polymers
Article Transient Thermal Tensile Behaviour of Novel Pitch-Based Ultra-High Modulus CFRP Tendons
Giovanni Pietro Terrasi 1,2,*, Emma R. E. McIntyre 2, Luke A. Bisby 2, Tobias D. Lämmlein 1 and Pietro Lura 3,4
1 Mechanical Systems Engineering Laboratory, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland; [email protected] 2 BRE Centre for Fire Safety Engineering, Institute for Infrastructure & Environment, School of Engineering, University of Edinburgh, The King’s Buildings Edinburgh, EH93JL Scotland, UK; [email protected] (E.R.E.M.); [email protected] (L.A.B.) 3 Concrete and Construction Chemistry Laboratory, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland; [email protected] 4 Institute for Building Materials (IfB), ETH Zurich, 8092 Zurich, Switzerland * Correspondence: [email protected]; Tel.: +41-587-654-117; Fax: +41-587-656-911
Academic Editors: Alper Ilki and Masoud Motavalli Received: 24 October 2016; Accepted: 6 December 2016; Published: 21 December 2016
Abstract: A novel ultra-high modulus carbon fibre reinforced polymer (CFRP) prestressing tendon made from coal tar pitch-based carbon fibres was characterized in terms of high temperature tensile strength (up to 570 ◦C) with a series of transient thermal and steady state temperature tensile tests. Digital image correlation was used to capture the high temperature strain development during thermal and mechanical loading. Complementary thermogravimetric (TGA) and dynamic mechanical thermal (DMTA) experiments were performed on the tendons to elucidate their high temperature thermal and mechanical behaviour. The novel CFRP tendons investigated in the present study showed an ambient temperature design tensile strength of 1400 MPa. Their failure temperature at a sustained prestress level of 50% of the design tensile strength was 409 ◦C, which is higher than the failure temperature of most fibre reinforced polymer rebars used in civil engineering applications at similar utilisation levels. This high-temperature tensile strength shows that there is potential to use the novel high modulus CFRP tendons in CFRP pretensioned concrete elements for building applications that fulfill the fire resistance criteria typically applied within the construction industry.
Keywords: carbon fibre reinforced polymers (CFRPs); high elastic modulus; tensile properties; high temperature behaviour; dynamic mechanical thermal analysis (DMTA); thermogravimetric analysis (TGA)
1. Introduction In the last decade, the precast concrete industry has been progressively developing a range of advanced thin-walled concrete elements utilizing high-performance, self-consolidating concrete reinforced (or prestressed) with high-strength, lightweight, and non-corroding carbon fibre reinforced polymer (CFRP) grids or tendons [1–4]. Due in particular to the stress corrosion resistance of CFRP reinforcements, the concrete cover is not determined by corrosion protection or carbonation issues, as is the case for steel reinforcement and prestressing, and the concrete cover in these bending elements can therefore be reduced to a minimum (e.g., 10–20 mm), leading to lighter and more durable precast elements. The concrete cover size is now determined by static considerations (reception of the compressive stresses due to bending and of the splitting tensile stresses in the prestress transfer zone of prestressing tendons at the beam ends) and by the mismatch in thermal expansion coefficient
Polymers 2016, 8, 446; doi:10.3390/polym8120446 www.mdpi.com/journal/polymers Polymers 2016, 8, 446 2 of 15 between CFRP (transverse to fibre direction [1]) and high-performance concrete [5]. One example of this trend is a new type of precast CFRP pretensioned concrete element intended as load-bearing panels for glass/concrete building facades [1]. Adequate load carrying capacity during exposure to fire is crucial for applications of CFRP precast concrete structural elements in both the interior and exterior of buildings. Adequate fire resistance is a legitimate concern for these novel precast CFRP reinforced high strength concrete elements, since it is known that the tensile strength of CFRP and the bond strength between CFRP reinforcing tendons and concrete deteriorate comparatively quickly at elevated temperature [6,7]. It is also widely known that high-strength concrete show an increased propensity for heat-induced explosive spalling failure when subjected to fire [8]. The tensile and bond strength decreases experienced in a fire situation, and their impacts on the load-carrying capacity of reinforced and prestressed concrete structures, remain largely unknown particularly for prestressing applications. The spalling behaviour of high-strength concrete can be effectively mitigated by advanced concrete mix design incorporating a suitable quantity and type of polypropylene microfibre [9]. This paper provides insights into the high temperature thermal and tensile strength behavior of novel UHM CFRP tendons that have the potential to effectively reinforce/prestress high-strength concrete elements. The high modulus of the carbon fibers in particular allows considerable load transfer to the fibers in concrete at low strains of the composite (assuming an adequate and durable bond), making these materials suited for reinforcement and prestressing applications in durable, thin-walled concrete precast elements. A tensile testing machine equipped with a thermal chamber was used to test the novel prestressing tendons under both transient and steady-state high temperature conditions (between 300 and 570 ◦C). Digital image correlation (DIC) enabled the assessment of strains in the CFRP during elevated temperature testing. The aim of these experiments was to characterize the high-temperature tensile response of a specific, novel ultra-high modulus (UHM) pitch-based carbon fibre reinforced polymer (CFRP) tendon and to compare the observed behaviour against that of conventional CFRP prestressing tendons based on polyacrylonitrile (PAN) carbon fibres, as well as to conventional prestressing steel wires. Additional thermo-mechanical analysis (DMTA) and thermogravimetric (TGA) experiments were undertaken to shed further light on the high temperature tensile strength behaviour of the novel reinforcing bars.
