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Josipa Bošnjak
Materials Testing Institute, University of Stuttgart, Germany

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Journal article
Published: 01 December 2020 in Journal of Building Engineering
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This paper presents a series of tests conducted to study the post-fire (residual) performance of reinforced concrete beams with critical (under-designed) lap splices. Furthermore, a 3D numerical modelling approach is validated against the experimental results. Two types of beams, namely Type 1 - with continuous reinforcement, and Type 2 – with critical lap splice, were investigated. Three specimens of each beam type were tested: (i) in ambient conditions (reference tests), (ii) in residual state after exposure to standard ISO 834-1 fire of 60 minutes, and (iii) in residual state after exposure to standard ISO 834-1 fire of 90 minutes. Under ambient conditions, the beam with continuous reinforcement failed due to yielding of tension reinforcement and resulted in a ductile load-displacement curve. On the other hand, the beam with critical lap splice failed due to bond splitting and resulted in a brittle load-displacement behaviour. After exposure to fire, beams with continuous reinforcement showed a nominal effect of fire on residual behaviour, while the beams with critical lap splice displayed a drastic reduction in their capacity and poor performance after exposure to fire. It is shown that the chosen numerical approach realistically reproduces the experiments.

ACS Style

Akanshu Sharma; Josipa Bošnjak; Margaritis Tonidis. Post-fire performance of RC beams with critical lap splices – Experimental investigation and numerical validation. Journal of Building Engineering 2020, 34, 102045 .

AMA Style

Akanshu Sharma, Josipa Bošnjak, Margaritis Tonidis. Post-fire performance of RC beams with critical lap splices – Experimental investigation and numerical validation. Journal of Building Engineering. 2020; 34 ():102045.

Chicago/Turabian Style

Akanshu Sharma; Josipa Bošnjak; Margaritis Tonidis. 2020. "Post-fire performance of RC beams with critical lap splices – Experimental investigation and numerical validation." Journal of Building Engineering 34, no. : 102045.

Journal article
Published: 21 September 2019 in Construction and Building Materials
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In this work, experimental and numerical studies are performed to investigate the influence of elevated temperature on the residual (post-heating) behavior of masonry. Two different types of solid brick units namely, clay bricks and calcium silicate bricks, were used in the study. The tests were performed on the individual brick units, the mortar as well as the masonry prisms (brickwork bricks and mortar). The specimens were subjected to elevated temperature reached at a relatively low heating rate and the target temperature was retained for a minimum of 2 h to ensure uniform heating of the specimen followed by slow cooling to room temperature. The residual performance of the specimens was tested to verify the influence of temperature on the behavior of masonry. The numerical (3D finite element) modeling of the specimens was performed and temperature dependent microplane model was used as the constitutive law for the brick and mortar. The numerical modeling approach has been validated and shown to be able to realistically simulate the residual behavior of masonry after exposure to elevated temperature. The results of experimental and numerical study show a relatively moderate influence of temperature on the strength but a strong influence on the stiffness of the masonry system.

ACS Style

Josipa Bošnjak; Serena Gambarelli; Akanshu Sharma; Amra Mešković. Experimental and numerical studies on masonry after exposure to elevated temperatures. Construction and Building Materials 2019, 230, 116926 .

AMA Style

Josipa Bošnjak, Serena Gambarelli, Akanshu Sharma, Amra Mešković. Experimental and numerical studies on masonry after exposure to elevated temperatures. Construction and Building Materials. 2019; 230 ():116926.

Chicago/Turabian Style

Josipa Bošnjak; Serena Gambarelli; Akanshu Sharma; Amra Mešković. 2019. "Experimental and numerical studies on masonry after exposure to elevated temperatures." Construction and Building Materials 230, no. : 116926.

Journal article
Published: 07 March 2019 in Engineering Structures
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In this work, the residual bond performance of reinforcement in concrete under elevated temperatures is experimentally investigated using the cuboidal (square prismatic) concrete specimens. The test reinforcement was placed at different cover distances with an aim to study the influence of elevated temperature on bond pullout and bond splitting behavior. It was found that under ambient conditions as well as after exposure to elevated temperature, concrete cover has a strong influence on the absolute value of the bond strength and failure mode. The thermal degradation of bond strength is found to be similar to that of compressive or tensile strength of concrete for pure bond or splitting failure respectively. The level of confinement provided by the concrete cover to the rebar reduces with increasing temperature, while the width of splitting cracks increases with increasing temperature. The bond stiffness gradually reduces with the increase in exposure temperature for both pullout and splitting failure. The details and the results of the experimental investigations are reported in this paper.

