Saija Varjonen, Jussi Mattila, Jukka Lahdensivu, Matti Pentti

Conservation and Maintenance of Concrete Facades Technical Possibilities and Restrictions

Tampere 2006

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FOREWORD Modern architecture built in 1960’s-1970’s forms an essential part of European cultural heritage. From the cultural sustainability point of view, recent heritage and especially suburban landscapes are endangered by long-term repair activity. In many cases, repair measures applied as part of standard maintenance change the original nature of buildings considerably. This Guideline deals with the conservation and maintenance of concrete suburban areas with special emphasis on mass produced concrete panels. The aim of the Guideline is to be a tool that makes it easier to understand the technical aspects related to conservation cases of concrete. The Guideline is part of MAC

2006, Modern

Architecture Conservation – Research and Training Project, which is a European research project to study the prerequisites of conservation of modern suburban architecture. The project received a grant from the European Commission Culture 2000 Framework Program in 2005 and

was implemented within one year starting from September 2005. The research was conducted at Tampere University of Technology (TUT), Institute of Structural Engineering. The report was written by Saija Varjonen, M.Sc. (Civ.Eng.), and the commentary committee was comprised of Prof. Matti Pentti, Dr Jussi Mattila and Jukka Lahdensivu, Lic.Tech. The work on the Guideline was supervised by a Project Group, which was comprised of Claes Caldenby of Chalmers University of Technology, Joseph King of ICCROM, Jukka Lahdensivu of TUT, Tommi Lindh of the National Board of Antiquities, Jussi Mattila of TUT, Mona Schalin of Kati Salonen & Mona Schalin Architects Ltd, Esko Sistonen of Helsinki University of Technology, Saija Varjonen of TUT and Ola Wedebrunn of The Royal Danish Academy of Fine Arts.

Contact information: Tampere University of Technology Institute of Structural Engineering P.O. Box 600 FI-33101 Tampere FINLAND Tel. +358 (0)3 3115 11 Fax. +358 (0)3 3115 2811 Email. forename.surname@tut.fi www.tut.fi/rtek

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TABLE OF CONTENTS

GENERAL ASPECTS OF SUBURB CONSERVATION ..............................................................................5

INTRODUCTION ............................................................................................................................................5 SUBURBS AS A TARGET OF CONSERVATION ..................................................................................................5 TECHNICAL ASPECTS RELATED TO CONSERVATION ......................................................................................6

Deterioration in Concrete .....................................................................................................................6 Protective Measures .............................................................................................................................6 Procedure for Determining Eligibility for Conservation.........................................................................6

DEGRADATION OF CONCRETE ................................................................................................................7

CORROSION OF STEEL ................................................................................................................................7 Carbonation ..........................................................................................................................................7 Chloride Contamination ........................................................................................................................8 Active Corrosion ...................................................................................................................................8

DISINTEGRATION OF CONCRETE...................................................................................................................9 Disintegration by Freeze - Thaw Exposure ....................................................................................... 10 Formation of Ettringite ....................................................................................................................... 10 Alkali Reactivity of Aggregate ............................................................................................................ 11

MALFUNCTION IN THE MOISTURE BEHAVIOUR OF STRUCTURES .................................................................. 11 REDUCED BEARING CAPACITY OF STRUCTURAL MEMBERS OR WEAKENING OF FIXINGS OR TIES.................. 12 DELAMINATION OF TILES ........................................................................................................................... 12 DEGRADATION OF COATINGS .................................................................................................................... 12 DEFORMATION AND CRACKING .................................................................................................................. 13

MEASURES TO RETARD DETERIORATION .......................................................................................... 14

GENERAL ................................................................................................................................................. 14 MONITORING OF CONDITION ..................................................................................................................... 14 RENEWAL OF SURFACE TREATMENT ......................................................................................................... 14 PATCH REPAIR ......................................................................................................................................... 15 ELECTROCHEMICAL REPAIR METHODS ...................................................................................................... 16 MIGRATING CORROSION INHIBITORS ......................................................................................................... 17 DEMOLITION AND REBUILDING ................................................................................................................... 17 MEASURES THAT ALTER APPEARANCE ...................................................................................................... 17

PROCEDURE TO DETERMINE CONDITION (CONDITION INVESTIGATION) ...................................... 18

DETERMINING THE STRUCTURES AND MATERIALS ...................................................................................... 19 EVALUATION OF POTENTIAL DEGRADATION MECHANISMS AND TYPES OF MALFUNCTION.............................. 19

Reinforcement Corrosion Due to Carbonation or Chlorides in Concrete .......................................... 19 Disintegration of Concrete ................................................................................................................. 21 Decrease in Bearing Capacity ........................................................................................................... 22 Other Malfunctions ............................................................................................................................ 22

PREREQUISITES FOR CONSERVATION ............................................................................................... 23

INSTRUCTIONS FOR PRO-ACTIVE MAINTENANCE ............................................................................ 25

USEFUL LITERATURE ............................................................................................................................. 27

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General Aspects of Suburb Conservation

