The Challenges in Rebar Testing

Interview conducted by Stuart MilneJun 10 2015

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.

What are the main testing challenges facing rebar manufacturers, suppliers and consumers?

Due to the growth being seen globally within the construction industry, we have found that rebar manufacturers and suppliers are encountering increased production volumes, the need for higher strength and larger diameter rebar, an increased requirement for stainless and coated grades, and more construction applications that require mechanical couplers. A combination of these factors are impacting our customers, as they have an increased pressure to be able to test to new or additional requirements, and increase the capability of their testing programmes to be able to withstand the demand.

Global construction is set to increase over the next decade. What issues must be considered by those involved?

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.

Rebar Bend Test

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.

Gripping irregular rebar specimens is an increasing problem. What solutions do Instron offer to combat this issue?

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.

To mitigate these issues, Instron has developed rebar application specific jaw faces with proper tooth patterns that effectively prevent slippage and jaw breaks, whilst allowing for the safe and easy removal of broken test pieces. The grips were designed so that internal components are shielded from falling scale produced during rebar elongation, which has the benefit of reducing maintenance costs and possible down time. Another benefit in the design is that a single set of jaw faces accepts a wide range of specimen diameters so that the user doesn’t have to frequently change the jaws, saving time when testing a wide variety of specimens.

What challenges do bent specimens present during testing and how do you overcome these?

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.

Instron’s DuraSync grips are able to accommodate specimens with moderate to severe bends. These grips feature a synchronizer that overcomes side-loads whilst maintaining axial alignment. The clamping force is adjustable, which helps to reduce jaw breaks and there is no need to reset (or re-centre) the grip jaws between tests. Please see the video below for a demonstration of our DuraSync grips being used whilst testing a bent rebar specimen:

What safeguards do Instron offer against violent failures during testing?  

Rebar specimens can exhibit violent failures with significant recoil during testing. Over 60G of acceleration is common which can also lead to increased system wear. Standard rebar sizes require test system capacities up to 2,000 kN (400,000 lbf), whereas larger, higher-grade bars require capacities up to 4,000 kN (800,000 lbf).

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.

What other challenges can Instron offer a solution for?

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.

What benefits does your testing software offer the user?

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.

How do you see this sector progressing over the next decade?

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.

About Thomas Wodcke

Thomas Wodcke is Product Engineer and Applications Specialist for Instron’s Industrial Products Group, focusing on metals and construction applications. Thomas holds a BEng degree from Coventry University in Automotive Engineering, and initially joined Instron as a result of their graduate intake programme, before quickly climbing the ranks into an Applications Engineer position within the Dynamic product team.  Thomas specialises in evaluating the perceived technical risk involved in challenging customer applications requirements, and developing the best solutions in order to overcome these.