Fracture behavior of concrete reinforced with basalt fibers

Transcription

1 Fracture behavior of concrete reinforced with basalt fibers Mohammed Ishtiyaque #1, M.G. Shaikh *2 #*Applied Mechanics Department, Govt. College of Engineering, Aurangabad -Maharashtra 1 ishtiyaque2011@rediffmail.com, 2 mohummadshaikh@gmail.com Abstract In present study an attempt is made to study the effect of addition of basalt fibers on various fracture properties of concrete. Normal strength concrete was cast by using water to cement ratio as 0.38 and a total of five mixtures were prepared by using basalt fibers (0.25, 0.5, 0.75 &1% by volume of concrete) into those concretes. Three point bending test were conducted on notched beams to study fracture properties such as fracture energy, fracture toughness (K IC ) and critical tip opening displacements (CTOD C ) of different concrete mixes. Test results showed an improvement in tensile strength and fracture properties of basalt fiber reinforced concrete mixes when compared with control mix. Tensile strength was enhanced by 11 % with addition of 0.25% basalt fibers. Fracture toughness was also enhanced by 402% & 269 % with addition of 0.25% and 0.75% basalt fibers. However inclusion of basalt fibers resulted in reduction in the workability and compressive strength of concrete mixture with increasing fiber contents. 25 % decrease in compressive strength was observed with addition of 1% BF when compared with control mix. Keywords Basalt fibers, Fracture energy, Fracture toughness, Critical tip opening displacement. I. INTRODUCTION Basalt is a volcanic igneous rock which exhibits good performance in terms of strength, temperature range, and durability. Basalt fibers (BF) are obtained from basalt rocks through melting process. Basalt fibers have good resistance to chemical attach, impact load and fire with less poisonous fumes. Due to its lower cost and high tensile strength BF could be a suitable replacement for steel, glass, and carbon fibers in many applications. Previous research works on use of BF in concrete is limited. Concrete is the most widely used construction materials all around the world. Concrete is strong in compression, is full of defects like voids, air pockets, lack of bond between aggregates & cement paste and cracks. Therefore it is highly heterogeneous material with low tensile strength, low strain capacity in tension and low cracking resistance. Hence it is prone to fracture and fails in a brittle manner without warning when tensile stresses exceed tensile strength of concrete [1], [2]. Addition of fibers in concrete matrix improves its various fracture properties such as fracture energy, fracture toughness and critical crack mouth opening displacement. Fibers control initiation and propagation of cracks in concrete through bridging action during both micro and macro-cracking of the matrix. This mechanism increases the demand of energy for cracks to propagate. Therefore the overall fracture behavior of concrete is improved [3]-[5]. The purpose of present study was to evaluate the various fracture parameters such as fracture energy (G f),fracture toughness (Critical stress intensity factor-k IC) and critical crack mouth opening displacement (CMOD C) of normal strength concrete reinforced with different volume fractions of basalt fibers (0%,0.25,0.5,0.75 and 1%) by volume. A. Materials and Mix Proportions II. EXPERIMENTAL STUDY The materials used in this research includes basalt coarse aggregates with a particle density of 1560 kg/m 3,Godavari river sand with a particle density of 1750 kg/m 3,cement,super plasticizer and basalt fibers. Maximum particle size of the coarse aggregate, river sand was 20mm and 4.75 mm respectively. 79

2 The type of cement used in all concrete mixes was Portland pozzolana confirming to IS: (Part 1) of Ultra Tech brand and its properties are presented in Table I. A naphthalene based super plasticizer was also used. Basalt fibers with a density of 2.63 g/cm 3 and a length of 24 mm were used throughout the study at varying contents. Physical and mechanical properties of BF, provided by the manufacturer, are presented in Table II. TABLE I PHYSICAL AND MECHANICAL PROPERTIES OF CEMENT Physical Properties Specific gravity 3.15 Standard consistency (%) 32 Initial setting time (minutes) 270 Final setting time (minutes) 350 Volume expansion (mm) 1.0 Fineness (m 2 /kg) days compressive strength (MPa) 55 Length (mm) Diameter (µm) TABLE II PROPERTIES OF BASALT FIBERS Tensile Strength Density Elastic (MPa) (g/cm 3 ) Modulus (GPa) Thermal Conductivity w/mk B. Sample Preparation & Testing Procedure In order to determine the effect of BF addition on compressive strength and split tensile strength, cube specimens with dimensions of 150 mm were cast. Density of fresh concrete was determined by using 150mm cube specimens. In addition to this, 15 notched beam specimens (3 for each mixture) with dimensions of 100 mmx120 mmx525 mm and with a notch of depth 30 mm and thickness 3mm were tested by three point loading to determine the fracture energy (G f), critical stress intensity factor (K IC) and critical mouth opening displacement (CMOD c), as illustrated in Fig.1.Before performing hardened concrete tests at 90 days, all specimens were cured in water at 27 C 0. Compressive and split tensile strength each was determined on three 150mm cubic specimens for each mixture as per IS: The notches in the beam specimens were formed using 3 mm thick steel plates in the wooden moulds. The deflection, evaluated in the proximity of the notch on the lower side of the test specimen and crack mouth opening displacement (CMOD) of the notch was measured using distance sensors mounted on stand and steel frames. During the test, load, deflection and CMOD were recorded continuously using data logger. The experimental setup for fracture test is shown in Fig.1. Thus from the recorded data, load versus deflection and load versus CMOD curves were obtained for each specimen. The fracture energy (G f) was determined by using the procedures recommended by RILEM 50-FMC-Technical Committee. Critical stress intensity factor (K IC) and CMOD c were determined using the procedures recommended by RILEM 89 FMC-Technical Committee and two parameter model of Jenq and Shah (1989). 80

