Acid Sulfate Soils

ACID SULFATE SOILS

Concrete Structures - Advice For Design AndConstruction

REVIEW REPORT, EDITION 2 - JUNE 1997

ASS - Concrete S t ruc tures

RTA Techno logy June 1997i

SUMMARY

The use of the terms Acid Sulfate Soils (ASS) and Potential Acid Sulfate Soils (PASS)has increased in the last few years. For many engineers in both design andconstruction, as well as project managers, the knowledge, implications and actionsrequired to deal with ASS and PASS is not well understood. This report is aimed atsupplying the necessary knowledge to enable correct decisions to be made forstructural concrete in ASS and PASS environments. The report :

• gives a brief environmental and geological briefing for the target readers (Sections1 to 4),

• describes associated deterioration of concrete structures (Section 5),

• reviews methods of exposure classification (Section 6),

• recommends an exposure classification method (Section 7),

• details factors to be considered when designing and specifying concrete structuresin contact with acid sulphate soils (Sections 8 to 10),

• reviews and recommends protective coatings and other protection methods(Sections 11, 12), and

• recommends procedures with a flowchart for dealing with the soils (Section 13).

A list of selected papers, standards, manuals, etc is given in the Reference section forreaders who need further details regarding any aspects of the report.

DISCLAIMER

The Roads and Traffic Authority of NSW and its employees or agents involved in the preparation andpublication of this Document do not accept any contractual, tortious or any other form of liability forthe contents of this Document or for any consequences arising from its use. Anyone using theinformation contained in this Document shall apply and rely upon their own skill and judgement.


RTA Techno logy June 1997i i

Table of Contents

1 INTRODUCTION 1

2 BACKGROUND 1

3 IDENTIFIED LOCATIONS OF ASS IN NSW 2

4 IDENTIFICATION OF HAZARDS TO STRUCTURES 2

5 DETERIORATION OF CONCRETE STRUCTURES IN ASS 3

5.1 TYPES OF DETERIORATION 35.2 WHAT IS THE “PH” 45.3 DETERIORATION DUE TO ACIDITY 45.4 DETERIORATION DUE TO SULFATES 5

6 EXPOSURE CLASSIFICATIONS 5

6.1 GENERAL 56.2 ‘92 AUSTROADS BRIDGE DESIGN CODE 66.3 AS 3600 CONCRETE STRUCTURES 66.4 AS 3735 CONCRETE STRUCTURES FOR RETAINING LIQUIDS 66.5 AS 2159 PILING - DESIGN AND INSTALLATION 76.6 OTHER CLASSIFICATIONS 8

7 RECOMMENDED EXPOSURE CLASSIFICATIONS AND DESIGN REQUIREMENTS 9

8 STRUCTURAL DESIGN 13

8.1 CONCEPTUAL DESIGN 138.2 EFFECT OF PROTECTIVE MEASURES ON PILE ULTIMATE CAPACITY 13

9 SACRIFICIAL CONCRETE LAYER 13

10 CONCRETE MIXES 14


RTA Techno logy June 1997i i i

10.1 MEASURES AGAINST SULFATE ATTACK 1410.2 MEASURES AGAINST ACID ATTACK 15

11 PROTECTIVE COATINGS 15

11.1 GENERAL 1511.2 SURFACE PREPARATION - AN ESSENTIAL ELEMENT 1611.3 TYPE OF COATING SYSTEMS 1711.4 COATING SYSTEM LIFE 18

12 OTHER PROTECTION METHODS 18

13 RECOMMENDED PROCEDURES 19

REFERENCES 21

APPENDIX A 23

APPENDIX B 25

APPENDIX C 26


RTA Technology Page 1 o f 27 June 1997

1 INTRODUCTION

The RTA has recently issued a Policy and Procedures Manual and a GuidelinesManual addressing environmental issues and risks posed by acid sulfate soils (ASS)and potential acid sulfate soils (PASS)24, 25. The overall policy for projectdevelopment, construction, maintenance and decommissioning of roadworks in areascontaining such soils is given in the manuals.

The above manuals focus on the impact of RTA works on the environment where acidsulfate and potential acid sulfate soils exist, and the identification, classification,treatment, monitoring and management of such soils.

This report provides information and advice to structural designers and projectmanagers to help ensure that concrete structures in naturally occurring ASS and PASSenvironments have the required durability. The information and advice form the RTApolicy on this topic. This report does not cover pavements, concrete pipes or otheracid sulfate or potentially acid sulphate environments such as waste.

2 BACKGROUND

Potential acid sulfate soils are naturally occurring soils which contain iron pyrite (ironsulphide, [FeS2]) or pyritic material in unoxidised state.(The pH of PASS is generallybetween 6 to 7.)

Acid sulfate soils are naturally occurring soils containing pyrite, or chemicalprecursors of pyrite, which have begun to oxidise through exposure to oxygen. Whenwater passes through ASS, sulphuric acid is leached out (the pH of ASS can be as lowas 3.5).

The sulphuric acid reacts with the minerals in the soil to change soil properties. If thesoil has insufficient buffering capacity to neutralise the acid, the soil-water, groundwater and drainage water will all become acidic and will contain dissolved aluminium,iron and heavy metals.

Engineering operations on potential and acid sulfate soils, such as excavation,dredging and draining accelerate the exposure of pyritic material to air. Theseoperations can speed up the production of acidic waters to many times the natural rate.

