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Part C: Assessing the aggressive chemical environment

C1. GeneralThis Part describes the occurrence of chemicals in theground that are potentially harmful to concrete andgives procedures that lead to assessment of theAggressive Chemical Environment for Concrete(ACEC) Class of the ground.

The discussion on the occurrence of aggressivechemicals includes reference to some types in landcontaminated by the activities of man. The scheme forground assessment is, however, restricted to naturalground, ground mildly contaminated by some commonmanmade chemicals and fills derived from both ofthese. Ground containing excessive amounts ofmanmade chemicals (for example resulting in anacidity of less than pH 2.5) or rarely encounteredsubstances are not catered for by the ACECclassification and will require specialist investigationand assessment.

The various stages in the ground assessment anddecisions affecting concrete are set out in Figure A1 ofPart A. The detailed steps involved in groundassessment are shown in Figure C1. The terminologyused in the various boxes of these figures is explainedlater.

BRE Digests prior to SD1:2001 used a primaryclassification of ground into five sulfate classes. Theyalso gave incremental rules for modification of theseprimary classes to account for other factors that affectthe severity of chemical attack, including:• groundwater acidity and mobility• concrete geometry, curing conditions and type of

use.Sometimes, however, these recommendedmodifications were overlooked or incorrectly applied bydesigners and specifiers.

A new approach to classification of aggressive groundconditions was therefore adopted in SD1:2000 that iscontinued with slight modification here. The derived

ACEC class takes direct account of the type of site, thesulfate concentration, and the groundwater acidity andmobility.Factors which are specific to the concrete construction,such as type of element, section thickness, curingconditions, application of hydrostatic pressure and theintended working life, are taken account of separately inParts D to F when specifying concrete quality to meet theassessed ground conditions.

Differing site assessment procedures are given here fornatural ground, for brownfield locations that may containaggressive chemical residues, and pyritic ground. Theprocedure for the latter is specifically included owing torecently found severe TSA in highway sub-structuresembedded in pyrite-bearing Lower Lias Clay fill:generation of sulfate due to oxidation of the pyritefollowing ground disturbance was found to be a majorfactor.

Three key changes have been made to the procedure forground assessment as compared with the previous editionof SD1:• The limits of the Design Sulfate Classes based on 2:1

water/soil extract tests on soil have been reduced,making this classification route more conservative(see Box C7).

• There is no need to take high magnesium levels intoaccount for natural ground – the ‘m’ suffix DesignSulfate Classes now only apply to brownfieldlocations. This is because, in the UK natural groundconditions, magnesium levels are invariably wellbelow values which may significantly affect concrete.

• The concentrations of sulfate, magnesium and otherrelevant chemicals in water and water/soil extracts areexpressed in mg/l instead of g/l.

Minor changes include re-naming the former Highly Mobilegroundwater as Flowing water and catering for presenceof this in the Aggressive Chemical Environment forConcrete classification (Table C1) for some types ofgroundwater.

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Steps of procedure

1. Carry out Desk Study and Walk Over of site to identify type of site, eg Brownfield, and any ground conditions which may be aggressive to concrete

Refer to Special Digest

Sections C4.2 and C4.3

2. Carry out Ground Investigation to determine: a) Groundwater mobility (Static, Mobile, Flowing) b) Concentrations of aggressive chemicals in soil and groundwater, including: - sulfates - sulfides (especially in pyritic ground) - water soluble magnesium - acids (indicators pH, chloride and nitrate ions)

Section C4.5 and C4.6Section C3

Section C5.1

3. Determine Design Sulfate Class for site or site locations

Step 3 of Sections C5.1.1, C5.1.2 and C5.1.3

4. Determine Aggressive Chemical Environment for Concrete (ACEC) Class for the site or site locations from Table C1or C2, taking into account: - Design Sulfate Class, - type of site (Natural ground or Brownfield) - water mobility - pH

Section C5.2 and Tables C1 and C2

8. Proceed to concrete specification in Parts D, E and F of Special Digest

Continue as Stage 3 of Figure A1.

20-11-04Figure C1: Procedure for assessing ground environments that are aggressive to concrete.

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C2. Principal constituents of aggressiveground and groundwater

This Section describes the chemical agents commonlyencountered in natural ground and brownfield locationsthat are aggressive to concrete. It does not discussother types of chemical activity or groundcontamination.

C2.1 Sulfates and sulfidesSulfates commonly occur both in the solid part of theground (soil, rock or fill) and in groundwater. Sulfatescan also be derived by oxidation of sulfides, such aspyrite (FeS2), by natural processes such as weathering,sometimes aided by construction activities. It is,therefore, necessary to consider the distribution ofsulfides, as well as sulfates in ground, which may beaffect buried concrete. It should be noted, however,that sulfides usually provide no hazard to concrete inthe absence of oxygen and mobile water. Box C1 liststhe main sulfur species found in the UK, most of whichare either sulfates or sulfides.

An overview of the role and occurrence of sulfates andsulfides is given here. A more detailed discussion isgiven in Chapter 3 of the Thaumasite Expert GroupReport [1].

C2.1.1 Natural groundIn UK natural ground, sulfates most commonly occur inthe form of hydrated calcium sulfate (gypsum).Significant amounts of magnesium sulfate (epsomite)and sodium sulfate (Glauber’s salt) may also bepresent.

Calcium sulfate has limited solubility, producing amaximum concentration of SO4 in water at normalground temperatures of about 1400 mg/l. Magnesiumsulfate and sodium sulfate are much more soluble so, ifpresent in the ground in sufficient quantities, willdissolve to produce sulfate concentrations many timesgreater. More rarely, sulfate may also be present inrelatively insoluble forms, as in the mineral barite(barium sulfate). Such minerals do not usually presenta hazard to concrete.

The likelihood of sulfates being present in naturalground depends on the geological strata, the

weathering history of those strata and the groundwaterflow patterns. The geological strata most likely to havesubstantial sulfate concentrations are ancient sedimentaryclays, including Mercia Mudstone (Keuper Marl), LowerLias Clay, Kimmeridge Clay, Oxford Clay, Wealden Clays,Gault Clay and London Clay. In addition to the above,sulfates may be found in locally significant concentrationsin a wide range of other natural strata ranging fromCarboniferous mudstones to Recent alluvium and peat.

The sulfate-bearing strata of greatest national significanceare shown in Figure C2. However, it is important to notethat:• In most geological deposits (the Mercia Mudstone

being a notable exception) only the weathered zone(generally the upper 2 m to 10 m) is likely to have asignificant quantity of sulfates present. In mostaffected strata, the sulfate-bearing zone can thereforebe distinguished by brown colouration characteristic ofweathered clay, compared to the dark-grey colour ofunweathered clay, shale or mudstone that maycontain sulfide minerals.

• Within the weathered zone, sulfate concentrationsmay vary substantially laterally and vertically. It isusual for the top metre or so of undisturbed ground tobe very low in sulfates owing to leaching by rainfall.Also common are high concentrations of sulfateswhich have accumulated at the base of the tree rootzone, at depths of 2 to 3 m, and near the bottom of theweathered zone, at typical depths of 3 to10 m.

In all of the geological strata listed above, except MerciaMudstone, unweathered material at some metres depthmay contain sulfides, particularly pyrite. In their naturalenvironment, it may take thousands of years for suchsulfides to be converted to sulfates by weathering. Butsulfides can be converted relatively rapidly to sulfuric acidand sulfates if exposed to air and water by constructionactivities or by the presence of mobile groundwater - seeBox C2.

C2.1.2 Brownfield locations Fill materials found on sites, or brought in duringconstruction, may contain substantial quantities of sulfatesand occasionally sulfides. The characteristic red shalegenerated from the self-combustion of colliery spoil (seeFigure C3) often contains variable amounts of commonsulfates which originated from pyrite present in some Coal

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Measures strata. Other fill materials that may containsulfates include accumulations of old blastfurnace slag,oil shale residues in the Lothians and clinker from theold-style chain grate power stations or from refuseincineration. Furnace bottom ash (fba) and pulverizedfuel ash (pfa) from the current power generationprocess contain only small amounts of calcium sulfate.

Some sulfates in brick rubble may arise from thebricks, but more significant quantities may be present ifit has adhering plaster (containing gypsum), or if thebricks came from the demolition of old chimneys.

There may also be unusual sulfates, such asammonium sulfate, in soil and ground water as a resultof past industrial use and agriculture.

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Box C1: Sulfur mineral species found in UK ground

Anhydrite CaSO4 Found in evaporite rocksBarytes BaSO4 Common vein mineral in rocksCelestine SrSO4 Rare find, eg Mercia MudstoneEpsomite MgSO4.7H2O Found in evaporite rocksGypsum CaSO4.2H2O Common in soils and rocksJarosite KFe3(OH)6(SO4)2 Weathering product of pyriteMarcasite FeS2 Nodules in chalk and limestoneMirabilite NaSO4.10H2O Found in evaporite rocks(Glauber’s salt)Pyrite FeS2 Common in soils and rocksPyrrhotite FeS Rare find in soils and rocksOrganic sulfur Common in peat

Box C2: How sulfides are converted to sulfates in disturbed groundThe process of oxidation of sulfides to sulfates in initiallyunweathered geological material is complex and may involvebacterial action but can be simply expressed as follows:• In the presence of oxygen in air or groundwater, pyrite (FeS2)

may be oxidised to form red-brown ferric oxide (Fe2O3) or yellow-brown hydrated ferric oxide Fe(OH)3 together with sulfuric acid (H2SO4).The latter is the initial source of sulfate ions and acidity.