2. Materials Tested New pitch-based high modulus carbon fibres with an attractive performance/price ratio are increasingly available for the construction materials market. Examples of such fibres are given in [9], describing industrial-grade fibre rovings composed of 12,000–16,000 single carbon filaments, with an elastic modulus of 640–790 GPa and a tensile strength varying between 2600 and 3200 MPa. Their cost is approximately 50 Euro/kg, which is twice the cost for conventional high strength carbon fibres. Considering that the cost of a carbon fibre reinforced polymer (CFRP) tendon with diameter 5.4 mm is approximately 4 Euro/m when using conventional high strength carbon fibres, we can estimate the price impact of the new high modulus fibres. If we assume that the novel UHM tendons can be produced by pultrusion, the resulting pultruded tendon price would increase approximately to 4.7 Euro/m, which is a 17% increase in price (at a 20%–30% lower tensile capacity). The high tensile strength, corrosion resistance, and low density (2.15 g/cm3 [10]) of these fibres make them attractive for structural engineering applications. The high modulus of the fibres in particular allows considerable load transfer to the carbon fibres in hybrid material systems at low strains/deformations of the composite (assuming an adequate and durable bond). The drawback of the pitch-based fibres is a comparatively low failure strain for the novel fibres and therefore for tendons manufactured from these fibres; this is between 0.3% and 0.4% [10] and must be carefully considered in design of an FRP prestressed structure when using these materials. It is noteworthy that prestressing steel has drastically Polymers 2016, 8, 446 3 of 15 different tensile behaviour, and is typically considered an ideally elastic-plastic reinforcement in design, Polymers 2016, 8, 446 3 of 15 with a plastic limit strain of between 0.1% and 0.2%. The study presented in this paper was focused on a novel type of CFRP tendon, for possible use in civil engineering concrete prestressing applicationsapplications [[11],11], composed of 66% by volume Mitsubishi DIALEADDIALEAD™™ K13916 fibres fibres [[9].9]. The The novel novel CFRP CFRP pres prestressingtressing tendons tendons have have a a diameter of 5.3 mm Polymers 2016, 8, 446 3 of 15 (with aa standardstandard deviationdeviation of of± ±0.10.1 mm)mm) andand werewere produced produced batch batch wise wise by by a tape-layinga tape-laying method method [12 [12]] at aat totala total length Thelength study of 3.2of presented m3.2 per m per tendon. in tendon. this Topaper accomplish To was accomplish focused this, on unidirectionalthis, a novel unidirectional type of pre-impregnated CFRP pre-impregnated tendon, for possible (prepreg) (prepreg) use layers werelayersin laid were civil between engineeringlaid between two pinsconcrete two and prestressing thenpins wrappedand then applicat in wra theionspped hoop [11], in direction composed the hoop with of direction 66% a carbon by volume with fibre rovingMitsubishia carbon to press fibre themrovingDIALEAD™ transversely to press them K13916 and transversely form fibres a [9]. cylindrical andThe formnovel geometry. aCFRP cylindrical pres Thetressing geometry. commercial tendons The have preimpregnated commercial a diameter preimpregnated of fibre 5.3 mm rovings usedfibre (with hadrovings an a standard epoxy usedpolymer deviationhad an matrix epoxyof ±0.1(huntsman mm)polymer and were matrix XB3515/AD5021 produced (huntsman batch [wise13 XB3515/AD5021]), by and a tape-laying were hardened [13]), method forand [12] 1.5 were h at ◦ ◦ 100hardenedCat followeda total for length1.5 byh at 2of h100 3.2 at m°C 140 per followed Ctendon. before by To demolding. 2accomplish h at 140 °C Thethis, before tendons’unidirectional demolding. surface pre-impregnated The was tendons’ subsequently (prepreg) surface coated was layers were laid between two pins and then wrapped in the hoop direction with a carbon fibre withsubsequently a lamination coated epoxy with (araldite a lamination LY 5052 epoxy [13]), (araldit and, toe LY promote 5052 [13]), a strong and, bond to promote with the a surroundingstrong bond concrete,roving the to tendons press them were transversely sand coated and by form broad-casting a cylindrical silicageometry. sand The (with commercial a mean diameterpreimpregnated of 0.5 mm). with fibrethe surrounding rovings used concrete,had an epoxythe tendons polymer were matrix sand (huntsman coated by XB3515/AD5021 broad-casting [13]), silica and sand were (with a The coating was hardened for two additional hours at 125 ◦C. The resulting novel UHM CFRP meanhardened diameter for of 1.5 0.5 h atmm). 100 °C The followed coating by was2 h at hardened 140 °C before for demolding.two additional The tendons’ hours surfaceat 125 was°C. The prestressing tendons are shown in Figure1a. resultingsubsequently novel UHM coated CFRP with prestressinga lamination epoxytendons (araldit are eshown LY 5052 in [13]), Figure and, 1a. to promote a strong bond with the surrounding concrete, the tendons were sand coated by broad-casting silica sand (with a mean diameter of 0.5 mm). The coating was hardened for two additional hours at 125 °C. The resulting novel UHM CFRP prestressing tendons are shown in Figure 1a.
(a)(b)
Figure 1. (a) Novel sand coated ultra-high modulus (UHM) pitch-based carbon fibre reinforced Figure 1. (a) Novel sand coated ultra-high(a)( modulus (UHM) pitch-basedb) carbon fibre reinforced polymer (CFRP)polymer and (CFRP) (b) conventional and (b) conventional polyacrylonitrile polyacrylonitrile (PAN)-based (PAN)-based CFRP tendon. CFRP tendon. Figure 1. (a) Novel sand coated ultra-high modulus (UHM) pitch-based carbon fibre reinforced Thesepolymer tendons (CFRP) behave and ( blinear) conventional elastically polyacrylonitrile until tens (PAN)-basedile failure (FigureCFRP tendon. 2), have an average tensile These tendons behave linear elastically until tensile failure (Figure2), have an average tensile strength of 1561 MPa (with a standard deviation of ±68.05 MPa from six tensile tests) and an average strength ofThese 1561 tendons MPa (with behave a standard linear elastically deviation until of tens±68.05ile failure MPa from(Figure six 2), tensile have an tests) average and antensile average E11 modulus (measured following [14]) of 509 GPa (±13.49 GPa). The resulting Design Tensile E modulusstrength of (measured 1561 MPa (with following a standard [14]) deviation of 509 of GPa ±68.05 (± 13.49MPa from GPa). six tensile The resulting tests) and Designan average Tensile Strength11 (DTS) is 1400 MPa. The DTS was calculated from the results of the six tensile tests shown in StrengthE11 modulus (DTS) is (measured 1400 MPa. following The DTS [14]) was of calculated 509 GPa from(±13.49 the GPa). results The of resulting the six tensileDesign testsTensile shown Figure 2 using the average measured strength of 1561 MPa (Table 1), the standard deviation of 68.05 in FigureStrength2 using (DTS) the is 1400 average MPa. The measured DTS was strength calculated of fr 1561om the MPa results (Table of the1), six the tensile standard tests shown deviation in MPa Figureand assuming 2 using the a normalaverage measureddistribution strength of the of te 15nsile61 MPa strengths (Table 1), (which the standard is one deviationpossible ofassumption 68.05 of 68.05 MPa and assuming a normal distribution of the tensile strengths (which is one possible followingMPa and[15]). assuming The DTS a normalwas defined distribution as the of average the tensile tensile strengths strength (which minus is one 2.3 possible standard assumption deviations, assumption following [15]). The DTS was defined as the average tensile strength minus 2.3 standard correspondingfollowing [15]). to the The 2.1 DTS percentile. was defined as the average tensile strength minus 2.3 standard deviations, deviations,corresponding corresponding to the 2.1 to percentile. the 2.1 percentile.
Figure 2. Tensile stress versus strain response of new UHM CFRP tendons (6 specimens labelled Figure 2. Tensile stress versus strain response of new UHM CFRP tendons (6 specimens labelled K13916 S1 to S6), compared with a conventional CFRP prestressing tendon [8]. K13916 S1 to S6), compared with a conventionalconventional CFRP prestressingprestressing tendontendon [[8].8].
Polymers 2016, 8, 446 4 of 15
Table 1. Comparison of the tensile properties of UHM CFRP tendons versus standard CFRP tendons. Polymers 2016, 8, 446 4 of 15 Fibre volume E [14] E σ σ σ Material 11 calc. 11 exp. 11 max calc. 11 max exp. 11 failure exp. Table 1. Comparison of the tensilecontent properties (%) of (GPa)UHM CFRP tendons(GPa) versus[14] (MPa)standard CFRP(MPa) tendons. (%) UHM CFRP tendon K13916 66 502 509 2112 1561 0.30 Fibre volume E11 calc. [14] E11 exp. σ11 max calc. σ11 max exp. σ11 failure exp. Standard CFRP tendonMaterial UTS5631 [1] 65 150 156 3096 2471 1.54 content (%) (GPa) (GPa) [14] (MPa) (MPa) (%) UHM CFRP tendon K13916 66 502 509 2112 1561 0.30 FigureStandard3 aCFRP shows tendon the UTS5631 brittle [1] failure that 65 was observed 150 during 156 tensile 3096 testing of 2471 the CFRP tendons 1.54 at the loaded end of the resin-cast anchorage that was used to grip the tendon during testing. A close-up Figure 3a shows the brittle failure that was observed during tensile testing of the CFRP tendons of theat brittle the loaded CFRP end failure of the surface resin-cast at theanchorage same location that was is used given to ingrip Figure the te3ndonb. This during failure testing. mode, A and the resultingclose-up tensileof the brittle capacity CFRP of failure the tendons, surface wasat the clearly same influencedlocation is given by the in complexFigure 3b. stress This failure state at the loadedmode, end and of the the resinresulting cast tensile anchorage capacity system of the [tend11].ons, A combination was clearly influenced of tensile by stress the complex along the stress tendon axis, astate superimposed at the loaded compression end of the resin (clamping) cast anchorage stress system transverse [11]. A to combination the tendon of axis, tensile and stress a peak along in shear stressthe at tendon the entrance axis, a ofsuperimposed the tendon compression to the anchorage (clamping) caused stress a premature transverse to and the brittle tendon shearing-off axis, and a the CFRPpeak at this in shear location. stress A at potential the entrance solution of the totend theon pronounced to the anchorage strength caused anisotropy a premature and and matrix-bond brittle problemshearing-off of the UHM the CFRP fibres at [ 15this] would location. be A to potentia designl a solution tensile anchorageto the pronounced system strength and corresponding anisotropy test and matrix-bond problem of the UHM fibres [15] would be to design a tensile anchorage system and setup which leads to smaller transverse stress peaks. Analysis of bending tests of under-reinforced corresponding test setup which leads to smaller transverse stress peaks. Analysis of bending tests of concreteunder-reinforced beams prestressed concrete with beams these prestressed UHM tendons with these (Empa, UHM unpublished tendons (Empa, results) unpublished showed results) that when designingshowed for that the tensilewhen designing failure of for the the CFRP tensile tendon, failure tensile of the stresses CFRP tendon, in the outermosttensile stresses tendon in reachedthe levelsoutermost around 1900tendon MPa reached at flexural levels around failure of1900 the MPa beam. at flexural failure of the beam.
