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IiA 7246 | Issue | 5 December 2014

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Structural Guidance Note 2014 SGN 08Typical structural defects in 20th century concrete buildings:

an introduction



Contents

Page

1 Introduction 2

1.1 Pre-war (1890 – 1914) 2 1.2 Inter-war (1914 – 1945) 3 1.3 Post-war (1945 – 1970) 3 1.4 1970 – 1990 4 1.5 1990 to the present 4 1.6 Construction timeline 5 1.7 Design code development 6

2 Methodology of appraisal 7

3 Cracking in concrete 8

4 Defects 10

4.1 Design and detailing deficiencies 11 4.2 Construction-related defects 13 4.3 Durability-related defects 15 4.4 Materials-related defects 20 4.5 Accidental damage 24 4.6 Cladding-related defects 25

5 Testing 28

5.1 Types of concrete test 28 5.2 Determining concrete strength 30 5.3 Determining reinforcement 30

6 Key references 32

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1.2 Inter-war (1914 – 1945)After the First World War, there was a large demand for housing, and steel,timber and bricks, as well as skilled labour, were scarce. Concrete became a

popular building material and was increasingly used for bridges, buildings andstadia.

Cement was in short supply, driving a demand for alternative binders such asGGBS (Ground-granulated blast-furnace slag), pumice and burnt clay.

With lessons learned from the Highpoint I block built in North London in 1934(designed by Lubetkin & Tecton and engineered by Ove Arup), Highpoint II(1936) embodied a significant step in reinforced concrete construction: box-frameconstruction, where the internal crosswalls and floors became structural allowingthe front and rear elevations to be highly glazed.

Concrete structures of this period had very thin walls by present-day standards.

The shortage of good quality materials often led to deficient substitutes, andinnovations such as high alumina cement (HAC), used to speed up early strengthgain, were to prove problematic in later years.

Woodwool slabs were introduced as an inner face insulation sheeting and soon became permanent shuttering.

As with the pre-war period, inter-war reinforced concrete buildings may be ofheritage significance, and this should be understood before undertaking any workon them.

1.3 Post-war (1945 – 1970)As happened after the First World War, there was a significant demand forhousing in the post-World War 2 period. This led to rapid construction and thedevelopment of “system-build ing” by competing contractors.

Materials were effectively “rationed ” by licensing, and construction was largelylimited to housing, schools and industry. A shortage and increase in the cost oftimber saw building details that would normally be timber replaced with concrete.

Box-frame construction remained popular, particularly in local authority housingdevelopments. Lean construction was commonplace and thin walls and slabs can

be expected when dealing with buildings of this period.The introduction of CP114-2: 1948 : Code of practice for the structural use ofreinforced concrete in buildings led to improvements in the quality ofconstruction. Greater control was placed on the grading of aggregate, batching,vibration and compaction from this time.

Woodwool slabs became very popular in the early 1950s.

While prestressed concrete was introduced in England in 1936 and used forunderground munitions stores during the Second World War by the British WarOffice, it wasn’t widely used until the 1950s when it became popular in the designof bridges and shell structures.

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The demand for rapid construction in the 1950s saw the employment of precastconcrete panels as loadbearing walls (although early examples date back to theearly 20th century). This form of construction is commonly referred to as large

panel system (LPS) blocks or “system - builds”. This coincided with the

development of sealants and mechanical jointing details. Reema and Wates aretwo of the most well-known system-builders. The London County Council was akey promoter of system-builds utilising precast concrete, and by the early 1960sEngland was noted for the high quality of its precasting.

The partial collapse of a high-rise residential LPS tower (Ronan Point) in Londonin 1968 highlighted problems with the joints between panels and a lack ofrobustness. As a result, the Ministry of Housing and Local Governmentcommissioned investigations into the cause of the disaster, and it instructedowners of LPS dwellings to appraise all blocks over six storeys. In the decadesthat followed many reports and guidance documents (mainly written by theBuilding Research Establishment) on the appraisal of LPS buildings were

produced. Shell structures were also fashionable in this period, the most famous example inEngland being the Commonwealth Institute in Kensington, completed in 1962.

1.4 1970 – 1990Lean construction continued to be commonplace and thin walls and slabs can beexpected in buildings from this period.

Limit state codes were introduced in the UK in 1972, bringing about “modern ” design practice. The largest change was the approach to shear design, which had

not been well understood up to this point.During these decades several “in-service ” problems arose from constructionmethods that had been adopted during the post-war period. A few well publicisedcollapses of concrete structures using HAC concrete led to its ban in the 1970s.Most collapses have since been related back to poor construction details and if the

presence of HAC is suspected, specialist advice should be sought from AT&R.However if there are no obvious signs of deterioration, and if the concrete is well

protected from water, then it may be possible to conclude that the building willcontinue to perform adequately.

In 1977 the use of calcium chloride as an accelerator was banned, as it was

recognised that the presence of chlorides can promote corrosion.Industry knowledge of the behaviour of concrete in fire rapidly developed in the1980s, leading to improved and enhanced guidance for fire design.

