Fibre Reinforced Facade Encyclopedia: Finishes
An exposed aggregate finish is produced by removing the thin layer of cement paste at the cast face so the decorative stone in the mix becomes the visible surface. The paste is removed either chemically, using a surface retarder applied to the mould that stops the face setting so it can be washed off, or mechanically by abrasive blasting. The stone does the work: the colour, scale and sparkle of the facade are decided by aggregate selection, not by the cement.
With 10 mm aggregate, a light exposure reveals about 1 mm of stone and reads as a sanded surface at three metres. A deep exposure reveals 3 to 4 mm and reads as pebble from across the street.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Mechanism | Removal of surface cement paste | Chemical retarder and wash, or abrasive blast |
| Light exposure | 0.5 to 1 mm of paste removed | Aggregate tips just visible |
| Medium exposure | 1 to 2 mm | About one third of the stone showing |
| Deep exposure | 3 to 6 mm | Needs 10 mm or larger aggregate |
| Aggregate size | 5 to 14 mm typical | Larger stone needs a thicker face coat |
| Face coat on GRC | Typically 5 to 10 mm premix | Carries the aggregate; sprayed backing behind it |
| Cost premium | Typically 15 to 40 percent over off-form | Retarder, washing bay, water treatment, stone cost |
| Programme | Typically 1 to 2 days per panel | The wash window is fixed by the retarder |
| Colour behaviour | Very stable | Stone does not fade; the matrix weathers |
How the finish is achieved
The chemical route is the one used for high quality architectural work. A surface retarder is applied to the mould face before casting. It inhibits hydration of the cement it touches, so after the panel has gained enough strength to demould, the outer millimetre or two of matrix is still soft while the body has set normally. That soft paste is washed off with water and a brush or low-pressure lance. Retarders are supplied in graded strengths, each corresponding to a nominal depth, so the retarder grade sets the depth, not how hard the operator scrubs.
The timing window catches people out. Wash too early and the aggregate is undermined and drops out; wash too late and the retarded paste has hardened and comes off in patches. The window is measured in hours and moves with ambient temperature, so runs cast on a hot afternoon and a cool morning are not washed on the same clock. That makes exposed aggregate a production control problem, and it is why panels are sequenced and logged.
The mechanical route removes paste by abrasive blasting instead. It reaches a similar look at light and medium depths, needs no retarder, wash bay or timing window, but abrades the stone itself, so it suits hard granites and quartz better than soft limestone or marble. Acid etching removes only a fraction of a millimetre, so it is a surface texture rather than an aggregate exposure; the sandblasted and acid etched finish pages compare the three.
Aggregate selection
Because the stone is the finish, aggregate selection is an architectural decision rather than a supply decision.
- Colour: granites, basalts, quartz, marble chip, river gravel and crushed recycled glass are all used, and the matrix behind the stone still shows, so a pale stone in grey cement reads very differently from the same stone in white
- Size: larger stone reads at greater distance and needs a deeper exposure and a thicker face coat; 5 to 7 mm suits close range, 10 to 14 mm a facade read from the street
- Shape: crushed stone gives faceted sparkle and keys well into the matrix; rounded gravel gives a softer read but plucks out more easily at deep exposures
- Gradation: single-size aggregate gives an even, deliberate pattern, a graded mix a more natural read that is harder to hold consistent
Exposed aggregate on GRC
Sprayed glass fibre reinforced concrete has no coarse aggregate in it, so an exposed aggregate GRC panel is made in two layers. A premix face coat, typically 5 to 10 mm thick and carrying the decorative stone, is placed against the retarded mould face first, and the structural sprayed GRC is built up behind it while the face is still green so the two bond as one panel.
That construction sets two limits. The aggregate cannot be much more than half the face coat thickness or it will not be properly embedded, which caps usable stone size, and the exposure has to stop well short of the interface with the sprayed backing. The practical ceiling on a thin GRC skin is therefore shallower than on precast concrete. The face coat must also be pigmented and batched to the same discipline as the rest of the mix, because at light exposures a good deal of matrix is still visible between the stones.
Cost, durability and specifying
In service the finish is durable and forgiving. The stone does not fade, the texture breaks up run-off so streaking is less pronounced than on a smooth face, and small chips disappear into it. The trade-offs are that the open surface holds more dirt and is harder to clean graffiti from, which is why an anti-graffiti coating is often specified at ground level even though a coating changes the surface reading.
State the aggregate by quarry, name, nominal size and gradation, the matrix cement type and colour, and the nominal exposure depth in millimetres and the method. Then sample: a hand sample to prove the combination, a control panel of at least 600 by 600 mm made by the production method, and a full-size first-off panel viewed vertically outdoors. Present the control as two or three pieces showing the acceptable extremes, the mechanism described on the colour consistency page.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (face coats, finishes and sampling)
- PCI MNL-117 Manual for Quality Control for Plant Produced Architectural Precast Concrete
- PCI Architectural Precast Concrete, colour and texture selection guide
- AS 3610.1 Formwork for concrete, surface finish classes
- AS 2758.1 Aggregates and rock for engineering purposes, concrete aggregates
- AS 1530.1 combustibility is a property of the tested panel, not the finish (check the edition of the NCC adopted in your jurisdiction)
Related terms
Questions specifiers ask
How is exposed aggregate actually made?
Either chemically or mechanically. A surface retarder on the mould face stops the outer millimetre or two of cement setting, and that soft paste is washed off after demoulding to leave the stone proud. Alternatively the set surface is abrasive blasted to cut the paste away. Retarder grade or blast setting controls depth.
Can GRC be given an exposed aggregate finish?
Yes, by casting a premix face coat of typically 5 to 10 mm carrying the decorative aggregate, then spraying the structural GRC behind it while the face is green. Stone size and exposure depth are both limited by the face coat, so exposures are shallower than on precast.
What depth of exposure should I specify?
Light at 0.5 to 1 mm reads as a sanded surface close up, medium at 1 to 2 mm shows roughly a third of the stone, and deep at 3 to 6 mm reads as pebble and needs 10 mm or larger aggregate. Specify the depth in millimetres and confirm it on a control sample.
Why must aggregate come from a single source?
Because quarry faces change colour and gradation over time and the aggregate is the finish. Securing and storing the whole project’s stone from one production run at the outset is the most effective colour control measure available.
Is exposed aggregate harder to clean?
Yes. The open surface holds more dirt than a smooth panel and graffiti is harder to remove. At ground level an anti-graffiti coating is commonly specified, accepting that it slightly changes the appearance of the surface that was chosen.
Choosing stone for an exposed aggregate facade? We make face coat samples from the actual quarry material so the decision is made on the real mix. Request a face coat sample
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.