Fire Behaviour of Circular Concrete Columns with

Fire Behaviour of Circular Concrete Columns with RestrainedThermal ElongationJoao Paulo Rodrigues, Luis Laim Manfred Korzen,

Journal of Advanced Concrete Technology, volume ( ), pp.12 2014 289-298

Transport Properties of Fire-Exposed ConcreteWilasa Vichit-Vadakan, Elizabeth A.KerrJournal of Advanced Concrete Technology, volume ( ), pp.7 2009 393-401

An advanced transient concrete model for the determination of restraint in concrete structures subjectedto fireMartin Schneider, Ulrich SchneiderJournal of Advanced Concrete Technology, volume ( ), pp.7 2009 403-413

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Journal of Advanced Concrete Technology Vol. 12, 289-298, September 2014 / Copyright © 2014 Japan Concrete Institute 289

Scientific paper

Fire Behaviour of Circular Concrete Columns with Restrained Thermal Elongation João Paulo C. Rodrigues1*, Luís M. Laím2 and Manfred Korzen3

Received 25 February 2014, accepted 22 August 2014 doi:10.3151/jact.12.289

Abstract Most of the previous studies on reinforced concrete columns with elastically restrained thermal elongation were carried out on square, rectangular or ‘+’-shaped cross sections. The number of fire resistance tests on circular reinforced con-crete columns with elastically restrained thermal elongation is still very small. In order to examine the influence of sev-eral parameters on the behaviour in fire of this type of columns several fire resistance tests were carried out. The pa-rameters tested were the load and restraint level, slenderness of the column and longitudinal reinforcement ratio. In the fire resistance tests the specimens were exposed to the ISO 834 standard fire curve and the critical time (fire resistance) and temperature and failure modes were determined. The test results showed that the spalling phenomenon may occur in circular columns and so reducing its fire resistance. The increasing of the load level led to a reduction while the increas-ing of the longitudinal reinforcement ratio or the decreasing of the slenderness of the columns led to an increasing of their fire resistance. The restraint level might not be much relevant on the fire resistance of circular reinforced concrete columns.

1. Introduction

The reinforced concrete structures under fire action have generally satisfactory behaviour. However, its per-formance could be much improved if it was well-known the effect of all parameters that have influence on fire resistance of this sort of structures. So the axial load ratio, fire exposure condition, axial and rotational re-straint level, cross-sectional shape, longitudinal rein-forcement ratio, slenderness, load eccentricity, concrete covering, spalling phenomena and concrete strength are several of the parameters that have recently been studied (Lie et al 1984; Lin et al 1992; Dotreppe et al 1997; Kodur et al 2004).

Firstly, Klingsch et al (1977) observed that columns of higher eccentricities and smaller applied load levels led to higher fire resistances, since the predicted resis-tance of the columns at ambient temperature are in these cases more conservative. In addition, larger cross-sections and a great number of steel reinforcement bars, distributed along the side of the column, increased the fire resistance. Secondly, as evidenced by Lie and Lin (1985), the full restraint against the axial thermal elon-gation of reinforced concrete columns has a little influ-

ence on their fire performance. Another important thing was that the restraint on thermal elongation can be bene-ficial for the columns’ fire performance if the surround-ing structure is able to redistribute part of the load. Thirdly, Ali et al (2004), Xu and Wu (2009) concluded that increasing the loading level decreased the restrain-ing forces and column failure times. As well as that it was also observed for normal-strength concrete that increasing the restraining degree increased the degree of spalling (Ali et al 2004) and can induce significant addi-tional axial forces in columns (Wu and Xu 2009). Fi-nally, Franssen and Dotreppe (2003) stated that use 12 or 20 mm diameter longitudinal reinforcement had no difference on surface spalling and despite this phe-nomenon the values obtained for fire resistance were relatively high.

