Why Concrete Deteriorates & How to Remedy it

Experience has shown that in the vast majority of cases concrete deterioration in the UK over the last 50 years has primarily involved corrosion of the reinforcement and consequent spalling and delamination of the concrete surfaces. In most cases, distress has initially been non-structural and essentially surficial, which generally only effected appearances, although the spalling surfaces have represent a potentially significant Health and Safety risk, in terms of falling debris. If the deterioration has been allowed to continue, however, structural distress has developed, either within specific elements, or structures as a whole. The following paragraphs give a very brief summary of the typical processes of deterioration involved and are intended to aid the lay-reader to understand the reasoning behind the programme of investigations carried out.

Reinforced concrete is a composite material generally comprising coarse and fine aggregates set in a cementitious matrix and reinforced with mild steel bars or rods. The cementitious matrix, generally a type of Portland cement is highly alkaline (pH values of fresh concrete in the range 12 to 13) which reacts with the steel surfaces to produce a passivating layer or film surrounding the reinforcement. Whilst the alkalinity of the concrete matrix remains high the passive film remains

intact and deleterious corrosion of the reinforcement is unlikely, under normal circumstances. Once exposed to the atmosphere, which is essentially acidic the alkalinity of the concrete is neutralised, inwards from the exposed surfaces. The carbon dioxide in the atmosphere reacts with the alkali hydroxides within the concrete matrix to produce various carbonate compounds (and a reduction in pH to around 8 to 10), hence the term carbonation. Once carbonation has extended into the concrete to the level of the reinforcement the pH around the steel reduces and the passive film subsequently deteriorates. Potentially deleterious corrosion of the reinforcement can then occur.

Building Research Establishment (BRE) Digest 444: 2000 gives the following empirical formula for the “parabolic ingress rate” of carbonation:

d = k tn

Where d = the carbonation depth,
k = a constant,
t = time,
n = an exponent lower than 1, often taken as 0.5

The rate constant, k, depends on a number of factors including:

• cement type and content;
• water:cement ratio;
• aggregate type;
• duration, relative humidity and temperature during a controlled curing period;
• degree of compaction;
• environmental conditions including relative humidity, temperature and the local concentration of carbon dioxide.

Generally, for average Portland cement concrete exposed externally, carbonation depths of between 3mm and 6mm would be expected at 5years of age, increasing to between 5mm and 8mm at 10years and between 10mm and 15mm at 50years. For the same concrete exposed internally values would be expected to be significantly higher due to drier exposure conditions and potentially higher concentrations of CO2 in the atmosphere, i.e. for average Portland cement concrete exposed externally for 5years, carbonation depths of between 3mm and 6mm were recorded, whereas for internally exposed specimens, corresponding depths increased to between 5mm and 12mm.

Under normal circumstances whilst the pH level of the concrete matrix around the steel remains high, corrosion of the reinforcement is unlikely. However, in a concrete containing excessive chloride, present either as an original mix constituent (e.g. calcium chloride added as an accelerator or salt contamination of the aggregates or the use of saline rather than fresh mixing water) or as a subsequent contaminant from an external source (e.g. de-icing salts or sea water, both via either airborne spray or direct contact) severe and localised corrosion of the steel can occur regardless of carbonation.

Chloride contamination has the added complication that provenance and cement type can both significantly effect the amount of chloride available for deleterious reaction with the steel. The chemical analysis generally carried out indicates total (acid soluble) chloride and cannot differentiate between ‘combined’ (present as an intrinsic matrix or aggregate constituent) or ‘free’ (freely available for deleterious reactions) chloride. For example, in the case of chloride present at mixing, whether by deliberate addition, saline mix water or contaminated aggregates, a proportion of the chloride could become combined within the hydrated cement phases and therefore not freely available for corrosion reactions, until the matrix becomes altered, e.g. through the processes of carbonation.

