The use of admixtures to waterproof concrete: Part 1

The use of admixtures to waterproof concrete: Part 1

Terms such as watertight, waterproof and damp-proof are often used as if they are interchangeable and all mean the same. What is required is agreement on what these words actually mean so that the numerous admixtures in this market can be correctly categorised.

Watertight simply means that water cannot freely flow through the concrete, for example, through an unfilled bolt hole, a poorly sealed service penetration or a crack.

Damp-proof means that water by the action of capillary suction will not pass through the concrete. However, water can be in the form of a liquid or vapour and ordinary concrete has little resistance to the passage of vapour. Vapour will penetrate through even the densest of substances, though only very slowly, so to be categorised as damp-proof, an admixture must demonstrate ultra-low absorption properties.

To be waterproof therefore, the concrete must be watertight and damp-proof.

The European standard for concrete admixtures, BS EN 934-2, has a category for ‘water-resistant’ admixtures. However, it must be recognised that European Standards have absolutely nothing to do with ‘quality’ or efficacy; they are purely there to categorise products and group them into boxes for control purposes. Indeed, given the very low threshold to be achieved, most superplasticising admixtures meet this requirement!

The 2009 revision to BS8102, which deals with waterproofing of below ground structures, unfortunately did nothing to address this issue, though more about this standard will be discussed in the second article.

If we look at America, again there is no standard, though the American Concrete Institute issued a report dealing with waterproofing admixtures. In this, they are classified as either PRAN (Permeability Reducing Admixture No Hydrostatic Pressure) or PRAH (Permeability Reducing Admixture Hydrostatic Pressure) systems.

Great emphasis is made by many manufacturers of the ability of their admixture to achieve a better a ‘permeability coefficient’. What is permeability? It is the movement of water through a ‘saturated’ substance. Hardly what we need in a basement! The typical test for permeability quantifies the depth of penetration (mm) of water under pressure applied to the face of a sample of concrete, NOT the ‘flow’ of water per se. It really only measures the surface density of the sample. Many researchers have identified correctly that ‘absorption’ is the primary mechanism of water movement through a section of concrete(1) and it is the speed of the movement of water through concrete that is very important as regards durability and resistance to corrosion.

Ideally, any such integral waterproofer would satisfy BOTH criteria – PRAN and PRAH – but very few materials achieve this.

So the first problem is that there is no standard that says what a waterproofing admixture is or what it must achieve, though in 1987 the BRE(2) was commissioned by the British Standards Institution to test such admixtures marketed at the time. Fifteen different ‘waterproofing’ tests were carried out on the nine products put forward for testing. Only one admixture outperformed the plain concrete in all the tests and all the other products tested. However, several proved so ineffective that they were shown to be worse than the control in more than 10 of the 15 tests!

For specifiers this is an issue because under EU law they are not allowed to specify just one product, so we end up with terminology such as ‘or similar’ or ‘as equal and approved’. This allows ‘value engineering’ to have a field day. We should be honest about this term as ‘value’ has nothing to do with this exercise. In most cases, for ‘value engineering’ read ‘the cheapest we can get away with’ and/or ‘the maximum we can increase our profit’.

Hence many articles such as: “Officials cry foul over glaring mismatch in stone colours on key project. It was supposed to be a stunning focal point for a £97m millennium scheme to revamp a historic courtyard in the British Museum. Officials were confident that Sir Robert Smirke’s 150-year-old Great Court would be beautifully set off by its new glass roof and a revamped south portico, splendidly restored with traditional Portland stone.

“Yesterday the scheme, paid for with Lottery money, was embroiled in controversy as the museum’s managing director claimed it had been deceived by a stonemason who used cheaper French stone for the portico.

“Experts estimate that the difference in cost was around £100,000. Museum officials were accused of ‘dereliction of duty’ by English Heritage after it was revealed contractors used the wrong kind of stone to build the South Portico feature of the London museum.”

Another example is the Grenfell Tower disaster, with one newspaper reporting that leaked documents showed the tower’s fireproof cladding was “downgraded to save £293,000”.

Architects and consulting engineers are professionally qualified people who take care to specify what is required. It is unfortunate that in many cases the role of the architect and engineer, as recently reported in the Architects’ Journal, has been diminished and “compliance with regulations and specifications, instead of being a minimum standard, becomes the maximum necessary to meet the contract requirements, with numerous specialists and ‘value engineers’ engaged to see how compliance can be achieved by doing less”.

So what can specifiers do to try and prevent this dumbing down of standards? At best, they can specify a named material, with the “or at least equal and approved” notation and then specify the performance requirements for the product to be used. There will be more on this in the next article in April.

1. Capillary absorption by concrete. Dr Andrew Butler, TRL. Concrete July/August 1997.

“Clearly ‘permeability’ is not a good measure of resistance to chloride penetration. Calculations of the water penetration depth during wetting showed that the speed of capillary absorption is of the order of 10-6m/s – a million times faster that permeability.

“Permeability is a measure of flow under an external pressure and is a property of saturated materials: the narrower the pores in saturated concrete, the lower its permeability. The narrower the pores, the greater the resultant capillary pressure and so the greater the influx of water…”

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