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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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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
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
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