(a) (b)
Figure 3. (a,b) Brittle failures of two UHM CFRP tendons at the load entrance of the resin-cast metallic Figure 3. (a,b) Brittle failures of two UHM CFRP tendons at the load entrance of the resin-cast metallic barrel anchor used to anchor the tendons during the tensile tests. barrel anchor used to anchor the tendons during the tensile tests. According to the rule of mixtures for unidirectional fibre reinforced polymers [15], and Accordingconsideringto that the rulethe Mitsubishi of mixtures DIALEAD™ for unidirectional K13916 fibre carbon reinforced fibres polymershave a guaranteed [15], and consideringtensile that thestrength Mitsubishi of 3200 DIALEAD MPa [10],™ theK13916 theoretical carbon tensile fibres strength have a of guaranteed the UHM tensileCFRP tendons strength is of 0.66 3200 × 3200 MPa [10], the theoreticalMPa = 2112 tensile MPa, strengthwhich is of35% the higher UHM than CFRP the tendonsaverage istensile0.66 ×strength3200 MPa measured = 2112 in MPa the, tests which is described above. This is likely due to the observed premature failure mode next to the anchors (see 35% higher than the average tensile strength measured in the tests described above. This is likely due to Table 1). A similar approach shows that the experimentally-determined tensile elastic (E11) modulus the observed premature failure mode next to the anchors (see Table1). A similar approach shows that of the novel tendons (509 GPa) is accurately estimated with the rule of mixtures [15], which suggests the experimentally-determinedto 0.66 × 760 GPa = 502 GPa. tensile elastic (E11) modulus of the novel tendons (509 GPa) is accurately estimatedThe with thermomechanical the rule of mixtures behaviour [15], which of suggeststhe new to UHM 0.66 × 760tendons GPa =was 502 compared GPa. against Theconventional thermomechanical CFRP tendons behaviour used for of theconcrete new UHMpre-tensioning tendons wasapplications compared [1,6,11]. against The conventional more CFRPconventional tendons used CFRP for tendons concrete have pre-tensioning a nominal diameter applications of 5.4 mm [1, 6(±0.1,11]. mm) The and more were conventional produced by CFRP tendonspultrusion. have a nominalPAN based diameter carbon fibres of 5.4 in mm the (conven±0.1 mm)tional and tendons were were produced TENAX™ by pultrusion. UTS 5631 [16] PAN at a based carboncarbon fibres fibre in volume the conventional fraction of 65%. tendons These weretendons TENAX had an™ epoxyUTS poly 5631mer [16 resin] at matrix a carbon consisting fibre volume of fractionBakelite of 65%. Ruetapox These VE tendons 4434 resin had [17]. an epoxy A sand polymer coating was resin also matrix applied consisting to the surface of Bakelite of these Ruetapox bars by VE in-line spray-coating silica sand onto a coating of epoxy resin (araldite LY 5052 [13]), applied after 4434 resin [17]. A sand coating was also applied to the surface of these bars by in-line spray-coating the primary pultrusion and curing process, as described previously, and resulting in a coating silica sand onto a coating of epoxy resin (araldite LY 5052 [13]), applied after the primary pultrusion approximately 0.5 mm thick. The CFRP tendons’ average nominal tensile strength was 2471 MPa and curing process, as described previously, and resulting in a coating approximately 0.5 mm thick. (standard deviation of ±168 MPa from 45 tensile tests), with an elastic modulus E11 [14] of 156 GPa. The CFRPThe tensile tendons’ stress-strain average relationship nominal tensile for the strength reference was CFRP 2471 tendons MPa (standard was linear-elastic deviation to of failure±168 MPa from(Figure 45 tensile 2), and tests), displayed with an an elastic average modulus ultimateE 11strain[14 ]of of 1.54% 156 GPa. [1]. Both The CFRP tensile tendons stress-strain studied relationship herein for the reference CFRP tendons was linear-elastic to failure (Figure2), and displayed an average
Polymers 2016, 8, 446 5 of 15 ultimate strain of 1.54% [1]. Both CFRP tendons studied herein have a nominal mass of 0.123 kg/m, which is about half the mass per unit length of a standard 6 mm diameter cold-drawn steel prestressing Polymers 2016, 8, 446 5 of 15 wire [18], and about 47% of the mass per unit force. A sample of the reference CFRP prestressing tendonhave is showna nominal in mass Figure of1 0.123b. kg/m, which is about half the mass per unit length of a standard 6 mm diameter cold-drawn steel prestressing wire [18], and about 47% of the mass per unit force. A sample 3. Experimentalof the reference Programme CFRP prestressing tendon is shown in Figure 1b.
The3. Experimental primary aim Programme of the study presented herein was to experimentally characterize the transient elevated temperature tensile behaviour of the novel pitch-based carbon fibre reinforced epoxy tendons describedThe in theprimary previous aim of section.the study It presented was of interestherein wa tos compareto experimentally the response characterize of the the novel transient tendons elevated temperature tensile behaviour of the novel pitch-based carbon fibre reinforced epoxy with the behaviour [6] of the conventional normal modulus CFRP prestressing tendons based on tendons described in the previous section. It was of interest to compare the response of the novel Polyacrylonitriletendons with(PAN) the behaviour carbon fibres.[6] of the Two convention sets of high-temperatureal normal modulus experiments CFRP prestressing were performed tendons for the newbased UHM on Polyacrylonitrile tendons in a materials (PAN) carbon testing fibres. frame Two equipped sets of with high-temperature a high temperature experiments environmental were chamber.performed The experiments for the new UHM consisted tendons of in 11 a transientmaterials elevatedtesting frame temperature equipped with tensile a high tests—In temperature which the CFRPenvironmental was loaded to chamber. a sustained The loadexperiments and then consisted heated underof 11 transient constant elevated load until temperature failure—Followed tensile by two steadytests—In state which high the temperature CFRP was loaded tensile to tests a sustained of the new load UHM and then tendons heated at under 300 ◦C, constant in which load the until tendons werefailure—Followed heated to a constant by two temperature steady state and high then temper loadedature under tensile constanttests of the temperature new UHM tendons until failure. at 300 The°C, in total which length the tendons of each were tensile heated specimen to a consta wasnt 810temperature mm, which and then led toloaded a free under testing constant length of 500 mmtemperature between until the failure. 140 mm long steel barrel anchorages (Figure4). The crucial issue of gripping The total length of each tensile specimen was 810 mm, which led to a free testing length of 500 the tendons, which is far from straightforward for this type of tendon, was addressed by epoxy mm between the 140 mm long steel barrel anchorages (Figure 4). The crucial issue of gripping the bondingtendons, high which strength is far steelfrom straightforward sockets (Grade for ETG this 100type [of19 tendon,]) to both was tendon addressed ends. by epoxy After bonding spraying a polytetrafluoroethylenehigh strength steel (PTFE)sockets separating(Grade ETG layer 100 on [1 the9]) anchorages’to both tendon inner ends. surfaces, After the spraying CFRP tendona specimenspolytetrafluoroethylene were centered and (PTFE) fixed separating with a silicone layer on dropthe anchorages’ at the load inner entrance surfaces, of thethe CFRP steel anchorages.tendon A low-viscosityspecimens were lamination centered epoxy and fixed resin with (Araldite a silicone LY dr 5052op at [13 the]) wasload thenentrance cast of into the thesteel conical anchorages. interior of the cylindricalA low-viscosity steel socketslamination [5]. epoxy Half resin of the (Araldite steel anchorages LY 5052 [13]) were was ground then cast to into a conical the conical shape interior at the load entranceof the (Figures cylindrical3a and steel4 )sockets in order [5]. to Half make of the this stee regionl anchorages of the socket were ground softer, to which a conical gave shape better at the results whenload performing entrance (Figures tensile tests3a and at 4) ambient in order temperature to make this region [5]. An of M36the socket ISO metric softer, screwwhich gave thread better was cut results when performing tensile tests at ambient temperature [5]. An M36 ISO metric screw thread into the external surface of the steel anchorages in order to fix them to steel adaptors that were then was cut into the external surface of the steel anchorages in order to fix them to steel adaptors that clamped into the wedge action grips of the materials testing frame, as shown in the highlighted area of were then clamped into the wedge action grips of the materials testing frame, as shown in the Figurehighlighted5a. area of Figure 5a.
