There's NO such thing as WATERPROOF CONCRETE - or is there?

Internationally renown Los Angeles swimming pool designer & builder Paolo Benedetti of Aquatic Technology Pool & Spa, lecturer, educator, published author and industry expert, discusses the truths behind claims of "waterproof concrete."

Lately there has been a lot of misinformation provided about "watertight" or "waterproof" concrete. This misinformation is the result of people confusing the two terms PERMEABILITY and WATERPROOF.

Let's first outline some basic scientific facts (per the American Concrete Institute - ACI):
1. CONCRETE IS PERMEABLE (ACI),

THEREFORE,

2. CONCRETE CANNOT BE WATERPROOF
(without taking additional measures such as special admixes or surface coatings - ACI).

3. THE DENSITY OF THE PLACED CONCRETE AND (the original) WATER CONTENT OF THE MIX DESIGN - ONLY AFFECT IT'S PERMEABILITY.

False Claims? or Just Confused?
To claim that a dense concrete vessel is watertight or waterproof is a scientifically false statement. It may in fact have a low permeability (the ability of the water to pass through the concrete), but it will still have some measurable level of permeability. You may be able to fill it, and it might not appear to lose any water, but it is still permeable - and therefore losing some water. The mere fact that water can pass into the concrete, illustrates that it cannot be WATERTIGHT or WATERPROOF. It is physically impossible (without the assistance of admixes), regardless how dense the concrete is.

The higher the level of hydrostatic pressures upon the surface of the concrete, the greater the distance the water will permeate the concrete. Given concrete of the same density (PSI,MPA) and therefore the same level of permeability, here is an example of how permeability can affect "water tightness" of a concrete structure:
a. Under water at the upper elevations of a large concrete dam, there is barely any hydrostatic pressure upon the surface of the concrete. The water will penetrate and saturate the concrete only to a certain depth.
b. At the base of the dam, where there exists extreme levels of hydrostatic pressure, the water will penetrate a greater distance into the concrete wall. This occurs even when the concrete has the same density as the upper elevations.

In the case of the Hoover dam, water is actually seeping through the concrete walls at the lower elevations! This is because ALL concrete is permeable. The hydrostatic pressure is actually driving the water through the permeable matrix of the concrete.

Increase the Density
Increasing the density of the concrete can only reduce the permeability of the concrete, but it cannot eliminate it. Microscopically, there are a multitude of passages through the concrete. There are small cracks around the aggregates. There are small fissures in the concrete from the shrinking that occurs during it's curing stage. There are voids where the water in the original mix once was.

Methods to increase the density of concrete, and thereby reduce it's permeability are achieved by:
1. reducing the water in the original mix (utilize super-plasticizers)
2. increasing the "fines" in the original mix (silica fume particles are 100 times finer than cement particles)
3. practicing approved curing methods
4. proper placement and compaction of the wet concrete
5. employing specialty "waterproof" (densifying) admixes

Silica Fume
Because of their small size in relation to cement particles, the silica fume particles can fill in tiny voids that occur between the cement & aggregate particles. The silica fume particles will react with the free lime that is released during cement hydration. The end products are calcium silicate hydrates (CSH). These CSH particles replace the weaker lime that is normally found in concrete. In field use of silica fume as a component of a mix design will reduce permeability by 20 times, over a mix design without silica fume.

"Waterproof" Admixes
The use of "waterproof" admixes (e.g. Xypex, Kryton, etc.) merely make the concrete so dense, that it becomes for all intensive purposes, impermeable. The shell is technically waterproof, because the concrete is impermeable. The presence of water with these admixes, actually promote additional crystalline growth - sealing the permeable microscopic voids, a term called "hydroscopic self-healing." Mind you, this is occurring on a microscopic level, so these crystalline structures will not heal structural or shrinkage cracks. However, poorly placed, improperly cured, or too much water in the concrete will defeat any benefits of these admixes.

Untreated 1. CONCRETE (UNTREATED) A control sample of concrete was sheared through at 50 mm below the top surface. The sheared face shows some of the by-products of cement hydration with which Xypex reacts. Precipitated calcium hydroxide together with cubic and rhombic particles are visible. (courtesy of www.Xypex.com)

Initiation 2. XYPEX CRYSTALLIZATION (INITIATION) Taken at 50 mm within a Xypex-treated concrete sample, this photograph shows the initiation of the Xypex crystalline reaction after Xypex Concentrate was applied to the surface. (courtesy of www.Xypex.com)

Mature 3. XYPEX CRYSTALLIZATION (MATURE) This photo was taken 26 days after the application of Xypex Concentrate at a depth of 50 mm into the concrete sample. A dense, fully developed crystalline structure has formed within the capillary tracts of the concrete to completely block the flow of water. (courtesy of www.Xypex.com)

Permeability Testing
The problem with the widely utilized Rapid Chloride Permeability Test, is that it does not measure the depth of the chloride penetration or how rapidly the chloride ions reached a specific depth. An accurate measurement of permeability must be able to measure BOTH the depth and rate of penetration.

Alternate tests that provide these measurements do not take into account any atmospheric or environmental conditions that may increase the penetration (hydrostatic pressure), as these tests were created by highway departments to measure the quality of in-place concrete. The surface of a concrete slab, overpass, or guard railing has virtually ZERO hydrostatic pressure upon it.... only the thickness of the rainfall!

Because ALL concrete is permeable to some degree, we should all be striving to achieve maximum density in our placed concretes. This lessens the permeability and the chances of reinforcement corrosion - but, it does not eliminate the risk.

Waterproofing / Watertight
The ONLY means to create a WATERPROOF or WATERTIGHT vessel, is to eliminate permeability - which by concrete's nature is scientifically impossible. If you are willing to forgo additional waterproofing measures, then you are also accepting an unknown level of permeability into your concrete structures. The level of permeability may be minuscule, but it is an UNKNOWN that I am not willing to accept.

To actually claim that a vessel is WATERPROOF or WATERTIGHT, without the additional use of waterproof barriers is false. The concrete is still permeable, maybe it is less permeable than vessels built by others, but it is not water proof or watertight! The only means to achieve a waterproof or watertight vessel is to 100% eliminate the permeability.

The only way to stop the permeability, is to keep the water away from the concrete. This is where waterproofing measures come into play. Curing sprays do not make a vessel waterproof - they only help reduce the permeability, so do not buy into the "waterproof" sales pitch.

Because waterproofing cannot be applied to the reverse of a concrete vessel shot against earth, it is imperative that proper placement, compaction, mix design and curing procedures be followed. The addition of waterproofing admixes will reduce the permeability of the concrete from the reverse side, protecting the reinforcing steel and thereby increasing the structure's lifespan.

Be sure that you use the correct terms:

INCREASED DENSITY REDUCES PERMEABILITY AND INCREASES DURABILITY (but it is not waterproof).

WATERPROOF MEANS THAT WATER CANNOT / DOES NOT ENTER (i.e. PERMEATE) THE CONCRETE.

(Scanning electron microscope images & descriptions used with permissions and courtesy of www.Xypex.com)

Paolo Benedetti - Aquatic Artist"Creating water as art."™Aquatic Technology Pool & Spa©www.aquatictechnology.com