Investigating Sulphate Attack in Floor Slabs

1960's bungalow. Sulphate attack in floorslab?
1960’s bungalow. Sulphate attack in floorslab?

We were asked if we could carry out another investigation into potential sulphate attack in a floor slab this week. The scenario was that the property, a 1960’s bungalow in a Nottinghamshire village, had recently been subject to a homebuyers survey report, which raised concerns with regard to potential sulphate attack in the solid concrete floors. The Chartered Surveyors report contained the usual caveats, “Inspection of the floor was limited due to existing floor coverings” etc. However, the report contained the following statement… “It is noted that some floors to the property are uneven and require further investigation. This type of unevenness felt within the floor is an indication there may be a problem with sulphites in the base to the floors.” Unusual I thought, since sulphites are more usually found in food and wine rather than floor slabs. However, we got the crux of the problem and explained the potential implications to our client when we received the call.

Straight edge highlights degree of heave in floorslab.
Straight edge highlights degree of heave in floorslab.

Sulphate attack is caused when sulphates contained within the concrete aggregate contain sulphates or the hardcore sub-base below the concrete contains sulphates. These sulphates react with the Tricalcium Aluminate in ordinary portland cement (OPC) to form Ettringite, a crystal that expands as it grows, in the process often causing  substantial damage to the concrete floor slab. Moreover there can be consequential structural damage as heave within the floor can displace internal walls built off the floor or indeed displace brickwork in the outer perimeter walls.

In terms of the investigation process we are looking to establish a number of things.

  1. How significant is the heave or cracking in the floor slab?
  2. Is there consequential structural damage?
  3. Is there a damp proof membrane installed between the floorslab and the hardcore?
  4. Sampling of both concrete and hardcore for laboratory analysis.
  5. Measurements to aid providing the client with a budget figure for floor replacement should sulphate attack be confirmed.
Verdigris to copper radiator pipes and localised cracking
Verdigris to copper radiator pipes and localised cracking

On internal visual inspection it was immediately obviously that there was substantial heave to the floorslab in a number of rooms and we pulled up two carpets for a closer inspection. It was obvious to us that there was high probability that the floorslab had failed due to sulphate attack. Sulphate attack is expedited by the present of moisture and we also noted that there were copper central heating pipes running through the concrete floorslab and also the the central heating boiler pressure was low. This raised alarms that a central heating system leak may well be contributing to floor moisture levels. Verdigris on the copper pipes indicated copper corrosion caused by being in direct contact with the highly alkaline concrete.

Bitumin oversite removed and coring through concrete slab for sampling.
Bitumin oversite removed and coring through concrete slab for sampling.

We set about sampling both the concrete and the hardcore because in our experience, the hardcore is often the prime culprit and the source of sulphates. Obviously, if there was a DPM installed then this barrier between the hardcore and the concrete slab significantly reduces the risk of sulphates coming into contact with the concrete. However, the concrete itself could contain aggregate with a high sulphate content, hence why both the concrete and hardcore are sampled and analysed for sulphates. Additionally we have a cement content analysis carried out on the concrete because this helps us determine the ratio of sulphate to cement, an important factor in determining the severity of failure.

Hardcore material on left and concrete on right.
Hardcore material on left and concrete on right.

We cored through the floor, initially cutting through an oversite of poured bitumen, which of course acts as a surface applied damp proofing barrier, this in itself told us that we would not find a polyethylene DPM installed. DPM’s started to be installed in floors around 1965 so this property would have been at the very front end of installations had one been installed.

The concrete itself was of very low quality and only circa two inches thick  so it didn’t take long to  cut through. We packed both a sample of concrete and retrieved a sample of hardcore material before making good the hole with a quick setting concrete mix; we did note that the hardcore material was extremely damp, adding to our concerns regarding a potential subfloor leak. Carpets were replaced and stretched with a knee kicker and we dropped the samples off at our laboratory. We’ve not actually obtained the results yet but we have warned of the very strong possibility that the heave found within these floor slabs is caused by sulphates. The reason for our pessimism, is the obvious visual damage seen to the floors and the poor quality of the hardcore material, which appeared to be crushed builders rubble rather than the usual fly ash or blast furnace waste that can cause this problem. Crushed builders rubble is a low quality hardcore fill material that often contains sulphates.

