超低温作用下预应力混凝土的有效预压性能研究

With deterioration of our country’s natural environment such as fog, the energy structure is forced to have an adjustment and a lot of receiving and storage facilities for liquefied natural gas (LNG) start to be constructed. All of them adopt prestressed concrete structures to meet the requirements with impermeability. But unlike exposure to room temperature, the effective preloading behavior will vary due to the action from various cryogenic temperatures and cryogenic freezing-thawing cycles with the temperature range from room temperature to the LNG’s temperature (about -165 ℃), and thus existing possible for the loss of their due anti-cracking function. At home and abroad the mechanical properties of steel and concrete exposed to ultralow temperature have been discussed, but the effective preloading behavior of prestressed concrete exposed to ultralow temperature not covered. It is the first condition to ensure that there has not exsisted the cracking of the concrete during cryogenic action for such facilities to reserve a huge amount of flammable and explosive LNG, so it is necessary and urgent to study the changing regularity of the effective preloading behavior for prestressed concrete exposed to ultralow temperature. Based on the existing corresponding research results and the actual working conditions from this kind of tanks, the research range and influencing factors are first determined, then through discussing the effects of various main factors on the effective preloading behavior using the method which depends mainly on experimental research with an assistance from finite element calculation, the gerneral changing regularity of the effective preloading behavior for prestressed concrete under the action of such temperature as the LNG and so on is analyzed and summarized. These research results will have an especially important technological and economic significance for safely designing and reliably estimating the prestressed concrete structures used for LNG’s receiving stations and huge tanks.

我国自然环境的恶化如雾霾等迫使能源结构调整，兴建液化天然气接收和储存等设施。它们均采用预应力混凝土结构以满足其密闭要求，但不同于常温环境，其有效预压性能因其遭受常温至液化天然气温度（约-165℃）间各种低温及低温冻融作用而变化，进而可能丧失其应有的防裂功能。国内外对钢材和混凝土超低温受力性能进行了探讨，但对超低温作用下预应力混凝土有效预压性能方面尚未涉及。确保这类储存易燃易爆且量极大的设施超低温运行中混凝土不出现裂缝是其首要条件，故开展超低温作用下预应力混凝土有效预压性能研究必要且迫切。基于已有的研究结果和这类储罐实际情况确定其研究范围和影响因素，采取以试验为主辅以有限元计算方法探讨影响有效预压性能的各种主要因素作用，分析总结液化天然气等超低温作用下预应力混凝土有效预压性能变化一般规律。这对于液化天然气混凝土接收站及其巨型储气罐类预应力混凝土结构设计及其安全评估具有特别重要的技术经济意义。

大量兴建的液化天然气接收站及其大型储存罐等预应力混凝土结构必然遭受各种可能的超低温工况，所施加的预压应力将因其发生变化可能丧失应有的防裂功能，故对其研究十分必要。基于已有的研究结果和实际工程情况确定研究范围和影响因素，以试验为主辅以有限元计算方法对其进行探讨。为合理地设计其试验研究方案，先由458个试件试验考察作用的低温（-40℃至-180℃）及混凝土的含水率（1.5%至5.5%）和强度等级（C30至C50）等关键影响因素的耦合作用性能，再据此进行共226个预应力混凝土试件的试验，考察的主要影响因素包括作用的低温（0℃至-160℃）、初始预压预应力水平（0.2至0.7）、混凝土的强度等级（C40至C70）和含水率（2.5%至5.0%）以及超低温冻融循环作用的温度区间和冻融次数等。结果表明，各种初始预压应力水平、作用的超低温以及混凝土强度等级和含水率情况的预应力混凝土超低温作用下有效预压应力均有较大的损失，但它们的影响程度存在明显的差异性。其中，初始预压应力水平和作用的超低温的影响较为显著。不仅单一影响因素下预应力混凝土超低温作用下有效预压应力损失率随之变化存在明显的差异，而多影响因素耦合作用下的变化规律将变得更加复杂。作用的超低温在其降温和恒温时的影响还存在明显的差异。恒温时将出现有效预压应力损失率有所恢复特性；经历超低温冻融循环作用的预应力混凝土有效预压应力损失率与初始预压应力水平、作用的超低温以及混凝土的强度等级间密切相关、且其变化关系较为复杂，但随冻融循环作用时间均呈现周期性波浪状变化特性，且若干次冻融循环作用后便趋于稳定；基于试验结果辅以有限元计算分析还回归出大量的预应力混凝土超低温下有效预压应力损失率等计算表达式。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。