Appropriate specifications
The overall success of a protective coating scheme can be influenced by many factors and starts with a well-prepared specification. The specification is an essential document that is intended to provide clear and precise instructions to the contractor on what is to be done and how it is to be done. It should be drafted by someone with appropriate technical expertise, and it should be clear as to what is required, and what is practical and achievable.
[top]General
The specification should be written in a logical sequence, reflecting the order in which the various processes are to take place starting with surface preparation, going through each paint or metal coat to be applied and finally dealing with specific areas, e.g. welds. It should also be as brief as possible, consistent with providing all the necessary information. The most important items of a specification should include the following:
- The method of surface preparation and the standard required
- The maximum interval between surface preparation and subsequent priming.
- The types of paint or metallic coatings to be used, supported by relevant standards.
- The method/s of application to be used.
- The number of coats to be applied and the interval between coats.
- The wet and dry film thickness for each coat.
- Where each coat is to be applied (i.e. shops or site) and the application conditions that are required, in terms of temperature, humidity, etc.
- Details for treatment of welds, connections, etc.
- Rectification procedures for damage, etc.
Specifications may be either ‘performance’ based or ‘prescriptive’. Performance based types allow the specifier to choose those treatments that will achieve a desired life to first maintenance whilst the prescriptive types set out detailed requirements and include specific processes and coatings e.g. The National Highways Series 1900[1]
[top]Protection systems for buildings
There are many sources of information to provide details of surface treatments and coatings to enable a suitable specification to be prepared. This exercise should take into account the relevant industry standards, environmental conditions, coatings manufacturers products and contractors facilities. There may be several alternatives for any particular structure and the optimum system should provide the required durability at least cost.
To assist the specifier, a set of 10 tables of ‘standard’ systems for buildings have been prepared based upon industrial experience and modern practices. The systems consider the different environmental categories that occur in interiors, exteriors and special conditions including swimming pools and steelwork in perimeter wall configurations. An example for internal steelwork that is visible is shown below. Refer to ‘standard’ systems for buildings for notes to the table.
| System number | IV-C3-A | IV-C3-B | IV-C3-C |
|---|---|---|---|
| Structure life | 50+ | 45 | 40 |
| Coating life | 40+ | 25 | 20 |
| Nearest equivalent BS EN ISO 12944-5[2] | - | C3.07 | C3.06 |
| Surface preparation to BS EN ISO 8501-1[3] | - | Blast clean to Sa 2½ | Blast clean to Sa 2½ |
| Factory applied coatings | Hot-dip galvanize to BS EN ISO 1461[4] (note 2) |
i) Zinc phosphate epoxy primer 80μm (note 1) ii) High build epoxy MIO 100μm |
i) High build zinc phosphate epoxy primer 120μm (note 1) ii) High solid aliphatic polyurethane finish 60μm |
| Site applied coatings | (note 3) | Recoatable polyurethane finish 60μm | - |
Notes to tables:
- The thickness values given for primers are the total thickness used and may include a prefabrication primer. For example, 80µm can be in one coat or as 20µm prefabrication primer plus 60µm post fabrication primer.
- For steel profiles over 6mm thick the minimum average thickness of galvanized coatings to BS EN ISO 1461[4] is 85µm.
- To enhance durability and improve the aesthetic appearance, the hot-dip galvanized coating can be treated with a mordant wash or ‘T’ wash followed by a vinyl primer at 40µm and a vinyl finish at 60µm.
[top]Protection systems for bridges
The majority of steel bridges are protected according to the requirements of the National Highways, and Network Rail standard specifications (see below). Alternative coating systems and methods may be specified for other bridges but the same standards and principles of good coating practices should similarly be applied.
[top]National Highways
The National Highways requirements for new structures are described in the Manual of Contract Documents for Highway Works (MCDHW):
- Volume 1: Specification for Highway Works, Series 1900 Protection of Steelwork Against Corrosion[1]
- Volume 2: Notes for Guidance on the Specification for Highway Works, Series 1900: Protection of Steelwork Against Corrosion[5].
These documents consider the environment, accessibility, required durability of the systems and finish colour. The factors to be taken into account when selecting an appropriate system are described, and a summary table of suitable protective systems for bridges (Table 19/2B) is presented below.