3 Fig. 1. Experimental Set up for fracture test TABLE III MIX PROPORTIONS & PROPERTIES OF FRESH CONCRETE MIXTURES Concrete MIxtures Cement w/c Ratio Coarse Aggregate River Sand SP Basalt Fiber Content By Volume (%) Slump (mm) Fresh Density C B B B B A. Workability III. RESULTS AND DISCUSSION Slump values of the concrete mixes are presented in Table III.It is found that addition of basalt fibers reduces the workability of concrete as some quantity of mixing water and cement paste is utilized for coating the surface of fibers. Hence higher doses of super plasticizer were required for basalt fiber reinforced concrete mixes. Density of fresh concrete was determined by using 150mm cube specimens. Density of fresh concrete mixes is also shown in Table III. B. Physical Properties The average values of bulk dry density are presented in Table IV. C. Compressive, Tensile Strength & Fracture Properties The average values of compressive and split tensile strength of concrete mixes are given in Table IV.The compressive strength of the mixtures varied between & MPa. Where the lowest value belongs to B100 mixture. The split tensile strength of the mixtures varied between 2.14 & 3.05 MPa.Where the highest value belong to mix B25 and lowest value for B100 mixture. 81

4 The average values of fracture energy and fracture toughness of concrete mixes are given in Table IV.The fracture energy of the mixtures varied between 0.16 & 1.37 N/mm.Where as the highest value belong to mix B25 and lowest value for mixture B75.Load versus mid-span deflection curves for prismatic specimens are shown in Fig.2. The fracture energy obtained on normal strength concrete was compared to the fracture energy obtained by other researchers for self compacting concrete and normal strength concrete. (Fig.4). The fracture toughness of the mixtures varied between & MPa mm 1/2.Where as the highest value belongs to mix B25 and lowest value for mixture B100. Load versus CMOD curves for prismatic specimens are shown in Fig.3.. TABLE IV MECHANICAL TEST RESULTS OF CONCRETE MIXTURES Concrete Mixtures V f a (%) f c b (MPa) f t c (MPa) G f d (N/mm) K IC e (MPa.mm 1/2 ) CMOD c f (mm) C 0 B B B B a Volumetric fraction of fibers b Average compressive strength c Average splitting tensile strength d Fracture energy e Critical stress intensity factor for mode I cracking f Critical crack mouth opening displacement Fig. 2 Load-deflection curves in fracture test 82

5 Fig. 3 Load-CMOD curves in fracture test Fig. 4 Comparison of fracture energy obtained by authors and other researchers IV. CONCLUSIONS The main conclusions of the study may be summarized as follows: The addition of 1% BF resulted in 25% and 22% reduction in the compressive and splitting tensile strength over the control mix, respectively. As fiber percentage increased, the probability of these fibers balling together and leaving voids in the matrix was greater. The addition of BF resulted in reduction in the workability of concrete mixture with increasing fiber contents. Addition of BF resulted in significant enhancement of splitting tensile strength of concrete mixtures by about 11% over the control mix. Fracture toughness was significantly enhanced by using BF. The maximum increase in fracture toughness was observed in mix B25 and B75 as 402% and 269 % respectively, when compared to control mixture. Fracture energy was also enhanced by using BF. Basalt fiber reinforced concrete mixtures showed higher ultimate loads at 0.25% BF content. The maximum increase in fracture energy was observed in mix B25. 83

6 ACKNOWLEDGMENT T Technical support of S.J. Karkare sir, proprietor of Sigma Industries, Pune (M.S.) for providing devoted data-logger along with distance sensors and help from technicians of concrete technology & strength of material laboratories of Government Engineering College, Aurangabad, Maharashtra, India and time to time encouragement by Nissar khan sir, in charge, heavy structures laboratory, IIT Mumbai are greatly appreciated. REFERENCES [1] B. Karihaloo, Fracture mechanics and structural concrete, Longman Scientific & Technical, New York: John Wiley & Sons, [2] S. Shah, S. Swartz, and C. Ouyang, Fracture mechanics of concrete: Applications of fracture mechanics to concrete, rock and other quasibrittle materials, New York: Wiley-Inter-Science Publication, [3] D. Dias, and C. Thaumaturgo, Fracture toughness of geopolymeric concretes reinforced with basalt fibers, Cement and Concrete Composites, vol. 27, pp , [4] K. Nihat, Abrasion resistance and fracture energy of concretes with basalt fiber, Construction and Building Materials, vol. 50, pp , [5] M. Di Ludovido, Prota, and G. Manfrendi, Structural upgrade using basalt fibers for concrete confinement, J. Compos Constar, vol. 14, pp , [6] D. Zongcai, and L. Jianhui, Mechanical behaviors of concrete combined with steel and synthetic macro-fibers, International Journal of Physical Sciences, vol. 1, pp , Oct [7] M. Pajak, and P. Tomasz, Flexural behaviour of SCC reinforced with different types of steel fibers, Constructions and Building Materials, vol.48, pp , June [8] U. Abbas, Materials development of steel-and basalt fiber-reinforced concretes, M. Eng. thesis, Norwegian University of Science And Technology, [9] RILEM Recommendation, Determination of fracture parameters of plain concrete using three-point bend tests, Mater Struct, vol. 23, pp , [10] ACI Committee 544, Measurement of properties of fiber reinforced concrete, ACI Material Journal, vol. 85, pp , [11] D. Hannant, Fiber reinforced concrete, Oxford: An imprint of Elsevier, [12] RILEM Committee 50 FMC, Determination of fracture energy of mortar and concrete by means of three-point bend tests on notched beams, Mater Struct. (RILEM Paris), vol.18, pp ,