Explosive Spalling of Ultra-High Performance Fibre Reinforced Concrete Beams Under Fire

Charles Kahanji, Faris Ali and Ali Nadjai

FireSERT, Built Environment Research Institute, School of the Built Environment, Ulster University, Shore Road, BT37 0QB, United Kingdom

E-mails: Kahanji-c@email.ulster.ac.uk; f.ali@ulster.ac.uk; a.nadjai@ulster.ac.uk

ABSTRACT

Ultra-high performance fibre reinforced concrete (UHPFRC) is a new class of concrete with favourable mechanical properties over ordinary concrete. A great amount of research relating to the performance of UHPFRC at room temperature has been conducted, but its behaviour at elevated temperatures has not been sufficiently explored. This study presents the outcomes of an experimental study on stressed singly reinforced UHPFRC beams subjected to ISO 834 fire curve for 60 minutes. The beams were reinforced with steel fibres of 2% and 4% volumetric ratio. A distinct load (25kN, 50kN and 75kN), corresponding to 20%, 40% and 60% of beam’s ambient temperature ultimate strength was imposed on the beam prior to the fire test and was kept constant throughout the heating duration. An additional beam containing polypropylene (PP) fibres (along with steel fibres) was tested to investigate the effectiveness of PP fibres in reducing explosive spalling. Exposing the concrete beams to fire caused severe explosive spalling. Spalling was more prevalent in beams containing 2% fibres. The beams under the 50% load level spalled significantly more than the other two load categories and had the least fire resistance. The beam containing PP fibres recorded no spalling, demonstrating the effectiveness of PP fibres in eliminating spalling.

Keywords – Ultra High-performance fibre reinforced concrete; ISO 834; Fire resistance; Concrete beams; steel fibres; polypropylene; spalling

1. INTRODUCTION

Fire in buildings has the potential to cause substantial loss of serviceability that may lead to structural collapse. The risks associated with fire are not only limited to structural damage but can cause injuries and loss of lives of occupants and emergency rescue teams. Due diligence must, therefore, be paid to the material selection that can offer optimum safety which can resist fire long enough to allow for timely evacuation of occupants. Concrete is one of the main materials used in

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construction industry. Being a non-combustible material, concrete performs exceptionally well in the event of fire by preventing the spread of fire to other compartments. However, explosive spalling under fire has been observed and reported as being one of the most hazardous behaviour of concrete in fire. Explosive spalling can have a detrimental effect on the load bearing capacity of structural components. The past years have seen an emergence of special concrete to meet the ever sophisticated challenging architectural designs. Ultra-high performance fibre reinforced concrete (UHPFRC) is one such type of concrete that has recently been developed. UHPFRC have exceptional mechanical properties in comparison to normal strength concrete (NSC) and “conventional” high-performance concrete (HPC). Among them, high compressive strength (over 150MPa), enhanced ductility, increased durability, toughness and improved freeze-thaw resistance [1-6]. UHPFRC typically has low water-binder ratio and does not contain coarse aggregates which gives rise to a densely packed concrete matrix [7, 8]. Owing to the tiny particles of materials used in casting UHPFRC, the resulting solid material is of lower porosity with virtually no pores leading to the surface. The drawback of concrete with lower porosity is their susceptibility to spalling when exposed to rapidly rising temperatures similar to a standard fire. The actual spalling mechanism is still an area undergoing research but it’s widely attributed to the build-up of pressure in enclosed concrete voids or capillary pores when concrete is heated. As temperatures rise, vapour pressure in the capillary pores rises and because these pores are blind (fully enclosed), with no gateway to the external surface, when the vapour pressure exceeds the concrete’s tensile strength, the concrete fractures into pieces and this process is usually accompanied by violent noise and shrapnel of concrete are scattered at high speeds. This can be dangerous to both occupants as well as rescuers and firefighters. Studies have however shown that adding polypropylene (PP) fibres to concrete with a dosage of between 1-3kg/m3 can eliminate spalling [9-11].

The behaviour and performance of HPC under fire is well documented [12-17]. HPC generally have lower fire endurance than NSC and are more susceptible to spalling due to the high packing density [18, 19]. However due to the three main factors that distinguish the UHPFRC from “conventional” high strength concrete (microfibers, the absence of coarse aggregate, and low permeability), it may be unsafe to design UHPFRC structures for fire resistance, based on the properties and parameters of HPC. It has also been observed that most of the fire-related research have largely centred on NSC and “conventional” HPC. The majority of fire tests on UHPFRC materials have been carried out with small elements such as cylinders, cubes, prisms and panels [20-27]. The results of such tests showed severe spalling but the elements containing polypropylene fibres did not spall. On the other hand, only a handful of fire tests on full-size UHPFRC elements have been conducted. Lee et al. [27] tested two identical UHPFRC reinforced columns measuring 500x500x3428mm3

to a standard fire. The columns had a hybrid combination of steel (0.5% vol.), nylon (0.2% vol.) and polypropylene fibres (0.2% vol.) and a constant axial load was

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applied throughout the fire duration of 3 hours. Both columns achieved fire endurance of over 3 hours and remained intact with only minor spalling, which was attributed to the presence of polypropylene fibres. Pimienta et al. [21] conducted fire tests on two industrial prestressed I-shaped beams made from commercially marketed UHPFRC, Ductal©-AF. One beam had a 50kN imposed load while the other was unloaded. Both beams did not record significant material loss. The unloaded beam had minor spalling while the loaded beam recorded no spalling at all. In terms of fire endurance, the unloaded beam was intact throughout the 2-hour fire duration. The loaded beam failed in bending after 36 minutes and failure was attributed to pre-stressing cables.