Figure 4. (a) Setup to cast epoxy grout anchorages; (b) Six CFRP specimens with testing anchors Figure 4. (a) Setup to cast epoxy grout anchorages; (b) Six CFRP specimens with testing anchors installed. installed.
Polymers 2016, 8, 446 6 of 15 Polymers 2016, 8, 446 6 of 15
FigureFigure 5. (a )5. Test (a) Test setup setup with with DIC DIC camera camera in in front front ofof Instron 600 600 LX LX tensile tensile testing testing machine machine equipped equipped withwith environmental environmental chamber; chamber; the upperthe upper tendon’s tendon’s anchorage anchorage clamping clamping device device is highlighted;is highlighted; (b ()b Detail) showingDetail a CFRPshowing specimen a CFRP with specimen three attached with thermocouplesthree attached andthermocouples the DIC patch and alignment/numbering the DIC patch for strainalignment/numbering measurement. for strain measurement.
A high-capacity materials testing frame at the University of Edinburgh’s BRE Centre for Fire A high-capacity materials testing frame at the University of Edinburgh’s BRE Centre for Fire Safety Engineering (Edinburgh, Scotland, UK), which includes an in-line environmental chamber, Safetywas Engineering used for the (Edinburgh, tensile tests Scotland,(Figure 5). UK), The whichsetup consisted includes of an an in-line Instron environmental 600 LX static hydraulic chamber, was useduniversal for the tensile testing tests machine (Figure (Instron,5). The Norwood, setup consisted MA, USA) of anwith Instron a tension/compression 600 LX static hydraulic load capacity universal testingof machine600 kN, equipped (Instron, with Norwood, an Instron MA, CP103790 USA) with high atemperature tension/compression environmental load chamber capacity (Instron, of 600 kN, equippedNorwood, with MA, an Instron USA) capable CP103790 of cove highring temperature a temperature environmental range from −70 chamber to +620 °C (Instron, (temperature Norwood, MA, USA)stability capable ±2 °C, heating of covering rate 10 a temperature°C/min). The anchor rangeages from of− the70 tensile to +620 specimens◦C (temperature were clamped stability using± 2 ◦C, heatingspherically rate 10 ◦seatedC/min). wedge The anchoragesaction grips ofwith the tensile150 kN specimens capacity wereboth clampedabove and using below spherically the seatedenvironmental wedge action chamber. grips with 150 kN capacity both above and below the environmental chamber. The transient thermal tensile tests were performed at a crosshead stroke rate of 0.5 mm/min up The transient thermal tensile tests were performed at a crosshead stroke rate of 0.5 mm/min up to the predefined prestress level, and three representative prestress levels were studied; namely to the predefined prestress level, and three representative prestress levels were studied; namely 40%, 40%, 50%, and 60% of the tendon’s design Ultimate Tensile Strength of 1400 MPa. These prestress 50%,levels and 60% are ofrepresentative the tendon’s designof the Ultimatelong-term Tensilein service Strength stresses of 1400expected MPa. for These CFRP prestress tendons levels in are representativepre-tensioned of the precast long-term concrete in serviceelements stresses [1] in pr expectedactice. The for CFRP CFRP tendons tendons were in pre-tensioned thus stressed precastto concretecorresponding elements service [1] in practice.stresses of The560, 700, CFRP and tendons 840 MPa, were with thusthree stressedor four specimens to corresponding tested at each service stressesstress of level, 560, 700,and then and heated 840 MPa, at a withrate of three 10 °C/min or four under specimens a sustained tested load atcontrol each mode stress until level, failure and then heated(or at until a rate theof maximum 10 ◦C/min stable under temperature a sustained of 570 load ± 2 control°C of the mode environmental until failure chamber (or until was thereached). maximum stable temperatureFor the two of steady 570 ± state2 ◦ Ctests, of thethe environmentalUHM specimens chamberwere prestressed was reached). to a small stress of only 100 ForMPa theduring two heating steady up state to 300 tests, °C, theagain UHM at a heating specimens rate wereof 10 °C/min, prestressed and then to a loaded small stressto failure of only under a crosshead stroke controlled loading at a rate of 0.5 mm/min. 100 MPa during heating up to 300 ◦C, again at a heating rate of 10 ◦C/min, and then loaded to failure One K-type ThermoCouple (TC) (denoted as TC2, Empa, Dübendorf, Switzerland) was under a crosshead stroke controlled loading at a rate of 0.5 mm/min. positioned in the gas phase at the center of the testing chamber, and was used to control the Onechamber’s K-type temperature. ThermoCouple Three (TC)additional (denoted type asK TCthermocouples2, Empa, Dübendorf, (denoted as Switzerland) TC1, TC3, and was TC4) positioned were in thetied gas onto phase the at CFRP the center specimen of the using testing 0.3 mm chamber, steel wires: and was TC1 usedat 95 tomm control and TC the3/TC chamber’s4 at 190 mm temperature. from Threethe additional top inner surface type K of thermocouples the temperature (denotedchamber (see as TCFigure1, TC 5).3 ,These and TCadditional4) were TCs tied were onto used the to CFRP specimen using 0.3 mm steel wires: TC1 at 95 mm and TC3/TC4 at 190 mm from the top inner surface of the temperature chamber (see Figure5). These additional TCs were used to monitor the temperature of the tendon and the uniformity of temperature within the chamber and on the test specimens. Polymers 2016, 8, 446 7 of 15
A digital single lens reflex (DSLR) camera of type Canon 650d (18 Megapixel CMOS, 50mm fixed focal length lens, Canon Inc., Tokyo, Japan) was used for DIC analysis to measure the axial deformations of the tendons during the tensile tests (see Figure5). The camera was used to acquire high-resolution digital images of the sand-coated surface of the CFRP tendons, at a rate of 0.2 Hz. A custom-coded Matlab module called GeoPIV (Version RG, Queen’s University, Kingston, ON, Canada) was used for the DIC analysis; this was developed by Take et al. at Cambridge University, UK, and Queen’s University, Canada [20]. The initial DIC analysis was carried out with 32 × 32 pixel patches (~2.3 mm square in the images based on an average scale factor amongst the tests). Data from this analysis displayed considerable scatter, so an additional DIC analysis was performed using 64 × 64 pixel patches, which significantly improved the output. Thirteen patches were used in a vertical formation on each CFRP bar, as shown in Figure5b. The initial stress vs. DIC-derived strain behaviour showed an increase in stiffness during the loading phase, before starting the transient tensile thermal experiments. This indicated minor out-of-plane movement of the test specimen in the early stages of loading (i.e., during seating of the specimens). Linear extrapolation, using a tangent determined in the tensile stress range between 300 and 700 MPa, was used to correct for this error. Thermogravimetric analysis (TGA) (A Netzsch TG 209 F1, Netzsch GmbH, Selb, Germany) was performed on the UHM tendon material to characterise its thermal degradation in an unstressed state. Two uncoated samples of the CFRP tendon core material (discs of 3 mm diameter and approximately 1 mm height) were cut with a diamond saw from a tendon that had previously had its sand coating mechanically removed. The two samples were cleaned with acetone , had starting weights of 12.2 mg, and were conditioned in a storage room at 23 ◦C and 50% Relative Humidity (RH) before performing the TGA tests. A Netzsch TG 209 F1 Iris thermos-microbalance was used, with tests performed in air at a heating rate of 20 ◦C/min over the range from 25 to 900 ◦C. The testing precision was ±0.01 mg. Dynamic Mechanical Thermal Analysis (DMTA) was also performed on two specimens of the novel UHM tendons, to provide further insights into the thermomechanical response of the novel material and, crucially, to determine its glass transition temperature (Tg) by various possible definitions. A dynamic mechanical thermal analyser EPLEXOR 500 (Gabo Qualimeter GmbH, Ahlden (Aller), Germany), with built-in displacement sensors and a 500 N load cell, was used. For each 50 mm long specimen, two DMTA tests were performed in a 3-point bending mode in accordance with [21] (displacement controlled, 40 mm span). The static base load was chosen to induce a 0.2% tensile strain at mid-span, over which a dynamic displacement causing ±0.082% strain amplitude was superimposed at a frequency of 1 Hz. The temperature range was between 20 and 210 ◦C, at a heating rate of 2 ◦C/min. The sand coating was removed from the original tendons at their support locations and the specimens were stored in lab conditions before testing (1 month in a 23 ◦C/50% RH climate). The Tg values were calculated both using an E’ (storage modulus) onset definition, and using a peak tan δ definition [21].
