and surrounding areas)
you are reading this, the chances are you have been told you need a
shale test, in-fill test floor test
or bore test.
I have provided some
useful information on shale tests for you.
If you have any queries
or want to book a shale test, or simply want some advice, please feel
free to contact me :-
helpful guide to shale and in-fill testing in domestic properties
This guide has been
written in order to give estate agents, mortgage brokers and other
professionals within the property market, a source of
information, regarding shale and in-fill testing.
It has long been noticed that there is an awful lot of confusion
regarding all aspects of this type of Investigation.
We quite often get people saying that samples "don't need testing
because it’s not shale, and we were only asked for a shale
We do try to explain that it is not just shale that can cause problems,
and other materials have to be checked out also.
However in some cases people still insist that the samples are not to
When this happens, we simply provide a report which states they
requested us not to test the samples.
This invariably leads to them ringing us back up at zero hour, asking
us to test them, because "the lender has insisted on testing before
monies are offered".
This either delays the sale or costs them more money to pay for the
swift turnaround analysis.
Many people simply want to know what the test entails, are we going to
make a mess etc.
In this report, I have tried to answer all the most frequently asked
questions at a basic level. If a more detailed explanation is required,
just give me
a ring and I will be more than happy to help.
Firstly, in our local area, the test is referred to as a "shale test".
Whilst Cleveland shale is one type of material that we look for, it is
not the only material that has been used that can cause problems in
If we were located anywhere else in the UK, the test would be known as
Other variations include bore test, floor test, floor survey, sub-floor
fill test and concrete test.
WHY IS A TEST REQUIRED?
The use of hardcore in
house construction first became
common in the immediate post-war period, when construction materials
were in short
Solid floors, comprising a concrete floor slab over hardcore, largely
superseded suspended timber floors that were typically used in the
Also, waste materials, such as burnt colliery spoil and blast-furnace
slag were promoted by government as being appropriate materials for use.
Unfortunately, little or no guidance on the selection and use of
suitable materials was available in the early post-war years and there
use of deleterious materials.
Hardcore, derived from many industrial by-products, was included in the
construction of hundred of thousands of domestic properties from the
1940's to around the late 1970's / early 80's.
The legacy has been a continuing occurrence of damage to floor slabs
and abutting walls due to deleterious hardcore.
(Hardcore is a fill material used in building construction to raise
ground levels and provide a dry, firm and level base on which to cast a
ground floor slab or ‘over-site’ concrete beneath
Remediation of such damage is rarely covered by household insurance, as
the cause is deemed to be a latent construction defect.
Thus, in most cases, owner-occupiers have had to pay (typically
£10-20k) out of their own pockets for repair. - NB This
is new - in accordance with local government white papers
regarding the green environment, in
circumstances it may not be necessary to carry out a full removal, but
that is something you will have to talk to us about as and when we know
what the situation for your property is.
This situation has promoted a blight of numerous housing
cases have occurred, and now prospective buyers and professionals
concerned with house purchase and mortgages have naturally been very
cautious when dealing with properties that may have problems.
To this end, an in-fill Investigation is often required by a mortgage
lender to determine whether a particular property has an already
potential future problem.
Each dwelling needs to be considered on its own merits (just because
Fred's house at number 42 passed doesn't automatically mean that John's
at number 44 will also pass).
MANY HOLES ARE DRILLED?
That depends on a
number of factors.
The nor.m is two to three, but
there are exceptions.
The problems we face are mainly access related.
If a property has no attached or Integral garage, and has laminate or
other specialist flooring throughout, most lenders will happily accept
one borehole is allowable, e.g. in a cupboard that has not been
In other cases, we may carry out a borehole in the main property, and
find something that doesn't need testing, then, find something that
lab. testing in an integral garage. In this case, we would excavate
hole in the main property if possible.
Even in a house that has no access problems, there is still a limiting
factor as to the number of holes excavated.
Ideally we would excavate lots of
The problem is we are bound by financial constraints.
Fortunately, mortgage lenders realise this and usually accept what we
can get for them.
On the rare occasion they want extra holes, it is often necessary to
bring in a specialist to lift and replace the laminate etc, this would
costs; as would having to drill and test samples from extra boreholes.
The test is carried out
INSIDE the property.
