A source of information on all aspects of mundic testing in Cornwall and parts of Devon.
Since the decline in use of natural stone at the beginning of the twentieth century most domestic and small commercial properties in Cornwall and South Devon have been built with concrete blocks or mass concrete. Until the mid-1950’s concrete blocks were often locally made, sometimes by individual builders, and shuttered concrete was mixed on site.
Before the second world war concrete products were rarely transported more than 20 km. Because concrete was made where it was needed and as transport was difficult and costly, aggregates were sought locally. Now all blocks are factory produced and mass concrete is supplied ready-mixed from facilities where manufacture is strictly controlled.
In many parts of Cornwall and South Devon ample supplies of cheap and often suitably graded aggregate were available as waste materials from the region’s metalliferous mining industry. The use of mining and, more particularly, ore processing wastes as aggregates, is central to the problem of accelerated concrete degradation in the region. It is widely called “the mundic block problem”. Mundic is an old Cornish word for the common sulphide mineral pyrite.
House near Camborne. Irregular branching cracks (many recently repaired) in render overlying seriously degraded blockwork. The aggregate is a sulphide-rich copper-arsenic mining waste typical of the northern part of the Camborne – Redruth mineralised district.
Concrete core and chisel samples are received from the surveyor for a Stage 1 concrete screening test. The samples are laid out in numerical order for logging into the database.
Weighing & Measuring
Cores in process of being weighed and measured.
Close up of core examined under microscope.
Finished test showing final report sent back to surveyors.
Accelerated deterioration is generally associated with the in situ oxidation of pyrite (and other sulphide minerals) in mine waste aggregates and sulphuric acid attack on the cement. Deterioration is occasionally so severe that concrete becomes structurally unsafe and some properties have had to be demolished. In 1994 the Royal Institute of Chartered Surveyors (RICS) published guidelines for the testing of concrete block and shuttered mass concrete from properties built prior to the mid-1950’s. This original guidance note has since been up-dated and amended.
The testing regime so established has been designed to classify the concrete, primarily to satisfy lending institutions that a property is fit to mortgage on normal terms. Much more detailed information on mundic concrete degradation processes, aggregate identification and the distribution of aggregate types in the Southwest of England can be viewed by downloading the following 136 page document (warning large file 9.7Mb, right-click and choose Save As).
The image below shows the progressive degradation of concrete made with lead ore processing waste from Wheal Mary Ann Mine, near Liskeard.
The accepted main mechanism of degradation involves the oxidation of sulphide minerals, mainly pyrite, in mine waste and other mineralised aggregates. Three main mechanisms for pyrite oxidation have been recognised.
1. Oxidation by a sequence of chemical reactions.
2. Bacteriological oxidation.
3. Electrochemical oxidation.
It has been suggested that surface area, availability of water, temperature, pH, oxygen concentration and the presence of certain trace elements may influence the rate of pyrite oxidation.
You can download a document describing the processes of degradation in much more detail by clicking here. (The document is an extract from A compendium of concrete aggregates used in South West England by A.V. Bromley, © Petrolab 2002.)
The principal purpose of the staged examination procedure (Stage 1, Stage 2, Stage 3) is to classify the concrete material into one of the defined classes.
The principal purpose of the staged examination procedure (Stage 1, Stage 2, Stage 3) is to classify the concrete material into one of the classes defined in the RICS Guidance Note. This classification requires: a) identification of the aggregate type, and b) assessment of the condition of the concrete. Both coarse and fine aggregates (sand fraction) are grouped according to the principal rock or material aggregate types identified. Examples of the aggregate types in each group are shown below.
GROUP 1: Considered Stable
China clay waste
Quartz-rich waste is an important by-product of china clay extraction from the St Austell and Dartmoor granite plutons. At present it is the most widely used aggregate for concrete block manufacture in the region. In the past china clay wastes from the St Just area of the Land’s End, the Tregonning – Godolphin and Bodmin Moor granites were also used for concrete block manufacture.
Because the china clay wastes of Southwest England are all produced from granites of restricted composition, by very similar extraction and processing methods, they are mineralogically very similar.
The major component is glassy grey quartz (qz). Minor components include tourmaline or schorl (to), partly kaolinised and sericitised alkali feldspar (fs), muscovite, pale brown lithium mica. Topaz and fluorite are common in china clay wastes from topaz granite which makes up part of the western lobe of the St Austell pluton.
All-in china clay waste aggregates are generally graded between < 100 mm and 5 mm – 10 mm. All minerals are strongly liberated. Quartz is characteristically equidimensional with rough, pitted surfaces.
