Architectural concrete: surface categories and sample panel

Sites rarely argue about whether a wall is load-bearing. They argue about whether it looks right. The client sees blotches, the contractor sees ordinary concrete as it comes out of the mould. Both are right, because nobody wrote down beforehand how that surface was meant to look. In architectural concrete the surface is the product — either it is described in numbers before production, or it becomes a negotiation at handover.

This article explains how architectural concrete differs from ordinary as-cast concrete, what categories of surface quality exist, and what has to be agreed before an element reaches the casting table.

What "architectural" actually means

Architectural concrete is concrete whose surface appearance is planned and defined at design stage. Not every surface left raw after striking qualifies — only one produced under a defined technological regime, in order to achieve the intended texture, colour and uniformity.

The effect can be reached in four ways: leaving the surface as cast, colouring the concrete throughout, working the hardened surface mechanically (grinding, bush-hammering, sandblasting, polishing), or exposing the aggregate. Whichever is chosen, the same factor decides the outcome: repeatability of production conditions.

Three categories instead of the word "nice"

Requirements for the surface are grouped into three categories, matched to how exposed the element is.

BA1 — low requirements. Surfaces whose appearance is secondary: basement walls, underground car parks, plant rooms.

BA2 — medium requirements. Ordinary visible surfaces: stairwells, corridors, common areas.

BA3 — high requirements. Representative elements: facades and exposed walls in interiors.

Behind each category sit four separately described parameters — texture, porosity, colour uniformity and formwork class. Only these turn "make it look good" into something that can be checked.

Texture: what may show at the formwork joints

The texture category describes above all what happens at the joints between formwork panels. F1 allows cement paste leakage up to about 20 mm wide and 10 mm deep, and plane offsets up to 10 mm. F2 brings those down to about 10 and 5 mm and adds organisational requirements: the same type of formwork, an agreed way of sealing the joints, and evenly applied release agent. F3 means leakage up to about 3 mm wide, offsets up to 5 mm, and the need to design the formwork itself — panel layout, joints, seals — as a task in its own right.

Worth knowing: a sharp arris carries a radius of about 3 mm for production reasons. The perfectly sharp edge does not exist; if the design calls for one, a chamfer with a triangular fillet has to be planned and written down explicitly.

Porosity: numbers instead of impressions

Blowholes are the most common source of complaints, because everyone judges them differently. So they are measured by area, on a standard control field of 500 × 500 mm, counting pores between 2 and 15 mm in diameter.

With absorbent formwork the limits are tighter: 3000, 2000 and 1000 mm². From P2 upwards the interaction between the concrete mix, the release agent and the formwork has to be checked; at P3, changes to the mix composition and the use of recycled water and aggregate are ruled out.

Colour: why it varies and what helps

Colour differences between elements are a technological phenomenon, not a workmanship defect. They depend on curing time and the moment of striking, on the temperature of the mix, the table and the surroundings, on the consistency of the concrete, on how the release agent was applied, and — in shift work — on who made the element. The colder it is, the greater the risk of blotchy or darker surfaces.

The colour uniformity categories set out what is acceptable. RZ1 allows lightening and darkening, but neither rust nor dirty run-off streaks. RZ2 allows uniform, large-area changes of shade, but rules out mixing different types of formwork facing and requires a mixing time of at least 60 seconds. RZ3 is the strictest and in practice forces a single source for every constituent throughout production.

The sample panel — the cheapest insurance

At BA1 a sample panel is optional, at BA2 it is recommended, at BA3 it is required. It is the one moment when both sides look at the same surface and agree that this is how the whole is to look. The cost of one sample element is a fraction of the cost of a dispute over a facade; at P3, at least two sample panels are recommended.

Why precast makes it easier

Architectural concrete cast on site cures in conditions nobody controls: the formwork stands in rain and sun, the temperature shifts through the day, and the striking date follows the programme. In the plant the element is made on a casting table, indoors, with a repeatable mix and a constant curing time. That does not guarantee an identical shade, but it removes most of the reasons why shades drift apart.

Precast also forces one design decision earlier than in-situ work does: the panel layout. Erection joints will be visible, so it is better to plan them as part of the facade composition — possibly supported by dummy joints — than to discover their pattern after erection.

What the categories do not cover

The requirements described apply to surfaces that reproduce the formwork. They do not apply to surfaces shaped while the concrete sets, such as trowelled ones, nor to surfaces worked after hardening. For those, the scope and method of assessment have to be agreed separately — ideally, again, on a sample panel.

The full guidance together with comparison photographs is collected in our guide to architectural concrete, available in the downloads section. The parameters of our elements are described on the architectural concrete product page, and an example built in this technology is the facades of the Aura building in Poznań.