There are two types of floor construction that are encountered; solid
concrete floor slabs and suspended timber floors.
The Investigation for both
types is essentially the same.
If the floor is of a timber construction, the joists are located, and a
circular saw is used to cut a number of floorboards. The number is
though more boards may be needed to be removed if there is a large void
the timber floor.
(If M.D.F type boards are present, we either take a full board up if
practicable or cut a sufficient square as appropriate). Occasionally,
an extra cost
is incurred due to technical considerations. - This would be discussed
full before any works took place.
Once the boards are removed, it will be seen that there is a concrete
A hole in the region of 4 - 5" diameter is drilled in the concrete to
expose the underlying in-fill.
The in-fill is visually identified and its depth is determined.
Samples are taken, then the findings - including any anomalies,
presence or otherwise of a damp proof membrane etc. are recorded.
Once all this has been done, the hole is back-filled with arisings from
the borehole (and inert gravel if more fill is needed). The hole is
concreted up and the floor boards re-instated.
A solid concrete floor is treated much the same. Quite often, Concrete
floors have "thermoplastic" tiles - designed to stop "cold bridging",
screed of one description or another.
In the case of the tiles, we attempt to take each one up without
breaking it, so it can be glued back down after the test. If we do
severely break a
tile, (some are glued down so well, they have to be broken to be
removed), we do
carry a selection of spares, but we cannot guarantee we can provide a
If screed is present instead of tiles, we will concrete the hole up to
finished floor level. In this case the area of the test can be clearly
this isn't generally an issue as the floor is usually covered in some
way, with a
carpet for example.
Polythene is placed over the hole, in order to protect carpet and
underlay until the concrete cures.
The in-fill under an attached or integral garage is also often
investigated. The same procedure applies, but in this case, the area of
test is often
unlikely to be hidden under carpet, and you will see a small round area
of different coloured concrete to the rest of the floor.
We explain this and get
permission where necessary before proceeding in these circumstances.
IT MESSY AND NOISY
It is certainly noisy.
You can imagine, we are chiselling
concrete, and that’s not quiet. The actual noise only lasts
minutes generally, but it could be much longer (or shorter).
We always advise people with pets to make arrangements for them to be
elsewhere - even if this is just in
another room. This is especially important with small animals.
Messy? Well that is a subjective opinion. There is certainly a lot of
dust kicked up (we are after all chiselling through concrete). But we
utmost to contain it.
For example, we use an industrial vacuum cleaner, and the nozzle is
pointed at the chisel tip while we are drilling through the
This way, over 95% of the dust is removed at source, However, we cannot
guarantee there will be no dust whatsoever.
We do provide dust sheets in the area in which we work, although some
people like to cover their furniture with their own dust sheets prior
to our arrival.
As a quick guide; to date, (touch wood) we have never had anyone
complain about the mess (that is because we are honest with them from
What we have had on many occasions is people tell us how pleasantly
surprised at the LACK of mess the shale test has caused.
In general, the on-site
test takes about an hour and a half.
duration depends on a number of factors Including access, density,
depth and hardness of concrete, density and depth of In-fill.
The time taken It may be as little as 45 minutes, or as much as 3.5
hours, but the typical duration is 1.5 hours.
DO YOU KNOW
WHETHER SAMPLES NEED TO BE SENT OFF?
From the moment we walk
into a property, we are looking for
Even just walking across a floor can reveal important information.
note of the way the concrete behaves as we drill through it, any
are present and a host of other things.
If we find any problems, or anything other than materials that are
accepted as being "stable", we will need to send samples off to the
Materials that are accepted as being stable include Dolomite, Clean
gravels and natural clays.
If there are anomalies in the floor, even these may have to be sent off.
Materials that are routinely sent off include, but are not limited to;
Cleveland shale, colliery spoil, slag, certain sands and gravels, ash
There are a number of
different mechanisms that cause
damage, depending on the material type.
Let's first look at Cleveland shale;
Cleveland shale is a "mud-rock". Millions of years ago it was clay.
Over time, and subjected to temperature and pressure changes (a little
bit like a
mild metamorphosis - a process called diagenesis), it changed to a rock.
