I have no experience of pyrite contamination and what follows may be if little direct relevant to the debate.
Also I'm an architect, not an engineer - I call on engineers for advice even on single storey extensions, never mind houses or apartments.
That means I 'm not an expert on structures, but it also means I am competent to comment on all matters in relation to buildings - I also ask good questions. ;-)
One thing is bothering me about all of these instances of foundations and walls cracking.
I accept that there may be pyrite contamination in the hardcore/fill/aggregate, but has anyone got a handle on how bad it needs to be to cause this level of damage to the building fabric?
Citing pyrite as the sole source of cracking and settlement in spec-built houses and apartments may in fact be correct in one or even all these cases, but I have come across cracking and settlement for other reasons.
In my experience:
- inadequate design
- poor detailing and
- poor workmanship
are the usual causes of settlement and cracking.
Design and Detailing:
The work of the designers begins at site assessment stage, with the selection of where to build the buildings and we have seen heated debates in the recent past about building on flood plains north of Dublin City.
Ground conditions can cause significant problems for substructures, and their design must address adequately any issues arising from groundwater, contaminants already in the soil or geological features.
Some parts of Dublin have underground watercourses and/or underground hollows and building on or near them can involve piling the foundations with ground beams between the piles.
This can be expensive, not just for the foundation work where costs can be offset against the labour of digging and preparing trench foundations, but also for ground floor slabs.
Poor ground may have to be excavated and backfilled and this filled material, even if properly selected, placed layered and compacted will be liable to settlement.
If the depth of fill exceeds 1.2M or so, the engineer is likely to recommend the use of precast slabs for the ground floor of the lowest apartment.
This has cost implications in terms of detailing for radon gas and services penetrations and pipe connections.
Materials and Workmanship:
The following non-exhaustive list of activities all have to be done to specified standards or else cracking may occur:
- selection of the aggregate
- selection of the cement
- selection of admixtures
- water quality
- mixing of the concrete
- the water ratio
- time to placement
- distance to placement
- the placement and vibration
- allowing curing before imposing loads
- temperature of the works while they cure
The correct production and placement is only part of it and the aggregate is only part of that.
In other words, while contaminated aggregate can cause problems in itself, there are many other sources of cracking.
If the ground is too poor for strip footings, special foundations will be needed to transmit the load of the superstructure to a bearing stratum.
Before you get anywhere near the site with a digger, the engineer and architect have to assess and agree the structural system and detail the building accordingly.
Preparation
Before you get anywhere near placing concrete, even with a straightforward house foundation, the excavation and preparation of the ground has to be carried out, which inter alia includes;
- the identification, excavation and filling in of soft spots
- the laying of graded hardcore in layers depths to engineers spec
- the compacting of the hardcore
- the blinding of the hardcore with sand or lean mix
- the blinding of the foundation trenches with lean mix
- the placement of reinforcement to allow correct cover
- the reinforcement of the base of the foundation
The Pour
After that you need to carefully commence the work:
- the pouring of the concrete itself
- distance from source if pumped
- height from outlet to poured level
- vibration of the concrete to eliminate air pockets
- aeration of the concrete to help protect the steel
- adequate curing time to develop strength.
- use of concrete of the correct strength
- keeping it at the right temperature while curing
The temptation on large sites can be to pump to the maximum distance and/or drop concrete from too high a level.
These should be resisted because they can disrupt to the mix matrix [sand/cement/aggregate/water] and/or cause cavities forming within the setting concrete.
As you can see all of this can be very complex work and the rate at which work can proceed on site is determined to a degree by the minimum time taken to build in concrete - if adequate time isn't allowed, the work will suffer.
Bigger Building Work:
On a medium-to-high-rise apartment block, more sever construction issues can arise.
The extra loads from the higher structures [including wind loads] may mean that reinforced foundations or piles are required regardless of the ground conditions.
On large building sites the scale and complexity of the project can involve several different pours [piles, ground beams, slabs] proceeding simultaneously in separate areas of the site.
Bigger building projects inter alia requires significant additional supervisory work by the contractor as well as specialist knowledge and skills over and above those of the small house builder.
Investigative Work
Establishing whether or not pyrite is present is relatively affordable and can point to one possible cause of cracking and settlement, if discovered.
However if the providers of foundations to the affected properties did not carry out the work compliantly, cracking and settlement may occur for reasons totally unrelated to presence of pyrite in the mix.
Assessing all the other factors could be a long and expensive process and not one to be undertaken lightly by a house or apartment owner.
Where an apartment block is affected, or where significant numbers of houses in an estate are affected, the owners should consider pooling their resources to fund a professional investigation team.
To help form an opinion in relation to this matter the following matters should be assessed:
- the level of investigative work undertaken by the designers prior to site
- the detail design of specialist foundations to support the superstructure
- the level and quality of testing or concrete cubes and materials
- the level of on site check-in and testing of materials supplied separately
- the existence of warranties or undertakings from suppliers of materials
- the level of contractor's personnel involved with supervising the concrete work
- the retention of a competent site engineer by the developer, contractor or engineering firm
Local excavations may be required to be carried out on:
- an absolute basis involving extensive excavations or
- a statistical basis where only a few key corners of buildings are exposed in a large scheme
These will allow professionals to visually inspect and assess empirically [through testing or coring] whether or not the foundations were properly sized, placed and constructed.
Conclusion and Caveat
Inadequate design, site preparation, site supervision, poor detailing, materials and/or workmanship are possible causes or contributory causes to cracking and settlement in buildings.
It may be helpful if all of these other issues are explored by professionals acting for the injured party where pyrite is suspected of being the prime cause of the problems,
This post is not exhaustive and the comments herein are justified and are made as fair comment in the public interest based on known issues in the building industry.
This post is not intended to raise unnecessary concerns amongst persons already worried about their future and the future of what they thought was their lifetime investment, their home.
This post shows some avenues of investigation and redress that any competent solicitor or building professional should consider when assessing any instance of cracking or subsidence in a building.