4. Results Selected results of the transient thermal tensile tests are summarized in Table2. At 50% of DTS the average failure temperature of the new UHM tendons was 409 ◦C (from four repeat tests with a standard deviation of ±8.3 ◦C), while at 60% of DTS the average failure temperature decreased to 395 ◦C (three tests, ±9 ◦C). The failure temperature in these tests was defined as the average temperature of the measuring thermocouples attached to the tendon, at the moment when loss of prestress occurred; this was initiated by the successive tensile failure of the near-surface fibres (which were at this point ‘dry’ due to the epoxy matrix having pyrolysed). For the lowest prestress level of 560 MPa (corresponding to 40% of DTS), failure was not reached during any of the tests. Thus, after holding the load constant for 10 min at a temperature of 570 ◦C, it was decided to increase the tensile stress from 560 MPa up to failure (at a crosshead displacement rate of 1 mm/min) to determine the remaining tensile strength at that temperature. The resulting average remaining tensile strength was 644 MPa (±18 MPa); this corresponds to 46% of DTS. Polymers 2016, 8, 446 8 of 15
remaining tensile strength at that temperature. The resulting average remaining tensile strength was 644 MPa (±18 MPa); this corresponds to 46% of DTS. Polymers 2016 8 In the ,two, 446 steady-state tensile tests, the UHM specimens were prestressed to 100 MPa and8 then of 15 heated to 300 ± 5 °C. A crosshead displacement controlled loading was then applied until failure. The firstTable specimen 2. Results offailed transient at 1004 thermal MPa tensile (72% tests of DTS) of prestressed, whereas UHM the tendons. second n.a.failed means at 1037 not applicable. MPa (74% of DTS). Constant stress (MPa) (% of DTS) Mean failure temperature (◦C) (Stdv) CFRP strain at failure estimated by DIC (%) Table 2. Results560 (40%) of transient thermal Notensile failure tests at 570 of◦C- prestressed UHM tendons. n.a. means not 700 (50%) 409 (8.3) 0.18%–0.22% applicable.840 (60%) 395 (9.0) n.a. Constant stress (MPa) (% of DTS) Mean failure temperature (°C) (Stdv) CFRP strain at failure estimated by DIC (%) In the560 two (40%) steady-state tensile tests, No failure the at UHM 570 °C specimens were prestressed - to 100 MPa and 700 (50%) ± ◦ 409 (8.3) 0.18%–0.22% then heated840 to (60%) 300 5 C. A crosshead displacement 395 (9.0) controlled loading was n.a. then applied until failure. The first specimen failed at 1004 MPa (72% of DTS), whereas the second failed at 1037 MPa (74%Figure of DTS). 6 shows typical tensile strain versus temperature curves for one of the specimens loadedFigure at 50%6 shows of DTS typical (Specimen tensile strain 50c), versus with temperaturethe strains curvescaptured for oneusing of theDIC specimens by measuring loaded the at 50%increase of DTS in spacing (Specimen between 50c), with two thepatches strains (e.g., captured ‘13-1’ usingdenotes DIC the by average measuring strain the measured increase in between spacing betweenpatches no. two 13 patches and no. (e.g., 1 in ‘13-1’ Figure denotes 5b). It theis no averageteworthy strain that measured these data between are smoothed patches based no. 13 on and a no.LOcal 1 in regression Figure5b). (LOESS) It is noteworthy model [22]. that theseThe longitudin data are smoothedal tendon based strain on increases a LOcal regressionlinearly during (LOESS) the modelinitial loading [22]. The phase longitudinal to reach tendon the predefined strain increases prestress linearly level during corresponding the initial to loading the initial phase horizontal to reach theplateau predefined in the data prestress in Figure level 6. corresponding Thereafter, the to te themperature initial horizontal is increased plateau at a rate in the of data10 °C/min in Figure until6. ◦ Thereafter,failure, and the the temperature resulting strains is increased are evident. at a rate Given of 10 C/minthat the until fibres failure, in the and UHM the resulting specimens strains are arecarbon, evident. it was Given expected that the that fibres there in would the UHM be a specimenssmall strain are reduction carbon, it during was expected the heating that phase therewould of the beexperiment a small strain (since reduction the bars’ duringcoefficient the heatingof thermal phase expansion of the experiment in the fibre (sincelongitudinal the bars’ direction coefficient CTE of11 −6 thermalis slightly expansion negative, in i.e., the − fibre1.2 × longitudinal 10−6 in the range direction between CTE11 50is and slightly 125 negative,°C [10]). This i.e., − expectation1.2 × 10 inis ◦ theconfirmed range between by the data 50 and up to 125 temperaturesC[10]). This of expectation approximately is confirmed 250 °C. Figure by the 6 dataclearly up shows to temperatures a turning ◦ ofpoint approximately in the data 250in theC. Figuretemperature6 clearly range shows betw a turningeen 270 point and in320 the °C, data where in the the temperature strains begin range to ◦ betweenincrease on 270 further and 320 heating.C, where For all the specimens strains begin prestressed to increase at 50% on furtherof DTS heating.and investigated For all specimens with DIC prestressedduring testing, at 50%this oftemperature DTS and investigated range was determined with DICduring to be between testing, 260 this and temperature 360 °C based range on was the ◦ determinedminimum strain to be betweenvalue observed 260 and af 360ter theC based onset on of the heating. minimum Based strain on valuethe TGA observed data (see after Figure the onset 7), ofthis heating. range indicates Based on the TGAonset data of matrix (see Figure thermal7), this decomposition. range indicates It theis noteworthy onset of matrix that thermalthe TC decomposition.readings during It testing is noteworthy are surface that temperatures, the TC readings and during do not testing necessarily are surface represent temperatures, the tendon and core do nottemperature. necessarily The represent specimen the tendon failure core was temperature. signaled Theby specimenrapidly increasing failure was strains signaled starting by rapidly at ◦ increasingapproximately strains 360 starting °C, somewhat at approximately below the 360 averageC, somewhat failure temperatures below the average reported failure in temperaturesTable 2. The reporteddecomposition in Table of2 .the The epoxy decomposition from the ofbars’ the epoxysurface from inwards, the bars’ combined surface with inwards, creep combined of the epoxy with creepmatrix of inside the epoxy the bars, matrix ma insidey explain the this bars, difference. may explain this difference.