BRE Digest 444: Part 2: 2000 indicates a ‘Negligible’ risk of chloride induced corrosion, in dry uncarbonated concrete, where values for chloride ion by weight of cement are less than SAY 0.2% for ingressed chloride and less than SAY 0.4% for original contaminants present at the time of mixing. The risk category significantly worsens in the case of the latter where the concrete is damp (in the case of ingressed chloride the concrete is presumably damp, at least intermittently) and, for both cases where the carbonation front has encroached upon the reinforcement. Carbonation can both reduce the threshold level for corrosion initiation and increase the probability of corrosion for a particular chloride concentration by reducing the pH and the chloride binding capacity of the cement paste.

The assessment of chloride provenance can be aided by the preparation and analysis of incremental depth rather than bulk samples. Concrete samples (most commonly drilled dust samples) carefully prepared to include material from selected depths beneath an exposed surface (SAY for example A: 5mm to 25mm; B: 25mm to 50mm; C: 50mm to 75mm; etc.) can be analysed separately to identify any variations with depth. A consistent decrease in chloride with depth from the surfaces would be suggestive of chloride ingress from an external source after setting and hardening of the concrete. The analysis of samples from sheltered locations, away from any likely sources of external contamination (e.g. beneath asphalt toppings, above splash/spray zones and on leeward elevations) could indicate whether or not the concrete was likely to have contained any chloride at the time of mixing and casting. Incremental depth sampling would also enable a comparison between chloride contamination and the depth of cover to reinforcement, i.e. has chloride contamination from an external source extended inwards to the depth of reinforcement?

For the above reasons the concrete under investigation has been tested in-situ for depths of carbonation and depths of cover to the reinforcement together with laboratory analyses of samples for the contents of chloride.

In some cases, e.g. road, bridge and car park decks together with associated elements can be subjected to further testing as follows:

The corrosion of steel in concrete is an electrochemical process. The reinforcement generally exhibits cathodic (positive) and anodic (negative) areas, with the anodic portions potentially deleteriously corroding to cause the classic symptoms of surface delamination and spalling etc. In some instances the measurement of parameters including electrical potential (1/2 cell potential and corrosion rate measurements), electrical resistance and resistivity of the surface concrete can be used for the identification and evaluation of corrosion condition. Measurements taken at regular intervals, in a grid pattern across the surface of a concrete element can be used to identify relatively anodic and cathodic areas within the reinforcement and areas of concrete more, or less capable of acting as an electrolyte, which is linked to corrosion rate. Such measurements when plotted graphically, in the form of colour coded contour maps can be a particularly useful diagnostic tool. Selected areas can then be subjected to further investigation, for example exploratory cutting-out for direct reinforcement inspection. It should be noted, however, that these methods can be sensitive to various factors including temperature (both air and surface temperatures), concrete moisture contents and reinforcement electrical continuity. Measurements should collectively be used, together with other data to interpret potential corrosion condition only at the time of measurement. Measurements should ideally be taken at regular time intervals to assess potential corrosion condition at different times of the year when controlling parameters such as temperature and concrete moisture content will be different.

It is important always to approach any structure with an open mind. The concrete, together with any other associated materials have also, therefore, been closely inspected and the exposure conditions assessed in order to identify any distress not consistent with the above and, therefore, requiring further investigation and additional testing.

Further information, with particular reference to concrete, its durability, deterioration and assessment can be sourced within a large number of publications and documents, including those listed below.

General Discussion

Assuming that a thorough and appropriate survey has been carried out, and having diagnosed the cause/s of deterioration, BS EN 1504-9 gives guidance, as described below in the following Tables, on the “principles and methods for remediation” of both “defects in concrete” and “reinforcement corrosion”.