| System type | Access type | Metal Coating | 1st Coat | 2nd Coat | 3rd Coat | 4th Coat | Minimum total dft of paint system (μm) |
|---|---|---|---|---|---|---|---|
| I | R | - | Zinc Rich Epoxy Primer (2 pack) | MIO HB QD Epoxy undercoat (2 pack) | Epoxy Acrylic (2 pack) or Polyurethane (2 pack) or Organic Modified Polysiloxane (2 pack) finish | - | 275a |
| Item 109 (50 μm) | Item 112 (125 μm) | Item 167, 168, 169 or 185 (50a μm) | |||||
| - | Zinc Phosphate HB QD Epoxy (2 pack) | MIO HB QD Epoxy undercoat (2 pack) | Epoxy Acrylic (2 pack) or Polyurethane (2 pack) or Organic Modified Polysiloxane (2 pack) finish | - | 300a | ||
| Item 111 (75 μm) | Item 112 (125 μm) | Item 167, 168, 169 or 185 (50a μm) | |||||
| II | D | - | Zinc Rich Epoxy Primer (2 pack) | MIO HB QD Epoxy undercoat (2 pack) | MIO HB QD Epoxy undercoat (2 pack) | Epoxy Acrylic (2 pack) or Polyurethane (2 pack) or Organic Modified Polysiloxane (2 pack) finish | 400a |
| Item 109 (50 μm) | Item 112 (125 μm) | Item 112 (125 μm) | Item 167, 168, 169 or 185 (50a μm) | ||||
| - | Zinc Phosphate Epoxy (2 pack) | HB Glass Flake Epoxy (2 pack) | Epoxy Acrylic (2 pack) or Polyurethane (2 pack) or Organic Modified Polysiloxane (2 pack) finish | - | 525a | ||
| Item 110 (25 μm) | Item 123 (400 μm) | Item 167, 168, 169 or 185 (50a μm) | |||||
| - | Zinc Phosphate HB QD Epoxy (2 pack) | MIO HB QD Epoxy undercoat (2 pack) | MIO HB QD Epoxy undercoat (2 pack) | Epoxy Acrylic (2 pack) or Polyurethane (2 pack) or Organic Modified Polysiloxane (2 pack) finish | 425a | ||
| Item 111 (75 μm) | Item 112 (125 μm) | Item 112 (125 μm) | Item 167, 168, 169 or 185 (50a μm) | ||||
| III | R or D | - | Zinc Rich Epoxy Primer (2 pack) | MIO HB QD Epoxy undercoat (2 pack) | - | - | 200 |
| Item 109 (50 μm) | Item 112 (125 μm) | ||||||
| - | Zinc Phosphate HB QD Epoxy (2 pack) | MIO HB QD Epoxy undercoat (2 pack) | - | - | 225 | ||
| Item 111 (75 μm) | Item 112 (125 μm) | ||||||
| - | Water based epoxy primer for blast cleaned internal surfaces (2 pack) | Water based epoxy undercoat/sheen finish for internal use (2 pack) | - | - | 225 | ||
| Item 113 (100 μm) | Item 114 (100 μm) | ||||||
| IV | R or D | Hot-dip Galvanizing | ‘T’ Wash | Zinc Phosphate Epoxy (2 pack) or MIO HB QD Epoxy undercoat (2 pack) or Extended Cure Epoxy MIO (2 pack) | MIO HB QD Epoxy undercoat (2 pack) or Extended Cure Epoxy MIO (2 pack) | Epoxy Acrylic (2 pack) or Polyurethane (2 pack) or Organic Modified Polysiloxane (2 pack) finish | 225ab |
| Item 155 (or sweep blast) | Item 110, 112 or 121 (25c μm) | Item 112 or 121 (125 μm) | Item 167, 168, 169 or 185 (50a μm) |
Notes:
a) Min. dft increased by 50 μm if item 185 finish is specified.
b) Min. dft increased by 125 μm if items 112 or 121 specified for 2nd Coat.
c) Min. dft increased by 100 μm if items 112 or 121 specified for 2nd Coat.
Key:
R = Ready
D = Difficult
HB = High Build
MIO = Micaceous Iron Oxide
QD = Quick Drying
dft = dry film thickness
For the purposes of maintenance painting, new structures are described as either ‘Ready Access’ where there are limited restrictions for working, or ‘Difficult Access’, where a structure crosses a busy motorway or railway. The minimum requirements for the durability of coating systems are currently as follows:
- No maintenance for 12 years.
- Minor maintenance from 12 years.
- Major maintenance after 20 years
For specification of the colour, reference is made to the BS 4800[6] range, description and any special finish e.g. gloss/low sheen.
[top]Network Rail
Network Rail’s requirements for protective treatments to be used on bridges are given in the following documents.
NR/L3/CIV/039[7] defines the requirements for the assessment, certification, registration and specification of protective coatings and sealants for use on Network Rail’s infrastructure. NR/L3/CIV/040[8] defines the requirements for the selection and use of protective coating systems, and NR/GN/CIV/002[9] supports NR/L3/CIV/040[8] by providing guidance and information on the selection, and application of such systems.
A summary table of the main systems for new works is reproduced here. Other systems are available, e.g. systems suitable for the interior of box girders. Refer to NR/L3/CIV/039[7] for full details.
