Durability of Glass, Polypropylene and Steel Fiber Reinforced Concrete

Transcription

1 Durability of Glass, Polypropylene and Steel Fiber Reinforced Concrete Tara Rahmani 1 Behnam Kiani 2 Farzaneh Sami 3 Bastam Najafi Fard 4 Yaghoob Farnam 5 Mohammad Shekarchizadeh 61 ABSTRACT This paper investigats the effects of addition of polypropylene, glass, and steel fibers on durability properties of fiber reinforced concrete. The properties include compressive and flexural strengths, electrical resistivity, water penetration, gas permeability, and scaling resistance to deicing chemicals. Fibers are added at volume fractions of 0.125%, 0.125%, and 0.5% respectively. The results show that polypropylene and glass fibers improve the electrical resistivity by 61.9% and 11.9% respectively. Moreover, the water penetration and gas permeability of steel fiber reinforced concrete are increased compared to plain concrete. In addition, the scaling resistance also is improved more for steel fiber reinforced concrete. KEYWORDS Durability, fiber reinforced concrete, electrical resistivity, water penetration, gas permibility, and scaling resistance 1 University of Tehran, School of Civil Engineering, Tehran, Iran 34469, Phone , Fax , Tara_rahmani@ut.ac.ir 2 University of Tehran, School of Civil Engineering, Tehran, Iran 34469, Phone , Fax , cmi@ut.ac.ir 3 University of Tehran, School of Civil Engineering, Tehran, Iran 34469, Phone , Fax , cmi@ut.ac.ir 4 University of Tehran, School of Civil Engineering, Tehran, Iran 34469, Phone , Fax , cmi@ut.ac.ir 5 University of Tehran, School of Civil Engineering, Tehran, Iran 34469, Phone , Fax , Bayas3@yahoo.com 6 University of Tehran, School of Civil Engineering, Tehran, Iran 34469, Phone , Fax , shekarch@ut.ac.ir

2 T. Rahmani, B Kiani, F. Sami, B. Najafi Fard, Y. Farnam and M. Shekarchizadeh 1 INTRODUCTION Experimental research results have been showed considerable improvement in the post-cracking behavior of concretes containing fibers. Therefore, compared to plain concrete, fiber reinforced concrete is much tougher and more resistant to impact [Mehta & Monteiro 2001]. In addition to strength chracteristics, concrete should have adequate durability to perform in accordance with its intended level of functionality and serviceability over an expected or predicted life cycle. Durable concrete must potentially have the ability to withstand expected exposed deteriorative conditions. In terms of deterioration of concrete due to physical or chemical causes, the penetration of fluids or gases through the concrete are nearly always involved. The overall susceptibility or penetrability of concrete structures, especially when compounded by additional environmental or exposure challenges, is the key to its ultimate serviceability and durability [Mindess et al. 2002]. According to Balaguru and Ramakrishnan [1986], steel fiber reinforced concrete had lower water absorption and lower permeability than plain concrete. Miloud [2005], however, found that addition of steel fiber to concrete increases water and gas permeability, irrespective of the fiber amount or fiber length. Balaguru and Shah [1992] indicated that permeability of polymeric fiber reinforced concrete and plain concrete are also increased. Hoseini et al. [2009] reported that fiber reinforcement results in a drop in the permeability of concrete under mechanical stress. This was likely due to a change in the crack profile in the presence of fibers whereby a multitude of closely spaced microcracks form instead of appearance of a few large cracks. The objective of the present study is to investigate the effect of fibers on durability properties of concrete. To achieve this goal, polypropylene, glass, and steel fibers by volume fractions of 0.125%, 0.125%, and 0.5%, respectively, were studied. The effect of these fibers on compressive and flexural strength, electrical resistivity, water penetration, gas permeability, and scaling resistance to deicing chemicals were investigated. 2 MATERIALS AND MIX PROPORTIONS The cement used in all mixes was ordinary Portland cement which complies with ASTM type II. The W/C ratio for all mixtures is equal to Natural siliceous sand with a maximum nominal size of 4.75 mm was used as fine aggregate in mix proportions. Fine aggregate had a fineness modulus of 3.31 and specific gravity value of 2.62 gr/cm 3. Two sizes of crushed natural stone with maximum nominal size of 9 mm and 19 mm were used in this study as coarse aggregate. Specific gravity of coarse aggregates was 2.66 and 2.67 respectively. Three different types of fibers were used to investigate the effect of fibers on durability characteristics of concrete. The properties of fibers are shown in Table 1. Table 1. Properties of the different fibers used Property Polypropylene Steel Glass Length (mm) Diameter (mm) Aspect ratio (l/d) Specific gravity Tensile strength (MPa) Geometry Fibrillated Hooked Fibrillated In this study, four mixture proportions were considered; polypropylene, glass, and steel fiber reinforced concrete mixtures together with control mixture were prepared. Results are compared with control mixture results, which proportioned without any fibers. The details of mixture proportions are presented in Table 2. Volume fractions of various fibers used in the mixtures are also given in Table 3. 2 XII DBMC, Porto, PORTUGAL, 2011

