Challenges in Dealing with Corrosion in

Heritage Buildings

Peter Johnsson Hyder Consulting

David West International Conservation Services

Abstract Corrosion of hidden metal components in the facades of heritage buildings is a major cause of damage to

these buildings. The spalling of various facade cladding materials caused by the corrosion products

frequently presents a major risk to public safety. The need to undertake repairs to address this risk is

complicated in heritage buildings by the requirement to minimise the works on the historic fabric in order to

lessen the impact on the heritage significance of the building. A common outcome can be works to repair

visible deterioration rather than a more systematic approach to resolve the problem. As a result, underlying

corrosion that may not yet have caused spalling or other visible manifestations is left in place without

treatment.

This paper uses recent façade repair works carried out on several heritage facades to illustrate both the

challenges, and the opportunities for creative approaches to investigation of the problems and

implementation of repairs. New strategies for investigation and managing of deterioration and repair into the

future are proposed.

Introduction The investigation and repair of heritage buildings requires a different mindset from ‘everyday’ engineering

inspections. Traditionally, diagnostic engineers seek to thoroughly investigate a building or structure to

determine the mechanisms responsible for any observed deterioration and the full extent of these problems.

Following diagnosis, a prediction of the extent of affected would be made and a thorough programme of

repair would be undertaken, addressing all the causes and potential future issues across a structure.

Whilst a thorough approach was also recommended for heritage buildings, the effectiveness of techniques

used to diagnose and quantify hidden problems requires an understanding of corrosion mechanisms and a

significant amount of experience.

On the other hand, thorough investigations are not always possible, as owners are reluctant to spend their

budgets on ‘preventative investigations’. The result is that investigations are usually undertaken in the short

period following a failure and on a limited budget. Therefore, the opportunity to develop new techniques and

refine existing techniques to create a definitive specialist service that provides as many answers as possible

about corroding steel elements in particular would prove attractive.

What is Heritage? Heritage items may be described broadly as things that “give us a sense of living history and provide a

physical link to the work and way of life of earlier generations” (NSW Heritage Office). In Australia, there are

differing approaches to the conservation of heritage items, depending on whether they are natural, cultural or

built heritage. This paper is concerned with items of built heritage, which may comprise buildings,

engineering structures, or ruins.

The Federal and State Governments have all legislated for the protection of built heritage, and have

established a range of government bodies and departments to implement this protection. The provisions of

these Acts vary, so that there is no one single procedure applicable across the country. Indeed, depending

on the ownership and location of a built heritage item, differing requirements may apply for two buildings

located next to each other.

However, the key elements of each piece of built heritage protection legislation are generally similar, and

typically include the following components:

• Establishment of a mechanism for recognising significant items of built heritage (a register or list)

• A process for evaluating the significance of built items, usually based around the provisions of the

Australia ICOMOS “Burra Charter”, as well as additional guidelines

• Setting of requirements for protection, conservation and maintenance of significant items of built

heritage

• Identification of procedures for provision of management and advisory services on the part of the

Government enacting the legislation

Essentially, each Government agency establishes and maintains a register of built heritage items, and

implements policies to facilitate the care of these assets on behalf of the community.

In addition, in most States, Local Government is empowered by Planning legislation to establish registers of

built heritage items considered to be of local significance, and to manage the conservation of that

significance through planning processes.

The Burra Charter The Australia ICOMOS Burra Charter, 1999 is a document used in Australia to give guidance for the

conservation and management of places of cultural significance (cultural heritage places). It outlines the

procedures and approaches considered to be appropriate in undertaking work on heritage items.

It is appropriate to detail some of the definitions of alternative types of work to heritage items at this point, as

defined in the Burra Charter:

� 1.4 Conservation means all the processes of looking after a place so as to retain its cultural significance.

� 1.5 Maintenance means the continuous protective care of the fabric and setting of a place, and is to be distinguished from repair. Repair involves restoration or reconstruction.

� 1.6 Preservation means maintaining the fabric of a place in its existing state and retarding deterioration.

� 1.7 Restoration means returning the existing fabric of a place to a known earlier state by removing accretions or by reassembling existing components without the introduction of new material.

