Guest article from Joe Malone BSc(Hons) ICIOB
…obtaining accurate moisture measurement in the field can be fraught
with difficulty and pitfalls…… You need to adopt a pragmatic approach in the
field, understand the limitations of your test equipment and thoroughly review
build specifications and manufacturers product information
Background
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Figure 1. New
development with pumped calcium sulphate floor screeds
Source: Author's Own
|
Malone Associates Ltd, were recently instructed by a large National
developer to help resolve a dispute relating to one of their London
developments. Essentially we were asked to investigate issues relating to
ongoing dampness within a newly laid floor slab within a flat in a new low rise
block. The history surrounding this matter was that a client had purchased the
flat then noticed air blisters under the Amtico flooring in her lounge. The
Amtico was taken up and a square section of the floor screed was excavated to
see what was going on. My understanding was that moisture was found sat on top
of the plastic resilience layer, the floor was patch repaired and further
drying was being carried out and monitored until such time as readings were
obtained that were low enough to justify laying the finished flooring.
We
were unsure what methods of testing and monitoring was being carried out but
had early concerns that the wrong type of equipment was being used. On
initial enquiries as to the type of floor screed used we were informed that
Calcium Sulphate floor screed has been laid, specifically a Supaflo pumped screed by Cemex. It is
worth noting at this point that the terms applied to Calcium Sulphate floor
screeds can get confusing because they can also be commonly referred to as
‘Calcium Sulphate screed’, ‘Gypsum’ screed or ‘Anhydrite’ screed. All three
terms refer to the same generic product.
Construction - Inspection
of the plans we were sent would indicate that the block is constructed of a
103mm brick outer leaf, a 100mm insulated cavity and a 140mm Metal section
fully insulated frame inner leaf. With cement particle board outer, and 2 inner
layers of 12.5mm plasterboard with taped joints. Importantly we could see that
the floors appeared to be constructed of a 250mm poured in-situ concrete floor slab overlaid
with a 10mm resilient layer and a 45mm screed.
Site Observations - We
attended site in November and after initial visual inspection assessed the flat
for condensation risk. To that end the following readings were recorded.
Relative humidity = 47%, Ambient air temperature 19.2OC, Dew point
temperature = 7.2OC. The
floor temperature was recorded at 17OC or roughly 10OC above dew point temperature. These results proved conclusively that there was
no active condensation damp problem in the flat on the day of inspection.
It could
be seen on initial visual inspection of the floor screed which was stripped of
finished Amtico flooring and attempts had been made to strip the screed of
flooring Adhesive and primer. This is a difficult process and it was clear that
there were still patchy areas of adhesive residue across the lounge floor. We
also noted a large patch repair area to the floor and noted the dissimilarity
between the existing screed and the repair material.
 |
Figure 2.
Aggregate contained within this repair section gave concerns relating to
compatibility
Source: Author's Own
|
It was initially assumed by
site staff that the repair section was calcium sulphate and therefore
compatible with the rest of the floor. When I asked for this to be confirmed
the staff member who repaired the floor confirmed that it was in fact concrete
and as such this repair section is incompatible with the existing floor
screed and would need to be excavated
and replaced. This is because the
Tricalcium Aluminate present in ordinary Portland cement (OPC), contained
within the concrete repair section, will react with the sulphate in the floor
screed and cause ettringite to form. I’m sure you will all recognise this
problem as Sulphate attack, which is an expansive reaction and could well lead
to problems with the finished floor in the near future.
Test Methods
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Figure 3. Tramex
CME used for taking readings in floor screed.
Source: Author's Own
|
We
noted the presence of a Tramex CME (Concrete Moisture Encounter) within the
flat and it became apparent that floor moisture readings were being obtained by
this means. This is standard industry practice because these meters are
specifically calibrated for concrete and concrete floor screeds. Indeed, we use
a Tramex CME ourselves but we are aware that the tool has some limitations when
it comes to a reliable diagnostic process. We took electronic moisture readings
across the whole of the lounge floor to obtain comparative readings and our own
readings tended to confirm and support the previous statement made by the
developer, to the effect that moisture levels were highest nearer the patio
doors leading to the balcony. We received our highest Tramex reading of 7.5% at
this point, which would tend to indicate that the floor was significantly damp.
We also noted a very high Tramex reading of 6% to the concrete repair patch. We
were looking to obtain readings of 0.5% total moisture content (TMC) or <=75%
equilibrium relative humidity readings (ERH).
