The effect of compressive stress on thermal and hygric properties of Portland cement mortar in wide temperature and moisture ranges

Abstract

Basic thermal and hygric parameters of cement mortar — namely, the thermal conductivity, the linear thermal and hygric expansion coefficients, moisture diffusivity, and water vapor permeability — are determined in dependence on the applied mechanical load inducing compressive stress in the samples ranging from 0% to 90% of compressive strength. The measurements are performed in wide temperature and moisture ranges, from 20°C to 1000°C and from the dry state to the saturation water content. The measured results show that both thermal and hygric parameters of cement mortar are affected only by compressive stress of 90% of the compressive strength of the material and higher, in a significant way. Scanning electron microscope images and porosimetric measurements reveal that the most probable reason for the observed differences in thermal and hygric parameters is the appearance of cracks with a typical width of 1–2 μm.

Introduction

Thermal and hygric properties of concretes, cement mortars and cement pastes are measured at room temperature in standard conditions, in most cases. For many applications of concrete in building structures, is it quite sufficient as usual environmental exposure is within the range from about −20°C to +50°C. However, some concrete structures can be exposed to elevated or high temperatures during their lifetime. Fire resistance problems of concrete structures can be considered probably as the most important example in this respect, but special industrial applications of concrete, such as in blast furnaces, nuclear safety-related structures or heat pipes also become of great significance. In such conditions, high temperature values of thermal parameters have to be determined in order to analyze the behavior of the particular structures already in the design phase in a proper way.

We will introduce several examples of measurements of thermal parameters in high temperature range determined by various investigators. Thermal expansion of cement paste in the temperature range to 800°C was measured by Philleo [1] and Harada et al. [2], thermal expansion of various concretes up to 1000°C for instance by Schneider [3] or Komarovskii [4]. Specific heat of cement paste was determined by Harmathy [5], specific heat of various types of concrete by Harmathy and Allen [6], Hildenbrand et al. [7]. Thermal conductivity of cement paste was measured by Harmathy [5], thermal conductivity of concrete by Schneider [3], Harmathy and Allen [6], Hildenbrand et al. [7]. A more comprehensive survey of the thermal properties of concrete at high temperatures measured until ∼1992 can be found by the reader in the book by Baz̆ant and Kaplan [8].

Hygric parameters of cement pastes and concretes were measured at normal conditions very frequently. For instance, moisture diffusivity of concrete was measured by Baz̆ant and Najjar [9], water permeability of cement paste by Powers and Brownyard [10], water permeability of concrete by El-Dieb and Hooton [11], Halamickova et al. [12], etc. On the other hand, there are very few measurements of hygric parameters in non-standard conditions; for instance, Baz̆ant and Thonguthai [13] studied moisture diffusivity for temperatures above 100°C.

Concrete structures are practically always exposed to mechanical load producing compressive or tensile stress in the material. However, the effect of mechanical load on thermal and hygric properties was investigated very seldom, until now. We can mention the work by Baz̆ant et al. [14], where the influence of crack on drying permeability of concrete in room temperature conditions was studied, as one of very few examples.

In this paper, we study the main thermal and hygric properties of Portland cement mortar, namely, the thermal conductivity, the linear thermal expansion coefficient, the moisture diffusivity, the water vapor permeability and the linear hygric expansion coefficient in dependence on compressive stress up to 90% of compressive strength, which was imposed on material samples prior to the measurements. Thermal conductivity is determined for a dry material in the temperature range to 800°C, and for a moist material with moisture content ranging from dry stage to maximum water saturation in the room temperature conditions. The linear thermal expansion coefficient is measured for a dry material up to 1000°C. The hygric transport parameters are measured for characteristic values of the moisture content in room temperature conditions, with the linear hygric expansion coefficient at room temperature in a wide moisture range up to maximum water saturation.