Performance evaluation of reactive powder concrete with polypropylene fibers at elevated temperatures

Reactive Powder Concrete (RPC) is a type of ultra-high strength concrete. Due to its dense microstructure, is vulnerable to explosive spalling at elevated temperatures. Remarkable application of RPC in special structures throughout the world has drawn the attention to understand the performance of RPC at elevated temperatures, which has not been investigated extensively yet. The main objective of this work was to evaluate the performance of RPC at elevated temperatures from 200 C to 800 C, by obtaining residual mechanical properties after exposure. The study aims to find an optimum fiber dosage for spalling protection of RPC. To improve the mechanical properties, RPC incorporating fiber dosage from 0.1% to 0.9% is studied. The thermal deterioration of RPC is assessed using ultrasonic pulse velocity, water absorption and sorptivity. Results shows that 0.1% fiber dosage is enough to control spalling of RPC up to 800 C. To enhance the residual properties of RPC exposed to elevated temperatures, it is recommended to use fiber dosage of 0.5%. The study also includes microstructural analysis of RPC subjected to elevated temperatures, to assess and evaluate the formation of pores and cracks.

Construction and Building Materials 169 (2018) 499–512 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat Performance evaluation of reactive powder concrete with polypropylene fibers at elevated temperatures Parameshwar N Hiremath, Subhash C Yaragal ⇑ Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore 575025, India h i g h l i g h t s  Performance of RPC with different fiber dosages at elevated temperatures is investigated  RPC with at least 0.1% polypropylene fiber dosage, checks spalling at elevated temperatures  0.5% fiber dosage, has shown superior fire endurance characteristics a r t i c l e i n f o Article history: Received 31 July 2017 Received in revised form 18 February 2018 Accepted March 2018 Keywords: Reactive powder concrete Polypropylene fibers Elevated temperature Mechanical properties Microstructure a b s t r a c t Reactive Powder Concrete (RPC) is a type of ultra-high strength concrete Due to its dense microstructure, is vulnerable to explosive spalling at elevated temperatures Remarkable application of RPC in special structures throughout the world has drawn the attention to understand the performance of RPC at elevated temperatures, which has not been investigated extensively yet The main objective of this work was to evaluate the performance of RPC at elevated temperatures from 200 °C to 800 °C, by obtaining residual mechanical properties after exposure The study aims to find an optimum fiber dosage for spalling protection of RPC To improve the mechanical properties, RPC incorporating fiber dosage from 0.1% to 0.9% is studied The thermal deterioration of RPC is assessed using ultrasonic pulse velocity, water absorption and sorptivity Results shows that 0.1% fiber dosage is enough to control spalling of RPC up to 800 °C To enhance the residual properties of RPC exposed to elevated temperatures, it is recommended to use fiber dosage of 0.5% The study also includes microstructural analysis of RPC subjected to elevated temperatures, to assess and evaluate the formation of pores and cracks Ó 2018 Elsevier Ltd All rights reserved Introduction Concrete is a composite material consisting of various ingredients, that are entirely different in their properties from each other It is very difficult to assess the fire resistance of concrete, due to different thermal characteristics of each ingredient The most vencher part is presence of moisture and porosity of the concrete The utilization of High Strength Concrete (HSC) for last few decades, throughout the world, has proved itself to be promising construction material [1] But in case of fire performance, some research studies have shown that HSC has disadvantage to resist fire, i.e., it is more prone to explosive spalling, due to low permeability and high brittleness when compared to normal strength concrete [2] The same observation was made in case of High Performance Concrete (HPC) due to dense microstructure and very ⇑ Corresponding author E-mail address: subhashyaragal@yahoo.com (S.C Yaragal) https://doi.org/10.1016/j.conbuildmat.2018.03.020 0950-0618/Ó 2018 Elsevier Ltd All rights reserved low permeability seems to be a disadvantage, in the situations where HPC is exposed to fire [3] From earlier studies, it is proven that HPC, is susceptible to spalling or even explosive spalling when subjected to rapid rise in temperature during fire exposure Spalling of concrete depends on many parameters such as ingredients of mix, type of aggregate, rate of temperature exposure, thermally induced mechanical stress, density of concrete, moisture content etc The main two reasons for explosive spalling of HPC are, thermal stress induced by rapid temperature rise and water vapour which may cause high pore vapour pressure To overcome spalling of concrete, addition of fibers, especially polypropylene fibers to concrete is well known fact in the field of construction Addition of polypropylene fiber has been proved to be very efficient in reducing spalling of concrete at elevated temperatures Polypropylene fibers melt at temperature of 170 °C, whereas spalling occurs between 190 °C and 250 °C [4] Presence of polypropylene fibers reduces the internal vapour pressure and eliminates the chances of spalling under fire The length of fibers has significant effect in 500 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 preventing explosive spalling of concrete Using 12 mm length of polypropylene fibers efficiently mitigates the explosive spalling when compared to mm length [5] This is in agreement with findings of [6] who reports that as length of fiber increases such as 12, 19 and 30 mm, it proves to be more effective in preventing spalling when compared to shorter polypropylene fibers of length mm and mm From literature, it is learnt that adding polypropylene fiber in HSC was good to reduce chances of spalling, but however some of researchers have reported that this has an adverse effect on strength RPC is a new emerging construction material in the modern era RPC with its high strength and durability properties, has been gradually replacing HSC and HPC, especially in special structures like long span bridges, tall structures and nuclear power plants As it is becoming commonly used, the chances of being exposed to high temperature also increases in the event of accidental fires However, so far only a few researchers have reported on its performance at elevated temperatures RPC has dense microstructure this seems to be a disadvantage in the situation where the fire endurance is a necessity The absence of voids does not relieve the internal stress that creates a major problem This problem can be solved by addition of polypropylene fibers to the mix However only a few studies have been carried out on RPC subjected to elevated temperatures and they also revealed contrary results, necessitating further research An experimental investigation made in 2015 [7,8] found that, plain RPC spalled under high temperature and spalling starts at 360 °C, whereas RPC with polypropylene fibers shows no spalling The reason behind spalling and performance of RPC prepared with different fiber dosages under elevated temperatures remains to be investigated Furthermore, mechanical properties including compressive strength, split tensile strength and weight loss of RPC, exposed to elevated temperatures are also of great concern from the serviceability requirements Experiments indicate