co1 concrete example asr

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tel +44 (0)1209 219541 email [email protected] Edwards Offices, Gweal Pawl, Redruth, Cornwall TR15 3AE

Registered in England & Wales · Company No. 4777735

A Bridge

A Client

Concrete Petrographic Report CO1Example

http://www.petrolab.co.uk/

Petrographic Report - Concrete A Client

Contents

1 Sample details..........................................................................................12 Petrographic description – C1...................................................................2

2.1 Aggregates.............................................................................................................22.2 Binder....................................................................................................................32.3 Voids......................................................................................................................42.4 Fractures and degradation....................................................................................42.5 Composition - C1...................................................................................................5

3 Petrographic description – C2...................................................................63.1 Aggregates.............................................................................................................63.2 Binder....................................................................................................................83.3 Voids....................................................................................................................103.4 Fractures and degradation..................................................................................113.5 Composition - C2.................................................................................................13

4 Summary.................................................................................................144.1 Concrete C1........................................................................................................144.2 Concrete C2........................................................................................................14

5 Images....................................................................................................155.1 Sample C1...........................................................................................................165.2 Sample C2...........................................................................................................19

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Petrolab document control

Client A Client

Report title A Bridge

Analysis required Detailed petrographic examination in accordance with ASTM C-856 on two concrete cores.

Client reference -- Client contact A Client

Report ID (version) CO1 Report date Example

Prepared by R Garside BA MSci Checked by J Strongman Msci ARSM

Limitations

This report relates only to those samples submitted and specimens examined and to any materials properlyrepresented by those samples and specimens. This report is issued to the Client named above for the benefit ofthe Client for the purposes for which it was prepared. It does not confer or purport to confer on any third party anybenefit or right pursuant to the Contracts (Rights of Third Parties) Act 1999.

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Petrographic methods of investigation for hardened concrete

Preliminary examination

The submitted sample / specimens are examined as received, and after careful washing to removeloose debris, using a Nikon SMZ-U stereomicroscope. The microscope has a continuous zoom rangeof 7.5x to 75x and it is equipped with a 150W continuous ring, fibre optic illuminator. It is useful for thepreliminary examination of materials and it is possible to discriminate and sometimes identify featuresas small as 100 µm in size. The microscope has a trinocular head and can be used for low powerphotomicrography. The main purpose of the preliminary examination is to identify the degree ofconsistency or variability in the submitted sample / specimens and any superficial evidence ofdeterioration. This facilitates the selection of the most appropriate specimen(s) for a full petrographicexamination when multiple specimens of the same concrete type are submitted.

Sample preparation

Following preliminary examination, the selected specimen is cut in half parallel or perpendicular with itsaxis (to suit the objectives of the investigation). One half of a cut specimen may be carefully ground,finishing with #600 grit carborundum powder. At least one standard petrographic thin section isprepared from each selected specimen. Additional thin sections may be prepared from a largespecimen in order to investigate variation in the concrete or to suit the objectives of the investigation.The cut specimen, used for section preparation, is typically vacuum impregnated with a low viscosityepoxy resin containing a yellow fluorescent dye. A high resolution, low magnification digital image ofeach thin section is prepared using a film scanner. This type of image is useful for illustrating themesostructure of the concrete.

Microscopic examination

High-power microscopy is undertaken using a Nikon research polarising microscope. The thin section(s)may be examined in both plain and cross-polarised light, and reflected ultraviolet light creatingsecondary fluorescence. Photomicrographs are taken using a Nikon high resolution digital camera fittedto the trinocular head of the microscope.

Quantitative investigations

Modal analysis to determine the concrete composition is performed using a Pelcon 64 channelelectromechanical point counter using stepping and traverse intervals of 0.5 mm. One thousand pointsare counted from each thin section (provided the area of the sample represented on the thin-section issufficient). Water-cement ratios for uncarbonated concrete can be estimated by the petrographiccomparison of the fluorescent intensity of the paste with that of standard mortars viewed under carefullycontrolled reflected ultraviolet illumination. An estimated mix design may be calculated for standardPortland-type cement without cement substitutes.