Introduction

The growth of European suburban areas was fast in the 1960’s and 1970’s. Migration from the countryside into towns and changes in social structure created demand for fast and massive housing production. Concrete quickly reached a dominant position in the construction of facades and balconies due to the rapid development of the prefabrication techniques of precast panels. Large suburbs were built which changed the former pre-war townscape remarkably. Nowadays, modern urban architecture clearly forms an essential part of European cultural heritage. One of the major problems related to recent suburban developments is that many people do not regard them as heritage. When assessing qualities of cultural sustainability, there is a need to better integrate the concern for modern heritage into the larger sustainable development context. Seen from the point of view of cultural sustainability, suburban landscapes are primarily endangered by long-term repair activity. Although the stock of buildings with concrete panel facades built in the 1960’s and 1970’s is relatively young, the repair need of these structures is high because their unexpectedly rapid deterioration. The degradation of reinforced concrete facades cannot be retarded efficiently enough by appearance-saving light methods if the structures are of poor quality, or if repairs are postponed too much. Therefore, in most cases, the repair measures applied as part of standard

maintenance will, sooner or later, change the original nature of the buildings and townscapes remarkably. This Guideline deals with the maintenance and renovation of suburban areas from the conservation point of view. Here, conservation is considered the application of such maintenance measures that preserve the original appearance of a structure. The Guideline provides a tool that makes it easier to understand the technical aspects of conservation and how time affects conservation possibilities by introducing measures that allow evaluating the chances of maintaining the original appearance of a particular building throughout its lifetime.

Fig 1. Modern Lasnamäe suburb, in Tallinn Estonia.

Suburbs as a Target of Conservation

Unlike in Central and Southern Europe, most of the building stock in Northern Europe, like in Scandinavia, is relatively young. Older buildings are therefore often considered so valuable that their preservation is taken for granted. However, in the case of modern suburban buildings built in the 1960's and 1970's, the situation is very different. There are often huge amount of similar houses that are not valued very highly. Therefore, it may be unnecessary to conserve everything- It is rather possible to select part of this huge mass for conservation. This special situation with a large building stock makes it a good place to investigate which buildings are sensible to select to be conserved.

This Guideline helps choose the buildings or areas where conservation is sensible technically and economically. By this it is meant that it is not necessarily sensible to invest huge resources in the conservation of buildings where deterioration is progressing rapidly, if there are similar buildings which can be maintained with minimal effort because of their good technical quality. The need of conservation and related issues are not the subject of this publication. Decisions concerning this will require totally different kind of analysis that is based purely on architectural values.

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Fig 2. Modern Pihlajamäki suburb in Helsinki, Finland (Photos: M. Schalin).

Technical Aspects Related to Conservation

Deterioration in Concrete The service-life of reinforced concrete structures, like concrete panel facades, is often strongly limited by deterioration due to several mechanisms. Therefore, some kinds of repair measures become, sooner or later, necessary to maintain the aesthetics and safety of the structures. If the quality of a structure is poor and its exposure is severe, and if maintenance is neglected, heavy repairs are usually needed to restore the technical performance of the structure. Such repairs often change the original appearance significantly. Therefore, it is important to take into account the properties of the structure as well as the effects of deterioration in determining the need of maintenance in the future, when the eligibility of a target building for conservation is evaluated. The deteriorative mechanisms are dealt with in the chapter on “Degradation of Concrete”.

Protective Measures By applying suitable relatively light measures well in advance, it is usually possible to prevent damage or at least retard the propagation of deterioration to prolong the service-life significantly and postpone the need of heavy repair. Therefore, it is important to be aware of the availability and impact of maintenance-type repair methods. These are discussed in the chapter on “Measures to Retard Deterioration”.

Procedure for Determining Eligibility for Conservation The durability of a concrete structure, the possible propagation of deterioration and the remaining service-life determine the need for repair. These factors cannot be determined by visual inspection if the damage is not already severe. They can only be established by an investigation procedure, which should be carried out by a skilled investigating engineer whenever repairs are considered. The procedure is described in the chapter on “Procedure for Determining Condition”.

Fig 3. Modern Frölunda suburb in Gothenburg, Sweden.

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Degradation of Concrete

Concrete facades as well as all other concrete structures exposed to European outdoor climate are affected by several degradation mechanisms such as reinforcement corrosion and disintegration by different mechanisms. Degradation may limit the service life of structures and, therefore, the possibility of retaining the present or original appearance of buildings and suburbs. This is why it is important to know the basics of the degradation mechanisms of concrete to be able to successfully use suitable renovation measures. Recognition and description of different types of degradation mechanisms, which limit the service life of a structure, will be presented below.

Potential problems in structures may be caused either by different kinds of deterioration mechanisms or by malfunction of structures, for example problems with moisture behaviour. Usually, degradation, like the corrosion of reinforcing steel, may proceed for relatively long in a structure before becoming visible. Degradation may result in, for instance, aesthetic problems or even reduced bearing capacity of structures. From experience it is known that defective moisture performance of joints and different connection details may cause localised damage thereby accelerating the propagation of deterioration.

Fig 4. Stained and degraded coating causing aesthetic problems, Pärnu KEK, Estonia.

Corrosion of Steel

One of the most common degradation mechanisms causing the need to repair concrete facades, and concrete structures in general is the corrosion of reinforcement due to carbonation or chlorides. Reinforcing bars within concrete are normally well protected from corrosion due to the high alkalinity of concrete pore water. Corrosion may start when the passivity is destroyed, either by chloride penetration or due to the lowering of the pH in the carbonated concrete.

Carbonation Carbonation is a chemical reaction where atmospheric carbon dioxide diffuses into the pore system of concrete and reacts with the alkaline hydroxides of concrete. Due to carbonation, the pH of the concrete decreases from the level of 13 to the level of 8.5 and the passivity of reinforcing

steel loses its passivity. This initiates active corrosion if there is enough moisture and oxygen available in the concrete. Carbonation begins at the surface of a newly constructed member and propagates slowly as a front at a decelerating rate deeper into the structure (Fig 5). The speed of propagation is influenced mainly by the quality of concrete (amount of cement and porosity of concrete) as well as moisture exposure. Heavy moisture exposure, for example due to rainfall, slows down carbonation because water blocks the pores from CO2. Therefore, for example, carbonation is much slower on the upper surfaces of balcony slabs than on the soffits, which are sheltered from rain.

