This paper summarizes the research that is being carried out at the Universities of Coimbra and Rio de Janeiro, on fibre reinforced concretes at high temperatures. Several high strength concrete compositions reinforced with fibres (polypropylene, steel and glass fibres) were developed. The results of compressive tests at high temperatures (300 °C, 500 °C and 600 °C) and after heating and cooling down of the concrete are presented in the paper. In both research studies, the results indicated that polypropylene fibers prevent concrete spalling.

Rodrigues, J. P. C.

Santos, S. O.

Toledo, R.

Velasco, R. V.

Acta Polytechnica

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

1 IntroductionIn the last 20 years, concrete has been developed to per-form better in all situations. The changes made in mixturecomposition (lower water content, use of superplasticizers,optimization of grain size distribution, use of particles withpozzolanic activity, addition of fibers, etc.) have led to strikingimprovements in many properties such as strength, rheologyof fresh concrete, ductility and compactness. Compactnessusually results in better durability, but it may also lead to thebrittle behavior of high-performance concrete (HPC) in fireconditions. Under certain thermal and mechanical stresses,HPC may spall [1].Spalling is the violent or non-violent breaking off of layersor pieces of concrete from the surface of a structural elementwhen it is exposed to high and rapidly rising temperatures,as experienced in fires [2]. It results from two concomitantprocesses: (i) the thermo-mechanical process associated withthe thermal expansion/shrinkage gradients that occur withinthe element being heated, and (ii) the thermo-hydral processthat generates high-pressure fields of gas (water vapour andtrapped air) in the porous network [3].Fires have occurred in several tunnels in Europe in thepast years. They have caused loss of life, as well as significantdamage to the concrete structure. Some of these tunnels,such as the Channel Tunnel and the Great Belt Tunnel, werebuilt recently. This shows that spalling phenomenon is notyet sufficiently understood for it to be prevented in the con-struction process.This paper intends to contribute to the understand-ing of concrete behavior at high temperatures, and to thecharacterization of the spalling phenomenon, through thedevelopment of concretes with improved fire behavior. APortuguese study and a Brazilian study are described, whichwere carried out to develop fiber concrete compositions withbetter fire behavior.2 Experimental studies2.1 Portuguese testsStudies are in progress in the Laboratory of Testing Mate-rials and Structures of the University of Coimbra to develophigh-strength concretes (HSC) with improved fire behavior.Three compositions of concrete are presented in this paper:one without fibers, one with both steel and polypropylenefibers, and a third one with glass fibers. Compressive strengthtests were carried out at high temperatures on cylindricalspecimens of these three concrete compositions [4].Concrete compositionsTable 1 provides details of the three compositions. Allcompositions contained Portland cement (CEM) type II42,5R, superplasticizer (SP) SIKA 3002 HE, limestone filler(LF) and four different aggregates: fine sand (FS) with afineness modulus of 1.87, coarse aggregate (CA) with maxi-mum size of 9.5 mm and two calcareous crushed stone (CS1and CS2 with maximum sizes of 12.5 mm and 25 mm,respectively).The compositions differ only in the fiber type, or lackof them. The first composition (HSC) had no fibers, whilethe second contained DRAMIX RC ZP305 steel fibers (SF)30 mm in length, 0.55 mm in diameter, a length/diameterratio of 55, tensile strength of 1100 MPa, together with©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 29Acta Polytechnica Vol. 49  No. 1/2009Compressive behaviour at HighTemperatures of Fibre ReinforcedConcretesS. O. Santos, J. P. C. Rodrigues, R. Toledo, R. V. VelascoThis paper summarizes the research that is being carried out at the Universities of Coimbra and Rio de Janeiro, on fibre reinforced concretesat high temperatures. Several high strength concrete