ACI 224R-01 superdes ACI 224R-90 and became effective May 16, 2001.Copyright © 2001, American Concrete Institute.
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Control of Cracking in Concrete Structures
ACI 224R-01
The principal caus of cracking and recommended crack-control proce-dures are prented. The current state of knowledge in microcracking and fracture of concrete is reviewed. The control of cracking due to drying shrinkage and crack control in flexural members, overlays, and mass con-crete construction are covered in detail. Long-term effects on cracking are considered and crack-control procedures ud in construction are pre-nted. Information is prented to assist in the development of practical and effective crack-control programs for concrete structures. Extensive ref-erences are provided.
Keywords : aggregates; anchorage (structural); bridge decks; cement-aggregate reactions; concrete construction; concrete pavements; concrete slabs; cooling; corrosion; crack propagation; cracking (fracturing); crack width and spacing; drying shrinkage; shrinkage-compensating concrete;heat of hydration; mass concrete; microcracking; polymer-modified concrete;prestresd concrete; reinforced concrete; restraint; shrinkage; temperature;tensile stress; thermal expansion; volume change.
CONTENTS
Chapter 1—Introduction, p. 224R-2
Chapter 2—Crack mechanisms in concrete, p. 224R-2
2.1—Introduction
2.2—Compressive microcracking 2.3—Fracture
Chapter 3—Control of cracking due to drying shrinkage, p. 224R-113.1—Introduction
3.2—Cau of cracking due to drying shrinkage 3.3—Drying shrinkage
3.4—Factors controlling drying shrinkage of concrete 3.5—Control of shrinkage cracking 3.6—Shrinkage-compensating concrete
Chapter 4—Control of cracking in flexural members, p. 224R-174.1—Introduction
4.2—Crack-control equations for reinforced concrete beams 4.3—Crack control in two-way slabs and plates
4.4—Tolerable crack widths versus exposure conditions in reinforced concrete
4.5—Flexural cracking in prestresd concrete
4.6—Anchorage-zone cracking in prestresd concrete 4.7—Crack control in deep beams 4.8—Tension cracking
Reported by ACI Committee 224
Mohamed Abou-Zeid
David W. Fowler *Edward G. Nawy *John H. Allen Grant T. Halvorn Randall W. Poston *James P. Barlow Will Hann *Royce J. Rhoads Merle E. Brander *M. Nadim Hassoun Andrew Scanlon Kathy Carlson Harvey Haynes *Ernest K. Schrader *David Darwin *Paul Hedli Wimal Suaris *Fouad H. Fouad *
Tony C. Liu
Zenon A. Zielinski
Florian Barth Chairman
Robert J. Frosch *
Secretary
*Members of ACI 224 who assisted in revisions to this report.
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224R-2ACI COMMITTEE REPORT
Chapter 5—Long-term effects on cracking,
p. 224R-24搞笑图文
5.1—Introduction
5.2—Effects of long-term loading
5.3—Environmental effects
5.4—Aggregate and other effects
5.5—U of polymers in improving cracking characteristics
Chapter 6—Control of cracking in overlays,
p. 224R-25
6.1—Introduction
6.2—Fiber-reinforced concrete (FRC) overlays
6.3—Latex- and epoxy-modified concrete overlays
6.4—Polymer-impregnated concrete (PIC) systems
6.5—Epoxy and other polymer concrete overlays
Chapter 7—Control of cracking in mass concrete, p. 224R-28
7.1—Introduction
7.2—Methods of crack control
7.3—Design
7.4—Construction
7.5—Operation
Chapter 8—Control of cracking by proper construction practices, p. 224R-34
8.1—Introduction
8.2—Restraint相视而笑
8.3—Shrinkage
8.4—Settlement
8.5—Construction
8.6—Specifications to minimize drying shrinkage
8.7—Conclusion
硒酵母胶囊瘦脸小技巧Chapter 9—References, p. 224R-39
禀告是什么意思
9.1—Referenced standards and reports
9.2—Cited references
9.3—Other references
CHAPTER 1—INTRODUCTION
Cracks in concrete structures can indicate major structural problems and detract from the appearance of monolithic construction. There are many specific caus of cracking. This report prents the principal caus of cracking and a detailed discussion of crack-control procedures. The report consists of eight chapters designed to help the engineer and the contractor in developing crack-control measures.
This report is an update of previous committee reports (ACI Committee 224 1972, 1980, 1990). ACI Bibliogra-phy No. 9 supplemented the original ACI 224R (1971). The Committee has also prepared reports on the caus, evaluation, and repair of cracking, ACI 224.1R; cracking of concrete in di-rect tension, ACI 224.2R; and joints in concrete construction, ACI 224.3R.
