MICROSTRUCTURAL DEVELOPMENT IN FIRE DAMAGED CONCRETE - A CASE STUDY V P Chatterjee, S K Chaturvedi, S K Gupta, M Kumar and P S Sharma National Council for Cement and Building Materials, Ballabgarh, Haryana, India Abstract The present paper deals with study of a damaged concrete exposed to fire. The evaluation of the concrete core obtained from a basement indicated loss of hardness of the concrete from 6.5 to 3.5, when measured in Mohs hardness scale. The air void distribution was found to increase by almost 20%. Micro fractures were widened from 35 micron to 625 microns maximum. In fact three types of micro fractures viz. honey comb, web and dendritic were developed and the fractures were discontinuous in nature and propagated deep inside the concrete along with air voids of various shapes and sizes. The fire damaged concrete samples showed honey combing and web fractures joined with the air voids resulting brittleness in the concrete. The hydrated products of cementitious phases and siliceous aggregates have undergone enthalpy changes when subjected to high temperature. The concrete consisted of fine aggregate of river sand and two types of coarse aggregates i.e. Biotite- Granite and Hypersthene-Granite which were found to be partially weathered. The microstructural and compositional changes of feldspars constituting aggregate as observed above also indicated the exposure of concrete to temperature beyond 700 0 C. The damage resulted in micro fractures, stretching and elongation of mineral grains and higher percentage of strained quartz found as in petrographic investigations. The above evaluation indicated that the structure damaged by exposure to fire is not safe for living. Introduction Petrographic examination of concrete provides rapid means of diagnosing numerous concrete problems. Petrographic investigations of concrete that has been exposed to fire can exhibit a number of abnormal features and accordingly quantification of damages in terms of morphometric changes and microstructure can easily be assessed. Cracking, paste hardness, aggregate colour changes and condition and reinforced response all have special importance with respect to the effects of heat. Aggregate colour changes may be the most obvious sign of fire exposure. Numerous microstructural changes are also developed in both coarse and fine aggregates. All these parameters envisage carrying out the study and are included in the present paper. Experimentation A random sample of concrete core (core length 18 cm & diameter 10 cm) containing coarse and fine aggregates and cement paste was evaluated. The core sample was steel grey in colour with vitreous luster. The sample was found to be massive and compact but not very hard and loosely packed. Compatibility of the aggregates (both coarse and fine) and cement was also poor. Hardness of the sample was determined using Mohs hardness scale. Taking hardness into consideration sample was cut along the length into two equal halves. First half was used for selecting the samples at 2 cm variation to study the air void distribution, cement hydration, correlation between cement and aggregate etc. Coarse aggregate samples used in the concrete were separated out for detailed petrographic evaluation.thin sections of the selected rock types were prepared. Thin sections of four samples containing coarse and fine aggregates and cement mortar were prepared for detailed microscopic studies to establish the relation between the aggregates and mortar. Four polish sections of different locations were prepared to study the products of adverse reactions (if any), behavior of cement paste within the coarse aggregate, microstructural and morphological changes developed in the sample.
The studies are based on ASTM-C-457, C-856, IS- 2386 pt VIII and IS -343. The studies were carried out in transmitted and reflected light in NIKON-POL-600E Microscope. Analyses were carried out in Image Analysis System attached to the Microscope. For DTA analyses, samples from 0-60 mm, 60-120 mm and 120-180 mm segments were selected from second half of the concrete core. Samples of each segment were crushed separately and coarse aggregates were removed completely. Rest of the samples were crushed and ground to -100mesh.The resulting samples were used for thermal analysis, which was carried out on a simultaneous DTA/TG thermoanalyser. Results and Discussions The hardness of the mortar was originally 6.5, which was reduced to 3.5. Hardness test was carried at different points of the core to arrive at the exact hardness. It was found that there was slight variation in hardness in the cement mortar adjoining to coarse aggregate than the cement mortar outside. It was observed that two rock types (Biotite -Granite and Hypersthene-Granite) were used as coarse aggregate. River sand was used as fine aggregate. Description of the rock types used in the concrete, thin sections containing both aggregates and mortar and polish sections of the concrete are given in the following paragraphs. Modal composition of the rocks is given in Table -1 and granulometric analyses of quartz is given in Table- 2. Biotite-Granite This is a medium grained textured rock. The major mineral constituents are quartz, biotite and plagioclase-feldspar. Accessory minerals are orthoclase-feldspar, hypersthene and iron oxide. (Fig.1) Subhedral to anhedral quartz grains are well graded and inhomogeneously distributed. Granulated quartz grains are developed as small clusters in the rock. The strained quartz percentage is about 28% and their undulatory extinction angle varies from 32 0 to 36 0. Lath to tabular shaped biotite grains are highly fractured and twisted. Biotite