This research paper presents the findings of an experimental study on the fire performance of UHPFRC beams tested under three loading levels and exposed to the ISO 834 [28] fire curve. In order to obtain an elaborate understanding of fire behaviour of UHPFRC, the experimental programme was divided into three series; A, B and C. Series A consisted of 3 identical beams that were reinforced with steel fibres of 2% volumetric ratio. Series B also had 3 identical beams but contained steel fibres of 4% ratio. To understand the effectiveness of polypropylene fibres in eliminating spalling, series C contained one beam that had a hybrid combination of steel fibres (2%) ratio and polypropylene fibres (4kg/m3).

1.1 Research significanceThere are many factors that affect the behaviour of concrete in fire and these include, aggregate type, mix proportion, water-cement ratio and age of concrete. On account of the superior mechanical properties of UHPFRC over ordinary concrete such as high compressive strength and light weight, the material has the potential for wide range applications especially in structures requiring high strength materials such as high rise buildings, tunnelling, nuclear reactor etc. It can also be used as a non-structural material in applications such as cladding for building façade and flooring. In most of these applications, the structures may be at risk of fire and the spalling phenomenon of UHPFRC can have devastating consequences on the structural integrity. Currently, there are no design guidelines for fire resistance of UHPFRC in major design codes including the American Concrete Institute (ACI) Code and the Eurocode. It is, therefore, essential that research and studies are conducted to understand the behaviour of UHPFRC at elevated temperatures.

2. THE EXPERIMENTAL PROGRAMME

2.1 MaterialsThe constituent materials used to cast the beams are shown in Table 1. All the beams were reinforced with Dramix steel fibres, manufactured by Bekaert. These are straight fibres with a length of 13mm and a diameter of 0.2mm. The fibres are cold drawn steel wires with a tensile strength greater than 2000 MPa. One beam, in

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addition to having been reinforced with steel fibres, also contained polypropylene fibres.

Table 1: Constituent materials

Constituent Type Specific Mass (kg/m3)

Cement CEM I 52.5N 967Silica fume Larsen 60%<1μm 251

Sand ≤0.6mm 675HRWR Larsen Chemcrete HP3 77

Steel fibres Dramix® OL 13/.02 2% = 1584% = 316

Polypropylene Larsen Monofilament 4Water w/b ≈ 0.20 244

2.2 Casting and CuringThe materials shown in Table 1 were mixed in a 150-litre concrete pan mixer. The dry mix of cement, sand and silica fume were first mixed with water of 0.2 water/binder ratio for a duration of 10minutes. This was followed by the addition of superplasticiser (HRWR) and the mix components were allowed to mix for a further 10 minutes. Finally, steel fibres (including PP where necessary) were and all the components were mixed for a further 5 minutes. The wet concrete was then poured into the moulds which were placed on a vibrating table. Nine 50mm cubes which were used to determine the compressive strength of the beams were cast alongside each beam. The freshly cast concrete beams and the accompanying cubes were covered with a damp hessian material and left to set for 24 hours. The moulds were then stripped the following day and the beams were submerged wholly in a tank containing water at ambient temperature. The beams from series A and B remained in the curing tank for at least 90 days while the series C beam remained in the curing tank for 28 days and tested the next day. The beams were thereafter transferred to a conditioning room where they remained for at least 3 months until the day of testing, in conformity with BS EN 1363-1 [29] . The temperature and relative humidity of the conditioning room were set at 21-25°C and 40% to 60% respectively.

2.3 Design and Details of Test SpecimensThe investigation consisted of an experimental programme on seven UHPFRC beams of 2000mm length, 200mm depth and 100mm width. Each beam was reinforced with traditional 16mm diameter steel reinforcement bars in the tension region and an all-round cover of 20mm was provisioned. The beam notations shown in Table 2, contain 3 letters RLF, followed by a numerical value of either 2 or 4. The numerical value represents the steel fibre dosage of 2% and 4% used in the respective beams. The two-digit number (20, 40 and 60) at the end represents the imposed load ratio as a percentage of the beam’s ambient ultimate bending strength.

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The ultimate strength of the beams was obtained experimentally from the UHPFRC beams previously tested at ambient temperature [30]. This was supported by moment capacity computation based on Eurocode 2 [31]. The beams attained the test day compressive strengths of over 150MPa.

Table 2: Detailed description of beams

Series Fibre Type Beam IDLoad Ratio[%]

Applied load [kN]

Beam age

[Days]

fcu - 28 days[MPa]

fcu - test day

[MPa]

Steel fibres (2%)RLF2-20 20 25 205 117.8 157.5

A RLF2-40 40 50 234 131.1 163.4RLF2-60 60 75 238 155.0 178.7

Steel fibres (4%)RLF4-20 20 25 245 167.1 162.3

B RLF4-40 40 50 248 148.5 166.8RLF4-60 60 75 252 155.9 173.8

C PP (4kg/m3) + steel fibres (2%) RLF2P-40 40 50 29 100.6 100.6

At 28 days, the three accompanying cubes were tested to determine the beams’ 28-day compressive strength. The other 3 cubes were tested to determine the “test day” compressive strength. The two strength values are tabulated in Table 2. The test day compressive strength for series A and B beams were all greater than 150 MPa. Only the beam with polypropylene had compressive strength value below 150MPa. The low strength values could be attributed to the addition of polypropylene fibres. The RLFP2-40 was cured for 28 days only while the rest remained in the curing environment for at least 3 months. Polypropylene fibres do not enhance the structural properties of concrete and may, in fact, lead to a reduction in strength. Due to their physical configuration and lower strength, the polypropylene-cement paste interfaces in the concrete matrix were most likely the weakest spots, leading to reduced bonds strength and ultimately contributed to the reduction in compressive strength.