that, despite the positive effects of polypropylene fibers in enhancing the residual strength of the heated RPC, an overdose of fiber could have an adverse influence on the RPCs thermomechanical properties A proper dosage of fibers to improve the spalling resistance of RPC depends on the mix proportion and the geometry of the fibers In case of elevated temperatures resistance performance, of RPC remains a concern, more so in relation to explosive spalling Earlier researchers have investigated that RPC is vulnerable to explosive spalling under elevated temperatures, which seriously jeopardizes the safety of RPC applications Yang et al [9] studied performance of RPC under elevated temperature in the range of 400 °C–800 °C Results show considerable reduction in strength and elastic modulus values due to elevated temperatures Liu and Huang [10], have reported that the residual strength of RPC at elevated temperatures decreases significantly at temperature beyond 300 °C when compared that of RPC at room temperature The reduction in strength is mainly because of the pore pressure mechanism in RPC that prevents water vapour from free transport within and its escape from the matrix, when exposed to elevated temperatures Pore pressure mechanism is caused due to dense microstructure and mainly due to disconnected pores Explosive spalling occurs when the pore pressure in the matrix accumulates to a threshold, exceeding the tensile strength of concrete Kalifa et al [11] suggested that mixing polypropylene fibers could reduce the pore pressure of concrete and decreases the risk of spalling and also as fiber content increases pore pressure decreases Essential problem associated in understanding, spalling of RPC including pore characteristics, pore size distribution, pore pressure and factors related to explosive spalling are yet, not exhaustively studied However, dense microstructure of RPC prevents evaporation and escape of free water from the interior portion of RPC specimen at elevated temperatures Due to its low permeability and discontinuous pore network, the risk of explosive spalling has jeopardized the safety of RPC structure This hinders the commercial development and application of RPC in the field of modern constructions Therefore, the physical parameters like weight loss, colour change, crack development, mechanical properties such as compressive strength, split tensile strength and water absorption of RPC at elevated temperatures is required to be investigated The effect of different fiber dosage on spalling has not been reported for RPC Since, the RPC is more likely to spall than HSC, it is necessary to investigate and understand the spalling behavior of RPC and recommend optimum fiber dosage to prevent spalling without compromise on fresh and hardened properties The effect of elevated temperatures up to 800 °C on fiber reinforced RPC has been the scope of this study Mechanical properties such as compressive strength, split tensile strength and physical parameters like weight loss, crack development at different temperatures were determined Durability properties like water absorption and sorptivity have also been studied This study also investigates the degradation of microstructure and its effect on residual mechanical properties of RPC after exposure to elevated temperatures To identify the deuteration portion at microstructural level, RPC specimens were subjected to Scanning Electron Microscope (SEM) analysis The SEM results strengthen and reinforce the reason behind reduced mechanical properties after exposure The previous researcher’s results on mechanical properties of RPC containing polypropylene fibers are not in agreement with each other This is due to differences in curing condition of specimen, material used for RPC production and the way of experimentation The optimum fiber dosage to prevent explosive spalling and simultaneously maintaining the residual strength to the expected range for RPC is yet to be investigated in detail Therefore, the focus of present investigation is to determine minimum dosage of polypropylene fibers required to mitigate spalling and to possess acceptable residual strength levels Experimental program 2.1 Materials RPC is composed of cement, silica fume, quartz powder and silica sand with very low w/b ratio to achieve required workability High range water reducing admixtures are used RPC is cement based concrete mixture In the present study, Ordinary Portland Cement of 53 grade was used which complies with IS:122692013 The chemical and physical properties of cement are shown in Tables and respectively Silica fume is the second basic important ingredient of RPC which fills the voids of micro particulates in the cement It also produces secondary hydrates products by pozzolanic reaction from the results of primary hydration Undensified silica fume was used in the present study, which complies with ASTM C1240-03 a and IS:15388-2003 Chemical composition of undensified silica fume is presented in Table The particle size of silica fume is extremely very fine of size 0.1 mm The physical properties of silica fume are tabulated in Table Quartz powder is the finest material compared to cement The particle size of quartz powder ranges from 10 mm to 45 mm It acts as a filler material in the mix proportion of RPC The chemical and physical properties of quartz powder are tabulated in Tables and respectively Silica sand is largest particle size material in mix proportion of RPC In the present study silica sand was used with particle size ranging from 150 mm to 600 mm The sand confirms to zone IV grading requirement as per IS: 383-2016 To maintain the required workability, a second generation polycarboxylic ether polymer, high range water reducing superplasticizer Master 501 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 Table Chemical composition of RPC ingredients Constituents Cement Silica Fume Quartz Powder Chemical compositions (%) CaO SiO2 Al2O3 MgO Fe2O3 TiO2 SO3 59.67 0.16 – 20.21 95.89 >99 9.08 0.22 0.17 2.02 0.11 – 3.64 0.10 0.56 0.45 – – 2.41 – – Table Physical properties of RPC ingredients Mix Constituents Physical properties Specific surface area/Particle size Cement Silica Fume Quartz Powder Silica Sand 330 (m /kg) 20,000 (m2/kg) 10–45 mm 150–600 mm Specific Gravity Density (kg/m3) Colour LOI 3.15 2.26 2.60 2.60 3210 300 700 1630 Light grey Dark grey Milky white Yellowish-white 1.45 1.10 – – Glanium Sky 8276 was used in the present study which meets the requirements of IS: 9013-1999 Polypropylene fibers of length 12 mm were used in the present study 2.2 Mix proportion and specimen preparation The mixing method adopted in producing RPC is quite involved when compared to conventional concrete production [12,13] The only change is the addition of polypropylene fibers and increment of superplasticizer dosage to maintain required flowability Mix proportion of RPC adopted is presented in Table The mixing sequencing of RPC as followed is, dry mixing of all ingredients in first stage, later in the second stage addition of half volume of water containing half the amount of superplasticizer In the present study, same mixing sequence was adopted [12] with one more value-added ingredient that is polypropylene fibers These fibers were added at the end of third mixing stage with increased mixing time by two minutes in total mixing duration The entire mixing process is completed within 14 Normal pan mixer with mixing speed of 80 RPM was used After completion of mixing, fresh mix of RPC was poured in 100 mm cube moulds and compacted on vibration table to remove air voids After one day of setting, cubes were removed from moulds and kept for curing under water for 28 days 2.3 