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This study investigated the synergy in shear response of ultra-high-performance fiber-reinforced concrete (UHPFRCs) containing different contents of long and short smooth steel fiber reinforcements at high strain rates.
Journal of Science and Technology in Civil Engineering NUCE 2019 13 (1): 12–20 SHEAR RESISTANCE OF ULTRA-HIGH-PERFORMANCE CONCRETE REINFORCED WITH HYBRID STEEL FIBER SUBJECTED TO IMPACT LOADING Pham Thai Hoana , Ngo Tri Thuongb,∗ a Faculty of Building and Industrial Construction, National University of Civil Engineering, 55 Giai Phong road, Hai Ba Trung district, Hanoi, Vietnam b Faculty of Civil Engineering, Thuy Loi University, 175 Tay Son street, Dong Da district, Hanoi, Vietnam Article history: Received 23 August 2018, Revised 29 September 2018, Accepted 18 December 2018 Abstract This study investigated the synergy in shear response of ultra-high-performance fiber-reinforced concrete (UHPFRCs) containing different contents of long and short smooth steel fiber reinforcements at high strain rates Shear resistance of two ultra-high-performance mono-fiber-reinforced concrete (UHP-MFRCs): L15S00 (containing 1.5 vol.-% long and 0.0 vol.-% short fiber) or L00S15, and one ultra-high-performance hybrid-fiberreinforced concrete (UHP-HFRCs): L10S05 (containing 1.0 vol.-% long and 0.5 vol.-% short fiber) at high strain rates of up to 272 s−1 was investigated using a new shear test setup by an improved strain energy frame impact machine (I-SEFIM) The L10S05 generated high synergy in shear strength, shear peak toughness at static rate and high synergy in shear strain, shear peak toughness at high strain rates Moreover, all the investigated UHPFRCs were sensitive to the applied strain rates, especially in term of shear strength Keywords: UHPFRCs; shear resistance; synergy effect; strain-rate dependent; impact https://doi.org/10.31814/stce.nuce2019-13(1)-02 c 2019 National University of Civil Engineering Introduction Ultra-high-performance fiber-reinforced concrete (UHPFRC) is a potential material for wide use in protective structures for aeronautics, nuclear industry, and military buildings as a safeguard against impact or blast loading, owing to its superior mechanical characteristics such as very high compressive strength [1], high tensile strength, ductility [2], and energy absorption capacity [3] Nevertheless, the application of UHPFRCs to civil infrastructures is still very limited because of their relatively high fiber contents and cost [4, 5] It is necessary to reduce the fiber contents as well as the cost of the UHPFRCs, without sacrificing their high mechanical resistance and work ability Several methods have been carried out to reduce the fiber content and cost of UHPFRCs, which may be listed as follows: (1) increasing mechanical interfacial bond strength between fiber and matrix by utilizing deformed steel fiber geometries [6]; (2) generating synergistic responses by blending of long and short fibers reinforcements [5]; and (3) enhancing the physical and chemical bond strength between the fiber and matrix by maximizing packing density of the matrix [7] Among the various methods, blending long and short fibers has been proven as one of the most effective methods, owing to a combination of various features from those different fiber reinforcements [8, 9] For example, ∗ Corresponding author E-mail address: trithuong@tlu.edu.vn (Thuong, N T.) 12 Hoan, P T., Thuong, N T / Journal of Science and Technology in Civil Engineering the shorter reinforcements can effectively restrict the development of micro-cracks while the longer reinforcements can bridge macro-cracks [10] Even though the mechanical properties of ultra-high-performance hybrid-fiber-reinforced concrete (UHP-HFRC) have been intensively investigated by many researchers, researchers have mostly focused on the compressive [9–12], tensile [5, 13, 14], and flexural [15, 16] properties of UHPHFRCs rather than their shear resistance [16] Moreover, most previous studies have focused on the quasi-static properties [9, 12, 13] rather than the impact behavior [5, 10, 11, 14, 16] Wu et al [10] used the split Hopkinson press bar (SHPB) testing to investigate the static and dynamic compressive strength of UHP-HFRCs and found that the UHP-HFRC containing 1.5% fiber volume content (1.5 vol.-%) long and 0.5 vol.-% short steel fiber reinforcements exhibited higher compressive strength than those containing only 2.0 vol.