F.S. Rostásy

Changes of pore structure of cement mortars due to temperature

Abstract Changes of pore structure of cement mortars caused by high temperatures (up to 900 °C) as well as by extremely low temperatures (down to − 170 °C) could be proved by means of mercury porosimetry. While high temperatures lead to an increase of the total volume of pores > 40 A low temperatures do not. But in both cases a coarsening of the pore structure occurs. The usefulnes of the test method for detecting structural defects caused by a temperature treatment was shown exemplarily by establishing an experimental relation between the pore volume and the residual strength after low temperature cycles.

Behaviour of mortar and concrete at extremely low temperatures

Abstract The behaviour of hydrated cement paste, mortar and concrete specimens (age ∼ 110 days) was investigated at low temperatures and after cyclic temperature-time histories. The relative changes of thermal strains, compressive and tensile splitting strength as well as the alteration of pore structure were studied. The strength in the frozen state increases with decreasing temperature and rising moisture content. If the frozen water-saturated mortar or concrete is reheated to room temperature a loss of strength is registered. The loss of strength increases with the number of temperature cycles. Storing the specimens at rel. hum. below 85% no losses were registered and no irreversible expansion occured. Water-saturated mortar showed after only 12 cycles an irreversible positive strain of 2,7%, indicating internal damage which was also proved by mercury porosimetry. Thus, the moisture content seems to be one of the decisive factors affecting the sensitivity of mortar and concrete subjected to extremely low temperatures.

A method for the calculation of the chemical composition of the concrete pore solution

A method for the calculation of the chemical composition of the concrete pore solution is given. It is based on the solution of a non linear system of equations, built up by the solubility products of the difficult soluble phases, the ion product of water and the equilibrium constant of the reaction between CaOH+ and Ca2+ in water. An approximation algorithm for the determination of the activity coefficients of CaSO4 and Ca(OH)2 is incorporated in the calculation. The model was tested on data presented by Moragues et al. and Andersson et al..

Transient creep of concrete under biaxial stress and high temperature

Understanding concrete behavior under transient high temperature is an important aspect of structural fire research. Consequently, many investigations have been performed under uniaxial loading conditions, but, only a few under biaxial states of stress. This report has two primary goals. The mechanical behavior of unsealed concrete and mortar was studied under biaxial stresses and transient temperature. Furthermore, the influence of the concrete composition was investigated for quartzitic concrete and mortar in thermal expansion tests and biaxial transient creep tests. Based on the test results a model of the transient creep of concrete under multiaxial loading is proposed. In combination with a suitable high temperature failure model, this model is applicable for concretes with different compositions.

A model for the calculation of combined chemical reactions and transport processes and its application to the corrosion of mineral-building materials: Part I. Simulation model

Abstract The numerical simulation of corrosion processes can be a valuable tool for the planning of durable structures exposed to chemical attack and for planning of their maintenance and repair. In this paper, a simulation model—incremental in time and space—of corrosion of mineral-building materials is described. The model combines the calculation of transport processes with the quantitative simulation of chemical reactions. The following transport processes are incorporated: capillary suction, diffusion of solved species (including the influence of the diffusion potential), water vapor diffusion, and diffusion of solved species through the inner surface of cracks. The reactions are simulated by the repeated calculation of the thermodynamic and kinetic stable phase assemblage. Due to the thermodynamic module, the simulation does not depend on the specific reaction. Additional modules are incorporated for the actualization of transport parameters and material strength as well as for the expansion in the case of sulphate attack.

Stress-strain-behaviour of concrete at extremely low temperature

Abstract 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.

PapersA model for the calculation of combined chemical reactions and transport processes and its application to the corrosion of mineral-building materials: Part I. Simulation model

Abstract The numerical simulation of corrosion processes can be a valuable tool for the planning of durable structures exposed to chemical attack and for planning of their maintenance and repair. In this paper, a simulation model—incremental in time and space—of corrosion of mineral-building materials is described. The model combines the calculation of transport processes with the quantitative simulation of chemical reactions. The following transport processes are incorporated: capillary suction, diffusion of solved species (including the influence of the diffusion potential), water vapor diffusion, and diffusion of solved species through the inner surface of cracks. The reactions are simulated by the repeated calculation of the thermodynamic and kinetic stable phase assemblage. Due to the thermodynamic module, the simulation does not depend on the specific reaction. Additional modules are incorporated for the actualization of transport parameters and material strength as well as for the expansion in the case of sulphate attack.

Compressive strength and deformation of steel fibre reinforced concrete under high rate of strain

Abstract The influence of high strain rates on the strength and deformation of fibre reinforced concrete (0,75% and 1,50% by volume hooked and straight fibres) in uniaxial and eccentric compression tests is reported. The tests were performed on normal weight concrete with a mean strength of 25 MPa (water-cement ratio 0.89). The results show that the dynamic strength ( ϵ dyn / ϵ stat ≈ 10 4 ) is about 20 per cent higher, at which the ultimate strain also increases by 10 to 20 per cent. The failure of the fibre concrete specimens occurred in a much more ductile manner compared to plain concrete; this was found for static and impact loading to the same extent. The residual static strength of eccentrically loaded concrete is slightly diminished by a dynamic preloading.

On prediction of relaxation of colddrawn prestressing wire under constant and variable elevated temperature

Abstract During the development of a gas-cooled high-temperature reactor HTR 500 in the Federal Republic of Germany, it became necessary to examine the relaxation behaviour of high strength colddrawn prestressing wires under stationary and transient elevated temperature. Isothermal relaxation tests were caried out at ten temperatures between 20 and 175°C with at least two tests per temperature. In anisothermal relaxation tests, three different temperature histories with initial and multiple activations were investigated. An expression could be achieved to describe the observed isothermal relaxation. On the basis of the initial activation behaviour an equation was developed to describe the temperature dependent relaxation during heat-up. Based on the results gained in this investigation a principle of superposition was proposed to calculate the resultant relaxation for a given temperature history.

NONLINEAR STATIC ANALYSIS OF END-FITTINGS FOR GFRP-PRESTRESSING RODS

Abstract This contribution affords an insight into a current research project about end fittings for pretensioned GFRP rods. The purpose of the present paper is to show that the ADINA-FE-program can be modified by the user at reasonable expense for the analysis of structures and materials whose properties cannot be reproduced adequately by the original version of the program. For the analysis of anchorage elements for pretensioned GFRP rods a modified elastic-plastic material model for 2/D-elements and a bond-link-element have been added to the program maintaining completely its modular structure. The modifications are explained and the stiffness matrix for the bond-link-element is derived. With this modified program different types of end fittings are analyzed with special regard to bond between the GFRP rod and the surrounding synthetic resin material. A comparison of the results of the FE-analysis shows good correspondence to test data.