A great experimental research program has been car-ried out during the last years in the Laboratory of Test-ing Materials and Structures at Coimbra University, Portugal, in order to investigate the structural perform-ance of concrete columns (Martins and Rodrigues 2010) and shear connectors in composite steel and concrete structures (Rodrigues and Laim 2011) under fire condi-tions by using mainly transient state test method. Hence, in this paper the results of several experimental tests to characterise the behaviour of circular reinforced con-crete columns in fire are presented. The experimental programme was carried out in a new experimental set-up to test building columns with restrained thermal elongation. This experimental system allows varying degrees of axial and rotational stiffness and load levels to be applied to the columns being tested. In this sense, it was looked for to analyse the effect of the load and restraint levels, the slenderness of the column and the longitudinal reinforcement ratio on the fire resistance of

1Professor, ISISE - Institute Sustainability and Innova-tion on Structural Engineering, University of Coimbra, Portugal. *Corresponding author, E-mail: [email protected] 2Postdoc, ISISE - Institute Sustainability and Innovation on Structural Engineering, University of Coimbra, Portugal. 3Senior Researcher, BAM Federal Institute for Materials Research and Testing, Berlin, Germany.

J. P. C. Rodrigues, L. M. Laím and M. Korzen / Journal of Advanced Concrete Technology Vol. 12, 289-298, 2014 290

these columns. Other important goal of previous (Mar-tins and Rodrigues 2010) and this research is to provide experimental data for developing and calibrating a new numerical model capable of simulating and studying the importance of spalling in concrete columns. Finally, the experimental and numerical results will be the basis of an analytical study for the development of possible rec-ommendations to the designers and even guidelines to the available design rules.

2. Experimental tests

The experimental tests on the circular reinforced con-crete columns were conducted in the Laboratory of Test-ing Materials and Structures of the University of Coim-bra, in Portugal. The experimental programme consisted of ten fire resistance tests with restrained thermal elon-gation. Two different stiffness values of the surrounding structure were adopted, namely, 13 and 128 kN/mm (values obtained by experimental tests). Four columns were subjected to fire with the lowest stiffness of the surrounding structure and six others with the highest one. The values of stiffness of surrounding structure were chosen to simulate real structures. The lower value of 13 kN/mm means practically absence of thermal re-straint, and the stiffness of 128 kN/mm (the highest value possible to test in this experimental set-up) tried to simulate a common two-storey building of 3 x 4 bays of 6 m span. 2.1. Test set-up In Fig. 1 the test layout is illustrated, showing the test set-up for fire resistance tests on columns with re-strained thermal elongation. A wide-ranging experimen-

tal research has already been carried out in this facility for at least the past four years, namely, on steel (Correia and Rodrigues 2012; Correia et al 2011), concrete (Mar-tins and Rodrigues 2010; Rodrigues et al 2010) and steel-concrete composite columns (Pires et al. 2012; Correia and Rodrigues 2011). This system allows testing columns of different materials, cross-sections, slender-ness, and varying stiffness of the surrounding structure and load levels.

A two-dimensional reaction frame (1) consisted of two HEB500 columns and an HEB600 beam of S355 steel class was used to support a hydraulic jack (2) which applied a compression load to the columns. This hydraulic jack had a maximum capacity of 3 MN and was controlled by a servo hydraulic central unit W+B NSPA700/DIG2000. In addition, the stiffness of the surrounding structure of the columns under test was realized by a three-dimensional restraining frame with four HEB300 columns and four identical beams of S355 steel class, orthogonally arranged (3). The stiffness could be varied by changing the position of the columns or the height of the beams cross-section. During these experimental tests, only the position of the columns was changed. Additionally, above the specimen between the hydraulic jack and the beams of the three-dimensional restraining frame a compression load cell (4) was mounted in order to monitor the load during the whole test.

The specimens were heated with a vertical modular electric furnace (5). This furnace was 1500 mm x 1500 mm x 2500 mm in internal dimensions and capable to heat up to 1200 ºC (two modules of 1000 mm height with a 90 kVA power supply and one module of 500 mm height with a 45 kVA power supply) and follow fire curves with different heating rates, including the ISO 834 fire curve. Therefore, only a limited part of the specimens could be directly heated, i.e. from 250 mm to 2750 mm along the height.