Passing into waterways and ground water, the sulphuric acid affects plant growth,aquatic life, animal and human health, and degrades engineering structures.

In addition to the deterioration risk for engineering structures in acid sulfate soils, theunconsolidated estuarine sediments containing PASS may cause uneven subsidenceunder relatively low loads, causing structural problems.



In the remainder of this report, ASS will be taken to refer to both ASS and PASSunless otherwise noted.

3 IDENTIFIED LOCATIONS OF ASS IN NSW

ASS may be found at any level up to 5 metres above Australian Height Datum alongthe NSW coastal plain. They may be covered by other sediments.

Reported findings of locations containing ASS in NSW include the floodplains of thefollowing rivers :

• Clarence River

• Clyde River

• Hawkesbury River

• Hunter River

• Macleay River,

• Manning River

• Myall River

• Nambucca River

• Richmond River

• Shoalhaven River

• Tweed River

The above list only covers those sites which have been identified at the time ofpreparation of the RTA Policy on ASS and PASS.

4 IDENTIFICATION OF HAZARDS TO STRUCTURES

Much legislation has been put in place to protect the environment from deteriorationdue to the effects of ASS. Under legislation, acid sulfate soils are those which containgreater than 0.1% sulphide and net acid generation potential greater than 0.0.

The RTA Policy and Procedures Manual requires the identification of acid sulfate soilsat an early stage of project planning. Assessment and treatment procedures togetherwith a procedure for preparing management plans have been set out in that manual.

In obtaining the information necessary to prepare the environmental management plan,much of the information required for the concrete durability design will be obtained.

The identification of ASS is carried out in various stages. For the purpose of thisreport, only two simple methods of identification are noted.



The two methods are : -

(i) visual indicators of ASS,(ii) on-site pH measurements.

Visual indicators of ASS at a site may include yellow efflorescence on the soil surface,sulphurous smell, iron staining and iron flocculants in streams.

On site pH measurements of streams and fresh ground water samples, and of 1:5 soil :water samples provide a good indication of the likely severity of the acid sulfateproblem.

Where the soils contain enough sulphides, the sulfate content of ground watercollecting in construction pits, wells or boreholes may increase over a period of weeksto several times the original value. After the backfilling of the construction pits, thesulfate content soon drops to the previous level, since the supply of air has beeninterrupted . This explains why water samples taken from the construction pit areusually higher in sulfates than those obtained from exploratory drilling. Protectivemeasures based on the higher sulfate content of water samples obtained from theconstruction pit would be excessively conservative and expensive since the formationof sulfates is in this case local and transient.

Moving water is particularly dangerous to concrete. In stagnant water, the dissolvedsalts will tend to combine with the components of the hardened cement paste. Forexample, the sodium sulfate content of ground-water will react with the calciumhydroxide in cement to form gypsum. The pores of concrete are sealed to a certainextent by the precipitated gypsum. As a result, a natural protective layer is developedon and near the concrete surface. Also, in moving water the aggressive acid sulfatesmay be replenished, whilst in stagnant water the acid sulfates become exhausted withtime. In cohesive soils (clay) the seepage rate of ground-water is of the order 10-5 m/swhile in granular soils, rates a hundred or even a thousand times higher are possible.In such soils, higher rates of deterioration should be anticipated.

5 DETERIORATION OF CONCRETE STRUCTURES IN ASS

5.1 Types of Deterioration

In general terms, depending on the predominant chemical reaction, deteriorationprocesses can be classified generally into three groups, namely:

• leaching, which removes part or all of the hardened cement paste from concrete;

• deterioration by exchange reactions and by the removal of readily solublecompounds from the hardened cement paste;

• swelling deterioration, largely due to the formation of new, stable compounds inthe hardened cement paste



5.2 What Is The “pH”

The acid or alkaline character of a liquid depends on its content of H+ (hydrogen ions)and OH- (hydroxyl ions). There are hydrogen and hydroxyl ions in water. Thecondition of any aqueous solution is described by its hydrogen ion concentration.

The pH value is -log10 (hydrogen ion concentration). In fully neutral water thenumber of H ions which cause acidity is equal to the number of OH ions, which causealkalinity. Any increase in the number of the one type of ion in water is accompaniedby a corresponding decrease in the number of the other type. The neutral pH level is 7.Decreasing numbers from 7 to 0 indicate increase in acidity. Increasing numbers from7 to 14 indicate increase in alkalinity.

A small change in the pH value is significant since the scale is logarithmic.

Approximate classification for pH values

pH

0-1-2-3 4-5-6 7 8-9-10 11-12-13-14

Character ofwater

Acidic Mildly acidic Neutral Mildlyalkaline

Alkaline

pH content H+ ions predominant OH- ions predominant

5.3 Deterioration Due To Acidity

Acids in concentrations common in natural waters tend to dissolve the carbonate layeron the surface of concrete, preventing further carbonation and promoting thereby theleaching of lime from the interior of concrete. Concrete will deteriorate because thecalcium hydroxide of concrete and the acids attacking it form water soluble saltswhich are subsequently leached. Beside the general leaching effect of acids , sulphuricacids may give rise to sulfate swelling as well.

The rate of acid corrosion of any concrete is controlled by the nature of the acid, theconcentration of free hydrogen ions (the pH), and by the solubility of the calcium saltsformed by exchange reactions with the salts dissolved in the water. These calciumsalts, if soluble, are leached from the concrete.