China clay waste is normally regarded as a stable and durable aggregate. Rare instances of defective concrete made with china clay waste are usually explained in terms of stale cement, inadequate cement content, prolonged poor maintenance or chemical attack from flue gases or acid groundwater.
Coarse china clay waste is widely used in structural concrete at the present time. It was formerly used in mass concrete walls of domestic properties in a restricted area west of the St Austell granite, notably Indian Queens and St Columb Road. This aggregate is made up from composite fragments including tourmalinised and kaolinised granite, quartz – tourmaline veinstones, quartz – feldspar porphyry and rhyolite.
Coarse grained granite
Specimen location: Penryn
The main components of the aggregate are slightly kaolinised and sericitised alkali feldspar and plagioclase and quartz, with subordinate amounts of muscovite, biotite, chlorite and tourmaline. Small amounts of vein quartz, quartz – tourmaline and quartz – haematite veinstones also occur occasionally.
This aggregate is from the southern part of the Carnmenellis granite. It consists of a mixture of unweathered “blue” granite and slightly weathered “brown” granite. The patchy pervasive limonitisation is a result of natural weathering. It does not indicate in situ alteration of the aggregate.
Coarse-grained granite aggregates from the Carnmenellis pluton are found in concrete blocks and mass concrete in Falmouth, Penryn and surrounding villages and less commonly in the Camborne – Redruth area. In mass concrete they are sometimes found in combination with beach or river sand or even mine waste fine aggregates.
Granite aggregates from the St Austell and Bodmin Moor granites are found occasionally in blockwork in the St Austell, St Blazey and Bodmin areas. Dartmoor granite aggregates were used extensively in South Devon.
Granite aggregates are stable. There are no records of concrete degradation associated with their use.
Fine grained granite
Specimen location: Liskeard
Fine-grained, two-mica granites were used extensively as concrete aggregates in East Cornwall. They came mainly from the Hingston Down granite stock, near Gunnislake, though it is possible that some aggregates were produced from fine-grained granites in the Bodmin Moor pluton. The granite is made up from feldspars and quartz with subordinate biotite, muscovite, chlorite and tourmaline. Some granite from the Hingston Down stock carries sulphide minerals, though usually in very low concentrations.In the near surface zone the granite is often pervasively oxidised as a result of natural weathering and many fragments are strongly impregnated with secondary brown limonite. The cement in this concrete has a strong brown colour but this is not the result of in situ oxidation of the aggregate. It is caused by sliming of limonite dust from weathered aggregate during handling or mixing of the concrete. In spite of its appearance the concrete is sound. No problems have been reported with the use of this aggregate.
Crushed basic and metabolic igneous rocks (e.g.epiodiorite, serpentinite)
Metamorphosed dolerite and gabbro
Specimen location: Falmouth
This aggregate is from the Crousa Downs tectonic unit of the Lizard ophiolite complex. The main source was probably the former West of England quarry at Porthoustock. The quarry is developed in the root zone of the sheeted dyke complex and the product includes gabbros and various fine to medium-grained dolerite dyke rocks. White plagiogranite, amphibolite, granulite and mafic mylonite from high strain zones occur in minor amounts.
The major mineralogical components are plagioclase feldspar (+ saussuritised feldspar), diopside and hornblende. Ilmenite, sphene, chlorite and carbonates occur in minor amounts. Finely comminuted ferromagnesian silicates (diopside and hornblende) give the cement a characteristic greenish colour.
The gabbro and dolerite commonly carry small quantities of sulphide minerals including pyrite, pyrrhotite, chalcopyrite, pentlandite and niccolite though concentrations are usually much less than 0.1%.
This aggregate was used exclusively in Falmouth and Penryn and nearby villages. It is usually found alone as an all-in aggregate in concrete blocks, with a maximum size of 5 mm or 10 mm. Less commonly, it is blended with china clay waste or granite.
There are no known problems associated with the use of the Lizard aggregate. Occasionally, however, it was blended with sulphide-bearing tin gravel from the Carnon Valley in concrete that has suffered degradation as a result of in situ sulphide oxidation.
Specimen location: Newlyn
Penlee or Gwavas Quarry, near Newlyn, was developed in a large, lensoid intrusion of fine to medium-grained dolerite. The Penlee intrusion lies wholly within the contact metamorphic aureole of the later Land’s End granite. As a result of metamorphism the dolerite was converted to a fine to medium-grained plagioclase feldspar + green hornblende + sphene ± biotite assemblage with characteristic decussate texture. The dolerite was also pervasively mineralised with a complex suite of sulphides including pyrite, chalcopyrite, pyrrhotite, stannite and molybdenite. Total sulphide mineral content is commonly between 1% and 2%.
