The Repair of Reinforced Concrete
developments in the repair of reinforced concrete include modern electrochemical
techniques that can minimise the interference with the structure,
an important factor in building restoration.
Hoover Building, Perivale, London (built 1932-35): a spectacular
example of reinforced concrete construction, now occupied
generally think of concrete as a modern building material, yet it
is one of the oldest and most durable building materials. Its earliest
known use was for a hut floor in former Yugoslavia, dating from 5600BC:
later, more notable examples included the Great Pyramid at Giza and
the Parthenon in Rome.
the Romans experimented with bronze reinforcement, reinforced concrete
as we know it today dates from the mid 19th century following the
introduction of Portland cement concrete in 1854 when it was patented
by Joseph Aspeden in Wakefield. Steel reinforced boats and plant tubs
were made in the 1850s, and a patent was taken out in 1854 by William
Wilkinson for a method of building fireproof buildings using strips
of iron embedded in mass concrete. Wilkinson showed that he understood
where tension steel was needed in his flat ceilings, where wire ropes
were embedded following the line of tension in the upper parts of
beams over supports and in the lower parts in the mid-span.
has the advantage of having the tensile strength that concrete lacks,
and is highly compatible in its chemical and physical characteristics
as we will see later. The matching of thermal expansion coefficients
is critical to the versatility of reinforced concrete.
masonry and brick, reinforced concrete structures deteriorate under
attack from external elements such as freeze-thaw damage (the expansion
of frozen moisture within the structure as it thaws), and erosion.
In a composite man-made material such as concrete there are additional
mechanisms caused by the greater complexity of its composition. Of
particular concern today is the alkali silica reaction in the concrete
and the corrosion of the reinforcing steel, both of which are affected
by the alkalinity of Portland cement concrete. Portland cement is
made by burning constituents which include lime in a kiln and grinding
the result to a fine powder. This produces a highly alkaline material
which reacts with water and hardens. When it is added to coarse and
fine aggregate and mixed with water, the cement combines with the
aggregate and hardens to form concrete. The hardening process (hydration
reaction) is complex and continues over many months if not years,
depending on the amount of water in the mix. There must be excess
water for workability and a pore network therefore develops as it
dries out. Excess calcium hydroxide and other alkaline hydroxides
are present in the pores and a solution of pH 12.0 to 14.0 develops
(pH 7.0 is neutral; values below indicate acidity, and alkalinity
above). It is this pore network and the solutions it contains that
are critical to the durability of the concrete.
SILICA REACTIVITY (ASR)
ASR occurs if the wrong aggregates are used in the mix. Some silicaceous
minerals, including quartzes and opals, react with water in a high
alkaline environment to form silica gel, a material used to absorb
moisture. As silica gel swells when it absorbs moisture, the material
can cause concrete to crack, and white, weeping deposits of silica
appear. In many cases ASR is superficial and harmless, but it is unattractive
and difficult to treat. The most effective remedy is to dry out the
if not most types of concrete incorporate some material which is susceptible
to ASR. However very few structures show signs of significant ASR
damage, as the reactive aggregate components which cause the problem
are consumed in the process. Those areas of the United Kingdom where
ASR is prevalent are now well known and the quarries responsible have
OF REINFORCING STEEL
Although the alkalinity within the concrete pore structure can lead
to ASR, the high pH value also provides a protective coating of oxides
and hydroxides on the surface of the steel reinforcement. Without
this layer, which is known as a 'passive' film, the steel would be
exposed to the air and moisture in the pores, leading to rapid corrosion.
It is the main chemical reason why reinforced concrete is a durable
construction material. The layer is durable and self repairing, and
it can last for hundreds of years if the alkalinity is maintained.
However, the passive layer itself can be attacked by chlorides in
salt and the alkalinity of the concrete can be reduced by reaction
with atmospheric carbon dioxide, a process known as 'carbonation'.
Carbon dioxide, which is present in the air in proportions of around
0.3 per cent by volume, dissolves in water to form a mildly acidic
solution. Unlike other acids that may chemically attack and etch the
surface of the concrete, this acid forms within the pores of the concrete
itself where the carbon dioxide dissolves in any moisture present.