5.4 Deterioration Due To Sulfates

The sulfates most detrimental to Ordinary Portland Cement are those of ammonium,calcium, magnesium, and sodium. Potassium, copper and aluminium sulfates are lessharmful. Barium sulfate and lead sulfate which are insoluble in water do not affectconcrete.

Damage to concrete is caused by an expansive chemical reaction between tricalciumaluminate C3A in the cement and sulfates in solution which produces both gypsum andcalcium sulphoaluminate (ettringite). The crystals of ettringite occupy a largervolume than the original compounds. The larger volume leads to concrete expansion,cracking, and disintegration.

The aggressiveness of soil containing sulfates is specified in terms of SO3 content andrecently in terms of SO4 content. However SO3 can be converted into SO4 by thefollowing relationship :

SO3 = 0.83 SO4

6 EXPOSURE CLASSIFICATIONS

6.1 General

There are a number of codes, standards and other references which deal with exposureclassifications. This section reviews the available classifications and aims to giveguidance when selecting exposure classifications for ASS of various aggressiveness.

According to the exposure classifications, concrete quality, chemical contentrestrictions, cover, and other requirements are determined.

In determining the classification of structural members in ASS, thedesigners/specifiers in conjunction with the Project Managers should weight thepossible changes in the environment, and hence classification, over the design life ofthe structure.

Since changes to the pH with time is dependant on many factors, there is no directvalid laboratory method capable of measuring potential pH. However, by carefulstudy of various factors and examination of test results (tests in Appendix B ofreference 24), experts would be able to predict potential changes to the pH within areasonable range. It is suggested that such estimate should be requested from thegeotechnical consultant.

For example, if pH of ground water is measured at 7 at the investigation stage, butother tests have shown that the soil is potential acid sulfate soil, this means that pH cansignificantly drop as the soil becomes disturbed or drained. Therefore constructionmethods, future development, and other factors which result in draining or disturbing



potential acid sulfate soil should be considered when determining the exposureclassification.

The permeability of soil, discussed earlier, is another factor to be taken into accountwhen determining the exposure classification. In this section and associatedappendices, soils with permeability less than 10-5 m/s are referred to as lowpermeability soils (eg clay) and soils with higher permeability are referred to as highpermeability soils (eg sand). Free water streams come under the category of highpermeability soils.

It is important that designers and specifiers ensure that appropriate and completeinformation is reported by the responsible investigation parties, evaluated and thenused when selecting an exposure classification.

6.2 ‘92 AUSTROADS Bridge Design Code

Classifications are given in the order of increasing aggressiveness from A, B1, B2 toC. “U” classification is used for other exposures subject to special consideration.

For ASS, the exposure classification “U” is to be used since the specific environmentfor such soils is not included in any of the classifications A to C. The Code requiresthe designers/specifiers to identify such exposures and specify requirements to ensuredurability.

It is the designer’s responsibility to then draw limits and requirements for thisparticular exposure. The Code broadly considers that permeable soils with a pH < 4.0or ground containing more than one gram per litre ( 1000 mg/l or 1000 ppm) of sulfateions as aggressive.

6.3 AS 3600 Concrete Structures

The approach and classifications used this Standard are similar to that of‘92 AUSTROADS.

The commentary for the Standard 8 lists some references for guidance to limits andrequirements to be specified for exposure classification “U”.

6.4 AS 3735 Concrete Structures For Retaining Liquids

Four basic exposure classifications in order of increasing aggressiveness from A to Dare given in the Standard. The classification is in line with ‘92 AUSTROADS andAS 3600 but with an additional classification D in the absence of classification U.Also comprehensive guidance is given in a separate Supplement 10.

It should be noted that for all concrete surfaces in exposure classification D, theStandard requires such surfaces be isolated from the attacking environment.

The exposure classifications are determined for a range of environments which are:



1. Fresh water 4. Corrosive liquids, vapours and gases2. Sewage and waste water 5. Other liquids3. Sea water 6. Ground water

The applicable item for ASS is No. 6, in which a broad range of classifications isgiven and reference made to the Supplement for assistance. The applicable item,together with material from the Supplement, is rearranged and detailed inAppendix A.

The exposure classification for the surface of a member is to be determined from theStandard and from AS 3600 for the most severe environment, or use, to which theconcrete will be subjected during its operational life. However in the case of ASS, AS3735 requirements are more detailed than AS 3600 and hence overrule.

For ASS, the above means that the exposure classification needs to be determined forboth sulfate aggressiveness and for acidity and the higher classification from the two isto be used in accordance with the qualifications given in the Standard (eg the use ofSulfate Resistant cement and/or the use of limestone aggregates).

The Standard recognises the following as methods for obtaining such concrete:

• the use of sulfate resistant cements (superseded cement classification Type D)

• the use of pozzolanic material (eg fly ash) blended with Ordinary Portland Cementie blended cements

• the use of a waterproofing agent with Ordinary Portland Cement.

Sulfate resistant concrete and RTA preferred methods for obtaining sulfate resistanceare discussed in detail in later sections.

6.5 AS 2159 Piling - Design and Installation

This Standard uses different exposure classifications to ‘92 AUSTROADS and theother Australian Standards reviewed above. The classification is self explanatory andcomprises Non-aggressive, Mild, Moderate, Severe and Very Severe.

The relevant classification for ASS is rearranged and summarised in Appendix B.The Standard uses sulfate expressed as SO3 . To maintain consistency of this reportand to enable comparisons, the SO3 is converted in the appendix to SO4 (this is doneby using the equation SO3 = 0.83 SO4).