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Fig 5. Carbonation propagates as a front into the structure.

Carbonation cannot be seen. The determination of the carbonation depth of concrete always requires sampling and laboratory testing. The most typical carbonation depths of concrete facades constructed in the 1960’s and 1970’s are nowadays around 10– 20 mm if the concrete quality is normal. The depth of the concrete cover varies a lot, but quite often the reinforcement is in carbonated concrete. From this it can be concluded that the corrosion problems are actually caused by insufficient cover depths of reinforcement.

Chloride Contamination The passivity of steel may be destroyed also by the presence of chlorine ions derived either from the environment or from the use of contaminated constituents of concrete. Chlorides act as catalysts to corrosion and, therefore, the corrosion process may proceed rather quickly. The threshold value of chloride contamination for corrosion to start in non-carbonated concrete is

extremely low, i.e. around 0.03 - 0.07 weight percent. Whereas carbonation-induced corrosion leads to more or less uniform corrosion, chloride attack usually causes localised corrosion, i.e. small areas suffering from severe corrosion. This is known as pitting corrosion and can result in severe reduction in steel diameter. Chloride-induced corrosion becomes highly accelerated when carbonation reaches reinforcement depth. This means that the extent of visible corrosion damage may increase strongly in a short time.

Active Corrosion Once the passivity is destroyed either by carbonation or by chloride contamination, active corrosion may start in the presence of moisture and oxygen. The rate of corrosion depends strongly on the moisture content of concrete and proceeds significantly only if the relative humidity of concrete exceeds 80 %. Due to corrosion, the diameter of steel bars becomes smaller and their tensile capacity is weakened. Thus, besides aesthetic problems, corrosion may also cause a safety hazard. Corrosion may run for a long time before it can be noticed on the surface of the structure. Eventually, corrosion products accumulate on the steel surface and occupy a 3 to 6 times larger volume than the original metal. This generates an internal pressure, which leads to cracking or spalling of the concrete cover (Fig 6, Fig 7). Visible damage appears first on the spots where the concrete cover is thinnest. The service-life of concrete with respect to carbonation-induced corrosion consists of two phases: the carbonation and corrosion phases (Fig 8). It must be noted that, for instance, moisture exposure effects carbonation and corrosion differently. In moist conditions, the carbonation reaction is slow whereas the corrosion reaction proceeds actively.

Fig 6. Corrosion products generate a pressure, which leads to spalling of the concrete cover.

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Fig 7. Spalling of the concrete cover.

Fig 8. The service-life of a concrete structure consists of two phases: carbonation and corrosion phases.

Disintegration of Concrete

Concrete is a very brittle material. It can stand only extremely limited tensile strains without cracking. Internal tensile stresses due to expansion processes inside concrete may result in internal cracking and, therefore, disintegration of concrete (Fig 9). Concrete may disintegrate as a result of several phenomenona, such as frost weathering, formation of late ettringite or alkali-aggregate reaction, which results in internal expansion.

Fig 9. Internal cracks caused by freeze-thaw action. Picture shows polished section of concrete taken from a concrete facade.

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Disintegration by Freeze - Thaw Exposure Concrete is a porous material whose pore system may hold varying amounts of water, easily more than 10 % of its volume. As the water in the pores freezes, it expands about 9 % by volume, which creates hydraulic pressure in the system. If the level of water saturation of the system is high, the overpressure cannot escape into air-filled pores and thus causes mechanical damage to the internal structure of the concrete resulting finally in total loss of strength. Frost resistant concrete can be produced by air-entrainment, which means that a sufficient amount of permanently air-filled so-called protective pores are created in the concrete by adding a suitable agent to the concrete mix. The idea of the protective pores is that the pressure from the freezing dilation of water can escape into these air-filled pores (Fig 10). Most of the concrete cast in the 1960’s and 1970’s is not frost-resistant, which is why its moisture behaviour and level of moisture exposure have a strong impact on probability of frost damage. Because the propagation of frost damage requires that the pores are almost totally filled with water, a reduction in moisture exposure

is an efficient way to reduce the risk of damage. For instance, the risk of frost-damage to a balcony slab can be reduced by proper waterproofing.

Fig 10. Sufficient air voids in the cement paste ensure frost resistance. The arrows represent the flux of water in freezing concrete.

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Formation of Ettringite The formation of ettringite is a chemical reaction caused by sulphate minerals that occur in hydrated cement. Ettringite may form in concrete as a result of excessive thermal treatment during the curing of concrete or if the concrete is for some reason contaminated by sulphates. An ettringite reaction involves a strong volume expansion of reaction products, i.e. swelling, since the volume of ettringite is roughly 300-fold compared to the volume of the reactants. The forming ettringite mineral crystallises onto the walls of the air-filled pores whereby the volume of protective pores and the frost resistance of concrete decrease. An ettringite reaction leads to concrete degradation either as a result of frost weathering or as the pressure created by the filling of pores causes cracking of the concrete. Finally, the deteriorated concrete loses its strength. The formation of ettringite requires that the concrete is very moist at least periodically.