Figure 6.6. DIC-measured tensile strain versus temperaturetemperature curves for the transient thermal tests of Specimen 50c subjected to aa prestressprestress ofof 700700 MPaMPa (50%(50% ofof DTS).DTS).
Polymers 2016, 8, 446 9 of 15
The strain at failure of the tendons (0.18%–0.22% for a sustained stress of 700 MPa, as shown in Table 2) has been reduced based on the expected quasi-static strain at 700 MPa to filter out the bar movement during the loading phase. On this basis, the strain calculations assume that the distance between the camera lens and the foremost surface of the bar is unchanged from the initial calibration carried out at the start of the experiment; therefore it is expected that the results will contain some small calibration errors, and as such should only be considered as an illustrative estimate. Figure 8 shows one UHM CFRP specimen prestressed at 700 MPa (50% of DTS) immediately before and after failure (at 410 °C in this case). At temperatures above 400 °C, the epoxy matrix is completely decomposed and the ‘dry fibres’ are carrying the applied loads (albeit with some charred matrix residue remaining). The fibres relax after failure and the cross section physically opens (Figure 8 right). The sudden increase in strain is clearly visible comparing these two images using
Polymersthe auxiliary 2016, 8, horizontal446 blue lines drawn. 9 of 15 The relative retained mass versus temperature measured for the novel UHM-CFRP in the TGA is givenThe instrain Figure at failure7. The ofthermal the tendons degradation (0.18%–0.22% of the epoxy for a sustainedresin (which stress makes of 700 up MPa, 23% asof shownits initial in Tablemass) 2)starts has beenat approximately reduced based 300 on °C the and expected ends at quasi-static approximately strain 680 at °C.700 TheMPa authors to filter hypothesize out the bar movementthat the first during mass thedrop loading represents phase. the On pyrolysis this basis, of the the strain epoxy calculations matrix, and assume the second that the (starting distance at betweenapproximately the camera 570 °C) lens is and caused the foremostby char/residue surface oxidation,of the bar iswhile unchanged the oxidation from the of initialthe carbon calibration fibres Polymerscarriedthemselves 2016out, 8at(the, 446the remaining start of the 77% experiment; of the mass therefore of the it tendon) is expected follows that fromthe results about will 680 contain°C [23], some9 ofand 15 smallcontinues calibration until 900 errors, °C (the and end as suchof the should TGA test). only be considered as an illustrative estimate. Figure 8 shows one UHM CFRP specimen prestressed at 700 MPa (50% of DTS) immediately before and after failure (at 410 °C in this case). At temperatures above 400 °C, the epoxy matrix is completely decomposed and the ‘dry fibres’ are carrying the applied loads (albeit with some charred matrix residue remaining). The fibres relax after failure and the cross section physically opens (Figure 8 right). The sudden increase in strain is clearly visible comparing these two images using the auxiliary horizontal blue lines drawn. The relative retained mass versus temperature measured for the novel UHM-CFRP in the TGA is given in Figure 7. The thermal degradation of the epoxy resin (which makes up 23% of its initial mass) starts at approximately 300 °C and ends at approximately 680 °C. The authors hypothesize that the first mass drop represents the pyrolysis of the epoxy matrix, and the second (starting at approximately 570 °C) is caused by char/residue oxidation, while the oxidation of the carbon fibres themselves (the remaining 77% of the mass of the tendon) follows from about 680 °C [23], and continues until 900 °C (the end of the TGA test).
Figure 7. Thermogravimetric (TGA) plot (mass loss vs.vs. temperature) for the UHM CFRP material.
The strain at failure of the tendons (0.18%–0.22% for a sustained stress of 700 MPa, as shown in Table2) has been reduced based on the expected quasi-static strain at 700 MPa to filter out the bar movement during the loading phase. On this basis, the strain calculations assume that the distance between the camera lens and the foremost surface of the bar is unchanged from the initial calibration carried out at the start of the experiment; therefore it is expected that the results will contain some small calibration errors, and as such should only be considered as an illustrative estimate. Figure8 shows one UHM CFRP specimen prestressed at 700 MPa (50% of DTS) immediately before and after failure (at 410 ◦C in this case). At temperatures above 400 ◦C, the epoxy matrix is completely decomposed and the ‘dry fibres’ are carrying the applied loads (albeit with some charred matrix residue remaining). The fibres relax after failure and the cross section physically opens (Figure8 right). The sudden increase in strain is clearly visible comparing these two images using the auxiliary horizontalFigureFigure blue8. 7. CFRP Thermogravimetric lines specimen drawn. prestressed (TGA) plot at 700 (mass MP lossa (Specimen vs. temperature) 50d) immediately for the UHM before CFRP (a )material. and after failure (b).
Figure 8. CFRP specimen prestressed at 700 MPaMPa (Specimen 50d) immediately before ( a) and after failure (b).
The relative retained mass versus temperature measured for the novel UHM-CFRP in the TGA is given in Figure7. The thermal degradation of the epoxy resin (which makes up 23% of its initial mass) starts at approximately 300 ◦C and ends at approximately 680 ◦C. The authors hypothesize that the first mass drop represents the pyrolysis of the epoxy matrix, and the second (starting at approximately 570 ◦C) is caused by char/residue oxidation, while the oxidation of the carbon fibres themselves Polymers 2016, 8, 446 10 of 15
(thePolymers remaining 2016, 8, 446 77% of the mass of the tendon) follows from about 680 ◦C[23], and continues10 until of 15 900 ◦C (the end of the TGA test). The DMTA experiments of two specimens of the novel UHM tendons gave more insight into The DMTA experiments of two specimens of the novel UHM tendons gave more insight into their their thermo-mechanical response (Figure 9). The storage modulus E’ (primary axis) versus thermo-mechanical response (Figure9). The storage modulus E’ (primary axis) versus temperature temperature is depicted next to the tangent of the phase shift angle δ (on the secondary axis) versus is depicted next to the tangent of the phase shift angle δ (on the secondary axis) versus temperature. temperature. The two specimens were measured in two consecutive DMTA heating runs between 20 The two specimens were measured in two consecutive DMTA heating runs between 20 and 210 ◦C. and 210 °C. The results of the second heating run produced essentially the same E’ versus The results of the second heating run produced essentially the same E’ versus temperature and tan δ vs. temperature and tan δ vs. temperature curves as in the first run, and are not reproduced in Figure 9. temperature curves as in the first run, and are not reproduced in Figure9.
FigureFigure 9.9.Results Results of of DTMA DTMA analysis, analysis,E’ and E’ and tan δtanversus δ versus temperature temperature for the novelfor the UHM novel CFRP UHM tendons. CFRP tendons.
