Thermal behaviour of unstressed and stressed high

Journal of Structural Fire Engineering Thermal behaviour of unstressed and stressed high strength concrete containing polypropylene fibers at elevated temperature Duncan Cree, Prosper Pliya, Mark F. Green, Albert Noumowé,

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Article information: To cite this document: Duncan Cree, Prosper Pliya, Mark F. Green, Albert Noumowé, (2017) "Thermal behaviour of unstressed and stressed high strength concrete containing polypropylene fibers at elevated temperature", Journal of Structural Fire Engineering, https://doi.org/10.1108/JSFE-07-2016-0014 Permanent link to this document: https://doi.org/10.1108/JSFE-07-2016-0014 Downloaded on: 25 July 2017, At: 14:18 (PT) References: this document contains references to 26 other documents. To copy this document: [email protected] The fulltext of this document has been downloaded 11 times since 2017* Access to this document was granted through an Emerald subscription provided by Token:Eprints:JTTBAHZNHZPENJBZREB8:

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Thermal behaviour of unstressed and stressed high strength concrete containing polypropylene fibers at elevated temperature Duncan Cree


Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, Canada

Thermal behaviour

Received 27 July 2016 Revised 3 January 2017 Accepted 16 January 2017

Prosper Pliya Department of Civil Engineering, University of Cergy-Pontoise, Cergy-Pontoise, France

Mark F. Green Department of Civil Engineering, Queen’s University, Kingston, Canada, and

Albert Noumowé Department of Civil Engineering, University of Cergy-Pontoise, Cergy-Pontoise, France

Abstract Purpose – The purpose of this paper is to evaluate high strength concrete (HSC) containing polypropylene fibers (PP-fibers) at high temperature under a compressive load.

Design/methodology/approach – The use of PP fibers in HSC is known to reduce and at times

eliminate the risk of spalling. HSC containing 0, 1 and 2 kg/m3 of PP-fibers were subjected to various temperatures from 20°C to 150°C, 300°C and 450°C and evaluated in a “hot condition”. One group of specimens was in a non-stressed condition during heating (unstressed hot), while a second group was subjected to an initial preload of 40 per cent of the room temperature compressive strength during the heating (stressed hot). Findings – Results showed that stressed concrete containing PP-fibers had lower thermal gradients (the temperature difference between the surface and center temperatures as a function of radial distance) and a decrease in relative porosity. However, the compressive strength of stressed specimens was improved with or without fibers as compared to that of the unstressed HSC. The increased stress levels due to concrete expansion at elevated temperature were also reported. The PP-fibers did not have a significant effect on the compressive strength of stressed concrete as compared to the unstressed state. Originality/value – This paper reports the compressive strength of PP-fibers in HSC at elevated temperature with and without a pre-load.

Keywords SEM, Temperature, Concrete, Thermal gradient, Polypropylene fibers, Hot strength Paper type Research paper

1. Introduction Elevated temperature behavior of concrete has been explored by several studies; Phan et al. (2001), Anderberg (1997), Ulm et al. (1999), Kalifa et al. (2001), Bilodeau et al. (2004), Zeiml et al. (2006), Noumowé (2005) and Pliya et al. (2011). High strength concrete (HSC) is known

Journal of Structural Fire Engineering © Emerald Publishing Limited 2040-2317 DOI 10.1108/JSFE-07-2016-0014


