FRACTURE STUDIES ON CONCRETES WITH HYBRID STEEL FIBERS

Transcription

1 BEFIB212 Fibre reinforced concrete Joaquim Barros et al. (Eds) UM, Guimarães, 212 FRACTURE STUDIES ON CONCRETES WITH HYBRID STEEL FIBERS Murat Aral *, Ozkan Sengul *, Canan Tasdemir * and Mehmet A. Tasdemir * * Faculty of Civil Eng., Istanbul Technical University Ayazaga Campus, Maslak, Istanbul, Turkey tasdemirme@itu.edu.tr, web page: Keywords: Steel fiber, polypropylene fiber, mechanical behavior, fracture energy, optimization. Summary: The main obective of this work was to provide more information on the properties of hybrid fiber reinforced concretes. Meso steel fibers, macro steel fibers and micro polypropylene fibers were used in the study. Three different steel fibers with and/or without hooked ends were added to the mixtures. The volume fraction of each steel fiber was variable, but their total volume fractions were kept constant at 2 %. Twelve concrete mixtures with or without different fibers were produced, these mixtures were; a control mixture without any fibers, a mixture produced using only polypropylene fibers, and 1 mixtures containing both steel fibers and polypropylene fibers. In all these mixtures, the volume fraction of polypropylene fiber was kept constant at.5 %. Partial replacement of aggregate by steel fiber was based on one to one volume basis. Compressive strength, splitting strength and modulus of elasticity of the specimens were obtained. Fracture energies were also determined. In order to determine the optimum amounts of fibers, a multi-obective simultaneous optimization technique containing desirability functions was performed. 1 INTRODUCTION Concrete is a composite material consisting of cement paste, aggregates and interfaces between them. Concrete usually have low tensile strength due to its brittle nature. Loads such as dynamic loading may easily result in initiation and propagation of cracks in concrete. In today s concrete technology, very high strength concretes can be easily produced. One of the disadvantages of such concretes, however, is the brittleness. As the concrete strength increases, the brittleness also increases, which may limit the application of high strength concretes. One way of controlling cracking and reducing the brittleness (i.e. increasing ductility) of high strength concretes is to utilize short steel fibers in concrete. Uniformly and randomly distributed short steel fibers in concrete can increase energy absorption, toughness, impact resistance, tensile strain capacity and ductility of concrete. The main contribution of steel fibers to concrete can be seen after matrix cracking. If a proper mixture is designed, after the matrix cracking, randomly distributed short fibers in the matrix act as crack arresters by bridging mechanism, undergo a pull-out process, delay crack formation and limit crack propagation [1]. Debonding and pulling out of fibers from the matrix require more energy; therefore, a substantial increase occurs in toughness. Fiber type, aspect ratio (length/diameter), volume fraction, orientation of fibers in the matrix and pull-out resistance of fibers, as well as matrix properties influences the performance of steel fiber reinforced concretes [2]. Use of steel fibers in modern concrete technology is increasing due to the ability to control cracking of concrete. By optimizing the materials and mixture proportions, steel fiber reinforced concretes can be tailored for specific applications such as pavements and slabs on grades, industrial floors, shotcrete, self compacting concrete, precast elements, cladding, repairing and retrofitting of reinforced concrete structures [3]. Shrinkage and thermal cracking may also be controlled using different types of fibers [1, 4].

