Concrete’s Nature

No matter the method by which it is applied, concrete is a fascinating material.

The history books tell us that it’s been in use for thousands of years – as far back as 6500 BC, when it was used by Bedouins to make cisterns in which they collected and kept water underground in desert climates.

The ancient Greeks used concrete, too, as did the Assyrians and especially the Romans, who turned its use into a practical art form that saw the fabrication of the aqueducts still crossing the countryside in various places across western Europe – and of the Pantheon, a spectacular concrete structure that to this day is one of the city of Rome’s greatest attractions.

For more than 8,000 years, people have worked, experimented and tinkered with mixtures of various substances, always striving to make concrete a better building material. But one thing nobody has ever been able to do is make concrete waterproof without using additives to make it so. Watertight, yes. But waterproof? Nope.

DEFINING DISTINCTIONS

The difference between those two terms, “watertight” and “waterproof,” have long been important. but they’re even more critical now because there’s so much misinformation out there about what the words actually mean. My preference is to look at this as a simple misunderstanding – one that can be resolved by consulting with the experts.

According to the American Concrete Institute (ACI), the world’s leading authority on the nature and use of the material, concrete is permeable – that is, given sufficient pressure, water will pass right through it. They also report that concrete, absent special additives and/or surface coatings, can never be called waterproof.

To be sure, concrete can be made so dense that water has a tough time penetrating its surface and moving through its mass, but even in those cases, untreated concrete is still permeable. Playing with water-to-cement ratios and the mix design can, for example, make concrete very dense, but the only thing to be accomplished in this way is making concrete less permeable rather than impermeable and therefore completely resistant to water penetration and passage.

In that light, any claim that a dense concrete vessel is waterproof is scientifically, provably false. A shell may in fact have an exceedingly low permeability, but it will still be penetrated at some measurable level. You can fill a shell and might not detect any water loss, but that shell, untreated, is still permeable and ultimately will lose some water other than through evaporation or splash-out. Perhaps that volume won’t be much, but it’s moving through the concrete just the same, no matter how dense the material may be.

Broad Agreement Other trade entities whose professionals work with watershape shells recognize that concrete is not inherently waterproof. The Tile Council of North America, for one, requires positive and demonstrable waterproofing measures be employed before subsequent layers (such as the tile that is their primary interest) may be applied to a vessel. — P.B.

To repeat: This mere fact – that water can pass into and eventually right through it – means that concrete cannot be waterproof, end of discussion. It is physically impossible (again, without the assistance of admixes), no matter the care and skill with which it has been placed.

The key factor here is hydrostatic pressure: As it increases, water will penetrate sooner and pass through its mass more rapidly. To understand what that means, consider Hoover Dam: At the top of that structure, near the water level, the hydrostatic pressure is negligible, permeability is minimal and penetration may even stop. But at the base of the dam – one of the world’s tallest – the pressures are incredibly high because of the mass of the water column.

Even assuming Hoover Dam’s concrete has the same composition and density at top and bottom, it’s simply a fact that water will penetrate the concrete at a much faster rate at the base of the dam, where hydrostatic pressure is at its most extreme. It’s true: As spectacular and well-engineered as is Hoover Dam, water seeps right through the concrete at lower elevations.

This is because all concrete is permeable and the hydrostatic pressure drives water through the permeable microscopic matrix of the dam’s concrete.

MAKING A DIFFERENCE

Again, increasing the density of the concrete reduces the permeability of the concrete, but it cannot eliminate it.

Think about it on the microscopic level: As the concrete cures, multiple pathways through the matrix develop: small cracks form around the aggregates, small fissures appear in the concrete during hydration and voids develop where the water existed in the original mix.

You can increase the concrete’s density by reducing the water in the original mix (and using super-plasticizers as a substitute); by increasing the “fines” in the original mix (for example, by using silica fume particles, which are a hundred times finer than cement particles and more readily fill voids); by using recommended and approved curing methods; by properly placing and compacting the wet concrete; and by including specialty densifying (that is, waterproofing) admixes.

Waterproofing admixes such as Xypex (Xypex Chemical Corp, Richmond, British Columbia, Canada) are inserted into the concrete batch and work by filling and effectively sealing gaps in the concrete matrix, making it impermeable from both sides. At left is a photomicrograph of untreated concrete components. In the middle is a sample to which Xypex has just been applied, while at right that reaction has matured, revealing a crystalline structure that will grow within the capillaries of the concrete and block the flow of water.