Other testing

In some circumstances, additional (non-petrographic) testing using an appropriate chemical or physical testing procedure may be undertaken by an approved laboratory. When undertaken, the test certificate and the results obtained are provided in an appendix to the main body of the report.

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1 Sample details

Two samples of concrete were supplied by A Client on xx/xx/xxxx for petrographicexamination. The samples were reported to be from a concrete bridge structure.Sample details, as received, are shown in the table below.

Table 1 · Samples received

Report no. Sample reference Type

C1 Core location Concrete core, Ø100 mm

C2 Core location Concrete core, Ø100 mm

The investigation requested was a detailed petrographic description and quantitativemodal analysis, to assess concrete quality and any evidence of degradation, inparticular any evidence or potential for Alkali-Silica Reaction (ASR). A thin section wasmade parallel to the core axis in the area of the core which showed the greatestevidence of degradation, or, when there was no immediately evident damage, the baseof the core as this would be most likely to show the presence of ASR.

Two types of concrete were identified. Petrographic observations are recorded in thetables below with additional explanatory text beneath. A summary of the concretetype(s) is provided followed by annotated images of the samples.

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2 Petrographic description – C1

2.1 Aggregates

Table 2 · Aggregate details

Type crushed aggregate and natural sand

Coarse aggregate crushed limestone Abundance 47.4%

Major components bioclastic limestone

Minor components calcareous shell fragments

Size range 3 mm to 20 mm Average 10 mm

Grading good Rounding angular – sub-angular

angular morphologies predominate

Shape mainly equant Sphericity low

Fine aggregate beach sand Abundance 22.1%

Major components vein quartz, calcareous shell fragments

Minor components limestone, ferruginous sandstone, stable silicate minerals (plagioclase feldspar, K-feldspar, pyroxene), altered basalt, iron oxides, calcite, chert, zircon, glauconite, volcanic glass

Size range 50 µm to 1 mm Average 300 µm

Grading good Rounding sub-angular - rounded

sub-rounded morphologies predominate

Shape mainly equant Sphericity moderate

Percentage passing 600 µm 80 (visual estimate)

Sample Aggregate microfractures Secondary alkali-silica gel

C1 none seen none seen

The coarse aggregate is a crushed limestone and the fine aggregate is a beach sand.Both aggregates are generally considered stable. The fine aggregate contains a minoramount of chert and volcanic glass (2.7 % of the aggregate total, modal analysis) whichare considered potential alkali-silica reactive components1. However the aggregate inthis sample shows no evidence of alkali-silica gel development or associated radialmicrofracturing.

There is no evidence of any other aggregate instability or reaction between aggregateand the binder.

1 The Diagnosis of Alkali-Silica Reaction, Report of a working party. Appendix D. British Cement Association, 1992. Where rocks or mineral types are noted as 'reactive', it does not necessarily imply that damage has been caused by ASR when these have been used.

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2.2 Binder

The binder is Portland-type cement, probably OPC. The sample appears to becompletely uncarbonated except for trace patchy carbonation in the surface 10 mm ofthe concrete.

The optical properties of the uncarbonated paste are summarised in the table below.

