The Investigation and Repair of Historic

NSW Heritage Office The Investigation and Repair of Historic Concrete was written by Susan Macdonald, Assistant Director of the NSW Heritage Office. Susan studied architecture and has considerable experience in heritage conservation, having previously worked in the Architectural Conservation Team of English Heritage. She has a particular interest in the conservation of 20th century buildings and materials. She is former Secretary of DOCOMOMO UK and a former member of the executive committee of ICOMOS Australia.

Acknowledgements Thank you to all those who have contributed to the development of this technicat resource. A number of people provided expert comments and contributions to the text, including the members of the Heritage Council's Technical Advisory Group. And special thanks to John Broomfield and Kevin Hunt who also assisted with illustrations. NSW Heritage Office 3 Marist Place Parramatta NSW 2150 Locked Bag 5020 Parramatta NSW 2124 Ph: (02) 9873 8500 Fax: (02) 9873 8599 [email protected] www.heritage.nsw.gov.au Bottle kilns similar to those used in production In the 19th century. photographed in 1889. first began Disclaimer to be produced by Australian companies in the 1880s. including the Cullen Bullen Lime and Cement Company at Portland. NSW. Any representation, statement, opinion or advice, Photograph courtesy of the Cement & Concrete Association expressed or implied in this publication is made in good of Australia faith but on the basis that the State of New South Wales, its agents and employees are not liable (whether by reason of negligence, lack of care or otherwise) to any person for any damage or loss whatsoever which has occurred or may occur in relation to that person taking or not taking (as the case may be) action in respect of any representation, statement, or advice referred to above. ISBN 1 876415673 H003/03 May 2003

NSW Heritage Office

Front Cover: One of the most significant uses of concrete in 11:> This work is copyright of the NSW Heritage Office. Australia was in the construction of the Sydney Opera House. Apart from any use as permitted under the Copyright Act 1968, Precast segments, post-tensioned together formed the ribs of no part may be reproduced by any process without written the roof shells. The building was officially opened on permission from the NSW Heritage Office. Requests and 20 October 1973. enquiries concerning reproduction and rights should be Photograph courtesy of the Government Printing Office col/ection, addressed to the NSW Heritage Office, Locked Bag 5020. State Library of NSW Parramatta, NSW 2124. ~&qo.5c>~~ MAc..... The Investigation and Repair of Historic Concrete

This information sheet provides guidance on the repair of reinforced-concrete buildings. It looks at the causes of concrete deterioration and explains how to approach the repair of reinforced-concrete buildings. It also strongly recommends that a specialist structural engineer be consulted for independent expert advice. Such an engineer will need to have the necessary expertise in structural assessment, investigation and repair. A glossary is provided to explain some of the more technical terms used in this information sheet.

Contents

A brief history of concrete in Australia 4

Understanding the material and the structure 5

Concrete: Composition and characteristics

How and why concrete deteriorates 6

Concrete decay

Causes of deterioration in

An approach to repair 10

Assessment and investigation

Repair

Maintenance

Balancing technical performance, life-cycle costs and conservation

Further information 17

Glossary 18 The Investigation and Repair of Historic Concrete A brief history of concrete in Australia

Despite being thought of as a modern material, concrete has been in use for thousands of years. Lime-based hydraulic (those set by chemical reaction with ) have been in use since Roman times. Some ancient mortars still survive today, including examples used in the Pantheon in Rome and aqueducts in France. The word concrete comes from the Latin concretus, which means "mixed together" or compounded.

The use of these hydraulic mortars declined after the Reinforced concrete construction was introduced into Romans but was revived during the Middle Ages. Australia in the early 19th century and by 1909 there The technology survived until the eighteenth century were about six specialist companies using patented when John Smeaton's experiments in England produced systems of reinforced concrete. The first reinforced the first real advancement in concrete technology since concrete structure in Australia was the Johnstons Creek early Roman times. This signalled Ihe beginnings of the and Whites Creek Sewer Aqueduct built in 1895 in modern concrete industry. Annandale. It was constructed using Monnier's system of reinforcement (under license from the French Lime concrete was the precursor to modern concrete company). This structure was recently repaired and still and was made with natural cements. In Australia the serves its original purpose for sewerage. The aqueduct early use of what was described as concrete was, in is listed on the State Heritage Register in recognition of fact, mainly lime concrete. Natural cements were initially its significance as one of the first major reinforced imported to Australia from the 1840s and then from the concrete structures built in Australia. 1860s were manufactured in Australia. Australia was quick to take up reinforced concrete. The first modern concrete was made with Portland A number of reinforcing systems for floors and roofs cement. Portland cement was manufactured from were used, including straps and external forms. limestone or chalk, together with clay or shale. It was By t 915 the use of reinforced concrete began to patented by Joseph Aspdin in 1824 and named after its be regulated . resemblance to Portland stone. It began to be produced in Australia in the 1880s. Towards the end of the It was not until the 1950s that , nineteenth century modern concrete (using Portland which uses tensioned steel to increase strength, began cement instead of lime) began to be used for floors, to be used in Australia. After WWII structural steel was foundations and infrastructure projects. in short supply and prestressed concrete provided a substitute. The ice tower at Warragamba Dam is one of the earliest examples. Reinforced concrete The use of concrete has changed dramatically over the The principles of reinforcing concrete to provide both last 100 years, and continues to evolve as tensile and compressive strength were understood in understanding about the material improves. ancient times. However, it was not until the nineteenth century that a number of European and North American inventors developed and patented reinforcing methods. The sewer aqueducts constructed at Johnstons Creek and This resulted in the widespread introduction of a fully Whites Creek in Annandale were Australia 's first major reinforced fledged concrete industry. concrete structures. They were built by Carter Gummow & Co. in 1895 using Joseph Monnier's reinforced concrete technique , under license from the French company. Photograph courtesy of the Cement & Concrete Association of Australia. NSW Heritage Office Understanding the material and the structure

Just as the design of a reinforced-concrete building varies enormously according to its date, so does the construction method, materials and standards of workmanship. All these factors affect how the building deteriorates and may result in different problems, depending on when the building was constructed. It is thus imperative to understand the characteristics of a reinforced-concrete structure, in order to diagnose problems correctly and identify appropriate repair techniques.

