应力作用下水泥基材料碳化和渗透特性研究
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摘要
混凝土的耐久性一直是水泥混凝土科学界和工程界最为关注的问题之一，近年来其研究已经从单一劣化因素转向多种劣化因素对混凝土耐久性的影响。由于实际工程中的混凝土结构均承受不同类型的荷载和带有不同宽度的微裂缝，故本文考虑选取外荷载引起的压应力对普通混凝土渗透性的影响，弯曲应力对砂浆、净浆碳化深度的的影响以及在带有裂缝状态下水泥基材料的渗透和自愈现象开展了部分实验研究，同时通过对混凝土、砂浆及净浆系列水泥基试件的碳化和渗透实验比较，从微观角度对碳化和渗透、自愈现象的一些机理也做了初步研究。
通过水的渗透试验可以观察到有外部压应力的存在会使混凝土的渗透性有明显改变。在应力比小于0.6的情况下，混凝土的渗透系数会随着应力比的增加而减小，减小的规律可用式K=K_0·e-~(aη)表示。当应力比大于0.6以后，混凝土的渗透系数会明显增加，但增加幅度未超过一个数量级。混凝土的渗透稳定性则表现为渗透系数会随渗透时间增加而减小，减小的趋势是起初的40～60小时内下降最快，其后愈来愈平缓，其规律可用公式K=(a+ct)／(1+bt)来表示。
带有裂缝试件水的渗透试验表明：单位时间通过裂缝的渗透水量会随着渗透时间增加而减小，在渗透实验开始的三周内减小幅度最大，其后趋缓。水力梯度、裂缝宽度、渗流时间、试件品种均对自愈程度有比较大的影响。对试件裂缝中沉积的白色物质做SEM、XRD分析得出其成份主要为CaCO_3，CaCO_3堵塞裂缝应该是引起自愈现象的主要原因；CaCO_3的生成包括表面反应和扩散反应两个阶段，这在一定程度上解释了自愈现象先快后慢和自愈幅度砂浆＞混凝土＞净浆。
外部弯曲应力会加速水泥石和水泥砂浆的碳化过程，这可能与受拉区域碳化收缩受到限制有关：弯曲应力作用下水泥石和水泥砂浆的碳化动力学过程可用式x=C_0e~(an)t~b描述。根据渗透实验、孔结构分析的结果可以得出：碳化降低了水泥石和水泥砂浆总孔隙率并细化了孔径，其原因是由于碳化生成的碳酸钙在毛细孔中沉积，将大的毛细孔分割成小孔所致。因此碳化将会降低水泥石和水泥砂浆的渗透性，渗透性的降低与水灰比有关，水灰比越大其降低的幅度也越大。
The durability of concrete is one of the key problems in cement and concrete field that many scholars are concerned with . In recent years, the focus of durability research is transferred from single degradation factor to multi ones. As we know, concrete structures in reality are always under various stresses or with microcracks of different width. So in this paper, compressive stress tensile stress and microcracks are introduced into water permeability and carbonation experiment, with the object to study the relationship between those factors and durability indexes. SEM XRD and MIP methods were also used to study the relevant mechanisms.
The results from water permeability test indicated that the permeability coefficient of concrete was significantly different with or without compressive stress. The permeability coefficient(K) decrease with the increase of stress when the stress is below 60% of the ultimate compressive strength, and the decreasing trend can be expressed by the function K=Ko ean. When the stress acted on concrete specimens exceed 60% of ultimate compressive strength, the coefficient(K) will increase obviously but the largest change is still within the order of 10-9 cm/s.The time-dependant characteristic can be expressed by the function K=(a+ct)/(l+bt). The coefficient K decreased significantly in the first 40~60 hours of the water permeability test, and the decreasing rate became slow as the time goes on.
The permeability behaviour of specimens(concrete, mortar and paste) with microcrack(0.1~0.35mm width) is different from those without microcracks. Water volume flowing across microcrack per hour will reduce with the increase of permeating time and the reducing is most significant in the first three weeks. In addition, water gradients, crack width, permeability time, specimen's kinds can also influence the self-sealing degree. XRD and SEM analysis showed that the main composition of the white substance which formed in cracks is CaCO3, which can block cracks and is probably the main cause of self-sealing. Two processes which called surface reaction process and diffuse reaction process are defined in the formation process of CaCO3.The phenomenons in self-sealing experiment can be explained by those two processes.
