收缩开裂混凝土.doc

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1、Shrinkage, Cracking and Deflection -the Serviceability of Concrete Structures 收缩，开裂和变形 混凝土结构使用的可靠性 RI Gilbert 里吉尔伯特 Professor and Head, School of Civil and Environmental Engineering土木及环境工程学校校长兼教授The University of New South Wales, Sydney, NSW, 2052 新南威尔士大学，悉尼，新南威尔士州 2052 号Email: i.gilbertunsw.edu.au

2、电子邮件： i.gilbert unsw.edu.au ABSTRACT摘要 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 des

3、ign. 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 and sp

4、acing 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 by Standards A

5、ustralia to revise the serviceability provisions of AS3600-1994, particularly those clauses related to shrinkage. 本文讨论收缩对混凝土结构可靠性的影响，它概述了为什么收缩是重要的，它的主要影响，即对结构最终的开裂程度和挠度大小的影响，以及在设计中应该注意什么？有一种模型可以预测在普通混凝土、高强度混凝土、不稳定性普通混凝土和钢筋混凝土中的收缩应变，无论有没有外部约束的情况下，都可以用这种模型来解释。描述解析过程是为了估算弯曲裂缝及直接拉伸裂缝的最终宽度和间隔，并呈现出一个简化的过程

6、，它包括影当计算长期收缩时的影响。该文件还包含一个概要的注意事项，目前正在由澳大利亚国家标准工作组制定，以校正混凝土使用可靠性条款AS3600-1994尤其是那些涉及到收缩的条款。KEYWORDS关键词 Creep; Cracking; Deflection; Reinforced concrete; Serviceability; Shrinkage.徐变;开裂;变形;钢筋混凝土;可靠性;收缩。 1.1 . Introduction引言 For a concrete structure to be serviceable, cracking must be controlled and de

7、flections must not be excessive. 为了使混凝土结构更加耐用，开裂要在控制范围内同时变形量绝不能超过限度It must also not vibrate excessively.，振动也必须不过度。有效荷载作用下混凝土结构中，混凝土的收缩在许多方面都起着重要的作用。Concrete shrinkage plays a major role in each of these aspects of the service load behaviour of concrete structures.The design for serviceability is pos

8、sibility the most difficult and least well understood aspect of the design of concrete structures.混凝土可靠性的设计可能是混凝土结构各个方面中最困难的和最不好理解的。 Service load behaviour depends primarily on the properties of the concrete and these are often not known reliably at the design stage.有效荷载的特点，主要取决于混凝土的性能，在设计阶段这些性能往往是不

9、确定的。 Moreover, concrete behaves in a non-linear and inelastic manner at service loads.此外，在有效荷载作用下，混凝土具体表现为一个非线形和非弹性的方式。The non-linear behaviour that complicates serviceability calculations is due to cracking, tension stiffening, creep, and shrinkage.这种非线性性能使混凝土的可靠性计算复杂化，主要是由于裂缝、拉伸硬化、徐变和收缩。 Of these,

10、 shrinkage is the most problematic其中，收缩是主要问题。约束收缩会导致时效性开裂运输，并逐步降低拉伸硬化的有利影响。 在受弯构件中It results in a gradual widening of existing cracks and, in flexural members, a significant increase in deflections with time.它将导致已有的裂缝逐渐加宽并使变形量随时间大量增加。The control of cracking in a reinforced or prestressed concrete st

11、ructure is usually achieved by limiting the stress increment in the bonded reinforcement to some appropriately low value and ensuring that the bonded reinforcement is suitably distributed. 控制钢筋或预应力混凝土结构中的开裂通常是通过限制连接钢筋中的应力相适应地以一个低位值增长，并确保连接钢筋对称分布。 在Many codes of practice specify maximum steel stress

12、increments after cracking and maximum spacing requirements for the bonded reinforcement.许多实践规范中，明确规定了在开裂和连接钢筋达到允许的最大间距之后钢筋的最大应力。 However, few existing code procedures, if any, account adequately for the gradual increase in existing crack widths with time, due primarily to shrinkage, or the time-depe

13、ndent development of new cracks resulting from tensile stresses caused by restraint to shrinkag但是，现有的规范的规程，如果有的话，足以充分解释现有的裂缝的宽度随时间的增加而增长，主要归因于收缩，或新的裂缝随时间的发展源于限制收缩导致的拉应力。 For deflection control, the structural designer should select maximum deflection limits that are appropriate to the structure and