Concrete: Composition and characteristics

Concrete is made up of sand or stone, known as aggregate, combined with cement paste to bind it. Aggregate can be of various sizes. It is broadly categorised as fine (commonly sand) and coarse (typically crushed stone or gravel). The greater proportion of concrete is aggregate which is bulky and relatively cheaper than the cement. As much of the constituents of concrete come from stone, it is often thought that concrete has the same qualities and will last forever. Concrete has been called arlificial stone, cast stone, reconstructed stone and reconstituted stone. However, concrete must be thought of as a distinct material to stone. It has its own characteristics in terms of durability, weathering and repair. Concrete has high compressive strength, but weak tensile strength. To overcome this deficiency when concrete is used as a structural building material , reinforcement (originally iron and later steel) is included Concrete was used for lighthouses and marine engineering in areas where tension occurs. This also helps to works, such as Bradleys Head Lighthouse in Sydney Harbour. combat early shrinkage and subsequent diurnal thermal It was the first pre-cast concrete lighthouse in Australia. expansion and contraction. Constructed in 1905 by Gummow Forrest & Company, it is the earliest example of the use of reinforced concrete in maritime Steel and concrete have similar coefficients of situations and utilises both in-situ and pre-cast sections. expansion and contraction and therefore work well Photograpll by David Nutley together as a composite material. Concrete is usually mixed wet, poured into , which supports its wet weight, distributed around the placed reinforcement, and then compacted or vibrated to expel Reinforced concrete air. It then sets or hardens by chemical reaction. Concrete is nearly always reinforced with steel embedded within it. Reinforced concrete can be manufactured in a casting yard (). But for buildings, it is more commonly made on site (in situ concrete). Its performance and appearance are therefore dependent on the individual materials of which it is composed, workmanship during construction, and on subsequent environmental conditions and maintenance. Improved quality control and mix designs, along with a now recognised understanding of some of the earlier degradation problems, has greatly reduced the chances of poor quality construction and premature degradation. Unfortunately, a number of historic concrete structures exhibit problems that are related to their date of origin. Practice methods of the day, such as workmanship and understanding about best practice, can lead to concrete decay, as can materials in use at the time, including The houses forming the striking streetscape of types of reinforcement, cement matrix and water 258-266 Johnston Street, Annandale, were built by well known content. Sydney builder John Young in the 18805. was used in the footings and turrets. Photograph courtesy of Anthony Mitchell .-- ~

The Investigation a nd Repair of Historic Concrete How and why concrete deteri orates

It is imperative to establish the cause of the concrete degradation or steel reinforcement before repairs are carried out. Poor understanding of the cause of the p roblem can result in inappropriate repairs.

Concrete d ecay

Concrete is a relativ ely durable and robust building material, but it can be severely weakened by poor manufacture or a very aggressive en vironment. Concrete degradation can be a cause for concern on its own, or in reinforced structures it may lead to deere ased protection to the steel. This in turn encourages corrosion of the steel, often followed by cracking and spallin g of the concrete. Deterioration of con crete is due to either: • chemical degradation of the cementitious matrix; • corrosion of the r einforcement steel; • physical damage (impact, abrasive and fire damage). The most important causes of concrete deterioration are described below.

Causes of d eterioration in reinforced concrete

Design proble ms There are a number of design and specification problems that can result in reinforced-concrete deterioration.

Causes of the problem Results

Poor reinforcement details, for example congested or Leads to cracking, poor compaction, loss of inadequate reinforc ement, inadequale cover to alkaline enVironment, and voids around the steel reinforcement

Poor detailing of fi xings, window frames, handrails, Water penetration, localised cracking, and balcony supports, and exp ansion joints weakness

Long, slender com ponents Excessive flexing may lead to cracking

Inadequate design for creep Deflection due to strain under continued that can result in cracking

Decorative finishes , such as acid etching, Results in varying depth of cover around the bush hammering, and fluting steel and localised corrosion

Poor drainage Leads to water ponding and localised corrosion/degradation

Incorrect concrete grade for purpose Can produce concrete that is too weak/too strong

Mixes that result in high drying shrinkage Can result in cracking

Mixes that are per meable to chloride ions Chloride induced reinforcement corrosion

Centen nial Park Reservoir No 1. I showing mass concrete groined arch es and brick columns. The reservoir was completed in 1898 afteT nearly five years of construct ion time and was describ ed as "by far the largest of its kind in the Southern He misphere". The reservoir is still operating. Image RS/PI83 L P, courtesy of Sydney Water Corporation and Sydney Catchment Authority Historjcal Research and Archives Facmty 0 NSW Heritage Office

Workmanship Care and attention during construction is crucial to the long-term durability of reinforced concrete. Current building codes, including AS 3600, give guidance appropriate to the exposure conditions, and on mix design (particularly the water/cement ratio and amount of cement binder), additives, compaction, detail, and thickness of cover. The type of cement binder is also now well defined. However, this has not always been the case and durability problems of earlier Typical reinforcement corrosion resulting from inadequate depth Of concrete covering to the reinforcement. concrete are summarised in the following table. Photograph by Susan Macdonald

Causes of the Problem Result

Poor mixing Leads to inhomogeneous concrete, localised weakness, and reinforcement corrosion

Incorrect water/cement ratios Can lead to variable strength, inadequate durability, increased drying shrinkage, excessive permeability

Poor compactionlvibration Results in honeycombing, voids, excessively permeable concrete, localised reinforcement corrosion

Varying and inadequate cover depths around Leads to localised reinforcement corrosion, the steel penetration of damaging substances

Poor curing techniques Results in shrinkage cracks, increased permeability, poor durability

Premature stripping of formwork Can result in cracking

Materials Lack of knowledge about the importance of careful selection and specification of materials and the use of additives has created a number of durability problems Typical cracked concrete section due to poorly compacted for historic concrete structures. They can be the concrete. result of the use of the following. Photograph by Kevin Hunt

Causes of the Problem Result

Too low cement content Results in weakened and poor durability concrete

Too high cement content Can result in excessive shrinkage/poor workability and cracking

Additives, such as Chloride ions destroy the protective passive- oxide layer on the steel

High alumina cement Weakening concrete in wet environments

Too finely-ground cements Causing excessive shrinkage and cracking that compromises the permeability of the concrete

Poor quality aggregates Resulting in -aggregate reaction, poor workability of the concrete, poor compaction, high drying shrinkage and weak concrete

Contaminated aggregates Resulting in corrosion of steel and degradation of concrete in extreme cases

Poorly shaped and badly graded aggregates Results in poor workability often necessitating extra water or vibration during forming which can lead to segregation, bleeding etc

Incorrect water/cement ratios Giving rise to weak concrete, loss of durability, increased permeability to gases and chloride ions .-

The Investigation a nd Repair of Historic Concrete

EnvlronmentaI influences These play an impo rtant part in reinforced -concrete deterioration and include the following .