The restraining forces generated in the column due to the heating were measured by a load cell of 3 MN lo-cated inside a void steel cylinder of high stiffness (6). This cylinder was rigidly connected to the restraining frame by means of M24 class 8.8 bolts as well as all connections between the elements in the experimental set-up. A massive steel cylinder, rigidly connected to the testing column, entered in a void steel cylinder and due to the thermal elongation of the column compressed the load cell.

The axial displacements and rotations on the top and bottom of the column were measured by displacements transducers, LVDT (7), orthogonally arranged in three different points, forming a deformation plan (Fig. 2).

2.2. Test specimens The tested columns were 3000 mm tall and two different circular cross-sections, one with 250 mm and the other with 300 mm in diameter (Fig. 3). Two steel plates, steel class S355 (with a nominal yield strength of 355 MPa Fig. 1. Experimental set-up for fire resistance tests on

columns.


and a tensile strength of 510 MPa, according to EN1993-1.1 2004), measuring 450 mm x 450mm x 30 mm each, were still connected by welding to the longi-tudinal steel reinforcement bars and a steel hook at the ends of columns, as it can be seen in Fig. 4. All test specimens had six longitudinal steel reinforcing bars, which were 12 mm (four columns) or 20 mm (six col-umns) in diameter (Table 1). For each specimen the transversal reinforcement was performed by 6 mm or 8 mm diameter stirrups with a spacing of 100 mm until 700 mm from the supports, and a spacing of 150 mm in the central part (Fig. 5). It is noticed that the 6 or 8 mm diameter stirrups were used with the 12 or 20 mm di-ameter reinforcing bars, respectively. The concrete cov-ering related to the stirrups for all tested columns was 30 mm. These and some other characteristics of the specimens tested are summarised in Table 1. It can be seen that the reference C25-12re-70LL-13K corre-

sponds to a 250 mm diameter circular column (C25) reinforced with 12 mm diameter longitudinal steel rein-forcing bars (12re). Additionally, the designation 70LL means that the column was axially loaded to 70% of the design value of buckling load of the column at ambient temperature. This load was calculated on basis of the nominal curvature method, as established in EN 1992-1-1 (2004). The designation 13K indicates that the value of the axial stiffness of the surrounding structure was 13 kN/mm.

All reinforcing bars used in the test specimens were made of A500NR steel and all specimens presented a similar concrete composition with calcareous aggregate with a compressive strength of approximately 27 MPa at 28 days old and so corresponding to a C20/25 class ac-cording to NP EN 206-1 (2007). The concrete had the reference C20/25-S3-XO(P)-D22-CL0.4 and was fabri-cated with Portland cement type II/A-L 42.5 R, a maxi-mum aggregate size of 22 mm and a water to cement ratio of 0.60 (S3 class).

All specimens were tested after 90 days of casting

Fig. 2. Scheme of the test set-up for fire resistance tests on columns.

Fig. 4. Detail of the connection between the steel rein-forcing bars and the steel end plates.

Fig. 3. Test columns.

Fig. 5. Scheme of the test columns: longitudinal (a) and cross-sectional (b) views.


concrete, in order to reproduce as faithful as possible the moisture conditions of the real columns, and so reduce the influence of that on the spalling and consequently on the behaviour of the column in the fire test.

