The most important threats For the durability of concrete constructions are, in random order:
1. Chemical attack of the concrete. 2. Physical attack of the concrete.
3. Electro-chemical attack of the reinforcement steel, causing secondary deterioration of the concrete.
4. Construction errors.
5. Calamities.
Categories 1. and 2. are being taken together as direct causes of deterioration of the concrete itself.
Category 3. represents the most widespread cause of concrete deterioration. This attack mechanism
operates in an indirect way and eventually leads to rebar corrosion. The expanded corrosion products are the immediate cause of concrete deterioration through spalling.
Category 4. involves errors and flaws in design,
Petrographical analysis is used to diagnose the occurance of Alkali Silica Reaction and ellringite formation. It can also determine the susceptibility for future occurrence of these reactions. Photo NEBEST
by.
construction and usage (failure or deterioration due to, For example, poor construction techniques or inadequate workmanship). Category 5. involves calamities like damage caused by Fire, overload or collisions with transportation vessels. The last two categories are not relevant For this paper.
Direct attack of concrete
The locus of causality of the chemical or physical attack of concrete can be placed internally or externally with respect to the material. An external cause of attack can be either chemical or physical and is initiated out of the construction's environment.
An internal cause of attack is always a chemical threat where the 'aggressive' species are present as of construction, that is 'from day one'. Chemical attack mechanisms include:
Alkali Aggregate Reaction -
AAR
concerns a reactiondo.<:o,mo.mo_ 57 The Fair Face of Concrete
of chemical species (alkalis: sodium and potassium) with susceptible aggregates. The most important form is called ASR, short for Alkali Silica Reaction, where low-crystallinity silica modifications, like chert, react with the alkalis to form silica gel. The expansive reaction of the gel with water can cause concrete deterioration. This process mostly occurs as a result of internal causes, but penetration of alkalis from deicing salts can be considered an external cause, which is fairly rare in architectural constructions and therefore not further elaborated in this paper.
Ettringite formation -A reaction of chemical species (gypsum) with specific clinker ((3A) from the cement.
The reaction product, ettringite, is a highly expanded substance causing concrete deterioration. Ettringate formation mostly involves internal causes, but penetration of sulphates can be considered an external cause. The water consumption with this reaction is typically very high and therefore this attack mechanism is only seen with very humid -e.g.
continuously wet- constructions, which are uncommon for architectural structures and therefore not further discussed in this paper.
Acid attack -This involves a reaction of the concrete with acids from the environment resulting in a dissolution of the cement stone by the acid and total degradation of the concrete. This attack mechanism is always due to external causes and only seen in industrial environments and sewers, and therefore not further discussed in this paper.
Physical attack mechanisms are always external causes
of
concrete deterioration and include:Wear and erosion -Wear. and erosion are caused by mechanical loading of the concrete surface inducing, on a micro scale, tensile forces above the strength of the concrete. This mechanism always involves external causes. This attack mechanism is only seen in heavy-load situations or industrial piping and therefore not further discussed in this paper.
Frost damage -Freezing water will become ice involving an expansion of volume by 9%. This can cause concrete deteriorction when occurring inside of the concrete but is, in its pure form, rather rare in the Netherlands. In combination with penetrated deicing salts there is a risk of lock up
of
water in between two layers of 'frozen' concrete. On subsequent phase transition of the locked-up water, spalling could occur.This mechanism always involves external causes. It can be avoided by usage of correct concrete mixtures during fabrication, including special consideration to the water-to-cement ratio which needs to be limited to a maximum of 0.45 or 0.55 in presence of air entraining agents.
Indirect aHack of concrete
Differentiation between an internal or external locus of causality with respect to the conc~ete can be made for indirect attack mechanisms, as done with the direct mechanisms, only the distinction between the two is less useful in this case. The indirect attack of
Exposed reinforcement steel and rebor corrosion as a consequence of acidification of the rebar environment caused by carbondiaxide penetration (carbonation) Photo: NEBEST bv.
Investigation of odvancement of carbonation fronts under the surface, and chloride content of the concrete.
Photo NEBEST bv.
concrete is always due the external causes with exception
of
the presence of bound-in chlorides, mostly added as calciumchloride ((0(12) as an accelerating additive for cement hydration. The tolerance for chlorides in new constructions in the Netherlands is presently limited to 0.4 weight % relative to mass of cement, which is considered to be a safe limit for concrete durability.Reinforcement steel in concrete is well protected against corrosion. On the one hand the concrete cover itself s a barrier to chemical processes. On the other hand the high alkalinity of the concrete
environment, with pH values of typically 13, is ideal for the steel. The reinforcement is covered with a thin layer of iron-oxides and hydroxides which prevent the iron substrate from further corrosion -the so called passive layer or film. The speed of corrosion
of
steelin normal concrete is therefore not relevant.