2 The main obective of this study presented herein is to provide more information on the properties of hybrid fiber reinforced concretes. Concretes containing different types of steel fibers were prepared and their properties were obtained. Effects of curing conditions were also obtained. A multi-obective simultaneous optimization technique was performed. The details of these studies are given below. 2 EXPERIMENTAL DETAILS 2.1 Materials Same ordinary Portland cement (OPC 42.5) and silica fume were used in all the concrete mixtures. The aggregates used were siliceous sand (-.25 mm), crushed limestone fines (-4 mm), natural sand (-4 mm) and crushed limestone no I (4-16 mm). The aggregate grading and maximum particle size of aggregate were kept constant in all the mixtures. The specific gravities were 2.65 g/cm 3 for the crushed limestone and limestone fines; and 2.63 g/cm 3 for the siliceous sand. A superplasticizer was used in concretes to reduce the water content and maintain approximately the same workability. Three types of steel fibers and a polypropylene fiber were used in the study. Two of the steel fibers have hooked ends while the other one has straight ends. Some properties of the steel fibers are shown in Table 1. Polypropylene fibers were 2 mm in length and 16 microns in diameter. The specific gravity of this fiber is,91 kg/dm 3 and available as fiber bundles. 2.2 Mixtures Table 1: Properties of steel fibers Fiber code Length Diameter Aspect Tensile strength (mm) (mm) ratio (MPa) O Z R Twelve concrete mixtures including; a reference mixture without any fibers (coded as NC), a mixture containing only polypropylene fibers (coded with PP) and 1 different mixtures containing hybrid fibers, were produced (Table 2). The letters O, Z and R indicate the fiber types used in the mixtures. Numbers following these letters denote the fiber contents. Table 2: Fiber contents in the concrete mixtures Concrete code Polypropylene Steel fibers (%) fiber (%) O Z R Total NC PP O Z R OZ OR ZR OZR R1-O.5-Z Z1-O.5-R O1-Z.5-R

3 In all the concretes containing fibers, the polypropylene fiber content was constant and it was.5% by volume. For the concretes containing hybrid fibers, the amounts of each steel fiber varied. However, the total steel fiber content in these concretes was also the same and kept at 2% for all the concretes with the hybrid fibers. As a result, the total fiber content was 2.5% for the concretes containing hybrid fibers. The cement content was kept constant in all the concretes. The amount of silica fume was also the same and it was 1 % of the cement by weight. The water/cement ratio was also the same for all the concrete mixtures. Superplasticizer was used at approximately 1.5% - 2% in all the mixtures. Table 3 shows details of the mixtures. Material (kg/m 3 ) Table 3: Concrete mixture proportions and some properties of the fresh c oncretes NC PP O Z R OZ OR ZR OZR R1- Z,5- O,5 Z1- R,5- Z,5 O1- R,5- Z,5 Cement Silica fume Water Superplasticizer 8,8 8,9 11,8 11,7 11,9 11,7 11,8 11,7 5,9 11,9 11,5 11,4 Crushed stone Crushed sand Natural sand Silica sand Polypropylene fiber Steel fiber: O Steel fiber: Z Steel fiber: R Unit weight Water/cement,3,3,3,3,3,3,3,3,3,3,3,3 ratio Slump (cm) Concrete prisms of 1x1x5 mm, cylinders of 15 mm diameter and 3 mm height and discs of 15 mm diameter and 6 mm height were prepared from each of these mixtures. All the specimens were demoulded 24 hours after casting and they were stored in water at 2 o C until testing. In addition, a second concrete series were tested to investigate the effect of curing. Water/cement ratio of the second series were.11 obtained by high cement content and a high-range water reducing admixture. Silica fume was also used in these mixtures. The amount of this admixture varied between 11 % and 12 % by weight of cement for different composite mixtures to maintain the same workability. For these mixtures, after demoulding, the three different curing methods were applied: i) steam curing was as follows; an initial moisture curing for one day, followed by steam curing for 3 days at 9 C and further in a water tank saturated with lime at 2 C until testing day. ii) The period of hot curing was similar to the steam curing, however, temperature of 2 C was applied to the specimens. During this high temperature curing regime, specimens were wrapped by the two layered wrapping material. The 3

