Valuation reports often state that a valuation of a building is made on the assumption that the structure contains no deleterious materials. What is meant by this phrase?
In fact the Royal Institution of Chartered Surveyors recommends that all valuations should contain the following paragraph:
We have not arranged for any investigation to be carried out to determine whether or not any deleterious or hazardous material has been used in the construction of this property or has since been incorporated and we are therefore unable to report that the property is free from risk in this respect. For the purpose of this valuation we have assumed that such investigation would not disclose the presence of any such material in any adverse condition.
This recommendation is made because a valuation is not a structural survey, and also because even a structural survey will not necessarily disclose the presence of certain substances which can put the structure or its occupants at risk. Deleterious (harmful) materials are therefore the construction industry’s mistakes; materials which were initially thought to be satisfactory but which have proved in practice to be inadequate or dangerous.
A distinction should be made between such materials and ordinary materials which are quite satisfactory in themselves but which fail through faulty or inappropriate application, poor building practice or bad workmanship. An example is a slate roof being laid at too shallow a pitch, so that rainwater creeps back over the top edge of the slates and into the building.
The more common deleterious materials are:
- Asbestos
- High alumina cement
- Calcium chloride
- Woodwool slabs
- Galvanised wall ties
These are pernicious materials because the presence of some of them is not apparent by mere observation and can be detected only by chemical analysis.
Asbestos
It is now thought asbestos fibres released into the atmosphere from break-up or wear and tear of asbestos in buildings are carcinogenic. The most hazardous form is thought to be the so-called “blue” asbestos, but other types may be dangerous. Nevertheless, this material is a hazard only if it is breaking up or deteriorating or if it needs to be removed. The removal of blue asbestos does require very careful techniques to avoid hazard to demolition workers and this can be expensive.
Asbestos has been used as an insulating material in the form of lagging around heating pipes and installations, for sound and heat insulation throughout the building, as a roof covering on industrial buildings and in sprayed form to make structural steelwork heat resistant in case of fire. Asbestos can be sealed by various plastic products to prevent any possible break-up and release of fibres. It is only if asbestos is found within a building or needs to be removed, or the presence of blue asbestos is discovered that problems are caused.
High alumina cement
High alumina cement (HAC) is a form of quicksetting and heat-generating cement. It has been used in the building industry probably since the 1920s but its use was more widespread in the industrialised building techniques used between 1960 and 1972. However, even during this period, its presence in buildings constructed at the time is the exception rather than the rule.
HAC had the advantages that it hardened and gained strength quickly and thus speeded up production off-site of pre-cast components. Also, when used on site, setting time would be reduced to enable site work to proceed more quickly. All cement generates a certain amount of heat when setting owing to chemical reaction, but HAC generated more heat than usual and was used sometimes on site in severe weather conditions when ordinary concrete might have frozen during the setting process leading to severe weaknesses.
The problems associated with the used of HAC came to general attention in 1973-74 following the collapse of the roof of the Stepney Swimming Pool and earlier problems in 1973 at the Camden School for girls and Leicester University. Generally, buildings constructed after 1974 can be considered safe, since the widespread publicity relating to HAC during 1973 and 1974 meant that its use was no longer specified.
Over a period of time, concrete containing HAC undergoes a chemical change (particularly in damp or humid conditions) known as “conversion”, which means that it becomes porous and loses strength. After conversion, the concrete becomes stable and no further reduction in strength should occur.
Its presence in buildings is sometimes extremely difficult to detect. For example, a building may have a steel frame, but the floors could be constructed of pre-cast concrete beams spanning between horizontal steels, which then have concrete poured on top on site to make a level floor, and ceiling finishes applied to the underside. In such a case, the beams might be totally concealed, the manufacturer of the beams may no longer be in business; the architect and consulting engineers responsible for the design of the building, and the district surveyor, may not have adequate records, or even worse, may never have known of the exact composition of these beams.
Where it is possible to obtain the consent of the owners or occupiers, buildings can be tested for the presence of HAC by taking samples from critical areas and sending them to a laboratory for analysis. This process normally takes about 10 days. It will be appreciated, however, that not all areas from which it is desired to take samples will be accessible and compromises may have to be made.