Environmental c ondition Result

Carbon dioxide and acidic gases Lower pH around the steel that enables corrosion to progress

Water Can introduce depassivating chloride ions into concrete

Freeze thaw in co Ider zones Breakdown of surface, progressive cracking, water penetration to reinforcement that enables corrosion to progress

Salt ingress Marine salt introduces depassivating chloride ions into the surface of the concrete

Chemical attack Chemical attack by chlorides can cause corrosion of steel or sulphates that can cause degradation of the cementitious matrix

Vibration Causes cracking, spalling, and delamination

Impact damage Causes physical weakening of structural components, exposure of steel reinforcement, cracking etc

Physical dama ge Concrete structures can also be affected by physical damage, such as: • excessive loads • impact damage • abrasion • fire damage.

Lack of maintenance and poor repair When concrete fi rst began to be used as a major structural building material, it was promoted as a material that needed little, or no mainten ance. However lack of maintenance is a major contributor to re inforced-concrete deterioration.

Sources of Design and Materials Environmental Physical damage deterioration workmanship

Potential Cement type/quality Poor detailing Presence of CO, Inadequate causes Mix design and acid gases design of 1 Insufficient cover to deterioration Poor aggregate reinforcement Freeze thaw Impact selection / reactivity I Salt and Vibration Incorrect chemical attack water:cement (de-icing salts, Settlement ratio ground water) Seismic Mechanical strength Biological growth of aggregate Change of use Poor vibration Weathering increased floor Additives or and compaction loadings contaminants Thermal movement Wind Fire damage

Inadequate maintenance

Poor repairs

Inadequate maintenance

The principal sources and causes of concrete deterioration , Table by Susan Maedo nald 0 NSW Heritage Office

Corrosion Corrosion of the steel reinforcement The major cause of deterioration in reinforced concrete Most reinforced concrete deterioration is related structures is corrosion of the reinforcing steel. Corrosion to reinforcement corrosion either directly or indirectly requires air and water to take place. The key to as the result of other degradation. Reinforcement durability in concrete design is to have a mix and cover corrosion will usually result in the development of which ensures that the reinforcement is always well expansive rust products (hydrated iron oxides) that put protected from these elements. the concrete into tension around the steel. This leads to cracking, and eventually spalling of the concrete. However, when corrosion occurs the same steel that Degradation of the concrete matrix can result in loss of gives reinforced concrete its tensile strength, is capable protection to the reinforcement steel and corrosion. of rupturing the concrete. This is the result of the The two mechanisms - chemical degradation of the expansion in volume which takes place when steel cementitious matrix and corrosion of the steel corrodes. reinforcement - may be initiated independently but There are two major causes of steel corrosion that can become inter-related. occur even without other degradation to allow water Cement paste is usually highly alkaline, containing and to track to the steel: mobile hydroxyl ions within the pore water that is always and chloride attack. present. These hydroxyl ions (OH)" in the pore water coat the surface of the embedded steel reinforcement The corrosion process producing a strong passive-oxide layer. It is when this oxide layer is broken down, by the ingress of aggressive Corrosion of steel in concrete is an electrochemical ions such as chlorides (Clr, or not maintained by the reaction in which the major constituent of steel (iron) high (due to atmospheric carbon dioxide or goes into solution as iron ions with a flow of electrons attack by acidic gases), that corrosion can occur. (electrical flow). This site is called the anode. Electrons The effectiveness of the passive layer in providing are produced in this self-sustaining process and flow long-term corrosion protection to the steel is through the reinforcement towards cathodic sites - dependent on: where they react with oxygen and water from outside to • the constituents of the concrete; produce additional hydroxyl ions. This is known as the • the thickness and quality of the concrete forming the cathodic reaction. There is no external source of cover; electrons: the anode produces electrons at the same • the durability and permeability of the concrete forming rate the cathode consumes them. This process is the cover; shown in the figure below. • the homogeneity of the concrete forming the cover; The most common causes of steel corrosion are the • environmental conditions. loss of alkalinity around the steel as a result of There are many good texts on corrosion of carbonation and the presence of chlorides at the steel reinforcement, some of which are included in Further surface. Although both processes produce different Information. forms of corrosion, carbonation and the presence of chlorides result in the loss of the protective passive layer around the steel and the commencement of the electrochemical reaction explained above. Both are also directly related to the durability of the concrete and its ability to prevent acidic gases destroying the alkalinity of the concrete, or to prevent chloride ions from migrating to the steel. Harmful corrosion can occur in the presence of dissimilar metals that are linked by an electrolyte in the concrete; for example when zinc-coated steel is linked to uncoated steel or where there are copper pipes or I I aluminium fittings in the concrete. -

Carbonation occurs when atmospheric carbon dioxide Anode Reactions Cathode Reaction reacts with the pore water in the concrete creating Fe _ FeZ' .. 28" '/202 ... H:P ... 2e~ -+ 20H- , which reacts with the lime in solution Fe2' ... 20H- -+ Fe(OHh

(Ca(OH), reducing the alkalinity of the concrete from 4Fe(OH)z ... O2 + 2~ - 4Fe(OHb 12.5 to less than 9.0. Carbonation is not harmful to - 2F~0a .~ ... 4H~ (RUSn concrete but when the carbonation front reaches the level of the steel reinforcement, corrosion will The corrosion of concrete-encased steel is an electrochemical commence. reaction. Illustration courtesy of John B. Broomfield Chlorides may have been added to the concrete at the time of construction as a calcium chloride accelerator (used in cold weather, or to speed up turnaround of moulds and formwork). or cast in from poorly-washed marine aggregates, or may have ingressed from sea water, wind-blown marine spray or saline groundwater. Chlorides attack the passive oxide layer of the steel but do not affect the alkalinity of the concrete. The Investigation and Repair of Historic Concrete An approach to repair

Concrete repair is today a major industry. It is also a specialist activity. However, the issues surrounding the conservation of our historic concrete structures are only just beginning to be addressed. Unlike some traditional materials, concrete and other modern materials do not yet have well developed, universally implemented conservation methodologies for their investigation, repair or long-term maintenance. In addition, when new repair methods are being developed, conservation needs are rarely considered.