Parameters studied Effect of different dosages of polypropylene fibers in preventing explosive spalling of RPC has been focused in the present study The polypropylene fibers content is varied from 0.1% to 0.9% The performance of RPC prepared with different fiber dosages at elevated temperatures of 200 °C, 400 °C, 600 °C and 800 °C, are investigated using digitally programable electric muffle furnace as shown in Fig The rate of heating was °C/min The retention period at elevated temperature, is taken as 30 for all temperature levels The microstructural investigation was carried out on RPC samples exposed to different temperature levels Physical parameters, such as colour change and crack development at different elevated temperatures were recorded by physical observation Fig Electric Muffle Furnace 2.4 Parameter evaluated In this study an attempt was made to study the performance of plain RPC at elevated temperature RPC after exposure to elevated temperatures, under goes changes in physical and chemical properties Generally assessment of fire damaged structure, usually starts with visual observation of colour change, crack development and spalling of concrete [14] Changes in physical properties including colour change and crack development have been considered here for assessment after exposure The variation in colour change and initiation of crack for RPC mixes prepared with different fiber dosages have been evaluated by careful inspection The weight loss at a given temperature was measured from three specimens Average percentage of weight loss were determined for RPC specimen prepared with various fibers dosages The weights of samples were taken before and after exposure to elevated temperatures for weight loss evaluation Portable Ultrasonic Nondestructive Digital Indicating Tester (PUNDIT) measurement is one of the quick methods that indicates, the qualitative degree of damage Deterioration of RPC after exposure to elevated temperatures, is assessed by using PUNDIT The UPV test is conducted on 100 mm cubes as per IS:13311 (Part 1):1992 Compressive strength test was conducted on cubes of 100 mm as per IS: 516-1959 The resid- Table Mix proportion of RPC Cement Silica fume Quartz powder Water/binder ratio Superplasticizer dosage 900 kg/m3 20% 20% 0.18 1.5 to 2.5% 502 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 ual compressive strength is evaluated for each temperature level Compression testing machine of capacity 2000 kN was used in the present study Split tensile test was conducted on exposed 100 mm RPC cube specimen The testing procedure as per IS: 5816-1970, was fallowed The most important essential parameters governing the concrete durability is penetration of water, gas and ions which mainly depends upon micro structure and porosity of concrete It is well known that RPC consists of dense microstructure The development of pores and micro cracks under elevated temperatures has a major impact on durability properties like water absorption and sorptivity The water absorption test was carried out on exposed 100 mm RPC cubes as per BS 1881-122 RPC cubes after exposure to elevated temperature were cooled down to room temperature Later weight of the cubes were taken and immersed under water for 24 h After 24 h, the cubes were removed and kept outside till it reaches to surface saturated condition then weight of cubes were taken The same procedure was followed for all RPC cubes with different fiber dosages, which were subjected to elevated temperature exposure Sorptivity test determines the rate of capillary rise in absorption of water as a function of time when only one surface of the specimen is exposed to water ingress by capillary suction, during initial contact with water Before conducting sorptivity test, four side surfaces of exposed RPC cubes were sealed with paraffin wax to ensure free water movement only through the bottom surface Then RPC specimen were kept on plastic strip in a tray such that the free water level was about mm above the bottom surface of specimen in contact with water The mass of water absorbed per unit area before immersion and subsequently after intervals of min, 10 min, 20 min, 30 min, 60 min, 180 min, 360 and 1440 was determined Test setup of sorptivity is as shown in Fig Three cubes were used for each test 2.5 Microstructure analysis In the present study SEM was used to understand the morphology of RPC after exposure to different elevated temperatures The presence of polypropylene fiber in RPC at low temperature exposure, that created channels through melting process of polypropylene fiber was confirmed by secondary electron images at high magnification using SEM The SEM was aided with Energy Dispersive X-ray Spectroscopy (EDS) which facilitates understanding the elemental composition and atomic weight of chemical compounds developed in RPC, when exposed to elevated temperatures Based on the presence of chemical compounds such as Ca, Si, Al and S and their atomic weight, the ratio of Si to Ca was determined A detailed microstructural investigation was carried out to understand the structural arrangements of hydrated and unhydrated products when RPC specimen were exposed to different elevated temperature levels Fig Water sorptivity Setup Results and discussion 3.1 Performance of plain RPC at elevated temperatures Current study includes performance of plain RPC at elevated temperatures The results are as shown in Fig and Table From results, it can be observed that as temperature increases strength also is observed to increase The increase in strength observed from 100 to 350 °C is around 20% The increase in strength may be due to rapid hydration of unreacted cement and silica fume which produce large amounts of hydrated products Thermoactive nature of quartz powder also participates in the hydration process when RPC is exposed to elevated temperatures, that leads to formation of dense hydrated products When plain RPC is exposed to 400 °C, explosive blasting of RPC was observed as shown in Fig Therefore, the results of residual strengths of RPC, have been reported in the above Table up to 350 °C 3.2 Physical observation The damage to the fiber reinforced RPC, after being exposed to elevated temperature can be roughly detected by observing the RPC surface Fig 5(a)–(c) shows RPC surface with different fiber dosage exposed to various elevated temperature levels (200 °C, 400 °C, 600 °C, 800 °C), along with the one at ambient temperature From Fig 5(a)–(c) it can be observed that, at 200 °C, there is no colour change and visible crack development on RPC surface At 400 °C, formation of light micro cracks was observed on RPC surface composed of 0.1% fiber dosage The RPC cubes composed of 0.5% and 0.9% have not shown any crack development at 400 °C of exposure RPC exposed to 600 °C has shown visible cracks on surface of all RPC cubes composed of different fiber dosages Among three different fiber contents, the RPC prepared with low fiber dosage (i.e 0.1%) has shown considerable surface cracks, compared to other RPC prepared with high fiber dosages From Fig it can be observed that as fiber content increases number of cracks appears to get reduced The presence of small pores were observed on the surface of RPC specimen composed of 0.1% fiber content at 600 °C The presence of these pores were less in number on surface of RPC specimen composed of 0.5 and 0.9% fiber dosages The slight colour change was observed in this temperature range Colour of cubes turns to slight pink reddish and the intensity of colour decreases as fiber dosage increases When RPC cubes were exposed to 800 °C, the development of crack was more pronounced, especially for RPC cubes produced with 0.1% fiber