Consideration of non linear creep strain of silicious concrete on calculation of mechanical strain under high temperatures as a function of load history
Abstract
Consideration of non linear creep strain of silicious concrete on calculation of mechanical strain under high temperatures as a function of load history
Ulrich Schneider1, Martin Schneider2*, Jean-Marc Franssen3
1Univ. of Techn. Vienna, Karlsplatz 13/206, 1040 Wien, Austria, ulrich.schneider+
2Univ. of Techn. Vienna, Karlsplatz 13/206, 1040 Wien, Austria,
3NFSR Belgium, Univ. of Liege, 1, Ch. Des Chevreuils, 4000, Liège, Belgium,
Keywords: concrete, fire, calculation method, nonlinear creep function
Introduction
The paper is given of a extended model for concrete in compression at elevated temperature that incorporates elastic, plastic and creep strain as a function of temperature and stress history. The mechanical strain calculated as a function of elastic strain, plastic strain and creep strain depends on the stress history on the load level during fire exposure.
In following will be shown a model based of an earlier model presented by some of the authors [1]. It will concluded by a creep function as a function of load history considering the influences of load history of Young’s modulus and compression strength.
A comparison is given between experimental results with cylindrical specimens and calculated results [2].
Relationships
The load factor presents the load history as a function of stress during heating and compression strength.
This will be used in elastic branch of stress-strain-relationship, must be ≤0,3. Assumed in calculation of elastic strain this is usable.
The Young’s modulus depends of load history and temperature.
If grater then 0,3 then the young’s modulus decrease and based on experimental results.
The compression strength also depends on . Next figure shows a function of load history and heating. The maximum of is also 0,3.
If =0 the compression strength calculates in according to Eurocode 2.
In the elastic branch of material law a hardening process is shown. This process is included in the calculation of mechanical strain.
Otherwise the load factor considered in the creep function . This allowing the increase of elasticity due external load while heating by the function g(,T).
Results
The calculation of mechanical strain considering the extended material law and will be compared with measuring results. The results was from an research project [2]. The specimens had following geometric dimensions.
Cylinder with 80 mm of diameter and a high of 300 mm. The experimental heating rate is 2K/min. The compression strength of siliceous concrete at 20°C is 38 MPa.
The experiment considering different load functions while heating.
The calculation with load history and temperature has a very high accordance to measurements.
Outlook
To calculate structures subjected to fire are used material law with specific properties of every materials. The creep function for siliceous concrete depends of load history during heating briefed in this model. The load history depends of restrain stress and could use in this model.
This model could be used to develop a concrete model considering enthalpy inside. This energy could describe the relation between crack opening and closing subjected to fire and restrain stress and load history. Current this will be researched.
References
[1]Schneider, U.; Morita, T.; Franssen, J.-M.: A Concrete Model Considering the Load History Applied to Centrally Loaded Columns Under Fire Attack, Fire Safety Science – Proceedings of the Fourth International Symposium, Ontario, 1994
[2]Schneider, U.: Ein Beitrag zur Frage des Kriechens und der Relaxation von Beton unter hohen Temperaturen, Habilitationsschrift, Braunschweig 1979
Date: 08.10.2018Seite 1