The Penlee aggregate was widely used in blockwork in Penzance and Newlyn and surrounding villages and occasionally in Marazion, St Ives, Carbis Bay and Hayle. Its use in structural concrete was more widespread and it is found in bridges, for example, all over West Cornwall.
This specimen is from blockwork in a house at Newlyn. The aggregate consists mainly of fine and medium-grained metadolerite (fmd, mmd). Vein quartz (qz), various veinstones and biotite hornfels, from above the intrusion, are found in minor amounts. Sulphide minerals, mainly pyrite (py), chalcopyrite and pyrrhotite, occur as disseminated crystals and stringers in metadolerite and as liberated grains in the cement matrix. Note that some aggregate fragments show slight pervasive limonitisation and have incomplete limonitic crusts (lm) that merge into narrow haloes of oxide-impregnated cement.
The performance of this aggregate is not well-understood. It is often strongly oxidised but its condition usually owes more to stockpile weathering than in situ alteration. There are no well-documented cases of general degradation in domestic properties. An industrial building, constructed in 1941 with Penlee blockwork and poorly-maintained, shows classic symptoms of general accelerated degradation, included rectilinear cracking in the overlying mortar render, detachment of render and gypsum exuding from the inner exposed surface of the concrete.
Current RICS Guidelines advise that concrete made with this aggregate may be assigned to Class A if it has less than 1.5% equivalent pyrite and shows no obvious evidence of aggregate-related degradation.
Specimen location: Gunnislake
The specimen is from blockwork in Gunnislake though the aggregate is probably from South Devon. It is quarried dolerite. As usual, the primary assemblage of clinopyroxene + plagioclase feldspar + olivine is partly replaced by saussuritised plagioclase + green hornblende + chlorite + leucoxene.
This aggregate is made up from a mixture of medium-grained dolerite, with relict ophitic texture (mgd) and fine-grained aphyric and plagioclase-phyric rocks (fgd).
There are also small amounts of vein quartz and calcite and chloritic veinstones. The aggregates commonly carry traces of pyrite and other sulphide minerals but always in quantities that are too small to give cause for concern. Pervasive limonitisation (lm) of some fragments is a result of natural weathering. It does not indicate in situ oxidation of the aggregate.
Finely comminuted ferromagnesian minerals give the cement a characteristic greenish colour associated with many dolerite aggregates.
This material is usually found as an all-in aggregate, with maximum size of 5 mm to 10 mm, in concrete blocks. Occasionally, it is blended with china clay waste, limestone or furnace clinker.
No problems have been reported with this material.
Partly serpentinised picrite
Specimen location: Polperro
The former Clicker Tor Quarry, Liskeard was developed in a serpentinised cumulate picrite. The primary assemblage is cumulus olivine + intercumulus clinopyroxene + abundant granular magnetite. It is moderately to completely altered with the primary minerals replaced by serpentine + tremolite + stilpnomelane. There are also occasional fragments of serpentine + carbonate veinstones. The aggregate is easily recognised by its dark green colour and the distinctive mottled texture of large composite fragments. The aggregate was used extensively in the Liskeard – Looe area of East Cornwall.
The picrite was used as all-in aggregate concrete blocks and as coarse aggregate in mass concrete. Occasionally, it was blended with lead ore processing waste (Wheal Mary Anne) to make concrete blocks.
The aggregate is normally regarded as stable and there are no known problems associated with the use of the material in concrete blocks. There are rare instances of degradation of mass concrete. Degradation is possibly the result of in situ aggregate expansion caused by hydration of residual olivine. Olivine is a high temperature mineral that alters readily to secondary serpentine as a result of hydrothermal processes and weathering, though normally this takes place over geological time scales.
Olivine is unstable in strongly alkaline solutions at slightly elevated temperatures. It is possible that a combination of high alkali cement and local heating during setting provided conditions under which hydration of olivine could occur. Alteration of even a small of part of the olivine would be enough to cause expansion and structural weakening of the concrete and this may have been exacerbated by the crystallisation of secondary hydrated magnesium silicates in voids. The picrite was formerly used in the region as coarse aggregate in high strength concrete, notably in railway bridges. No evidence has been found which suggests it is unstable in these structures.
Furnace clinker or coking breeze
Specimen location: Falmouth
Furnace clinkers are widely used as concrete aggregates throughout the region. Ample supplies were available from steam-raising furnaces at former mines, from coking plants, small coal-fired power stations and other industrial sources.
This specimen is from blockwork in a house at Falmouth.