2.3. Test procedure These fire resistance tests were performed in two stages. Firstly, the specimens were axially loaded up to the tar-get force at a rate of 2.5 kN/s. The load levels tested were 30, 70 and 100% of the design value of buckling load of the columns at ambient temperature. These load-ing levels intended to simulate different serviceability load conditions of the columns when they are inserted in a real building structure. Note that one column with 100% of load level was tested, in order to determine the safety level of this type of elements in fire situation and at most severe mechanical loading, in spite of the au-thors have been aware of being a very special limit sce-nario. The HEB300 beams of the three-dimensional restraining frame above the test specimen were, in a first step, not connected to the respective HEB300 columns with the purpose of axially applying the load on the test column. In other words, during the initial load applica-tion, the vertical displacements of these beams were allowed as a slide, guaranteeing vertical rigid body movement of the upper beams and ensuring that the defined axial load was directly applied to the test specimen. Then, when the load reached the desired level, the connections between the upper beams and the pe-ripheral columns of the restraining frame were material-ized by means of M24 class 8.8 threaded rods (Fig. 6) in order to simulate the axial restraint to the thermal elongation of the test column as clamped boundary con-ditions at top and bottom. Note that all elements of the restraining frame were connected by steel treaded bolts, as well as the concrete column to the restraining frame by means of two steel end plates, as stated before. Therefore, two stiffness values of the surrounding struc-ture were tested: 13 kN/mm and 128 kN/mm, corre-sponding to a span of 6 and 3 m of the beams, respec-tively (Fig. 1, n.3).

Finally, at the second stage the test specimen was

heated according to the standard fire curve ISO 834-1 (1999). It is noticed that the columns were only heated 2500 mm of their length, from 250 mm to 2750 mm height, because they had to be connected outside the furnace. During the heating the load was kept constant and the test was considered terminated when the axial force of the specimen was lower than the initial applied load before the test starting.

Three type K (chromel-alumel) thermocouple probes were used to measure the furnace temperature along its height and fourteen type K wire thermocouples were used to measure the temperature in the specimens at five cross-sections and different depths, as represented in Fig. 7. One thermocouple was placed in the center of sections S1 and S5, whereas four thermocouples were placed in sections S2, S3 and S4. In this sense, one of these four thermocouples was welded to the longitudinal reinforcement bars (θSi - T1) and the others were embed-ded in the concrete at different depths: one near the sur-face (θSi - T2), another in the center (θSi - T4) and a third one midway between them (θSi - T3). It was just used a thermocouple in sections S1 and S5 and in their center because these sections were outside the furnace and their temperature could only increase by thermal con-

Table 1. Characteristics of the test columns.

Cross-section Longitudinal reinforcement

Column reference Diameter (mm)

Ac (mm2)

Number anddiameter

(mm)

As (mm2)

ReinforcementRatio As/Ac (%)

Elastic axial stiffness,

kC (kN/mm) Slenderness

C25-12re-70LL-13K C25-12re-70LL-128K 250 49087 6φ12 679 1.38 542 58

C25-20re-70LL-13K C25-20re-70LL-128K C25-20re-30LL-128K

C25-20re-100LL-128K

250 49087 6φ20 1885 3.84 616 58

C30-12re-70LL-13K C30-12re-70LL-128K 300 70686 6φ12 679 0.96 763 48

C30-20re-70LL-13K C30-20re-70LL-128K 300 70686 6φ20 1885 2.67 837 48

Fig. 6. Detail of the connection between the top of col-umns and the beams of the three-dimensional restrain-ing frame.


duction. At last, the three thermocouples that measured the furnace temperature were placed at 750, 1750 and 2500 mm from the bottom of the test specimen. The data acquisition was done by a TML data logger, model TDS-530.

2.4. Results and discussion 2.4.1. Temperature distribution Figure 8 presents, as an example, the furnace tempera-tures as a function of time of all fire resistance tests with the stiffness of the surrounding structure of 128 kN/mm. The evolution of temperatures inside the furnace over time followed nearly the ISO 834 fire curve and was very uniform in all fire resistance tests, meaning that the tests are comparable.

In Fig. 9 it is presented, as an example, the evolution of temperatures in cross-section S4 of the test C25-12re-70LL-13K as well as the furnace temperature and the standard fire curve ISO 834. Each measuring point in the cross-section was used for assessing an average temperature, taking into account the influence areas defined by the thermocouples embedded in the concrete. As a result of this average temperature in each study cross-section, it was possible to establish the tempera-ture profile in the vertical direction of the columns for different time instants (Fig. 10).