as shown in Fig 5(a) The RPC specimen have Fig Compressive strength of plain RPC at elevated temperatures 503 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 Table Compressive strength results at elevated temperatures Temperature (°C) 100 150 200 250 300 350 Mean (MPa) 105 111 115 119 123 126 fiber dosage inefficient to reduce spalling of concrete The vapour pressure created inside the concrete at elevated temperature leads to formation of micro cracks These small width cracks grew to large widths at higher elevated temperatures 3.3 Weight loss Fig Explosive blasting of plain RPC at elevated temperatures shown, major colour change when exposed to 800 °C From Fig (a), it can be observed that colour of cubes turns to grey reddish The presence of more surface voids was observed on RPC composed of low fiber dosages Spalling was not observed in entire exposure condition at elevated temperatures This indicates that 0.1% of polypropylene fiber is sufficient to prevent explosive spalling of RPC up to 800 °C From the above observation, it can be concluded that, as fiber content increases, leading to decreases in surface cracks and pores The thick cracks were observed on RPC samples composed of low fiber dosage at elevated temperatures This is may be due to low 27 ˚C 200 ˚C Fig shows the variation of average percentage loss in weight with elevated temperature for RPC specimen of different fiber dosages From Fig 6, it can be observed that as temperature increases, there is an increase in percentage weight loss in all RPC specimen prepared with different dosages In case of RPC prepared with 0.1% fiber dosage show, lower percentage of weight loss at elevated temperatures compared to other RPC mixes prepared with 0.5% and 0.9% This is because at temperatures above 170 °C, polypropylene fibers melt and create channels inside the concrete RPC with high dosage of fiber, suffer more weight loss due to fiber evaporation at high temperatures At temperatures, above 600 °C, these fibers turn to molten state and evaporates As fiber content increase, the rate of evaporation also increases Therefore, there is an increase in percentage weight loss with increase in fiber dosage at elevated temperatures The weight loss predominantly occurs, due to loss of water in all three forms, namely free water, adsorbed water and chemically bounded water From results, it can be observed that there is no significant difference observed between RPC mixes prepared with different fiber dosages at elevated temperature of 200 °C 400 ˚C 600˚C 800˚ C (a) 27 ˚C 200 ˚C 600˚C 400 ˚C 800˚ C (b) 27 ˚C 200 ˚C 400 ˚C 600˚C (c) Fig Fiber dosage (%) (a) 0.1 (b) 0.5 (c) 0.9 800˚ C 504 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 Fig Average percentage weight loss at different elevated temperatures At 400 °C the percentage weight loss, is more for RPC mix prepared with 0.9% fiber dosage As fiber content increases the percentage weight loss is also increasing with temperature [15] The same trend can be observed at 600 and 800 °C At 800 °C, maximum variation in weight loss was observed between RPC mixes prepared with different fiber dosages There seems to be approximately 5% and 7% variation in weight loss with 0.5% and 0.9% fiber dosage when compared to 0.1% fiber dosage There is a sudden increase in weight loss beyond 400 °C The rate of weight loss increase drastically as fiber dosage increase beyond 400 °C From the above observations, it can be stated that the apparent bulk mass loss occurs between 400 °C and 800 °C This loss may be due to evaporation of water and in addition beyond 400 °C there may be decomposition of hydrated and unhydrated compounds The process of dehydration starts beyond 400 °C, which release considerable amount of chemically bound water creating the interior pores At higher temperature levels, polypropylene fibers have been completely vaporized Hence cumulative effects of these parameters make RPC specimen to suffer considerable amount of weight under elevated temperatures 3.4 Ultrasonic pulse velocity This test is a qualitative one, used to evaluate the quality of concrete and this technique is sensitive to degradation phenomena including internal cracking and other deuteration due to thermal treatment UPV test was carried out to determine the severity of damage, when RPC specimen were exposed to elevated temperatures Fig shows results of RPC specimen prepared with various fiber dosages subjected to elevated temperatures From Fig it can be observed that, as fiber dosage increase, the UPV also increases at room temperature In case of 0.1% fiber dosage, the UPV results show sudden drop in velocity beyond 200 °C For the case of 0.1% fiber dosage, the value of UPV decreases continuously with increase in temperature up to 400 °C The smaller the relative UPV value, more is the severity of the damage The reduction in UPV value in 0.1% fiber dosage, is due to insufficient fiber content which is unable to release vapour pressure from core of concrete, that leads to micro-cracks As number of cracks increases there will be chances of discontinuous matrix and formation of voids This leads to reduction in UPV values The RPC mixes, prepared with 0.5 and 0.9% fiber dosages have shown higher UPV values compared to RPC mix prepared with 0.1% fiber dosage for 200 °C and 400 °C Values for the case of 0.1% fiber dosage, shows reduction in velocity from 4.78 to 2.00 km/s with increase, in temperature from 27 °C to 800 °C and consequently gradual deterioration in the quality of concrete It is observed that, beyond 200 °C the UPV values decrease rapidly, due to sharp deterioration in the physical state of exposed RPC Beyond 400 °C the UPV values of RPC mixes prepared with 0.5% and 0.9% fiber dosages have shown a large decrease This is may be due to evaporation of fibers that create channels which leads to increase of internal micro cracks As number of microcracks increase, the quality of concrete decreases directly This leads to higher reduction in UPV values for cases of 0.5% and 0.9% fiber dosage when compared to the case with 0.1% fiber dosage for 600 °C and 800 °C The lowest UPV value, was observed for RPC specimen prepared with 0.9% fiber dosage, and at 800 °C Formation of pores and cracks through melting of fibers in case of higher dosage of fibers, under elevated temperatures leads to physicochemical changes in cement paste and thermal incompatibility between cement paste and aggregate which is, believed to be responsible for the deterioration in mechanical properties At 800 °C, the UPV values for 0.1% dosage, is quite higher than for the specimen prepared with 0.9% This is due to less number of cracks developed inside the concrete after evaporation but in case of 0.5% and 0.9% fiber dosage, the number of microcracks in specimen seems to be more, hence there is sharp decrease in UPV values at 800 °C From the above discussion, it was observed that as fiber dosage increases, UPV value also increases at room temperature as well as at 200 °C Beyond 200 °C there is a sharp decrease in UPV values for 0.1% and 0.9% fiber induced RPC specimen After 400 °C, also 0.5% fiber embedded RPC specimen have shown better UPV values, that fall in the range of good quality of concrete as per Table 3.5 Compressive strength RPC belongs to HSC category, the dense microstructure of RPC does not allow release of pressure, created by vapour under elevated temperatures This may lead to development of internal cracks in the cement matrix zone of RPC Which in turn may cause spalling of concrete, so to reduce this event from happening, different polypropylene fiber dosages were added to RPC mixes to study their performance at elevated temperatures Compressive