compositions reinforced with fibres (polypropylene, steel and glass fibres) weredeveloped. The results of compressive tests at high temperatures (300 °C, 500 °C and 600 °C) and after heating and cooling down of theconcrete are presented in the paper. In both research studies, the results indicated that polypropylene fibers prevent concrete spalling.Keywords: Concrete, high temperatures, compressive strength, spalling, polypropylene fibres, steel fibres, glass fibres.CEM[kg]CS1[kg]CS2[kg]CA[kg]FS[kg]LF[kg]W/C SP[%CEM]PF[kg]SF[kg]GF[kg]HSC 400 600 321 230 470 200 0.3 2.9 – – –HSCSPF 400 600 321 230 470 200 0.3 11.6 1 70 –HSCGF 400 600 321 230 470 200 0.3 11.6 – – 1.5Table 1: Concrete compositions (per m3)DURO-FIBRIL polypropylene fibers (PF), 31 m in diameterand 6 mm in length (HSCPSF); the third composition con-tained VIMACRACK glass fibers (GF), 12 mm in length and14 m in diameter (HSCGF).Table 2 shows the resistance classes for each concretecomposition, obtained in compressive strength tests at roomtemperature, after 28 days, carried out according to the Euro-pean Standard EN 206-1 (2000). This table also incorporatesthe values corresponding to a compression load of 0.7 fcd,applied to the specimens during the heating process, in thecompressive strength tests at high temperatures. This loadwas intended to simulate the maximum load that the concreteis usually subjected to in building structures.When exposed to high temperatures, the polypropylenefibers form pathways that allow the steam to escape, therebyreducing the pressure accumulated in the porous networkof the concrete element, while the steel fibers increase theductility of the concrete, making it more mechanically andthermally resistant. Glass fibers were included in a concretecomposition to assess to what extent these fibers can replacethe polypropylene and steel fibers, taking into account theconcrete strength at high temperatures.SpecimensThe specimens were cylinders 75 mm in diameter ()and 225 mm in height (h), with h/ ratio 3.Five type K thermocouples (Cromo-Alumel) 0.5 mm in di-ameter were placed within the specimen and on its surface tomeasure the temperature during high temperatures tests.The location of the thermocouples was defined accordingto the recommendations of RILEM TC-200 HTC [5].Test procedureThe test system used is presented in Fig. 1. It consisted ofan AMSLER compression machine with a capacity of 5 MN(a), a cylindrical furnace with an internal diameter of 90 mmand 300 mm in height, capable of temperatures up to 1200 °C(b) and a TML TDS-601 datalogger for data acquisition(force, displacement, and furnace and specimentemperatures) (c).The test procedures followed the RILEM TC-200 HTCrecommendations [5]. A load equal to 70 % of the designvalue of concrete compressive strength at room temperature(0.7 fcd) was initially applied to the specimen. When theload level was reached, the specimen was heated at a rate of3 °C/min, until the desired level of temperature was reached.Three maximum temperatures were tested: 300 °C, 500 °Cand 600 °C. The temperature was deemed to be reachedwhen the average temperatures registered by the three speci-men thermocouples on the surface matched the temperatureof the furnace. The specimen was then kept at that tempera-ture for an hour to stabilize. The temperature difference30 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 49  No. 1/2009fc[MPa]fcm[MPa]fck[MPa]Resistance Class Compression load (0.7 fcd)[kN]HSC73.0471.33 63.33 C50/60 130.572.2568.69HSCPSF75.7976.21 68.21 C55/67 140.677.1875.65HSCGF65.1964.16 56.16 C45/55 115.857.0470.23Table 2: Compressive resistance classesFig. 1: Test systeminside and outside the specimen, given by thermocouples atthe same level, was also ascertained. This difference shouldnot be greater than 20 °C.ResultsFig. 