Mechanism of Internal Frost Damage

内部霜冻损害机制(普林斯顿大学乔治·W·谢勒)弗罗斯特每年给美国的基础设施造成价值数十亿美元的破坏，一个世纪以来一直是密集研究的主题，但破坏的原因仍存在争议。这项工作的第一个目标是确定水泥和混凝土孔隙中的水结晶造成损害的机制。理论论证表明，主要问题不是水转化为冰时体积的增加，而是冰晶体直接施加在孔壁上的压力。传统上，混凝土中引入气孔是为了防止冻害，人们普遍认为，气孔的作用是为结冰时膨胀而转移的水提供水槽。然而，有理由相信，空洞更重要的作用是为成核提供场所；空洞中形成的冰然后从周围的毛孔中吸水，水中的吸力压缩身体。在这项研究中，我们将研究晶体成核的位置，以及晶体通过孔洞生长过程的动力学和热力学。这项工作的第二个目标是开发改进的方法来保护混凝土免受霜冻破坏。为此，我们会在空隙中引入成核剂以促进成核，并会操控空隙周围的气孔结构以控制冰所产生的吸力；这两个因素都将有助于降低冰晶对混凝土施加压力的风险。为了模拟冻结过程和应力的发展，有必要知道当身体冻结，毛孔逐渐被冰堵塞时，孔隙网络的渗透性。我们将使用一种新的弯曲梁方法来测量冻结过程中渗透率的变化。其原理是，弯曲薄板会导致顶面收缩，底面拉伸；如果板是饱和的多孔体，那么气孔中的液体在板的顶部附近被压缩，并在底部附近处于张力状态。弯曲引起的压力梯度导致液体流过孔隙，直到压力达到平衡。由于维持板的恒定挠度所需的力随着孔压的平衡而减小，因此可以分析力随时间变化的测量结果来确定物体的渗透性，就像我们使用多孔玻璃、水泥和石头所演示的那样。在目前的工作中，样品将被逐步冻结，并将确定冰对人体刚度和渗透性的影响。

Mechanism of Internal Frost Damage (George W. Scherer, Princeton University) Frost does billions of dollars worth of damage to the infrastructure of the US every year, and has been the subject of intensive study for a century, but the cause of the damage is still in dispute. The first goal of this work is to identify the mechanisms by which the crystallization of water within the pores of cement and concrete produces damage. Theoretical arguments suggest that the primary problem is not the increase in volume as water transforms into ice; rather, it is the pressure exerted directly on the walls of pores by ice crystals. Air voids are conventionally introduced into concrete to protect against frost damage, and it is widely thought that their role is to provide sinks for water displaced by the expansion as ice is formed. However, there is reason to believe that the more important role of the voids is to provide sites for nucleation; ice formed in the voids then sucks water out of the surrounding pores and suction in the water compresses the body. In this research, we will study the sites where crystals nucleate, and the kinetics and thermodynamics of the process by which the crystals propagate through the pores. The second goal of the work is to develop improved ways to protect concrete from frost damage. To that end, we will introduce nucleating agents into the voids to promote nucleation, and will manipulate the pore structure surrounding the voids to control the suction exerted by the ice; both of these factors will help to reduce the risk that ice crystals will exert stress on concrete. To model the freezing process and the development of stress, it is necessary to know the permeability of the pore network as the body freezes, and the pores become progressively blocked by ice. We will use a novel beam-bending method to measure the changes in permeability during freezing. The principle is that bending a thin plate causes the top surface to contract and the bottom surface to stretch; if the plate is a saturated porous body, then the liquid in the pores is compressed near the top of the plate and put into tension near the bottom. The pressure gradient caused by bending leads to flow of the liquid through the pores until the pressure equilibrates. Since the force needed to sustain a constant deflection of the plate decreases as the pore pressure equilibrates, measurements of the change in force with time can be analyzed to determine the permeability of the body, as we have demonstrated using porous glass, cement, and stone. In the present work, the sample will be progressively frozen, and the effect of ice on the stiffness and permeability of the body will be determined.