外文翻譯--混凝土結(jié)構(gòu)使用的可靠性

1、Electronic Journal of Structural Engineering, 1 ( 2001) 15Shrinkage, Cracking and Deflection-the Serviceability of Concrete StructuresR.I. GilbertProfessor and Head, School of Civil and Environmental Engineering The Uni

2、versity of New South Wales, Sydney, NSW, 2052Email: i.gilbert@unsw.edu.auABSTRACTThis paper addresses the effects of shrinkage on the serviceability of concrete structures. It outlines why shrinkage is important, its m

3、ajor influence on the final extent of cracking and the magnitude of deflection in structures, and what to do about it in design. A model is presented for predicting the shrinkage strain in normal and high strength con

4、crete 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 and spacing of b

5、oth 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 considerat

6、ions currently being made by the working group established by Standards Australia to revise the serviceability provisions of AS3600-1994, particularly those clauses related to shrinkage.KEYWORDS Creep; Cracking; Deflect

7、ion; Reinforced concrete; Serviceability; Shrinkage.1. IntroductionFor a concrete structure to be serviceable, cracking must be controlled and deflections must not be excessive. It must also not vibrate excessively. Con

8、crete 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 understood aspect of the d

9、esign 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 inelas

10、tic manner at service loads. The non-linear behaviour that complicates serviceability calculations is due to cracking, tension stiffening, creep, and shrinkage. Of these, shrinkage is the most problematic. Restraint to

11、 shrinkage causes time-dependent cracking and gradually reduces the beneficial effects of tension stiffening. It results in a gradual widening of existing cracks and, in flexural members, a significant increase in defl

12、ections 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 ensuring

13、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

14、code procedures, if any, account adequately for the gradual increase in existing crack widths with time, due primarily to shrinkage, or the time-dependent development of new cracks resulting from tensile stresses cause

15、d by restraint to shrinkage.For deflection control, the structural designer should select maximum deflection limits that are appropriate to the structure and its intended use. The calculated deflection (or camber) must

16、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 e eJSE JSE

17、JSEInternationalElectronic Journal of Structural Engineering, 1 ( 2001) 17arise because shrinkage has not been adequately considered by the structural designer and the effects of shrinkage are not adequately modelled i

18、n the design procedures specified in codes of practice for crack control and deflection calculation.The advent of shrinkage cracking depends on the degree of restraint to shrinkage, the extensibility and strength of th

19、e 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, and reduced by creep, is at all ti

20、mes 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. Fu

21、rthermore, the relief offered by creep decreases with age. The existence of load induced tension in uncracked regions accelerates the formation of time- dependent cracking. In many cases, therefore, shrinkage cracking i

22、s inevitable. The control of such cracking requires two important steps. First, the shrinkage-induced tension and the regions where shrinkage cracks are likely to develop must be recognised by the structural designer.

23、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 Shrinkage? Shrinkage of

24、 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

25、shrinkage. Some high strength concretes are prone to plastic shrinkage, which occurs in the wet concrete, and may result in significant cracking during the setting process. This cracking occurs due to capillary tension

26、 in the pore water. Since the bond between the plastic concrete and the reinforcement has not yet developed, the steel is ineffective in controlling such cracks. This problem may be severe in the case of low water cont

27、ent, 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 during

28、 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

29、sealed specimen. Concrete shrinkage strain, which is usually considered to be the sum of the drying and chemical shrinkage components, continues to increase with time at a decreasing rate. Shrinkage is assumed to appro

30、ach a final value, , as time approaches infinity and is dependent on * sc ?all the factors which affect the drying of concrete, including the relative humidity and temperature, the mix characteristics (in particular,

31、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 strengt

32、h concrete is smaller 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 ( MPa), AS3600 sugg