The sulfate limits of this Standard approximate those of AS 3735. However directmatching for the two different classifications of the two standards is not appropriatesince specified minimum concrete strengths and cover differ. AS 2159 refers toAS 3735 for design of concrete which is exposed to severe and very severe sulfateenvironments. AS 2159 also uses separate tables for acid and sulfate exposure. Thislimits the usefulness of the standard in ASS conditions.



6.6 Other Classifications

In a report recently published by the Building Research Establishment 14, exposureclassifications are given differently to the above reviewed standards and codes.

The significance of the classification is the progressive selection of exposureclassification and the relationship between various exposures. The SO4 and pH ofsoils and natural ground water are measured and classified accordingly.

Where pH is greater than 5.5, classification is dependant only on the SO4concentration. There are five basic classes for SO4 and two sets of modifications tothese classes to be considered progressively according to types of exposure and typesof structure. Various cement types are also specified within the basic classification.

Where pH is less than 5.5, sites are classified first on basis of SO4 concentration asabove and then reclassified on basis of pH.

The BRE method of determining the SO4 content uses a two stage process. The initialstage uses a simple method to detect the presence of SO4. If this method gives a resultabove a threshold, then a more accurate test is applied to determine the SO4 content fordesign purposes. The procedures use either :

a) undiluted ground water or

b) ground water diluted to 2 : 1

As Australian tests use either undiluted ground water or ground water diluted to 5 : 1,the recommendations in this report are based on undiluted ground water.

Since the scope of the BRE report covers other structures as well as bridges and roadstructures, only classifications and modifications applicable to bridge and roadstructures in ASS are referred to in this report. Appendix C includes the rearrangedclassification tables.

Table C/1 determines the exposure classification in accordance with SO4 content andmodifies the classification according to the member size and the mobility of theground water.

Table C/2 shows the changes to be made to the classification determined from TableC/1 according to the pH and the nature of the ground water.

This last modified classification is used to determine the requirements for cement type,minimum cement content, maximum free water - cement ratio and concrete protection.



7 RECOMMENDED EXPOSURE CLASSIFICATIONS AND DESIGNREQUIREMENTS

Based on the above available information, Tables 1a and 1b below relate exposuresapplicable to bridge foundations in ASS to the classifications of ‘92 AUSTROADS.Exposures are given in term of B1, B2, C, and U. The exposure classification type Ais not used as B1 is the minimum requirement for members in soil or water under ‘92AUSTROADS requirements.

Except for exposure classification ‘U’ in Tables 1a and 1b, concrete quality, coverand other durability requirements are determined according to the exposureclassification in the normal fashion of ‘92 AUSTROADS.

For exposure classification U, recommended design requirements are given in Tables2a and 2b. The two tables relate exposures applicable to bridge foundations in ASS tothe classifications of ‘92 AUSTROADS with additional measures and qualifications.

Design requirements B1, B2, C indicate equivalent concrete requirements to thatspecified for the relevant exposure classification of ‘92 AUSTROADS.

Design requirement C1 indicates design requirement C with the addition of fullisolation of the concrete surface from the aggressive environment.

For retaining and culvert structures, high permeability soil condition and its relevantdesign requirements should be always used. This is due to the nature of constructionof these structures requiring draining and granular fill.



Table 1a : Recommended Exposure Classification in Terms of ‘92 AUSTROADSFor LOW Permeability Soil

SO4 Equivalent Exposure Classifications in terms of ‘92 AUSTROADS

(mg/l or ppm) pH

≤ 3.5 > 3.5

≤ 4.5

> 4.5

≤ 5.5

> 5.5

< 400 U U B1 B1

400 - 1500 U U B1 B1

1500 - 3000 U U B1 B1

3000 - 6000 U U U U

> 6000 U U U U

Table 1b : Recommended Exposure Classification in Terms of ‘92 AUSTROADSFor HIGH Permeability Soil

SO4 Equivalent Exposure Classifications in terms of ‘92 AUSTROADS

(mg/l or ppm) pH

≤ 3.5 > 3.5≤ 4.5

> 4.5≤ 5.5

> 5.5

< 400 U U B2 B1

400 - 1500 U U C B2

1500 - 3000 U U U U

3000 - 6000 U U U U

> 6000 U U U U



Table 2a : Design Requirements for Exposure Classification Type U of Table 1a ForLOW Permeability Soil

SO4 Design Requirements (See Notes : Table 2 below)

(mg/l or ppm) pH

≤ 3.5 ≤ 4.5> 3.5

> 4.5≤ 5.5

> 5.5

< 400 B2 B1

400 - 1500 B2 B1

1500 - 3000 B2 B1

3000 - 6000 C B2 B2 B2

> 6000 C1 C B2 B2

Notes : Table 2a

1. Table 2b is to be used for retaining and culvert structures.

2. Design requirements B1, B2, C indicate equivalent concrete requirements to that specifiedfor the relevant exposure classification of ‘92 AUSTROADS.

3. Design requirement C1 indicates design requirement C with the addition of full isolationof the concrete surface from the aggressive environment.

4. Environments under the dark horizontal line require sulfate-resisting blended cement.(Refer to section 10).

5. Environments to the left of the dark vertical line require require blended cement concretescontaining calcareous aggregate with an increased concrete cover unless designrequirement C1 is achieved. (Refer to section 8.)