2 shows a graph plotting the variation of concretecompressive strength vs the maximum temperature that thespecimens were subjected to in the experimental tests.In tests carried out up to 300 °C, all specimens had a slightloss of strength. The HSC samples had a strength loss of 5 %while the HSCPSF and HSCGF samples lost 10 %.In the 500 °C tests, the HSC and HSCPSF specimens lostabout 30 % while the HSCGF samples lost approximately26 % of their strength at room temperature.In the maximum temperature tests, 600 °C, all the speci-mens collapsed before temperature stabilization. This mayindicate that the initial compressive load applied to the speci-mens was excessive for this temperature.There was a less explosive failure in the HSCPSF speci-mens, which confirms that steel fibers achieve a more ductileconcrete.Explosive spalling was not observed in any of the heat-ing-cooling tests, only some cracks. Cracking was worse fortests up to 500 °C. At this temperature, the HSCSPF speci-mens suffered exfoliation at the top.In 300 °C tests the HSC specimens showed only somesurface cracking.2.2 Brazilian TestsTests to assess the stress-strain behavior of polypropylenefiber reinforced high performance concrete at ambient tem-perature (27 °C) and at high temperatures of 400, 650 and900 °C, were carried out in COPPE’s Laboratory of Structuresat the Federal University of Rio de Janeiro, Brazil [6].The residual compressive strength and elastic moduluswere determined after the concrete was subjected to a heatingand cooling process.The role of polypropylene fiber in controlling the spallingof high performance concrete (HPC) was also investigated.Concrete compositionThe concrete was made with Portland cement (CPIII-40),sand with a fineness modulus of 2.70, crushed syenite witha maximum size of 9.50 mm and a specific weight of2.70 g/cm3, naphthalene sulfonate based superplasticizer (SP)with a total amount of solid particles of 40 % and silicafume. The polypropylene fibres (PF) were 40 mm long andhad a specific weight of 0.91 kg/dm3 and an elasticmodulus of3500 MPa.©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 31Acta Polytechnica Vol. 49  No. 1/2009Fig. 2: Concrete compressive strength vs maximum temperatureSeries Cement[kg]Silica[kg]Sand[kg]Agg.[kg]Water[l]SP[l]Fibres[kg]C65 365 37 780 857 156 8.30 –C85 414 42 694 895 151 8.49 –C65PF0.25 365 37 780 857 156 8.30 2.28C65PF0.5 365 37 780 857 156 8.30 4.56C85PF0.25 414 42 694 895 151 8.49 2.28C85PF0.5 414 42 694 895 151 8.49 4.56Table 3: Mix proportions for the HPC tested (per m3)The mixtures were produced so as to reach compressivestrength levels of 65 MPa (C65) and 85 MPa (C85) at 28 days.They were reinforced with 0.25 % and 0.5 % by volume ofpolypropylene fibers (C65PF0.25/C85PF0.25 and C65PF0.5/C85PF0.5 series, respectively).The mix proportions of the mixtures are summarized inTable 3.Specimens and test procedureThe compression tests were carried out on cylindricalspecimens 100 mm×200 mm. A 2500 kN MTS compressionmachine was used, at a loading rate of 0.00078 mm/s. Threesamples were tested for each mixture. Average values weretaken as representative for each analyzed mixture.Prismatic specimens (150 mm×260 mm×100 mm) wereused in the spalling and total porosity studies. They wereheated in a computer-controlled electric furnace at a rate of10 °C/min. Three maximum temperatures (400 °C, 650 °Cand 900 °C) were chosen. Once the peak temperature wasreached it was maintained for one hour and then cooled toroom temperature at a rate of (0.4–0.5) °C/min.Total porosity was measured by means of water absorptiontests on cylindrical specimens (60 mm in height, with a diam-eter of 25.4 mm) extracted from the prismatic samples.ResultsTypical compressive stress-strain curves for the plain andPF reinforced HPC matrices at room temperature and afterexposure to high temperatures are presented in Fig. 3.At room temperature, the addition of small volume frac-tions of PF to the concrete did not significantly change thebehavior in compression of the matrices.The results for the specimens that were subjected to tem-peratures of 400 °C show that the PF HPC specimens suffereda more pronounced loss of strength and rigidity than theplain HPC specimens. At 650 °C and 900 °C all concretespecimens experienced a similar strength and rigidity loss,regardless of the presence of fibers in the mixtures.The poor behavior of the PF concrete specimens is associ-ated with the melting of polypropylene fibers at about 170 °C.A slight dilatation of about 10 % [3] of the polypropyleneoccurs