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3. FIRE TESTS PROCEDURE

Each beam was tested in the furnace while loaded for a period of one hour. The simply supported beam was placed on half-round steel rollers on top of the furnace with a clear span of 1750mm. Only the 1500mm of the 2000mm length of the beam fell within the internal chamber of the furnace of which only half the depth (bottom half) was exposed to direct fire as shown in Figure 1. The top half of the beam was unexposed to heating and the loading was applied from the top surface. A four-point load conforming to Figure 1 was implemented. A 100 kN capacity load cell was used to record the applied load. The applied load was transmitted to the steel spreader beam and subsequently relayed onto the test beam through two 30mm diameter steel rollers spaced 400mm apart. The load was applied in increments of 2.5kN for the 25kN load and 5kN increments for the 50 and 75kN loads. There was at least a 2-minute interval between successive loads, which was set aside to stabilise the loads and deflection reading and this was also used for purposes of monitoring crack formation. Once the final load was attained, the load was left to stabilise for at least 30 minutes before the furnace burners were switched on. Efforts were made to keep the load constant throughout the test.

Figure 1. Compressive layout of the beam

3.1 Furnace The tests were carried out in a furnace with an internal chamber measuring 1500mm x 1500mm x 1500mm. The furnace was designed to test material's ability to withstand exposure to fire (high temperatures). The test data enables material's performance elements, like load thermal transmittance and carrying capacity to be analysed. The three vertical walls and floor surface of the furnace were lined with special high temperature insulating fire bricks while its front was fitted with a shutter sliding door made from alumina fibreboard. The furnace top was covered with two specially made concrete slabs with provision to accommodate the test sample beam between them and ensuring that the heat flow was unhindered. The beam specimen and slab interface and the slab-wall interface were lined with ceramic fibre blanket insulation. The fibre blanket used had the ability to withstand temperatures of up to

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1260°C. In order to monitor the actual beam deflection, it was essential to leave sufficient play between the top cover slab and beam interface so that the downward displacement movements of the beam were not restrained. Ceramic fibre blanket was the ideal material to fill the intentionally created gaps. The test specimen beam was supported on two steel half-round rollers which were welded to the top of the furnace frame, above the walls. The furnace chamber had a total of 7 burners but for this particular project, only 4 burners were used. The inner chamber had five thermocouples which monitored the furnace temperature to ensure that it conformed to the ISO 834 curve. Figure 2 shows the schematic view of the beam showing the half-depth portion of the 1.5m length of the beam that was exposed to elevated temperature. Figure 3 shows the view of the beam inside the furnace before the test, showing clearly the three sides of the 1500mm beam length that were to be heated.

Figure 2: Schematic side view of the furnace showing the position of the beam

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Figure 3: View of the beam inside the furnace before fire test

3.2 Test Instrumentation Two linear variable differential transformers (LVDTs) were attached to the beam to measure the deflection. Due to the complex loading system on the rig, it was not practically possible to position the LVDTs on the midspan of the beam. Arising from that, the two LVDTs were each positioned 300 mm from the mid-span. Thermocouples of type K-310 with stainless steel sheath material were fixed at various locations of the beams to monitor the temperature distribution across the beams’ depth and on the rebars. The exact positions for the six thermocouples, marked TC1, TC2, TC3, TC4, TC5 and TC6 are shown in Figure 4. These locations include the bottom surface, rebar, the beam centre (50mm depth) and the top surface. As spalling on the bottom surface of the beam was highly anticipated, there was a likelihood of thermocouples at such locations being detached from the surface and thereby reading the furnace air temperature instead. Three thermocouples (TC1, TC2 and TC3) were therefore positioned on the bottom surface to measure the surface temperature of the beam. It was anticipated that out of these three thermocouples, at least one would remain firmly attached to the surface throughout the fire test to give the reading of surface temperature. One thermocouple (TC4) was attached to one of the two rebars and the remaining two were fixed inside the concrete beam core centre (TC5) and on the top surface (TC6) respectively.

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Figure 4: Thermocouples locations

4. RESULTS AND DISCUSSION

All the seven beams were successfully tested and the recorded results relating to failure mode, endurance time are displayed in Table 3. Each tested beam from series A and B was scheduled to run for 60 minutes but where failure resulted in the break-up of the beam onto the furnace floor, the test was terminated as a precautionary measure. The latter scenario only occurred in beams with highest loading levels (RLF2-60 and RLF4-60). The RLF2P-40 beam was to be tested until failure because its aim was to study the influence of polypropylene fibres on the beam’s fire endurance. Figure 5 shows the RLF2-40 beam inside the furnace after the fire test, showing clearly the extent of spalling.