-% of long or short fibers, at both static and high strain rates Millard et al [16] used drop-hammer techniques to investigate the dynamic increase factor (DIF) under both flexural and shear loading of UHP-HFRCs The results showed that the beam containing vol.-% long and short steel fibers produced the lowest dynamic increase factor (DIF) under flexural loading, whereas there is no significant strain rate enhancement in the case of shear loading Tran et al [5] investigated the synergistic response of blending fibers in UHPC under high rate tensile load using a strain energy frame impact machine (SEFIM) They have reported that the blending of long and shorter steel fibers in UHPC generated notable synergistic effects on the tensile response of UHP-HFRCs, especially at high strain rates Until now, there is still little available information about the effect of fiber hybridization on the shear resistance of UHPFRCs, especially at high strain rates This study aims to understand the influence of synergistic response and strain rates on the shear resistance of UHPFRCs using the new shear test method, recently developed by Ngo et al [17], that is capable of measuring the shear-related hardening response of UHPFRCs, accompanied by multiple microcracks The first one of the two main objectives in this study is to examine the synergistic responses on the shear resistance of UHP-HFRCs and the second objective is to investigate the strain rate effect on the shear resistance of UHPFRCs Experimental program Three series of prism shear specimen named as L15S00 (containing 1.5 vol.-% long and 0.0 vol.-% short fiber), L00S15 (containing 0.0 vol.-% long and 1.5 vol.-% short fiber), and L10S05 (containing 1.0 vol.-% long and 0.5 vol.-% short fiber) with the same UHPC matrix were prepared and tested Each specimen series consists of specimens, leading to the total of 18 prism specimens with the same size of 50 × 50 × 210 mm3 2.1 Material and specimen preparation The composition by weight ratio of Ultra-high-performance (UHPC) matrix is listed in Table while the properties of long and short smooth steel fibers are listed in Table The silica sand and the silica fume are first to dry mixed for mins The cement and the silica powder are then added and mixed in approximately more mins The water and superplasticizer are slowly added with mins interval and mixed continuously until the mixture showed adequate workability Finally, the fibers are carefully poured by hand into the mixture while the mixer machine kept rotating for mins Detail of the mixing procedure can be found in the previous work [17] The UHPFRC mixture is cast into plastic molds by a scoop without vibration before storing in the laboratory temperature for 48 h The specimens are demoded and cured in the hot water tank at 90 13 Hoan, P T., Thuong, N T / Journal of Science and Technology in Civil Engineering Table The composition of UHPC matrix by weight ratio Cement (Type I) Silica fume Silica sand Silica powder Super-plasticizer Water 0.25 1.10 0.30 0.067 0.2 Table Properties of smooth steel fibers Fiber type Diameter, d f (mm) Length l f (mm) Density, ρ (g/cc) Tensile strength, µu (MPa) Elasticmodulus, E (GPa) Short smooth steel fiber Long smooth steel fiber 0.2 0.2 13 19 7.90 7.90 2788 2580 200 200 ± 2◦ C in 72 h All specimens were tested at the ages of 28 days The compressive strength of UHPC matrix was 189 MPa according to [18] shows shear machine at high strain rates A shear setup Fig.Fig shows thethe shear testtest machine at high strain rates A shear testtest setup 2.2 Test and procedure with the same specimen boundary conditions as the static shear with thesetup same specimen sizesize andand boundary conditions as the static shear testtest waswas employed in improved strain energy frame impact machine employed ininvestigate an an improved strain energy frame machine to to In order to the synergistic responses and theimpact strain rate effect on(I-SEFIM) the(I-SEFIM) shear resistance of UHPFRCs, shear tests were conducted at both staticat andhigh strain rates Static shear tests investigate the shear resistance of UHPFRCs athigh high strain rates detail of investigate the shear resistance of UHPFRCs strain rates TheThe detail of were carried out on three specimens of each specimen series, which were denoted by the “-S” notation folshear impact system could found elsewhere [19] shear stress shear impact system could be be found elsewhere [19] TheThe shear stress waswas lowing the name