The aggregate consists of the usual mixture of hyaline and hypohyaline (silicate + oxide + glass) clinker (hvc), carbonaceous-vesicular (cvc) and laminated clinker (lc) and incompletely burned coal (pbc). Replacement and rimming by red iron oxides is the result of processes related to combustion. It is not evidence of in situ oxidation. The abundant fine clinker debris that gives the cement in many clinker concretes a characteristically “dirty” appearance, is a result of fragmentation of fragile carbonaceous-vesicular clinker during handling or mixing of the concrete.
Clinker aggregates normally perform well. Problems are sometimes caused by the use of clinkers that contain high proportions of unburned coal, lime sulphide minerals such as marcasite or if the coal was supplemented by inhomogeneous waste materials.
Sintered pulverised fuel ash
Specimen location: St Ives
This example is probably Lytag, manufactured by sintering pulverised fuel ash. It is readily identified by the spherical and subspherical shapes of the aggregate fragments and their high vesicularity. The zonal structure with reddish brown, oxidised rims and almost black cores is characteristic.
The aggregate is found in low fines, lightweight insulation blocks, usually in internal walls. There are no known problems associated with its use but its high porosity suggests it may be susceptible to degradation in prolonged wet regimes.
Furnace clinker¦china clay waste (Groups 1-4¦1-1)
Specimen location: Hayle
Furnace clinker and copper smelter slag are common aggregates in the Hayle area. The sources are a former coal-fired power station and important copper smelting works that operated during the eighteenth and nineteenth centuries. This specimen is from mass concrete foundations. The aggregates are furnace clinker and china clay waste in approximately equal proportions.
The china clay waste consists principally of liberated quartz (qz) with subordinate amounts of tourmaline, kaolinised feldspar (fs) and micas. The clinker aggregate consists of hyaline and hypohyaline-vesicular (hvc), carbonaceous vesicular clinker and small amounts of partly-burned coal. The greyish brown appearance of the cement is caused by fine debris removed from fragile vesicular clinker fragments during handling and mixing the concrete.
The aggregates are well-graded and the concrete is sound though clinker-rich concretes often perform badly under wet conditions in footings and foundations. This is partly because of enhanced permeability due to abundant vesicular clinker and the tendency of partly burned coal to swell and crack, especially under conditions of cyclic wetting and drying.
Specimen location: Crantock
A wide variety of beach and estuarine gravels were used as concrete aggregates along the north coast of Cornwall. This material is probably from the Gannel Estuary, south of Newquay.
The aggregate is made up from unweathered and weathered grey and brown mudstones (md), fine-grained sandstones (sdst) and subordinate vein quartz (qz). Stable silicate minerals and calcareous shell fragments are important in the finer size fractions.
In this concrete the absence of iron oxide impregnation haloes round altered aggregate fragments indicates that their condition is the result of natural weathering before the concrete was made. Normally, these gravel aggregates are considered safe but there are instances of severely degraded concrete made with this type of material. The degradation mechanism is associated with in situ oxidation of very fine-grained disseminated pyrite in some mudstone pebbles. The sulphide mineral has sometimes survived unaltered through the weathering cycle. In some poorly-maintained concrete it has oxidised in place, causing expansion of mudstone pebbles and severe degradation of the concrete.
If there is any evidence of degradation in concrete containing these materials, they should be carefully examined in polished section to establish if they carry disseminated pyrite.
Specimen location: Helston
The gravel beaches which extend along Mount’s Bay from Porthleven to Gunwalloe Church Cove were widely exploited as sources of aggregate for concrete blocks and mass concrete. The gravel aggregate is found mainly in towns and villages bordering Mount’s Bay including Marazion, Porthleven and Helston.
This well-rounded aggregate is lithologically complex and its composition varies slightly in different localities. The major components are yellowish and reddish brown chert (ch), from offshore Eocene gravels, white vein quartz (qz) and fine-grained mudstones (md) and siltstones of local origin. Quartz-dominated veinstones (vs), altered fine-grained basic igneous rocks and calcareous shell fragments are other important components.
The gravel is usually found as an all-in aggregate with maximum size of 5 mm or 10 mm, though much coarser-grained material is sometimes used in mass concrete. Occasionally, in inland areas such as Breage and Porkellis, it is blended with mining waste or furnace clinker.
The aggregate usually performs very well though, because it often has low fines content, concrete made with this material may have very high interconnected porosity. Under wet conditions, in some footings and foundations, such concrete has degraded because of prolonged recrystallization and leaching of the cement.
Because the major components of the aggregate (chert, quartz) are very hard it is difficult to recover intact diamond drill cores, even from perfectly sound concrete.
Specimen location: Looe
Beach gravels, derived mainly from Lower Devonian mudstones and fine sandstones, were widely used as aggregate in blockwork and mass concrete in East and West Looe, Polperro and neighbouring villages.