• If calcium carbonate (CaCO3) is present, the H2SO4 will further react withit to produce calcium sulfate which crystallises as gypsum(CaSO4.2H2O).

• In the presence of calcium, up to 1400 mg/l of sulfate ions (SO4) mayremain in solution in groundwater. If there is insufficient calciumcarbonate to neutralise the sulfuric acid, the groundwater may becomeacidic. The latter condition is rare in the most commonly encounteredpyritic clays as these generally contain abundant calcium carbonate. Itcan, however occur in certain strata, such as some Carboniferousmudstones

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Figure C2. Principal sulfate and/or sulfide-bearingstrata in England and Wales. North of theindicated line much of these strata are covered byglacial deposits which, if partly derived from theindicated strata, may also contain sulfides andsulfates.

Figure C3. Coal mining areas of Great BritainWhere sulfate-bearing coal mining wastes andmetal processing slags are most likely to beencountered.

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C2.2 AcidsC2.2.1 Mineral acids Sulfuric acid is the only mineral likely to be found innatural groundwater. As noted in Section C2.1.1, thisacid may result from the oxidation of pyrite. Acidicconditions from the oxidation of pyrite are reported infills derived from Carboniferous mudstone and OxfordClay [2]. There have also been cases of pH levels lessthan 3.5 on recently drained marsh land, resulting frompyrite-bearing peaty soils being exposed to oxygen.However, in much of UK there is sufficient calciumcarbonate available in the ground eventually toneutralise any sulfuric acid by forming calcium sulfate(gypsum).

Residual pockets of sulfuric acid may be also found onsites previously used for industrial processes and, inexceptional circumstances, hydrochloric (see SectionB3.3) or nitric acid could also be found.

C2.2.2 Humic acid Natural groundwater may be mildly acidic owing to thepresence of humic acid (which results primarily fromthe decay of organic matter), This acids is not highlyionised and will not produce a pH below about 3.5.

C2.2.3 Carbonic acid and aggressive carbondioxideCarbonic acid (H2CO3) is a weak acid that forms whencarbon dioxide dissolves in water. Rainwater istherefore the common source. As it is readilyneutralised by reaction with calcium carbonate in theground, it will generally only be encountered inrelatively pure soft waters such as those flowing fromuplands of non-calcareous rock.

In respect of aggressiveness to concrete, theparameter ‘aggressive carbon dioxide’ is used as ameasure of the potential for water containing dissolvedCO3 to dissolve calcium hydroxide and other solubleparts of the cement paste. As is explained in SectionB4, only part of CO3 dissolved in groundwater isavailable to attack concrete as some (often most) isalready utilised in bicarbonates (eg Ca(HCO3)2 ) andsome is reserved for ‘stabilising’ such bicarbonates.

Appropriate sampling and test procedures fordetermination of aggressive carbon dioxide are given inpr EN 13577: 1999. Part F of this Special Digest

indicates that levels greater than 15 mg/l are a potentialproblem to the inner surface of pipes and culverts carryinga flow of water. EN 206-1:2000 categorises levels of 15-40mg/l as slightly aggressive, 40-100 mg/l as moderatelyaggressive and greater than 100 mg/l as highlyaggressive. Higher levels of aggressive carbon dioxide willbe associated with low pH, but pH cannot be used as theprincipal indicator since pH will be affected by anypresence of humic and mineral acids.

C2.3 Magnesium, calcium, sodium and potassium ions

These elements are important, as they constitute theprincipal source of cations that support sulfate anions insolution in groundwater and collectively control thestrength of sulfate solutions available to attack concrete.Additionally, presence of magnesium inherently modifieschemical reactions in sulfate attacked concrete (seeSection B3.1).

All four elements are prevalent in UK natural ground, butonly determination of magnesium ion content is a routinepart of site investigation (see Section C5.1.2).

C2.4 Ammonium ionsAmmonium sulfate, (NH4)2SO4 is used in agriculture as afertiliser. However, there is no evidence that harmfulconcentrations of ammonium sulfate occur in groundarising from the normal use. Only rarely have potentiallydamaging concentrations been found, eg resulting fromspillage of the material around fertiliser stores. Ammoniumions may also be present in brownfield sites subject toformer industrial use, especially gasworks.

Specialist advice should be sought if high concentration ofammonium is suspected: determination is not a routinepart of assessment of ground for concrete.

C2.5 Chloride ionsChloride is a common element in natural soil andgroundwater, particularly in the form of sodium chloride(NaCl) or common salt. An obvious source is inlandpenetration of seawater. In some regions underlain byhalite-bearing Mercia Mudstone it may come from naturalbrine seepages. As a legacy of man’s activities, chloridesare extensively found in industrial wastes, particularlythose associated with chemical production. Sodiumchloride is also widely found adjacent to roads owing to itsuse as a de-icing salt.

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C3. Presence and mobility of groundwaterThe rate of chemical attack of concrete depends on theconcentration of the aggressive ions and the ease withwhich they can be replenished at the reaction surfacein the concrete. The replenishment rate will be relatedboth to the porosity and permeability of the soiladjacent to the concrete and to the presence andmobility of the groundwater in the surrounding area.Definitions of water mobility used in this Special Digest,and procedures for establishing them, are given below:

C3.1 Static groundwaterStatic groundwater is confined to locations where theground is either permanently dry, or contains water buthas low permeability (ie little water movement ispossible). The mass permeability in the latter case willgenerally be less than 10-7

m/s (see BS 8004: 1986,Figure 6). A typical example would be clayey soils withtight fissures and no included sand or silt horizons.

In winter and spring, when water tables are generally attheir highest, the presence of Static groundwaterconditions can be established on a proposedconstruction site by either digging a trial pit or drilling aborehole to the intended full depth of concrete. If nowater has seeped into the trial pit or borehole within 24hours, the water conditions can be declared to beStatic. Alternatively, a standpipe piezometer can beinstalled, for example by embedding it in a sandcolumn in a borehole and sealing over the top 0.75 mwith bentonite pellets. If this remains dry, or a variablehead test (see BS 5930) indicates a groundpermeability of less than 10-7

m/s, the water conditionscan be confirmed as effectively Static.

At times of low water table (generally early summer tomid-autumn), it will often be difficult to prove Staticgroundwater conditions from seepage tests. At thesetimes of year the absence of water at proposedconcrete depths must not be taken as the soleevidence of Static groundwater conditions. Eitherrequest an appropriately qualified professional (such asa geotechnical engineer, engineering geologist orhydrologist) to prove a case for Static groundwaterconditions from a consideration of the site geology andhydrology, or assume for concrete design purposesthat the more conservative condition of Mobilegroundwater exists.

Some construction activities can greatly increase masspermeability of ground and may, therefore, change anatural site condition of Static groundwater to a Mobilegroundwater condition. The likelihood of this happeningshould be considered in the ground assessment.

C3.2 Mobile groundwaterThe term Mobile groundwater is defined to cover thefollowing range of conditions:• Water held in pores and structural discontinuities in

the soil, and which is free to flow into an excavation togive a standing water level. The ground permeabilitywill generally be greater than 10-7

m/s. • Water which is percolating slowly through the ground,

say at less than 1 m per day. • Still water in ponds, sumps, or similar accumulations

of free water.

The presence of Mobile groundwater may be seasonal.At times of year when groundwater levels are high, Mobilegroundwater conditions can be confirmed by either digginga trial pit or drilling a borehole to the relevant depth andleaving it temporarily open. Surface protection is neededto prevent ingress of rainfall and for safety of personnel. IfMobile water is present, there will be some seepage intothe trial pit or borehole within 24 hours, but often the waterintake will be much more rapid.

In early summer to mid-autumn, when groundwater levelsare generally low, these simple field tests may fail todetect seasonally adverse mobile groundwater conditions.However, it will often be apparent from a consideration ofthe site geology and hydrology that the seasonaloccurrence of Mobile groundwater is likely. On no accountshould the conclusion be drawn that groundwaterconditions are Static merely because there is an absenceof mobile groundwater at one particular time or season.

The rapidity and position of groundwater seepagedetected in trial pits or boreholes does not further affectthe groundwater mobility classification (except in the caseof Flowing water as indicated below). However, theseseepage characteristics should be reported as part of thesite investigation findings, as this may have a bearing onthe type of Additional Protective Measures (see SectionD6) to be adopted for protecting concrete from chemicalattack.

