维基百科

混凝土的潜变与收缩

混凝土的潜变与收缩是混凝土的两种物理性质，是应用混凝土时需要考量的变因。混凝土如果长时间受应力，便会缓慢变形，移除持续施予的力之后，变形只有部分会恢复，可以恢复的变形称为弹性变形，不能恢复、永久的变形称为潜变[1]。而在没有受力的情况下，混凝土水含量变多或变少也会使混凝土的体积变化，称为膨胀与收缩。

机制 编辑

潜变 编辑

混凝土的潜变为其中水泥的矽酸钙水合物（C-S-H）在持续受应力下缓慢流动引起。

水泥浆浆体由固体的水泥凝胶（cement gel）组成，水泥凝胶又由矽酸钙水合物形成的薄片以及薄片之间大量的空隙（大约占凝胶40%至55%的体积）组成。[2] 这些空隙中含有会蒸发的层间水，水会对孔壁施加分离压力（disjoining pressure），并削弱薄片与薄片之间的结合力。[3] 混凝土受力造成薄片之间滑动可能就是潜变的原因。

同样的应力下，潜变（永久变形）通常是弹力变形（去除载荷后可恢复的变形）的三倍。[1]

收缩 编辑

混凝土中的水泥硬化过程会产生收缩。水泥中的矽酸钙水合物薄片间的水分减少之后，薄片与薄片之间的间隔因为缺乏水分的流体静拉力（hydrostatic tension）支撑而缩小，导致混凝土体积变小。 混凝土的收缩依水分减少的方式或发生收缩的部位不同可以分为：

  • 干燥收缩（Drying Shrinkage）：水分蒸发到混凝土外部导致总体的体积收缩。[4]只要将干燥收缩的混凝土泡水一段时间就可以回复部分体积[5]。干燥收缩的机制与潜变关系密切[6]
  • 塑性收缩（Plastic Shrinkage）：混凝土表面与内部蒸发速率不一致[7]，导致收缩的速率也不一致，造成开裂的现象，称为塑性收缩。
  • 自体收缩（Autogenous Shrinkage）：水分被混凝土中的水泥吸收，导致孔隙中的水分减少而收缩。水灰比过低的混凝土特别容易因自收缩而开裂。[8]

并非因为水分变化造成体积变小的收缩：

变因 编辑

如果在混凝土中混入足够多的骨料的话，可以抑制水泥凝胶的潜变和收缩，因为骨料的蠕变较不明显。[10] 孔隙水含量小或孔隙湿度低时的潜变较小[11]，而完全干燥的混凝土不会潜变。

混凝土加载（受应力）时，材料年龄越大，潜变越小[12]，这是因为混凝土会随著时间越来越干燥的关系。大部分的应变都发生在受力前期，大约有四分之一到三分之一的潜变发生在第一个月，并且大约一半到四分之三的总潜变会发生在持续载荷的前半年。[1]

理论上温度越高，潜变的程度也要越大，但实务上升温增加的潜变会与混凝土失去水份减少的潜变互相抵消。升温会加速层间水与其他物质发生水化，使混凝土孔隙中的水分减少。没有水分施加的分离压力，矽酸钙薄片与薄片之间就难以滑动，使增加的潜变抵消。[13]

另外，干燥过程的水分变化越剧烈，从湿润到完全干燥中间的潜变总量也会越多。较大试体干燥较缓慢，因水分含量变化引起的潜变较小，所以潜变总量会较少，收缩总量也较少。[14]

本构方程 编辑

混凝土的本构方程中可以出现的变量有温度、材料年龄、水化程度、孔隙湿度（与环境湿度有关）和孔隙水含量。这些变量被称为状态变量（state variables），是可以用来描述材料中任何一点的点属性的变量。[15] 试体尺寸虽然也会影响潜变，但不列在本构方程中。环境湿度是影响孔隙湿度的条件之一，因为本构方程中已经包含孔隙湿度了，所以环境湿度就不会出现在本构方程中。[16]

