Never Make Safety Vulnerable For Any Reasons

An alarming change to common practices in coupler length are sweeping across India.

Couplers, or Mechanical Splices as are they are also called, are becoming a common method for connecting reinforcing bars together. However, from what we can tell, the way that couplers work is still greatly misunderstood by the vast majority of the Structural Engineering Industry. Therefore, the majority of Structural Engineers happily take the lead of the various standards organisations around the world who have written standards to empirically determine the performance of a system, but this has led to 2 key problems that jeopardizes the structural integrity of our concrete structures.

1. The samples are prepared by the supplier of the system and are usually prepared and installed perfectly. For obvious reasons, this perfect installation is not always possible on site due to manufacturing tolerances and site conditions.

2. The global standards and certification bodies that exist today, are mainly focussed on the initial proof of concept of a system rather than the on-going quality control. Even those bodies that have some element of on-going quality control do not simulate the on-site conditions.

In short, there is no “policing” of the quality control at site level.

For these reasons, most certified suppliers build in some additional robustness to allow for the issues that commonly occur on site, namely:

1. Imperfect thread lengths. There is a tolerance on the thread length that can easily result in the engagement of the 2 bars being 45% : 55% instead of the tested 50% : 50%.

2. Gaps between the rebars. In the case of prefabricated pile cages, it is impossible to have all the bars in exactly the same plane and as such a gap of up to 20mm between 2 bars could easily occur, again reducing the engagement of the thread.

As you can see, both of these will result in the same issue, that the engagement of the thread on the end of the rebar into the coupler will be compromised and a smaller engagement will occur. Thread length is defined to ensure that the amount of load transferred to each thread does not exceed the stripping capacity of that thread. If the stripping capacity is exceeded, then the failure mechanism will change to a stripping failure.

We therefore need to provide a sufficient safety factor against this that will also cover the typical onsite issues shown above.

In India, we have seen an alarming trend where people are marketing a “2D” coupler system where the length of the coupler has been reduced by about 20% to 2x bar diameter. It does pass all the necessary tests in the standard but does not perform well when you try to mimic the onsite problems listed above. We are sure it is only a matter of time before similar products appear in other countries as people strive for cost reduction, but we need to be aware that in this case, cost reduction is directly reducing the safety of the system.

Picture above showing shorter couplers 40mm failed tensile test.
Picture above showing Moment couplers 40mm passed tensile test.

We understand these issues and recently conducted some comparative tests and found that the 2D coupler strips with a bar gap of only 15mm, whereas the longer standard coupler that is marketed by certified suppliers continues to work properly well beyond this limit. This means that the total safety factor on the “2D” coupler is less than 20% even if the installation is performed perfectly.

Never Compromise On Safety!
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Impact Of Coupler Strength With Reduced Embedment

  • Gaps between the bars of greater than 15mm fail with the shorter coupler, but still pass with the Moment coupler.
  • 40mm coupler with a reduced engagement of just 19% will result in a failure using the shorter coupler.
  • There is no factor of safety against failure of typical installation issues.

We should not be comfortable with this reduction in total structural safety and strongly urge structural engineers to investigate these topics more thoroughly and ensure the future robustness of our built environment for future generations.

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Committed to delivering excellence in the construction industry, Halfen Moment Group has selected 10 essential building products which have significantly improve the construction site safety, efficiency, cost and speed. These selected top 10 products have been vastly used by our clients and proven to
i) reduce construction time,
ii) improving construction quality,
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iv) reducing of total construction costs.

Mechanical Splices

 1) Having rebar congestion problem during lap splicing? Do you know you can easily eliminate the consequences of 40D rebar of wastage, by saving substantial amount of time and cost? Click to learn more about our Moment® JoinTec Coupler

2) Do you know that you can connect precast concrete elements by using coupler? Used together with Moment’s proprietary 100MPa grout, this coupler is tested to meet seismic requirements and can achieve bar break technology. Click to learn more about our Moment® Half Grout Coupler

3) Ever encounter a site situation where rebar threading is not possible for further rebar connection? A simple step by step execution of cold forged gripping the coupler sleeve against the rebar makes it possible!
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4) Looking for a specific coupler for repair works, especially where one bar is already fixed in concrete with insufficient space? No threading is required and easy to install! Click to learn more about MBC Coupler

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5) Is the connection between the construction junctions of your precast elements strong enough? Would you like to have a stronger connection with more flexibility? Click to learn more about Moment® Wire Loop Box

6) Looking for a quick, less labor and economical way of solution for connection to foundation or linking columns together? Immediately loadable, hence bracing is not required! Click to learn more about Halfen Column Shoe

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7) Still using the expensive site drilling method to install a support bracket? Or would you prefer to install a channel via casting, with unlimited adjustment along the line of the channel, and yet safer and more reliable? Better safe than sorry, Click to learn more about HALFEN Cast-In Channel

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8) Is your reinforced concrete structure vulnerable to corrosion or early deterioration? Would you like to maintain the proper distance between rebar and form face, yet with a 75% reduction of time? Click to learn more about our Moment® Fast Spacer

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The connection of precast components on construction sites has always been a difficult challenge due to manufacturing tolerances and the need to transfer loads through the structure; even more so when you factor in a required resilience to a seismic events. Precast components are inherently stiff, so it is essential that the connection components are robust enough to allow the structure to absorb the energy of the earthquake, whilst providing enough robustness to avoid collapse.

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