Multi-Storey Car Parks

Multi�Storey Car Parks – Investigation, Repair & Maintenance

Aston University – June 2009

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The commonest construction material for multi�storey car parks in the UK is steel reinforced

concrete. Durability problems associated with reinforced concrete are typically the result of either

loss of alkalinity due to carbonation of the cover or the ingress of significant levels of chloride

ions from some external source. As reinforced concrete multi�storey car parks spend much of

their lives exposed to the elements in addition to wet and salty motor cars, it is little wonder that

they suffer so many of these problems so visibly.

By employing a combination of experience, good engineering sense and the findings of other

workers as reported in the literature, it is usually possible to identify the areas in a reinforced

concrete structure where destructive re�bar corrosion is most likely to be occurring.

Where the primary cause of corrosion is the ingress of chloride ions then clearly any detail where

salt�laden water can gather and concentrate is at greater risk. Little can be achieved without

actually examining the structure and performing simple tests to target the risk areas and select

appropriate techniques for further examination. This is not possible without a full understanding

of the mechanisms of carbonation and chloride ion ingress.

Similarly, those areas under the greatest threat of loss of structural integrity can usually be

identified in advance so allowing any investigation to be correctly targeted. Where both the risk

of corrosion and the loss of structural integrity coincide, the potential for serious failure is so high

that the prospective investigator must ensure the problem can be adequately identified and

quantified.

Having identified what is corroding, why it is corroding and how fast it is corroding it is then

necessary to decide what, if anything, can be done to stop the corrosion and make�good the

damage.



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When selecting a method of repair it is essential to know whether the 'owner' expects the

renovated structure to be:

� better than new;

� as good as new;

� not quite as good as new but better than it was;

� prevented from going worse, or

� made to degrade more slowly.

All too often the last option is specified and the first option expected. All these approaches are

valid in the correct context and a large part of the skill in concrete repair is in correctly identifying

and matching methods and service requirements, usually within the confines of highly restricted

budgets.

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Multi�story car parks can be split into two basic types. Those that are “add�ons”, and those that

are income generating. If a major shopping centre is being constructed, it will require parking for

shoppers. The parking is an “add on”, in that it is not the main purpose of the development. It is

likely to be built under a design and build package, sub contracted and made up of standard

details. Aesthetic considerations are important, as the tenants of the development are likely to

demand a high specification finish. Because aesthetics are important the car park is likely to be

maintained to a reasonable standard.

If a car park is being built to generate income, it will be close to an attraction, such as an airport

or city centre. Again it is likely to be constructed under a design and build contract, but the basic

requirement will be to get the car park constructed and open as rapidly as possible to generate

income. Aesthetics are only important if there is competition from other parking structures.

Because aesthetics are of lower importance, and maintenance costs money, these are less likely

to be maintained. Construction of new income generating car parks is less common than

construction of new add�ons.

The basic aim of any car park, is to provide a safe and secure place to park cars. As such the

main requirement is to provide as much column free space as possible, with as low head room



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as possible. This leads to large spans and lightweight decks. The design and build approach

also produces standardized elements. This combined with a need for rapid construction tends to

favour simply supported precast elements that can be lifted into place with a minimum of fuss.

Typically to provide a running surface, with appropriate falls an insitu concrete topping is applied,

which can provide additional structural capacity by increasing the overall depth of the elements,

and by acting compositely with beams.

As insitu concrete is required for the surfacing a natural alternative would be to construct the car

park entirely out of insitu reinforced concrete. One common approach is to use waffle slabs.

These have significant weight savings compared with insitu flat slabs. Lift slab construction is

also an alternative. Columns are cast in place, and floors are cast at ground level and winched

up the columns into their final resting place. These are then pinned into place with simple

connections.

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The boom in car park construction coincided with the boom in concrete construction in general,

i.e. the 1960’s. A typical design life for a building structure is 50 years, hence increasing

numbers are reaching the end of their theoretical design life. In the interim period better

understanding of structural issues such as punching shear and robustness have come about,

and indeed corrosion of steel in concrete has only been studied since the early 1970’s. Due to

the fact they are lightweight and the design is typically standardised, rather than performed in

detail for each structure, car parks tend to be lively and so structural movement, if it has been

considered at all, does not tend to follow standard prescribed patterns. This produces a large

number of lightweight structures nearing the end of their design life, with oversimplified structural

designs, and limited consideration of corrosion, that are likely to be cracked and leaking.

