Tensile testing on TMT bars is a critical quality control procedure to determine their mechanical properties, including yield strength, ultimate tensile strength, and percentage elongation. This is typically conducted using a Universal Testing Machine (UTM) and is mandatory for reinforcing steel as per BIS Code IS 1786 in India
Purpose
The tensile test measures a TMT bar's ability to withstand stress and deform before fracturing. It provides crucial data to confirm the bar's quality and suitability for various construction applications, especially in high-load-bearing and seismic zones
Elastic phase: Initially, the bar stretches proportionally to the load. If the load were removed, the bar would return to its original shape.
Yield phase: The bar begins to deform plastically, or permanently. This is the yield point, which can be clearly visible for some steels or identified using the 0.2% offset method for others.
Necking and fracture: After reaching its ultimate tensile strength (the maximum stress it can withstand), the bar begins to form a localized reduction in area known as "necking" and eventually fractures
As per Indian Standard (IS) 1786:2008, the quality of TMT bars is confirmed through bend and re-bend tests, which assess the material's ductility and resistance to strain aging. The bend test confirms the bar's ability to undergo plastic deformation without fracture, while the re-bend test measures its resilience after aging
The bend test evaluates the bar's ductility by bending a sample 180 degrees over a specified mandrel.
Preparation: Take a straight test sample of TMT bar.
Bending: Apply continuous and uniform pressure to the midpoint of the bar until a 180-degree bend is achieved. The bending must be done gradually, without vibration or sudden shocks. The bar is bent against a mandrel with a diameter specified by IS 1786, which varies depending on the bar's grade (e.g., Fe 550D) and nominal size (diameter).
Observation: Visually inspect the outer curved surface (the tension zone) of the bent portion for any signs of rupture or cracks.
Result: The bar passes the test if no cracks or ruptures are visible to a person with normal vision.
This test checks the bar's resistance to strain aging, which can cause embrittlement and reduce ductility over time. The re-bend test should use a new sample, not the one used for the bend test.
First Bend: Bend a test sample 135 degrees using a mandrel as per IS 1786.
Aging: Place the bent sample in boiling water ( 100- degree Celsius) for 30 minutes, then allow it to cool naturally to room temperature.
Second Bend: Re-bend the cooled sample in the opposite direction by applying pressure to the original bend until the included angle is 157.5 degree raised to the composed with power.
Observation: Visually inspect the re-bent portion for any visible cracks or ruptures.
Result: The bar passes if there are no signs of cracking or rupture.
As per the Bureau of Indian Standards (BIS), the test for mass per meter run of steel reinforcement bars is outlined in IS 1786:2008, High Strength Deformed Steel Bars and Wires for Concrete Reinforcement. This field test determines if the actual weight of the steel bar is within the permissible tolerance of its nominal weight.
The mass per meter run (also called "unit mass" or "unit weight") is the weight of a steel bar per unit of length, typically expressed in kilograms per meter (kg/m). The test compares the actual mass of a measured sample with the theoretical mass calculated using the bar's nominal diameter.
Step-by-step procedure
A minimum of four samples should be cut from different bundles of the same diameter reinforcement bars. The sample length is typically 1 meter, but a minimum of 0.5 meters is also acceptable.
Clean the sample. Remove any loose rust, dirt, or other foreign materials from the bar using a wire brush. Excess rust can affect the weight and lead to an inaccurate measurement.
Measure the length. Measure the length of the cleaned bar accurately, in meters. Since the bar might not be perfectly straight, measuring from multiple sides and taking the average is recommended.
Weigh the sample. Use a calibrated electronic weighing machine to weigh the sample bar precisely. Record the weight in kilograms.
Calculate the mass per meter run. Divide the weight of the bar by its measured length to get the mass per meter run.
Calculate the theoretical mass. The theoretical or nominal mass is determined using the bar's diameter. The formula is derived from the density of steel (approx. 7850 kg/m³) and the cross-sectional area of the bar.
Nominal mass (kg/m)=𝐷2/162
Here, D is the nominal diameter of the bar in millimeters.
Dimensional tolerance testing for TMT bars is conducted as per the Indian Standard code IS 1786:2008. The standard defines acceptable variations for the nominal diameter and nominal mass of the bars to ensure structural integrity and quality.
Rolling margin (Diameter tolerance)
This is the permissible variation in the nominal diameter of the bar during the manufacturing (rolling) process.
Permissible limits: The standard specifies different tolerance percentages depending on the bar's size.
Test procedure:
Measure the actual diameter of the TMT bar at multiple points along its length using a vernier caliper or micrometer.
Calculate the average of these measurements to find the average actual diameter.
Compare the result with the standard limits to check for compliance
Importance of tolerances
Structural design: Undersized bars can compromise structural safety by reducing the ability to carry the intended load.
Cost implications: Bars that are significantly overweight or underweight can affect project costs. Paying for a higher tonnage of steel than required is inefficient, while using underweight bars may mean the structure has insufficient reinforcement.
Proper bonding: The correct diameter is critical for ensuring a strong bond between the steel and the concrete. Deviations can result in poor grip and reduced load transfer capacit
The microstructure test, or etching test, of TMTbars is a crucial quality control measure. It verifies that the unique microstructure a hard outer layer and soft inner core was formed correctly during the thermomechanical treatment process. This distinct structure gives TMT bars their superior strength and ductility
The importance of the etching test
TMT bars are produced through a heat treatment process involving rapid quenching with water jets after hot rolling. This process cools the surface rapidly, forming a hardened layer of tempered martensite, while the inner core cools slowly, allowing it to remain soft and ductile, with a microstructure of ferrite and pearlite.