Cored hole made good prior to refitting carpets.
Cored hole made good prior to refitting carpets.

Our report will contain commentary on any structural implications found and of course will confirm or disprove whether the heave and cracking was caused by sulphates. We will of course also provide additional commentary relating to supplementary factors, such as the leaking central heating  system. A budget figure to replace internal floor slabs came in at around £15k, so this is a costly problem to remedy  when found.


19 responses to “Investigating Sulphate Attack in Floor Slabs”

  1. Andy avatar

    Hi, Just wondering what the outcome of this was. I have a floor which is similar in terms of age and condition in a known Sulphate problem area so trying to gather as much information as possible!.


    1. Joe Malone avatar
      Joe Malone

      The last blog details some of the work being completed Andy.

  2. Paul avatar

    We have the same same problem … At another 1960 built bungalow in Ripley Derbyshire.

    Advice would be much appreciated.

    1. Joe Malone avatar
      Joe Malone

      Advice with regards to what in particular Paul?

  3. Paul avatar

    How deep should we break out to where the new membrane should be placed ?

    I’ve got building control involved, also. They are coming out next week.



    1. Joe Malone avatar
      Joe Malone

      Paul, you’re not supplying enough information or context with which to answer this question. Presumably you are digging up a floor affected by sulphate attack. How much you dig out depends on what is contaminated. What testing have you had done for sulphates? Hopefully both the concrete and the hardcore were tested?

  4. Oliver Castledine avatar
    Oliver Castledine

    To what extent do you think the leaking heating system caused the attack? Without the leak was it possible the the sulphates could have sat inertly in the base and concrete?
    Why on earth were the pipes buried? Surely the slightest expansion from slab or pipe was going to end in defect?

  5. john Hartnett avatar
    john Hartnett

    just viewed a 1960,s bungalow in Pensarn north wales did not see any problems with the bungalow but the garage gave me cause for concern the floor had several large crack & was lifting in places with weeds growing through the floor,my first thoughts were red ash ! what do you think?

    1. Joe Malone avatar
      Joe Malone


      It’s almost impossible to provide useful commentary based on such limited information. Could be sulphate attack, or even ground heave, if the slab is particularly thin. Garage floor slabs are generally of a poorer quality, generally no DPM installed to provide a barrier between the hardcore fill or ground, both of which will contain sulphates.

      Regards. Joe

  6. Susan Jensen avatar
    Susan Jensen

    Joe, I have been researching sulphate damage to solid floors as a late 1950 house I own has several bumps to the floor. There is not other damp in the property or signs of damage to walls. but it’s a problem I feel I should address. So I’d like some advice if that’s okay.

    Is sulphate damage harmful to health?

    Does sulphate damage have to be assessed by a qualified surveyor or would a local builder be able to do that assessment.

    If sulphate damage is found to be present does the whole slab have to be replaced or is it possible to repair the areas where the bumps are?

    Are there other problems that can cause bumping to the slab?

    I’m a retired lady on my own and want to gather as much information as possible before I commit to high cost repairs, so would be grateful for any advice.

    Thanks Sue

    1. Joe Malone avatar
      Joe Malone

      Hi Sue,

      In answer to your questions:

      1. Sulphate attack is not damaging to health and you can dispel any fears on that count.

      2. Sulphate attack can only be confirmed after sampling the floor and subsequent laboratory analysis of those samples. A local builder would probably have no clue as to what must be done in regard to testing, but then many Chartered surveyors also have little experience of the testing procedure. Often surveyors understand the visible identification of heave in the floor, and the mechanism of sulphate attack, but their experience doesn’t extend beyond this. Around 80% of cases we investigate turn out not to be sulphate attack but failure by some other mechanism, curling, settlement or ground heave are all common problems. Movement in the floor is often so small as to not warrant further investigation as the floor is still perfectly serviceable and useable.

      3. If found, then yes, the whole slab would need replacing generally. However, if there are rooms that are still serviceable then these can be left, with the known risk that the problem may re-surface at a later date in those rooms which retain the original concrete flooring.

