1

TECHNIQUES FOR THE PROTECTION AND REPAIR OF REINFORCED CONCRETE STRUCTURES

1. INTRODUCTION

Many of the recent structures existing in Portugal are made of reinforced concrete. For example, 45%

of the residential buildings are made with it. Initially, in the 60’s to 80’s, when this material started to be

extensively used, people thought it was eternal, so durability was not an issue to be considered in

design and construction.

Now it is understood that structures made of reinforced concrete get old and have a limited lifetime,

usually of 50 years. Without durability criterion that lifetime can short to only 20 or 30 years, if subjected

to severe aggressive environments. To achieve the 50 years without any big repair intervention new

regulation, namely the EC2, is very strict in terms of durability, specifying minimum reinforcement cover

and concrete class in accordance with environmental exposure.

This summary refers to this theme, studying the new Standard EN 1504 “Products and Systems for the

Protection and Repair of Concrete Structures”, summing up the information exposed in it and

explaining it, constituting a state-of-art of this subject.

2. STANDARD EN 1504

The European Standard EN 1504 was elaborated by the Technical Committee CEN/TC 104 and it

defines the principles that shall rule the protection and repair of concrete structures. This standard

covers the following aspects [1]:

• Assessment of the condition of the structure.

• Identification of the causes of deterioration.

• Determining the objectives of protection and repair.

• Selection of the appropriate principle(s) of protection and repair.

• Selection of methods.

• Definition of properties of products and systems.

• Specification of maintenance requirements following protection and repair.

The Standard is organised in different parts, in accordance with the following:

2

• Part 1: General scope and definitions.

• Part 2 to 7: Specifications related with various kinds of interventions.

• Part 8: Quality control and evaluation of conformity.

• Part 9: General principles for the use of products and systems.

• Part 10: Site application of products and systems and quality control of the works.

One important issue that this standard introduces is the classification, in accordance with the damage,

of all the causes for reinforced concrete structures deterioration, as it is exposed in the next chapter.

3. REINFORCED CONCRETE DETERIORATION

In this chapter the various causes of reinforced concrete deterioration are exposed, with a proposal of

the methods suitable for an intervention of prevention or repairing of the respective damage. Those

methods are named and explained in a further chapter of this summary.

1. Deterioration due to concrete defects

1.1. Mechanical actions

1.1.1. Impact – Methods for repairing: M1.3, M1.4, M3.1, M3.2, M3.3, M5.1

1.1.2. Overload – Methods for repairing: M1.3, M1.4, M3.1, M3.2, M3.3, M3.4, M4.1, M4.2,

M4.3, M4.4, M4.5, M4.6, M4.7

1.1.3. Movement – Methods for repairing: M1.3, M1.4, M3.1, M3.2, M3.3, M3.4, M4.1, M4.2,

M4.3, M4.4, M4.5, M4.6, M4.7

1.1.4. Explosion – Methods for repairing: M1.3, M1.4, M3.1, M3.2, M3.3, M5.1

1.2. Chemical reasons

1.2.1. Alkali-aggregate reaction – Methods for repairing: M1.3, M1.4, M2.1, M2.2, M2.2,

M2.4, M3.1, M3.2, M3.3, M3.4, M5.1

Fig. 1 – Mapped cracking due to alkali-aggregate reaction (left), gel formed near the aggregate (right)

3

1.2.2. Aggressive agents – Methods for repairing: M1.1, M1.2, M1.3, M1.4, M1.5, M1.6,

M1.7, M2.1, M2.2, M2.3, M2.4, M6.1, M6.2

1.2.3. Biological activities – Methods for repairing: M5.1, M5.2, M6.1, M6.2

1.3. Physical actions

1.3.1. Freeze / thaw – Methods for repairing: M2.1, M2.2, M2.2, M2.4

1.3.2. Thermal – Methods for repairing: M1.3, M1.4, M1.5, M3.4, M5.1

1.3.3. Salt crystallization – Methods for repairing: M1.1, M1.2, M1.3, M1.4, M1.5, M1.6, M1.7,

M2.1, M2.2, M2.3, M2.4, M6.1, M6.2

1.3.4. Shrinkage – Methods for repairing: M1.3, M1.4, M1.5, M3.4, M5.1

1.3.5. Erosion – Methods for repairing: M1.6, M3.1, M3.2, M3.3, M3.4, M5.1, M5.2

1.3.6. Wear – Methods for repairing: M1.6, M3.1, M3.2, M3.3, M3.4, M5.1, M5.2

2. Deterioration due to reinforcement corrosion

Fig. 2 – Crack due to corrosion (left), spalling due to rebar corrosion (right)