Fig. 11. Ettringite mineral crystals formed onto the walls of the air-filled pores decrease the volume of protective pores and the frost resistance of concrete. The shown area is about 0.6 mm x 0.5 mm (Photo: A. Koskiahde, Betonialan Ohuthiekeskus FCM Oy)

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Alkali Reactivity of Aggregate An alkali-aggregate reaction is an expansion process between alkali and aggregate grains in hardened concrete. Due to the alkalinity of hydrated cement, certain types of aggregate may degrade producing expansive products. The reaction requires that the cement contains an abundance of alkalis (Na, K), the aggregate includes minerals with low alkali resistance, and the moisture content of the concrete is sufficiently high. Alkali-aggregate reactions are divided into three general types: alkali-silicon, alkali-carbonate and alkali-silicate reactions depending on the reacting aggregate. An alkali-aggregate reaction is a common cause of concrete cracking in areas where alkali-

susceptible aggregates are used – usually relatively young sediment-type aggregates containing siliceous minerals such as opal, chert, chalcedony, tridymite, cristobalite or strained quartz. Dense deep-seated aggregates generally have high chemical resistance. A concrete structure affected by an alkali-aggregate reaction is typically stained by surface moisture, exhibits irregular pattern cracking and swelling, and has a gel-like reaction product oozing out of the cracks. The damage caused by the alkali-aggregate reaction resembles the cracking due to frost weathering and both often appear simultaneously.

Malfunction in the Moisture Behaviour of Structures

A malfunction in the moisture behaviour of structures in itself is an important degradation mechanism. It may also initiate or accelerate other degradation, like corrosion or disintegration, by causing accumulation of excessive moisture on concrete. Therefore it is crucially important to pay regard to these problems in maintenance as early as possible.

Typical problematic details are joints between structural members exposed to rainfall, connections between façade and roof and windows, cappings and signs attached to structures, etc.

Fig. 12. Frost damage from extremely severe moisture exposure due to faulty balcony glazing.

Fig. 13. Local damage caused by deficient details.

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Reduced Bearing Capacity of Structural Members or Weakening of Fixings or Ties

The safety of structures may be essentially affected by deterioration of structures, for instance, due to frost damage or corrosion of reinforcement. There are usually also many embedded brackets, etc. in the structures, which are constructed using prefabricated panels. These parts may also be exposed to deterioration. It is worth mentioning that the

condition of these brackets cannot usually be established by means of a visual inspection. In the early years of precast technology, the panels were cast and fastened by methods which were later found not to be durable, for example fixings and brackets which were not made of stainless steel.

Delamination of Tiles

Many kinds of tiles (ceramic, clay brick, natural stone) have been used in concrete facades. They are generally attached to the concrete surface by a bond between the tiles and concrete (concrete is cast on the tiles) or by fixing mortar (tiles are fixed to hardened concrete). Tiles are usually durable, but weathering may weaken their bond. In the case of impermeable tiles (ceramic and natural stone) accumulation of moisture behind the tiles is possible. This can lead to disbonding due to frost damage. It is also possible that the concrete substrate disintegrates. If the cover depths behind the tiles are small, tiles can be disbonded by corrosion.

Fig. 14. Delamination of ceramic tiles.

Degradation of Coatings

The typical service-life for an ordinary coating on a concrete facade is somewhere around 20 years. Because coatings have traditionally been used on concrete structures purely for aesthetic reasons, normal degradation is usually not a big problem. Whenever the appearance of the surface is no longer appealing enough, it is simply recoated. That is a relatively straightforward procedure provided that the concrete structure itself is in good condition. It must be noted that even if the coating seems to be in good condition, the structure underneath may be deteriorated.

Fig. 15. Degradation of coating is typical in concrete facades and balconies.

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Deformation and Cracking

Outdoor concrete structures are exposed to continuous deformation due to changes in temperature and moisture content. It can be evaluated that the magnitude of this deformation is roughly 1 mm per metre of structure. As stated already earlier in this chapter, concrete is a very brittle material. If free deformation of a structural member is not possible, it easily leads to cracking.

It is important to note that cracking can result from various reasons, such as normal or over-loading, accidents, shrinkage, degradation (corrosion and disintegration), thermal and moisture deformation, deformation of foundation or supporting members, etc. In some cases cracking is only an aesthetic problem, but sometimes cracking may also accelerate degradation. The repair measures depend strongly on the reason behind the cracking and have to be considered case by case.

Fig. 16. Cracking in concrete panels. On the left the cracking is caused by freeze-thaw action.

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Measures to Retard Deterioration

General

When the general aim is to preserve the original appearance of a concrete structure for as long as possible, it is important to apply well in advance measures that can protect the structure from deterioration processes, or at least retard them as much as possible. If this kind of pro-active maintenance is neglected, heavy repairs such as overcladding, which changes appearance, will often become necessary sooner or later. This chapter describes the alternative repair measures which do not change the appearance of the building. The following issues will be considered with different repair methods:

- Prerequisites to use a specific method, - Positive and negative effects of certain

repair methods, and

- Feasibility and life-cycle economy of the method.

It is important that the feasibility of the maintenance and repair techniques to be used are considered by a professional structural engineering on the basis of the results of the condition investigation. The service-life of a repair method depends mainly on three factors: the proper selection of a repair in each case, the work performance and environmental circumstances. The service lives mentioned below represent a successful implementation of repair where all the above factors have been considered.

Monitoring of Condition

It is not always necessary to apply repair measures to the structure or building immediately although a deterioration process is clearly going on. Sometimes better and more economical results can be achieved by postponing repair measures. For example, a structure may still look good but some corrosion damage is expected due to minimal cover depths. If that does not cause a safety hazard and the surface is such

that it allows easy repair of damage, the repair can wait until the damage has occurred. Monitoring of a structure means that the state of its deterioration process is followed to detect any unexpected damage. It is also important to keep an eye on the structure to ensure that no significant changes in its exposure conditions for the worse take place.