Table 3 · Uncarbonated binder

Cement type Portland-type Cement (probably OPC) Modal volume 29.8%

Replacement local ettringite replacement

Optical properties of least altered paste

Colour dark brown to pale fawn Isotropy weakly anisotropic

Texture irregularly mottled Disseminated calcite abundant

Dye absorption variable, moderate to high Fluorescence variable, moderate to high

Portlandite

In paste abundant Distribution irregular

Maximum size (µm) 60 Shape granular and branching anhedral grains

Replacement calcite (common)

Aggregate margins common Distribution irregular

Maximum size (µm) 80 Shape monolayers and outgrowths

Replacement calcite, ettringite (rare)

Cement clinker grains

Alite common Maximum size (µm) 100

Max. birefringence not determined Colour pale yellow

Hydration CSH-replaced Reaction rims none seen

Belite rare Maximum size (µm) 30

Max. birefringence not determined Colour muddy brown

Hydration CSH-replaced Reaction rims diffuse opaque rims

Ferrite present Maximum size (µm) 50

Alteration strongly oxidised Colour dark red-brown - opaque

Clinker grain clusters


Matrix dark brown glassy phase, granular iron oxide

Hydration completely hydrated Reaction rims none seen


Matrix sparse, medium brown glassy phase

Hydration strongly hydrated Reaction rims diffuse opaque rims

Alite + belite none seen

The mineralogy and texture of the uncarbonated paste are characteristic of PortlandCement concrete made at a moderate water/cement ratio (≥ 0.45). The uncarbonated binder shows evidence of minor local ettringite replacement.

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2.3 Voids

The main features of the air void system are summarised below.

Table 4 · Voids

Void system

Modal volume 0.7% Shape spherical – irregular

Size range 100 µm to 5 mm Average size 500 µm

Interconnectivity generally isolated

Secondary minerals

ettringite present

Alteration at void walls local ettringite replacement

There are traces of ettringite in voids and the surrounding binder, particularly in areas ofslightly higher porosity, however there is no evidence of significant sulphate attack.

2.4 Fractures and degradation

Table 5 · Fractures

Anastomosing microcracksAbundance none seen

Peripheral fissures

Abundance rare

Distribution at margins of aggregate fragments, no specific lithologies

Maximum aperture 60 µm Average aperture 35 µm

Secondary minerals

Mineral 1 granular calcite Abundance rare

Mineral 2 portlandite Abundance present

Mineral 3 ettringite Abundance present

Alteration at crack walls local ettringite replacement

Fractures related to degradation

Abundance none seen

Peripheral fissures may form as a result of aggregate shrinkage and as a result ofdissolution or ettringite replacement of portlandite outgrowths and monolayers.

The samples show no evidence of microfracturing related to sulphate replacement ofthe binder or other aggregate-related degradation.

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2.5 Composition - C1

The volumetric composition of the concrete was determined by modal analysis. Onethousand points were counted from the thin section.

Sample C1

Coarse aggregate 47.4%

Fine aggregate 22.1%

Binder 29.8%

Voids 0.7%

Water-cement ratio was estimated by comparing the fluorescent intensity of the pasteunder high intensity reflected ultraviolet illumination with that of standard mortars. Arange of values between 0.4 and 0.5 were obtained. A preferred water-cement ratio of0.45 has been used to estimate the mix design.

Table 6 · Estimated (theoretical) mix design

ComponentVolume

%Density(kgm3)

Estimated mix designw/c0.45

Coarse aggregate 47.4 2650 Coarse aggregate 1256 kgm3

Sand aggregate 22.1 2650 Sand aggregate 586 kgm3

Binder 29.8 3120 Cement 387 kgm3

Voids 0.7 Water 174 kgm3

Total 100.0

Density 2403 kgm3

Fluorescent intensity 0.4-0.5 Slump without plasticiser 30-60 mm

Preferred water/cement ratio 0.45 28-day strength 51.7 MPa

Maximum aggregate size, mm 20 crushed Calculated cement content 16.1 %

The estimate assumes that no air entrainment agent or plasticiser is present.

NOTE: It must be emphasised that the estimation of concrete mix design by petrographic methods is subject to many potential errors. It should not be used as a substitute for appropriate and approved methods of chemical analysis and physical testing.

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3 Petrographic description – C2

3.1 Aggregates

Table 7 · Aggregate details

Type crushed aggregate and natural sand

Coarse aggregate crushed limestone Abundance 53.0%

Major components bioclastic limestone, carbonaceous limestone (silicified limestone?)

