Mechanisms of Freezing Damage to Concrete

冻融循环可能会导致混凝土内部开裂或表面结垢，从而损坏混凝土。造成这两种损害的物理机制显然不同，但细节存在争议。这个项目的目的是检查造成损害的基本机制，并测试预防它的新方法。传统上，混凝土是通过引入表面活性剂来防止冻害的，这种表面活性剂会产生直径在50-100微米左右、间距约为200微米的气孔。当水在水泥浆体的孔隙中形成时，这些气孔充当水槽，水可以流入其中；这可以防止在水转变为冰的过程中膨胀产生的破坏性水力压力。然而，当冰在周围的中孔中形成时，空隙不能保护糊料免受结晶压力的影响。我们打算研究结晶压力和液压压力对于内部裂纹的相对重要性。为了控制结晶的发生，我们将向空气空隙中引入成核剂，以便冰优先在那些开放的空间形成，在那里它可以在不产生压力的情况下生长。如果有效，这种成核剂可以与产生空隙的表面活性剂一起加入混凝土中。为了检查混凝土表面的结垢，我们将采用一种新的实验方法。在一块水泥浆体的表面建立一个液体池，冷却系统直到冻结发生。随着冰的形成，由板的翘曲揭示的内应力发展；通过用光学探头测量挠度，我们可以研究应力发展的动力学。初步实验揭示了几种应力产生机制：孔隙液体相对于固体浆体的热收缩，孔隙水向身体外表面的冰转移，以及表面冰和膏体之间的热膨胀不匹配。这些机制的相对重要性将被量化，我们将探索空气空隙、成核控制和其他将损害降至最低的方法的有效性。这项研究将作为博士论文的主题，并将通过将部分研究作为高级论文和独立研究项目整合到本科课程中。冻结/融化测试将使用W.R.格雷斯公司实验室的特殊设施和专业知识。我们还将与Leo Pel教授(埃因霍温)合作，获得混凝土样品中冻结过程的磁共振成像。

Freeze/thaw cycles can damage concrete by causing internal cracking or superficial scaling. The physical mechanisms responsible for these two types of damage are apparently different, but the details are controversial. The purpose of this project is to examine the fundamental mechanisms responsible for the damage, and to test novel methods for preventing it. Concrete is conventionally protected from frost damage by introducing surfactants that create air voids with diameters on the order of 50-100 um and spacing of about 200um The voids serve as sinks into which water can flow when ice forms in the pores of the cement paste; this prevents development of destructive hydraulic pressures created by the expansion as water transforms to ice. However, the voids do not protect the paste against crystallization pressure when ice forms in the surrounding mesopores. We intend to examine the relative importance of crystallization pressure and hydraulic pressure with respect to internal cracking. To control the occurrence of crystallization, we will introduce nucleating agents for ice into the air voids, so that ice forms preferentially in those open spaces, where it can grow without creating stress. If effective, such nucleating agents could be incorporated into concrete along with the surfactants that create the voids. To examine surface scaling of concrete, we will employ a novel experimental approach. A pool of liquid is established on the surface of a plate of cement paste, and the system is cooled until freezing occurs. As the ice forms, internal stresses develop that are revealed by warping of the plate; by measuring the deflection with an optical probe, we can study the kinetics of stress development. Preliminary experiments have revealed several stress-generating mechanisms: thermal contraction of the pore liquid relative to the solid paste, transfer of pore water toward the ice at the exterior surface of the body, and thermal expansion mismatch between the superficial ice and the paste. The relative importance of these mechanisms will be quantified, and we will explore the effectiveness of air voids, nucleation control, and other methods for minimizing the damage. This research will be the subject of a doctoral thesis, and will be integrated into the undergraduate curriculum by having parts of the study done as senior theses and independent research projects. Freeze/thaw testing will use special facilities and expertise in the labs of the W.R. Grace Corporation. We will also collaborate with Prof. Leo Pel (Eindhoven) to obtain magnetic resonance imaging of the freezing process within the concrete samples.