2.1. Carbonation – Methods for repairing: M1.1, M1.2, M1.3, M1.4, M1.5, M1.6, M1.7, M2.1, M2.2,

M2.3, M2.4, M7.1, M7.2, M7.3, M7.4, M8.1, M9.1, M10.1, M11.1, M11.2, M11.3

2.2. Stray currents – Methods for repairing: M2.1, M2.2, M2.3, M2.4, M8.1, M9.1, M10.1, M11.1,

M11.2, M11.3

2.3. Corrosive contaminants

2.3.1. At mixing (chlorides) – Methods for repairing: M2.1, M2.2, M2.3, M2.4, M7.2, M7.5,

M8.1, M9.1, M10.1, M11.1, M11.2, M11.3

2.3.2. From external environment (chlorides) – Methods for repairing: M1.1, M1.2, M1.3,

M1.4, M1.5, M1.6, M1.7, M2.1, M2.2, M2.3, M2.4, M7.5, M8.1, M9.1, M10.1, M11.1,

M11.2, M11.3

2.3.3. Other contaminants

4

4. INTERVENTION

The Standard helps defining the kind of intervention to adopt, exposing the various options possible. It

also specifies numerous aspects to consider in the design and execution of the intervention. The

methods defined in the Standard for protection and repairing of concrete structures are organised in

accordance with the principles to achieve. In the next table those principles and methods are exposed

[1]:

Principle Methods

M1.1 Impregnation

M1.2 Surface coating with and without crack bridging ability

M1.3 Locally bandaged cracks

M1.4 Filling cracks

M1.5 Transferring cracks into joints

M1.6 Erecting external panels

P1 [PI] – Protection against ingress

M1.7 Applying membranes

M2.1 Hydrophobic impregnation

M2.2 Surface coating

M2.3 Sheltering or overcladding P2 [MC] – Moisture control

M2.4 Electrochemical treatment

M3.1 Applying mortar by hand

M3.2 Recasting with concrete

M3.3 Spraying concrete or mortar P3 [CR] – Concrete

restoration

M3.4 Replacing elements

M4.1 Adding or replacing embedded or external reinforcing steel bars

M4.2 Installing bonded rebars in performed or drilled holes in the concrete

M4.3 Plate bonding

M4.4 Adding mortar or concrete

M4.5 Injecting cracks, voids or interstices

M4.6 Filling cracks, voids or interstices

P4 [SS] – Structural strengthening

M4.7 Prestressing – (post tensioning)

M5.1 Overlays or coatings P5 [PR] – Physical resistance

M5.2 Impregnation

M6.1 Overlays or coatings P6 [RC] – Resistance to chemicals M6.2 Impregnations

M7.1 Increasing cover to reinforcement with additional cementitious mortar concrete

M7.2 Replacing contaminated or carbonated concrete

M7.3 Electrochemical realkalisation of carbonated concrete

M7.4 Realkalisation of carbonated concrete by diffusion

P7 [RP] – Preserving or restoring passivity

M7.5 Electrochemical chloride extraction

P8 [IR] – Increasing resistivity M8.1 Limiting moisture content by treatments, coatings or sheltering

P9 [CC] – Cathodic control M9.1 Limiting oxygen content (at the cathode) by saturation or surface coating

P10 [CP] – Cathodic protection M10.1 Applying electrical potential

M11.1 Painting reinforcement with coatings containing active pigments

M11.2 Painting reinforcement with barrier coatings P11 [CA] – Control of anodic

areas M11.3 Applying inhibitors to the concrete

5

Before the execution, planning must be accomplished to achieve good results. The Standard refers

which preparation works must be done in accordance with the method adopted. The most important

works are:

• Preparation of the concrete surface (cleaning, roughening, concrete removal).

• Preparation of reinforcement surface (cleaning).

