Core Testing – Pyrrhotite

A qualified field technician will be on-site within 2-3 days after initial contact to perform the core sampling work.

• A minimum of two (2) core samples are extracted from two different foundation walls per standard-size residential building or outbuilding. (Addition/detached garage foundations require to be tested and reported independently from the main building given that the concrete was placed at different times and/or circumstances).
• The samples are drilled out of a 4-inch diameter cylinder (the typical specimen used for the petrographic examination is a 3¾ to 4 in. diameter core).
• When sampling is done from the basement, core drilling is executed all the way through the wall to confirm the presence or absence of waterproofing on the exterior surface of the foundation.
• Core samples are physically labeled and documented (chain of custody form) regarding the location of sample collection from the concrete foundation with control numbers matching written report.
• Cores are securely identified, sealed and packaged for an immediate shipment to our lab.
• Resulting cavities are filled with high quality non-shrink grout.
• Reasonable access must be provided prior to technician’s arrival . Example: Moving furniture / home decor / rugs, etc.

Since pyrrhotite is a sulfide mineral composed of iron (Fe) and sulfur (S), measuring sulfur concentration in the coarse aggregate is a critical part of the Concrete Core Analysis.  A high correlation exists between sulfur concentration and pyrrhotite concentration in rock aggregates.  The correlation is even higher in CT and MA rock aggregates known for their high pyrrhotite proportion relative to other sulfide minerals such as pyrite, chalcopyrite, pentlandite, etc.  In general terms, high sulfur concentrations indicate high pyrrhotite concentrations in CT and MA rock aggregates.  Inversely, low sulfur indicates low pyrrhotite.

THE 0.1% STANDARD

European standard EN 12620 has placed a sulfur concentration limit in coarse aggregate if it is to be used in concrete.  The standard states that the sulfur concentration must not exceed 0.1% in weight of coarse aggregate if pyrrhotite is present.  The US Army Corps of Engineers has also recently put out similar recommendations.

Methodology

Total sulfur in concrete analysis is performed using LECO infrared combustion sulfur analysis in accordance with the relevant guidelines outlined in standard NQ 2560-500/2003, 6.2.1, A.2, A.3.2. and is carried out on a portion of each core sample as received by SEDEXLAB INC. (approx. 1000 grams).  The LECO sulfur analysis provides sulfur concentrations by weight of concrete.  These results are then converted to a sulfur concentration by weight of coarse aggregate.

Sedexlab petrographers perform the mineralogical and microstructural assessment of aggregates and concrete highlighting potential problems related to expansive reactions due to the oxidation of iron-sulfide minerals. The main objective of the petrographic examination is to identify and quantify the mineral pyrrhotite in the coarse aggregate.

From each received concrete core sample, a disk-shaped section is saw-cut and polished to a precise mirror-like finish.  Our petrographers examine these polished sections under polarized light microscopes with the purpose of identifying pyrrhotite’s distinctive optical properties in reflected light.   Approximately 100 coarse aggregate particles are screened for pyrrhotite in two polished sections combined with the purpose of determining pyrrhotite surface ratios relative to coarse aggregate particles.  The pyrrhotite concentration is then derived from these surface ratios and from the previously calculated concentration of sulfur in the coarse aggregates.  The result is an average concentration of pyrrhotite by weight of coarse aggregate (%w).

The Analysis includes:

• Concrete core descriptions (concrete condition, coarse and fine aggregate description, steel reinforcements, moisture barriers, etc.).
• Determination of percentage of coarse aggregate particles containing pyrrhotite.
• Determination of percentage of pyrrhotite per unit mass of coarse aggregate.
• Determination of evidence of oxidation of pyrrhotite (replacement iron oxides).
• Petrographic composition of coarse aggregate and fine aggregate.
• Composition of iron sulfide minerals in the coarse aggregate (pyrrhotite, pyrite, chalcopyrite, pentlandite, marcasite, etc.).

Petrographic examinations are conducted in accordance with relevant guidelines outlined in ASTM C 856, “Standard Practice for Petrographic Examination of Hardened Concrete.”

Our clients not only appreciate the thoroughness of our laboratory analysis, but they also value our ability to deliver the information accurately with a realistic perspective. Sedexlab’s Concrete Core Analysis report provides clarity in layperson’s terms to all parties involved.

Results are clearly presented and summarized in tables, bullet points, and graphs with concluding statements that reflect well-balanced and experience-based professional judgment. The well laid-out 9-page report includes a summary of results & analytic observations and comes complete with an appendix detailing petrographic examinations, core descriptions, chemical analysis, photographs, and calculation methodology.

The reports can be used for home disclosures, property tax relief, and claims to the captive insurance group for Connecticut homeowners (CFSIC).

Results and reports are produced 10-14 days from the time we receive the core samples. The PDF analysis report is sent via email to the purchaser and/or approved representative(s).

Pyrrhotite is a naturally occurring iron sulfide mineral commonly found in regional rock formations. Untested rock aggregates were extensively mined out of these rock formations and were widely distributed, for years and even decades to regional concrete producers.

