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When it comes to the challenge of waterproofing and gas proofing a below-ground structure, how do modern waterproofing products perform as an effective gas protection offering?
The answer to this question depends on several aspects, including the types of ground gases or contaminants that are present, as well as the contamination levels that are outlined in the site report that should be performed on any such project.
Saying this however, ground gas and contamination is an expansive subject and can be quite different to the waterproofing industry.
British Standard 8102:2009 is the ‘Code of practice for protection of below ground structures against water from the ground’, and it defines three “Grades” of waterproofing depending on the desired internal environment:
The British Standard also outlines three “Types” of waterproofing system that can be used, either standalone or in combination, in order to achieve these Grades:
Consideration should be given to combining these Types of waterproofing in any scenario where the assessed risks are deemed too high, or where the consequences of failure are unacceptable.
When it comes to considering protection against both water and gas, the first paragraph of Section 4 of the British Standard states that:
“it is essential for the success of any project involving below ground structures that strategies for dealing with groundwater, soil gases and contaminants are considered.”
Consideration of gasses and contaminants should be considered at the earliest stages of a waterproofing design.
Consequently, when required, Newton will always recommend waterproofing products that possess third-party tested gas resistance. In turn, these systems may also be verified by an independent specialist depending on the gasses that are present.
In waterproofing, the safest designs always seek to combine two or more forms of protection, with one of those forms always being a Type C system, supported by the Type A and B products.
The important next step is to be able to assess the level of gas protection that each waterproofing element provides.
Protecting Against Radon
The most important document to consider for radon protection is the BR 211 ‘Radon: Guidance on protective measures for new buildings’.
In previous editions, BR 211 stated that if a basement structure was “tanked” with a product of equal performance to a 1200-gauge DPM then it would provide sufficient protection. As a result, Newton has third-party test results for Radon resistance on some of our external waterproofing membranes.
However, in Section 6.12 of the most recent 2015 edition, BR 211 now recognises that:
“Below-ground waterproofing and radon management are specialist activities that can conflict. It is recommended that dual protection systems are designed and installed by specialists who are suitably qualified in both waterproofing and radon management.”
Newton’s cavity drain membranes are third-party tested for radon resistance, and we developed the patented Newton PAC-500 system for high-risk radon sites (discussed in Section 6). By partnering with gas protection specialists Prestige Air, the PAC-500 system combines Type C waterproofing with a positive air curtain (PAC) ventilation system that controls both water and gas in any below-ground structure, whether it is new or existing, domestic or commercial.
Protecting Against Methane & Carbon Dioxide
British Standard 8485:2015 is the ‘Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings’, and when it comes to basement structures, in Section A4 it states:
“In general terms, a basement constructed of dense, cast in situ concrete appropriately designed to BS EN 1992 to provide integral waterproof protection normally represents the most impermeable of structural forms. Combining this with a waterproof membrane to create a fully tanked basement provides the best basement solution for preventing the ingress of ground gases.”
As with the previous editions of BR 211, “tanking” is being used here as a catch-all term for waterproofing. In this case however, the word indicates what we would call an external or internal Type A waterproofing membrane.
However, as waterproofing designers, we must consider the gas protection capabilities of our entire design, including all three Types of waterproofing and the structure itself.
Assessing the Gas Capabilities of a Waterproofing Design
British Standard 8485:2015 provides all of the tools needed to accurately assess the gas capabilities of our waterproofing designs.
The first step is knowing the Characteristic Situation (CS) value of our site – this will be established during your site investigation and ranges from CS1 (very low risk) to CS6 (such high risk that some sites cannot be built on).
Once this is known, we can use Tables 3, 4, 5, 6 and 7 from the British Standard to:
For full details on the scoring system and the points that can be allocated to different design elements, please download the full article from this page.
Utilising Waterproofing Products for Gas Proofing
There are several Newton products that have been tested and meet the standards for use as protection against gas below-ground. However, with gas proofing as in waterproofing, the most effective designs combine multiple forms of protection, and always include a Type C cavity drain membrane system.
