CONTROL OF CRACKING IN CONCRETE STRUCTURES. Includes all amendments and changes through Errata, March 24, View Abstract. Product. CONTROL OF CRACKING IN CONCRETE STRUCTURES. Includes all amendments and changes through Reapproval Notice, View Abstract. Product. ACI R October 1, | Author: RAJ_ | Category: Fracture, Concrete, Fracture Mechanics, Strength Of Materials, Reinforced Concrete.
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242r Science Physics ACI This material may not be reproduced or copied, in whole or part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of ACI. The technical committees responsible for ACI committee 224 and standards strive to avoid ambiguities, omissions, and errors in these documents. In spite of these efforts, the users of ACI documents occasionally find information or requirements that may be subject to more than one interpretation or may be incomplete or incorrect.
ACI committee documents are intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. 2224r who use this publication in any way assume all risk and accept total responsibility for the application and use of this 224e.
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Fouad Ralf Leistikow Randall W. The causes of cracks in concrete structures are summarized. The procedures used to evaluate cracking in concrete and the principal techniques for the repair of cracks are presented.
The key methods of crack repair are discussed, and guidance is provided for their proper application. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains.
The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom.
Reference to this document shall not be made in contract documents. Chapter 2—Evaluation of cracking, p. 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 electronic or mechanical device, printed, written, or oral, 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.
They may affect appearance only, or they may indicate significant structural distress or a lack of durability. Cracks may represent the total extent of the damage, or they may point to problems of greater magnitude.
Their significance depends on the type of structure, as well as the nature of the cracking. For example, cracks that are acceptable for buildings may not be acceptable in water-retaining structures.
Good crack repair techniques depend on knowing the causes and selecting appropriate repair procedures that take these causes into account; otherwise, the repair may only be temporary. Successful long-term repair procedures must address the causes of the cracks as well as the cracks themselves.
This report is intended to serve as a tool in the process of crack evaluation and repair of concrete structures. The causes of cracks in concrete are summarized along with the principal procedures used for crack control. Both plastic and hardened concrete are considered.
The importance of design, detailing, construction procedures, concrete proportioning, and material properties are discussed. The techniques and methodology for crack evaluation are described. The need to determine the causes of cracking as a necessary prerequisite to repair is emphasized. The selection of successful repair techniques should consider the causes of cracking, whether the cracks are active or dormant, and the need for repair.
Criteria for the selection of crack repair procedures are based on the desired outcome. Twelve methods of crack repair are presented, including the techniques, advantages and disadvantages, and areas of application for each.
Cracks are categorized as occurring either in plastic concrete or hardened concrete Kelly ; Price In addition to the information provided herein, further details are presented in ACI R and articles by Carlson et al. Additional references are cited throughout the chapter. Due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete.
This results in shallow cracks of varying depths that may form a random, polygonal pattern, or be essentially parallel to one another Fig. They range from a few inches to many feet in length, and are spaced from a few inches millimeters to as much as 10 ft 3 m apart. Plastic shrinkage cracks begin as shallow cracks, but can become full-depth cracks later in the life of the concrete.
Plastic shrinkage cracking is usually associated with the rapid loss of moisture caused by a combination of factors that include high air and concrete temperatures, low relative humidity, and high wind velocity at the surface of the concrete. Concrete with lower amounts of bleed water, such as those containing mineral admixtures especially silica fume have a greater tendency to undergo plastic shrinkage cracking than concrete with a greater tendency to bleed.
Because plastic shrinkage cracking is due to a differential volume change in the plastic concrete, successful control measures require a reduction in the relative volume change between the surface and other portions of the concrete.
Steps can be taken to prevent rapid moisture loss due to hot weather and dry winds ACI R, These measures include the use of fog nozzles to saturate the air above the surface and the use of plastic sheeting to cover the surface between finishing operations.
Windbreaks to reduce the wind velocity and sunshades to reduce the surface temperature are also helpful. It is good practice to schedule flatwork after the windbreaks have been erected. During hot, windy weather with low humidity, it is sometimes advisable to reschedule 224r concrete placement or to initiate concrete operations at night.
During this period, the plastic concrete may be locally restrained by reinforcing steel, a previous concrete placement, or formwork. This local restraint may result in voids, cracks, or both, adjacent to the restraining element Fig. When associated with reinforcing steel, settlement cracking increases with increasing bar size, increasing slump, and decreasing ack Dakhil et al.
The degree of settlement cracking may be intensified by 224e vibration or by the use of leaking or highly flexible forms.
