200 mm). Flexural cracking in reinforced concrete slabs is controlled by limiting the center- to-center spacing of bars in each direction to the lesser of 2.5 times the thickness of slab or 20 in (500 mm). In fully prestressed slabs, similar to beams, the
32、maximum tensile stress in the concrete due to short-term service loads is limited to 3 fi. For partially prestressed slabs, the incremental steel stress should not exceed 22 ksi (1 50 Design and Construction Practices to Mitigate Cracking 11 ma) and the center-to-center spacing of reinforcement incl
33、uding bonded tendons, is not to exceed 20 in. (500 mm). It should be noted that the extensive Nawy demonstrated that the maximum crack spacing in two-way reinforced concrete slabs should not exceed 12 in. (300 mm), otherwise yield line wide cracks would be prematurely generated. Hence, the AC1 318 C
34、ode limits the maximum spacing to twice the slab thickness. FLEXURAL CRACKTNG AND CRACK CONTROL IN PRESTRESSED PRETENSIONED AND POST-TENSIONED BEAMS The increased use of partial prestressing, allowing limited tensile stresses in the concrete under service and overload conditions while allowing non-p
35、restressed steel to carry the tensile stresses, is becoming prevalent due to practicality and economy. Consequently, an evaluation of the flexural crack widths and spacing and control of their development become essential. Work in this area is relatively limited because of the various factors affect
36、ing crack width development in prestressed concrete. However, experimental investigations support the hypothesis that the major controlling parameter is the reinforcement stress change beyond the decompression stage. Nawy, et al, have undertaken extensive research since the 1960s on the cracking beh
37、avior of prestressed pretensioned and post-tensioned beams and slabs because of the great vulnerability of the highly stressed prestressing steel to corrosion and other environmental effects and the resulting premature loss of prestress.0* Serviceability behavior under service and overload condition
38、s can be controlled by the design engineer through the application of the criteria presented in this section. A. Mathematical Model Formulation for Serviceability Evaluation 1. Crack Spacing Primary cracks form in the region of maximum bending moment when the external load reaches the cracking load.
39、 As loading is increased, additional cracks will form and the number of cracks will be stabilized when the stress in the concrete no longer 12 Nawy exceeds its tensile strength at further locations regardless of load increase. This condition is important as it essentially produces the absolute minim
40、um crack spacing which can occur at high steel stresses, to be termed the stabilized minimum crack spacing, The maximum possible crack spacing under this stabilized condition is twice the minimum, to be termed the stabilized maximum crack spacing. Hence, the stabilized mean crack spacing, acs , is e
41、valuated as the mean value of the two extremes. The total tensile force T in Fig. 3 transferred from the steel to the concrete over the stabilized mean crack spacing can be defined as where Y= c1= T = asp Co (94 a factor reflecting the distribution of bond stress maximum bond stress which is a funct
42、ion of Co = sum of reinforcing elements circumferences The resistance R of the concrete area in tension this is the direction for which crack control check is made. Reinforcement area A, per unit width (22b) Q. ,1 = active steel ratio = . 12(dbi +CI) where c1 is clear concrete cover measured from th
43、e tensile face of the concrete to the nearest edge of the reinforcing bar in direction 1. w = crack width at face of concrete caused by flexural load at the service level (in.) Subscripts 1 and 2 pertain to the directions of reinforcement. Detailed values of the fracture coefficients for various bou
44、ndary conditions are given in Table 1. Using SI units, the expression in Eq. 21 becomes Design and Construction Practices to Mitigate Cracking 21 where f, is in MPa and all the terms for the grid index GI in Eq. 21 are in mm. A graphical solution of Eq. 2 is given in Fig. 8 for rapid determination o
45、f the reinforcement size and spacing needed for crack control where fy = 60,000 psi (414 Mpa) and f, = 40% fy = 24,000 psi (165.5 Ha). C. Tolerable Crack Widths in Concrete Structures The maximum reasonable crack width that can be tolerated in a structural element without distress depends on the par
46、ticular function of the element and the environmental conditions to which the structure is liable to be subjected. Table 1 from the AC1 Committee 224 Report3 on cracking serves as a reasonable guide on the tolerable crack widths in concrete structures under the various environmental conditions that
47、are normally encountered. The crack control equation and guidelines presented are important not only for the control of corrosion in the reinforcement but also for deflection control. The reduction of the stiffness EI of the two-way slab or plate due to orthogonal cracking when the limits of tolerab
48、le crack widths in Table 1 are exceeded, can lead to excessive deflection both short-term and long-term. Deflection values several times those anticipated in the design, including deflection due to construction loading, can be reasonably controlled through camber, and control of the flexural crack w
49、idth in the slab or plate. Proper selection of the reinforcement spacing s1 and s2 in the perpendicular directions, as discussed in this section, and not exceeding 12 in. center to center, can maintain good serviceability performance of a slab system under normal and reasonable overload conditions. Long-term Effects on Cracking In most cases, the magnitude of crack widths increases in long-term exposure and long-term loading. The increase in crack width can vary considerably in cases of cyclic loading, such as in bridges, but the width increases at a decreasing rate with 22 Nawy t
