FINITE ELEMENT MODELLING OF SELF COMPACTING CONCRETE BEAMS WITH DIFFERENT COVERS SUBJECTED TO...

International J.of Multidispl.Research & Advcs. in Engg.(IJMRAE), ISSN 0975-7074, Vol. 4, No. I (January 2012), pp. 207-224

FINITE ELEMENT MODELLING OF SELF COMPACTING CONCRETE BEAMS WITH DIFFERENT COVERS SUBJECTED

TO THERMAL LOADS

ANAND. N, PRINCE ARULRAJ. G AND XAVIER OLI ELAVAN. P

Abstract

Fire resistance of structures is an important safety aspect which is to be considered in the design of

buildings. The resistance of materials to fire has been studied by few researchers in the past. The

behavior of concrete subjected to higher temperature is not yet completely understood. The Indian

standard for plain and reinforced cement concrete IS 456:2000 has a clause for fire resistance. As per

this clause, fire resistance is a function of cover to the reinforcement. The fire resistance depends on

many other parameters such as grade of concrete, grade of steel, density of concrete etc. Self

compacting concrete has been recently accepted as a promising material that can be used in structural

elements with congested reinforcement. An attempt has been made in this study to understand the

thermal behaviour of self compacting concrete beams with different covers for various heating

conditions. During the present study, the concrete beams were analysed under temperature loads for

deflection, stresses and strain using the software ANSYS 11. Since, the behaviour of concrete is non-

linear; the non-linear behaviour properties are incorporated in the model and the model is solved using

a coupled thermal structural analysis method. Meshing has been done using an optimized modeling

procedure where every element is created and placed in position so that consistent support conditions

and loading points can be maintained. Reduction of strength and increase in deflection values were

observed when temperature loads were introduced in the model geometry. Increasing the cover

resulted in excessive deflection which can be interpreted as spalling in real cases.

---------------------------------------- Keywords : Furnace, Cover, SCC, Room temperature, Deflection, Peak load

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ANAND. N, PRINCE ARULRAJ. G AND XAVIER OLI ELAVAN. P 208

1. INTRODUCTION

The fire safety of any building depends to a great extent on its fire resistance, which in turn

depends on the combustibility and fire resistance of its main structural elements. The

strength of the concrete and steel will decrease as the temperature increases. When the

strength of the steel at an elevated temperature reduces to that of the ultimate stress in the

steel, flexural collapse will occur. The study of the behavior of concrete at elevated

temperature assumes great importance in recent times because the accumulated annual loss

of life and property due to fire is comparable to the loss caused by earthquakes and cyclones.

This necessitates development of new codes for fire-resistant design which can be achieved

only through extensive studies on the structural behavior of concrete under high temperature

conditions. Concrete protects the steel inside against the harmful effects of the external

environment. One of the most important of these effects is high temperature. Concrete cover

plays a major role in protecting the reinforcement against high temperature. The choice of

finite element modeling for the purpose of this study is mainly based on the limitations of the

experimental investigations such as cost of the experimental setup, limitations in casting

large specimens, risks involved in handling heated specimens and the prolonged time

necessary to carry out investigations. These limitations and risks are eradicated by use of the

software ANSYS which has been accepted and widely used for such research investigations.

Ansys modeling enables to carry out a detailed study of various cases in a short period of

time, provided, the model is validated with the help of few experimental investigations.

2. REVIEW OF LITERATURE

El-Hawary et al (1997) investigated the effect of fire exposure time and the concrete cover

thickness on the behaviour of R.C. beams subjected to fire in shear zone and cooled by

water. Eight reinforced concrete beams of size 180 x 20 x 12 cm were investigated. The

beams were divided into two groups. Group (1) consisted of four beams with a cover

thickness of 2 cm and group (2) consisted of four beams with a cover thickness of 4 cm.

FINITE ELEMENT MODELLING OF SELF COMPACTING… 209

Each group was subjected to a temperature of 650°C for different periods of time, i.e. 0, 30,

60. 120 min. The compressive strengths of concrete beams were determined nondestructively

using a Schmidt hammer the next day after exposure to fire. Then the beams were tested by

applying two transverse loads incrementally. Strains and deformations were measured at

each load increment. Cracking loads, crack propagation and ultimate loads were recorded for

each beam. The behaviour of the beams exposed to fire in the shear zone were found to be

highly affected by the fire exposure time and the change of the cover thickness.

