Product conversations tend to use these four words loosely, as if they all meant "lets water through" or "does not let water through." They measure different phenomena under different conditions, and a specification that picks the wrong one can pass a material that fails the actual exposure, or reject one that would have worked.
Porosity: how much void volume exists
Porosity is the fraction of the concrete's volume that is not solid. It is a static property: how much empty space exists in the paste, aggregates, and air voids. Total voids in hardened concrete can be measured by methods such as ASTM C642, which determines density, absorption, and voids from oven-dry and saturated mass comparisons.
Porosity tells you the size of the reservoir. It does not tell you how fast anything moves through the connections between voids, which is the property that usually controls durability.
Permeability: flow under pressure
Permeability describes how easily a fluid moves through connected pores under a pressure gradient. It is the property that matters for water held against concrete: foundations below the water table, tanks, marine structures at depth.
Permeability depends on both porosity and connectivity. Two concretes with identical porosity can differ enormously in permeability if one has well-connected capillary pores and the other has blocked or disconnected pathways. That is why low permeability, rather than low porosity, is the goal in most durability specifications.
Absorption: uptake by immersion
Absorption measures how much water concrete takes up when immersed, expressed as a mass or volume change. ASTM C642 is a common method. Absorption answers "how much can this concrete soak up?" but not "how fast under real exposure?" because full immersion is a more aggressive condition than most surfaces ever see.
Sorptivity: the rate of capillary uptake
Sorptivity measures the rate at which concrete draws in water by capillary suction when only one face is in contact with water, which is how most real concrete actually meets water: one wetted surface, no pressure head. ASTM C1585 defines the measurement. Sorptivity is the property most directly connected to splash-zone wetting, rain on horizontal surfaces, and meltwater sitting on a slab.
Why the distinction matters
Consider three ways to describe "resists water," all technically true for a given material, each answering a different question:
- Low absorption: "it does not soak up much when fully immersed."
- Low sorptivity: "it does not rapidly pull in water by suction."
- Low permeability: "it does not pass much water under pressure."
A treatment that reduces surface absorption might do little for pressure-driven flow through a connected pore system. A test showing reduced uptake in one condition does not establish the same reduction in another. This is why credible technical data sheets identify the exact test used, and why reading the TDS carefully matters more than the marketing headline.
The same discipline applies to test interpretation in general: our article on what ASTM C1202 actually measures covers a well-known example of a test whose name oversimplifies what it measures.
What this means for product conversations
When a treatment or mix is described as reducing "permeability," the useful questions are:
- Which property was actually measured, and by which method?
- On what substrate and in what condition?
- Compared against what control?
- Under what conditions does that reduction apply?
These questions are not hostile; they are the same ones an engineer would ask. A supplier who can answer them precisely is demonstrating that their claims trace to real measurements.
The terms in one table
| Term | What it measures | Typical test method | The question it answers |
|---|---|---|---|
| Porosity | Fraction of volume that is void | ASTM C642 (voids) | How much empty space exists? |
| Permeability | Flow under pressure | Various pressure methods | How much fluid passes under pressure? |
| Absorption | Uptake by immersion | ASTM C642 | How much water does it soak up when soaked? |
| Sorptivity | Rate of capillary uptake (one face wetted) | ASTM C1585 | How fast does it pull in water by suction? |
Keep the table when a product conversation starts using the words loosely: ask which row the claim is about, and which method measured it.
Where each property dominates in real exposures
Matching the property to the exposure is the practical payoff of the vocabulary:
- Sorptivity governs horizontal surfaces meeting rain, meltwater, and splash: sidewalks, decks, curbs, aprons. Most surfaces meet water this way, one face at a time, with no pressure.
- Permeability governs anything holding back water: foundations below grade, tanks, marine structures at depth, retaining walls.
- Absorption is a material characterization: useful for comparing mixes and treatments, less directly connected to any single field condition.
- Porosity underlies all of them; air void systems (deliberately entrained air) are porosity working for the concrete, not against it.
A product claim about "reducing water penetration" on a sidewalk surface is a sorptivity-class claim; the same claim about a foundation wall is a permeability-class claim. The evidence for each is measured differently, which is why a single test cannot serve both conversations.
Why marketing collapses the terms, and how to un-collapse them
"Reduces permeability" is the phrase most often borrowed because it sounds most structural. The discipline when you hear it:
- Ask which method measured it, on what specimens, against what control.
- Match the measured property to your actual exposure: is your problem pressure-driven flow, capillary uptake, or vapour? The test that matches your condition is the one that counts.
- Beware percentages without units and methods: "86% more water resistant" is not a sentence with test context; "the treated specimen gained 1.8 g versus 13.4 g for the untreated control under [method]" is (our PRO Tek evidence walkthrough shows the format applied to real numbers).
Suppliers who volunteer the method, specimen, and control are demonstrating exactly the precision this vocabulary exists to support.
What this article does not tell you
It does not provide threshold values for any exposure class; those come from the applicable specifications and standards for the project. It also does not evaluate any specific product.
Where to go next
How Water Moves Through Concrete covers the physical mechanisms, and How to Read a Concrete Product Technical Data Sheet applies this vocabulary to product documents. For the test-evidence landscape, see Freeze-Thaw Testing vs Deicer Scaling Testing.