The distinctive major components are purple mudstones (pmd) and green mudstones (gmd) from the Dartmouth Beds and vein quartz (qz). Grey mudstones, fine-grained grey sandstone, granite, altered dolerite, calcareous shell fragments, stable silicate minerals and brown iron oxides occur in minor amounts in the fine size fractions.
Aggregate used in blockwork was screened and usually has a maximum size of 10 mm to 15 mm. Poorly-graded all-in aggregate, with maximum size of 100 mm to 200 mm is commonly found in mass concrete foundations.
There are no records of aggregate-related degradation associated with this material. However, some mass concrete and blocks were made with very low cement content and have suffered local degradation as a result of strong recrystallisation and leaching of the cement.
China clay waste river/estuarine gravel
Specimen location: Truro
Formerly, much china clay waste was discharged into streams and rivers draining from the strongly kaolinised St Austell granite. These “white rivers” became badly silted in their lower courses and needed constant dredging. China clay waste-dominated sands and fine gravels from dredging operations in Charlestown Harbour and at Ruan Lanihorne were used for concrete block manufacture.
The aggregates are dominated by liberated grains of grey, glassy, granitic quartz (qz) with subordinate tourmaline (to) and other stable silicate minerals. There are usually a few pelitic rock pebbles and fragments of vesicular furnace clinker. These materials are never found in unmodified china clay wastes. China clay-contaminated sand and gravel aggregates from Charlestown Harbour are common in blockwork in St Austell, St Blazey and Truro. Material from the river at Ruan Lanihorne was transported to Penryn by barge. It is common as aggregate in blockwork in the Falmouth area.
The aggregate is stable and there are no problems associated with its use. It is usually mistaken for china clay waste because of its composition. Slight rounding and the presence of occasional rounded pelitic rock pebbles and furnace clinker should serve to identify this aggregate.
Note the irregularly-shaped masses of neat cement (ncb) suggesting the binder might have been slightly stale when used.
Contaminated river gravel
Specimen location: Truro
This problematical gravel aggregate is probably from the Carnon Valley though it could be from another river system that was contaminated by hard rock mining waste.
It consists of a mixture of rounded pebbles and angular fragments and it is characterized by extreme lithological complexity.
The components include fine-grained pelitic hornfels (ph), vein quartz (qz), quartz – chlorite veinstones (qcv), quartz – tourmaline veinstones (qtv) and chloritised coarse-grained granite (cg) and greisen. Minor components in the finer size ranges include stable silicate minerals and iron oxides. Note that this specimen also contains a fragment of wood and some hyaline laminated clinker (hlc).
Some fragments shows pervasive haematisation (he) or limonitisation (lm) but the absence of limonitic crusts and haloes of iron oxide-impregnated cement suggests alteration occurred in response to natural low temperature hydrothermal processes or weathering. This aggregate is sulphide-free and there is no evidence that it has undergone any significant in situ alteration.
Mine waste-contaminated gravels that are usually by-products of alluvial tin extraction are generally found as aggregates in Falmouth, Penryn and the Carnon Valley itself. Occasionally they occur in Truro, both in blockwork and mass concrete. The difficulty in assessing these materials lies in their variable lithologies and sulphide contents. Some aggregates, like this one, are safe. Although they are very complex and inhomogeneous they have negligible sulphide content. Others are potentially deleterious (Group 1-5/2-2) or have caused degradation because they carry abundant sulphides in mine tailings contaminants.
Contaminated river gravel (Group 1-5/2-2)
Specimen location: St Blazey
The specimen is from a concrete block. The aggregate is a river gravel. It is probably a by-product of alluvial tin extraction from a stream draining the southeastern part of the St Austell granite and its metamorphic aureole.
The aggregate consists of rounded to sub-angular pebbles of fine-grained pelitic hornfels (ph), fine-grained quartz – tourmaline and quartz – chlorite veinstones and vein quartz (qz). Stable silicate minerals make up part of the finer size fractions.
Most of the aggregate fragments have crusts or patinas of secondary iron oxides and this is responsible for the dark brown colour of the cement. Note that the neat cement ball (ncb) is not coloured by iron oxides. Much oxidation is ascribed to natural weathering though some veinstone fragments carry relict pyrite and chalcopyrite that has survived weathering. The sulphides are generally too fine grained and corroded to be easily recognised under a low power stereomicroscope.
Petrographic study showed that areas of white cement contained abundant secondary gypsum indicating that sulphide minerals in the aggregate have oxidised in place and attacked the binder. The presence of residual sulphides suggests potential for further degradation. The aggregate, though a river gravel, should be assigned to Group 2 on lithological grounds. The concrete in the specimen is a Class B material.
