COROSSION PROPERTIES OF CONCRETE CONTAINING MICRO SILICA

Transcription

1 International Journal of Advancements in Research & Technology, Volume 2, Issue4, April COROSSION PROPERTIES OF CONCRETE CONTAINING MICRO SILICA Mr SARATH S 1, Mr R VANDHIYAN 2 & DR.E B PERUMAL PILLAI 3 1 P G student,department of Civil Eng., PSNA College of Engineering & Technology, Dindigul. 2 Associate prof, Department of Civil Eng PSNA College of Engineering & Technology, Dindigul. 3 The Principal, R M K CET, Tiruvallur District, Tamilnadu sarassarath@gmail.com, vandhianr@gmail.com ABSTRACT Concrete is a uniform a mixture of aggregates, binder material and water. The concrete thus formed is weak in many aspects or properties such as permeability, durability corrosion resistance, etc. All over the past and present researches were going on regarding development of concrete having good strength and which overcomes the above shortcomings. This is achieved by incorporating certain materials in the concrete mix. One such material that can be added in concrete to enhance various properties is micro silica. When micro silica is added to concrete by partially replacing (10%) the binder material, the hardened concrete seems to have more strength. This is achieved because micro silica particles are of very small size (1/100 th size of cement particle) and these particles enters to very minute pores in concrete and fills the voids thereby making the concrete more denser and stronger. Since its fills up the pores, there will be very less and small amount of pores and internal passages in the structure thus retarding the movement of water and other corrosion causing salts to be in contact with the embedded reinforcements. The project aims at studying the corrosion properties of micro silica incorporated concrete. This includes the study of bond strength in concrete reinforcement interface, permeability of water and other salts in to the concrete and flexural behavior of reinforced silica incorporated concrete beam.

2 International Journal of Advancements in Research & Technology, Volume 2, Issue4, April INTRODUCTION 1.1 GENERAL One of the important and most widely used artificial engineering material is concrete. It consists of hard inorganic materials called aggregates such as gravel sand crushed stone slag etc cemented together with binding material and water. When these materials are mixed together so as to form a workable mixture it gives considerable freedom to the designers and engineers to mould the structural element to any desired shape or form. In most structural applications, concrete is employed primarily to resist compressive stresses which is the one of the most important and useful properties of concrete. The compressive strength as well as many other properties are inversely proportional to the mass ratio of the amount of water and cement used in the mix. The lower this ratio stronger the concrete. Ultra High Performance Concrete (UHPC) is a recent development in material technology and far superior to conventional cement concrete and Other high strength concrete. UHPC can be considered as a logical development of cement concretes in which the ingredients are proportioned and selected to contribute efficiently to the various properties of cement concrete in fresh as well as in hardened states. One major remarkable quality, in the making of UHPC is the virtual elimination of voids in the concrete matrix, which are mainly the cause of most of the ills that generate deterioration. Important governing factors for UHPCs are strength, long term durability, serviceability as determined by crack and deflection control, as well as response to long term environmental effects. UHPCs are concretes with properties or attributes which satisfy the performance criteria. Generally, concretes with higher strengths and attributes superior to conventional concretes are desirable in the construction industry. UHPC can be considered as a logical development of cement concretes in which the ingredients are proportioned and selected to contribute efficiently to the various properties of cement concrete in fresh as well as in hardened states.

3 International Journal of Advancements in Research & Technology, Volume 2, Issue4, April MICRO SILICA Micro silica, also referred to as silica fume or condensed silica fume, is a byproduct material that is used as a pozzolan. This byproduct is a result of the reduction of high-purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy. Silica fume rises as an oxidized vapor from the 2000 C (3630 F) furnaces. When it cools it condenses and is collected in huge cloth bags. The condensed silica fume is then processed to remove impurities and to control particle size. Condensed silica fume is essentially silicon dioxide (usually more than 85%) in noncrystalline (amorphorous) form. Since it is an airborne material like fly ash, it has a spherical shape (Fig. 3-8). It is extremely fine with particles less than 1 μm in diameter and with an average diameter of about 0.1 μm, about 100 times smaller than average cement particles. Condensed silica fume has a surface area of about 20,000 m2/kg (nitrogen adsorption method). For comparison, tobacco smoke s surface area is about 10,000 m2/ kg. Type I and Type III cements have surface areas of about 300 to 400 m2/kg and 500 to 600 m2/kg (Blaine), respectively. The relative density of silica fume is generally in the range of 2.20 to 2.5. Portland cement has a relative density of about The bulk density (uncompacted unit weight) f silica fume varies from 130 to 430 kg/m3 (8 to 27 lb/ft3). Silica fume is sold in powder form but is more commonly available in a liquid. Silica fume is used in amounts between 5% and 10% by mass of the total cementitious material. It is used in applications where a high degree of impermeability is needed (Fig. 3-9) and in highstrength concrete. Silica fume must meet ASTM C ACI 234 (1994) and SFA (2000) provide an extensive reviewof silica fume. 1.3 CONCRETE CONTAINING MICRO SILICA Micro silica, also known as silica fume, has been used as a concrete property enhancing material and as a partial replacement for Portland cement for over 25 years. Micro silica is a by-product in the production of silicon metal or ferrosilicon alloys.. Micro silica is a mineral admixture composed of very fine solid glassy spheres of silicon dioxide (SiO 2 ). Most micro silica particles are less than 1 micron ( inch) in diameter, generally 50 to 100 times finer than average cement or fly ash particles. Micro