of each series, whereas the dynamic shear tests were carried out on three remaining obtained from dynamic strain attached on of the obtained twotwo dynamic gauges attached on the the surfaces theseries specimens offrom each series, which were strain denoted by gauges the “-H” notation following thesurfaces name ofofeach Fig shows thewhile static shear test system The shear test setup,was recently proposed byfrom Ngo et al transmitter while the shear strain of the specimen was measured transmitter bar,bar, the shear strain of the specimen measured from the the [17], was employed in the universal test machine (UTM) to implement the static shear test Details of relative displacement marked points a fixed a moved relative displacement of of marked points on on a fixed gripgrip andand a moved gripgrip by by a a the shear test setup could be found in [17] The speed of machine displacement was maintained as high-speed camera system, as applied shown in Fig The speed of cell applied wasthe high-speed camera system, asThe shown in load Fig measured The speed applied loadload was mm/min during static shear testing was by aofload installed inside controlled by capacity of coupler and types of energy frame: the coupler with UTM, while the was by linear displacement (LDVTs) controlled by displacement thethe capacity of recorded coupler andtwo types ofvariable energy frame: thetransducers coupler with attached to the bottom surface of the specimen by an aluminum frame, as can be seen in Fig capacity high strength steel energy frame were used in this study 800800 kNkN capacity andand high strength steel energy frame were used in this study Figure Static Static shear testtest setup Fig.1 shear Fig.1 Static shear test Impact shear test setup Fig.Fig 2.Figure Impact shear testtest setup Impact shear setup setup setup Fig shows the shear test machine at high strain rates A shear test setup with the same specimen size andResults boundary conditions as the static shear test was employed in an improved strain energy frame Results impact machine (I-SEFIM) to investigate the shear resistance of UHPFRCs at high strain rates The The shear stress-versus-strain ofelsewhere UHPFRCs the different strain rates is two The shear stress-versus-strain of UHPFRCs atshear the different strain rates is detail of shear impact system could be found [19].atThe stress was obtained from shown in Fig 3, while their shear parameters listed in Table The equations shown in Fig 3, while their shear parameters listed in Table The equations 14 areare to calculate thethe shear strength, shear strain capacity, strain rates, andand shear peak to calculate shear strength, shear strain capacity, strain rates, shear peak toughnesscan be be referred in [19] Generally, thethe shear resistance of UHPFRCs toughnesscan referred in [19] Generally, shear resistance of UHPFRCs increased as as thethe applied strain rates increased, although the the shear parameters increased applied strain rates increased, although shear parameters Hoan, P T., Thuong, N T / Journal of Science and Technology in Civil Engineering dynamic strain gauges attached on the surfaces of the transmitter bar, while the shear strain of the specimen was measured from the relative displacement of marked points on a fixed grip and a moved grip by a high-speed camera system, as shown in Fig The speed of applied load was controlled by the capacity of coupler and types of energy frame: the coupler with 800 kN capacity and high strength steel energy frame were used in this study Results The shear stress-versus-strain of UHPFRCs at the different strain rates is shown in Fig 3, while their shear parameters are listed in Table The equations to calculate the shear strength, shear strain capacity, strain rates, and shear peak toughness can be referred in [19] Generally, the shear resistance of UHPFRCs increased as the applied strain rates increased, although the shear parameters were strongly dependent on the combination of fiber reinforcements The L10S05 exhibited the highest shear strength (τmax ) and shear peak toughness (T sp ) at static rate The average τmax of L00S15, L10S05, and L15S00 are 18.2, 24.4, and 20.8 MPa, while T sp of those are 0.51, 0.89, and 0.76 MPa, respectively Their γmax are 0.045, 0.050, and 0.054 as listed in Table However, the shear strength (31.9 MPa) of L15S00 is significantly higher than those of the L10S05 (30.1 MPa) and the L00S15 (26.80 MPa) at high strain rates In addition, the L10S05 produced