Xudong Shi et al (2004) carried out an experimental investigation on flexural members

exposed to fire with different concrete cover (CC) thickness (10–30 mm). Investigation was

carried out to determine the influence of the CC on the fire resistance of reinforced concrete

flexural members. Fire load was applied through the bottom and the side faces. From the test

results, it was found that the bottom CC had significant influence on the fire resistance

however the extent of this influence decreased with an increase in the CC thickness. Thus, it

is improper to excessively increase the bottom CC thickness to improve fire resistance. The

lateral CC was found to have insignificant effect on the fire resistance compared to the

bottom CC. A concept of the equivalent CC thickness is proposed in this paper to predict the

effect of the CC on the flexural capacity of reinforced concrete members exposed to fire.

Esref Unluoglu et al (2007) investigated the mechanical properties of structural

reinforcement steel after the exposure to high temperatures. Plain steel, reinforcing steel bars

embedded in mortar and plain mortar specimens were prepared and exposed to 20, 100, 200,

300, 500, 800 and 950 °C temperature for 3 h individually. Deformed steel bars with

diameters of 10, 16 and 20mm were used during the study. Tensile strength of the

reinforcements taken from cooled specimens, variations in the yield strength, ultimate

strength were determined. A cover of 25 mm was found to give protection against

temperature up to 500°C. However, when this temperature exceeded, the reinforcing steel

was found to loss its strength. It was observed that 25 mm cover thickness was not sufficient

to protect the mechanical properties of reinforcing steel when exposed to temperatures over


500°C. Considering the reports that, temperatures during fire accidents could reach a value

as high as 1093°C, adequate concrete cover thickness has to be provided to protect the

reinforcements.

Ilker Bekir Topcu et al (2011) studied the changes in the mechanical properties of the

reinforcement steels having covers in the range 3cm to 5cm inside the mortar specimens

after exposure to high temperatures. In order to ensure covers in the range 3cm to 5cm, 76 ×

76 × 310 and 116 × 116 × 350 mm sized reinforced mortar specimen were prepared. These

reinforced mortar specimens were exposed to 20, 100, 200, 300, 500 and 800°C

temperatures. After the exposure, the steel rods were taken out of these mortar specimen and

the mechanical properties were determined. Stress–strain curves of the steel bars exposed to

several temperatures were drawn. The yield and ultimate strengths of the steel bars were also

determined. The results of the study have shown that the larger the covers resulted in better

protection of steel bars against high temperatures.

3. ESTIMATION OF THE INPUT PARAMETERS

In order to model the reinforced self compacting concrete specimens subjected to elevated

temperature, stress strain relationship for self compacting concrete and time temperature

graphs are to be given as input in the Ansys model. Self compacting concrete mix of grade

M25 concrete was designed. Cubes and cylinders were cast. Compressive strength and

density of the cubes were found. The cylinders were used to derive the stress-strain

behaviour of SCC using uniaxial compression test. Time-temperature curves were obtained

for the specimens heated inside the electrical furnace using an IR-thermometer for the

following cases.

a. Specimens were kept inside the furnace and heated from room temperature to 9000C

b. Specimens were placed in the furnace after achieving a furnace temperature of 9000C

and the specimens were kept inside the furnace for durations of 30 minutes, 60 minutes

and 90 minutes


Time-temperature relationship for the specimens that are heated from room temperature

to 9000C is shown in Fig 1 and Fig 2 gives the Time-temperature relationship for the

specimens that are kept for 90 minutes at 900°C.

Figure 1: Time-temperature Curve for Specimens Heated from

Room-temperature to 9000C (1173 K)

Figure 2: Time-temperature Curve for Specimens that are kept for

90minutes at 9000C (1173 K)


Similar curves have been drawn for other heating conditions. These time-temperature curves

have been fitted with suitable curves and equations have been derived which were used as

input in the Ansys model. Table 1 gives the details of the relationship between time and

temperature for the various heating conditions.