4 International Journal of Advancements in Research & Technology, Volume 2, Issue4, April silica in concrete contributes to strength and durability two ways: As a pozzolan, micro silica provides a more uniform distribution and a greater volume of hydration products. [Ref:8] As a filler, micro silica decreases the average size of pores in the cement paste [Ref:8] Micro silica for use in concrete is available in slurry or dry forms. In either form, micro silica is a very reactive pozzolan when used in concrete due to its fine particles, large surface area, and the high silicon dioxide content. There are several effects on the properties of fresh and hardened concrete when micro silica is used along with fly ash and chemical admixtures. In fresh concrete, micro silica affects the water demand and slump. The concrete water demand increases with the increased amounts of silica fume, due primarily to the high surface area of the micro silica. Fresh concrete containing micro silica is more cohesive and less prone to segregation than concrete without micro silica. Since micro silica is used with other admixtures, such as waterreducing or high-range water-reducing admixtures, the slump loss is actually due to the change in chemical reactions. Micro silica is also known to affect the time of setting and bleeding of fresh concrete. Mechanical properties of silica-fume concrete, such as creep and drying shrinkage, have been known to be lower than that of concrete without micro silica. At 28 days, the compressive strength of micro silica concrete is significantly higher than concrete without silica fume. Micro silica is also linked to the decrease of permeability, chemical attack resistance, and enhancement of the chloride ion penetration resistance of concrete. [Ref 8] Concrete containing micro silica makes concrete a better performing material, allowing designers to use it efficiently in increasingly slender structures. Architects prefer using these type of concrete in high-rise construction in order to design thinner floor slabs and slimmer columns, in addition to it being aesthetically more appealing. Some contractors favour the use of this because they can remove the form work earlier. In addition to reducing creep and shrinkage, increasing the use of silica containing concrete in high rise construction increases the stiffness of the structure. As a result, deflections of concrete members are reduced.

5 International Journal of Advancements in Research & Technology, Volume 2, Issue4, April FEATURES OF CONCRETE CONTAINING MICRO SILICA Some of the salient features of micro silica containing which make it superior to other concretes are : High early compressive strength High tensile and flexural strength High modulus of elasticity Low permeability to chloride and water intrusion Discontinues internal pores Enhanced durability and toughness Superior resistance to chemical attack Higher bond strength High electrical resistivity 1.5 CORROSION IN CONCRETE STRUCTURES Corrosion of embedded steel is one of the major causes of concrete deterioration in reinforced concrete structures. This type of corrosion results when a corrosive species, water and air, penetrate through the concrete pores to the steel s surface. The key to inhibiting rebar corrosion is to restrict the permeability of concrete. Reinforcing steel in concrete is normally protected from corrosion by the passive film formed at the steel/concrete interface inside the alkaline cementitious matrix. However, this passivation can be eliminated either by a decrease in the ph value (ph < 9) due to carbonation, or by the presence of chloride salts, which initiates an expansive corrosion of the reinforcing steel and eventually damages the surrounding concrete. Concrete structures such as bridges, buildings, sanitary and water facilities, and other reinforced concrete structures might suffer severe damages due to corrosion of the reinforcing steel. Damages caused by the consequent cracking and spalling of the concrete cost billions of dollars each year. Steel corrosion reduces the cross section area as well as the continuity of the surface of steel. Such reductions lower the tension strength of steel and decrease the bond strength due to a slip between steel and the surrounding concrete, and, consequently deteriorate the member strength In addition to the economic losses incurred, public safety is also jeopardized, even culminating in loss of lives due to incidents like collapsing of bridges and structures. Experimental investigations and mix design