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C3.3 Flowing groundwaterGroundwater is to be regarded as ‘Flowing’ when itpercolates through the ground under a permanenthead in substantial quantity and at a relatively rapidrate, say greater than 1 m/day. Flowing groundwatermay be expected on a site that contains verypermeable soil and is sloping or could be subject to ahydraulic head from a nearby hill or embankment.

Seepage flow of water into a borehole or trial pitexcavated on a site below the water table (ie under atemporary head) does not necessarily prove a Flowinggroundwater condition. However, the condition canoften be inferred from an overall consideration of thequantity and rate of flow, the type of soil and thesurrounding topography. Specialist hydrological advicemay be required if site evidence is meagre or difficult tointerpret.

For some use of concrete, the definition of Flowinggroundwater is extended to cover water that is flowingin surface conduits or streams.

Only in two cases is Flowing water intrinsically cateredfor in the precautions recommended for concrete:• In Table C1, a step up of ACEC class is

recommended when the Flowing water ispotentially aggressive because it is ‘Pure’ or has asignificant level of aggressive carbon dioxide.

• In the Design guides of Part F, for specific precastconcrete products, an internal lining isrecommended when water and sewer servicescarry flowing water (not specifically groundwater)that contains a significant level of aggressivecarbon dioxide.

However, more generally, Flowing water should beregarded as contributory factor, which can exacerbateall forms of chemical attack and be a vehicle that mighttransport aggressive chemicals to a site from adjacentland.

Box C3: Important facts to note about the presence andmobility of groundwater in relation to chemical attack ofconcrete are:• The presence and mobility of groundwater may vary seasonally.• Highest groundwater levels may be expected in winter and

spring, the lowest levels in late summer.• Groundwater mobility may vary with depth and must be

established for the full depth of a concrete construction.• Permeable silty and sandy soils in which water is present

generally provide little or no resistance to the movement ofwater carrying dissolved chemicals.

• Accumulations of free water, for example in a pond, will readilyfacilitate the movement of dissolved chemicals.

• Fissured clay and clay fills, which may have free water presentin fissures and voids, generally have a relatively lowpermeability that allows only slow movement of water carryingdissolved chemicals. This may be detrimental to concrete overa period of time.

• Clays in which fissures and other discontinuities are absent orare tightly closed have very low permeability but generallyremain fully saturated with pore water at all seasons owing tocapillary action. This pore water may allow some limitedmovement of chemicals by diffusion through the liquid phase,but in general the quantity of chemicals reaching the concretewill not be sufficient to cause significant chemical attack.

• Ground on, or at the foot of, slopes or retaining structures maybe subject to enhanced flow of groundwater owing to thegravitational ‘head’ of water.

• Civil engineering works, such as road construction, office andfactory developments, and large housing developments candisrupt natural drainage. This may affect flows in rivers andstreams, and sub-surface groundwater movements and levels.In differing circumstances this may lead to increased or reducedflows. The consideration of the effects of site drainage inrelation to structures and foundations is essential and, inparticular, the presence of porous carrier drains which maydivert water into the area of the foundations – see Section D6.6.

• Care is needed to avoid concrete being exposed to aggressiveconditions in a ‘sump’ environment. In several cases of seriousdeterioration to concrete from TSA in the foundations ofhighway bridges [1], a major contributory factor was that thefoundations had been constructed in excavations that weresubsequently backfilled with pyritic clay and also subject toingress of water.ReminderWhen you are uncertain as to whether the groundwater is Staticor Mobile – eg owing to lack of site data or knowledge ofchanges to ground permeability that may result fromconstruction - then assume a Mobile groundwater condition.

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C4. Site investigation for aggressiveground conditions

C4.1 IntroductionThis section describes the site studies, sampling andtesting needed to assess whether ground conditionsare potentially aggressive to concrete. It isrecommended that the investigation comprisesequentially a desk study, a walk-over survey, and aground investigation using trial pits and/or boreholes toassess visually the ground profile and to takerepresentative samples of soil and groundwater forchemical analysis. The chemical agents to bequantified in particular are those listed in Section C2as commonly encountered: sulfates, sulfides, acids(pH) and magnesium. In brownfield locations, chlorideand nitrate should also be quantified as respectiveindicators of hydrochloric and nitric acids. Thepresence of all of these is specifically taken intoaccount by the ACEC classification here.

A list of recommended test methods and sourcedocuments for the chemical analysis of soils andgroundwater are given in Appendix C1.Chemical agents aggressive to concrete that areencountered only rarely, such as ammonium salts,should also be investigated if their presence issuspected, for example from past use of the site.However, specialist advice should be sought withregard to their detection and appropriate concretespecification. Additionally, aggressive carbon dioxideshould also be determined in respect of the type ofFlowing ground-water conditions indicated in SectionC3.3.

Investigation of contaminated land requiresconsideration of other hazards, as well as chemicalsaggressive to concrete. Such matters are beyond thescope of this Special Digest. BS 10175: 2001 andseveral recent reports from CIRIA, DETR and theEnvironment Agency [4, 5, 6, 7, 8, 9]

are recommended forguidance on the additional requirements required forplanning, executing and interpreting site investigationsand risk management of development on contaminatedland.

For all locations an appraisal should be made of thegroundwater conditions and, in particular, whether

concrete could be exposed to Mobile or Flowinggroundwater (see Section C3).

The site investigation should be carried out by suitablyexperienced persons. The level of detail should be broadlyrelated to the importance of the proposed construction, thecomplexity of the site and the level of assurance requiredfor risk management.

Appendix C2 gives guidance on more comprehensive siteinvestigation that may be needed when investigatingcases of sulfate attack on concrete.

C4.2 Desk studyAn initial desk study should be carried out to identify andreview relevant existing information. In particular, evidenceshould be sought of any aggressive chemicals and ofpotentially aggressive substances such as pyrite.Guidance on desk studies is given in BRE Digest 318;Boxes C4 and C5 list items specific to aggressive ground.In respect of pyrite, particular note should be taken of greyor black-coloured alluvial deposits, overconsolidated clays,mudrocks, Coal Measures, slates and schists. A list ofgeological formations known to contain pyrite is given inBox C6. Also listed are typical pyrite contents (by % mass)quoted for samples taken from the localities indicated. Thegeological formations are not necessarily confined tothese locations and pyrite contents may vary substantiallyfrom indicated values.

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Box C4: Possible sources of information to be consideredin a desk study:• published topographical maps;• aerial survey photographs;• geological maps and memoirs (particularly ‘Drift’ maps);• soil survey maps;• site investigation records for past developments or

construction on adjacent sites;• data from the Environment Agency or local authorities on

regional water levels and flooding;• data from British Geological Survey.

Further sources of information are listed in Annex B ofBS 5930:1999 and TRL Report 192 [10].

Box C5: The following list of topics may be relevant to deskstudy assessment of the risk of chemical attack of concrete:

• bedrock and superficial (drift) geology (particularly importantas an indicator of pyritic ground, see Section C2.1);

• location and type of previous development, particularly ofindustry which might have left aggressive waste materials;

• topography from ground contours, including changes whichmight indicate placing of fill;

• records of flooding;• location of existing natural and man-made drainage systems

including, streams, ditches, trench drains and field drains;• reworking of pyrite-rich clays;• use of colliery spoil or mine waste.

Box C6: UK geological formations known to contain sulfides (derived from Table 3.2 of TEG Report [1] –see this for individual references )

Geology Location of samples % pyriteColliery spoil UK 0 – 12Carboniferous Limestone Shales Yorks & Derbyshire 5 – 10Coal Measure shales England 0.7 – 1.4Carboniferous Culm Measures Devon 2.4Namurian mudstones Derbyshire 0 – 6Rhaetic mudstones,Westbury Formation S. Glamorgan 4 – 6Stonefields Slate GlosLower Lias Clay S W England 5 – 8Upper Lias Clay Northants 3 – 5Whitby Shale Teesside 3 – 9Oxford Clay Oxon, Cambs 3 – 5Oxford Clay E & S England 5 – 15Kimmeridge Clay Dorset 4Weald Clay SussexSandgate Beds SE England 0.5 – 0.9Gault S England 0.7 – 1.0Bracklesham Beds SE EnglandHeadon Beds SE EnglandBarton Clay HantsBembridge Beds Isle of WightLondon Clay SE England 0 – 4Recent alluvial deposits DerbyshireMarsh peat Fen district

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C4.3 Site inspection (walk-over survey)The aim of the walk-over survey is to examine thesurface of the site for evidence of conditions that mightcontribute to a chemical/ hydrological environmentaggressive to concrete. Particular attention should bepaid to the following:• examine any exposures of natural ground and

record details of geological materials and organicdeposits such as peat (which may be acidic);

• inspect and record details of any former structuresand waste materials;

• compare surface topography with previous recordsto check for the presence of fill, erosion or cuttings;

• consider the effects of changes to the topographyas a result of new construction;

• note the presence of slopes which, by providing ahead to water, could enhance water mobility;

• note water levels, direction and rate of flow inwatercourses;

• note position of wells or springs;• note the nature of vegetation in relation to soil type

and wetness of the ground. Unusual greenpatches, reeds, rushes or willow trees oftenindicate wet ground.

