UDK 691:620.1 ISSN 1580-2949 Original scientific article/Izvirni znanstveni članek MTAEC9, 49(4)563(2015) FRACTURE PROPERTIES OF PLAIN AND STEEL-POLYPROPYLENE-FIBER-REINFORCED HIGH-PERFORMANCE CONCRETE LASTNOSTI LOMA NAVADNEGA IN VISOKOZMOGLJIVEGA BETONA, OJAČANEGA S POLIPROPILENSKIMI VLAKNI Piotr Smarzewski, Danuta Barnat-Hunek Lublin University of Technology, Faculty of Civil Engineering and Architecture, Nadbystrzycka Str. 40, 20-618 Lublin, Poland p.smarzewski@pollub.pl Prejem rokopisa - received: 2014-08-05; sprejem za objavo - accepted for publication: 2014-09-22 doi:10.17222/mit.2014.180 The aim of this research was to establish the fracture properties of high-performance concrete (HPC) containing two widely used types of fibers. The experimental investigation consisted of the tests on cubes, cylinders and notched prismatic samples made of plain HPC and fiber HPC (FHCP) with variable contents of steel or/and polypropylene fibers ranging from 0.25 % to 1 %. Extensive data on compressive, splitting and flexural tensile behaviors, modulus of elasticity and fracture energy were recorded and analyzed. The experimental investigations showed that HPC in fracture mode I exhibit brittle/softening behavior. The FHPC materials showed a more ductile behavior compared to that of the HPC materials. Fiber bridges cracked on the fracture surface during the loading and delayed cracking, thus the element did not break. The results of the bending tests showed an extended post-peak softening behavior. The shape of the descending branch was dependent on geometrical and mechanical properties as well as the quantity of the fibers used. The results of the research were evaluated and it was shown that the fibers contributed considerably to the structural integrity and stability of the HPC elements, thereby improving their durable service life. Keywords: hybrid fibers, steel fibers, polypropylene fibers, high-perfomance concrete, mechanical testing, fracture properties Namen tega članka je bil ugotoviti lomne lastnosti visokozmogljivega betona (HPC), ki je vseboval dve vrsti vlaken. Eksperimentalno delo je bilo na kockah, valjih in zarezanih prizmatičnih vzorcih, izdelanih iz navadnega HPC in z vlakni ojačanega HPC (FHCP) s spremenljivo vsebnostjo od 0,25 % do 1 % jeklenih in/ali polipropilenskih vlaken. Ugotovljeni in analizirani so bili obsežni podatki o vedenju pri tlačni, cepilni in upogibni natezni obremenitvi, o modulu elastičnosti in energiji loma. Preizkusi so pokazali, da prelom HPC v načinu I izkazuje krhko/mehčalno vedenje. FHPC-materiali so pokazali bolj duktilno vedenje v primerjavi s HPC-materiali. Vlaknasti mostovi so pokali na površini loma med obremenitvijo in so zadrževali pokanje, zato se element ni porušil. Rezultati upogibnega preizkusa so pokazali povečano mehčanje po vrhu. Oblika pojemajočih korakov je bila odvisna od geometričnih in mehanskih lastnosti, kot tudi od količine uporabljenih vlaken. Rezultati preiskav so bili ocenjeni in pokazalo se je, da vlakna občutno prispevajo k strukturni integriteti in stabilnosti HPC-elementov in s tem podaljšajo dobo uporabnosti. Ključne besede: hibridna vlakna, jeklena vlakna, polipropilenska vlakna, visokozmogljiv beton, mehanski preizkusi, lomne lastnosti 1 INTRODUCTION geometric properties. With each type of fiber certain pro- perties of concrete can be improved. In order to improve High-performance concrete (HPC) is a material mechanical properties, especially the tensile and flexural frequently used in the building industry due to its strengths and long-term concrete shrinkage, steel fibers durability. Concrete technology has developed at a rapid are usually used. Low-modulus polypropylene fibers can pace over the last two decades and the material perfor- reduce early-age