ACI Bazant Jeffrey J. Brooks Ronald G. Jason Weiss Secretary Mario A. Chiorino Marwan A. Daye Walter H.
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Bazant Jeffrey J. Brooks Ronald G. Chiorino Marwan A. Daye Walter H. Dilger N. Mueller Lawrence C. Novak Klaus A. Shiu Carlos Videla Domingo J. This guide describes the effects of numerous variables on shrinkage and creep of hardened concrete, including mixture proportions, environment, design, and construction.
This document is aimed at designers who wish to gain further information about factors changing shrinkage and creep but does not include information on the prediction of shrinkage and creep or structural design issues associated with shrinkage and creep. Keywords: creep; drying shrinkage; strain. Section 1. This document does not include information on the prediction of shrinkage and creep or structural design issues associated with shrinkage and creep.
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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. This document also provides references that provide direction for those wishing to seek additional information about shrinkage and creep.
Various terms are shown in Fig. The values of total strain, shrinkage, and creep are time-dependent. Shrinkage and creep may occur in three dimensions; however, most research suggests that total strain, shrinkage, and creep occur in each dimension independently. Thus, changes in length will be consistently used throughout this document, rather than changes in volume. As shown in Fig. Shrinkage does not include changes in length due to temperature variations, but depends on the environment and on the configuration and size of the specimen.
Shrinkage strain is usually measured by casting companion load-free specimens identical to the loaded concrete specimens used to measure the total strain. These companion specimens are cast from the same concrete batch, have the same dimensions, and are stored in the same environment as the loaded concrete specimens.
It is common to describe shrinkage in microstrain or millionths, as the value of strain Cement paste Fig. The figure shows that the concrete undergoes autogenous shrinkage before drying.
Once drying commences at time t0, drying shrinkage occurs. Upon loading, both drying and basic creep occurs in the drying specimen. Less commonly, it is termed basic shrinkage or chemical shrinkage. Autogenous shrinkage was almost never considered as a factor in research on shrinkage and creep before , and it has become a greater factor with the increased use of high-performance concrete. Factors affecting, and the prediction of autogenous shrinkage, are outside the scope of this report.
As development of research continues in this area, ACI Committee will present additional information. Autogenous shrinkage is usually small for many normal compressive strength concretes and can usually be neglected.
For normal-strength concrete, it is usually assumed that the entire shrinkage strain is from drying shrinkage, and any contribution from autogenous shrinkage is neglected. Because drying shrinkage involves moisture movement through the material and moisture loss, drying shrinkage depends on the size and shape of the specimen.
Due to the relationship of drying shrinkage to water loss, it may be expected to reach a final value; although, this is difficult to be confirmed experimentally due to the long duration of the drying process in normal size specimens RILEM TC ; Al-Manaseer, Espion, and Ulm ; Bazant A final value has been documented for specimens of hardened cement paste thin enough to dry to an equilibrium water content Wittman et al.
Factors affecting, and the prediction of carbonation shrinkage, are outside the scope of this report. Plastic shrinkage is outside the scope of this report. Few research studies have closely recorded the magnitude of swelling and studied the factors affecting the magnitude of this phenomenon. Experimentally, it is obtained by subtracting from the total strain the shrinkage strain measured on load-free companion specimens with the same size and shape as the loaded specimens and placed in the same environment.
It is dependent on the duration of the load application and strain reading procedures. The separation of this initial component of the load-induced strain is made for convenience, and it may be determined using standardized procedures for the experimental determination of a static elastic modulus corresponding to the strain in a short interval after load application CEB ; RILEM TC CSP ; Bazant and Baweja ; Carreira and Burg ASTM C is often used to determine this value.
In this test, the initial strain corresponds to a load duration of 0. Although often done by researchers, the committee recommends that the strain should not be separated into initial and creep strains, due to the loading rate factors that affect the estimated initial strain at loading.
It is obtained from the load-induced strain by subtracting the initial strain defined in Section 1. The creep strain may be several times greater than the initial strain. Creep strain may be subdivided into a drying and a nondrying component, termed drying and basic creep, respectively. It represents the creep at constant moisture content with no moisture movement through the material, and is consequently independent of the specimen size and shape.
To determine basic creep, it is necessary to measure the deformations of a set of sealed specimens under constant load and to determine the total strain; and, if autogenous shrinkage cannot be neglected, deformations of companion sealed, load-free specimens should be measured.
It has not been determined whether basic creep approaches a final value, even after 30 years of measurement of sealed specimens Bazant ; CEB Three sets of specimens are required to determine the drying creep: a loaded set that is allowed to dry to determine the total strain, a loaded set of sealed specimens to determine basic creep, and a load-free set at drying to determine the total shrinkage strain Carreira and Burg This is mathematically described in Eq.
