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Actions Shares. Embeds 0 No embeds. No notes for slide. The cur- rent state of knowledge in microcracking and fracture me- chanics is discussed. The control of cracking due to drying shrinkage and crack control for flexural members, layered systems and mass concrete are covered in detail.
Long- term effects on cracking are considered, and crack control procedures used in construction are presented. Informa- tion is provided to assist the engineer and the constructor in developing practical and effective crack control pro- grams for concrete structures. Keywords: adiabatic conditions; aggregates: air entrainment; an- chorage structural ; beams supports ; bridge decks; cement-ag- gregate reactions; cement content; cement types; compressive strength: computers; concrete construction; concrete pavements; concrete slabs; concretes; conductivity: consolidation; cooling; crack propagation; cracking fracturing ; crack width and spacing: creep properties; diffusivity; drying shrinkage; end blocks; expan- sive cement concretes; extensibility; failure; fibers; heat of hydra- tion; insulation; joints junctions ; machine bases; mass concrete; microcracking; mix proportioning; modulus of elasticity; moisture content; Poisson ratio; polymer-portland cement concrete; pozzo- lans; prestressed concrete; reinforced concrete; reinforcing steels; restraints; shrinkage: specifications; specific heat; strain gages; strains; stresses; structural design; temperature; temperature rise in concrete ; tensile stress; tension; thermal expansion; volume change.
ACI Committee Reports, Guides, Standard Practices , and Com- mentaries are Intended for guidance in designing, planning, executing, or inspecting construction, and in preparing speci- fications Reference to these documents shall not be made in the Project Documents.
If items found in these documents are desired to be part of the Project Documents, they should be phrased in mandatory language and incorporated into the Proj- ect Documents. Copyright 0 , American Concrete Institute.
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 Contents Chapter 1 - Introduction, page R-2 Chapter 2 - Crack mechanisms in concrete, page R-2 2. They can expose reinforcing steel to oxygen and moisture and make the steel more susceptible to corrosion. While the specific causes of cracking are manifold, cracks are normally caused by stresses that develop in concrete due to the restraint of volumetric change or to loads which are applied to the structure.
Within each of these categories there are a number of factors at work. A successful crack control program must recognize these factors and deal with each of them, in turn. This report presents the principal causes of crack- ing and a detailed discussion of crack control pro- cedures. The body of the report consists of seven chapters designed to help the engineer and the con- tractor in the development of effective crack control measures.
This report is an update of a previous committee report, issued in In the updating pro- cess, many portions of the report have undergone sizeable revision, and the entire document has been subjected to a detailed editorial review.
Chapter 2, on crack mechanisms, has been completely rewritten to take into account the experimental and analytical work that has been done since the completion of the first committee report. Chapter 6, on crack control in concrete layered systems, is new to the report and deals with a form of concrete construction that was in its infancy at the time the first report was drafted.
Individual chapters on crack control in re- inforced and prestressed concrete members have been condensed into a single chapter, Chapter 4, on crack control in flexural members.
The resulting pre- sentation is more concise and, hopefully, more useful to the structural designer. Chapter 5, on long-term effects, details some interesting findings on the change of crack width with time. Chapters 3, 7, and 8, which consider drying shrinkage, mass concrete, and construction practices, respectively, have been expanded and updated to take into account the most recently developed procedures in these areas. In ad- dition, new sections have been added to Chapters 7 and 8 which provide specific guidance for the devel- opment of crack control programs and specifications.
The committee hopes that this report will serve as a useful reference to the causes of cracking and as a key tool in the development of practical crack con- trol procedures in both the design and the construc- tion of concrete structures.
References 1. Of special in- terest during the early work was the realization that the behavior of concrete, under compressive as well as tensile loads, was closely related to the formation of cracks. Under increasing compressive stress, mi- croscopic cracks or microcracks form at the mortar- coarse aggregate boundary and propagate through the surrounding mortar, as shown in Fig.
During the first decade of research, a picture de- veloped that closely linked formation and propaga- tion of these microcracks to the load-deformation be- havior of concrete. Prior to load, volume changes in cement paste cause interfacial cracks to form at the mortar-coarse aggregate boundary. Bond cracking increases until the load reaches approximately 70 percent of the compressive strength, at which time microcracks begin to propa- gate through the mortar.