14、its intended use. 为了控制挠度，结构设计者应选择“最大挠度限制值”以适应结构和符合预期的使用，计算的挠度（或弯曲度）不能超过这些限制。现行的规范给出的最大挠度的选择和挠度的计算是比较笼统的。Codes of practice give general guidance for both the selection of the maximum deflection limits and the calculation of deflection.但是，在大多规范中，简化的挠度计算过程是从钢筋混凝土简支梁的试验中获得的。当遇到更为复杂的结构时，这些计算过程往往给出粗糙的不准确的

15、预测。再者，就是现存的规范规程不能给出有用的关于如何充分塑造在挠度计算时徐变和收缩的时效影响的方法。Serviceability failures of concrete structures involving excessive cracking and/or excessive deflection are relatively common. 可靠性失效的混凝土结构，包含过多开裂和/或过度弯曲是较为常见的。 Numerous cases have been reported, in Australia and elsewhere, of structures that complied

16、 with code requirements but still deflected or cracked excessively. 无数的案例报告证实，在澳大利亚和其他地方，结构符合规范要求，但仍然出现过度弯曲和开裂。 In a large majority of these failures, shrinkage of concrete is primarily responsible. Clearly, the serviceability provisions embodied in our codes do not adequately model the in-service b

17、ehaviour of structures and, in particular, fail to account adequately for shrinkage. 在大部分的这些破坏中，混凝土收缩是负主要责任。显然，这些包含于我们规范中的可靠性条文不能恰当的建立正在使用的结构的性能模型， 特别是难于恰当解释混凝土的收缩。 The quest for serviceable concrete structures must involve the development of more reliable design procedures. 追求可靠的混凝土结构必须涉及更可靠的设计过程的形

18、成和发展。 It must also involve designers giving more attention to the specification of an 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.它也必须涉及到设计者更多地重视适当的混凝土配合比的规定，特别是对于徐变和收

19、缩特征的组合，同时也要求在建设过程中有合理的工程的投入。 High performance concrete structures require the specification of high performance concrete (not necessarily high strength concrete, but concrete with relatively low shrinkage, not prone to plastic shrinkage cracking) and a high standard of construction, involving suitab

20、ly long stripping times, adequate propping, effective curing procedures and rigorous on-site supervision. 高性能混凝土结构要求规格的高性能混凝土（不一定是高强度混凝土，但混凝土的相对收缩率低，不容易产生塑性收缩裂缝）和高标准的建设，相称地涉及较长的脱模时间，足够的支撑和严格的现场监督。 This paper addresses some of these problems, particularly those related to designing for the effects of

21、 shrinkage. 本文讨论这些问题中的一些，特别是那些影响混凝土收缩的设计。 It outlines how shrinkage affects the in-service behaviour of structures and what to do about it in design.它概述了收缩如何影响现役结构，和在设计中应该做什么，It also provides an overview of the considerations currently being made by the working group established by Standards Austral

22、ia to revise the serviceability provisions of AS3600-1994 1 , particularly those clauses related to shrinkage.它还提供了一个概述的注意事项，一个目前正在由澳大利亚国家标准工作组确定的标准，澳大利亚国家标准修改可靠性规定AS3600 - 1994 1 ，特别是那些有关收缩的条文。 2.2 .可靠性的设计 Designing for Serviceability可靠性可 When designing for serviceability, the designer must ensure

23、that the structure can perform its intended function under the day to day service loads. Deflection must not be excessive, cracks must be adequately controlled and no portion of the structure should suffer excessive vibration. 当设计可靠性时，设计者必须确保整个结构，在日常荷载的一天天作用下能够完成它预期的功能。弯曲绝不能过量，裂缝必须得到足够的控制以及没有任何部分的结构

24、应受到过度振动。收缩导致时效性的收缩收缩开裂，从而降低了混凝土结构的刚度，并因此在可靠性设计的各个方面产生不利因素。 Deflection problems that may affect the serviceability of concrete structures can be classified into three main types:弯曲变形在混凝土可靠性设计方面的影响可以被归纳为三个主要的类型： (a) Where excessive deflection causes either aesthetic or functional problems. （一） 过度弯曲导致审

