STUDY ON PERFORMANCES OF STEEL FIBER REINFORCED C55 GRADE SELF-COMPACTING CONCRETE

Transcription

1 STUDY ON PERFORMANCES OF STEEL FIBER REINFORCED C55 GRADE SELF-COMPACTING CONCRETE Beixing Li (1), He Gao (1), Gong Cui (1) and Jin Zha (1) (1)Key Laboratory of Silicate Materials Science and Engineering of Ministry of Education, Wuhan University of Technology, China Abstract Based on the requirement for the construction application of high strength steel fiber reinforced self-compacting concrete (SFR-SCC) to the composite joint section between steel and concrete box girders of the super-long span hybrid girder cable-stayed bridge in the Edong Yangtze River, the effect of the volume fractions of short steel fiber on the workability and compressive strength of C55 grade SFR-SCC was investigated, and the differences in the mechanical properties, crack resistance, flexural toughness and durability between the C55 grade SFR-SCC and normal C55 grade SCC were tested. The results indicated that the incorporation of 0.6~0.8vol% steel fiber significantly increased the compressive strength of the SFR-SCC, and didn t affect the non-vibration compressive strength while meeting the self-compacting. Furthermore, the 0.6vol% steel fiber significantly increased the split tensile strength, flexural strength and elastic modulus, and decreased the sensitivity of plastic shrinkage crack and drying shrinkage value as well as enhanced the toughness of the SFR-SCC. Moreover, the SFR-SCC has high chloride penetration resistance and freeze-thaw durability. Key words: steel fiber; high strength self-compacting concrete; mechanical properties; crack resistance; durability 1. INTRODUCTION The Edong Yangtze River bridge with a main span of 926m is a hybrid girder cable-stayed bridge with two-tower and double-cable planes. Due to the importance and complexity of durability of the concrete in the composite joint section between steel and concrete box girders, it requires some special performances, such as self-compactability, high strength, low shrinkage and creep, high anti-cracking, improved toughness and impact resistance, as well as high durability, a steel fiber-reinforced self-compacting concrete (SFR-SCC) was introduced in the construction of the composite steel-concrete joint. Self-compacting concrete (SCC) is featured in its fresh state by high flowability and rheological stability. SCC has excellent applicability for elements with complicated shapes and congested reinforcement. SCC reinforced with steel fibers enhances its application 586

2 because the mechanical performance of concrete is improved. The addition of steel fibers into self-compacting concrete may take advantage of its high performance in the fresh state to achieve a more uniform dispersion of fibers. The compactness of the SCC matrix, due to the higher amount of fine and extra-fine particles, may improve interface zone properties [1], and consequently also the fiber-matrix bond, leading to enhanced post-cracking toughness and energy absorption capacity. SFR-SCC is more ductile and tougher than conventional SCC and has demonstrated higher residual strengths [2]. Workability of SFR-SCC is directly influenced by the type and content of fibers used, as well as the SCC matrix. The ratio of the length of fiber to its diameter is the aspect ratio (L/D). Higher aspect ratio and volume concentration (V f ) of fibers improve the performance of SFR-SCC in the hardened state but also adversely affect its workability [3]. Thus, the product of these two parameters, which is termed the fiber factor, has become a key index in comparing different SFR-SCC mixtures. The main goal of this study was to produce an optimized C55 strength grade SFR-SCC in terms of workability and hardened properties suited to casting the composite steel-concrete joint of the super-long span hybrid girder cable-stayed bridge. The research findings will help engineers to better understand the overall performance of SCC and SFR-SCC. 2. EXPERIMENTAL PROCEDURES 2.1 Materials An ordinary Portland cement from Huangshi Huaxin cement stock company, type P O 42.5 according the Chinese Standards GB , was used. Its Blaine fineness was 345 m 2 /kg and its specific gravity was 3.1 g/cm 3. The compressive strength of the cement is 27.4MPa and 52.6MPa at 3-day and 28-day, respectively. A C-type first grade fly ash in conformity with GB [4] from Wuahn Yangluo power plant was used as mineral admixture. Its Blaine fineness was 620 m 2 /kg and its specific gravity was 2.2g/cm 3. Local well-graded natural sand with a fineness modulus of 2.9 was used as fine aggregate. Its bulk density, water adsorption and fraction passing 75-μm sieve were 2630 kg/m 3, 1.2% and 1.5%, respectively. Coarse aggregate was local crushed limestone, with maximum size of 20 mm with a bulk density of 2710 kg/m 3. The superplasticizer (SP) was a polycarboxylic-acid type, commercially branded as BASF SP-8CR, produced by the Construction Chemicals Division of BASF, Shanghai, China. It is a retarding-type high-range water reducer. The solid content, ph and specific gravity of the admixture were 20%, 6.5%, and 1.04 g/cm 3, respectively. Short steel fibers having a dumbbell shape, type GS-2000, were employed. The fibers had a length of 20mm, a diameter of 0.4mm, an aspect ratio of 50. The density, modulus of elasticity and tensile strength of the fibers were 7.80 g/cm 3, 220 GPa and 1150 MPa, respectively. 2.2 Mix proportions Table 1 shows the mixture proportions of a normal-slump concrete (NC), a SCC and three SFR-SCC mixtures. The proportions of the NC mixture are already used for the manufacture of the prestressed box girders of the Edong Yangtze River bridge, which was a high-performance concrete developed by Wuhan Technology of University. Each mixture had a constant water/binder (w/b) of 0.31 and a constant fly ash amount of 20% by weight. If required, additional superplasticizer was added to achieve similar slump flow for all SCC mixtures of 650 ± 25 mm. 587