In Fig. 9 it can be seen that the temperature in the column core and on the longitudinal steel reinforcing bars did not exceed 300 and 600 ºC, respectively, while the temperature in the furnace was higher than 800 ºC for over 90 minutes. A large thermal gradient is ob-served from the surface to the centre of the concrete column (thermocouples θS4 – T2, θS4 – T3 and θS4 – T4), reaching around 600 ºC. However, the specimen tem-perature along its height was almost the same inside the furnace, observing a mean range from 80 to 40 ºC which corresponding respectively to 30 and 120 minutes of test run (Fig. 10). It should be remembered that the high

thermal gradients near the ends of the column are due to the fact that the first 250 mm both at the bottom and top end of the column were outside of the furnace or were insulated by ceramic wool. The maximum temperature of the concrete columns was reached at the top layer in the furnace as it was expected.

2.4.2. Restraining forces A similar development in terms of the evolution of the axial restraining forces generated in columns was ob-served during the tests for the different stiffness of the surrounding structure. The restraining forces in columns increased up to a maximum and then decreased up to failure (Figs. 11 - 13) mainly because of the loss of me-chanical properties of the steel and concrete as a func-tion of temperature and apparently of the transient thermomechanical interaction strain (also called tran-sient creep strain). The failure criterion adopted was the

Fig. 8. Furnace temperatures as a function of time.

Fig. 9. Evolution of temperature in cross-section S4 of the test column C25-12re-70LL-13K as a function of time.

Fig. 10. Specimen temperature distribution along its height of the test column C25-12re-70LL-13K at different time instants of the fire test.

Fig. 7. Location of the thermocouples in the specimens.


time (critical time) when the axial forces reached again the value of the initial applied force, i.e. P / P0 = 1. Ta-ble 2 summarises the main results obtained from all fire resistance tests specially the maximum restraining forces (Prest) and axial elongations (δmax) of heated col-umns, the critical time (tcr), and the mean concrete ( cθ ) and steel temperature ( sθ ) in column at the time of failure. The mean steel temperature in column was cal-culated by taking the average of temperatures which were read by the thermocouples welded to the longitu-dinal reinforcement bars (θS2 – T1, θS3 – T1 and θS4 – T1), whereas the mean concrete temperature in column was calculated in function of the mean temperatures in sec-tions S1, S2, S3, S4 and S5 and assuming a linear varia-tion of the temperature along the height of column. Fi-nally, those mean temperatures in cross-sections were determined by averaging the temperatures which were read by the thermocouples embedded in the concrete (θSi – T2, θSi – T3 and θSi – T4) and taking into account their influence areas (A2, A3 and A4), as illustrated in Fig. 7.

An important conclusion to be drawn in relation to the temperatures in concrete columns is that the mean temperature in the longitudinal steel reinforcement bars may be a better indicator to estimate when the column fails than the mean temperature in the concrete, since the temperature in the cross-section is not uniform and the cross-section area can change during the test due to spalling or detachment of concrete. As it can be seen in Table 2, most of the times, when the stiffness of the surrounding structure or the initial load level increased the mean steel temperature at critical time decreased, as it was expected. This was not always observed because the severity of spalling was not the same in all tested columns. Some ones did not present any spalling at all, but when this phenomenon occurred even some longitu-dinal steel reinforcement bars of some columns re-mained directly exposed to fire, as it is also indicated in Table 2. This is very hard to understand because there is not a direct link between this phenomenon and the load

Table 2. Main results of the fire resistance tests.

