11.02 Selection and testing

Categories: Stone Cladding

Introduction
The selection of stone requires consideration of a large number of factors which may be grouped as follows:

• Material characteristics

The stone may be initially chosen for its aesthetic qualities but it must have also have suitable mechanical properties and durability, be available is sufficient quantity at an affordable price and in the required sizes, image.
• Joint design

Jointing between the individual stones will depend on the type of stone selected and method of fixing, image.  Joints may be sealed, or left open as in a rainscreen, in which case they may or may not contain a baffle. The form of joint selected may affect the overall appearance of the wall and may be decided at an early stage.  However, final joint design cannot be finalised until a stone has been selected.  For instance, sealants have to be compatible with the stone and porous stones may be susceptible to staining from the sealant.  Appropriate test methods for assessing the susceptibility of the stone to staining can be carried out in accordance with ASTM C 1248 or ASTM D 2203 depending on the stone type.  The results from these tests may in fact preclude the use of such stones.
• Fixing design

Selection of a particular stone will depend on the method of fixing and fixing spacing, image. These cannot be finally designed until the stone has been selected; however, a preliminary fixing design is required before the stone can be selected. The final design of fixings needs to be considered once the panel size, thickness and finish have been tested for durability and strength in the orientation that they are designed to be used.

The selection of stone thus has to proceed in parallel with the design of fixings and joints.
 

Preliminary selection
The preliminary choice of a stone is dependent upon cost, availability, colour, surface finish and workability. It is also important to check at the preliminary selection stage that the maximum panel size determined by the cladding layout and proposed fixing assembly will not exceed what can be processed from the quarried blocks.

It is necessary that close collaboration between the construction professionals and stone producers takes place during the preliminary selection procedure to avoid testing stone that is visually correct but structurally weak in the orientation that it is to be used on the building.

In the case of sedimentary rocks (e g. limestone) the size of the stone panels is limited by the depth of the bedding layer.  It is also a prerequisite that this type of stone should be orientated such that the stone is used in the natural bedding plane. An igneous or metamorphic rock is not restricted by the presence of bedding layers but is controlled by the presence of natural joints within the rock.

The process of communication between the construction professionals should answer the list of questions below before considering any stone for testing:

• Does the chosen stone have the required design strength ?
• Is the desired surface finish achievable ?
• What is the most suitable thickness of the stone for use ?
• Can the size of stones for cladding panels be cut from the quarried blocks ?
• Can the quarry guarantee the quantity and quality of the material ?
• Can the quarry guarantee the delivery time for the project ?

Durability and strength data on the stone should be gathered either from the quarries or by viewing similar stone facades.  Stone is a natural material liable to variation in strength within a single bed.  In some quarries, these beds are sufficiently regular in character for selection to present no difficulty.  In other quarries, the variations are such that individual blocks from which the units are cut will need to be considered on their merits.
 

Testing
Testing may be carried out on the stone(s) chosen as a result of the preliminary selection process to determine their suitability as an engineering material.  The design criteria within BS 8298 cover stone cladding slabs up to 700 x 1000mm of specific thickness; however, panels limited to 900 x 600mm of specific thickness do not require additional testing.  Panels outside this size or where durability is considered an issue because of the thickness of the panel or surface finish, BS 8298 requires panels to be selected by test and calculation.

The selection and testing procedure specified within this document gives a full representative test regime to follow for selection of stone for such panels. Preliminary tests are carried out to establish the suitability of a stone within the quarry and production tests are carried out as part of an on-going quality assurance check to monitor the consistency of the stone properties.  During the testing procedure it may become apparent that a stone has a more than adequate strength and large factors of safety can easily be assumed within the design.  Alternatively a stone may be loaded almost to its full strength for the design to work.  However, one can design to accommodate the strength or weathering qualities of a stone rather than discard a stone that does not seem suitable at first.  This can be achieved by either consistent use of smaller panels, thicker panels, improved support conditions or a combination of these factors.