1.5 1990 to the presentBy now, design codes were well established and many design/construction-relateddefects had come to light, prompting changes in design or material specification.

High-tech architecture saw concrete frames that were exposed to the eye butinboard of a façade envelope (commonly glass) providing protection from rain

and external conditions. Stability will be provided by designed moment frame orstability walls/cores (ie they should not be reliant on masonry infills for stability!)

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1.6 Construction timelineDating a building is an important step when surveying and assessing the building.It may be possible to ascertain its age from a distinct architectural style.

Additionally, the availability of various structural materials, components andsystems and their periods of common use have changed over time. Of course, buildings may at some point have been altered or extended.

Understanding the date and therefore likely form of construction and details canalso help to form an idea of what the hidden defects are likely to be.

Table 1. Periods when various forms of construction were used.

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1.7 Design code developmentRegulations for reinforced concrete were introduced in 1915, although the firstnational design code for concrete structures was not published until 1934. Prior tothe development of codes, designs were either proprietary or in accordance withtext books. Table 2 is an extract from TR70: Historical approaches to the designof concrete buildings and structures [6]. This document provides useful referenceinformation, including on the development of design codes, materials standardsand historic design methodology.

Table 2. Publication dates for main codes [6]

Date Design code

1915 London County Council Reinforced Concrete Regulations.

1934 Code of practice for reinforced concrete.*

1938 Code of practice for the design and construction of reinforced concrete structures for the storage of liquids.

1948 CP114. The structural use of normal reinforced concrete in buildings.

1950 CP114.100 – 114.105. Suspended concrete floors and roofs (including stairs).

1957 CP114. The structural use of normal reinforced concrete in buildings (Revised version of1948 code).

1959 CP115. The structural use of prestressed concrete in buildings.

1960 CP2007. Design and construction of reinforced and prestressed concrete structures forthe storage of water and other aqueous liquids (imperial units).

1962 BS1926. Ready-mixed concrete.

1965 CP116. The structural use of precast concrete.1969 CP114. The structural use of reinforced concrete in buildings: Part 2. Metric units.

1969 CP116. The structural use of precast concrete: Part 2. Metric units.

1970 Addendum No 1 to CP116:1965 and CP116: Part 2: 1969. Large-panel structures and structural connections in precast concrete.

1970 CP2007. Design and construction of reinforced and prestressed concrete structures forthe storage of water and other aqueous liquids (metric units).

1972 CP110. Code of practice for the structural use of concrete. Part 1. Design, materials andworkmanship. Part 2. Design charts for singly reinforced beams, doubly reinforced beamsand rectangular columns. Part 3. Design charts for circular columns and prestressedbeams.

1976 BS5337. Code of practice for the structural use of concrete for retaining aqueous liquids.

1981 BS5328. Methods of specifying concrete, including ready-mixed concrete.

1984 BS6349. Maritime structures.

1985 BS8110. Structural use of concrete. Part 1. Code of practice for design and construction. Part 2. Code of practice for special circumstances. Part 3. Design charts for singlyreinforced beams, doubly reinforced beams and rectangular columns.

1987 BS8007. Code of practice for design of concrete structures for retaining aqueous liquids.

* The 1934 code was issued by the Department of Scientific and Industrial Research and hence is often referred to as “TheDSIR Code”

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2 Methodology of appraisalA common misconception is that modern structures have been “engineered ” andthat they will have been correctly designed, say, for lateral stability.

Unlike older buildings of loadbearing masonry where the structure is often ondisplay and load paths are relatively simple to visualise and understand, modernframed buildings often have their structure concealed behind architecturalfinishes. Visualising the structural skeleton may therefore be difficult.

Without drawings it is very difficult, if not impossible, to differentiate betweenstructural concrete walls and concrete walls “added in ” by the architect. Also, thefaçade may be contributing to the overall building stability; the old London StockExchange was a typical example of such a case.

In addition, without drawings you will not immediately know the amount, size or

type of reinforcing steel and you may not know if it comprises embedded steelsections or individual bars. Also, the concrete strength and mix properties may not be known. There are various destructive and non-destructive tests, from whichinvestigators can gain greater confidence in the original design as well as thecurrent condition. Testing will need to be carefully considered, based on thereasons for your current assessment (ie what are you trying to confirm or justify)and access to the building to undertake testing (ie whether it is occupied, orowned by the client commissioning your appraisal, etc).

We therefore tend to rely heavily on archival drawings, if they exist. Howevereven with access to these it is advisable to break away the cover concrete inseveral locations to expose the reinforcement, so as to confirm that what is on the

drawing was actually built.A word of caution: don’t take things at face value . Brick walls may be renderedand painted, giving the same appearance as concrete walls, and structuralelements may seem thicker than they actually are due to the application of cementrender or screeds. Some targeted physical investigation may be required.