Principle	Principle Definition	Methods Based on the Principle
“Principles and Methods Related to Defects in Concrete”
Principle 1 (PI)	Protection against Ingress   Reducing or preventing the ingress of adverse agents, e.g. water, other liquids, vapour, gas, chemicals and biological agents.	1.1:  Impregnation Applying liquid products which penetrate the concrete and block the pore system.   Surface coating with and without crack bridging ability. 1.3  Locally bandaged cracks. 1.4  Filling cracks. 1.5  Transferring cracks into joints 1.6  Erecting external panels 1.7  Applying membranes
Principle 2 (MC)	Moisture Control   Adjusting and maintaining the moisture content in the concrete within a specified range of values.	2.1  Hydrophobic impregnation. 2.2  Surface coating. 2.3  Sheltering or overcladding. 2.4  Electrochemical treatment Applying a potential difference across parts of the concrete to assist or resist the passage of water through the concrete.  (Not for reinforced concrete without assessment of the risk of inducing corrosion).
Principle 3 (CR)	Concrete Restoration   Restoring the original concrete of an element of the structure to the originally specified shape and function.   Restoring the concrete structure by replacing part of it.	3.1  Applying mortar by hand. 3.2  Recasting with concrete. 3.3  Spraying concrete or mortar. 3.4  Replacing elements.
Principle 4 (SS)	Structural Strengthening   Increasing or restoring the structural load bearing capacity of an element of the concrete structure.	4.1 Adding or replacing embedded or external reinforcing steel bars. 4.2 Installing bonded rebars in preformed or drilled holes in the concrete. 4.3 Plate bonding. 4.4 Adding mortar or concrete. 4.5 Injecting cracks, voids or interstices. 4.6 Prestressing - (post-tensioning)
Principle 5 (PR)	Physical Resistance   Increasing resistance to physical or mechanical attack.	5.1 Overlays or coatings 5.2 Impregnation.
Principle 6  (RC)	Resistance to Chemicals   Increasing resistance of the concrete to surface deterioration’s by chemical attack.	6.1  Overlays or coatings 6.2  Impregnation.

Principle	Principle Definition	Methods Based on Principle
“Principles and Methods Related to Reinforcement Corrosion”
Principle 7 (RP)	Preserving or restoring passivity   Creating conditions in which the surface of the steel reinforcement can maintain or return to a passive condition	7.1:  Increasing cover to the reinforcement with additional cementitious mortar or concrete. 7.2:  Replacing chloride-contaminated or carbonated concrete 7.3  Electrochemical realkalisation of carbonated concrete 7.4:  Realkalisation of carbonated concrete by diffusion 7.5:  Electrochemical chloride extraction
Principle 8 (IR)	Increasing resistivity   Increasing the electrolytic resistivity of the concrete	8.1:  Limiting moisture content of the concrete by surface treatments, coatings or sheltering
Principle 9 (CC)	Cathodic control   Creating conditions in which potentially cathodic areas of reinforcement are unable to drive an anodic reaction	9.1:  Limiting oxygen content (at the cathode) by saturation of the concrete or surface coating
Principle 10 (CP)	Cathodic Protection   Polarising the steel reinforcement cathodically so as to reduce the rate of anodic reaction	10.1:  Impressed current systems 10.2:  Sacrificial anode systems
Principle 11 (CA)	Control of anodic area   Creating conditions in which potentially anodic areas of reinforcement are unable to take part in the corrosion reaction	11.1  Painting reinforcement with coatings containing active pigments 11.2:  Painting reinforcement with barrier coatings 11.3:  Applying anodic inhibitors to the concrete
NB:  Various methods included above may contain products and systems not covered by the 1504 series of European standards. Inclusion of methods in this table does not imply approval or confirmation of their effectiveness.

In our opinion, the successful repair and refurbishment of any structure should, subject to future design-life requirements ideally return the various concrete elements to a better-than-new condition; the “as-built” condition of any deteriorated and distressed structure, now proposed for refurbishment, was such that failure has occurred within it’s useful life.

In our opinion, a structure of this type, in this condition, could be repaired and refurbished, using the above principles and the ‘state-of-the-art’ technologies available today with the aim of providing an indefinite additional life-in-service.

The remedial strategy could range from a simple ‘make-safe’ (with or without holding repairs) strategy, to a high-Specification, ‘one-stop’ strategy, with an allowance for a limited number of maintenance re-visits, generally to SAY re-apply surface coatings.

The former would obviously suit a limited budget and / or where the future life of a structure was either limited or uncertain. Such a strategy would allow for the elevations to be ‘made-safe’ from the risk of falling debris with an option for simple ‘holding-repairs’, to extend the safe condition of the elevations for up to SAY 5 years. This safe condition could obviously be further extended, with periodic re-visits, assuming that the elements concerned were and remain structurally sound, until the structure is either re-developed, or refurbished.

