Effect of Fire on RC Slab

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 04 | September 2016 ISSN (online): 2349-6010

Effect of Fire on RC Slab Poorna S M. Tech. Student Department of Civil Engineering FISAT, Kerala, India-683577

Reshma Prasad Assistant Professor Department of Civil Engineering FISAT, Kerala, India-683577

Abstract Concrete is a construction material used for buildings, bridge decks and also used for other special purposes. Concrete has now become a very common construction material. The main advantage of this building material is that it can be designed as per the requirements. In the modern industry, grade of concrete used varies from M20 to M70. Even higher grades are designed if needed. When the concrete is used in special purposes the chance of fire exposure also increases. In case of unexpected fire, the properties of concrete changes and hence it is important to know the deflection rate of the concrete structures and the various effects of fire on concrete. Numerous studies were conducted on concrete both experimental and analytically to understand its behaviour under fire. In this study the thermal behavior of reinforced concrete slab exposed to fire is studied and presented. The paper mainly focuses on the percentage deflection of RC slab when exposed to elevated temperature. The RCC slabs were modeled using ANSYS14.5, to show the behavior of slab at elevated temperature with M25, M70 and M100 grade of concrete and with cover of 30mm,40mm and 50mm. Analysis was also carried out to study the load-deflection pattern and percentage of deflection with and without heat with a pressure of 0.1N/mm2. In stage one, 9 specimens were modeled to show the effect of different grade of concrete with different cover. The heat is applied on the basis of ISO 834 curve. The result showed that the deflection of a slab decreases as the cover provided increases. It was also found that the deflection of the slab decreases as the grade of concrete increases. From the result incurred it is seen that the analysis showed minimum deflection for M100 with cover 50mm. Hence it can be observed that, it is very essential to consider the effect of fire while designing special structures such as blast furnaces etc.

Keywords: Fire on RC Slab, Modal Analysis, ISO 834 Curve, Thermal Behaviour _______________________________________________________________________________________________________ I.

INTRODUCTION

Reinforced Concrete (RC) has been the most popular construction material used worldwide in the past century. It has proven to be a wonderful construction material that possesses almost all of the desirable properties such as excellent insulation from environment, durability, low cost, ease of construction, ability to mould in any given shape to name a few. Even from structural aspects, reinforced concrete construction serves its intended purpose extremely well, if properly designed and constructed. The most substantial consequence of fire on a concrete slab is the stiffness and strength degradation which may lead to collapse. It is important to study how the concrete property changes according to temperature. The compressive strength of the concrete decreases when temperature increases but the ultimate strain of the concrete increases with temperature. The tensile strength of the concrete also decreases with an increase in temperature. A tensile stress can also be obtained up to a temperature of 5000C.The modulus of elasticity of the concrete decreases with an increase in temperature. The reduction of the modulus of elasticity is due to the weak bonds formed in the microstructure of the cement paste when the temperature increases and is the result of the onset of rapid short-term creep.

II. LITERATURE REVIEW Significant researches was carried out on effect of fire on RC slab Faris ali et al (2008) performed an experimental investigation on explosive spalling and deformation of 6 full-scale simply supported reinforced concrete slabs subjected to conventional fire curve (BS476) and severe hydrocarbon fire curve. Each slab was loaded with 65% of its BS8110 design load and was heated from the bottom side only. Temperature profile was recorded at 3 depths within the slabs and the moisture content was also measured before and after the tests. The deflection of the slabs was recorded at the middle of the 3 meters span. The second part he represents a Finite Element Modeling of the slabs using DIANA software. The concrete slabs were modeled including the embedded reinforcement to conduct a non-linear transient structural analysis taking into account cracks appearance and creep. A comparison between the experimental and the FEM was analyzed. A discussion of the current state of our ability to predict the effects of fire on building structures has been presented. The emphasis has been on the degree to which fire dynamics, heat transfer, and structural analysis can be coupled.

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III. FINITE ELEMENT MODELING RC SLAB Modeling and analysis of RC slab is performed in ANSYS 14.5. The analysis process consists of loading the control specimen with elevated temperature (as per ISO curve 843) and with a pressure of 0.1N/mm 2 load and to study the deflection. Different test specimens were studied with different grade of concrete and with different cover.

The two convergence criteria used for the analysis are force and displacement. The displacement converges, but the force does not converges. Therefore, the convergence criterion for force is dropped and the tolerance value for displacement criteria is changed to 0.05. A small criterion must be used to capture correct response. This criterion is used for the remainder of the analysis. The program then gives a message specifying that the model has a significantly large deflection, exceeding the displacement limitation of the ANSYS program.