Renewal of Surface Treatment

When the surface treatment is renewed the basic properties and the damage state of the old structure remain the same as before the repair. It is important to realise that physical damage of a structure cannot be repaired just by applying a new coating. Most harmful deteriorative processes require a lot of moisture. Therefore, reducing moisture exposure is the main procedure for retarding expansion of damage. It can be realised by special coatings, by renewing joints, and by improving the moisture behaviour of structural details in general. Suitable coatings can reduce the passage of potentially deleterious substances, like moisture,

and hence slow down deterioration. Certain types of coatings prevent the penetration of liquid water into concrete without hindering the escape of water vapour from it. Standard Painting is mainly an aesthetic measure and it is suitable when the old concrete structure itself is in good condition. It should be remembered that even if the surface of the structure seems to be in good condition, this does not mean that there could not be serious deterioration inside. The most significant deterioration mechanisms become visible only after they have progressed very far. The typical service-life of an ordinary coating on a concrete facade is around 10 - 20 years.

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Patch repair of local visible damage and renewal of elastic joints are usually connected to standard painting procedure.

Fig. 17. Repair of local visible damage by patch repair and renewal of elastic joints are usually connected to standard painting procedure.

Protective Coatings may be utilised if the structure is still in good shape but there is risk of future damage. Concrete that is not air-entrained or is made with alkali-susceptible aggregate or where the carbonation front has advanced near the reinforcement are examples where a protective coating may be a good solution for retarding deterioration. It should be pointed out that protective coatings may not function as expected if the concrete is already damaged. A typical service-life for a protective coating on a concrete façade is around 20 – 25 years. Impregnation means treatment of surfaces by a material, which penetrates into concrete and provides it with water-repellent properties without changing the colour or forming a film on the surface. Impregnation can be made with a liquid, gel or cream type material, which usually contains silane. Impregnation reduces the amount of water absorbed by concrete e.g. from rainfall. However, it is important to realise that when the surface turns non-absorbing, the amount of water penetrating the surface locally, for example through cracks and other leakage points, will increase.

Patch Repair

Patch repair is a traditional way of repairing local damage in all kinds of concrete structures. It can be a light method in connection with coating renewal, but also involve heavy repair. In the heavy alternative, also areas where the damage is still unseen are repaired. The basic idea of a patch repair is to remove deteriorated concrete and expose corroding steel, and then replace the removed concrete with new material. Cast concrete, shotcrete or, most usually, special repair mortar is used in the work. The surface of the repaired area is finished by different methods depending on the surface type. In the case of painted plain concrete surfaces the whole façade may be treated with finishing mortar and then recoated. Patch repairs are usually exposed to nearly the same environmental and other loads as the structure before the repair, particularly if no protection, like a protective coating, is applied as part of the repair. Since the materials used in the repair work are usually cementatious like concrete, the repaired area will be subjected to nearly the same harmful processes as the surrounding structure.

Fig. 18. Patch repair is used to repair local damage.

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It is important to note that the service-life of a repaired structure does not depend merely on the durability of the repaired areas but also essentially on the durability of other parts of the structure that have not yet been repaired. In many cases, the degradation of parts other than the repaired ones will determine the actual service-life of repaired structures. Patch repair is a feasible method when the amount of damage is very limited, like slight corrosion damage or small local incipient disintegration and the repair work is easy and simple. A good rule of thumb is that local repair should be applied only to local damage. Prior to planning of repair, a careful condition investigation has to be always carried out to find out the real extent of the damage to be repaired. In principle, patch repairs are suitable when the aim is to conserve the present appearance of the building. If the surface cannot be coated or it is highly textured, it is possible that the repaired

areas stand out too clearly from the original surface. A typical service-life for a thorough patch repair on a concrete façade is around 20 - 30 years.

Fig. 19. The repaired areas stand out clearly from the original surface in concrete panels with an exposed aggregate surface.

Electrochemical Repair Methods

Widespread corrosion of reinforcement is one of the main causes of the repair of concrete structures. If patch repair is applied in cases where corrosion is no longer local, a lot of money has to be spent on chipping off concrete. In other case, the result of the repair will probably not prove very durable. With electrochemical methods, it is necessary to remove only those parts of concrete that have already cracked or spalled. There are three different electrochemical maintenance methods available: realkalisation, chloride removal, and cathodic protection. They all have the same operational principle: by means of an external conductor, an electric current is passed through the concrete to the reinforcement. The final result of the current is that corrosion is halted either by removing the aggressive chlorine ions from concrete (chloride removal), by restoring the alkalinity of concrete (realkalisation), or by permanent potential shift (cathodic protection). In realkalisation and chloride removal, the current is applied for a limited time (from one week to a month) after which the anode system is removed from the surface. The concrete's protective properties are restored and may provide protection for many years. Of course, the possibility of further chloride ingress or carbonation has to be eliminated.

In cathodic protection, the anode system is installed permanently, and a very small current is applied constantly to the reinforcement to prevent corrosion. The anode system always changes the surface to some extent, because it is applied across the surface either in the form of a conductive coating, rods embedded in drilled holes or a titanium mesh in shotcrete. Cathodic protection is an alternative for chloride removal. It is applicable in cases where partial changes in appearance are allowed and very long-term corrosion protection is pursued.

Fig. 20. Temporary anode system is attached to the old façade surface and is coated with wet-sprayed cellulose.