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to contain a denser microstructure than normal strength concrete. When HSC is heated at elevated temperatures, internal moisture has difficulty to travel through the dense microstructure to the concrete surface. This results in an increase of water vapor build-up in the internal pores of the concrete and the possibility of spalling or explosive phenomenon with risk of thermal instability increase. When concrete is subjected to elevated temperatures, the term “thermal instability” is used interchangeably when either spalling phenomenon occurs. Concrete thermal instability could be the result of vapor pressure buildup mechanisms and/or restrained thermal dilatation mechanisms according to Anderberg (1997) and Ulm et al. (1999), respectively. Parameters such as water content, porosity, permeability and aggregate type have an influence on thermal instability. An effective method to reduce the hazard and improve concrete thermal stability is the addition of polypropylene fibers (PP-fibers) as was observed by Kalifa et al. (2001), Bilodeau et al. (2004), Zeiml et al. (2006), Noumowé (2005) and Pliya et al. (2011). Thermal differential analyses conducted by Kalifa et al. (2001) have shown that PP-fibers melt at approximately 160°C170°C and vaporize around 340°C. Melting of PP-fibers produces expansion channels and lowers internal vapor pressure at the interior of the concrete which reduces the likelihood of explosive spalling. Vapor pressures within concrete specimens with and without PP-fibers were measured by several authors Kalifa et al. (2001) and Bilodeau et al. (2004). Kalifa et al. (2001) established that the addition of fibers decreased the vapor pressures in concrete during heating. In addition, the peak pressure dropped with the increase of fiber amounts and became relatively constant when greater than 1.75 kg/m3 of fibers were added. Development of heat flow within preloaded stressed concrete specimens containing PP-fibers has not been studied to a great extent. In general, the thermal behaviors of cylindrical concrete specimens can be evaluated using thermocouples at the center and at the surface of the specimens (thermal gradient). For instance, Noumowé et al. (2006) studied the high temperature behavior of unstressed conventional and self-compacting HSC. The addition of PP-fibers was observed to modify the thermal gradients in both of these concrete types and improved the thermal stabilities. Similarly, Kanéma et al. (2011) investigated the thermal behavior of unstressed normal and HSC. The results showed that spalling was due to low concrete permeability, water content and an increased thermal gradient within the concrete. The evolution of the temperature within the concrete was primarily a result of the type of concrete, specimen size and heating rate. More recently, Debicki et al. (2012) evaluated the spalling behavior of unstressed high performance concrete with and without PP-fibers. Although the authors’ determined the pore pressure to be the principal source for spalling, the internal thermal gradients were also found to participate in spalling of the specimens. In the literature, four models based on loading and heating conditions are proposed to evaluate the elevated temperature resistance of concrete specimens: (1) stressed (a pre-load is applied to the concrete) and tested ‘hot’ (the strength of the concrete is determined at an elevated temperature); (2) unstressed and tested “hot”; (3) stressed and load maintained at elevated temperature but properties evaluated at room temperature after heating; and (4) unstressed at high temperature and tested at room temperature after heating. The common study has been the latter, to measure the post-fire residual strengths at room temperature after cooling from elevated temperatures on unstressed concrete specimens. Hot strength data for concrete-containing PP-fibers is limited because of the sophisticated


test equipment required which may or may not hold a load while heat is generated. Hot compressive strength should also be investigated because concrete submitted to elevated temperatures must maintain their structural carrying capabilities throughout the duration of a fire. For example, HSC without fibers and under a compressive load at elevated temperature was found to have an increase in the risk of spalling as reported by Phan et al. (1997). The spalling phenomenon was illustrated by Diederichs et al. (1988) during the heating of stressed test cube specimens. The addition of PP-fibers is generally added to concrete to prevent explosive spalling; however, in some cases, HSC was shown to be susceptible to spalling under a compressive load even with PP-fibers, as given by Hertz (2003). Investigations by Malhotra (1956), Castillo and Durrani (1990) and Kim et al. (2013) have been carried out on the effect of temperature on the mechanical properties of stressed concrete. Malhotra (1956) have demonstrated smaller strength reductions of stressed specimens compared to stressed-free specimens within a temperature range of 200°C-600°C. This reduction was believed to be due to retardation of crack development by the imposition of a compressive stress. Kim et al. (2013) studied the effects of temperature and PP-fibers on the mechanical properties of normal and HSC submitted to 20 and 40 per cent of the maximum load at room temperature. They concluded that the residual mechanical properties of concrete with PP-fibers were increased in stress level and excessive stresses were determined to shorten the life of concrete. At a stressed level of 20 per cent, the residual strength was higher in the order of 13-17 per cent than those of specimens without stress. When the level of stress was increased to 40 per cent, the specimens were destroyed in compression during the heating process. Few studies have reported the behavior of stressed (pre-loaded) concrete containing PP-fibers during a heating phase (hot condition), while the literature suggests that the tendency of explosive spalling on HSC under a compressive loading is contradictory. Therefore, the objective of this work was to investigate the effect of high temperature on HSC containing PP-fibers in an unstressed or stressed condition. The results were compared to HSC without PP-fibers. Variables considered were the volume fractions of PP-fibers in the concrete (0.11 and 0.22 per cent) and three temperatures – 150°C, 300°C and 450°C – were selected using a heating rate was 15°C/min and compared to those at 20°C. The concrete behavior was defined by temperature measurements (surface and center) which provided data on water vaporization, melting of the PP-fibers and calculation of the thermal gradients. In addition, compressive strengths were evaluated, and because of concrete expansion from heating, increased stress levels were observed. Porosity measurements were conducted to evaluate how the melting of the PP-fibers influenced the strength of the unstressed and stressed concretes. The scanning electron microscopy was used to understand microstructure changes before and after heating. 2. Materials and methods 2.1 Specimen preparation The elevated temperature tests were carried out on HSC cylindrical specimens measuring 102 203 mm (diameter height). Three specimens were used to measure the room temperature strength, while one specimen was used for each elevated temperature test and loading condition for nine specimens. The concrete mixture was prepared from Portland Limestone Cement, Type GUL according to the Canadian Standards Association (2013) CSA A3000-13 with a density of 2,417 kg/m3. Coarse aggregates were river rounded granite pea stone with a high content of calcareous aggregates and fine river sand. Both fine and coarse aggregates were dried at 105°C for 24 hours to remove moisture. To increase the workability