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Figure 5: RLF2-40 beam inside the furnace after fire test

Table 3: Endurance and failure mode results

Beam IDLoad ratio[%]

Applied Load[kN)

Failure Time[min]

Failure mode

Fire endurance [min]

RLF2-20 20 25 N/A Flexure 60

RLF2-40 40 50 49 Shear 30

RLF2-60 60 75 53 Flexure 30

RLF4-20 20 25 N/A Flexure 60

RLF4-40 40 50 52 Shear 30

RLF4-60 60 75 54 Flexure 30

RLF2P-40 40 50 66 Flexure 60

Figure 6 andFigure 7 show the beams from series A and B after fire tests showing extensively the severe levels of explosive spalling in each beam and their failure modes. A quick comparison of the corresponding beams from both streams shows stark similarities in failure patterns except that series A beams spalled significantly more than series B beams. For example, the shear crack in RLF2-40 and RLF4-40 that caused failure in both beams occurred at almost identical locations.

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Figure 6: Series A - beams with 2% fibre content (RLF2-20, RLF2-40 and RLF2-60) after fire test

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Figure 7. Series B-beams with 4% fibre content (RLF4-20, RLF4-40 & RLF4-60) after fire test

4.1 Failure modesThe beams were inspected for cracks during the loading process and prior to heating. Failure was assumed to have occurred upon hearing fracture sound accompanied with the sudden leap in deflection value. The imposed load was immediately removed but heating continued unless the beam had split into two. BS EN 1363 [29] provides for deflection as one of the failure criteria for flexural loaded elements where an element is assumed to have lost its loadbearing capacity if the limiting deflection exceeds L2/400d, which is 38mm for the test beams in this study. Beams with a 20% load ratio (RLF2-20 and RLF4-20) were the only ones that never failed but they were weakened significantly due to spalling. Application of 25kN and 50kN loads didn’t produce any visible cracks prior to heating. However, the deformation (deflection) of the RLF2-40 was significantly higher than the RLF2-20. At 50 minutes, for instance, the RLF2-20 had attained a deflection of 18mm while the deflection of the RLF2-40 was almost 50mm. This means that the entire depth of the

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middle portion of the beam was exposed to direct heating with a temperature of over 900°C. Exposure to heating of the previously unexposed surfaces of the beam reignited explosive spalling, and this resulted in the beam losing much of its section and load carrying capacity. Failure of both beams with 60% loading level (RLF2-60 and RLF4-60) was brittle in nature. Both beams snapped into two pieces with very little signs of imminent failure beforehand. There were few cracks that formed after the application of 75kN load prior to heating. However, the level and extent of cracks that formed during the fire test could not be fully quantified due to excessive spalling in the tension region of the beam. The failure pattern for corresponding beams in series A and B were similar, the only variation was the degree of spalling. Figure 8 shows the furnace floor before the test and after fire test with spall fragments lying on the floor. Also showing is the close up photo detailing the extent of shear failure and damage to concrete cover.

(a) (b)

(c) (d)

Figure 8.(a) furnace floor before the test; (b) furnace floor after test with spall fragments; (c)shear failure in RLF2-40; (C) exposed rebars in RLF2-40 beam due to loss of cover

4.2 DeflectionTwo sets of deflection values for each beam are presented Table 4. These are the initial and final deflections values. The initial deflection was due to the constant imposed load and this value was taken just before the start of heating. The final deflection was recorded at the end of the fire test at exactly 60 minutes or at the time of failure for the beams that failed before the 60-minute mark. The final deflection was therefore due to the combination of imposed load and thermal load.

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Table 4: Deflection of beams before and after fire test

Series Fibre content Beam IDInitial

deflection [mm]

Final deflection

[mm]RLF2-20 1.9 35.5

A 2% RLF2-40 2.0 76.0RLF2-60 4.3 35.5RLF4-20 0.7 28.3

B 4% RLF4-40 2.2 74.8RLF4-60 4.7 33.9

C 2% steel+PP RLF2P-40 3.2 43.3

4.2.1 Deflection of beams with 2% fibres

Deflection comparisons of four beams with 2% fibres (RLF2-20, RLF2-40, RLF2-40 and RLF2P-40) are graphically presented in Figure 9. In the first 30 minutes of heating, all the four curves seem to follow a rather linear pattern. The hybrid beam maintained an almost linear profile throughout the 1-hour test. The RLF2-40 recorded the highest deflection and the least endurance. It failed at 49 minutes in shear but did not break into two and the loading was removed immediately but the heating went on uninterrupted until the end of the test. The reason it recorded the deflection twice as high as the other two beams was due to severe spalling which reduced its cross-section area and ultimately its load-carrying capacity, as detailed in section 4.4. RLF2-60, on the other hand, failed by snapping into two pieces at about 53 minutes. The furnace was immediately switched off to prevent damage to instruments that were on top of the slab covers.

Figure 9: Deflection of beams with 2% fibres over the entire heating duration

4.2.2 Deflection of beams with 4% fibres

The deflection patterns of series B beams shown in Figure 10 are quite similar to series A beams. At any given time, the beams with higher loads had higher

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deflection rates. The RLF4-20 beam did not fail but its behaviour shows the rate of deformation increasing rapidly towards the end of the test signalling imminent failure. The RLF4-40 beam behaved in a similar manner as the corresponding series A beam (RLF2-40). It can be seen from the graph that the RLF4-40 beam deformed more towards the end of the test when spalling had intensified. The beam spalled severely affecting its load-carrying capacity. As expected for higher loads, RFL4-60 posted higher rates of deformation but snapped suddenly into two pieces after 54 minutes.