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Historical car parks are likely to have been constructed to designs that would be considered to

be sub standard by current codes. A good example of this is parapets. Originally they may have

been intended as edge protection for pedestrians, or simply to provide an aesthetic improvement

to the exterior of the car park. Today’s codes expect them to withstand vehicle impact. Similarly

the requirement to include nominal shear links into beams only came into being in 1967, and

robustness only became a structural concern after the Ronan Point incident where a gas



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explosion caused progressive collapse of a corner of a block of flats. Even standards from as

late as 1985, are reported thus:

“The ‘generally applicable’ safeguards against progressive collapse in BS8110 are not sufficient

for flat slabs and can result in an increased risk of an initial punching failure dragging down more

of a flat slab structure.”

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In addition to the structural concerns the materials used were often not of highest quality. The in�

situ concrete used for toppings was often a low cement mix with a relatively high water cement

ratio to enable easy placing, and was likely to have been poorly cured, this results in a large

number of car parks suffering from plastic cracking occurring within weeks of casting. Numerous

examples also exist of inadequate compaction resulting in honeycombing, and poorly placed

reinforcement giving low cover.

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Most car parks are likely to have been constructed with some form of waterproofing on the top

deck, typically asphalt. This will degrade with time, as a result of UV exposure, traffic and

general weathering. No waterproofing material will last forever.

The joints are unlikely to have been formally designed, and even if this is not the case the

movement probably will not represent that in the design calculations. Typical joint sealants are

hot poured bitumen onto an appropriate backing material (including, historically, asbestos rope).

These are unlikely to provide any significant length of service. Even properly designed joints

which are inadequately maintained will pick up dirt and debris which will result in failures.

Bearings for simply supported beams will often become fixed after a significant period of time,

such that they no longer allow the required movement. A common problem with one type of pre�

cast car park is that the beams continue to move and the bearings transfer this movement into

the corbels. This results in the corbel supporting the beam becoming damaged.

Drainage on car parks is often poorly detailed and maintained, for example, inadequate falls to

the outlets. This results in ponding, which in turn is often addressed by coring through the deck

to allow discharge onto lower levels without consideration of the structural consequences. In



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some cases, to maximise the clear space available the drainage runs through the middle of

columns. Once again inadequate maintenance leads to these becoming blocked.

The basic problem with water in a car park is that during the winter months it is contaminated

with deicing salts. These percolate into the concrete and cause corrosion of the reinforcement.

Standing water, failed joints blocked drainage and any cracking all exacerbate this problem, but

even without these the chlorides will drip off cars and penetrate the concrete.

Steel in concrete normally does not corrode due to the high levels of alkalinity present, however

if the level of alkalinity drops or chlorides get into the concrete, corrosion can occur. Once

chlorides reach the level of reinforcement in sufficient quantities corrosion will initiate.

Additionally, calcium chloride may have been deliberately cast into concrete as a set accelerator

and this was only discontinued in 1977 once the link with corrosion had been identified.

The other cause of corrosion of steel in concrete is due to carbonation, where atmospheric

carbon dioxide penetrates into the concrete. Once this reaches the depth of the reinforcement

corrosion can commence. If calcium chloride is present in the mix, carbonation can release the

chemically bound chlorides so that they can participate in the corrosion process.

In addition to the loss of structural reinforcement due to corrosion, the volume of rust is

significantly bigger than the steel it replaces. As a result concrete delaminates, resulting in loss

of concrete section, and a risk of falling debris. Where concrete in the deck delaminates it

produces tripping hazards.

One further cause of distress that affects users of the car park is as a result of water passing

through concrete. As water percolates through cracks and defects in concrete calcium hydroxide

in the concrete dissolves into the water. As the alkaline water passes out of the concrete it

carbonates, and calcium carbonate precipitates as a calcite deposit. This can result in the

formation of stalactites and stalagmites which are unsightly. It also results in highly alkaline

water dripping onto cars, which can result in degradation to the paintwork.

