PROTECTIVE COATINGS FOR CONCRETE BRIDGES



Protection of structures is very important for increasing the life of the structure. There is a growing national concern about premature deterioration of our concrete infrastructure projects such as bridges, aqueducts etc. requiring early repair and/or replacement. Use of protective coatings will increase the durability and hence the life of these structures. As most of these structures are made up of concrete and steel, the role that coatings play in protecting the surface of concrete is highlighted. The need, functions, requirements and factors governing the selections of protective coatings are discussed. This report covers various aspects related to protection of concrete bridges.

Keywords: Protective coatings, concrete bridges, combination systems.


1.0 INTRODUCTION


Bridges are one of the oldest structures built by man as an arrangement made to cross an obstacle in the form of a low ground or a stream or a river or over a gap without crossing the gap. These structures plays a vital role in the development of the country by forming an important part of the infrastructure for both roadways and railways. Across the globe, billions of dollars are spent annually in the construction of new bridges and also on the repair and restoration of distressed bridges. The repair works can be reduced by coating the bridges with protective coating. Hence for increasing the life of the bridges protective coatings are used. Different types of coatings are used for the protection of concrete bridges. Coatings are generally used for enhancing the life and aesthetic appeal of the structure. Coatings are any mixture of film forming materials plus pigments, solvents and other additives, which when applied to a surface and dried, yields a film that is functional and often decorative.


As far as concrete bridges are concerned, the term coating is often used to describe more heavy-duty materials, like the epoxies or polyurethane, because their prime function is protection rather than decorative.


2.0 NEED FOR PROTECTIVE COATINGS


The design of concrete structures such as bridges in aggressive environment involves consideration and design solutions for a wide array of functions and service requirements, many of which are inter related. Corrosion of embedded steel in concrete is one of the major design concerns in structures exposed to hostile environments in general and concrete bridges in coastal areas in particular.
Usually, the problem becomes real when the designers of concrete structures in coasted areas, ignore or lack of understanding of carbon dioxide, chlorides ion penetration into concrete and the corrosion process. Thus the longevity and durability of these structures are compromised.
Worldwide awareness about the use of physical barriers ie. surface coatings is growing to keep carbon dioxide, chloride ions and other aggressive harmful agents out of the concrete. This triggered the development of specialty protective coatings to protect bridges and other atmospherically exposed reinforced concrete structures from attack by acidic gases, chloride ions, carbon dioxide, sulphates, oxygen, water etc.


3.0 FUNCTIONS AND REQUIREMENTS OF COATINGS.


The coatings has two main functions

i) To prevent the surface against dampness and weathering and thereby increasing the life of the structures.
ii) To provide the surface with an attractive finish.
The coatings should have the following requirements.
iii) Good adhesion to the surface to be coated
iv) Low water absorption
v) Good abrasion resistance and high toughness
vi) Low susceptibility of cracking
vii) Excellent weathering resistance
viii) Low susceptibility to staining
ix) Good resistant against growth of fungi, algae moss etc.

4.0 TYPES OF COATINGS


Coatings provide protection to the structure by forming a relatively thick film on the surface.

They generally consist of binder, pigments, fillers and a carrier. The binder is a polymer which can be of various types and which dictates the major properties of the system including resistance to carbonation and to some extent flexibility.
Polymers which have been in use as coatings include chlorinated rubbers, urethanes, epoxies, acrylics and vinyl etc.
Chlorinated rubber coatings : These have been available from many years.
They have:-

-Good moisture resistance
-Carbonation resistance
-Degree of flexibility
The demerits of chlorinated rubber are
-Poor UV resistance
-Poor dirt shedding properties.

Polyurethane coatings may be formulated to produce flexible, anti carbonation coverings, with excellent UV resistance and abrasion resistance. Their main draw back is limited breath ability and sensitivity to water.

Epoxies have excellent chemical and abrasion resistance as well as good anti carbonation properties. Their main disadvantage is their tendency to chalk and yellow, when subjected to UV and also limited breath ability and flexibility.

Acrylic and met acrylic resins have been successfully used for a number of years as a base polymer for a wide variety of masonry coatings. Because of their low film build, clear acrylics do not give good resistance to carbonation. They are durable and have excellent dirt shedding and UV properties. However, they have limited flexibility. This problem can be solved by using flexible acrylics.