Figure 10: Deflection of beams with 4% fibre content over the heating duration

4.2.3 Deflection of beams with the same load levels

Comparisons of beams with 2% and 4% fibres with the same load levels are presented graphically in Figure 11, Figure 12 and Figure 13. The aim was to ascertain the influence of fibre dosage on the deformation behaviour of beams. Generally, there are no consistent patterns which can be drawn from the graphs. However, for beams with 60% loading, the curves were almost identical throughout the test. For the other two loading categories, the deflections quite matched each other in early stages of tests before diverging in the latter stages of the test with the 2% beams undergoing increased deformation. The discrepancy could be attributed to the spalling levels which were more severe in 2% beams. Increased spalling resulted in a decrease in cross-sectional area, which eventually led to more displacement. It can also be seen from Figure 12 that the RLF2P-40 beam which never recorded any spalling, had an almost linear load-deflection profile during the 60-minute interval. Compared with the other two beams under the same loading level, this gives an indication of how spalling affects the deformation behaviour of the beams.

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Figure 11. Deflection pattern of beams with 20% loading level

Figure 12. Deflection pattern of beams with 40% loading level

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Figure 13. Deflection patterns of beams with 60% loading levels

4.3 Thermal BehaviourCalculation of temperature within the structural member is an important design parameter stated in BS EN 1991-1-2 [32]. Direct computation of temperature distribution in the concrete material is not quite straight forward due to various parameters that affect its properties. This is, even more, challenging in new concrete material like UHPFRC, more so that they are prone to spalling; a phenomenon that is not quite definitive and happens randomly.

4.3.1 Steel reinforcement

Figure 14 presents temperature distribution in the reinforcement bars for all the seven beams monitored by TC4 thermocouples, which were attached to the rebar during the casting process and left to set with the beam. Three beams; RLF2-20, RLF2-40 and RLF4-40 had their concrete cover partially exposed in some areas due to spalling. Visual examination of the RLF4-40 beam showed loss of concrete cover of reinforcement on many spots (more than other beams) and this explains why it recorded the highest temperature. The RLF4-60 recorded the least rebar temperature throughout the entire test. It was also the beam that spalled the least from among the series A and B beams. At the time it failed, its rebar temperature was 250℃ lower than the beam with the highest rebar temperature (RLF4-20). The lower rebar temperature could be attributed to the less severe spalling which kept the concrete cover almost intact. Spalling is a random phenomenon, some sections of the beam had spalling depths of up to 30mm. Some of these affected areas coincided with the position of the rebars (exposing the rebars to direct heat), while others did not. Such random nature of spalling is the contributing factor to the discrepancies in temperature profiles in the beams rebars.

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Figure 14. Temperature development in the rebar

4.3.2 Beam Centre Temperature

The temperature profile at the centre of the 7 beams as measured by their respective thermocouples (TC5) are presented in Figure 15. The general picture is that RLF2P-40 recorded the least temperature at the centre throughout the test with a maximum of just over 150°C after 60 minutes compared to temperatures above 200°C for other beams. This could be attributed to the absence of spalling as this meant that heat had to be transmitted through the entire thickness from the exposed surface to the centre where the thermocouple was fixed. Another explanation could be due to the high levels of moisture content in the beam with hybrid fibres as it was not conditioned.

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Figure 15. Temperature development of the centre of beam

4.3.3 Top surface

The development of temperature of the top, unexposed surface, was monitored by TC-6 thermocouples. The resulting curves for all the 7 beams are shown in Figure16. It can be seen that the temperature for all the beams in the first 20 minutes remained constant at room temperature. The 20 minutes that followed, the temperature rose marginally but didn’t go beyond the 50°C mark except for the RLF2P-40 beam. After 40 minutes, the deflection of most beams was nearing 20mm and the top surface was getting into close proximity with the furnace heat. The RLF2-40 failed after 49 minutes, attaining a deflection of over 50mm, at that instant the top-surface was almost inside the furnace and exposed to the flames, and this explains why the temperature rose rapidly to over 300°C in the last 10 minutes. For all the other beams, the temperature of the unexposed face was below 100℃ way below the 180K maximum temperature rise stipulated in EN 1992-1-2 [33].

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Figure 16. Temperature development of the top surface, unexposed to heating

4.3.4 Bottom surface

Three thermocouples, TC1, TC2 and TC3 were attached to the bottom surface of the beams at different locations to read the temperature during the entire fire test. The temperature profiles for the surface of the beam, monitored by one thermocouple are presented in Figure 17. Also included is the average gas temperature of the furnace chamber which was the same for all the tests since it followed the ISO 834 fire curve. Visual inspections at the end of the tests revealed that most of the thermocouples were detached from the concrete surface primarily due to spalling. Spalling commenced after 10 minutes and peaked around the 20 th minute. The worst affected were the RLF2-40 and RLF4-40 beams, where all three thermocouples in each beam were detached. From Figure 17, the temperature readings of both thermocouples rose dramatically after 15 minutes, providing some indication as to when they were detached from the concrete surface and started to read the gas temperature. All the three thermocouples on the bottom surface of the series C-beam, RLF2P-40, which did not spall, were intact at the end of the test. The graphs further show that the RLF2P-40 beam had the least temperature at every stage when compared to others. This could be due to higher amounts of moisture in the beam.