Tensile stress will accelerate the carbonation of paste and mortar, which can be explained by the reason that shrinkage of carbonation is confined by tensile stress, and
the function x = C0eantb can describe this process. Carbonation can lower the total
porosity and lessen the aperture in the cement paste. It may be the deposition of CaCO3 in capillary that divide the large capillary into small one. Carbonation can also lower the water permeability of samples. The bigger the W/C ratio, the large the decrease rate of permeability.
通过水的渗透试验可以观察到有外部压应力的存在会使混凝土的渗透性有明显改变。在应力比小于0.6的情况下，混凝土的渗透系数会随着应力比的增加而减小，减小的规律可用式K=K_0·e-~(aη)表示。当应力比大于0.6以后，混凝土的渗透系数会明显增加，但增加幅度未超过一个数量级。混凝土的渗透稳定性则表现为渗透系数会随渗透时间增加而减小，减小的趋势是起初的40～60小时内下降最快，其后愈来愈平缓，其规律可用公式K=(a+ct)／(1+bt)来表示。
带有裂缝试件水的渗透试验表明：单位时间通过裂缝的渗透水量会随着渗透时间增加而减小，在渗透实验开始的三周内减小幅度最大，其后趋缓。水力梯度、裂缝宽度、渗流时间、试件品种均对自愈程度有比较大的影响。对试件裂缝中沉积的白色物质做SEM、XRD分析得出其成份主要为CaCO_3，CaCO_3堵塞裂缝应该是引起自愈现象的主要原因；CaCO_3的生成包括表面反应和扩散反应两个阶段，这在一定程度上解释了自愈现象先快后慢和自愈幅度砂浆＞混凝土＞净浆。
外部弯曲应力会加速水泥石和水泥砂浆的碳化过程，这可能与受拉区域碳化收缩受到限制有关：弯曲应力作用下水泥石和水泥砂浆的碳化动力学过程可用式x=C_0e~(an)t~b描述。根据渗透实验、孔结构分析的结果可以得出：碳化降低了水泥石和水泥砂浆总孔隙率并细化了孔径，其原因是由于碳化生成的碳酸钙在毛细孔中沉积，将大的毛细孔分割成小孔所致。因此碳化将会降低水泥石和水泥砂浆的渗透性，渗透性的降低与水灰比有关，水灰比越大其降低的幅度也越大。
The durability of concrete is one of the key problems in cement and concrete field that many scholars are concerned with . In recent years, the focus of durability research is transferred from single degradation factor to multi ones. As we know, concrete structures in reality are always under various stresses or with microcracks of different width. So in this paper, compressive stress tensile stress and microcracks are introduced into water permeability and carbonation experiment, with the object to study the relationship between those factors and durability indexes. SEM XRD and MIP methods were also used to study the relevant mechanisms.
The results from water permeability test indicated that the permeability coefficient of concrete was significantly different with or without compressive stress. The permeability coefficient(K) decrease with the increase of stress when the stress is below 60% of the ultimate compressive strength, and the decreasing trend can be expressed by the function K=Ko ean. When the stress acted on concrete specimens exceed 60% of ultimate compressive strength, the coefficient(K) will increase obviously but the largest change is still within the order of 10-9 cm/s.The time-dependant characteristic can be expressed by the function K=(a+ct)/(l+bt). The coefficient K decreased significantly in the first 40~60 hours of the water permeability test, and the decreasing rate became slow as the time goes on.
The permeability behaviour of specimens(concrete, mortar and paste) with microcrack(0.1~0.35mm width) is different from those without microcracks. Water volume flowing across microcrack per hour will reduce with the increase of permeating time and the reducing is most significant in the first three weeks. In addition, water gradients, crack width, permeability time, specimen's kinds can also influence the self-sealing degree. XRD and SEM analysis showed that the main composition of the white substance which formed in cracks is CaCO3, which can block cracks and is probably the main cause of self-sealing. Two processes which called surface reaction process and diffuse reaction process are defined in the formation process of CaCO3.The phenomenons in self-sealing experiment can be explained by those two processes.