Minor components greywacke, sandstone

Size range 2 mm to 20 mm Average 10 mm

Grading good Rounding angular – sub-angular

angular morphologies predominate

Shape mainly equant Sphericity low

Fine aggregate beach sand Abundance 18.7%

Major components vein quartz, calcareous shell fragments

Minor components limestone, ferruginous sandstone, stable silicate minerals (plagioclase feldspar, K-feldspar, pyroxene), altered basalt, iron oxides, calcite, chert, zircon, glauconite, volcanic glass

Size range 50 µm to 1 mm Average 300 µm

Grading good Rounding sub-angular - rounded

sub-rounded morphologies predominate

Shape mainly equant Sphericity moderate

Percentage passing 600 µm 80 (visual estimate)

Sample Aggregate microfractures Secondary alkali-silica gel

C2 common - generally gel-filled, iron oxide filled near base of sample

common, sometimes carbonated

The coarse aggregate is a crushed limestone and the fine aggregate is a beach sand.The fine aggregate contains a minor amount of chert and volcanic glass (2.5 % of theaggregate total, modal analysis) and the coarse aggregate contains a minor amount ofgreywacke (< 5 % of the aggregate total, visual estimate). These are consideredpotential alkali-silica reactive components1 but show no evidence of initiating ASR in thethin section.

In this sample < 20 % coarse aggregate (visual estimate) shows evidence of internalfracturing and the development of pervasive cracking and alkali-silica gel formation. Thebinder around these aggregate fragments also shows significant gel replacement, inpatches Ø < 2 mm.

In this sample the coarse aggregate most commonly associated with fracturing andsurrounding gel-filled binder is the carbonaceous limestone. This aggregate type is nottypically considered a high risk ASR aggregate, as ASR is usually attributed to thepresence of microcrystalline or highly strained quartz. Although the limestone does notappear to be obviously silicified in thin section, it does contain small strained and finely

1 The Diagnosis of Alkali-Silica Reaction, Report of a working party. Appendix D. British Cement Association, 1992. Where rocks or mineral types are noted as 'reactive', it does not necessarily imply that damage has been caused by ASR when these have been used.

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crystalline quartz grains and the abundant opaque carbonaceous material may alsocontain microcrystalline silica which is not distinguishable under the microscope. Theaggregate fragments are harder than typical limestone which would also suggest thatthey have been silicified.

Silicified limestone is not widely considered a high risk aggregate and only a smallnumber of cases of reaction with this aggregate have been reported in the UK. It ispossible that some unusual environmental conditions, such as high alkali cement, mayhave caused the aggregate to react.

There is no evidence of any other aggregate instability or reaction between aggregateand the binder.

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3.2 Binder

The binder is Portland-type cement, probably OPC. The core is completelyuncarbonated except for a strong carbonation layer around some fractures and patchycarbonation of alkali-silica gel.

The optical properties of the uncarbonated paste are summarised in the table below.

Table 8 · Uncarbonated binder

Cement type Portland-type Cement (probably OPC) Modal volume 27.7%

Replacement local ettringite, alkali-silica gel, iron oxide

Optical properties of least altered paste

Colour pale fawn to dark brown Isotropy weakly anisotropic

Texture irregularly mottled Disseminated calcite abundant

Dye absorption variable, moderate to high Fluorescence variable, moderate to high

Portlandite

In paste abundant Distribution irregular

Maximum size (µm) 50 Shape granular and branching anhedral grains

Replacement calcite (common)

Aggregate margins common Distribution irregular

Maximum size (µm) 110 Shape tabular crystals and outgrowths

Replacement calcite, ettringite (rare)