Table 1: Time-Temperature Equations Adopted for ANSYS Modeling

CONDITION EQUATION

270C to 9000C Temp = 265.6*Time0.2483

90 minutes at 9000C Temp = (66.3*Time) + 940



4. METHODOLOGY

4.1 Solid65

SOLID65 is used for the 3-D modeling of concrete. This element is capable of cracking in

tension and crushing in compression. The element is defined by eight nodes having three

degrees of freedom at each node such as translations in the nodal x, y, and z directions. The

geometry details of SOLID 65 element is shown in Fig.3

4.2. Solid70

SOLID70 is used for modeling concrete with thermal load. This element has a 3-D thermal

conduction capability. The element has eight nodes with a single degree of freedom,

temperature, at each node. The element is applicable to a 3-D, steady-state or transient

thermal analysis. The element also can compensate for mass transport heat flow from a

constant velocity field. The geometry details of SOLID 70 element are shown in Fig 4.


Figure 3: Geometry of Solid 65 Element

Figure 4: Geometry of Solid 70 Element


4.3. Link 8

LINK8 is used for modeling the rebars. Link 8 is a spar which may be used in a variety of

engineering applications. The 3-D spar element is a uniaxial tension-compression element

with three degrees of freedom at each node: translations in the nodal x, y, and z directions.

The geometry details of LINK 8 element are shown in Fig 5.

Figure 5 : Geometry of Link 8 Element

5. VALIDATION OF ANSYS MODEL

As software packages require to be validated for checking accuracy of results, ANSYS

results need to be validated with experimental results before carrying out detailed analysis.

Validation was done using the results of the experimental investigation carried out by Kumar

A and Kumar V (2003) and Moetaz M. El-Hawary et al (1996)

Table 2 shows the details of the specimen used by Kumar A and Kumar V

Table 2: Salient Features of Test Carried out by Kumar A and Kumar V

Size of the

specimen (m)

Temperature

range Time duration

Time-

temperature

curve

Rate of

heating

Rate of

cooling/

coolant

Beam(3.96x0.2

x0.3)

Furnace

temperature

1hr,1.5hr,2hr,

2.5hr

IS 3809-

1979 -

Natural

air


Table 3 shows the details of the specimen used by Moetaz M. El-Hawary et al et al

Table 3: Salient Features of Test Carried out by Moetaz M. El-Hawary et al

Size of the

specimen

(m)

Temperature

range

Time

duration

Time-

temperature

curve

Rate of

heating

Rate of

cooling/

coolant

Beam

(1.8x0.12

x0.2)

650°C (furnace

temperature)

0, 30min,

60min,

120min

Furnace

temperature

curve

-

Sprayed with

water

immediately

The beams used by Kumar A and Kumar V and the beam used by Moetaz were modeled in

Ansys and the comparison between the results of the experimental investigation and the

Ansys results are given in Table 4 and Table 5.

Table 4: Comparison of Results of the Experimental Investigation of Kumar A & Kumar V with

Ansys Results

Condition Deflection Ultimate Load % of Error

Experi

ment Ansys

Experi

ment Ansys Deflection Load

Units mm mm Tonne Tonne

Reference

specimen 27 28.84 16 17.45 6.82 9.06

Specimen heated

for 60 min as per

IS 3809 & cooled

by air

29 30.54 13 13.97 5.31 7.46


Table 5: Comparison of Results of the Experimental Investigation of El-Hawary et al with

Ansys Results

Condition Deflection Ultimate Load % of Error

Experi

ment Ansys

Experi

ment Ansys

Deflect

ion Load

Units mm mm Tonne Tonne

Reference specimen 6.75 7.11 5.95 6.54 5.33 9.91

Specimen kept in

the furnace at 650°C

for 30 minutes and

cooled by water 9.60 10.54 5.25 5.88 9.79

12.0

From Table 4 and Table 5 it can be seen that the percentage of error between the Ansys

model and the experimental investigation is within the acceptable limits.