shrinkage and help control the pheno-mance has been significantly improved. Initially, the menon of the spalling of concrete during fire. One of the attention of the researchers was only focused on recent concepts is the hybridization of fibers, the opti-increasing the compressive strength. Nowadays, HPC mum combination of several kinds of fibers with with a compressive strength exceeding 100 MPa can be different properties to create a complex composite with a readily designed and manufactured. However, the brittle- very high resistance to cracking in a wide range of crack ness of concrete increases with an increase in its width.10 The hybrid-fiber-reinforced concrete is mainly strength. The higher the strength of concrete, the lower is used in underground waterproof projects, road and its ductility. Fibers are added to the matrix as a reinforce- bridge engineering and seismic structures. A lot of re-ment to control the cracking, to increase the ductility and search revealed that a hybrid of steel and polypropylene to improve the general ductility of a material.1-4 Fiber- fiber in concrete exhibits composite advantages of the concrete research has been conducted for over fifty two-fiber material properties, improves the interface years5,6 and future directions for its development are still condition between cement and aggregate, enhances the being set.7-9 Nowadays, there are numerous types of medium continuity of concrete, and constraints the fibers made of different materials that are of different occurrence and development of concrete cracks.11-15 The basic parameter for measuring the fracture process in quasi-brittle materials is the fracture energy (Gf). The most common way to measure the fracture energy of concrete materials is the method proposed by RILEM TC 50-FMC.16 In order to analyze and compare the fracture behavior of high-performance concrete (HPC), steel-fiber-reinforced high-performance concrete (SFHPC), polypropylene-fiber-reinforced high-performance concrete (PFHPC) and hybrid (steel and poly-propylene)-fiber-reinforced high-performance concrete (HFHPC), by varying the type, proportion and content of the fibers, it is necessary to evaluate the experimental fracture energy up to the value that is limited by the long sloping curve. This study evaluated the effects of the type and content of the fibers on the mechanical properties and fracture of FHPC. For this purpose, three-point bending tests on notched samples were carried out in accordance with RILEM TC 89-FMT17,18 and the experimental fracture-energy values calculated up to the threshold criteria were compared. 2 MATERIALS, SAMPLES AND THE TEST PROGRAM 2.1 Details of the materials and sample specifications The experimental examination was carried out on cubes, cylinders and notched prismatic samples made of HPC and FHPC with variable contents of steel fibers (SFHPC), polypropylene (PFHPC) or the hybrid (HFHPC). The following tests were conducted: compres-sive strength, splitting tensile strength, modulus of elasticity, and the three-point bending tests to determine the effects of the fiber type and its content on the com-pressive-tensile behavior, deflection and fracture energy. Sample specifications used in the test program are shown in Table 1. 