Llandough Hospital the date of this cracking in the wall of the eastern ward is unknown. The cracks were hidden with battens faced with formica sheet


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Llandough Hospital - the date of this cracking in the wall of the eastern ward is unknown. The cracks were hidden with battens faced with formica sheet.

  • Llandough Hospital - the date of this cracking in the wall of the eastern ward is unknown. The cracks were hidden with battens faced with formica sheet.



Llandough Hospital was written up in the 1980s. Other sites with sulphate problems which I was involved with in the 1980s/90s were at Swindon, Hucclecote, Camarthen and piles in the London Clay. At these sites, the material was in situ and had a lower calcite content than the Dublin rocks - bacteria clearly played a significance role.

  • Llandough Hospital was written up in the 1980s. Other sites with sulphate problems which I was involved with in the 1980s/90s were at Swindon, Hucclecote, Camarthen and piles in the London Clay. At these sites, the material was in situ and had a lower calcite content than the Dublin rocks - bacteria clearly played a significance role.

  • In Dublin to date we have little evidence that bacteria are important in the more alkaline rocks. t is believed the calcareous nature of the material may affect the rate at which the initial expansion occurs and hence the time frame for distress in buildings.



Working on the basis that this is a briefing, not a lecture, in the time available I’m only going to mention some key facts and offer you a simplified approach to a complex problem.

  • Working on the basis that this is a briefing, not a lecture, in the time available I’m only going to mention some key facts and offer you a simplified approach to a complex problem.

  •  

  • Much of the detail and the research evidence behind the key facts is set out in Ground Chemistry: Implications for Construction (1997) and Implications of Pyrite Oxidation for Engineering Works (2014) which discusses a number of cases from the Dublin area.



Pyrite? What’s the problem?

  • Pyrite? What’s the problem?

  • Heave

  • Sub-structure concrete

  • NRA, SR 21 and IS 398

  • Concrete blocks

  • Concrete pipes

  • Summary



Pyrite – iron sulphide – is present in most rocks, particularly such sedimentary rocks as the mudstones and limestones found in the Dublin Basin.

  • Pyrite – iron sulphide – is present in most rocks, particularly such sedimentary rocks as the mudstones and limestones found in the Dublin Basin.

  •  

  • It may form as visible lumps/cubes or as microcrystals (less than a micron/a thousandth of a millimetre across). Frequently the microcrystals are drawn together to form framboids (raspberry-like clusters), which are still far too small to be seen by the naked eye.



Framboid and octahedral pyrite on a fractured surface in the London Clay

  • Framboid and octahedral pyrite on a fractured surface in the London Clay



As long as the pyrite is isolated from moisture and oxygen, there is no problem. The difficulties start when the overburden stress is reduced and/or the rock is quarried and the pyrite exposed.

  • As long as the pyrite is isolated from moisture and oxygen, there is no problem. The difficulties start when the overburden stress is reduced and/or the rock is quarried and the pyrite exposed.

  •  

  • Moisture and oxygen react with iron sulphide

  • to produce

  • ferrous sulphate and sulphuric acid

  •  

  • Pyrrhotite – a particularly unstable form of pyrite - reacts about a hundred times more quickly than pyrite.



Cubes/lumps of pyrite have a relatively small specific surface (surface area : volume) compared with a cluster of microcrystals (sugar lump/sugar grains) hence the chemical reaction is slower. Unbroken pyrite cubes are often displayed in museums – in Dublin some can be seen in the walls of Christchurch.

  • Cubes/lumps of pyrite have a relatively small specific surface (surface area : volume) compared with a cluster of microcrystals (sugar lump/sugar grains) hence the chemical reaction is slower. Unbroken pyrite cubes are often displayed in museums – in Dublin some can be seen in the walls of Christchurch.

  •  

  • With quarried material it is dangerous to separate pyrite into reactive and non-reactive forms. During blasting, crushing, handling and compacting, the cubes/lumps may be fractured, increasing their specific surface hence they react much more quickly.



Fractured pyrite lump

  • Fractured pyrite lump



When oxidation of the iron sulphide begins, the ferrous sulphate formed has a volume some 200-300% greater than the initial components. In mudstones, the creation of the ferrous sulphate rim may crack open the rock fragment.

  • When oxidation of the iron sulphide begins, the ferrous sulphate formed has a volume some 200-300% greater than the initial components. In mudstones, the creation of the ferrous sulphate rim may crack open the rock fragment.





When mudstones are quarried, stress relief results in a dilation of the material along the bedding planes or clay-rich bands (laminations). This facilitates both the ingress of the oxidising agents (moisture and oxygen) and the movement of sulphate-rich solutes.

  • When mudstones are quarried, stress relief results in a dilation of the material along the bedding planes or clay-rich bands (laminations). This facilitates both the ingress of the oxidising agents (moisture and oxygen) and the movement of sulphate-rich solutes.