3 Durability of fiber reinforced concrete 2.1 Mixing Procedure Table 2. Mix proportions for all mixtures Cement Fine aggregate Coarse aggregate Water (kg/ m 3 ) (kg/ m 3 ) (kg/ m 3 ) (kg/ m 3 ) 12.5 mm 19 mm Mixture No. Table 3. Dosage of different fibers Fiber type Volume fraction (%) Fiber dosage (kg/ m 3 ) C0-0 0 PF Polypropylene SF Steel GF Glass All mixtures were mixed in a conventional pan mixer with a capacity of 150 lit. At First, the coarse aggregate, fine aggregate, and 20 percent of mixing water were placed in the mixer. After 1 min of mixing, the mixer was stopped for 1 min. Then, the remainder of the mixing water and also cement were added and mixed for 2 min. Finally, fibers were dispersed by hand to achieve a uniform distribution throughout the concrete and mixed for 2 min. In the case of steel fibers, to help fiber dispersion, the fibers were saturated with 1-2 litres of mixing water for about 10 min. After mixing, the workability of concrete was determined using slump and inverted slump tests.the freshly mixed fiber reinforced concrete was placed in the moulds and consolidated by a vibrating table. Then, each of the specimens was allowed to stand for 24 h before demoulding, afterward stored in water at 23 ± 1 ºC until the time of testing. 3 MECHANICAL PROPERTIES 3.1 Compressive Strength Compressive strength test was carried out according to BS For each type of fiber, three 150 mm cubic specimens were tested at the age of 3, 7, and 28 days. The average results are shown in Fig. 1. The reduction in compressive strength of polypropylene and glass fiber reinforced concretes can be attributed to the fact that larger specific surface area of these fibers increases the air content in concrete which can reduce the compressive strength [Wu & Wenhui 2006]. Figure 1. Compressive strength of concrete specimens XII DBMC, Porto, PORTUGAL,

4 T. Rahmani, B Kiani, F. Sami, B. Najafi Fard, Y. Farnam and M. Shekarchizadeh 3.2 Flexural Strength Flexural strength test was performed on prismatic specimens with dimensions of mm using the third point loading procedure according to ASTM C1018. Three specimens per mix were tested at the age of 28 days. The average results are presented in Fig. 2. According to the results, the flexural strength of concrete improved with addition of polypropylene, glass, and steel fibers, achieving 9.8%, 10.3%, and 27.3% improvements, respectively. 4 DURABILITY PROPERTIES 4.1 Electrical Resistivity Figure 2. Flexural strength of concrete specimens The electrical resistivity of concrete is being increasingly used to indirectly evaluate concrete characteristics such as the chloride ion diffusivity, the degree of concrete resistivity to severe environments, and its aggressiveness. This parameter may also provide useful information regarding the rebar corrosion performance in concrete. The electrical resistivity of the specimens was measured based on AC Impedance Spectrometry (ACIS) method in this study. In this method, a fresh cement paste (R~0) was used in order to provide the proper electrical connection between copper plate and concrete specimen. The test was carried out on 100 mm cubic specimens at the age of 3, 7 and 28 days. The avearage values obtained over three specimens are reported as electrical resistivity of concrete. The electrical resistivity results of three diffrent fiber reinforced concrete are presented in Fig. 3. Figure 3. Electrical resistivity of concrete specimens 4 XII DBMC, Porto, PORTUGAL, 2011

