Concrete Rehydration after Heating to Temperatures of up to 1200 C

Transcription

1 Concrete Rehydration after Heating to Temperatures of up to 1200 C Markéta Chromá 1 Pavel Rovnaník 2 Dita Vořechovská 3 Patrik Bayer 4 Pavla Rovnaníková 5 ABSTRACT The research presented in this paper involves the study of the possible regeneration of cement binder in concrete subjected to high temperatures. An ongoing experimental program is simulating the heating of a concrete structure by a fire during which the temperature in the structure gradually increases. The experimental program is designed to analyze concrete behaviour after heating to C and is investigate investigating the consecutive reactions of phases (which originate after heating) with water. The laboratory specimens ( mm) are manufactured from fine-grained concrete with a water to cement ratio of w/c = After heating and cooling, the specimens are placed under the following conditions for a time period of 2 months: (i) laboratory conditions (t = 22 ± 2 C, R.H. = 55 ± 5 %), (ii) water (t = 22 ± 2 C) and (iii) laboratory conditions (t = 22 ± 2 C, R.H. = 55 ± 5 %) with periodical immersion of the specimens in water. To investigate the mechanism of concrete regeneration mercury intrusion porosimetry and X-ray diffraction (XRD) analysis are performed. In order to quantify the changes in concrete properties, the specimens are tested in compression and three point bending. The results of the research could make an important contribution to decisions made concerning the reconstruction of concrete structures affected by fire. In suitable cases it would be possible to regenerate parts of a concrete structure instead of totally rebuilding it, which would significantly decrease repair costs. KEYWORDS Concrete, Temperature, Rehydration, XRD, Porosimetry. 1 Brno University of Technology, Faculty of Civil Engineering, Brno, CZECH REPUBLIC, chroma.m@fce.vutbr.cz 2 Brno University of Technology, Faculty of Civil Engineering, Brno, CZECH REPUBLIC, rovnanik.p@fce.vutbr.cz 3 Brno University of Technology, Faculty of Civil Engineering, Brno, CZECH REPUBLIC, vorechovska.d@fce.vutbr.cz 4 Brno University of Technology, Faculty of Civil Engineering, Brno, CZECH REPUBLIC, bayer.p@fce.vutbr.cz 5 Brno University of Technology, Faculty of Civil Engineering, Brno, CZECH REPUBLIC, rovnanikova.p@fce.vutbr.cz