Column reference P0 (kN)

Pmax (kN)

Prest (kN)

Pmax / P0

kS / kCR

(min)θc

(ºC) θs

(ºC)δmax

(mm)

Observation of spalling

C25-12re-70LL-13K 363 378 15 1.041 0.024 110 382 502 3.89 No C25-12re-70LL-128K 363 400 37 1.102 0.236 104 368 521 1.31 No C25-20re-70LL-13K 624 694 70 1.112 0.021 163 484 558 6.71 No

C25-20re-70LL-128K 624 829 205 1.328 0.208 147 511 567 2.18 Yes C25-20re-30LL-128K 267 663 396 2.480 0.208 194 503 729 2.36 Yes

C25-20re-100LL-128K 891 993 102 1.114 0.208 119 390 410 1.82 No

C30-12re-70LL-13K 458 483 25 1.054 0.017 152 373 471 2.21

Yes, but the rebars did not remain

directly exposed to fire

C30-12re-70LL-128K 458 547 89 1.194 0.168 198 448 402 1.09 No C30-20re-70LL-13K 766 810 44 1.057 0.016 154 322 690 5.09 Yes

C30-20re-70LL-128K 766 975 209 1.273 0.153 185 424 537 2.15 Yes

Fig. 11. Restraining forces as a function of time – influ-ence of load level (serviceability load).

Fig. 12. Restraining forces as a function of time – influ-ence of longitudinal reinforcement ratio and restraint level.

Fig. 13. Restraining forces as a function of time – influ-ence of slenderness.


level or the axial restraint level. So, some of the conclu-sions made ahead on this paper did not check in all ex-perimental tests because of the spalling phenomenon in columns. This phenomenon and the effect of its severity on the structural behaviour of these columns are also discussed further ahead.

In Fig. 11, it can be observed that an increase of the serviceability load yields a significant decrease of the fire resistance and the restraining forces generated by the heated columns. The specimen C25-20re-30LL-128K had an increase of 64 % and 2.35 times in critical time and restraining forces, respectively, comparing to the specimen C25-20re-100LL-128K. It can also be seen that for high serviceability loads on the columns, the restraining forces tend to be more affected than the critical time, since the reduction was higher in the re-straining forces than in the critical time respectively from the specimen C25-20re-70LL-128K to the speci-men C25-20re-100LL-128K.

In relation to the restraint level, it was found that in-creasing this parameter by a factor about ten led to a slight decrease in the critical time, about 8 % (Fig. 12). On the other hand, the critical time was increased ap-proximately by 45 % due to an increase by 13 % of the axial stiffness of the columns, kC, specifically, increas-ing by 2.78 times in cross-sectional area of the longitu-dinal steel reinforcement bars. As well as that, an in-crease both in restraint level and longitudinal rein-forcement ratio led to an increase in restraining forces, which in this case ranged from 7 to 23 % of the service-ability load applied on the column, as it can be seen in Fig. 12.

Another main influence parameter of behaviour of these reinforced concrete columns in fire is the slender-ness or even the effective elastic axial stiffness of the column, only changing the column diameter. The results obtained in this research make it possible to state that increasing the slenderness of the columns may lead to a significant reduction of critical times. This reduction may be in the order of 50 % increasing by only 20 % in the slenderness of the column as it was observed by the critical times of the specimens C30-12re-70LL-128K and C25-12re-70LL-128K (Fig. 13). This is a conse-quence on the thermal conductivity of concrete and hence columns with larger cross-sections take much further time to heat. However, the spalling phenomenon may be a big problem, because it can be expected that columns with larger cross-sections have one fire resis-tance and due to the spalling in concrete they actually might not have that behaviour (C30-12re-70LL-13K). It seems that the restraint level does not influence much the critical time of the reinforced concrete columns and the appearance of spalling phenomenon at least with this type of concrete.

Figure 14 shows the restraining forces of some col-umns in function of the mean steel and concrete tem-peratures of the column. In this graph it can be observed that the mean concrete temperature was slightly higher

in the specimen C25-12re-70LL-13K than the specimen C25-12re-70LL-128K when these reached failure, and the opposite was observed in relation to the mean steel temperature (nevertheless the difference is small in both cases). It can be conclude that these columns axially loaded by 70% of their buckling load, reach the critical time when the mean steel and concrete temperature are about 510 and 375 ºC, respectively. It is also noticed that for temperatures above 400 ºC in the longitudinal steel reinforcement bars, the structural stability of these columns begins to be compromised, which is in accor-dance to the degradation of the mechanical properties of steel established in EN 1992-1-2 (2003).

