CHARACTERIZATION OF INDIGENOUS STEEL BARS FOR USE IN ... · Property Fe 500 1 20.2 % proof stress...

International Journal of Application or Innovation in Engineering & Management (IJAIEM) Web Site: www.ijaiem.org Email: [email protected]

Volume 9, Issue 7, July 2020 ISSN 2319 - 4847

Volume 9, Issue 7, July 2020 Page 40

ABSTRACT Concrete intended to take high compressive stresses and not tensile stresses in the civil engineering structures. The steel bars are generally used in reinforced concrete structures to compliment concrete to provide the required tensile strength, rigidity, and durability. Because of the importance and role of steel bars in reinforced concrete structures, accurate information on the steel’s physical and chemical properties are essential at the design stage. Theraw materials from different sources are used for manufacturing of steel bars. As a result, chemical composition, crystalline structure, and mechanical properties of these bars can vary from batch to batch are unavoidable. This variation also exaggerates when the steel produced from scrap. The use of indigenous materials in the construction industry lowers transportation costs, reduces custom taxes and reduces the risk of uninterrupted supplies of materials. It also supports local businesses improvement, local employment and also feeds money into the economy of the county. However, the quality of local material should be on par with the requirements of the quality standards. This study investigates the level of conformity of steel bars manufactured in Bhutan. Generally, the performance of steel bars characterized by checking the specific properties like chemical composition, strength, and elongation. For this purpose, an investigation carried out on the percentage of the chemical composition of elements, yield strength (YS), and tensile strength (TS) properties as per the procedure specified in national as well as international standards. A total of 124 and 169 numbers of random steel samples were collected periodically from the factory site. They were used for investigation of chemical composition and testing of physical properties of different sizes viz-16mm, 20mm, 25mm, 32mm, and 36mm, respectively. These samples were tested in the quality control laboratory of the manufacturer for assessing the physical and chemical parameters for its use in the Hydroelectric Power Project. The chemical composition and physical properties of these locally manufactured steel have met the specifications of national and even international standards. Hence, they are confirmed to fit for use in Hydroelectric Power Projects and other structures. Keywords: Chemical composition, Yield Stress or strength (YS), Tensile Strength (TS), Percentage Elongation.

1.0 INTRODUCTION Variations in chemical composition and amount of residual elements are unavoidable in the steel making process. This variation is primarily caused due to the use of raw materials from different sources and the use of scraps in steel bars' manufacturing process. The small variation in the chemical composition of steel bars influences the mechanical behavior too. In this view, the characterization of the indigenously available high strength TMT bars (Fe500) has been taken up. The TMT bars having diameters 16 mm, 20 mm, 25 mm, 32 mm, and 36 mm were analyzed for chemical and mechanical properties and compared with the prescribed requirements given in IS 1786- 2008. The chemical composition of reinforcing steel bars was determined using optical emission spectrometry (OES). Further, the mechanical properties

CHARACTERIZATION OF INDIGENOUS STEEL BARS FOR USE IN

HYDROELECTRIC PROJECTS IN BHUTAN

N. V. Mahure1, Mandalozu Raja2, Chenga Tshering3 and Man Singh Rai4 1 Scientist E, Central Soil and Materials Research Station, New Delhi

2Scientist D, Central Soil and Materials Research Station, New Delhi

3 Assistant Executive Engineer, Punatsangchhu II Hydroelectric Project Authority, Bhutan

4Manager, Quality Control Department,Perfect TMX TMT, Bhutan




such as mass per meter length, yield strength (YS), tensile strength (TS), and percentage elongation at fracture (%PE) are also evaluated.