Table 2b : Design Requirements for Exposure Classification Type U of Table 1bFor HIGH Permeability Soil

SO4 Design Requirements (See Notes : Table 2 below)

(mg/l or ppm) pH

≤ 3.5 > 3.5≤ 4.5

> 4.5≤ 5.5

> 5.5

< 400 C1 C

400 - 1500 C1 C

1500 - 3000 C1 C C B2

3000 - 6000 C1 C1 C C

> 6000 C1 C1 C1 C1

Notes : Table 2b

1. Design requirements B1, B2, C indicate equivalent concrete requirements to that specifiedfor the relevant exposure classification of ‘92 AUSTROADS.

2. Design requirement C1 indicates design requirement C with the addition of full isolationof the concrete surface from the aggressive environment.

3. Environments under the dark horizontal line require sulfate-resisting blended cement(refer to section 10).

4. Environments to the left of the dark vertical line require blended cement concretescontaining calcareous aggregate with an increased concrete cover unless designrequirement C1 is achieved. (Refer to section 8.)

5. Modification to the table for retaining and culvert structures

• Calcareous aggregate shall not be used

• B2 becomes B2 plus full isolation

• C becomes C1

• C1 : no change



8 STRUCTURAL DESIGN

8.1 Conceptual Design

The conceptual design of a bridge or any other structure in ASS should cater forassociated deterioration and environmental risks.

Suitable designs in such cases would incorporate minimum excavation. For exampledriven piles would be preferred over in-situ bored piles. Also the possibility ofextending the piles above the ground to the headstocks or having above ground pilecaps ( hence avoiding excavation) should be considered.

8.2 Effect of Protective Measures on Pile Ultimate Capacity

If the shaft of a concrete pile is covered by a special casing, liner or coating, it isnecessary to take into account the reduction in the shaft resistance for that section ofthe pile which is protected. The overall bearing capacity of friction piles is likely to begreatly reduced compared to end-bearing piles. Where cast-in-situ piles have to beinstalled in ground which is highly aggressive over the full depth of the piles, fullysleeved pile shafts on expanded bases of inert aggregate can be used, but the bearingcapacity may be less than when the pile bases are formed with concrete.

9 SACRIFICIAL CONCRETE LAYER

Under conditions of stagnant soil water, the use of reactive aggregate can mitigate thedegree of chemical attack on the concrete. For example, the appropriate use of goodquality limestone aggregate neutralises part of the acid that would otherwise attack thecement paste.

There are certain advantages in having a coarse aggregate which is not completelyimmune to attack. One of the advantages is that the concentration of acid in theaggressive water can be reduced more rapidly if it reacts with both the aggregate andthe cement.

Since limestone-aggregate concrete is capable of maintaining a reasonably smoothsurface during erosion, it can be practicable to design for relatively thick sacrificiallayers. Based on Hughes et al, the rate of erosion for 0.0016% acid concentration (pHof approximately 3.5) was about 0.75 mm per year, so for a life of 20 years and asafety factor of 2, the required sacrificial layer would be 30 mm.

When total cover, comprising the design cover plus the sacrificial layer thickness,exceeds 80 mm, the use of supplementary mesh reinforcement in the cover zone isrequired.



The use of limestone aggregate without an increase in concrete cover is disastrous.Therefore it is crucial that when limestone aggregate is specified for ASS anappropriate increase in concrete cover, as a sacrificial layer, should be made. Theincrease of concrete member thickness due to such a layer should not be accounted forwhen calculating design capacity.

10 CONCRETE MIXES

10.1 Measures Against Sulfate attack

To resist sulfate attack it is essential that the concrete is dense and well compacted.Low concrete permeability and choice of cement type is more important than highcharacteristic strength.

It has been reported that a more mature concrete is far more resistant than an immatureconcrete exposed to sulphate attack. It is highly recommended for the concrete to befully cured and matured before exposure to sulphate.

In neutral to alkaline environment, some resistance to sulfate attack is obtained whenthe tricalcium aluminate , C3A, content of the cement is kept low. The traditionalsulfate resisting Portland cements with low C3A have been and are still used in suchenvironments.

In acidic environments, the efficiency of Portland cement of low C3A by its own isquestionable. Work done by the CSIRO and others has demonstrated that the additionof supplementary cementitious materials to Portland cement improves sulfateresistance in neutral to acidic environment. It has been reported in severalexperimentally-based papers that the use of blended cements gives protection againstsulfate attack that is superior to that provided by sulfate resisting Portland cement.This is regarded as being due to the lowering of C3A content by replacement ofPortland cement, the refined pore structure, and the reduction of calcium hydroxiderequired for the formation of gypsum in the attack mechanism.

The use of blended cements referred to above has some qualifications. Blendedcements of Portland cement and fly ash has good sulfate-resisting properties onlywhen the fly ash content exceeds 25% of the total cementitious content of the concrete.The use of fly ash proportions exceeding 40% is not recommended for general use,because of placement difficulties and a lack of data on performance.

Similarly, blended cements with high slag content not less than 65% are consideredsulfate-resisting. The higher the slag content up to a maximum of 75%, the higher theconcrete resistance to sulfate attack. The use of such high slag content cement shouldbe restricted to concrete surfaces which will be either permanently in a wet conditionor permanently isolated from air. This restriction guards against the high risk ofdeterioration by carbonation of such concretes when exposed to carbon dioxide presentin the air.



Blended cements with 5-15 % silica fume also have good sulfate-resistingcharacteristics.