Understanding significance Similarly, many architects unversed in concrete deterioration go directly to the manufacturers of Understanding the significance of a building is concrete repair products for advice, rather than to an fundamental to the development of an appropriate independent engineer. Repairs may be based on a conservation policy. This will provide vital information cursory investigation - often a free quotation - and with upon which decisions can be made that affect the little information supplied as to the nature or even the building's performance or appearance. For instance, extent of the repair work proposed. if the surface finish is an important architectural characteristic of the building and contributes directly to Unfortunately, this often results in an inadequate its aesthetic significance, then a repair method should analysis of the problem that does not identify the cause be selected that preserves the surface finish. On the or the extent of the deterioration and may not have other hand, if the aesthetic value is in the form, and the considered the overall structural stability or strength of surface has been considered as a sacrificial material, the building. Apart from the dangers of not considering then the overall architectural composition may override the structural adequacy of the building, such proposals the importance of retaining the original surface material. tend to address the symptom rather than the cause, and there is often little modification to the proprietary Of course, efforts should be made to prevent the system to cope with the specific conditions. removal or damage of significant fabric. However, any decision to alter the fabric will need to achieve a In addition. the qualities or character of the building in balance between the preservation of the essential or terms of its design or heritage significance may not significant character of a building as a whole, and the have been fully understood. The repair option is unlikely conservation of individual components or materials. to have been selected with this in mind. This can often lead to inadequate repairs that may be detrimental to the architectural or historic significance of the building. Understanding the problems For any building, adequate physical investigation will Concrete buildings (like any other building type) require always include visual and non-destructive inspection appropriate assessment, investigation, testing and and in some cases destructive investigations may be repair pOlicies. Concrete is a structural material and required. An assessment of damage against repair therefore structural appraisal will be required where options, available resources and life-span requirements signs of deterioration are evident or where a change of will influence the repair programme. building use is proposed, particularly where this involves increase or redistribution of loading. Life cycle It is generally acknowledged that the architect is nowadays a co-ordinator of specialists, and it is Best conservation practice aims to achieve the frequently necessary to seek specialist advice for certain longest life expectancy that is practical and economic. aspects of a project, be it new build or conservation. As with other more traditional construction forms, this Reinforced concrete has been the realm of specialist does not necessarily mean that a one-off repair is contractors ever since it began to be used regularly at essential or the best approach. A longer-term managed the end of the nineteenth century. programme of repairs and maintenance may be better suited to the problems at hand and the finances of In the same way that the first specialist contractors of the building owner. reinforced concrete were licensed to use the various patented systems, specialist contractors today hold licenses for the various methods of concrete repair, Other issues such as desalination, realkalisation or cathodic Besides the technical, commerCial, and conservation protection. Even traditional patch repair methods aspects already discussed, the social or occupational virtually always use proprietary products, bought off the conditions should be considered. Buildings may be in shelf with various standard-use requirements forming occupation before, during and after repairs. part of a warrantied system. This is a result of the This introduces other potential problems: disruption to licensing of alternative repair methods, and the the tenants, noise and dust pollution. These must be introduction of proprietary repair mixes in the addressed when the repair and maintenance strategy is early 1980s. being formulated. Relocating or temporarily re-housing EmphasiS on the contractor/specialist has resulted in occupants is not always practical. Suffice to say that a tendency for concrete repair to be approached via the particular social situation may influence the selection the contractor, who is likely to recommend a repair of the most appropriate repair technique. based on the company's own licensed system.

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Assessment and investigation

Signs of deterioration indicate the reinforced concrete structure may be at risk, and a structural assessment will be required to determine its safety, stability and strength. A suitably qualified professional engineer should carry out structural appraisal. The structural assessment will highlight any problems and requirements for structural works that will need to be incorporated in the overall repair programme. To determine the most appropriate repair treatment, the causes of the problems must be identified. The methodology is the same as that for the investigation of any other heritage building and includes the usual stages described below.

Selection of the appropriate specialist consultant The services of an experienced structural engineer will be required, and in some cases a concrete or corrosion specialist may also be necessary. 80th should be independent so that there is no conflict of interest between the aims of conservation and the choice of the best repair method for the project.

Gathering historical information It is important to determine changes to the concrete, its environment, and its function through its history and also to establish as far as possible the future function, known environmental conditions, and the available Australia Square was designed by Harry Seidler, and its 50-storey tower was completed in 1967. It was the first fully structural maintenance resources. Most importantly, changes in lightweight concrete building in Australia and at the time of chloride levels, levels of carbonation, and chemical and construction was the tallest lightweight concrete structure in the physical characteristics are useful in understanding world. Seidler conferred with famous Italian structural engineer, patterns of deterioration. General historical information Pier Luigi Nervi, on the concrete design of the circular tower. about the building is always essential for understanding Photograph by NSW Heritage Office how and why a material has been used, and also for assessing the significance of the building. Information gleaned from structures of similar age and condition and in comparable environments can be extremely Diagnostic investigation useful. Contemporary publications can also increase In order to determine the cause of the problem and the understanding about design and construction nature of the concrete's resistance to deterioration, it practices generally. will be necessary to carry out non-destructive, and sometimes destructive, testing in conjunction with the Physical inspection and examination phYSical examination. It is also important to identify through investigation areas of latent damage, which can A thorough physical investigation will be required to be far more extensive than the visible damage. The identify and diagnose problems and to quantify and types of tests that will probably be required are specify the repair works. Mapping defects such as described below. AS1 012-92 Methods of Testing cover concrete, spalling, delamination, cracking, depth Concrete is the appropriate Australian Standard for of friable surface layers, and other defects will offer such tests. information about the nature and extent of the problem. Chloride-ion concentration levels and profiling is An experienced consultant measured by dissolving dust samples or crushed core can determine much from samples in acid and then determining the concentration the patterns of patent using standard wet chemical methods. Profiling is damage, such as achieved by taking samples at different depths (usually reinforcement corrosion, every 25mm), and this data is related to the location of alkali-aggregate reaction the reinforcement. This provides information on the and structural damage. extent and risk of corrosion due to the presence of Such a survey will also chlorides. assist in estimating the costs of rehabilitation. Depth of cover to steel reinforcement and location of reinforcement is carried out using calibrated cover meters or radar. These non-destructive methods provide an indication of the depth of cover, and its location, which can be compared against the depth of carbonation and used to help estimate the potential Examination of edge from the wall cavity using latent damage. Cover meters are difficult to use and a fibreoptic borescope. require an experienced operator and interpreter. Photograph by Kevin Hunt The Investigation and Repair of Historic Concrete