In this revision of the report, Chapter 2 on crack mechanisms has been revid extensively to reflect
the interest and attention given to aspects of fracture mechanics of concrete during the 1980s. Chapter 3 on drying shrinkage has been rewritten. Chapter 4 has been revid to include updated information on crack-width predictive equations, cracking in partially prestresd members, anchorage zone cracking, and flexural cracking in deep flexural members. Chapter 6 on concrete overlays has been reorganized and revid in modest detail to account for updated information on fiber reinforcement and on polymer-modified concrete. Chapter 7 on mass concrete has been revid to consider structural conquences more extensively.
CHAPTER 2—CRACK MECHANISMS IN
CONCRETE
2.1—Introduction
Cracking plays an important role in concrete’s respon to load in both tension and compression. The earliest studies of the microscopic behavior of concrete involved the respon of concrete to compressive stress. That early work showed that the stress-strain respon of concrete is cloly associated with the formation of microcracks, that is, cracks that form at coar-aggregate boundaries (bond cracks) and propagate through the surrounding mortar (mortar cracks) (Hsu, Slate,
Sturman, and Winter 1963; Shah and Winter 1966; Slate and Matheus 1967; Shah and Chandra 1970; Shah and Slate 1968; Meyers, Slate, and Winter 1969; Darwin and Slate 1970), as shown in Fig. 2.1.
During early microcracking studies, concrete was considered to be made up of two linear, elastic brittle materials; cement paste and aggregate; and microcracks were considered to be the major cau of concrete’s nonlinear stress-strain behavior in compression (Hsu, Slate, Sturman, and Winter 1963; Shah and Winter 1966). This picture began to change in the 1970s. Cement paste is a nonlinear softening material, as is the mortar constituent of concrete. The compressive non-linearity of concrete is highly dependent upon the respon of the two materials (Spooner 1972; Spooner and Dougill 1975; Spooner, Pomeroy, and Dougill 1976; Maher and Dar-win 1977; Cook and Chindaprasirt 1980; Maher and Darwin 1982) and less dependent upon bond and mortar microcracking than originally thought. Rearch indicates, however, that a sig-nificant portion of the nonlinear deformation of cement paste and mortar results from the formation of microcracks that are veral orders of magnitude smaller than tho obrved in the original studies (Attiogbe and Darwin 1987, 1988). The smaller microcracks have a surface density that is two to three orders of magnitude higher than the density of bond and mortar microcracks in concrete at the same compres-
sive strain, and their discovery reprents a significant step towards understanding the behavior of concrete and its constituent materials in compression.
The effect of macroscopic cracks on the performance and failure characteristics of concrete has also received considerable attention. For many years, concrete has been considered a brittle material in tension. Many attempts have been made to u principles of fracture mechanics to model the fracture of concrete containing macroscopic cracks.
The field of fracture mechanics was developed by Griffith (1920) to explain the failure of brittle materials. Linear elastic fracture mechanics (LEFM) predicts the rapid propagation of a microcrack through a homogeneous, isotropic, linear-elastic material. The theory us the stress-intensity factor K that --``````-`-`,,`,,`,`,,`---
CONTROL OF CRACKING IN CONCRETE STRUCTURES
224R-3
reprents the stress field ahead of a sharp crack in a struc-tural member which is a function of the crack geometry and stress. K is further designated with subscripts, I, II, and III, depending upon the nature of the deformation at the crack tip. For a crack at which the deformation is perpendicular to the crack plane, K is designated as K I, and failure occurs when K I reaches a critical value K I c, known as the critical stress-intensity factor. K I c is a measure of the fracture tough-ness of the material, which is simply a measure of the resis-tance to crack propagation. Often the region around the crack tip undergoes nonlinear deformation, such as yielding in metals, as the crack grows. This region is referred to as the plastic zone in metals, or more generally as the fracture process zone. To properly measure K I c for a material, the test specimen should be large enough so that the fracture process zone is small compared with the specimen dimensions. For LEFM to be applicable, the value of K I c must be a material property, independent of the specimen geometry (as are other material properties, such as yield strength or compressive strength). Initial attempts to measure K I c in concrete were unsuccessful becau K I c depended on the size and geometry of the test specimens (Wittmann 1986). As a result of the heterogeneity inherent in cement paste, mortar, and concrete, the materials exhibit a significant fracture-process zone and the critical load is preceded
by a substantial amount of slow crack growth. This precritical crack growth has been studied experimentally by veral rearchers (John and Shah 1986; Swartz and Go 1984; Bascoul, Kharchi, and Maso 1987; Maji and Shah 1987; Castro-Montero, Shah, and Miller 1990). This rearch has provided an improved understanding of the fracture process zone and has led to the development of more rational fracture criteria for concrete.
