The subgrade (ground Conditions) should be assessed, looking at factors that will impact on the design of paving base such as the plasticity and the height of the water table. The control of surface and ground water will ensure that the Portland Stone paving performs well, so it is important that the foundation and the area surrounding the paving is adequately drained. The Portland Stone has excellent frost resistance, but a build-up of salts from contaminated surface water or poor drainage could result in salt crystallisation that could cause accelerated weathering. See BS 7533 – 101 Section 6 and Annex D.
The design of the base will be influenced by the ground conditions and the likely loadings and traffic on the paving. There are a number of differing options for the base including concrete bases or graded aggregate, but the control of any ground movement and ground water should be the key consideration. See BS 7533 - 101 Section 6 and Annex D. Bases that may causing staining, such as bituminous should be avoided.
We would suggest that our Portland Stone was best suited to Traffic Categories 1 to 4, which is pedestrian areas only to urban footways with no planned vehicle use and pedestrian areas used by light commercial vehicles, emergency, and maintenance vehicles.
For applications involving heavier loadings, we would suggest that a concrete or reinforced concrete base laid to a fall could be the best option. Further information on the base including the minimum thickness is set out in Table 24 in BS 7533.
It is important that the bedding does not contain any materials (for example builders’ sand) that could leach into and stain the Portland Stone. Ideally, they should be as permeable as possible. The stones should be fully bedded when laid, ensuring the edges are fully supported. See BS 7533 - Section 101 5.3 & 5.4.
The optimum size for Portland Stone external paving is either 400mmx400mm, 600mmx400mm, 600mmx600mm or gauged widths (max. 500mm), random lengths. All these sizes are suitable at 30mm thickness for most pedestrian only application, and the gauge widths random length due their lower wastage will have a lower carbon footprint and lower price. Larger stone sizes are readily available but may require lifting equipment for installation. Larger stone sizes will result in a higher wastage therefore increasing the carbon footprint and price. To calculate the thickness guidance is given in BS 7533 – 101 Section 6.6 & Annex F, but we would suggest that the minimum thicknesses for a 600mm x 600mm Portland Stone is 40mm for Traffic category 2, 50mm thick for traffic category 3 and 75mm thick for traffic category 4, subject to an engineer clarification that these are suitable for the intended loadings.
We suggest that our stones are cut vertically to the bedding plane rather than the normal horizontally, as the stone is free of horizontal weaknesses and stones shell formation are more visually pleasing.
The joint width will be dependent of whether the surface is bound or unbound. We would suggest that unbound is probably only suitable to pedestrian use and fine washed sand or Portland stone dust can be brushed into joints varying from 2 to 5mm. The pointing mortar can be, 1: 2: 8 or 9, white cement, lime, and Portland Stone Dust. Lime mortars may be appropriate in some applications.
For a bound surface, typically a Portland stone dust and white cement would be laid in a 6mm joint +/-1.5mm. See BS 7533 – 101 Section 5.5.
All Albion Stone’s Portland Stones are regularly tested for their slip resistance, and with the recommended honed finish have excellent slip resistance in dry and wet situations.
Portland Stone has a relatively high water absorption, so in certain circumstances algae may form on the stone surface and reduce the slip resistance. We suggest that the stones are regularly brushed with a stiff broom to discourage the algae growth. If the build-up becomes a problem, gentle jet washing is recommended for bound surfaces and a suitable algaecide in accordance with the manufacturer’s instructions can be used for unbound surfaces, ensuring the runoff is controlled, see the Cleaning & Maintenance section below.
We do not recommend the application of de-icing salts or compounds onto our stones, and particularly onto frozen stones where a rapid localised rise in temperature can cause spalling on the face of porous natural stones. The situation is then further aggravated by the ingress of this salt and water solution into the stone which can crystallise, causing further spalling on the surfaces. There are underground heating options that have been successfully used with stone pavers that could be consider for sensitive locations. It should be noted that if snow and/or ice has been removed by manual methods, then care should be taken to avoid unintentionally making the paving more slippery.
Resistance to Pedestrian traffic
Albion Stone’s Portland Stones are regularly tested to BSEN 14157 for abrasion resistance and the results typically range between 22 and 26. Although this suggests that some are suitable for the most intensive applications such as railway stations, we would suggest that moderate applications with footfalls of 5 million pedestrians are more appropriate for our stones.
The Portland Basebed tends to perform less well in the salt crystallisation tests, and as salts can sometimes be washed into paving, we do not recommend these stones for paving applications. We are also aware that many paving projects particularly threshold steps in Central London are Basebed and have demonstrated excellent service over several hundred years.
There are a number of differing types of pedestals that can be used for external paving. They allow the easy access to the void under the stones and it is the stone’s weight rather than grout and bedding that holds them in place. It is important to ascertain that the pedestals provide enough support for the intended loadings based on the stone’s flexural strength.
Portland Stone Steps and other Landscaping features
Portland Stone can be laid as treads laid on a similar base and laying course to the paving and risers seated on top of the tread, set against mortar, and secured with a stainless-steel pin. Monolithic solid Portland Stone steps can also be laid similar to the treads, either step on step, or step behind step. If these monolithic steps are bridging a void, then the stone’s strength should assess against the likely loadings.
The pallets of finished stone are wrapped in recyclable plastic to protect the stone during the transportation, unloading and limited site storage, not for prolonged external storage.
All stone need to be carefully handled on site by experienced fixing operatives. The designer should also ensure that the relevant lifting and manual handling regulations are considered at the design stages.
The best method of achieving satisfactory finished work, free from unsightly staining, mortar accretions and smearing, is prevention and good workmanship from an experienced Portland Stone Fixing Contractor. Care should be taken to avoid damage and suitable protection may be required, but if a replacement stone is required, Albion Stone will attempt to complete the work as expediently as possible.
After initial laying, the paving may dry inconsistently and a small proportion of organic compounds that are water soluble may migrate to the surface of the stone.
This slight discolouration will normally settle down very quickly and will not require any additional cleaning. We do not recommend sealing the stone for external paving.
In protected areas, the stone may require light brushing with water and a suitable mild detergent such as- Lithofin Power Clean, followed by a clean water rinse in order to remove air borne dirt. For any further contamination to these areas, contact the Lithofin Technical Line for guidance on cleaning these areas
Areas beneath trees and persistently damp sheltered areas may be subject to algae growth, particularly during the first two years of installation. This growth can be controlled by brushing with a stiff broom or gentle jet washing, ensuring the grouting is not dislodged, and/or the application of a specific stone- algae, spore remover -Lithofin outdoor cleaner. Once this deep clean is completed the spore can be inhibited with the use of the product Lithofin Algex. Please ensure that runoff from any cleaning products is carefully controlled.
As with cladding, any defective or leaking rainwater pipes that spill onto Portland Stone paving will over a period of time, cause a decline in the condition of the affected units and should be rectified. For any further contamination issues such as rust staining, you can contact the Lithofin Technical Line for guidance on cleaning these areas
Albion Stone, a fourth generation family business pride ourselves on having a helpful and skilled workforce as well as modern and traditional manufacturing process to enable us to produce the highest quality Portland Stone with minimal environmental impact.