By Norah Crammond, Centre for Concrete Construction, BRE/file/05.pdf · By Norah Crammond, Centre...
Transcript of By Norah Crammond, Centre for Concrete Construction, BRE/file/05.pdf · By Norah Crammond, Centre...
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Three-year Findings from the BRE Thaumasite Field Trial By Norah Crammond, Centre for Concrete Construction, BRE Researchers at the Building Research Establishment in the UK have recently excavated buried concrete specimens from a field trial site on Lower Lias Clay in central England. The specimens had been exposed to Class 3 sulfate-bearing groundwater at ground temperatures of 10 -14 °C for a period of three years and as a consequence, many had undergone the thaumasite form of sulfate attack (TSA). Conventional sulfate attack, leading to the formation of ettringite and/or gypsum did not play a role in the deterioration process. An initial air-cure was found to be beneficial in that the cast-in-situ concretes were attacked much more than their precast counterparts. A selection of the cast-in-situ concretes were examined using X-ray diffraction, chemical analysis and microscopy studies and the main findings are discussed in this presentation. A variety of binder/aggregate combinations were investigated and a clear distinction could be made after three years between TSA-susceptible and TSA-resistant mixes. An approximate 'depth of attack' measurement has been determined for each of the susceptible mixes. All concretes made with blended cements containing pfa, ggbs (including BRECEM), microsilica and metakaolin performed satisfactorily irrespective of specimen or aggregate type. It was noticed however, that several of the unattacked concretes showed high total sulfate values (in excess of 6% by weight of binder) in their outer surfaces. This was a surprising result, which has been investigated further in this presentation. All the cast-in-situ concretes made with PC and PLC had undergone TSA to varying degrees whereas the SRPC cast-in-situ concretes were only significantly attacked if they contained a carbonate aggregate. TSA requires a source of carbonate ions, which is usually supplied through the use of a carbonate aggregate but, in this study, it has occurred in Portland cement concretes containing an all-in siliceous aggregate. Evidence points to bicarbonate ions dissolved in the groundwater as being an additional source. The author recommends that in future investigations of concrete, the concentration of bicarbonate ions in groundwater should be measured whenever possible and that the use of total sulfate determinations as the sole technique for diagnosing sulfate attack should be reviewed.
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Three Year Findings from the BRE Thaumasite Field Trial
Norah CrammondInternational Seminar on TSA, June 24th 2003Centre for Cement and Concrete,University of Sheffield
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1998
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1998
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1998
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Details of Site
Class 3 Sulfate conditions (1.4 - 3.0 g/l) - Ca, Mg and Na
Temperature at specimen depth between 10 and 13 °C
Groundwater level always above buried specimens
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Concrete Mixes Binders include PC, SRPC, PLC, BRECEM and PC combined with ggbs, pfa, microsilica and metakaolinAggregates include flint gravel, calcareous limestone and dolomitic limestone
Main mixes:Cement content 310 - 320 Kg/m3
Free water/ binder ratio 0.53 - 0.58Outlier mixes:
Cement content 270 - 290 Kg/m3
Free water/ binder ratio 0.72 - 0.78NB: Main mixes don’t comply with recommendations in SD1
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2001
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2001
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Laboratory Analyses
Visual and physical assessment
X Ray Diffraction
Optical microscopy
Sulfate determinations with depth
Scanning electron microscopy (SEM)
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2001
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Coring cube for laboratory analysis
Ø100
250
250 mm
Cube core drilled
Face exposed toin-situ Lower Lias Clay
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Core split into two - Half A and Half B
250
Ø100
Face exposed to in-situLower Lias Clay
Half B
Half A
Face exposed to in-situLower Lias Clay
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Half A for Optical Microscopy and SEM
Half A
Thin sections
Face exposed to in-situ Lower Lias Clay
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Half B for Total Sulfate Determinations
Cut 10 - 7 mm
Cut 27 - 14 mm
Cut 5Mid section(bulk)