Finally, it is common that the engineer will want to know the concretecompressive strength for the building being appraised. However, knowledge ofstrength is not always important, ie where no change of use or loading is planned.Compressive testing can be disruptive and expensive, and there will alwaysremain a degree of uncertainty as it is impracticable to get a large number of cores

from all of the building elements. Judgement will be needed to balance the impacton the building versus the degree of uncertainty in the results. This is to be thesubject of a forthcoming stand-alone internal guidance note.

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3 Cracking in concreteReinforced concrete structures have to crack to fully mobilise the steelreinforcement, so understanding the importance of cracking is key. Cracks andcrack patterns have different characteristics, depending on the underlying cause.Most of the crack types listed below are not typically considered to be structuraldefects, although they may have serviceability or aesthetic implications.

Fig 1. Reasons that cracking may occur [12].

Fig. 2. Theoretical examples of types of cracks [10].

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Fig 3. Theoretical examples of crack patterns [12].

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4 DefectsDefects in concrete structures can be broadly divided into five categories:

design and detailing deficiencies construction-related defects durability-related defects materials-related defects accidental damage (ie fire, earthquake, impact, poor structural alterations).

Further details on the more commonly encountered, structurally significantdefects are included below, including links to more detailed referenceinformation. Where information is available, the period during which the defectsmight be encountered and the likelihood of encountering them have beenincluded, although this information is not always readily available.

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4.1 Design and detailing deficienciesPossible design deficiencies in concrete buildings include:

lack of robustness lack of shear reinforcement lack of tying or bearing of precast units (including stairs) incorrect detailing (particularly at joints) inadequate assessment of critical load paths.

Large panel system (LPS) robustness Indicators: There may be no visibleindicators under normal service loads.

Period: LPS structures were built up to theearly 1970s.

Consequences: Inadequate tying between precast elements gave poor robustness performance.

Insufficient numbers of ties between innerand outer panels can lead to panel failure.

Corrosion of in situ stitch reinforcementcan lead to joint failure.

Method of assessment: Destructive andnon-destructive tests along with risk-basedappraisals; refer to BRE Report 511

Details : The partial collapse of aresidential tower block at Ronan Point

brought to everyone's attention the fataldesign flaw. Immediately following, theMinistry of Housing and LocalGovernment instructed owners of LPSdwellings to appraise all blocks of over sixstoreys. Note that blocks of six storeys orless may not have been strengthened.

Further reference: Handbook for the structural assessment of large panel system(LPS) dwelling blocks of accidentalloading. [14]

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&docid=RUk8vINPmI5jNM&tbnid=cVF0bamiYtjPRM:&ved=0CAUQjRw&url=http://www.petitionbuzz.com/petitions/ronanpointmemorial&ei=gkitU_CQLuPiywOKn4H4BA&bvm=bv.69837884,d.bGQ&psig=AFQjCNHGGhKIbOfhXUYc0UTUlcFbVws0YQ&ust=1403951603319206

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Lack of shear reinforcement Indicators: There may be none, ordiagonal shear cracks may form.

Period: before 1985, particularly before

1972 Consequences: Shear failure is brittle. It

can be sudden and lead to partial or totalcollapse.

Method of assessment: non-destructivetesting and opening up to verify shearreinforcement; design check on capacity

Details : Before 1969, minimum shearreinforcement was not required. With theintroduction of CP114 in 1969, nominalshear reinforcement was required for nearlyall components except slabs, footings andminor members. Shear reinforcementdesign was based on levels of permissiblestress, and little guidance was given on

punching shear resistance.

In 1972, CP110 introduced a new approachand provided more guidance on slabs. Theintroduction of BS8110 in 1985 broughtshear reinforcement design towards today’sstandards.

Further reference: Historical approachesto the design of concrete buildings [6].

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4.2 Construction-related defectsConstruction-related defects may include:

honeycombing (inadequate grading or poor compaction) and grout loss lack of cover (from poor control during construction, such as misalignment of

formwork or bars) poor quality concrete or inadequate care during the curing process inadequate formwork (sagging, grout loss, etc).

They may also include defects that have primarily aesthetic implications:

colour variations steps in the surface, from misalignment of formwork and blow holes staining (eg from rust on the formwork prior to casting, from impurities in the

concrete mix, or from pollutants in the atmosphere) reinforcement ripple.

Further information on visual defects in concrete is available on the VisualConcrete pages of the Structural Skills Network.

Honeycombing and grout loss Indicators: lack of fines

Likely locations: various but mostcommonly in the lower portions ofstructural elements where placing theconcrete was difficult

Consequences: Shallow areas arecosmetic, but deeper areas may lead to asignificant reduction in protection to thereinforcement.

Method of assessment: visual inspection

Details : Honeycombing can result frominadequate grading of aggregate and/or

poor compaction of the concrete.

Alternatively it can be caused by groutleakage at construction or formwork joints

where they have been inadequately sealed. Further reference: Diagnosis of

deterioration in concrete structures .[10].