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As stated above, drainage can often be rectified by coring through the deck in areas where

standing water is a problem. This is often done without due regard to the structural

consequences of cutting reinforcement. A similar approach is often taken with services, such as

lighting and CCTV, where cabling needs to be run across the car park, or where induction loops



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are fitted to detect traffic in the car park. These can involve cutting 50mm deep slots in the deck

and ramps. If the car park serves another development, this can encroach into the car park to

provide plant rooms and offices, both of which affect the loading and thermal gradients within the

structure.

Finally as problems are identified with structural elements such as parapets, repairs and

upgrades can be attempted which are not thoroughly considered. It is all very well to bolt barrier

upgrades to the deck, but this may transfer the failure mode from a parapet being pushed off on

impact to significant damage to the deck or support structure.

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The environment provided by good quality concrete for the embedded steel reinforcement is one

of high alkalinity (generally >pH 13), produced by the hydroxides of sodium, potassium and

calcium released during the various hydration reactions. In addition, the bulk of surrounding

concrete acts as a physical barrier to most of the substances that may lead to degradation of the

reinforcement.

Provided this environment is maintained, the steel remains passive and any small breaks in the

stable protective oxide film are soon repaired. However as previously discussed if the alkalinity

of the surroundings is reduced, for example by reaction with atmospheric carbon dioxide

(carbonation), or if depassivating chloride ions are made available at the surface of the steel

then corrosion may be initiated, resulting in loss of steel section and spalling of cover concrete.

It is also possible to lose the passive oxide film in conditions of low oxygen availability, such as

may be encountered in buried or submerged structures, although rates of metal loss are

negligible.

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Inadequate cover is invariably associated with areas of high corrosion risk due to both

carbonation and chloride ingress. By surveying the surface of a structure with an

electromagnetic covermeter and producing a cover contour plot, the high�risk areas can be

easily identified. A cover survey of newly completed structures would rapidly identify likely

problem areas and permit additional protective measures to be taken.

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While this remains an ill�defined area, two forms of crack are of interest when evaluating the

condition of a reinforced concrete structure; those present before the onset of corrosion which

might assist the corrosion processes (large shrinkage and movement cracks), and those

produced as a direct consequence of corrosion (expansive corrosion products leading to

cracking and spalling).

It should be remembered that concrete is intrinsically a cracked material and only those cracks

above a critical width which intersect the steel are liable to assist the corrosion processes.

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As previously discussed, chloride ions can enter concrete in two ways, i) they may be added

during mixing either deliberately as an admixture or as a contaminant in the original constituents,

or ii) they may enter the set concrete from an external source such sea water.

Once chloride ions have reached the reinforcement in sufficient quantities they will depassivate

the embedded steel by breaking down the protective oxide layer normally maintained by the

alkaline environment. The concentration of chloride ions required to initiate and maintain

corrosion is dependant upon the alkalinity and it has been shown that there is an almost linear

relationship between hydroxyl ion concentration and the respective threshold level of free

chloride.

In practice the evaluation of free (unbound) chloride and hydroxyl ion levels is impractical on a

regular basis. It is more usual to employ total chloride ion levels to evaluate the likely corrosion

state of the steel with the assistance of empirically determined relationships obtained from

similar structures. Samples are easily obtained by drilling holes and collecting the dust but there

is generally a spatial difference between the site where the dust sample was collected and the

position where a half�cell measurement was taken or a corroded bar exposed.

This partly explains the problems that have been encountered when trying to identify simple

relationships between chloride levels, half�cell potentials and reinforcement corrosion.

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Carbon dioxide present in the atmosphere combines with moisture in the concrete to form

carbonic acid. This then reacts with the cement hydration products resulting in a reduction in the



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alkalinity of the concrete. The rate at which this neutralisation occurs is influenced by factors

such as moisture levels and concrete quality. The depth of carbonation in a structure can be

quite easily established by the use of phenolphthalein indicator on freshly exposed material.

