CORDIS Archive

Durability assessment of sandwich panels intended to be used for roof coverings and wall finishes

1. CONFORMITY WITH THE WORK PROGRAMME

This topic falls under the Competitive and Sustainable Growth Programme, generic activity Measurement and Testing. Specifically, it is related to Objective GROW-2000-6.2.1 Methodologies to support standardisation and Community policies for which expressions of interest have been called. It is also relevant for Key Action 1, Innovative Products, Processes and Organisations.

This research can be considered in the frame of the Construction Products Directive (CEE/89/106). Within the CPD-mandates, M/121 "Internal and external wall and ceiling finishes" and M/122 "Roof coverings, rooflights, roof windows and ancillary products", the durability aspects of products are an important issue which need to be reflected in harmonised standards, preferably through test methods.

There is a lack of appropriate EN or EN ISO standards to determine or assess the lifetime of sandwich panels. The development of test methods to determine durability aspects of sandwich panels would be used to support CEN/TC128/SC7 and SC11 standardisation activities, which needs to produce harmonised standards in accordance to the above mentioned EC-mandates. The test method is therefore strongly related to EC policies.

2. KEYWORDS

CEN - Standardisation - CPD - Mandates - Harmonised standard - Test method - Roof coverings - Wall claddings - Sandwich panels - Durability assessment.

3. SUMMARY OF OBJECTIVES AND JUSTIFICATION

The objective of the research would be to support the on-going CEN standardisation activities within CEN/TC128/SC7 and SC11.

The objectives are:

• to define environmental, thermal and humidity loads which influence the durability of metal sheets and (metal faced) sandwich panels.
• to develop performance tests for metal sheets and all sandwich panels, especially within the scope of the SC11 product standard (metal faced with insulation core).
• to carry out intercomparisons of the results of the newly developed test methods in a number of nationally recognised and accredited testing establishments in order to verify repeatability, reproducibility and pan European use in accordance with the approach on Conformity Assessment.

CEN/TC128/SC7 will develop two harmonised product standards, covering self-supporting and fully supported metal sheets. For these standards, a supporting test method is needed to determine:

• corrosion resistance of the metal sheets

The research results will also be valuable for CEN/TC128/SC11 standardisation activities.

The SC11 Working Group "Durability" has been active for some time and a draft product standard, incorporating relevant requirements, has been established. The TC requires inexpensive test methods to determine:

• deterioration of mechanical strength and structural safety
• physical ageing of the core insulation material
• ageing of the bond between faces and core
• deterioration of mechanical strength due to impact and cyclical loading [foot traffic]

The proposed test methods assess the integrity of sandwich panels under variable temperature and humidity conditions, and also variable load conditions. The tests are therefore directly related to the safety of the product in its intended use. For applications where the product is free standing or self-supporting as in a ceiling, the mechanical strength is totally dependent on the composite integrity of the panel being maintained.

For these test methods, research has to determine:

• agreed levels for classification ^(x)
• the repeatability and reproducibility of each test method across EU Member States in different laboratories.

^(x) The standard, based upon the results from this research, might state threshold values below which metal sheets or sandwich panels are not considered safe for their intended use.

4. BACKGROUND

Currently CEN/TC128/SC7 "Roof covering products from metal sheet" and SC11 "Self-supporting double skin metal faced insulating sandwich panels" are drafting harmonised standards in view of the CPD Mandates M/121 "Internal and external wall and ceiling finishes" and M/122 "Roof coverings, Rooflights, Roof windows and Ancillary products".

The SC7 and SC11 harmonised European standards should include indications concerning the working life of the product in relation to the intended use and the method for its assessment. The working life is the period of time during which the essential characteristics of the product meet or exceed minimum acceptable values, without incurring major costs for repair or replacement.

The CPD calls for European standards to be expressed as far as practicable in product performance terms. Preferably, the durability of construction products should be assessed using performance-based methods.

The development of the test methods to determine the durability of metal sheets and sandwich panels is necessary, because no European test methods are available, although national Finnish and Spanish standards, which cover some aspects, have been identified.

Reference documents: see additional information.

5. ECONOMIC AND SOCIAL BENEFITS

Economical benefits

The industry for roof covering and wall cladding products addresses the national markets by means of their national representation; as for the R&D; of the industry, firms are organising themselves on European level internally or externally by means of networking.

Until recently, the industry for roof covering and wall cladding products has been confronted with a variety of specifications, both based on national standards and voluntary technical specifications.

The industry-manufacturers of the products have requested European Standards for the products to specify product specifications. These are also requested by the market.

CEN/TC128 has already established important results, having published a number of product standards since it was established in 1987.

The experts in CEN/TC128 believe that the harmonised European Standards in the frame of the Construction Products Directive will remove all product related technical restrictions.