Table3 summarizes the Tg values determined following a ‘Peak tan δ’ definition [21] and a more Table 3 summarizes the Tg values determined following a ‘Peak tan δ’ definition [21] and a more conservative ‘Tg onset’ definition (used in [6]). As already noted, each sample was retested with the conservative ‘Tg onset’ definition (used in [6]). As already noted, each sample was retested with the same DMTA protocol to observe whether an increase in Tg would take place. This would be a clear same DMTA protocol to observe whether an increase in Tg would take place. This would be a clear sign of a post-hardening of the epoxy matrix of the tendon during the first heating cycle. No such sign of a post-hardening of the epoxy matrix of the tendon during the first heating cycle. No such◦ shift in Tg was observed, and the Tg values measured for the second DMTA run were between 0.9 C shift in Tg was observed, and the Tg values measured for the second DMTA run were between 0.9 °C and 2.3 ◦C lower than in the first measurement run (see Table3); this is within the uncertainty of the and 2.3 °C lower than in the first measurement run (see Table 3); this is within the uncertainty of the instrumentation used (±3 ◦C). The DMTA results for the novel UHM tendons are similar to those instrumentation used (±3 °C). The DMTA results for the novel UHM tendons are similar to those obtained in [1,5] for the conventional CFRP tendons (properties in Table1 and further elaborated in [ 1]). obtained in [1,5] for the conventional CFRP tendons (properties in Table 1◦ and further elaborated in The average Tg onset for the conventional tendons was observed to be 121 C[6], and the average Tg [1]). The average Tg onset for the conventional tendons was observed to be 121 °C [6], and the (peak tan δ) was 135 ◦C[1]. average Tg (peak tan δ) was 135 °C [1].
Table 3. Tg values for UHM CFRP tendons measured during first and second heating runs of DMTA testing. Table 3. Tg values for UHM CFRP tendons measured during first and second heating runs of DMTA testing. ◦ ◦ Sample/Run Tg (onset E’) ( C) Tg (max tan δ)( C)
1/1Sample/Run Tg (onset116 ± E’)1 (°C) Tg (max tan 142 δ) ±(°C)1 1/2 1/1 115116± ±1 1 142 ± 139 1 ± 1 2/1 1/2 117115± ±1 1 139 ± 140 1 ± 1 ± ± 2/2 2/1 115117 ±1 1 140 ± 139 1 1 2/2 115 ± 1 139 ± 1 5. Discussion 5. Discussion The high-temperature tensile strength results of the novel UHM CFRP tendons (Table2) are consideredThe high-temperature to be comparatively tensile good strength when compared results of to the the novel high temperatureUHM CFRP performancetendons (Table of typical 2) are FRPconsidered reinforcements to be comparatively for concrete structures. good when A literature compared review to the in [high7] concluded temperature that, atperformance temperatures of betweentypical FRP 250 andreinforcements 400 ◦C, most for carbon/epoxy concrete structures CFRP composites. A literature lose review about half in of[7] their concluded original that, tensile at temperatures between 250 and 400 °C, most carbon/epoxy CFRP composites lose about half of their original tensile strength. The novel UHM CFRP tendons investigated in the present study have a design tensile strength of 1400 MPa, and their failure temperature at a prestress of 50% of the design
Polymers 2016, 8, 446 11 of 15
Polymersstrength. 2016 The, 8, 446 novel UHM CFRP tendons investigated in the present study have a design tensile11 of 15 strength of 1400 MPa, and their failure temperature at a prestress of 50% of the design tensile strength ◦ wastensile found strength to be was 409 foundC (Table to be2), 409 which °C (Table is at (or 2), above)which is the at upper(or above) end the of theupper accepted end of temperature the accepted temperaturerange for this range stress for value. this stress value. A comparison of of the the tensile tensile strength strength of of the the no novelvel UHM UHM tendons tendons with with the the thermal thermal behaviour behaviour of standardof standard (PAN-based) (PAN-based) CFRP CFRP tendons tendons [6] [6 versus] versus that that obtained obtained from from prior prior tests tests on on deformed high tensile steel prestressing wires of type BS 5896 [[24]24] is shown in Figure 1010.. The Eurocode 1992-1-2 [25] [25] high-temperature design tensile strength reductio reductionn curve for Class A cold drawn prestressing steel wires is also included in Figure 10 forfor comparison.comparison. The secondary axis of Figure 10 also shows the retained mass of thethe novelnovel CFRPCFRP materialmaterial duringduring thermogravimetricthermogravimetric analysis.analysis. Highlighted are the ◦ start of the epoxy matrix decomposition, at approximately 300 °C,C, and and the start of the carbon fibre fibre ◦ oxidation, at around 680 °C.C.
Figure 10. High-temperature tensile strength test results for novel UHM CFRP reinforcing tendons, Figure 10. High-temperature tensile strength test results for novel UHM CFRP reinforcing tendons, compared against conventional CFRP tendons [6], BS 5896 steel prestressing wires [24], and guidance compared against conventional CFRP tendons [6], BS 5896 steel prestressing wires [24], and guidance from Eurocode 1992-1-2 [25]; also including TGA mass loss curve for the novel UHM CFRP. from Eurocode 1992-1-2 [25]; also including TGA mass loss curve for the novel UHM CFRP.
The first observation arising from Figure 10 is that typical steel prestressing wires also suffer The first observation arising from Figure 10 is that typical steel prestressing wires also suffer considerable reductions in tensile strength at elevated temperatures. The figure also confirms the considerable reductions in tensile strength at elevated temperatures. The figure also confirms the expectation that the Eurocode 1992-1-2 [25] tensile strength reduction recommendations are expectation that the Eurocode 1992-1-2 [25] tensile strength reduction recommendations are somewhat somewhat conservative (by approximately 60 to 100 °C based on the limited results presented conservative (by approximately 60 to 100 ◦C based on the limited results presented herein) with respect herein) with respect to the testing methods and modern steel prestressing materials used, for to the testing methods and modern steel prestressing materials used, for example, in [24]. This was example, in [24]. This was previously verified by others [6]. previously verified by others [6]. Figure 10 shows that both UHM and conventional CFRP tendons [6] also experience Figure 10 shows that both UHM and conventional CFRP tendons [6] also experience considerable considerable reductions in tensile strength under exposure to elevated temperatures. On average, reductions in tensile strength under exposure to elevated temperatures. On average, the PAN-based the PAN-based normal modulus CFRP tendons described in [1,6] are more sensitive to elevated normal modulus CFRP tendons described in [1,6] are more sensitive to elevated temperature than a BS temperature than a BS 5896 steel prestressing wire [24]; for example experiencing failure at a 5896 steel prestressing wire [24]; for example experiencing failure at a temperature about 70 ◦C lower temperature about 70 °C lower than the steel at the a stress level of 1000 MPa. However, the main than the steel at the a stress level of 1000 MPa. However, the main objective in the current study was objective in the current study was the characterization of the elevated-temperature strength the characterization of the elevated-temperature strength behaviour of the novel UHM CFRP tendons, behaviour of the novel UHM CFRP tendons, which—If considered relative to their design tensile which—If considered relative to their design tensile strength—Showed a better normalized retention strength—Showed a better normalized retention of high temperature performance than the of high temperature performance than the conventional PAN-based CFRP tendons. The design conventional PAN-based CFRP tendons. The design tensile strength of the normal modulus CFRP tensile strength of the normal modulus CFRP tendons is 2000 MPa [1], while that of the new UHM tendons is 2000 MPa [1], while that of the new UHM tendons is only 1400 MPa. The novel UHM CFRP tendons show a failure temperature of 409 °C at a sustained stress level of 50% of their design tensile strength (700 MPa), while the conventional CFRP tendons fail at an average temperature of