应用 编辑

参考资料 编辑

  1. ^ 1.0 1.1 1.2 Gambali.Ajay Kumar; Shanagam.Naveen Kumar. Creep of Concrete (PDF). International Journal of Engineering Development and Research. 2014, 2 (4): P.3800 [2021-08-07]. （原始内容存档 (PDF)于2015-04-13）. 
  2. ^ Z. P. BaZant、F. H. Wittmann (编). Creep and Shrinkage in Concrete Structures (PDF). 约翰威立. 1982: P.164~165 [2021-08-04]. ISBN 0 471 10409 4. （原始内容存档 (PDF)于2021-08-04） （英语）. The paste consists of solid cement gel and contains numerous capillary pores. The cement gel contains about 40 to 55% of pores in volume, has an enormous pore surface area (roughly 500 m2/cm3), and is made up of sheets of colloidal dimensions (of average thickness about 30 A, with average gaps about 15 A between the sheets). The sheets are formed mostly of calcium silicate hydrates and are strongly hydrophylic. 
  3. ^ Z. P. BaZant、F. H. Wittmann (编). Creep and Shrinkage in Concrete Structures (PDF). 约翰威立. 1982: P.164~165 [2021-08-04]. ISBN 0 471 10409 4. （原始内容存档 (PDF)于2021-08-04） （英语）. Because the pores of cement gel are micropores of subcapillary dimensions they cannot contain liquid water or vapour; but they do contain evaporable water (water that is not chemically bound in the hydrates), which is strongly held by solid surfaces and may be regarded as (hindered) absorbed water or interlayer water. This water can exert on the pore walls a significant pressure called the disjoining pressure the value of which depends on temperature and the degree of water saturation of capillary pores. 
  4. ^ Neenu S.K. Types of Shrinkages in Concrete and its Preventio. THE CONSTRUCTOR. [2021-08-10]. （原始内容存档于2021-08-10）. Drying shrinkage is caused by the loss of surface -absorbed water from the calcium silicate hydrate ( C-S-H) gel and also due to the loss of hydrostatic tension in the small pores. 
  5. ^ Neenu S.K. Types of Shrinkages in Concrete and its Preventio. THE CONSTRUCTOR. [2021-08-10]. （原始内容存档于2021-08-10）. A part of this shrinkage caused can be recovered by immersing the concrete in water for a specified time. This is termed as the moisture movement. 
  6. ^ Neenu S.K. Types of Shrinkages in Concrete and its Preventio. THE CONSTRUCTOR. [2021-08-10]. （原始内容存档于2021-08-10）. This shrinkage is mainly due to the deformation of the paste, though the aggregate stiffness also influences it. 
  7. ^ Ravindra K.DhirOBE, Jorgede Brito, Rui V.Silva, Chao Qun Lye. Sustainable Construction Materials. 爱思唯尔. 2019: 9.4.2 Plastic Shrinkage [2021-08-10]. ISBN 978-0-08-100985-7. （原始内容存档于2021-08-10） （英语）. Plastic shrinkage occurs in a freshly mixed concrete, with loss of water by evaporation from its surface, after placing and before hardening of the concrete. This can lead to plastic shrinkage cracking if the rate of evaporation is higher than that of the bleeding water rising to the surface of the concrete. 
  8. ^ Shen Jianxi. Comprehensive Renewable Energy. 爱思唯尔. 2012: 6.14.2.3.2 Autogenous shrinkage [2021-08-14]. ISBN 978-0-08-087873-7. （原始内容存档于2021-10-29） （英语）. Autogenous shrinkage is the uniform reduction of internal moisture due to cement hydration, which is typical of high-strength concrete. Autogenous shrinkage contributes significantly to concrete cracking when the water–cement (w/c) ratio is less than 0.4 . The use of concrete with a somewhat higher w/c ratio can mitigate this problem. However, the strength and impermeability of concrete will be decreased if the w/c ratio is increased. 
