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A guide to Mundic Block

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.

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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.

Problems

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.

Types

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'.

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'.

Furnace Residue

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.

Reactive Silica

Such as flint found in beach gravel generally found in mass concrete.

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 performance assessment.

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 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 2

The examination will identify and classify results that cannot be defined by the above test and determine Classes 'A/B' and 'B'.

Stage 3

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 deterioration.

Tests are carried out in laboratory conditions to simulate extreme weathering.

Current statistics indicate that 75% of properties prove successful when subjected to this examination.

Successful results are reclassified as 'Class A/B'.

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 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.

Private Sector Housing,
Council Offices,
Luxstowe House,
Liskeard,
Cornwall,
PL14 3DZ

0300 1234 151