Clay roof tile durability
Dr Andrew Smith, Head of Sustainability and Construction Materials at Lucideon, discusses a new single frost-resistance test standard for the whole of Europe.
In late 2013, a revision to the clay roof tile product standard, EN 1304 – Clay Roofing Tiles for Discontinuous Laying, was published, incorporating a reference to the revised EN 539-2 Test for Frost Resistance. Until then, frost resistance testing for clay roof tiles in Europe was based on four different test methods (Methods A–D), each of which was related to a specific geographical region of Europe.
In the UK and Ireland, test Method D was used to assess the frost resistance of clay roof tiles for the climatic conditions typical of the area. The use of multiple test methods for a single property was seen as detrimental to pan-European trade – member states, along with local construction rules and regulations, would only acknowledge frost-resistance test certificates relating to their geographical region. Prior to the new test method, if manufacturers sought pan-European sales coverage, they were required to test the same roof tile properties four times, to different methods but for the same property.
It has taken more than 10 years to develop a single frost resistance test method to replace the four geographically related procedures. The frost resistance of a clay roof tile is now based on the number of damage-free cycles the products achieve during the assessment. Each of the European geographical regions now has a defined number of cycles that act as a threshold for compliance with frost resistance in their specific climatic zone.
For the past 25 years, Lucideon has worked closely with the UK’s clay roof tile manufacturers in testing their products to show compliance with the European Product Standard EN 1304. The change in the standard did not come out of the blue – we have worked closely with colleagues from CTMNC in France who have been at the forefront of the development of the new standard and have undertaken numerous trials to help develop and refine it, not least in aligning the results of the new test method with those of the old. This ensures that frost resistance today means the same as it did five years ago, when it comes to the laboratory test method assessment process.
In 2006, with the revision of EN 539-2, the new ‘single European’ frost-resistance test was introduced as a fifth option to the standard, to enable manufacturers and test houses throughout Europe to become familiar with the new test method and to allow evaluation with the existing regional methods.
Out with the old
For the UK, nearly everything has changed. The table on the previous page shows a comparison between the old UK method and the new Single European Test Method. For a start, we now only test six tiles per set rather than up to 21 plain tiles. Before testing, the tiles are now soaked by ‘progressive immersion’ over five days, each day the depth of the water increases by 1/5 of the height of the tile. This provides a greater level of saturation for some tiles as the progressive nature of the immersion results in capillary uptake of water into the fine pore structure. Each tile is now tested individually, rather than as an installed roof system and, prior to testing, a linen or hessian cloth is fixed to the rear side of the tile before being placed in the frost rig. Finally, freezing is undertaken in air and thawing is achieved by the tiles being immersed in water. The freeze-thaw temperature profile is now controlled by a thermocouple embedded in a calibration tile, rather than just on air temperature measurements.
New tests, new equipment
In order to accommodate the fundamental change to the frost-resistance testing methodology, Lucideon collaborated with Butler Refrigeration to design and commission the construction of a new freeze-thaw test rig. As with all bespoke testing rigs, it was a steep learning curve. We took the opportunity to design a rig that could accommodate 16 sets of six clay roof tiles simultaneously, but would also be capable of testing natural stone construction products that share the same fundamental methodology.
The new rig is composed of four water tanks. The upper two header tanks are for conditioning and freeze-thawing. One of the lower tanks is for conditioning, where the progressive immersion takes place and products can be stored under water prior to testing, and the other lower, freeze-thaw tank is where the tiles are subjected to the cyclical freezing and thawing process.
One of the prerequisites for the design was that the water would be recycled, enabled by the header tanks. This saves more than 1,000 litres of water per cycle, and gives a higher degree of control over the temperature of the water, as heating elements are present in both the conditioning tank and freeze-thaw header tank.