Crushed slate from the great quarry at Delabole has been widely used as a concrete block aggregate. It is found mainly in North Cornwall, between Wadebridge and Camelford, though occasionally it was used in West Cornwall, for example at St Agnes.
The aggregate is easily recognized by its distinctive greenish grey colour, uniform lithology and strong penetrative cleavage. The slate is often weakly mineralised. It often has traces of pyrite and sometimes chalcopyrite and pale brown, low iron sphalerite that occur in discrete, cleavage-parallel lenses. The sulphide mineral content is normally very low, much less than 0.1%.
It is usually found as an all-in aggregate with a maximum size of 5 mm or 10 mm. Fine slate dust imparts a characteristic pale greenish grey colour to the cement.
Delabole roofing slates are very durable and even in polluted atmospheres they may last for more than one hundred years. The material is stable as a concrete aggregate. There are no known instances of degradation associated with its use.
Specimen location: Plymouth
Devonian limestone aggregate, from quarries in South Devon, is found in concrete blocks in East Cornwall and in the Plymouth area.
It is used alone as an all-in aggregate and it is also found blended with china clay waste, dolerite, various gravels or furnace clinker.
The main components are fine-grained light and dark grey recrystallised limestone. Calcite (ca) and calcite – haematite veinstones (chv) occur in minor amounts.
Concrete made with this aggregate commonly has distinctive pink or pinkish brown cement caused by sliming of earthy red haematite from the veinstones.
There are no known problems associated with the use of this material.
Note the fine-grained dolerite aggregate used in the mortar render.
Specimen location: Praa Sands
The aggregate in this concrete block is a granite-hosted ore processing waste. It is probably a heavy medium separation (HMS) reject from the former South Crofty Mine. However, the widespread occurrence of similar materials throughout the County suggests there may have been other sources of similar aggregate (there was definitely a block-making plant near St Ives that used this type of material, probably from Wheal Reeth).
The main components of the aggregate are tourmalinised and haematised granite (thg), quartz – tourmaline (qtv) and quartz – haematite (qhv) veinstones and quartz (qz). Stable silicate minerals and iron oxides occur in minor amounts. Sulphide minerals are absent or present only in trace amounts.
There are no known problems associated with the use of this type of aggregate. Though it is clearly an ore processing waste, it is assigned to RICS Group 1-6.
This concrete is made with tin mining waste from the metamorphic aureole overlying the western flank of the St Austell granite. The host rocks include finely banded quartz – tourmaline hornfels (qth), quartz – tourmaline veinstones (qtv) and vein quartz. Granite and quartz – haematite veinstones (qht) occur in minor amounts. The mineralisation is characterised by extremely low sulphide mineral concentrations. Sulphides are very rarely found in the aggregate. Because the principal components are made from resistant tourmaline and quartz the aggregate is very stable. There are no recorded instances of aggregate-related degradation and for this reason it is classified as a Group 1-6 material. Pervasive limonitisation of some veinstone fragments is the result of low temperature hydrothermal alteration or weathering before the concrete was made (note the absence of an iron oxide impregnation halo round the large limonitised quartz – tourmaline veinstone fragment).
Sometimes this aggregate is found in poorly-constructed mass concrete walls that show evidence of degradation related to cement recrystallisation and leaching. Chimney flues were often cast in the mass concrete and flue gas related degradation is very common in areas surrounding chimneys.
Specimen location: Tywardreath
Natural highly vesicular, hyaline pumice has been imported into the United Kingdom as an aggregate for use in low strength thermal insulation blocks. It is probably from the Lipari Islands.
Concrete made with this aggregate is found occasionally throughout the region in non-loading bearing internal walls.
The aggregate is easily recognised by its white or pale grey colour, glassy texture and high vesicularity. It is usually found alone as an all-in aggregate with a size range between about 1 mm and 5 mm or 10 mm. Occasionally, it is blended with a small amount of fine, quartz-dominated china clay waste sand, suggesting that some pumice blocks were manufactured locally.
There are no known problems associated with the use of this material when it is used in appropriate circumstances.
GROUP 2: Considered potentially deleterious
Impure chert and black mudstone aggregates from Lower Carbonifeous Meldon Chert and Slate Formations are found exclusively in Launceston. They are found in blockwork and mass concrete. The chert and mudstone were rarely used alone. It was blended with china clay waste, granite, dolerite, furnace clinker and various gravels. It is common to find three or even four aggregates from different sources in the same concrete.