Depth of carbonation - this is a simple on-site chemical test, where the concrete is broken out in representative areas and sprayed with a solution of in alcohol and water. It can also be carried out in the laboratory using split core samples. Areas of concrete that have not carbonated turn the solution pink, whilst carbonated areas do not change the solution. The average depth of the cover to reinforcement is then compared to the depth of the carbonation. However, this important test requires the skills of an experienced consultant to carry out and interpret the results. Petrographic, chemical and X-ray diffraction examination of core or spalled samples for visual The concrete to the main arches of the Johnston's Creek inspection and determination of mix design (types of aqueduct was repaired USing a dry shotcrete process , which is a aggregates, cement content), freeze/thaw, alkali-silica spray applied method. reaction, sulphate attack, and other damage. Photograph by Lianne Hall The number and location of core samples required is structure specific. Tests and sampling should be Current repair methods and implemented in a manner that causes least disruption to preventative treatments the fabric. Taking destructive samples should be agreed with the architect and may need statutory approval. Patch repair There are a number of other useful diagnostics that may Patch repair is the earliest form of concrete repair and, form part of an investigative package including apart from the introduction of technically advanced corrosion risk surveys (using half cell, linear polarisation materials and improved application methods, is still the and resistivity testing), permeability (measuring diffusion same process. Patch repair is the most common rate) impact, ultrasonic, radar and radiography (to method and is required wherever there is patent detect laminations and other defects, steel condition damage and thus is likely to be required as part of and location). most repair programmes. Before patching, the building must be adequately Interpretation of results supported as the structural capacity may be temporarily The skill and knowledge of an experienced specialist in reduced during repair. Patch repair is carried out by specifying, supervising and interpreting these results is mechanically breaking out the damaged, cracked, paramount. spalled, carbonated or chloride-contaminated concrete to fully expose the steel reinforcement. The steel must be thoroughly cleaned to the requisite Australian Repair and rehabilitation The repair techniques currently available for concrete range from 'do nothing' through to 'rebuild', with a number of measures in between. It is unlikely that either of the above extremes is ever appropriate for a heritage building unless the damage is either very minor, or severe enough to jeopardise public safety. In most cases a repair method, or combination oj repair and rehabilitation methods somewhere in between, will be the most appropriate.

Repair

As with any conservation project, there will be a number oj different repair options available, each with advantages and disadvantages. The choice will be determined by: • the results oj the investigation works • the severity oj the deterioration • the risk of future deterioration • required service life • the practicality of applying the repair option • aesthetics • suitability for a heritage building • cost. Most repair proposals for reinforced concrete structures include treatments that arrest or reduce future deterioration, and many products and systems are marketed as a package. When selecting the repair technique and any preventative treatments the following In this repair the patch repairs were carefully matched by sight should all be considered: significance of the building, to the existing concrete using a latex mould to replicate the technical defects, required performance and any special board-mar1

Standard and is then usually coated with a proprietary subsequent cracking). Patches which appear a close bar primer to protect the reinforcement steel. Severely match when dry, can appear as blemishes on the corroded reinforcement (where there is a loss of 10% or building when wet from rain. In time, the new concrete more of the section) may require replacement by will weather and blend in more closely with the existing splicing in new sections if the loss of section is concrete. Regard for cutting out to obvious joint lines excessive. (such as those provided by board-marked concrete) and mimicking the existing surface texture will assist in Replacement of reinforcement should be done with the replicating the original finish. Traditional stone-repair same material to avoid bimetallic corrosion. Therefore methods can provide a precedent for sensitive the replacement for plain steel with galvanised steel concrete repair. should be avoided. Trial patches are essential when attempting to replicate The broken-out concrete is reinstated using either: an existing surface finish. Sample patches should be • a site-batched cementitious repair mortar, usually viewed wet and dry and should represent the different polymer modified to help reduce permeability, textures and colour ranges found around the structure. shrinkage and adhesion; • a pre-bagged cementitious repair mix. Coatings As the general principle in the repair of heritage Protective coatings for concrete have been in use in buildings is to replace like materials with like materials, one form or another virtually since concrete first came cementitious-based mixes should be used in preference into use. However, it is only in the last 15 years that to epoxy-based repair materials. coatings have become an important part of the repair Large surface area repair work may utilise spray-applied and maintenance industry. For exposed concrete the concrete. In some cases, the affected component may proposal to coat a building has obvious implications be shuttered and the low shrinkage repair concrete can and constitutes a major change in appearance. be poured, pumped, or grouted in. In addition, the application of a coating on any building introduces a maintenance commitment and requires Patch repairs should be cut out and reinstated as full access to the building each time it is renewed. discreetly as possible, taking care to observe joints, A repair programme that relies on the coating for its board marks or other distinguishing design markings. performance will be reliant on the effective life of the The concrete is cut out to form right-angled or slightly coating: adequate provision for future maintenance and re-entrant edges, avoiding feather edges. For deeper repairs must be considered. patches stainless steel pins should be considered to assist in securing the new patch repair.

Repair mixes In the majority of cases today, patch repairs are carried out with pre-bagged repair mortars. Whilst these products have the advantages of providing a quality controlled, ready-made product, they will be unlikely to match the existing concrete of exposed concrete buildings and will result in unsightly patches that will inevitably reinforce the argument lor an opaque coating. It is possible for manufacturers to produce pre-bagged, colour-matched mortars for a particular site. It is usually the case that more than one colour will be required as the original mix may have varied around the building. However, it is possible that the new colours will weather over time differently to the parent concrete, particularly if different aggregates and cements are used. Site-batched mortars and were the norm until the 1980s. They are cheaper than pre-bagged mixes and provide the opportunity to match the existing concrete colour. However site-mixed mortars necessitate careful site supervision of the mixing process to ensure consistency and quality control. Information about the original aggregates and cement type may be available. It is often possible to consult the original documentation of old buildings or, for buildings of more recent origin, the architect themselves. Although there is sometimes a difference between the original specification and what actually happened on site, original documents are invaluable for providing a guide to the materials, and thus can help in developing an appropriate repair mortar. They provide insight to the architect's intentions regarding the concrete, the methods used on site, and the reasons for the selection The Grace Building in York Street is Sydney's finest example of of the building material. the skyscraper gothiC style. It was constructed between 1928-30 It can be difficult to achieve a perfect match between by Morrow and Gordon and is considered technically significant because of the unusual reinforced concrete slab and beam the original concrete and the repair mortars, which tend construction and the glazed terracotta cladding. It is listed on to be modified with additives to improve performance the State Heritage Register. (enhancing durability and reducing shrinkage and Photography by NSW Heritage Office r------~------~ ------