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Migrating Corrosion Inhibitors

The general purpose of corrosion inhibitors is to retard the rate of the corrosion reaction in carbonated or chloride-contaminated concrete by hindering some of the sub-reactions of corrosion. Migrating inhibitors are liquids, which are applied to the surface of an existing structure from where they travel slowly by diffusion to the surface of

the steel. Inhibitors are consumed with time and will only work up to given level of activation. So far, it has been impossible to develop migrating inhibitors that have a sufficient inhibitive effect also in practise.

Demolition and Rebuilding

Sometimes, when there is severe damage in a structure, it may prove most convenient to demolish and rebuild the structure. This is true especially in the case of prefabricated structures because their demolition is usually not too

complicated. The new panels may imitate the original appearance of a façade. The service-life of demolished and rebuilt precast panels is up to 100 years.

Measures that Alter Appearance

There are several repair methods for concrete facades that alter their appearance. Most of them use overcladdings or overlays. These repair methods are usually utilised in cases of widespread damage in the structure. Overcladdings stop or slow down the progress of the damage. Overcladding can be accomplished by ventilated structure using metal sheets, fibre-cement or polymer boards, brickwork, etc, or by rendering

over additional thermal insulation. Cementitious overlays like shotcrete or gunite (mortar applied with an ejector) are used especially on structural members. The service-life of overcladdings or overlays is around 30-50 years or even more. Because these methods alter appearance, they are not dealt with in the guideline in more detail.

Fig 21. Overcladding may change the original appearance of the building considerably.

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Procedure to Determine Condition (Condition investigation)

The repair measures required for the maintenance of a target building depend strongly on its technical condition, i.e. the types of deterioration processes going on, their current stage, and how quickly they will cause problems. Therefore, the technical suitability of a target building for conservation is determined in practise by a systematic façade condition investigation. Technical suitability for conservation means here the easiness of retaining the building's appearance through maintenance. Sometimes it is almost impossible to avoid, for example, overcladding because of the poor shape of the façade. The next chapter of this Guideline presents a systematic method for evaluating the prerequisites related to the structure and its exposure conditions that affect its eligibility for preservation. The practical aim of a condition investigation is to provide the property owner, designing engineers and architects information to help them decide on the repair needs and possibilities and to select the repair methods best suited for each facility, as well as to schedule repair measures so that heavy repair methods can be avoided. The content of the condition investigation must be such that set goals are met. Generally speaking, condition investigations are made to determine the remaining service-life of examined structures, their need of repair and their safety. To achieve that, the investigation must reveal any significant damage to structures and defects in their performance. This requires determining the existence of damage, its extent, location, degree, impacts and future propagation by damage types. Because major technical and financial decisions are made on the basis of a condition investigation, it should be reliable. If repairs are done without a proper condition investigation, many kinds of difficulties can be faced either

during the repair work or during later maintenance of the repaired structure. If an unsuitable repair method is selected, the service life of the structure will be shortened. If the reason for the damage does not become clear, it cannot be eliminated by the repair and, therefore, degradation may continue despite the repair measures. To ensure the reliability of a condition investigation, the investigating engineer should have good knowledge of the performance and properties of examined structures and understand the degradation mechanisms, defects, deficiencies and repair methods at issue.

Fig. 22. Field and laboratory tests are essential parts of a condition investigation.

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Determining the Structures and Materials

The first phase of every condition investigation is to study what kind of structure or structures are under investigation. This means knowing what kind of structural system the object has and what are the materials and layers it has been constructed of. It is self-evident that no successful investigation can be carried out if the investigator does not know the type and the nature of the object. Information on structures and their materials is usually gathered from original construction documents and by visual inspection, which also produces information about visible damage, e.g. the amount of damage that certainly exists. Naturally, as far as the documents are concerned, it has to be taken into account that the structures have not necessarily been constructed exactly according to the documents. Sometimes the dissimilarity between the structure and the document representing it may be remarkable.

It is also important that different types and parts of structures, as well as structures in different exposure conditions, are distinguished from each other and investigated as separate subject groups.

Fig. 23. Construction documents are an important information source, but they cannot be trusted absolutely.

Evaluation of Potential Degradation Mechanisms and Types of Malfunction

The second phase of a condition investigation involves recognising and determining the state of all potential degradation mechanisms that endanger the performance and durability of a structure as well as structural and exposure factors that affect their progress. The potential problems may be caused either by different kinds of deterioration mechanisms or by malfunctions of structures, for example problems with moisture performance potentially initiating or accelerating the progress of deterioration processes.

The mutual importance and the combined effects of different deterioration mechanisms should also

be evaluated carefully. It is self-evident that factors related to the bearing capacity and safety of fixings are the most important issues to be investigated. Examination of design documents provides preliminary information about the degradation risk of different types of structures. They are studied to get information about the quality of the concrete used (strength, type of cement, use of air-entrainment, etc). The moisture performance of the structure can also be evaluated from documents.

Reinforcement Corrosion Due to Carbonation or Chlorides in Concrete Initial data for the study of the corrosion of reinforcing steel is gathered from the documents and by visual inspection. By visual inspection it is possible to estimate: - the amount and location of visible damage

(spalls, cracks, rust stains or spots),

- the depth of concrete cover in damaged spots, and

- the moisture behaviour that affects the rate of active corrosion.

20

Fig. 24. Corrosion damage in balcony parapets.

