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INTERNATIONAL SEMINAR: The Thaumasite Form of Sulfate Attack of Concrete

CONTROL OF THAUMASITE FORMATION IN CONCRETE: CURRENT APPROACH AND RESEARCH NEEDS

Dr Ewan A Byars

Centre for Cement and Concrete University of Sheffield

This paper discusses the current approach taken in BRE Special Digest 1 to the specification of concrete for thaumasite attack resistance, with respect to the lessons learned from the research at Sheffield University raises questions on the issues of:

i) The effects of source of aggressive chemicals (clay or pure solution) on rate of attack of concrete

ii) The potential to afford an additional protective measure by better backfill compaction (reduction of surrounding clay permeability)

iii) The need for a specification on minimum cement contents

iv) The current specification for aggregate carbonate contents and the need for an additional clause that considers alternative carbonate sources and

v) The potential need to reclassify SRPC with respect to Thaumasite resistance

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Control of ThaumasiteFormation in Concrete

Current Approach and Research Needs

Dr Ewan ByarsCentre for Cement and Concrete

University of Sheffield

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Occurrence of ThaumasiteIn March 2002, the TEG reported a significant number

of new cases of thaumasite formation in cementitious construction, including:

•• 16 Highways Agency cases (Gloucester/Wiltshire)• 6 cases associated with sulfate bearing brickwork• 1 Highways Agency case in Co. Durham• 2 internal sand/cement render cases contaminated with

gypsum• 2 cases of slab heave on sulfate-bearing fill• 1 kerb failure in a drainage adit• 1 set of harbour steps exposed to seawater

Notably, 2 cases were observed where the concrete was made with siliceous aggregates

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Current SpecificationsOverview of SD1, Pt. 2

1. Assess Aggressive Chemical Environment for Concrete (Table 2)

2. Define cement type to be used (Table 3)3. Determine Aggregate Carbonate Range (Table 4)4. State Structural Performance Level (Table 5) and

member size5. Use 1, 2 and 4 to determine Design Chemical Class

(Table 7)6. Use output from 5 and 3 in Table 6 to obtain max W/C

and min cement content

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Aggressive Chemical Environment

Important FactorsPresence of sulfates - generally salts of calcium, magnesium and sodium Presence of sulfides – particularly pyrite which may oxidise after excavation and prior to backfill to produce sulfuric acid then sulfate salts on neutralisation with clay minerals or concrete surfaceMobility of groundwater – for transportation and refreshment of aggressive species at concrete surfaceAcidity of groundwater

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Mobility of GroundwaterCurrent Definitions

Static – permanently dry or clay permeability < 10-6m/s

Mobile – water can flow through soil, permeability > 10-6m/s

Highly Mobile – water is flowing through soil e.g. under hydraulic head

Research Questionsi) What is the effect of different degrees of clay density after

compaction on backfill impermeability?ii) How does this relate to rate of thaumasite formation?iii) Can we provide contractors with with backfill compaction

guidelines as an Additional Protective Measure ?

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ExampleClay Density X Attack Rate Y

How does attack rate vary with degree of compaction ?

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Aggressive Chemical Environment for Concrete (ACEC)

The sulfate, sulfide, acid and mobile groundwater values in the current SD1 lead to:

7 Design Sulfate Classes with 16 sub-ACEC classes (Table 2)

These, along with structural and member size details, feed in toTable 7 in SD1 Part 2 to give concrete Design Chemical Classes (DC)

then using Table 6, with aggregate carbonate content details, gives target values of W/C and minimum cement content

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Aggressive Chemical Environment for Concrete (ACEC)

Insert table 2

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Design Chemical Class(based on Tables 3,4 SD1 Pt 2)

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Concrete Quality

Research QuestionDo we need to specify minimum cement content ?

Low-water concrete (made with maximum aggregate content and appropriate use of superplasticizers and mix proportioning) can be made at the relevant w/c ratios and suitable workability with most gravel aggregates and some crushed rock aggregates.

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Aggregate Carbonate Ranges

Research QuestionEffect of carbonate sources other than from aggregate?

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Digest 363 Class 2 SolutionOPC

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Digest 363 Class 2 SolutionPLC

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Digest 363 Class 3 SolutionOPC

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Digest 363 Class 3 SolutionPLC

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Digest 363 Class 3 SolutionPFA

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Digest 363 Class 3 SolutionSRPC

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Digest 363 Class 3 SolutionGGBS

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Digest 363 Class 4B SolutionPFA

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Digest 363 Class 4B SolutionSRPC

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Digest 363 Class 4B SolutionGGBS

Siliceous Agg. Carbonaceous Agg.

Results from Sheffield Study

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Cementitious Groups(from Table 3, SD1 Part 2)

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Cementitious Groups

SRPC GGBS

Research QuestionShould Sulfate Resisting Portland Cement be reclassified in terms of

its resistance to thaumasite formation ?

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Special Digest 1Features :

16 ACEC Classes3 structural performance levels3 concrete member thicknesses128 Design Chemical Classes corresponding to the above

Practical QuestionsIs this an overly complex approach to a final product whose cement content varies by only 100kg/m3 and W/C by 0.2 ?

Could the methodology be simplified by having less categories?

Should we specify concrete W/C ratio in increments of less than 0.5 for finer tuning ?

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Control of ThaumasiteFormation in Concrete

Recommendations relating to SD1from the Sheffield Study

Clarify the relative aggressivity of clay conditions versus solution chemistry and adjust Table 2 (needs further research)Provide details of the effects of backfilled clay permeability in terms of compacted densities for control of water mobility (needs further research)Clarify the effects of carbonate from aggregates versus other sources and adjust Table 4 (needs further research)Revise Table 3 cementitious grouping to take into account the susceptibility of SRPC and possibly other cementitiouscombinations to thaumasite attack (from current research)

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Control of ThaumasiteFormation in Concrete

Final Conclusions1. To date, concrete containing 45% GGBS has provided

much better resistance to both thaumasite and acid attack than other uncoated cementitious combinations

2. All concrete coated with bitumen has remained unaffected by thaumasite attack