Nevertheless, the protective environment can change into a relatively aggressive environment either by
d~'.(Q,mo.mo_ 58 The Fair Face of Concrete
penetration of chlorides -mostly from sea water, sea breeze or deicing salts- or by the penetration of carbondioxides from the atmosphere. The carbondioxide effectively neutralizes the high alkalinity of the concrete after which the pH drops to values below 10. After the 'neutralized' zone has reached the rebar the passive film becomes unstable and corrosion activity starts. The presence of chloride ions in the pore water solution near the rebar also causes instability of the passive film, even at high pH values.
When the rebar actively corrodes, the steel is
transformed in iron-oxides and hydroxides (rust). The corrosion products are expanded as compared to the original steel volume, and therefore induce tensile stresses in the concrete. After a certain amount
of
corrosion products have formed, the concrete starts cracking and eventually spalling can occur. Due to the corrosion-initiated concrete deterioration the
aesthetical demands For the construction are no longer met. Furthermore, the concrete cover is damaged and effectively removed and the reinforcement steel is damaged, resulting in an effectively reduced diameter of the rebar which eventually can cause the construction to mechanically Fail even with a 'normal' load.
Concrete repair
With corrosion-initiated concrete deterioration, traditional concrete repair is essentially aimed at restoring the structural integrity and reassuring the protection of the reinforcement steel, thereby arresting Further development of concrete deterioration. Defective concrete -either material near the rebar
contaminated with chlorides above a certain threshold level or concrete covering that is
carbonated up to or beyond the rebar- is removed by cleaning the reinForcement steel and concrete surface, recasting the concrete section and taking preventive measures against new concrete attack. This can be achieved by coating the surFace, applying extra layers
of
concrete cover at certain surfaces, or usage of polymer concrete repair materials. Evidently the visual quality of the surface is not a primary objective in such a case. When the aesthetical aspects matter, as is the case with works of architecture, this traditional approach is no longer adequate.Three relatively new techniques, which are based on the idea to eliminate the main cause of damage by reversing the electro-chemical process of corrosion, have been tested and used for the past two decades.
These so called electro-chemical repair methods hove proven to be able to solve this problem, and are:
Cathodic Protection (CP) -By applying an anode and an electrical current from the anode to the
reinForcement steel the corrosion is arrested.
CP
is a very effective way to stop corrosion with a permanent system, but the anode and electrical wiring are not supposed to alter the visual quality of theconstruction. ThereFore
CP
has a limited applicability to architectural works: only for those constructions which can be protected From a non-visible sideCP
can be used.
Chloride Extraction (CE) -Also called chloride removal or desalination, this method is effective when penetration of chlorides is the cause of rebar
corrosion. Through application of a temporary anode system an electrical current is applied from anode to
Traditional repair methods typically involve complete removal of all defeclive concrete behind the rebor. II is mostly a very severe measure that compromises the integrity of the material.
Photo NEBEST by.
do_<::o,mo.mo_ 59 The Fair Face of Concrete
the reinForcement steel. Due to the electrical current, negatively charged ions, like chloride, move to the external anode and are removed with the anode system after treatment. Typically the treatment will take 4 to 8 weeks. The system is technically very similar to CP but the used current densities are approximately a hundred times higher, and the system is only installed temporarily.
Realkalisation IRA) -When carbonation of concrete is the cause of rebar corrosion RA is effective. Through application of a temporary anode system an electrical current is applied From anode to the reinForcement steel. Due to the electrical current, positively charged ions, like sodium, move to the reinForcement steel where hydroxyl ions are formed, in effect Forming NaOH which will typically increase the
pH
at the rebar to values of 14 or higher.Typically the treatment will take
1
to 2 weeks. The system is technically very similar to CP but, again, the used current densities are approximately a hundred times higher and the system is only temporary.Conclusions
Chloride extraction and realkalisation are obviously interesting techniques For repair of works of architecture, but even more these techniques have proven to be efficient and cost effective in some cases were 'traditional' repair would have meant general surFace chiselling and major concrete repair.
Furthermore, these techniques provide optimal durability with minimal annoyance from vibrations, dust, noise and so on. For concrete repair of architectural works chloride extraction and
realkalisation are important maintenance techniques which can arrest rebar corrosion without altering the visual appearance of the construction.
Anthony WM. van den Hondel is a consultant with NEB EST b~ an engineering and consultancy firm in Groot Ammers, the Netherlands.
de.<:'o,mo.mo_ 60 The Fair Face of Concrete