The detailed design of an appropriate refurbishment strategy to satisfy the latter, using the available technologies can also be tailored to suit specified limits and / or requirements, for example, in terms of budget, longevity and appearance using a combination of one or more of the techniques discussed, in general terms below.

Conventional or Traditional Patch-Repair

Strategy
For conventional or traditional concrete patch-repair all of the defective concrete, defined as all carbonated and/or chloride contaminated concrete in contact with the steel, should be removed, the steel cleaned and treated, and the concrete then reinstated using proprietary concrete repair materials and good practice.

NB: If areas of steel were to be left encapsulated within deteriorated concrete, as defined by conventional concrete repair criterion, further deterioration could take place and subsequent distress could possibly occur within the designed life-to-first-maintenance.

This strategy would satisfy BS EN 1504: Part 9, Principle 7 (“Preserving or restoring passivity”) and in particular principle 7.2 (“Replacing chloride-contaminated or carbonated concrete”).

For chloride contamination, as indicated above, BRE Digest 444: Part 2 recognises values, by weight of cement, in excess of 0.2% for "ingressed" chloride and 0.4% for "cast-in" chloride as carrying an elevated risk of inducing reinforcement corrosion.

A conventional or traditional concrete patch-repair strategy, depending upon the prognosis discussed above and the level of Specification should last for between 5years and 15years. Further information concerning concrete repair can be sourced within various publications and documents, including those listed.

Placement / Installation of Materials

Repair materials can be formulated in a number of different ways so that their placement and installation can be tailored to suit particular site or structural conditions. Products are available for hand placement, spraying, or self-leveling / compacting, i.e. flowable mortars or concretes.

Note:
No matter what processes have been involved in the deterioration of the concrete the above conventional patch-repairs or reinstatement will have to be carried out at least to the areas of physically damaged, disrupted and / or delamination. The various methods discussed below address the areas where the reinforcement is encapsulated within currently ‘sound’, but carbonated / chloride contaminated concrete, without the need to remove this concrete. These methods, therefore, limit the quantity of relatively expensive, disruptive and time-consuming cutting-out and subsequent patch-repair needed to achieve the required / specified finished product.

Electrochemical Rehabilitation.

General
The corrosion of steel in concrete is an electrochemical process with anode and cathode reactions as illustrated below:

Figure i: Anodes and Cathodes

The anode reactions are as follows:

1. Fe > Fe2⁺ + 2e^-
2. Fe²⁺+ 2OH- > Fe(OH)₂
3. 4Fe(OH)₂ + o₂ + 2H₂O > 4Fe(OH)₃ > 2Fe₂O₃.H₂O + 4H₂O (RUST)

The cathode reaction is as follows:

1. ½O₂ + H₂O + 2e^- > 2OH^-

Reactions at the anode produce rust, which expands to produce the classical symptoms of surface spalling.

A patch-repair strategy involving only those areas of physically damaged, disrupted or delaminated concrete, only addresses the anodes, leaving the cathodes untreated (except for the effects of any subsequently applied coatings), although the concrete in these areas is potentially similarly deteriorated with respect to carbonation and/or chloride contamination. The reinforcement within a patch-repair will become a cathode with the surrounding, former cathodes becoming anodes, thus causing the onset of “incipient anode” corrosion surrounding the patch-repairs, as illustrated below.

Figure ii: Incipient Anodes

Electrochemical treatments artificially modify the polarity of an existing reinforcement system, with the steel maintained, at least for the period of the treatment, as a cathode.

 

Figure iii: Sacrificial Anodes

The use of sacrificial anodes, fixed with electrical continuity to the reinforcement, installed either within patch-repairs (1), and / or within areas of ‘sound’ but carbonated / chloride contaminated concrete (2), or fixed externally (3), can prevent, or at least minimize, the risk of incipient anode corrosion.

It should be noted that although the purchase costs for anodes type (1) and (2) may be similar, the installation costs will be significantly different. The former are installed into the repairs, with the bulk of the installation costs within those for the repair, but for the latter they have to be installed into separate, drilled holes, in some circumstances, i.e. where chloride levels are ‘Extremely High’, on an orthogonal grid pattern, with close centres.