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Lack of cover Indicators: The reinforcement pattern may be reflected in colour variations on theconcrete surface (although this might notalways be due to low cover). Localisedcracking and rust staining due to corrosionmay subsequently occur.

Consequences: reduced durability, fire protection and bond, possible risk fromfalling concrete

Method of assessment: A covermeter can be used to estimate the cover, althoughresults should be verified with localisedopening-up.

Details : — .

Further reference: Historical approachesto the design of concrete buildings [6].

Reinforcement ripple Indicators: shallow troughs reflecting the pattern of reinforcement

Likely locations: skip-floated concrete;lightweight aggregate concrete

Consequences: While reinforcement rippleis not typically considered to be a structuraldefect, it can have an alarming appearance;it is therefore considered to be an aesthetic,not a structural or durability problem.

Method of assessment: visual inspection

Details : Reinforcement ripple is a surfaceirregularity on concrete slabs, in the formof shallow troughs over the lines ofreinforcement.

It is believed to be caused by vibrations setup in the reinforcement by the method ofcompacting the concrete, which results inadditional compaction around the bars.

Further reference : Concrete advice Note9. Reinforcement ripple. The ConcreteSociety, 2003 .

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Corrosion from carbonation Indicators: spalling, cracking

Likely locations: all externally exposedsurfaces or internal surfaces in moist

conditions Consequences: Carbonation results in loss

of protection to the steel reinforcement,resulting in a risk of corrosion expansionand associated spalling of concrete.

Method of assessment: Phenolphthaleincan be used to test the depth of carbonationon freshly exposed concrete faces(localised break-out).

Details : Carbonation is the reaction of CO 2 in the atmosphere with calcium hydroxidein the cement paste. The reaction producescalcium carbonate and lowers the pH of theconcrete from a protective alkalineenvironment, to about pH9. At this pH, the

protective oxide layer breaks down andcorrosion becomes possible.

The rate of penetration of CO 2 into theconcrete is affected by the quantity ofmoisture in the atmosphere. The optimumcondition for carbonation-inducedcorrosion is sheltered, moist exposure or inwetting and drying conditions, particularlywhere cover to reinforcement is low.

Further reference: Diagnosis ofdeterioration in concrete structures .[10].

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Corrosion from chlorides Indicators: spalling, cracking, corrosion ofreinforcement

Likely locations: Corrosion of

reinforcement is accelerated by chlorideions from internal sources such as calciumchloride (a once commonly-usedaccelerator, see Section 4.4) or the use ofunwashed marine aggregates, and fromexternal sources such as de-icing salts or amarine environment.

Consequences: corrosion of reinforcementand related spalling (loss of section)

Method of assessment: laboratory testingof dust samples (see Section 5.1.3)

Details : Chloride-induced corrosion cantypically be differentiated fromcarbonation-induced corrosion as it ischaracterised by local, rapidly corrodingareas of bars (pitting); carbonation-inducedcorrosion tends to be general.


Freeze – thaw Indicators: surface scaling and spalling;note that frost attack may produce a similarcrack pattern to alkali-silica reaction(ASR); it can sometimes be distinguished

by the presence of spalling.

Likely locations: concrete subjected tofreezing when saturated

Consequences: loss of section, reduceddurability

Method of assessment: visualexamination, petrographic examination(see Section 5.1.1) .

Details : Surface scaling and spalling canoccur when water held in the capillary

pores of cement paste freezes at lowtemperatures. Ice formation results inexpansive forces, which can be greater thanthe local strength of the concrete. Severitymay be greater where exposed to de-icingsalts.


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Chemical attack: internal sulphateattack, or delayed ettringiteformation (DEF)

Indicator: cracking

Likely locations: heat-cured precast, andin situ massive concrete exposed tomoisture. There is a concern that DEF mayoccur in large concrete structures due to theheat of hydration as the concrete cures.DEF is not a common phenomenon.

Consequences: loss of durability due todelayed internal expansive reactions

Method of assessment: visual assessment, petrographic examination (see Section5.1.1)

Details: The mineral ettringite is normallyformed at an early age during the concrete

curing process at ambient temperatures.Delayed formation of ettringite occurswhen early high temperatures prevent itsnormal formation. Its gradual formation inthe cooled, set concrete is expansive andcan lead to cracking and can increase therisk of problems such as freeze – thawattack. The right conditions must be

present for the delayed reaction to occur,including the presence of water.

Further reference: BRE IP11/01. Delayedettringite formation: in situ concrete .

Chemical attack: external delayedthaumasite sulphate attack (TSA)

Indicators: cracking, deflection

Likely locations: concrete in wet ground,eg foundations, with sulphates in thegroundwater. TSA is a rare phenomenon.

Consequences: loss of strength.

Method of assessment: visual assessment, petrographic examination (Section 5.1.1)

Details : TSA is a rapid form of sulphateattack which occurs in the presence ofsulphates in the groundwater. TSA occursat low temperatures in wet ground. Thethaumasite mineral forms, graduallyreplacing the cement paste matrix, andcauses the concrete to soften andeventually disintegrate.