4 first layer wrapping material was a high temperature resistant plastic sheet and the second one was an aluminium foil, iii) Standard curing in a water tank saturated with lime at 2 C. 2.3 Testing Compressive strength and modulus of elasticity of the concretes were determined on the concrete cylinder specimens. Disc specimens were used for the splitting tensile strength tests. Fracture energies of the concretes were obtained at the age of 28 days by performing three-point bending tests on the concrete prisms. Notches of 4 mm depth were made on the mid-span of the prisms by using a diamond saw. Figure 1 shows the bending test set-up. P 1 mm 6 mm a 4 mm 5 mm LVDT S=4 mm L=5 mm 5 mm Figure 1: Bending test set-up For the plain concretes, the deflection rate at the mid-span of the beams was kept constant at.2 mm/min. The beams with steel fiber fibers were tested at the deflection rate of.5 mm/min up to a deflection of.5 mm. Then the rate was increased to.1 mm/min up to 1 mm deflection. The load was applied by an Instron 55R closed-loop testing machine of 1 kn capacity, and the deflections were measured simultaneously by using three linear variable displacement transducers (LVDTs). The load versus deflection curve for each beam was obtained by recording the average of three measurements taken at the mid span. The area under the load versus deflection at mid span curve as in Figure 2 was described as a measure of the energy required for each deflection [5]. Deflection of 1 mm was chosen to calculate the fracture energy. Figure 2: Typical load versus deflection at mid span curve 4

5 Fracture energy of the specimens can be calculated as follows: G F W mg where G F is the fracture energy (N/m), W is the area under load versus deflection curve (N/m), m is the weight of the specimen, g is the gravitational acceleration (9,81 m/s 2 ), o : is the deflection of the specimen at failure, and A lig : is the effective cross-section of the specimen. The net flexural strength is calculated by using the equation given below: f fnet A lig 3Pl 2 B ( D a where; f fnet is the net flexural strength (MPa), P is the maximum load carried by the specimen (N), l is the span (mm), B is the width of the specimen (mm), D is the height of the specimen (mm) and a o is the depth of the notch (mm). The relative ductility of the mixtures were also calculated as follows; (3) G RD G F, FRC where, RD is the relative ductility, G F,FRC is the fracture energy of fiber reinforced concrete (N/m), and G F,FRC is the fracture energy of normal concrete without any fibers (N/m). 3 EFFECT OF FIBERS The results of the compressive strength, splitting tensile strength, modulus of elasticity, net flexural strength and fracture energy tests are shown in Table 4. Concrete codes Compressive strength (MPa) Modulus of elasticity (GPa) F, NC ) 2 Table 4: Test results Splitting tensile strength (MPa) Net flexural strength (MPa) Fracture energy (N/m) Relative ductility NC , PP , O ,1 Z ,7 R ,6 OZ ,4 OR ,3 ZR ,1 OZR , R1-O.5-Z ,4 Z1-O.5-R ,4 O1-Z.5-R ,2 (1) (2) 3.1 Compressive Strength Compressive strengths of the concretes are shown in Table 4 and also in Figure 3. The 28-day cylinder compressive strength of the reference concrete without any fibers is 82.5 MPa. Compressive 5

6 NC PP O Z R OZ OR ZR OZR R1-O.5-Z.5 Z1-O.5-R.5 O1-Z.5-R.5 strength of the concrete containing polypropylene fibers is 91.5 MPa, which is approximately 1% higher compared to that of the normal concrete. With a length of 2 mm and diameter of 16 microns, these fibers can be classified as micro fibers. Polypropylene fibers can be effective in reducing shrinkage cracks. Although strength increase was obtained for the concretes containing polypropylene fibers, it is known that contribution of these fibers to strength is usually minimal. An increase such as 1% in the compressive strength of concrete containing polypropylene fibers may be due to the natural scatter in the test results and more results are needed to clarify the effect of the fiber used on the compressive strength of concrete. As seen from Table 4 and also in Figure 3, compressive strengths of steel fiber reinforced concretes are almost the same of slightly higher than normal concrete. These strength increases are more pronounced especially for the concrete containing the fiber of 6 mm length and.16 mm diameter. Micro cracks start to form in concrete under loading. These cracks, then increase both in number and length. During loading, micro- and meso-dimensional fibers can act like a bridge between these cracks and restrict the opening of the cracks [6]. The short steel fibers (fiber coded as O) used in these mixtures may be classified as meso fibers and with a uniform distribution in the concrete; they may control the initiation and development small cracks. Thus, it is possible to increase compressive strength. The effect of the macro and meso-fibers on compressive strength, however, is limited compared to micro fibers. Compressive strength (MPa) Concrete mixture code Figure 3: Compressive strength of the concretes 3.2 Modulus of elasticity Modulus of elasticity of the concrete cylinders was also determined by uniaxial compressive testing. As shown in Table 4, effect of fibers on modulus of elasticity of concrete is small. Some of the fiber reinforced concrete mixtures even have slightly lower modulus of elasticity. Previous studies also indicate that the effects of fibers on modulus of elasticity of concrete are negligible [7]. 3.3 Splitting tensile strength Splitting strength of the concretes containing steel fiber is higher than that of the normal concrete. When single type fiber reinforced concretes are compared to those with hybrid steel, it can be concluded that the hybrid fiber reinforced concretes are perform better. 6

