Tolerances In Concrete Construction

Every argument about whether concrete work is acceptable comes down to the same question: acceptable against what.

Concrete is a site-cast material and it carries tolerances that are much coarser than people expect. Those tolerances are published, they are not a matter of opinion, and knowing them settles most disputes before they start.

What tolerances exist for

Not for appearance. For function.

A column out of position changes the load path. A slab out of level affects the floor finish and the drainage. A wall out of plumb affects the facade fixing. Reinforcement out of position changes the capacity and the [cover](/blog/concrete-cover-and-why-it-decides-durability).

Which means the tolerance on any element is set by what depends on it, and different elements carry very different allowances.

The categories that matter

Position. How far an element can be from where the drawing puts it. Columns, walls and footings each have their own allowance, and cumulative error across a grid is treated separately from individual error.

Plumb and alignment. For columns and walls, expressed both as a deviation over a storey height and as a total deviation over the full building height. The total is not the sum of the storeys.

Level and flatness. Two different things, frequently confused. Level is whether the slab is at the right height. Flatness is whether it is smooth over a short distance. A slab can be perfectly level and unacceptably wavy, and the finish requirement decides which one matters. A polished finish or a raised floor system has far tighter flatness requirements than a carpeted office.

Dimensions of the member itself, including thickness, which affects capacity directly.

Reinforcement position, including cover and spacing.

Openings and cast-in items, where the tolerance is usually set by whatever connects to them. A holding down bolt group has a tight tolerance because a base plate has to fit over it, which is the whole substance of [steel fabrication tolerances and site fit](/blog/steel-fabrication-tolerances-and-site-fit).

Where the surprises are

Slab level is allowed to vary more than most clients expect. A floor that reads as visibly out of level over a long room can still be inside tolerance, and the fix is in the floor finish rather than in the slab.

Column position tolerance is larger than the facade needs. Which is why the facade is set out from a survey of the actual structure rather than from the design grid, and why that survey is a programme item rather than an afterthought.

Cumulative deviation up a building. Each storey can be within tolerance and the top of the building can still be noticeably off the theoretical line.

Surface finish is specified, not assumed. A class of finish has to be nominated. An off-form concrete wall intended to be seen and specified as an ordinary finish will be disappointing and compliant.

Measuring it properly

Most disputes come from bad measurement rather than bad concrete.

Level is measured against a datum, over defined spans, with a survey instrument or a defined straightedge method depending on what is being assessed. Plumb is measured over the full height of the element rather than to a convenient point. Flatness is measured over the span the standard nominates, not over whatever distance suits the argument.

And it is measured against the specified requirement, which may be tighter than the standard default. A project specification can and frequently does nominate tighter tolerances for specific elements, and where it does, that is the benchmark.

When something is out

The question is not whether it is out, it is what depends on it.

Out and it does not matter. Accept it and record it.

Out and something has to adapt. Adjust the following trade: pack the steel, grind the floor, adjust the facade setout. Usually the cheapest resolution, and it needs the engineer to confirm the adaptation does not compromise anything.

Out and the capacity is affected. An engineering assessment, and either a demonstration that it is still adequate or a remedial design. A column out of position transfers load eccentrically and that eccentricity is calculable.

Out beyond what can be resolved. Removal and rebuild, which is rare and does happen.

The wrong response is to modify the element on site without an assessment, which is how a tolerance problem becomes a structural one.

Catching it before the pour

Every position, plumb and reinforcement tolerance is checkable while the formwork is up and the steel is exposed.

Which is the entire argument for [pre-pour inspection](/blog/construction-supervision-pre-pour-inspections). A setout error found before the pour is a half hour fix. The same error found afterwards is a negotiation.

ACSES provides structural engineering and construction inspection across Sydney. Get in touch.