Cement clinker grains


Max. birefringence not determined Colour pale yellow

Hydration CSH-replaced Reaction rims none seen


Max. birefringence not determined Colour muddy brown

Hydration CSH-replaced Reaction rims diffuse opaque rims

Ferrite present Maximum size (µm) 75

Alteration strongly oxidised Colour dark red-brown - opaque

Clinker grain clusters


Matrix dark brown glassy phase, granular iron oxide

Hydration completely hydrated Reaction rims none seen


Matrix sparse, medium brown glassy phase


Alite + belite rare Maximum size (µm) 165

Matrix medium brown glassy phase, granular iron oxide


The mineralogy and texture of the uncarbonated paste are characteristic of Portland Cement concrete made at a moderate water-cement ratio (≥ 0.5). The uncarbonated

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binder shows evidence of minor local ettringite replacement. It also shows evidence of patchy alkali-silica gel replacement around aggregate fragments and iron oxide replacement at the base of the core, presumably in proximity to corroded reinforcement.

The main features of the carbonated paste are summarised below.

Table 9 · Carbonated binder

Transition zones

Carbonation fronts sharp, irregular

Width of transition zone 100 µm – 300 µm

Replacement sequence 1 portlandite 2 paste 3 cement clinker grains

Fine-grained paste

Abundance present Distribution around microcracks and areasof carbonated gel

Size range < 1 µm to 5 µm Texture uniform, compact

Relict clinker grains present

Residual gel present Granular iron oxide present

Maximum size 50 µm Maximum size 50 µm

Microporosity low Pore size 5 µm To 25 µm

Secondary minerals ettringite, alkali-silica gel

Coarse-grained paste

Abundance present Distribution patches next to corroded reinforcement

Size range 5 µm to 25 µm Texture microgranular

Relict clinker grains present (alite)

Residual gel present Granular iron oxide present

Maximum size 100 µm Maximum size 100 µm

Microporosity high locally Pore size 5 µm to 50 µm

Secondary minerals ettringite, iron oxides

Snowflake calcite none seen

The carbonated paste around gel-associated fractures is mainly fine-grained andstrongly recrystallised. The texture is typical of natural atmospheric carbonation alonghigh porosity zones. Carbonated paste around the iron oxide filled binder is coarsegrained, suggesting that fractures penetrating down to the reinforcement have acted aspathways for leaching in addition to facilitating corrosion of the steel.

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3.3 Voids

The main features of the air void system are summarised below.

Table 10 · Voids

Void system

Modal volume 0.6% Shape spherical – irregular

Size range 50 µm to 5 mm Average size 500 µm

Interconnectivity locally highly connected

Secondary minerals

Voids in uncarbonated paste Voids in carbonated paste

ettringite rare granular calcite rare

portlandite rare ettringite rare

alkali-silica gel rare alkali-silica gel rare

iron oxides rare

Alteration at void walls local ettringite or alkali-silica gel replacement

There are traces of ettringite in voids and the surrounding binder, particularly in areas ofslightly higher porosity, however there is no evidence of significant sulphate attack.Voids around the edge of gel filled binder or fractures show evidence of gel linings andvoids next to the iron oxide filled binder also contain iron oxides.

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3.4 Fractures and degradation

Table 11 · Fractures

Anastomosing microcracksAbundance none seen

Peripheral fissures

Abundance rare

Distribution around coarse aggregate fragments, no specific lithologies

Maximum aperture 30 µm Average aperture 15 µm

Secondary minerals

Mineral 1 ettringite Abundance present

Mineral 2 portlandite Abundance present

Mineral 3 alkali-silica gel Abundance rare

Mineral 4 iron oxides Abundance rare

Mineral 5 granular calcite Abundance rare

Alteration at crack walls ettringite, iron oxide binder replacement

Fractures related to degradation

Abundance common

Distribution Type 1 – throughout core sample, no specific orientation Type 2 – at base of sample, generally sub-parallel to surface

Maximum aperture 1 – 1.2 mm; 2 – 100 µm Average aperture 1 – 200 µm; 2 - 25 µm

Crack walls usually closely matching in binder and aggregate, occasionally non-matching in carbonaceous limestone aggregate