2 M. Chromá, P. Rovnaník, D, Vořechovská, P. Bayer and P. Rovnaníková 1 INTRODUCTION The behaviour of concrete structures at extreme temperatures is presently the subject of research concerning the safe operation of various types of structures (fire safety in tunnels and at chemical factories, industrial and high-rise buildings, power plants, etc.). Concrete exposed to high temperatures and its properties after heating is a subject under investigation by many authors. Arioz [2007] studied the effect of heating concrete up to temperatures of 200 to 1200 C on its strength, bulk density and surface texture. Visible damage was present on the surface for specimens heated to 600 C and higher. At a temperature of 1200 C, the binder in the concrete completely disintegrated. Strength significantly decreased with increasing temperature. Matesová [2007] compared the strength of concrete tested immediately after heating and cooling with that of concrete tested several months after cooling. Significant strengthening occurred for concrete specimens exposed to a laboratory environment for several months after heating. It was found that concretes with higher w/c ratios are more affected than concretes with lower w/c ratios. Shui et al. [2009] analyzed the rehydration of cement paste heated to temperatures of 300 to 900 C. Hydrated cement paste was ground to a grain size of < 75µm and heated to temperatures of 300 to 900 C with 2.5 hours of exposure at the maximum temperature. The specimens were mixed with water after cooling and the obtained cement pastes were cast into a mould ( mm). The initiation, time of hardening and compressive strength after 3, 7 and 28 days were further evaluated for the cement pastes. The initiation and time of hardening decreased with increasing temperature; compressive strength increased with time. The highest strengths were reached for specimens heated to a temperature of 800 C after 28 days. They had 60 % of the strength of the hydrated paste that was not heated. Alonso & Fernandez [2004] studied the microstructural changes in an OPC cement paste subjected to temperatures of 100 to 750 C using 29 Si MAS-NMR, thermogravimetry and XRD. The rehydration process in the specimens after placing them in a saturated chamber (100 % R.H.) at room temperature for 3.5 months was also studied. The presented experiments show the ability of concrete to rehydrate significantly in a certain environment during a long time period after heating. To be more specific, it has been found that the rehydration of concrete placed into water for a time period of 2 months after heating up to temperatures of 200 to 1200 C leads to a significant increase in compressive and flexural strength at temperatures ranging from 600 to 1000 C. These experiments indicate that it makes sense to investigate this field in more detail and find an appropriate explanation regarding the mechanism of concrete strengthening and stiffening after heating. 2 EXPERIMENT Laboratory specimens ( mm) made of fine-grained concrete (in the next text only concrete) with a water to cement ratio of w/c = 0.45 were manufactured. The compounds in the mix, namely CEM I 42.5 R cement according to ČSN [1993] from the cement works at Mokrá and 3 fractions of quartz sand according to ČSN [1994], were mixed in weight ratios of 1:1:1:1. At the same time, specimens ( mm) made of cement paste were prepared for use in the study of phase composition before and after heating using X-ray diffraction (XRD) analysis. All of the manufactured specimens were unmoulded 24 hours after casting under laboratory conditions (t = 22 ± 2 C, RH = 55 ± 5 %) and placed into water for another 27 days. Afterwards, the specimens were dried to a constant weight at 60 C [Mendes et al. 2009] and heated in a muffle furnace to maximum temperatures of 200, 400, 600, 800, 1000 and 1200 C with a heating rate of 5 C/min and with 120 min of exposure to the maximum temperature. 2 XII DBMC, Porto, PORTUGAL, 2011

3 Concrete Rehydration after Heating After cooling, the specimens were placed under the following conditions for periods of 2 months: (i) laboratory conditions (t = 22 ± 2 C, R.H. = 55 ± 5 %), (ii) water (t = 22 ± 2 C) and (iii) laboratory conditions (t = 22 ± 2 C, R.H. = 55 ± 5 %) with periodical immersion of the specimens in water (every Monday, Wednesday and Friday for 3 minutes). Each configuration (defined by the maximum temperature, time and conditions after heating) was tested on three specimens. In order to quantify the changes in concrete properties, the specimens were tested in compression (compressive strength) and three point bending (modulus of rupture) immediately after cooling and after 2 months of being placed under all conditions (i)-(iii). Undamaged specimens after cooling and after 2 months of being placed in water were subjected to pore structure analysis using mercury intrusion porosimetry. Powdered cement paste specimens were subjected to XRD analysis to detect the presence of crystalline phases also after cooling and after 2 months of being placed in water. 3 RESULTS AND DISCUSSION 3.1 Compressive Strength The effect of placing concrete under various conditions after heating on its compressive strength is shown for individual maximum temperatures in Fig. 1 (left). It follows from the figure that both placement in water and the periodical immersion of specimens in water have a significant effect on the rehydration of concrete after heating primarily to 400, 800 and 1000 C (specimens placed in water) and to 600 and 800 C (specimens periodically immersed in water) compared to specimens tested immediately after cooling. The strength of specimens placed under laboratory conditions decreases compared to specimens tested immediately after cooling. 3.2 Modulus of Rupture The effect of placing concrete under various conditions after heating on its modulus of rupture is shown for individual maximum temperatures in Fig. 1 (right). A significant increase in the modulus of rupture has been found for specimens heated to 600, 800 and 1000 C for both specimens placed in water and specimens periodically immersed in water compared to specimens tested immediately after cooling. Figure 1. The effect of placing concrete under various conditions after heating on its compressive strength (left) and modulus of rupture (right) is shown for individual maximum temperatures. XII DBMC, Porto, PORTUGAL,