Thermal behaviour


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of all mixtures, BASF Glenium 7700 superplasticizer (high-range water-reducing admixture) was added to the mix. Commercially available PP-fibers were Duomix® fire (M6) fine monofilaments with a length of 6 mm, diameter of 18 mm and a melting temperature of 160° C-165°C. The tensile strength was 300 MPa and modulus of elasticity was 3.5-3.9 GPa based on the manufacturers’ data sheet. The fibers are supplied in clusters and during concrete mixing are dispersed throughout the concrete. HSC specimens were produced with and without PP-fibers. Three batches of HSC were fabricated and are referred to as follows: the reference concrete with no fibers (C), concrete with 0.11 per cent PP-fibers (CP1), equivalent to 1 kg/m3; and concrete with 0.22 per cent PP-fibers (CP2) equivalent to 2 kg/m3. All mixtures had the same water/cement (w/c) ratio of 0.35 and a slump between 180 and 190 mm. The amount of superplasticizer was adjusted to maintain a constant workability. Table I provides details of the mixture proportions for HSC with and without fibers. The moisture contents at 28 days were included with their respective standard deviations (6). The samples were de-molded after one day and sealed in plastic bags at ambient temperature to prevent moisture loss. Heating and testing was conducted after 28 days of curing. 3. Experimental study Hot temperature tests were conducted in an electric resistance Instron box furnace, series 3119 chamber which is able to reach a temperature of 600°C. The furnace was controlled with a Eurotherm 2408 temperature controller. Samples were tested at ambient temperature and at elevated temperatures of 150°C, 300°C and 450°C. The specimens were heated at a rate of 15°C/min from 20°C up to their respective target temperatures and held until the center temperature was equal to the surface temperature by 2°C. This heating rate was selected based on an ordinary fire scenario having a heating rate in the range of 10°C-20°C/ min as was conducted by Hertz (2003). The choice of target temperature was made to compare the behavior of the cementitious matrix and PP-fibers during heating: 150°C prior to PP-fibers melting, 300°C after melting and 450°C after the vaporization phase. All tests were conducted at elevated temperature in an unstressed or stressed condition. Three type-K thermocouples (TC) were used to monitor the temperature in the furnace (TC-1), cylinder surface temperature (TC-2) and cylinder core temperature (TC-3), as shown in Figure 1. The thermocouple at the center of the specimen was positioned in place during casting and was located at mid-height and mid-diameter. The water porosity tests were conducted on five representative samples obtained from the broken cylinders. The specimen pieces were first oven dried at 80°C for 24 hours until no weight change was observed and further immersed in distilled water to obtain saturated specimens. The total porosity measurements were obtained by first weighing the dry samples in air, weighing immersed saturated specimens in water and weighing specimens in a saturated but surface dry condition. Compressive strength measurements were carried out in accordance with ASTM C39-14 (2014) at a loading rate of 0.5 MPa/s in position control Components (kg/m3)

Table I. Mix proportions for HSC with different fibers additions

Concretes C CP1 CP2

Water

Cement

Gravel

Sand

Super

F-PP

Slump (mm)

Moisture content (%)

Density (kg/m3)

166 166 166

475 475 475

888 888 887

888 888 887

0.3 0.4 0.4

0 1 2

180 190 190

3.8 6 0.2 4.4 6 0.2 4.5 6 0.2

2,417 2,418 2,417


mode for both unstressed and stressed heated cylinders. Room temperature strength measurements were made after 28 days of curing on three specimens and averaged. Prestressing was done at 40 per cent of the room temperature strength or 24 MPa. Microstructure analysis was conducted using a high-resolution scanning electron microscopy (SEM) model Quanta 650 FEG with an accelerating voltage of 15 keV. Relatively small, flat broken samples selected randomly from compression tests were analyzed without any special preparation.