The etching test, also known as the "Ring Test," confirms the formation of this ideal composite structure.
Hard outer ring: A darker outer ring of tempered martensite is revealed upon etching.
Soft inner core: A lighter inner core of ferrite and pearlite should be visible.
Uniformity: The ring must be continuous, concentric, and of uniform thickness (typically 7–10% of the bar's diameter) to pass the test.
Sample preparation:
A small cross-sectional disc (25–30 mm thick) is cut from a representative TMT bar.One side of the disc is polished to a mirror-like finish using a series of progressively finer emery papers (e.g., grades 120 through 1500) and polishing equipment.Care must be taken to avoid overheating the sample, which can alter the microstructure. Coolants should be used during the cutting and polishing processes.
Etching:
The polished surface of the sample is dipped into a chemical etchant, most commonly Nital solution.Nital is a solution of nitric acid (5–10%) in ethyl alcohol (90–95%).The sample is submerged for a specific duration, which can vary depending on the steel and the etchant's concentration. A cloudy finish on the metal surface indicates the optimal etching time.
Rinsing and drying:After etching, the sample is immediately rinsed in water to stop the chemical reaction.It is then dried and prepared for examination.
Microstructural examination:The etched sample is examined under a metallurgical microscope at various magnifications (e.g., 50x to 1500x).
What the test reveals:The results of the etching test provide critical insights into the quality and heat treatment of the TMT bar.
Optimal microstructure: A clear, uniform, and concentric ring structure indicates that the bar was properly thermomechanically treated and possesses the desired combination of a strong, hard outer surface and a soft, ductile core.
Inadequate heat treatment: An improperly formed ring, a non-uniform structure, or a lack of a clear distinction between the core and the case may indicate manufacturing defects, such as improper quenching.
Mechanical properties: The microstructure directly correlates with the mechanical properties. An analysis can confirm that the bar has the necessary strength and ductility for construction applications. For instance, a very thick martensitic layer can make the bar brittle, while too much pearlite in the core can reduce strength
These methods expose TMT bar samples to aggressive conditions in a controlled environment to simulate long-term corrosion in a shorter time frame.
Salt Spray (Fog) Test:
TMT bar samples are placed in a special chamber and exposed to a fine mist of a 5% sodium chloride (salt) solution at a constant temperature, typically 35°C. The test assesses how long the bars can resist rust formation when exposed to saline or coastal environments.
Alternate Immersion Test:
This test involves repeatedly immersing TMT bars in and out of a corrosive solution, such as artificial seawater, to simulate tidal or fluctuating environmental conditions.
Sulfur Dioxide Test:
In this test, samples are placed in a cabinet containing sulfur dioxide gas and a controlled level of humidity. This simulates industrial atmospheric pollution, which can lead to acid rain and accelerate corrosion.
Impressed Current Method:
An accelerated electrochemical test where a DC power supply is used to pass a current through the TMT bar, forcing a much faster corrosion rate than would occur naturally. This is particularly useful for simulating corrosion in marine environments over decades.
A chemical test on TMT bars involves analyzing their elemental composition to ensure they meet specified quality and performance standards. This is primarily done using a spectrometer, which provides a detailed breakdown of the elements present
Primary elements analyzed
The chemical composition of TMT bars directly influences their strength, ductility, and corrosion resistance. Quality standards, such as IS 1786 in India, set specific limits on the percentage of each element.
Carbon (C):
The carbon percentage is a key determinant of the bar's hardness and strength. Higher carbon content increases strength but can make the steel more brittle and reduce weldability. High-quality TMT bars have a controlled, lower percentage of carbon to ensure a better balance of strength and ductility.
Sulfur (S) and Phosphorus (P):
These are considered harmful impurities in steel. High levels can lead to brittleness and a reduction in the steel's load-bearing capacity. Standard chemical tests verify that the percentages of sulfur and phosphorus are within permissible limits.
Sulfur & Phosphorus (S+P):
A combined maximum percentage for sulfur and phosphorus is also a standard quality parameter to check for overall impurity levels.
Other elements
For micro-alloyed or corrosion-resistant grades of TMT bars, other elements are also tested to verify their properties.
Manganese (Mn): Used to increase the strength and hardness of steel.
Copper (Cu) and Chromium (Cr): These elements are added to enhance the bar's resistance to corrosion.
How the test is performed
A cross-sectional sample of a TMT bar is cut, typically 50–60 mm long.
The sample is then ground and polished on one side to prepare it for analysis.
The prepared sample is placed in a spectrometer, which analyzes the element percentages by firing an electrical spark at the sample.
The spectrometer measures the light emitted and provides a detailed result of the chemical composition, often in just 10–15 seconds.
Other methods, such as X-ray Fluorescence (XRF) spectroscopy or Electrochemical Testing for corrosion resistance, may also be used for more detailed analysis.
Importance of chemical testing
Quality assurance: Verifies that the manufacturer has used the correct materials and followed the proper manufacturing process.
Performance: A balanced chemical composition ensures the TMT bar has the necessary combination of high strength, weldability, and ductility for the intended construction purpose.
Durability: Controlled impurity levels and the presence of micro-alloying elements ensure the bars are resistant to corrosion, particularly in humid or coastal environments.
Compliance: Confirms that the bars adhere to national and international standards, such as those set by the Bureau of Indian Standards (BIS).