This chapter is divided into two ctions. The first ction on compressive microcracking prents the current knowledge of the respon of concrete and its constituent materials under compressive loading and the role played by the various types of microcracks in this process. The cond ction discuss the applicability of both linear and nonlinear fracture mechanics models to concrete. A more comprehensive treatment of the fracture of concrete can be found in ACI 446.1R.
2.2—Compressive microcracking
During early microcracking rearch, a picture devel-oped that cloly linked the formation and propagation of microcracks to the load-deformation behavior of concrete. Before loading, volume changes in cement paste cau inter-facial cracks to form at the mortar-coar aggregate bound-ary (Hsu 1963; Slate and Matheus 1967). Under short-term compressive loads, no additional cracks for
m until the load reaches about 30% of the compressive strength of the con-crete (Hsu, Slate, Sturman, and Winter 1963). Above this value, additional bond cracks are initiated throughout the matrix. Bond cracking increas until the load reaches about 70% of the compressive strength, at which time the microc-racks begin to propagate through the mortar. Mortar crack-ing continues at an accelerated rate, forming continuous cracks parallel to the direction of compressive load, until the concrete is no longer able to sustain the load. The ont of mortar cracking is related to the sustained, or long-term, compressive strength. Derucher (1978) obtained a somewhat different picture of the microscopic behavior of concrete using the scanning electron microscope (SEM). He subjected dried concrete specimens to eccentric compressive loading within the SEM. He obrved that microcracks that exist
Fig. 2.1—Cracking maps and stress-strain curves for concrete loaded in uniaxial compression (Shah and Slate 1968).
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224R-4
ACI COMMITTEE REPORT
before loading are in the form of bond cracks, with exten-sions into the surrounding mortar perpendicular to the bond cracks. Under increasing compression, the bond cracks widen but do n
江西中考ot propagate at loads as low as 15% of the strength. At about 20% of ultimate, the bond cracks begin to propagate, and at about 30%, they begin to bridge between one another. The bridging is almost complete at 45% of the compressive strength. At 75% of ultimate, mortar cracks start to join one another and continue to do so until failure.In general, microcracking that occurs before loading has little effect on the strength of compressive strength of the concrete.In studies of high-strength concrete, Carrasquillo, Slate,and Nilson (1981) concluded that it was more appropriate to classify cracks as simple (bond or mortar) and combined (bond and mortar) and that the formation of combined cracks consisting of more than one mortar crack signaled unstable crack growth. They obrved that the higher the concrete strength, the higher the strain (relative to the strain at peak stress) at which this unstable crack growth is obrved.They obrved less total cracking in high-strength concrete than normal-strength concrete at all stages of loading.
Work by Meyers, Slate, and Winter (1969), Shah and Chandra (1970), and Ngab, Slate, and Nilson (1981) demon-strated that microcracks increa under sustained and cyclic loading. Their work indicated that the total amount of micro-cracking is a function of the total compressive strain in the concrete and is independent of the method in which the strain is applied. Suaris and Fernando (1987) also showed that the failure of concrete under constant amplitude cyclic loading is cloly con
放逐之路nected with microcrack growth. Sturman, Shah,and Winter (1965) found that the total degree of microcracking is decread and the total strain capacity in compression is incread when concrete is subjected to a strain gradient.
Since the early work established the existence of bond and mortar microcracks, it has been popular to attribute most, if not all, of the nonlinearity of concrete to the formation of the microscopic cracks (Hsu, Slate, Sturman, and Winter 1963; Shah and Winter 1966; Testa and Stubbs 1977; Car-rasquillo, Slate, and Nixon 1981). A cau and effect rela-tionship, however, has never been established (Darwin 1978). Studies by Spooner (1972), Spooner and Dougill (1975), Spooner, Pomeroy, and Dougill (1976), and Maher and Darwin (1982) indicate that the degree of microcracking can be taken as an indication of the level of damage rather than as the controlling factor in the concrete’s behavior.