Mortar cracking continues at an accelerated rate until the material ultimately fails. For concrete in uniaxial tension, experimental work indicates that major microcracking begins at about 60 percent of the ultimate tensile strength.
In general, it has been agreed that the micro- cracking that occurs prior to loading has very little effect on the strength of concrete. However, work by Brooks and Neville 2. Their study shows that upon drying, the strength of test speci- mens first increases and then decreases. They postu- late that the initial increase is due to the increased strength of the drier cement paste and that the ulti- mate decrease in strength is due to the formation of shrinkage induced microcracks.
Work by Meyers, Slate, and Winter2. Their work indicates that the total amount of microcracking is a function of the total compressive strain in the concrete and is independent of the method in which the strain is applied. Sturman, Shah, and Winter2. At about the same time that the microcracking studies began, investigators began applying fracture mechanics to the studies of concrete under load. The field of fracture mechanics, originated by Griffith2.
Since concrete has for many years been considered a brittle material in tension, fracture mechanics is con- sidered to be a potentially useful analysis tool for concrete by many investigators. The classical the- ory serves to predict, the rapid propagation of a macrocrack through a homogeneous, isotropic, elas- tic material.
The theory makes use of the stress in- tensity factor, KI , which is a function of crack geom- etry and stress. Failure occurs when KI reaches a critical value, KIc , known as the critical stress-in- tensity factor under conditions of plane strain. KIc is thus a measure of the fracture toughness of the ma- terial. To properly measure KIc for a material, the test specimen must be of sufficient size to insure maximum constraint plane strain at the tip of the crack.
For linear elastic fracture mechanics LEFM to be applicable, the value of KIc must be a material constant, independent of the specimen geometry as are other material constants such as yield strength.
Shah, and F. Later in- vestigations evaluated the crack resistance of paste, mortar and concrete in terms of the fracture tough- ness, itself. They found that KIc in- creased with age, and decreased with increasing air content for paste, mortar, and concrete. The effec- tive fracture toughness of mortar increased with in- creasing sand content, and the fracture toughness of concrete increased with an increase in the maximum size of coarse aggregate.
Additional work by Naus,2. These observations lead to the possibly erroneous conclusion that fracture me- chanics may not be applicable to concrete. Because certain size requirements must be met, before frac- ture mechanics is applicable, these results may only indicate that the test specimen did not satisfy all of the minimum size requirements of linear elastic frac- ture mechanics.
The balance of this chapter describes some of the more recent studies of crack mechanisms in concrete and gives a somewhat different picture from that presented in the previous committee report. These studies utilized rela- Carino found that polymer impregnation did not in- tively thick, soft coatings on the coarse aggregate to crease the interfacial bond strength, but did increase reduce the bond strength.
Since these soft coatings the compressive strength of concrete. He attributed isolated the aggregate from the surrounding mortar, the increase in strength to the effect of the polymer the effect was more like inducing a large number of on the strength of mortar, thus downgrading the im- voids in the concrete matrix.
Two other studies2. Darwin and Slate2. They found that a large reduction in interfacial bond strength causes no change in the initial stiff- ness of concrete under short-term compressive loads and results in approximately a 10 percent reduction in the compressive strength as compared to similar concrete made with aggregate with normal inter- facial strength see Fig. They also found that the lower interfacial strength had no appreciable ef- fect on the total amount of microcracking.
However, in every case, the average amount of mortar crack- ing was slightly greater for the specimens made with coated aggregate. This small yet consistent dif- ference may explain the differences in the stress- strain curves. The importance of mortar, and ultimately cement paste, in controlling the stress-strain behavior of concrete is illustrated by the finite element work of Buyukozturk2. Perry and Gillott 2. Their results indicate that reducing the inter- facial strength of the aggregate decreases the initiation stress by about 20 percent, but has very little effect on the discontinuity stress.
They also ob- served a 10 percent reduction in the compressive strength for specimens with low mortar-aggregate bond strength. Mortar Fig. However, his finite element model could not dupli- cate the nonlinear experimental behavior of the physical model using the formation of interfacial bond cracks and mortar cracks as the only nonlinear effect.
Maher and Darwin2.
ACI Manual of Concrete Practice, 2006. Part 2: ACI 224R-01 to ACI 313R-97