4 M. Chromá, P. Rovnaník, D, Vořechovská, P. Bayer and P. Rovnaníková 3.3 Pore Structure The distribution of pores and changes to it after exposure to elevated temperature are presented in Figure 2 (left). The specimens investigated mainly contained pores smaller than 1 µm in diameter. The smaller pores cause a rapid increase in vapour pressure due to the dehydration of CSH gel upon heating [ref], and for this reason, and consequently crack formation and spalling result in concrete heated to elevated temperatures. The differences in cumulative pore volumes for specimens heated up to 400 C are not significant. However, data measured at above 400 C indicate that the modification of heating temperature significantly affects pore characteristics. Above 400 C, the cumulative pore volume and pore size gradually increased and at 1200 C only pores larger than 10 µm can be found in the structure. The increased volume of pores with a higher radius results in a decrease in compressive strength. The distributions of pores after exposure to elevated temperatures and placement in water for 2 months are presented in Figure 2 (right). Pore distributions are similar to those measured immediately after cooling except for specimens heated to temperatures of 600 and 800 C. Pores at these temperatures are only slightly increased compared to those obtained for temperatures up to 400 C. This results in a relative increase in compressive strengths and moduli of rupture compared to specimens measured immediately after heating and cooling. Figure 2. The cumulative intruded volume of pores immediately after heating and cooling (left) and after 2 months of placement in water (right). The results for total porosity observed immediately after heating and cooling and after 2 months of placement in water for different exposure temperatures are presented in Figure 3. The differences in the total porosity after heating up to 400 C and cooling are not significant; they are within the range of standard deviations. Above 400 C, the total porosity gradually increases the value at 1200 C is 1.8 times higher compared to the value at 400 C. After rehydration the total porosity is similar in the specimens preheated up to 800 C and increases further at higher temperatures. By comparison of the total porosity before and after hydration it can be seen that the values obtained are very close except in the case of specimens heated to 600 and 800 C, in which instance the total porosity before rehydration is higher. 4 XII DBMC, Porto, PORTUGAL, 2011

5 Concrete Rehydration after Heating Figure 3. Total porosity. 3.4 XRD analysis The qualitative composition of specimens and changes in the content of individual phases are summarized in Tables 1 and 2 for specimens measured immediately after heating and cooling and specimens tested after 2 months of placement in water, respectively. Only a comparison of the relative contents of individual phases in all specimens has been performed. Table 1. Phase composition immediately after heating and cooling. phase/ specimen ettringite monosulphate gypsum anhydrite II silicocarnotite yeelimite portlandite calcite vaterite free lime C3S C2S C3A mayenite C4AF Ref ? C ? C ? C ? C ? + +? C ? C ? Legend: +++, ++, + relative content of phase (regarding other specimens) phase is not present?...presence of phase is uncertain X-ray diffractograms of the reference specimen and the heated specimens at various temperatures are shown in Fig. 4a. Typical reflections associated with ettringite (C 3 A 3CŜ H 32 ), monosulphate (C 3 A CŜ H 12 ), gypsum (CŜH 2 ), portlandite (CH), calcite (CaCO 3 ), alite (C 3 S), larnite (C 2 S) and brownmillerite (C 4 AF) have been found in the reference sample. After heating, some reflections disappeared, such as those corresponding to ettringite, monosulphate and gypsum, which were identified in the initial specimen but not in the samples heated to 200 C and higher. Gypsum is decomposed to anhydrite II at temperatures of 600 to 800 C and anhydrite II is further partly transformed to CaO and SO 3. A reduction of diffraction peaks related to portlandite is also observed at temperatures above 400 C, which proves the dehydroxylation of CH and the formation of CaO. For temperatures above 800 C the diffraction peaks of CH are absent. The peaks of calcite increase XII DBMC, Porto, PORTUGAL,

