Thomas Wodcke, Product Engineer and Applications Specialist for Instron's Industrial Products Group, talks to AZoM about the challenges faced in rebar testing and the solutions Instron can offer.

The global growth rate projections will mean new bridges (and repairs or replacement of existing bridges), roadways, dams, tunnels, mass transit systems, water and sewage systems, airport, hotels, stadiums, and shopping centres - all of which require reinforcement bar. Portable Steel Bar Bending Machine

During construction, rebar is embedded within the concrete in order to improve the tensile strength, and because it has thermal expansion properties that are similar to concrete, rebar allows the structure to expand and contract without compromising it. As a result, product testing standards become crucial in order to ensure the quality of the rebar being used.

Rebar’s irregular surface geometry and the scaling that occurs during elongation present several gripping challenges; the teeth of the grip jaws initially clamp onto the ridges of the specimen, preventing engagement and leading to slippage, whilst an aggressive tooth pattern or too much clamping force can cause premature specimen failure. Additionally, surface scale can build up in the teeth of the jaws and lead to slippage, or the scale can get into the mechanical parts of some grip designs and cause premature damage.

Rebar specimens are often cut from coiled material and must be straightened prior to tensile testing. As a result, specimens are not always perfectly straight. Instron has several solutions for testing bent specimens; for example, our wedge grips are able to accommodate slight or minor bends and provide both effective and economic testing of the most common rebar.

It is important to note that the clamping force is directly proportional to the amount of tensile load applied, and that too much clamping force can cause premature failure. For severely bent rebar, side-acting grips therefore can provide the best solution.

Currently, Grade 100 and No. 20 bars are being added to standards such as ASTM A615 and can produce tensile loads of 3100 kN (approx.700,000 lbf). Instron manufactures robust hydraulic grips and load frames that safely and effectively absorb the released energy preventing unwanted damage to the testing system and enhancing operator safety. Grip and load frame actuation are sealed from dirt and debris and are able to absorb the high levels of energy released upon specimen failure.

Measuring strain is another testing challenge when testing rebar specimens. When a manual extensometer is used long gauge lengths are required, and the extensometer must attach to the uneven surface of rebar without slipping, whilst remaining attached through maximum force of failure for automatic elongation results.

Instron’s rebar extensometer solves all three of these issues and has the added benefit of being uniquely identified by the software, preventing accidental use of an incorrect extensometer. Instron also offers an automatic extensometer with an adjustable gauge length that accommodates a wide range of specimen diameters and gauge lengths. It also features automatic clamping and release, which improves operator safety and reduces test variability between operators. Additionally, it can also remain attached through peak load or failure allowing for automatic recording of elongation results.

ISO and ASTM Standards require common rebar tensile results. However, the nomenclature is different. For instance, when referring to Yield Point, ISO uses Upper Yield Strength (ReH), whilst ASTM uses Yield Point (Drop of Beam or Halt of Pointer). Instron’s materials testing software automatically calculates the test results, reducing both operator and system variability.

For testing programs that require the generation of time consuming reports, Instron’s Bluehill 3 materials testing software can automatically produce customised testing reports based on the results calculated and requested operator inputs. This can save a great deal of time and also ensure consistency between operators and tests.

Bluehill 3 allows for method control with our prompted method feature. This allows for control over what can be accessed by the operator protecting the integrity of the test method. This feature is perfect for developing and integrating standard operating procedures and demonstrating them easily and effectively.

Percentage elongation after fracture is a common calculation that is required when testing rebar. This is automatically calculated in the testing software but you may also need to remove the extensometer to protect it from unnecessary damage during failure. Due to the violent nature of fracture exhibited by rebar it may be unsafe to do this manually as operators are required to enter the test space whilst the test is still running. What’s more is that the extensometer should be removed after the peak load when necking has begun to occur. To counter this problem, Instron’s materials testing software allows for the test to be paused automatically to ensure operator safety. If an automatic extensometer is being used, the software can communicate with the device to disengage from the specimen with no need for operator input.

According to the Global Construction 2020 Report (Global Construction Perspectives and Oxford Economics), the global construction industry is on pace to grow by almost $5 trillion over the next 7 years. This is an increase from $7.2 trillion to just over $12 trillion, with construction output expected to grow by 70% by 2025. Housing is the largest global sector for construction, whilst the global infrastructure market is also expected to show the highest growth in the next decade. Global sports events, nuclear and renewable energy and other key sectors will also have an impact on increased demand.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

Stuart graduated from the University of Wales, Institute Cardiff with a first-class honours degree in Industrial Product Design. After working on a start-up company involved in LED Lighting solutions, Stuart decided to take an opportunity with AZoNetwork. Over the past five years at AZoNetwork, Stuart has been involved in developing an industry leading range of products, enhancing client experience and improving internal systems designed to deliver significant value for clients hard earned marketing dollars. In his spare time Stuart likes to continue his love for art and design by creating art work and continuing his love for sketching. In the future Stuart, would like to continue his love for travel and explore new and exciting places.

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