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Figure 17. Temperature distribution of bottom surface of the beams during the 1-hour heating

4.4 Explosive Spalling The heating of all series A and B beams was characterised with severe explosive spalling which can be attributed to the dense-packed nature of UHPFRC. In this experimental programme, fine sand (with size particles no greater than 0.6mm) were the largest particles in the concrete matrix. On the other hand, the silica fume used accounted for the smallest granular material size particle of less than 1µm. Silica fumes filled up the voids between the sand particle and the material was thus bound together by hydrated cement. This resulted into the densely packed concrete matrix. Explosive sounds of spalling started after 10 minutes by which time the furnace gas temperature had reached 675°C. From Figure 17, which profiles the surface temperature of the beams, all the six beams from series A and B had attained a minimum surface temperature of about 275°C. For series A beams, RLF2-40 beam was the worst affected by spalling, losing 6 kg as fragments of spalling, as a consequence, its load carrying capacity was severely reduced. The least affected in terms of spalling was the RLF2-60, shedding off 1.5kg in spalling fragmentation. The possible explanation for this is that the as the beam was being loaded up to 75kN, multiple cracks formed in the beam and with increased temperature strain, these cracks became even wider in width. These cracks created escape routes for gaseous vapour, markedly reducing the vapour pressure inside the concrete, more so around the vicinity of cracks. The disparities in amounts of spalling between RLF2-20 and RLF2-40 can be explained in terms of their displacement behaviour. Both loads (25kN and 50kN) were not enough to cause significant cracks in the

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beams prior to heating. However as heating progressed, the rate of deflection of RLF2-40 was higher resulting in regions of the upper part of the beam, previously unexposed, coming in direct contact with the flame. Towards the end of the test, there was re-emergence of explosive spalling sounds which was as a result of the previously unexposed upper region coming into direct heating. Series B beams followed a similar pattern as their counterpart beams in series A, with decreased spalling. The decrease in spalling observed in series B beams has been explained in section 4.6.2.

4.4.1 Spalling and moisture loss

Each beam was weighed before and after the fire test. Spall fragments that were scattered on the furnace floor were carefully collected and weighed separately after the furnace had cooled down. For all the beams, the total mass of the beam after the fire test (including the spall fragment mass), was less than the mass of the beam before the fire test. Since concrete is a non-combustible material, it can be assumed that the difference was due to moisture loss. Moisture loss was therefore computed the using the three parameters (mass before test, beam mass after test and spall fragment mass). The spalling percentage was computed from the spall fragments and the total mass. The mass data for each beam is tabulated in Table 5.

Table 5: Weight of beams and spalling

Beam ID

Beam mass

before test [kg]

Beam mass after test [kg]

Spall fragments

[kg]

Total Mass (fragments + beam after

test)[kg]

Spalling[%]

Moisture loss[%]

RLF2-20 99.3 89.68 4.13 93.81 4.4 5.5RLF2-40 98.7 87.19 6.04 93.23 6.5 5.5RLF2-60 101.1 94.00 1.50 95.50 1.6 5.5RLF4-20 109.39 101.68 2.86 104.54 2.7 4.4RLF4-40 105.23 97.04 3.24 100.28 3.2 4.7RLF4-60 105.5 99.60 0.90 100.50 0.9 4.7

RLF2P-40 100.8 93.45 0 93.45 0 7.3

As stated previously, the RLF2P-40 beam was not placed in the conditioning room and was tested a day after being removed from the curing water tank. It, therefore, had considerably high amounts of moisture compared to series A and B beams. Therefore, despite showing 100% resistance to spalling, the RLF2P-40 beam recorded the highest loss in moisture (7.3%). The mass loss was mainly as a result of the loss of free water in concrete which was dissipated through evaporation. All the beams from series A lost about 5.5% while series B lost an average of 4.5%. The higher percentage mass loss it recorded, therefore, reflects the higher levels of moisture content it retained prior to testing.

Figure 18 presents the graphical comparisons of levels of spalling of each beam. Comparisons of test data of corresponding beams with same loading levels across series show that spalling was more prevalent in series A beams

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Figure 18. Spalling comparisons of beams

4.5 Hybrid Reinforced UHPFRC Beam – RLF2P-40As previously mentioned, the RLF2P-40 beam was reinforced with hybrid fibres (2% steel fibres and 4kg/m3 PP). Khoury and Willoughby [9] recommended a dosage of between 1-3kg/m3 for fire protection. The study further reported that even though a higher dosage between 3-9kg/m3 is quite effective in reducing spalling, it presents workability problems. However, a dosage of 4kg/m3 was used in this study and it did not affect the concrete workability. The beam was tested in the furnace like all other beams in series A and B and a constant load level of 40% was applied during the test. The test results of series A and B beams have identified RLF2-40 beam, with 2% fibres and an imposed load of 40%, as the beam that was most severely affected by spalling and had least fire endurance. The design of RLF2P-40 was based on the RLF2-40 beam where the latter’s constituent materials were altered with the addition of polypropylene fibres for purposes of studying their influence in eliminating spalling and improving the beam’s fire endurance. The beam had high moisture content levels prior to testing. Increased moisture content leads to increased spalling. Polypropylene fibres with trade name Fiberflex, manufactured by Larsen were used in this experimental study. These were monofilament fibres with a melting point of 160°C. During fire test, no explosive spalling sounds were heard throughout the test. At 60 minutes, the beam showed no immediate signs of failure and posted a deflection of 26mm. The test was continued beyond the 60-minute mark in order to

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determine the actual time of failure. The beam went on to fail 6 minutes later through bending in stark contrast to the two corresponding beams (RLF2-40 and RLF4-40) with same load levels beams which failed in shear. As can be observed from Figure19, the beam emerged out of the test intact with no loss of material through spalling or sloughing. Polypropylene fibres, therefore, played a critical role of eliminating spalling completely despite the test beam possessing huge levels of moisture content. Spalling in series A and B beams commenced when the surface temperature of concrete attained a temperature of about 275°C. At this temperature, polypropylene fibres would have long been melted thereby creating connected channels through which gaseous vapours under high pressure would escape to the concrete surface, averting any possible explosions. The results show that even when the moisture content in concrete is at its highest levels, polypropylene can still effectively eliminate spalling.