When the clay was deposited, some of it was deposited in layers -
laminations - and in some of these laminations; other chemicals were
laid down with
Even though the clay is now a rock, the laminations and the chemicals
are still present.
These chemicals have an ability to react with water, and grow crystals.
As the crystals grow, they start to force the laminations apart,
causing expansion of the rock itself.
As the rock expands, the "extra volume" needs to go somewhere, and it
can either "go up or out" and usually does both.
The result is that floor slabs can be pushed up, and supporting walls
can be pushed off underlying foundations. Additionally, any walls built
the slab can be moved in such a way to give them eccentric loading that
they weren't designed for, not to mention problems with sub-floor
Now, the amount of chemicals that can grow crystals within the body of
the rock, will determine how much expansion can take place.
A lot of chemicals can lead to large volume expansion. If none of these
chemicals are in the laminations, then you can expect no expansion.
Slags are another problematic material. They are a by-product of the
steelworks industry, and some slags have
a two fold problem.
One is that they contain substances collectively known as water soluble
sulphates (also spelt sulfates).
This problem will be addressed in a short while, but first let's look
at the other problem associated with some metaliferous slags.
They have an ability to expand, which can cause the same expansion type
problems as shale expansion. They also tend to then totally collapse
into a powder.
Thus they have caused an expansion problem and damaged the concrete,
and then they collapse leaving a void. Clearly the presence of a void
can give lack of support to an already significantly damaged concrete.
The next problem is THE
most common problem we encounter. Sulphate attack is the most common
factor regarding problems associated with deleterious In-fill.
In our region, Cleveland shale is the big name on the streets, but in
fact even in this region, a property is more likely to fail from high
sulphate concentrations in the in-fill, than it is from potential shale
It is such a major problem country-wide, literally dozens of papers
have been written on the problem.
(Compare that with two or three written on
Cleveland shale expansion).
What are sulfates (also spelt sulphates) and what is sulfate attack?
Sulfates are salts in which the negatively charged ion (anion) SO4 2–
compound with a metal positively charged ion (cation) such as Ca2+.
In hardcore we are concerned primarily with sulfates that are readily
soluble in water and which can therefore be readily transported to
Such sulfates include gypsum (calcium sulfate, CaSO4), epsomite
(magnesium sulfate, MgSO4), and Glauber’s salt (sodium
Sulfate-bearing materials which have been used in the past as hardcore
in domestic properties include burnt colliery spoil (red ash or red
shale), furnace bottom ash (black ash or just ash), blast-furnace slag,
pyritic shales, and demolition debris including brick rubble,
containing gypsum plaster.
The amount of soluble sulfates present in the material is a vital
factor in determining the potential for sulfate attack on concrete.
Where a concrete slab overlies moist sulfate-bearing hardcore, the
sulfates in the hardcore may migrate into the concrete where they react
constituents in the cement matrix.
Two sulfate attack mechanisms have been identified as affecting concrete
slabs and oversite concrete:
(i) Conventional or ettringite form of sulfate attack. In this type of
attack, sulfates and water react with the tricalcium
aluminate found in Portland cement to form a calcium sulfo-aluminate
hydrate known as ettringite.
This type of sulfate reaction has long been known and most published
guidance on sulfate attack has addressed this mode.
The formation of ettringite can be destructively expansive since it has
a solid volume greater than the original constituents and typically
myriad acicular (needle-shaped) crystals that can collectively generate
internal stresses in the concrete.
(ii) The thaumasite form of sulfate attack (TSA). This form of sulfate
attack was first recognised in the UK in the 1990s
and has since been found in several floor slabs and in over-site
reaction product is the mineral thaumasite which is a calcium silicate
Since the calcium silicate hydrates provide the main binding agent in
Portland cement, this form of attack weakens the concrete as well as
causing some expansion and, in advanced cases, the cement paste matrix
is eventually reduced to a mushy, incohesive mass.
Whilst several chemically different types of concrete have been used in
construction over the years, the type used for floor slabs has
invariably been one based on the use of Portland cement, and of these
the majority have employed the commonly available ordinary Portland
This type of cement is the most vulnerable to sulfate attack as it
imparts to concrete, abundant amounts of both calcium aluminate
calcium silicate hydrates that are readily attacked by sulfates.
MISTAKES AND QUESTIONS
"This house has stood
for forty years with no problems.
