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A guide to Mundic block

What is "Mundic"?

The word mundic describes the deterioration of concrete. This is due to the decomposition of mineral constituents within the aggregate. Many properties built in the South West of England used concrete blocks made of waste rock from mining, quarrying and beach gravel. The waste rock was combined with beach sand, china clay waste or mine processing residue.

The production of blocks using these materials took place from the turn of the twentieth century until the 1950s. Mass production of concrete blocks became common after this time. This did not completely eradicate the use of local materials in blocks and foundations until the early 1960s.

Types of materials used

Sulphide Minerals

Often found in mined or quarried rock. These can oxidise under damp atmospheric conditions with the production of sulphuric acid. This attacks the cement causing weakness and expansion - 'Mundic Decay'.

Fine Grained Rocks

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 - known as '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.

Who can carry out testing for mundic block?

Cornwall Council do not provide a testing service. There are many consultants who provide examination and testing services within Cornwall. Search online for 'mundic testing in Cornwall' or look in the telephone directory.

Testing for mundic block

The test consists of a two-stage analysis and a performance assessment.

Preliminary Screening Test

50mm diameter drill holes, a “core”, are taken from the external walls and foundations. Cores will be taken from internal walls and the chimney if they are accessible.

The examination will identify if the concrete is made up of suitable materials and hence Class A.

In some cases, after the Preliminary Stage, where concrete cannot be placed into 'A' or 'C', further testing is needed.

Stage 2

The stage 2 examination will identify and classify results that cannot be defined by the above test and determine Class 'B'.

Stage 3

The stage 3 examination will assess the performance of the aggregate material with the core samples taken.

Applied to 'Class B' material following the stage two investigation. Only in cases where the surveyor and the petrographer, are satisfied that the property's structural condition and examined core material does not show visible deterioration.

Testing carried out in laboratory conditions simulates extreme weathering.

Successful results are re-classified as 'Class A/B'.


Examination and classification results in that:-

Up to September 2015

  • 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 this appears sound it could cause potential problems
  • Class C - Concrete is unsound and repair needed.

After September 2015

  • Class A1 (formerly Class A) - Sound concrete satisfactory condition.
  • Class A2 (formerly Class A/B) – Concrete considered sound subject to adequate protection and maintenance.
  • Class A3 (new classification) – This would formally have been Class A/B . Likely to have undergone the Stage 3 Expansion Testing.
  • Class B (unchanged) – Concrete contains more than 30% possible problem aggregates. It appears sound but could cause potential problems.
  • Class C (unchanged) – Those found to be unsound from examination.
    Results show up to 80% of properties have passed in Class A (A1 after September 2015) at the preliminary screening stage. About 5% have gone to Class C. The remaining 15% have undergone Stage 2 examination and many have been re-graded Class A/B (A2 from September 2015). A small number have also been re-graded to Class A/B (A3 after September 2015) after having undergone expansion testing).

With Class C materials examinations should be made by a Structural or Civil Engineer.

  • Class A1 - A3 are mortgageable (subject to lender)
  • Class B and C are un-mortgageable

The information on this page has been developed in to a bite size guide:

Download our Mundic block bite size guide

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