the highest value in terms of the shear strain and the shear peak toughness Their values of γmax and T sp are 0.088 and 1.40 MPa for the L00S15, 0.107 and 1.91 MPa for the L10S05, and 0.06 and 1.12 MPa for the L15S00 Failure of the specimens is shown in Fig All specimens failed with two major shear cracks, accompanied by the formation of multiple-micro cracks In addition, the number of cracks at high strain rates (Fig 4(a)) was significantly higher than at static rates (Fig 4(b)) Discussions 4.1 Synergistic effect of blending long and short fiber on shear resistance of UHP-HFRCs The synergy evaluation of UHP-HFRCs using Eq (1) is shown in Fig The Eq (1) defines synergy as the amount by which the performance of a hybrid system exceeds that of each monocomponent system as the same fiber volume content [5]: S = (V f ) (V f ) (V f ) ) − max(Rmono,a , Rmono,b Rhybrid,a+b (V f ) (V f ) max(Rmono,a , Rmono,b ) (1) (V f ) (V f ) (V f ) where Rhybrid,a+b is the shear resistance of UHP-HFRC reinforced with fiber a and b, Rmono,a , Rmono,b are the shear resistance of ultra-high-performance mono-fiber-reinforced concrete (UHP-MFRC) containing fiber a and b, respectively Notably, the UHP-HFRCs and UHP-MFRCs have the same total fiber volume content, V f A positive value of “S” indicates that the hybrid system performs better than the mono system or the sum of individual fibers As can be seen in Fig 5, the UHP-HFRC containing 1.0 vol.-% long fiber and 0.5 vol.-% short fiber (L10S05) exhibited the positive synergy values for the shear strength (τmax ), shear peak toughness (T sp ), but the negative synergy value for the shear strain capacity (γmax ), at static rate Whereas they produced the best synergy in the Tsp, at high strain rates Specifically, the synergy values for τmax , γmax and T sp of L05S10 were 0.175, −0.075, and 0.160 at the static rate, and −0.056, 0.218, and 0.367 at the high strain rates, respectively The reason for the synergy effect of the UHPFRCs at static 15 the L10S05 produced the highest ininterms ofofof the shear strain and the shear L00S15, 0.107 andTheir 1.91 MPa for L10S05, and 0.06 and 1.12 MPa for the the L10S05 produced the highest value in terms the shear strain and the shear the L10S05 produced the highest value the shear strain and the shear peak toughness values ofof the and T 0.088 and 1.40 MPa for the max peak toughness values of value and Tterms 0.088 and 1.40 MPa for the max spsp peak toughness Their values max and Tare are and 1.40 MPa for the spare L00S15, 0.107Their and 1.91 MPa for the L10S05, and0.088 0.06 and 1.12 MPa for the peak toughness Their values of and T are 0.088 and 1.40 MPa for the L15S00 peak toughness Their values of and T are 0.088 and 1.40 MPa for the max sp max sp peak toughness Their values of and T are 0.088 1.40 L00S15, 0.107 and 1.91 MPa for the L10S05, and 0.06 and 1.12 MPa for the max sp L00S15, 0.107 andand 1.91 MPa forfor thethe L10S05, andand 0.06 andand 1.12 MPa forfor thethe L00S15, 0.107 1.91 MPa L10S05, 0.06 1.12 MPa L15S00 L00S15, 0.107 and 1.91 MPa for the L10S05, and 0.06 and 1.12 MPa for the L00S15, 0.107 and 1.91 MPa for the L10S05, and 0.06 and 1.12 MPa 0.107 and 1.91 MPa for the L10S05, and 0.06 and 1.12 MPa forfor thethe L15S00 L15S00 30L00S15, 30 30 L15S00 L10S05-S L00S15-S 30 30 30L15S00-S SP1 SP1 SP1 L15S00 L15S00 L15S00 Hoan, P T., Thuong, N T / Journal of Science and Technology in Civil Engineering L10S05-S 4040 40 L00S15-H stress (MPa) Shear(MPa) (MPa) stressstress ShearShear Shear stress (MPa) Shear stress (MPa) (MPa) stress Shear (MPa) stress (MPa) stress ShearShear 3040 40L00S15-H L00S15-H 40L00S15-H 30 L00S15-H L00S15-H L00S15-H 3030 30 2030 30 30 20 2020 20 1020 20 20 10 1010 10 40 -1 SP1(263 s ) -1 SP1(263 s ) SP2(270 s -1 ) -1 -1 -1 SP2(270 s -1 s ) s )) SP1(263 SP1(263 SP1(263 SP3(262ss -1)) -1 -1 SP3(262 -1 s ) ss-1 )) SP2(270 SP2(270 SP2(270 sSP1(263 )-1 -1 -1 SP1(263 SP1(263 s )s )s ) -1 -1 -1 s ) s ) SP3(262 SP3(262 SP3(262 sSP2(270 ) -1 -1 SP2(270 SP2(270 s -1 ) s ) s ) -1 -1 SP3(262 (MPa) (MPa) stressstress ShearShear stress (MPa) Shear(MPa) stress stress (MPa) Shear Shear (MPa) stress Shear stress (MPa) Shear (MPa) Shear stress b) b) L10S05-S L10S05-S b) L10S05-S b)L10S05-S L10S05-S b) (b) L10S05-S 40 b) L10S05-S b) L10S05-S b) L10S05-S L10S05-H 4040 40 -1 SP3(262 SP3(262 s )s )s ) 40 SP1(247 s -1 ) SP1(247 s -1 ) SP2(254 s -1 ) SP2(254 s-1-1 ) -1 -1 SP3(223 SP1(247 SP1(247 SP1(247 s -1s) s ) ) s -1) SP3(223 s ) -1 -1 s ) s -1 ) SP2(254 