Surely if there was going to be a problem it would have shown itself by
The answer is "not necessarily”
Firstly, these reactions can be very slow, so you may not have noticed
them yet. Also, the crystals that grow in the shale body and the
reaction between sulphate and concrete all rely on one major factor:-
The hardcore doesn't have to be saturated but water must be available.
A common source is groundwater that is drawn up through the finer
fraction of the hardcore by capillary action. Occasionally the source
may be surface water floods, or leaking drains and water pipes.
So if there has been an absence of water, you will not see that
anything sinister has manifested itself.
But, that does not mean that the POTENTIAL
for future damage is not
there (which is what mortgage lenders want to know).
"This house has a damp proof membrane below the slab, so sulphate
attack won't occur".
Sulfate migration from hardcore into concrete may be impeded by the use
of a separating membrane.
Polythene sheets, installed primarily as a damp-proof membrane (DPM),
began to serve this purpose from the mid 1960s, and became almost
universal for concrete slab construction on sulfate-bearing hardcore by
DPMs installed in the 1960s were typically only 500 gauge (125 micron)
thick, and doubts have been cast by some practitioners on their
as a barrier to sulfate migration.
Also, membranes have been known to perforate during the construction of
the floor, and some have not been overlapped sufficiently.
To this end, limits at present are imposed as to the upper allowable
sulphate concentrations that can be contained by the presence of a damp
DAMAGE LOOK LIKE
The first visible sign
of is usually some unevenness in the
may be accompanied by appearance of cracks in the concrete screed and
finish that are at first narrow, but which widen with time.
The unevenness of the floor will typically progress into an obvious
heave and there may be increasing difficulty in using internal doors as
of the lower edge fouling the floor.
However, lifting is generally highest in the centre of the room
resulting in a doming in the floor that is detectable with an
adequately long straight
The doming is commonly accompanied by a radiating or map pattern of
cracking, with some cracks making vertical steps in the floor of the
order of several millimetres.
Externally, the pushing out of walls by the sideways expansion
initially causes horizontal cracks to appear in mortar courses at or
near to DPC level.
With time these typically show a horizontal step in the masonry with
the courses above being thrust out (over-sailing or lipping) relative
the courses below.
There may also be irregular displacement of blocks of the wall adjacent
to the DPC particularly near to corners.
A further indication of sulfate attack may be efflorescence on the
outer face of a wall which has mortar filled cavities below DPC level.
Many load bearing interior walls will be carried down to their own
foundations and behave similarly to external walls. In some cases,
partition walls will have been constructed using a floor slab as bottom
walls may be uplifted by doming of the slab, causing deformation and
diagonal cracking particularly near to door openings.
Door openings and frames
may distort from square, making doors difficult to open or close.
Hopefully, your test
will come back favourable. However, if
you are in
the unfortunate position of having a house with in-fill problems, you
may be required to carry out remedial works in order to sell it.
The type of remedial works required varies from job to job.
What is important is that you must get a reliable and trustworthy
builder who has experience and a very good knowledge of what is
Far too many builders assume they know what is required and are quite
happy to tell you things like they can "save you money by leaving the
kitchen in, and just digging under the slab".
So make sure you check out your prospective builders carefully.
Make sure they are going to remove the kitchen, downstairs bathroom etc
and not just leave them in and excavate under them.
Ask them what they are going to do if the interior supporting walls are
built off the slab. They should be telling you that the walls will be
by other means, and the slab and all underlying material will be
Don’t let them tell you they have tell you that the slab and
material has to be left intact in these areas.
If a builder gives a quote without wanting to see the report or knowing
the depth of fill, alarm bells should start ringing; a builder needs to
know the depth of the fill so he can work out what volumes need to be
Then he can estimate things like how long it will take, how many skips
he will need, how much inert material to put back etc.
Some builders in the past have not taken out all the deleterious
You need to make sure they are going to remove all the in-fill down to
the natural material.
Ask them if they get the local building inspector involved, in order to
ensure compliance with current local building regulations.
I hope that this report has been of some use to you.
I have tried to ensure that it is accurate at the time of writing.
If you have any questions at all, please don’t hesitate to
this on Delicious
Shale Determination Services: Tel 01642
mob. 07989 962610