Polymers 2016, 8, 446 12 of 15 tendons is only 1400 MPa. The novel UHM CFRP tendons show a failure temperature of 409 ◦C at a sustained stress level of 50% of their design tensile strength (700 MPa), while the conventional CFRP tendons fail at an average temperature of 334 ◦C when loaded at 50% of their design tensile strength (i.e., 1000 MPa) [6]. For a sustained stress level of 1000 MPa, both tendon types fail in the same temperature range between 300 and 334 ◦C. Considering the shallower gradient of the loss of tensile strength of the novel UHM tendons with temperature, at 300 ◦C the average tensile strength of the tendons is 1021 MPa (73% of their design DTS). At 570 ◦C, the epoxy matrix of the tendons is almost fully decomposed, and therefore incapable of transferring shear stresses [15] amongst the individual carbon fibres (see the TGA curve in Figure 10). However, the average remaining tensile strength of the novel tendons is 644 MPa, which corresponds to 46% of DTS. It is noteworthy that the maximum surface temperature of a CFRP tendon in an intact prestressed concrete slab of 75 mm total thickness, and subjected to an ISO 834 fire from one side, can be expected to reach between 450 and 475 ◦C after 90 min [8]. For this type of fully prestressed slab [1] CFRP tendon prestress is typically 40% of the tendon design DTS (i.e., 560 MPa for the UHM tendons in the current study). This is less than the tendons’ remaining tensile strength at 570 ◦C (644 MPa, Stdv. 18 MPa). Thus, this suggests that—at least with respect to the high temperature tensile strength performance—there is the potential to use the novel UHM CFRP tendons in CFRP pre-tensioned concrete elements for building applications [1,6] with up to 90 min of fire resistance. The other important ‘fire-performance’ related issue that cannot be overlooked in this context is the understanding of the bond behaviour at high temperatures between the novel sand-coated UHM tendons and the concrete. For such prestressed concrete elements, CFRP anchorage regions with a limited temperature increase are essential. Combined experimental and numerical studies on these issues are being undertaken via a collaborative project between The University of Edinburgh and Empa. It is widely considered that the upper temperature limit for CFRP used in civil engineering structural applications is given by the Tg of the polymer resin matrix used in the CFRP composite profile fabrication [26]. For FRP composites used in infrastructure applications, Tg is commonly measured in DMTA experiments and determined using a Tg-onset definition. This definition is shown in Figure9, where the retained storage modulus E’(T) of the specimen is plotted versus the temperature for the CFRP tendons used in the current study. The Tg-onset value for the CFRP tendons was 115 to ◦ 117 C (Table3). For comparison, the Tg value for the novel UHM tendons was also determined with a tan δ peak-definition (also shown in Figure9), which gave—As anticipated in [ 27]—Somewhat higher values of 139–142 ◦C (Table3). While it is clear that the loss in matrix modulus (and strength) for temperatures over Tg-onset influences the decrease of the remaining tensile strength of the CFRP tendon with temperature, this reduction is limited to approximately 540 MPa in the temperature range 115 to 300 ◦C (Figure 10). This suggests that a considerable portion of the short-term tensile strength of the novel UHM CFRP tendons can be used at temperatures well above Tg of the tendons’ epoxy matrix (if the anchorage zones of the tendons are not subjected to high temperatures). This is a potentially important finding when designing for fire resistance of a UHM CFRP reinforced or prestressed concrete structural element. Thermogravimetric analysis in air was used to determine if the temperature of onset of resin decomposition (e.g., approximated with a tangent intersection as Td-onset in Figure 10), could be used to provide an indication of the temperatures which might lead to a considerably reduced transient thermal tensile strength of the UHM CFRP tendons. The thermal degradation was correlated and is displayed as the normalized reduction of the specimen’s mass [28] in Figure 10. It can be observed that the thermal degradation of the epoxy resin starts at approximately 300 ◦C, and ends at approximately 680 ◦C, while the oxidation of the carbon fibres begins at about 680 ◦C and continues ◦ ◦ until the test finishes at 900 C. The comparison of the estimated Td-onset value of 410 C versus the temperatures needed to cause tensile rupture suggests that Td-onset does not provide a particular Polymers 2016, 8, 446 13 of 15 indication of a necessarily ‘critical’ temperature for the tendons when stressed to reasonable service levels. Interestingly, at temperatures higher than Td-onset the reduction in transient thermal tensile strength is limited. This suggests that at such high temperatures the carbon fibre bundle (tendon) does not experience any significant stress transfer between the fibres via the decomposing epoxy matrix. The composite action is therefore lost and the ‘dry’ fibres maintain the tensile strength in the range of 640 to 700 MPa. Clearly, this only holds for the stress levels and testing procedures used in the current study, and additional research on a variety of CFRP materials and sustained stress levels is necessary for verification and corroboration.
6. Summary and Conclusions A novel ultra-high modulus CFRP prestressing tendon made from coal tar pitch-based carbon fibres was characterized in terms of high-temperature tensile strength (up to 570 ◦C) with a series of transient thermal and steady temperature tensile tests. Digital image correlation was used to capture the high-temperature strain development during thermal and mechanical loading. Complementary thermogravimetric (TGA) and differential mechanical thermal (DTMA) experiments were also performed on the tendons to elucidate their high temperature thermal and mechanical behaviour. The high-temperature tensile strengths determined for the novel UHM CFRP tendons are considered to be outstanding when compared to the high temperature performance of FRP reinforcements for concrete structures or conventional steel prestressing wires. A literature review in [7] revealed that at temperatures between 250 and 400 ◦C, most carbon/epoxy CFRP composites lose about half of their original tensile strength. The novel UHM CFRP tendons investigated in the present study have a design tensile strength (DTS) of 1400 MPa; their failure temperature at a prestress of 50% of the design tensile strength is 409 ◦C. Conventional PAN-based CFRP tendons investigated by the same authors [6] failed at an average temperature of only 334 ◦C when loaded to 50% of their design tensile strength. It is noteworthy that at 300 ◦C the average tensile strength of the novel UHM tendons studied was still 1021 MPa (73% of their design tensile strength). This shows that a considerable part of the short-term tensile strength of the novel UHM CFRP tendons can be used at temperatures well ◦ ◦ above the Tg of the tendons’ epoxy matrix (115 C). At 570 C, the epoxy matrix of the tendons was almost fully decomposed and therefore incapable of transferring shear loads amongst the individual carbon fibres. This was confirmed using TGA experiments. However, the average remaining tensile strength of the novel tendons was 644 MPa, which corresponds to 46% of their design tensile strength. This relatively high remaining tensile strength at 570 ◦C shows that there is potential to use the novel UHM CFRP tendons in CFRP pre-tensioned concrete elements for building applications that fulfill the fire resistance criteria (e.g., for an ISO 834 design fire scenario) [1,6,8]. Additional research is needed to confirm this conclusion.
Acknowledgments: We are grateful to Michel Barbezat for performing the DMTA measurements and to Empa for financial support. The UK Royal Academy of Engineering is acknowledged for the support of Bisby and for granting a distinguished visiting fellowship to Terrasi and The University of Edinburgh. Ove Arup and Partners Limited are acknowledged for their ongoing support of Bisby. Cristian Maluk is acknowledged for his generous support in discussions on the fire performance of CFRP prestressed concrete elements. Author Contributions: Giovanni Pietro Terrasi was responsible for the design and for performing the experiments and for their evaluation and interpretation. The production, anchorage and tensile testing of the CFRP specimens at ambient temperature were performed by Tobias D. Lämmlein. DIC measurements and analysis were undertaken by Emma R. E. McIntyre, who generously supported the main experiments. Luke A. Bisby and Giovanni Pietro Terrasi initiated the study, and with Pietro Lura they wrote the paper, regularly and critically reviewed and coordinated the project and performed the result analysis. Furthermore, they were very involved in the literature analysis. Conflicts of Interest: The authors declare no conflict of interest. Polymers 2016, 8, 446 14 of 15
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