To begin with, the third-party certified Newton 403 HydroBond-GB (gas-barrier) meets with Section C2 ‘Available Membranes’ of British Standard 8485, and exceeds the performance levels of the 0.4mm polythene sheet which is recommended by the standard for above-ground scenarios (and which is not suitable for below-ground use).
Newton 403 HydroBond-GB also has lap tests for methane and carbon dioxide, and it is important that the figures for the tests (along with any other discontinuities such as pile heads or foundations) are taken into account and averaged over the whole basement area.
Designing for Defects
In waterproofing, the legal precedent set by the ‘Outwing vs Weatherald’ high court case states that:
“overlapping self-adhesive membranes cannot be expected to achieve a total or absolute watertight bond capable of resisting penetration by water pressure.”
Therefore, regardless of a products’ laboratory testing, we must expect defects in such membranes and ensure that our design accounts for this. Consequently, best practice in waterproofing is to combine systems, and the safest designs emphasise both a correctly designed reinforced concrete structure and an internal Type C waterproofing system.
Issues of Ventilation
Table 6 of British Standard 8485 outlines how additional gas protection points can be achieved by using external ventilation. However, this clashes with waterproofing best-practice, where we design to account for a full head of groundwater pressure coming to bear against the structure at some point in its design life.
Such a scenario would mean that any ventilation would fill with water, and yet despite this discrepancy, it is theoretically possible for a below-ground concrete car park with just a Newton RASCOtank Type B waterproofing system to achieve 6.5 gas protection points (according to Table 5 and Part (e) of Table 6, as shown below).
In waterproofing terms, Newton always specifies a Type C system where the consequences of failure are unacceptable, and, in our opinion, the gas protection points available for a Grade 3 basement in Table 5 of BS 8485 should not be attributed without the use of Type C waterproofing.
The ‘Positive Air Curtain’ Solution
Despite the problematic statements within BS 8485:2015, generally one would not normally specify a single form of waterproofing, especially in scenarios where there is both water and gas present.
For a Type C system to form an effective gas barrier, all of the laps need to be sealed and the condensation strip removed, which places much greater importance on the internal dehumidification and ventilation systems.
A third-party tested Type C membrane, such as Newton 508R, is also required, which has been tested against methane, carbon dioxide and radon.
However, where the greatest protection is required, the Newton Type C System can be upgraded into a Newton PAC-500 positive air curtain.
The Newton PAC-500 System is a patented, serviceable and high-performance solution that combines Type C basement waterproofing and ground gas mitigation, and can be accepted by the design team to achieve further gas protection points from Table 6 of British Standard 8485.
Overall, the British Standard recommendations and requirements for gas protection and waterproofing (as outlined by BS 8485:2015 and BS 8102:2009) must be considered in combination.
Whilst the requirements of BS 8485 are perfectly designed for above-ground, there are ways in which these recommendations fall short when viewed in the context of protecting a below-ground structure against both gas and water.
Despite this, for those wishing to protect their below-ground structure, there are multiple options through which they can achieve the required protection, in accordance with both BS 8485 and BS 8102 – whether they plan on using external, gas-tested membranes, a correctly designed concrete structure, an internal PAC-500 positive air curtain, or a combination of methods.
Regardless of the chosen method, it is a necessity that the installation is verified by an independent gas specialist, as outlined by CIRIA 735, ‘Good practice on the testing and verification of protection systems for buildings against hazardous ground gases’.
Fortunately, the gas specialist who wrote the site report is often able to undertake this role, and Newton can also supply a list of verification companies from the British Verification Council who can fulfil this responsibility.
Finally, as well as all of the gas considerations that have been discussed in this paper, it is important not to forget potential contaminants in the ground such as VOCs (volatile organic compounds).
With this in mind, Newton has a third-party testing letter outlining the suitability of Newton 403 HydroBond-GB for resistance to VOCs. And whilst we have not specifically undertaken submersion and vapour testing for the chemicals outlined in the CIRIA 748 document, ‘Guidance on the use of plastic membranes as VOC vapour barriers’, the solutions that have been outlined and discussed in this paper are still suitable.
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Our staff are able to provide guidance for projects of all sizes, whether you require some general advice about damp or waterproofing, or support with technical drawings and specifications.