Assessment of post-fire reinforced concrete structures: Determination of depth of temperature penetration and associated damage

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1 Concrete Repair, Rehabilitation and Retrofitting II Alexander et al (eds) 2009 Taylor & Francis Group, London, ISBN Assessment of post-fire reinforced concrete structures: Determination of depth of temperature penetration and associated damage C. Alonso Institute of Construction Science Eduardo Torroja, Madrid, Spain ABSTRACT: The assessment of concrete structures affected by the fire should allow understanding what happened during fire and developing rational criteria for the evaluation of the security of the structure. The result of the assessment should be used to take the decision on further repair, partial or total demolition of the structure. Two main goals to be answered during the assessment analysis are: 1) The determination of the depth of concrete exposed to temperatures that produce irreversible alteration of concrete components, and 2) the mechanical strength decay of concrete that can affect the load bearing capacity of the element or the structure itself. The assessment of the integrity of a concrete structure in the post fire situation involves addressing the damaged of each component. To differentiate between concrete chemically affected by the fire or physically damaged is an important issue. In this paper the depth of concrete of a structure exposed 48 h in fire and a tall building, >100 m (Windsor Tower) 18 h exposed to fire are used as examples of real fires. In-situ inspection and laboratory tests of concrete components have been performed. The heterogeneity in the distribution of the damage is identified and the depth of concrete affected by the fire is determined. 1 INTRODUCTION Reinforced concrete is considered a material that shows an acceptable resistance to high temperatures, which allows using concrete elements without the need of any additional protection. However, long periods of exposition of reinforced concrete to high temperatures introduce physical-chemical changes in its properties that lead to mechanical strength decay which produces losses in the safety of the structure, (Piasta et al (1984a), Alonso & Fernandez-Municio, (2004, 2008). The assessment of deteriorated concrete structures after fire is needed in order to identify the level of damage induced by the chemical and physical processes taking place, in all the components at high temperatures and also during cooling, which contribute to the loss of mechanical strength, Tay & Tam (1996), Khoury (1992), Chan et al (2000). The residual structural capacity has to be accurately addressed when the safety of the structure is in risk, in order to define the best strategy for repairing or to decide on its demolition. The low thermal diffusivity of concrete guarantees a slow propagation of the chemical transformations of the components of concrete, paste and aggregates, which also need time for fully developing conversions at each specific temperature. Due to these reasons strong thermal gradients appear, that induce internal mechanical pressures in the concrete mass, which allows the development of cracks both during heating and cooling, Chan et al (2000). Besides, when the temperature gradients are high, the pressures accumulated inside the pores increase due to the water vapour evolution, so the risk of spalling of external concrete layers is possible, leaving new surfaces of concrete exposed directly to the external high temperatures of the fire, Bazant & Kaplan (1996). The physical-chemical transformations that take place in concrete components during fire are well known and also very well addressed when the characterisation is made in the laboratory in concrete samples that have been allowed to reach the steady state at each temperature; an acceptable correlation with loss of mechanical properties is found, Rostasy et al (1987). It is more difficult to assure a good correlation of real fire scenarios using equivalent time curves from codes and the level of damage in concrete, Schneider (1990). However, it is more complicated when the fire takes place in a real structure, as many factors are contributing to the damage and no clear relationship can be found between the maximum temperature and the residual concrete strength or load bearing capacity of the structure. 471