Vinyl binder used in coatings are often made from a copolymer of vinyl chloride and vinyl acetate.

They have:-

Good alkali resistance
-Degree of flexibility

Table shows the properties of different polymers.

Table 1 Comparison of performance of various resin systems

Resin Type Alkali resistance Water resistance Flexibility Breath ability UV Resistance Carbonation Resistance Cl and SO4 resistance
Alkyl No Yes Yes Yes No No No
Vinyl Yes No Yes Yes No No No
Styrene to Polymer Yes Yes No Yes No No No
Chlorinated rubber Yes Yes Yes No No Yes No
Pure aliphatic acryl ate Yes Yes Yes Yes Yes Yes Yes
Polyurethanes Yes No Yes No Yes Yes No
Epoxies Yes Yes No No No Yes Yes





5.0 FACTORS GOVERNING THE SELECTION OF COATINGS


There are several factors which govern the selection of material to be used as protective coating for concrete bridges. Some of the key points to be looked at are:

¢ Compatibility with concrete
¢ Chloride ion resistance
¢ Carbon dioxide diffusion resistance
¢ Resistance to moisture
¢ Resistance to acidic gases and vapour
¢ Resistance to liquid driven mechanisms.
¢ Breath ability
¢ UV resistance
¢ Aesthetics
¢ User friendly or easy application
¢ Flexibility
¢ Economy
¢ Long life and durability.
¢ Environment friendly
¢ Alkali resistant


6.0 APPLICATION OF PROTECTIVE COATINGS


Whenever coatings are to be applied on any surface, first the surface has to be made free from friable material, grease, oil, any other foreign material etc ie the surface has to prepared so that the coating has good adhesion on the surface.




6.1 Surface preparation for concrete structures.

The long term performance of a coating is significantly influenced by its ability to adhere properly to the material to which it is applied. Hence it is essential that, the coating has good adhesion over it for long-term protection.
Concrete surface requires adequate treatment. The surface preparation includes.

1. Removal of surface contaminants:

Surface contaminants like oil or grease are to be removed with an alkaline cleaner or detergent followed by fresh water washing and drying. Through drying is necessary before applying any coating.
2. Removal of protrusion.
Dry/wet abrasive blasting, powder grinding are used to remove protruding surface, imperfection etc.
3. Removal of efflorescence

Efflorescence is a white powdery deposit of soluble salt present in the concrete, which needs to be removed by through wire brushing.
4. Removal of chemical contaminants

Acidic surfaces are neutralized with an alkaline cleaner and washed with freshwater. Alkaline surfaces are treated with dilute weak acid.
5. Filling of voids

Voids are formed on the surface of concrete while vibration process or curing. Voids are to be sealed with sealers.
6. Roughening of surface

This is required to improve coating adhesion. It is carried out by air blasting.
7. Removal of algae, moss

These are visible and can be removed by steam cleaning followed by day brushing. Application of a fungicide and were brushing again after drying will prevent re-growth.



6.2 Application.

If proper grade of coating is selected and applied properly in proper quantity, it is definite to obtain best results from the coating. The common designation for the series of waiting. The common designation for the series of coatings applied to a surface is primer coat, intermediate coat and top coat. The general functions of coats in a coating system are:
Primer coat provides:-

1. Adhesion to surface

2. Inhibition of corrosion if inhibitive pigments are present.

Intermediate coat provides

1. Additional thickness
2. Chemical resistance
3. Adhesion between primer and top coat.
Top coat provides
1. Weather and chemical resistance, wear and abrasion resistance.



7.0 COATINGS FOR CONCRETE


Concrete is the most versatile man made construction material. It is so popular on account of its durability characteristics. However, due to several internal and external factors, the concrete tends to deteriorate over a period of time. Permeability is one of the most critical factors that affect the durability of concrete. The ingress of water through the body of the concrete as well as passage of several chemicals attacks the embedded steel reinforcement leading to the deterioration of the concrete.
The major problems encountered in concrete structures can be summarized as follows:

a) Carbonation and chloride attack, which will lead to the corrosion of steel bar
and subsequent weathering of concrete.

b) Attack by acidic gases or solutions, which also leads to the corrosion of steel bar and spalling.

c) Water permeation which indirectly helps the process of carbonation and corrosion.

d) Weather erosion leading to localized problems.

