A guide to Mundic Block
Last updated: 13/09/2013
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Concrete screening for construction materials
Many properties in the South West of England are constructed
from concrete blocks laid onto mass concrete foundations. The main
reason 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 1950s 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 1960s.
Some of these local materials used as aggregates in concrete
construction can cause deterioration and mechanical weakening of
the building form. Lack of cement can cause deterioration too.
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'.
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'.
Clinker, Coking Breeze and Slag from metal smelters, gasworks
and laundries. If the coal has not been adequately burnt it can
expand when wet causing cracking.
Such as flint found in beach gravel generally found in mass
Testing for mundic block
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. Supplementary stage
three expansion testing was added in 2005.
The test consists of a two-stage analysis and a stage three
Preliminary Screening Test
The screening test involves taking a number of 50 mm diameter
drill holes where a “core” is taken from the external walls,
samples of foundations and, where accessible, internal walls and
These are examined in a laboratory and determine the category as
- Class A - Sound concrete satisfactory
- 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
- 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.
The examination will identify and classify results that cannot
be defined by the above test and determine Classes 'A/B' and
The examination will assess the performance of the aggregate
material with the core samples previously taken.
It can be applied to 'Class B' material following the stage two
investigation when, in the opinion of the surveyor and the
petrographer, they are satisfied that the property's structural
condition and examined core material do not indicate visible
Tests are carried out in laboratory conditions to simulate
Current statistics indicate that 75% of properties prove
successful when subjected to this examination.
Successful results are reclassified as 'Class A/B'.
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 regraded Class A or A/B.
Class C materials it is recommended that examination be made by
a Structural or Civil Engineer.
The Council does not provide a testing service. You will
see that some links have been provided to some consultants who
undertake examinations and provide test reports, these links will
open in a new page. This is not a comprehensive list.
The information concerning local consultants is provided to help
and assist you. The Council accepts no liability for any works or
reports undertaken or provided by any of these firms.