2.2 Mixture design and the sample-production process The following components were used in the recipes for the concrete mixtures: Portland cement CEM I 52.5 N-HSR/NA, two types of the coarse aggregate - grano-diorite or granite, quartz sand, water, condensed silica fume and superplasticizer. The maximum size of the coarse aggregate was 8 mm. The silica fume in the form of powder had a specific surface of 15-30 m2/g. Baumix steel fibers, hooked-end, with a 1100 MPa tensile strength, 200 GPa modulus of elasticity, 50 mm length and an aspect ratio of 50 were used. Polypropylene fibers Baucon had a length of 12 mm and a modulus of elasticity of 3.5 GPa. The tests on the CEM I 52.5 N-HSR/NA cement were carried out according to the Polish standards.19 20 A chemical analysis was performed, the cement composition was determined and the results obtained are shown in Tables 2 and 3. The determination of the particle-size distribution for the granodiorite and granite aggregates as well as quartz sand was performed in line with the standard.21 Table 2: Chemical composition of cement in mass fractions, w/% Tabela 2: Kemijska sestava cementa v masnih deležih, w/% Cement component Content, w/% SiO2 20.92 Al2O3 3.50 Fe2O3 4.38 CaO 64.69 MgO 1.20 SO3 3.07 Na2O 0.22 K2O 0.38 Cl 0.082 Ignition loss 1.27 Ash 0.26 Total 99.97 Table 3: Cement technical parameters Tabela 3: Tehnični parametri cementa Cement characteristics CEM I 52.5 N-HSR/NA Specific surface area (cm2/g) 4433 Water demand (%) 30 Commencement of bonding (min) 120 End of bonding (min) 180 Volume stability according to Le Chateliere (mm) 2.00 Compressive strength after 2 d (MPa) 27.7 Compressive strength after 28 d (MPa) 57.1 Tensile strength after 2 d (MPa) 5.29 Tensile strength after 28 d (MPa) 8.23 Table 1: Sample specifications Tabela 1: Specifikacija vzorcev Label HPC1 SFHPC HFHPC1 HFHPC2 HFHPC3 PFHPC HPC2 Fiber type - Steel (S) Hybrid (S +P) Hybrid (S +P) Hybrid (S +P) Polypropylene (P) - Lf/mm - 50 50/ 12 50/ 12 50/12 12 - Vf/Vc 0 1 0.75 + 0.25 0.5 + 0.5 0.25 + 0.75 1 0 Number of the tested samples Compression test 3 3 3 3 3 3 3 Splitting tensile test 3 3 3 3 3 3 3 Modulus of elasticity test 3 3 3 3 3 3 3 Three point bending test 3 3 3 3 3 3 3 Table 4: Mixture proportions Tabela 4: Razmerja me{anic Material Symbol, unit HPC1 SFHPC HFHPC1 HFHPC2 HFHPC3 PFHPC HPC2 Cement c/(kg/m3) 670.5 670.5 670.5 670.5 670.5 670.5 670.5 Quartz sand 0/2 mm 5/(kg/m3) 500 500 500 500 500 500 500 Granodiorite 2/8 mm a1/(kg/m3) 990 990 990 - - - - Granite 2-8 mm a2/(kg/m3) - - - 990 990 990 990 Silica fume s//(kg/m3) 74.5 74.5 74.5 74.5 74.5 74.5 74.5 (sf/c)/%o 11 11 11 11 11 11 11 Superplasticizer sp/(L/m3) 20 20 20 20 20 20 20 sp/(c + sf)/% 2.7 2.7 2.7 2.7 2.7 2.7 2.7 Water w/(L/m3) 178 178 178 178 178 178 178 w/(c + s/) 0.24 0.24 0.24 0.24 0.24 0.24 0.24 Steel fiber wfs/(kg/m3) - 78 58.5 29.25 19.5 - - Vfs/% 0 1 0.75 0.5 0.25 0 0 Polypropylene fiber wfp/(kg/m3) - - 2.25 4.5 6.75 9 - Vfp/% 0 0 0.25 0.5 0.75 1 0 In order to attain the same workability, the ISOFLEX CX 793 superplasticizer based on polycarboxylate ethers with a density of 1.065 g/cm3 at 20 °C was used in all the concrete mixtures. The HPC and FHPC mix designs per cubic meter of the concrete used in the experimental program are shown in Table 4. The mixtures were prepared using a typical concrete mixer with a capacity of 100 L. The mixing procedure was as follows: quartz sand and coarse aggregate were homogenized together and mixed with half a quantity of water. Then, cement, silica fume, the remaining water and, finally, the superplasticizer were added. After the components were thoroughly mixed, the fibers were gradually added by hand to obtain homogeneous and workable mixtures. Molds coated with an anti-adhesive substance were filled with the concrete batch that was compacted on a vibrating table. After the compacting, the samples were covered with a foil to minimize the loss of moisture. All the samples were stored at a temperature of about 23 °C until removing them from the moulds after 24 h and they were placed into a water tank for 7 d to cure. After 7 d the samples were removed from the tank to cure in the laboratory conditions for up to 28 d. 2.3 Test equipment and solutions Compressive and splitting tensile tests were carried out after 