Here it reacts with the alkaline calcium hydroxide forming insoluble
calcium carbonate. The pH value then drops from more than 12.5 to
about 8.5. The carbonation process moves as a front through the concrete,
with a pH drop across the front. When it reaches the reinforcing steel,
the passive layer decays when the pH value drops below 10.5. The steel
is then exposed to moisture and oxygen and is susceptible to corrosion.
inside the building frequently carbonates totally without any sign
of deterioration as the concrete dries out, leaving the steel exposed
to air but not moisture. Problems are seen externally where concrete
is exposed to the elements and in certain situations internally, such
as kitchens and bathrooms, where the concrete is susceptible to condensation
or water-leakage. External facades are particularly vulnerable, especially
where cladding panels have poorly placed handling steel that is near
the surface. Carbonation does not have to penetrate far and the concrete
quality may be of poor quality.
DUE TO CHLORIDE
Salt causes corrosion by a different mechanism. When dissolved in
water sodium chloride forms a versatile, highly corrosive solution
of sodium ions (Na+) and chloride ions (Cl-). Salt is used for de-icing
roads and its presence in sea water is a major problem for reinforced
concrete structures. The very mobile chloride ions disperse through
concrete pores in solution and where they come into contact with the
reinforcing steel they attack the passive layer. Steel oxidises in
the presence of air and water to form rust which has a volume of up
to 10 times that of the steel consumed. As concrete has a low tensile
strength it will crack when as little as a tenth of a millimetre of
steel has been consumed. Horizontal cracks form, causing corners to
'spall' and surfaces to 'delaminate' as the reinforcement's concrete
cover becomes detached and falls away in sheets. The consequence can
be seen on the underside of road bridges and many buildings and structures
beside the sea.
Corrosion of steel reinforcement occurs by an electrochemical process
which involves exchanges of electrons similar to that which occurs
in a battery. The important part of the mechanism is the separation
of negatively charged areas of metal or 'anodes' where corrosion occurs
and positively charged areas or 'cathodes' where a harmless charge
balancing reaction occurs (Figure 1). At the anode the iron dissolves
and then reacts to form the solid corrosion product, rust. The rust
is formed at the metal/oxide interface, forcing previously formed
oxide away from the steel and compressing the concrete, causing it
If corrosion of steel in concrete is suspected, a deterioration survey
must be carried out to identify the cause, mechanism and extent of
corrosion. An inadequate investigation can lead to higher costs and
inadequate repairs. There are certain tests which are specific to
the corrosion assessment of steel in concrete, relying on the electrochemical
nature of the corrosion process. These are half-cell potential measurement,
resistivity measurement and corrosion rate measurement. Further examination
of these techniques is beyond the scope of this article, and a reading
list is provided below for further reference.
The obvious thing to do when confronted with corrosion damage is to
cut out the damaged areas, replace any steel weakened by section loss
and put back good quality concrete. However there are several problems
with this approach:
out the area of damage may leave many areas that are about to crack
a result of the electrochemical nature of the corrosion process,
repairs can actually lead to an acceleration of corrosion in adjacent
areas, especially with chloride-induced corrosion, as the removal
of the corroding anode also cause the loss of the protective cathodes
around it and new anodes form when the material is renewed
repairs may be visually intrusive as it is very difficult to match
the concrete used for repair to the colour and texture of the original,
and it is almost impossible to get the new material to weather in
the same way
concrete removal requires substantial temporary support, adding
to the complexity of the project as well as expense
and barriers can be very effective if the amount of chloride at
the depth of the reinforcement is below the chloride threshold or
if the depth of carbonation is less than the cover depth. Penetrating
sealers such as siloxy silanes have been shown to help to dry out
concrete if leaks are repaired and the amount of direct water on
the concrete is reduced. These are colourless and penetrate the
surface leaving the appearance unaffected.
silanes are not suitable for carbonated concrete. Anti-carbonation
coatings must be crack-bridging surface coatings to keep out carbon
dioxide. However, coatings membranes and sealers are all useless if
corrosion has already begun and direct water impingement is not minimised.
Coatings, penetrating coatings and barriers can also be effective
in slowing or stopping ASR by drying out the concrete.
