Investigation on Effects and Properties of Concrete Structure after Exposed to Fire

Abstract

The integrity and strength of the materials used in reinforced concrete structures are highly influenced by their exposure environment. Exposing the building materials to high temperature changes their performance. The change in the performance is a result of the change in the material properties. When subjecting a building material to fire, the occurring damage depends on the severity of the fire in terms of the fire temperature and time of exposure. This study investigates the effects of fire exposure on the set of RC beams. Through controlled experiments, varying temperatures from 3000C to 7000C are applied to RC beams, analyzing changes in compressive strength, spalling, and microstructure by using rebound hammer, UPV meter and applying two-point loading in UTM. Results indicate a significant reduction in compressive strength with increasing fire temperature, attributed to thermal degradation of cement paste and aggregate. Additionally, changes in microstructure, such as pore structure alterations and formation of cracks, are observed. Also, few of RC beams were strengthened using GFRP sheets as a U- shaped wrapping to entire beam and U- shaped wrapping at certain intervals. The strengthened specimens are again tested in UTM for two-point loading, the results of the tests showed the feasibility of rehabilitating RC beams exposed to fire using GFRP plates. U-shaped wrapping to entire beam showed the good performance.

Introduction

I. INTRODUCTION

Reinforced Concrete (RCC) structures serve as the backbone of modern infrastructure, providing strength, durability, and stability. However, when exposed to fire, these structures face significant challenges that can compromise their integrity and safety. The post-fire strengthening and retrofitting of RCC beams are critical processes aimed at restoring their structural capacity and ensuring continued functionality. This thesis investigates these processes through a comprehensive review of existing literature, aiming to provide insights into effective methodologies, challenges, and future directions in this field. Fire-induced damage to RCC beams arises from a combination of thermal effects and material behaviour. High temperatures during a fire can led to the deterioration of concrete properties, including strength and modulus of elasticity, as well as the degradation of steel reinforcement. Literature reveals that the severity of damage depends on various factors such as fire duration, temperature, and the structural configuration of the beams. In response to fire-induced damage, researchers and engineers have developed numerous strengthening and retrofitting techniques to enhance the resilience of RCC beams. Fiber-Reinforced Polymer (FRP) composites have emerged as a popular choice due to their high strength-to-weight ratio, corrosion resistance, and ease of application. Studies have shown that externally bonded FRP sheets or wraps can effectively restore the flexural and shear capacities of fire-damaged beams, providing an efficient solution for rehabilitation. Steel plate bonding is another commonly employed method for strengthening fire damaged RCC beams. Literature indicates that bonding steel plates to the surface of the beams can significantly increase their load- carrying capacity, particularly in enhancing flexural strength. Moreover, steel plates offer advantages such as cost-effectiveness and versatility in application, making them a viable option for retrofitting damaged structures.

II. LITERATURE REVIEW

1. Sesha P. Ratnam , Srinivasa K. Rao Research Scholar, Department of Civil Engineering, Andhra University, Visakhapatnam, Andhra Pradesh- 530 003, India Exposure of reinforced concrete (RC) buildings to an accidental fire may result in cracking and loss in the bearing capacity of major components like columns, beams and slabs. It is challenging for structural engineers to develop efficient repair and rehabilitation techniques that enable RC members to restore their structural integrity after being exposed to intense temperatures for a considerable period of time. Therefore, this study was carried out to generate experimental data on repair techniques and performance of heat affected RC beams after repair. Data is presented from tests conducted on RC beams. Initially RC beams were exposed to high temperatures by furnace in which rate of heating is as per standard ISO 834 furnace and repaired using two different repair materials 1 and 2. Non-destructive tests (NDT) were conducted for all the beams after a curing period of 28 days and again after temperature exposure      of 100 to 700°C with increments of 100°C each for 3 h duration. After heating, the fire affected beams were repaired with repair materials 1 and 2. Thereafter, these test specimens were tested by NDT followed by flexure test. The load deflection behaviour of RC beams repaired with repair material 1 and 2 have been studied and presented.
2. E. Bhargayi, N. Girl Bahu and V. Sowjana vani Generally during fire accidents in buildings, columnsand beams are exposed to fire and these structures are expected to perform well during such extreme conditions. This shows the importance of the study of concrete structures when they are exposed to fire. The objective of this limited study is to provide an overview of the effects of size and fire on columns and beams. In meeting this objective previous investigations done by various researchers are summarized.
3. Salim Barbhuiya and Abdul Munim Choudhury (2015) Studied the size effect of RC beam column connections under cyclic loading. Ordinary Portland cement of 53 Grade is used. Reinforcing steel of diameters 20 mm, 12 mm and 8 mm are used, considering three types of beams–column connections with some specific deficiencies. Cyclic load is applied with displacement-controlled load. Parameters studied are energy dissipation. From the study it is concluded that size effect is more pronounced in specimens exhibiting brittle mode of failure.
4. Mohammed Mansour Kadhum et.al In this paper, the authors discussed a study in which some mechanical properties and deflection behavior of rectangular reinforced concrete beams under the effect of fire flame exposure is presented. The properties investigated were compressive strength and load-deflection behavior of rectangular reinforced concrete beams under the effect of fire flame exposure. The concrete specimens and beams were subjected to fire flame temperatures ranging from (25-800) °C at different ages of 30, 60 and 90 days, three temperature levels of 400, 600 and 800°C were chosen for exposure duration of 2.0 hours. Authors found that,