6 M. Chromá, P. Rovnaník, D, Vořechovská, P. Bayer and P. Rovnaníková in intensity above 400 C and gradually disappear at the higher temperatures due to the calcite decomposition to CaO and CO 2. The partial increase in the intensity above 400 C is probably caused by the partial transformation of decomposed portlandite into calcite [Stepkowska et al 2004]. Free lime (CaO) is identified in the specimens heated to 800 C and higher; it originates from the transformation of portlandite, calcite and anhydrite II. At temperatures of above 1000 C the peaks of sulphate crystalline phases are observed silicocarnotite (Ca 5 (SiO 4 ) 2 SO 4 ) and yeelimite (Ca 4 Al 6 O 12 SO 4 ). Also, the presence of mayenite (C 12 A 7 ) and peaks with increasing intensity related to larnite and brownmillerite are evident. Table 2. Phase composition after 2 months of placement in water. phase/ specimen ettringite monosulphate gypsum anhydrite II silicocarnotite portlandite calcite free lime C3S C2S C3A mayenite C4AF gismondine? katoite CAH + CASH (except for gismondine) Ref ? C ? C ? C ? C? ? C ? C ? ++ + Legend: +++, ++, + relative content of phase (regarding to other specimens) phase is not present?...presence of phase is uncertain The XRD diffractograms corresponding to the rehydrated specimens are presented in Fig 4b. The specimens, after heating up to 800 C and being rehydrated, contain reflections of ettringite, and peaks related to portlandite and calcite are observed in all specimens. After rehydration, peaks of free lime are not present in the specimens heated to 800 C and higher due to the rehydration of lime into portlandite, which can carbonate to form calcite. After rehydration, katoite (C 3 AH 6 ) has been identified in the specimen preheated to 1000 C and CAH + CASH in the specimen preheated to 1200 C. The occurrence of other crystalline phases in all specimens is similar to that before rehydration. 4 CONCLUSIONS The experimental works presented in this paper involved the study of the possible regeneration of cement binder in concrete subjected to high temperatures. It is shown that water treatment of concrete specimens preheated to temperatures of 200 to 1200 C leads to a significant increase in compressive strength and modulus of rupture. This has been found for both the constant placement of specimens in water and their periodical immersion in water. The results are supported by microstructural analyses porosimetry and XRD analysis. The results show that it is possible to partly regenerate concrete that has been subjected to high temperatures, e.g. that are prevalent during a fire. 6 XII DBMC, Porto, PORTUGAL, 2011

7 Concrete Rehydration after Heating Figure 4. XRD immediately after heating and cooling (a) and after 2 months of placement in water (b). ACKNOWLEDGMENTS This outcome has been achieved with the financial support of the Czech Ministry of Education, project No. 1M0579, within the activities of the CIDEAS research centre. REFERENCES Alonso, C & Fernandez, L. 2004, Dehydration and rehydration processes of cement paste exposed to high temperature environments, Journal of Materials Science 39, pp Arioz, O. 2007, Effects of elevated temperatures on properties of concrete, Fire Safety Journal 42, pp ČSN EN , Cement. Složení, jakostní požadavky a kritéria pro stanovení shody. Část 1: Cementy pro obecné použití (Cement. Composition, qualitative requirements and criteria for conformity assesment). In Czech. ČSN Křemenné písky. Základní technické požadavky (Siliceous sands. Basic technical requirements). In Czech. Matesová, D. 2007, Effect of exposure time after heating and w/c ratio on residual strength of concrete: pilot studies, in XIth International Conference on Ecology and New Building Materials and Products, Telč:VUSTAH, Czech Republic, pp Mendes, A., Sanjayan, J.G. & Collins, F. 2009, Long-term progressive deterioration following fire exposure of OPC versus slag blended cement pastes, Materials and Structures 42, pp Shui, Z., Xuan, D., Chen, W., Yu, R. & Zhang, R. 2009, Cementitious characteristic of hydrated cement paste subjected to various dehydration temperatures, Construction and Building Materials 23, pp Stepkowska, E.T., Blanes, J.M., Franco, F., Real, C. & Pérez-Rodríguez, J.L. 2004, Phase transformation on heating of an aged cement paste, Thermochimica Acta 420, pp XII DBMC, Porto, PORTUGAL,