SP2(254 SP2(254 s ) -1 -1 SP1(247 SP1(247 s ) -1s ) SP1(247 s -1 )-1 -1 SP3(223 SP3(223 SP3(223 s ) s -1) s )-1 SP2(254 SP2(254 SP2(254 s -1 )s ) s ) L10S05-H 30 40L10S05-H L10S05-H 40L10S05-H 40 30 L10S05-H L10S05-H 3030 30L10S05-H 20 30 30 3020 -1 c) c) L15S00-S L15S00-S c) L15S00-S c) (c) L15S00-S c) L15S00-S L15S00-S 40 c) c) L15S00-S L15S00-S c) L15S00-S L15S00-H 4040 40 -1 -1 SP3(223 SP3(223 SP3(223 s )s ) s ) L15S00-H 30 40L15S00-H 4040 L15S00-H 30 L15S00-H (MPa) (MPa) stressstress ShearShear stress (MPa) Shear (MPa)(MPa) stressstress ShearShear (MPa) stress Shear (MPa) stress Shear (MPa) Shear stress a) L00S15-S a) L00S15-S a) L00S15-S a) L00S15-S a) (a) L00S15-S L00S15-S 40 a)a) a) L00S15-S L00S15-S L00S15-S L00S15-H (MPa) stress (MPa) Shear (MPa) stressstress ShearShear Shear stress (MPa) Shear stress (MPa) (MPa) stress (MPa) stress Shear (MPa) stress ShearShear 40 (MPa) stress (MPa) Shear (MPa) stressstress ShearShear Shear stress (MPa) Shear stress (MPa) (MPa) stress Shear (MPa) stress Shear Shear stress (MPa) stress (MPa) Shear (MPa) (MPa) stressstress ShearShear Shear stress (MPa) Shear stress (MPa) stress Shear (MPa)(MPa) stress (MPa) stress ShearShear L00S15-S L15S00-S 25 30 2530 SP2 SP1 30 30 30 SP2 SP1 3025 SP2SP1 30 30 30 L10S05-S 25L00S15-S 25 25L15S00-S SP3 L10S05-S SP2 L10S05-S SP2 SP2 SP3 SP3 L15S00-S SP1SP1 30 SP1SP1 SP1 L00S15-S L15S00-S SP1 SP1 SP1 SP1 30 30L00S15-S 30 30 30 30 30 20 SP3 202530 25 SP3 2520 SP3 SP2SP2 2525 25 25 L10S05-S 25 25 SP2 L10S05-S SP2 SP2 SP2 SP2 SP2 L00S15-S L15S00-S SP2 L00S15-S L15S00-S SP1SP1SP1 SP1 SP1 L00S15-S L15S00-S 20 20L10S05-S 20 SP1SP1 SP1 SP1 SP3SP3SP3 25 25 SP3SP3 SP3 SP3 25 25 25 25 SP3 25 SP3 SP2SP2SP2 15 25 25 SP2 SP2 15 SP2SP2 2015 SP2 SP2 20 2020 20 2020 20 20 SP3SP3SP3 SP3 15 15 15 SP3 SP3SP3 SP3 SP3 20 20 20 20 20 2020 20 10 10152015 15 15 1515 15 15 1510 10 10 10 15 15 15 15 15 15 1515 15 5 10 10 10 10 10 10 10 10 10 5 10 10 10 10 10 10 1010 10 5 05 5 0 5 5 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 0.05 strain up to peak g 0.05 0.1stress, g 0.15 0.25 5 0Shear strain 0.05up to peak 0.1 stress, g0.15 0.2 5 Shear strain 0.05 up to peak 0.1 stress,0.15 0.2 5 5Shear Shear strain up to peak stress, g Shear strain up to peak stress, g Shear strain up to peak stress, g 0 0 0 0 0.050.05 0.10.1 0.1 0.15 0.150.15 0.20.2 0.2 0 0.05 0.050.05 0.10.1 0.1 0.15 0.150.15 0.20.20.2 0 0.05 0.05 0.15 0.05 0.10.10.1 0.15 0.15 0.20.20.2 0 0.05 Shear strain up touppeak stress, Shear strain to stress, Shear strain to peak stress, Shear strain tostress, peak stress, Shear strain uppeak to stress, peak stress, Shear strain to peak stress, strain up to peak g g g strain upup to peak g g g strain upup to up peak stress, g g g 0 0Shear 0 Shear 0 Shear 0.05 0.1 0.10.1 0.150.15 0.15 0.20.20.2 0 0.05 0.05 0.10.10.1 0.15 0 0.050.05 0.05 0.150.15 0.20.2 0.2 0 0.05 0.050.05 0.1 0.1 0.1 0.15 0.150.15 0.2 0.2 0.2 Shear strain up tostress, peak stress, Shear strain to stress, peak stress, Shear strain up to peak stress, Shear strain up to g g g Shear strain upup to up peak g g g Shear strain upuptoto peak stress, gg g Shear strain uppeak to peak stress, Shear strain to peak stress, Shear strain peak stress, L15S00-H L15S00-H 3030 30L15S00-H 2030 30 30 20 SP1(224 s -1 ) SP1(224 s -1 ) -1 SP2(243 s ) -1 -1 -1 SP2(243 s ) SP1(224 SP1(224 SP1(224 s -1 ) s )s ) SP3(232 s -1 ) -1 -1 -1 SP3(232 s ) -1 SP2(243 SP2(243 SP2(243 sSP1(224 ) s -1)s )-1 SP1(224 SP1(224 s -1s) ) s ) -1 -1 -1 s )s )-1 SP3(232 SP3(232 SP3(232 sSP2(243 )-1 -1 SP2(243 SP2(243 s s) ) s ) -1 -1 SP3(232 SP3(232 SP3(232 s -1s) ) s ) 2020 20 2020 20 1020 20 20 10 10 20 20 2010 1010 10 1010 10 010 10 10 10 10 10010 10 10 0.05 0.050.05 0.1 0.1 0.150.15 0.2 0.2 0.1 0.1 0.15 0.15 0.2 0.2 0 0.05 0.05 0.1 0.1 0.15 0.15 0.2 0.2 0 0 0.05 Shear strain up to peak stress, g Shear strain up touppeak stress, g g Shear strain up to peak stress, g Shear strain up to peak stress, g Shear strain to peak stress, Shear strain up to peak stress, g 0 0 0 00 0.05 0.10.10.1 0.15 0.15 0.20.20.2 0.05 0.15 0.050.05 0.10.1 0.1 0.15 0.150.15 0.20.2 0.2 0 0.05 0.050.05 0.10.1 0.1 0.15 0.150.15 0.20.20.2 0 0.05 0 0.05 Shear strain tostress, peak stress, Shear strain to peak stress, Shear strain uppeak to stress, peak stress, Shear strain up touppeak stress, Shear strain to peak stress, Shear strain to stress, strain up to peak g g g strain upup to up peak stress, g g g strain upup to peak g g g 0 Shear 0 Shear 0 00Shear 0.050.05 0.1 0.1 0.1 0.15 0.150.15 