grains are broken into small pieces along the weak planes. Prismatic plagioclase grains are mostly fresh. Few plagioclase phenocrysts and subhedral orthoclase grains are fractured, shattered and partially altered. Subhedral hypersthene grains are highly fractured, shattered and altered. Fig.1: Distribution of biotite, orthoclase-feldspar, plagioclase-feldspar, microclinefeldspar, hypersthene, iron oxide & quartz grains in coarse aggregate. (At 5X, X-nicols) Hypersthene-Granite This is a coarse grained textured partially weathered rock. The major mineral constituents are plagioclase-feldspar, hypersthene, biotite, orthoclase-feldspar and quartz. Accessory minerals are microcline-feldspar and iron oxide. Prismatic plagioclase grains with corroded margins are highly fractured and partially altered. Subhedral hypersthene grains are also highly fractured, shattered,
twisted and altered along the weak planes. Tabular to lath shaped biotite grains are highly altered, fractured and iron leached. (Fig. 2) Subhedral orthoclase grains are mostly fractured and saussoritised. Subhedral quartz grains are well graded and homogeneously distributed. Granulation is too severe in quartz grains. The strained quartz percentage is about 34% and their undulatory extinction angle varies from 37 0 to 39 0. Tabular microcline grains are highly corroded on the margins and fractured. Subhedral iron oxide grains are corroded on the margins. Fig.2 Distribution of quartz, orthoclase-feldspar, plagioclase-feldspar, microclinefeldspar, hypersthene and iron oxide grains in the coarse aggregate. (At 5X, X-nicols) Table 1 MODAL COMPOSITION OF THE COARSE AGGREGATES (Results in %) Sl. M I N E R A L S No. Rock Type Quartz Plagioclase -feldspar Biotite Hypersthene Orthoclase -feldspar Microcline -feldspar Iron oxide 1 Biotite 32 24 28 6 7-3 - Granite 2 Hypersthene- Granite 12 29 15 24 14 4 2 Table 2 GRANULOMETRIC ANALYSES OF QUARTZ (Results in µm) Sl. No. Rock Type Minimum Maximum Average 1 Biotite - Granite 64 987 516 2 Hypersthene -Granite 76 896 470 The mafic minerals present in the rocks of the concrete were highly fractured and shattered as observed in the thin sections. Feldspar grains were highly distorted and altered along the weak planes. Dendritic, pitted and bridging structures were developed in most of feldspar grains. Margins of few feldspar phenocrysts were completely reacted.bands of micro grains were developed on the margins of the phenocrysts with discontinuous micro fractures in the adjacent mortar. Few quartz grains were highly fractured and shattered. In the mortar, it was observed that micro needle shaped grains were developed due to alkali-silica- reaction. Polish Sections of Hardened concrete and DTA analysis In the polish sections, it was observed that the coarse and fine aggregates were uniformly distributed in the cement paste. Air voids of different shapes and sizes with highly corroded margins were present
in the sample. There was large variation in size of air voids.most of air voids were hollow in nature. In few instances the voids were filled with micro globular grains. This effect was observed in most of the big size voids ( >2mm). Sagging in small voids was observed. These refilled voids were evenly distributed in the sample. Few micro size capillaries discontinuous in nature were also present in sample. The macro size capillaries were also observed in the centre of core of the sample. The capillaries were mostly open in nature with smooth margins. Capillaries and air voids were not inter related with each other.(fig 3) Fig. 3: Open Air Void with light colour bands containing Grains on the margins. (At 5X) Cement phases were completely hydrated. Elongated rod shaped gels containing micro grains were uniformly distributed throughout the cement paste portion of the sample. Impingement of the cement was observed in feldspar and quartz grains. Dendritic structure is commonly developed by cement intruded in coarse aggregate grains along the weak planes. Pitted and bleb shaped structures were developed in the mafic constituents of the coarse aggregates. Numerous clusters of honey combing and web fractures were developed in the concrete. These clusters were uniformly distributed through out the sample. The fractures were mostly discontinuous in nature with shallow depth. However, opening of the fractures were increased to a large extent. Development of pink to red colour was observed in few coarse aggregate grains. Depth of the paste carbonation was deep in the sample. Cement paste content of the sample was moderate. Air voids were randomly distributed through out the core. Effect of adverse reactions such as alkali-silica-reaction was feeble. Cement paste volume of the sample was high. But bonding of the concrete was not proper. Aggregate volume was on higher side in the sample. The concrete became highly porous and brittle due to fire. Condition of embedded item was poor in the concrete. The distribution of microstructural components such as joints and other discontinuities were present in the sample. Extent of the cement hydration was completed as grains of cement phases were almost absent. Only traces of belite grains in highly deformed form were observed in higher magnification (50X, objective). When water to cement ratio was attempted to establish, it was observed that the distribution pattern was non uniform in nature. However, it was beyond the scope of the instrument. Hence no conclusions were made.similarly air content also shown a moderate result. Majority of the above mentioned microstructures were developed in the coarse and fine aggregates and cement mortar due to increase in temperature due to fire.