2 The most common requirement for in-situ investigation is the estimation of concrete strength, through the assessment of defined properties, adopting the criteria that variations in strength are taking place between non affected and damaged concrete. The assessment of the residual capacity of concrete structures exposed to fire is a quite complex task, mainly due to the heterogeneity of the concrete, but also associated with the irregular distribution of the damage induced by the fire. The possible approach is to find an average response of the concrete cover both using destructive and non destructive analysis, in order to interpret the overall response of the concrete after fire, Feliceti (2008). In-situ non-destructive techniques are widely employed for distinguishing between damaged and apparently non affected areas of concrete elements, such as resilience, hammer-tapping, ultrasonic pulse etc. Benedeti (1998), Benedetti & Mangoni (2004a), Fellicety (2004), Calavera et al (2005), Colombo & Felicetti (2006). However, the determination of the penetration of the damage and the differentiating between the type of damage (chemical, physical or mechanical) are important to ascertain the consequences of the thermal gradients, which require the use of concrete cores for laboratory testing, usually for mechanical strength, petrography analysis and thermal alteration of cement paste. More feasible methods are being adopted, including infra-red thermal imaging, digital camera colorimetric etc., Fellicety (2004b), Colombo & Felicetti (2006), Short & Purkiss (2004), Zhang et al (2002). Chemical analysis of cement paste, e.g. thermogravimetric, TG and X Ray Diffraction tests, are also employed in order to determine the alteration of hydrated cement paste components that are related to mechanical strength decay, Alonso (2006a,b), Alonso & Fernandez-Municio (2007). Also, attempts have been made to assess cracking using petrographic thin sections and Scanning Microscopy, SEM, Tay & Tam (1996), Rilley (1991), Piasta (1984b), Cioni et al (2001). Alonso et al (2005). In Alonso (2006a) the identification of indicators of damage to assess reinforced concrete structures affected by fire is considered, and a protocol to identify the level of damage in the concrete is given. However, the isolated use of any method does not give reliable results because of the gradient and nonhomogeneity of the damage that cause differences between the external and internal strength. In addition the assessment of post-fire reinforced concrete structures is complex due to the overlapping of chemical and physical phenomena inducing different type of damages, Alonso (2006a,b). In this paper the assessment of damage of concrete in post-fire structures that have suffered different scenarios of fire are considered, one underground structure 48h in fire and a tall building, Windsor Tower, 18h in fire. In-situ inspection and laboratory tests have been performed. The heterogeneity in the distribution of the damage is identified, and the depth of concrete affected by the fire is determined, that has allowed identifying the gradient of temperatures reached in the concrete. The depth of concrete for the critical temperature of 500 C has been determined. Finally, a protocol for assessment of concrete structures affected by fire is given including non destructive and destructive methods and micromacrostructure analyses of the damage. 2 DESCRIPTIONS OF THE STRUCTURES The Windsor Tower in Madrid, >100 m tall, was involved in a major fire on the 13th of February 2005, of 18 hours duration. The fire caused an extensive structural damage of the upper floors of the building. The fire of this building caused intense interest among researchers seeking a better understanding of the performance of concrete structures under fire, due to the nature of the construction of the building, a large concrete frame with steel perimeter columns. The standard structure of the tower was built around a central reinforced concrete core, with columns of and cm size, that housed lifts and stairways, Calavera et al (2005), Fletcher et al (2006). The other type of building construction considered was an underground power station for distribution of electricity, containing three basements. The roof of the first basement was built with prestressed beams. A reinforced concrete slab 3 m thick was constructed between the second and third basement. Each floor has columns with different dimensions, and cm, spaced 4 to 5 m. Some intersecting concrete beams, cm, complete the structure. The fire was initiated in the first basement and reached the second and third basements. The duration of the active fire in the third basement was 48 hours. 3 IN-SITU INSPECTION OF POST-FIRE CONCRETE STRUCTURES 3.1 Visual inspection Visual inspection was used to assess apparent damage, for instance the distribution of zones of exploding concrete, as those shown in figure 1, in beams and slab of the underground structure. Also the corners of the columns are weak points for explosion, as shown in figure