The Investigation and Repair of Historic Concrete

There are a number of different types of coatings and Expert advice is required to design, install and monitor a impregnations for concrete, including water repellents, cathodic-protection system. Because of the potential paints and surface sealers, that are adequately different levels of corrosion activity in different parts of a described elsewhere (see Further Information). building, considerable skill is required to design a The decision to apply a coating needs to be carefully system that ensures comprehensive protection is balanced against cost, long-term performance, achieved where required. This means regular monitoring future funds for maintenance, aesthetics, technical and adjustment of the system may be necessary. performance and environmental conditions. There are A system can be designed to make it possible to instances where coating a building may be inappropriate introduce ICCP with minimal intervention to the fabric, and ineffective against the specific deterioration although areas of visible damage will require patch phenomena, and there may be other methods of repair repairs. However, there is limited experience with this and preventative treatment, such as cathodic method in Australia. protection, that are more suitable.

There are still some uncertainties about the Realkalisatlon effectiveness of coatings in providing long-term Realkalisation is a comparatively recent technique and protection to chloride-contaminated concrete that one that has not been widely used in Australia. There is cannot be properly treated. Despite the appfication of as yet insufficient data to prove its performance in the the coating, there may be enough residual moisture and long term. It is an electrochemical process that uses a chlorides in the concrete to maintain corrosion activity, DC current passing between the reinforcement steel and or new activity may be initiated due to changes in a temporary anode (usually placed on the surface of the oxygen and moisture levels. New corrosion can also be component in question). The DC current produces set up adjacent to patched concrete where there is hydroxyl ions at the steel reinforcement that replaces chloride contamination. the alkalinity of the concrete formerly lost through carbonation. The reintroduction of the hydroxyls around Cathodic protection the steel enables the re-formation of the protective, Impressed current cathodic protection (ICCP) is an passive layer around the steel. It is appropriate for electrochemical-repair and prevention technique and the concrete where carbonation is Ihe cause of the only method that halts corrosion of the reinforcement deterioration. steel. It is the subject of numerous texts (see Further Information) and is now a well-understood and well­ documented repair technique for carbonated and, more commonly, for chloride-contaminated concrete. Similar in approach to realkalisation and desalination (described below), it passes sufficient, continuous current between the steel reinforcement and a At the time of its opening in 1964, the Gladesville Bridge was the permanent anode (fixed onto the surface in the form of longest single-span concrete arch bridge in the world. The design a coating or mesh and gunnite application, or into the and construction of the bridge - pictured here at the opening - were of great international interest. World renown engineer, concrete) of sufficient strength to stop the anodic M.E.L Freyssinet was asSOCiated both in design and on-site reaction. As corrosion cannot occur unless the anodic supervision. and cathodic reaction both operate, corrosion is halted. Photograph by Paul Percival, courtesy of the Australian Photographic Agency, State Library of NSW NSW Heritage Office

The radical Williamson House in Mosman (nicknamed the "Igloo House") was designed in the t 950s by Harry Seidler. It had a light concrete body set on circular concrete columns on a waterfront cliff and unusual twin-arched garages with thin-shelled concrete construction. It was the first Australian house to use flat-slab construction, with special reinforced concrete developed by Sydney engineer Peter Miller that did away with extensive masonry foundations. The house is listed on the State Heritage Register. Photograph by Max Dupain

This process is patented and thus in the hands of Corrosion Inhibitors specially licensed contractors. Realkalisation typically takes about one week to perform. It is theoretically non­ Although known for many years, corrosion inhibitors destructive and reduces patch repair to those areas of are only now beginning to be actively marketed as a visible damage. preventative treatment in a concrete repair program. They delay or reduce the rate of corrosion by forming a passive film on the steel. They can be either applied to Electrochemical chloride removal or extraction the surface of the concrete (migratory inhibitors), Electrochemical chloride removal or extraction, also incorporated in a repair mix, applied to the steel during known as desalination is another electrochemical repair or introduced in pellet form in holes in the technique that has only recently been introduced into concrete (vapour phase inhibitors). It is claimed that Australia. As with realkalisation, a DC current is passed they are able to function in heavily chloride­ between the steel reinforcement and a temporary contaminated concretes (up to 2% chloride by mass surface anode, and is used to remove harmful salts of cement). such as chlorides from the concrete. Negatively charged Corrosion inhibitors are attractive from a conservation ions are repelled from the negatively charged steel. point of view as they are almost invisible on application, With sufficient pore water always present in the although their long-term visual effect is unknown. concrete, the chloride ions are forced from the steel Their effectiveness in preventing corrosion of steel reinforcement outwards towards the temporary anode re inforcement depends on correct diagnosis of the where, over some five to six weeks, they are collected electrochemical corrosion problem. However, the in the electrolyte and removed along with the migratory ability of some application systems is not well temporary anode. proven in the long term, with little data and no case The advantage of this method in conservation terms is histories to indicate their long-term effectiveness and the minimal disruption to the surface of the concrete. effects on the concrete. However, like rea lkalisation, desalination is licensed to specialist contractors and there is little long-term data Conclusion available to determine the life cycle of the treatment or any potential side effects. The above list of techniques is not exhaustive and it is more than likely that a repair strategy will involve a There is some concern about the long-term effects of combination of methods. However, when selecting these electrochemical techniques, given the lack of data repair techniques, aspects must be considered other about their long-term performance and potential side than just the technical. The more recent methods do not effects. Their use should therefore be approached with have long-term data to prove their ongoing performance caution for heritage buildings. or long-term effects and should be carefully considered before being used on heritage buildings. The Investigation and Repair of Historic Concrete