2.0 IMPORTANT CHARACTERISTICS OF REINFORCING BAR Good strength, bond with concrete, thermal expansion characteristics (similar to concrete), and bendability are the main qualities that make steel rebars more effective in producing high quality reinforced concrete structures. The bond between steel bars and concrete mainly depends on the shape, geometry of ribs. Steel rebars are generally round in cross-section with protrusions or ribs on to the bar surface. Besides strength, the durability of the reinforced structures depends upon rebar quality. Durability is the ability of the structure to maintain safety and serviceability criteria during its intended design life. Corrosion of rebars in concrete structures is one of the main causes that could impair strength and durability. Hence, the chemical composition of rebar plays an essential role in this respect. Apart from the strength and chemical composition, other properties like bendability and weldability characteristics of rebars are also important in construction. Bendability is essential from giving requisite shape to the rebar to suit the demand of the structures. Sometimes, welding of high diameter rebars is resorted to reducing congestion. The weldability of rebar is also an essential factor for fixing embedded parts in the concrete before pouring. The enhancement of rebar strength by changing chemical composition (for example, an increase in carbon content) has adverse effects on ductility and weldability. Therefore, balancing of conflicting requirements is required in fixing the characteristics of rebar to achieve an optimum balance between strength, ductility, durability and cost.

3.0 REQUIREMENTS OF Fe500 AS PER IS 1786 – 2008 (AMENDMENT- NOV., 2012) The requirements of chemical and mechanical properties of high strength deformed bars (Fe500) are given in Table 1 and Table 2 respectively.

Table 1: Chemical Properties of High Strength Deformed Bars (Fe500) Sl. No.

Grade of Steel Constituent Max. Percentage by mass

Max. Percent Variation (Over Specified Maximum Limit)

1

Fe 500

Carbon (C) 0.300 0.020 2 Sulphur (S) 0.055 0.005 3 Phosphorus (P) 0.055 0.005 4 Sulphur +Phosphorus 0.105 0.010

NOTES: 1. Carbon Equivalent (CE):

a. CE 0.53 (when micro-alloy/low alloys are used) Where b. CE (carbon equivalent) 0.42 (when micro-alloys/low alloys are not used)

Where 2. Sum of (Nb + V + B +Ti) shall not exceed 0.30 % (individually or in combination) 3. Low alloy steel may also be produced by adding alloying elements like Cr, Cu, Ni, Mo and P, either individually or in

combination, to improve allied product properties. However, the total content of (Cr + Cu + Ni + Mo + P) shall not be less than 0.40 %. In such low alloy steels when phosphorus (P) is used, it shall not exceed 0.12 % and when used beyond the limit given above, the carbon (C) shall be restricted to a maximum of 0.15 %, and in such case the restriction to maximum content of sulphur (S)+ phosphorus (P) as given in Table 1 and the condition of minimum alloy content 0.40 percent shall not apply.

4. Generally Nitrogen (N) content< 0.012% or 120 ppm. Higher nitrogen contents up to 0.025 percent (250 ppm) may be

permissible provided sufficient quantities of nitrogen binding elements, like Nb, V, Ti, Al, etc. are present. For ascertaining the nitrogen binding elements, following expression may be used, where all elements are in ppm.




Table 2:Mechanical Properties of High Strength Deformed Bars (Fe500) Sl. No.

Property Fe 500

1 0.2 % proof stress or yield stress (YS), Min, N/mm2 500.0 2 0.2 % proof stress or yield stress (YS), Max, N/mm2 - 3 Ratio of Tensile Strength (TS) / yield stress (YS) ratio, N/mm2

(ratio of tensile strength to the 0.2 percent proof stress or yield stress of the test piece)

≥ 1.08, but TS not less than 545.0N/mm2

4 Percentage Elongation (% PE), min. on gauge length 5.65√A, where A is the cross-sectional area of the test piece

12.0

4.0 METHODOLOGY

The random samples of high strength TMT bars (Fe500) were collected periodically during the year 2014 to 2019. The characterization of these steel rods was conducted by testing at the Quality Control Laboratory (QCL) of the manufacturing site in Bhutan. The manufacturer used Thermex Quenching technology for manufacturing, and the quality specifications maintained as per IS standards.

