A guide to Mundic Block

What is "Mundic"?

The word mundic is used to describe a cause of deterioration in concrete due to the decomposition of mineral constituents within the aggregate. Many properties in the South West of England are built using concrete blocks produced from waste rock worked from mining, quarrying and free supplies of beach gravel. The mine waste rock was a coarse aggregate with fine mix aggregates produced from beach sand, china clay waste or mine processing residue.

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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.

 A printable guide to Mundic Block

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.

Cornwall Council do not provide a testing service.  However, there are a number of consultants who provide examination and testing services within Cornwall. Search online for 'mundic testing in Cornwall' or look in the telephone directory.

The current 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 making 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 the chimney.

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

In some cases, after the Preliminary Stage 1 when concrete cannot be placed into 'A' or 'C', it is recommended that further testing is carried out.

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 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'.


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 appearing sound could cause potential problems.
  • Class C are 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) – The development of a Stage 3 Expansion Testing that was formerly designated as Class B, would have formerly been classified as Class A/B but will now be designated as A3.
  • Class B (unchanged) – Concrete contains more than 30% possible problem aggregates although appearing sound could cause potential problems.
  • Class C (unchanged) – Those that are found to be clearly unsound from examination.

A large number of properties have been examined and the results indicate some 80% 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 regraded Class A/B (A2 from September 2015). A small number will also be regraded to Class A/B (A3 after September 2015) after having undergone expansion testing).

With Class C materials it is recommended that examination be made by a Structural or Civil Engineer.

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

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