The distinctive colour change, from deep pink in unaffected concrete to clear in the carbonated

region, is sufficiently accurate for most practical purposes provided a number of measurements

are obtained to allow for local variations.

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The micro�climate to which the reinforced concrete member is exposed directly affects the

likelihood and extent of reinforcement corrosion. Factors such as chloride ion levels and pH have

already been discussed but the most important aspect of the local environment is the moisture

level. Carbonation, chloride ion ingress, resistivity and corrosion rate are all greatly influenced by

the degree of saturation.

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Chloride induced corrosion is a distinct function of temperature, humidity and cycles of wetting

and drying. Where chloride ions have been present in the concrete from the time of mixing, the

time to initiation may be zero. The main purpose of monitoring is to try and identify corrosion

sites that have not yet reached the point where cracking and spalling have occurred. By

combining the information obtained from the many methods available, it is possible to isolate the

high�risk regions with a good degree of confidence, e.g.

High chloride + large negative half�cell potentials + low cover + low resistivity = high corrosion

risk.

Greater accuracy is achieved by calibrating the structure through the use of water jetting as an

exploratory tool in areas which have shown both low and high risks of corrosion as a result of

testing. Where extreme conditions of very low or very high corrosion risk have been identified

there is probably no need for actual corrosion rates to be determined.

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The site detection of re�bar corrosion by direct means is completely dominated by the half�cell

techniques where potentials are measured between the steel and a standard reference electrode



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and then related to the likely corrosion state of the re�bar. While the technique has remained

largely unchanged, the equipment has developed significantly.

The traditional and simply constructed copper/copper sulphate electrode with its leaky and

reactive electrolyte is gradually being replaced by commercially produced silver/silver chloride

types. Data loggers are replacing note pads and the electrodes are being mounted in arrays and

wheels to permit rapid surveying of large areas.

The apparent simplicity of the half�cell survey technique can lead to a great many problems.

Widely spaced readings, dry concrete, surface laitance and inadequate or poorly maintained

equipment have all been known to produce meaningless or misleading results. Furthermore, any

metal/electrolyte junction in contact with the measuring circuit, such as a wet galvanised clip or a

silver bracelet on a damp arm, can impose an additional potential leading to much confusion.

A significant development of the standard half�cell survey is the down�hole or in�depth technique

which makes use of chloride sampling holes to produce a three�dimensional or cross�sectional

map. In this way, the half�cell survey is not limited to the layer of reinforcement nearest the

surface and the condition of otherwise inaccessible re�bars can be evaluated.

While the application of half�cell techniques becomes easier through a mixture of technology and

ingenuity, the interpretation of the data produced remains complex and repetitious. Less reliance

is being placed upon the ASTM guidelines which are based on American experience and may

not always be relevant under different conditions of temperature and humidity. While such basic

risk bands are a useful starting point, they are not a substitute for a full and proper

understanding of the principals of the corrosion of steel in concrete.

Any use of alternatives to conventional plain steel reinforcement, for example epoxy coated,

galvanized or stainless steel rebar, requires more care to be applied when carrying out and

interpreting half�cell potential surveys and there is a clear need for guidance in the selection of

inspection techniques for such structures.

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There is a degree of interest in obtaining resistivity measurements to assist in the interpretation

of the half�cell survey. Very simply, the higher the resistance of the concrete, the smaller the

magnitude of corrosion current for a given potential difference.



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Resistivity can be measured in�situ using the Wenner four�probe technique, provided care is

taken to avoid highly resistive surface layers and the close proximity of adjacent re�bars, both of

which may disrupt the measurements.

For an area where half�cell measurements indicate corrosion activity, a resistivity of over 12

kN/cm would suggest little or no corrosion to be occurring while a resistivity of less than 5 kN/cm

is consistent with a high corrosion risk.

As an alternative to the Wenner method, resistivities can be obtained from concrete cores. The

advantages of this technique are that it is less prone to measurement errors and readings can be

obtained for the cores in the as�received, saturated and air�dry condition so that the sensitivity to

moisture content may be evaluated.