6 International Journal of Advancements in Research & Technology, Volume 2, Issue4, April RESULTS OF MATERIAL TESTS Basic material results containing concrete also. Micro silica can be incorporated in concrete in two ways: SL NO TEST VALUE (a) addition to binder material І TEST FOR CEMENT 1 Consistency 31 2 Specific gravity Fineness 0.33 П TEST FOR FINE AGGREGATES 1 Specific gravity Fineness Bulk density 1693Kg/m 3 Ш TEST FOR COARSE AGGREGATES 1 Specific gravity Fineness Bulk density 1527Kg/m 3 (b) replacement to binder material From former studies and researches, it showed that replacement gives an increased strength than addition. It is clear that 10 % replacement of the binder material gives high strength. Further replacement retards the strength of concrete. Concrete S C FA CA M20 Ordinary M silica EXPERIMENTAL SET UP 3.MIX PROPORTION (W/C= 0.53 ) As my work is on the study of micro silica containing concrete it is necessary to incorporate the mix design for micro silica Since the study deals with corrosion, certain small set ups must be made ready to create an artificial environment to induce corrosion. Some of the required set ups are briefly explained below: (a) IMPRESSED CURRENT VOLTAGE METHOD It is one of the most widely used methods for inducing corrosion in reinforcement. In

7 International Journal of Advancements in Research & Technology, Volume 2, Issue4, April this method a standard amount of voltage (6-24 volt) is applied to the reinforcement. For this a cell is created with the help of a DC battery. The positive terminal of the battery is connected to the reinforcement which is to be corroded and the negative terminal is connected to a stainless steel plate. Now the concrete containing the reinforcement thatt is to be corroded and the stainless steel plate are to be immersed in 3.5 % of NaCl solution. A point that is to be kept while setting up this apparatus is that the reinforcrmrnt that is to be corroded should not come in contact with the salt solution. On applying the voltage for pre determined time we can obtain corrosion. (b) HALF CELL POTENTIAL METER corrosion, half cell potentiometer is used. A half cell potential meter is basically a chemical cell containing an electrode, electrolyte. The electrode used is copper rod and the electrolyte used is copper sulphate solution. The cell is placed on the top of the concrete surface in which the corroded steel is present. The lower part of the cell thus formed must be permeable so that the blue color of copper sulophate solution comes in contact with the concrete surface. Thus a complete cell is formed. The cell can be said to be complete only when a multi meter is used in the circuit. The positive end of multi meter is connected to the the reinforcement whose potential is to be measured and the negative end is connected to the half cell. Various potential can be calculated by varying the position of the half cell potentiometer on the surface of concrete. With the help of the readings taken using multi meter and using the Faradays law for mass loss, we can calculate amount of corrosion that have taken place in reinforcement inside the concrete. We need to calculate the amount of corrosion that has happened to the reinforcement embedded in the concrete due the process of accelerated corrosion. In order to determine the amount of

8 International Journal of Advancements in Research & Technology, Volume 2, Issue4, April (c) PREPARATION OF TEST SPECIMENS (CYLINDERS) 5. EXPECTING RESULT The aim of my project is ultimately concentrated on finding the bond strength that is existing between the concrete and the reinforcement. For this the guidelines from IS 2770 (part1) was followed. The cylinder was cast in such a manner thatt a bottom cover of 50 mm was given. After that a rod was centered to the cylinder and a 50 mmm ht concrete was poured. Now the balance rod was covered using a pvc pipe of more than 25 cm ht. the aim of providing the pvc pipe was to avoid the unnecessary area of contact. It helps in avoid water to have contact with reinforcement. Specimens are prepared for non corroded ordinary concrete, corroded ordinary concrete, nonn corroded microsilica concrete, corroded microsilica concrete. The final outcome of this investigation is to bring out the performance of reinforcement in various corroded environments when micro silica is partly used as abinding material.. The pull out test will give the variation in bond strength on the corroded and control specimensof ordinary as well as micro silica contaiing concrete. REFERENCES: 1. M Mazloom, J J Brooks ; Effect of silica fume on mechanical properties of high strength concrete ; Cement and concrete composites 26, M Davraz, L Gunduz ; Reduction of alkali silica reaction risk in concrete by micronized amorphous silica ; Construction and building materials 22, Oguzhan K, Bahar D ; Corrosion behavior of reinforcing steel embedded in concrete produced with finely ground pumice and silica fume ; Construction and building materials 24, Shamsad Ahamad ; Techniques for nducing accelerated corrosion in steel in concrete ; The arabian

9 International Journal of Advancements in Research & Technology, Volume 2, Issue4, April journal for science and engineering 34, Fernanda da Silva, Jefferson ; A study of steel bar reinforcement corrosion in concretes with SF and SRH using electrochemical impedance spectroscopy ; Materials research 9, Safwan A. Khedr, Mohamed Nagib ; Characteristics of silicafume concrete ; ACI manual of concrete practice, part 3, Dr.G.S.Thirugnanam; Performance Evaluation of Protective Coatings on Corrosion resistance in Transmission Line Tower foundations ; International journal of applied engineering research 1, R Siddique, M Iqbal Khan ; Supplementary cementing materials; Engineering materials 2011 (pg )