Pyrite

Swelling of the backfill under the concrete slab may occur if conditions are favourable to the oxidation of pyrite. Pyrite testing consists in identifying the types of rock making up the backfill to determine whether swelling could occur. Testing does not determine the quantity of pyrite. Almost all rock types contain pyrite. There is negligible potential for damage if good quality rock is used. In conclusion, pyrite is not a problem in itself; rather, it is mainly the quality of backfill that should be considered.

In applicable cases, we also check whether the concrete slab is affected by sulfation and whether the natural soil on which the building was constructed has a potential for pyrite-related swelling.

The Canada Mortgage and Housing Corporation (CMHC) and the Société d’habitation du Québec (SHQ) recommend Protocol CTQ-M200 for pyrite testing. The protocol determines the applicable procedures and methods, as well as the skills required for each step. Multitest complies with CTQ-M200. The time required to take a sample is approximately 45 minutes and the premises are left clean and dust free.

1. Visual survey
   The Multitest technician prepares a report on observable damages that may have been caused by pyrite inside and outside the building.
2. Sampling
   The laboratory technician drills a 250 mm (6 inch) diameter hole through the concrete slab to collect a sample of the concrete, the backfill and, if possible, the soil underneath the backfill. The hole in the concrete slab is immediately repaired using cement.
3. Laboratory analysis
   The laboratory geologist checks to see whether the concrete has been affected by chemical reactions and whether the underlying soil has a swelling potential. The geologist then determines the swelling potential of the backfill and determines whether problems may be expected in the future.
4. Conclusion
   Based on our visual survey and laboratory report, Multitest determines whether remedial work is imperative or not.
5. Written report
   We provide a recapitulative report that includes our recommendations, our visual survey and the laboratory’s technical expertise.

In accordance with Procedure CTQ-M200, backfill is assessed according to the Petrographic Swelling Potential Indicator (PSPI). The PSPI does not indicate the percentage of pyrite. It is an index that ranges from 0 to 100. Values closest to 0 are best.

As pyrite within the rocks oxidizes slowly, observable damage will appear slowly and progressively. It usually takes at least 25 years for the damage to become apparent as the pyrite granules oxidize over time and its concentration in the rock declines.

Pyrite is an iron sulfide contained in backfill (crushed stone) and sometimes in the natural soil under the concrete slabs (floor) of buildings. It can cause damage to the concrete slab or the foundations.

The first phenomenon results from the fact that pyrite may oxidize when favourable conditions (oxygen and moisture) exist, thus resulting in the formation of gypsum crystals. These crystals increase in volume, which leads to swelling of the backfill material and potential cracking of the concrete slab—which may or may not heave. If the backfill is relatively deep, such as street-level garages, lateral thrusts could result in damage to the foundation walls.

A second phenomenon may occur: sulfation of the concrete slab. The chemical solutions formed in the backfill by oxidation are sometimes absorbed by the concrete slab, which could result in cracking, with or without heaving, without affecting the building foundations.

Finally, there is a third phenomenon: swelling of the natural soil located under the property. As certain quarries on Montréal’s South Shore contain swelling clayish rocks (mainly shale) due to the presence of pyrite, the same is true for the bedrock under some buildings on the South Shore. Major damages are quite rare in these cases, however.

Damage to concrete slabs or the foundations generally becomes apparent 10 or more years after the building’s construction. Damage progresses slowly and could extend over 50 to 70 years. This situation does not pose a threat to the environment or to occupants’ health. However, cracks in the concrete slab or foundations can cause increased humidity in the building’s basement and lead to the growth of mould that could affect air quality.

Heaving (bumps) in concrete slabs may be considered non-harmful if they are less than 10 mm (half an inch). Obviously, the occupant’s degree of tolerance and the building’s use are determining factors. Heaving of more than 10 mm could result in vertical pressure to the subfloor and partitions (dividing walls) of the basement, cause cracking, and possibly affect the floor above.

The foundation walls will not be damaged by swelling caused by pyrite as the thickness of the backfill is usually less than 300 mm (12 inches). The laboratory geologist’s conclusions are generally more conservative when the backfill is thicker than 500 mm (20 inches). Damage to street level garages could be more extensive for two reasons: there is often more than 500 mm of backfill under the concrete slab and builders often use backfill of lesser quality than that used for the basement.

