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Insitu testing of drossbach tubes  


Pat Mohan
New Member
Joined: 3 years ago
Posts: 1
29/01/2021 6:59 am  

We were recently asked by a peer reviewer to considering insitu testing of drossbach tubes. I understand CSI do the testing however it is costly.

Are other engineers asking for drossbach tubes to be tested on site and if so, what quantities?

Does SESOC consider it necessary?

Are there any other methods to test without using SCI scanning?

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Andy Reid
New Member Customer
Joined: 3 years ago
Posts: 1
03/02/2021 5:14 pm  

Hi Pat,

This is a really valid line of query. It would be good to read a “best practice” article about use of drossbachs. Not only filling and QA of, but also confinement and non-contact splice issues.

Personally, I make sure: starterbar and duct length is conservative
                                    contractor understands that he must, and has the equipment available to, fill from the bottom up
                                    top of drossbach has a clear “stand tube” that gives a visual confirmation that ducts are 100% full

I’ve never used CSI, nor do I know exactly what they can offer.

Hopefully someone more expert can chime in? (Jason Ingham?)                                

Nic Brooke
Member Admin
Joined: 3 years ago
Posts: 24
04/02/2021 3:50 pm  

If the question relates to assessment of existing structures, there is some guidance in the Yellow Chapter version of C5:

Care is required during grouting of drossbach ducts and grout sleeves to ensure that the connections are sufficiently filled. If such connections are employed in critical connections of a structure being assessed, it is recommended that appropriate investigations are undertaken to ensure the connections are adequately filled. This can be achieved by drilling a pilot hole near the top of the duct to check for voids or using recently
available radar/ultrasound techniques. Assessment of the capacity of inadequately filled drossbach connections requires significant judgement, but may be based on Equation C5.6 provided the total length of competent grout exceeds 100 mm and 10 times the anchored bar diameter, i.e. 10𝑑b (fib 2012a). Premature bar buckling is likely to occur if a significant continuous length of duct is left unfilled (greater than 4𝑑b), and bars in such unfilled ducts should be treated as ineffective in resisting seismic demands.

This obviously leaves some answers a little vague – unfortunately there is little information at the moment to fill the gaps.

There is currently a substantial piece of work being undertaken by a joint SESOC/Concrete New Zealand Learned Society working group that is aiming to develop guidance on both specification, installation, and later assessment of drossbach connections. Keep an eye out for updates!

Jamie Nguyen
Active Member
Joined: 3 years ago
Posts: 6
05/02/2021 4:48 pm  

We have also discussed this in our office and with reputable concrete specialists.

We are trialling a new QA procedure on our next project by measuring the volume of grout.
We know the drossbach void volume minus reinforcing bar volume and just need to match it approximately to the grout used.

This should give a good us a good indication of whether or not we are close to achieving filled drossbachs.

Typically poor contractors will just shutter the bottom of the panels and fill with grout which is a horrible way of doing it as you don’t know how much grout is lost to filling gaps under the panels.

Will see how it goes 🙂