Cracks in aggregate cracks commonly run through and possibly propagate in carbonaceous limestone aggregate fragments

Secondary minerals

Mineral 1 ettringite Abundance common

Mineral 2 alkali-silica gel Abundance common

Mineral 3 iron oxides Abundance common

Mineral 4 granular calcite Abundance present

Alteration at crack walls ettringite, alkali-silica gel, iron oxide binder replacement

Peripheral fissures may form as a result of aggregate shrinkage and as a result ofdissolution or ettringite replacement of portlandite outgrowths and monolayers.

Fractures related to development of alkali-silica gels mostly propagate into the thinsection from outside the area, though there are a few microfractures which appear tooriginate in carbonaceous limestone fragments. These discontinuous fracturespermeate into the surrounding paste approximately perpendicular to the aggregatesurface and can be traced for up to at least 20 mm. Fractures in the binder generallyhave matching walls with discontinuous linings of alkali-silica gel and/or lining of fibrousettringite. Maximum fracture apertures are < 1.2 mm but are typically 200 µm in width.The gel in the majority of fractures is uncarbonated, except for the wider fractures andareas with gel replacing the binder. It appears to be at least partly contemporaneouswith the ettringite, since it both overgrows ettringite in fractures and has ettringiteforming on its surface.

The most likely cause of the ASR development is due to the concrete beingcontinuously wet which allows the alkalinity of the pore fluid to increase to a point where

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any microcrystalline and/or highly strained quartz reacts. The presence of sulphatereplacement in the uncarbonated binder is further evidence that the concrete has beenwet for prolonged periods. The presence of carbonated gel and evidence that ASRfracturing has caused reinforcement corrosion suggests that some of the gel is relativelyold. However, it is not possible to determine whether alkali-silica reaction in the concretehas ceased or is ongoing.

Observations of the core sample show evidence of corrosion and associated expansionof steel reinforcement at the base. This has produced multiple radiating surface-parallelmicrofractures which are iron oxide-filled and there is local iron oxide impregnation ofthe binder (< 3 mm from sample base). The cause of corrosion is due to air ingress andsubsequent carbonation. The process is a classic feedback system: as the steelbecomes more corroded it expands further creating more fractures which penetrate tothe surface allowing further air ingress and corrosion.

The core sample provided is almost completely uncarbonated and the concrete isextremely dense. It is considered unlikely that surface carbonation has penetrated to thedepth of the reinforcement and, therefore there must have been an initial fracturemechanism to create a pathway to the reinforcement. From the evidence of carbonatedalkali-silica gels in fractures it seems likely that the initial fracturing was caused by ASR.

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3.5 Composition - C2

The volumetric composition of the concrete was determined by modal analysis. Onethousand points were counted from the thin section. The concrete appears to be slightlyunevenly mixed as there are patches of high cement content with abundant unhydratedclinker and other areas with a higher water content.

Sample C2

Coarse aggregate 53.0%

Fine aggregate 18.7%

Binder 27.7%

Voids 0.6%

Water-cement ratio was estimated by comparing the fluorescent intensity of the pasteunder high intensity reflected ultraviolet illumination with that of standard mortars. Arange of values between 0.45 and 0.55 were obtained. A preferred water-cement ratio of0.5 has been used to estimate the mix design.

Table 12 · Estimated (theoretical) mix design

ComponentVolume

%Density(kgm3)

Estimated mix designw/c0.5

Coarse aggregate 53.0 2650 Coarse aggregate 1405 kgm3

Fine aggregate 18.7 2650 Sand aggregate 496 kgm3

Binder 27.7 3120 Cement 338 kgm3

Voids 0.6 Water 169 kgm3

Total 100.0

Density 2406 kgm3

Fluorescent intensity 0.45-0.55 Slump without plasticiser 30-60 mm

Preferred water/cement ratio 0.5 28-day strength 45.5 MPa

Maximum aggregate size, mm 20 crushed Calculated cement content 14.0 %

The estimate assumes that no air entrainment agent or plasticiser is present

NOTE: It must be emphasised that the estimation of concrete mix design by petrographic methods is subject to many potential errors. It should not be used as a substitute for appropriate and approved methods of chemical analysis and physical testing.