Figure 19. Hybrid beam, RLFP2-40 containing steel and polypropylene fibres after fire test

4.6 Fire Endurance of BeamsAn analysis of test data and visual observations during and after experimental tests points to some possible factors that might have an influence on fire endurance of beams and these are explained briefly in the sections below.

4.6.1 Effect of spalling

A closer scrutiny of beams of same constituent materials shows a direct relationship between the degree of spalling and fire endurance. The two beams that spalled severely (RLF2-40 and RLF4-40) across series A and B had the least fire endurance. While other factors such as loading levels and the amount of fibres had a cumulative effect on endurance, spalling as a single factor can have a devastating effect on the load-bearing capacity of a flexural element. Complete elimination of spalling prolonged the load bearing capacity and endurance of the beam as evidenced from the beam with hybrid fibres, RLF2P-40.

4.6.2 Effect of amount of fibre

A clear relationship between fibre content and endurance was observed. The beams with 4% steel fibre contents had slightly higher endurance and crucially less spalling

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compared to corresponding 2% beams. Steel fibres increase the tensile strength of concrete and studies have shown that it can prevent spalling to some degrees [34]. The amount of fibres in concrete has a direct correlation on the tensile strength of concrete. Therefore, the beams with 4% fibres could have higher tensile strength than those with 2% fibres. The resulting increase in tensile strength could have led to the beam to withstand the vapour pressure that built up inside concrete due to heating. This, in turn, resulted in reduced spalling in those beams and ultimately led to increased fire endurance.

4.6.3 Effect of loading levels

Three loads of 25kN, 50kN and 75kN were imposed on the beams with each load having a different outcome in terms of endurance and other responses. As expected, the endurance was higher in beams with lower load levels. As previously stated application of the 25kN and 50kN loads did not result in any visible cracks on the beams prior to heating, while the 75kN load did cause some cracks in the tension region. Results showed that spalling flourished in beams which had little or no cracks prior to heating.

5. CONCLUSIONS

The experimental programme successfully tested UHPFRC beams with 2% and 4% steel fibres with compressive strengths of up to 178 MPa and among these was one beam with hybrid fibres of steel and polypropylene. The beams were subjected to an ISO 834 fire curve in the furnace for one hour under three different loading levels of 20%, 40% and 60% of designed capacity. The key findings are summarised below:

The beams imposed with the 20% load level from both series A and B were the only beams that did not fail, although they spalled significantly.

The beams under the load level of 40% (RLF2-40 and RLF4-40) recorded the least fire endurance and failed in shear. This loading category was the worst affected by spalling and even lost the concrete cover in some regions

The 60% load level caused the beams to fail catastrophically through bending. This loading category recorded the least spalling. The cracks that formed prior to heating acted as escape channels for gaseous vapour to migrate from the inside of concrete matrix to the surface, thus reducing pore pressure.

The beams containing fibres of 2% dosage were more susceptible to spalling compared to those with 4%. The extra steel fibres create high tensile and bond strengths in the concrete matrix which can withstand pore pressure and reduce spalling.

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The beam with hybrid fibres (RLF2P-40) did not spall despite having high moisture content. PP fibres are therefore very effective in eliminating spalling in stressed beams exposed to elevated temperature.

ACKNOWLEDGMENTS

Funding for this research was supported by the Vice Chancellor’s Research Scholarship (VCRS) of the Ulster University.

REFERENCES

[1] A. E. Naaman and K. Wille, "The path to ultra-high performance fiber reinforced Concrete (UHP-FRC): Five decades of progress," in Proceedings of Hipermat 2012 3rd   International Symposium on UHPC and Nanotechnology for High Performance Construction Materials, 2012, .

[2] K. Wille, A. E. Naaman, S. El-Tawil and G. J. Parra-Montesinos, "Ultra-high performance concrete and fiber reinforced concrete: achieving strength and ductility without heat curing," Mater. Struct., vol. 45, pp. 309-324, 2012.

[3] K. Wille, A. E. Naaman and G. J. Parra-Montesinos, "Ultra-high performance concrete with compressive strength exceeding 150 MPa (22 ksi): a simpler way," ACI Mater. J., vol. 108, pp. 46-54, 2011.

[4] I. H. Yang, C. Joh and B. Kim, "Structural behavior of ultra high performance concrete beams subjected to bending," Eng. Struct., vol. 32, pp. 3478-3487, 11, 2010.

[5] S. Yang and B. Diao, "Influence of curing regime on the ductility of ultra-high performance fiber reinforced concrete (UHPFRC)," in Anonymous American Society of Civil Engineers, 2009, pp. 1-7.

[6] C. Magureanu, I. Sosa, C. Negrutiu and B. Heghes, "Mechanical Properties and Durability of Ultra-High-Performance Concrete," ACI Materials Journal, vol. 109, pp. 177, March 2012.

[7] F. de Larrard and T. Sedran, "Optimization of ultra-high-performance concrete by the use of a packing model," Cem. Concr. Res., vol. 24, pp. 997-1009, 1994.

[8] K. Wille, A. E. Naaman and S. El-Tawil, "Optimizing Ultra-High-Performance Fiber-Reinforced Concrete," Concr. Int., vol. 33, pp. 35-41, 2011.