Lastly, Fig. 15 possibly provides a general idea of how different the structural behaviour of a circular col-umn can be from a square column under fire conditions. Note that the results of the square reinforced concrete columns were taken from previous works done by the authors (Martins and Rodrigues 2010). Only the results of these columns are compared since only these ones presented the same boundary conditions (loading and support) and slenderness. It is quite interesting to point out that the critical times were slightly higher in the

Fig. 14. Restraining forces as a function of the mean steel (a) and concrete (b) temperatures of the column.

Fig. 15. Comparison of restraining forces in square and circular reinforced concrete columns.


circular columns than in the square columns, even when concrete spalling occurs (Table 2). Therefore, in the presence of this phenomenon, whereas the edges of the square columns are the main zone affected, exposing the longitudinal steel reinforcement bars directly to fire; any zone in the circular columns can be affected (exposing or not the steel bars), which may be an important factor to take into account the choice of the cross-section shape.

2.4.3. Axial deformation and rotations Concerning the displacements and rotations at the ends of the columns (top and bottom) they were very small as expected for concrete columns. As example, in Fig. 16 it is presented the variation of the axial deformation of the test columns C25-20re-70LL-13K and C25-20re-70LL-128K as a function of time so as to show the differences in the thermal elongation of columns for the different restraint levels. It could be concluded that the maximum axial elongation of the 250 mm diameter columns be-tween the surrounding structure with the lowest stiffness was about three times higher than when it was adopted the surrounding structure with the highest stiffness. In relation to the 300 mm diameter columns that difference was about two times. In addition, it can be seen by Ta-ble 2 that there was mostly a very little difference in the maximum axial deformation between the 250 and 300 mm diameter columns for the same stiffness of the sur-rounding structure. Finally, in Figs. 17 it can be ob-served that the rotations on the ends of the column C25-20re-70LL-13K were both similar and in Figs. 16 and 17 it can be seen that the maximum axial displacement of this specimen due to thermal elongation was about 6.7 mm and the very low maximum rotation of 0.008

rad. Please note that no lateral deformations of the col-umns have been measured because the fire tests were finished when the axial forces reached again the value of the initial applied load (instant with small deforma-tions of the columns) and didn't make sense to measure beyond this point. In addition, there was the risk of de-tachment of concrete pieces from the surface of the col-umns, making hard the lateral displacement measure-ments.

Figure 18 shows the restraining forces as a function of the axial displacements of the columns C25-20re-70LL-13K and C25-20re-70LL-128K, as example. It can be seen the hysteresis phenomenon in the structural behaviour of the reinforced concrete columns under fire conditions, in other words, they presented different re-straining forces respectively during the increasing and decreasing of their axial displacements, as it was ex-pected. It is assumed that one of the reasons for this phenomenon is due to the different behaviour of the upper restraining beams during increasing and decreas-ing of the axial restraining forces, since the temperature of those beams slightly increased by conduc-tion/convection (due to the hot air which came out from the furnace) during these tests. The fact that the effect of the hysteresis phenomenon has been bigger in column C25-20re-70LL-128K than in column C25-20re-70LL-13K may have resulted from the axial restraining forces in the former column have been higher than in the last one.

2.5. Columns after test The main observation made after the fire resistance tests was that even the circular reinforced concrete columns may present local or global detachment of concrete pieces from the surface of the column (as it can be seen in Fig. 19), as well as, large cracks along the column’ height. It is well-known that among other parameters the spalling phenomenon depends essentially on the concrete moisture and porosity, the compression level of the concrete, the concrete compressive strength and the cross-section shape of the specimens. In this study, the compression level of the concrete and the stiffness of the surrounding structure are assumed to be the most responsible parameters, i.e. the concrete crushing is as-sumed to be the main failure mode responsible for the

Fig. 16. Axial deformation in columns C25-20re-70LL-13K and C25-20re-70LL-128K as function of time.