28 d, in accordance with the standards,22,23 using 100 mm cubes. A Walter-Bai AG servo-hydraulic closed-loop testing machine with a capacity of 3 MN was used. The test method for the static modulus of elasticity of concrete in compression was performed on the cylinders with a 150 mm diameter and a height of 300 mm, measuring the deformation stress of the samples in the range from 0.5 MPa to 30 % of the concrete compressive strength. The examination was conducted by means of a Walter-Bai AG press and a modulus-measuring device with an extensometer in line with the recommendations of ASTM.24 Three-point bending tests were also performed after 28 d on a MTS 809 axial/torsional testing system machine, in accordance with RILEM TC 89-FMT,17 using 80 mm X 150 mm x 700 mm prismatic samples (Figure 1a). In the mid-span of each sample, a single notch was made with a concreting flat iron sharpened at its tip, with a thickness of 3 mm and a depth of 50 mm, in order to locate the cracks. Before the testing, the samples were provided with plaques for fixing the clip gauge thereon. Figure 1: Three-point bending test: a) sample geometry and dimensions, b) experimental-set up Slika 1: Tritockovni upogibni preizkus: a) geometrija vzorca in dimenzije, b) eksperimentalni sestav The tests were conducted by imposing a displacement rate of 0.05 mm/min. In order to measure the crack-mouth-opening displacement (CMOD), a strain gauge consisting of elastic plates separated by means of a non-conductive cube was used (Figure 1b). It was provided from the outside to the system for an automatic detection of the CMOD values corresponding to the loads applied. 3 RESULTS AND DISCUSSION 3.1 Compressive strength The cube compressive peak strength of each sample, the mean values of the compressive strength, the standard deviation and the coefficient of variation are given in Table 5. The cube compressive strength was insignificantly affected by adding steel and polypropylene fibers; however, a higher decrease in the compression strength was observed when the percent of the polypropylene-fiber volume added was higher. This was mainly due to the low modulus of elasticity of the propylene fibers and to some difficulties in dispersing the fibers in the mixtures. It should be noted that at a 1 % fiber volume in PFHPC, it had 73 % of the strength of the HPC2 fiber-free cube made of the same aggregate, whereas at the polypropylene-fiber volume of 0.75 % and the steel fibers of 0.25 %, the strength of the cube made of HFHPC was about 95 % of the HPC2 strength. Table 5: Cube compressive strength Tabela 5: Tla~na trdnost kock Mixture designation Compressi-ve strength MPa Mean value MPa Standard deviation MPa Coefficient of variation % 146.4 HPC1 152.3 151.0 4.1 2.7 154.4 158.0 SFHPC 152.2 154.9 2.9 1.9 154.6 142.5 HFHPC1 143.1 144.7 3.4 2.3 148.6 132.5 HFHPC2 138.2 133.9 3.8 2.9 130.9 124.1 HFHPC3 120.1 122.3 2.0 1.7 122.7 94.8 PFHPC 95.8 94.6 1.3 1.4 93.2 130.6 HPC2 125.3 129.5 3.8 2.9 132.6 3.2 Splitting tensile strength The cube peak-splitting tensile strength for each sample, the mean values of the splitting tensile strength, the standard deviation and the coefficient of variation are shown in Table 6. The addition of the fibers significantly affected the cube splitting tensile strength; however, a higher increase was observed at a higher percent of the steel fibers added. It was noted that at a 1 % steel-fiber volume in SFHPC, the cube strength increased by 55 % compared to the HPC1 fiber-free cube. A similar increase in the strength was observed for HFHPC at a 0.75 % steel-fiber volume and 0.25 % polypropylene-fiber volume. Lower increases in the strength of 12 % were obtained at a 1 % polypropylene-fiber volume in PFHPC compared to the HPC2 fiber-free cube. Table 6: Cube splitting tensile strength Tabela 6: Natezna trdnost cepljenja kock Mixture designation Splitting tensile strength MPa Mean value MPa Standard deviation MPa Coefficient of variation % 8.9 HPC1 9.3 8.9 0.4 4.7 8.5 14.0 SFHPC 13.8 13.8 0.1 1.0 13.7 13.8 HFHPC1 13.3 13.5 0.2 1.7 13.5 10.4 HFHPC2 9.6 10.0 0.4 3.8 10.1 9.5 HFHPC3 9.3 9.3 0.2 1.9 9.2 7.4 PFHPC 7.7 7.6 0.2 3.1 7.9 7.1 HPC2 6.7 6.8 0.3 4.5 6.5 3.3 Modulus of elasticity The cylinder modulus of elasticity for each sample, the mean values of the modulus of elasticity, the standard deviation and the coefficient of variation are shown in Table 7. The value of the cylinder elasticity modulus was only slightly affected by adding the steel fibers and it increased with their volume fraction added. This was mainly due to the high modulus of elasticity of the steel fibers. It should be noted that at a 1 % fiber volume in SFHPC, the cylinder modulus of elasticity was 4 % higher than that of the HPC1 fiber-free cylinder. At a lower content of the steel fibers, the modulus value was gradually decreased and at a 0.25 % fiber volume in the cylinder made of HFHPC3, the modulus value was lower by 10 % compared to that of the HPC2 standard concrete. Table 7: Cylinder modulus of elasticity Tabela 7: Modul elasti~nosti valjastih vzorcev Mixture designation Modulus of elasticity MPa Mean value MPa Standard deviation MPa Coefficient of variation % HPC1 38381 38645 38087 38371 279 0.7 SFHPC 39767 39466 39986 39740 261.1 0.7 HFHPC1 34266 33987 34562 34272 287.5 0.8 lHFHPC2 32890 32355 32096 32447 404.9 1.2 HFHPC3 29870 29264 29763 29632 323.4 1.1 PFHPC 29641 29455 29179 29425 232.5 0.8 HPC2 32322 32780 32542 32548 229.1 0.7 Figure 2: Typical experimental load-deflection curves for the FHPC notched prismatic samples Slika 2: Zna~ilne krivulje obremenitev - deformacija prizmati~nih FHPC-vzorcev z zarezo 3.4 Flexural tensile behavior The behavior of the HPC ordinary concrete samples during the bending test was almost linear-elastic up to the peak-load values, then the curve was sloping until the complete separation of the samples into two parts. The FRC samples showed a trilinear variation with the significant cracking between the first crack load and the peak load. The typical experimental load (F) - deflection (^) curves for the selected samples SFHPC, PFHPC and HFHPC - recorded during the bending tests, are shown in Figure 2. The examination was carried out in two stages. During the first stage, the crack-mouth-opening displacement (CMOD) and deflection were measured until the cracking of the beams along the whole height. After dismantling the strain gauge only deflection was recorded. A typical generic curve of the FRC samples is characterized with a linear curve up to the first crack, and then with the non-linear behavior up to the peak load. After reaching the peak load, the load-carrying capacity declines; however, the higher the loss of the strength, the lower is the steel-fiber content. As micro-cracks grow and join into larger macro-cracks, the long hooked-end fibers become engaged in crack bridging. Compared to the high-performance concrete, the peak load increases when steel fibers are used as shown in Figure 3 and Table 8. Figure 3: Typical experimental load-deflection curves for HPC and HFHPC Slika 3: Zna~ilne eksperimentalne krivulje obremenitev - deformacija za HPC in HFHPC In particular, with respect to the typical curves for SFHPC, the peak load is increased by 17 %, compared to that of HPC1. An even higher increase of 92 % was obtained for the HEHPC2 batch (from 0.5 % of steel fibers and 0.5 % of polypropylene) compared to HPC2. The PEHPC samples with a low modulus of fibers of 1 % achieved similar peak-load values for the HPC2 samples. A very high coefficient of variation