More general guidance on the walk-over survey isgiven in BS 5390 Annex C and BRE Digest 348.Following the walk-over survey, an assessment shouldbe made of the presence and distribution of conditionslikely to be aggressive to concrete. These data shouldbe used to plan the ground investigation.

C4.4 Visual description of the groundThe starting point for an investigation of chemicalagents in the ground that may be aggressive toconcrete is a good visual description of the groundprofile to the full depth of concrete construction.Laboratory testing can give precise values for chemicalcontents at particular locations but will not necessarilybe fully representative.

A visual assessment may detect local concentrations ofpotential hazardous minerals, such as gypsum(CaSO4.2H2O), pyrite (FeS2) and marcasite (FeS2), andfeatures that affect the transmission of groundwater.The soil description can be accomplished by means oftrial pits or boreholes, described in BS 5930 and BREDigests 381, 383 & 411. Guidance on the occurrence

and identification of sulfate and sulfide minerals is given inAppendices A, B and C of HA 74/00 [11].

The ground description should particularly note thefollowing features relevant to assessment of theaggressivity of chemical environment:• Soil particle size and composition.• Soil colour: a dark-grey or black colour of

unweathered mudrocks and clays generally indicatesthat they originated in anaerobic conditions conduciveto the formation of pyrite; a dark-grey, blue or blackcolouration of clay generally indicates that it isunoxidised; brown colouration of clay generallyindicates that it is in a weathered, oxidised state.

• Soil structure: this gives information on the state ofweathering and ease of groundwater transmission.

• Presence of any visible sulfate or sulfide minerals butnote that pyrite is often finely disseminated and is notidentifiable, even with a hand lens.

• Presence of any visible calcium carbonate in the formof amorphous nodules, fossil fragments or calcitecrystals. This can be also be detected byeffervescence of the soil when it is tested with dilutehydrochloric acid (5% HCl).

C4.5 Sampling and testing soilsSamples for the required chemical tests can be takenusing standard site investigation techniques incorporatingtrial pits and boreholes – see BS 5930, BS 10175 andBRE Digests 381, 383 and 411. Precautions should betaken to protect site workers and site neighbours if thedesk study indicates the presence of substances harmfulto health. To avoid contaminating the samples, theminimum amount of water should be added to the holeduring boring, preferably none.

Representative samples should be taken for sulfateclassification from key depths in the ground in each test pitor borehole, bearing in mind the preliminary structuraldesign concept and the likely distribution and sulfides asindicated in Section C2.1. One test might be consideredsufficient for a house foundation to be installed at 1 mdepth and two tests if it is to be founded at 3 m depth. Formore substantial foundations and piles, samples should betaken at about 1 – 2 m intervals ensuring they include atleast one from any obvious change of stratum. Thenumber of pits or boreholes will depend mainly on thesize, topography and complexity of the geology of the site,

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as evidenced by the findings of the desk study andwalk-over survey.

The mass of samples required for chemical testingshould be as given in BRE Report BR 279 Section 4 orBS 1377: Part 1:1990 Section 7, that is 100 g for finegrained soils, 500 g for medium-grained soils and 3 kgfor coarse-grained soils. In fine-grained soils, samplesshould be obtained preferably by driving a tube into theground. After extraction of the tube, the ends of thesample should be sealed to restrict loss of moistureand intrusion of air and thereby minimise oxidation ofpyrite. The samples should be stored in a cool, darkplace, at a temperature of between 2°C and 4°C, andbe tested as soon as possible: the maximum delayshould be three weeks. The conditions and duration ofstorage prior to testing should be recorded and given tothe site appraiser together with the test data. Materialselected for laboratory testing should be taken from thecentre of block and core samples to avoid the effects ofsurface oxidation and contamination by different wateror soil.

Recommended test methods for the chemical analysisof aggressive soils are given in Appendix C1.

C4.6 Sampling and testing groundwaterIf there is Mobile groundwater on a site, indicated byvisible seepage into a trial pit or borehole, this shouldalways be tested for aggressive chemical content. Thisis because groundwater may have a concentration ofdissolved chemicals greater than are present in theimmediately surrounding solid ground owing totransportation from a distant source.

Groundwater samples can be obtained by collectingseepage into a trial pit or borehole. Water seeping fromthe base or sides of a trial pit can be collected in acontainer such as a clean, sealable sample jar. Careshould be taken to avoid water that has entered the pitdirectly from rainfall or surface run-off. A note of anyvisible seepage and the direction(s) from which itcomes will help in a groundwater mobility assessment.In ground of lower permeability (<10-7

m/s – see BS8004 1986, Figure 6) a groundwater sample can bestbe obtained from a standpipe piezometer installed in aborehole, backfilled with sand and sealed over thetopmost metre with bentonite/cement pellets. After

reaching equilibrium, such a piezometer will also indicatethe height of the water table. Additionally, the permeabilityof the ground can be determined in the piezometer by avariable head test (see BS 5930).

The concentration of some chemicals in groundwater, forexample sulfates, may vary seasonally, probably beinggreatest in the late summer when groundwater is reducedin volume. Also, it is possible for groundwater in boreholesto be found to contain different concentrations of solublesulfates at different depths. In such circumstances, itshould be noted that groundwater samples taken after theboring is completed may contain water from severaldifferent strata.

Controlled procedures should be used for obtaining,handling and storage of groundwater samples. Samples of0.5 to 1 litres should be obtained. They should be stored ina clean, well-filled, sealed container, kept at lowtemperature to minimise changes due to bacterial action(4°C recommended) and analysed as soon as possible.

The acidity/alkalinity of water can be tested on site usingpH test strips or a portable meter. For Flowinggroundwater requiring the determination of aggressiveCO2, use the sampling procedure given in prEN 13577:1999. Recommended test methods for the chemicalanalysis of aggressive groundwater are given in AppendixC1.

It is strongly recommended that, at the same time assamples of groundwater are taken, an assessment ismade of its mobility with reference to the definitions inSection C3. Knowledge of the mobility of any groundwateron the site is an essential prerequisite for the ACECclassification of the ground. This is because groundwaterdetermines the ease with which an aggressive chemicalcan have access to the concrete. Also, the mobility of anygroundwater must be known for some categories ofconcrete construction, since one of the recommendationsfor the protection of concrete foundations may be toaddress the site drainage.

SD1:2005 Part C - Draft 1 v8: 14/12/04 13

C5. Classification of site locations forchemicals aggressive to concreteThe Aggressive Chemical Environment for Concrete(ACEC) is introduced here to take into account sulfateconcentration and other factors related to theenvironment in which the concrete is to be placed, forexample the mobility and pH of the groundwater. Forthe higher Sulfate Classes on brownfield locations, themagnesium ion concentration is also taken intoaccount. More cautious action limits in respect ofacidity are applied to brownfield locations as comparedto natural ground.

C5.1 Groundwater and soil analysesFour different categories of site have been identified asrequiring specific procedures for investigation foraggressive ground conditions:• Natural ground locations, except those

containing pyrite. These are the most commonlyencountered locations. They are described inSection C5.1.1 and Figure C4

• Natural ground locations that contain pyrite.These are locations where the ground containspyrite which, if disturbed, may oxidise to sulfates –see Section C5.1.2 and Figure C5

• Brownfield locations, except those containingpyrite. – see Section C5.1.3 and Figure C6.

• Brownfield locations that contain pyrite. Theseare brownfield locations where the ground containspyrite which, if disturbed, may oxidise to sulfates –see Section C5.1.4.

The category of a site or individual site location shouldbe provisionally established by desk study (see SectionC4.2). If no desk study has been carried out, it shouldbe assumed that pyrite may be present in the groundand the site testing procedures given in Section C5.1.3and C5.1.4 should be followed.The precautions recommended in this Special Digestapply only to concrete placed in ground where the pHis greater than 2.5. Only in very exceptionalcircumstances in the UK are pH readings below 2.5encountered.

C5.1.1 Natural ground locations, except thosecontaining pyriteThis is a location that is known from the Desk study ora preliminary ground investigation not to be either aBrownfield one or to contain pyrite in any strata that

may be encountered by construction. Several locationsmay need to be separately classified for chemical agentsaggressive to concrete if the site is extensive and/or theground conditions are complex.

The analytical tests required for classification (seeFigure C4) are water-soluble sulfate content (SO4 mg/l)and the pH. Classification should be carried out, whereverpossible, by using samples of both soil and groundwater.For soil, the sulfate analysis, expressed as SO4, should beon a 2:1 water/soil extract and the pH analysis on a 2.5:1water/soil extract.