Fig. 17. Rotations at the ends of column C25-20re-70LL-13K as function of time.

Fig. 18. Restraining forces in columns C25-20re-70LL-13K and C25-20re-70LL-128K as a function of the axial displacements.


collapse of the columns, taking into account the failure criterion adopted in this study. Furthermore, the distance from the electric resistances of the furnace to the col-umn may also have a slight influence due to the thermal radiation. In this sense the columns with larger diame-ters could be further affected by the thermal radiation and consequently by the spalling phenomenon (Table 2).

The experimental tests indicated that the spalling phenomenon could occur in any part of circular columns,

whereas in square or rectangular columns normally oc-curs on the corners (Martins and Rodrigues 2010). An-other important consequence resulting from the high temperature levels in the columns was the reduction of the column cross-section along time. The detachment of concrete pieces on the specimen surface could occur for days after the tests (Fig. 20). So, this point may be very important when the residual strength of reinforced con-crete columns after fire is studied.

3. Conclusions

This paper presented and discussed the results of an experimental investigation on the fire resistance of cir-cular reinforced concrete columns with restrained ther-mal elongation. This study showed that concrete spalling can occur in circular columns and even when the concrete compressive strength is low, in spite of the fact that its effect may be weaker than in square/rectan-gular columns.

The results of this experimental research lead to the following conclusions about the effect of the tested pa-rameters on the behaviour of the columns subjected to fire:

The load level has a considerable influence on the performance of these columns in fire. With an increase in the load level, a significant reduction of the critical times was observed, in other words, from 30 to 70 % of the load level applied on column it was observed a re-duction of 64 % in the critical times.

In relation to the stiffness of the surrounding structure it could be verified that the performance of the columns were very little affected by this parameter. In fact, in-creasing this parameter from 13 to 128kN/mm led to a maximum decrease of about 10% in the critical times.

Finally, increasing the longitudinal reinforcement ra-tio or decreasing the slenderness of the column in-creases significantly the critical times of the reinforced concrete columns. The critical time was increased ap-proximately by 45 % due to increase by 2.78 times in longitudinal reinforcement ratio for instance. And in-creasing by only 20 % in the slenderness of the column the reduction of critical times may be in the order of 50 %.

Acknowledgments The authors would like to thank the Portuguese Founda-tion for Science and Technology (FCT) for its financial assistance under the framework of research projects REEQ/499/ECM/2005 and PTDC/ECM/65696. References Ali, F., Nadjai, A., Silcock, G. and Abu-Tair, A., (2004).

“Outcomes of a major research on fire resistance of concrete columns.” Fire Safety Journal, 39, 433-445.

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Fig. 19. Spalling phenomenon observed in the columns C30-20re-70LL-13K (a), C30-20re-70LL-128K (b) and C25-12re-70LL-13K (c) after the fire resistance tests.

Fig. 20. Column C30-12re-70LL-13K one (a), three (b) and seven (c) days after the fire test.


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Notation: Ac cross-sectional area of the concrete An influence area of thermocouple n in cross-

section of the column As cross-sectional area of the longitudinal steel

reinforcement bars kc axial stiffness of the column kS axial stiffness of the surrounding structure P axial compression force Pmax maximum axial compression force P0 initial applied load or serviceability load of the

column Prest maximum restraining forces in the column tcr critical time

si skl − distance between the column sections Si and Skδ axial elongation of the column δmax maximum axial elongation of the column

cθ mean concrete temperature of the column at the instant of failure

sθ mean steel temperature of the column at the instant of failure

siθ mean temperature in section i θSi - Tn temperature measured by thermocouple n in

section i

Documents

Fire Behaviour of Circular Concrete Columns with