  9. ^ Yves F. Houst. Carbonation Shrinkage of Hydrated Cement Paste. CANMET/ACI International Conference on Durability of Concrete. 1997,. supplementary papers (Fourth): pp.481–491. [2021-08-15]. （原始内容存档于2021-08-15）. 
  10. ^ Z. P. BaZant、F. H. Wittmann (编). Creep and Shrinkage in Concrete Structures (PDF). 约翰威立. 1982: P.164~165 [2021-08-04]. ISBN 0 471 10409 4. （原始内容存档 (PDF)于2021-08-04） （英语）. The main intrinsic factors are the design strength, the elastic modulus of aggregate, the fraction of aggregate in the concrete mix, and the maximum aggregate size. Increase of any of these factors causes a decrease of creep as well as shrinkage. This is because the aggregate does not creep appreciably and has a restraining effect on creep and shrinkage. 
  11. ^ Z. P. BaZant、F. H. Wittmann (编). Creep and Shrinkage in Concrete Structures (PDF). 约翰威立. 1982: P.169 [2021-08-04]. ISBN 0 471 10409 4. （原始内容存档 (PDF)于2021-08-04） （英语）. Creep at constant pore water content is less for a smaller pore water content or a lower humidity in the pores. 
  12. ^ Z. P. BaZant、F. H. Wittmann (编). Creep and Shrinkage in Concrete Structures (PDF). 约翰威立. 1982: P.168 [2021-08-04]. ISBN 0 471 10409 4. （原始内容存档 (PDF)于2021-08-04） （英语）. Creep decreases as the age of concrete at the instant of loading increases (this is actually the effect of the increase in the degree of hydration). 
  13. ^ Z. P. BaZant、F. H. Wittmann (编). Creep and Shrinkage in Concrete Structures (PDF). 约翰威立. 1982: P.169 [2021-08-04]. ISBN 0 471 10409 4. （原始内容存档 (PDF)于2021-08-04） （英语）. Creep also increases with increacing temperature, but this effect is offset by the fact that a temperature increase also accelerates hydration which in turn reduces creep. 
  14. ^ Z. P. BaZant、F. H. Wittmann (编). Creep and Shrinkage in Concrete Structures (PDF). 约翰威立. 1982: P.169 [2021-08-04]. ISBN 0 471 10409 4. （原始内容存档 (PDF)于2021-08-04） （英语）. Another important non-local extensive factor which is not a state variable is the size of specimen or structural member. The drying process in a larger specimen is slower, and consequently the creep increase due to drying is less, i.e. creep is less for a larger specimen. Similarly, shrinkage is less for a larger specimen and it is also less for a higher environmental humidity. 
  15. ^ Z. P. BaZant、F. H. Wittmann (编). Creep and Shrinkage in Concrete Structures (PDF). 约翰威立. 1982: P.168 [2021-08-04]. ISBN 0 471 10409 4. （原始内容存档 (PDF)于2021-08-04） （英语）. Among the extensive factors we must distinguish the local from the external ones. The former, also called the state variables, are those which can be treated as a point property of a continuum. They are the only ones which can legitimately appear in a constitutive equation. Temperature, age,degree of hydration, relative vapour pressure (humidity) in the pores, and pore water content represent state variables affecting creep. 
  16. ^ Z. P. BaZant、F. H. Wittmann (编). Creep and Shrinkage in Concrete Structures (PDF). 约翰威立. 1982: P.168 [2021-08-04]. ISBN 0 471 10409 4. （原始内容存档 (PDF)于2021-08-04） （英语）. On the other hand, the size of specimen and the environmental humidity are not admissible as state variables in a constitutive equation even though they have a great effect on creep of a concrete specimen. Properly, the environmental humidity must be considered as the boundary condition for the partial differential equation governing pore humidity. It is the pore humidity, not the environmental one, which directly affects creep and can appear in the constitutive equation. 
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