The movement of charged ions and the separation of anodes and cathodes
along the steel creates some of the problems but also offers us some
solutions to the corrosion of steel in concrete, as corrosion can
be stopped by making all the steel a cathode (Figure 1). This is done
by putting an external anode on the surface or embedding it in the
concrete (Figure 2). The DC power supply, known as a transformer rectifier,
will then pass current between the anode and the reinforcing steel.
electrochemical rehabilitation approach can be used in three different
ways: cathodic protection; electrochemical chloride migration or 'desalination';
In this process the anodes, power supply and control systems are permanent,
and a range of anodes can be used (Figures 3 and 4). The aggressive
anodic reaction is isolated to a corrosion resistant anode while the
harmless cathodic reaction occurs at the surface of the steel reinforcement.
This process creates additional hydroxyl ions, rebuilds the passive
alkaline layer and repels chloride ions.
has been used on hundreds of reinforced concrete structures around
the world and has potential for the conservation of historic brick
and stone masonry, terracotta and statuary where steel and iron has
been used to provide reinforcement or a structural frame.
CHLORIDE MIGRATION (DESALINATION)
This process uses a temporary anode, power supply and monitoring system
to apply 50 volts direct current to the steel. The positive charge
repels the negatively charged chloride ions and rebuilds the passive
layer over a period of four to six weeks. Although less well proven
than CP, the technique has been used to successfully treat more than
50 structures in the UK, continental Europe and North America.
This system is the equivalent of desalination for carbonated structures.
It relies on the principle that the hydroxyl ions produced at the
cathode re-alkalise the concrete from the reinforcement outwards.
This is linked with a wet anode at the surface that contains calcium
carbonate, which moves under electro-osmotic pressure and re-alkalises
the concrete from the surface inwards.
are more than one hundred re-alkalisation projects completed in the
UK and on the continent. One of the earliest was the renovation of
the Hoover Factory beside the M40 at Perivale, NW London (see illustration
at top of page).
specialist treatments require expert advice to check that the structure
is suitable and that the best system is applied. There must be steel
continuity, separation between steel and anodes and reasonable concrete
quality before these techniques can be considered as cost effective
and technically sound for a particular structure.
INHIBITOR REPAIR TECHNIQUES
A recent development is the impregnation with chemical corrosion inhibitors
which are widely used in the power generation, chemical and manufacturing
industries. Recently, attempts have been made to introduce these chemicals
into hardened concrete. If successful, then these could be good, relatively
simple methods of increasing the life span, reducing maintenance and
providing a 'minimum intervention' method of slowing or stopping corrosion.
Corrosion of steel in concrete can be seen to be a significant problem
for many reinforced concrete structures if moisture is present. If
there is no salt to cause corrosion in the short term, carbonation
will affect most structures over the centuries. If the structure cannot
be kept dry then there is a range of techniques that can be used depending
on the structure, its condition and the cause and extent of the problem.
techniques can reduce the amount and extent of patch repairs, and
leave the appearance unchanged with probes embedded in the concrete
or a surface coating, depending on requirements and conditions. Chemical
impregnation with corrosion inhibitors is also under investigation
as a further option.
silica reaction is a chemical attack of the aggregates in the presence
of the alkalinity of the concrete and moisture. If the concrete can
be kept dry then ASR will be minimised. Most ASR damage is unsightly
rather than structurally dangerous.
- JP Broomfield, 'Assessing Corrosion Damage on Reinforced Concrete
Structures' in Corrosion and Corrosion Protection of Steel in Concrete, Edited
by R Narayan Swamy, Sheffield Academic Press, 1994
- Cathodic Protection of Reinforced Concrete - Status Report, Report
No. SCPRC/001.95, Society for the Cathodic Protection of Reinforced
Concrete, London, 1995
- CC Stanley, Highlights in the History of Concrete, British Cement Association, Crowthorne, Berks, 1986
Concrete Society Technical Reports:
- No 26 Repair of Concrete Damaged by Reinforcement Corrosion,
- No 36 Cathodic Protection of Reinforced Concrete, 1989
This article is reproduced from The Building Conservation Directory, 1996
DR JOHN BROOMFIELD is an independent consultant specialising in the corrosion
of steel in concrete. He has worked with the Strategic Highway Research
Programme (SHRP) in Washington DC (1987-1990) and Taywood Engineering
Limited, with whom he designed some of the first cathodic protection
systems to be installed on concrete structures in the UK and the Far East.
He is an active member of a number of corrosion standards bodies in Europe and the
US and has published over 20 papers in the field.
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