Maintenance Balancing technical performance, life-cycle costs Until the 1970s when the relevant concrete codes of and conservation practice began to acknowledge issues of durability and long-term performance, reinforced concrete was There will inevitably be some 'trade-off' between the believed to be a maintenance-free building material. It is now broadly acknowledged that this is not the case. various technically correct repairs and conservation which seeks to minimise the impact of the repair on the Apart from all the usual maintenance issues such as building. Each particular case must be individually drainage and roofing, reinforced concrete requires long­ considered. term care to counter potentially harmful environmental conditions and to cope with any material problems. To assess the financial implications of the different repair options, it is important to assess the life-cycle As the rate of deterioration is time dependent it may cost. The capital cost of a repair plan may represent differ across the structure. At the time of inspection not anything from less than 10% to 100% of the actual all the defects may be evident. Ongoing maintenance repair. It may be better to introduce a significant is therefore needed for periodic examination and testing maintenance strategy to upgrade the building regularly. to determine whether further repairs are necessary. Or it may be better to spend more on the capital cost of Regular condition surveys of reinforced concrete the initial repair and reduce the expense of ongoing buildings should be carried out, including cover, maintenance, for instance, when scaffold or access to carbonation and chloride content measurements (if the high rise buildings is involved. building is likely to be at risk from chlorides). Inspections should be made in order to identify any It is not possible for this Information Sheet to identify latent damage or druminess. Maintenance is an the most appropriate approach for repairing every important part of the repair strategy and necessitates building, but it is important to highlight the necessity of a continued and long-term commitment to care for evaluating life-cycle costs of the repair instead of simply the building. calculating the capital cost, which is unfortunately the usual practice. It is important when selecting suitable Preventative maintenance measures may involve repair and maintenance options to examine the various changes to the building's appearance and may require repair options against their expected service life, the statutory approval. The decision to introduce impact on the building, life cycle costs and future preventative measures, such as coatings, will also resources. introduce new materials that will require maintenance themselves. Any decision should be carefully assessed The aim of the repair should be to retain or reveal the in the manner discussed for repair. significance of the building. Decisions should be based upon a thorough understanding of the building, its Regular maintenance checks of concrete should include problems and the level of resources available to inspection of the repairs to ensure that they are fulfilling maintain the selected repair method in the future. their function. Repair materials have their own maintenance cycles that need to be respected. A maintenance program should include upkeep of repairs to ensure their continued effectiveness, as well as that of the original fabric. As with any building type, anticipatory maintenance inspections will prevent problems developing and enable regular upkeep works to be planned. Reacting to problems after they have occurred results in a larger repair programme and unexpected costs.

The tower to produce ice for the concrete mixers on the construction Site of Warragamba Dam was one of the first structures to use prestressed concrete. which was used as a replacement for structural steel when it became scarce after WWII. This photograph ot the central mix plant and the ice plant was taken In 1955. The plant has now been demolished. Image W550628-8, RSP3, courtesy of Sydney Water Corporation and Sydney Catchment Authority Historical Research and Archives Facility NSW Heritage Office Further information

Broomfield, J.P. 1997, Corrosion of Steel in Concrete: Understanding, Investigation and Repair, E & FN Spon , London. Bronson, S., and Jester, T. (eds.) 1997, Special Issue: Mending the Modern, APT Bulletin 28:4, Association for Preservation Technology, Washington, D.C. Cowan, H. 1998, From Wattle and Daub to Concrete and Steel: the Engineering Heritage of Australia's Buildings, Melbourne University Press, Melbourne. De Jonge, W., Doolaar, A. 1997, The Fair Face of Concrete: Conservation and Repair of Exposed Concrete, Preservation Dossier No. 2, DOCOMOMO International Eindhoven, The Netherlands. Heiman, J. L. 1985, The Durability of Cast-in-situ Reinforced Concrete, National Building Technology Centre, Department of Housing and Construction, Chatswood, NSW. Lewis, M. 1988, 200 Years of Concrete in Australia, Concrete Institute of Australia, Sydney. Macdonald, S. 2003, Building Pathology: Concrete, Blackwell SCience, Oxford. Slaton, D. and Foulks, G. (eds.) 2000, Preserving the Recent Past 2, Historic Preservation Education Foundation, Washington D.C. Slaton, D. and Shiffer, RA (eds.) 1995, Preserving the Recent Past, Historic Preservation Education Foundation, Washington D.C. Standards Australia 1994, Concrete Structures­ Commentary AS 3600 Supp 1 - 1994. Standards Australia 1996, Concrete Structures - Commentary AS 3600 Supp 1 - 19941 AMDT 1 - 1996, Standards Australia, Sydney. Standards Australia 1996, Guide to Concrete Repair and Protection HB 84: 1996, Standards Australia, Sydney. Standards Australia 2001, Concrete Structures AS 3600 - 2001 , Standards Australia International, Sydney. Standards Australia 2002, Concrete Structures AS 3600 - 20011Amdt 1 - 2002, Standards Australia International, Sydney. Standards Australia 1992-2000, Methods of Testing Concrete AS 1012, Standards Australia International, Sydney. Standards Australia, Standards of New Zealand, ACRA & CSIRO 1996, Guide to Concrete Repair and Protection, Sydney. Warren Centre for Advanced Engineering Studies 1989, Economic Recycling and Conservation of Structures, UniverSity of Sydney. The Investigation and Repair of Historic Concrete Glossary