4.1 Mechanical Tests The samples were machined to standard tensile test pieces, and the tensile test was conducted using 1000 kN capacity Universal Testing Machine (UTM). The guidelines of IS 1608 and IS 2062 are adopted for determining Yield Strength (YS), Tensile Strength (TS) and Percentage Elongation (% PE). The calculated results were then compared with the minimum specified values in IS 1786- 2008. Test results were analyzed using the following equations: Tensile Strength (TS) =

Yield Strength (YS) =

Percentage Elongation (% PE) =

Effective Cross-sectional Area(A) = Where = Initial gauge length of the test piece& = Final gauge length of test piece

= Mass of test piece in kg = Length of test piece in meter

Length and diameters of the test pieces were measured before and after the test by Vernier caliper.

4.2 Chemical Analysis The chemical composition of the steel bar was determined by using Optical Emission Spectrometer (OES). Each steel bar sample is to be tested, cut into 50mm length pieces. The test surface of these pieces was ground by using a grinding machine until achieving a smooth, flat, and impurity-free surface. This test was carried out at the Quality Control Laboratory of the Manufacturer. In chemical analysis, nearly thirty main and residual elements were estimated apart from the Iron (Fe). The main elements apart from Iron (Fe) were Carbon (C), Sulphur (S), Phosphorus (P), Copper (Cu), Silicon(Si), and Manganese (Mn). Other elements present in residual amounts like Chromium (Cr), Nickel (Ni), Niobium (Nb), Aluminium (Al), Boron (B), Tungsten (W), Molybdenum (Mo), Vanadium (V), Titanium (Ti), Calcium (Ca), Zink (Zn), Nitrogen (N), Lead (Pb), Tin (Sn), Arsenic (As), Zirconium (Zr), Bismuth (Bi), Lanthanum (La), Cerium (Ce), Antimony (Sb), Selenium (Se), Tellurium (Te) and Tantalum (Ta).

5.0 RESULTS AND DISCUSSIONS




The results of mechanical properties and chemical compositions conducted on 169 and 124 random samples of high strength TMT bars (Fe500) were discussed in detail in the subsequent paragraphs. The obtained results of the chemical composition with particular references to the main elements Carbon (C), Sulphur (S), Phosphorus (P), Copper (Cu), Silicon (Si), Manganese (Mn), Chromium (Cr), Molybdenum (Mo), Vanadium (V), Nickel (Ni), and Carbon Equivalent (CE) are presented graphically and compared with prescribed requirements of IS 1786-2008.

5.1 Mechanical properties 5.1.1. Mass per Length (w):

The measured mass per length of different sizes viz 16 mm, 20 mm, 25 mm, 32 mm, and 36 dia mm bars is at par with the acceptable range as per IS 1786- 2008. The details of the acceptable mass per length is as follows:

Diameter (mm) 16 20 25 32 36 Mass per meter (kg) 1.58 2.47 3.85 6.31 7.99

5.1.2. Strength Characteristics and percentage elongation.

The experimental yield strength results of 169 samples obtained are presented in Figure 1 and compared with the minimum requirement of 500 MPa as per IS 1786- 2008. The yield strength results for all the steel bars higher than the minimum requirement and ranging from 501 MPa to 612 MPa with the mean yield strength of 545.91 MPa.

Figure 1: Results of Yield Strength compared with Minimum Prescribed Limit

The experimental tensile strength results of 169 samples obtained are presented in Figure 2 and compared with the minimum requirement of 545 MPa as per IS 1786- 2008. The tensile strength results for all the steel bars higher than the minimum requirement and ranging from 578.88 MPa to 739.95 MPa with the mean tensile strength of 650.63 MPa.

Figure 2: Results of Ultimate Tensile Strength compared with Minimum Prescribed Limit

The tensile strength to yield strength (TS/YS) ratio is calculated for all the samples and presented in Figure 3. The TS/YS ratio of all the steel bars samples is ranging from 1.12 to 1.30, which is more than the required minimum value of 1.08 as per the IS 1786-2008. The mean (TS/YS) ratio is about 1.19, with a standard deviation of 0.04.




Figure 3: Ratio of Ultimate Tensile Strength to Yield Strength compared with Minimum Prescribed Limit

The percent elongation (%PE) is calculated for all the samples and presented in Figure 4. The %PE of all the steel bars samples is ranging from 16.78 to 25.59, which is more than the required minimum value of 12% as per the IS 1786-2008. The mean %PE of about 20.34, with a standard deviation of 1.80.