Some of the more attentive
readers may have already spotted the potential problem with the use of the
Tramex on a Calcium Sulphate floor screed… Did you note that I said they were
calibrated specifically for concrete? Calcium sulphate is not concrete and
therefore any readings obtained with the Tramex can only be referred to as a
relative reading. Tramex’s own product information states that ‘The Concrete Encounter utilises “state of the art” electronic
technology to provide the flooring industry with an accurate and simple to use
non-invasive handheld instrument for nondestructive testing (NDT) of Moisture
Content (MC) in concrete and comparative moisture readings in, gypsum and other
floor screeds.’ Moreover, the
Tramex is only reasonably reliable when used on virgin concrete that is clean
and dust free. Once primers, adhesives and other coatings have been applied to
the floor then the Tramex becomes even less reliable. The floor in question had
too much surface contamination for the Tramex to ever be reliable even it was
concrete as opposed to Calcium Sulphate flooring.
Actions
Because the floor repair
section was to be excavated and repaired we decided to test the floor using the
calcium carbide meter. For those of you not familiar with calcium carbide
testing, a small amount of test material is weighed and placed into the pressure
vessel. This is then mixed with calcium carbide powder and the chemical
reaction within the vessel releases acetylene gas that pressurizes the vessel
and causes a reading to register on the gauge. The amount of acetylene produced
is directly proportional to the amount of moisture present in the sample and
this is the most reliable site test in use for establishing the moisture
content of concrete and masonry products.
 |
Figure 4. Repair section excavated and showing a TMC of
1.8% (Tramex showed 6%)
Source: Author's Own
|
The concrete repair section was indeed still damp, though this was a
null point due to its excavation. We simply tested this area to complete our
understanding of what moisture contents were actually present right across the
floor compared to readings obtained with the Tramex CME. In fact the calcium
sulphate screed proved to be bone dry with readings no higher than 0% TMC
obtained.
 |
Figure 5. Tramex
showed 7.5%, actual TMC = 0%
Source: Authors Own
|
We
believe that the Amtico flooring was initially laid too early but subsequently
after removal of the Amtico the developer has dried the floor thoroughly but
been chasing their own tail due to false positive readings obtained from the
Tramex.
After
the floor slab was repaired we provided the client with ongoing advice for
monitoring the deep repair section that was replaced but there are only three
reliable ways of obtaining reliable results for moisture content in floor
slabs, these are:
1. Calcium carbide testing
2. Testing equilibrium relative humidity (ERH)
using Hygro sleeves.
3. Testing equilibrium relative humidity (ERH)
using a floor humidity box.
Since
options 1 & 2 involve destructive testing, then the only option available
for ongoing moisture tests in the flat is option number 3. A floor humidity box
should be placed on the large repair section and sealed with double-sided Butyl
tape. We recommend that the humidity level inside the box is then recorded
after a 72 hour period. If the ERH reading is 75% or less then you are free to
proceed with the installation of finished flooring. Manufacturers information
on the calcium sulphate screed used for the repair section indicated that the
screed could take up to 50 days to dry so clearly an initial period of drying
would be required before monitoring was started.
Ongoing Tests
The
developer chose to use a ‘Tramex Hygrohood’ floor humidity box sealed in place
with double sided butyl tape for ongoing moisture measurement of the repaired
floor section. However this proved what we already know, in that readings of
<=75% ERH are extremely difficult to achieve in floor screeds. Four weeks
later I was called back to reassess the flooring after the developer failed to
get ERH readings below 77%. Also you must remember that that ERH readings are
relative to temperature as well as moisture yet product information makes no
reference to the temperature benchmark at which the ERH reading should be
taken. We generally recommend that rooms are maintained at circa 20oC
when screeds are being dried out and this is a general recommendation that is
often found; it therefore makes sense to recommend that an ERH reading of
<=75% should be obtained with a room temperature of 20oC. The
Hygrohood shows a reading of 77% ERH at 21.5oC but if we drop the
room temperature to 20oC then the ERH increased to circa 80%.
The client
asked if we could test with calcium carbide again and it was simply a matter of
understanding what depth the latex screed could be laid to. This allowed us to
take a series of shallow drillings to obtain enough material for calcium
carbide testing. The test again proved that the repaired floor section was now
completely dry and the developer could
move forward with installing the latex screed. The drilled areas will be made
good by the latex screed.
All this
proves that obtaining accurate moisture measurement in the field can be fraught
with difficulty and pitfalls. Moreover, I think some of the guidance
information is poor; particularly the guidance that an ERH of <=75% should
be achieved, perhaps <=80% ERH would be more realistic because once that
figure was achieved at a room temperature of 20oC the floor proved
to be dry. You need to adopt a pragmatic approach in the field, understand the
limitations of your test equipment and thoroughly review build specifications
and manufacturers product information.
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Figure 6. Tramex
showing 77% ERH at 21.5oC. The repair section proved to be dry.
Source: Author's Own
|
Joe
Malone
Director
(Malone Associates Ltd)
First Published on 6/1/15 at www.buildingdefectanalysis.co.uk
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