The cause and exact extent of corrosion damage are determined by laboratory and field tests. The assessment of the amount of steel in active corrosion state at various depths is based on samples of carbonation depths and cover depths of reinforcement. In the field, the distribution of cover depths is measured by a covermeter separately from each group of structures. In the laboratory, it is possible to determine the penetration of carbonation from core samples by using a phenolphthalein indicator, which colours the non-carbonated concrete purple. It must be noted that the carbonation front usually varies a lot across the structure. Therefore, several samples must be taken to get a correct view of carbonation. Chloride contamination is measured from drilled powder samples, for example, by titration. In the case of concrete structures exposed to chloride, such as bridges, chloride content is determined from samples taken from different depths to find out the penetration depth. In the case of mixed-in chlorides (added on purpose to accelerate the hardening process) the amount is equal throughout the concrete and usually exceeds the critical chloride content significantly.

The extent of corroding reinforcement can be estimated by comparing the carbonation or chloride depth distribution of concrete to the cover depth distribution of rebars according to Figure 26. It shows, for example, that rebars located closer than 12 mm to the surface may be corroding.

Fig. 25. The phenolphthalein indicator leaves the carbonated concrete colourless.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

depth [mm]

n

carbonation

cover depth

Fig 26. An example of histograms of measured carbonation and cover depth distributions.

21

Disintegration of Concrete The assessment of disintegration of concrete requires examining two basically different issues, i.e. whether the concrete is susceptible to this type deterioration (frost resistant, contains susceptible aggregate or harmful sulphates) and whether it has already suffered damage. The existence, degree and extent of degradation must be investigated by several different methods. Visual inspection is only capable of revealing far advanced damage. It is used to assess the extent and location of visible

degradation and various signs indicating degradation such as type of cracking pattern of concrete surfaces, efflorescence from cracks, warping of precast panels (Fig. 27) and compression of panel joints (Fig. 28). Field investigation of disintegration consists of core sampling and careful hammering of concrete surfaces. Far advanced damage can be detected and its extent evaluated by hammering a concrete surface with a heavy hammer.

Fig. 27. Warping of precast panels caused by freeze-thaw deterioration.

The actual damage state due to disintegration is usually determined in a laboratory from samples indirectly by tensile strength tests or directly by, so-called thin-section analysis. This means that the micro-structure of the concrete is studied with an optical microscope. This requires that a very thin (20 μm) sample is prepared from the core sample. The assessment of the propagation of a disintegration process in concrete is considerably more difficult compared to that of steel corrosion. Concrete structures found to be susceptible to disintegration do not necessarily show any damage in practise. Besides material properties, the moisture stress level also affects essentially the occurrence of damage. In general, it can be stated that the disintegration process usually accelerates with time. This is because the propagation of damage increases cracking, which increases permeability. Thus, the progress

of damage usually raises the moisture content of concrete. The progress of disintegration should be monitored by regular testing, for example by tensile strength tests.

Fig. 28. Compressed panel joints.

22

Decrease in Bearing Capacity One of the most important tasks of a condition investigation is to find out any possible decrease in the bearing capacity of structural members or weakening of the structural fixings or ties of a structure (for example weakening of the ties of sandwich panels due to corrosion). The condition of structural members, fixings and ties is examined in the field usually from drilled or

chiselled holes by visual inspection. It should be pointed out that if the condition seems to be good, examinations have to be carried out in several spots and also in the most exposed areas to confirm the initial result. The proving of bad condition requires examinations in much fewer spots.

* * *

Other Malfunctions Other malfunctions are usually related to flaws in the moisture behaviour of the structure. They are usually discovered by means of visual inspection. Moisture measurements cannot usually determine moisture performance, because the moisture content of structures varies significantly depending on seasonal changes and recent weather conditions.

Visual inspection to evaluate moisture behaviour includes examination of defects in elastic joints and other exposed joints, connections and details, performance of possible ventilation inside the structure, defects in paints and coatings, defects in facade tilings (ceramic, clay brick or natural stone tiles), harmful cracking or deformations in concrete, defects due to the use of the structure (for example normal wear), etc.

23

Prerequisites for Conservation

There are several factors which determine how easy it is to maintain the original appearance of a concrete façade or other concrete structure. In many cases the greatest problem is the degradation of structures which necessitates repair measures. In certain cases the damage is so extensive that it is almost impossible to avoid using repair methods which change appearance significantly. Naturally, if unlimited financial resources are available for maintenance, it is often possible to use complex and expensive repair methods or even demolish and reconstruct the façade to look the same as it was originally. However, today's construction techniques do not always enable to achieve just the same result as originally. The patina formed during the decades of use is not possible to restore by any means. The purely technical factors which usually determine the difficulty of conserving a concrete façade are listed in the following table. The attributes leading to a case which can be easily conserved are collected in the upper part of the table. The attributes that make conservation difficult are at the bottom of the table. Often the main factor determining the choice of the proper repair method is the state of degradation of structures. The alternatives are described in the first column. If there is no damage present or even expected in the near future, the measures

required for conservation are much easier than when widespread damage has occurred. In certain cases damage may be so extensive that it is almost impossible to avoid heavy repair methods. Another technical factor determining the difficulty of conservation is the type of concrete surface. The alternatives are described in the second column. Generally speaking, it could be said that in case of painted plain concrete there are many more options for conserving the surface than, for example, in the case of exposed aggregate concrete. Exposed aggregate concrete is usually almost impossible to repair with light methods, like patch repair, if the purpose is to achieve a surface similar to the one before the repair. It is important to note that the feasibility or ease of conservation depends also on other than technical factors. The requirements for final surface quality play a crucial role. If, for example, it is acceptable that the repaired areas may stand out from the final surface after patch repair, the options are much wider than if the surface is expected to have a uniform coating. It must be stressed that the table, or this Guideline in general, do not deal with the evaluation of the need of conservation. That is done on the basis of architectural values.