Cut 30 - 7 mmCut 4
7 - 14 mm
250
Face exposed to in-situLower Lias Clay
Face exposed to in-situLower Lias Clay
Half B
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Three-year visual assessment
Precast concretes generally OK
Insitu concretes - variable surface attack
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Cast-in-situ cubes: Depths of attack Average depth of attackMain mix
Binder type Carbonate SiliceousPC 7mm <1mm
PLC 8mm 6mmSRPC 6mm No attack
30% pfa No attack No attack70% ggbs No attack No attackmicrosilica No attackmetakaolin No attack
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Three-year assessment of attacked cast-in-situ specimens
Conventional sulfate attack has not been detected
In all cases of attack, TSA was the deterioration process identified
Composition of the thaumasite varied depending on aggregate type
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Magnesian Limestone aggregate concretes
Inte
nsity
0
100
200
300
400
500
600
700
800
900
2-Theta - Scale15.56 15.6 15.7 15.8 15.9 16.0 16.1 16.2 16.3 16.4 16.5
Cube 3
Cube 23
Cube 73
Cube 15
Cube 71
Cube 83
Cube 55
RED - PORTLAND CEMENT
BLUE - PORTLAND LIMESTONE CEMENT
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Siliceous aggregate concretesIn
tens
ity
0
100
200
300
400
500
600
700
800
2-Theta - Scale15.43 15.5 15.6 15.7 15.8 15.9 16.0 16.1 16.2 16.3 16.4
Cube 1
Cube 69
Cube 21
Cube 13
Cube 53
Cube 81
Cube79
Cube 1
RED – PORTLAND CEMENT
BLUE – PORTLAND LIMESTONE CEMENT
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In siliceous aggregate cubes:
Carbonate from aggregate
And also from groundwater
Figure A1: Core 1 - top corner of in situ clay side face.Photomicrograph ( 1.84mm across) under crossed polars showing adistinctive halo of thaumasite occurring around a shell fragment.
Figure A2: Core 1 - top corner of in situ clay side face.Photomicrograph ( 0.9mm across) under crossed polars showingbright specks of popcorn calcite deposition within an area of Zone 4TSA
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Details of Site
Class 3 Sulfate conditions (1.4 - 3.0 g/l) - Ca, Mg and Na
Temperature at specimen depth approximately 10°C
Groundwater level always above buried specimens
Bicarbonate alkalinity as CaCO3 - 0.41 and 0.49g/l
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Average depth of attack % SO4 in outer 7mmMain mixBindertype Carbonate Siliceous Carbonate Siliceous
PC 7mm <1mm 14.5 12.0PLC 8mm 6mm 16.5 11.5
SRPC 6mm No attack 12.5 15.030% pfa No attack No attack 11.0 7.5
70% ggbs No attack No attack 6.0 4.5microsilica No attack 12.0metakaolin No attack 6.5
Cast-in-situ cubes:Depths of attack and sulfate profiles
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Pfa concrete - 11% SO4Main mix + carbonate
PLC concrete - 16% SO4Outlier mix + carbonate
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Distribution of sulfates with depth
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Pfa concrete - 11% SO4
PLC concrete - 16% SO4
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Inte
nsity
0
100
200
300
400
500
600
2-Theta8 9 10 11 12 13 14 15 16
PLC - red
pfa - black
XRD of surface of cubes
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Mix type Features %CaO %Al2O3 %SiO2 %SO3
30% pfamain mix
Cracked, highrelief, amorphous
27.0 5.7 4.3 17.0
PLCoutlier
mix
Cracked, highrelief
27.0 6.5 2.6 17.8
PLCoutlier
mix
Low relief, cryst-aline background
27.0 3.5 8.2 14.4
Standard ettringite 27.2 8.2 0.0 19.4
Standard thaumasite 27.0 0.0 9.6 12.9
Composition of sulfate-bearing phases
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Conclusions 1TSA was the only mode of sulfate attack identifiedAfter 3 years a distinction can be made between TSA-resistant and TSA-susceptible concrete mixesAn initial air-cure is beneficialAll concretes made with blended cements containing pfa, ggbs (including BRECEM), microsilica and metakaolinperformed satisfactorily irrespective of specimen or aggregate typeAll the cast-in-situ concretes made with PC and PLC had undergone TSA to varying degreesThe SRPC cast-in-situ concretes were only significantly attacked if they contained a carbonate aggregate
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Conclusions 2Outlier worse than main mixesFaces exposed to in-situ clay generally worseWith siliceous aggregate PC concretes,the thaumasite is more Al-rich and CO3-poorIn future, concentrations of bicarbonate ions in groundwater should be measured as these can contribute towards TSAHigh levels of sulfate in surface of concretes irrespective of whether attacked or not - present in an amorphous phase closer in composition to ettringite than to thaumasiteDiagnosis of sulfate attack using total sulfate determinations as the sole analytical technique may need to be reviewed