The chemical classification of a given site location shouldbe carried out in the following five steps:Step 1: Determine the ‘characteristic’ values for sulfateconcentration in tests on (a) soil samples and (b)groundwater samples. It is important to test groundwatersamples if these are obtainable from the location asgroundwater is generally the agent by which aggressivechemicals reach the concrete.All samples to be used for sulfate classification should becarefully taken, handled and tested, as described inSection C4.(a) Soil samplesIf only a small number of soil samples have been testedfor water-soluble sulfate using the 2:1 water/soil extracttest, the highest measured sulfate concentration (mg/lSO4) should be taken as the characteristic values.However, if the water-soluble sulfate results for soil varywidely, it may be appropriate to test further samples toobtain a more representative data set. In a data set wherethere are five to nine results available for the location, themean of the highest two of the sulfate test results shouldbe taken as the characteristic value for water-solublesulfate (mg/l SO4). In a data set where there are 10 ormore results available, the mean of the highest 20% of thesulfate test results (rounded to 100 mg/l) should be takenas the characteristic value.(b) Groundwater samplesThe highest determined sulfate concentration (mg/l SO4)of the samples (rounded to 100 mg/l) should be taken asthe ‘characteristic’ values for the groundwater at a givenlocation.

Step 2: Determine the basic Sulfate Classescorresponding to the Step 1 characteristic values forgroundwater using columns 1 and 3 of Table C1 and forsoil using columns 1 and 2 of Table C1.

SD1:2005 Part C - Draft 1 v8: 14/12/04 14

Editor’s noteSubscript all ‘4’ in SO4

Yes

No Are groundwater samples available ?

Groundwater samples should be taken and tested wherever physicallypossible

See Section C5.1.1 for further information

Option 2For Mobile groundwater or Flowing water select column 6of Table C1

Use tests in Appendix 1 on groundwater samples to determine:(a) soluble sulfate g/l SO4(b) pH

Consider all soluble sulfate and pH resultsfor groundwater and find 'characteristic' values for site or location.(see C5.1.1, Steps 1(b) and 4)

Find Sulfate Class equivalent to characteristic values of soluble sulfate in groundwater using columns 1 and 3 of Table C1 = Result 2

Take the highest of Results1 and 2 as the Design Sulfate Class for the site or location(see C5.1.1, Step 3)

Use Appendix C1 tests on soil samples to find: (a) Water-soluble sulfate g/l SO4 in 2:1 water / soil extract(b) pH in 2.5:1 water / soil extract

Consider all water-soluble sulfate and pH results for soil and find 'characteristic' values for site or individual locations.(see C5.1.1, Steps 1(a) and 4)

Option 1For Static groundwaterselect column 5 of Table C1

Figure C4. Procedure for determining ACEC classification for locations on natural ground sites, except for ones where soils may contain pyrite

20-11-04

For each site location, select samples of various site materials from key depths(see Sections C4.5, C4.6 and C5.1.1)

Find Sulfate Class equivalent to characteristic values of water-solublesulfate in soil using columns 1 and 2 of Table C1 = Result 1

For adopted Design Sulfate Class, select row of Table C1 correspondingto characteristic pH of location.ACEC Class can now be foundfrom column 7 of Table C1

For adopted Design Sulfate Class, select row of Table C1 correspondingto characteristic pH of location.ACEC Class can now be foundfrom column 7 of Table C1

SD1:2005 Part C - Draft 1 v8: 14/12/04 1

Step 3: Adopt a value for Design Sulfate Class for thesite location from a consideration of the SulfateClasses for groundwater and soil determined in Step 2.If only one type of sample (groundwater or soil) wastested, the Class determined for this type may be takenas the Design Sulfate Class for the location.If both types of sample (groundwater and soil) weretested, the highest of the two determined SulfateClasses should be taken as the Design Sulfate Classfor the location.

Step 4: Determine the ‘characteristic’ values for pH byconsidering the values obtained from tests on soil andgroundwater. For both soil and groundwater, take therespective lowest measured values of pH if only asmall number of samples have been tested. Otherwisetake the respective means of the lowest 20% of the pHresults. The characteristic value of the pH should thenbe taken as the lowest of the pH determinations for thesoil and groundwater.

Step 5: Determine the ACEC Class for the sitelocation. Starting with the Design Sulfate Class, addthe characteristic value of the pH to chose theappropriate pH range for the assessed mobility ofgroundwater (Static or Mobile) - see Figure C4 andTable C1. The ACEC classification is explained furtherin Section C5.2.

As well as the outcome of this classification procedure,the results of all the individual chemical analyses,including the location and depth of the samples, shouldbe made available to the engineer and concretespecifier.

Box C7: Technical notes on limits for sulfate classes

• The division between Classes 2 and 3 for groundwater isrelated to the maximum solubility of calcium sulfate (1.44g/ lSO4). Higher sulfate concentration in groundwater confirmsthe presence of more soluble sulfates, usually magnesiumor sodium. Other divisions between Sulfate Classes aredrawn somewhat arbitrarily.

• The limits of Design Sulfate Classes based on 2:1 water/soilextracts have been lowered relative to previous Digests thecorrelation between old and new limits (in terms of g/l SO4)being as follows:

Sulfateclass

New limitsg/l SO4

Old limitsg/l SO4

DS-1 <0.5 <1.2DS-2 0.5 - 1.5 1.2 – 2.3DS-3 1.6 - 3.0 2.4 –3.7DS-4 3.1 - 6.0 3.8 - 6.7DS-5 >6.0 >6.7

This consequence of this adjustment is to make theground classification based on soil tests moreconservative, eg some soils that were previously classifiedas DS-2 would now be considered as being DS-3.

The change stems from findings of numerous research groundinvestigations carried out by BRE and others followingdiscoveries of the thaumasite form of sulfate attack (TSA) in theconcrete foundations of highway structures. [1] In the largemajority of cases, the sulfate class limits based on 2:1 water /soil extract tests on soil have been found to be substantiallylower than sulfate class based on sulfate in groundwater. Notsurprisingly, therefore, they were inconsistently low compared tothe actual occurrence of TSA. The new limits bring sulfateclassification based on 2:1 water / soil extract tests into paritywith the groundwater based tests.

5

SD1:2005 Part C - Draft 1 v8: 14/12/04 16

C5.1.2 Natural ground locations that contain pyriteTo classify site locations where ground materials(natural ground or ‘clean’ fill derived from naturalground) may contain sulfides, such as pyrite, it isessential to take account of the ‘total potential’ sulfatecontent which might result from oxidation followingground disturbance. The extra test requirements,compared with the procedure given in Section 6.1.1,are (see Figure C5):(i) Determine the ‘total’ sulfate content (‘AS’ as % SO4)by the ‘acid-soluble’ method – see Appendix C1.(ii) Determine the ‘Total sulfur’ present (‘TS’ % S).(iii) Calculate the ‘Total Potential Sulfate content’(TPS as % SO4) from the stoichiometric equation:TPS % SO4 = 3.0 x TS % S.

This gives a conservative estimate of the TotalPotential Sulfate since any sulfur within organic matterand minerals such as barite, both of which are moreinert than pyritic sulfur, are included.(iv) Determine, for each individual sample, the amountof Oxidisable Sulfides (‘OS’ expressed as % SO4) inthe suspected pyritic ground by subtracting the acid-soluble sulfates (AS % SO4) from the Total PotentialSulfate content (TPS % SO4):OS % SO4 = TPS % SO4 - AS % SO4

If the amount of Oxidisable Sulfides is greater than0.3% SO4 in a significant number of samples, pyrite isprobably present. This can be confirmed by X-RayDiffraction (XRD) and Scanning Electron Microscopy(SEM) analysis. It can also be quantified directly by theAcidified Chromium Reduction method – see TRLReport 447 [7].

If it is concluded that pyrite is present in significantamounts, the sulfide content of the ground materialmust be taken into account if concrete is to be exposedto disturbed material which might be vulnerable tooxidation. This should be done in four additional stepsas compared with C5.1.1:

Step 6: Determine the characteristic values of the TotalPotential Sulfate content (TPS % SO4) for the sitelocation from a consideration of the results of severaltests on the pyritic ground. In a data set where five tonine results are available for the location, the mean ofthe highest two of the TPS values should be taken asthe characteristic value (rounded to 0.1% SO4 ). In a

data set where 10 or more TPS results are available, themean of the highest 20% should be taken as thecharacteristic value.

Step 7: Determine the Sulfate Class equivalent to thecharacteristic value of the Total Potential Sulfate contentusing columns 1 and 6 of Table 2.

Step 8: Compare the Sulfate Class for Total PotentialSulfate with the Sulfate Classes determined (in SectionC5.1.1) for groundwater and water-extract tests on soil.The highest of these Sulfate Classes should then be takenas the Design Sulfate Class for the site location. Alimitation can be applied if the Sulfate Class for the TotalPotential Sulfate is initially found to be Sulfate Class 5, butSulfate Classes for groundwater and the water-extractstests are Sulfate Class 3 or less. In this case, the DesignSulfate Class for the site location can be limited to SulfateClass 4.

The reason for this limitation is that the procedure forsulfate classification based on Total Potential Sulfate isoften highly conservative as not all the pyrite in soil will beoxidised and only a part will be taken into solution bygroundwater. Some reliance is therefore placed on thefindings of field studies of disturbed pyritic clay that hasundergone oxidation. These have shown a maximumSulfate Class for groundwater in pyritic clay subject toprolonged oxidation to be Sulfate Class 4.