Thermal behaviour

4. Results and discussion 4.1 Moisture content The water content of concrete at 28 days for C, CP1 and CP2 was 3.8 6 0.2, 4.4 6 0.2 and 4.5 6 0.2 per cent, respectively. A slight increase in water content was observed in concrete containing PP-fibers. According to Savastano and Agopyan (1999) steel, aggregates and impermeable materials have shown to accumulate water at the interface between the fiber surface and the cement paste. A study conducted by Ritchie and Rahman (1974) on concrete bleeding containing PP and steel fibers showed water was retained around the fibers, resulting in a lower bond strength. The amount of bleeding varied with the fiber size, type and content. PP polymer is naturally hydrophilic; therefore, it tends to attract water onto its surface. Perhaps, applying a hydrophobic coating on the surface of the fibers would reduce the overall water content of the concrete containing PP-fibers. 4.2 Thermal properties The results for the temperature evolution as a function of time are given in Figure 2 for heating up to 450°C. The surface temperature profiles for concretes C, CP1 and CP2 for both unstressed and stressed situations remained relatively consistent. At the beginning and end of the heating profiles, the surface temperatures were in agreement with the furnace air temperature. After about 5 minutes of heating, the deviation was more pronounced because of the difference in a larger thermal mass of the concrete compared to air. Short plateaus occurred for the center temperature profiles between 179°C-235°C (dependent on loading condition and fibers content) after 50 minutes of heating which were indicative of moisture vaporization within the concrete. These plateaus also occurred at testing temperature of 300°C but were not visible at 150°C. Irrespective of the loading condition, water vaporized at a higher temperature for concrete without PP-fibers which may have been due to lower moisture contents. Interestingly, HSC

Upper platen Concrete specimen Thermocouples

TC-1 TC-2 T

TC-3

Lower platen

Box furnace

Figure 1. Schematic of specimen with instrumented thermocouples in the box furnace


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Figure 2. Evolution of surface (Ts) and center temperatures (Tc) as a function of time during heating to 450°C for unstressed (a) and stressed (b) specimens

Figure 3. Evolution of thermal gradient as a function of surface temperature during the heating to 300°C for unstressed (a) and stressed (b) specimens

under stress produced a vaporization plateau at a lower temperature than for unstressed concretes. Pre-stressing may have driven the moisture toward the interior of the specimen, thereby increasing the moisture content at its center. As anticipated, the temperature rise at the center of the concrete cylinder was slower compared to the surface and furnace temperatures because of the low thermal conductivity of the concrete. No surface crack or spalling was observed during the various heating and loading cycles. A slight decrease at the center temperature occurred for C, CP1 and CP2 for both unstressed and stressed specimens. For example, as given in Figure 2(a), the unstressed concrete C was decreased by 1°C when it reached 236°C. CP1 was decreased from 194°C to 188°C for a drop of 6°C, while CP2 was reduced from 186°C to 179°C for a drop of 7°C. Similarly, for the stressed concretes as illustrated in Figure 2(b), C and CP1 was decreased by 1°C when they reached 163°C and 175°C, respectively, whereas CP2 was reduced from 155°C to 145°C for a drop of 10°C. As temperature increased, the pore pressure increased and, as a consequence, raised the vapor pressure. As the concrete was stressed and pores were closed, the hot liquid water under pressure was released into water vapor thus decreasing the temperature of the gas. This phenomenon can be explained by The First Law of Thermodynamics and the adiabatic process for an ideal gas. When an adiabatic or expansion cooling occurs, the temperature of a gas drops, which is due to air, quickly expanding in volume and creating a rapid release of gas under pressure. This occurrence is more prevalent from CP2 concrete, as it contained the most water. The evolution of the thermal gradient with respect to surface temperature up to 300°C is given in Figure 3 for unstressed and stressed specimens. The thermal gradient was calculated by dividing the temperature difference between the surface (Ts) and the center


