Waterproofing of Concrete – Admixtures
Guest Post: ‘Waterproofing’ of Concrete – Admixtures
– Bob Cather BSc (Hons) CEng FIMMM – past President of The Concrete Society
The question of whether to use a ‘tanking’ membrane or a specialist waterproofing admixture to create a waterproof concrete structure is a regular feature in the overall design of concrete structures, be it basements, tunnels, water tanks, swimming pools etc.
How Concrete Admixtures Work
There are very many ‘waterproofing’ admixtures on the market world-wide that can be added to the concrete at mixing intended to achieve enhanced resistance to water migration. Across the range of products there are many common features and differences. The promotion of these systems through the pages of this journal and elsewhere, often come with attractive claims for performance, ease of use and back-up ‘warranties’.
Basement structures are often complex entities that can provide significant restraints to design of appropriate waterproofing
Although the different products may claim different composition and or mode of action – pore-lining repellency, crystal forming pore blocking, organic pore plugging under pressure – the presentation of claims, technical back-up, warranties have much in common across the majority of the products offered.
This discussion paper seeks to bring together the key issues and claims and presents some evaluation of the materials and invites a rigorous debate of benefits and expectations.
This is not intended to be a specific approval or rebuttal of individual claims on specific individual products. Although there is reference made to some technical documents, the materials, the claims and the issues are relevant worldwide.
Two headline views, derived below, is that the use of such admixtures alone is very unlikely to substantially improve the structural concrete’s ability to resist the passage of liquid water and any claimed improvement to the resistance to water vapour, is more uncertain. An important distinction is made, here and later in this paper, between water penetration through ‘structural concrete and that through a structure constructed in concrete.
BBA Certification For Admixtures
A source of independent assessment of products outside the national standards is certification by Agrément or other technical approval bodies.
These certificates generally record that concretes containing the special admixture produce concrete with lower water permeability compared to a reference concrete. The certificates are more ambivalent about effects of adopting these modified concretes on the overall water exclusion of a below ground structure.
‘Failures’ often occur at the joints, junctions and boundaries
Water Leakage Through Cracks and Service Penetrations
It is common experience, drawn from real projects that water leakage through a concrete structure is predominantly not through the body of well compacted concrete, but rather where the concrete isn’t – at cracks, at joints at service penetrations, etc. Where the concrete itself is of poor quality – exhibiting poor compaction or having inclusions – there may be water leakage but the admixtures seeking to ‘improve’ the concrete matrix cannot be expected to provide benefit in such circumstances.
Thus although the concrete containing the admixture may be shown to have higher resistance to water ingress, this does not mean the structure itself will be more resistant. The argument that structural quality concrete has a very high degree of inherent ‘waterproofness’ is supported by many so-called ‘permeability’ tests that have been carried out (3). Establishing a true ‘flow’ condition in a laboratory test is extremely difficult for concrete representing the low permeability mass typical of structural concrete.
There is also some doubt as to whether the apparent improvement in water permeability property of the concretes tested by the certification bodies, is actually from the ‘active’ ingredient and not due to other changes between the control concrete and the test concrete. The table below summarizes the data from some issued assessment certificates. It shows and contrasts the composition and performance detail declared.
A first comment to make is that, according to guidance on permeability (3), all of the permeability results above – test and control – would be classified of concrete of low permeability (concrete permeability < 10 -12) . There is a further degree of uncertainty about the conclusiveness of these tests, if the permeability results shown in the table for the notionally similar control specimens.
If we have good quality concrete, that is good enough in terms of permeability is there really benefit in making a concrete that is say, five times better?
For some of the products above e.g. ‘δ’ or ‘ε’ the differences in permeability could plausibly derive from the substantial reduction in w/c ratio between control and test. The link between w/c ratio and permeability is well established.
From the above arguments there seems little benefit in adopting a special formulation waterproofing admixture to achieve better resistance against liquid water ingress.
These arguments typically come up in a contractor suggested alternative to a membrane based waterproofing design. But, if the concrete itself can be waterproof enough – with judicious attention to joints and penetrations – why opt for a membrane at all? Confidence, habit, vapour control, durability? The factors and guidance involved with these wider issues are, at present rather less well defined and conclusions rather more difficult to draw.
Water Vapour Permeability Considerations
We would instinctively conclude perhaps, that whilst concrete can exclude liquid water it may permit passage of water vapour. It is true that making measurements of water vapour permeability is easier to achieve than with liquid water. The major uncertainty with consideration of water vapour is to decide what level of vapour ingress can be permitted. The terminology used in some design guides do not help – ‘dry’ or ‘totally dry’ – define the required transmission performance of the concrete. Additionally the design for control of water vapour ingress is to a greater extent influenced by the nature of the use of the internal space and the mechanical services – dehumidification, ventilation etc. This has been recognized by the revision to BS 8102 has removed the previous Grade 4 environment.
The Agrément certificates for some of the waterproofing admixtures has attempted to put a performance requirement on the concrete that would meet vapour resistance needs of the superceded BS8102. However, on the values stated and allowing for typical concrete construction sections, it can be shown that the control concrete without the admixture can meet the vapour transmission requirements.
Hence the conclusion at present is that individual project needs and perceived risks will need to be considered in deciding whether there is benefit in adopting one of these admixtures in a concrete-alone solution or to design on the basis of a tanking membrane system.
The Use of External Tanking Membranes for Below Ground Concrete Structures
Warranties For Concrete Admixtures
The admixture manufacturer offer of warranties was mentioned earlier in this note. These can outwardly be attractive in establishing confidence in the waterproofing option. In precis terms the warranty will offer to oversee the detail of waterproofing design, have some contribution to site practice and significantly, warrant that if there is a leak they’ll come back and ‘fix’ it.
On those projects that we’ve encountered a warranty in place, the arrangement appears to work to a large degree. However typical warranty documents have potentially significant issues. Returning to ‘fix’ the leaks only covers the act of grouting or patching or whatever is required. The supplier will normally be expected to be provided free (in both senses) access to the defect. This may be difficult and have significant on-cost if the leak becomes apparent only after the space has been fitted out with plant and other equipment. Leaks may not become apparent until sometime after the construction has been completed. A further common limitation is that the cost of the ‘fix’ will only be covered up to the cost of the admixture originally supplied.
These warranties may well be a positive contribution for particular projects but it is recommended that the warranty terms are vigorously reviewed.
There is hopefully a wider and more detailed debate to be had on the issues raised in this paper. There is certain to be a spread of views, experiences and expectations.
Newton Waterproofing Systems thank Bob Cather for his submission of this article.