Step 9: Determine the ACEC Class of the ground from therow of Table C1 that correlates first with the DesignSulfate Class, second with the water conditions, andthird with the characteristic value of pH.

Box C8: Practical notes concerning pyritic ground

• Concrete in pyritic ground which is initially low in solublesulfate does not have to be designed to withstand a highpotential Sulfate Class unless it is exposed to ground whichhas been ‘disturbed’ to the extent that contained pyrite mayoxidise and the resultant sulfate ions reach the concrete.This may prompt redesign of the structure or change to theconstruction process to avoid ground disturbance, forexample by using precast or cast-in-situ piles instead ofconstructing a spread footing within in an excavation.

• The sole determination of the ‘acid-soluble sulfate content’as employed in some recent European standards will notdetect pyrite, which might be oxidised to sulfates as a resultof ground disturbance.

SD1:2005 Part C - Draft 1 v8: 14/12/04 17

Editor’s noteSubscript all ‘4’ in SO4

Yes

Yes

Is there a possibility of sulfides in ground, eg pyrite in unweathered clay?

Yes

No

No

No Take the highest of Results1 and 2, as the DesignSulfate Class.

Groundwater samples should be taken and tested wherever physically possible

Consider all soluble sulfate and pH results for groundwater and find'characteristic' values for the site or location

Find Sulfate Class equivalent to characteristic values of soluble sulfatein the groundwater using columns1 and 3 of Table C1 = Result 2

Will concrete be exposed to disturbed ground in which pyrite may oxidise to sulfate ?

For each individual sample of the pyritic ground subtract the result of the Acid Soluble test ( AS as % SO4 ) from the result of the Total Potential Sulfate test ( TPS as % SO4 ) to calculate the amount of Oxidisable Sulfides (OS as % SO4 ). ie OS =( TPS - AS)

Are the values of OS > 0.3% SO4 for a significant number of samples?

This indicates that pyrite is presentwhich may oxidise if ground is disturbed.From a consideration of Total Potential Sulfate tests on pyritic ground find the characteristic value of TPS (SO4 %) for the site or location


Option 2For Mobile groundwater or Flowing water select column 6of Table C1

For adopted Design Sulfate Class, select row of Table C1 correspondingto characteristic pH of location. ACEC Class can now be found from column 7 of Table C1


Yes

No Are groundwater samples available ?

Use tests in Appendix 1 on groundwatersamples to determine:(a) soluble sulfate content g/l SO4(b) pH

Use tests in Appendix 1 on soil samples to find:(a) Water-soluble sulfate (WSS as SO4 g/l) in 2:1 water/soil extract(b) Acid-soluble sulfate (AS as % SO4) (c) Total Potential Sulfate (TPS as % SO4) = 3 x Total Sulfur (TS % S) (d) pH of 2.5:1 water / soil extract

Find Sulfate Class equivalent to characteristic values of water-solublesulfate using columns 1 and 2 of Table C1 = Result 1

Find Sulfate Class equivalent to characteristic value of Total Potential Sulfate (TPS as SO4 %) using columns 1 and 4 of Table C1 = Result 3

Take the highest of Results1, 2 and 3 as the DesignSulfate Class. But if Result 3 isthe highest, then limit it to DS-4

20-11-04Figure C5. Procedure for determining ACEC classification for sites or locations where disturbance of pyrite-bearing natural ground could result in additional sulfate

See Section C5.1.2 for further informationFor each site location, select samples of various site materials from key depths(see Sections C4.5 and C4.6 and C5.1.2)

SD1:2005 Part C - Draft 1 v8: 14/12/04 18

C5.1.3 Brownfield locations, except thosecontaining pyriteThe following points should be noted:• A brownfield location is defined as a site or part of

a site that has been subject to industrialdevelopment, storage of chemicals (includingagricultural use) or deposition of waste, and whichmay contain aggressive chemicals in residualsurface materials or in ground penetrated byleachates. Where the history of a site is not known,it should be treated as a brownfield site until thereis evidence to classify it as natural.

• The type of chemicals present and theirconcentration, though of significant to concretespecification, may not be such as to earn theemotive title ‘contaminated land’.

• This Special Digest does not seek to cover use ofconcrete in heavily contaminated land, for instancethe lower bound for guidance in respect of groundacidity is pH 2.5.

• This Section is for sites or site locations that areknown from the Desk study or a preliminary groundinvestigation not to contain pyrite in any strata thatmay be encountered by construction.

• Several locations on a site may need to beseparately classified for chemical agentsaggressive to concrete if the site is extensiveand/or the ground conditions are complex.

The assessment of chemical aggressiveness(Figure C6 is similar in the initial stages to theprocedure given in Steps 1 and 2 of Section C5.1.1,but with an additional consideration of the level ofmagnesium present when the sulfate level is greaterthan 3000 mg/l in either the water extract or thegroundwater.

The additional procedures for magnesium whenapplicable are:• Step 1a: Determine the ‘characteristic’ values for

magnesium concentration, in tests on (a) soilsamples and (b) groundwater samples. Assessvariable data in the same way as advised forsulfate.

• Step 2a: Determine the ‘basic’ Sulfate Class forthe site location using columns 1, 4 and 5 of Table

C2 for groundwater, and using columns 1, 2 and 3 forsoil.

For ground containing suspected of containing mineralacids of industrial origin, an additional procedure, Step 10,is recommended prior to taking account of the mobility ofthe groundwater.

Step 10The pH of the samples should first be considered (FigureC6). If a significant number of these are lower than pH 5.5,the amounts of chloride and nitrate (NO3) should also bedetermined in addition to sulfate content. Substantialpresence of chloride and nitrate ions on a brownfieldlocation indicates that hydrochloric and nitric acids may bepresent in the ground. The effect of hydrochloric and nitricacids on concrete is likely to be similar to that of sulfuricacid so, for classification purposes, their chemicallyequivalent sulfate concentration should be calculated andadded to any actual soluble sulfates present (as SO4 mg/l)in the respective samples:

SO4 equivalent of Cl = Cl x 1.35 mg/lSO4 equivalent of NO3 = NO3 x 0.77 mg/l.

This procedure should not be used to assess thesusceptibility of reinforcement to corrosion.

Adjusted characteristic values of sulfate may then bederived and from these adjusted Sulfate Classes for soiland groundwater. The Design Sulfate Class for the localitymay be taken as the highest of these adjusted SulfateClasses.

The ACEC Class of the ground can then be found from therow of Table C2 which correlates first with the DesignSulfate Class, second with the water conditions, and thirdwith the characteristic value of pH.

C5.1.4 Brownfield locations containing pyriteIf the desk study indicates that there is a possibility thatthe ground materials on the brownfield location haveoxidisable sulfides (such as pyrite in unburnt colliery spoil),then the additional procedures given in Steps 6 to 8 ofSection C5.1.2 and Steps x 1a and 2a in Section C5.1.3.should all be carried out. The Design Sulfate Class for thelocation should then be taken as the highest of the SulfateClasses derived by the differing procedures.

The ACEC Class of the ground can then be found from therow of Table C2 which correlates first with the DesignSulfate Class, second with the water conditions, and thirdwith the characteristic value of pH.

SD1:2005 Part C - Draft 1 v8: 14/12/04 19

Editor’s noteSubscript all ‘4’ in SO4& ‘3’ in NO3

No Is pH < 5.5for location ?

No

Yes

Yes

Is Cl or NO3present at [1] location?

Yes

No

Yes

No

Are groundwater samples available ?

No

For groundwater is SO4 > 3.0 g/l ?

Yes

Determine Mg g/las Appendix 1

Determine Mg g/las Appendix 1

20-11-

Use tests in Appendix 1 on soil samples to find:(a) Water-soluble sulfate g/l SO4 in 2:1 water/soil extract(b) Cl and NO3 in 2:1 water / soil extract(c) pH of 2.5:1 water / soil extract

For (a), is water-soluble sulfate > 3.0 g/l SO4 ?

