Effect of Fire On Flexural Strength Of Reinforced Concrete Beam

Effect of Fire On Flexural Strength Of Reinforced Concrete Beam

1Rahul P. Chadha, 2A R Mundhada

1 Final Year Student (M.E. Structure), 2 Professor Department Of Civil Engineering

Prof. Ram Meghe Institute of Technology & Research Badnera, Amravati-444701, Maharashtra, India

Abstract

Going vertical is the order of the day. Most of us in metropolitan cities invariably end up living in a high-rise apartment complex. Tier-2 cities too are not far behind, for a significant number of new developments are multi-storey apartment complexes. With habitat becoming increasingly dense, one needs to ask : Are the high -rise buildings that are the future of the urban landscape equipped to deal with emergencies? One such emergency is fire, from which no structure, however well-built, is immune. This work also gives an insight in to behavior of RCC structures in fire. The experimentation has been done to find out the effect of fire on flexural strength of reinforced concrete beams. After heating, these specimens were allowed to cool at room temperature & some samples were quenched with water for rapid cooling and then tested for flexural strength.

Simultaneously, theoretical investigation of various parameters in relation to fire was carried out.

Index terms Reinforced Concrete, Beams, Fire resistance, Flexural strength, Spalling , C25-Beam with clear cover 25mm, C30-Beam with clear cover 30mm

1. Introduction

   We are all aware of the damage that fire can cause in terms of loss of life, ho mes and livelihoods. A study of 16 indus trialized nations (13 in Europe plus the USA, Canada and Japan) found that, in a typical year, the nu mber of people killed by fires was 1 to 2 per 100,000 inhabitants and the total cost of fire da mage a mounted to 0.2% to 0.3% of GNP. UK statistics suggest that of the half a million fires per annum attended by firefighters, about one third occur in occupied buildings and these result in around 600 fata lit ies (almost all of which happen in dwe llings). The loss of business resulting fro m fires in co mmerc ia l and office buildings runs into millions of pounds each year. The extent of such damage depends on a number of factors such as building design and use, structural performance, fire e xt inguishing devices and evacuation procedures. Although fire safety

   standards are written with this e xpress purpose, it is understandably the safety of people that assumes the greater importance. Appropriate design and choice of materia ls is crucial in ensuring fire safe construction. Codes and regulations on fire safety are updated continually, usually as a result of research and development.

   Most local and overseas studies on concrete under elevated temperatures have not consider the effects of water quenching during firefighting operations. Though concrete and steel rein forcing bars are non- combustible, both have been shown to degrade in strength during and after e xposure to the high temperatures of a fire. However, there appears to be little data available in respect of the effects of water quenching on the fire perfor mance of high strength concrete in a building fire.

   The aim of this dissertation was to increase the awareness of the structural engineering fie ld to the concepts behind structural design for fire safety. The development of simplified design tools that predict the fire performance of structural ele ments is of utmost importance to practicing structural engineers. These tools address structural fire performance fro m an applied design approach similar to those which e xist for the effects of wind and earthquake loads. Extensive research has been published on the performance of structural steel in fire conditions, and simplified design tools already e xist to describe its behavior. However, such tools do not exist for reinforced concrete structures where research has been focused on the materia l properties of concrete in fire conditions rather than structural performance.

2. Experime ntal Work

The specimens for testing were RCC bea ms. Forty two RCC bea ms were cast with similar c ross – sectional details, length and grade of concrete and clear cover provided to reinforcement. Six specimens were tested for the Fle xura l strength using UTM before heating at norma l temperature and the result were tabulated. Twelve specimens (6 specimen of 25mm c lear cover & 6 specimen of 30mm c lear cover) each were heated in the electrica l furnace at 550°C for 1 hour and 2 hour respectively without any disturbance. Same procedure was repeated for 12 specimens each for

750°C and 950°C. After heating, specimen were kept aside for norma l cooling at atmospheric temperature. Three mo re samples of clear cover 25mm were heated for 2 hours at 750°C but were quenched with water & rapidly cooled. The bea ms of size 150x150x700(a ll dimensions in mm) were kept on the UTM with setup to check for fle xura l strength. Point load was applied at a constant rate for a ll the specimens.

Results & Discussions

Ther mogravi metric Analysis (TGA)

Thermo gravimetric analysis consists of finding change in weight of a materia l with increase in temperature. Th is plot is called a Thermogra m.. This technique allows to find out the temperature range in wh ich a materia l will re ma in stable and the temperature at which it would undergo decomposition.

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C 251hr C 30 1hr

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Flexural streng th testing on UTM

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C 30 2 hr. C 25 2 hr.

C 30 1 hr. C 25 1 hr.

Effects of Wate r Quenching on Reinforced Concrete Structures under Fire

Temperature v/s Strength

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Quenching

Simu ltaneously, theoretical investigation of various

parameters in re lation to fire will be carried out.

Effect of duration afte r heating beam at 950°C

1. Conclusion

   Based on the results of this experi mental work the following c onclusions can be dr awn:

   High temperature is one of the most important

   physical deteriorat ion processes that influence the durability of concrete structures and may result in undesirable structural fa ilures.

   When exposed to high temperature, the physical structure of the Re inforced concrete bea ms change considerably.

   The general conclusion is that majority of fire damaged RCC structures are repairable. But the effect of elevated temperature above 900°C on the reinforced concrete beams was observed that there is significant reduction in fle xura l strength.

   The effect of fire on the reinforced concrete beams

   heated at 750°C cooled rapidly by quenching in water and normally cooled in the atmospheric temperature were studied and it is observed that the strength of rapidly cooled bea ms is high.

   The fle xu ral strength for beams exposed to fire at 550°C & 750°C for 60 and 120 minutes were less than that for the reference beam by about 34.84% and 44.37% respectively. But for 950°C there is significant decrease in fle xu ra l strength by about 61.99% and 64.24% respectively.

   The reductions in strength for beams e xposed to fire with a cover thickness of 25mm & 30mm is almost same at 550°C say 35% less than that for the reference beam ,but 60% and 47% less than that for the reference beam at 750°C and for 950°C it is 64% and 61% respectively.

   By heating the reinfo rced concrete beams, we ight loss is negligible say 4% till 750°C but there is a significant we ight loss say 12% at 950°C.

   Some spalling of concrete was observed in the beam e xposed to fire for 2hr at 950°C at the time of re moval fro m furnace, wh ich increased with time under normal weathering conditions.

   To sum up, up to 550°C, the weight l oss for RCC is negligible & the flexural strength reduces by 1/3rd. No cracking, spalling or scaling is obser ve d up to this stage. The fire affec te d structure up to this point onl y re quires rapi d cooling & re pairs. At @ 750°C, there is a further drop in weight & fle xur al strength, cracks do appe ar but there is har dly any spalling or scaling. The fire affecte d structure at this point re quires rapi d c ooling & retrofitting. Factor of safety will come down but the structure will be service able. Beyond this stage, all the par ameters drop alar mingly. Weight l oss at 950°C excee ds 10% , flexur al strength c omes down by 2/3rd, major cracking, spalling & scaling could be obser ve d. The fire affec te d porti on at this stage may re quire major retrofitting or re pl ace ment & c ant be relie d

   upon. Gre ater cover & faster cooling provi de relief.

2. Future Scope

   Due to the paucity of funds, the experimentation had its limitations. To increase the database size, similar d issertation work can be ta ken up in future for temperatures of 650°C, 850°C & 1050°C. If charts are prepared by combining the two works, they will be more representative in nature & will help in predicting the drop in fle xura l strength for intermediate te mperature values.

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Resistance Of Reinforced Concrete Flexural M embers Journal Of Structural Engineering © A SCE / AUGUST 2004 / 1225