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  • Writer's pictureSi Shen

Construction quality control and tolerances

Construction quality control at its core is risk management. The quality control measures put into place are essentially mitigation measures to deal with the risk of failure of compliance to the design. The level of strictness in quality control is typically connected to the criticality of the structure being built, which ties into its consequence of failure – a structure with severe consequences requires tighter control measures.

Tiers of construction quality control

I am specialised in concrete structures, so let me use the rules given in BS EN 13670 – 2009 as an example. It classifies the strictness of quality control into three levels called ‘Execution Classes’. A higher Execution Class means a tighter set of control measure must be in place to ensure compliance with tolerance specifications.

  • · Class 1 does not need very tight control, and only need visual inspection by and random measurements required. This class is suitable only for non-critical structural elements, and when they fail they do not result in significant damage.

  • · From Class 2 up, a defined and systematic checking procedure must be in place. Most commonly seen structures should fall under this category.

  • · Class 3 requires the most detailed inspections and measurements for all load bearing structural elements. The inspection must also be done by an independent inspector.

When exactly to use each Execution Class depends on the National Annex, regulations, and are tied into the three tiers of reliability in Eurocode 0. A lot of critical infrastructure projects and some high-spec private sector structures belong to Class 3.

Construction tolerance

There are two Tolerance Classes as stated in BS EN 13670:

  • Class 1: the ‘normal’ class. This class is what has been incorporated in Eurocode design procedures, and reflected in the material reduction factor – by considering the construction to be non-perfect, the effectiveness of the material is reduced down from its theoretical value.

  • Class 2: the ‘advanced’ class. This class has tighter tolerance specifications, and can lead to improved effectiveness of material, hence justifying a lower material reduction factor.

Typical allowable tolerance for various elements are listed below. Note that these are just ‘ballpark’ figures and the actual allowable tolerance may vary depending on the specific geometry of the elements.

· Dimensional tolerance: 10mm to 30mm depending on size; reinforcement placement tolerance: 10mm to 25mm

· Walls/columns: 15mm to 25mm for verticality, 15mm for deviation of centrelines between continuous elements, 15mm to 30mm for ‘wonkiness’ of each element

· Beams/slabs: 20mm for positional tolerance, 15mm for bearing alignment

What if I mess up?

What if I mess up? i.e. what if the element is built out of tolerance? Firstly, if it is not an easy fix on site, the non-conformity (i.e. out-of-tolerance) should be recorded and reported to the designer, who will assess the severity of the non-conformity. Remedial measures are normally taken for non-severe cases, to strengthen the member and make it acceptable. In rare and extreme cases, where there is no way round, the member has to be demolished and replaced. Normally, where this has to be done, there is usually a compelling reason behind and there is absolutely no other way to make it work.

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