Use tests in Appendix 1 on groundwatersamples to determine:(a) soluble sulfate content g/l SO4(b) Cl g/l and NO3 g/l content(c) pH

Consider all sulfate, Mg and pH results for groundwater and find 'characteristic' values for site or location

Find Sulfate Class equivalent to characteristic values of soluble sulfate and Mg in groundwater using columns1, 4 and 5 of Table C2 = Result 2

Take the highest of Results1 and 2 as the basic Sulfate Class for the site or location

Respectively for test results on soil and groundwaterCalculate SO4 equivalent of Cl (Cl x 1.35 g/l)and/or SO4 equivalent of NO3 (NO3 x 0.77 g/l)and add to corresponding characteristic values for soluble SO4.Find Sulfate Classes for soil and ground water equivalent to these adjusted characteristic values = Results 3 and 4. Use the highest of Results 3 and 4 to find the Design Sulfate Class for the site or location


Option 2For Mobile groundwater orFlowing water select column 8 of Table C2

Note [1] Significant values of Cl and NO3 indicate that hydrochloric and nitric acids may be present.These can be allowed for by adjusting the determined soluble sulfate content.A moderate presence of chlorides is not of concern, provided that the pH > 5.5

Groundwater samples should be taken and tested wherever physically possible

Find Sulfate Class equivalent to characteristic values of water-solublesulfate and Mg in soil using columns 1, 2and 3 of Table C2 = Result 1



See Section C5.1.3 for further information

Figure C6. Procedure for determining ACEC classification for locations on Brownfield sites, except for ones where soils may contain pyrite

Consider all water-soluble sulfate, Mg and pH results and find 'characteristic' values for site or individual locations. (see C5.1.3)

For each site location, select samples of various site materials from key depths(see Sections C4.5, C4.6 and C5.1.3)

SD1:2005 Part C - Draft 1 v8: 14/12/04 20

C5.2 Aggressive Chemical Environment forConcrete (ACEC) ClassificationThe Aggressive Chemical Environment for Concrete(ACEC) classification is set out in Tables C1 for naturalground locations and C2 for brownfield locations. The process of ACEC classification of a location startswith the classification of the ground into one of fiveDesign Sulfate Classes. The route through this sulfateclassification (see Section C5.1 and Figures C4 to C6)depends on the type of ground location and presenceor absence of substances including magnesium ions,pyrite and, for pH less than 5.5, chloride and nitrateions. Having established the appropriate DesignSulfate Class, modifications are applied which relate tothe mobility and pH of groundwater. Mobile water (seeSection C3.2) and low pH (see Section C2.2) are bothadverse ground conditions that lead to the designationof a more severe ACEC class. Static water is a morebenign condition that allows for a less severe ACECclass.

An overview of the procedure to determine and applythe ACEC classification is set out in Figure C1; detailedsteps for the three main categories of site location aregiven in Figures C4, C5 and C6.

SD1:2005 Part C - Draft 1 v8: 14/12/04 21

Table C1: Aggressive Chemical Environment for Concrete (ACEC) classification for natural ground locations a

Sulfate GroundwaterDesign Sulfate

Class forlocation

2:1 water/soilextract b

Groundwater Total Potentialsulfate c

Staticwater

Mobilewater

ACEC classfor location

1 2 3 4 5 6 7SO4

mg/lSO4

mg/lSO4

%pH pH

≥2.5 AC-1s >5.5 d AC-1 d

DS-1

<500 <400 <0.24

2.5-5.5 AC-2z>3.5 AC-1s >5.5 AC-22.5-3.5 AC-2s

DS-2

500 - 1500 400 – 1400 0.24-0.6

2.5-5.5 AC-3z>3.5 AC-2s >5.5 AC-32.5-3.5 AC-3s

DS-3

1600 - 3000 1500 - 3000 0.7-1.2

2.5-5.5 AC-4>3.5 AC-3s >5.5 AC-42.5-3.5 AC-4s

DS-4

3100 - 6000 3100 - 6000 1.3-2.4

2.5-5.5 AC-5>3.5 AC-4sDS-5

>6000 >6000 >2.4

2.5-3.5 ≥2.5 AC-5Notesa. Applies to locations on sites that comprise either undisturbed ground that is in its natural state (ie is not brownfield – see

Table C2) or ‘clean’ fill derived from such ground.b. The limits of Design Sulfate Classes based on 2:1 water/soil extracts have been lowered relative to previous Digests –

see Box C7).c. Applies only to locations where concrete will be exposed to sulfate ions (SO4), which may result from the oxidation of

sulfides such as pyrite, following ground disturbance.d. For Flowing water that is potentially aggressive to concrete owing to high purity or an aggressive carbon dioxide level

greater than 15 mg/l - increase the ACEC class to AC-2z.

Explanation of suffix symbols to ACEC Class • Suffix s indicates that the water has been classified as static.• Concrete placed in ACEC Classes that include the suffix z have primarily to resist acid conditions and may be made

with any of the cements or combinations in Table D2.

SD1:2005 Part C - Draft 1 v8: 14/12/04 22

Table C2: Aggressive Chemical Environment for Concrete (ACEC) classification for brownfield locations a

Sulfate and magnesium GroundwaterDesign Sulfate

Class forlocation

2:1 water/soilextract b

Groundwater TotalPotentialSulfate c

Staticwater

Mobilewater

ACEC classfor location

1 2 3 4 5 6 7 8 9SO4

mg/lMgmg/l

SO4

mg/lMgmg/l

SO4

%pH d pH d

≥2.5 AC-1s>6.5 AC-15.5-6.5 AC-2z4.5-5.5 AC-3z

DS-1 <500 <400 <0.24

2.5-4.5 AC-4z>5.5 AC-1s >6.5 AC-22.5-5.5 AC-2s 5.5-6.5 AC-3z 4.5-5.5 AC-4z

DS-2 500-1500

400-1400

0.24-0.6

2.5-4.5 AC-5z>5.5 AC-2s >6.5 AC-32.5-5.5 AC-3s 5.5-6.5 AC-4

DS-3 1600-3000

1500-3000

0.7-1.2

2.5-5.5 AC-5>5.5 AC-3s >6.5 AC-42.5-5.5 AC-4s

DS-4 3100-6000

≤1200 3100-6000

≤1000 1.3-2.4

2.5-6.5 AC-5>5.5 AC-3s >6.5 AC-4m2.5-5.5 AC-4ms

DS-4m 3100-6000

>1200 e 3100-6000

>1000 e 1.3-2.4

2.5-6.5 AC-5m>5.5 AC-4sDS-5 >6000 ≤1200 >6000 ≤1000 >2.42.5-5.5 ≥2.5 AC-5>5.5 AC-4msDS-5m >6000 >1200 e >6000 >1000 e >2.42.5-5.5 ≥2.5 AC-5m

Notesa ‘Brownfield’ locations are those sites, or parts of sites, that might contain chemical residues produced by or associated

with industrial production - Section C5.1.3.b. The limits of Design Sulfate Classes based on 2:1 water/soil extracts have been lowered relative to previous Digests –

see Box C7).c Applies only to locations where concrete will be exposed to sulfate ions (SO4), which may result from the oxidation of

sulfides such as pyrite, following ground disturbance.d An additional account is taken of hydrochloric and nitric acids by adjustment to sulfate content – see Section C5.1.3.e The limit on water-soluble magnesium does not apply to brackish groundwater (chloride content between 12000 mg/l

and 17000 mg/l). This allows m to be omitted from the relevant ACEC Classification. Sea water with chloride contentabout 18000 mg/l and stronger brines are not covered by this Table.

Explanation of suffix symbols to ACEC Class number• Suffix s indicates that the water has been classified as static.• Concrete placed in ACEC Classes that include the suffix z have primarily to resist acid conditions and may be made

with any of the cements in Table D2.• Suffix m relates to the higher levels of magnesium in Design Sulfate Classes 4 and 5.

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Box C9: Incremental rules used in Tables C1 and C2 to adjust the ACEC Class of a sitewith respect to each Design Sulfate Class:

For natural ground with Static groundwater• For Design Sulfate Classes 2, 3 & 4, the ACEC class has been decreased by 1.• For pH > 3.5 there is no further change.• For pH < 3.5 the ACEC class has been increased by 1.• The suffix s has been added.

For natural ground with Mobile groundwater• For pH > 5.5 there is no change.• For pH < 5.5 the ACEC class has been increased by 1 and suffix z has been added.

For brownfield sites with Static groundwater• For Design Sulfate Classes 2, 3 & 4, the ACEC class has been decreased by 1.• For pH > 5.5 there is no further change.• For pH < 5.5 the ACEC class has been increased by 1.• The suffix s has been added.

For brownfield ground with Mobile groundwater• For pH > 6.5 there is no change to the ACEC class.• For pH 5.5 - 6.5 the ACEC class has been increased by 1 and suffix z has been added.• For pH 4.5 - 5.5 the ACEC class has been increased by 2 and suffix z has been added.• For pH < 4.5 the ACEC class has been increased by 3 and suffix z has been added.

For sites with Static water, two adjustments will often be applicable and these may cancel out.

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Appendix C1: Recommended testprocedures for ground aggressive toconcreteUK test methods for the chemical analyses of aggressivesoil and groundwater have traditionally beendocumented in BS 1377: Part 3: 1990. Because thisStandard did not cover some tests needed for groundinvestigation in respect of concrete, it was supplementedin 1995 by procedures detailed in BRE Report BR 279.This updated some test procedures covered by BS 1377to include modern techniques, such as determination ofsulfate in aqueous solutions by cation exchange and ionchromatography.

Currently, however, both these documents are in need offurther revision to accord, for example, with the latestwidespread practice of determining elements in solutionby use of inductively coupled plasma atomic emissionspectroscopy (ICP-AES), rather than gravimetricanalysis. The larger test laboratories currently follow in-house procedures using this latter approach, rather thanBS 1377 and BR 279. The list of recommended test methods in Box C10 isbased on recent BRE experience and the outcome of areview of test procedures for determination of sulfurspecies, reported in TRL Report 447 [12]. This list is notexclusive, however, and other methods may be usedprovided they can be demonstrated to be appropriate.