3 Table 1: Mixture proportions of concretes Component (kg/m 3 ) NC SCC SFR- SCC SFR- SCC SFR- SCC Cement Fly ash Natural sand Crushed aggregate Water Superplasticizer Steel fiber (% by volume) (0.6%) 54.6 (0.7%) 62.4 (0.8%) w/b ratio (by weight) Test methods Fresh properties tests All workability tests were conducted near the mixer after waiting for 30 seconds of mixing. Slump tests were conducted for traditional normal-slump concretes in accordance with GB/T Various workability tests, as mentioned as follows, were carried out for the SCC and SFR-SCC mixtures in accordance with the Self-Compacting-Concrete Committee of EFNARC [5,6]. Slump flow test Filling ability and flowability of the SCC and SFR-SCC mixtures were tested using the slump flow test. The slump flow is the mean diameter of the horizontal spread of the concrete mass, after lifting the slump cone. Targeted minimum slump flow for the research was 625 mm. T 50 time was also recorded during the slump flow test. T 50 time is the time required by the concrete mass to spread to 50cm diameter, indicating the filling ability of the mixture. Targeted T 50 for the project is 3 to 7 seconds. During the slump flow test, there was no restriction offered to the freely flowing SCC/SFR-SCCC mixtures. Hence, the flow spread and T 50 time recorded during this test were referred to as an unrestricted slump flow and unrestricted T 50 time. J-ring test Passing ability of the mixture was tested using a J-ring apparatus, to simulate actual congestion of reinforcement in the composite steel-concrete joint section. The ring used for the test includes 17 rebars of 10 mm diameter set equidistant, which leaves a net spacing of 40 mm between rebars. A J-ring test is performed by lifting the slump cone and allowing SCC/SFR-SCC to flow radially outward through the J-ring. Flow of SCC/SFR-SCCC is obstructed by the bars, thereby creating a difference of level in the concrete (quantified as the J-ring value) that is inside the J-ring and the one that has passed through it. The targeted J-ring value was 10 to 15 mm depicting a satisfactory passing ability. Slump flow and T 50 time was also measured during the J-ring test, which indicated the restricted slump flow and restricted T 50 time. 588