This specimen is from blockwork. The main aggregate, making up about 70% of the total, consists of a mixture of black, impure chert (ch), dark grey mudstone (md) and occasional fragments of weathered, pale grey mudstone with limonite staining. Fragments of milky white vein quartz (vqz) occur in minor amounts.
The second aggregate is probably china clay waste consisting mainly of liberated, grey, glassy quartz grains (qz) with subordinate tourmaline, micas and kaolinised feldspar.
Both chert and mudstone aggregates sometimes contain substantial amounts of very fine-grained disseminated, often framboidal pyrite though this is nearly always too small for recognition with a low power stereomicroscope.
Though this material appears completely sound there have been problems of concrete deterioration associated with this material, especially when it is blended with furnace clinker. Oxidation of disseminated pyrite is presumed to be responsible for degradation.
If there is no evidence of aggregate-related or other degradation, the mudstone/chert aggregate may be re-grouped to Group 1-6 and concrete that appears sound may be assigned to Class A.
Mining and ore processing wastes dominated by pelitic and chlorite-rich hornfels are common aggregates in the Camborne – Redruth area. They are derived from massive tin – copper mining operations along the northern flank of the Carn Brea granite ridge. Such aggregates are responsible for almost all general concrete degradation in Camborne, Redruth and nearby villages such as Portreath and Illogan. Similar aggregates from the St Day mineralised district are found in Truro and Falmouth.
Though all of these aggregates are considered potentially deleterious at the present time there is increasing evidence that some low sulphide, chloritic hornfels-dominated materials are probably very stable.
This aggregate, probably an ore processing waste, is made up from fine-grained pelitic hornfels (ph), fine-grained chlorite-rich hornfels, derived from fine-grained basic igneous rocks (ch), quartz – chlorite veinstones (qcv) and vein quartz (qz). Quartz – tourmaline and quartz – tourmaline – chlorite veinstones, fluorite, stable silicate minerals and iron oxides occur in minor amounts.
Pyrite, chalcopyrite and arsenopyrite are the main sulphide minerals. Secondary copper sulphides, including bornite, chalcocite and covellite are also found. Sulphide concentration varies between traces and about 0.5%, exceptionally they may reach 1%. Sulphide minerals normally occur as locked crystals in veinstones and occasionally as liberated grains. Fine, disseminated pyrite may also occur in pelitic and chloritic hornfels. This specimen has low sulphide mineral content though millimetre size arsenopyrite crystals (asp) occur in some veinstone fragments. In the Camborne – Redruth area this type of mine waste aggregate is found alone in blockwork, usually with a maximum size of 10 mm to 20 mm, and in mass concrete. In mass concrete it is found in walls and foundations, either alone or in combination with furnace clinker, granite or various gravels. Some of the most serious concrete degradation in Camborne is found in mass concrete with very coarse, low fines aggregate of this general type.
Lead ore processing waste
Specimen location: St Newlyn East
This well-known aggregate is found mainly in concrete blocks, especially in Perranporth and Newlyn East. It occurs occasionally in Newquay, Truro and surrounding villages.
The major components are cleaved, dark and pale grey mudstone (md), black pyritic mudstone and vein quartz (qz). The latter often contains abundant limonite (lm).
Minor components include quartz – chlorite (ch) and quartz – sericite veinstones and calcite.
Sulphide minerals include galena, pale brown sphalerite and pyrite in veinstones and as liberated grains. Very fine-grained disseminated pyrite occurs in dark mudstones but usually it is too fine-grained to be seen under a low power stereo- microscope. Residual sulphide mineral concentrations are commonly about 0.2%.
The aggregate is often strongly limonite-encrusted though it is not always clear whether this is a result of in situ oxidation or if it is a consequence of natural weathering before the concrete was made. Iron oxides are usually dispersed through the cement, giving a characteristic brown colour.
The material was generally used as all-in aggregate, graded between about 100 mm and 5 mm or 10 mm, in concrete blocks. Similar material is occasionally found as coarse aggregate in mass concrete footings and foundations, in combination with fine aggregate from the same source, china clay waste or beach gravel.
The aggregate is responsible for most general concrete degradation in the Perranporth area. Deterioration is believed to result from in situ oxidation of disseminated pyrite in mudstone fragments and bulk expansion of the aggregate. Direct sulphate attack on the cement has an insignificant role in degradation. Secondary sulphate minerals are rare.
The source of this deleterious aggregate has been traced to a block making plant located at the former Wheal Mary Anne Mine, near Menheniot. The plant is still recognizable though the site is now used for other industrial purposes.
The condition of concrete made with this aggregate is very variable. Identical blocks in some properties remain completely sound while in others they have degraded to a condition where demolition has been necessary. Concrete made with this aggregate generally has a low sulphide mineral content, typically < 0.5% pyrite equivalent. Most of the pyrite is present as very finely disseminated grains in mudstone wallrocks and is not visible under the stereomicroscope.