Experimental work by Spooner (1972), Spooner and Dougill (1975), Spooner, Pomeroy, and Dougill (1976), and Martin,Darwin, and Terry (1991) indicates that the nonlinear compres-sive behavior of concrete is strongly influenced by the nonlinear behavior of cement paste. As illustrated in Fig. 2.2, cement paste under compression is not an elastic, brittle material as stated in the past, but a nonlinear material with a relatively high strain capacity. The nonlinear behavior of cement paste can
be tied to damage sustained by the paste, even at very low stress.Using a cyclic loading procedure, Spooner (1972), Spoon-er and Dougill (1975), and Spooner, Pomeroy, and Dougill (1976) demonstrated that both paste and concrete undergo mea-surable damage at strains (0.0004) at which an increa in bond and mortar microcracking cannot be detected. The level of damage can be detected at low loads by using an energy method and by a change in the initial modulus of elasticity for each load cycle. The process of damage is continuous up to failure.
The physical nature of damage that occurs in cement paste,like that in concrete, appears to be related to cracking. This point was first made by Spooner, Pomeroy, and Dougill (1976) bad on volumetric strain measurements and then by
Fig. 2.2—Stress-strain curves for cement paste, mortar, and concrete; w/c = 0.5 (Martin,Darwin, and Terry 1991).
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CONTROL OF CRACKING IN CONCRETE STRUCTURES
224R-5
Yoshimoto et al. (1972) and Yoshimoto, Ogino, and Kawakami (1976) who reported the formation of “
hair-shaped” and “void-shaped” cracks in paste under flexure and compressive loading. The relationship between nonlinear deformation and cracking in cement paste is now firmly es-tablished by the work of Attiogbe and Darwin (1987, 1988). Studies of the stress-strain behavior of concrete under cyclic compressive load (Karsan and Jirsa 1969; Shah and Chandra 1970) indicated the concrete undergoes rapid deterioration once the peak stress exceeds 70% of the short-term compres-sive strength of the concrete. In their study of cyclic creep, Neville and Hirst (1978) found that heat is generated even when specimens are cycled below this level. They attributed the heat to sliding at the interfacial boundary. The work of Neville and Hirst, along with the work of Spooner, suggests that it can be possible that the heat measured is due to some microscopic sliding within the paste.
Several studies have attempted to establish the importance of interfacial bond strength on the behavior of concrete in compression. Two studies emed to indicate a very large effect, thus emphasizing the importance of interfacial strength on concrete behavior in compression (Shah and Chandra 1970; Nepper-Christenn and Nieln 1969). The studies ud relatively thick, soft coatings on coar aggregate to reduce the bond strength. Becau the soft coatings isolated the aggregate from the surrounding mortar, the effect was more like inducing a large number of voids in the concrete matrix.
Two other studies (Darwin and Slate 1970; Perry and Gillott 1977) that did not isolate the coar aggregate from the mortar indicated that interfacial strength plays only a minor role in controlling the compressive stress-strain behavior of concrete. Darwin and Slate (1970) ud a thin coating of polystyrene on natural coar aggregate. They found that a large reduction in interfacial bond strength caus no change in the initial stiffness of concrete under short-term compressive loads and results in about a 10% reduction in the compressive strength, compared with similar concrete made with aggregate with normal interfacial strength (Fig. 2.3). Darwin and Slate also monitored microcracking. In every ca, however, the average amount of mortar cracking was slightly greater for specimens made with coated aggregate. This small yet consistent difference may explain the differences in the stress-strain curves. Perry and Gillott (1977) ud glass spheres with different degrees of surface roughness as coar aggregate. Their results also indicate that reducing the inter-facial strength of the aggregate decreas the compressive strength by about 10%.
Work by Carino (1977), using polymer-impregnated concrete, corroborated the last two studies. Carino found that polymer impregnation did not increa the inter-facial bond strength but did increa the compressive strength of concrete. He attributed the increa in strength to the polymer’s effect on mortar strength, therefore downgrading the importance of interfacial bond.
The importance of mortar in controlling the stress-strain behavior of concrete is illustrated by the finite-element work of Buyukozturk (1970) and Maher and Darwin (1976, 1977). Buyukozturk (1970) ud a finite-element reprentation of a physical model of concrete. The model treated mortar (in compression) and aggregate (in compression and tension) as linear elastic materials while allowing cracks to form in the mortar and at mortar aggregate boundaries. Buyukozturk simulated the overall crack patterns under uniaxial loading. His finite-element model, however, could not duplicate the full nonlinear behavior of the physical model using the for-mation of interfacial bond cracks and mortar cracks as the only nonlinear effects. Maher and Darwin (1976, 1977) have shown that a very clo reprentation of the actual stress-strain behavior can be obtained using a nonlinear reprentation for the mortar constituent of the physical model.
Fig 2.3—Stress-strain curves as influenced by coating aggregates (Darwin and Slate
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1970).
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