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 Thaumasite
Formation in Concrete
Current Approach and Research Needs
Dr Ewan Byars
Centre for Cement and Concrete
University of Sheffield
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Centre for
Cement and Concrete
Occurrence of Thaumasite
Centre for
Cement and Concrete
In March 2002, the TEG reported a significant number
of new cases of thaumasite formation in
cementitious construction, including:
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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 Specifications
Centre for
Cement and Concrete
Overview 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 size
5. 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
Centre for
Cement and Concrete
Important Factors
ƒ Presence of sulfates - generally salts of calcium, magnesium
and sodium
ƒ 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 surface
ƒ Mobility of groundwater – for transportation and refreshment
of aggressive species at concrete surface
ƒ Acidity of groundwater
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Mobility of Groundwater
Current 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 Questions
i)
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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Centre for
Cement and Concrete
Example
Clay Density X
Centre for
Cement and Concrete
Attack Rate Y
How does attack rate
vary with degree of
compaction ?
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Aggressive Chemical Environment
for Concrete (ACEC)
Centre for
Cement and Concrete
The sulfate, sulfide, acid and mobile groundwater
values in the current SD1 lead to:
ƒ 7 Design Sulfate Classes with 16 sub-ACEC classes (Table 2)
ƒ These, along with structural and member size details, feed in to
Table 7 in SD1 Part 2 to give concrete Design Chemical
Classes (DC)
ƒ 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)
ƒ Insert table 2
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Centre for
Cement and Concrete
Centre for
Cement and Concrete
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Design Chemical Class
(based on Tables 3,4 SD1 Pt 2)
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Centre for
Cement and Concrete
Concrete Quality
Centre for
Cement and Concrete
Research Question
Do 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 Question
Effect of carbonate sources other than from aggregate?
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Centre for
Cement and Concrete
Digest 363 Class 2 Solution
OPC
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Digest 363 Class 2 Solution
PLC
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Digest 363 Class 3 Solution
OPC
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Digest 363 Class 3 Solution
PLC
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Digest 363 Class 3 Solution
PFA
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Digest 363 Class 3 Solution
SRPC
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Digest 363 Class 3 Solution
GGBS
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Digest 363 Class 4B Solution
PFA
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Digest 363 Class 4B Solution
SRPC
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Digest 363 Class 4B Solution
GGBS
Siliceous Agg.
Carbonaceous Agg.
Results from Sheffield Study
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Centre for
Cement and Concrete
Cementitious Groups
(from Table 3, SD1 Part 2)
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Centre for
Cement and Concrete
Cementitious Groups
SRPC
Centre for
Cement and Concrete
GGBS
Research Question
Should Sulfate Resisting Portland Cement be reclassified in terms of
its resistance to thaumasite formation ?
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Special Digest 1
Centre for
Cement and Concrete
Features :
ƒ
ƒ
ƒ
¾
16 ACEC Classes
3 structural performance levels
3 concrete member thicknesses
128 Design Chemical Classes corresponding to the above
Practical Questions
Is 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 Thaumasite
Formation in Concrete
Centre for
Cement and Concrete
Recommendations relating to SD1
from the Sheffield Study
ƒ Clarify the relative aggressivity of clay conditions versus solution
chemistry and adjust Table 2 (needs further research)
ƒ Provide details of the effects of backfilled clay permeability in terms
of compacted densities for control of water mobility (needs further
research)
ƒ Clarify the effects of carbonate from aggregates versus other
sources and adjust Table 4 (needs further research)
ƒ Revise Table 3 cementitious grouping to take into account the
susceptibility of SRPC and possibly other cementitious
combinations to thaumasite attack (from current research)
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Control of Thaumasite
Formation in Concrete
Centre for
Cement and Concrete
Final Conclusions
1. 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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