� 1.8 Reconstruction means returning a place to a known earlier state and is distinguished from restoration by the introduction of new material into the fabric.

� 1.9 Adaptation means modifying a place to suit the existing use or a proposed use.

In essence, the critical issue in approaching works required on an item of heritage significance is

understanding the cultural significance. For example, a chair might be significant because it is a rare

example of a particular type of chair, and the design of this type of chair was very influential in the

development of chairs. In this case, the form and appearance of the chair is of high significance. However,

a chair might be significant because it was used by a famous person at an important event. In this case, the

associations of the chair are of high significance. Alternatively, a chair might be significant because it was

made by an important person using materials salvaged from another place of significance. In this case, the

materials of the chair are of high significance. In each case, conservation of the chair is likely to require

different approaches as a result of the differing significance of the chair.

Generally, conservation is based on a respect for the existing fabric, use, associations and meanings. It requires a cautious approach of doing as much as necessary but as little as possible (referred to within the Burra Charter and the heritage profession as ‘damanbalap’.

Typically, traditional techniques and materials (or those used at the time of construction) are preferred for the

conservation of what is defined as “significant fabric”. This means those parts of a building that define the

‘significance’, whether they be particularly decorative or just represent the building techniques used around

the time of construction, which may be significant in themselves.

In some circumstances current techniques and materials may be acceptable, as long as they offer

substantial performance benefits. In these cases, demonstration of the benefits through testing or

performance in use is generally required to validate the selection of the materials and/or techniques. In

particular, it is important to show that the approach is either reversible without impact on the significance of

the item, or that there are no deleterious effects on the significance of the item.

The removal of “significant fabric” is generally not acceptable unless the material is unrepairable, and there

are substantial risks to public safety or damage to other portions of the item.

Reconstruction, something that is common in normal engineering repair, is only acceptable where there is

already substantial accumulated damage or alteration to a building.

It is important to note that conservation of the fabric of significant heritage items may use different

approaches to those used at the time of original construction, as well as to those that might be used to repair

or reconstruct buildings not considered to be of heritage significance. An example of this would be the use

of soluble adhesives to re-adhere lifting paint from the surface of decorative paint schemes on the interior of

a significant building. Originally, the wall would have been painted; a conventional repair approach would

involve removal of the lifting paint, preparation and repainting. Conservation of the significant decorative

paint scheme could involve re-adhering of lifting paint using soluble adhesives to stabilise (preserve) the

significant fabric.

What Types of Facades Could Have Hidden Metal Components? Mild steel components have been used extensively in heritage facades, generally as a fixing for individual

façade cladding units, or as the skeleton for the façade cladding material to be fixed to. This use of mild

steel occurred prior to the research into and understanding of the corrosion mechanisms that corrosion

engineers have undertaken over the past 100 years. It is well known that most of the problems with

corrosion of reinforcement that we have encountered in reinforced concrete construction have come to light

over the past 30-40 years. A number of these problems are directly comparable to the techniques used in

heritage buildings using cementitious grouts and lime mortars in conjunction with embedded steel

components particularly.

Figure 1: Terracotta standard construction

Case Histories

1. Glazed Terracotta Faience

Background

The building was constructed circa 1928 and is of the Inter war Commercial Palazzo Style. It has historic,

aesthetic and architectural significance. The building is rare for its use of grey 'granite' terracotta. It is a

significant component of the surrounding 1920's streetscape and is significant as a landmark building.

The building has glazed terracotta (faience) cladding over a concrete-encased steel framed structure.

The faience cladding has localised areas of cracking and spalling associated with corrosion of structural steel

elements behind and movement of the building structure.

Investigation Techniques

The investigation involved removal of a number of pieces of terracotta, which identified one of the main

causes of deterioration as being the corrosion of mild steel fixings and support structure embedded in the

cementitious backing mortar used to fill the terracotta pieces once they had been assembled in place.

The limitation was identifying how many areas were affected, as the following photos clearly illustrate. If a

problem is identified in one element – i.e. visible cracking – is it reasonable to assume that the same problem

will occur for the remaining elements, even though the problem is not visible upon inspection?, If not, then

how much time will have to pass before the other elements have the same problem, if they ever do?

