Results

Updated July 6th, 2023

The following table shows the number of homes tested by the UConn team to date. The number of samples include all individual samples from all homes; every home typically has between 3 and 7 samples tested, with the average home having 5 samples. Each sample consists of approximately 150 g of homogenized powder, which is tested for total sulfur, sulfate and sulfide in triplicate (see Our Method). The average of triplicate analyses is then calculated for each sample and reported in the testing report.

 Home Description No of homes No of samples No of ST  analyses No of WD-XRF analyses
 NN (No damage – No pyrrhotite) 57 260 780 780
 NP (No damage – Pyrrhotite detected) 49 225 675 675
 DP (Damage – Pyrrhotite detected) 67 232 696 696
TOTAL 173 717 2151 2151

Note: The home description was adopted by the UConn Team and is not related to any official classification for evaluation or prioritization purposes.

For statistical analysis, homes were categorized in three classes:

  • NN: Homes with no observable cracks associated with pyrrhotite presence, and for which the measured sulfide concentration was below the quantification limit of the WDXRF method (<0.25% pyrrhotite by weight of concrete)
  • NP: Homes with no observable cracks associated with pyrrhotite presence, and for which the measured sulfide concentration was above the quantification limit of the WDXRF method (>0.25% pyrrhotite by weight of concrete)
  • DP: Homes with observable cracks associated with pyrrhotite presence, and for which the measured sulfide concentration was above the quantification limit of the WDXRF method (>0.25% pyrrhotite by weight of concrete)

Figure 1 below shows the distribution of total sulfur (ST) by weight percent of concrete for the samples in each home class (NN, NP, DP)

Fig. 1: Distribution of total sulfur (ST) in NN samples, NP samples and DP samples

NN samples with no visible damage and no pyrrhotite had a total sulfur median concentration of 0.17 wt.%, which agrees with the amount of sulfur added through the addition of Portland cement to a typical concrete mix. Samples with higher concentrations of total sulfur may have higher content of hydrated cement, or may have trace amounts of other sulfur sources, such as sulfide.

The large majority of NP samples with pyrrhotite and no visible damage had a total sulfur concentration between 0.3 and 0.5 wt.%.

DP samples from homes with observed damage all had pyrrhotite concentrations exceeding 0.30 wt.%. From this dataset, it may be concluded that homes with total sulfur concentration lower than 0.30 wt.% have a 0% statistical chance to have visible damage.

Figure 2 below shows the distribution of the pyrrhotite concentration calculated on the basis of the amount of sulfide.

Fig. 2: Distribution of pyrrhotite (a) in NP samples (no damage) (b) in DP samples (damage)

Comparing the NP and DP datasets, the following can be observed:

  • The average concentration of pyrrhotite in the NP dataset (0.66 wt.%) is significantly lower than the average (1.19 wt.%) in the DP dataset.
  • A few NP samples have high pyrrhotite concentrations, exceeding the average pyrrhotite concentration of DP homes but have no visible damage.
  • A considerable number of DP samples have concentrations similar to the average of samples in homes with no visible damage; therefore, a lower pyrrhotite concentration of equal or less than 0.5 wt.% does not guarantee the absence of visible damage.

Further analysis was performed by taking the average concentration of pyrrhotite in each home and evaluating the influence of the age of the home on the basis of the year of construction.

Figure 3 shows that the age of homes with visible damage (DP) is considerably higher.

Fig. 3: Age of (a) homes classified as NP (no damage) (b) classified as DP (damage)

Every home with observed damages was older than 20 years, with only 6 homes between 21 and 25 years of age, the average age being 35 years. In contrast, NP homes had an average age of 25 years.

Still, there were 18 homes older than 25 years of age and no visible damage, and 6 homes that were 21 and 25 years of age and showed damage. The pyrrhotite and total sulfur concentrations in these home categories were further evaluated as shown in Figure 4.

Fig. 4: Distribution of pyrrhotite (a) and total sulfur (b)
in homes with no damage and age less than 25 years, homes with damage and age less than 25 years, homes with no damage and age more than 25 years and homes with damage and age more than 25 years

Comparing the newer homes (age less or equal to 25 years), it is clear that the pyrrhotite and total sulfur concentrations are generally lower in the homes with no visible damage, potentially indicating variations in the rock mined during different time periods.

For older homes, there is no statistical difference in the pyrrhorite concentration between homes with and without visible damage. Thus, the absence of visible damage in older homes cannot be attributed to lower amount of pyrrhotite; there are additional factors beyond age and amount of pyrrhotite that resulted in lack of damage in these homes. There are 18 homes in this dataset, which will be investigated further.

Fig. 5: Distribution of pyrrhotite (a) and total sulfur (b) in CT and MA homes with pyrrhorite detected

As shown in Figure 5, there was no statistically significant difference in the datasets from homes located in CT versus MA, so that these datasets will continue to be evaluated as a single dataset.