Where strengths have to be accurately known because low factors of safety are required to make the design feasible, or where limited data are available for the selected stone(s) then a more extensive test programme may be required.  Conversely a modified test regime involving fewer tests may be more appropriate for a stone that is not stressed so close to the point of failure or where strength and durability characteristics are well documented.

It is important while planning the selection and testing programme to take account of the time-scale involved so as not to delay construction. The required lead-in time for testing may vary depending on the engineer’s knowledge of the proposed stone’s characteristic strengths and weaknesses.  However, because of the inherent complexities of natural stone it is often necessary to test the stone for project specific applications.  As a guide the table below sets out the minimum time scale required to conduct various tests, including sample preparation. These tests are described in Section 11.03 and Section 11.04 as appropriate. Additional time will be required to obtain samples from the quarry.
 

Preliminary tests
Preliminary tests are only carried out when specifying exterior stone for large projects, or where the unique design of the building necessitates. The preliminary testing should be carried out at least 3 to 6 months before the stone is to be installed on site, allowing sufficient time for further selection and testing of an alternative stone or modification of the stone specification if the initial stone is found to be below the specified minimum requirements.  Stone provided for preliminary tests should be taken from different locations within areas intended for quarrying during the production phase of the project, and as indicated on the submitted map of the quarry.  A sample of stone produced for testing purposes should include 20 specimens and the minimum number of specimens required should be 20% greater than the number of tests specified to account for breakages.

Stone tests are specific to a particular bed within a quarry, therefore it is important to establish that the specimens submitted for testing are part of the same sample block from a particular level of quarry from which the contract supply will be produced.  The range of tests to be performed on the specimens of stone are given in the table below. Full details of durability and strength tests are covered in detail in Section 11.03 and Section 11.04 respectively.

The schedule of preliminary tests is based on North American practice and relates to testing of stone that will be mechanically fixed as part of a rainscreen or curtain wall construction.  The schedule to adopt for any project will depend on the scale of the project, historic test data available and the likely stress the stone will be subjected to during the life of the building.

The full schedule of tests are outlined in steps (a - h).  For preliminary testing it is necessary to test from (a - d) inclusive together with test (e or f) depending on the orientation of the stone on the building.  Where durability of the stone is an issue because of the large volume of stone being used, or the environmental conditions the stone will be subjected to, then aged-strength test (g) may need to be specified.
 
 

This section is only concerned with tests to determine basic material properties. Requirements for tests on cladding panels and fixings as part of the overall cladding design are described in Section 11.08.

If the preliminary results indicate the stone to have sufficient strength and durability for its purpose, only at this stage would it be appropriate to test the long term behaviour of the stone via the freeze-thaw and thermal stability tests.  These durability tests are laboratory based tests used to give an indication of the long term performance of the stone.  However, a true representation, and one that has an advantage over laboratory tests, would include an inspection of various buildings which have been constructed using the same stone and exposed to similar environmental conditions.  Inspection may not be possible for some of the more ‘exotic’ stones that have not been previously used in the UK, but where applicable will act as a good indicator of the performance of the stone in the long term.
 

Production tests
Production testing should be used as part of an on-going quality check of the quarried stone.  Current good practice is to number each quarried block as it is extracted with a number that it retains throughout the quarrying, testing and cutting process.  Each stone block should be stored at the quarry in the orientation that it was extracted to enable the production testing to be conducted systematically

Production tests should be performed on every 15 m³ of stone throughout the quarrying operation, unless otherwise agreed.  The water absorption test (a) or bulk specific gravity test (b) are used as early indicators of a stone’s possible weakness. If there is a discrepancy between the production and the preliminary results it is at the discretion of the engineer to conduct further testing of adjacent blocks of stone to find the extent of the weakness through the quarried stone and if appropriate modify the specification accordingly before further production of the stone commences

If the preliminary test data show a wide variation of results then it would be appropriate to carry out a strength test (c, d or e), whichever is considered most appropriate in the orientation and thickness that the stone is to be used.
 

As production tests continue a stock-pile of the stone will be cut, shaped and collated ready for transportation to site.  This procurement route will limit the potential for substandard stones being transported to site with the possibility of the panels failing prematurely