These pyrrhotite-containing aggregates are the probable cause of the premature deterioration of residential concrete foundations reported in many cities and towns in Massachusetts and Connecticut.

When exposed to water and air, pyrrhotite particles in rock aggregates break down to form acidic-, iron-, and sulfate-rich by-products that are larger in size.  This expansion initiates distress and cracking in the concrete, providing more pathways for water and air to penetrate and react with more pyrrhotite-bearing aggregates.  This compounding expansive reaction can eventually lead to a loss of concrete structural integrity. The problems, sometimes developing within the first 10 years, often begin to appear after 15 to 20 years or more.

According to the Geological Society of America, rock aggregate in the failing concrete foundations in CT and South Central MA was largely mined from a single quarry in Willington (CT), within a stratified metamorphic unit mapped as Ordovician Brimfield Schist.

The Known Source; Becker’s Quarry & J.J. Mottes Concrete Plant

Pyrrhotite-bearing rock aggregates identified in the crumbling concrete foundations in Connecticut and Southern Massachusetts originated from Becker’s Quarry in Willington, CT. The Quarry was affiliated with the now-defunct JJ Mottes Concrete Company.

The laced concrete was poured from 1983 to 2015 by JJ Mottes who at the time operated a concrete manufacturing plant in Stafford Springs, only a few miles from Becker’s Quarry.  A state commission had determined that approximately 95,000 homes were built during that time span within a 50-mile radius of the plant, but the exact number of projects J.J. Mottes supplied concrete for is unknown because of a lack of documentation.  The Special Commission had determined that those who are located within 20 miles of the plant are the most at-risk.

The economic and social impacts on the region are immense. Officials have already identified approximately 40 cities and towns in CT affected by pyrrhotite-laced foundations while 35 have been identified in MA so far.  Officials in MA have no clear idea of how many homes are affected but one estimate used by CT officials says that 10,000 homes may potentially be affected in MA.

 Other unconfirmed Source(s) in Central MA

MA Authorities express real concern that other producers of deleterious aggregate may exist in their State and may have unknowingly supplied the aggregate to regional concrete producers. Recent testing has identified pyrrhotite-laced foundations, outside the presumed radii of operations of JJ Mottes, well north of MA Turnpike I-90 in Central MA.   Indeed, petrographic examinations have revealed that these pyrrhotite-bearing aggregates differ significantly in nature from those produced at Becker’s quarry in CT suggesting that these rocks were mined out of at least one other quarry presumably located in Worcester County.  Unfortunately, many questions are still unanswered concerning the extent and the time period of the distribution.

Not officially.

Although the undesirable nature of pyrrhotite in concrete is well known, there is no precise value issued to this date, by any U.S. State or Federal laws, as to the maximum authorized pyrrhotite concentration in coarse aggregate if it is to be used in concrete.

However, concentrations of around 0.3% pyrrhotite by weight of coarse aggregate have been reported to represent the minimum levels linked to visual indications commonly associated with pyrrhotite-laced concrete.  Our own case history data as well as information shared by some of our peers in Canada and the U.S. lend support to the validity of such levels. Is 0.3% pyrrhotite by weight of coarse aggregate the threshold limit above which it is unsafe?  Unfortunately, until more research and case history data reveal with more accuracy the minimum level, this question remains unanswered.

A ”trace” of pyrrhotite and the 0.1% sulfur standard

SEDEXLAB defines a ”trace” as a non-deleterious amount of pyrrhotite in concrete coarse aggregates.  SEDEXLAB and most international experts refer to the ”0.1% sulfur standard” described in aggregate standard EN 12620.  The internationally recognized standard states that coarse aggregate with a sulfur concentration that does not exceed 0.1% in weight meets the standard’s acceptability requirements for the aggregate’s use in concrete even if some pyrrhotite is present.  These very small sulfur levels reflect negligible amounts of pyrrhotite which are visually confirmed by our petrographers when performing petrographic examinations using reflected light microscopy.

To this date, of all the core samples we tested since 2013 that contained a trace of pyrrhotite (sulfur not exceeding 0.1%), none have presented any notable deterioration commonly associated with pyrrhotite oxidation.

Here are the main telltale signs of premature deterioration of foundation walls caused by pyrrhotite-induced expansive reactions in concrete:

• Map cracking or pattern cracking

Intersecting cracks that extend in depth that vary in width from fine and barely visible to open and well-defined.  These cracks are the prime indicator of pyrrhotite-related distress.

• Horizontal cracking

Horizontal cracks will generally move along a wall, to the connecting wall, compromising a wide span of your foundation area of support. These cracks are generally more serious in nature and require immediate professional inspection whether they are pyrrhotite-related or not.

• Spalling on surface

Spalling concrete is a problem where part of the surface peels, breaks, or chips away. Also known as scaling, it is the result of a weak surface that is susceptible to damage.