3 to the fire, 2) followed by a sudden increase of the temperature of the wire, and 3) breaking some wires due to loss in ductility of the steel. Also some brittle fracture in reinforcements, as shown in figure 2, indicates that the temperature has reached values >500 C. Figure 1. Damage in concrete elements after fire, spalling in prestressed and reinforced beams and in concrete slab. 3.2 Non-destructive in-situ testing The most employed ND method for assessing the extension of the damage of concrete structures after fire is the ultrasonic pulse velocity. The ultrasound pulse test performed on the columns of the underground structure pointed out the heterogeneities in the damage within the column itself and between columns, as shown in figure 3. The ultrasound rate varies with: a) the height of the column, indicative of the heterogeneous distribution of the damage, b) the presence of cracks and c) the distance to the crack. These measurements put in evidence that sharp variations in cracking intensity correspond well with low ultrasound rate measurements, however, the penetration of the damage and the depth of chemical alteration components due to fire cannot be deduced directly from these measurements. Figure 2. Damage in a corner of the columns of the Windsor Tower. Changes in colour and break of aggregates, oxide layer covering the external bars and brittle fracture in a stirrup (Calavera et al (2005)). One concern in the inspection is to identify the moment of the different explosions, if they occurred during fire, at high temperature, or during cooling. Although this is not an easy task from a simple observation, some aspects in the concrete can help: The pink colour change in the paste and siliceous aggregates of the concrete of Windsor Tower suggested a temperature in the area around ºC, Alonso (2006b). Besides, the explosion zones can leave the reinforcement exposed, if it is fully covered with a layer of oxide of brown-red or black colour, is typical of corrosion induced at high temperature, >500ºC. The pretressed beams, which are very sensitive to fire and temperature gradients, can be subjected to explosions that partially or totally destroy the beams, including breaking the presetressed wires. The explosion could develop in several steps: 1) first a concrete explosion due to thermal and pore pressure stresses in concrete that leaves the wires exposed directly 3.3 Concrete and reinforced samples Cores of concrete in the selected zones defined from in-situ test should contribute to the final determination of the damage penetration. The extraction of a full core is, in most of the cases, quite complicated due to the gradient of damage, chemical alteration and generation of cracks. The observation inside the hole of the column, after the core extraction, can help to identify the propagation of cracks, as those shown in figure 4-left. The carbonation depth should be also determined in-situ at different points of the structure and in the cores, using phenophtalein indicator, in order not to misunderstand with portlandite transformation induced by the fire, Alonso (2006a,b). Rate (m/s) Heavy cracked cracks< No cracks 2-cracks< heavy cracked Column Height (cm) Figure 3. Ultrasound pulse velocity variation in columns of the underground building. Effect of cracking and height. 473

4 % CSH Floor P12 Floor P13 Floor P19 Floor P14 Figure 4. Concrete core from a column and hole showing cracks (left). Steel wire covered with oxide and brittle fracture due to high temperature exposition (left). Reinforcement samples should be obtained to determine the loss of mechanical properties and the embrittlement, as shown in figure 4 (right) Depth (cm) Figure 5. Depth of cement paste alteration at several concrete depths, from TG tests. 4 MECHANICAL AND CHEMICAL CHANGES IN CONCRETE SAMPLES Concrete samples should help to determine the mechanical strength and to identify the temperature depth penetration that has induced chemical changes in concrete components. 4.1 Mechanical strength losses The determination of the mechanical losses due to the action of fire is a complicated task, as the damage does not distribute homogenously. Besides, a gradient of concrete with different levels of alteration of chemical components exists. In addition, most of the time it is not possible to get a full core, as the case shown in figure 4, with the external zone broken; however the inner part can be used for the determination of mechanical strength of the mass of the concrete not affected by the fire. 4.2 Methods for the determination of chemical and physical transformations in cement paste The CSH gel of hydrated cement paste is the main component and responsible for mechanical strength, Alonso & Fernandez-Municio (2008). The dehydration of CSH identified with the TG test at a weight loss between 100 to 350 C does not take place at ambient temperature, as happens with portlandite transformation due to carbonation, Alonso (2006a,b). The CSH transformation allows identifying the depth of concrete exposed to temperatures up to 350ºC, as shown in figure 5 for columns of the Windsor Tower of different floors, given a maximum depth for CSH transformation up to 3 cm, while in the underground building up to 7 cm, Alonso (2006a,b). Figure 6. Microscopy of