6. ANSYS MODELING

Seventeen SCC beams were modeled during the present investigation. The sizes of all beams

were 500x100x100 mm. All beams were reinforced with two number of 6mm diameter

Fe415 rods at top and three number of 6mm diameter Fe415 rods at bottom. The first beam

is a reference specimen. The second and third beams were heated from room temperature to

900°C. The second beam had a cover of 20mm and the third beam had a cover of 25mm.

Both these beams were cooled by air. The fourth and fifth beams were kept at a temperature

of 900°C for 30 minutes. The fourth beam had a cover of 20mm and the fifth beam had a

cover of 25mm. Both these beams were cooled by air. The sixth and seventh beams were


kept at a temperature of 900°C for 60 minutes. The sixth beam had a cover of 20mm and the

seventh beam had a cover of 25mm. Both these beams were cooled by air. The eighth and

ninth beams were kept at a temperature of 900°C for 90 minutes. The eighth beam had a

cover of 20mm and the ninth beam had a cover of 25mm. Both these beams were cooled by

air. The tenth and eleventh beams were heated from room temperature to 900°C. The tenth

beam had a cover of 20mm and the eleventh beam had a cover of 25mm. Both these beams

were cooled by spraying water. The twelfth and thirteenth beams were kept at a temperature

of 900°C for 30 minutes. The twelfth beam had a cover of 20mm and the thirteenth beam

had a cover of 25mm. Both these beams were cooled by spraying water. The fourteenth and

fifteenth beams were kept at a temperature of 900°C for 60 minutes. The fourteenth beam

had a cover of 20mm and the fifteenth beam had a cover of 25mm. Both these beams were

cooled by spraying water. The sixteenth and seventeenth beams were kept at a temperature

of 900°C for 90 minutes. The sixteenth beam had a cover of 20mm and the seventeenth beam

had a cover of 25mm. Both these beams were cooled by spraying water. The load deflection

curves for the SCC beams that are heated from room temperature to 900°C and for the beams

that are kept at a temperature of 900°C for 30, 60 and 90 minutes are shown in Fig. 6, Fig.7,

Fig.8 and Fig.9 respectively. These specimens were cooled by air.


Figure 6: Load-Deflection Curves for Beams Heated from Room Temperature to 9000C and

Cooled by Air

Figure 7: Load-Deflection Curves for Beams Heated for 30 Minutes Under 9000C and Cooled by

Air


Figure 8: Load-Deflection Curves for Beams Heated for 60 Minutes Under 900°C and Cooled by

Air

Figure 9: Load-Deflection Curves for Beams Heated for 90 Minutes Under 9000C and Cooled by

Air


The load deflection curves for the SCC beams that are heated from room temperature to

900°C and for the beams that are kept at a temperature of 900°C for 30, 60 and 90 minutes

are shown in Fig. 10, Fig.11, Fig.12 and Fig.13 respectively. These specimens were cooled

by spraying water.

Figure 10: Load-Deflection Curves for Beams Heated from Room Temperature to 9000C and

Cooled by Spraying Water

Fig. 11: Load-Deflection Curves for Beams Heated for 30 Minutes Under 9000C and Cooled by

Spraying Water


Figure 12: Load-Deflection Curves for Beams Heated for 60 Minutes Under 9000C Cooled by

Spraying Water

Figure 13: Load-Deflection Curves for Beams Heated for 90 Minutes Under 9000C Cooled by

Spraying Water


The comparison of the deflections of the SCC beams that are subjected to elevated

temperatures is given in Table 6.

Table 6: Comparison of Deflection Values of SCC Beams

Type of

specimen

Type of

cooling

Deflection at

peak load for

beams with

20mm cover

Deflection at

peak load for

beams with

25mm cover

% increase

in deflection

Reference 2.769 3.105 12.13

Room temperature to

900°C

Air 2.805 4.153 48.06

30 minutes at 900°C Air 2.861 4.273 49.35

60 minutes at 900°C Air 2.892 4.295 48.51

90 minutes at 900°C Air 2.935 4.331 47.56

Room temperature to

900°C

Water 4.556 4.939 8.41

30 minutes at 900°C Water 2.908 4.428 52.27



It is seen from Table 6 that the deflections for the beam with a cover of 25mm are larger than

that for the beam having a cover of 20mm. The larger deflection for the reference specimen

is due to the reduction in the effective depth of beam as a result of larger cover. The reason

for larger deflection for the specimen subjected to elevated temperature may be due to

spalling of concrete and reduction in effective depth. Deflections of water cooled specimens


are more than that of the air cooled specimens. The specimen heated from room temperature

to 900°C has the largest deflection when compared with the reference specimen. The

increase in the deflection for the specimen having a cover of 20mm and heated from room

temperature to 900°C is 64.5% higher than the reference specimen when cooled by water.

