Deterioration of Concrete Exposed to Evaporative Transport Conditions

混凝土中的大多数劣化机制要求外部介质通过连通的孔隙系统进入混凝土。这种有害物质可能是直接侵蚀混凝土的硫酸盐，导致钢筋腐蚀的氯化物，或者是可以引发碱集料反应或导致冻融破坏的水。变质率在很大程度上取决于所讨论的液体的渗透率。能够量化这一比率对于预测结构的劣化率和使用寿命至关重要。目前大多数模型依赖于扩散作为主要的质量传递性质。然而，蒸发运输可以产生高得多的穿透率，从而使基于扩散的模型非常不准确。当结构的一部分暴露在湿气中，而附近的一部分暴露在大气中时，就会发生蒸发输送。这就产生了一个蒸发表面，它倾向于把水吸进混凝土，并以一种取决于水从表面蒸发速度的速度向这个表面吸去。大多数暴露在水中的结构都符合这种描述；从海洋中伸出的桥墩、平地上的楼板和土挡土墙就是其中的几个例子。申请人先前使用磁共振成像（MRI）进行的研究表明，蒸发输送是一个非常复杂的机制，需要进一步研究才能将其引入使用寿命模型。此外，评估混凝土的劣化率需要一种新的方法，这种方法依赖于机械性能，而不是像膨胀这样的间接测量。申请人打算使用混凝土的拉伸性能作为力学性能恶化的量化介质。由于混凝土中的大多数劣化机制涉及内部膨胀应力的发展，而内部膨胀应力本质上是拉伸的，因此拉伸强度是量化损伤程度和残余强度的最合适的力学性能。拟议的研究将通过进一步评估混凝土中的蒸发传输现象，开发一种暴露方案来创建基于蒸发传输的劣化，并设计一种基于拉伸性能劣化的评估方案，将这些问题联系在一起。总的来说，拟议的研究计划将更好地理解在世界各地的混凝土结构中发现的非常常见的传输机制。有人可能会说，蒸发运输是最普遍的恶化机制。能够量化其对劣化的影响，再加上标准化的暴露和测试制度，将导致更准确的使用寿命预测。这将允许更可持续的结构设计，大大减少材料的使用和生命周期成本。

Most deterioration mechanisms in concrete require that an outside agent enter the concrete through the connected pore system. This deleterious material could be sulfates that attack the concrete directly, chlorides that cause corrosion of the reinforcing steel, or simply water that can initiate alkali-aggregate reactions or contribute to freeze-thaw damage. Deterioration rates are highly dependent upon the rate of penetration of the liquid in question. Being able to quantify this rate is critical toward predicting deterioration rates and thus service lives of structures. Most current models rely on diffusion as the primary mass transport property. However, evaporative transport can produce a far higher rate of penetration, thus making a diffusion-based model highly inaccurate. Evaporative transport occurs when part of a structure is exposed to moisture while a nearby portion is exposed to the atmosphere. This creates an evaporation surface that tends to draw water into the concrete and toward this surface at a rate that depends upon the evaporation rate of the water from the surface. Most structures exposed to water match this description; bridge piers protruding from the ocean, floor slabs on grade, and earth retaining walls are a few examples. Prior research by the applicant using magnetic resonance imaging (MRI) has shown that evaporative transport is a very complex mechanism that needs further study before it can be introduced into a service life model. In addition, evaluating the deterioration rate of concrete requires a new approach that relies on mechanical properties as opposed to indirect measurements like expansion. The applicant intends to use the tensile properties of concrete as a quantification medium for mechanical property deterioration. Since most deterioration mechanisms in concrete involve the development of internal expansive stresses that are tensile in nature, tensile strength is the most appropriate mechanical property for quantifying the magnitude of damage and the residual strength. The proposed research will tie these issues together by further evaluating the phenomenon of evaporative transport in concrete, developing an exposure regimen to create evaporative transport based deterioration, and devising an evaluation protocol based on the degradation in tensile properties. In general, the proposed research program will provide a better understanding of a very common transport mechanism found in concrete structures throughout the world. One could argue that evaporative transport is the MOST prevalent mechanism in deterioration. Being able to quantify its effects on deterioration, coupled with a standardized exposure and testing regime, would result in much more accurate service life predictions. This would allow for more sustainable design of structures, dramatically reducing material usage and lifecycle costs.