strength at room temperature and residual compressive strengths at differ- 505 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 Fig UPV results of RPC exposed to different elevated temperatures Table Velocity criterion for concrete quality grading [IS:13311 (Part 1):1992] Sl No Pulse velocity (km/s) Concrete quality grading Above 4.5 3.5–4.5 3.0–3.5 Below 3.0 Excellent Good Medium Doubtful ent elevated temperatures, for RPC with different fiber dosages, are presented in Table and in Fig Normalized residual strength variation of RPC at elevated temperatures is also shown in Fig Increase in fiber dosage has shown increase in compressive strength at room temperature At 200 °C, polypropylene fiber dosage of 0.1, 0.5 and 0.9% has shown 3, and 4% higher compressive strength compared to RPC at room temperature respectively The increased compressive strength is due to strengthened hydrated cement paste after evaporation of free water at 200 °C, which leads to greater Van der waals forces that cause cement gel layer to come closer to each other [16] At 400 °C exposure, the compressive strengths have increased drastically for all RPC mixes as seen from Fig RPC mix prepared with 0.1, 0.5 and 0.9% fiber dosage have shown 6, 13 and 13% increase in compressive strengths respectively when compared to RPC at room temperature Increase in strength is mainly attributed to further hydration process, with catalyzed hydration through non-reacted cement products, in the presence of steam upshot under autoclaving effect formed in pastes, which is considered as advancement in chemical bonding process [17] Among three different fiber dosages, 0.5% and 0.9% have shown considerable higher compressive strengths up to elevated temperature of 400 °C From Fig it can be observed that up to 400 °C as fiber content increases strength is also observed to increase Porosity of concrete has a significant impact on pore vapour pressure Polypropylene fibers melt at temperature less than 300 °C, which results in an increase in concrete porosity and creation of more escape routes leading to reduction in bond water vapour pressure However, melting of polypropylene fibers causes thermal incompatibility between the aggregate and cement paste which leads to increase in free space and creates a thermal shock absorber The melting of polypropylene fibers is beneficial for evaporation of water vapour and improves the compressive strength of RPC, up to 0.5% Fiber dosage of 0.5% in RPC has shown best performance at elevated temperatures Beyond 600 °C the residual compressive strength decreases significantly for all RPC mixes prepared with different fiber dosages This decrease is due to the transformation of calcium hydroxide to calcium oxide in the range of 400 °C to 500 °C and reduction and disintegration of Calcium Silicate Hydrated between 400 °C and 600 °C [17] Among three fiber dosage, RPC with 0.5% fiber content has shown highest residual compressive strength This may be due to right fiber dosage addition leads to proper and proportionate channels for release of vapour pressure there by reducing the chances of spalling However, at higher fiber dosage than optimum, due to more pores, the strength suffers more The residual strength of RPC with 0.1% fiber dosage after exposure to 600 °C was 77.3% of its original strength at 27 °C, that is 22.7% reduction in compressive strength For RPC with 0.5% fiber Table Compressive strength results of RPC at elevated temperatures Temperature (°C) Fiber dosage (%) 0.1 27 200 400 600 800 0.5 0.9 Strength MPa Normalized Strength Strength MPa Normalized Strength Strength MPa Normalized Strength 105 108 111 82 71 1.00 1.03 1.06 0.78 0.68 108 112 122 98 82 1.00 1.04 1.13 0.91 0.76 113 118 128 88 68 1.00 1.04 1.13 0.77 0.60 506 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 Fig Compressive strength results of RPC at elevated temperature Fig Normalized compressive strength of RPC at elevated temperature dosage, the residual strength was 90% of its original strength at 27 °C, which was much more than residual strength obtained for RPC with 0.1% fiber dosage The reduction in compressive strength was about 10% for RPC composed of 0.5% dosage at 600 °C RPC with 0.9% fiber dosage at 600 °C has shown 22% reduction in compressive strength which is almost same as that of 0.1% fiber dosage at 600 °C This shows that minimum fiber dosage 0.1% is not efficient to maintain residual strength within acceptable range as well as 0.9% fiber dosage seems to be over dosage for RPC under elevated temperatures Results indicate that, 0.5% fiber dosage effectively serves the purpose of reducing explosive spalling as well as maintain reasonable residual strength The same observation was made in the study of polypropylene reinforced concrete at elevated temperature [18] RPC mixes exposed to 800 °C have shown considerable strength loss The residual compressive strength obtained for RPC with 0.1% fiber dosage at 800 °C was around 68% which indicates 32% loss in strength However, for 0.5% fiber dosage, the residual strength being 76% of its original strength at room temperature For 0.5% fiber dosage, there was only 24% reduction in strength which is comparatively better than 0.1% fiber dosage The drastic reduction in residual compressive strength (40%) was observed for RPC with 0.9% fiber content This may be due to adverse effect of over dosage of fibers that leads to creation of large number of channels due to evaporation of fibers at high temperature These channels propagate and enlarge in size causing early failure of concrete The reduced strength at 800 °C is due to transformation of quartz from a to b form that cause the volumetric expansion of the RPC at approximately 571 °C, which results in reduction of bonding between the aggregate and cement paste [19] The other strong reason for reduced strength at 800 °C is the decomposition of calcium hydrate gel that causes severe deterioration of RPC [20] 3.6 Split tensile strength The split tensile strength results of RPC with fiber dosage of 0.1, 0.5 and 0.9% at room temperature and at elevated temperatures are presented in Table The variation of split tensile strength is also shown in Figs 10 and 11 At 200 °C the split tensile strength of RPC increases considerably when compared to RPC at 27 °C From Fig 10 it can be observed that, as fiber content increases strength is also increasing up to 400 °C This shows positive impact of polypropylene fibers in strength enhancement of RPC under tensile loading All three-different fiber dosage i.e 0.1, 0.5 and 0.9% have shown 26, 28 and 18% increase in split tensile strength at 200 °C respectively 507 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 Table Split tensile strength results of RPC at elevated temperatures Temperature (°C) Fiber dosage (%) 0.1 27 200 400 600 800 0.5 0.9 Strength MPa Normalized Strength Strength MPa Normalized Strength Strength MPa Normalized Strength 4.14 5.25 6.30 3.62 1.70 1.00 1.26 1.50 0.87 0.41 7.56 9.80 11.85 6.90 3.42 1.00 1.28 1.56 0.91 0.45 9.25 10.99 13.45 7.27 2.86 1.00 1.18 1.45 0.78 0.30 Fig 10 Split tensile strength of RPC after exposure to elevated temperature Fig 11 Normalized split tensile strength at elevated temperature The increased split tensile strength was also observed when RPC specimen exposed to 400 °C There was about 50, 56 and 45% increase in tensile strength observed for fiber dosage of 0.1, 0.5 and 0.9% respectively The drastic increase of strength of RPC for all fiber dosages is due to development of secondary hydrated and conversion of remaining unhydrated cement grains that rapidly hydrated producing secondary hydrated gel with active participation of silica fume at this