7 Load (N) 3.4 Flexural strength Flexural strength of the concrete with polypropylene fibers is slightly lower than normal concrete. Addition of steel fibers, however, increased the flexural strength significantly. The net flexural strength of the normal concrete was 6. MPa, while flexural strength of concrete containing R type fibers has reached a value of 21.8 MPa, which is approximately 4-fold increase. The test results indicate that for the hybrid steel fiber reinforced concretes, higher flexural strengths were obtained with increasing fiber aspect ratio. 3.5 Fracture energy Fracture energy is calculated using the area under the load - deflection curve. Fracture energy, or toughness, is the energy required during the bending test and it is used for describing the post-peak response of concrete. As shown in Table 4, fracture energies of the concrete and the concrete containing polypropylene fibers are almost the same. Fracture energies of steel fiber reinforced concretes; however, are much higher. Load - deflection curves of some of the concretes are shown in Figure 4. In order to better express the effect of the fibers and simplify the figure, only some of the concretes are included in this figure. For each mixture, a typical test result was used in the figure. 12 R 9 Z 6 OZR 3 NC O PP Deflection (mm) Figure 4: Load - Deflection curves of some mixtures The results show that the effect of fiber aspect ratio on fracture energy is significant. Fracture energy of normal concrete without fibers is 18 N/m while that of the mixture containing R coded fiber is 1113 N/m, which is approximately 1 times higher than the reference concrete. Among the steel fiber reinforced concretes, the mixture coded as Z1-O.5-R.5 has the lowest fracture energy. However, compared to normal concrete, fracture energy of this steel fiber reinforced concrete is about 26 times higher. The increase in the fracture energy is caused by the fiber pull-out and fiber debonding during the fracture process. Under loading, fibers bridge the cracks and more tortuous crack propagations takes place, and as a result, fracture energy increases. High fracture energies are the most important features of steel fiber reinforced concretes and due to their high ductilities; these concretes have a wide area of applications. Depending on the high energy absorption characteristics, steel fiber-reinforced concretes are preferred for impact resistance [8]. 7

8 NC PP O Z R OZ OR ZR OZR R1-O.5-Z.5 Z1-O.5-R.5 O1-Z.5-R Relative Ductility Figure 5 shows the relative ductility of the concretes produced in this study. As mentioned above, relative ductility is the ratio of the fracture energy of fiber reinforced concrete to that of the plain concrete without any fibers. As seen in Figure 5 and also in Table 4, the relative ductility of the concrete containing polypropylene fibers is equal to plain concrete. The relative ductilities of steel fiber reinforced concretes are approximately 29 to 94 times higher than the reference concrete. The concrete containing the fiber with the highest aspect ratio (concrete code R) has the highest ductility. The result obtained for the hybrid steel fiber reinforced concrete OZR is the second highest value. The relative ductility is a simple way of comparing different concretes. As this ratio increases, concrete exhibits more ductile behavior. The results obtained confirm the high ductility of steel fiber reinforced concretes. 1 Relative ductility Concrete mixture code Figure 5: Relative ductility of the concretes 4 EFFECT OF CURING 4.1 Fracture energy Effects of curing on hybrid steel fiber concretes were also investigated. Three types of curing i) normal curing at 2 C, ii) steam curing at 9 C, and iii) hot curing at 2 C were applied. These curing conditions were selected to represent different curing conditions applied in the industry. Figures 6, 7 and 8 show the effects of curing conditions and the type of the steel fibers on the mechanical behavior of the concretes. As seen in these figures, each mixture shows the brittle behavior of a typical matrix. It is seen that the use of high strength steel fibers improves the mechanical performance and increases the fracture energy. Especially after the first crack, the formation of strain hardening in the ascending branch of the curve is a typical indication of high performance cementitious composites. The high temperature curing regimes activate pozzolanic reactions; as a result fracture energies of the concretes increases significantly. The concretes with normal strength hooked-end steel fibers have lower peak load and steeper gradients of the softening branch compared to the mixtures with high strength hooked end steel fibers and the higher fracture energies were obtained. These results are similar to those mentioned above. 8

