An Investigation into the Thermo-Mechanical Response of High Strength Concrete Elements to Combined Thermal and Impact Loads

Student thesis: Doctoral Thesis


This research project studies the effect of polypropylene fibres and steel fibres on the behaviour of concrete subjected to combined thermal and impact loads. Buildings and structures are vulnerable to extreme load catastrophic events such as fires, accidental collisions and explosive blasts. It is often the situation that a structure is subject to a combination of these extreme loads during these events. Situations with a combination of thermal and impact loading include a building fire with subsequent partial collapse of an upper floor, an accidental impact leading to a subsequent fire or a fire near explosive materials (gas tanks, fertiliser storage).
Concrete is a widely used building material due to its strength, durability, and overall versatility. When concrete is exposed to high temperatures from a fire its strength and stiffness decreases, it may crack or spall and lead to potential structural collapse. Weakened concrete structures under fire conditions are more susceptible to complete collapse if subject to a subsequent impact load. This research project aims to investigate the resistance of reinforced concrete columns subject to combined compressive, thermal and impact loads. The use of polypropylene and steel fibres within concrete is investigated as potential solutions for improving the thermal and impact resistance of the concrete structures.
Experimental studies presented in this thesis examine the response of concrete columns to a combination of compressive, thermal and impact loads. Polypropylene fibres were incorporated into the concrete to improve thermal stability, while steel fibre improved the residual mechanical properties. Three concrete compositions were studied: a control mix without fibres, a polypropylene fibre mix, and a mix containing both polypropylene and steel fibre mix. The dosages of the polypropylene and steel fibre content investigated was 2.5 kg/m3 and 157 kg/m3. A total of sixty concrete specimens were manufactured and experimentally tested (30 no. 100x100x500 mm and 30 no. 150×100x500 mm).
The combined compression, impact and thermal load experiments were successfully achieved through the design and manufacture of a bespoke rig which applied compression via a hydraulic press, heat via ceramic heaters and an impact load via a free-falling 7.5 kg drop-weight to concrete columns. The columns, set with 200 kN of compressive load from the hydraulic press were subject to heating-cooling cycles from room temperature to set temperatures of 200°C, 250°C, 300°C, 350°C and 400°C, at a rate of 10°C.min-1. Once the
desired temperatures were acquired the 7.5 kg free-falling drop-weight was released from a height of 1 meter, impacting the specimens at the mid-span. Temperatures, displacements and strains were recorded as a result of the combined loads.
Results from the experimental trials showed that mid-span deflection due to combined thermal and impact load reached 16 mm at 400°C heating. No significant mass loss occurred between 105°C and 400°C temperature range. The presence of steel fibres does not influence the mass loss of HSFRC. Modulus of elasticity does not degrade significantly up to 250°C; however rapid deterioration occurs in 250-400°C, and reaches to 10.6 GPa at 400°C.
Computational verification was undertaken via transient, non-linear, coupled finite element analysis, using LUSAS Analysis was performed on concrete columns with the load, geometric, boundary condition and material parameters from the experimental trials. A good agreement is shown between the computational and experimental results, replicating the displacement and strain trends observed with temperature rise.
This research project demonstrates that concrete structures subject to high thermal loads are more susceptible to complete failure from subsequent impact loads than without initial heat loads. The project has also shown that the use of polypropylene and steel fibres within concrete can improve the resistance to high thermal loads and subsequent impact loads. This has been achieved through a series of computationally validated novel experimental trials informed by detailed prior research in the field. The study has also provided a sound methodology for testing concrete and other structural samples to combined compressive, thermal and impact loading. These outcomes will prove valuable for designing future concrete structures to withstand high temperature and impact loads and can contribute towards to development of design codes and guidance.
Date of Award25 Jul 2023
Original languageEnglish
SupervisorLaurie Clough (Supervisor), Nikos Nanos (Supervisor) & David Begg (Supervisor)

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