25、美或功能上的问题。 (b) Where excessive deflection results in damage to either structural or non-structural element attached to the member. （二） 过度弯曲导致结构或与构件相联系的非结构性元素的损害。 (c) Where dynamics effects due to insufficient stiffness cause discomfort to occupants. （三） 力学上的效果，由于刚度不足导致居住者的不适。 3.3 . 收缩的Effects of Shri

26、nkage影响If concrete members were free to shrink, without restraint, shrinkage of concrete would not be a major concern to structural engineers.如果混凝土构件可以自由地收缩，而不接受约束，那么混凝土收缩就不会是结构工程师主要关心的，However, this is not the case.然而，事实并非如此。混凝土构件的收缩通常受到它的支点或其相邻结构的约束The contraction of a concrete member is often res

27、trained by its supports or by the adjacent structur混凝土，与混凝土相连接的钢筋也限制了它的收缩Bonded reinforcement also restrains shrinkage.与，Each of these forms of restraint involve the imposition of a gradually increasing tensile force on the concrete which may lead to time-dependent cracking (in previously uncracked

28、regions), increases in deflection and a widening of existing cracks.每个这些约束的形式都涉及一个强加于混凝土的逐步增加的拉伸力，可能导致混凝土时效性开裂（在先前无开裂区域）挠度的增加以及开裂缝的加宽，对收缩的约束可能是导致混凝土中难看的裂缝的最普通的原因 Restraint to shrinkage is probably the most common cause of unsightly cracking in concrete structure对，In many cases, these problems arise

29、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.在许多情况下，出现这些问题是因为收缩并没有得到结构设计者的充分的考虑，以及在按照现行规范制定的开裂控制和变形计算过程下设计时

30、，收缩的影响得不到到充分的蓝本。The 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.收缩裂缝的出现取决于对收缩的约束程度、拉伸时混凝土的强度和延展性以及拉伸徐变和存在于构件中的荷载导致的拉力。Cracking can only be av

31、oided if the gradually increasing tensile stress induced by shrinkage, and reduced by creep, is at all times less than the tensile strength of the concrete.如果逐渐增大的由收缩引起的且随徐变增长而减少的拉应力始终低于混凝土的抗拉强度，那么裂缝只能回避。Although the tensile strength of concrete increases with time, so too does the elastic modulus a

32、nd, therefore, so too does the tensile stress induced by shrinkage.尽管混凝土抗拉强度的增加随时间增加，弹性模量和收缩引起的拉应力也随时间增长而增加。Furthermore, the relief offered by creep decreases with age.此外，由徐变导致的拉应力的减少与时间成反比。在未开裂区由荷载导致的拉力的存在The existence of load induced tension in uncracked regions accelerates the formation of time-d

33、ependent cracking.在加快了时效性开裂的形成，因此，In many cases, therefore, shrinkage cracking is inevitable.在许多情况下，收缩裂缝是不可避免的。The control of such cracking requires two important steps.控制这种裂缝需要两个重要步骤。First, the shrinkage-induced tension and the regions where shrinkage cracks are likely to develop must be recognised

34、 by the structural designer.第一，收缩所致的拉力和所在区域可能形成的收缩裂纹必须被结构设计师所识别。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.第二，在这些区域内必须有足够数量和配置的锚固钢筋，以确保该裂缝保持良好和结构依然可靠。 3.1 What i

35、s Shrinkage3.1什么是收缩？ Shrinkage of concrete is the time-dependent strain measured in an unloaded and unrestrained specimen at constant temperature. 混凝土收缩是在恒温下未受荷载且无应变的试件上测量出的时效性应变。It is important from the outset to distinguish between plastic shrinkage, chemical shrinkage and drying shrinkage.从一开始区分塑

36、性收缩、化学收缩和干燥收缩是很重要的。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.一些高强度混凝土容易于产生塑性收缩，多发于潮湿的混凝土，并有可能在混凝土放置的过程中产生相当多的裂缝。This cracking occurs due to capillary tension in the pore water.这些裂缝的

37、产生由于孔隙水中毛细张力。 Since the bond between the plastic concrete and the reinforcement has not yet developed, the steel is ineffective in controlling such cracks.因为塑性混凝土和钢筋中的粘结还没有形成，钢筋在控制这种裂缝时是无效的。This problem may be severe in the case of low water content, silica fume concrete and the use of such concrete