9 Load (N) Load (N) Curing regimes enhance the microstructure of the composites and as a result the fracture energy has been greatly improved. It can be concluded that increasing the curing temperature increased the peak load significantly and this resulted an increase in fracture energy while the slope of the ascending branch of the curves remained almost constant. It is shown that, depending on the high temperature curing condition, fracture energy of the steel fiber reinforced concrete increased substantially Z SN OZ RSN O RN N PN Displacement (mm) Figure 6: Load - displacement curves at normal curing at 2 o C Z SV OZ RSV O RV N PV Displacement (mm) Figure 7: Load - displacement curves at steam curing at 9 o C 9

10 Load (N) Z SZ OZ RSZ O RZ N PZ Displacement (mm) Figure 8: Load - displacement curves at hot curing at 2 o C 4.2 Microstructural studies To compare the effects of high temperature curing on the microstructure, polished plane and thin sections of samples were prepared. The pictures of polished sections under the UV light indicates that there is a map cracking associated with 2C. 2C and 9C curing levels did not present any cracks, and the volume of surface connected porosity is low. The pore system of 2C and 9C were also not connected with each other. It was observed that the cracks in the sample subected to 2C were filled with calcium hydroxide. In thin section of composites cured at 2C the interfacial transition zone in the composites with the dense microstructure is characterized by a direct link between the steel fiber and the matrix. The CH near the steel fiber reacts with ultrafine silica fume, forming dense C-S-H which fills in the spaces at the interfacial zone, producing increased bond between the matrix and steel fiber. The increased bond allows transfer of stress efficiently between the matrix and steel fibers. Evidence of possible thermally-induced damage such as delayed ettringite formation was also checked. No delayed ettringite formation was observed at hot curing conditions during the petrographic investigations. 5 OPTIMIZATION Optimization is a procedure for obtaining the best possible option and it is an important tool for decision making process. This procedure is based on defining performance criteria (obective functions), independent and dependent variables and formulation of the parameters based on constraints. For the optimization of concrete mixtures; the performance criteria may be the strength, permeability properties, fracture properties, durability characteristics, cost or other concrete properties. The independent variables, for example, may be the amounts of constituent materials. The responses (dependent variables), which are based on the independent variables, are compared to the performance criteria. In the optimization of concrete mixtures, concrete properties such as the compressive strength obtained for a given mixture design may be one of the responses. Optimization usually involves considering several responses simultaneously, such as high strength and low cost. In order to optimize several responses, multi criteria optimization techniques were used in this study. A useful approach for the optimization of multiple responses simultaneously is to use desirability functions which reflect the levels of each response in terms of minimum and maximum 1