temperature The increase in strengths is may be due to bonding effect between fiber and matrix as we can observe from Fig 10 with increase in fiber dosage strength is also observed to increase This indicates positive influence of fiber effect on tensile strength enhancement of RPC up to 400 °C The tensile strength of RPC at 400 °C increases due to autoclave effect and the creation of shorter and stronger siloxone elements that causes an increase in strength in temperature range of 250–350 °C [19] The drastic decrease in tensile strength of RPC after 600 °C was observed for RPC mixes with various fiber dosages The RPC with 0.5% fiber dosage has shown less deterioration in tensile strength The residual tensile strength is around 91% of its original strength at 27 °C [21] For RPC with 0.9% fiber dosage the residual strength is less when compared to 0.5% fiber dosage and it is around 78% of its original strength at 27 °C The lowest tensile strength of RPC with high dosage of polypropylene fibers is possibly be due to dense network of melted channels created through evaporation of fibers under elevated temperature that accumulate at a single 508 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 place, initiating cracks That leads to sudden failure of RPC specimen under tensile loading The similar strength deterioration trend was observed when RPC specimen were exposed to 800 °C The reduction in strength is mainly because of chemical degradation and microcracking due to excessive pore pressure and thermal incompatibilities between aggregate and cement paste The other reason for large strength deterioration is phase changes of quartz aggregates At high temperature more than 600 °C transformation of quartz from a to b form causes the volumetric expansion of RPC at approximately 571 °C, which results in reduction of bonding between the aggregate and cement paste [22] RPC with 0.5% fiber dosage has shown good residual strength when compared to RPC with 0.1% and 0.9% fiber dosages The mean high strength is due to proper bonding between aggregate and cement paste without major cracks developed due to channels created through melting of polypropylene fibers However, after exposure to 800 °C, still the residual strength is around 45% of its original strength at 27 °C RPC with 0.9% fiber dosage has shown reduced split tensile strength (30%) compared to RPC with 0.5% fiber dosage The addition of higher content of fibers creates continuous and dense channel that leads to propagation of micro cracks to large scale This phenomenon causes deterioration of concrete under low tensile loading condition Th reduction of tensile strength at 600 °C and 800 °C due to pores and channels created due to evaporation of bond water and melting of fibers, which increase the internal defects of RPC matrix and also weaken the bonding between cement paste and aggregate This is considered as adverse effect of over dosage of polypropylene fibers On the contrary polypropylene fiber has positive effect, if and only when optimum fiber content is embedded in RPC The maximum reduction of tensile strength at 800 °C is due to phase change of quarzitic material As temperature increases, the tetrahedral chains of quartz molecules gets elongated and reorient, leading to significant volume increase, that causes radial cracking around the perimeter of the particle in heated specimen [23] 3.7 Water absorption The most important essential parameters governing the concrete durability is penetration of water, gas and ions which mainly depends upon micro structure and porosity of concrete It is well known that RPC consists of dense microstructure The development of pores and micro cracks under elevated temperatures has a major impact on durability properties like water absorption and sorptivity Hence water absorption studies on exposed RPC specimen were carried out, to assess the properties like intrinsic porosity and permeability of concrete RPC cubes after exposure to elevated temperatures were cooled down to room temperature Later weight of the cubes were taken and immersed under water for 24 h After 24 h, the cubes were removed and kept outside till it reaches to surface saturated condition then weight of cubes were taken The quantity or volume of moisture, that enters concrete, depends on the concrete permeability and interconnectivity between pores Results of water absorption of RPC specimen with different fiber dosages, and exposed to elevated temperatures are presented in Fig 12 It is observed that, as temperature of exposure increases water absorption is also increasing At 200 °C, there is no much difference between water absorption values obtained for RPC with different fiber dosages At 400 °C percentage of water absorption for RPC specimen with 0.9% fiber dosage has shown a little higher value compared to RPC specimen with 0.5% and 0.1% The increase in water absorption, is may be due to penetration of more water through surface voids and channels created by melted polypropylene fibers Strength enhancing chemical compounds starts to decay from 450 °C The decomposition of these compounds creates porosity in internal matrix RPC specimen after exposure to 600 °C has shown considerable amount of water absorption The results also show that as fiber content increases, the percentage of water absorption is also observed to increase This is obvious as fiber content increases, the rate of fiber evaporation also increases at elevated temperatures, which leaves more number of channels and enhances interconnected voids through which water easily penetrates into the body of concrete, thus increasing the water absorption value The presence of molten fiber and channels created by melting of fibers are confirmed through SEM images at different magnifications which is discussed in Section 3.9 Among the three fiber dosages, RPC with 0.9% fiber content has shown higher percentage of water absorption after exposure to elevated temperatures Also, the rate of water absorption increased rapidly beyond 400 °C for all fiber contents The percentage of water absorption till 400 °C was lower than 4%, this may be due to the presence of molten polypropylene fibers blocking the channels and not allowing water to enter the concrete core However, at higher temperatures, due to melting of fibers more channels are created through which water has easy access into the body of concrete At 800 °C, the RPC with 0.9% fiber dosage has 9.7% of water absorption, which is comparatively higher than RPC specimen prepared with 0.5 and 0.1% fiber dosages The bunches of inter connected channels created through melting of fibers, makes the concrete much more porous, which is the reason for higher water absorption values 3.8 Water sorptivity After RPC exposure to elevated temperatures, the water sorptivity of the specimen were assessed to determine the inner concrete properties, since the test is directly related to the formation of pores and cracks in the heated RPC specimen Fig 13, shows results of water absorption per unit area for RPC with 0.1% fiber dosage at different elevated temperatures The rate of water sorptivity increased sharply with increase in temperature From Fig 13 it can be observed that, at 200 °C the rate of sorptivity increases up to 20 and then on it gradually decreases In case of 400 °C the rate of sorptivity increases till 30 This is likely due to more surface damage of RPC at 400 °C that leads to propagation of surface cracks These cracks allow water to penetrate inside the body of concrete increasing sorptivity When RPC specimen were exposed to 600 °C there is colour change and formation of visible hair cracks on surface of RPC specimen The sorptivity results at 600 °C, show decrease in rate of sorptivity after 20 This is because the heated RPC specimen absorb more water to fill the voids and pores, and also shrinkage cracks created due to thermal effect within short period of time Later the rate of water absorption reduces due to saturation condition attained inside the body of concrete From the Fig 13 it is further observed that, the high rate of sorptivity was obtained for the RPC exposed for 800 °C It reaches to 29.11 Â 10À4 mm/min0.5 within 10 duration, after which sorptivity sharply drops From the overall sorptivity results it can be concluded that, the rate and total sorptivity of RPC specimen after exposure can be attributed to the effect of moisture loss and crack development which in turn is due to thermal incompatibility between cement paste and aggregate under elevated temperatures Fig 14 shows sorptivity results of RPC with 0.5% fiber dosage, after exposure to elevated temperatures At 200 °C, the sorptivity value is gradually increasing till 30 where it attains maximum value of sorptivity 1.22 Â 10À4 mm/min0.5, which is comparatively less than RPC with 0.1% fiber dosage at 200 °C This is due to the P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 Fig 12 Water absorption for RPC at elevated temperatures Fig 13 Sorptivity results of RPC with 0.1% fiber dosage at different elevated temperature Fig 14 Sorptivity variation of RPC (0.5% fiber dosage) with temperature 509 510 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 presence of high percentage of fiber dosage at the exterior portion of cubes, that creates dense matrix not allowing the moisture ingress inside As temperature increases, sorptivity is also observed to increase At 400 °C the rate of sorptivity increases up to 20 and attains maximum sorptivity value of 3.96 Â 10À4 Then rate of sorptivity decreases gradually The increased rate of sorptivity at initial duration is due to melting and evaporation of fibers located on the exterior portion of specimen, leaving maximum number of channels on the surface These surface channels created by fibers allow water to ingress to some more extent with passage of time Later the dense matrix of RPC with its interface connectivity with remaining fibers and molten fibers that block the channels from outside to inside there by arresting water ingress This is the likely reason for slowdown of rate of sorptivity after 20 of duration At 600 °C, the rate of sorptivity increased to 5.67 Â 10À4 mm/ min0.5 within 10 This value is comparatively higher than sorptivity at 400 °C with 10 duration The possible reason for this is, presence of large number of channels on the surface and interior cracks created by melting of fibers that allow moisture at a faster rate so that, the maximum value of sorptivity was attained within a short duration of time The same observation was made in case of 800 °C Among all, the maximum sorptivity was obtained for 800 °C at duration of 10 The total sorptivity value for RPC with 0.5% fiber dosage has shown comparatively reduced value than that for the RPC with 0.1% fiber dosage This observation indicates that 0.5% fiber dosage is relatively better than, that for the case of 0.1% fiber dosage, as far as durability is concerned Fig 15, shows sorptivity results of RPC with 0.9% fiber dosage It is observed that at 200 °C, the rate of sorptivity is very slow and it has reached 0.73 Â 10À4 mm/min0.5 with long duration of 60 This is due to dense fiber matrix interface on the surface of the specimen that control moisture ingress But in case of 400 °C the sorptivity value increases suddenly up to 2.52 Â 10À4 mm/min0.5 with in short duration of time 10 Later, it has shown gradual decrease in rate of sorptivity At 600 °C, the maximum sorptivity value was 6.86 Â 10À4 mm/min0.5, which is comparatively higher than the sorptivity value for RPC with 0.5% fiber dosage at 600 °C This is due to high percentage of fiber dosage, that leads to evaporation of fibers and creates maximum number of melted channels These channels permit moisture ingress Hence the maximum sorptivity results were obtained for RPC with 0.9% fiber dosage Finally, at 800 °C the initial rate of sorptivity is observed 11.36 Â 10À4 mm/min0.5 which is higher than sorptivity results of RPC with 0.5% fiber dosage and less than RPC with 0.1% fiber dosage The higher sorptivity compared to 0.5% fiber dosage is because of more number of melted channels that allows water to penetrate in the body of concrete While the lower sorptivity results compared to 0.1% fiber dosage is because of less deterioration of interior structure of concrete due to reduced spalling mechanism and pore pressure 3.9 Microstructure analysis The microstructure of RPC with different fiber dosages at various temperatures from 200 °C to 800 °C are shown in Figs 16– 19 The RPC specimen have shown dense microstructure with closed arrangements of hydrated compounds at 200 °C, which can be seen in Fig 16 The same dense matrix have been observed for all RPC specimen with different fiber dosages The SEM images of RPC with 0.5% and 0.9% have shown presence of polypropylene fibers and these fibers are closely knit with cement paste at 200 °C The high strength was observed for all RPC specimen with different fiber dosages at 200 °C The increase in compressive strength is due to unreacted silica fume that reacts with cement and hydrates: SO2 serves as a catalyst and accelerates the hydration reaction by producing C-S-H, which enhances the compressive strength of RPC [24] At temperature of 400 °C the microstructure of RPC is as shown in Fig 17 RPC prepared with 0.5% and 0.9% have shown molten channels created by melting of polypropylene fibers The compressive strength of RPC at 400 °C has shown increase in strength when compared to 200 °C This may be due to dense internal structure of RPC and at this temperature quartz powder serves as a catalyst and accelerates the reaction, which results in formation of dense C-S-H structure This observation is confirmed through EDS analysis i.e., Si/Ca ratio of RPC exposed to 400 °C has shown higher value compared to RPC at room temperature Hence increased compressive strength was observed at this temperature Microstructure of RPC cubes are exposed to 600 °C, revels that there is decomposition of CH and considerable number of cracks occurs due to thermal expansion of the cement paste which causes local break down of bond between cement and aggregate Hence the reduction in compressive strength was observed for RPC at 600 °C The presence of channels and cracks developed due to thermal expansion was observed in SEM images of RPC at 600 °C as shown in Fig 18 From figure, it is observed that RPC with 0.1% fiber dosage has shown deuteration of interface zone between aggregate and cement paste The presence of micro cracks devel- Fig 15 Sorptivity variation of RPC (0.9% fiber dosage) with temperature 511 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 0.1% 0.5% Polypropylene fiber Polypropylene fiber 0.9% Fig 16 Microstructure of RPC at 200 °C with different fiber dosages 0.5% 0.1% Melted fiber channels 0.9% Melted fiber channels Melted fiber channels Fig 17 Microstructure of RPC at 400 °C with different fiber dosages 0.1% Cracks in ITZ 0.5% Micro cracks in interface 0.9% Bunch of melted fiber channels Fig 18 Microstructure of RPC at 600 °C with different fiber dosages 0.1% Weak ITZ 0.5% 0.9% Interaction of channels Fig 19 Microstructure of RPC at 800 °C with different fiber dosages oped due to vapour pressure were more In case of RPC with 0.5% fiber dosage has shown less number of microcracks and dense structure of cement and aggregate interface as shown in Fig 18 This may be the reason for