Suprenant and Malisch demonstrated that the addition of fibers can reduce the formation of settlement cracks. The following items will reduce settlement cracking: Fortunately, aggregate particles provide internal restraint that reduces the magnitude of this volume change to about 0. On the other hand, concrete tends to expand when wetted the volume increase can be the same order of magnitude as that observed due to shrinkage. These moisture-induced volume changes are a characteristic of concrete.
If the shrinkage of concrete could take place without restraint, the concrete would not crack.
224R-01: Control of Cracking in Concrete Structures (Reapproved 2008)
It is the combination of shrinkage and restraint provided by another part of the structure, by the subgrade, or by the moist interior of the concrete itself that causes tensile stresses to develop. When the tensile strength of the material is exceeded, concrete will crack.
Cracks may propagate at much lower stresses than are required to cause crack initiation ACI In massive concrete elements, tensile stresses are caused by differential shrinkage between the surface and the interior concrete. The higher shrinkage at the surface causes cracks to develop that may, with time, penetrate deeper into the concrete.
The magnitude of the tensile stresses induced by volume change is influenced by a combination of factors, including the amount and rate of shrinkage, the degree of restraint, the modulus of elasticity, and the amount of creep.
The amount of drying shrinkage is influenced mainly by the amount and type of aggregate and qci cement paste cement and water content of the mixture. As the quantity of aggregate increases, the shrinkage decreases Pickett The higher the stiffness of the aggregate, the more effective it is in reducing the shrinkage of the avi that is, the shrinkage of concrete containing sandstone aggregate may be more than twice that of concrete with granite, basalt, or highquality limestone Carlson The higher the water and cement contents, the greater the amount of drying shrinkage U.
Bureau of Reclamation ; Schmitt and Darwin ; Darwin et al. Surface crazing alligator pattern on walls and slabs is an example of drying shrinkage on a small scale. Crazing usually occurs when the surface layer of the concrete has a higher water content than the interior concrete. The result is a series of shallow, closely spaced, fine cracks. A procedure that will help reduce settlement cracking, as well as drying shrinkage in walls, is reducing the water content of the concrete as the wall is placed from aaci bottom to the top U.
Using this procedure, bleed water from the lower portions of the wall will tend to equalize the water content within the wall. To be successful, this procedure needs careful control of the concrete and proper consolidation. Shrinkage cracking can be controlled by using contraction joints and proper 2224r of the reinforcement.
The reduction or elimination of subslab restraint can also be effective in reducing shrinkage cracking in slabson-ground Wimsatt et al.
In cases where crack control is particularly important, the minimum requirements of ACI may not be adequate. These points are discussed in greater detail in ACI R, which describes additional construction practices designed to help control the drying shrinkage cracking that does occur, and in ACI Thus, it is a problem often associated with high-strength concretes.
Autogenous shrinkage occurs without the loss aco moisture from the bulk concrete. These temperature differences result in differential volume changes. When the tensile stresses due to the differential volume changes exceed the tensile strength, concrete will crack.
Temperature differentials and the accompanying volume changes due to the dissipation of the heat of hydration of cement are normally associated with mass concrete which can include large columns, piers, beams, and ac, as well as damswhereas temperature differentials due to changes in the ambient afi can affect any structure.
Cracking in mass concrete can result from a greater temperature on the interior than on the exterior. The temperature gradient may be caused by either the center of the concrete heating more than the outside due to the liberation of heat during cement hydration or more rapid cooling of the exterior relative to the interior. Both cases result in tensile stresses on the exterior and, if the tensile strength is exceeded, cracking will occur. The tensile stresses are proportional to the temperature differential, the coefficient of thermal expansion, the effective modulus of elasticity which is reduced by creepand the degree of restraint Dusinberre ; Houghton The more massive the structure, the greater the potential for temperature differential and restraint.
Procedures to help reduce thermally induced cracking include reducing the maximum internal temperature, delaying the onset of cooling, controlling the rate at which the concrete cools, and increasing the tensile strength of the concrete.
These and other methods used to reduce cracking in massive concrete are presented in ACI When one portion of a structure is subjected to a temperatureinduced volume change, the potential for thermally induced cracking exists. Designers should give special consideration to structures in which some portions are exposed to temperature changes, while other portions of the structure are either partially or completely protected.
A drop in temperature may result in cracking in the exposed element, whereas increases in temperature may cause cracking in the protected portion of the structure. Temperature gradients cause deflection and rotation in structural members; if restrained, serious stresses can result Priestley ; Hoffman et al.