The choice of protective treatment depends upon the life requirement of the structure, the environment, and maintenance requirements.
| Reference number | Title | Surface preparation and profile | Coats (stripe coats omitted) | Estimated cost £/m2 (2008) | |||
|---|---|---|---|---|---|---|---|
| A | B | C Intermediate coat |
D Top coat | ||||
| N1 | Thermally sprayed metal / epoxy | Sa3 70 to 100μm |
Aluminium or Zinc 100μm min. |
Epoxy sealer (Item 7.1.1) 25μm max. |
High solids epoxy primer (Item 7.1.4) 150μm min. or Epoxy intermediate coat (Item 7.2.2) 150μm min. |
Anti-graffiti paint: Polyurethane coloured finish (Item 5.6.3) 50μm min. or Acrylic urethane (Item 7.3.1) 50μm min. or Polysiloxane (Item 7.3.4) 50μm min. |
35 |
| N2 | Epoxy glass flake | Sa2½ 70 to 100μm |
Epoxy blast primer (Item 7.1.2) 25μm min. |
- | Epoxy glass flake (Item 7.2.3) 400μm min. |
28 | |
| N4 | Epoxy MIO | Sa2½ 70 to 100μm |
Epoxy blast primer (Item 7.1.2) 50μm min. or Zinc rich epoxy blast primer (Item 7.1.3) 50μm min. |
High solids epoxy primer (Item 7.1.4) 100μm min. or Epoxy intermediate coat (Item 7.2.2) 125μm min. |
Epoxy intermediate coat (Item 7.2.2) 125μm min. |
25 | |
Ideally, the performance data for a protective coating product/system (usually obtained from the results of accelerated ageing tests) will indicate a nominal 25 year service life to first maintenance. However, products and systems that have previously been tested under the 3000 hours (15 years) criteria in accordance with Issue 4 (RT/CE/S/039 (RT98)[10]) may continue to be used. The time to first major maintenance assumes that the protective treatment is applied by suitably competent operatives such as those certificated under the ICATS scheme. The environment is classified in accordance with BS EN ISO 12944-2[11]. The corrosivity categories (C grades) for exterior environments are designated as:
- C2 - Low
- C3 - Medium
- C4 - High
- C5 - Very High
Generic descriptions of these exterior environments are provided in NR/GN/CIV/002[9].
For colour, top coats are normally required to have a Class A Match to BS 4800[6] or BS 381[12] shade.
[top]References
- ↑ 1.0 1.1 Manual of Contract Documents for Highway Works: Volume 1 - Specification for Highway Works, Series 1900 Protection of steelwork against corrosion, August 2014, TSO
- ↑ BS EN ISO 12944-5: 2019, Paints and varnishes, Corrosion protection of steel structures by protective paint systems, Protective paint systems, BSI
- ↑ ISO 8501-1: 2007, Preparation of steel substrates before application of paints and related products. Visual assessment of surface cleanliness. Rust grades and preparation grades of uncoated steel substrates and of substrates after overall removal of previous coatings, ISO
- ↑ 4.0 4.1 BS EN ISO 1461: 2009, Hot dip galvanized coatings on fabricated iron and steel articles. Specifications and test methods. BSI
- ↑ Manual of Contract Documents for Highway Works: Volume 2: Notes for Guidance on the Specification for Highway Works, Series 1900: Protection of Steelwork Against Corrosion, August 2014, TSO
- ↑ 6.0 6.1 BS 4800: 2011, Schedule of paint colours for building purposes. BSI
- ↑ 7.0 7.1 7.2 7.3 NR/L3/CIV/039: Specification for the assessment and certification of protective coatings and sealants (Issue 5), Network Rail, 2009
- ↑ 8.0 8.1 8.2 NR/L3/CIV/040: Specification for the use of protective coating systems (Issue 2), Network Rail, 2019
- ↑ 9.0 9.1 9.2 NR/GN/CIV/002: The use of protective coatings and sealants (Issue 5), Network Rail, 2009
- ↑ RT/CE/S/039: Specification RT98L - Protective treatment for rail track infrastructure, Issue 4, February 2002
- ↑ BS EN ISO 12944-2: 2017, Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Part 2: Classification of environments, BSI
- ↑ BS 381C SET:1996 (R2002), Specification for colours for identification, coding and special purposes, BSI
[top]Resources
- Steel Buildings, 2003, The British Constructional Steelwork Association Ltd.
- Chapter 12 – Corrosion Protection
[top]Further reading
- D.Deacon & R.Hudson (2012), Steel Designer’s Manual (7th Edition), Chapter 36 - Corrosion and corrosion prevention, The Steel Construction Institute.
- D.A. Bayliss & D.H.Deacon (2002), Steelwork Corrosion Control (2nd edition), Spon Press.
[top]See also
- Corrosion of structural steel
- Influence of design on corrosion
- Surface preparation
- Paint coatings
- Standard corrosion protection systems for buildings
- Metallic coatings
- Inspection and quality control