5. TECHNIQUES FOR THE PROTECTION AND REPAIR OF REINFORCED CONCRETE

The Standard EN1504 names the methods to be used in accordance with the principle to achieve with

the intervention. Those methods are based on techniques. The same technique is repeated in various

methods depending on the aimed principle. In this summary we will explore those techniques.

1. Superficial protection

1.1. Impregnation

This technique is used to limit the access of aggressive elements without interfering with the structure

aspect. There are simple impregnations, made of low density resins, that penetrate and fill the pores of

the surface of the concrete, diminishing its superficial permeability and increasing resistance. There is

also the hydrophobic impregnation, that uses silans and siloxans. In this case the pores are not filled.

Simply, inside of them, a hydrophobic compound repeals moisture and, so, low pressure water is not

absorbed. In the use of this technique the risk of evaporation of the product during application must be

considered and, also, it shall penetrate at least 2mm.

Methods in which the impregnation is used: M1.1, M2.1, M5.2, M6.2, M8.1

Fig. 3 – Impregnation of a surface (left), surface protection with paint (right)

6

1.2. Surface coating

This technique provides superficial protection, since it consists on a layer put on top of the substrate

surface. There are two kinds of coating. One is painting, consisting on a thin layer (0.1 to 1.0mm) that

control carbonation, chloride penetration, chemical attack. There is also the mineral or mixed mineral

and polymers coating, usually a layer with 1 to 5mm thick, that provide physical resistance to the

surface, lowers its permeability and covers low width cracks. In this coatings a minimum permeability

should be specified so that moisture won’t accumulate near the surface, deteriorating it.

Methods in which the surface coating is used: M1.2, M2.2, M5.1, M6.1, M8.1, M9.1

1.3. Membranes

This is a special type of surface coating, whose main feature is being very flexible and totally

impermeable. Their use should consider the risk of moisture accumulation near the surface, if any

exists within the concrete.

Methods in which the surface coating is used: M1.3, M1.7, M6.1, M9.1

1.4. Overcladding – mortar or shotcrete

This technique consists on providing extra cover to reinforcement and protection to surface, with

collocation of new layer over the surface with a thickness from 5mm to 60mm or more. If it is a very

thick layer (over 60mm) it should be used small diameter debris to reduce the effect of shrinkage. Also

some admixtures can be used to diminish the chemical vulnerability, increase elasticity and

workability… such as fly ash or polymers. The use of overcladding should be preceded by a pre-

humidification of the surface of the substrate. Also the adhesion guaranteed between new and old

materials should be superior to 1.0MPa.

Methods in which the overcladding is used: M2.3, M5.1, M6.1, M7.1

1.5. Physical external protection

This kind of protection consists on installing, externally to the structure, material such as, for example,

precast concrete panels or composite plates. Depending on the chosen material, it can provide great

protection against penetration of aggressive elements or physical degradation due to use, erosion or

impacts.

Methods in which the physical external protection is used: M1.6, M2.3

7

Fig. 4 – Membrane application (left), physical external protection with fibre glass pates (right)

2. Prevention against corrosion

2.1. Steel protection against corrosion

There are two kinds of steel protection: rebar coatings or the use of corrosion inhibitors. The first either

contains active pigments with anticorrosive properties or acts as a barrier isolating the steel from water,

which is, for example, the case of resins. In the other hand, the inhibitors are used in the concrete and

they act in the anode reconstructing the protective layer of steel or they act in the cathode. Their

effectiveness is still questioned nowadays.

Methods in which the steel protection against corrosion is used: M11.1, M11.2, M11.3

2.2. Cathodic protection

The cathodic protection consists on installing a system that forces the rebars to act as cathodes. It can

be passive if it consists on the installation of sacrificial anodes with lower electric potential than the

steel, in electric contact with the rebars. Otherwise, it can be active if electric current originated in a

external font is imposed to a system that is formed by the rebars and a neutral anode that is collocated

on the surface of the element. This is a very effective way of protecting the steel against corrosion due

to chloride penetration.