Figure 4: Results of Percent Elongation compared with Minimum Prescribed Limit

5.2 Chemical composition The main elements such as Carbon (C), Sulphur (S), Phosphorus (P), Copper (Cu), Silicon (Si), Manganese (Mn), Chromium (Cr), Molybdenum (Mo), Vanadium (V), Nickel (Ni) and Carbon Equivalent (CE) discussed below:

5.1.1. Carbon (C) In steel bars, the content of carbon determines the strength and hardness of steel. The higher the carbon content greater is the hardness, strength, and wear resistance of the steel. However, with increasing carbon content, ductility, weldability, and toughness are reduced. Comparison of obtained results with the maximum permissible limit plus allowable variation as specified in IS 1786 are presented graphically in Figure 5.

Figure 5: Results of Percentage of Carbon Content




The estimated percentage of the carbon content of steel bars is in the range of 0.072 - 0.280% with a standard deviation of 0.045. All the 124 test sample results observed fall within the maximum permissible limit of 0.30% plus 0.02% variation as per IS 1786- 2008.

5.1.2. Sulphur (S) In steel, it is an undesirable impurity. Sulphur decreases notch impact toughness, reduces weldability, surface quality, and decreases ductility. It generally appears as sulphide inclusions in the steel, which decreases its strength. The results of percent sulphur content compared with the maximum permissible limit plus allowable variation as per IS 1786 are presented graphically in Figure 6.

Figure 6: Results of Percentage Sulphur Content

The results of the percentage of the sulphur content of steel bar samples range from 0.014 to 0.055% with a standard deviation of 0.007. All the 124 test sample results observed fell within the maximum permissible limit of 0.055% plus 0.005% variation as per IS 1786- 2008.

5.1.3. Phosphorus (P) In steel bars, phosphorus increases hardness and strength but decreases toughness, ductility, and makes the steel bars brittle. The results of percent phosphorus content compared with the maximum permissible limit plus allowable variation as per IS 1786 are presented graphically in Figure 7.

Figure 7: Results of Percentage of Phosphorus Content

The results of the percentage of the phosphorus content of steel bar samples range from 0.025 to 0.055% with a standard deviation of 0.006. All the 124 test sample results observed fall within the maximum permissible limit of 0.055% plus 0.005% variation as per IS 1786- 2008.




Figure 8: Results of Percentage of sum of Sulphur and Phosphorus Content

The percentage of the sum of phosphorus and sulphur content results compared with the maximum permissible limit plus variation are presented graphically in Figure 8. The results of the percentage of the sum of phosphorus and sulphur content for steel bars range from 0.047 to 0.105% with a standard deviation of 0.011. All the 124 test sample results observed are falling within the maximum permissible limit of 0.105% as per IS 1786- 2008.

5.1.4. Copper (Cu) In steel bars, copper often found as a residual agent. It is also added to the product for enhancing hardening properties and increase corrosion resistance.

Figure 9: Results of Percent Copper Content

The IS 1786- 2008 has not given any prescribed limit for percent copper content. The results of percent copper content are presented graphically in Figure 9. However, the results of percent copper content for steel bars ranges from 0.001 to 0.068% with a standard deviation of 0.013. All samples were found to meet the requirement of 0.85%, according to BS 4449.

5.1.5. Silicon (Si) In steel bars, silicon is one of the main deoxidizers. It improves strength, elasticity, acid resistance, and results in coarser grain sizes.

Figure 10: Results of Percentage of Silicon Content




The IS 1786- 2008 has not given any prescribed limit for percent silicon content. The results of percent of silicon content are presented graphically in Figure 10. However, the results of percent silicon content for steel bars are ranging from 0.019 to 0.290% with a standard deviation of 0.042. All samples were found to meet the requirement according to ASTM A706 and ISO 6935-2 of maximum silicon content of 0.55% and 0.60%, respectively.