4 2.3.2 Plastic shrinkage crack propagation In order to initiate cracks in the concrete, a steel form of 600mm 600mm 63mm with 14 bolts of 10mm in diameter mounted regularly at each four sides of the rigid frame was used to restricts possible drying shrinkage of concrete. In order to avoid frictions between concrete and bottom plate of the restrict ion steel form, a Teflon sheet was placed at the bottom of the form, and a paraffin paper was further placed on it. The apparatus used for the evaluation of crack propagation is shown in [7]. Concrete was placed in the restrict ion steel form and cured with a cover of acrylic board for 2 hours under a constant temperature of 30 and relative humidity of 60%. Subsequently, it was exposed to an air flow with a velocity of 8 m/s under a constant temperature of 30 and relative humidity of 60%. Then the age of crack initiation, number of cracks, crack length and crack width were measured until the age of 24 hours. Parameters representing crack propagation characteristics at the age of 24 hours (at the end of test ) may be the average crack area, number of cracks per unit area and total crack area per unit area. The details of crack parameters calculation and evaluation methods are shown in [7] Hardened properties tests (1) Mechanical properties Standard concrete cubes measuring 150 mm in size, and standard beams of in 150mm square cross section and 300mm long were manufactured for each mixture. Cube compressive strength, split tensile strength and beam compression-modulus of elasticity were conducted to determine the mechanical properties of the mixtures according to the Chinese Standards JTG E It should be noted that the NC mixture specimens were mechanically vibrated whereas the SCC and SFR-SCC mixtures were cast without any mechanical vibration. These specimens were cast in steel forms and wet cured at 95 % percent relative humidity and 20. At different curing times (7 and 28 days), three specimens were used for each mechanical test. (2) Flexural toughness Two most common methods (ASTM C1018 [8] and JSCE SF-4 [9]) are used to measure the toughness of steel fiber reinforced concrete subjected to bending. The tests were carried out using a 2000KN Instron 1346 servo-hydraulic universal testing machine. During the test, a prismatic specimen of 100mm 100mm 400mm was placed on a simple support with clear span of 300mm, and then subjected to a third-point loading under displacement control at a rate of 0.01 mm/min. Load and deformation data were recorded during the tests and stored by the data acquisition system for processing at a later stage. Toughness indexes I 5, I 10 and I 30, etc., are then calculated by taking the ratios of the area under load-deflection curve up to a certain multiple of first-crack deflection (according to the deflections of 3, 5.5 and 15.5 times first-crack deflection, respectively ) and the area up to the occurrence of first crack. Unlike the ASTM C 1018, JSCE SF-4 provides just a single value of toughness. For a given load-deflection curve, toughness is the area under the load deflection curve measured up to a deflection of span/150 (2 mm). (3) Drying Shrinkage Free drying shrinkage test is carried out on prismatic specimens of 100mm 100mm 515mm according to JTG E After 1 day of wet curing, the demolded specimens were stored at constant temperature (20±3 ) and constant relative humidity (60±5%) while 589

5 measuring drying shrinkage at different curing times. (4) Durability Chloride ion permeability was measured using cylinders 100mm in diameter and 50 mm in height by RCM method based on chloride ion migration forced by electric current conducted by Tang Luping and the architecture material institute of German Aachen Industry University. The test was carried out according to Chinese Standard CECS and the non-steady-state diffusion coefficient of rapid migration (D RCM ) was achieved [7]. The frost resistance was measured by test method for resistance of concrete to rapid freezing and thawing on concrete prism specimens (100mm 100mm 400mm), wet cured for 28 days. The test was carried out according to the Chinese Standard DL/T The dynamic elastic modulus of the concrete specimens were measured every 50 cycles in order to evaluate the relative dynamic modulus (RDM) of elasticity. 3. RESULTS AND DISCUSSION 3.1 Fresh properties Observation during and after the tests showed that fibers were uniformly distributed, randomly oriented, and without any signs of balling or clustering. Table 2 presents the various workability test results of different concrete mixtures. All the SCC and SFR-SCC mixtures achieved the minimum target level of unrestricted slump flow, that is, 625 mm. None of the mixtures showed segregation, bleeding, or halo-formation. The unrestricted slump flows of SFR-SCC mixtures were as good as that of the SCC mixture. Comparatively larger slump flow could be attributed to the slightly more cementitious material content and larger superplasticizer dosages in the SFR-SCC mixtures. In this study, short steel fibers had almost no effect on the unrestricted slump flow. On the other hand, the restricted and unrestricted slump flows for SFR-SCC mixtures with the short steel fibers were almost the same, indicating better flowability of the mixture. Kinetic performance, that is, filling ability measured in terms of T 50 time of various SCC and SFR-SCC mixtures are presented in Table 2. The results indicate that all the SCC and SFR-SCC mixtures satisfied the targeted unrestricted T 50 time criteria of 3 to 7 seconds. SCC mixture had almost the same unrestricted and restricted T 50 time, while all SFR-SCC mixtures had restricted T 50 times much greater than the unrestricted T 50 time. A higher fiber volume content in the case of the SFR-SCC3 mixture tended to significantly increase the restricted T 50 time, but did not affect the unrestricted T 50 time. In other words, the addition of steel fibers significantly lowered the restricted filling ability. Among all the SCC and SFR-SCC mixtures, only SCC and SFR-SCC1 had a satisfactory J-ring value (that is, within the targeted range) of 15 mm. The J-ring value of SFR-SCC3 with the highest fiber volume content was largest, indicating lowest passing ability. From the workability test results, it is quite clear that SFR-SCC1 mixture can be considered within acceptable ranges of self-compactability, because its passing ability and filling ability is best among all the SFR-SCC mixtures. 590