(Tc) of the concrete cylinder by its radius as was performed by Kanéma et al. (2011) and Debicki et al. (2012). The maximum thermal gradients for the unstressed and stressed specimens occurred at approximately 200°C and 250°C, respectively. The thermal gradient tends to increase with the addition of PP-fibers because of the improved porous networks. Generally, an increase in thermal gradient will produce large thermal stresses. Although the furnace was maintained at a constant heating rate, the interior of the concrete between the surface and the center of the cylinders with and without fibers experienced different heating rates as represented by the thermal gradient (e.g. slope) changes in Figure 4 for a heating temperature of 450°C. The thermal gradient profiles for the unstressed and stressed concrete without fibers are divided into five stages (e.g. Stages I-V) shown by the short vertical dashed lines in Figure 4(a) and (b), respectively. A high positive thermal gradient in Stages I and II indicated that the surface temperature was increasing faster than the core temperature. The thermal gradient in Stages II and III indicated a reduced heating rate between the surface and the core temperature. During Stages III and IV, a final temperature increase was observed at a maximum thermal gradient of 29°C/cm because of heating and moisture vaporization in the core. From Stages IV to V, a steep thermal gradient indicated a sharp temperature change between the core and the surface in which the core temperature was increasing faster than the surface temperature. Regardless of the loading condition, adding PP-fibers increased the thermal gradient after the melting temperature of the PP-fibers which can be related to a more open-pore structure. In general, stressed concretes with and without PP-fibers had lower thermal gradients than the unstressed concrete possibly due to a decrease in the pore structure. Loading concrete may reduce the amount of internal micro-cracks and micro-voids available for transferring heat from the surface to the core. Figure 5(a) shows the evolution of compressive loading compared to the displacement of the press plate during the heating phase of 450°C. The compressive strength increases to a peak of 58 per cent of the initial load (e.g. 24 MPa). During this variation of stress, the displacement of the press plate remained constant. The stressed concrete cylinder specimens without fibers showed an increase in a maximum height displacement of 3.2 mm after approximately 58 minutes of heating. The change in diameter was not measured because of difficulty in instrumentation of the cylindrical specimens. In the position control mode employed, the ram load was held constant, while the change in position was measured. This dilation enforced the fact that concrete expands, as it is heated even under a stress. The variation of the compressive load during heating was observed for the various temperature fields in Figure 5(b). As heating evolved and concrete expanded, the stress level within the concrete increased as time and temperature increased. At a low temperature of 150°

Thermal behaviour

Figure 4. Evolution of thermal gradient as a function of surface temperature during the heating to 450°C for unstressed (a) and stressed (b) specimens


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Figure 5. Evolution of compressive stress during heating for the (a) displacement of the press plate at 450° C and (b) due to heating at 150 C, 300°C and 450°C in a stressed condition

Figure 6. Evolution of surface and center temperatures and compressive stress during heating to 450°C for stressed C (a) and CP1 (b) concretes

C, the stress on the press plate was maximum at about 29 MPa after 40 minutes, while the greatest stress of 39 MPa occurred at 450°C after 58 minutes. The peak stress observed for specimens tested at 150°C and 300°C were not as remarkable as those tested at 450°C. These differences were thought to be related to the lower temperatures reached by the concrete. The variation of compressive loading and the evolution of surface and center temperatures of the specimen heated at 450°C are compared in Figure 6(a) for concretes without fibers and Figure 6(b) for concretes with PP-fibers. The addition of PP-fibers into concrete in the framework of this study was shown to have a greater influence on the behavior of the concrete. According to Figure 6(a), the highest stress occurred when the surface temperature was 350°C and the center temperature was 230°C as illustrated by the dotted vertical line. With the addition of 1 kg/m3 to the concrete mix, the greatest stress occurred at 37 MPa after 54 minutes, while the surface and center temperatures were 340°C and 220°C, respectively, as given in Figure 6(b). The highest stress was observed to be created by the expanding concrete. In addition, the surface and center temperatures were slightly lower for CP1 as compared to C concretes. This would indicate, once the PP-fibers vaporized, the hollow channels aided in dilatation of the confined concrete. In this way, the hollow networks formed weakened the concrete microstructure from increased stress concentrations. This can be explained by a decrease in concrete material, a disconnected concrete microstructure and increased micro-crack formation along the depressions created from melting of the PP-fibers, as depicted in Figure 11(d). Piasta et al. (1984) investigated concrete thermal expansion during heating. This thermal expansion was strongly related to the nature of the aggregates. For example, at a


temperature of 100°C, the thermal expansion ranged from 0.1 per cent for all concretes studied to 0.35 per cent for limestone aggregate concrete. At 400°C, the thermal expansion was 5.8 per cent for siliceous aggregates concrete. The thermal expansion of the concrete, when prevented, can generate high compressive stresses. The results of this study showed an increase in the compressive stress of concretes when under load during heating. The maximum stress reached during heating at 450°C was about 63 per cent (38 MPa) of its strength at room temperature. This variation could explain the reason for the spalling phenomenon observed by Kim et al. (2013) when preloaded to 40 per cent under ISO-834 curves. Water content also has a role in explosive spalling.