was observed for the steel-fiber-reinforced batch, which indicated that the decisive factor in obtaining the peak-load values was the amount of the longitudinally oriented fibers. Table 8: Three-point bending tests Tabela 8: Tritočkovni upogibni preizkusi Mixture designation Peak load kN Mean value kN Standard deviation kN Coefficient of variation % HPC1 7.5 7.0 7.9 7.5 0.4 6.0 SEHPC 12.1 7.1 7.1 8.8 2.9 32.9 HEHPC1 7.3 10.6 8.2 8.7 1.7 19.6 HEHPC2 6.2 12.3 11.0 9.8 3.2 32.7 HEHPC3 8.5 7.0 6.8 7.4 0.9 12.5 PEHPC 5.1 5.0 5.4 5.2 0.2 4.0 HPC2 5.0 5.0 5.3 5.1 0.2 3.4 /fcL = 2 Fl l 2b( h - a o)2 (MPa) where b = 80 mm, h =150 mm and l = 600 mm relate to the width, the height and the span of the samples tested, ac = 50 mm is the notch depth and (h - ac) is the distance between the tip of the notch and the top edge of a sample. The parameters /eq,2 and /eq,3 were evaluated up to the deflections of = ^l + 0.65 mm and Ö3 = ^l + 2.65 mm, where ^l is the deflection corresponding to Fl. The portion of the energy required by the fracture of the concrete corresponding to the OBA field in Figure 4 (D^oab) was not considered in assessing the equivalent flexural strength. Only the effect of the fibers was considered (ABCD - DfBz,2 and EEGH - DfBz,3 in Figure 4). The equivalent flexural strength was calculated from the following expressions: feq,2 /en,3 3l D f BZ,2 2b(h-a 0)2 0.5 3l Df BZ,3 2b(h-a 0)2 2.5 (MPa) (MPa) (2) (3) The residual flexural tensile strengths /r,i and /r,4 at the mid-span deflections of 0.46 mm and 3.00 mm, respectively, were computed according to: /r,1 = /r,4 = 3 F l 2b( h - a 0)2 3 Fr,4 l 2b( h - a 0)2 (MPa) (MPa) (4) (5) 3.5 Fracture properties In order to describe the EHPC post-cracking enhancement, different toughness indexes were proposed. RILEM proposed the concept of an equivalent flexural tensile strength, feq.25 Recently, RILEM proposed the concept of a residual flexural tensile strength, /r, which is more readily assessed.26 According to RILEM, recently recommended /eq,2 or /r,i are used in the verification of the serviceability limit states, and /eq,3 or /r,4 are applied in the ultimate limit-state analysis.27 These parameters are also used to determine the stress-strain curves which are useful in modeling the EHPC postcracking behavior. With regard to the experimental curves, the load at the limit of proportionality, Fl, the corresponding strength, /fct,L, the equivalent flexural strengths, /eq,2 and /eq,3, and the residual flexural strengths, /r,i and /r,4, relating to the mid-span deflections of 0.46 mm and 3.00 mm, were evaluated. The load at the limit of proportionality Fl is equal to the highest value of the load recorded up to a deflection of 0.05 mm. The strength corresponding to the limit of proportionality can be computed using the following equation: Figure 4: Evaluation of flexural-tensile-strength parameters according to RILEM TC 162-TDE Slika 4: Vrednotenje parametrov upogibne natezne trdnosti skladno z RILEM TC 162-TDE The energy dissipation in the fractured concrete is the most advantageous characteristic of FHPC due to the addition of the fibers to the material. The fracture energy (Gf) was computed as the area under the stress-displacement curves. Assuming a linear stress distribution in relation to the fracture depth, the tensile stress was calculated according to the following formula: 3 Fl ^ = ^-^ (MPa) (6) 2b(h h - a 0)2 where F is the load recorded during the three-point bending test. The fracture energy was computed up to the predetermined load-deflection point based on the following formula: G F = add (N/mm) (7) The fracture energy for the FRC samples needs to be computed in relation to the specified value of the displacement. A reliable cut-off point can be selected at a 10 mm