Care should be taken when specifying sulfate resisting cement in acidic environment.Calling for AS 3972 type SR cement for use in ASS without further qualificationswill not guarantee the required resistance. A sulfate resisting blended cementwith proportions as above and quality as specified in RTA QA SpecificationB80 23, should be clearly stated and used.

10.2 Measures Against Acid Attack

The erosion of concrete surfaces by acidic water is affected much less by the type ofcement than by the quality of the concrete. However, the lower porosity of blendedcements is generally regarded as being beneficial in reducing the rate of acid attack. Itis recommended that blended cements incorporating fly ash and/or slag as indicatedabove be used.

The use of limestone aggregate in a sacrificial concrete layer has been discussedpreviously. In the case of piles installed in an impermeable clay soil, acid or sulfateattack only penetrates the concrete to such a small extent that the incorporation ofsome extra centimetres of dense “sacrificial concrete”, may obviate the need forspecial cements. However in other circumstance where higher permeability soil ispresent, both measures in addition to others may be necessary.

11 PROTECTIVE COATINGS

11.1 General

Protective coating is a specialised field and outside the scope of this report. Thepurpose and the scope of this section is for general reference only.

Coating of steel members is very well established and a wide range of references andstandards are available for use including RTA specification B220. Concrete coatingsare covered in many references but not to the extent of steel coatings.

Concrete coatings should be considered as one line of defence in an aggressiveenvironment along with other measures. Thus, as a general rule, using coatings doesnot mean relaxation of concrete quality and composition discussed in earlier sections.

The selection of a coating type is a complex process and is affected by several factors.These factors include :

• The type of surface to be coated and its condition

• Surface orientation (eg horizontal, vertical)

• Construction methods



• The environment and level of aggressiveness

• Duration and changes of exposure

• Temperature at time of curing of coating

• System life required

• Maintenance frequency and methods, if feasible

Therefore, attempting to recommend a general coating system for a specifiedenvironment is not appropriate since the environment is not the only factor. An expertopinion should be sought on a case by case basis.

11.2 Surface Preparation - An Essential Element

Surface preparation and conditions under which the coating is applied are extremelycritical. Therefore, such processes should have appropriate supervision to ensure theefficacy of the coating system.

Minimum requirements for surface quality and cleanliness are usually specified by thecoating supplier in terms of methods of preparation to be used. This is to ensure thebonding of the coating to the concrete surface. Mechanical cleaning such as blastingis usually required.

In ACI 515.1R-79, there are a few quick and simple methods to measure thecleanliness of a surface, three of which are reported herein

Dusty condition. Wipe the surface with a dark cloth. If a white powder is on thecloth, the surface is considered to be too dusty and therefore unsatisfactory for somecoating systems

Oily condition. Sprinkle water on the dried concrete surface. If the water spreads outimmediately instead of standing as droplets, it may be concluded that the surface is notcontaminated by oils or dust.

Laitance. The presence of laitance may be detected by scraping the surface with aputty knife. If a loose powdery material is observed, excessive laitance is present.Adhesion could be adversely affected by this laitance.

The repair of defects in concrete which will be covered with a coating requires specialattention. Patches should be allowed to cure and full bond achieved prior to coatingapplication. If poor adhesion and/or unsound patching is suspected, then the patchshould be removed and replaced with a new sound patch prior to coating application.

The dryness of the concrete surface is also critical. A maximum moisture content of 5to 8 % is usually recommended by suppliers. Moisture content is considered excessiveif moisture collects at the bond line between the concrete and the coating materialbefore the coating has cured. This may be evaluated by taping a 1 m x 1 m clear,



polyethylene sheet to the concrete surface and determining the time required formoisture to collect on the underside of a polyethylene sheet. This can be comparedwith the curing time of the coating reported by the supplier. However the use of aprimer facilitates the process as its curing time is normally shorter than the maincoating.

11.3 Type of Coating Systems

There are a number and variety of coating systems. The basis of the coating and thecuring process differentiate the various systems. The following are the recommendedgeneric types of coating systems suitable for application to concrete.:

1. Chlorinated Rubber Systems

2. Epoxy Systems

3. Vinyl Ester Systems

4. Bituminous Systems

Epoxy coatings have generally satisfactory acid resistance where pH>3.0. They arenot suitable where pH <3.0.

Coal tar epoxies should not be used for any coating purposes. Their use has not beenapproved by the Australian Paint Approval Committee (previously known asGovernment Paint Committee) for health hazard reasons.

A bituminous coating provides an economical alternative to proprietary paints forcoating concrete elements which will be buried. Bitumen has good resistance to acidattack and water penetration and adheres well to properly prepared concrete surfaces.Application procedure should be as steps 1 and 2 below followed by a priming coat ofcutback bitumen or thinned bitumen emulsion. Two spray applications of cutbackbitumen or bitumen emulsion (with at least one day between coats) should then beapplied. Care should be taken when placing granular fill around bitumen coatedconcrete as bitumen when cured is usually not as hard or abrasion resistant asproprietary paints. Bitumen coating of culverts should ideally be carried out insitu toavoid potential coating damage during transport/installation.