0.2 0.2 0.2 0.05 0.150.15 0.20.2 0.2 0 0.05 0.050.05 0.05 0.1 0.10.1 0.150.15 0.15 0.20.20.2 0 0.05 0.05 0.10.1 0.1 0.15 Shear strain uppeak to peak stress, Shear strain uptoto peak stress, Shear strain to peak stress, Shear strain up tostress, peak stress, Shear up to peak stress, Shear strain to stress, peak stress, Shear strain up to g g g Shear strain up peak stress, gg g Shear strain upup to up peak g g g (d) L00S15-H (e) L10S05-H (f)strain L15S00-H a) L00S15-H b) L10S05-H c) c) L15S00-H a) L00S15-H b) L10S05-H L15S00-H a) L00S15-H b) L10S05-H c) L15S00-H L00S15-H L10S05-H L15S00-H a)a)L00S15-H b)b)L10S05-H c)c)L15S00-H Fig.Fig Shear stress-versus-strain of UHPFRCs at different strain rates Shear stress-versus-strain of UHPFRCs at different strain rates a) L00S15-H b) L10S05-H c) L15S00-H a) L00S15-H b) L10S05-H c) L15S00-H a) L00S15-H b) L10S05-H c) L15S00-H Fig Shear stress-versus-strain of UHPFRCs at different strain rates Fig.3.3.Shear Shear stress-versus-strain atdifferent different strainrates rates Figurestress-versus-strain Shear stress-versus-strain ofUHPFRCs UHPFRCs atat different strainstrain rates Fig ofofUHPFRCs Fig stress-versus-strain strain rates Fig 3.Shear Shear stress-versus-strain ofUHPFRCs UHPFRCs atdifferent different strain rates Fig 3.3.Shear stress-versus-strain ofofUHPFRCs atatdifferent strain rates rates (b) High strain rates a)Static Static rates High strain rates a)(a)Static rates b) b) High strain rates a) Static rates b) High strain rates Staticrates rates Highstrain strainrates rates a)a)Static b)b)High Figure Typical failure of shear UHPFRCs specimens Fig Typical failure of shear UHPFRCs specimens a) Static rates b) High strain rates Fig Typical failure of shear UHPFRCs specimens a) Static rates b) High strain rates a) Static rates b) High strain rates Fig Typical failure of shear UHPFRCs specimens Fig.4.4.Typical Typicalfailure failureofofshear shearUHPFRCs UHPFRCsspecimens specimens Fig Failure of the specimens is shown in Fig All specimens failed withtwo two Fig Typical failure ofnot shear UHPFRCs specimens Fig Typical failure ofin shear UHPFRCs specimens ofFig the specimens is shown Fig All specimens with Typical failure of UHPFRCs specimens rates Failure was Failure different from the high strain rates is shear really clear but likely relatedfailed tofailed the difference in of the specimens is shown in Fig All specimens with two Failure ofthe thespecimens specimens isshown shown inFig Fig 4.All Allspecimens specimens failed with twoatIn Failure of is in failed with two major shear cracks, accompanied by the formation of multiple-micro cracks crack propagate mechanism in the UHPFRC specimens under different applied strain rates Unlike major shear cracks, accompanied the in formation of multiple-micro cracks In Failure ofcracks, specimens shown Fig specimens failed with two Failure ofthe the specimens isby shown information Fig 4.All All specimens failed with two major shear accompanied by the of multiple-micro cracks In Failure of the specimens isisshown information Fig 4.4.the All specimens with two major shear cracks, accompanied by the formation of multiple-micro cracks In the static rates, the micro and macro cracks almost happen at same time owingfailed to thecracks extreme load major shear cracks, accompanied by the of multiple-micro In major shear cracks, accompanied the formation multiple-micro cracks InIn major shear cracks, accompanied the formation of multiple-micro cracks speeds The difference inaccompanied the strain-rate sensitivity characteristics the long and short fiber might major shear cracks, bybyby the formation ofofof multiple-micro cracks In 6 be another reason for the different synergy effect between static and high applied strain rates The 66 synergy response of the L10S05 under shear loading, in this study, was the same as those under direct 6 tensile loads at high strain rates Tran et al [5] investigated the synergy response of the L10S05, under static and high strain rate direct tensile loads, reported that the L10S05 exhibited the negative effects 16 Hoan, P T., Thuong, N T / Journal of Science and Technology in Civil Engineering Table Test results Shear strength, τmax Strain rate Test series L00S15-S Specimen SP1 SP2 SP3 Type s−1 Static 0.000667 Average SD L10S05-S SP1 SP2 SP3 Static Average SD L15S00-S SP1 SP2 SP3 Static Average SD L00S15-H SP1 SP2 SP3 High rates Average SD L10S05-H SP1 SP2 SP3 High rates Average SD L15S00-H SP1 SP2 SP3 High rates