Figure 2: Corroded fixing of dentil at level 10 and remaining dentils

Diagnosis

Due to the nature of the significant fabric, it is not necessarily acceptable to extend repair works into areas

that do not currently show any signs of deterioration. This leaves that the possibility that areas may remain

that will be at risk in the future, possibly even soon after repair. How does a remedial engineer or

conservation specialist justify ‘repairing’ a building that may pose a future risk in a relatively short period of

time?

Repair Options / Process

As the facade is symmetrical, the current strategy for repairing the facade is to erect scaffold to one half of

the facade and deconstruct the most severely affected elements. The expected outcome of this process is

threefold:

1 To better understand the construction and mechanisms responsible for the deterioration and predict the extent of the problems;

2 To obtain samples of the terracotta facade pieces that can be taken off site to allow custom replicas of the elements to be fabricated;

3 To finalise repair and reconstruction methods.

It is expected that there will be a significant delay before the replica elements and repair methods are

finalised. Once the repair and reconstruction details are determined and replicas are available, the process

to deconstruct and investigate the other half of the facade will commence, with the repairs and reconstruction

of the damaged sections of the facade to follow shortly after.

Figure 3: Cracking of terracotta elements and corrosion of embedded steel frame

However, this approach does not resolve the unknown condition of the mild steel elements behind terracotta

units that currently show no signs of cracking. As it is not acceptable to deconstruct the entire façade and re-

erect the terracotta cladding using new fixings, the owner of the building will have to take on the risk of

monitoring the condition of the cladding, with the potential for repeated cycles of repair, and no timescale for

an endpoint.

2. Limestone Clad Facade

Background

The building has a limestone clad façade with mild steel cramps used to fix the stones in place. It was built

circa 1935 and is a steel framed building with reinforced concrete floors and infill panels of brick clad in

limestone. Its style is inter-war Art Deco.

The initial inspection occurred after a piece of limestone detached from the face and fell into the street below.

The following investigation was relatively brief and determined that there was an issue with corrosion of

hidden fixings in the overhanging features. This illustrates the need to identify these areas ahead of the time

when the corrosion causes such a risk. The presence of a crack in the limestone unit was a visible indicator

of the presence of corrosion to the mild steel fixings behind.

Investigation Techniques

Following emergency removal of the affected elements, a visual inspection again served as the basis of this

quantification and specification of repairs.

The inspection was conducted using a large boom lift parked on one of the city’s main streets with significant

time constraints. The locations of defects were marked up on elevations and these served the basis of the

repair methods and quantities used by the contractor to price the works.

The limited investigation and uncertainty over the construction led to some scope changes during

construction, but these were overcome with the appropriate input from the project team.

Figure 4: Cracked limestone piece and spalled area showing corroding fixing cramp

Figure 5: Façade Elevation

Diagnosis

On the basis of the limited access for investigation, it was not possible to determine the full extent of issues

that needed rectification. Therefore assumptions had to be made on the actual extent. Potentially,

excessive areas could have been removed or alternatively, too little may have been treated. Techniques

used to detect corrosion in these types of elements are largely interpretive.

The limestone cladding units themselves were generally in good condition. However, there were spalls and

cracks at the overhanging decorative features and some in ashlar blocks. These spalls and cracks were

caused by corroding mild steel fixings (due to water penetration from flat surfaces above the decorative

features into the interface between the stone and the backing concrete). There was some correlation with

the presence of voids around the steel fixings, and a strong correlation with the amount of water penetration.

It was predicted that eventually, all of the steel restraint fixings to the limestone cladding are likely to begin to

corrode, leading to the potential for cracking and spalling. The timescale for this is probably measurable in

decades, and possibly even in centuries, but will present an ongoing risk to public safety.

Figure 6: Overhanging features after removal of cladding from areas with suspect fixings

Repair Options / Process

Therefore, it was decided that as all fixings of limestone cladding on the overhanging features were likely to

have been subjected to a corrosive environment, particularly at the interface between the concrete and

limestone, the specified scope would be to remove the cladding from these overhanging features to expose

the corroded fixings, replace these with stainless steel fixings and reinstall the cladding back to the concrete

core of the features.

