液化天然气作用下混凝土冻融循环性能

With increase in requirement for cleansing energy resource, at present a lot of the receiving stations and huge tanks for liquefied natural gas ( LNG ) are being programed and some of them have being run or started to be constructed. Their structures are made from the prestressing concrete because the concrete is very economical compared with other structural materials for such equipment structures and can meet with various requirements from such equipment. Clearly, the prestressing concrete structures of the receiving stations and huge tanks for LNGs would undergo the action of various ascenting-descenting temperature cycles between natural environment and LNG temperatures. As the freezing point temperatures of water stored in concrete holes generally descent with the decrease in the dimension of concrete holes, there exists the action of freezing-thawing cycles for the concrete exposed to ultralow temperatures. At home and abroad the research on the freezing-thawing cycle behavior of concrete mainly focuses on such temperature action condition as under natural environment temperatures, this is to say that the temperature generally exceeds -30℃, and only the strength property of concrete undergoing freezing-thawing cycle action is discussed. But the mechanical properties of prestressing concrete structures, to a large degree, depend on the deformation property of their concrete undergoing freezing-thawing cycle action. It can be seen that the accident from the LNG tanks due to their huge storage quantity and natural gas' inflammable and explosive properties is wondrously fearful and catastrophic. Therefore, it is very important and necessary for developing a research on the freezing-thawing cycle action behavior of concrete exposed to ultralow temperature. Based on the existed corresponding research results and actual working conditions of prstressing concrete structures from the LNG action, the research range and influencing factors are first determined, then through discussing the regularities of strength and deformation of concrete undergoing freezing-thawing cycle action under influence of these selected important factors using the method which depends mainly on experimental research with an assistance from finite element calculation, the compressive constitutive relationship of concrete undergoing freezing-thawing cycle action exposed to LNG temperature is given. These research results will have an especially important technological and economic significance for the concrete structural safety design of LNG's receiving stations and huge tanks.

我国清洁能源需求剧增促使液化天然气接收站和其大型贮气罐等设施近年开始大规模地兴建。其混凝土结构将遭受常温至液化天然气温度（约-165℃）间各种可能升降温循环作用，而混凝土孔隙中水的冰点温度随孔径减小而降低，也即不同超低温下混凝土都存在冻融循环作用。国内外对其性能研究基本集中于自然环境作用下即温度多不低于-30℃，并仅探讨其冻融循环强度。而预应力混凝土结构受力性能与混凝土变形有很大关系。这类设施贮气量极大、又天然气易燃易爆，一旦发生事故后果极其严重。故开展混凝土超低温冻融循环性能研究必要且迫切。基于已有的研究结果和实际遭受液化天然气混凝土结构情况确定其研究范围和因素，采取试验为主辅以有限元计算方法探讨各种主要因素作用下混凝土超低温冻融循环的强度和变形变化规律，建立适用于液化天然气作用混凝土冻融循环本构关系。这对于液化天然气混凝土接收站和其大型贮气罐类混凝土结构设计具有特别重要的技术经济意义。

随我国能源贮备战略实施和清洁能源天然气需求剧增，促使液化天然气接收站及其大型储存罐等设施大量地兴建。其预应力混凝土结构将遭受常温至液化天然气温度间各种可能升降温循环的冻融作用，预应力的有效预压效果不仅与遭受超低温冻融作用混凝土的强度有关，对其变形性能也极为敏感。故对其研究必要且迫切。基于已有的研究结果和实际遭受液化天然气作用混凝土结构情况确定研究范围和因素，以试验为主辅以有限元计算方法对其进行探讨。共进行了802个混凝土试件的试验，考察的主要影响因素包括混凝土的强度等级（C40、C50和C60）和含水率（0.0%~5.5%）以及超低温冻融循环作用的温度区间（上限温度20℃~-30℃、下限温度-40℃~-190℃）和冻融次数（0次~58次）等。结果表明，混凝土的强度等级和含水率以及超低温冻融循环作用的温度区间和冻融次数均未影响其宏观表象，其破坏形态也均相似、基本上呈对顶锥状，但强度等级和含水率较高的试件下限温度时破坏呈明显的脆性特征、且随作用的温度区间下限温度降低更加显著，而上限温度时则与之相反。所探讨的影响因素均对混凝土超低温冻融循环作用的强度、弹性模量、峰值应变以及应力-应变关系等性能产生明显的影响，且存在明显的相互耦合影响作用，表现出的变化规律极其复杂、也不尽相同，并且上下限温度时呈现出明显不同的受力特性。各种影响因素下混凝土的单次冻融循环作用损伤指标和累积冻融循环作用损伤指标也存在明显的差异和复杂的变化规律。超低温冻融作用不同于自然环境温度冻融，较少的冻融次数混凝土便显示出性能的恶化，混凝土的强度等级和含水率较高以及温度区间的下限温度较低时将更加明显。基于试验结果辅以有限元计算分析还回归出大量的混凝土超低温冻融循环作用性能计算表达式。这些研究结果对于液化天然气接收站和其大型贮气罐等超低温环境混凝土结构设计以及已建的这类设施安全性能评估都具有特别重要的技术经济意义。