Stress-strain-behaviour of concrete at extremely low temperature

The application of prestressed concrete for the construction of storage vessels for liquefied natural gas (LNG) requires concise knowledge of the behaviour of concrete at extremely low temperatures. The tests reported herein show that the strength of concrete increases with the decrease of temperature. This increase is the more pronounced the greater the moisture content of concrete at test. While at room temperature concrete exhibits a ductile behaviour, the stress-strain-behaviour is changed at low temperatures towards increasing brittleness. A relation between the compressive strength, the strain at maximum stress and the thermal strain was found in the tests as a function of temperature.     

Thermal expansion of Portland cement paste,mortar and concrete at high temperatures

Results of measurements of thermal expansion of Portland cement paste, mortar, concrete and a dolomitic rock aggregate are presented. Test temperatures ranged from 27 to 871°C. Cement paste contracted when subjected to temperatures from 204 to 871°C. Thermal expansion of mortar and concrete was dominated by thermal expansion characteristics of the mineral aggregate. However, expansion was moderated by contraction of the cement paste matrix at elevated temperatures. Average coefficients of expansion, over specific temperature ranges, are given for the materials tested. Equipment developed to control automatically the rate of heating and record strain vs. temperature relationships is described.     

Thermal deformation of loaded concrete at low temperatures. 3: Lightweight concrete

Longitudinal and transverse thermal deformation has been measured on lightweight concrete cylinders during a thermal cycle, from room temperature up to −90°C. Oven-dry samples show an almost linear, isotropic, reversible and load independent dilatational behaviour. Water-saturated samples behave in a different way; when load-free, thermal deformation is nearly isotropic, although highly non linear (between −10°C and −60°C) and ending with an irreversible expansion. When axially loaded (10 MPa), lightweight concrete shows a strongly anisotropic behaviour. Volumetric thermal deformation, for a given moisture content, does not seems to depend on applied load.     

Effect of elevated temperatures on geopolymer paste, mortar and concrete

Geopolymers are generally believed to provide good fire resistance due to their ceramic-like properties. Previous experimental studies on geopolymer under elevated temperatures have mainly focused on metakaolin-based geopolymers. This paper presents the results of a study on the effect of elevated temperature on geopolymer paste, mortar and concrete made using fly ash as a precursor. The geopolymer was synthesized with sodium silicate and potassium hydroxide solutions. Various experimental parameters have been examined such as specimen sizing, aggregate sizing, aggregate type and superplasticizer type. The study identifies specimen size and aggregate size as the two main factors that govern geopolymer behavior at elevated temperatures (800 °C). Aggregate sizes larger than 10 mm resulted in good strength performances in both ambient and elevated temperatures. Strength loss in geopolymer concrete at elevated temperatures is attributed to the thermal mismatch between the geopolymer matrix and the aggregates.     

Thermal deformation of loaded concrete at low temperatures, 2: Transverse deformation

Transverse thermal deformation of concrete cylinders has been measured during a thermal cycle, up to −90°C. Oven dry concrete shows an almost linear, reversible and isotropic behaviour. Load-free water-saturated concrete exhibits a complex strain behaviour, characterized by dilatation during cooling (from −10°C to −60°C) and an irreversible expansion after reheating. Longitudinal loading (15 MPa) enhances the aforementioned behaviour. A qualitative explanation of such effects is outlined.     

Short-term creep of concrete at elevated temperatures

Results of a study of time-dependent deformation of concrete over a period of five hours at room and elevated temperatures (22 to 649°C) are described. The influences of load, temperature and material variables are examined and data compared with several mathematical models commonly used to describe concrete creep behavior. Information obtained from this study will help in evaluation of the design of concrete structures to withstand fire and other high temperature environments.     

Stress-strain behaviour of concrete and mortar at high rates of tensile loading

The Split-Hopkinson-Bar technique was used in the investigation on tensile stress-strain behaviour of concrete and mortar at high stress rates (5–30 N/mm²ms).The single loading tests showed that the impact tensile strength was higher than the static one, and that impact strains at the maximum stress were larger than static strains.The impact fatique tensile tests indicated an increase of strains in the course of fatigue loading and increasing fatique life with decreasing maximum stress level.These phenomena are discussed with the aid of fracture mechanics concepts and explanations for differences in the behaviour of concrete and mortar are suggested.     

Analysis of composite steel and concrete columns at high temperatures

The growing demand for knowledge about the effect of fire on structures has stimulated research worldwide. This article presents experimental results of short, composite columns made of steel and concrete when submitted to high temperatures in furnaces, with and without axial compression loading, as well as a numerical analysis of the temperature distribution in these columns. The columns were modeled as concrete‐filled tubes with three thicknesses and two diameters considered. In addition, standard fire temperature–time curves were obtained experimentally for use in the numerical calculations. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermal and oxidation chemistry of coal at low temperatures

The chemical reactions which occur when Illinois No. 6 (hv C) and Rawhide (SBB C) coals are thermally pretreated at 100°C and when Illinois No. 6 coal is subsequently oxidized at 100°C with O₂ have been studied using in-situ FT-i.r. differende spectroscopy. Significant spectroscopic changes were seen. Vacuum drying at 100°C resulted in the decomposition of carboxylic acid species to form a variety of new carbonyl species (in Rawhide) and decarboxylated or decarbonylated coal (Illinois No. 6). Oxidation of predried Illinois No. 6 coal leads to the formation of new carbonyl species. The assumption that drying does not alter the chemical composition of coal may not be correct. Thus, overall spectroscopic (and chemical) changes observed in moderate temperature reaction studies may depend upon sample pretreatment, drying and storage. In addition, the time/temperature profile used in a reaction study may affect the overall changes observed by altering the relative contribution of the different reactions.     

Fundamental studies of NOx storage at low temperatures

A commercial NO_x storage catalyst (Pt, BaO and alumina containing) was investigated by temperature programmed desorption (TPD) experiments in the temperature range 100–400 °C. The catalyst stored a substantial amount of NO_x at 100 °C using NO + O₂. Nitrites or loosely bound NO species are suggested for this storage, since no NO was oxidised at this low temperature. In addition, the released NO_x during the temperature ramp consisted of mainly NO and at lower temperatures the NO₂ dissociation is limited. Water and CO₂ was found to decrease the storage substantially, 92% for the NO + O₂ adsorption at 100 °C. The total storage for 60 min using NO₂ + O₂ at 200 °C was similar when introducing CO₂ and H₂O. However, the initial total uptake of NO_x was decreased. Initially we probably formed loosely bound NO_x species, which likely are strongly influenced by water and CO₂. After longer time periods are barium nitrates probably formed and they can remove the carbonates by forming stable nitrates, thus resulting in the same total uptake of NO_x.     

Creep of concrete at elevated temperatures and boundary conditions of moisture

First test results are presented concerning the creep of concrete at elevated temperatures up to T = 130 °C, attained on both drying specimens and those in a vapour-saturated environment. The experiments were realized by new test techniques, which were developed on the basis of a critical analysis of former creep tests.The creep deformations of unsealed specimens heated up after loading are in the scattering range of results known so far. However, sealed specimens under vapour-saturated conditions for T = 130 °C as opposed to T = 20 °C leads to creep deformations about 10 times higher. The results are discussed and conclusions drawn.