Average SD MPa DIF 0.045 0.048 0.042 0.045 0.003 1.0 0.59 0.51 0.42 0.51 0.08 1.0 1.0 0.049 0.050 0.051 0.050 0.001 1.0 0.93 0.93 0.79 0.89 0.08 1.0 20.33 20.98 20.99 20.8 0.4 1.0 0.060 0.051 0.053 0.054 0.005 1.0 0.84 0.72 0.73 0.76 0.06 1.0 235 260 270 257 26.93 26.11 27.22 26.8 0.6 1.48 1.44 1.50 1.47 0.080 0.104 0.079 0.088 0.014 1.77 2.31 1.75 1.94 1.44 1.59 1.15 1.40 0.22 2.86 3.15 2.28 2.76 272 254 223 230 29.81 29.71 30.80 30.1 0.6 1.24 1.22 1.26 1.2 0.105 0.078 0.136 0.107 0.029 1.95 1.56 2.71 2.1 1.65 1.27 2.81 1.91 0.80 2.04 1.66 3.68 2.5 224 243 232 232 30.00 33.10 32.59 31.9 1.7 1.44 1.59 1.57 1.5 0.059 0.052 0.069 0.060 0.008 1.09 0.96 1.27 1.1 0.93 0.85 1.59 1.12 0.41 1.22 1.11 2.08 1.5 0.000667 0.000667 0.000667 0.000667 DIF 18.63 17.92 18.02 18.2 0.4 1.0 24.80 23.57 24.80 24.4 0.7 Shear peak toughness, T sp DIF 0.000667 MPa Shear strain at peak stress, γmax in term of post-cracking strength (σ pc ), but highly effective in terms of tensile strain capacity (εc ) and peak toughness (T p ) 17 Notably, the UHP-HFRCs and UHP-MFRCs have the same total fiber volume content, Vf A positive value of “S” indicates that the hybrid system performs better than the mono system orT.the sum of individual fibers Hoan, P T., Thuong, N / Journal of Science and Technology in Civil Engineering 0.6 Static rate High strain rates Synergy coefficent 0.4 0.367 0.218 0.2 0.175 0.160 -0.056 -0.2 -0.075 Shear stress Shear strain Peak toughness Shear parameters Figure Synergistic response of UHP-HFRCs Fig Synergistic response of UHP-HFRCs 4.2 rate effect on shear of UHPFRCscontaining AsHigh canstrain be seen in Fig 5, resistance the UHP-HFRC 1.0 vol.-% long fiber DIFs, ratio between dynamic and static responses, of the shear parameters (τmax , for γmax ,the and 0.5The vol.-% short fiberthe (L10S05) exhibited the positive synergy values T sp ) of UHPFRCs at high strain rates (up to 272 s−1 ) are plotted in Fig 6, including DIFs for shear shear strength (maxshear ), shear peak toughness (Tshear negative synergy value sp), but strength (Fig 6(a)), strain capacity (Fig 6(b)), and peak the toughness (Fig 6(c)) Generally, the UHPFRCs were found to be sensitive to the applied strain rates As the strain increased from the for the shear strain capacity (max), at static rate Whereas they produced the best static rate (0.000667 s−1 ) to the high strain rates (up to 272 s−1 ), the DIFs of τmax of the L00S15, L10S05, 1.47,strain 1.20, and 1.50, Specifically, while the DIFs ofthe γmaxsynergy were 1.94,values 2.10, andfor 1.10, synergy in and theL15S00 Tsp, atwere high rates max, respectively Those DIFs of T sp , which is shown in Fig 6(c) were 2.76, 2.50, and 1.50 3 DIF of shear strength DIF of shear strength DIF of shear strength 1.51.5 1.5 L15S00 L15S00 L15S00 1.201.201.20 1 L00S15 L00S15 L00S15 L10S05 L10S05 L10S05 1.94 2 1.94 1.94 2.102.102.10 L15S00 L15S00 3 L15S00 0 L00S15 L10S05 L15S00 L00S15 L10S05 L10S05 L15S00 L15S00 L00S15 (a) Shear strength strength a)a)Shear strength a)Shear Shear strength L15S00 L15S00 L15S00 2.762.762.76 2.502.502.50 2 1.501.501.50 1 0 L00S15 L10S05 L15S00 L00S15 L10S05 L10S05 L15S00 L15S00 L00S15 Types of UHP-HFRCs Types of UHP-HFRCs Types of UHP-HFRCs L00S15 L00S15 L00S15 L10S05 L10S05 L10S05 1.101.101.10 1 0.50.5 0.5 4 1.501.501.50 DIF of shear strain DIF of shear strain DIF of shear strain 1.471.471.47 L00S15 L00S15 L00S15 L10S05 L10S05 L10S05 DIF of shear peak toughness DIF of shear peak toughness DIF of shear peak toughness 2 Types of UHP-HFRCs Types of UHP-HFRCs Types of UHP-HFRCs (b) Shear strain strain b)b)Shear strain b)Shear Shear strain 0 L00S15 L10S05 L15S00 L00S15L10S05 L10S05L15S00 L15S00 L00S15 Types of UHP-HFRCs Types of UHP-HFRCs Types of UHP-HFRCs (c) Shear peak toughness peak toughness c)c)Shear peak toughness c)Shear Shear peak toughness Figure Strain rate effect on shear resistance of UHPFRCs Fig rate effect resistance Fig 6.6.Strain rate effect ononshear resistance ofofUHPFRCs Fig 6.Strain Strain rate effect onshear shear resistance ofUHPFRCs UHPFRCs The average DIF (1.50) of the L15S00 for τmax at high strain rate up to 272 s−1 was found to be Fig the experimental shear strength (exp )and calculated shear Fig 7plots the experimental shear strength ((exp )exp calculated shear Fig 7plots plots the experimental shear strength )and and calculated shear significantly lower than those of tensile strength The DIF of the tensile strength (σ pc ) of UHPFRC strength (cal )ofof UHPFRCs strain rates the cal was calculated strength (1.5 (cal )cal atathigh strain rates InInwhich, cal calculated containing vol.