The increase in the deflection for the specimen having a cover of 25mm and heated from

room temperature to 900°C is 59.07% higher than the reference specimen when cooled by

water. The specimens which are heated from room temperature to 900°C have largest

deflections. Since it takes nearly 400 minutes for the specimen to reach 900°C from room

temperature, the specimens are exposed to severe conditions for a larger period than the

other specimens. Hence the deflections are more in specimens that are heated from room

temperature to 900°C.

CONCLUSIONS

Initial deflections were observed for the SCC beams subjected to elevated temperatures even

before the introduction of structural loads. The SCC beams with a cover of 25mm heated

from room temperature to 900°C had 49.35% more deflection than that of the SCC beams

with a cover of 20mm when cooled by air. The SCC beams with a cover of 25mm heated

from room temperature to 900°C has 52.27% more deflection than that of the SCC beams

with a cover of 20mm when cooled by spraying water. Higher cover resulted in higher

deflections in all the cases of heating. The increase in deflection may be due to spalling of

concrete cover thus exposing the reinforcement to higher temperature. Improving the cover

will improve the performance of R.C.C elements only up to a certain limit. Beyond this limit,

spalling of concrete will takes place resulting in exposure of reinforcements to higher

temperatures. Since the ratio d’/d (d’-Effective cover, d-Effective depth) will be an important

factor in deciding the effectiveness of cover, the detailed Ansys analysis is further required

to obtain the optimal cover for maximum protection of reinforcement against high

temperature.


REFERENCES

[1] Kumar. A and Kumar. V, “Behaviour of RCC beams after exposure to elevated temperatures”, IE(I) Journal – CV, Vol. 84, November 2003, pp 165-170

[2] El-Hawary. M.M., Ragab. A.M., Abd El-Azim. A and Elibiari. S., “Effect of fire on flexural behaviour of RC beams”, Construction and Building Materials, Vol. 10, No. 2, 1996, pp 147-150

[3] El-Hawary. M.M., Ragab. A.M., Abd El-Azim. A and Elibiari. S., “Effect of fire on shear behaviour of RC beams”, Computers and structures, Vol. 65, No. 2, 1997, pp 281-287

[4] Xudong Shi, Teng-Hooi Tan, Kang-Hai Tan and Zhenhai Guo., “Influence of Concrete Cover on Fire Resistance of Reinforced Concrete Flexural Members”, Journal of Structural Engineering, Vol. 130, No. 8, August 2004, pp 1225-1232

[5] Esref Unluoglu, Ilker Bekir Topcua and Burcak Yalaman, “Concrete cover effect on reinforced concrete bars exposed to high temperatures”, Construction and Building Materials, 21 (2007), pp. 1155–1160

[6] Ilker Bekir Topcu, Ahmet Raif Boga and Abdullah Demir, “Influence of cover thickness on the mechanical properties of steel bar in mortar exposed to high temperatures”, Fire and Materials, Volume 35, Issue 2, March 2011, pp. 93–103

Anand. N

Asst. Professor, School of Civil Engineering, Karunya University, Coimbatore, India Email : [email protected]

Prince Arulraj. G

Prof and Dean of Civil Engineering, SNS College of Technology, Coimbatore, India Email : [email protected]

Xavier Oli Elavan.P

Asst. Professor, Dept of Civil Engineering, Hindustan College of Engineering and Technology, Coimbatore, India Email : [email protected]

FINITE ELEMENT MODELLING OF SELF COMPACTING CONCRETE BEAMS WITH DIFFERENT COVERS SUBJECTED TO...

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