Current and Expected State of Damage Difficulty of the type of Concrete Surface

to be Conserved

No damage present or expected Disintegration: - concrete is frost-resistant and - no alkali-silica reactivity Corrosion: - cover depths are sufficient and

- concrete contains no chlorides No structural problems Only local damage present or expected - i.e. damage is not expected to expand in the future Deterioration is possible, but no wide spread visible damage is present and there are no structural problems Disintegration: - concrete is not frost-resistant and/or - alkali-silica reaction is possible Corrosion: - cover depths are insufficient and concrete is carbonated and/or

- concrete contains chlorides near surface Widespread damage and/or serious structural problems

Painted plain concrete (Fig. 29) Concrete surface can be coated Structured surfaces:

- Uncoated plain concrete (Fig. 30)

- Brushed concrete (Fig. 31)

- Exposed aggregate concrete (Fig. 32, 33)

- Ceramic tile finishing (Fig. 34)

- Pigmented uncoated concrete

- Patterned concrete (Fig. 35)

- Special surface textures (Fig. 36)

DIF

FIC

UL

TY

IN

CR

EA

SE

S

24

Fig 29. Painted plain concrete Fig 30. Uncoated plain concrete

Fig 31. Brushed concrete Fig 32. Exposed aggregate concrete

Fig 33. Exposed aggregate concrete Fig 34. Ceramic tile finishing

Fig 35. Patterned concrete Fig 36. Special surface textures (Photo: M. Schalin)

25

Instructions for Pro-Active Maintenance

The durability of the early concrete facades in suburbs is highly insufficient in many cases. Therefore, they are often vulnerable to further deterioration also when conserved. When a decision to conserve a façade or some other structure has been made, it is important to launch all sensible measures to protect the structure from deterioration. It is also vital that the feasibility of the maintenance and repair techniques to be used is determined by a professional structural engineer specialised in the maintenance of concrete structures. The decisions should be made on the basis of the results of the condition investigation. Generally speaking, the most dangerous enemy of reinforced concrete structures is simply moisture. All significant deterioration mechanisms need quite a lot of water to proceed and cause problems. The most useful rule of thumb to prolong technical service-life is to promote measures that decrease the moisture exposure of structures and/or speed up drying. One example is freeze-thaw deterioration. A vast majority of the concrete cast in the 1960’s and 1970’s is not frost-resistant. Therefore, moisture behaviour and the level of moisture exposure have a strong impact on the probability of frost damage. Because the propagation of frost damage requires pores to be almost totally filled with water, a reduction in moisture exposure by suitable surface treatment or perhaps by enhancing the performance of drainage or window flashings, is an efficient way of reducing

the risk of damage. For instance, in a balcony slab, the risk of frost-damage can be reduced remarkably by applying proper waterproofing. The reduction of the moisture can be done by special coatings, by renewing joints, and by improving the moisture behaviour of structural details in general. Defective moisture performance of joints and different connection details may cause localised damage thereby accelerating the propagation of deterioration (Fig 37). One of the most important things is to keep the water drainage systems in a good condition.

Fig. 37. Local damage caused by a deficient detail.

26

27

Useful Literature

Broomfield, J. Corrosion of steel in concrete, understanding, investigation and repair. E&FN Spon. 1997. Glendinning, M. and Muthesius, S. Tower Block: Modern Public Housing in England, Scotland, Wales, and Northern Irelandí Yale University Press 1994 Kalm, M. and Ruudi, I. (editors) Constructed happiness-Domestic Environment in the Cold War Era. Estonian Academy of Arts, Proceedings 16. Tallinn 2005 Mattila, J. On the Durability of Cement-Based Patch Repairs of Finnish Concrete Facades and Balconies. Doctoral Thesis. Tampere University of Technology 2003 MacDonald, S. Concrete: Building Pathology. Blackwell Publishing 2002 Pigeon, M. and Pleau R. Durability of concrete in cold climates. E&FN Spon. 1995. Pentti, M. The Accuracy of the Extent-of-Corrosion Estimate Based on the Sampling of Carbonation and Cover Depths of Reinforced Concrete Façade Panels. Doctoral thesis. Tampere University of Technology 1999

The Research Reports of Institute of Structural Engineering 1998 – 2006 137 Kylliäinen, M., Talonrakentamisen akustiikka. TTY 2006. 205 s. 42 €. 136 Varjonen, S., Mattila, J., Lahdensivu, J., Pentti, M., Conservation and Maintenance of Concrete

Facades - Technical Possibilities and Restrictions. TUT 2006. 27 p. 135 Heinisuo, M., Ylihärsilä, H., All metal structures at elevated temperatures. TUT 2006. 54 p. 37

app. 34 €. 134 Aho, H., Inha, T., Pentti, M., Paloturvallinen rakentaminen EPS-eristeillä. TTY 2006. 106 s. + 38

liites. 42 €. 133 Haukijärvi, M., Varjonen, S., Pentti, M., Julkisivukorjausten turvallisuus. TTY 2006. 25 s. + 111

liites. 132 Heinisuo, M., Kukkonen, J., Design of Cold-Formed Members Following New EN 1993-1-3. TUT

2005. 41 p. 34 €. 131 Vinha, J., Korpi, M., Kalamees, T., Eskola, L., Palonen, J., Kurnitski, J., Valovirta, I., Mikkilä, A.,

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129 Vinha, J., Valovirta, I., Korpi, M., Mikkilä, A., Käkelä, P. Rakennusmateriaalien rakennusfysikaaliset ominaisuudet lämpötilan ja suhteellisen kosteuden funktiona. TTY 2005. 101 s. + 211 liites. 42 €.

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