Further reference: Deterioration of

cement-based building materials: lessonslearnt [13].

http://civildigital.com/wp-content/uploads/2014/06/1typicaldamage.jpg

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In addition, the following defects that are typically but not necessarily durability-related may be observed:

Efflorescence and lime leaching Indicators: white patches or deposits,

stalactites on the surface Consequences: typically an aesthetic

problem, although significant leakage may be symptomatic of a more severe problemwith adverse effects on durability

Method of assessment: visual assessment

Details : Water leaking through theconcrete can dissolve calcium hydroxidefrom the matrix. On contact with theatmosphere (generally at cracks or joints),

this is precipitated on the surface as a whiteresidue. Significant leakage can result inthe formation of stalactites. Lightefflorescence on younger structures mayresult simply from water drying fromconcrete.


Rust-staining and rust-spots

Indicators: rust stains or spots on concretesurface

Consequences: typically an aesthetic problem only

Method of assessment: check on cover,carbonation and chemical analysis ofsamples to rule out a more serious problem

Details : When reinforcement has beenfixed for some time before concreting, rustfrom the reinforcement may get washedonto the formwork and stain the concretesurface.

Rust spots are typically from tyingwires/nails left on formwork, tying wirenot bent back in, or from iron compoundsin aggregates (eg pyrites).


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4.4 Materials-related defectsMaterials-related defects include:

high alumina cement (HAC) calcium chloride Aggregate-related defects such as aggregate swelling and shrinkage, softening

and alkali-silica reaction (ASR).

Types of products:

autoclaved aerated concrete (AAC) woodwool slabs.

High alumina cement (HAC)

A view of the end of a typical precast HAC concreteI-beam that has been exposed by removing bricksfrom a cavity wall — from High alumina cementconcrete, Moss & Dunster.

Indicators: can be hard to detect, possiblydark colour of concrete, age of structure,shape of element

Period: 1922 – 1975; the height of use was1950 – 1970.

Likely locations: precast prestressedconcrete beams

Consequences: loss of strength andincreased susceptibility to corrosion

Method of assessment: Chemical analysiscan be used to determine the type of

cement, based on alumina content.Methods to determine the residual strengthare discussed in Section 5.2.

Details : HAC was promoted after WWIIdue to its resistance to the effects ofsulphate, and the speed at which it couldreach peak strength — within 24 hours of

pouring. However, this high strength is dueto the cement being in an unstable form.With time, it converts to a more stable,weaker concrete, leaving the concrete

much more porous than before.In addition, its increased porosity makes itmore vulnerable to chemical attackalthough exposure of HAC elements toaggressive chemical environments is rare.

Further references: ROGERSON, R, et al. High aluminacement concrete in buildings.

BATE, S. High alumina cement concrete inexisting building superstructures. BRE.

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Calcium chloride Indicators: spalling, cracking, corrosion ofreinforcement

Period: 1890 – 1975; the height of use was

1950 – 1970. Consequences: corrosion of reinforcement

and related spalling (loss of section).

Method of assessment: petrographicexamination (see Section 5.1.1) , chloridetest.

Details : Calcium chloride was commonlyused as an accelerator until it was no longer

permitted, as directed in an amendment toCP110 in 1977. Excessive chlorides in themix cause a reduction in the alkalinity,leading to corrosion of reinforcement.

Further reference: Historical approachesto the design of concrete buildings [6] .

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Alkali-silica reaction/alkali-aggregatereaction (ASR/AAR)

Indicators: a network of cracks — “map”cracking, possibly with evidence of exudedgel products; note that frost attack may

produce a similar crack pattern, however itcan sometimes be distinguished by the

presence of spalling.

Period: 1930 – 1980, particularly 1960 – 1980 . Relatively few structures have beenidentified with ASR and even fewer ofstructural significance.

Consequences: typically surface cracking;internal cracks may cause loss of strength.

Method of assessment: visual inspection, petrographic examination (see 5.1.1)

Details : ASR is a reaction between certainforms of silica contained in aggregates, andalkalis present in Portland cement. Itoccurs only in the presence of moisture.The products of the reaction are usually ofgreater volume, resulting in an expansionand causing random “map” cracking. De-icing salts can contribute to ASR.

Further references: INSTITUTION OF STRUCTURAL ENGINEERS. Structural effects of alkali- silica reaction, IStructE, 1992, with 2010addendum .

BUILDING RESEARCH ESTABLISHMENT. Digest 330 Part 1, Alkali-silica reaction in concrete, 2004.

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Autoclaved aerated concrete (AAC) Indicators: hairline cracking, excessive in-service deflections

Period: 1920 – 1980; the height of use was

1950. Likely locations: roof planks, beam and

block floors

Consequences: excessive roof deflectionsresultin in ponding of rainwater.; someevidence of corrosion initiation

Method of assessment: petrographicexamination (see Section 5.1.1)

Details : AAC is formed by aerating a mixof fine inert mineral particles and bonding

them together with Portland cement. Thisis steam-cured and combined withreinforcement to form lightweight roof,floor and wall panels.