• Bowing walls

Noticeable inward movement of a concrete wall, often associated with horizontal cracking and requiring immediate professional inspection.

• Rust-like staining on the surface of the walls.
• Efflorescence (whitish powder) in the vicinity of the cracking surface

Ongoing Monitoring of Concrete Foundations

We recommend the use of photographs as well as a written log to monitor the progress of deterioration.  We suggest marking and measuring methods to monitor how the cracks expand. For example, a standard crack gauge may be used to accomplish this task.

Although certain measures may mitigate to some extent the risk of future concrete expansion, at present no measure other than foundation replacement is known to reverse or eliminate pyrrhotite-Induced concrete deterioration.

• Sealing Cracks

Cracks allow moisture and air to penetrate the concrete and might accelerate any potential reactions.  Fill in cracks with a concrete repair product or a flexible polymer sealant. If the cracks are still active, a flexible sealant will work better than a rigid one.

• Reducing Ground Level Humidity

Surface drainage should be the first line of defense in every residential moisture protection system. Groundwater can be controlled to a great extent by reducing the rate at which rainwater and surface runoff enter the soil adjacent to a building. Roofs typically concentrate collected rainwater at a building’s perimeter where it can cause groundwater problems.  Water that is drained quickly away from a building at the ground surface cannot enter the soil and contribute to below-grade moisture problems.

Ground-level humidity can be reduced by:

• Repositioning gutter spouts to divert water away from the foundations.
• Modifying the slope of the ground around the foundations.
• Sealing the asphalt covering at foundation joints.
• Planting beds located next to the building walls should always be well-drained to avoid concentrating moisture along the foundation line.

• French Drain

Subsurface drainage systems can collect and divert groundwater away from the walls and floor of a basement. The most common method of keeping groundwater away from basement structures is to provide a perimeter drain or footing drain (French drain) in the form of perforated, porous, or open-jointed pipe at the level of the footings. Perimeter drains artificially lower the water table below the elevation of the floor.  Crushed stone or gravel should always be placed above and below perimeter drains to facilitate water flow.

When possible, the existing French drain should be assessed to verify proper functioning. This drain can gradually block after a long period of time.

• Waterproofing Membranes

Waterproofing is the treatment of a surface or structure to prevent the passage of liquid water under hydrostatic pressure. When combined with effective subsurface drainage, a waterproofing membrane can provide good performance. In wet climates, or on sites with high water tables, fluctuating water tables, or poor drainage, a waterproofing membrane should be used in addition to subsurface drains

Since pyrrhotite is a rock aggregate problem, our concrete analysis focuses on identifying and quantifying pyrrhotite relative to the coarse aggregate, one of the components of concrete mix.

Sedexlab conducts a series of independent laboratory procedures of which two (2) represent the bulk of the analysis.

1) Sulfur Analysis – Determination of the concentration of sulfur in the coarse aggregate

2) Petrographic Examination – Determination of the presence (or absence) of pyrrhotite and determination of its concentration in the coarse aggregate. 

• Sulfur Analysis

Since pyrrhotite is a sulfide mineral composed of iron (Fe) and sulfur (S), measuring sulfur concentration in the coarse aggregate is a critical part of the Concrete Core Analysis.  A high correlation exists between sulfur concentration and pyrrhotite concentration in rock aggregates.  The correlation is even higher in CT and MA rock aggregates known for their high pyrrhotite proportion relative to other sulfide minerals such as pyrite, chalcopyrite, pentlandite, etc.  In general terms, a high sulfur concentration will indicate a high pyrrhotite concentration for CT and MA rock aggregates.

From each received core sample, approximately 1000 grams of concrete are submitted for a total sulfur analysis using LECO infrared combustion which provides the concentration of sulfur by weight of concrete.  The results obtained are then converted to a concentration of sulfur by weight of coarse aggregate.

The 0.1% Standard

European aggregate standard EN 12620 has placed a sulfur concentration limit if the coarse aggregate is to be used in concrete.  According to the standard, the sulfur concentration must not exceed 0.1% in weight of coarse aggregate if pyrrhotite is present.  The US Army Corps of Engineers has also recently put out similar recommendations.

• Petrographic Examination

The main objective of the petrographic examination is to identify and quantify pyrrhotite in the coarse aggregate.  The analysis also allows for identifying the evidence of pyrrhotite-induced expansive reactions such as pyrrhotite oxidation and replacement iron oxides (secondary minerals).

From each received concrete core sample, a disk-shaped section is saw-cut and polished to a precise mirror-like finish.  Our petrographers examine these polished sections under polarized light microscopes with the purpose of identifying pyrrhotite’s distinctive optical properties in reflected light.

Approximately 100 coarse aggregate particles are screened for pyrrhotite in two polished sections combined with the purpose of determining pyrrhotite surface ratios relative to coarse aggregate particles.  The pyrrhotite concentration is then derived from these surface ratios and from the previously calculated concentration of sulfur in the coarse aggregates.  The result is an average concentration of pyrrhotite by weight of coarse aggregate (%w).