Thermal behaviour

4.3 Porosity The results for the relative porosity are provided in Table II. The porosities after drying at 80°C, for C, CP1 and CP2 concretes were 11.6 6 0.8 per cent, 12.8 6 0.8 per cent and 13.6 6 0.8 per cent, respectively. The relative porosities were calculated by dividing the porosity (PT) for each heating cycle by the porosity (P80) obtained at 80°C. The porosity was influenced by an increase of PP-fibers content in the unheated concretes. For example, by comparing concrete without fibers (e.g. C) to CP1 and CP2, the porosity of concretes increased by 10 and 17 per cent, respectively, in their unheated state with the addition of 1 kg/m3 and 2 kg/m3 of fibers. This additional porosity created by the presence of PP-fibers was related to adsorbed water on the fibers surface. The moisture accumulated on the surface and created voids between the fiber/cement interfaces after concrete hydration. The moisture content of concrete was also measured and interestingly showed an increase of 16 and 18 per cent as fiber content increased from 1 kg/m3 to 2 kg/m3, respectively, as shown in Table I. The addition of PP-fibers at room temperature can be viewed as modifying the HSC microstructure. The evolution of relative porosity as a function of heating temperature is given in Figure 7. The porosity of all concrete specimens with or without PP-fibers was observed to increase after

Relative porosity [%] (PT/P80) Concretes

150

Unstressed (°C) 300

C CP1 CP2

112 107 101

115 118 108

450

150

Stressed (°C) 300

450

136 132 127

111 104 102

125 115 108

137 129 122

Table II. Evolution of water porosity

Figure 7. Evolution of relative water porosity as a function of heating temperature for unstressed (a) and stressed (b) specimens


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heating. For example, the relative porosity of unstressed C concretes increased from 112 per cent at 150°C to 136 per cent at 450°C, while the preloaded concretes increased from 111 per cent at 150°C to 137 per cent at 450°C. This translated to a 21 and 24 per cent increase in relative porosity for concretes without fibers, respectively. The results suggested that the effect of pre-stressing on porosity was not as remarkable for concrete without fibers. As PP-fibers were not present, the increase in porosity was due to various physical and chemical concrete transformations during heating as opposed to changes from fibers additions. The results imply the effect of stress on porosity was not significant for concrete without fibers. The influence of PP-fibers was also noted for both unstressed and stressed studies. The relative porosity of concretes with fibers increased less rapidly than that of concrete without fibers. Fibers thus consume heat in latent form which could limit the increase of the pore volume. Comparing unstressed and stressed concretes, the relative porosities vary little for each temperature studied. At each test temperature, the relative porosity for unstressed and stressed concretes tended to decrease with PP-fibers additions. A stressed condition during heating may reduce the amount of internal micro-cracks and micro-voids which has an impact on the thermo-hydro transfer as provided during the evolution of the temperature at the center of the specimens in Figure 2. The evolution of water porosity as a function of heating temperature is shown in Figure 8 for unstressed (a) and stressed (b) specimens. The water porosity was observed to increase with an increase in furnace temperature for both unstressed and stressed specimens. At 80° C, both unstressed and stressed concrete C, CP1 and CP2 had porosities of 11.6, 12.8 and 13.6 per cent, respectively. This indicated loading and heating concrete to 80°C did not affect the porosity of the concrete. Unstressed concrete heated from 150°C to 300°C increased in porosity by 2, 11 and 7 per cent for C, CP1 and CP2, respectively. In the same way, stressed concrete increased in porosity by 13, 11 and 5 per cent. This suggests that around the melting temperature of the PP-fibers, the porosity was not affected to a great extent by the loading regime. Overall, the concrete without fibers increases in porosity because of concrete material degradation as temperature increased. The porosity of concrete with fibers is amplified due to concrete degradation as well as melting of the fibers. 4.4 Compressive strength The results for the relative compressive strength are summarized in Table III. The compressive strengths at room temperature for C, CP1 and CP2 concretes were 60.0 6 1.0 MPa, 55.9 6 3.7 MPa and 56.4 6 0.6 MPa, respectively. The relative compressive strengths were calculated by dividing the compressive strength (fcT) for each heating cycle by the compressive strength (fc20) obtained at room temperature (after drying at 80°C). The room

Figure 8. Evolution of water porosity as a function of heating temperature for unstressed (a) and stressed (b) specimens