displacement.28 However, only a fracture dissipated up to a deflection of 3 mm seems to be interesting from the design viewpoint29 and such a deflection value was adopted while computing energy in this work. The results of the strength parameters and the fracture energy are included in Table 9. Referring to the data given in Table 9, it is noteworthy that the flexural-strength values obtained from Equations (2) and (3) lead to similar strength values in the cases of steel and hybrid fibers. The samples with the addition of polypropylene fibers only constitute an exception. For all kinds of fibers, the equivalent flexu-ral-strength values are lower than the strengths at the limit of proportionality given with Equation (1). The highest differences were observed for the polypropylene fibers. The obtained results emphasize the effect of the elasticity modulus on the variation in the HPC fracture properties. It can be seen that in FHPC the highest fracture energy was obtained with the addition of steel and polypropylene fibers, 0.5 % of each. The percentage volume of the steel fibers accounted for the fracture-energy increase. When decreasing the fiber content from 1 % to 0.75 %, the HFHPC-1 samples showed a decrease in the fracture energy by approximately 7 % compared to the SFHPC samples. The Gf values for the batches with the hybrid, steel-polypropylene fibers are due to the volume content of the low-modulus fibers. The higher the volume content of the low-modulus fibers, the lower are the Gf values obtained. The results of the fracture energy from Table 9 show that the ductility of the FHPC material at a high level of the strain depends largely on the capability of the fibers to bridge the cracks. Stiffer steel fibers provide a higher resistance to the loads whereas polypropylene fibers of a low modulus of elasticity provide a higher resistance to shrinkage and temperature stress. Therefore, it seems appropriate to combine the fibers of high and low modulus which can result in a longer period of durable service life. In the case of two batches of the HPC samples, a brittle fracture occurred through a separation of the elements into two parts (Figure 5). The FHPC samples with cracks underwent a significant deflection; however, not in all the cases the brittle Table 9: Tensile-strength parameters and experimental fracture energy Tabela 9: Parametri natezne trdnosti in lomna energija pri preizkusih Mixture designation FL/kN /fct,L/MPa /eq,2/MPa ./eq,3/MPa /R,1/MPa /R,4/MPa GF/(N/mm) SFHPC-1 12.1 13.5 6.4 12.3 5.7 12.6 32.3 SFHPC-2 7.1 8.0 7.0 6.9 7.9 6.1 17.1 SFHPC-3 7.1 8.0 7.4 7.5 7.9 6.1 19.0 Mean value 8.8 9.7 6.9 8.9 7.2 8.3 22.8 Standard deviation 2.9 2.9 0.5 3.0 1.3 3.7 8.3 HFHPC1-1 7.3 8.2 6.6 7.5 6.8 7.1 18.8 HFHPC1-2 10.6 11.9 6.1 9.7 6.8 10.6 24.9 HFHPC1-3 8.2 9.2 7.8 8.3 8.4 9.1 19.9 Mean value 8.7 9.8 6.8 8.5 7.3 8.9 21.2 Standard deviation 1.7 1.9 0.9 1.1 0.9 1.8 3.2 HFHPC2-1 6.2 7.0 6.1 6.6 6.2 6.1 16.6 HFHPC2-2 12.3 13.8 7.3 14.8 7.9 11.6 29.1 HFHPC2-3 11.0 12.4 7.1 11.1 5.6 10.5 28.3 Mean value 9.8 11.1 6.8 10.8 6.6 9.4 24.7 Standard deviation 3.2 3.6 0.6 4.1 1.2 2.9 7.0 HFHPC3-1 8.5 9.6 8.4 8.8 8.9 8.1 22.1 HFHPC3-2 7.0 7.9 6.6 5.9 7.5 6.5 14.8 HFHPC3-3 6.8 7.7 6.5 5.8 6.2 4.2 14.3 Mean value 7.4 8.4 7.2 6.8 7.5 6.3 17.1 Standard deviation 0.9 1.0 1.1 1.7 1.3 2.0 4.4 PFHPC-1 5.1 5.7 2.6 1.8 2.4 1.0 4.5 PFHPC-2 5.0 5.6 2.5 1.7 2.3 0.9 4.0 PFHPC-3 5.4 6.1 2.8 2.4 2.8 1.3 6.2 Mean value 5.2 5.8 2.6 2.0 2.5 1.1 4.9 Standard deviation 0.2 0.3 0.1 0.4 0.3 0.2 1.1 Figure 5: Notched HPC1 and HPC2 samples without fibers after the test Slika 5: Vzorci HPC1 in HPC2 z zarezo in brez vlaken po preizkusu Figure 6: Notched FHPC samples after the test Slika 6: Vzorci FHPC z zarezo po preizkusu destruction of the samples caused by a separation into two parts occurred ^^^re 6) as some bridges were formed by the fibers on the crack surface and limited the split. The samples with the fibers exhibited a more ductile behavior and the main crack ran from the crack-opening tip to the upper edge of the beam ^^^re 7). 