cement paste and aggregate alteration due to fire. SEM was used to confirm the type of damage. In the case of the concrete from the prestressed beams of the underground building, intense micro cracking in the mass of the concrete was observed, both in aggregates and cement paste, figure 6 (left); but the cement paste did not show chemical alteration. These damages are attributed to the explosions due thermal and pore pressures accumulated inside the concrete (Alonso, 2006a). The Backscattering Microscopy allowed identifying the degradation of cement paste and aggregates; loss of density of the material and bond between paste and aggregates are indicative that the depth of the concrete has been exposed to temperatures above 350 C, figure 6 (right). The presence of ettringite crystals indicate that the temperature reached values below 100 C at a specific depth, Alonso (2006b). The micro-hardness measurements in cement paste have allowed differentiation between different damaged zones and quality of concretes. The porosity has contributed; although to a lesser extent, to differentiating between levels of damage. Increases in porosity are generally associated with dehydration processes, but also due to the formation of microcracking, Alonso (2006a,b). 474

5 5 ANALYSES OF THE METHODOLOGIES FOR ASSESSMENT OF FIRE CONCRETE DAMAGE Although there are several techniques to determine the depth of concrete altered by the temperature, their use alone do not allow accurately determining the depth of concrete affected by the fire, and some of them are not able to discriminate between the type of damage, physical or chemical. None of the methods, neither NDT nor DT is able to determine the loss of mechanical strength even with the direct measurement of the strength, because of the presence of a gradient of damage in the concrete elements. Measurement of the ultrasound velocity has the advantage of a NDT technique able to differentiate among damaged and non-damaged zones. However, the determination of the depth of concrete cover affected is difficult from this method, as the ultrasound velocity variation in a fired concrete structure changes not only due to the dehydration of cement paste, but also because of the crack formation; however, the presence of cracks does not necessarily mean a fire alteration of the concrete components; in fact, most of cracks are due to the thermal stresses during fire, Alonso (2006a). Ultrasound velocity measurements taken in Calavera et al (2004) in Windsor Tower showed that the direct ultrasound velocity measurement data did not always allow discriminating with respect to the depth of damage, and laboratory tests on cores were needed to calibrate the measurements on-site to give an overestimation (up to a depth of 10 cm) of the depth at which the compressive strength of the concrete was considered irreversibly affected. Microscopic techniques allow differentiating between the microstructure changes in the paste, aggregates, the interface bonding loss and the cracking, but experience is needed to identify the origin of the damage. Thermo-gravimetric tests are able to identify the local chemical changes in cement paste and the level of dehydration of CSH, which is the component responsible for concrete strength development. Variations in CSH transformation and microstructure changes in concrete elements from different floors of the Windsor Tower indicated that the alteration of concrete due to fire penetrated up to a maximum of 3 cm, Alonso (2006b), which did not agree with the values predicted in Calavera et al (2005). Figure 7 shows the concrete depth of temperature penetration determined from microstructure testing. Relationships have been found between several indicators of concrete damage, including microhardness, porosity and CSH transformation. A transformation of at least 55% CSH is needed to induce relevant changes in the other properties, Alonso (2006). Tª ºC Figure 7. Mean temperature depth in concrete columns exposed to fire of Windsor Tower. Cover (mm) Depth (cm) Cover (mm) Figure 8. Isotherm in concrete columns of Windsor Tower exposed to fire. Determination of the depth of isotherm 500 C. The damage induced by the fire in the concrete elements is very heterogeneous; the reason is attributed to the fact that in a real scenario the fire is not homogenous within the structure, and even within the same floor or column. The surface temperature varies between one place and another and also the duration of the intensity of the fire, so that the effect of the concrete damage also will be different. In the case of Windsor Tower the extension of the damage varied from one floor to another, i.e. in floors 12 to 19 the differences were very relevant and the results showed that in the higher floors the depth of concrete affected by the fire did not penetrate more than 1.5 cm, while in the lower floors the penetration of damaged reached a depth of 3 cm. 475

