Public consultation opens on proposed changes to defective concrete blocks protocol

A revision of IS 465, the protocol which underpins the defective blocks redress scheme, has opened for public consultation.

Members of the public will be able to make an observation on the new protocol, which acknowledges pyrrhotite as the leading cause of the defective blocks crisis, under the consultation which runs until the end of May.

The revised IS 465 is significantly changed from the original protocol published in 2018 by the National Standards Authority of Ireland (NSAI).

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For the first time, pyrrhotite and iron sulfides are acknowledged as a leading risk factor in degrading concrete blocks, and laboratory tests will actively look for their presence when analysing core samples taken from affected buildings.

Here’s the science bit: When exposed to moisture and oxygen, deleterious minerals such as pyrrhotite undergo a chemical reaction and generate sulfuric acid which causes an ‘internal sulfate attack’ on concrete that degrades and weakens it and causes the patterned cracking that has been so widely observed in homes and other buildings in Inishowen and throughout north Donegal in particular.

Extensive analysis and testing was carried out on behalf of the NSAI as part of the review of IS 465 to better understand the causes of the crisis which has severely damaged tens of thousands of buildings in Donegal.

During controlled testing, blocks containing both pyrrhotite and mica were examined and damage was shown to be most severe in outer and inner leaves of homes, with rising walls affected to a lesser extent, albeit exhibiting moderate risks of deterioration.

Accelerated ageing tests confirmed that, where moisture and oxygen are present (above ground), blocks with pyrrhotite and mica can degrade over time. Tests found that blocks with mica alone don’t degrade as severely.

Tests carried out on samples taken from foundations found the initial stage of internal sulfate attack was under way, but accelerated ageing tests showed the concrete remained intact. This finding led to the conclusion that the overall risk of foundation deterioration is low, provided environmental conditions remain unchanged.

Future degradation

Significant emphasis is placed on the potential for future degradation of concrete blocks under the revised protocol, something that was not provided for in the same manner in the original protocol.

As part of the laboratory testing of core samples, a professional geologist will be required to classify the risk of future degradation where iron sulfides are present. Each part of the building (e.g. the rising wall, outer leaf, inner leaf or internal wall) will be classified using the highest risk factor from that sample.

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Under the revised protocol, where a building exhibits damage (e.g. cracking, disintegrated blocks, blown or missing render, displacement at window or door reveals) and core sample tests detect the presence of iron sulfide or potentially problematic rock types, then the engineer shall recommend the demolition and rebuild of all affected concrete block elements “as this is the only effective solution”.

In effect, it means a damaged building with a volume of pyrrhotite in its blocks will require demolition.

However, a building that exhibits damage where only mica is present, and where iron sulfides are absent and the risk of internal sulfate attack is determined to be negligible, may have the retention of some or all of its blockwork as long as the blocks retain sufficient compressive strength to carry the weight of the building and are protected from moisture leaching.

In a building like this, if the integrity of the concrete block is insufficient to bear the necessary loads, or protection against leaching by moisture movement cannot be provided, the engineer shall recommend the demolition of the affected part of the blockwork.

In a note, the IS 465 authors refer to a research paper based on samples taken from more than 1,800 properties that suggested homes with mica alone in their blocks (and no pyrrhotite) could be remediated without full demolition and with the retention of inner and/or outer leaf blocks - if the building envelope is robust.

Foundations

The IS 465 published in 2018 did not provide for any circumstance where foundations should be removed.

The revised protocol does provide limited grounds where the removal of damaged foundations should be considered, based on the presence of iron sulfides, pyrrhotite, cracking or evidence of internal sulfate attack.

Where abundant quantities of problematic rock types are found in laboratory tests, or where moderate quantities of pyrrhotite are present, the risk to the foundations is considered to be medium-to-high and consideration should be given to removing the foundations.

Notes regarding the section of the protocol that deals with foundations say there is currently no known occurrence of deterioration of foundation concrete showing damage similar to that seen in concrete blocks.

Because foundations are buried in the ground, even if that ground is saturated, it’s posited that the development of internal sulfate attack will be limited due to their limited exposure to oxygen.

Under ageing tests carried out, researchers considered the risk of internal sulfate attack progressing to the point where it would cause significant damage to the integrity of the foundations within the intended 50-year design period to be low.

Consultation

The revised IS 465 will remain open to public consultation until May 28. Registration with a valid email address is required.