A Mundic Block Guide

Many properties constructed in the South West are made from concrete blocks laid on mass concrete foundations. This is due to the lack of available raw materials to mould the conventional red clay bricks, therefore other materials had to be constructed from available resources in the area.

Building blocks were therefore produced from waste rock from the local mines, quarries and beach gravel from the coastline. The rocks from the mines were a coarse aggregate with finer aggregates from the beach sand, mining residue and china clay waste.

From the beginning of the 20th century blocks were made from these materials until the 1950’s when mass production of concrete blocks became more mainstream. However the use of the local materials were not completely phased out until the 1960’s.

There were problems with some of the local materials used in the concrete construction and signs of deterioration and mechanical weakening of the building can occur.


Sulphide Minerals – Usually found within mined or quarried rock. They can oxidise under damp conditions, which are common by the coast, producing sulphuric acid. This causes weakness and expansion of the cement, commonly called ‘Mundic Decay’.

Fine Grain Rocks – These are formed by sediments which have settled on the sea floor and are often quite soft. They can change in volume and delaminate when exposed to moisture which causes the fracturing of concrete and cement, named ‘Killas’.

Furnace Residue – Namely Clinker, Coking Breeze and Slag from metal smelters, gasworks and laundries. If the coal has not been burnt enough then it can expand when wet causing cracking of the material.

Reactive Silica – Such as flint found in beach gravel and sand which is found in mass concrete.

Testing for Mundic Block

The building materials must be tested to establish the condition of the dwelling; the test is made up of a two stage analysis and a stage three performance assessment. The present screening test was developed in 1994, and later revised in 1997, by the Royal Institution of Chartered Surveyors (RICS) with input by the Building Research Establishment and identifies major problems of concrete degrading. In 2005 the supplementary stage three expansion testing was added.

Preliminary Screening Test 

The test involves taking several 50 mm diameter drill holes, where a ‘core’ is taken from the external walls, the foundations, any internal walls and chimneys. These are then all examined in laboratory tests to determine the category, defined below:

Class A – Sound concrete satisfactory condition. The examination will identify that the concrete is composed of suitable materials and hence Class A.

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.

After the Preliminary Screening Test, if the concrete samples cannot be placed into ‘A’ or ‘C’, it is recommended that further testing be carried out.

Stage 2

This will identify and classify results that cannot be defined by the above test and determine Classes ‘A/B’ and ‘B’.

Stage 3 

This will assess the performance of the aggregate materials from the core samples previously taken. It can be applied to ‘Class B’ material following the Stage 2 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.

The tests are carried out in laboratory conditions to simulate extreme weathering, so they can determine the level of degradation that the material may be exposed to. 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 are sound and acceptable.

Class B are sound now but containing too much deteriorating material to be regarded as stable.

Class C are unsound and in need of repair.

A vast number of properties have been examined and the results indicate that 80% have passed in Class A 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 or A/B. For Class C materials it is then recommended that examination should be made by a Structural or Civil Engineer.