5 Durability of fiber reinforced concrete The results show that polypropylene fiber reinforcing improves resistivity, or reduces conductivity. Although being significant at an early age itself, the improvements in resistivity appear to be more pronounced at later ages. In addition, it is seen that hydration has a negative effect on conductivity [Banthia et al. 1992]. As we expected, steel fiber presence extremely decreased resistivity due to high electrical conductivity of steel fibers. Lower conductivity of polypropylene and glass fibers in comparison with steel fiber leads to increasing electrical resistivity of concrete. 4.2 Water Penetration Depth Water permeability test was done according to DIN-1048 Part I. The test was carried out on cylindrical specimens with 300 mm height and 150 mm diameter at the age of 28 days. Specimens were kept at water pressure of 1, 3, and 7 atmosphere bar for duration of 48, 24, and 24 hours sequentially. The water penetration depth was determined by splitting the cylinder and measuring the average depth of discoloration (due to wetting) taken as equal to the depth of penetration. Results are shown in Fig. 4. Figure 4. Water permeability of concrete specimens The results show that water penetration depth of polypropylene, glass and steel fiber reinforced concretes increases by 41%, 45%, and 51%, respectively, over plain concrete. This could be the consequence of several reasons. In fiber reinforced concrete, presence of fibers and their wall effect leads to increasing capillary effect. Indeed, fibers play the role of channels and conduct the water in concrete. Hence, water penetration depth increases. This phenomenon is more considerable about steel fibers because they have a stronger wall effect in comparison with other fibers [Liu et al. 2010]. Also, fibers act as a bridge and facilitate the interconnection between pores so that water permeability is increased [Miloud 2005]. 4.3 Oxygen Gas Permeability Permeability of concrete largely depends on the ease with which fluids, both liquids and gases, can migrate through the hardened concrete mass. Consequently, measurement of permeability provides an indicator of concrete durability so that permeability called as the key to durability [Neville 1995]. In this study, permeability of concrete to oxygen was determined by a method developed by Cembureau. In this method, the underlying principle is the Hagen-Poiseuille relationship for laminar flow of a compressible fluid through a porous body with small capillaries under steady-state condition. The relationship proposed by Hagen-Poiseuille for determining specific permeability coefficient can be written as Equation 1. XII DBMC, Porto, PORTUGAL,

6 T. Rahmani, B Kiani, F. Sami, B. Najafi Fard, Y. Farnam and M. Shekarchizadeh K = 2Q. p A( p a 2. L. η p 2 a ) (1) Where Q is volume flow rate of the gas (m 3 /s); A is cross-sectional area of the specimen (m 2 ); L is thickenss of the specimen in the direction of flow (m); is dynamic viscosity of the fluid at test temperature (N.s/m 2 ); p is the absolute value of inlet pressure (N/m 2 ); p a is outlet pressure, assumed to be equal to atmospheric pressure (N/m 2 ). By using oxygen as a gas and standard reference specimen with 50 mm thickness and 150 mm diameter, the relationship can be simplified to Equation 2. K oxygen Q. pa = 2 2 ( p p ) a (2) For each type of fiber, three specimens were tested at the age of 28 days. Oxygen permeability coefficient of the specimens would be obtained by evaluating the mean value of five pressure stages. The average results of gas permeability tests are presented in Fig. 5. Figure 5. Gas permeability coefficient of concrete specimens From the results, it can be drawn that the coefficient of gas permeability increases with adding fibers especially for steel fibers. The average value permeability coefficient for plain concrete is , while that of steel fiber reinforced concrete is To explain such increase in gas permeability of fiber reinforced concrete, it may be assumed that permeability is determined more by matrix properties than the fibers. In particular, the matrix-fiber interface has the largest content of pores and micro cracks that affect overal permeability. Also it is very important to mention that fibers act as ties between pores so that interconnections are created which allow gas flow to penetrate more easily inside the concrete structure [Miloud 2005]. 4.4 Scaling Resistance to Deicing Chemicals The deicer scaling resistance test was performed in accordance with ASTM C672. For each type of fiber, two specimens of mm were tested. The specimens were placed in moist storage for 14 days and stored in air for 14 days. Then, the surface of each specimen was covered with approximately 6 mm of a solution containing 4% calcium chloride. The specimens were exposed to 6 XII DBMC, Porto, PORTUGAL, 2011