11 desirability. A desirability function (d ) varies over the range of d 1. In case of maximizing and minimizing the individual responses, d i is defined by Eq. (1) and Eq. (2), respectively [9]: d d Y min f max f min f max f Y max f min f where d, Y, min f and max f are; the individual desirability function, the current, the lowest and the highest values of the th response included in the optimization, respectively. The power value t is a weighting factor of the th response. By using an overall desirability function (D) given in Equation (6), which is the geometric mean of the individual desirability functions (d ), the multi-obective optimization problem can be solved. 1 m 1 d2 d3... d m ) D ( d where m is the number of the responses. D takes values between and 1. The maximum value of D gives the optimum solution (the most desirable mixture) [9]. Mechanical properties (compressive strength, modulus of elasticity, splitting strength, flexural strength and fracture energy) of the concretes given in Table 4 were used for the optimization. The costs of the concretes were also included in the optimization. A simple approach was selected for obtaining the concrete costs. The unit costs and amounts of the each constituent material including the fibers were used for calculating the costs of the concretes. The mechanical properties obtained by testing and the costs calculated were used for calculating individual desirability functions as shown above. Mechanical properties were maximized while the cost was minimized. Using these individual values, overall desirability function for each concrete mixture was also obtained. Table 5 shows these desirability functions. Mixture code Compressive strength (d 1 ) Table 5: Desirability functions used in the optimization Modulus of elasticity (d 2 ) Individual desirability functions Splitting strength (d 3 ) Flexural strength (d 4 ) t t Fracture energy (d 5 ) Cost (4) (5) (6) Overall desirability function (D) (d 6 ) NC PP O Z R OZ OR ZR OZR R1-Z.5-O Z1-R.5-Z O1-R.5-Z

12 The maximum value of the overall desirability function is.689 obtained for the concrete containing R type fibers. Thus, among the 12 different concretes mixtures produced in this study, this concrete is the optimum mixture. This mixture has the maximum mechanical properties of mixtures, while the cost is minimal. Since the hybrid steel fiber reinforced concrete coded as R1-Z.5-O.5 has the second highest overall desirability value, this concrete is the second optimum mixture. In this study, 6 different responses were used for the optimization procedure. The multi criteria optimization technique applied is a useful method where different characteristics have to be considered together, and the method can be extended by including more responses such as different ages or different properties that should be maximized or minimized. 6 CONCLUSIONS The results obtained from this experimental study can be summarized as follows: 1) The addition of short steel fibers increased the compressive strength of concrete. Modulus of elasticity was not affected by the use of steel fibers. 2) Splitting tensile strength and net flexural strength of concretes containing hybrid fibers is higher compared to normal concrete. Higher flexural strength was used when the fiber aspect ratio is higher. 3) Fracture energies of the steel fiber reinforced concretes are substantially higher than the reference mixture, as expected. 4) The high temperature curing increases fracture energies of the concretes significantly. Increasing pozzolanic reactions due to high temperature may be the reason for this difference. 5) Multi criteria optimization is a powerful method for comparing different mixtures. For the optimization some properties can be maximized while some can be minimized. REFERENCES [1] A. Bentur and S. Mindess, Fiber Reinforced Cementitious Composites, Taylor&Francis (27). [2] ACI Committee 544, State-of-the-Art Report on Fiber Reinforced Concrete, ACI 544.1R-96 (1996). [3] A.M. Brandt, Cement-Based Composites: Materials, Mechanical Properties and Performance, E & FN Spon, London (1995). [4] P.N. Balaguru, and S.P. Shah, Fiber-Reinforced Cement Composites, McGraw-Hill (1992). [5] RILEM 5 FMC, Determination of The Fracture Energy of Mortar and Concrete By Means of Three-Point Bend Tests On Notched Beams, RILEM Draft Recommendation, Materials and Structures, 18 (16), (1985). [6] ACI Committee 544, Design Considerations for Steel Reinforced Concrete, ACI 544.4R-88 (1988) (reapproved 1994). [7] P.K. Mehta, and P.J.M. Monteiro, Concrete: Microstructure, Properties and Materials (21). [8] P.S. Song, J.C. Wu, S. Hwang, and B.C. Sheu, Statistical analysis of impact strength and strength reliability of steel polypropylene hybrid fiber-reinforced concrete, Construction and Building Materials, 19, 1 9 (25). [9] O. Sengul, and M.A. Tasdemir, Compressive Strength and Rapid Chloride Permeability of Concretes with Ground Fly Ash and Slag, Journal of Materials in Civil Engineering, 21, (29). 12