Evaluating the performance of chemical injection damp-proof courses.
Contrary to a vociferous minority view, rising dampness does occur and is not that uncommon in properties without physical damp-proof courses (just put a brick/mortar in water and watch it wick). Where rising dampness does occur and warrants remedial action then a chemical injection damp-proof course may be considered. Over the years there have been a number of problems of continued dampness following the insertion of a chemical injection damp-proof course. However, considerable argument can arise as to whether the problem is due to the damp-proof course, replastering or other factors: this tends to form the basis of related disputes.
Chemical injection damp proof-courses and replastering:
Injection damp-proof courses basically consist of the injection of a water repellent or pore blocking material into the base of a wall where it will form a diffuse band to 'control' the rising dampness. Unlike a physical damp proof course which provides an impervious barrier and hence an immediate cut in rising water, the chemical systems will generally only control the rising water -- not stop it. There are a number of physical factors related to the substrate and injection processes which govern why this is so. Thus, with chemical injection systems the lower part of a wall may always remain damp.
Therefore, to provide a non-spoiling decorative surface replastering is required. Removal of the old plasterwork removes potentially heavy contamination with hygroscopic/deliquescent salts which have accumulated over many years of rising damp and which, if left, may spoil new decorations. However, the new plasterwork has a very important function -- it must be capable of preventing hygroscopic/deliquescent salts passing to the new surface from contaminated underlying masonry. This function is extremely important because (a) walls take time to dry down, and (b) the lower part of the walls are always likely to remain damp because of the limitations of the chemical injection systems. Fundamentally, chemical injection damp-proofing is a system -- the injection of the damp proof course to 'control' the rising damp, and the replastering to prevent future spoiling -- the processes are inseparable.
Evaluation of the system:
The evaluation of a chemical injection system is a two part process, (i) evaluate the performance of the damp-proof course, and (ii) determine that the new render meets its required design function. The evaluation of the performance of the damp-proof course itself is based on the relationship between the distribution of water and salts in the wall. This means removing material from the wall and is therefore a 'destructive' method of moisture analysis, and not suited to general house purchase surveys -- how many vendors will allow you to drill holes in their walls prior to sale?
Looking first at moisture, the total moisture content of the material is potentially the sum of two components, the air dry (or hygroscopic) moisture content and free moisture content.
Air dry (hygroscopic) moisture:
In practice no material is truly dry, there is always some water present when it is in equilibrium with the atmosphere. This is known as the material's air dry moisture content. In the average clean brick or mortar it is not often above 1.5%, usually less. The air dry moisture content in most clean masonry will not cause an electrical moisture meter to respond. However, contamination with hygroscopic/deliquescent salts (eg, derived from rising damp) can cause the air dry moisture content to increase quite dramatically solely due to moisture absorption from the air by these salts.
The second component is the free moisture content which is only present if there is a source of water ingress such as rain penetration, rising dampness. The amount of free moisture will depend on the permeability, porosity and severity of the water ingress. The absence of free moisture indicates that a material is 'dry', that is not subject to water ingress from any source.
Ground water salts:
Not only is the distribution of moisture required, we also need to obtain details of past activity of rising dampness, and this can be obtained from the presence and distribution of ground water salts, basically chlorides and nitrates. Where a long term rising damp complex has existed, these salts will always mark the maximum height to which water has risen, and tend to form a concentration ('salt band') around the maximum height of rise. Thus, even if water ceases to rise the salts will still remain and 'mark' the maximum height once reached. It is the relationship between the distribution of moisture and salts that is the basis of the investigation.
Prior to any investigation of this nature it is essential to eliminate other potential sources of dampness such as condensation and the possibility of rain penetration which may interfere with the investigation.
Once this has been done, a vertical series of samples are removed from the wall by means of a slow speed drill and sealed immediately in air tight containers to prevent moisture loss (see here for method of sampling). In the laboratory moisture distribution (air dry and free moisture) are determined, the method being described in Building Research Establishment Digest 245, 'Rising dampness in walls: diagnosis and treatment'. The presence and distribution of soluble ground water salts are also analysed.
Table 1 shows a moisture and salt 'profile' taken from a wall that had been damp-proofed six years earlier. Free water is present up to 1200mm, and its distribution clearly shows it originates from the base of the wall. Soluble ground water salts are also present up to exactly the same height. Where both water ingress and salts are present to the same height it shows that the damp proof course is totally ineffective.
Table 2: Effective dpc:
Table 2 shows a further 'profile' from a damp wall subject of a dispute. In this case the presence of free water shows that water ingress is only occurring up to 100mm. However, salts were present up to 900mm and showed the maximum height to which the dampness had once risen. The analyses clearly showed that the rising damp had been extremely well controlled (free water only present at the very base of the wall). But note the very high air dry moisture content between 300 - 900mm; this was solely due to the hygroscopic nature of the heavy salt contamination. (Note: there are technical reasons why there is a negative free moisture content but for simplicity the results are recorded as 'nil').
It must be considered that optimistically walls take one month for every 25mm thickness to dry down (Building Research Establishment Digest 163). It is therefore essential for new plasterwork to hold back moisture and salts during the drying period, and indefinitely at the base of the wall because of the limitations of the damp-proof course itself. This is described in Building Research Establishment Defect Action Sheet 86, 'Replastering following the insertion of a damp-proof course'.
In the second case (Table 2) the render had failed to meet this function and salts had passed into the new plasterwork. All the dampness above 100mm was shown to be solely the result of the hygroscopic nature of the salt contamination, not an ineffective damp-proof course.
The Defect Action Sheet also gives guidance as to the type of sand/cement mix to be used; basically, a 1 : 3 cement to sand mix is specified, or an 'approved' renovating plaster. Analysis should be undertaken to determine the suitability of the mix to meet its required function. It should be noted that generally lime in a mix increases its permeability which is not desirable if the design function of the render is to be met; the use of high fines sand will also increase permeability. Similarly, materials such as bonding and lime plasters should not be used -- they are far too permeable and are unable to hold back moisture and soluble salts as required. However, consideration must still be given to the composition and quality of the substrate; clearly there are incidents where the substrate is unsuitable for application of strong cement based mixes.
Finally, don't be misled by the statement that 5% moisture content in a material is perfectly acceptable; in engineering bricks, for example, it can be extremely damp. Conversely, very high 'moisture' contents may not reflect water ingress (free moisture); it could be solely a salt problem such as that shown in the second case. Indeed, this was the basis of the dispute in the second case where a 'Carbide' meter was used on site and figures in excess of 5% moisture content were obtained; this was interpreted as a failed damp-proof course. However, a proper examination to produce the full moisture profile clearly showed that the dampness was due to hygroscopic salt contamination. It is situations like these that moisture/salt analyses are designed to identify, and eliminate the potential misinterpretation of results obtained on site by the use of 'Carbide' and other moisture meters.
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