  • What is frequently not appreciated is that many limestones contain microscopic bands of carbonaceous argillaceous material, reflecting a change in the depositional environment in which they accumulated. Although 1-2% pyrite is commonly found in these horizons, in the Dublin limestones more than 10% pyrite has been recorded in the thin clay layers.





The presence of the argillaceous bands facilitates the splitting of the rock with stress relief and when crushed/compacted. In addition, the clay minerals help the ingress of the oxidising agents by capillary movement.

  • The presence of the argillaceous bands facilitates the splitting of the rock with stress relief and when crushed/compacted. In addition, the clay minerals help the ingress of the oxidising agents by capillary movement.

  •  

  • Sulphuric acid released in the initial oxidation process moves within the fragment and can combine with calcium carbonate to form calcium sulphate (gypsum). This has double the volume of the initial components. Crystals are formed both within individual aggregate fragments and in the matrix.



Typical gypsum crystal from the London Clay as seen in museums and handled by students.

  • Typical gypsum crystal from the London Clay as seen in museums and handled by students.



From the initial formation, new growth extends outwards in the form of rosettes which become increasingly thick and push the rock apart.

  • From the initial formation, new growth extends outwards in the form of rosettes which become increasingly thick and push the rock apart.











Replacement fill

  • Replacement fill



 

  •  

  • Laminations – look for flaky fragments and see if there are any cracks on the side which can be opened with a knife. Alternatively, hit the fragment with a hammer and see if it splits along incipient laminations.

  • Look carefully for “fools gold” on exposed surfaces or clay-rich laminae.



In addition

  • In addition

  • Look for sparkling gypsum crystals/needles which have formed on the outer surfaces of the aggregate fragments. In limestone-rich rock, be careful not to confuse with sparkling cleaved calcite.

  • b)Look for signs of generalised or localised red/brown discoloration due to oxidation of the iron sulphides.





Particularly in dark rocks, it is very difficult to distinguish calcareous mudstone from argillaceous limestone by looking at a hand specimen.

  • Particularly in dark rocks, it is very difficult to distinguish calcareous mudstone from argillaceous limestone by looking at a hand specimen.

  • Technically, a limestone must have >50% calcite and a mudstone >50% argillaceous content.

  • Experts looking at 47 hand specimens from a core determined there were 19 limestones and 20 mudstones and were unable to clearly categorise 8 samples.

  • Based on a X-ray analysis, there were only 14 limestones – two of which had been classified as mudstone !



 

  •  

  • Averaging the pyrite contents for the two lithologies determined using X-rays, there was very little difference.

  • The highest pyrite content (6.0%) was found in a striped dark and pale mudstone and fine limestone with 21.3% calcite.

  • The second highest (5.6%) was in a limestone with 69.7% calcite.

  •  

  • It is not whether you call it limestone or mudstone which matters but, in addition to the amount of pyrite present,

  • The amount of clay minerals which facilitate capillary movement in argillaceous material, and

  • Whether crushing of a low permeability limestone may have resulted in the exposure of very fine fragments of pyrite.  



Dark coloured material should raise alarm bells……..

  • Dark coloured material should raise alarm bells……..

  • It is incorrect to assume that mudstones are the only “bad” material. Limestones may contain almost invisible argillaceous horizons, as well as pyrite cubes/lumps which can be broken down during processing.

  • c) One way of assessing whether argillaceous material is a significant component is to scratch the rock with a knife or screwdriver. Mudrocks indent, limestones don’t.

  • d) Does the material split on hammer impact, particularly the platy particles?



If aggregate fragments break down during compaction, the higher percentage of fines will accelerate the rate of oxidation of any pyrite present.

  • If aggregate fragments break down during compaction, the higher percentage of fines will accelerate the rate of oxidation of any pyrite present.

  • b) Do not over-compact sub-floor material; chemically induced heave will be exacerbated in a dense material.

  • c) Be careful with Cl 804/808 material. The grading is intended to produce a good dense aggregate, hence up to 20% of the material may be <0.5 mm; 7% may be silt or clay grade (<0.063 mm). The fine fraction generally contains a higher proportion of pyrite than the main aggregate.

  • d) Material placed beneath floors is best if it is clean (washed) and voided (to provided drainage and inhibit capillary rise).



Check the total sulphur values – ensure that adequate testing has been carried out on the material YOU are purchasing. See a range of results, not just an average. Judge the likelihood of problems on the average of the two highest TS results.

  • Check the total sulphur values – ensure that adequate testing has been carried out on the material YOU are purchasing. See a range of results, not just an average. Judge the likelihood of problems on the average of the two highest TS results.