This strategy would satisfy BS EN 1504: Part 9, Principle 10 (“Cathodic protection or prevention”) and in particular principle 10.2.

The life expectancy of sacrificial anodes is advised to be in the region of 10years to 15years, although it should be noted that the long term durability and effectiveness of this treatment, although expected to be good has not yet been proven. As a known technology for the protection of the hulls to steel ships, however, sacrificial anodes have been available for over 150 years and some permanent electrochemical installations or Cathodic protection systems have been designed to include sacrificial anodes, rather than externally applied paint or internally installed, discrete anode systems.

 

Figure iv: Permanent Impressed Current Cathodic Protection

Permanent impressed current installations or Cathodic protection (CP) systems are a well-proven technique for prevention of corrosion of metallic structures in aggressive environments. For reinforced concrete a permanent anode system is installed with a small current flow (10 to 20 mA/m2 ) used permanently to maintain the steel in a passive, cathodic state.

Various anode systems have been developed including surface applied conductive paint (1), activated titanium mesh within paint or cementitious overlays (2) and discrete titanium rods in drilled holes (3). This range of systems means that virtually any structure, or surface whether exposed or hidden can be protected using CP.

Monitoring and control can be achieved remotely by computer with the benefit that the corrosion-state is always under control.

This strategy would satisfy BS EN 1504: Part 9, Principle 10 (“Cathodic protection or prevention”) and in particular principle 10.1.

The condition of the structure after SAY 5 years would be significantly better than immediately following the repairs due to the additional beneficial effects of chloride removal and alkali evolution (re-alkalisation) within the concrete immediately surrounding the steel.

The life expectancy of a CP system would be 15 to 30 years with a minimum of maintenance, dependent on system components.

Further information can be obtained from BS EN 12696 and Concrete Society Technical Report No.73.

 

Figure v: Temporary Impressed Current Installations

Temporary impressed current electrochemical installations may be viewed as short term, high powered cathodic protection (CP), designed relatively rapidly to rehabilitate the cover concrete and the steel / concrete interface.

An anode system, usually consisting of an activated titanium mesh, or similar, installed onto the concrete surfaces and within a suitable electrolyte reservoir will be connected to the reinforcement. An electrical current of approximately ranging from 0.5A/m2 to 2A/m2 will then commonly be used to induce a migration into and out-of the electrolyte.

In the case of re-alkalisation, the migration of an alkali (usually sodium carbonate or potassium carbonate) into the concrete between the reinforcement and the surfaces will re-passivate steel encapsulated within carbonated concrete. An outward migration of free, unbound chloride contaminants, within the concrete between the reinforcement and the surfaces will also take place. This process is known as desalination or chloride extraction.

Renewal of alkalinity within the cover concrete can be achieved within 3 to 14 days with the removal of free, unbound chlorides within 1 to 3 months, depending upon the quality of the concrete and the extent of deterioration / contamination.

Following treatment, the anode system would be removed.

The suitability of a structure or element for these treatments will of course be dependent on a number of factors including size of sections, access to all deteriorated faces, degree and provenance of chloride contaminants and subsequent requirements for maintaining appearances.

These strategies would satisfy BS EN 1504: Part 9, Principle 7 (“Preserving or restoring passivity”) and in particular principles 7.3, 7.4 and 7.5.

The life expectancy of a temporary electrochemical treatment should be 10 to 15 years although it should be noted that the long term durability and effectiveness of these treatments, although expected to be good, has not yet been proven.

Further information concerning electrochemical treatments can be sourced within various publications and documents, including those listed.

Corrosion Inhibitors.

The prevention or limitation of corrosion of steel in concrete can be achieved by the use of corrosion inhibitors. Three generic types of corrosion inhibitors are available, namely calcium nitrite, sodium monofluorophosphate and amino alcohol.

These compounds, with pH levels of between 8 and 11 penetrate or migrate through the cover concrete, in either the liquid or vapour phases and are attracted towards embedded reinforcement where they form a protective film. The protective film limits anodic ionization at the steel surfaces and obstructs the available free oxygen, which prevents the cathodic part of the corrosion reaction. Potentially deleterious chloride ions can also be displaced from the steel surfaces.