Silica fume is of special interest here: Because of the material’s small size relative to cement particles, silica fume particles will indeed fill tiny voids that develop between the cement particles and the aggregate. It also reacts with free lime released during cement hydration to produce calcium silicate hydrates (CSH). The CSH particles replace the weaker lime that is normally found in concrete, which leads scientists to tell us that field use of silica fume as a component of a mix design will reduce permeability by 20 times over a mix design made without it.

The use of waterproofing admixes (such as Xypex, Kryton and others) is a much stronger approach: They make the concrete so dense that it becomes, for all intents and purposes, an impenetrable material – in which case the shell is effectively impermeable and therefore can be considered as waterproof.

In the presence of water, these admixes promote high levels of crystalline growth, a development that seals microscopic voids in a process called hydroscopic self-healing- which may sound like a bad New Age book but which really does take place within the matrix. It’s important to note, however, that these microscopic crystalline structures cannot heal structural or shrinkage cracks, which means that poor placement, improper curing or use of too much water in the mix can and will overwhelm the benefits of any of these admixes.

It’s possible to apply a surface sealer to a concrete surface and make it waterproof in the areas to which the material is actually applied, as is seen here with an application of Hydro Ban (Laticrete International, Bethany, Conn.) to the interior surfaces of a swimming pool. Although this works to keep water held within the pool from passing through the concrete, the back side of the shell, if left untreated, will offer water access to the shell and give it the opportunity (under adequate pressure) to move all the way through the concrete matrix until it contacts the inner surface of waterproofing agent.

All of this leaves the builder with questions to be answered about how effective any measures he or she might be taking at the batch plant will actually be in the field. The trouble is, permeability testing of the sort approved by ACI isn’t as helpful as it should be. Known as the “Rapid Chloride Permeability Test,” this assessment does not measure the depth of the chloride penetration or give any indication of how rapidly the chloride ions reach any specific depth.

To be accurate and adequate, measurement of permeability must be able to assess both the depth and rate of penetration – and this test does neither. Other tests are available, but they were designed by highway departments to measure the quality of poured-in-place concrete – that is, flat surfaces, walls and structures not subjected to any level of hydrostatic pressure beyond the thickness of occasional rainfall.

FACING FACT

So if we accept the scientific fact that all concrete is permeable to some degree, we should all strive to achieve maximum density in our placed concrete. This will lessen permeability and reduce opportunities for reinforcement corrosion, but it cannot eliminate water penetration or the risk it brings.

This leaves us with another fact: If we want to create waterproof vessels, the only way to do so is to eliminate permeability – which, given concrete’s nature, is categorically, scientifically impossible without using admixes or surface treatments. If you work with untreated concrete, in other words, then you are accepting an unknown, hard-to-determine level of permeability for your concrete structures.

That permeability may be minuscule in a well-crafted shell, but it is an unknown – a fact that many contractors, myself included, are unwilling to accept.

Another surface-applied approach to waterproofing comes from Basecrete Technologies (Sarasota, Fla.). In this case, the material is sprayed onto the surface, creating a rougher texture that allows subsequent applications to lock into the now-impermeable surface. But again, this seals just one side of the concrete shell.

So we do what it takes to stave off permeability – meaning we try to keep water from coming into direct contact with our cured concrete. Curing sprays do not make a vessel waterproof, even though they successfully reduce permeability. And they do absolutely nothing for the reverse side of a vessel, which is exposed to the earth that surrounds it. The upshot is that a still-porous structure may not even benefit from these sprays. Who’s to know?

So because waterproofing cannot be applied to the reverse of a concrete vessel (at least where it is shot against earth), it is imperative that proper placement, compaction, mix design and curing procedures be followed to ensure the greatest possible level of density. Beyond that, addition of waterproofing admixes will reduce the permeability of the concrete on both sides of the shell – against water within the vessel and in the surrounding soil – protecting the reinforcing steel and thereby increasing the structure’s lifespan.

But to claim that a vessel can be waterproof without the use of additional waterproofing systems is simply false. The concrete may be less permeable than it is in some other vessels, but it cannot be waterproof because permeability cannot be eliminated by density alone.

So let’s all start using the right terms in the right ways: A watertight vessel is one in which permeability has been reduced and durability has been increased as a result of the superior density that comes with correct material placement and processing. A waterproof one has been treated and prepared in such a way that water cannot penetrate the surface or pass through the concrete matrix; here, nothing less than impermeability is acceptable.

By definition, that’s all there is to the story.

Paolo Benedetti is principal at Aquatic Technology Pool & Spa, a design/build firm based in Morgan Hill, Calif. He may be reached at [email protected].