-% steel fibers was reported as about 3.0 at thethe high strain rate ofcalculated 21.4 s−1 strength ) UHPFRCs ofshort UHPFRCs athigh high strain rates Inwhich, which, the was was cal proposed equation and Kim (2017) [19], (2): bybyabyaproposed equation ofofNgo and Kim [19], asasEq (2): a proposed equation ofNgo Ngo and Kim (2017) [19], asEq Eq (2): 18(2017) 111 DIF DIF == = DIF max max max )(0.582 0.07023 0.582 (() )0.582 0.07023 0.07023 110 s s s 110 s/ s / s /110 110 110 s/ s / s /110 (2)(2)(2) oefficient 0.07023 in Eq (2) was kept for the L15S00 and justified to 0.06 fo he L00S15 and 0.048 for the L10S05, respectively, while the exponent (0.582 P T., Thuong, N T / Journal of Science Technology in Civil Engineering of all investigate as maintained AsHoan, demonstrated in Fig 7,andthe shear strength [5] could The lowerbe ratepredicted sensitivity of τby comparison the σ pc of UHPFRCs, was also max , in HPFRCs using the with emperical proposed by reported Ngo and Kim and explained by [19] owing to the lower inertial effect, in the shear specimen, of mortar matrix 2017) surrounding fibers Exp_L00S15 Exp_L10S05 DIF for shear strength Exp_L15S00 1.5 Cal_L00S15 Cal_L10S05 Cal_L00S15 0.5 0.00010.001 0.01 0.1 10 -1 Strain rate (s ) 100 1000 Figure Strain rate effect on shear resistance of UHPFRCs Fig Strain rate effect on shear resistance of UHPFRCs Fig plots the experimental shear strength (τexp ) and calculated shear strength (τcal ) of UHPFRCs at high strain rates In which, the τcal was calculated by a proposed equation of [19], as Eq (2): DIFτmax = γ˙ s < γ˙ ≤ 110/s 0.07023 × (˙γ)0.582 γ˙ > 110/s (2) where DIFτmax is the DIFs for the shear strength, γ˙ s is static strain rate (0.000667 s−1 in this study), and γ˙ is the applied shear strain rates Notably, the coefficient 0.07023 in Eq (2) was kept for the L15S00 and justified to 0.06 for the L00S15 and 0.048 for the L10S05, respectively, while the exponent (0.582) was maintained As demonstrated in Fig 7, the shear strength of all investigated UHPFRCs could be predicted by using the emperical proposed by [17] Conclusions The effects of blending fibers on the shear resistance of UHPFRCs at both static and higher strain rates were investigated using a new shear test method Specimens with the same size and boundary conditions were used at both static and high strain rates to minimize the potential effects of inertia and boundary conditions on the test results The following observations and conclusions can be drawn from this study: - All the investigated UHPFRCs were sensitive to the applied strain rate, especially the L15S00 - The L10S05 generated high synergy in shear strength, shear peak toughness at static rate, but high synergy in shear strain and shear peak toughness at high strain rates 19 Hoan, P T., Thuong, N T / Journal of Science and Technology in Civil Engineering Acknowledgement This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 107.01-2018.22 References [1] Wille, K., Naaman, A E., Parra-Montesinos, G J (2011) Ultra-High Performance Concrete with Compressive Strength Exceeding 150 MPa (22 ksi): A Simpler Way ACI Materials Journal, 108(1) [2] Wille, K., Kim, D J., Naaman, A E (2011) Strain-hardening UHP-FRC with low fiber contents Materials and Structures, 44(3):583–598 [3] Kang, S.-T., Lee, Y., Park, Y.-D., Kim, J.-K (2010) Tensile fracture properties of an Ultra High Performance Fiber Reinforced Concrete (UHPFRC) with steel fiber Composite 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fiber-reinforced concretes Construction and Building Materials, 171:901–912 [19] Ngo, T T., Kim, D J (2018) Shear stress versus strain responses of ultra-high-performance fiberreinforced concretes at high strain rates International Journal of Impact Engineering, 111:187–198 20 ... f ) where Rhybrid,a+b is the shear resistance of UHP-HFRC reinforced with fiber a and b, Rmono,a , Rmono,b are the shear resistance of ultra- high- performance mono -fiber -reinforced concrete (UHP-MFRC)... DIF of shear peak toughness DIF of shear peak toughness DIF of shear peak toughness 2 Types of UHP-HFRCs Types of UHP-HFRCs Types of UHP-HFRCs (b) Shear strain strain b)b )Shear strain b )Shear Shear... 0.1 0.15 Shear strain uppeak to peak stress, Shear strain uptoto peak stress, Shear strain to peak stress, Shear strain up tostress, peak stress, Shear up to peak stress, Shear strain to stress,