Hence protective coatings are given to the concrete structures for increasing the durability of the structures. The protective coatings applied over the concrete surface should withstand all the above attacks.
Various types of coatings are used for the protection of concrete structures. With the advancement in the polymer technology, materials are available which can be used as protective coatings. Some of the polymers available are Epoxy resin, Polyurethane resin, Acrylic resin, Polyester resin, silicone resin.
The table give the general comparison of various coatings employed in preventing the deterioration of concrete structures.
Table 2 Comparisons of various coatings
Properties Vinyl acetate copolymer Epoxy polyamide Aliphatic polyurethane Coal tar epoxy
Abrasion resistance Good Good Outstanding Good
Chemical resistance Good Good Good Good
Alkali resistance Good Good Good Good
Moisture permeability Low Low Medium Low
Weather resistance Fair Fair Excellent colour retention Chalking

Coatings applied to concrete are typically barrier coatings. They provide protection by becoming a physical shield or isolating the concrete from its environment ie it prevents the moisture and other chemicals from reaching the surface.
Thus coatings are given to the concrete.

¢ To protect it from chemical and physical attack
¢ To protect products stored or processed indirect contact with the concrete from contamination caused by dust from the substrate.
¢ To improve its appearance, case of maintenance.

8.0 PROTECTION OF CONCRETE BRIDGES


Concrete bridges undergoes deterioration due to ingress of water or chemical attack. Hence early repair and replacement is necessary before the destruction of the structure. If early repair and replacement is not done, it will not only drains the coffers of the nation, it also affects the time frames of the new projects, there by jeopardizing the progress of the country. Long-term bridge deterioration can only be effectively resolved through advanced technologies that have the superior ability to enhance and preserve both existing and newly erected structures.
The application of protective coatings, membranes and protective sealers to the bridge can serve as a barrier to corrosive environments. These products have to be chosen carefully and should be based on their long-term effectiveness and not based on their short-term costs.
As recently 10 years ago, bridge painting was a relatively simple operation with little emphasis on regulatory compliance, quality or life-cycle performance of materials.
For example, lead containing paints have been used to coat most of the bridges in the past. Many bridge structures are generally located in waterways or near wildlife habitat and this procedure now has a potential for impacting the environment.
Concrete sealers are typically used to prevent or slow the ingress of water chlorides, and other aggressive chemicals. Many sealers exist, including silanes, siloxanes and siliconates, epoxies, gum resins and mineral gums, linseed oil, stearates, acrylics silicates and fluorosilicates, urethanes and polyurethanes, polyesters, chlorinated rubber, silicones and vinyls.
Current bridge protection technology should meet the following criteria.

¢ Enhanced coating material durability and performance
¢ Design for corrosion control
¢ Cost control

Assessing the condition of concrete bridges and planning a repair and rehabilitation strategy is a complex task, but one made easier by several of the innovative technologies developed or evaluated under the strategic Highway Research Programme. SHRP has developed several publications and methods for assessment, protection, and rehabilitation of reinforced concrete structures.
Coating technologies of the past are designed to simply cover a surface area like nearing a jacket, coatings, in general, leave open pores beneath the coating surface exposed. Without this thin coating layer, no protection exists between the concrete structure and the outside elements when the coating cracks from thermal expansion, freeze-thaw cycles or simply wears away over time, the concreteâ„¢s open pores are once again exposed to the elements, and thus further degradation continues to occur.
Recent developments of advanced bridge protection technologies should provide a product that can be applied to new an existing bridge structures. A product should not only coat the surface profile, but should also saturate and penetrate into the pores of the concrete. Specialty rubbers can be added to the formulation to provide a strong tenacious physical bond. The product will form as part of the actual concrete structure itself ie. It will penetrate and become part of the physical concrete structure. This product would form a tough rubber matrix within the structure and provide additional structural strength to the bridge. Open pores in the concrete would be sealed, and small air pockets filled. Moisture, air, slats, chemicals and other damaging elements would be unable to penetrate the solid internal chemical network.
This newly formed rubber matrix would then reject the elements from penetrating into the core of the concrete structure where the metal frame work exists. This penetrating and bonding action further halts the progression of existing concrete deterioration and slows the progression of internal metal erosion.
Surface coating expansion rates are different than concrete expansion rates. Surface stress develops on coatings during seasonal freeze-thaw cycles.
This result produces micro fractures, product separation, peeling and cracking. This will further introduce exposure to the elements. Specially flexible rubber compounds would expand and contract with the structure during hot or cold conditions. The internally formed rubber matrix does not behave like a surface coating and, thus provides better protection during expansion due to seasonal freeze- thaw cycles. This specially rubber compound would also incorporate characteristics to improve overall flexible strength. In general, physical improvement would be noted in overall structural integrity of the bridge.
8.1 Combination systems
Systems have been formulated to provide maximum protection to reinforced concrete and prestressed concrete structures. These products were initially developed in North America where they have been in use for a number of years. One of the most successful combinations consists of a silane/siloxane acrylic blend primer with a pigmented acrylic top coat.
8.2 Functioning of the combination systems