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4 Summary

4.1 Concrete C1

1. One core sample of concrete was investigated to assess concrete quality and anyevidence of degradation. A thin section was prepared parallel with the long axis of thecore from an area near the base of the sample provided.

2. The cement appears to be Ordinary Portland Cement; there is no petrographic evidenceof any admixtures or air entrainment.

3. The estimated mix design appears to be within normal limits.

4. The coarse aggregate is composed of crushed limestone.

5. The fine aggregate is a beach sand.

6. Chert and volcanic glass make up circa 2.7 % of the total aggregate (modal analysis).Chert and volcanic glass are considered potentially alkali–silica reactive components.However, chert and glass in this sample show no evidence of alkali-silica reaction and/orfracturing.

7. There is no evidence of any other aggregate instability or aggregate binder reaction inthe sample.

8. The sample appears to be completely uncarbonated except for trace patchy carbonationin the surface 10 mm of the concrete.

9. Carbonation of the concrete is not a problem in itself as it does not significantly alter thestrength of the concrete. However, it is a potentially serious problem if the carbonationpenetrates to the depth of steel reinforcement, where it may facilitate corrosion andassociated expansion of the steel.

10. No reinforcement was observed in this sample.

11. There are traces of secondary sulphates in some voids and peripheral microfracturesand the surrounding binder, particularly in areas of slightly higher porosity, however thereis no evidence of significant sulphate attack.

12. The sample shows no evidence of microfracturing related to sulphate replacement of thebinder or other aggregate-related degradation.

13. The concrete in this sample shows no evidence of macroscopic fracturing relating todegradation and appears to be currently sound.

4.2 Concrete C2

1. One core sample of concrete was investigated to assess concrete quality and anyevidence of degradation. A thin section was prepared parallel with the long axis of thecore from an area which appeared to show the most evidence of degradation.

2. The cement appears to be Ordinary Portland Cement; there is no petrographic evidenceof any admixtures or air entrainment.

3. The estimated mix design appears to be within normal limits.

4. The coarse aggregate is composed of crushed limestone.

5. The fine aggregate is a beach sand.

6. The fine aggregate contains a minor amount of chert and volcanic glass (circa 2.5 % ofthe aggregate total, modal analysis) and the coarse aggregate contains a minor amountof greywacke (< 5 % of the aggregate total, visual estimate). Chert, volcanic glass andgreywacke are considered potentially alkali–silica reactive components. However, in thissample these components show no evidence of alkali-silica reaction and/or fracturing.

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7. Carbonaceous limestone aggregate fragments shows evidence of internal fracturing andthe development of radial cracking and alkali silica gels. Although limestone aggregatesare generally considered to be of low reactivity, there have been some cases wheresilicification of limestones has been identified as the source of deleterious ASR. Silicifiedlimestone is not widely considered a high risk aggregate and only a small number ofcases of reaction with this aggregate have been reported in the UK. It is possible thatsome unusual environmental conditions, such as high alkali cement, may have causedthe aggregate to react.

8. Fractures related to development of alkali-silica gels in the sample show patchydistribution through the core sample. The microfractures typically contain discontinuouslinings and fills of alkali-silica gel and fibrous ettringite, with gel usually overgrowing thesulphates (though in some places sulphate also overgrows gel). The binder around somecarbonaceous aggregate samples also shows significant gel replacement, in patches Ø< 2 mm.

