In the last month, the technical helpdesk has seen an increase in queries about ceramic ferrules and concrete – so I thought it worth sharing one of the common questions raised:
Although it may seem that ceramic ferrules used in the stud welding process have nothing to do with the concrete, this is not the case when you consider the common question: After they have been broken away from the stud to inspect the weld, is it necessary to remove broken ferrules from the deck prior to pouring the concrete slab?
This question was investigated by Nelson Stud Welding back in 2006 (Removal of Broken Ferrules-WTD dated 31/01/06) with the conclusion that the answer is NO – it is not essential that ferrules be removed from the metal deck before the concrete is poured. This was based on a number of points, some of which are detailed below:
- Ferrules are manufactured of fireclay with organic binders, which are then fired at high temperature resulting in an inert material that is not detrimental to the poured concrete.
- Broken ferrules have a similar density to NWC, in the region of 2250 – 2400kg/m³.
- The compressive strength of ferrules (approx. 65N/mm²) exceeds concrete strengths used in composite slabs and meets requirements of typical aggregate material used for making concrete.
As with aggregates, it is important that the broken ferrules are not collected into clumps or piles on the deck. Providing they are scattered evenly over the deck they should become wetted and mixed with the concrete as it is poured. The broken ferrules represent only a small quantity of material and should not interfere with the concrete placement or strength.
Whilst, as mentioned by Nelson, it is acceptable to leave broken ferrules on the deck to be added to the aggregate in the concrete, there are sometimes instances where the contract requires for these to be removed and disposed of. In these situations it is worth discussing this with the client / structural engineer to understand the reason, as the collection and removal of ferrules can be a timely and difficult process that impacts on programme, efficiency and cost.
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