5.1.6. Manganese (Mn) In steel, the presence of manganese has a significant impact on strength, ductility, and hardenability. It helps to reduce oxides and also counteract the presence of iron sulphide. Manganese content also helps in improving the abrasive resistance of reinforcing steel.

Figure 11: Results of Percent Manganese Content

The IS 1786- 2008 has not given any prescribed limit for percent manganese content. The results of the percent of manganese content are presented graphically in Figure 11. However, the results of percent manganese content for steel bars are ranging from 0.083 to 1.250% with a standard deviation of 0.131. All samples were found to meet the requirement according to ASTM A706 and ISO 6935-2 of maximum silicon content of 1.56% and 1.65%, respectively.

5.1.7. Carbon Equivalent (CE) Carbon Equivalent (CE) is an empirical value in weight percent. It relates the combined effects of different alloying elements used in the making of steels to an equivalent amount of carbon (Equation as above). As per IS 1786- 2008 and BS 4449, the alloying elements Carbon (C), Manganese (Mn), Chromium (Cr), Molybdenum (Mo), Vanadium (V), Nickel (Ni) and Copper (Cu) is taken into consideration for evaluation of carbon equivalent. CE is a numerical value that gives an indication of hardenability, hydrogen cracking susceptibility, and other properties that may also be linked to properties like hardness, such as toughness and strength of steel.

Figure 12: Results of Percent Chromium (Cr), Molybdenum (Mo), Vanadium (V), Nickel (Ni)




The results of percent Chromium (Cr), Molybdenum (Mo), Vanadium (V), Nickel (Ni) are presented graphically in Figure 12. The results of percent chromium content ranges from 0.001 to 0.068% with a standard deviation of 0.003, percent molybdenum content ranges from 0.001 to 0.010% with a standard deviation of 0.002, percent Vanadium content ranges from 0.001 to 0.008% with a standard deviation of 0.001, percent Nickel content ranges from 0.001 to 0.041% with a standard deviation of 0.008.

Figure 13: Results of Percent Carbon Equivalent Content compared with Maximum Prescribed Limit

The results of percent carbon equivalent are presented graphically in Figure 13. The results of percent carbon equivalent for steel bars are ranging from 0.189 to 0.420% with a standard deviation of 0.054. It is observed that all samples meet the maximum prescribed specification as per IS 1786- 2008.

6.0. CONCLUSION This study was taken up to monitor and characterize the quality of indigenously made steel bars for use in a hydroelectric project in Bhutan. The use of indigenous materials is beneficial and cost-effective as it cuts the freight cost, taxes. On the other hand, it also develops the socio-economics of the region. The TMT bars to be used in the hydroelectric project should have good strength characteristics, bond with concrete, thermal expansion characteristics (similar to concrete), etc. The bond between rebar and concrete depends on the shape, geometry of ribs. The durability of the structure depends upon the quality of the steel bars. Durability is the ability of the structure to maintain safety and serviceability criteria during its design life. The high strength steel rebars (Fe 500) used in this study found to have the following characteristics:

The bars are round in cross-section with remarkable protrusions or ribs on to the bar surface. It is presumed that the bond between rebar and concrete will be strong.

The mass per length of the bars is found as specified for their nominal sizes. The bars have higher yield strength, tensile strength, TS/ YS ratio, and percent elongation ratio than the

specified limits of IS 1786. The bars are more ductile and bendable. The bars are having the perfect balance of strength and flexibility. The chemical composition for main elements is found within the permissible limits. It is concluded that the

bars are having good weldability and also durable to withstand the surrounding environmental effects. Based on this study undertaken under the quality control/ quality assurance program, it is concluded that the high strength TMT steel bars (Fe 500), which is manufactured indigenously in Bhutan, is fulfilling the requirements of the standard specifications prescribed in national as well as international standards. Therefore, this steel is suitable for use in the Hydroelectric project in Bhutan. ACKNOWLEDGEMENT The authors would like to appreciate their working organizations for the constant encouragement and support rendered to them from time to time. Sincere gratitude extended to all the authors whose publications provided us directional information from time to time.