Tensile stress will accelerate the carbonation of paste and mortar, which can be explained by the reason that shrinkage of carbonation is confined by tensile stress, and
the function x = C0eantb can describe this process. Carbonation can lower the total
porosity and lessen the aperture in the cement paste. It may be the deposition of CaCO3 in capillary that divide the large capillary into small one. Carbonation can also lower the water permeability of samples. The bigger the W/C ratio, the large the decrease rate of permeability.
引文
[1] 卢木，混凝土耐久性研究现状和研究方向．工业建筑，1997，27(5)：1～6
[2] 李金玉，彭小平，邓正刚等．混凝土抗冻性的定量化设计．混凝土，2000，(12)：61～65
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[4] Sommer H著，冯乃谦译．高性能混凝土的耐久性．北京，科学出版社，1998
[5] 王媛俐．姚燕主编．重点工程混凝土耐久性的研究与工程应用．北京，中国建材工业出版社，2001：413-425
[6] Takafumi Sugiyama,T.W.Bremner, and Thomas A.H, Effect of stress on gas permeability in concrete. ACI Material Journal, 1996,93(5):443-450
[7] Ludirdija,D.R.L.Berger. Simple Method for measuring water permeability of concrete. ACI Material Journal, 1989,86 (5):433-439
[8] Hani R.Samaha and Kenneth C.Hover. Influence of microcracking on the mass transport properties of concrete. ACI Material Journal, 1992,89(4): 416-424
[9] 冯乃谦 著．实用混凝土大全．北京，科学出版社，2001．
[10] Nemkumar Banthia and Sidney Mindess. Water permeability of cement paste. Cement and Concrete Research, 1989,19:727-736
[11] R.H.Mills. Mass Transfer of water vapour through concrete. Cement and Concrete Research, 1985,15:74-82
[12] Min-hong zhang and Odd E.Gjorv. Permeability of high-strength lightweight concrete. ACI Material Journal, 1991,88(5):463-469
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[14] 水工混凝土试验规程(DL/T 5150-2001)．北京，中国电力出版社，1999
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[32] 蒋利学．张誉．混凝土部分碳化区长度的分析与计算工业建筑，1999，20(1)：4～7
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[34] 黄可信．吴兴祖．钢筋混凝土结构中钢筋腐蚀与保护．中国建筑工业出版社，1983
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carbonation problem. Chemial Engineering Science, 1991,(46)
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[37] Linhua Jiang,Baoyu Lin, Yuebo Cai. A model for predicting carbonation of high-volume fly ash concrete. Cement and Concrete Research,2000,30:699-702
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[52] 方憬，梅国兴，陆采荣．碳化对混凝土性能影响的研究．水利水电技术，1996(2)
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[55] 易成，谢和平，孙华飞，高伟．混凝士抗渗性研究的现状与进展．混凝土，2003(2)
[2] 李金玉，彭小平，邓正刚等．混凝土抗冻性的定量化设计．混凝土，2000，(12)：61～65
[3] Mehta P K,Shiessl P'Raupach M．混凝土系统性能与耐久性．第九届国际水泥化学会议综合报告论文集，南京化工学院，1993：240-345
[4] Sommer H著，冯乃谦译．高性能混凝土的耐久性．北京，科学出版社，1998
[5] 王媛俐．姚燕主编．重点工程混凝土耐久性的研究与工程应用．北京，中国建材工业出版社，2001：413-425
[6] Takafumi Sugiyama,T.W.Bremner, and Thomas A.H, Effect of stress on gas permeability in concrete. ACI Material Journal, 1996,93(5):443-450
[7] Ludirdija,D.R.L.Berger. Simple Method for measuring water permeability of concrete. ACI Material Journal, 1989,86 (5):433-439
[8] Hani R.Samaha and Kenneth C.Hover. Influence of microcracking on the mass transport properties of concrete. ACI Material Journal, 1992,89(4): 416-424
[9] 冯乃谦 著．实用混凝土大全．北京，科学出版社，2001．