Top and bottom portions of the core were chosen for judging the extent of damage. The top portion of the core was completely fractured with numerous cracks as affected by fire.dta curves of the two portions are presented in Fig-4.Top portion exhibited no endothermic peak for Ca(OH) 2 dehydroxylation indicating that this portion was exposed to fire at above 600 C,whereas sample from bottom exhibits the Ca(OH) 2 peak indicating that this portion of the sample was less damaged by fire. Fig-4: Typical DTA curve of the damaged / undamaged concrete sample The DTA curves of Core-a (fig.5) up to a depth of 120 to180mm from the top surface, reveals that the endotherm corresponding to Ca(OH) 2 dehydroxylation is negligible. Whereas in Core-b and Core-c corresponding to the depth of 60 to 120mm and up to 60mm, this endotherm is present. This observation suggests that the Core-c was exposed to more that 600 C.The effect of temperature on the strength of concrete is not significant up to 300 C. However, beyond 300 C a definite loss of strength takes place as shown in fig 6. Based on these, it is concluded that there is clear loss of strength in concrete up to the depth of 120mm from the exposed surface. Fig-5: Typical DTA curve of the concrete sample Fig-6: Compressive Strength of Concrete at High Temperature Conclusions The damaged concrete sample exposed to fire indicated that the coarse and fine aggregate as well as the mortar were severely affected due to increase in temperature. However, the temperature rise was not very high as indicated by the plagioclase grains present in the coarse aggregate. But orthoclase grains of both rock types were partially damaged both morphologically and compositionally. Orthoclase grains were partially melted also due to increase in temperature. Quartz grains present in the coarse and fine aggregate were highly strained, fractured and shattered. Few quartz grains were completely granulated due to increase in temperature. Granulation and fractures were also observed in
mica grains of the rocks. The mineralogical characters of the coarse aggregate indicated that the temperature rise during the fire was approximately in the range of 700 0 C to 1000 0 C. Majority of quartz grains of the fine aggregate were highly strained, fractured and shattered. Numerous quartz grains were severely granulated due to sharp increase in temperature. These granulated quartz grains were distributed as small clusters in the matrix of the concrete. Few micro quartz grains were completely melted. Distorted and deformed microstructures were developed in the mafic minerals of the fine aggregate. There was a clear indication of melting of cement paste and dehydration of the mortar. Percentage of open air voids increased. Open capillaries were also developed in the concrete, which also supports the effect of the fire. The above observations indicated that the concrete was exposed to fire for about seven to eight hours. The concrete was totally damaged by the fire. Based on above megascopic and microscopic investigations, it was recommended for removal of damaged concrete with fresh concreting. The DTA studies on concrete sample revealed that the extent of damage of concrete at different depths. This helped in rehabilitation by removing the damaged concrete. Acknowledgement The authors have freely drawn upon from the completed R &D and sponsored project report of NCB. The paper is being published with the permission of Director General, NCB, Ballabgarh, India. References 1 ASTM C 457, 1998. Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete, In: ASTM Vol.4.02 (concrete and aggregate s), Philadelphia, USA, American Society for Testing and Materials. 2 ASTM C 856, 2004. Standard Practice for Petrographic Examination of Hardened Concrete, In: ASTM Vol.4.02 (concrete and aggregate), Philadelphia, USA, American Society for Testing and Materials. 3 Chatterjee, S., and Gudmundsson, R., 1977. Characterization of entrained air bubble system in concrete by means of an image analyzing microscope, Cement and Concrete Research, Vol.7, 423-428. 4 Diamond, S., 1986. The microstructure of cement paste in concrete, 8 th International congress on the chemistry of cement, Rio de Janeiro 1:123-147. 5 Indian Standard IS: 2386 (pt VIII), 1977. Methods of Test for Aggregate for Concrete, Bureau of Indian Standards, N.Delhi. 6 Indian Standard, IS: 343, 1970. Specification for Coarse and Fine Aggregates from Natural Sources for Concrete, Bureau of Indian Standards, N.Delhi. 7 Ray, J.A., 1983. Things Pertographic examination can and can not do with concrete, Proceeding of the 5 th International Conference on Cement Microscopy, 66-85. 8 Ghosh,S.N.,Raina,S.J and Vishwanathan,V.N.,1978.Instrumental techniques for investigation of damaged concrete,indian concrete journal,vol.52,pp-147-149 9 Green,J.K.,1976.Some aids to the assessment of fire damage,concrete,vol.10,no.1pp-14-17. 10 NCB investigation on fire damage to a factory building (June1987), unpublished report.