It soon became clear that the cladding could not be removed intact, and as the supply of replacement

limestone was limited, the scope of work had to be reduced. This necessitated the identification of the

location of as many fixings as possible in the undamaged overhanging features. Impulse radar and

traditional metal detecting equipment was used and the results correlated to locate the likely locations of the

fixings.

The central overhanging feature was significant in that is contained a carved statuette or ‘relief’. It was

critical not to damage this carving and therefore locating the fixings was fundamental to removing the

statuette without damage.

The maps resulting from the impulse radar and metal detecting equipment surveys were useful in

determining the general locations of reinforcement, however were not able to be relied upon fully to remove

the statuette. It was subsequently decided to attempt to access the rear of the feature through the interior

brick wall. This revealed that what the radar ‘saw’ as a “frame” was actually the top of a reinforced concrete

box.

Figure 7: Radar images taken from the left side of the carved feature

Figure 8: Statuette before and after removal

As the flat upper surfaces of the overhanging decorative features presented a point of potential water entry, it

was required carefully redesign the detailing to ensure that water was shed from the surfaces in a manner

that was not detrimental to other areas of the facade.

Areas of repair included repair of cracked elements, brought about by building movement but also opening

up of areas that had corrosion of fixings and repair of these areas by indenting stone. All new fixings were of

stainless steel.

Other methods such as silane impregnation and traditional lime shelter coat were also used to limit the future

ingress of water.

The project also saw the reopening of the original quarry that supplied the stone for the building.

Figure 9: Repaired limestone element and completed façade

3. Glazed Terracotta Tiled Facade

Background

The building was designed and built during the mid 1950s, and became a seminal building on subsequent

high-rise design in Australia. It utilised novel construction and structural techniques.

Facing materials include glazed terracotta, marble, granite and mosaic tiles. This building is designed in the

Post-War International style.

Thermographic surveys carried out in 1983 revealed thermal discontinuities in the facing walls, and it was

assumed that an air gap exists between the tile facing, including its mortar bed and the base concrete wall.

In 1987 a report suggested that delamination of blocks and mortar assisted by expansion in reinforcement

due to rust could cause face of blocks to move outwards. This theory was not put to test by further

investigation of the areas surveyed by thermography and no further work was undertaken.

Another report in 1987 concluded that considerable physical deterioration had taken place, and could be

expected to accelerate.

Repairs were undertaken in 1997, but only focussed on localised areas of the terracotta tile clad facades at

the end of the each building. The repairs did not address the underlying cause of decay, namely corrosion of

embedded steelwork and deterioration of the tiles themselves.

The repairs undertaken were obtrusive and did not take into account the original material, with the glazed

terracotta tiles being replaced by concrete units which were then painted with a ‘matching’ matt blue paint.

The spalling glaze of the tiles was also chipped off and painted with the same matt finish paint.

Inspections after a change of building ownership in 2002 led to conclusions by the façade consultants

responsible that the facades would require over-cladding or re-cladding in the near future, as these were the

only options that could offer certainty of the structural performance of the façade. These approaches were

not considered appropriate with regard to conserving the historic significance of the building.

Investigation Techniques

The investigation involved rope access inspection of the facades, which noted the general extent of spalling

glaze and went some way to determining the extent of the corrosion damage.

Further knowledge might have given increased reassurance as to the current stability of the facades;

however, there was no effective technique to check the physical condition of corroding elements of the

façade and a risk of failure of the façade still existed.

With the owner's involvement, we were able to undertake intrusive investigations at a number of locations.

These were done in areas from public view. The anticipated deconstruction process was trialled in order to

determine its practicality.

Photo 10: Spalling tiles due to corrosion of shelf angles behind

The trials involved:

� Identify location and spacing of shelf angles up the façade.

� Locate representative (drummy/sound; repaired/original) whereas at shelf angle locations to open up

and inspect.

� Cut out length of mortar joint at shelf angle location to confirm corrosion to leading edge.

� Attempt to remove existing tiles without damage by saw cutting joints and careful dislodgement.

Repeat until successful.

� Assess nature of ‘drummy’ sound by fracture mode of tiles and/or mortar. Compare with ‘sound’

areas, both at shelf angles and away from them.

� Inspect saw cut interface for direction of any cracking.

� Breakout to reinforcement and shelf angle.

� Record shelf angle orientation.