Further reference: BUILDING RESEARCH ESTABLISHMENT. IP10/96. Reinforcedautoclaved aerated concrete planksdesigned before 1980.

Woodwool Indicators:

Period: 1920 – 1980; the height of use was1950.

Location: as permanent formwork, asinsulation panels

Consequences: loss of durability

Method of assessment: opening-up toinspect condition of concrete slabs formedon woodwool shuttering

Details : Woodwool slabs are made fromwood shavings bound together withcement. They have good insulation and fire

properties. When using woodwool as a permanent formwork, the concrete wastypically poorly compacted due to thecompressibility of the board, leading todurability issues .

Further reference:http://www.sandberg.co.uk/investigation-

inspection/inspection/woodwool.html

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4.5 Accidental damage

Fire Indicators: blackened surface, crackingand spalling, change in colour of concrete

Likely locations: various

Consequences: loss of strength (dependingon temperature of the fire)

Method of assessment: Colour change cansometimes be used to estimate thetemperature reached.

Details : Differential expansion of layers of

concrete and internal pressure as moisture becomes super-heated can result incracking and spalling.

Heating concrete above 300 ˚C reduces itscompressive strength linearly, with allstrength lost above about 1000 ˚C. Loss instrength may be associated with a changein colour. Strength in reinforcement is alsoreduced.

Further reference:CONCRETE SOCIETY. Technical Report

33 Assessment and repair of fire-damagedconcrete structures.

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Human intervention Indicators: core-drilled (rather thanformed) service openings, chases which cutreinforcement links, etc.

Likely locations: various

Consequences: loss of strength

Method of assessment: removal ofinternal finishes to expose concretesections for visual inspection; where loss ofsection occurs, carry out a design check onremaining section

Details : To allow full appraisal of aconcrete frame, the true dimensions of thestructural elements will need to beconfirmed. Columns and beams may berendered or disguised by decorative boards.Be especially suspicious when only one ortwo columns are clad and the others areexposed — the cladding may be hiding adefect or alteration.

4.6 Cladding-related defectsThe following defects are observed in the building cladding, but have beenincluded in this Note as problems often result from incorrect detailing in thedesign, in terms of their interaction with the concrete frame (ie no provision forcreep or thermal movements between two different materials).

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Failure of brick slips and panels:deflection of concrete frame

Indicators: loss of brick-slips, failure ofentire panels, vertical cracking in

brickwork

Likely locations: concrete-framed buildings with brick façades and nomovement joints at floor levels; common in

buildings pre-1980 when brick slips wereused on concrete-framed tower blocks tohide concrete slabs and provide acontinuous brick façade. Each level of the

brick façade bears on a concrete nib so thata storey-high panel of brick is supported atevery floor.

Consequences: compression failure of

brick panels, dislodging of brickslips, and possible collapse of areas of the outer brickwork

Method of assessment: visual inspectionto identify missing brick slips, bulging in

brick panels, and vertical cracks in the brickwork

Details : Over time, creep shrinkage of theconcrete frame occurs, shortening theheight of the building. The initial andseasonal movements in the brick cause the

panel to expand. Without movement jointsat floor locations, the brickwork is put intocompression, which it will not have beendesigned to take.

Where the panel is fully supported on thenib, compression cracks or buckling failureof the panel can occur.

Where the panel overhangs the edge of thesupporting nib (see next defect), the panel

puts the brick slips into compression anddebonding can occur. The panel is then freeto rotate at the base and head causing

panels to bulge outwards. Where there areinsufficient ties or where the restraintoffered by the ties has degraded, over timethe panel can bulge outwards and collapse.

Understanding the support condition of the brickwork is critical for even small façadechanges including overcladding withexternal wall insulation.

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Failure of brick panels: excessiveoverhang

Indicators: may be nothing obvious untillocal or global failure occurs; keyindicators will be loss of brick-slips, failureof entire panels, vertical cracking in

brickwork.

Likely locations: concrete-framed buildings with brick façades; common in buildings pre-1980 before the introductionof secondary steelwork to support thefaçade; each level of brick façade bears ona concrete nib so that a storey-high panel of

brick is supported at every floor.

Consequences: Loss of support of brick panels can lead to collapse of local or

larger area of brickwork. Method of assessment: visual inspection

to identify missing brick slips, bulging in brick panels, and vertical cracks in the brickwork; also steps in the face of brickwork where a section of brickworkmay have been pushed outward (ie base of

parapets)

Details : Construction tolerances oftenresulted in concrete nibs not always

providing the required support to the

brickwork panels (ie two-thirds supported).The gap between the face of the nib and the

brickslip was built up with mortar or eventhe doubling up of brickslips. This resultedin areas of the brickwork being gravitystacked over several storeys, which it willnot have been designed to do.

Of particular note is where parapets existand where there are no vertical joints in the

parapet brickwork to accommodatehorizontal expansion. Ratcheting over time

can reduce the original bearing length and,if this is insufficient to begin with, loss of

bearing can become a real problem.