[10] Nemkumar Banthia and Sidney Mindess. Water permeability of cement paste. Cement and Concrete Research, 1989,19:727-736
[11] R.H.Mills. Mass Transfer of water vapour through concrete. Cement and Concrete Research, 1985,15:74-82
[12] Min-hong zhang and Odd E.Gjorv. Permeability of high-strength lightweight concrete. ACI Material Journal, 1991,88(5):463-469
[13] Nemkumar Banthia and Sidney Mindess. Water permeability of cement paste. Cement and Concrete Research,1989,19:727-736
[14] 水工混凝土试验规程(DL/T 5150-2001)．北京，中国电力出版社，1999
[15] 顾慰慈 著．渗流计算原理及应用．北京，中国建材工业出版社，2000
[16] R.P.Khatri and V.Sirivivatnanon. Methods for the determination of water permeability ofconcrete. ACI Material Journal,. 1997,94(3):257-261
[17] Tarun R.Naik,Shiw S.Singh,Mohammad M.Hossain. Permeability of concrete containing large amount of fly ash. Cement and Concrete Research,1994,24(5):913-922
[18] A non. Permeability of concrete. Concrete Construction, 1989,34(10):870-872
[19] ASTM C1203-91. Standard method of test for electrical indication of concrete's ability to resist chloride ion penetration
[20] Kejin Wang,Daniel C.Jansen,Surendra RShah. Permeability study of cracked concrete. Cement and Concrete Research, 1997,27(3):381-393
[21] Carola Edvardsen. Water permeability and autogenous healing of cracks in concrete.ACI Material Journal, 1999,96(4):448-453
[22] C.-M.Aldea, S.P.Shah and A.Karr. Permeability of cracked concrete. Materials and Structures, 1999,32(6):370-376
[23] Mitsuru Saito and Hiroshi Lshimori. Chloride permeability of concrete under static and repeated compressive loading. Cement and Concrete Research, 1995,25(4):803-808,
[24] Kermani.A. Stressed concrete:Permeability of stressed concrete. Building Research and Information. 1991,19(6):360-366
[25] Nataliya Hearn,Rachel J.Detwiler, and Carmen Sframeli. Water permeability and microstructure of three old concretes. Cement and Concrete Research, 1994,24:633-640
[26] Corina-Maria Aldea. Extent of healing of cracked normal strength concrete, Journal of Materials in Civil Engineering,2000,(2):92-95
[27] 夏才初，杨林德，朱素平．不同应力水平下混凝土渗透性的试验研究．西部探矿工程，1996，6(11)：27～30
[28] ltaru.Horiguchi,et ai. The permeability of hybrid fiber reinforced concrete under compression and dry-wet chloride exposure. Transactions of the Japan Concrete Institute, 1999,21:195-200
[29] 刘祖华，梁发云．混凝土碳化研究现状评述．四川建筑科学研究，2000，26(3)：52～54
[30] J.P.Balayssac,CH.H.Detriche and J.Grandet. Effect of curing upon carbonation of concrete. Constuction and Building Material, 1995,9(2):91-95
[31] 金伟良，赵羽习．混凝土结构耐久性．科学出版社，2002，9：24～26
[32] 蒋利学．张誉．混凝土部分碳化区长度的分析与计算工业建筑，1999，20(1)：4～7
[33] V.G.papadakis et al. Fundamental Modeling and Experimental Investigation of Concrete Carbonation. ACI Materials Journal,1991,(88):363-373
[34] 黄可信．吴兴祖．钢筋混凝土结构中钢筋腐蚀与保护．中国建筑工业出版社，1983
[35] V.G.papadakis et al. Experimental Investigation and Mathematical Modeling of The
carbonation problem. Chemial Engineering Science, 1991,(46)
[36] 蒋利学，张誉．混凝土碳化区物质含量变化规律的数值分析．工业建筑，1999,29(1)：8-11
[37] Linhua Jiang,Baoyu Lin, Yuebo Cai. A model for predicting carbonation of high-volume fly ash concrete. Cement and Concrete Research,2000,30:699-702
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