� Record position of reinforcement in backing mortar and distance from structural concrete. Check for

cracking along the plane of the reinforcement.

� Test backing mortar for carbonation (Phenolphthalein required).

� Increase breakout for one metre length and inspect condition of reinforcement, shelf angle and fixings.

� Determine angle size and fixing details.

� Install new angle section to façade to test noise and practicability.

Individual tiles were removed by saw cutting around the tile and levering out the tile.

The following processes led to the ability to negotiate a cost plus contract. This had benefits to the client

with them allowed to set an upper limit budget.

Diagnosis

A number of deterioration mechanisms were identified during the course of the investigations,of which only

those associated with corrosion will be considered here.

The primary cause of chipping and spalling of the tile body at shelf angle locations was corrosion of the shelf

angles. Corrosion was generally located along the front edge of the angle section and the expansive

corrosion product caused some tiles to rotate, inducing forces into the adjacent tiles, resulting in chipping

and spalling at the edge of the tiles immediately above and below. In some locations, the corroding shelf

angles also resulted in the face of the tiles breaking off at the dovetails due to expansive pressures of the

corrosion product.

In addition, there was some corrosion of the reinforcement mesh in the cementitious backing mortar, along

with the fixings of this mesh to the concrete structure behind. This corrosion did not appear to have resulted

in any expansive corrosion products or debonding stresses.

It was considered that water penetration, combined with a relatively porous backing mortar, led to the

corrosion of the reinforcement mesh, fixings and shelf angles behind the tiles.

Testing of the mortar for carbonation and chloride content determined that these were generally not the

mechanisms responsible for the corrosion. Chloride content was measured by weight of concrete. Mortar

has higher cement content and thus the measured values were not considered to be above the threshold

concentration required for the initiation of corrosion.

Carbonation had generally not occurred in the mortar with the exception of localised carbonation at cracks

and exposed surfaces. This localised carbonation would allow the reinforcing mesh in these locations to

corrode.

Photo 11:

The in-fill mortar between the tiles and the reinforced concrete walls was reinforced by the mesh which was

tied back to the reinforced wall. The membrane action of these components might have prevented the

façade from excessive bowing outwards. If so, the integrity of the mortar and mesh system gave increased

reassurance as to the stability of the facades, provided the corrosion of the reinforcement was arrested.

It was later determined that the mesh only had a supplementary structural action. The level of corrosion was

also generally minor, except at voids. Therefore, it was not considered that this corrosion is the primary

cause of debonding of the mortar, but this is more likely to have been due to the expansion of tiles and

thermal effects.

Repair Options / Process

Cathodic protection (CP) of the reinforcing mesh and shelf angles was considered as a possible solution to

the corrosion problems; however it was clear from the investigations that this method would not be practical

as there was likely to be a large amount of discontinuity in the reinforcing mesh.

The shelf angles were considered to be an ongoing source of problems in the tiled façade. Repairs and

priming of the shelf angles in 1997 were unsuccessful in halting the corrosion.

It was concluded that effective rectification of the corrosion to the mild steel shelf angles would require

replacement of the existing shelf angles.

This was achieved by installing lateral restraint fixings to the rows of tiles above and below the shelf angles

and then removing the tiles at the shelf angle location. The existing shelf angle was then progressively

removed and the new angle was installed the next row above the existing angle.

The trials indicated that removal of the existing tiles without damage was not practical and most tiles at the

location of shelf angles were replaced with custom made matching tiles.

The extensive drumminess of the tiling system across the façade had led to concerns over the stability of the

system. It was confirmed that the delamination of the tile system was located at the mortar to structural

concrete. The removal and replacement of the reinforcement mesh and fixings was not considered to be

practical. The solution to this problem was to install additional lateral restraint and joints to accommodate

differential expansion / contraction of sub-structure and facade.

Photo 12: Main facades showing removal of deteriorated tiles and shelf angles

Where to from Now? There are several fundamental challenges that recur with heritage buildings containing mild steel elements in

the façade. The solutions to these challenges are likely to also offer opportunities for repair and

maintenance of older buildings not considered to have heritage significance.