REFERENCES [1] ASTM A706 / A706M - 16, “Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete

Reinforcement” [2] BS 4449- 2005, “Steel for the reinforcement of concrete - Weldable reinforcing steel - Bar, coil and decoiled

product – Specification’ [3] BS 4449-1997, “Specification for Carbon steel bars for the reinforcement of concrete (withdrawn during 2006)” [4] IS 1599-2012, “Method for bend test” [5] IS 1608- 2005, “Metallic Materials - Tensile Testing at Ambient Temperature” [6] IS 1786- 2008, “High Strength Deformed Bars and Wires for Concrete Reinforcement - Specification” [7] IS 2062 – 2011, “Hot Rolled Medium and High Tensile Structural Steel - Specification” [8] IS 9417 – 2018, “Welding - Cold-Worked Steel Bars For Reinforced Concrete Construction- Recommendations

For Welding” [9] ISO 6935-2- 2007, “Steel for the reinforcement of concrete - ribbed bars” [10] Lino R E, Marins A M F, Marchi L A, Mendes J A, Penna L V, Neto J G C, Caldeira J H P and Costa e Silva A L

V d, 2017, “Influence of the chemical composition on steel casting performance”, Journal of Materials Research and Technology, 217.

[11] Salman A and Djavanroodi F, 2018, “Variability of chemical analysis of reinforcing bar produced in Saudi Arabia”, IOP Conference Series: Materials Science and Engineering, Volume 348, International Conference on Materials Engineering and Applications, 14–16 January 2018, Bali, Indonesia.

[12] Wasiu Ajagbe, Abideen Ganiyu, Adefemi Adegbite1 and Oluwasola Akodu1, 2018, “Investigations on the chemical composition and tensile strength of steel bars in the Nigerian construction industry” IOP Conference Series: Materials Science and Engineering, Volume 513, 10th Asia Pacific Structural Engineering and Construction Conference 2018, 13–15 November 2018, Langkawi, Malaysia

AUTHORS N. V. Mahure acquired degree in civil engineering in the year 1987. He is presently working as Scientist Ein Central Soil and Materials Research Station (CSMRS), New Delhi, India. He had also served on deputation for three years in Punatsangchhu-II Hydroelectric Project, Bhutan as Head of Quality Control Department. He is having 30 years’ experience in concrete and construction technology, design of concrete mixes which includes high performance concrete, self-compacting concrete, roller compacted concrete etc., construction materials characterization, thermal studies for mass concrete and diagnostic investigations of structures. He has published more than 70 research papers, monographs in journals of National and International repute. Dr. Mandalozu Raja acquired PhD. from Indian Institute of Technology Delhi during the year 2019. He is presently working as Scientist Din Central Soil and Materials Research Station (CSMRS), New Delhi, India. He had also served on deputation for two years in Punatsangchhu-II Hydroelectric Project, Bhutan as Senior Research Office in Quality Control Department. He is having 20 years’ experience in concrete technology, characterizationof construction materials, concrete microstructure studies, thermal studies for mass concrete and diagnostic investigations of structures. He has published more than 3 research papers in journals of National and International repute. Chenga Tshering had his civil engineering education at College of Science and Technology, Royal University of Bhutan. Soon after graduation, he worked as an Assistant Engineer in the Quality Control Department for the construction of the prestigious 1020MW, Punatsangchhu-II Hydroelectric Project in Bhutan, which is the bilateral project between the Royal Govt. of Bhutan and Govt. of India. He is currently working as an Assistant Executive Engineer at the same organization. His area of interest is in the field of Hydropower construction particularly in concrete technology which includes High Performance Concrete, Self-Compacting Concrete, construction material properties, thermal studies for mass concrete and Alkali Aggregate Reaction (AAR). Man Singh Rai, is currently working as Manager, Quality Control Department at PERFECT TMX TMT, only steel manufacturing company in Chhukha, Bhutan. He Has Completed B.Sc. Physical Science (Chemistry & Mathematics) from Sherubtse College, Royal University of Bhutan. Besides looking after the quality control of steel his area of interest is in ISO internal Auditing, Quality assurance and Research & Development.

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