CEN/TC128 has developed a work programme in line with the CPD, covering the essential requirements as elaborated in the Interpretative Documents. Products that comply with the harmonised standards will be CE Marked and may be placed on the market throughout the European Union without any restrictions. Reduction of the barriers to trade in the EU should also improve trade.

Durability assessment, related to the performance characteristics as given in the CE Mandates M/121 and M/122, are to be dealt with in the harmonised European Standards. Preferably, CEN/TC128 would like to include performance-based criteria for durability assessment, as opposed to state-of-the-art descriptions, as requested by the European Commission in Guidance paper F.

The roofing and cladding products sold or used on the European market is supplied entirely from European suppliers/manufacturers, except for metallic products where about 30% comes from overseas.

Building practices throughout the European market have changed. One of the main requirements is now for buildings which are lightweight and which can be built quickly and to a high performance standard. Sandwich panels are the principal product used in these type of constructions and therefore fundamental to supporting the continuing growth of this construction sector and employment within it.

Note: In the UK, the growth rate for sandwich panels will be over 40% in 1999 and panels will be used in over 50% of lightweight buildings for the first time.

Safety benefits

Safety in use is a major area of importance for sandwich panels particularly because the use of the product has broadened dramatically in past few years. Historically, panels were used mainly for industrial buildings and internally in cold stores and food processing factories. Applications now cover all construction activities (offices, schools, hospitals and domestic use).

As a result, sandwich panels are becoming a commodity product. The risk that quality may be compromised and that the products may be used for the wrong end use applications is significantly greater.

Safety benefits and safety at the work place are relevant, because some of the test methods under consideration are directly related to durability (e.g. the relation between safety and durability of the adhesive between faces and core). CEN/TC128 has already established product standards for products such as safety hooks, walkways, etc.

Environmental benefits

Sandwich panels, which are factory engineered and include an insulation core, are essential to provide a healthy working and living environment and to reduce CO₂ emissions considerably.

Because of the insulation, sandwich panels have an important influence on the heating and air conditioning requirements of buildings, making them important for the reduction of scarce natural (fuel) resources.

Market interest

Industry, which is well represented in TC128/SC11 "Self-supporting double skin metal faced insulating sandwich panels", has already recognised the need for research. Most European countries are represented in SC11.

The introduction of the proposed test method should prevent the use of products whose long-term durability is doubtful in respect to the expected service life.

Sandwich panels are widely used throughout the European Community and their use is still increasing. A large number of European manufacturers of sandwich panels is represented in SC11 and they back this need for research.

This research supports European Trade by the development and improvement of test methods necessary for the development and implementation of the Construction Products Directive, which is related to trade.

6. SCIENTIFIC AND TECHNOLOGICAL OBJECTIVES

The intended uses (roof covering products and wall and ceiling finishes both for external and internal use) for a pan-European geographical region imply that a number of extreme conditions, regarding temperature and humidity, need to be taken into account.

1. Possible assessment methods

Durability assessment involves the performance testing of a product to determine the variation in its characteristics under a given action or cycle of actions. The most common types of performance testing are:

• Direct testing – the achievement of a certain level of performance is recognised as being sufficient to give an acceptable durability (e.g. fatigue tests)
• Indirect testing – the measurement of "proxy" characteristics that can be correlated to actual performance and hence durability (e.g. porosity for freeze-thaw resistance).
• Natural weathering/ageing tests – such tests either give a direct indication of durability (e.g. corrosion tests) or enable normal performance tests to be carried out after treatment, thus allowing the degradation in performance to be determined.
• Accelerated weathering/ ageing tests – as above, but with the normal ageing process speeded up to reduce the duration of the test.
• "Torture" tests – the product is subjected to conditions that are much harsher than those ever encountered in use.

Although these test methods would essentially be used for initial type testing purposes, correlation with other performance characteristics should also be established for factory production control purposes (e.g. thickness of coating for corrosion resistance assessment).

2. Description of performance characteristics to be assessed

2.1 Deterioration of mechanical strength and structural integrity

The potential change in mechanical strength of sandwich panels is particularly significant due to the composite (facing, adhesive and core) action of the panel.

Studies indicate that structural strength might be reduced by the combined effects of temperature and humidity, which affect the components in different ways.

Accelerated ageing tests, applying the principles of temperature and humidity cycling, should be used to indicate the long-term durability.

Reference documents: None available.

2.2 Ageing of the bond between faces and core

The adhesive bond between the (metal) faces and the core is vital to the satisfactory performance of the sandwich panels. The properties of the adhesive are fundamental.

This test method should simulate in a qualitative manner the forces and effects on an adhesive bond joint at metal-adhesive/primer interface. It should determine the environmental durability of adherent surface preparations. The method should be correlatable with service performance.