Table- 1 Material Properties (ANSYS Input) Material Properties Density 2400 kg/m3 Concrete Young’s Modulus 26.44GPa Poisson’s ratio 0.2

Fig. 1: The finite element (FE) model of the RC slab

IV. RESULTS AND DISCUSSIONS A slab size of 3300mmx1200mmx200mm with a cover of 30mm, 40mm and 50mm and with a grade of M25, M70 and M100 were modeled. The reinforcement provided in the slab was of 12mm and 10mm a pressure of .1N/mm2 was applied to the slab.

Heat was applied to the slab. The temperature applied is based on ISO 834 fire curve.

The analysis comprises of a total of 3 sets of models. The first set comprises of slab with M25 concrete and a varying cover of 30mm, 40mm and 50mm cover forms the first set. The second set have M70 with 30mm, 40mm and 50mm cover, and the final set consists of M100 with 30mm, 40mm and 50mm cover.

Fig. 2: M25 cover 30mm

Fig. 3: M25 cover 40mm

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Effect of Fire on RC Slab (IJIRST/ Volume 3 / Issue 04/ 032)

Fig. 4: M25 cover 50mm

Fig. 5: M70 cover 30mm

Fig. 6: M70 cover 40mm

Fig. 7: M70 cover 50mm

Fig. 8: M100 cover 30mm

Fig. 9: M100 cover 40 mm

Fig. 11: M100 cover 50mm

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Effect of Fire on RC Slab (IJIRST/ Volume 3 / Issue 04/ 032)

Interpretation of Results based on Grade Table – 2 Interpretation of Results based on Grade

Interpretation of results based on the total variation of deflection with respect to same grade of concrete but with different cover.

The total variation of deflection was expressed in terms of percentage. The first set shows M25 grade with different cover. Cover 30mm shows a deflection variation of 9.56 % when compared 40mm cover whereas cover 40mm shows a deflection of 8.23% when compared to cover 50mm. A total deflection of 17.08% decrease was experienced as we increase the cover. In the second set M70 with different cover was tabulated. At 30mm cover it was seen that the deflection was decreased by 12.49% when compared to 40mm cover, whereas the deflection changes to 12.75% when the cover was changed from 40mm to 50mm. And an overall variation of 23.66% decrease was experienced. In the third set M100 was tabulated against different cover. It was seen that a 14.4% decrease is experienced when the cover was changed from 30mm to 40mm, whereas a 15.11% variation is experienced with change in cover from 40mm to 50mm. An overall of 27.34% of variation decrease was experienced. It is seen that for a grade of concrete the maximum deflection is experienced for minimum cover. As we increase the grade of concrete the deflection is minimum.

Interpretation of Results based on Cover Interpretation of results based on the total variation of deflection with respect to same cover with different grade of concrete. The total variation of deflection was expressed in terms of percentage. The first set shows 30mm cover with different grade of concrete. M25 shows a deflection variation of 21.84% when compared M70 whereas M70 shows a deflection of 17.80% when compared to M100. A total deflection of 35.76% decrease was experienced as we increase the grade of concrete. In the second set 40mm cover with different grade of concrete was tabulated. At M25 it was seen that the deflection was decreased by 24.38% when compared to M70, whereas the deflection changes to 19.595% when the grade of concrete was changed from M70 to M100. And an overall variation of 39.198% decrease was experienced. In the third set 50mm cover was tabulated against different grade of concrete. It was seen that 28.03% decrease was experienced when the grade was changed from M25 to M70, whereas a 21.76% variation was experienced with change in grade from M70 to M100. An overall of 43.7% of variation decrease was experienced. The result shows that as we increase the grade of concrete with constant cover the slab with minimum cover experience more deflection. The deflection is minimum for maximum cover with higher grade of concrete.

Table – 3 Interpretation of Results based on Cover

V. CONCLUSIONS The behavior of one-way reinforced concrete slabs exposed to fire was investigated using ANSYS14.5. 11 simply supported oneway reinforced concrete slab models were considered with three different parameters namely: M25, M70 and M100 grade of concrete, different covers of 30mm, 40mm and 50mm with grade of steel 415N/mm2. The size of the slab 3300mmx1200mmx200mm.The temperature applied on the slab was based on the ISO834 fire curve. A pressure of 0.1 N/mm2

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was applied on the slab. The deflection were noted and studied. A slab M45 grade of concrete was also analyzed with and without heat. From the current study, the following conclusions could be drawn:  The normal strength concrete M25 and high strength concrete M70 and M100 exhibits a linear response.  The deflection of a slab decreases as we increase the cover. This is because concrete has the capacity to resist fire.  The deflection of the slab decreases as we increase grade of concrete. This is because the concrete becomes less permeable as we increase the grade of concrete.  From the above analysis it is concluded that M100 with a cover of 50mm shows a minimum deflection when exposed to high temperature and pressure.  Maximum deflection is shown by M25 grade with 30mm cover when it is exposed to high temperature and pressure.  As the deflection increases it affects the strength of the slab decreases.

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