These challenges include:

1. Investigation without damage

Investigation of heritage facades requires the use of techniques that minimise the damage to the fabric,

whilst providing clear information about the construction and condition of all elements. As the methods of

construction are often no longer current, it may be difficult to predict the configuration of elements. Current

NDT techniques do not fully capture these requirements and interpretation is still in the subjective stage.

The advances made in the diagnosis of concrete structures provide a basis that may lead to techniques that

could be used for heritage facades. There are many challenges with this and it will require ongoing

research.

With reference particularly to corroding fixings and elements, the technology does not quite exist to bridge

the electrochemical gap between two separate corroding elements, or to reliably ‘look’ from the outside.

Extrapolation of results is the key, determination of original section thickness from areas of damage and a

reliable method of checking whether this was applicable to any other areas was essential to providing the

answers required to allow a consultant to confidently recommend to a building owner that the future risks

have been minimised.

2. Validation of limited repairs

There is evidence of a shift in attitude amongst asset managers regarding the need for ongoing inspections

and maintenance of buildings. This shift has been primarily been triggered through consideration of liability.

However, it provides one part of an argument to validate a limited repair approach to problems such as the

corrosion of hidden steel elements in building facades.

What is still required to support this approach are:

• adoption of simple engineering and scientific models to explain the time-related onset of damage

arising from corrosion of a population of elements that communicate levels of liability to the building

owner

• understanding and reliable evidence for the time between first visible evidence of deterioration and

the likelihood of an unacceptable risk event occurring (i.e. piece falling into the street, water leakage

into the IT facility)

• adoption of whole of fabric programmed maintenance systems by asset managers (not just services

and finishes)

An additional benefit to asset managers responsible for portfolios of investment properties that arises from

the adoption of a limited repair approach is the ability to spread out the cost of repairs, and to budget for

future expenditure well in advance. This improves their ability to deliver acceptable returns on investment.

3. Monitoring of deterioration

The concept of monitoring deterioration is well understood. The value of a sequence of measurements of a

changing characteristic or property is much greater than a single observation.

Unfortunately, there is very limited appreciation of the value of monitoring condition with regard to the fabric

of buildings. Simple and reliable (repeatable) techniques that are easily reproducible over a long period of

time are likely to increase the acceptability of monitoring of building fabric.

The utopian dream for the subject of this paper would be a non-invasive remote technique for monitoring the

rate of corrosion of hidden steel elements in masonry facades.

4. Keeping records and information available

The change in ownership of buildings and personnel responsible for managing the buildings means that

problems are often re-investigated and solutions re-invented several times arising from the loss of memory

associated with the buildings. This could be addressed by:

• better filing / storage systems in the building (a building databank) – but history has shown that

these are only as good as the person managing them

• having a central repository for building records (run by local government) – but the privacy and

security issues seem insurmountable

• web databases, run either by consultants, building owners, or third party providers, such that the

records relating to a building are accessible via the web after provision of secure access – but how

is security maintained and who will pay for this?

It would be interesting to see a study quantifying the costs of not keeping records and information available.

5. Justifying the spend

Regardless of other issues, the fundamental driver for building owners in repairing a building will almost

always relate to the financial returns on their investment. Balancing the need to ensure public safety, the

requirement to conserve significant heritage items, and the drive to achieve acceptable financial returns from

the asset is a difficult exercise.

One of the challenges for us as we investigate buildings with deteriorating hidden metal elements is to

provide valid approaches to minimising and managing the costs for investigation and repair of these. This

may not always be possible, for often there are different methods of construction in different locations on the

same building, and different causes of the same type of deterioration, all of which do not become apparent

until after the conservation and repair work has commenced. However, owners will almost invariably seek to

have a clear understanding of the total cost of works prior to authorising these to commence.

In the case of heritage buildings, or those buildings with the potential to be considered as heritage items,

there is a value to the community in retaining the original appearance of the building. There is evidence,

both in Australia, and in other parts of the world, that heritage status can enhance the commercial value of

an older building. The passing of time increases the uniqueness and hence value of these older buildings.

The continued use of existing buildings also responds to the current drive for sustainability in the property

market. Ultimately, the possibility exists that instead of commissioning large new office towers as their

flagship headquarters building, major firms will aspire to occupy, and link their identity to, landmark heritage

buildings.