Mechanisms of Freezing Damage to Concrete

冻融循环会导致混凝土内部开裂或表面剥落，从而损坏混凝土。造成这两种损伤的物理机制显然不同，但细节仍有争议。该项目的目的是研究造成损害的基本机制，并测试新的方法来防止它。混凝土通常通过引入表面活性剂来保护免受霜冻损害，表面活性剂产生直径为50 - 100 μ m的空气空隙，空隙的间距约为200 μ m。空隙作为水槽，当冰在水泥浆的孔隙中形成时，水可以流入其中;这防止了当水转变成冰时由膨胀产生的破坏性液压的发展。然而，当冰在周围的中孔中形成时，空隙不能保护糊剂免受结晶压力。我们打算研究结晶压力和液压相对于内部开裂的相对重要性。为了控制结晶的发生，我们将冰的成核剂引入到空气空隙中，使得冰优先在那些开放空间中形成，在那里它可以生长而不产生应力。如果有效的话，这种成核剂可以与产生空隙的表面活性剂一起沿着掺入混凝土中。为了研究混凝土表面结垢，我们将采用一种新的实验方法。在水泥浆板的表面上形成一个液体池，并冷却系统直到发生冻结。随着冰的形成，内部应力的发展通过板的翘曲来揭示;通过用光学探针测量偏转，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糊状物的热收缩，孔隙水向身体外表面处的冰的转移，以及表面冰和糊状物之间的热膨胀失配。这些机制的相对重要性将被量化，我们将探讨空气空隙，成核控制和其他方法的有效性，以尽量减少损害。这项研究将是博士论文的主题，并将通过将部分研究作为高级论文和独立研究项目来整合到本科课程中。冻融试验将使用W.R.实验室的特殊设施和专业知识。格雷斯公司。我们还将与Leo Pel教授（埃因霍温）合作，获得混凝土样品冷冻过程的磁共振成像。

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.