Such information may be of greater value when attempting to predict the long�term performance

of a structure and when developing methods of modelling the corrosion processes in reinforced

concrete. Direct measurement of the circuit resistivity between the reference electrode and the

reinforcement can be helpful both in terms of corrosion modelling and in ensuring the validity of

the readings, particularly in areas of dry, highly resistive concrete.

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While the site measurement of corrosion rates may seem an important step in evaluating the

condition of reinforced structures, it may be of limited value unless directly associated with a

specific area, in known conditions and over a certain timescale. The true requirement is for an

indication of the loss of reinforcement cross�section with its direct consequences on the integrity

of the structure.

A range of laboratory�developed systems exist from very simple resistance techniques to

elaborate embeddable probes containing alkali�based reference electrodes capable of

measuring potentials, currents and pH. While an entire structure could never be effectively

monitored, a limited number of probes can be positioned in high�risk areas. The major technical

difficulty with such systems is ensuring that they are providing data that relates to the structure

and not just to the measuring device.

A number of laboratory�developed electrochemical methods have been adapted for use on site.

Such techniques include linear polarisation, electrochemical noise, AC impedance, harmonic

analysis and galvanostatic pulse analysis. While all these techniques perform well in the



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laboratory environment, it is still to be proven conclusively that they can be as reliable in the

more complex site environment.

There is a continuing interest regarding methods of modelling and calculating corrosion rates

from existing data such as half�cell potentials and resistivity, using a range of techniques from

simple two�dimensional calculations to complex three�dimensional finite element analysis. By

applying this approach to in�depth half�cell data it is possible to obtain corrosion rate information

for any layer of reinforcement.

True non�destructive techniques that have been evaluated for their ability to determine re�bar

cross�section include radar, ultrasonics, radiography, eddy currents and magnetic induction.

Such techniques were originally developed for use with homogeneous materials and generally

provide poor resolution when applied to the heterogeneous structure of concrete. If the

sensitivity of these methods can be improved, for example through the use of digital filtering and

enhancement, then they may develop into important site evaluation techniques.

Concrete Society Technical Report TR 60 gives details of many of these tests.

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Most current guidance recommends that car park structures should be inspected regularly. The

ICE recommends the production of “Life Care Plans”, the ODPM work recommends similar

routine inspections benchmarked against current condition and risk, the IStructE guidance

recommends that the handover package for new structures contains guidance on inspection

regimes. This would be expected under CDM regulations, and is what should be happening on

lifts, fire systems, lighting and electrical systems anyway.

In order to perform inspections there are a number of risks that need to be considered. As noted

previously, deterioration of concrete is governed by the ingress of aggressive agents such as

chlorides and carbon dioxide. This is likely to occur in the vicinity of drainage, in areas where salt

is tracked into the structure, or in areas where cracks or joints have allowed the aggressive

agent into the concrete. The areas of maximum structural stress are governed by the loading

patterns in the car parks. Fortunately the deterioration is not directly linked to the load and so

sudden structural failure is rare.



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There will be areas that are all but un�inspectable. In these locations it will be possible to

quantify the structural risk but it will not be possible to accurately quantify the deterioration risk,

in anything other than general terms regarding visible cracking and staining.

There will be many areas where the consequences of deterioration are structurally insignificant,

but from a health and safety point of view they may result in physical injury. A shallow spall in the

deck may have almost no structural impact, but will result in a trip hazard. Delaminating concrete

on the soffit would almost certainly be considered unacceptable but the probability of an isolated

area falling off at the same time as some unfortunate individual was passing underneath is

extremely small. Regardless of this both of these issues would still be considered by many as an

unacceptable risk.

Given the above, it becomes clear that it is in everyone’s interest to carry out regular inspections

of car parks.

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If no previous formalised inspection routine exists, it will be necessary to carry out a baseline

inspection. This will involve a review of all (any) previous inspection data, as�builts, and details of

modifications that have been undertaken. It is likely that significant gaps will exist in this

information, and these may be supplemented with hear say.

After reviewing the information available, it will be necessary to get appropriate personnel to

inspect the structure. There are many engineers who know much about structural inspections,

and there are many that know about deterioration in concrete. To inspect a car park you need

both these skills (or two engineers, who cover it).