By definition, the creep coefficient is dimensionless. The creep coefficient may be determined from compliance and from the nominal elastic modulus of the concrete, as shown in Eq.
Typical values of creep coefficient for long periods under sustained constant loading range from 1. The line through the curves indicates the initial elastic strain, which decreases with concrete age.
Care should be taken when using the creep coefficient due to the subdivision of the two components of the strain from which it is defined. Mechanisms and theories describing creep of concrete, however, are complex and not fully developed or understood. Several of the major theories are outlined below, and readers are directed to these sources for additional information.
A report issued by ACI Committee in ACI Committee summarized the different proposed mechanisms as: Viscous flow of the cement matrix caused by sliding or shear of the gel particles lubricated by layers of adsorbed water; Consolidation due to seepage in the form of adsorbed water or the decomposition of interlayer hydrate water; Delayed elasticity due to the cement matrix acting as a restraint on the elastic deformation of the skeleton formed by the aggregates; this component accompanies viscous flow and consolidation; and Permanent deformation caused by local fracture microcracking and crystal failure and recrystalization and formation of new physical bonds.
Neville, Dilger, and Brooks classified six basic theories: Mechanical deformation theory; Viscous flow; Plastic flow; Seepage of gel water; Delayed elasticity; and Microcracking. As a result of the previous definitions, the compliance is given by Eq. Further information on mechanisms influencing shrinkage and creep of concrete may be found in the references Bazant , ; Bazant and Carol ; Ulm, Le Maou, and Boulay This document is not intended to be a complete review of all references, but a guide document.
Bibliography No. While these references are not directly available from the American Concrete Institute, they may be found in cement and concrete or engineering reference libraries, such as that of the Portland Cement Association, and are considered invaluable basic resources for research in shrinkage and creep. Methods for reducing drying shrinkage are described below. It has been found Pickett that shrinkage of concrete SC is related to shrinkage of the cement paste SP and the volume of the aggregate content g according to Eq.
Suggested values of n are between 1. Most concrete used in general construction has aggregate volumetric fractions between 0. A rounder aggregate may result in a decreased paste content that will result in lower shrinkage. The combined influences of water content and cement content are shown in Fig.
Increasing the slump of concrete by adding water will tend to increase drying shrinkage. These actions will both result in the decrease of aggregate volume within a concrete. Based on Fig. The graph shows that the shrinkage of the concrete is not linearly related to the fractional volume of aggregate. Roper conducted extensive work on the effects of various cement compositions on the shrinkage of mortar specimens.
He found that cements with low quantities of sulfate may exhibit increased shrinkage. The shrinkage of concrete made with a high alumina content occurs more rapidly. The effect of admixtures on drying shrinkage is discussed by Brooks , where extensive discussions are made relating to other changes that are commonly made when using these admixtures such as changes in cement content or water use.
A line is drawn horizontally to determine the drying shrinkage. Concrete containing an aggregate with a high modulus of elasticity will tend to have a lower drying shrinkage than concrete containing an aggregate with a low modulus of elasticity, as shown in Fig. Some admixtures are specifically formulated to reduce shrinkage and are discussed by Nmai et al. Low humidity, wind, and high temperatures tend to increase the rate of drying leading to increased rates and magnitudes of drying shrinkage.
Unrestrained concrete stored in water will tend to swell. Various mathematical relationships have been used to relate drying shrinkage to relative humidity. The most commonly used formula relating drying shrinkage to relative humidity h, in percent, is shown in Eq. Because of the lack of data, testing may be appropriate for concrete in dry climates and desert regions and for the interior of heated buildings without humidifiers.
Their results are shown in Fig. The CEB-FIP Model Code presents equations to adjust the time rate of shrinkage and the ultimate shrinkage for concrete exposed to elevated long-term temperatures. These factors, which are listed in Sections 2.
These companion specimens are cast from the same concrete batch, have the same dimensions, and are stored in the same environment as the loaded concrete specimens. Get fast, free shipping with Amazon Prime. Upon loading, both drying and basic creep occurs in the drying specimen. Would you like to tell us about a lower price? Amazon Second Chance Pass it on, trade it in, give it a second life. Reference to this document shall not be made in contract documents. This document also provides references that provide
209.1R-05: Report on Factors Affecting Shrinkage and Creep of Hardened Concrete
ACI 209.1R-05 - Report on Factors Affecting Shrinkage and Creep of Hardened Concrete