  • b) Beware if Cl 804/808 is requested - the total sulphur of the fines (<0.063 mm) is likely to be much higher than the values the supplier will provide for the general aggregate.

  • c) If the total sulphur is below 0.1%S – no problem anticipated. Anything more than this needs careful consideration.



Acid soluble and water soluble sulphate values tell you about what is happening/has happened rather than what may happen in the future. They also are more likely to reflect the oxidation of the fine fraction rather than the pyrite within the fragments, which causes the main heave.

  • Acid soluble and water soluble sulphate values tell you about what is happening/has happened rather than what may happen in the future. They also are more likely to reflect the oxidation of the fine fraction rather than the pyrite within the fragments, which causes the main heave.

  • There is almost no acid soluble sulphate in fresh bedrock. Tests undertaken at the time of quarrying, crushing etc may give very low acid soluble sulphate contents. However, in a stockpile the material may exhibit gypsum growth within 8-12 weeks.



In samples from 1.5 m depth below Llandough Hospital, stored in a laboratory for 17 months, the ASS content increased from 0.43 to 1.92% SO4.

  • In samples from 1.5 m depth below Llandough Hospital, stored in a laboratory for 17 months, the ASS content increased from 0.43 to 1.92% SO4.

  • This prompted us to appreciate the necessity to establish total sulphur and hence the total potential sulphate which could form. We realised that by deducting the sulphur in the existing sulphates from the total sulphur, the sulphide remaining to be oxidised could be calculated.



??? Use suspended floors

  • ??? Use suspended floors

  • b) Don’t build internal walls on a continuous floor slab

  • c) Avoid under-floor heating as temperature accelerates chemical reactions

  • d) Avoid irregularity in sub-structure wall surfaces to minimise “gripping” by laterally expanding fill

  • e) 100 mm thick walls are vulnerable to sulphate attack



Sulphate-rich solutes moving from deleterious fill into concrete may result in deterioration of the sub-structure concrete/concrete block walls.

  • Sulphate-rich solutes moving from deleterious fill into concrete may result in deterioration of the sub-structure concrete/concrete block walls.

  • Growth of minerals in voids causes cracking – the pressure of crystallisation may be greater than the strength of the concrete.

  • b)Interactions between sulphates and the constituents of the cement result in expansion and a loss of integrity of the concrete.







Thaumasite exposed on a broken block

  • Thaumasite exposed on a broken block

  • See dropbox



Typically, before building commences, a site investigation is undertaken.

  • Typically, before building commences, a site investigation is undertaken.

  • One aspect is the assessment of the aggressivity of the ground based on such documents as BRE Special Digest 1 (2005). The consultant then advises on the amount of cement and/or admixtures required to reduce the effect of ground sulphates.

  • Too frequently insufficient account is taken of the increase of sulphates which will occur with oxidation due to exposure/lowering of the ground water level.

  • Remember ASS/pH may change with engineering works.



  • It would be assumed that imported hardcore to be placed next to concrete would be effectively inert.

  • To confirm this, specifiers/consultants would refer to the national Standards, eg NRA, SR 21.



NRA Series 800 was widely used in the building industry as well as for roadworks.

  • NRA Series 800 was widely used in the building industry as well as for roadworks.

  • In 2000 the chemical criterion for material placed within 0.5 m of concrete was WSS <1900 mg/l SO3 (c. 2300 mg/l SO4 ).

  • In May 2004 NRA changed this to an acid soluble sulphate criterion with an upper threshold of 0.2% but did not specify whether this was SO3 or SO4 (1.2 x SO3).

  • The 2004 Notes for Guidance recommended testing the ASS at least every 400 tonnes (approx 20 lorry loads). There was no caution that ASS may change with a change in the environment.



Appreciating the development of sulphates in crushed aggregate, the 2013 editions of both the 600 and 800 Series now recommend that material within 0.5 m of cement-bound materials should have a

  • Appreciating the development of sulphates in crushed aggregate, the 2013 editions of both the 600 and 800 Series now recommend that material within 0.5 m of cement-bound materials should have a

  • WSS of < 1500 mg/l SO4 and an

  • OS of <0.3% (as SO4).

  • Note: OS = (3 x TS) - ASS



SR 21:2014 recommends that if the TS content is

  • SR 21:2014 recommends that if the TS content is

  • < 0.1%S the material is acceptable;

  • > 1% S the material is unacceptable;

  •  between 0.1 and 1% S specialist testing/professional advice is required.

  • Be careful in choosing your professional advice.

  • Make sure you use some one with an appropriate background and experienced in sulphide/sulphur/sulphate issues - not a generalist.

  •  



SR 21 also states that petrographic assessment should identify if pyrrhotite is present.

  • SR 21 also states that petrographic assessment should identify if pyrrhotite is present.

