Fibre Reinforced Facade Encyclopedia: Properties
Water absorption is the mass of water a material takes up when immersed, expressed as a percentage of its dry mass. It is a proxy for pore structure, and pore structure is what determines almost everything about the long-term behaviour of a cementitious facade panel: freeze thaw resistance, staining and efflorescence, moisture movement, carbonation rate, and how quickly aggressive ions can reach anything embedded in the panel. It is one of the few numbers on a test certificate that predicts how a facade will look in twenty years.
Two panels with identical modulus of rupture can behave completely differently in service. The one with higher absorption stains, effloresces and moves more, because the water gets in and out of it more easily.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Sprayed glass fibre reinforced concrete | 8 to 13% by mass on immersion | Typical industry range; lower with good compaction |
| Premix glass fibre reinforced concrete | 10 to 15% by mass | Typical; higher entrapped air |
| Polymer modified cementitious mixes | Lower than equivalent unmodified mixes | The polymer film reduces capillary porosity |
| Apparent porosity, cementitious composites | 15 to 20% by volume | Typical; measured alongside absorption |
| FRP laminate (GFRP) | 0.1 to 0.6% by mass, 24 hour immersion | Typical; a well cured laminate absorbs very little |
| Glass fibre reinforced gypsum | High; interior use only | Gypsum is water sensitive and is not an external material |
| Normal weight concrete | 4 to 8% by mass | For comparison; typical |
| Test methods | BS EN 1170-6, ASTM C948, ASTM D570 | Immersion to constant mass, oven dry reference |
| Related checks | Freeze thaw cycling, efflorescence, moisture movement | All track with absorption |
What absorption is really measuring
A cementitious panel is not solid. It contains capillary pores left by the water that was needed for workability but not consumed by hydration, plus entrapped air from mixing and spraying. Water absorption measures the volume of that connected pore network, indirectly, by weighing how much water it holds.
That makes absorption a quality indicator as much as a material property. Two panels from the same mix will have different absorption if one was compacted properly and the other was not, if one was cured for seven days and the other for two, or if one has a higher water to cement ratio. A high absorption result on a batch coupon is therefore a signal to look at the process, not just a number to compare against a limit.
In a polymer laminate the mechanism is different. There is no capillary network to speak of; absorption is diffusion into the resin plus wicking along any fibre that is exposed at a cut edge or a void. Values are one to two orders of magnitude lower, typically 0.1 to 0.6 percent, and a result much above that indicates voids, an under-cured resin or unsealed cut edges rather than a property of the material.
What high absorption causes
- Freeze thaw damage: water in the pores expands about nine percent on freezing. In climates with freeze thaw cycles this is the dominant deterioration mechanism for a cementitious panel, and low absorption with a well distributed pore structure is the defence
- Efflorescence: soluble salts dissolve in the pore water, migrate to the evaporating surface and deposit as a white bloom. Less water movement means less efflorescence
- Staining and differential weathering: an absorbent surface holds dirt and takes up run-off from sills, copings and glazing, which produces the streaking pattern common on poorly detailed facades
- Larger moisture movement: a panel that takes up more water swells more and shrinks more as humidity changes, which increases the demand on joints and flexible anchors
- Higher thermal conductivity when wet: water conducts far better than the air it displaces
- Faster carbonation and ion transport: relevant wherever there is embedded steel, in a stud frame, an anchor or a cast-in fixing
Reducing it
Absorption is controlled in manufacture rather than corrected afterwards. The main levers are a low water to cement ratio, which reduces capillary porosity at the source; proper compaction, which in sprayed glass fibre reinforced concrete (GRC, GFRC) means roller compaction of each layer against the mould; the full seven day moist cure, because hydration products are what fill the capillary pores; and supplementary cementitious materials such as metakaolin or silica fume that refine the pore structure.
Polymer modification reduces absorption further, because the acrylic film blocks capillary paths. That is one of several reasons polymer modified mixes are used, but it has a fire consequence: a non-combustibility claim belongs to the tested formulation, not to the category. ShapeShell-RC is classified Not Deemed Combustible under AS/NZS 1530.1 by Ignis Labs, and any variation to the mix is a separate question for the fire engineer.
Applied treatments work differently. A silane or siloxane hydrophobic impregnation lines the pores rather than filling them, so the panel still breathes but capillary uptake of liquid water is greatly reduced. These treatments are consumable and have a finite service life, typically well short of the panel’s, so they belong in the maintenance plan rather than being relied on as a primary durability measure.
Specifying and testing
For cementitious composites, absorption and dry density are measured on the same coupon under BS EN 1170-6 or ASTM C948: the specimen is oven dried to constant mass, immersed to constant mass, and the mass gain expressed as a percentage. The pair of figures is more informative than either alone, because low density with high absorption indicates poor compaction while high density with high absorption indicates a porous mix.
For polymer laminates, ASTM D570 gives 24 hour and long-term immersion values. Long-term immersion matters more than the 24 hour figure for a facade, because the relevant question is whether the laminate reaches equilibrium at a low moisture content or continues to take up water, which would indicate voids or capillary wicking along fibres.
A sensible specification asks for absorption as part of the batch test record alongside modulus of rupture and limit of proportionality, sets a maximum appropriate to the exposure, and asks for a freeze thaw cycling result where the climate requires it. Asking for absorption without asking for the cure regime is asking for half the answer.
Standards and references
- BS EN 1170-6 Test method for glass-fibre reinforced cement, Determination of the absorption of water by immersion and determination of dry density
- ASTM C948 Standard Test Method for Dry and Wet Bulk Density, Water Absorption, and Apparent Porosity of Thin Sections of Glass-Fiber Reinforced Concrete
- ASTM D570 Standard Test Method for Water Absorption of Plastics
- BS EN 1170-8, cyclic weathering test for glass-fibre reinforced cement
- AS 1012.21 and AS/NZS 4456.14, water absorption test methods used in Australian concrete and masonry practice
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- AS 3600 Concrete structures, exposure classifications, as the framework for setting durability requirements
Related terms
Questions specifiers ask
What is a typical water absorption for GRC?
Roughly 8 to 13 percent by mass on immersion for well compacted sprayed material, with premix somewhat higher. It is measured alongside dry density on the same coupon, because the pair of figures together indicates compaction quality.
Why does water absorption matter for a facade?
It governs freeze thaw resistance, efflorescence, staining, moisture movement and the rate at which aggressive ions reach any embedded steel. It is the best single predictor of how a cementitious panel will look and behave after twenty years.
How much water does FRP absorb?
Very little, typically 0.1 to 0.6 percent by mass. A higher result usually indicates voids in the laminate, incomplete cure, or unsealed cut edges rather than a property of the material.
Does a hydrophobic treatment change the absorption figure?
Not the bulk figure. A silane or siloxane impregnation lines the pores and greatly reduces capillary uptake of liquid water at the surface, but it is a consumable treatment with a finite life and belongs in the maintenance plan.
How is absorption reduced during manufacture?
A low water to cement ratio, thorough roller compaction of each sprayed layer, the full seven day moist cure, supplementary cementitious materials, and polymer modification where the mix design allows it.
ShapeShell-RC is batch tested to the GRCA specification, with density and absorption recorded alongside modulus of rupture and limit of proportionality. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.