Research and development of these methods of concrete protection and rehabilitation have been undertaken on the continent and in the United States of America for a number of years. The technology was originally developed for the protection of metals exposed to atmospheric corrosion and was first used in conjunction with reinforced concrete in the USA in the early 1980’s.

The technology was subsequently introduced into the UK, with various products including; liquid, powder or slurry admixtures for fresh concrete; surface applied aqueous impregnation’s, gel injection’s and powder filled capsules for existing concrete; additives for various repair grouts and mortars. Specific Vapour Corrosion Inhibitors are also available in various forms including impregnated insulation foam or as paint coatings for the protection of exposed steelwork.

As with the electrochemical techniques detailed above, the use of corrosion inhibitors requires that only the detectable damage needs to be repaired. Concrete, which is carbonated, and/or chloride contaminated but otherwise sound can, in most cases be left in-situ.

This strategy would satisfy BS EN 1504: Part 9, Principle 9 (“Cathodic control”, i.e. principle 9.2) and Principle 11 (“Control of anodic area”, i.e. principle 11.3).

Although the life expectancy of these treatments should be at least 5 to 10 years, some products and applications may, in some circumstances, require regular re-treatments. For example, although liquid products, applied by brush, roller or spray would generally only require a single application, some gel injections or powder filled capsules, injected/installed into pre-drilled, corked or capped holes, could require re-application or renewal at regular maintenance intervals. In some environments, e.g. where warm, humid and/or salty, such maintenance, at least initially, could be as regular as 6 monthly, whilst the inhibitors penetrate, with subsequent intervals perhaps on a 2 to 3 year cycle. However, protection would be provided as long as the maintenance programme continued and gel injections or powder filled capsules perhaps have the advantage of potentially protecting reinforcement beneath hidden surfaces.

Surface Protection Systems

Although coatings can be applied simply for decorative purposes, surface treatments (including coatings) in the context of the concrete repair and refurbishment Industry have generally been applied as the first line of defense in a protection system, i.e. the treatments have been applied primarily to cover and / or seal the surfaces to ensure that the concrete does not continue to deteriorate as a result of further exposure to the environment.

The application of such treatments would satisfy BS EN 1504: Part 9, Principle 1 (“Protection against ingress”, i.e. principles 1.1, 1.2 and 1.3), Principle 2 (“Moisture Control”, i.e. principles 2.1 and 2.2), Principle 5 (“Physical Resistance/Surface Improvement, i.e. Principles 5.1 and 5.2), Principle 6 (“Resistance to Chemicals”, i.e. Principle 6.1) and Principle 8 (“Increasing Resistivity”, i.e. Principles 8.1 and 8.2).

Three main types of surface treatment are available:

Pore-liners. Hydrophobic impregnation treatments which line the pores and repel water, whilst allowing the concrete to ‘breath’.Pore-blockers. Impregnation materials applied partially or fully to fill the pores and seal the surfaces.Coatings and coating systems. Materials comprising cementitious pore fillers or renders, thin barrier coatings or breathable coatings.

Different types of coatings will be more or less appropriate to a specific application depending upon the environmental conditions prevailing and the requirements for the finished ‘product’.

A conventional concrete repairs strategy would normally require the use of a proprietary anti-carbonation coatings system to minimise further deterioration through carbonation. The coatings are formulated to allow the passage of water vapour, but to prevent the ingress of carbon dioxide and other deleterious substances such as chloride salts. These coatings in some cases may also produce a natural re-alkalising affect and should also allow the concrete to dry; perhaps modifying the potential, in the long-term, for further corrosion in the presence of chloride.

Following removal of the anode system installed as a part of a re-alkalisation or de-salination strategy surface coatings would normally be required to prevent further ingress of aggressive chemicals or leaching of alkalis which could re-activate corrosion. In this case the coating system would probably be similar to that used following a conventional concrete repairs strategy.

No additional surface coatings would be required after the installation of a CP system to limit further ingress of aggressive chemicals. However, some CP systems use anode components incorporated within coatings.