• The residual compressive strength ranged between (62 – 72 %) at 400 °C, (52 – 62%) at 600 °C and (38 – 49 %) at 800 °C.
• Large proportion of drop in compressive strength occurs at the first 1.0-hour period of exposure.
• Based on the results obtained, it was found that the shrinkage values increase with temperature increase.
• The temperature distribution through the thickness of beam that was found in this investigation is similar for all the beams which have the same thickness and exposed period to fire flame.
• After the beams were subjected to fire flame, two types of cracks developed. The first was thermal cracks, which appeared in honeycomb fashion all over the surface. The second crack originated at mid-span region due to bending from the applied load and called flexural cracks.
• It was noticed that the load deflection relations to specimens exposed to fire flame are flat, representing softer load- deflection behavior than of the control beams. This can be attributed to the early cracks and lower modulus of elasticity.
• At temperature of (400°C), both burning and subsequent cooling did not affect the mechanical properties of steel reinforcement; the effect was observed at 600 and 800°C. The residual yield tensile stress and residual ultimate stress was (90.6%, 78.8% and 89.8%, 81.4%) respectively.
• Modulus of elasticity of concrete is the most affected by fire flame temperature rather than compressive strength.

5. Ashok R. Mundhada, Arun D. Phophale et.al In this paper the author studied the effect of high temperatures on compressive strength of concrete. 90 concrete cubes of 150 mm size divided equally over three different grades of design mix concrete viz. M 30, M 25 & M 20 were cast. After 28 days’ curing & 24 hours’ air drying, the cubes were subjected to different temperatures in the range of 200°C to 800°C, for two different exposure times viz. 1 hour & 2 hours in an electric furnace. The heated cubes were cooled at room temperature for 24 hours & then subjected to cube compressive strength test. Mix design was carried out using the Ambuja method of design. The conclusions of the test were,

6. Dattatreya, B. Balkrishna Barathea In this paper the author did research work on studying the impact of fire on reinforcement provided in R.C.C structures of various types of buildings which are under blast or fire. The Behavior of Steel Reinforcement at           various elevated temperatures from 100° C to 1000°C was studied. The specimens for testing were TMT bars of 12mm diameter. 20 bars were cut to 30 cm size. Then the specimens were tested for mechanical properties using UTM before heating at normal room temperature and the properties were tabulated. 10specimens each were heated in the electric furnace at 100°, 300°, 600°, 900°C and 1000°C for an hour without any interference. After heating, out of 10 specimens for each temperature 5 samples were quenched in cold water for rapid cooling and the other 5 were kept aside for normal cooling at atmospheric temperature. These specimens later were tested for mechanical properties with UTM. The authors concluded that,

• Ductility of quickly cooled reinforced bars after heating to high temperature of 1000°C decreased which could be dangerous for a structure.
• Significant change in   ductility   was observed at high temperatures & near ultimate load.
• 
  A major problem caused by high temperatures was the spelling of concrete i.e. separation of concrete mass from concrete element which resulted in the exposure of steel reinforcement directly to high temperatures.
• If the duration and the intensity of fire are higher, the load bearing decreases to the extent of the applied load resulting in collapse of structure.

III. METHODOLOGY

A. Test on Fired Beam by using of (UTM)

1) Arrangement Of Two Point Loading

• Prepare the sample: Ensure the sample is properly prepared and positioned on the testing machine's bed or fixture. The sample should be aligned with the machine's axis and securely held in place to prevent movement during testing.
• Set the distance: Determine the distance between the two point loads based on the specifications of your testing procedure or the properties you want to measure. This distance will vary depending on factors such as sample size, material properties, and testing standards.
• Position the load points: Adjust the grips or fixtures of the testing machine to position the two point loads at the specified distance apart along the length of the sample. Ensure that the loads are applied symmetrically to avoid introducing any bending or twisting forces that could affect the test results.
• Calibrate the machine: Before applying any loads, calibrate the testing machine to ensure accurate measurement and control of the applied forces. This may involve zeroing the load cell, verifying the displacement measurement system, and checking the alignment of the load points.
• Apply the loads: Once the machine is calibrated and the sample is properly positioned, gradually apply the desired loads to the sample using the testing machine's controls. Monitor the applied force and any resulting deformation or displacement of the sample throughout the testing process.
• Record data: During the test, record relevant data such as applied force, deformation, and any other parameters specified by your testing procedure. This data will be used to analyze the sample's mechanical properties and performance.
• Evaluate results: After completing the test, analyze the recorded data to evaluate the sample's response to the applied loads. This may involve calculating mechanical properties such as tensile strength, elastic modulus, or fracture toughness, depending on the nature of the test.

Conclusion

1) From the performance of above test on the beams it concludes that there is no major temperature effect up to 300? to 400??. 2) At 500? colour beam changes and there is no spalling. 3) As a temperature rises at 600? minor spalling occur and cracks are form at temperature 700?, and the colour of beam changes into light brown and major cracks are found on beams. 4) Rebound number and compressive strength decreases with increase in temperature. 5) Range from 300? to 400? the compressive strength decreases 20% to 80% with the increase in temperature. 6) Maximum compressive strength decreases 80% at 700?. 7) The type of concrete observed from UTM test i.e concrete deteriorate, 8) As compare with the reference beam failure load decreases by 48% at 700? as the failure load decreases with increase in temperature. 9) Varying in temperature deflection increases and maximum deflection reduced at 700? 10) Regarding to strengthening technique U- shape wrapping to entire beam the failure load increases by 37%as compare to U- shape wrapping at intervals.

References

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Copyright

Copyright © 2024 Prof. S. V. Dhoke, Akshay Chikate, Abhijit Andhare, Anikesh Sable, Ashish Gabhane, Himanshu Bhabutkar, Yash Gandhe. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.