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894 Checking reinforcement before concrete pours

Report ID: 894

Published: Newsletter 59 - July 2020

Report Overview

A reporter shares their concerns about rebar positioning on site prior to concrete being poured.

Report Content

A reporter from a consulting firm has concerns about rebar being positioned on site prior to concrete being poured. As part of their services, they insist on visiting the site to view and comment on the installed reinforcement bars prior to concrete being cast. These range from RC ground beams on smaller projects, up to multi-storey RC framed buildings. They are aware that this is not standard practice with most other projects as the project Structural Engineer will not usually be requested to visit site to comment on installed reinforcement.

Typically there are significant issues with laps, cover, shear reinforcement incorrectly installed and spaced, spare or leftover bars, which should be impossible as every scheduled bar is needed, incorrect spacing, and incorrect layer directions (e.g. B1/B2 and T1/T2 are in the wrong directions). Generally, the response from the contractor on site is that Building Control have said it is fine and passed it.

What happens on all other projects that the Structural Engineer does not insist on visiting the site to comment on reinforcement? The reporter has little faith that the Main Contractor or the Building Control Inspectors, neither of which are typically Structural Engineers, will be picking up these issues.

Comments

Independent checking, ideally by the original designer, is the most straightforward way to check quality and accuracy of installation. However, there are also benefits in contractor detailing, where the detailing can be tailored to the contractor’s construction sequencing and thus potentially improving economy and site safety. There are also contractual systems where the designer is novated to the main contractor. Both of these approaches mean that the designer is not truly independent of the contractor. Conversely, there are main contractors who will employ Structural Engineers to check the reinforcement fixed by their subcontractors and who are independent of those fixing the reinforcement.

Generally, two safety issues stand out from this report:

  • No changes should be made unless sanctioned by the designer.
  • The observations reported are part of an unwelcome trend whereby designers fail to verify by inspection that what is being built is what they thought was being built.

Building Control Officers can only be ‘experts’ in Building Control, and whilst some may be structural engineers, it is not their primary role. They should not be relied upon to ensure contractor’s quality of work. Just because they have passed it, does not absolve the contractor or designer of their responsibilities.

It is essential that steel fixing work is undertaken by trained and knowledgeable persons under suitable supervision. Competent site work is a significant factor in ensuring the long-term integrity of the RC for its intended design life. For instance, a lack of cover or debris being present, too much cover, inadequate vibration compaction, inadequate ties of rebar resulting in movement during vibration, or misplaced and missing rebar, will often manifest themselves not on striking the formwork, but many years down the line.

In addition to poor quality control, there may be other causes for changes on site. Rebar detailing is a skill and incorrect detailing can mean bars are impossible to position accurately, or even fit in. Cases are known of contractors omitting bars where fit is impossible or where rebar density is so high that concreting is impossible. Guidance on good detailing is given in the IStructE Standard method of detailing structural concrete (Third edition).

A further issue on site is the need to check that the concrete mix supplied, especially when there are specialist mixes, comply with the specification.

 

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