Alkali aggregate reaction (abbreviated as MR) Creep The long-term shortening or deflection of the Reaction between the aggregates and the alkaline cement concrete as the strain increases under sustained stress, pasle, leading to Ihe development of expansive crystalline which usually has to be allowed for in the structural gel which is sufficiently strong to cause cracking of the design of the reinforced concrete. aggregate and of the concrete matrix. Sometimes also Delamination Separation of layers of concrete from the called alkali silica reaction, (ASR). main body of the material. Anode The positive pole of an electric circuit. In a Filler The aggregates which mixed with the binder (q. v.) cathodic protection system, a sacrificial material and water result in concrete. Typically categorized as introduced to act as the site of corrosion to inhibit coarse aggregate (crushed stone, gravel, etc.) and fine corrosion of the structure itself. aggregate (commonly sand). Binder The materials that comprise the cementing agents Galvanic action Occurs when two dissimilar metals are in concrete, mortars and renders. Cement is mixed with placed together in solution. The most active metal will water and added to aggregates (the filler - q.v.) to make become an anode and corrode as a current passes concrete. Historically, 'natural' cements such as the between them. This action is used to stop corrosion by volcanic ash (known as ), or lime or gypsum galvanising (coating steel with zinc) and in galvanic products, were used as binders. For concrete made cathodic protection. during the last century, Portland cement (q. v.) has been almost universally the binder, although other artificial Impressed current cathodic protection A method of cements are increasingly being used that employ recycled cathodic protection that uses a power supply and an inert industrial waste material such as ground granulated blast (or controlled consumption) anode to protect a metallic furnace slag. object or element by making it the cathode. Carbonation Loss of alkalinity in the concrete as a result In-situ concrete Concrete cast in its intended location, of depletion (brought about by the cf. precast concrete (q. v.). presence of atmospheric carbon dioxide, which with moisture forms carbonic acid). The reaction of calcium Latent damage Non-visible damage that is impairing, or hydroxide with the acid results in the formation of calcium will impair, the functionality of the structure and will , neutralising the in the pores and eventually require some form of remedial action. resulting in the loss of Ihe passivating oxide layer around Mass concrete A term generally synonymous with the embedded steel in the carbonated zone. The unreinforced concrete (q. v.) but also applied to massive carbonation front is the interface between the concrete elements and structures such as gravity dams, uncarbonated (virgin) concrete and the carbonated which may well have some reinforcement. concrete, as carbonation progresses inwards towards the steel. Carbonation is progressive but occurs at a reducing Oxidation The process of removing electrons from an rate with time. It progresses faster in zones of local atom or ion. The process: defects, such as cracking and poor compaction, than in Fe -> Fe" + 2e' the general body of (competent) concrete. Fe 2 • ---+ Fe3 • + e- Cathode The negative pole of an electric circuit. In a is the oxidation of iron to its ferrous (Fe') and ferric (Fe" ) cathodic protection system, the metal protected against oxidation state. Oxidation is done by an oxidizing agent, corrosion due to the presence of a sacrificial anode. of which oxygen is only one of many. Cement The binding material that is one the components Passivation The process by which steel in concrete is of concrete. Most commonly it is Portland cement (q. v.). protected from corrosion by the formation of a passive layer due to the highly alkaline environment created by the Chlorides As these occur in calcium chloride (used as a pore water. The passive layer is a thin, dense layer of iron cement-setting accelerator in the past) and sodium oxides and hydroxides with some content, that is chloride (in sea-water, wind-blown sea spray, and road initially formed as bare steel is exposed to oxygen and de-icing salt), they combine with water to form an water, but then protects the steel from further corrosion as aggressive agent leading to accelerated corrosion of it is too dense to allow the water and oxygen to reach the reinforcement. steel and continue the oxidation process. Corrosion 'Rusting' or formation of iron oxides and other Patent damage Visible damage in reinforced concrete compounds by electrolytic action when steel is exposed to decay. Damage can include cracking, spalling etc. water and oxygen. Aggravated by other aggressive agents such as acids or chlorides (q. v.). Rust occupies a larger pH Logarithmic scale for expressing the acidity or volume than the original iron, and consequently can cause alkalinity of a solution based on the concentration of cracking and spalling (q. v.) in the surrounding concrete. hydrogen ions; a neutral solution has a pH of 7, whilst a pH below 7 indicates an acid solution and a pH above 7 Cover The concrete between the reinforcement and the indicates an alkaline solution. Concrete has a pH of 12 to adjacent face of the element. It provides protection of the 13. Steel corrodes at pH 10 to 11. steel from corrosion (q. v.). The required thickness of cover and the quality of concrete mix used are influenced by the severity of exposure, and must be correctly chosen to ensure durability (q.v.). NSW Heritage Office

Pore (water) Concrete contains microscopic pores. Shrinkage Contraction of the cement paste as it These contain alkaline oxides and hydroxides of sodium, hardens, due to loss of moisture and changes to the potassium and calcium. Water will move in and out of the paste's internal structure. Some shrinkage is non­ concrete saturating, part filling and drying out the pores reversible due to these changes, while reversible according to the external environments. The alkaline pore shrinkage occurs as the concrete becomes wet in service water sustains the passive layer if not attacked by carbo­ and then dries again. Some materials that might otherwise nation or chlorides. find use as aggregates should be avoided if found to be 'shrinkable', as this property may damage the concrete in Portland cement Patented by Aspdin in 1824 and service. named after its resemblance to Portland stone. It is an artificial or manufactured material, although made from Spalling Detachment of lens-like pieces of surface limestone or chalk, together with clay or shale. These concrete, usually due to reinforcement corrosion and the contain alumina, silica, lime, iron oxide and magnesia, and production of expansive rust products that put the are ground to a fine powder, burnt in a kiln and then re­ concrete locally into tension, resulting in cracking and ground to a very fine powder which sets hard when mixed then spalling. with water. Unrein forced concrete Concrete that does not contain Post-tensioned concrete Prestressed concrete made by reinforcement. casting-in conduits or sheaths for prestressing steel that is tensioned and secured by anchorages once the concrete has cured. Profiling the conduits or sheaths produces a more efficient section, as noted below for pretensioned concrete. The conduits or sheaths are usually then grouted up to provide bond between steel and concrete, and to increase durability. Precast concrete Reinforced concrete cast in moulds as units or elements elsewhere than their final intended location, before being placed into position. Prestressed concrete Concrete that has had compressive stress applied to it by tensioned steel before it is put into service to carry loads. The prestressing steel may take the form of rods, wires, cables, or bars. Prestressing increases the strength of the element and can eliminate cracking in service. Pretensioned concrete Prestressed concrete made by tensioning the prestressing steel before the concrete is poured. This typically requires temporary anchorages to hold the ends of the steel, and stout moulds to resist the resulting compression forces exerted on them. Once the concrete has set, the anchorages are freed and the prestressing force is transferred as compression in the concrete. Pretensioning generally employs straight runs of steel, although sometimes it is profiled, following the pattern of the bending moment to give a more efficient use of the material. Reinforced concrete Concrete reinforced with metal rods, straps, wires or mesh that provides a composite material strong in tension and compression. The reinforcement is today most commonly mild or high-tensile steel but iron, annealed wire, and galvanised and stainless steel have all been used in various ways as reinforcement. In the future non-metallic, high-strength composites used as reinforcement may reduce or eliminate concerns over corrosion and durability. Repair action Taken to reinstate to an acceptable level the current functionality of a structure or its components that are either defective, degraded or damaged in some way.