The foundation walls (building structure) thus could be affected when the backfill thickness is high.

Yes! Pyrite testing concerns all properties, including those built after 1999, because, even though a standard on the use of DB certified backfill (i.e. negligible swelling potential confirmed by the quarry) was adopted in 1999, the National Building Code does not require the use of such backfill. The use of DB certified backfill is voluntary. Unless the owner has a certificate confirming that DB certified backfill was used, a pyrite test is recommended.

Since January 1, 2016 the Garantie de construction résidentielle (GCR) (residential construction warranty) requires the use of DB certified backfill for new residential buildings of 5 units or less. As of this date, only random checks of the delivery form of DB backfill provided by the builder are performed under the GCR to confirm that such backfill was used. Thus, if you do not have a valid DB certificate, there is no certainty the backfill will not cause potential damages due to pyrite; therefore, a test should be done. Click here for more details.

Experience in the real estate market has shown us that pyrite-related issues can have direct negative impact on property values, even if there are no apparent problems, as there is a potential for swelling (and thus damage).

Yes, it can—even if builders were not aware of the pyrite problem until 1997, when it became public knowledge. Under Québec law, the prescription period (i.e. timeframe in which you need to take action) is three years following discovery of the defect.

There is no limit as to the amount of time between the building purchase and the discovery of the defect; however, once a defect is discovered, the buyer must advise the seller without delay. We suggest you consult a lawyer in such a case.

Around one out of every four buildings tested (25%) have backfill that could potentially swell and cause damage. Not all these buildings will experience significant problems.

Most of the buildings affected are located on Montréal’s South Shore, the east (Pointe-aux-Trembles, Rivière-des-Prairies, Repentigny, etc.) and the west (Dollard-des-Ormeaux, Pierrefonds, Pointe-Claire, etc.). Many other areas are also affected on or around the island of Montréal, but to a lesser degree than those already mentioned. We regularly test for pyrite on the 82 municipalities of the Greater Montréal Area and often find affected buildings.

Excluding demolition and refinishing, remediation work costs $25 to $35 a square foot (e.g., between $8,000 and $12,000 for a one-car garage of 20’ x 10’). The work consists in replacing the concrete slab (floor) and backfill (crushed stone) down to the soil underneath. Backfill must be DB certified (click here for more details) and a vapour barrier membrane (e.g., polyethylene liner) must be placed over the backfill before the new concrete slab is poured.

We do not perform this type of work. However, we can refer you to contractors if necessary.

Pyrite testing consists in taking a sample of backfill (crushed stone) from under the concrete slabs (floor) to determine whether there is a potential for swelling that could result in damage to the building. In applicable cases, we also check whether the concrete slab is affected by sulfation and whether the natural soil on which the building was constructed has a potential for pyrite-related swelling.

No. We will contact you the business day before the test to confirm an appointment time.

Our technicians will be on site for around 45 minutes for 1 sample and 1 hour and 15 minutes for 2 samples.

In most cases, no. It may be worthwhile to have another test done in specific cases. Please call us if necessary.

Yes! Pyrite testing concerns all properties—including those built after 1999—as the National Building Code does not require builders to use non-swelling backfill (also referred to as “DB certified backfill”). For residential buildings constructed after January 1, 2016, please consult the “LEARN MORE” section in the pyrite testing section.

It is no noisier than a powerful vacuum cleaner.

We will keep your report for 25 years. The Petrographic Swelling Potential Indicator (PSPI) will not change over time, but it is possible that damages may appear or evolve.

The concrete slab is immediately repaired using cement. However, subfloors will not be repaired if we must drill into them.

A Petrographic Swelling Potential Index (PSPI) score of 10 or less indicates that no swelling of backfill is expected, while an PSPI score of 11 or more indicates a possibility of varying pyrite-related damages.

No. The concrete drill is electric and does not produce dust. The premises stay clean.

If the concrete slab is not accessible, we can drill in less visible locations such as inside a wardrobe, under a stove, etc.

We do not perform this type of work. However, we can refer you to contractors if necessary.