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Do you need a
Building or Structural Survey in Devon, Cornwall or West Somerset?
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Liam
Bunclark
Chartered
Surveyor
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Work
carried out includes
Home
Buyer Survey
Building Valuations
Major Defects Report
Building Surveys
Onsite Acquisition Advice
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Buildings Replacement Insurance Valuation
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Property
Advice Pages
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PROPERTY ADVICE
PAGES
Mundic Block
Concrete Screening
for Construction Materials
Introduction
Many properties in the South West of England are constructed
from concrete blocks laid onto mass concrete foundations.
The main reasons for the use of concrete blocks in this area is the
non-availability of suitable raw materials to form and mould conventional
red clay bricks. Blocks were produced from waste rock worked from mining,
quarrying and free supplies of beach gravel.
The mine waste rock was of a coarse aggregate with fine mix aggregates
produced from beach sand, china clay waste or mine processing residue.
The production of the blocks using these materials took place from the
turn of the twentieth century until the 1950's when mass production
of widespread concrete blocks became common.
This did not
totally eradicate the use of local materials in block and foundation
construction until the early 1960.s.
1 Problems
Several types of these local materials used as aggregates in concrete
construction can cause deterioration and mechanical weakening of
the building form.
Types
(1) Sulphide Minerals often found in mine or quarried rock.
These can oxidise under damp atmospheric conditions with the
production of sulphuric acid.
This attacks the cement causing weakness and expansion.
Commonly called . Mundic Decay .
(2) Fine Grained Rocks these are formed by sediments laid down
on the floor of oceans and can be quite soft.
They can change volume and delaminate under attack from
moisture fracturing the cement of the concrete.
This effect is called . Killas ..
(3) Furnace Residue Clinker, Coking Breeze and Slag from metal
smelters, gasworks and laundries. This can cause unburnt coal
which can expand when wet causing cracking.
(4) Reactive Silica such as flint found in beach gravel generally
found in mass concrete.
Other problems also can cause deterioration such as lack of cement
etc.
2 Testing of Concrete
To establish the condition of the building materials within the dwelling
suitable testing is required.
The present screening
test, was developed by the Royal Institution of Chartered Surveyors
with input by the Building Research Establishment in 1994 and revised
in 1997 and identifies major problems of concrete degrading.
The test consists of a two stage analysis.
Preliminary Screening Test
The screening test involves taking a number of 50mm diameter drill
holes from the external walls, samples of foundations and where
accessible internal walls and chimney.
These are examined in a laboratory and determine the category as
below.
Class A Sound concrete satisfactory condition.
Class A/B Concrete considered sound subject to adequate
protection and maintenance.
Class B Concrete contains more than 30% possible problem
aggregates although appearing sound could cause
potential problems.
Class C Those are found to be clearly unsound from examination.
The examination will identify that the concrete is composed of
suitable materials and hence Class A.
In dubious cases, after the Preliminary Stage 1 when concrete cannot
be placed into .A. or .C., it is recommended that further testing be
carried out.
Stage Two
The examination will identify and classify results that cannot be
defined by the above test and determine Classes .A/B. and .B.
3 Results
Examination and classification results in that:-
Class A and A/B. sound and acceptable.
Class B sound now but containing too much deteriorating material to
be regarded as stable.
Class C unsound and repair needed.
A large number
of properties have been examined and the results indicate some 80% have
passed in Class A at the preliminary screening stage, about 5% have
gone to Class C.
The remaining 15% have undergone Stage Two examination and many have
been re-graded Class A or A/B.
Class C materials it is recommended that examination be made by a Structural
or Civil Engineer.
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