The NSSS does not give a menu of coating specifications from which a surface treatment can be chosen. The requirements to be given in the Project Specification can either be given in performance terms or as prescriptive specifications for the surface preparation and any subsequent protective treatment. As the choice of coating system(s) has a significant effect on the fabrication, transport and erection processes, it has become more common for the Employer to provide a performance specification that then allows the Steelwork Contractor to select the actual products to be used. Section 10 of the NSSS has been written in performance terms. This procedure also allows the Steelwork Contractor to work with a preferred partner for supply of the protective treatment products. If any difficulties are then experienced with treatment, these are more readily resolved using the single sourcing assumed in the NSSS (see 10.1.1(ii)). This procedure also allows the Steelwork Contractor to match the performance of:
shop-applied treatments to fabricated steelwork,
coatings pre-applied by suppliers during manufacture of fasteners (see 10.5.3) and specialist products such as purlin systems,
work undertaken on-site to site weld areas (see 10.5.3), to rectify any damaged coatings and if top coats need to be applied there.
Guidance on various coatings including their life and maintenance characteristics is available in 'Steelwork Corrosion Protection Guides' and in several publications from Corus, BCSA, The Paint Manufacturers Association and the Zinc Information Centre.
The most fundamental issues to be considered with respect to protective treatments are:
the life expected of the structure,
the environmental conditions that will be experienced by the various parts of the structure, what maintenance regime for the protective treatment is practical,
whether the objective is protection against corrosion, fire or both,
whether there are architectural or engineering constraints on the choice of protective treatment, for example, choice of colour or surface appearance of top coats, or providing top coats that are readily cleanable.
The general choices are:
no treatment necessary, for example when steelwork is to be embedded in concrete (see 10.6),
surface preparation only with no further treatment (see 10.2), sprayed metal coatings (see 10.3),
hot-dip galvanizing (see 10.4),
paint treatments that may include fire-resistant intumescent coatings (see 10.5).
Suggested paint systems are given in the table at Annex B.
(i) — Requirement for surface preparation
The choice of surface preparation method is left to the Steelwork Contractor, as he needs to ensure that the surface is suitable for the application of any subsequent treatment. Occasionally, the Engineer may have additional particular requirements. For example, the surface preparation method can affect the final appearance of visually exposed steelwork, which could be important architecturally. In such cases it is often wise for the Engineer to ask for a trial sample to be used later as a benchmark. The provision of trial samples from procedure tests is useful generally for protective treatment systems that are also required to meet aesthetic criteria.
(ii) — Thickness and composition of sprayed metal coatings
The Engineer will need to be satisfied that the thickness and composition of metal spray is clearly specified. The decision about what metal can be influenced by the in-service conditions, the need for maintenance, and the likelihood of galvanic action between dissimilar metals placed in contact by the construction details. Guidance is provided in PD 6484: Commentary on corrosion at bimetallic contacts and its alleviation.
(iii) — Any requirements for galvanizing
Galvanizing is a popular option for many industrial structures and for exposed structures such as plant-supporting steelwork. The Galvanizers Association issues a Zinc Millennium Map that assists the specifier to relate performance requirements in terms of the risk of atmospheric exposure to the prescriptive requirements for galvanized coatings. Also, the Engineer and Steelwork Contractor need to be satisfied that the components are suitable for galvanizing in terms of design and materials. Again, the Galvanizers Association can provide published and verbal guidance on this, as can specialist galvanizing companies. There can be conditions that conspire to facilitate the occurrence of cracking of steelwork associated with galvanizing.
(iv) — Any requirements for paint treatment
The generic types of paint treatment are numerous, and for every generic type there are several manufacturers who can supply products. The most commonly specified options for off-site or shop protective treatments are:
organic paint treatments by brush or spray (these can be priming only, or complete or partially complete coatings),
duplex treatments with metallic and organic paint coatings.
The common options for site protective treatments are:
no further treatment,
hand touch-up of primers and applying finishing coats after erection,
complete treatment system on site after erection including surface preparation.
The system to be used for the protection for bolts must be compatible. This is generally bright zinc plating, galvanizing or sheradizing. The specifications for electroplated, galvanized and sheradized coatings on metal fasteners is given in BS EN ISO 4042, BS 7371-6 and -8 respectively. Compatibility between the treatment on the fasteners and that on the surrounding steelwork is important, and the Galvanizers Association provides a guide on Hot Dipped Galvanized Nuts & Bolts. If fasteners are protected with a film of oil this must be removed prior to painting.
Data sheets provided by paint suppliers will specify the required nominal paint thickness and acceptance criteria. A schedule of inspections at each stage of application should be planned.
This may be given in the paint manufacturer’s data sheets, the Project Specification or be part of the paint applicator’s quality system. These, together with the data sheets and the inspection plan, comprise the basic information needed for inclusion in the method statement required by 10.1.2.
(v) — Responsibility for touch-up of damaged areas and cleaning of surface treatments on site
If the painting contract is split, clear definition is required of the responsibility for cleaning down and repair of any damage that becomes apparent on site. It is usually more sensible and economic to have repairs carried out in the site-painting contract but, in either case, the paint system must be 'repairable'.
(vi) — Requirements for fire protective coating
There is an increasing usage of intumescent coatings for fire protection, and for these to be applied site. Hence, the Engineer should consider the potential advantages of specifying off-site application of fire protection that can lead to shorter and more reliable programme times, better control of quality, fewer trades on site and thus improved overall economy.
TABLE 1.6 — INSPECTIONS & TESTS - CHECKLIST
(i) — Inspections or tests to be carried out or witnessed by the Employer, Engineer or Inspection Authority
The Steelwork Contractor must be informed in the Project Specification if any of the tests required by the NSSS, or additional tests, are to be witnessed by the Employer, the Engineer or an Inspection Authority.
If the Employer intends to appoint an independent inspector, this requirement must be clearly defined because the timing and extent of testing can cause delay and increase costs (see 11.2).
(ii) — The period of advance notice required for these additional requirements
Neither party wants to waste time, so the period of advance notice required for these additional inspections or tests should also be stated.