during melting, which generates extra pore-pressurein the concrete and leads to a higher crack density than thatobserved in the plain concrete. The fiber beds left in the ma-trix after melting may also help in the local nucleation ofcracks as they have sharp angles, favoring the propagation ofmicrocracking.In this study, spalling was observed in C85 prismatic con-crete samples for the 400 °C temperature series. The spallingoccurred at about 200 °C–220 °C. The addition of 0.25 % byvolume of PF to the C85 matrix prevented concrete spalling.3 ConclusionsIn the Portuguese studies, it could be concluded that theinclusion of PF fibers in the concrete compositions preventedspalling. The specimens with steel and polypropylene fibersperformed better than those with glass fibers. A small detach-ment of surface concrete was observed in the glass fiberspecimens.A more explosive rupture occurred in specimens withoutfibers and in those with glass fibers. The benefit of steel fibersin controlling cracking was confirmed.The incorporation of shorter steel fibers and a smallamount of polypropylene fibers conferred greater strengthon the concrete specimens.In the Portuguese compression tests the concretes withglass fibers exhibited behavior quite similar to that of thespecimens with polypropylene and steel fibers. The loss ofstrength of HSCGF specimens at high temperatures wasonly slightly lower than for the HSCPSF specimens. It wasconcluded that glass fibers did not improve concrete perfor-mance at high temperatures. However, further studies withthis type of fiber are needed, since only a small number oftests were carried out in this study.In the Brazilian studies it was concluded that at high tem-peratures the PF HPC specimens showed a more pronouncedstrength and rigidity loss than the plain HPC specimens. Theincrease in porosity was also higher for the PF HPC. This be-havior can be associated with melting of PF at about 170 °C.Another conclusion from this study was that the additionof PF to the C85 matrix prevented concrete spalling.32 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 49  No. 1/2009T = 27 °CT = 400 °CT = 650 °CT = 900 °CT = 27 °CT = 400 °CT = 650 °CT = 900 °C(a)T = 27 °CT = 400 °CT = 650 °CT = 900 °CT = 27 °CT = 400 °CT = 650 °CT = 900 °C(b)Fig. 3: Compressive stress-strain curves at room temperature after exposure to high temperatures: (a) Concrete series C85; (b) Compos-ite series C85PF0.25Given these findings, we can state that PF preventsthe spalling of concrete. These fibers melt at approximately170 °C and are then partially absorbed by the microcrackedcement matrix, leaving a pathway for gas to dissipate and soreducing water vapor pore pressure.References[1] Kalifa, P., Menneteau, F., Quenard D.: Spalling andPore Pressure in HPC at High Temperatures, Cementand Concrete Research, Vol. 30 (1999), p. 1915–1927.[2] Khoury, G. A.: Effect of Fire on Concrete and ConcreteStructures. Prog. Struct. Engng Mater., Vol. 2 (2000),p. 429–447.[3] Kalifa, P., Chene, G., Galle, C.: High-Temperature be-haviour of HPC with Polypropylene Fibres. FromSpalling to Microstructure, Cement and Concrete Research,Vol. 31 (2001), p. 1487–1499.[4] Santos, S., Rodrigues, J. P.: Spalling on Concrete Struc-tures, Proceedings of the National Conference on StructuralConcrete, 2008, Guimarnes, Portugal.[5] Recommendations of RILEM TC 200-HTC : Mechani-cal Concrete Properties at High Temperature – Model-ing and Applications. Materials and Structures, Vol. 38(2005), p. 913–919.[6] Velasco, R. V., Toledo, R. D., Fairbairn, E. M. R., Lima,P. R. L., Neumann, R.: Spalling and Stress-Strain behaviourof Polypropylene Fibre Reinforced HPC after Exposure to HighTemperatures.Susana Otero Santose-mail: susana.otero.s@gmail.comJoao Paulo C. RodriguesFaculty of Science and Technology of University ofCoimbra, PortugalRomildo ToledoR. V. VelascoFederal University of Rio de JaneiroDepartment of Civil Engineering, COPPERio de Janeiro, Brazil©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 33Acta Polytechnica Vol. 49  No. 1/2009~

Compressive behaviour at High Temperatures of Fibre Reinforced Concretes

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