Many buildings containing HAC are reasonably safe, since, after conversion, the cement retains a certain amount of strength and, provided reasonable precautions are taken, problems need never arise. Particular care must be taken to ensure the concrete does not become damp in any way, and also it is essential to have the structure checked by a competent consulting engineer for its residual strength. Particular care must be taken to prevent water leaking into the building from whatever source, since after conversion cement is porous and leaks can lead to rusting of structural steelwork, steel reinforcing and other deteriorations.
Calcium chloride
Calcium chloride is simply a salt. As with all salts, its presence in concrete is generally undesirable since it tends to be hygroscopic and to attack structural steelwork or steel reinforcings. Salt in concrete is also occasionally found through the use of sea-dredged unwashed aggregates in the original mix.
Calcium chloride was sometimes deliberately introduced to the mix in order to speed setting. Cement should be allowed to cure slowly in order to gain maximum strength. If too great a proportion of calcium chloride was introduced to the mix, then setting could occur too quickly, meaning that the cement never reached its intended strength. Calcium chloride was also sometimes deliberately added to cement to increase heat generation and prevent freezing in severe weather, in a similar manner to HAC.
In limited quantities calcium chloride does no great harm but BS8110 Part 1, 1985 recommends that the maximum level should not exceed four parts per thousand by weight (0.4%).
Woodwool slabs
This is another material, the use of which was most widespread in the 1960s. Briefly, these slabs are made of wood shavings cemented together in the form of a thick board. Because of the air spaces between the shavings, the material, although quite rigid is lightweight.
Woodwool slabs were used generally for heat-or sound-insulation purposes. Their most dangerous use was as permanent shuttering for concrete poured in situ, since percolation of the concrete slurry into the spaces between the shavings tended to lead to cavitation of the concrete slab with consequential weaknesses and possible exposure of steel reinforcings to rusting. In other words, bubbles tended to form in the concrete, and drainage of water led in any case to a weaker setting strength.
When used in this way, the slabs were often left in place, either as the ceiling finish itself or covered in plaster or other ceiling finishes. When this happened the slab cannot of course be seen. Woodwool slabs were also used in industrial buildings for internal insulation. In this case they are perfectly satisfactory.
There are materials very similar in appearance to woodwool slabs, known as strawboard or fibreboard, which together with ordinary chipboard were sometimes used as support for flat or shallow-pitch roofs which were then covered in roofing felt. In this application, early forms of these materials were at risk if the roof leaked, since then, the effect is rather like pouring hot milk on Weetabix! The slab becomes soggy and can collapse.
For some years now, these materials have been banned by the Building Regulations and have been superceded by similar but inert materials.
Galvanised cavity wall ties
Cavity wall construction has been common in the United Kingdom since the second world war, and, until very recently, the two leaves have been tied together with galvanised steel ties. In certain places, walls constructed prior to 1955 have been found to have developed cracks owing to the rusting of these ties when the galvanised coating had broken down. Normally, the portion of the tie within the mortar mix rusted and, since when metal rusts it expands, cracking to the brickwork resulted. In severe cases, the only remedy is to cut out and renew the ties or to rebuild the outer skin of the wall.
Once again this particular defect is difficult to detect. Even if the wall is drilled to allow insertion of an optic probe into the cavity, the visible portion of the galvanised tie might well be satisfactory whereas the ends embedded in mortar are rusting. Without opening up for inspection it is almost impossible to be certain.
This article covers some of the more common deleterious materials: there are others which are generally less common. There are also allied areas such as the construction of buildings on the sites of old domestic refuse tips which might be carefully sealed to prevent entry of explosive methane gas generated by decomposing materials far below ground level. A site may contain dangerous or hazardous materials not revealed by normal investigation. These days such materials may even include stored radioactive waste.
The above is intended to provide general information on the occurance and implication of certain materials. It is stressed that expert advice must always be sought for any practical application of these general principles.