The exposed concrete surface are continuously attacked by a variety of aggressive chemicals present in the atmosphere like sulphur-di-oxide, carbon dioxide, oxygen, water, moisture, sulphates chlorides etc. Concentrations of each of the above in the atmosphere various from place to place depending on the environment. These airborne chemicals penetrate into the body of concrete through the pores and cause corrosion of reinforcement.
It is therefore necessary to protect the prestressed concrete and reinforced concrete from these attacks and the best way is to prevent them from entering the concrete. It is also important that any protective treatment given allows for excess water vapour present in concrete to evaporate out without itself rupturing due to vapour pressure. It should also not deteriorate on exposure to ultraviolet rays and weathering.
Combination systems are precisely designed to meet these needs. One such combination system is Dekguard S which consists of a silane/siloxane Primer and a pure aliphatic acrylate top coat. The low viscosity, low volatility and high penetration power silane/siloxane primer penetrates into the substrate and into the pores and bonds to the innerfaces of the pores providing a barrier against chlorides and water.
Further this primer does not block the pores and hence does not effect the breathability of concrete. The pure aliphatic acryl ate top coat provides high resistance to carbon dioxide, water and also ensures stability even under long term exposure to UV rays. Dekguard top coat has also self cleaning properties which enables the coating to look clean and neat even after years of exposure.
The Dekguard S system consisting of silane/siloxane primer and pure apliphatic top coat in combination protects the concrete surfaces from carbon dioxide, sulphur dioxide, oxygen, water, chlorides and acid gases while allowing water vapours from within to get out. It has been estimated through lab tests that the protection offered by this combination system (Dekguard S system) to concrete is equivalent to that given by over 500mm thick M30 grade concrete.
All these above mentioned advantages in addition to its easy application and availability in a range of aesthetic shades make this combination system (Dekguard system) an ideal choice for coating concrete bridges and other atmospherically exposed reinforced concrete structures.

9.0 CASE STUDY


Protection to concrete Bridge - Mandovi Bridge in Goa.
Mandovi Bridge, the bridge across river Mandovi which connects South Goa to North Goa was constructed during the early 1990s and has no protection against the attack of the coastal environment. The concern authorities after studying the properties of various types of polymer based coatings, selected combination system as the best protection proposed. This combination system consists of a silane/siloxane acrylic blend primer with a pigmented acrylic top coat. One such combination systems is Dekguard S manufactures by Fosroc Chemicals India Limited. The job included sand blasting of the super structure (girders) and providing it with a coat of Dekguard Primer.
Two layers of top coat using Dekguard S of silver grey colour was applied at a total wet film thickness of 175 microns at a coverage of 0.35ltrs per Sq.m using airless spray equipment. There total surface area of the protection provided to Mandovi Bridge is about 16,000Sq.m which is at present considered to be one of the major jobs in the filed of bridge protection in India. This job was executed during October 1998 to March 1999 as per the guidelines and specifications of the manufacture.


10.0 CONCLUSION


Now as a part of the gaining concept of repair and protection as a long term strategy towards the concrete repairs, a suitable coating is needed to be applied. The selection of suitable protective coatings saves enormous time and money by reducing the frequent maintenance and repair costs. The recently developed combination system possess all the properties of an ideal protective coating and hence can be used as an additional insurance measure against corrosion and premature concrete deterioration in general and concrete bridges in coastal areas in particular.



REFERENCES
a. Srinivasa Reddy.P, Protective coatings for concrete bridges “ A case study,
New Building materials and construction world, January 2001, pp32-36

b. Vijay Bhuskute, Futuristic Trends in Protective coatings, New
Building Materials and construction world, April 2001, pp 33-38

c. betterroads.com

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