7 Durability of fiber reinforced concrete this solution under a single cycle of freezing (for 16 to 18 hours) and thawing (for 6 to 8 hours) per day. Every five cycles the solution was changed, the surface of each test specimen was visually evaluated, and the surface mass loss was measured. The test was stopped when the visual rating reached 5. Visual rating of 5 is related to the specimens exhibited severe scaling such that coarse aggregates would be visible over entire surface. The average results are summerized in Table. 4. Table 4. Deicer scaling resistance of concrete specimens No. Mass loss (kg/m 2 ) 5C 10C 15C 20C 25C C PF GF SF It can be clearly seen that the deicer scaling resistance of fiber reinforced concretes are improved especially for steel fiber reinforced concrete. The reason for improvement is mainly that the concretes with fibers have much higher air content than those without fibers and therefore have much lower spacing factor [Quanbing & Beirong 2005]. 5 CONCLUSION Based on the results presented and discussed, following conclusions may be drawn: The flexural strength of the three fiber reinforced concretes considered in this study, were found to be 10-30% higher than plain concrete. The compressive strength of polypropylene and glass fiber reinforced concretes reduced due to an increasing in the air aontent of concrete. Polypropylene fiber reinforced concrete had the highest electrical resistivity compared to other types of fibers, at 28 days. Electrical resistivity of steel fiber reinforced concerte decreased significantly at all ages. The water penetration and gas permeability of concrete both increased with adding fibers due to increasing porosity. Steel fiber reinforced concrete had the highest gas permeability among the various types of fibers. Scaling resistance of all fiber reinforced concretes improved especially for steel fibers. REFERENCES Balaguru, P. & Ramakrishnan, V. 1986, Freeze-thaw durability of fiber reinforced concrete, ACI Journal, Vol. 83, Balaguru, P.N. & Shah, S.P. 1992, Fiber-Reinforced cement composites, McGraw-Hill, New York. Banthia, N., Djeridane, S. & Pigeon, M. 1992, Electrical resistivity of carbon and steel micro-fiber reinforced cements, Cement and Concrete Research, Vol. 22, Hoseini, M., Bindiganavile, V. & Banthia, N. 2009, The effect of mechanical stress on permeability of concrete: A review, Cement and Concrete Composites, Vol. 31, Liu, X., Chia, K.S. & Zhang, M.H. 2010, Water absorption, permeability, and resistance to chlorideion penetration of lightweight aggregate concrete, Construction and Building Materials, Vol. 25, Mehta, P.K. & Monteiro, P.J.M. 2001, Concrete: Microstructure, Properties and Materials, McGraw- Hill, New York. XII DBMC, Porto, PORTUGAL,

8 T. Rahmani, B Kiani, F. Sami, B. Najafi Fard, Y. Farnam and M. Shekarchizadeh Miloud, B. 2005, Permeability and porosity characteristics of steel fiber reinforced concrete, Asian Journal of Civil Engineering, Vol. 6, Mindess, S., Young, J.F. & Darwin, D. 2002, Concrete, Prentice-Hall, New Jersy. Neville, A. 1995, Properties of Concrete, Wiley & Sons, New York. Quanbing, Y. & Beirong, Z. 2005, Effect of steel fiber on the deicer-scaling resistance of concrete, Cement and Concrete Research, Vol. 35, Wu, Y. & Wenhui, Z. 2006, Effect of polypropylene fibers on the long-term tensile strength of concrete, Journal of Wuhan University of Technology, Vol. 22, XII DBMC, Porto, PORTUGAL, 2011