1、PDF外文：http:/ Journal of Structural Engineering, 1 ( 2001) 15 Shrinkage, Cracking and Deflection- the Serviceability of Concrete Structures R.I. Gilbert Professor and Head, School of Civil and Environmental Engineering The University of New South Wales, Sydney, NSW, 2052 Email: i.gilbert
2、unsw.edu.au ABSTRACT This paper addresses the effects of shrinkage on the serviceability of concrete structures. It outlines why shrinkage is important, its major influence on the final extent of cracking and the magnitude of deflection in structures, and what to do about it in design.
3、 A model is presented for predicting the shrinkage strain in normal and high strength concrete and the time-dependent behaviour of plain concrete and reinforced concrete, with and without external restraints, is explained. Analytical procedures are described for estimating the final width
4、 and spacing of both flexural cracks and direct tension cracks and a simplified procedure is presented for including the effects of shrinkage when calculating long-term deflection. The paper also contains an overview of the considerations currently being made by the working group established b
5、y Standards Australia to revise the serviceability provisions of AS3600-1994, particularly those clauses related to shrinkage. KEYWORDS Creep； Cracking； Deflection； Reinforced concrete； Serviceability； Shrinkage. 1. Introduction For a concrete structure to be serviceable, cracking
6、 must be controlled and deflections must not be excessive. It must also not vibrate excessively. Concrete shrinkage plays a major role in each of these aspects of the service load behaviour of concrete structures. The design for serviceability is possibility the most difficult and least well u
7、nderstood aspect of the design of concrete structures. Service load behaviour depends primarily on the properties of the concrete and these are often not known reliably at the design stage. Moreover, concrete behaves in a non-linear and inelastic manner at service loads. The non-linear behaviour tha
8、t complicates serviceability calculations is due to cracking, tension stiffening, creep, and shrinkage. Of these, shrinkage is the most problematic. Restraint to shrinkage causes time-dependent cracking and gradually reduces the beneficial effects of tension stiffening. It results in a gradual widen
9、ing of existing cracks and, in flexural members, a significant increase in deflections with time. The control of cracking in a reinforced or prestressed concrete structure is usually achieved by limiting the stress increment in the bonded reinforcement to some appropriately low value and ensur
10、ing that the bonded reinforcement is suitably distributed. Many codes of practice specify maximum steel stress increments after cracking and maximum spacing requirements for the bonded reinforcement. However, few existing code procedures, if any, account adequately for the gradual increase in existi
11、ng crack widths with time, due primarily to shrinkage, or the time-dependent development of new cracks resulting from tensile stresses caused by restraint to shrinkage. For deflection control, the structural designer should select maximum deflection limits that are appropriate to the structure
12、 and its intended use. The calculated deflection (or camber) must not exceed these limits. Codes of practice give general guidance for both the selection of the maximum deflection limits and the calculation of deflection. However, the simplified procedures for calculating eJSE International &n
13、bsp;Electronic Journal of Structural Engineering, 1 ( 2001) 16 deflection in most codes were developed from tests on simply-supported reinforced concrete beams and often produce grossly inaccurate predictions when applied to more complex structures. Again, the existing code procedures d
14、o not provide real guidance on how to adequately model the time-dependent effects of creep and shrinkage in deflection calculations. Serviceability failures of concrete structures involving excessive cracking and/or excessive deflection are relatively common. Numerous cases have been reported,
15、 in Australia and elsewhere, of structures that complied with code requirements but still deflected or cracked excessively. In a large majority of these failures, shrinkage of concrete is primarily responsible. Clearly, the serviceability provisions embodied in our codes do not adequately mode
16、l the in-service behaviour of structures and, in particular, fail to account adequately for shrinkage. The quest for serviceable concrete structures must involve the development of more reliable design procedures. It must also involve designers giving more attention to the specification of an
17、appropriate concrete mix, particularly with regard to the creep and shrinkage characteristics of the mix, and sound engineering input is required in the construction procedures. High performance concrete structures require the specification of high performance concrete (not necessarily high strength
18、 concrete, but concrete with relatively low shrinkage, not prone to plastic shrinkage cracking) and a high standard of construction, involving suitably long stripping times, adequate propping, effective curing procedures and rigorous on-site supervision. This paper addresses some of these prob
19、lems, particularly those related to designing for the effects of shrinkage. It outlines how shrinkage affects the in-service behaviour of structures and what to do about it in design. It also provides an overview of the considerations currently being made by the working group established by Standard