9. The most likely cause of the ASR development is due to the concrete being continuouslywet which allows the alkalinity of the pore fluid to increase to a point where anymicrocrystalline and/or highly strained quartz reacts. The presence of sulphatereplacement in the uncarbonated binder is further evidence that the concrete has beenwet for prolonged periods. The presence of carbonated gel and evidence that ASRfracturing has caused reinforcement corrosion suggests that some of the gel is relativelyold. However, it is not possible to determine whether alkali-silica reaction in the concretehas ceased or is ongoing.

10. There are traces of secondary sulphates, alkali-silica gel linings and iron oxides in somevoids, peripheral microfractures and the surrounding binder, particularly in areas ofslightly higher porosity, however there is no evidence of significant sulphate attack.

11. The core is completely uncarbonated except for a strong carbonation layer around somefractures and patchy carbonation of alkali-silica gel.

12. Carbonation of the concrete is not a problem in itself as it does not significantly alter thestrength of the concrete. However, it is a potentially serious problem if the carbonationpenetrates to the depth of steel reinforcement, where it may facilitate corrosion andassociated expansion of the steel.

13. Three 2 mm reinforcement bars were observed in the core sample at 35-42 mm depth.These pieces of reinforcement show no evidence of corrosion or microfracturing. Thecore also contains evidence of steel reinforcement at a depth of 60 mm from the surface.The reinforcement is extremely corroded and shows multiple radiating sub horizontalmicrofractures, the fractures are iron oxide filled and there is locally strong iron oxideimpregnation of the binder.

14. The cause of corrosion is due to air ingress and carbonation. The process has a positivefeedback because as the steel becomes more corroded it expands further creating morefractures, thus allowing further air ingress and corrosion of the steel.

15. Due to the evidence of degradation from both ASR and reinforcement corrosion in thesample provided, the concrete and the associated structure requires assessment fordamage and ongoing close monitoring. It is recommended that the condition of theconcrete in proximity to any reinforcement be ascertained because of the evidence ofcorrosion observed.

5 Images

Colour images (scanned images and annotated photomicrographs of each sample)begin over-page.

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5.1 Sample C1

A Sample C1

Photograph of sample as received (scale in cm).

Image A

Fuji S3 Pro digital camera

Daylight balanced oblique light

Thin section(s)

B Sample C1

Low magnification view of sample thin section. (Orientation arrow points towards drilling surface.)

Image B

Epson scanner

White cold cathode light

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Photomicrographs

shell

500 µm

shell

limestonevoid

quartz

C Sample C1

General view of sample showing all main phases.

Image C

Nikon Microphot-FXA petrological microscope

Plane polarised transmitted light

x40

shell

500 µm

shell

limestonevoid

quartz

D Sample C1

View of image C in cross polarised light.

Image D


Cross polarised transmitted light

x40

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ettringite

100 µm

void

ettringite

E Sample C1

Image showing minor ettringite in voids.

Image E



x200

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5.2 Sample C2

A Sample C2

Photograph of sample as received (scale in cm).

Image A

Fuji S3 Pro digital camera

Daylight balanced oblique light

Thin section(s)

B Sample C2

Low magnification view of sample thin section. (Orientation arrow points towards drilling surface.)

Image B

Epson scanner

White cold cathode light

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Photomicrographs

iron oxides

500 µm

iron oxides

limestonemicrofracture

quartz

C Sample C2

Image showing corrosion induced microfracturing and iron oxide impregnation of binder at the base of the core.

Image C



x40

gel

250 µm

carbonaceous limestone

quartz

D Sample C2

Image showing an area of alkali-silica gel completely replacing the binder around carbonaceous limestone aggregate.

Image D



x100

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ettringite

100 µm

microfracture

gel

gel

gel

E Sample C2

Image showing alkali silica gel and ettringite within microfractures. An area of gel replacing the binder is alsovisible.

Image E



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A BridgeCO1 Example


co1 concrete example asr

Documents

concrete c1

concrete c2

sample c1

sample c2

petrographic description

petrographic description

concrete cores

composition c1