      4. It’s worth noting Sue that where we find sulphate attack then there are often associated factors, leaking internal heating pipes being the most common. These pipes are often buried in the concrete, and because concrete is highly alkaline, it eats through the copper pipework causing them to perforate. We often have to also specify replacement of all ground floor heating pipes where they are found to be buried in the concrete.

  7. Tracey avatar

    Hi I am looking at purchasing a property and my surveyor has picked up high levels of damp and suggested that it may be as a consequence of sulphate damage as one of the floors is showing undulations on the floor and there is evidence of curling of skirting boards in another room. Some of the central heating pipes also appear to run under the concrete too. The property dates back to the 1950s and has had a couple of extensions since but the problem with the floor appears to be in the extension area which was carried out in the 1990s. I am trying to arrange a damp proof specialist to try and identify the route cause of the damp and also see if he can check for sulphate. Does it sound like sulphate is likely based on the info I have given?

  8. Kate avatar

    We have a house built in the 1950’s. when we bought it, it had a cork surface on the floors, this was removed and a bitumen surface layed.
    Recently in the lounge I have seen a bulge appear in the concrete,, there is only this one, could it be sulphate problems after nearly 70 years?.also I don’t think there are any pipes in the floor.
    Do you have any idea?
    Also I have read when clay absorbs water it expands and exerts an immense force on the concrete, usually caused by leak or freshwater, could we have an underground spring ?


  9. Joan avatar

    I am hoping to purchase a 1960 bungalow and the surveyor pointed out crystals on the garage floor in several places. Most of the floor had a covering over it. My daughter found this strange. The surveyor has suggested a different type of survey to establish the problem. Can this problem affect the rest of the property. It is a wood jilt bungalow with stone surround I am seriously having second thoughts on thisnow

  10. Chris Jones avatar
    Chris Jones

    What idiots at the Coal Board and councils decided that red ash would be good to use?
    The legacy is horrendous and there are no come backs on anybody?

  11. Jean Smallwood avatar
    Jean Smallwood

    I am in the process of buying a bungalow built about 1930.A builder has said that it has red ash sulphate damage because one of the solid floors has an indentation of about 11/2 inch deep. The survayor said thhat because he was unable to lift the carpet he couldn’t comment on the cause. How long is red ash potent and surely after all this time any damage would have been done. The damp proofing needs attention because it probably is bitumen and has decayed over the years

    1. Joe Malone avatar
      Joe Malone

      Hi Jean,

      Sulphate attack can occur at any time and often the process is incredibly slow because it accelerates in the presence of moisture. Often when we investigate this problem, we find associated leaks, from central heating pipework etc, particularly where heating pipes are buried in concrete floors.

      Sulphate attack doesn’t cause indentations, it causes heave, which is where the floor is lifted due to the expansive reaction in the floor slab, so from what you say, I’m not sure this is your problem.

      I also don’t understand your point about damp proofing, the floor won’t have a damp proof membrane due to its age. Damp proof courses in the walls are unrelated, and in any event, they simply don’t fail.

      If it has a bitumen oversite to the floor, then again, these don’t really fail, and if they do, you will notice obvious visual damage to the bitumen as they crack, or start to break up, they can easily be repaired.


      Joe Malone

  12. Richard avatar

    Interesting case study thanks. I’ve seen references to this being a defect affecting dwellings in the north midlands but any idea how wide spread the cases have been discovered. I have problem with a 1950’s property in West London. Laterally displaced brickwork below dpc, buried heating pipes and a heating system which loses pressure. I cant see the slab because it is covered in expensive engineered wood flooring.

  13. Michael Anscombe avatar
    Michael Anscombe

    Dear Joe
    I am having a sulphate floor test and visual inspection carried out at a property I had hoped to buy. The surveyor’s report says there are no symptoms of associated problems but due to the age and location of the property it advises a floor test be done.
    My question is this: will a favourable result and report following the test mean that I can purchase with confidence? I ought I be wary even if the report is ok ? I believe it will be a spot test in just one or two spots so presumably this is not necessarily representative of the entire ground floor space which may have variations from one spot to another.

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