ACS Style

Akanshu Sharma; Josipa Bošnjak; Saskia Bessert. Experimental investigations on residual bond performance in concrete subjected to elevated temperature. Engineering Structures 2019, 187, 384 -395.

AMA Style

Akanshu Sharma, Josipa Bošnjak, Saskia Bessert. Experimental investigations on residual bond performance in concrete subjected to elevated temperature. Engineering Structures. 2019; 187 ():384-395.

Chicago/Turabian Style

Akanshu Sharma; Josipa Bošnjak; Saskia Bessert. 2019. "Experimental investigations on residual bond performance in concrete subjected to elevated temperature." Engineering Structures 187, no. : 384-395.

Journal article
Published: 24 January 2019 in Fibers
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Addition of steel fibres to concrete is known to have a significant positive influence on the mechanical properties of concrete. Micro polypropylene (PP) fibres are added to concrete to improve its performance under thermal loads such as in case of fire by preventing the phenomena of explosive spalling. An optimum mixture of steel and micro PP fibres added to concrete may be utilized to enhance both the mechanical and thermal behaviour of concrete. In this work, systematic investigations were carried out to study the influence of elevated temperature on the mechanical properties and physical properties of high strength concrete without and with fibres. Three different mixtures for high strength concrete were used, namely normal concrete without fibres, Steel fibre reinforced concrete and Hybrid fibre reinforced concrete having a blend of hooked end steel fibres and micro PP fibres. The specimens were tested in ambient conditions as well as after exposure to a pre-defined elevated temperature and cooling down to room temperature. For all investigated concrete mixtures the thermal degradation of following properties were investigated: compressive strength, tensile splitting strength, bending strength, fracture energy and static modulus of elasticity. This paper summarizes the findings of the tests performed.

ACS Style

Josipa Bošnjak; Akanshu Sharma; Kevin Grauf. Mechanical Properties of Concrete with Steel and Polypropylene Fibres at Elevated Temperatures. Fibers 2019, 7, 9 .

AMA Style

Josipa Bošnjak, Akanshu Sharma, Kevin Grauf. Mechanical Properties of Concrete with Steel and Polypropylene Fibres at Elevated Temperatures. Fibers. 2019; 7 (2):9.

Chicago/Turabian Style

Josipa Bošnjak; Akanshu Sharma; Kevin Grauf. 2019. "Mechanical Properties of Concrete with Steel and Polypropylene Fibres at Elevated Temperatures." Fibers 7, no. 2: 9.

Journal article
Published: 13 January 2018 in Materials and Structures
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Temperature-dependent bond behavior has so far been investigated primarily using small cylindrical pull-out specimens in steady-state conditions. In this work, a new test setup is developed to investigate the bond behavior between reinforcing bars and concrete after fire. The setup uses beam-end specimens, which simulate the boundary conditions in flexural members more closely than the pull-out specimens. The major advantage is that different parameters can be easily varied. A previously conducted numerical study indicated the suitability of beam-end specimens to evaluate the bond performance after fire. In this work, pilot experiments were performed using beam-end specimens after exposure to the standard ISO 834 fire for 30 min. Accompanying reference specimens were tested at room temperature without exposure to fire. The details of the test setup and the results are reported in this paper. It is found that the existing test data obtained using pull-out specimens in steady conditions might not be conservative under realistic boundaries and loading. This result underlines the need to modify the experimental approach, in order to be able to realistically predict the performance of steel-to-concrete bond. In order to validate the thermo-mechanical numerical model employed in the previous numerical work, the tests were also numerically simulated. The results of the numerical analysis were found to be in good agreement with the experimental results.

ACS Style

Josipa Bošnjak; Akanshu Sharma; Christian Öttl. Modified beam-end test setup to study the bond behavior of reinforcement in concrete after fire. Materials and Structures 2018, 51, 13 .

AMA Style

Josipa Bošnjak, Akanshu Sharma, Christian Öttl. Modified beam-end test setup to study the bond behavior of reinforcement in concrete after fire. Materials and Structures. 2018; 51 (1):13.

Chicago/Turabian Style

Josipa Bošnjak; Akanshu Sharma; Christian Öttl. 2018. "Modified beam-end test setup to study the bond behavior of reinforcement in concrete after fire." Materials and Structures 51, no. 1: 13.