In some cases, the surfaces following repairs may not be suitable for the application of coatings. For example, rough surfaces or excessively voided surfaces may require pore-filling first, to prevent ‘pin-holing’. Rougher surfaces may require the application of thin, high-performance renders to produce the required surface for coating. These applications may also have a decorative effect, in terms of hiding or masking repairs.

As the first line of defence, the coatings system obviously bears the brunt of the various environmental factors which were probably a significant contribution to the deterioration and resultant distress which lead to the repair and refurbishment of the structure in the first place. The coatings will, therefore, be subjected to wear and tear and will require periodic maintenance.

The over-coating of existing coatings should always be carefully considered, with respect to adhesion and compatibility, new to old, together with the costs, disruption and hazards associated with the removal of the existing coatings. The adhesion of the existing coatings to the substrate, and that of new to old materials can be assessed using a programme of tensile bond, or pull-off testing. The materials manufacturer, selected to supply the new coatings should be able to advise on compatibility issues and the availability of ‘warranties’ for the use of their coatings in an over-coating scenario.

NB: Older coatings could contain potentially hazardous materials such as lead and / or asbestos and should always, consequently, be treated with care.

Further information concerning surface treatments can be sourced within various publications and documents, including those listed.

Structural Strengthening

In cases where the structural integrity of an element or structure has been called into question it may be cost effective to augment existing by installing additional reinforcement, perhaps using stainless. As an alternative, however, steel plate bonding or carbon-fibre could be used as external reinforcement.

 

The installation of additional, or replacement reinforcement would generally be most cost-effective within the cut-outs for concrete patch repairs, or where extensive cutting-out had taken place, i.e. where specific cutting-out would not be necessary.

The use of steel plate bonding or carbon fibre external reinforcement would generally be more cost-effective where elements were not significantly distressed. The former requires both industrial adhesives and the installation of ‘peel-off’ bolting whereas the latter would generally only require industrial adhesives. Carbon-fibre is also more flexible, available as either rigid plates or bandages (the latter allowing for the wrapping of elements), together with perhaps significant weight and space savings coupled with the benefit of generally easier and quicker installation.

Further information concerning structural strengthening can be sourced within various publications and documents, including those listed.

Over-Cladding / Curtain Walling Etc.

In some circumstances, e.g. a major internal and external refurbishment it may be appropriate or cost effective for the exposed and distressed concrete surfaces to be over clad. In this case, concrete repairs could be limited to a simple make-safe strategy, as discussed above. The over-cladding would essentially seal-in the concrete allowing it to dry-out and thus remove one of the key factors i.e. moisture in the corrosion reaction.

The concrete would continue to deteriorate in terms of carbonation and would remain potentially at risk from any chloride contaminants, although presumably sheltered from any further ingress from an external source/s. The absence of moisture, including condensation would prevent, or at least minimise future corrosion.

In addition to the above, over-cladding could have the advantage that the external appearance of a structure could wholly be updated / modernised, together with improvements in terms of insulation and the internal use of available space.

Potentially, however, over extended periods, a failure or lack of maintenance to weatherproofing details, together with the potentially deleterious effects of condensation upon already deteriorated concrete could give rise to the further deterioration and the development of distress which would be hidden from view.

Replacement of Elements
In some circumstances concrete elements may be found to be beyond economical repair in terms of the levels of distress and / or when weighed against the potential benefits of replacement using modern, alternative’s. In other cases, particularly in older structures where, for example, elements such as precast parapets in a car park have a direct or even in-direct health and safety contribution certain elements and their fixing details may be considered to be either unsatisfactory or not up to modern standards.

In these cases, it may be possible, or better, to re-cast elements using concrete, have replacement precast concrete units manufactured to match existing, have replacement plastic or glass-fibre units manufactured to match existing or to install steelwork.

Relative cost effectiveness would be dependant obviously upon the number of units concerned, their location and interrelationship with neighbouring units, together with any structural requirements (potentially temporarily overcome by propping) and health and safety issues.

Important Note

All materials employed in any refurbishment, regardless of detailed strategy should be of appropriate quality and should generally comprise tried and tested proprietary systems, manufactured under BBA or equivalent accreditation and installed by reputable Contractors covered by ISO 9002 (formerly BS5750) accreditation.