temperature strengths were slightly controlled by the presence of PP-fibers. Excess water accumulation between the fiber/cement and aggregate/cement interface transition zone can create voids, formation of calcium hydroxide (CH) and ettringite after concrete has hydrated. The addition of PP-fibers acted to reduce the mechanical properties of the concrete at room temperature. Figure 9 shows the evolution of the relative compressive strength as a function of temperature. Compressive strength of concrete with or without fibers decreased with rise in temperature. The behavior was observed to be the same for all concretes. The PP-fibers did not influence the evolution of compressive strength as a function of the temperature. As noted in the literature, the PP fibers do not have a major impact on the compressive strength of unheated concrete. The relative compressive strength of unheated C and CP2 concretes were 60 and 56 MPa, respectively, which translated to a 7 per cent decrease in strength because of a volume fiber fraction addition of 0.22 per cent. Correspondingly, water porosity measurements showed an increase in porosity with an increase in PP-fibers. These results confirmed the reduction in compressive strength. In general, the compressive strength of concretes with fibers remained less than that of concretes without fibers for all heating cycles studied. According to the literature, results of concrete with PP-fibers are contradictory. Several studies carried out by Suhaendi and Horiguchi (2006) and Xiao and Falkner (2006) showed a decrease of residual strength, while other studies by Kalifa et al. (2001) and Schneider et al. (1982) obtained improvements in residual strengths. This difference could be related to the experimental conditions, the cure conditions of the specimen and the heating rate. HSC under a 0 or 40 per cent stress improved in hot strength at 300°C compared to its room temperature properties, but the strength was reduced at 150°C and 450°C regardless of fiber additions. The compressive strength of concretes with or without fibers appeared to be better at lower temperatures when the unstressed and stressed conditions were compared.

Concretes

150

C CP1 CP2

67 77 70

Relative compressive strength [%] (fcT/fc20) Unstressed (°C) Stressed (°C) 300 450 150 300 110 102 97

93 90 75

75 80 81

109 102 102

Thermal behaviour

450 92 91 87

Table III. Evolution of compressive strength

Figure 9. Evolution of relative compressive strength as a function of heating temperature for unstressed (a) and stressed (b) specimens


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Figure 10. Evolution of compressive strength as a function of heating temperature for unstressed (a) and stressed (b) specimens

For instance, at 150°C, concretes C, CP1 and CP2 increased in strength by 12, 5 and 17 per cent when the HSC was under load compared to the unstressed specimens. However, all HSC specimens at 300°C and 450°C were not improved under stress. Concrete with 2 kg/m3 of PPfibers lost less compressive strength. The relative strength of concrete with and without a stress subjected to 450°C was 87 and 75 per cent, respectively. The results for the relative porosity of unstressed and stressed concretes at the same temperature were 127 and 122 per cent, respectively. Fibers not only improved the thermal stability of concrete but also limited damage to the material under certain conditions. Figure 10 illustrates the evolution of compressive strength as a function of heating temperature for various concretes studied. Between 20°C and 150°C, small hot strength losses were observed for both unstressed and stressed specimens. For example, unstressed C, CP1 and CP2 decreased in strength by 33, 23 and 30 per cent, respectively. Similarly, stressed specimens reduced by 25, 20 and 19 per cent, respectively. The greatest loss was for HSC containing no fibers, while the unstressed concrete specimens experienced a greater loss. Between 150°C and 300°C, a more significant hot strength gain was observed for both unstressed and stressed conditions. For instance, unstressed C and CP1 increased by 10 and 2 per cent, respectively; however, CP2 decreased by 2 per cent. Stressed cylinders improved by 10, 3 and 2 per cent, respectively. Typically when concreted is heated, steam and pressure from inside the cement paste initiate a condition called internal autoclaving between 100°C and 300°C. The process helps hydrate un-hydrated cement particles and a relatively steady or increase in strength is often observed as reported by Piasta et al. (1984). Beyond the melting temperature of PP-fibers (160°C-170°C), the fibers melted and produced a porous concrete. In general, empty pores tend to reduce the compressive strength; however, the autoclaving phenomenon seemed to take precedence within this temperature range. Beyond 300°C, there was a considerable drop in hot strength for all HSC specimens which can be explained by physical and chemical transformations taking place at elevated temperatures. Between 300°C and 700°C, dehydration and decomposition of the calcium silicate hydrate (CSH) gel occurred, the major compound responsible for strength as was reported by Piasta et al. (1984) and Schneider et al. (1982). At 450°C, unstressed concretes C, CP1 and CP2 tended to decrease by 7, 10 and 25 per cent, respectively, while stressed samples decreased by 8, 9 and 13 per cent, respectively. Overall, the concrete cylinders in the stressed condition had slightly better compressive hot strengths than the unstressed condition. A loaded member has the advantage of closed micro-cracks and an overall reduction in porosity.

4.5 Fractured specimens The failure mode of the HSC cylinders produced similar fractured shapes at all temperatures, whether unstressed or stressed and with or without fiber additions as shown in Figure 11. According to ASTM C39-14 (2014), the failures were categorized as either Type 1 [formation of a large cone on each end of the cylinder as shown in Figure 11(a)] or Type 2 [cone and a split type fracture as shown in Figure 11(b) and (c)].