Vinyl ester coatings are the most appropriate coating for the protection of concrete inacid soil conditions where pH<3.0. A complete system of any of the above systemswould include elements such as surface requirements, primer and one or more coats ofthe material. For example, the recommended procedure for application of a vinyl estersystem, in a high acidic environment is as follows :

1. Lightly brush blast concrete surface to remove laitance



2. Fill voids in concrete surface. Where many small voids are present , use acement render. An epoxy putty may be appropriate for filling a few, largevoids (greater than 2 mm)

3. Apply a moisture cured urethane primer, to seal surface

4. Apply high build vinyl ester coating, 1 coat with minimum dry film thickness2 mm.

Application of coatings should be by trained and experienced applicators only.

11.4 Coating System Life

The life of a coating systems varies from one system to another. Designers, specifiersand Project Managers should be aware of such variations and the relatively short lifeof coating systems compared with the design life of the structure. It is necessary toconsider the coating system life along with the other protection measures taken in anaggressive environment (eg sacrificial limestone concrete cover, etc)

The life of coating types as indicated earlier could vary from 10 to 20 years or evenmore depending on the severity of the conditions and the quality of surface preparationand coating application.

12 OTHER PROTECTION METHODS

Protection methods other than coatings may be required in some circumstances. Suchcircumstances include a prolonged and very aggressive environment capable ofpenetrating beyond the resistance of any concrete and coating combination.

In these circumstances, higher level of protection is required. There have been anumber of isolation methods previously used some of which are listed below.

• 10 mm gauge permanent mild steel casings with an inner liner of 1.5 mm thickheavy duty PVC membrane have been used for piles in extremely highconcentration of sulfates (13,800 to 28,800 ppm).

• In an environment of 3000 ppm of sulfate in addition to high concentration ofchlorides in the Middle East, PVC sleeves coated with nylon fabric wererecommended for pile protection.

• Packing a layer of limestone around the exposed face of the concrete componenthas been used to protect piles in acidic ground water.

• The precautionary measure of providing an extra sacrificial layer of concretearound the shaft of the pile is sometimes used together with other measures (it israrely relied upon by itself in very highly aggressive conditions)



• Local replacement of permeable soil around piles with an impermeable layer ofsoil.

13 RECOMMENDED PROCEDURES

The flowchart in the following page summarises procedures to be followed bydesigners, specifiers and/or Project Managers in the design, specification andconstruction of structures in ASS. The procedures are grouped in three main stages:-

the investigation stage,

the design and review stage, and

construction stage.

At each of these stages a number of steps is recommended. It is essential thatdesigners, specifiers and project managers communicate at all stages of the projectdevelopment so as to deliver adequate, buildable and economical structures.

Monitoring and evaluation of design, specification and construction methods ofstructures in ASS in a project will not only satisfy that project requirements but alsobenefit other projects.

Relevant sections of this report are referred to in the flowchart against most of therecommended steps.



Obtain briefing on the nature of ASS and PASS.

Actions Reference Section(s)

1, 2, 3

Gather monitored data from completed projects dealing with ASS / PASS

Obtain existing soil conditions for permeability, pH, and SO4 4, 5

Obtain expert advice on potential pH and SO4 over life time of structure 6

Determine exposure classifications and durability design requirements 7

Input into the overall structural design, as required 8

Input into the concrete section design and technical specification, as required 9

Specify special concrete mix requirements 10

Obtian expert advice on type of coatings and specify as required 11

Specify other protection method, as required 12

Monitor compliance with design and specification

DEALING WITH ACID SULFATE SOILS IN DESIGN AND CONSTRUCTION FLOWCHART

Investigation

Design and Review

Construction



REFERENCES

1. ‘92 AUSTROADS “Bridge Design Code”, Section Five-Code, AUSTROADS1992.

2. ‘92 AUSTROADS “Bridge Design Code”, Section Five-Commentary,AUSTROADS 1992.

3. ACI Committee 515, “Guide for the Protection of Concrete against ChemicalAttack by Means of Coatings and Other Corrosion Resistant Materials”, ACIManual Part 5

4. Al-Amoudi O.S., Maslehuddin M. and Saadi M.M., “ Effect of Magnesium Sulfateand Sodium Sulfate on the Durability Performance of Plain and BlendedCements”. ACI Materials Journal, V.92, No.1, Jan-Feb 1995.

5. AS 2159 “ Piling-Design and Installation”, Standards Australia, 1995.

6. AS 2159 Supp1 “Piling-Design and Installation-Guidelines”, Standards Australia,1996.

7. AS 3600 “Concrete Structures”, Standards Australia, 1994.

8. AS 3600 Supp1 “Concrete Structures-Commentary”, Standards Australia, 1990.

9. AS 3735 “Concrete Structures for Retaining Liquids”, Standards Australia, 1991.

10. AS 3735 Supp1 “ Concrete Structures for Retaining Liquids - Commentary”,Standards Australia, 1991

11. Bartholomew R.F., “The protection of concrete piles in aggressive groundconditions : an international appreciation”, symposium paper : RecentDevelopments in the Design and Construction of Piles. Institution of CivilEngineers, 1979.

12. Beal D.L. and Brantz H.L., “ Assessment of the durability characteristics of tripleblended cementitious materials”, Paper presented at Fly Ash, Silica Fume, Slagand Natural Pozzolans in Concrete, Fourth International Conference, Istanbul,Turkey, May 1992.

13. Biczok I., “Concrete Corrosion - Corrosion Protection”, Publishing House of theHungarian Academy of Sciences, Budapest, 1972.

