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.

Three Year Findings from the BRE Thaumasite Field Trial

Norah CrammondInternational Seminar on TSA, June 24th 2003Centre for Cement and Concrete,University of Sheffield

1998

1998

1998

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

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

2001

2001

Laboratory Analyses

Visual and physical assessment

X Ray Diffraction

Optical microscopy

Sulfate determinations with depth

Scanning electron microscopy (SEM)

2001

Coring cube for laboratory analysis

Ø100

250

250 mm

Cube core drilled

Face exposed toin-situ Lower Lias Clay

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

Half A for Optical Microscopy and SEM

Half A

Thin sections

Face exposed to in-situ Lower Lias Clay

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

Three-year visual assessment

Precast concretes generally OK

Insitu concretes - variable surface attack

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

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

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

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

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

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

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

Pfa concrete - 11% SO4Main mix + carbonate

PLC concrete - 16% SO4Outlier mix + carbonate

Distribution of sulfates with depth

Pfa concrete - 11% SO4

PLC concrete - 16% SO4

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

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

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

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