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4.6 Microstructure The microstructures of the HSC exposed to different elevated temperatures were studied by SEM at 20°C, 150°C, 300°C and 400°C. Concretes submitted to temperatures below 150°C had fibers that were intact, well distributed and bonded to the dense cement matrix, as shown in Figure 12(a) and (b). Although the fiber distribution efficiency is only a qualitative Figure 11. Typical HSC loaded to failure after heating at (a) 150°C, (b) 300°C and (c) 450° C in the stressed and unstressed condition with and without fibers

Figure 12. SEM of HSC containing PP-fibers at (a) 20°C and at (b) 150°C, hollow channels at (c) 300°C and (d) 450°C

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assessment, it is representative for all the specimens. After exposure to temperatures above 160°C-170°C, the fibers melted, leaving open hollow circular channels and longitudinal depressions [see Figures 12(c) and (d)]. These fine networks allowed internal water vapor and gases to escape, thus reducing the overall pore pressure build-up generally observed in HSC. It is understood that an increased PP-fibers content increases the porosity and lowers the pressure developed within the concrete.


5. Conclusions In this study, HSC with or without PP-fibers were evaluated in hot unstressed or stressed conditions at different elevated temperatures. Based on the results, the following conclusions can be made:

For both unstressed and stressed HSC, water vaporization plateaus took place at the center of the specimens. The plateau occurred at higher temperatures for concrete with lower moisture contents and unstressed conditions. The thermal gradient of HSC was increased after the melting temperature of the PP-fibers was reached which was due to a more open-pore structure. Stressed HSC with and without PP-fibers had lower thermal gradients than the unstressed concrete, possibly because of a denser structure. Although the thermal gradients ranged from 20°C to 50°C/cm at 450°C, no explosive spalling was observed. Confined (preloaded) HSC was shown to dilate during heating. Dilatation infers concrete expansion and eventual cracking. The porosity measurements showed increased concrete damage as temperatures raised. The porosity increased with heating and addition of PPfibers. The porous network propagated throughout the concrete specimens not only from polymer fiber melting but also from micro-cracking within the hollow channels of the cement paste due to stress concentrations. The compression strengths of concretes containing PP-fibers were affected more than the concretes without PP-fibers which was indicated by decreased strength values for all heating cycles studied. The presence of solid polymer fibers at room temperature and hollow channels at elevated temperature both tended to reduce the strengths. In the stressed condition, the hot compressive strengths of concretes with or without PP-fibers appeared to be better than the unstressed condition possibly due to closure of the micropores. Surface cracks were not visible and explosive spalling did not take place on any of the samples, possibly because of the water content of the concretes. HSC containing PP-fibers had higher moisture content than concrete without fibers. PP attracts water onto its surface. A coating material could be applied onto the surface of the fibers to reduce its hydrophobicity and thus water content of the concretes. SEM has shown the PP-fibers to be well dispersed throughout the matrix at ambient temperature. At elevated temperatures, the PP-fibers were melted and dissolved into the cement paste, leaving empty cavities and micro-cracks that allow internal water vapor and gases to escape.


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Savastano, H. and Agopyan, V. (1999), “Transition zone studies of vegetable fibre-cement paste composites”, Cement and Concrete Composites, Vol. 21 No. 1, pp. 49-57. Schneider, U., Diederichs, U. and Ehm, C. (1982), “Effect of temperature on steel and concrete for PCRV's”, Nuclear Engineering and Design, Vol. 67 No. 2, pp. 245-258. Suhaendi, S.L. and Horiguchi, T. (2006), “Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition”, Cement and Concrete Research, Vol. 36 No. 9, pp. 1672-1678. Ulm, F.J., Coussy, O. and Bazant, Z.P. (1999), “The chunnel fire. I: chemoplastic softening in rapidly heated concrete”, Journal of Engineering Mechanics, Vol. 125 No. 3, pp. 272-282. Xiao, J. and Falkner, H. (2006), “On residual strength of high-performance concrete with and without polypropylene fibres at elevated temperatures”, Fire Safety Journal, Vol 41 No. 2, pp. 115-121. Zeiml, M., Leithner, D., Lackner, R. and Mang, H.A. (2006), “How do polypropylene fibers improve the spalling behavior of in-situ concrete?”, Cement and Concrete Research, Vol. 36 No. 5, pp. 929-942. Further reading Behnood, A. and Ghandehari, M. (2009), “Comparison of compressive and splitting tensile strength of high-strength concrete with and without polypropylene fibers heated to high temperatures”, Fire Safety Journal, Vol. 44 No. 8, pp. 1015-1022. Corresponding author Duncan Cree can be contacted at: [email protected]

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