6 Finally the isotherms of the temperature depths inside the concrete columns were drown (figure 8), and in particular the isotherm for the depth of 500 C was determined, which can be used for further calculation of residual load bearing capacity. 6 CONCLUSIONS The use of a set of tests allows differentiating between several levels of damage and to identify indicators of damage in post-fire structures. The use of ultrasound pulse as a non destructive technique allows identifying damaged areas of the structure, but does not give the penetration of the damage. The TG test is an accurate method to determine the chemical degradation of the cement paste, The determination of % of CSH transformation in cement paste allows discriminating between damage and undamaged depths of concrete, Tª < 350 C. The microscopy analysis allow differentiating between sound and altered zones due to fire and is a good method to identify the presence of cracking and its distribution in the mass of the concrete. The depth of the damage is identified with this technique. The isotherm for 500 C can be determined from microstructure tests. The penetration of the damage inside the concrete in a real fire is heterogeneously distributed. ACKNOWLEDGEMENTS The author wants to thank MEC of Spain for the financial support of this work, PSE 11, HABITAT 2030, PSS REFERENCES Alonso, C. 2006a. Assessment of damage in concrete structures exposed to fire. Micro and macrostructural analysis. 4th Int. WSp. Structures in Fire, SIF 06, Aveiro: Alonso, C., Andrade, C., Menendez, E. & Gayo, E Microstructure changes in high and ultra high concrete exposed to high temperature environments. Quality of concrete structures and recent advances in concrete materials and testing. ACI, SP , Edt. P. Helene, E. Pazini, T. Holland & R. Bittencourt, edt Lindsay K. Kennedy: Alonso, C. & Fernandez-Municio, L Dehydration and rehydration processes of cement paste exposed to high temperature environments, J. Mater. Sci. 39: Alonso, C. 2006b. Influence of fire in the damages of concrete in the Windsord Building. Int Cong on Fire safety in tall buildings. Santander, Spain: Alonso, C. & Fernandez-Municio, L., Dehydration and rehydration processes in cementitious materials after fire. Correlation between micro and macrostructural transformations. fib workshop on Fire design of concrete structures. Coimbra: in press. Bazant, Z.P. &. Kaplan, M.F Concrete at high temperatures: Material properties and mathematical modells. Logman Grp. Ltd., England. Benedeti, A On the ultrasonic pulse propagation into fire damaged concrete, ACI structural J., 95 (3). Benedetti, A. & Mangoni, E., Damage assessment in actual fire situations by means of non-destructive techniques and concrete tests, WSp Fire design of concrete structures: what now?, what next? edt P. Gambarova, R. Felicetti, A. Meda and P. Riva, Milan: Chan, S.Y.N., Luo, X. & Sun, W Effect of high temperature and cooling regimes on the compressive strength and pore properties of high performance concrete, Construction and Building Mat. 14: Calavera, J., Izquierdo J.M. et al, Fire in the Windsor building. Survey of the fire resistance and residual bearing capacity of the structure after the fire. NIT INTEMAC, dec: Cioni, P., Croce, P. & Salvatore, W Assessing fire damage to R/C elements, Fire Safety J., 36: Colombo, M., & Felicetti, R New NDT techniques for the assessment of FIRE damaged concrete structures. 4th Int. WSp. Structures in Fire, SIF 06. Aveiro: Fellicety, R. 2004a. The drilling resistance test for the assessment of the thermal damage concrete. WSp Fire design of concrete structures: what now?, what next?, edt P. Gambarova, R. Felicetti, A. Meda & P. Riva, Milan: Fellicety, R. 2004b. Digital camera colorimetry for the assessment of fire-damaged concrete. WSp Fire design of concrete structures: what now?, what next?, edt P. Gambarova, R. Felicetti, A. Meda and P. Riva, Milan: Feliceti, R Recent advances and research needs in the assessment of fire damaged concrete structures, fib workshop on Fire design of concrete structures. Coimbra: in press. Fletcher, I., Borg, H., Hitchen, N. & Welch, S Performance of concrete in fire. A review of the state of the art with a case study of the Windsor Tower fire. 4th Int. WSp. Structures in Fire, SIF 06. Aveiro: Khoury, G.A Compressive strength of concrete at high temperatures: a reassessment., Magz of Conc Rs. 161: Piasta, Z. Sawicz, and L. Rudzinski, 1984a. Changes in structure of hardened cement pastes due to high temperature, Mater. & Struct. 17: Piasta, P. 1984b. Heat deformation of cement phases and microstructure of cement paste, Mater. & Struct. 17: Rilley, M.A Possible new method for assessment of fire-damaged concrete, Magz. of Conc. Rs., 43: Rostasy, F., Ehm, C. & Hinrichsmeyer, K Structural alterations in concrete due to thermal and mechanical 476

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