In respect of total sulfur determination for detection ofpyrite, the preferred method is high temperaturecombustion of a dried and ground specimen (<150µm) inan appropriate instrument, eg a modern carbon-sulfurdeterminator. All sulfur species present are evolved assulfur dioxide that is quantified by infra-red detectors.The procedure is described in BR 279 under the Ignitionin oxygen method. It is rapid (taking only a few minutes)and relatively low cost when carried out on numeroussamples. Generally the procedure has been found tohave an accuracy of the order of 1% provided anappropriate test procedure is used. This should includeuse of an appropriate ‘accelerator’ such as tungstentrioxide or vanadium pentoxide, efficient trapping of anywater vapour evolved and regular calibration usingstandard materials, including pyrite.

Warning – determination of total sulfur in specimenscontaining pyrite by the procedure given in Clause B.2 ofBS 1047 (as advocated by BS 1377: 1990) is notconsidered appropriate. It is understood that thisprocedure was aimed at determining the amount ofcomplex monosulfides found in blastfurnace slag. Pyriteis a divalent sulfide (FeS2) that is typically chemicallyrobust and is apparently not so readily dissolved by theapplied nitric and hydrochloric acids. Instances areknown where the total sulfur content of pyrite-bearingclays was under-measured by some 50% when usingthe BS 1047 procedure

Appendix C2: Guidance on comprehensivesite investigation of sulfate groundWhile the scope and procedures detailed in Sections C4and C5 should be appropriate for site investigationleading to routine specification of durable concrete in theUK, it does not provide the full understanding of sulfateground conditions which may be required for otherapplications or investigation of cases of sulfate attack onburied concrete. The principal lack in the given standardprocedures is a separate determination of the likelyconstituent sulfates (those of calcium, magnesium,sodium and potassium metals). Knowledge of these canbe important bearing in mind that the Mg, Na and Ksulfates are potentially more problematic than Ca sulfateowing to their high solubilities and different chemicalactivities, particularly in the case of Mg sulfate which isnotably aggressive to some types of concrete.

If a better understanding is required, a comprehensivesuite of soil and groundwater analyses can be carriedout to include determination of Ca, Mg, Na and K ions.Procedures for these are given in BR 279. It can also becarried out by ICP-AES analysis. Where cases of sulfateattack are being investigated and groundwater samplescan be obtained, it is additionally recommended thatcarbonate (or bicarbonate), chloride and nitrate contentsbe determined. The former will provide data on apossible source of external carbonates that can fuel TSAin the absence of carbonate aggregates, while theinclusion of all three will enable an ion balance check tobe made to provide an assurance that the principalconstituents of the groundwater have been accuratelydetermined. Such an ion balance check will require:

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(1) the quantities of the determined ions (mg/l) to bedivided by their atomic weights to find their relativenumbers in solution;

(2) the numbers of ions to be multiplied by theirvalencies as indicated by the respective superscriptsin the following SO4

-2, Cl-1, NO3-1, CO3

-2,Ca+2, Mg+2,Na+1, K+1;

(3) the algebraic sum of (2) to be expressed as apercentage by dividing by the total number of ions.

Any significant out-of-balance that can not be accountedfor by analytical errors may indicate the presence ofsome other ion which needs to be identified.

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Box C10: List of recommended source documents and test methods

Chemicaldeterminations recommended bythis Special Digest

Symbol(unit)

Recommended sourcedocuments

Recommended test methods

SoilpH in 2.5:1 water /soil extract pH BR 279 Electrometric method

BS 1377: Part 3 Section 9 Electrometric methodSoluble sulfate in2:1water / soil extract

WS (mg/l SO4)

BR 279 Procedures for gravimetric method, cation exchange, orion chromatography

BS 1377: Part 3:1990,Section 5

Gravimetric or ion exchange methods. (Values determined as mg/l SO3 should be multipliedby 1.2)

TRL Report 447: Test 1 Sulfate extraction procedure as BS 1377, butICP-AES used to determine sulfur in solution

Acid-soluble sulfate AS (% SO4) BR 279 Gravimetric method

BS 1377: Part 3:1990, Section 5

Gravimetric methods(Values determined as mg/l SO3 should be multipliedby 1.2)

TRL Report 447: Test 2 Preparation and extraction of sulfate as BS 1377,ICP-AES used to determine sulfur in solution

Total sulfur TS (% S) BR 279 Ignition in oxygen method, eg with sulfur-carbondeterminator

TRL Report 447: Test 4A Microwave digestion method

TRL Report 447: Test 4B Ignition in oxygen method, eg with sulfur-carbondeterminator

Magnesium in 2:1water / soil extract

WMg (mg/lMg) BR 279 Atomic absorption spectrometry (AAS) method

Commercial test lab in-houseprocedure

Sample preparation as BR 279, ICP-AES used todetermine magnesium in solution

Ammonium ion (mg/l NH4+) BR 279

Nitrate in 2:1 water /soil extract (mg/l NO3) BR 279

Chloride in 2:1 water/ soil extract (mg/l Cl) BR 279

BS 1377: Part 3, Section 7GroundwaterpH pH BR 279 Electrometric method

BS 1377: Part 3, Section 9 Electrometric methodSoluble sulfate GWS

(mg/l SO4) BR 279 Procedures for gravimetric method, cation exchangeor ion chromatography

BS 1377: Part 3, Section 5 Gravimetric or ion exchange methods (Values determined as mg/l SO3 should be multiplied by1.2)


Determination of sulfur by ICP-AES

Soluble magnesium GWMg (mg/l Mg)

BR 279 Atomic absorption spectrometry (AAS) method


Determination of magnesium in solution by ICP-AES

Ammonium ion (mg/l NH4+) BR 279

Nitrate ion (mg/l NO3) BR 279Chloride ion (mg/l Cl) BR 279

BS 1377: Part 3 Section 7Aggressive carbondioxide (mg/l CO2) pr EN 13577:1999

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References – Part C

[1] Department of Environment, Transport and the Regions. The thaumasite form of sulfate attack: Risks, diagnosis,remedial works and guidance on new construction. Report of the Thaumasite Expert Group. DETR, January 1999.

[2] Cripps J C and Edwards R L. Ground Chemistry Implications for Construction. Paper 2-2: Some geotechnicalproblems associated with pyrite bearing mudrocks. Ed Hawkins A B. Balkema. Rotterdam. pp 77-88, 1997.

[4] Department of Environment, Transport and the Regions. Handbook of model procedures for managingcontaminated land. Contaminated Land Research Report 11. DETR, 1999.

[5] Construction Industry Research and Information Association. Remedial Treatment of Contaminated Land. Vol III:Site investigation and assessment. Special publication 103, CIRIA, London, 1995

[6] Environment Agency. Guidance for safe development of housing on land affected by contamination. R & DPublication 66, Environment Agency, Bristol, 2000.

[7] Environment Agency. Secondary model procedures for the development of appropriate soil sampling strategies forland contamination: R&D Technical Report P5-066/TR, Environment Agency, Bristol, 2000.

[8] Environment Agency. Risks of contaminated land to buildings, building materials and services: A literature review,R&D Technical Report P331, Environment Agency, Swindon, 2000.

[9] Environment Agency. Assessment of and management of risks to buildings, building materials and services fromland contamination: A literature review, R&D Technical Report P5-035/TR/01, Environment Agency, Swindon, 2001.

[10] Perry J and West G. Sources of information for site investigations in Britain. Report 192. TRL, 1996.

[11] Highways Agency et al. Treatment of fill and capping materials using either lime or cement or both. Report HA74/00, Design Manual for Roads and Bridges, 2000.

[12] Reid J M, Czerewko M A and Cripps J C. Sulfate Specification for structural backfills. TRL Report TRL447:Transport Research Laboratory, 2001.

BRE Reports

BR 255. Paul, V. Performance of building materials in contaminated land. Garston, BRE, 1994.

BR 279. Bowley, MJ. Sulfate and acid attack on concrete in the ground: recommended procedures for soil analysis..Garston, CRC, 1995.

BR 447. Charles, JA, Chown, RC, Watts, KS, and Fordyce, G. Brownfield sites: ground related risks for building. Garston,CRC, 2002.

BRE Digests

318 Site investigation for low-rise building: Desk studies

348 Site investigation for low-rise building: The walk-over study

381 Site investigation for low-rise building: Trial pits

383 Site investigation for low-rise building: Soil description

411 Site investigation for low-rise building: Direct investigation

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British Standards Institution

BS 1377: Methods of test for soils for civil engineering purposes

Part 1:1990 General requirements and sample preparation

Part 3:1990 Chemical and electro-chemical tests

BS 5930: 1999 Code of practice for site investigation for civil engineering

BS 8004: 1986 Code of practice for foundations

BS 10175: 2001 Code of practice for the identification of potentially contaminated sites

pr EN 13577: 1999 Water quality - Determination of aggressive CO2 content.

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