14. Building Research Establishment Digest 363, “Sulfate and Acid Resistance ofConcrete in the Ground”, January 1996.

15. Environmental Impact Statement for State Highway 10-Pacific Highway,Chinderah Bypass. Report by GHD for RTA, 1991.



16. Fattuhi N.I. and Hughes B.P., “Effect of acid attack on concrete with differentadmixtures or protective coatings”, Cement and Concrete Research, vol 13, 1983pp 655-665.

17. Fidjestol P. and Frearson J. “ High-Performance Concrete Using Blended andTriple Blended Binders” .High Performance Concrete Proceedings, ACIInternational Conference, Singapore, 1994. ACI , SP 149-8.

18. Guirguis S. , “Durable Concrete Structures”, CIA Technical Note TN57, March1986.

19. Harrison W.H., "Durability of Concrete in Acidic Soils and Waters", Concrete ,February 1987.

20. Hughes B.P. and Guest J.E., “Limestone and Siliceous Aggregate ConcretesSubjected to Sulphuric Acid Attack”, Magazine of Concrete Research, Vol 30, No102, March 1978 pp 11-18.

21. Mangat P.S. and Khatib J.M., “Influence of Fly Ash, Silica Fume, and Slag onSulfate Resistance of Concrete”, ACI Materials Journal, Vol. 92 No. 5, Sept-Oct.1995.

22. Redner J. A., Randolph P. H. and Esfandi E., “ Evaluation of Protective Coatingsfor Concrete”, Paper from the Proceedings of SSPC 91 Protective Coatings forFlooring and Other Concrete Surfaces, 1991.

23. RTA B80 Concrete Work for Bridges.

24. RTA Guidelines “ Acid Sulfate Soil”, 1996.

25. RTA Policy and Procedures “Acid Sulfate Soil”, RTA, 1995.

26. White I. And Melville M.D., “Treatment and Containment of Potential AcidSulfate Soils”, CSIRO Technical Report No. 53, 1993.



APPENDIX A

Exposure Classification - Sulfate-containing Soils(AS 3735)

SO4 Content Ground water replenishment rate(ie soil permeability)

In Soil%

In watermg/l or ppm

Low (eg clay) High (eg sand)

<0.2 400 A2 B1

0.2 - 0.6 400 - 1500 B1 B2(or B1 with SR cement)

0.6 - 1.2 1500 - 30000 B1 B2,with SR cement

1.2 - 2.4 3000 - 6000 B2(or B1 with SR cement)

C,with SR cement

> 2.4 > 6000 B2,with SR cement

D

Notes : 1. SR cement: Sulfate-Resistant cement2. ppm: part per million



Exposure Classification -Acidic Soils(AS 3735)

Aciditymeasure

Ground water replenishment rate(ie soil permeability)

pH Low (eg clay) High (eg sand)

> 6.5 A1 B1

5.5 - 6.5 A2 B2

4.5 - 5.5 A2 B2, with calcareous aggregate andincreased cover to 125% of nominal

3.5 - 4.5 B1 C, with calcareous aggregate andincreased cover to 125% of nominal

< 3.5 B1, with calcareous aggregate andincreased cover to 125 %

D

Notes : 1. Calcareous aggregate is a limestone aggregate



APPENDIX B

Exposure Classification for Concrete Piles - Sulfate-Containing Soils(AS 2159)

SO4 Exposure Classification

In Soil%

In water (mg/lor ppm)

Low Permeability Soil(eg clay)

High Permeability Soil (eg sand)

<0.25 375 Non-aggressive Non-aggressive

0.25 - 0.62 375 - 1250 Non-aggressive Mild

0.62 - 1.25 1250 - 3125 Mild Moderate

1.25 - 2.5 3125 - 6250 Moderate Severe

> 2.5 > 6250 Severe Very SevereNotes : 1. ppm: part per million

Exposure Classification for Concrete Piles - Acidic Soils(AS 2159)

Aciditymeasure

Exposure Classification

pH Low Permeability Soil(eg clay)

High Permeability Soil (eg sand)

> 6.5 Non aggressive Non-aggressive

5 - 6 Non-aggressive Mild

4.5 - 5 Mild Moderate

4 - 4.5 Moderate Severe

< 4 Severe Very severe



APPENDIX C

Exposure Classification - Sulfate-containing Soils(Other Classifications from BRE report 14)

Table C/1

SO4 content inground water(mg/l or ppm)

Cast -in-situ concrete 140 mm to450 mm in thickness

Cast-in-situ concrete over 450 mmthickness,

and

Precast concrete members which havehad additional air curing after normalcuring cycle. (Several weeks air curing)

LowPermeability

Soil(eg clay)

HighPermeability

Soil (eg sand)

LowPermeability

Soil(eg clay)

High PermeabilitySoil

(eg sand)

< 400 1 1 1 1

400 - 1400 1 2 1 1

1400 - 3000 2 3 1 2

3000 - 6000 3 4 2 3

> 6000 4 5 3 4Notes : 1. ppm : part per million



Modification to Exposure Classification for Acidity(Other Classifications from BRE report 14)

Table C/2

Exposure ofFoundation

members

pH Changes in exposure Class(+ / - = increase / decrease the classdetermined from the previous table)

Low PermeabilitySoil

(eg clay)

High PermeabilitySoil

(eg sand)

Natural > 5.5 No change No change

ground water 3.5 - 5.5 No change +1

< 3.5 +1 +1

Wastes or > 5.5 No change +1

made-up 4.5 - 5.5 +1 +2

ground < 4.5 +1 +3

Documents

Acid Sulfate Soils