38、 in elements such as slabs with large exposed surfaces is not recommended.在混凝土含水量低的情况下，这个问题可能会很严重，所以硅粉混凝土和使用这种混凝土所建的有着大的暴露面的板的构件是不被推荐的。 Drying shrinkage is the reduction in volume caused principally by the loss of water during the drying process. 干缩是在干燥过程中主要由于水的损失而导致的体积的减小，Chemical (or endogenous) s

39、hrinkage 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 to be the sum of the drying and chemical shrinkage compone

40、nts, continues to increase with time at a decreasing rate.化学性（或内源性 ）收缩是水泥沙浆中的各种化学反应包括水化收缩所导致的，这种水化收缩是与密封试件中粘结剂水化的程度有关的。混凝土收缩应变，通常指的是干燥和化学收缩组件的总和，继续随时间以一个递减的速率增长，Shrinkage is assumed to approach a final value,收缩被假定接近一个最终值，, as time approaches infinity and is dependent on all the factors which affect

41、the drying of concrete, including the relative humidity 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随着

42、时间增长无限接近最终值同时取决于影响混凝土干燥的所有因素，包括相对湿度和温度，综合特性（尤其是粘结剂的种类和数量、含水量和水灰比、骨料的粗细比以及骨料的类型），以及构件的大小和形状。Drying shrinkage in high strength concrete is smaller than in normal strength concrete due to the smaller quantities of free water after hydrati干缩在高强度的混凝土中小于正常强度的混凝土，这是由于水合以后自由水的数量少的缘故。However, endogenous shri

43、nkage is significantly higher.但是，内源性收缩明显要高于后者。 For normal strength concrete ( 对于普通强度的混凝土（ 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 MPA ），AS3600建议在混凝土干燥以后的任何时间的设计干缩（包括干燥收缩和内

44、源性收缩）可由公式（ 1 ）估计。从 （1）wh 其中 is a basic shrinkage strain which, in the absence of measurements, may be taken to be 850 x 10 -6 (note that this value was increased from 700 x 10 -6 in the recent Amendment 2 of the Standard); k 1 is obtained by interpolation from Figure 6.1.7.2 in the Standard and depe

45、nds on the time since the commencement of drying, the environment and the concrete surface area to volume ratio. 是一项基本收缩应变，在无法测量的情况下，可取850 10 -6 （注：在最近标准的修订2里，此值是从700 10 -6 增加而来的）; k 1是由在标准里的图6.1.7.2按照插值法得到的且取决于干燥开始的时间，环境和混凝土表面积及容积率。 A hypothetical thickness, t h = 2A 2A / u e , is used to take this

46、 into account, where A is the cross-sectional area of the member and u e is that portion of the section perimeter exposed to the atmosphere plus half the total perimeter of any voids contained within the section. 一个假设的厚度，th = 2A/ ue,，应该习惯于充分考虑，其中A是构件的横截面面积，ue是暴露在大气中的那份横断面周长加上横断面内任何空隙率的总周长的一半。 AS3600

47、 states that the actual shrinkage strain may be within a range of plus or minus 40% of the value predicted (increased from 30% in Amendment 2 to AS3600-1994). In the writers opinion, this range is still optimistically narrow, particularly when one considers the size of the country and the wide varia

48、tion in shrinkage measured in concretes from the various geographical locatio AS3600在预测值的正负40 的范围陈述实际的收缩应变，可（从AS3600 1994修订2里的 30 增加而来），在笔者看来，这个范围仍然是有些乐观地狭隘，尤其是当人们考虑到该国的国土面积和不同的地理位置测量混凝土收缩的差异性。Equation 1 does not include any of the effects related to the composition and quality of the concrete.方程（1）

49、不包括与混凝土的成分与品质相关的任何影响，相同的ecs 值被预测与The same value of e cs is predicted irrespective of the concrete strength, the water-cement ratio, the aggregate type and quantity, the type of admixtures, etc. In addition, the factor k 1 tends to overestimate the effect of member size and significantly underestimate the rate of shrinkage development at early ag相同的混凝土强度，水灰比，骨料的种类和数量，外加剂的类型等无关，而且，因子k1倾向于高估了构件尺寸的影响而明显低估了收缩形成的早期阶段的影响。 T