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.

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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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March 11, 2020