Stone Federation publishes guidance about how to select the correct stone for structural stone projects.
This introductory document sets the context for the project and provides a series of definitions to help deliver clarity around the different building types that come under the banner of structural stone.
A Guide to Structural Stone is designed to be an ever-evolving library of resources equipping architects, structural engineers and clients with the information and inspiration required.
As part of this ever-evolving library of resources, we are delighted announce the launch of a new chapter of A Guide for Structural Stone which looks at how to select the correct stone for a structural stone projects.
Part 1: Selecting the correct stone
Scope
This is one of a series of documents that will be published by Stone Federation as part of an on-going process of delivering useful, correct and relevant information regarding the use of structural and load bearing stone.
This document has been produced as there are currently no British Standards in place that authoritatively cover this subject matter.
Introduction
We are at the beginning of an exciting phase for stone. Whichever part of the industry you occupy, the adoption of this technology will further promote its use. The nature of the material readily lends itself to changing aesthetic taste and technical use.
Approached in a considered and robust manner, the increased use of stone structurally should become a natural alternative to commonplace elements used in the design. SFGB will continue to explore and publish relevant guidance to assist the industry in its pursuit of this common goal.
When looking to build with stone to provide the structure of the building, it is even more important to ensure that the right type is chosen for the load-bearing requirements that are being placed upon it.
When used in a structural or load bearing capacity, the compressive load placed upon the stone will be the greatest force it encounters, so this data is vital in assessing a stone’s suitability for the project. This is unlike typical stone cladding, covered in BS8298, where the stone is usually supported at each floor level on a building and therefore experiences less compressive load.
However, in addition to compressive strength other characteristics particularly with respect to durability are also important and this will be covered by testing for properties such as water absorption, density and freeze/thaw resistance. These testing requirements are covered in a separate document.
When selecting the correct stone for the project, we would always encourage architects to look at surrounding stone buildings or research the stone types historically used in that area. These precedent studies are often more useful than some of the testing, particularly with respect to durability and they will also give an excellent insight into how the stone performs in service.
This approach will not only ensure that the material fits well into its location but will also help to reduce the carbon footprint of the project as historically, town and cities were shaped with stone quarried nearby.
Selection Process
Understanding the geological variations in natural stone is essential for any project, as these differences can affect both its appearance and performance. It is vital to be aware that larger stones that are likely to be used for loadbearing projects make the rejection of aesthetical geological characteristics typically found in the stone more difficult, as this has a significant impact on the price and carbon footprint.
The stone extraction company should have a good knowledge of their stone and, together with the stone specialist and the project team, should determine the acceptable geological variations, the most efficient sizes and any features that could impact its loadbearing performance. They will also be aware that some features, like cracks, may or may not be relevant and should be assessed by the stone specialist and extraction company.
At the design concept and pre-tender stages, a shortlist of possible stones should be assessed against defined performance criteria, ensuring that only stones with proven or thoroughly evaluated characteristics are included in the tender documentation.
The price of the stone needs to be viable when compared to other products so when selecting the stone, carefully consider the following key criteria: –
i. Evaluate colour, texture, suitability, availability, and carbon footprint (verified EPD).
ii. Indicative samples provide only a general overview of the stone’s geological characteristics. Full reference/range samples must be agreed upon by the project team prior to tendering.
iii. Confirm adequate block availability and production capability from the extraction site and the procurement time from block order through to site.
iv. Confirm the extraction location and transport implications to the manufacturing location and then onto the site.
v. Confirm adequate manufacturing capabilities in accordance with BS EN 771-6.
vi. Demonstrate appropriate quality and environmental management, and responsible sourcing credentials.
vii. Prepare a specification that considers unit sizes in relation to block sizes and ensures economic production in manufacturing and minimises waste.
viii. Review technical data and assessing whether additional testing is necessary.
Comprehensive technical data should be available for most stones, including a history of test results demonstrating consistency in their technical characteristics. Ideally, a proven history of use in the local vicinity will provide the design team with confidence in their stone selection. If technical data or historical use is not available, the stone can still be considered if it undergoes a rigorous programme of initial testing, production control testing, and careful sample selection, all overseen by a stone specialist.
i. Review the CE Marking information including the Declaration of Performance (DoP) and other relevant technical data to ensure that they are, at a minimum, in line with BS EN 771-6 and design the project based on these strength, durability, and geological characteristic.
ii. Evaluate the need for Production Control Testing (PCT) based on the technical data and history of use.
iii. Recognise that, in many cases, the DoP will be sufficient, however, initial testing may be required to ensure that samples are representative of the agreed project range.
iv. Conduct PCT as an additional measure to initial testing, at various stages of the project.
v. Implement a detailed sampling plan, complete with test reports for traceability.
Detailed guidance on the preferred approach to stone samples and testing (including PCT) is provided in the supporting Stone Federation testing document.
Stone samples, indicative and reference
Indicative stone samples.
The initial selection for a shortlist of stones to be used on the project should be made from indicative samples supplied from current extraction, normally from the producing quarry. These samples should be supported with historical technical information, showing test results in accordance with BS EN 771-6, together with a list of reference buildings in the vicinity of the proposed site.
NOTE Indicative samples show the typical colour and surface texture of the stone and should be normally approximately (600 × 600) mm. They cannot show all the natural variations in visual appearance, colour, texture and geological features that are present in all natural stones.
Samples should be labelled in accordance with BS EN 12440 and BS EN 12670, i.e. traditional name, petrological family, and the location of the quarry, including country of origin. The surface finish or grit size used should be clearly stated, to allow the specifier to accurately specify the stone.
Reference samples.
Reference samples (similar to those described in BS EN 1469 but considering clause 4.2 of this document) should be requested and include colour, texture, veining, shell content and the distribution, character and frequency of these, and all other features that are deemed to be part of the geological characteristics of the stone.
NOTE 1 The reference samples are normally held at the quarry or fabrication works and allow the designer to compare these samples with the production faces. A visit to the quarry also gives the designer an opportunity to look at the quantity and typical sizes of the block and to compare these with the project requirements.
The number and size of control stone samples should be sufficient to show the designer the range and distribution of the geological characteristics of the stone and should normally be made up of at least several square metres and a number of different panels and this may be needed in more than one orientation.
The reference samples should typically be viewed following the guidance presented in BS EN 1469 and compared with the indicative samples by the designer, with the assistance of a stone specialist, particularly for stone that does not have a proven history of use.
NOTE 2 To compare the reference samples with the actual production, look at the samples in an external environment in normal daylight conditions and in both dry and wet conditions.
Once approved, the reference samples should be located at the manufacturing site and form part of the PCT for the project. Duplicate reference samples should be retained at the quarry (where this is separate from the production site) or the project, and be recorded, approved and photographed by the designer before despatch.
NOTE 3 There are likely to be cost and even programme and sustainability implications if the approved reference samples exclude some geological characteristics typically found in the stone.
NOTE 4 Please see the testing document for the information on samples for testing.
If any dispute arises on site about the visual characteristics of the stone, the installed stone should be compared with the approved reference samples and viewed from at least 2 m.
If, for whatever reason, it is proposed to use a different stone, its suitability should be verified in accordance with BS EN 771-6.
This guidance is an excerpt from the new Selecting the Correct Stone chapter of A Guide to Structural Stone. The full chapter can be viewed at www.structuralstone.uk.