high residual strength of RPC with 0.5% fiber dosage after 600 °C The microstructure of RPC with 0.9% fiber dosage has shown good interface between aggregate and cement paste, but number of microcracks in channels created by melting of fibers are more However, this may be possible rea- 512 P.N Hiremath, S.C Yaragal / Construction and Building Materials 169 (2018) 499–512 son for reduced residual strength of RPC with 0.9% fiber dosage compared to 0.5% fiber dosage at 600 °C As temperature increases microstructure leads to deterioration continuously, in RPC with 0.1% fiber dosage has shown porous and weak interface zone between aggregate and cement paste at 800 °C as shown in Fig 19 This leads to reduction in compressive strength of RPC The minimum cracks and dense microstructure with limited number of melted channels were observed in case of RPC with 0.5% fiber dosage at 800 °C This may be the reason for considerable residual strength obtained with 0.5% fiber dosage The microstructure of RPC with 0.9% fiber dosage has shown more number of melted channels and cracks on the boundaries of melted channels at 800 °C as shown in Fig 19 At this temperature, the microstructure of RPC becomes quite disintegrated with rough grains The reduced strength at 800 °C is due to increase in width and number of cracks And, due to presence melted channels and cracks through boundaries of channels that deteriorates RPC rapidly, resulting in very low residual compressive strengths at 800 °C Conclusions In the present study, residual properties and prevention of spalling of RPC subjected to elevated temperature by using different polypropylene fiber dosages, is investigated Following, important conclusions are drawn from this study Spalling of RPC can be protected by using minimum fiber dosage of 0.1% and as fiber dosage increases risk of spalling reduces The increase in fiber dosage reduces the surface cracks and pores at elevated temperatures Weight loss of RPC increases as fiber content increases at elevated temperatures The UPV results show higher values for RPC with high percentage of fiber up to 400 °C Further as temperature increases beyond 400 °C, the velocities are decrease, as fiber dosage increases The residual mechanical properties such as compressive strength has shown increase in strength up to 400 °C and later there is sudden drop in strength As fiber dosage increases strength is also increasing up to 400 °C After 600 °C, the residual strength of RPC decreases as fiber dosage increases Split tensile strength of RPC has shown considerable increase in strength with increase of fiber dosage up to 400 °C However sudden drop in strength was observed after 600 °C The durability properties such as water absorption and sorptivity has shown RPC prepared with 0.5% fiber dosage, perform better in durability aspects compared to other fiber dosages Microstructural analysis of RPC revealed that, formation of dense microstructure and quantity of hydrated products increase up to 400 °C Later as temperature increases to 600 °C concrete starts to deteriorate by decomposition of hydrated products References [1] H.S Kim, S.H Lee, H.Y Moon, Strength properties and durability aspects of high strength concrete using Korean metakaolin, Constr Build Mater 21 (6) (2007) 1229–1237 [2] M Li, C Qian, W Sun, Mechanical properties of high-strength concrete after fire, Cem Concr Res 34 (6) (2004) 1001–1005 [3] S.Y.N Chan, X Luo, W Sun, Effect of high temperature and cooling regimes on the compressive strength and pore properties of high performance concrete, Constr Build Mater 14 (5) (2000) 261–266 [4] P Kalifa, G Chene, C Galle, High-temperature behaviour of HPC with polypropylene fibres: From spalling to microstructure, Cem Concr Res 31 (10) (2001) 1487–1499 [5] M.R Bangi, T Horiguchi, Effect of fibre type and geometry on maximum pore pressures in fibre-reinforced high strength concrete at elevated temperatures, Cem Concr Res 42 (2) (2012) 459–466 [6] Y.S Heo, J.G Sanjayan, C.G Han, M.C Han, Effect of fiber type, length and numbers of fibers per unit volume on spalling protection of high strength concrete in: In 1st International Workshop on concrete spalling due to Fire Exposure (2009, September) (pp 211–220) [7] Y Ju, L Wang, H Liu, K Tian, An experimental investigation of the thermal spalling of polypropylene-fibered reactive powder concrete exposed to elevated temperatures, Sci Bull 60 (23) (2015) 2022–2040 [8] M Canbaz, The effect of high temperature on reactive powder concrete, Constr Build Mater 70 (2014) 508–513 [9] W Zheng, B Luo, Y Wang, Compressive and tensile properties of reactive powder concrete with steel fibres at elevated temperatures, Constr Build Mater 41 (2013) 844–851 [10] C.T Liu, J.S Huang, Fire performance of highly flowable reactive powder concrete, Constr Build Mater 23 (5) (2009) 2072–2079 [11] P Kalifa, F.D Menneteau, D Quenard, Spalling and pore pressure in HPC at high temperatures, Cem Concr Res 30 (12) (2000) 1915–1927 [12] P.N Hiremath, S.C Yaragal, Influence of mixing method, speed and duration on the fresh and hardened properties of Reactive Powder Concrete, Constr Build Mater 141 (2017) 271–288 [13] C.M Tam, V.W Tam, K.M Ng, Optimal conditions for producing reactive powder concrete, Mag Concr Res 62 (10) (2010) 701–716 [14] O Arioz, Effects of elevated temperatures on properties of concrete, Fire Saf J 42 (8) (2007) 516–522 [15] J Xiao, H Falkner, On residual strength of high-performance concrete with and without polypropylene fibres at elevated temperatures, Fire Saf J 41 (2) (2006) 115–121 [16] K.M.A Hossain, High strength blended cement concrete incorporating volcanic ash: performance at high temperatures, Cem Concr Compos 28 (6) (2006) 535–545 [17] A.M Rashad, S.R Zeedan, A preliminary study of blended pastes of cement and quartz powder under the effect of elevated temperature, Constr Build Mater 29 (2012) 672–681 [18] M.V Mohod, Performance of polypropylene fibre reinforced concrete, IOSR J Mech Civil Eng 12 (1) (2015) 28–36 [19] Q Ma, R Guo, Z Zhao, Z Lin, K He, Mechanical properties of concrete at high temperature—a review, Constr Build Mater 93 (2015) 371–383 [20] B Demirel, O Kelesßtemur, Effect of elevated temperature on the mechanical properties of concrete produced with finely ground pumice and silica fume, Fire Saf J 45 (6) (2010) 385–391 [21] N Ayudhya, B Israngkura, Compressive and splitting tensile strength of autoclaved aerated concrete (AAC) containing perlite aggregate and polypropylene fiber subjected to high temperatures, Songklanakarin J Sci Technol 33 (5) (2011) [22] M.S Abrams, in: Compressive Strength of Concrete at Temperatures to 1600F, ACI, Detroit, MI, USA, 1971, pp 33–58 Publication Special 25 [23] T.G Nijland, J.A Larbi, Unraveling the temperature distribution in firedamages concrete by means of PFM microscopy: Outline of the approach and review of potentially useful reactions, HERON 46 (4) (2001) 253–264 [24] P.N Hiremath, S.C Yaragal, Effect of different curing regimes and durations on early strength development of reactive powder concrete, Constr Build Mater 154 (2017) 72–87  ... strength of RPC with high dosage of polypropylene fibers is possibly be due to dense network of melted channels created through evaporation of fibers under elevated temperature that accumulate at a... of concrete The vapour pressure created inside the concrete at elevated temperature leads to formation of micro cracks These small width cracks grew to large widths at higher elevated temperatures. .. elevated temperatures Liu and Huang [10], have reported that the residual strength of RPC at elevated temperatures decreases significantly at temperature beyond 300 °C when compared that of RPC at