6 Concrete Table 2: Summary of fresh concrete properties Slump Slump flow (mm) T 50 (s) (mm) Unrestricted Restricted Unrestricted Restricted J-ring value (mm) NC SCC SFR-SCC SFR-SCC SFR-SCC Plastic shrinkage crack propagation of fresh concrete Plastic shrinkage cracks are be caused by both water evaporation and cement hydration, and all plastic shrinkage occurs before setting. Table 3 shows the results of crack propagation in the wind condition in the end of test at 24 hours. Compared with the NC mixture, the SCC mixture has earlier age of crack initiation and more larger area of cracks. This may be attributed to hydration shrinkage due to relative increase of unit amount of cement and a decrease of unit amount of aggregate in the SCC mixture. Compared with the SCC, the amount of cracking was found to be lower for the SFR-SCC mixtures, and the average crack area of the SFR-SCC mixture is smaller than that of the SCC mixture. Moreover, the SFR-SCC mixture tends to crack later. Thus, the incorporation of steel fibers into SCC mixture improved its vulnerability to plastic shrinkage cracking. Table 3: Plastic shrinkage crack propagation of fresh concrete at the age of 24 hours Concrete Age of crack initiation /min Number of cracks /N Crack feature Average crack area /(mm 2 N -1 ) Number of cracks per unit area /(N m -2 ) Total crack area per unit area /(mm 2 m -2 ) NC Very fine SCC Fine SFR-SCC Very fine Mechanical properties The test results of compressive and split tensile strengths and elasticity modulus are summarized in Table 4. The 28-day compressive strength of the SFR-SCC mixtures varied between 79 and 81.9MPa, which obviously exceeded the target value of 70MPa. It should be noted that the compressive strengths of NC without any mechanical vibration were clearly lower than those with mechanical vibration. However, for all SCC and SFR-SCC mixtures, the mechanically vibrated compressive strengths were almost the same as those without any mechanical vibration, indicating a good slef-compactability of the mixtures. 591

7 Concrete Table 4: Summary of concrete mechanical properties Compressive strength of 7-day (MPa) vibrated Without vibrated Compressive strength of 28-day (MPa) vibrated Without vibrated Split tensile strength (MPa) Elastic modulus (GPa) 7d 28d 7d 28d NC SCC SFR-SCC SFR-SCC SFR-SCC Previous studies report a marginal to 20% increase in the compressive strength due to addition of fibers up to 1.5% by volume [5]. A comparison of 28-day compressive strengths of the SCC and all SFR-SCC mixtures, showed an increase from 12% to 16% due to the addition of steel fiber, which can be attributed to the better compaction and homogeneity of fiber distribution in SFR-SCC. The 28-day split tensile strength for the three SFR-SCC mixtures was 23% to 35% higher than that of the corresponding SCC mixture. Thus, fibers clearly increased the tensile strength of SCC. Past research has also reported an increase in split tensile strength of fiber-reinforced concrete [10]. Furthermore, a slight increase in the elastic modulus was observed in SFR-SCC mixtures over the corresponding non-fibrous SCC mixture. 3.4 Flexural toughness Typical load-deflection curves of SCC and SFR-SCC1 beams are given in Fig.1. For normal SCC, fracture occurred almost instantaneously once the peak load was reached, due to the tremendous amount of energy being released. For SFR-SCC1, the fiber bridging effect helped to control the rate of energy release. Thus, FRC maintained its ability to carry load after the peak. The most beneficial effect of adding steel fibers to plain concrete would be to increase the toughness of concretes. The toughness indexes according to ASTM C 1018 and the toughness values according to JSCE SF-4 of SCC and SFR-SCC1 are given in Table 5. It can be seen that for all toughness indexes, SFR-SCC1 has better toughness than those of SCC. At I 30, SFR-SCC1 was observed to be 170% more than SCC over the average toughness. Regarding the JSCE toughness, SFR-SCC1 has 202% larger toughness on average compared to that of SCC. 592