  • Unless the SEM/microprobe is used, pyrrhotite is very difficult to identify. This is probably one reason why pyrrhotite is considered “uncommon” in Ireland although geologically it would be expected to occur.

  •  

  • If pyrrhotite is present, the upper threshold for sulphur drops to 0.4% S. European Standards have lower values.

  •  

  • SR 21 does not draw attention to the fact that pyrrhotite oxdises very quickly and reverts to pyrite. It is likely that the high water soluble sulphate contents in some samples reflect this change.



IS 398 was produced to assist in determining whether or not a building has been damaged by reactive pyrite or is likely to be in the future.

  • IS 398 was produced to assist in determining whether or not a building has been damaged by reactive pyrite or is likely to be in the future.

  • The aggregate should be assessed against six criteria:

  •  





A pass category in Table 3 does not preclude the potential for both heave and damage to concrete/concrete blocks.

  • A pass category in Table 3 does not preclude the potential for both heave and damage to concrete/concrete blocks.

  • IS 398 should NOT be used to assess whether a material is acceptable for use as a new/replacement fill.



Most sub-structure walls are formed of concrete blocks founded on strip footings.

  • Most sub-structure walls are formed of concrete blocks founded on strip footings.

  •  

  • However, blocks are vulnerable to the ingress of sulphates from adjacent material, due to the amount of contained voids.

  • It is particularly important that the concrete aggregate does not contain sulphides.

  • In my opinion, the coarse aggregate used in the manufacture of blocks should have a total sulphur value of <0.2% S. The fine fraction of the aggregate (<0.5 mm) should have a total sulphur value <0.1% S.

  •  



Most concrete blocks consist of a rock aggregate, quartz sand and cement. In some cases, the inert quartz sand is replaced with fine limestone particles/dust.

  • Most concrete blocks consist of a rock aggregate, quartz sand and cement. In some cases, the inert quartz sand is replaced with fine limestone particles/dust.

  •  

  • If limestone fines are used instead of inert quartz sand, be very careful that they do not contain pyrite which, being in a dust form, would be particularly prone to oxidation.



 

  •  

  • Sulphur/sulphates released as a consequence of oxidation in the permeable concrete blocks may

  • react with the cement to reduce its long term integrity and/or

  • form crystals of gypsum, ettringite or thaumasite in voids - the pressure of crystallisation causing cracking of the block.

  •  



BRE notes that in concrete the presence of more than 4% sulphate by weight of cement does not automatically imply sulphate attack has taken place; it may only be a warning of potential attack in the future.

  • BRE notes that in concrete the presence of more than 4% sulphate by weight of cement does not automatically imply sulphate attack has taken place; it may only be a warning of potential attack in the future.

  • Note, this refers to sulphate attack due to the chemical interaction with the cement. In addition, crystals growing in voids result in damage to the integrity of the concrete.

  •  





When pyrite oxidises, ferrous sulphate and sulphuric acid are formed. This is an ongoing process.

  • When pyrite oxidises, ferrous sulphate and sulphuric acid are formed. This is an ongoing process.

  • 2. Measuring the total sulphur allows an indication of the total potential sulphate which may form, assuming all sulphur is present as iron sulphide and it is all oxidised. Oxidisable sulphide takes into account the sulphates already present.

  • 3. The amount of acid soluble sulphate present changes over time as the oxidation process continues. A very low value obtained when the material is freshly quarried is no guarantee that sulphates will not develop in the future.



4. Expansion results from the formation of ferrous sulphate and the reaction between sulphuric acid and calcium carbonate to form calcium sulphate (gypsum). Heating accelerates these reactions.

  • 4. Expansion results from the formation of ferrous sulphate and the reaction between sulphuric acid and calcium carbonate to form calcium sulphate (gypsum). Heating accelerates these reactions.

  • 5. As the gypsum crystals may “prop open” aggregate particles, the amount of expansion is not directly related to the volume of gypsum present.

  • 6. The pressure of crystallisation of gypsum is considerable and can raise floors, push walls outwards and lift entire buildings.



Concrete/concrete blocks and pipes in contact with the ground, may be affected by sulphates in the adjacent material.

  • Concrete/concrete blocks and pipes in contact with the ground, may be affected by sulphates in the adjacent material.

  • 8. When sulphate crystals grow in the hollows in concrete/concrete blocks there may be an initial apparent increase in strength. Subsequently, the pressure of crystallisation will crack the blocks such that they lose strength and integrity.

  • 9. If the aggregate in the concrete/concrete blocks contains pyrite, additional internal stresses develop when oxidation takes place.



10. Always ensure the supplier is aware of the end use of the product you are purchasing. They should have the expertise to supply you with a reliable material.

  • 10. Always ensure the supplier is aware of the end use of the product you are purchasing. They should have the expertise to supply you with a reliable material.



  • Many of the photographs used were taken by Sandberg





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