20、s Australia to revise the serviceability provisions of AS3600-1994 1, particularly those clauses related to shrinkage. 2. Designing for Serviceability When designing for serviceability, the designer must ensure that the structure can perform its intended function under the day to day service l
21、oads. Deflection must not be excessive, cracks must be adequately controlled and no portion of the structure should suffer excessive vibration. Shrinkage causes time-dependent cracking, thereby reducing the stiffness of a concrete structure, and is therefore a detrimental factor in all aspects
22、 of the design for serviceability. Deflection problems that may affect the serviceability of concrete structures can be classified into three main types: (a) Where excessive deflection causes either aesthetic or functional problems. (b) Where excessive deflection results in damage to either st
23、ructural or non-structural element attached to the member. (c) Where dynamics effects due to insufficient stiffness cause discomfort to occupants. 3. Effects of Shrinkage If concrete members were free to shrink, without restraint, shrinkage of concrete would not be a major concern to structur
24、al engineers. However, this is not the case. The contraction of a concrete member is often restrained by its supports or by the adjacent structure. Bonded reinforcement also restrains shrinkage. Each of these forms of restraint involve the imposition of a gradually increasing tensile force on the co
25、ncrete which may lead to time-dependent cracking (in previously uncracked regions), increases in deflection and a widening of existing cracks. Restraint to shrinkage is probably the most common cause of unsightly cracking in concrete structures. In many cases, these problems Electronic Journal of St
26、ructural Engineering, 1 ( 2001) 17 arise because shrinkage has not been adequately considered by the structural designer and the effects of shrinkage are not adequately modelled in the design procedures specified in codes of practice for crack control and deflection calculation. T
27、he advent of shrinkage cracking depends on the degree of restraint to shrinkage, the extensibility and strength of the concrete in tension, tensile creep and the load induced tension existing in the member. Cracking can only be avoided if the gradually increasing tensile stress induced by shrinkage,
28、 and reduced by creep, is at all times less than the tensile strength of the concrete. Although the tensile strength of concrete increases with time, so too does the elastic modulus and, therefore, so too does the tensile stress induced by shrinkage. Furthermore, the relief offered by creep decrease
29、s with age. The existence of load induced tension in uncracked regions accelerates the formation of time-dependent cracking. In many cases, therefore, shrinkage cracking is inevitable. The control of such cracking requires two important steps. First, the shrinkage-induced tension and the regions whe
30、re shrinkage cracks are likely to develop must be recognised by the structural designer. Second, an adequate quantity and distribution of anchored reinforcement must be included in these regions to ensure that the cracks remain fine and the structure remains serviceable. 3.1 What is Shri
31、nkage? Shrinkage of concrete is the time-dependent strain measured in an unloaded and unrestrained specimen at constant temperature. It is important from the outset to distinguish between plastic shrinkage, chemical shrinkage and drying shrinkage. Some high strength concretes are prone to plas
32、tic shrinkage, which occurs in the wet concrete, and may result in significant cracking during the setting process. This cracking occurs due to capillary tension in the pore water. Since the bond between the plastic concrete and the reinforcement has not yet developed, the steel is ineffective in co
33、ntrolling such cracks. This problem may be severe in the case of low water content, silica fume concrete and the use of such concrete in elements such as slabs with large exposed surfaces is not recommended. Drying shrinkage is the reduction in volume caused principally by the loss of water du
34、ring the drying process. Chemical (or endogenous) shrinkage results from various chemical reactions within the cement paste and includes hydration shrinkage, which is related to the degree of hydration of the binder in a sealed specimen. Concrete shrinkage strain, which is usually considered t
35、o be the sum of the drying and chemical shrinkage components, continues to increase with time at a decreasing rate. Shrinkage is assumed to approach a final value, *sc , as time approaches infinity and is dependent on all the factors which affect the drying of concrete, including the relative humidi
36、ty and temperature, the mix characteristics (in particular, the type and quantity of the binder, the water content and water-to-cement ratio, the ratio of fine to coarse aggregate, and the type of aggregate), and the size and shape of the member. Drying shrinkage in high strength concrete is s
37、maller than in normal strength concrete due to the smaller quantities of free water after hydration. However, endogenous shrinkage is significantly higher. For normal strength concrete ( 50cf MPa), AS3600 suggests that the design shrinkage (which includes both drying and endogenous shrinkage) at any time after the commencement of drying may be estimated from bcscs k .1 (1)