4 CONCLUSIONS The presence of steel fibers had an insignificant effect on the compressive strength of the high-performance concrete. Steel fibers increased the compressive strength by about 2.6 % at a 1 % fiber volume content. On the other hand, polypropylene fibers reduced the compression strength by about 37 % at 1 % fiber volumes. The addition of steel fibers in the amount of 0.5 % and the polypropylene amounting to 0.5 % caused an increase in the compressive strength by 3.4 % as compared to the standard high-performance concrete HPC2. The presence of the fibers had a significant impact on the splitting tensile strength of the high-performance concrete. However, the polypropylene fibers improved the splitting tensile strength by only 12 % at a 1 % fiber volume content, while the steel fibers at the same fiber volume content caused an increase in this parameter by 55 %. In the case of hybrid fibers the increase depended on the steel-fiber volume content and it was (52, 47 and 37) % for the (0.75, 0.5 and 0.25) % steel fiber volumes, respectively. The volume of the steel fibers was also a decisive factor regarding the modulus values for the high-perfor- Figure 7: Crack opening on the HEHPC2-1 sample ^fs = 0.5 %^fp = 0.5 %) Slika 7: Odpiranje razpoke na vzorcu HEHPC2-1 ^^ = 0,5 %^fp = 0,5 %) mance concretes. At the 1 % steel fiber volume there was a slight increase in the modulus by about 3.6 %. In all the other cases, there was a decrease in the modulus from 3 % to 12 %; the lower the decrease, the higher was the volume of the steel fibers of a high modulus. The above mentioned property was also affected by the type of the coarse aggregate. All the samples of the SEHPC, HEHPC and PEHPC fibers showed a typical three-line variation in the load-deflection curves at flexure. The curves mostly consisted of a linear branch up to the first crack, followed by non-linear behavior up to the peak and the extensive descending/strain softening region. The SEHPC samples showed the peak flexural load that is higher by 17 % compared to HPC1 at the 1 % fiber volume content. The PEHPC samples of a low elasticity showed a slight increase of about 2 % in the HPC2 peak load at the 1 % fiber volume content. The highest increase in the peak flexural load of 92 % was observed with the addition of the steel fibers in the amount of 0.5 % and polypropylene fibers in the amount of 0.5 %. The SFHPC samples had the residual flexural strength at 17 % of their peak-load values at a deflection of 3 mm. The HFHPC and PFHPC samples which contained the fibers of a low elasticity modulus showed an abrupt decrease in the load capacity immediately after the peak load, while the load losses ranged from 10 % for HFHPC1 to 473 % for PFHPC, depending on the volume fraction of the polypropylene fibers. Increasing the content of the polypropylene fibers reduces the fracture energy. The Gf values for the high-modulus fibers should be numerically higher than the corresponding values for the low-modulus fibers. However, the fracture energy is not a measure of the efficiency and effectiveness of the fibers in inhibiting the cracks. In the load-deflection curves it can be seen that similar extensibilities are caused by the low-modulus fibers and the high-modulus ones. Adding fibers to concrete has a significant effect on the splitting tensile strength, modulus of elasticity, flexural strength, fracture behavior, fracture energy and ductility. 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