大型隧道内层衬砌顶部出现.doc

大型隧道内层衬砌顶部出现裂缝的原因以及预防措施Roof Cracks in the Inner Shell of largeTunnels-Causes and Means of Avoiding ThemRupert Springenschmid, Markus Plannerer 大型隧道建成数年后，顶部有时会出现纵向裂纹，实际上这种现象是由于温度变化和收缩引起混凝土自身变形而造成的，并不是由于岩石压力所致。隧道内层采用防水膜和不加钢筋的混凝土衬砌能抵抗较宽的裂缝，而采用钢筋混凝土衬砌的隧道内层，当裂纹宽度超过0.3毫米时，应向裂纹灌浆，以免钢筋被腐蚀。 试验 对避免这种裂纹出现而采取的措施,进行了好几年的试验,即： * 在现有的隧道上(裂缝宽度的变 化，在 裂缝中提取岩心样品， 测量温度、湿度和会聚）； * 在实验室进行模拟试验,部分采 用新研制的设备(测量由于温度 和湿度的变化而引起的强制应 力以及由于湿度梯度引 起的变 形）； * 对自重荷载和强制力导致的垂直 应力、弯曲应力以及内应力进行 模拟计算。 有关的基础工作请参阅R. Breitenbueher 1、W.Fleischer2M. Mangold3的论文以及文献 4。文献5是关于顶部裂缝的综合报告以及结论。 在顶部模板拆除约12小时后，由于自重荷载而产生应力。该应力只能达到所需拆模强度3N/mm2的17左右。另外，还受到水合热引起温度升高而产生的强制应力的影响。这仅适用于相对喷射混凝土(和防水层）的浇注混凝土内层衬砌，不适用于环形断面（洞口范围）的明挖施工法。 硬化混凝土的零应力温度取决于水合热、新浇注混凝土温度以及形变特性值。零应力温度是混凝土变“硬”的温度，远高于使用期间的温度。内层衬砌逐渐冷却在切线方向产生垂直应力，但是对于形成裂缝来说，这个应力太小。急剧冷却（温度骤降）会产生显著的弯曲应力。与喷射混凝土相比，在温度较高下，浇注混凝土内缘部分先变硬（热积聚在顶部）。一侧变干引起的收缩显著地增加了弯曲应力，致使冬天顶部裂缝比夏天大。(图1) 结论 采取下列措施可以减小由于温度变化引起的强制应力5:* 采用应力随温度变化小的混凝土 组分。 如：在断裂范围内开裂温 度低的水泥1,温度应变系数小 的添加剂，拆模强度允许的气孔 混凝土，掺有粉煤灰的水泥（冬 季配方夏季配方)。* 避免高的12小时强度 (6N/mm2)。如果12小时内不拆 模，则不需要高的早期强度。* 避免高的新浇注混凝土温度。* 保持潮湿对混凝土进行后续处 理，使用自卸车。* 对顶部数米宽模板上的新浇注混 凝土进行冷却。这样，硬化的混 凝土从内到外(空气边缘)产生了应 力梯度(边缘压应力)。 为限制混凝土干缩，应该：* 新浇注混凝土的水含量小(用溶 剂)。* 水泥的碱含量不很高3。* 内层衬砌迎风表面(如公路隧道) 采用反应性树脂，使得混凝土变 干产生的收缩应力小。 文献：1 Breitenbuecher,R.: Zwangsspannungen und Rissbildung infolge Hydratationswaerme,Dissertation, TU Muenchen 1989.2 Mangold, M.: Die Entwicklung vonZwang-und Eigenspannungen in Betonbauteilen waehrend derHydratation. Dissertation, TU Muenchen 1994.3 Fleischer, W.: Einfluss desZementes auf schwinden und Quellenvon Beton. Dissertation, TU Muenchen 1992.4 Springenschmid, R.und Maidl, B.: Betoninnenschalen fue Tunnel anden Neubausrecken der Deutschen Bundesbahn, Beurteilung und Behandlung von Rissen. Muenchen/ Bochum, August 1991(unveroeffentlicht).5 Springenschmid, R., und plannerer, M.: Firstrisse in der Innenschale grosser Tunnel- Ursachenund Wege zur Vermeidung, Beton-und stahlbetonbau 92(1997),H. 3 ,S. 68-72, H, 4. S.109-112. Rupert Springenschmid 教授Markus Plannerer 特许工程师慕尼黑技术大学建筑材料学院，德国Munich BACK HOMEIn the roofs of large tunnels, longitudinal cracks are sometimes determined - often after years - which practically always are attributable to the internal deformations of the concrete through change in temperature and shrinking - in other words, not as a result of rock pressure. In the case of tunnels with waterproofing membrane and unreinforcedconcrete, even major gap widths can be tolerated whereas with regard to reinforced inner shells, cracks should be concreted as from a width of 0.3 mm so that the steel does not corrode. InvestigationsIn order to be in a position to undertake measures designed to prevent such cracks, investigations were carried out over a number of years namely * in existing tunnels(change in the gap width of the cracks, core samples in the crack, temperature, moisture and convergence measurements) * model tests in the laboratory partially making use of newly developed facilities (measurement of induced stresses resulting from changes in temperature and moisture as well as deformations caused by moisture gradients) and * model calculations of the normal, flexural and intrinsic stresses as a result of induced load and constraint.Related fundamental work is to be found in the dissertation by R. Breitenbach 1, W. Fleischer 2 and M. Mangold 3 as well as in 4. A comprehensive report on roof cracks with conclusion is presented in 5.In the roof, during stripping after roughly 12 h, stresses resulting from induced load occur, which only reach some 17% of a required stripping strength of 3 N/mm2. In addition, they are affected by intrinsic stresses caused by the rise in temperature as a result of hydration heat. This only applies to inner shells, which are concreted against shotcrete (and waterproofing), not for cut-and-cover methods with circular profile (portal zone).It depends on the hydration heat, the fresh concrete temperature and the deformation coefficients, which zero stress temperature is established in the hardening concrete. The zero stress temperature is the temperature at which the concrete becomes hard. It is far higher than the temperature during utilisation. A gradual cooling down of the inner shell brings about normal stresses in a tangentialdirection, which are too small for forming cracks. Rapid cooling down (speedy drop in temperature) causes considerable bending stresses, above all, if the concrete previously hardened during its placing against the inner edge zone at a considerably higher temperature (heat accumulation in the roof) than against the shotcrete. One-sided shrinkage through drying out considerably increases these bending stresses and leads to the roof cracks being further open in winter than they are in summer (Fig.1).图1导致顶部裂缝形成的荷载和产生的应力分布5ConclusionsThe induced stresses caused by changes in temperature can be kept small through the following measures 5: * utilisation of concrete compositions, which only lead to slight temperature stresses through: cement with low crack temperature within the failure limit 1, aggregates with lower temperature strain coefficient, air porous concrete as far as the stripping strength permits this, partial replacement of the cement through flyash (summer recipe/winter recipe) * avoidance of high 12 h strength (6 N/mm2). No stripping schedules below 12 h so that no high early strengths are required* avoidance of high fresh concrete temperatures* follow-up treatment of the concrete through keeping it moist, making use of back-up cars* cooling down the young concret