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5 Testing

5.1 Types of concrete testTo gain an understanding of the condition of the concrete, testing should be donein conjunction with a visual survey and a covermeter survey at test locations.

Factors to consider when determining frequency and locations of tests include:

external or internal environment presence of visible defects – extent of cracking, spalling, etc type of environment (external, marine, de-icing salts presenting higher

corrosion risk) exposure (eg south facing, horizontal versus vertical).

A wide range of concrete tests are available, including:

petrographic examination carbonation testing chloride testing half-cell potential resistivity linear polarisation resistance.

In most cases it will be appropriate to apply a range of tests based on whatinformation is required. Careful consideration should be given to selection of therange and number of tests, as it is often not possible or desirable to go back to dofurther testing.

5.1.1 Petrographic examinationA petrographic examination can be thought of as a concrete health check. Itinvolves the use of high power optical microscopes to examine samples ofconcrete to determine their mineralogical and chemical characteristics. Comparedto carbonation and chloride testing, this is expensive and takes more time to getthe results, so needs careful consideration when determining the number andlocation of tests required. However it is an important test to consider whenappraising a concrete structure.

The examination should only be entrusted to an experienced concrete petrographer and should provide description and commentary on the componentsof the concrete together with other features noted which are relevant to thestructural and durability performance of the concrete. This may include:

chemical attack, particularly sulfate or acid alkali-silica reaction (ASR) presence of high alumina cement (HAC) delayed ettringite formation (DEF) thaumasite sulfate attack (TSA) aggregate or cement paste shrinkage

frost attack carbonation

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microcracking aggressive leaching detection of unsound contaminants.

5.1.2 Carbonation testingThere are several common ways of performing the carbonation test. A small pieceof concrete may be broken away and a solution of phenolphthalein sprayed ontothe freshly exposed substrate. Alternatively, a hole approximately 25mm indiameter is drilled, the dust removed from the hole, and a solution of

phenolphthalein then sprayed onto the wall of the hole; this method should not beencouraged as it can give false results if the drill dust is not fully removed fromthe walls of the hole. Best results will be obtained from extracting a smalldiameter (25mm – 50mm) core, splitting it open along its length, and spraying thefracture surface. The alkaline concrete will turn pink but the carbonated concretewill not change colour, thus enabling the depth of carbonation from the surface to

be measured.

Fig 4. Phenolphthalein sprayed onto freshly exposed concrete

5.1.3 Chloride testingTesting can be performed on dust samples taken using a drill (noting the depth ofsample) or on samples from small (50mm) cores or lump samples, and then labtested to determine chloride level. Samples should be taken incrementally withdepth down at least to the depth of the reinforcement if chloride ingress from anexternal source is suspected. More than one hole may need to be drilled to obtainsufficient sample at each depth increment.

5.1.4 Cement content testingCement content determination is commonly required within test programmes butmay be of limited value, due to the significant uncertainty in the results obtained(probably no better than +/- 40 kg/m 3). In theory, cement content is required forinterpretation of chloride content measurements, but the high inherent variabilityin results means that assumed values are generally used. Cement contentmeasurements may provide an overall indication of concrete quality.

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5.1.5 Sulfate content testingSulfate content testing is commonly specified but is rarely of any practical useunless sulfate-related deterioration is suspected. Testing can be performed ondrilling dust samples (min 50g).

5.1.6 Half-cell potentialCorrosion of reinforcing steel is an electrochemical process and the behaviour ofthe steel can be characterised by measuring its half -cell potential. An electrodeforms one half of the cell and the reinforcing steel in the concrete the other. Thetest provides a measure of the likelihood of active corrosion at the time of testing.Care must be taken in interpretation of data and should be based on spacing ofcontours on an isopotential plot rather than the absolute readings.

5.2 Determining concrete strengthMethods of determining the in situ concrete strength include:

compressive testing of extracted cores rebound hammer (Schmidt hammer) ultrasonic pulse velocity (UPV) internal fracture/pull-out.

Obtaining reliable information on in situ concrete strength is a topic in itself andwill be dealt with in a subsequent Guidance Note.

5.3 Determining reinforcementThe strength of reinforcement can be estimated from the date of construction.Alternatively, a sample can be taken for testing, which requires removing a short length(refer to BS EN 15630-1 ).

There are several methods of obtaining the layout of reinforcement:

archive drawings covermeter (eg Ferroscan ) ground-penetrating radar (GPR).

It should be noted that while a comprehensive set of drawings and reinforcementdetails may exist, some opening-up work should be considered to verify that theas-built structure matches the drawings.

5.3.1 Covermeter (eg F er r oscan )The presence of reinforcement in concrete can be detected by the influence thatreinforcing steel has upon an electromagnetic field induced by the covermeter.

A covermeter may be used to determine the arrangement and position ofreinforcement together with the concrete cover to the bar and/or the indicative bardiameter.