Reference documents:

• ASTM D3762 Standard Test Method for Adhesive-Bonded Surface Durability of Aluminium (Wedge Test)

2.3 Deterioration of the insulation value

Deterioration of insulating value has been fully investigated by CEN/TC88. An agreed test for the aged value for all the core insulation materials when used with diffusion tight facings is given in the relevant horizontal standards.

The time-related change in the thermal performance of sandwich panels is particularly significant in the case of panels with foamed plastic core materials. The thermal resistance may decline slowly during the life of the product.

Because the exposed surface of the core material is greatly reduced due to the use of diffusion tight facings, the rate of rise of thermal conductivity should be reduced significantly.

Reference documents:

• ISO 8301:1991 Thermal insulation - Determination of steady-state thermal resistance and related properties - Heat flow meter apparatus
• ISO 8302:1991 Thermal insulation -- Determination of steady-state thermal resistance and related properties -- Guarded hot plate apparatus
• CEN/TC88 (draft) product standards (see additional information)

2.4 Fire resistance

Classification of fire resistance should done according to prEN 13501-2. The determination shall be done in accordance with EN 1363-1:1999 and EN 1363-2:1999 and also in accordance with prEN 1364-2 (ceilings) or prEN 1364-6 (non-loadbearing walls).

An agreed test assembly and temperature control method should be proposed, in close consultation with both CEN/TC127 and CEN/TC128 and intercomparitive testing should be performed in nationally recognised and accredited testing establishments in order to verify repeatability, reproducibility and pan European use in accordance with the approach on Conformity Assessment.

2.5 Impact resistance

Although an impact resistance test method is already under development (TC128 WI 00128061), which is intended to be used on internal and external wall panels, additional research should be proposed for the cyclical loading of roof and ceiling panels simulating regular foot traffic (walkability).

An agreed test assembly should be proposed, in close consultation with CEN/TC128 and intercomparitive testing should be performed in nationally recognised and accredited testing establishments in order to verify repeatability, reproducibility and pan European use in accordance with the approach on Conformity Assessment.

2.6 Corrosion aspects of metal faced sandwich panels

The test method should be intended to be used both for metal sheets used as roof coverings (fully supported and self-supporting) and those used as facings for sandwich panels with an insulation core.

Both types of roof covering or wall cladding products can be used internally and externally.

Reference documents:

• ISO/FDIS 12944-2 "Paints and varnishes - Corrosion protection of steel structures by protective paint systems. Part 2: Classification of environments"
• prEN ISO 14713 "Protection against corrosion of iron and steel in structures - Zinc and aluminium coatings - Guidelines"

TC128/SC7 European standards (see additional information)

7. TIME SCALE

The research results, i.e. test methods ready to be incorporated in or referred to in a product standard, need to be ready to be used in the draft harmonised standards which TC128/SC7 and SC11 are currently working upon.

According to the CEN/TC128 Business plan, a first draft for enquiry should be established by 2001-10.

The TC128 answers to the mandates M/121 and M/122 have been established in 1999. CEN/TC128/SC7 "Roofing products from metal sheet" and SC11 "Self-supporting double skin metal faced insulating sandwich panels" have already started the drafting of harmonised standards, a working document has been established or will be in the near future.

8. IMPORTANT ADDITIONAL INFORMATION

Reference documents:

CEN/TC128 WI00128061

Title working document: Self-supporting double skin metal faced insulating sandwich panels - Specifications

Document scope: This European standard specifies requirements for the standardisation of factory-made, self-supporting, double skin metal faced sandwich panels having an insulating core bonded to both facings, intended for discontinuous laying in the following applications: roofs, walls and wall cladding and within the building envelope, to maintain an internal climate, including use as partitions and ceilings.

CEN/TC128/SC7 draft and published European standards:

EN 501:1994 Roofing products from metal sheet - Specification for fully supported products of zinc sheet
EN 502:1999 Roofing products from metal sheet - Specification for fully supported roofing products of stainless steel sheet
EN 504:1999 Roofing products from metal sheet - Specification for fully supported roofing products of copper sheet
EN 505:1999 Roofing products from metal sheet - Specification for fully supported products of steel sheet
prEN 506 Roofing products from metal sheet - Specification for self-supporting roofing products of copper or zinc sheet
EN 507:1999 Roofing products from metal sheet - Specification for fully supported products of aluminium sheet
prEN 508-1 Roofing products from metal sheet - Specification for self-supporting products of steel, aluminium or stainless steel sheet - Part 1: Steel
prEN 508-2 Roofing products from metal sheet - Specification for self-supporting products of steel, aluminium or stainless steel sheet - Part 2: Aluminium
prEN 508-3 Roofing products from metal sheet - Specification for self-supporting products of steel, aluminium or stainless steel sheet - Part 3: Stainless steel

CEN/TC88 draft European product standards: