Stuart Eydmann and Hans Halstvedt revisit a subject previously discussed in Context 41.
Recent years have seen an increase in the specification of natural stone products for streetscape schemes as local authorities, enterprise companies and the general public have come to appreciate the durability and appropriateness inherent in the use of high quality traditional materials in sensitive and prestige locations. Associated with this trend is a greater awareness of the carefully detailed use of stone in other European countries where paving skills, protected by craft qualifications and apprenticeships, are held in the highest regard.
The current revival of the use of stone is to be greatly welcomed as our towns and cities once enjoyed an outstanding tradition of quality paving and it is only right that the surface of public spaces should enjoy the same level of attention being paid to the conservation of the buildings which surround them. However, in this newfound enthusiasm a number of fundamental issues are often overlooked or ill considered by designers, specifiers, contractors and clients.
This is a crucial factor in the specification of stone paving. Typical costs of buying and laying include £4550 per square metre for 70-90 mm Portuguese granite cubes and £65-70 for Caithness slabs. if more exotic materials are used, especially where they require sawing or carving, the cost can easily be over £100 per square metre. These figures compare unfavourably with £5 per square metre for an asphalt wearing course or £20 for high quality concrete paviours. However, when it is noted that the typical total costs of reconstructing an existing road in a built up area using bituminous products is in the region of £100 per square metre and £150 if natural stone is used the real difference in costs is 'only' in the order of around 50% more. Furthermore, it is being increasingly recognised that the maintenance costs of a well laid natural stone scheme will yield long term savings compared with a conventional road as its theoretical life is considerably longer and the natural stone products will retain a residual value in that they can be salvaged for re-use.
Given the cost implications of natural stone paving, it would be foolish to undertake schemes on a shoe-string budget as this will inevitably result in wasted investment if the appropriate quality standards have not been achieved. it is always better to maintain standards and save money by reducing the scale of the area to be improved.
As more traffic calming and streetscape projects are promoted, national guidelines for target budgets should be established for different types and standards of scheme in order that appropriate ranges of natural products can be considered at the outset.
Safety
Safety is also important. The use of stone setts for carriageways is often criticised or avoided on the grounds that such surfaces are slippery and dangerous due to their low polished stone value. Studies of the relative safety of setted and asphalt road surfaces have shown, however, that when two identical streets with the same geometry and traffic are compared it is evident that considerably less accidents take place on the setts. This has been explained by the fact that the vehicle speeds on the setts tend to be much lower than on the asphalt due, in part, to driver awareness of the potential danger. This conclusion justifies the specifying of stone for both traffic calming and conservation/aesthetic reasons.
Even where stone is specified, there remains a fundamental difference between modern British practice, where the setts and slabs are laid in mortar, and the traditional dry methods employed in the past and still extensively used throughout Europe.
In the traditional method, stones are laid as closely together as possible in ash, or course sand which is brushed into the joints until they are filled. For a period, this requires regular 'topping up' but in time the fill material is joined by other small debris which contributes to a natural set. As the stones are subjected to pressure from moving traffic the flexibility of the fill encourages them to rub together and binding becomes tighter. Removal and replacement is easy, quick and seamless and the detailing allows, indeed encourages, a degree of water penetration rather than run-off to the drainage system.
Laying setts in mortar produces an inflexible, impermeable surface which can crack under heavy traffic, and repairs and relaying are impossible without obvious scarring. Often there is discoloration and staining due to mortar spillage and any surface irregularities become permanently fixed. The use of mortar requires that the setts be laid with joints up to 50 mm wide to allow filling with semi-dry mortar, and this results in an appearance which contrasts greatly with the traditional close laying found in the best surviving old schemes. in some projects the water content of the mortar is increased to allow the mortar to be brushed into narrower joints as a liquid but this can reduce the mortar strength and increase the danger of staining
The perceived convenience of mortared construction is understandable but there are currently few national specifications or codes of practice relating to the pavement design, laying and testing of natural stone products in streetscape works apart from some British and EU standards for quarrying and production. Guidance can be taken from European publications but these are seldom available in English. Most designers have therefore developed their own specifications although these have met with varying degrees of success. There are many grey areas such as a fundamental disagreement as to whether the mortared pavement should be regarded as flexible or rigid and therefore whether concrete or bitumen construction should be used as a roadbase. Furthermore, with setts or cubes regularly subjected to the impact of traffic there is little understanding of the performance of different mortar mixes and the bonding between the setts and the mortar. The comparative curing times of different products is another issue which requires to be better understood since the waiting time required may result in traffic delays and an extended implementation period which can increase the contract costs dramatically.
If we are to advocate appropriate detailing and practice then we need to understand how our streets were constructed in the past and carry out more practical work on the ground to adapt the old ways to the demands of modern towns and cities.
Contractual Issues
The selection of contractors for the tender list can be problematic for streetscape schemes as the laying of cubes and setts is normally carried out by sub-contractors. The success or failure of a scheme is largely dependent on the sub-contractor and there is a problem in the lack of sufficiently qualified and experienced firms in Britain. The main contractor should be fully aware of the standards and skills associated with the laying of natural stone products and should not assume that a block-paving or slabbing subcontractor will be adequate. Furthermore, there is a tendency for the contractor to submit rates which are too low for the stone work this, leading to subsequent problems with workmanship and quality standards.