The principal components of the Wheal Mary Anne aggregate are cleaved grey mudstones (md), dark grey pyritic mudstones, vein quartz (qz) and colourless and pale yellow fluorite (fl).
Minor components include limonite (lm) chalcedonic quartz (cq), calcite, siderite and barite.
Sulphide minerals are pyrite, galena, minor chalcopyrite and sphalerite (in veinstones), and fine-grained disseminated, commonly framboidal pyrite (in mudstone). Total sulphide mineral concentrations are generally < 0.5%.
The aggregate is angular and graded between approximately 100 mm and 10 mm or 15 mm. It was used mainly as single source all-in aggregate but occasionally it is found blended with about 20% of picrite from the nearby Clicker Tor Quarry or with medium-grained dolerite.
During the 1920s and 1930s a small electricity generating plant in Falmouth used commercial and domestic wastes blended with coal in its furnaces. The furnace clinker was used as concrete aggregate in blockwork and mass concrete in a small area of Falmouth. There are several well-documented cases of severe concrete degradation associated with the use of this material.
The aggregate is usually poorly-graded. In addition to the normal products of coal burning, hypohyaline-vesicular (hvc) and carbonaceous clinker and partly burned coal (pbc), the aggregate contains broken glass and ceramic materials (cer), ferrous and nonferrous scrap and abundant partly calcined sea shells (shell fish were an important component of the local diet). Degradation is believed to be associated with:
(i) expansion of incompletely burned coal
(ii) hydration of lime in clinker
(iii) rehydration, carbonation and expansion of partly calcined sea shells
(iv) expansion and cracking of partly devitrified synthetic glass, and
(v) oxidation and expansion of ferrous scrap.
The incinerator waste aggregate often contains rock and mineral fragments, including granite and veinstones (vs) but it is uncertain if these were deliberately blended or if they are accidental contaminants.
The aggregate is recognised by poor grading and the presence of chalk white, partly calcined sea shell fragments, glass and ceramics and metallic scrap. It is now classified as a Group 2-3 material under RICS Guidance.
Concrete Condition & Class
The condition of the concrete is assessed during each stage
Concrete Condition & Class
The condition of the concrete is assessed during each stage and is based on any evidence of deterioration including, inter alia, sulphide decay, associated staining, matrix alteration, sulphate minerals and physical incoherence. Evidence of any ‘non-mundic’ forms of deterioration are included. At the conclusion of any stage, the condition of the concrete samples is designated as ‘Sound’ or ‘Unsound’ according to the following definitions.
Showing no, or only rare evidence of deterioration and in either case exhibiting properties which are considered unlikely to adversely affect future concrete performance, subject to regular protective maintenance.
Lacking physical coherence and/or showing common or abundant evidence of deterioration, also concrete too deteriorated to be sampled intact.
|Aggregate(s)||Concrete Condition||Concrete Class|
|Group 1 only||Sound||A₁
|Group 1 plus up to 30% of Group 2||Sound||A₂
|Less than 30% of Group 2||Sound||A₂, A₃
|Greater than 30% of Group 2||Sound||B
|Mainly Group 2||Unsound||C1
|Mainly Group 1||Unsound||C2
Class A₁ (previously A)
is for concrete containing only Group 1 aggregates (those considered stable and not likely to cause ‘mundic’ type deterioration). Properties containing Class A₁ concrete are generally considered suitable for mortgage purposes by the majority of mainstream lenders.
Classes A₂ & A₃ (previously Class AB)
Class A₂ is for concrete that currently appears sound but contains up to 30% Group 2 aggregates or for concrete from mass concrete footings containing more than 30% Group 2 aggregate that passes a density test. Class B concrete that passes the Stage 3 test criteria can be reallocated to Class A₃ (see Class B description below). Properties containing Class A₂ or A₃ concrete are generally considered suitable for mortgage purposes by the majority of mainstream lenders, but not all.
is for concrete that currently appears sound but, owing to the percentage of Group 2 aggregates, may retain potential for degradation with possible consequent loss of structural strength and integrity. Properties containing Class B concrete are generally considered unsuitable for mortgage purposes by the majority of mainstream lenders. A Stage 3 test is available for suitable Class B concrete that, if passed, allows the reclassification of the concrete to Class A₃.
is for concrete that is physically incoherent or displays common or abundant evidence of deterioration. Deterioration of the concrete, particularly in the case of concrete comprising mainly Group 1 aggregates, may not be due to ‘mundic’ type deterioration.