The inspection itself should look thoroughly at the car park. This should include looking for

defects in drainage, waterproofing and surfacing, evidence of deterioration of the concrete and

reinforcement, structural cracking and vulnerable areas, evidence of previous repairs, and

identifying areas that cannot be inspected simply. Issues such as lighting, parapets, pedestrian

separation and falls from height should also be considered.

Where necessary the output of this inspection may recommend additional testing to further

quantify the condition of the car park, including dust sampling for chlorides, carbonation testing,

half�cell testing, or even coring for strength or petrographic analysis.



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Once a baseline has been established less rigorous inspections can then be carried out. There

is no requirement for specialist engineering input to identify when drainage is blocked, or there is

standing water, and both these items may ultimately result in degradation of the concrete if not

addressed. Similarly a simplified routine inspection may be able to identify spalls in the concrete

which are indicative of greater problems.

The National Steering Committee on the Inspection Of Multi�Storey Car Parks has produced

guidance on inspections.

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Three questions need to be considered before structure management and the appropriateness

of remedial actions can be finalised:

1. What is the cause of the deterioration?

2. Can steps be taken to slow or stop the process?

3. What are the structural consequences of the existing and projected damage?

Much research and field trials have been directed at diagnostic techniques as illustrated earlier.

There are a wealth of proprietary repair materials and techniques such as cathodic protection

systems and realkalisation of concrete which are used to arrest the corrosion process, as

discussed later. However, very little research has been directed at the problem of strength

assessment of deteriorating structures.

This is reflected in the range of assessment codes which are available to practising engineers in

whom quantitative methods of predicting the loading effects give way to qualitative assessment

of the structural effects of deterioration.

Structural effects of corrosion generally manifest as loss of steel section or loss of bond between

reinforcement and concrete. The significance of such damage will be discussed in relation to

reinforced and post�tensioned concrete structures.

Assessment of the strength of reinforced concrete flexural members suffering corrosion to the

tension reinforcement is a common occurrence which is made difficult by the absence of strain

compatibility at the reinforcement level.



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Loss of strain compatibility arises because of a breakdown in bond between the steel and

concrete upon which the ultimate moment of resistance is dependent at all sections along the

member.

It is important to note that debonding may occur well in advance of significant loss of

reinforcement section and therefore incipient spalling presents the first contribution to a reduced

factor of safety against collapse.

Shear failures are generally brittle and can be triggered with little visual warning. Codified rules

for modelling shear action, even without deterioration, have no rational basis. Deterioration,

which is common in zones of high shear, i.e. the ends of decks adjacent to leaking expansion

joints, further complicates structural assessment. In trying to quantify the effects of corrosion

damage in zones of high shear it is often necessary to separate the different contributions of

reinforcement and concrete to shear resistance.

In many cases it is the loss of cover which has the most significant reduction on shear capacity

by eliminating dowel action. Increased risk of anchorage failure caused by loss of bond also

needs to be checked as this can be the trigger for collapse.

Assessment of pre�stressed concrete members with a lack of drawings and stressing records

presents a serious problem in establishing the level of pre�stress and hence load carrying

capacity. Combine this with concern over durability of the tendons in ducts that have not been

properly grouted and the paramount need for safety can result in decisions to strengthen or

replace being taken as a matter of prudence.

The structural consequences of deteriorating concrete and corroding reinforcement or tendons

are potentially disastrous if they are allowed to proceed unchecked.

By the careful application of an appropriately selected suite of these test procedures it is

possible to build up a detailed picture of the condition of even the most inaccessible reinforced

concrete elements and hence their structural integrity.

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The first and most regularly employed concrete repair option is to do nothing at all.



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Provided the structure and those who use it are not put at risk then many reinforced concrete

elements can be allowed to quietly degrade and disintegrate (hopefully subject to some form of

monitoring or periodic inspection), until their eventual replacement.

True repair starts with patching � the removal of cracked, delaminated and/or contaminated

concrete and reinstatement with a (usually) cementitious material. In their simplest form, patch

repairs may be little more than aesthetic exercises.