8 400 SCC SFR-SCC1 Dry shrinkage(10-6 ) NC SCC SFR-SCC Age(d) Fig.1: Typical load-deflection responses of concretes Fig.2: Drying shrinkage measurements up to 180 days Concrete Table 5: Flexural toughness of concretes Bending strength Average toughness indexes (MPa) I 5 I 10 I 30 Average toughness (N m) SCC SFR-SCC Drying shrinkage Fig. 2 shows the results of drying shrinkage obtained up to 180 days. The SCC and SFR-SCC mixtures with higher cementitious materials showed greater drying shrinkage than that of the NC mixture. Comparisons between SCC and SFR-SCC showed that the effectiveness of steel fibers addition in counteracting drying shrinkage of concrete was evident, because SFR-SCC exhibited 8.3% lower drying shrinkage after 6 months. 3.6 Chloride ion permeability Fig.3 shows the test results of chloride ion penetration of three concretes for curing 28-day and 56-day. The 28-day chloride ion diffusion coefficient (D RCM ) values of three mixtures are between 2.5 and m 2 /s, indicating moderate permeability. As curing age increased, the D RCM values was obviously decreased. It can be seen from Fig.5 that the D RCM values was slightly higher in the case of SFR-SCC1, followed by NC and SCC. The higher D RCM of SFR-SCC1 is attributed to the presence of a considerable amount of steel fibers. 3.7 Frost resistance The results of the freezing and thawing tests are shown in Fig.4. The results obtained up to 300 cycles showed that after 200 cycles, a significant trend towards low values of the RDM was detected in three concretes, especially for the NC and SCC mixtures (see Fig.4), thus 593

9 indicating a certain benefit related to the addition of steel fibers. Furthermore, it must be noted that the RDM of elasticity of the three concretes at 300 cycles ranged from 79% to 92%, indicating frost resistance higher than F300 grade d 56d DRCM (10-12 m 2 /s) RDM(%) NC SCC SFR-SCC NC SCC SFR-SCC Number of cycles Fig.3: Chloride ion diffusion coefficient of concretes Fig.4: RDM of elasticity of concretes 4. CONCLUSIONS Based on the findings of this study, the following conclusions can be made: (1) Highly workable and stable SFR-SCC can be made from locally available construction materials, which meets both the self-compaction and mechanical requirements. Incorporation of steel fibers decreased the workability of fresh concrete to some degree. However, addition of superplasticizer depending on the amount of fiber may improve the workability and self-compactability of fresh concrete. Optimum fiber contents for SFR-SCC mixture suitable for casting the composite joint section between steel and concrete box girders were: 0.6% by volume of short steel fibers (20mm long). (2) Test results of mechanical properties of SCC and SFR-SCC mixtures have confirmed the effectiveness of steel fibers in enhancing the compressive strength, tensile strength, flexural toughness of SCC. (3) The steel fiber addition proved to be very effective in preventing plastic shrinkage cracks and counteracting drying shrinkage of self-compacting concrete, which is usually a great problem for this material, rich in powders and poor in the coarse aggregate fraction. (4) The SFR-SCC durability is satisfactory, the chloride ion diffusion coefficient of SFR-SCC was low and the resistance to freezing and thawing was higher than F300 grade. The steel fiber addition to SCC slightly decreased resistance to chloride ion permeability and improved the frost resistance. REFERENCES [1] Corinaldesi V. and Moriconi G., Durable fiber reinforced self-compacting concrete, Cement and Concrete Research, Vol.34, , [2] Torrijos M.C., Barragán B.E. and Zerbino R.L., Physical-mechanical properties, and mesostructure of plain and fibre reinforced self-compacting concrete, Construction and Building Materials, 594

10 Vol.22, , [3] Grünewald S. and Walraven J. C., Parameter-study on the influence of steel fibers and coarse aggregate content on the fresh properties of self-compacting concrete, Cement and Concrete Research, Vol.31, , [4] GB , Standard of Fly ash for Cement and Concrete. Beijing: Standardization Administration of the People s Republic of China, 2005, 11pages. [5] Khayat K. H. and Roussel Y. Testing and performance of fiber-reinforced self-consolidating concrete, Materials and Structures, Vol.33, , [6] EPG guidelines, the European Guidelines for Self-Compacting Concrete Specification, Production And Use. May 2005, [7] CECS , Guide of Design and Construction to Durability of Concrete Structure. Beijing: China Architecture Press, 2005, 106pages. [8] ASTM C , Standard Test Method for Flexural Toughness and First-Crack Strength of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading). West Conshokocken: American Society for Testing and Materials, 1997, 8 pages. [9] Nataraja M. C., Dhang N., and Gupta A.P., Toughness characterization of steel fiber-reinforced concrete by JSCE approach, Cement and Concrete Research, Vol.30, , [10] Padmarajaiah, S. K. and Ramaswamy A., Comparative study on flexural response of full and partial depth fiber-reinforced high- strength concrete, Journal of Materials in Civil Engineering, Vol.14, ,