Methods in which the steel protection against corrosion is used: M10.1

Fig. 5 – Steel protection coating (left), cathodic protection of a bridge pier (right)

8

3. Techniques of repairing deteriorated concrete

3.1. Treatment of cracks

In general, if a crack width is greater than 0.3mm, it must be injected with a material that can, in

accordance with the Standard, be either able of transmitting forces, or be flexible enough to keep up

with crack movements or, else, be hidroexpansive, absorbing water and filling the crack. A mineral

product can be used, such as cemment grout or a polymer, such as epoxy resin. Cracks can also be

superficially sealed or reinforced with steel. The selection of the correct technique and product

depends upon knowing the reason for its existence and if it is expected further movements in it.

Methods in which the treatment of cracks is used: M1.4, 1.5, M4.5, M4.6

3.2. Patching repair of deteriorated surface with mortar or concrete

Due to spalling or extensive deterioration of the surface, it can exist superficial zones that leave the

steel unprotected against corrosion. So these areas must be filled and, to do it, it can be used mortar

or concrete made with small aggregates, after deteriorated concrete removal and surface preparation.

The material used should have admixtures to reduce shrinkage and increase workability, so that the

patch area won’t crack or result too much permeable. For that, polymers and fly ash can be added.

Sometimes the steel is very corroded and with significative section lost. In those cases new steel

should be added and forces must be transmitted between old and new rebars.

Methods in which patching of deteriorated surface is used: M3.1, M3.2, M3.3, M4.4, M7.2

4. Electrochemical treatment of contaminated concrete

If the carbonation depth if already very significant, there is the possibility of electrochemical

realkalisation, that provides new alkalis to the area surrounding the steel, increasing pH and so helping

to create a new protective layer. Also, if that’s the case, chloride can be removed from inside concrete.

Both techniques make use of simple installation, generically, consisting on a metal mesh outside

concrete, electrically connected to reinforcement, with imposed current. These must be kept for a few

days, but they are of simple installation, non intrusive and quite effective and durable.

Methods in which electrochemical treatment is used: M7.3, M7.4, M7.5

5. Strengthening of the structure

The Standard refers to the strengthening if structure’s safety is affected due to deterioration. In

particular, it refers to adding or replacing steel bars, to bonding external plats (steel or composite

material) or to the use of internal or external prestress.

Methods in which strengthening of the structure is used: M4.1, M4.2, M4.3, M4.7

9

6. Other techniques referred in the standard

Finally, some other techniques for intervention are possible and named in the Standard. This is the

case of electrochemical moisture control (M2.4), replacing of deteriorated elements (M3.4) and limiting

the oxygen content at the cathode by saturation or surface coating (M9.1).

6. CASE STUDY – REPAIRING AND PROTECTING OF A DETERIORATED CONCRETE FAÇADE

The façade repaired and protected belongs to a 10 storey building, in Lisbon, built in 1976. The façade

is composed of reinforced concrete elements. Those elements showed general signs of deterioration,

such as, spalling of surface concrete due to steel corrosion. Executed tests and visual observation led

to the conclusion that the concrete carbonation caused reinforcement corrosion and that element

geometrical conception led to accumulation of rain water.

It was decided a typical intervention with concrete patching repair of the deteriorated areas followed by

a superficial protection. The methodology adopted was:

a) Mapping of spalling areas or with exposed reinforcement.

b) Concrete removal with cutting disk or jackhammer, 20mm deeper than the reinforcement.

Fig. 6 – Reinforced concret element deteriorated (left), deteriorated concrete removed (right)

c) Mechanical surface preparation to create surface roughness.

d) Steel cleaning to remove rust and detect the need of replacement if section loss was significative.

e) Steel protection with cement base barrier, from the brand SikaMonotop 610.

f) Patching with tixotropic monocomponent mortar, made of cement, fly ash and resin, reinforced

with fibres (brand SikaMonotop 612). This mortar was applied by hand, in layers with a thickness

less than 20mm.

g) The adopted surface protection was a system that act as a membrane (Sikagarg Aquaprimer

552W (80µm), Sikagard 570 (110 µm) and Sikagard 550W (160 µm)).

10

During intervention a continuous scheme of quality control was implemented to guarantee the

effectiveness of protection.

Fig. 7 – Area being patched, in the first mortar layer (left), surface after repairing (right)

Fig. 8 – Products used for surface protection (left), quality control of surface protection (right)