Hopefully, as more schemes are completed indigenous sub-contractors will achieve higher standards in this field. Recognised vocational qualifications and the formation of an association of stone laying contractors would assist in the setting of standards for the industry.
It is important that adequate supervision is given by the designer who should also encourage the client, the main contractor and sub-contractors to strive for the highest quality finishes.
Strectscape work involves the local community and it is important that the construction team works with the local people, keeping them informed about the scheme in a pro-active manner rather than waiting for complaints which too often arise through a lack of consultation or communication.
In the past, granite or whin setts were used in the carriageways and Caithness or Yorkstone slabbing in the footways with heavy granite or whin kerbs. Many examples of this simple but pleasing approach to street design survive intact after more than 100 years of heavy pedestrian and traffic loadings. This tradition is still very relevant today but in many instances, such as shopping precincts or pedestrian areas, there is a desire to eliminate the kerb in order to provide shared surfaces. This raises aesthetic problems as designers often resort to unnatural lines of bollards in order to protect the slabs. Although experience gained from the use of manmade products shows that small slabbing elements can carry substantial traffic loadings we still require considerable research into the selection, dimensioning and laying of natural stones suitable for various traffic conditions.
Another design problem is the appropriateness of imported European materials and laying patterns. It is good conservation practice to use local resources but this is not always possible and stone has to be brought from outside the area. Granite paving is brought from Portugal, porphyry from Italy and quartzite slabs from Norway. The use of small cubes, usually of granite, laid in fan and arc shapes is common throughout Europe but was rarely traditional in the British isles where they are now being used with increasing frequency. This is proving controversial, particularly when the cubes themselves, and sometimes the craftworkers to lay them, are from abroad. Continental fan patterns are economic in terms of materials and exploit the arching effect of the traditional methods to the full and demonstrate how the adoption of the best traditional practices from abroad are justified where they are visually appropriate.
The design of schemes using natural materials should consider the future maintenance of the site. Public utilities will require access for the repair and maintenance of services and local authority cleansing and highways departments will have demands which have to be accommodated.
The Linlithgow project
In this project the design team of local authority officers and a consultant engineer were at pains to use appropriate traditional materials and laying methods where possible. A mortar free specification was adopted for work in the side streets and the main square which carry only a few hundred vehicles per day. Well graded whin dust with a maximum size of 6mm was selected for both the bedding layer and the jointing material for the laying of 70-90 mm Scottish diorite cubes laid in a fan pattern and reused whin setts laid in a traditional stretcher bond. The stones were laid with the minimum joint spacing and originally set proud before water and plate vibration was used to bed and compact them to the desired finished level. This method allowed final level adjustments to be made after the initial laying and further proved its worth when an area had to be removed to allow the laying of a new service duct. In a matter of hours the cubes were lifted, the duct laid, the trench backfilled and the cubes replaced without any visible signs that the work had been undertaken. The main disadvantages of the mortar-free method proved to be the 'topping-up' of the joints over the first two years by brushing in whin dust and some evidence of vehicle tracking in a localised area subjected to repeated traffic loadings.
After discussions with the highways authority it was not deemed permissible to use setts without mortar on the busy High Street which carries around eight thousand vehicles per day. The use of mortar brought particular problems as the conventional curing period of between 10 to 28 days was unacceptable as it would have lengthened the contract period considerably.
After considerable deliberation and trials, the design team agreed to employ a proprietary premixed semi-dry sand/cement bedding screed normally used for industrial flooring which retained workability for eight hours and achieved full strength after 48 hours. Another proprietary premixed brand of mortar was used for filling the joints. Precise amounts of water were added to the grouting material in a small hand mixer and plasticisers were included to allow the mortar to be brushed into the joints before hardening commenced in around 10 minutes. Accelerators gave the grout full strength in 48 hours and thus the curing period was virtually eliminated. Great care was taken to clean the setts before hardening started and although there was some immediate discoloration this was largely polished off by traffic in due course.
The mortared section does not have the traditional look of those areas where the setts have been laid in whin dust but fortunately there has been no cracking to date.
Project costs were constrained and therefore a baseline was established for the most economic selection and use of the stone. Scottish diorite cubes, re-used setts and Caithness slabbing were chosen as the main materials and £150 per square metre was set as an all inclusive target cost for the reconstruction of all carriageway and footway surfaces.
In the Linlithgow scheme maintenance has been facilitated by the avoidance of mortared joints where possible and the restriction of stone slabs to a maximum width of 600 mm to allow their lifting without the use of mechanical equipment. An important part of the aftercare was the development and publication of a streetscape manual providing specifications and guidance notes for routine maintenance and repairs for Council departments and public utilities.
The results have attracted interest from local authorities and agencies throughout the British Isles and the traditional specification is currently being repeated in schemes from Plymouth to Glasgow.
Conclusions
There is a need for the detailed analysis of the design of traditional paving of both successful and failed modern schemes. Work is required to collect detailed codes of practice, specifications and experience from other countries to further assist in further experimentation and the development of design guides and specifications suitable for streetscape works in the British Isles. Only then can we claim to be moving towards a rediscovery of the art and science of laying natural stone.