[9] G. A. Khoury and B. Willoughby, "Polypropylene fibres in heated concrete. Part 1: Molecular structure and materials behaviour," Magazine of Concrete Research, vol. 60, pp. 125-136, 03/01; 2015/12, 2008.

26

[10] G. A. Khoury, "Polypropylene fibres in heated concrete. Part 2: Pressure relief mechanisms and modelling criteria," Magazine of Concrete Research, vol. 60, pp. 189-204, 04/01; 2015/12, 2008.

[11] G. A. Khoury, "Effect of fire on concrete and concrete structures," Progress in Structural Engineering and Materials, vol. 2, pp. 429-447, 2000.

[12] P. Kalifa, G. Chéné and C. Gallé, "High-temperature behaviour of HPC with polypropylene fibres - From spalling to microstructure," Cem. Concr. Res., vol. 31, pp. 1487-1499, 2001.

[13] P. Kalifa, F. Menneteau and D. Quenard, "Spalling and pore pressure in HPC at high temperatures," Cem. Concr. Res., vol. 30, pp. 1915-1927, 12, 2000.

[14] P. Kalifa, G. Chéné and C. Gallé, "High-temperature behaviour of HPC with polypropylene fibres: From spalling to microstructure," Cem. Concr. Res., vol. 31, pp. 1487-1499, 10, 2001.

[15] V. Kodur, F. Cheng, T. Wang and M. Sultan, "Effect of Strength and Fiber Reinforcement on Fire Resistance of High-Strength Concrete Columns," J. Struct. Eng., vol. 129, pp. 253-259, 02/01; 2013/11, 2003.

[16] V. Kodur, "Spalling in high strength concrete exposed to fire: Concerns, causes, critical parameters and cures," in Anonymous American Society of Civil Engineers, 2000, pp. 1-9.

[17] F. Ali, A. Nadjai and S. Choi, "An experimental and numerical study on the behavior of high strength concrete columns in fire subjected to high restraint," Journal of Structural Fire Engineering, vol. 1, pp. 1-16, 2010.

[18] V. Kodur and R. Mcgrath, "Fire Endurance of High Strength Concrete Columns," Fire Technol., vol. 39, pp. 73-87, 01/01, 2003.

[19] V. Kodur and M. Dwaikat, "Effect of fire induced spalling on the response of reinforced concrete beams," International Journal of Concrete Structures and Materials, vol. 2, pp. 71-82, 2008.

[20] M. Behloul, G. Chanvillard, P. Casanova, G. Orange and F. F. F. France, "Fire resistance of ductal® ultra high performance concrete," in 1st Fib Congress, 2002, pp. 421-430.

[21] P. Pimienta, J. Mindeguia, A. Simon and M. Behloul, "Behavior of UHPFRC at high temperatures," in Designing and Building with UHPFRC, F. Toutlemonde and J. Resplendino, Eds. Hoboken, NJ USA: John Wiley & Sons, Inc., 2011, pp. 579-600.

[22] E. Klingsch, "Explosive spalling of concrete in fire," PhD Thesis, 2014.

[23] D. Hosser, B. Kampmeier and D. Hollmann, "Behavior of ultra high performance concrete (UHPC) in case of fire," in Proceedings of Hipermat 2012 3rd   International Symposium on UHPC and Nanotechnology for High Performance Construction

27

Materials, M. Schmidt, E. Fehling, C. Glotzbach, S. Fröhlich and S. Piotrowski, Eds. Kassel, Germany,: Kassel University Press, 2012, pp. 573-15.

[24] U. Diederichs and O. Mertzsch, "Behaviour of ultra high strength concrete at high temperatures," in Proceedings of Second International Symposium on Ultra High Performance Concrete Kassel, 2008, pp. 347-354.

[25] D. Heinz, F. Dehn and L. Urbonas, "Fire resistance of ultra high performance concrete (UHPC)–Testing of laboratory samples and columns under load," in International Symposium on Ultra High Performance Concrete, 2004, pp. 703-716.

[26] F. Dehn and G. Konig, "Fire resistance of different fibre reinforced high-performance concretes," in PRO 30: 4th International RILEM Workshop on High Performance Fiber Reinforced Cement Composites (HPFRCC 4), 2003, pp. 189.

[27] J. H. Lee, Y. S. Sohn and S. H. Lee, "Fire resistance of hybrid fibre-reinforced, ultra-high-strength concrete columns with compressive strength from 120 to 200 MPa," Magazine of Concrete Research, vol. 64, pp. 539-550, 06/01; 2016/01, 2012.

[28] International standard ISO 834-1, "Fire-resistance tests - elements of building construction - part 1: General requirements," International Organization for Standardization, Geneva, 1999.

[29] British Standards Institution. Fire Resistance Tests. General Requirements 2012.

[30] C. Kahanji, F. Ali and A. Nadjai, "Influence of curing regimes on flexural behaviour of ultra-high performance concrete beams," in Response of Structures Under Extreme Loading, East Lansing, Miami, USA, 2015, pp. 730-737.

[31] British Standards Institution, Eurocode 2: Design of Concrete Structures. General Rules and Rules for Buildings. London: BSI, 2004.

[32] British Standards Institution, Eurocode 1. Actions on Structures. General Actions. Actions on Structures Exposed to Fire. London: BSI, 2002.

[33] British Standards Institution, Eurocode 2. Design of Concrete Structures. General Rules. Structural Fire Design. London: BSI, 2004.

[34] V. Kodur, "Fiber reinforcement for minimizing spalling in High Strength Concrete structural members exposed to fire," ACI Special Publication, vol. 216, 2003.

 

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