Successful repair of carbonated or chloride�laden concrete requires the complete removal of

effected material adjacent to reinforcement. Chlorides are particularly difficult to deal with as

corrosion tends to occur in the areas of maximum chloride concentration which in turn protect

adjacent areas.

Repairs to areas of corrosion and delamination often result in subsequent corrosion to the

adjacent, previously protected areas � the so�called incipient anode effect. Patch repairing

chloride contaminated concrete can therefore become an expensive option, with large volumes

of sound but chloride contaminated material having to be removed with possible structural

implications.

The limitations of patch repairs, particularly with respect to chloride attack, have been important

in driving forward the development of alternative approaches to repair. A number of these

approaches are discussed below.

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Cathodic protection (C.P.) is a means of protecting steel from aggressive chloride�laden

environments. It is also less commonly used for carbonated concrete. The steel is maintained as

the cathode in an electrical circuit driven by either an impressed current or a galvanic (sacrificial)

anode. Cathodic protection systems must be carefully designed and account must be taken of

many different factors such as the aggressiveness of the environment; the area of steel to be

protected; the resistivity of the surrounding material; the positioning of any external metallic

objects which could be affected by the system; the type of anode used etc.

The initial design requirements and the application of a current throughout the service life of the

structure being protected can make cathodic protection expensive and complex in comparison to

conventional repair systems. A careful evaluation of cathodic protection systems relative to other



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repair options should be made in order to ascertain whether cathodic protection is the best route

to follow.

In addition to using cathodic protection as part of a repair strategy for corrosion damaged

concrete, the technique is also being developed for the protection of new structures, including

post�tensioned members. Because passive steel requires less polarisation to achieve protection,

stressed tendons can be included without the usual concerns of hydrogen embrittlement.

Continuing developments in the method of application and available anode systems, such as

sprayed conductive overlays, promise to extend the use of this technique in both new and

existing structures.

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Corrosion inhibitors are widely used in the protection of metals. There has long been interest in

their potential use for the protection of concrete reinforcement, generally as admixtures of

calcium nitrite.

A more recent development has been in the availability of inhibitor systems that can be applied

to the surface of reinforced concrete or injected into the body of the concrete and then migrate to

and protect the steel.

These materials are typically based on amino�alcohols and are capable of migrating through

concrete to form a film covering the surface of the reinforcement and thereby protect it. There is

particular interest in their use with pre�stressed and AAR�susceptible structures where the use of

cathodic protection may be undesirable. Recent formulations employing a silane backbone

appear to offer service lives of at least ten years when appropriately employed.

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Coatings can be used to protect concrete substrates in a number of ways. They can form an

impermeable barrier between the environment and the substrate and in this way afford almost

complete protection, assuming the coating is continuous. Coatings can also protect the substrate

by slowing down the rate of penetration of aggressive components from the environment into the

substrate, i.e. they provide partial protection, which may be adequate to allow the structure to

fulfil its design life.



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There are at present a large number of mainly organic coatings available to protect structural

materials such as reinforced concrete from many different service environments. In addition to

epoxies, polyurethanes and bituminous systems there is now an increasing use of penetrating

water repellent pore liners. Water repellents in current use are generally based on silanes and

siloxanes and guidelines have been produced for their application to U.K. highway structures.

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Water repellents based on organic silicon compounds offer a convenient route to enhancing the

durability of new and existing reinforced concrete structures, particularly where exposed to

chloride environments.

Alkylalkoxysiloxanes or siloxanes are produced when silane is reacted with water. Oligomerous

(less than 4 groups) and polymeric (greater than 4 groups) forms of siloxane can be produced,

but the large size of the polymeric form makes it difficult to apply and prone to remain tacky and

attract dirt.

Oligomerous siloxanes retain most of the advantages of silanes with regard to penetration and

moisture tolerance. They have the added advantage of a low vapour pressure under normal

conditions of application which results in far lower evaporation losses compared with silanes.

While conventional siloxanes still require an alkaline environment for full reaction to occur,

modified materials are now available with a suitable catalyst already added. Such materials will

react even in neutral materials provided moisture is present.

Multi-Storey Car Parks

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