Some covermeters (eg Ferroscan ) produce a full image of the reinforcementwithin the scanning areas. The images may be evaluated by viewing transverse or

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longitudinal sections, “slices ” at different depths, and by producing statisticalinformation.

Results should always be calibrated by physical measurement of cover (eg byopening up, or drilling down to the bar) in at least one location.

Limitations of the technique include:

Results for bar diameter are likely to need verifying/calibrating with someintrusive tests.

The detection depth is limited (consider GPR for greater depths). Results may be difficult to interpret in areas of congested reinforcement.

5.3.2 Ground-penetrating radarGPR systems use an antenna to send electromagnetic signals into a subsurface.Different materials will return different signals, by absorbing or reflecting energyto a different extent.

GPR has a wide range of uses and can be used with a variety of materials. Inconcrete, it is most likely to be used to:

measure the thickness and build-up of slabs and walls, for example it candifferentiate between slab and screed

rebar map reinforced concrete (up to 450mm deep) locate post-tensioning ducts.

Limitations of the technique include:

The rebar diameter cannot be detected.

5.3.3 ResistivityThe resistivity of concrete cover is a determining influence on the rate ofcorrosion of reinforcement. It can be estimated from measurements of the drop inelectrical potential between probes (typically in an array of four) placed in contactwith the concrete surface when a known alternating current is applied. It shouldgenerally be used in conjunction with measurement of half-cell potential.

Results can be adversely affected by a carbonated concrete surface layer; they arealso affected by temperature and moisture content.

5.3.4 Linear polarisation resistanceLPR measurements are generally used to determine the instantaneous corrosionrate of an electrode. The technique is based on the observation that within a small

potential range around the corrosion potential (E corr ), the relationship between thecorrosion potential (E corr ) and the logarithm of current density (I) is approximatelylinear. While instantaneous rates of corrosion can be measured and are of interestto determine mass of steel loss in either chloride-induced corrosion orcarbonation, usually the external environment may influence the accuracy of LPRmeasurements due to seasonal variations. Care and appropriate measurement

methodologies should be followed at all times.

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6 Key referencesThe list below contains useful general references. Note that references for specificdefects have been included alongside details of that particular defect in Section 4.

Arup appraisal guidance[1] 2014 SGN 05 : Typical floor systems in 19th and early 20th century

buildings: an introduction.

[2] 2014 SGN 06 : Typical defects in 19th century buildings: anintroduction.

General appraisal guidance[3] THE INSTITUTION OF STRUCTURAL ENGINEERS. Appraisal of

existing structures. Third edition. IStructE, 2010.

[4] CONCRETE BRIDGE DEVELOPMENT GROUP. Technical Guide2. Guide to testing and monitoring the durability of concretestructures. CBDG, 2002.

Historic concrete[5] SUTHERLAND J. et al . Historic concrete: background to appraisal.

Thomas Telford, 2001.

[6] THE CONCRETE SOCIETY. T echnical Report No 70. Historicalapproaches to the design of concrete buildings. CCIP, 2009.

[7] BUSSELL, MN. Institution of Civil Engineers No 11068 . The era of proprietary reinforcing systems. In Structures and buildings,historic concrete . ICE, 1996.

[8] BUSSELL, MN. Institution of Civil Engineers No 11069 . Thedevelopment of reinforced concrete: design theory and

practice. In Structures and buildings, historic concrete . ICE,1996.

[9] HISTORIC SCOTLAND. Historic concrete in Scotland. Part 1:History and development. Historic Scotland, 2013

Concrete deterioration[10] THE CONCRETE SOCIETY. Technical Report No 54 . Diagnosis of

deterioration in concrete structures. The Concrete Society,2000.

[11] WHITTLE, R. Failures in concrete structures. CRC Press, 2013.

[12] THE CONCRETE SOCIETY. Technical Report No 44 . The relevanceof cracking in concrete to corrosion of reinforcement. TheConcrete Society, 1995.

http://networks.intranet.arup.com/?document=6FF61BC1-BCD6-5076-80E6-993B2AFEE254http://networks.intranet.arup.com/?document=6FF61BC1-BCD6-5076-80E6-993B2AFEE254http://networks.intranet.arup.com/?document=72CE1E2A-B4DE-9EAE-229F-8D478B5D9B61http://networks.intranet.arup.com/?document=72CE1E2A-B4DE-9EAE-229F-8D478B5D9B61http://networks.intranet.arup.com/?document=72CE1E2A-B4DE-9EAE-229F-8D478B5D9B61http://networks.intranet.arup.com/?document=6FF61BC1-BCD6-5076-80E6-993B2AFEE254

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[13] BUILDING RESEARCH ESTABLISHMENT. IP4/03 . Deteriorationof cement-based building materials: Lessons learnt. BRE,2003.

[14] BUILDING RESEARCH ESTABLISHMENT. BRE Report 511 .Handbook for the structural assessment of large panel system(LPS) dwelling blocks of accidental loading. BRE. 2012.

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