Deterioration and recovery of FRC after high temperature exposure

Abstract

In general, physical properties of concrete deteriorate after high temperature exposure and this is a result of some morphological changes occur in microstructure of concrete. After heating some new phases occur in concrete and these have a tendency to react with water and carbon dioxide at the post heating stage. The most common one is expansive rehydration of CaO by turning into Ca(OH)₂ and this reaction should be avoided for concrete of which mechanical properties deteriorated after heating. Since, Ca(OH)₂ is a soluble product in water, water re-curing after high temperature exposure can be beneficial. Also using steel fibers in concrete may be useful to restrain the stresses occurred due to this expansion and polypropylene fibers can be used to reduce the pore pressure in concrete during heating. Therefore, 8 groups of 0.45 w/c ratio of concrete were produced by using different fibers and air entraining admixture. Cubic concrete specimens (15 cm on a side) were cast and only one face of the specimens was subjected to 1000 °C. K-Type thermocouples were placed in concrete specimens to monitor temperature change during heating. Two re-curing methods, air and water, were applied to specimens following the cooling period. SEM, EDX, XRD and TGA investigations were conducted to evaluate the morphological changes in concrete. Then effects of these changes on the residual mechanical properties of concretes were evaluated.

Introduction

Temperature increase in a material produces additional strains and stresses, which may affect mechanical behavior of the material. In cement-based materials there is another mechanism influencing the behavior of the material during temperature exposure. Cement-based materials, especially structural concrete, contain high amount of water molecules in their crystal structures. Breaking of water bonds due to increasing temperature may trigger morphological changes in microstructure resulting in deterioration of concrete. Also evaporated free water molecules cause high vapor pressure in the pores of concrete and sometimes this internal pressure reaches tensile strength of concrete. On the other hand, concrete is incombustible and performs satisfactorily for a long time under fire and no toxic fumes are emitted [1]. Therefore, concrete has good properties in terms of fire resistance. However, various kinds of deteriorations were observed on the concrete specimens, which were subjected to fire or high temperatures such as spalling, crazing, cracking, color changes, mass and strength losses due to aforementioned changes in its morphology and physical structure [[2], [3], [4], [5], [6], [7]]. Also, these deteriorations may be seen following the cooling period [7,8]. Because of the fact that, the stable microstructure of concrete turns to reactive phases (especially CaO and C₂S) after fire. These phases react with water or carbon dioxide and produce new formations in concrete, which may result in physical changes. As a result, a detailed repairing process such as replacing damaged layer of concrete with fresh concrete can be essential after fire.

During heating, spalling is mostly observed especially in high strength concrete [2,4,7,[9], [10], [11], [12], [13], [14]]. Using polypropylene fibers in concrete may reduce the spalling risk of concrete because they melt after 170 °C and leave micro channels which may facilitate evacuation of water vapor [7,10,11] also entrained air voids in concrete may improve the spalling resistance of concrete [7]. Although steel fibers were not found effective as polypropylene fibers in terms of spalling resistance [13,14] they can resist to severe fire conditions during heating and may prevent concrete from further cracking at high temperatures. Moreover, polypropylene fibers disappear after heating and leave voids and channels in concrete but steel fibers survive after heating and can also influence post fire behavior of concrete.

Concrete has lower thermal conductivity and therefore heating of it is relatively slow and in a short fire situation temperature rise in concrete may not result in severe damages. As the duration of heat exposure increases, the temperature in concrete slowly increases [3]. In most cases, only the cover part of a concrete member experiences high temperatures, which may cause severe deterioration in concrete. However, in the studies focused on the tests on small sized specimens, researchers mostly subject their specimens to high temperatures in all directions instead of one or two faces heating. In a real tunnel fire or a residential fire, one or two faces of structural members such as slabs and structural shear walls are directly subjected to heating. Therefore, one or two faces heating of a small sized specimen can better simulate the heating of a structural member in the event of fire [15].

After cooling, if water molecules diffuse in concrete these dehydrated components start to rehydrate and newly formed C-S-H, ettringites, Ca(OH)₂ and CaCO₃ can be observed following the cooling period [[16], [17], [18], [19], [20]]. These new formations can be the reason of recovery observed in physical and mechanical properties of concrete [7,[21], [22], [23], [24]]. Unfortunately, continuous concrete deteriorations can also be observed after cooling. Because of the fact that, CaO turns to Ca(OH)₂ and expands during this period [3] and its expansion may result in complete disintegration of already deteriorated concrete [17,18]. On the other hand, if concrete is directly subjected to water after cooling, Ca(OH)₂ particles produced during re-curing dissolve in water [25] and the further deterioration of concrete can be prevented.

In spite of the importance of recovery of fire-damaged concrete in terms of repair and rehabilitation, very few studies had been conducted on the subject. Most of the available data are in qualitative form and restricted to strength recovery only. Therefore, there is a need to conduct a comprehensive investigation to understand the effects of the rehydration process on the strength and durability recovery of fire-damaged concretes and also the contribution of fibers and chemical admixtures widely used in concrete today. For this purpose, an experimental study was organized to quantitatively investigate the effects of different fibers and air entraining agents on physical and mechanical properties of concrete. One-face heating conditions were also provided during heating processes of small size specimens. Air and water re-curing conditions were applied to heated specimens in order to investigate rehydration and carbonation processes at the post heating stage. Macrostructural and microstructural investigations, mechanical properties tests and measurements were conducted on concrete groups.