With external stone paving and roadways becoming ever more popular in Australia and around the world, what do today’s designers and architects have to keep in mind when deciding to use stone within their paving project?
Article: Jim Mann
We’ve heard the saying “everything old is new again” – and when it comes to using stone as a paving material, this adage is true. The Appian Way, built circa 300 BC, is a prime example and a testament to the durability and timeless nature of stone as external paving.
This ancient roadway has withstood the forces of nature and countless wars for over two thousand years with parts of the road still carrying vehicular traffic.
We now find stone used as prestige paving in nearly every major city in Australia and abroad. With external stone paving and roadways becoming ever more popular in Australia and around the world, what do modern designers and architects have to keep in mind when deciding to use stone within their paving project?
Load Bearing Capacity
Broken pavers are usually the first sign of a deteriorating pavement. To minimise the risk of failure under load and maintain the serviceability of the pavement, it is vital to determine the minimum breaking load required for the paving units in their intended location. Breaking load is governed by the inherent strength of the stone as well as the plan size and thickness of the paving unit. The Australian Standard for segmental pavers and flags (AS/NZS 4455.2-2010) sets performance requirements for paving units. Although the majority of the requirements are for concrete products, the table is a useful benchmark for the performance of stone pavers and flags.
Table 2.8 of the standard summarised below, sets minimum breaking load and dimension requirements for paving units to be installed within various locations. The standard differentiates between ‘pavers’ and ‘flags’ with a ‘flag’ defined as a large format solid paver with a gross plan area greater than 0.08m2 (e.g. greater than 200mm x 400mm).
The standard does not state how the requirements were derived but it is likely that they are based on the installation of pavers on a flexible base (e.g. sand). Installation of pavers on a rigid substrate (e.g. mortar bed) is likely to double the load bearing capacity of the paver.
While it may be obvious that a commercial vehicle imposes a greater load on a paver than a pedestrian, the traffic frequency and subsequent rate of live loads applied also increases the risk of failure. To accommodate these variables, the table recommends an increase in the minimum breaking load and unit thickness.
The table also sets a greater load carrying capacity for flags due to their larger format. As the size of the paving unit increases, the load imposed by a pedestrian or vehicle is less evenly distributed over the whole face of the unit and is imposed less as a compressive load and more as a flexural (bending) load. While the load applied to the paver may stay the same, the fact that the flexural strength of the paving material may only be 10% of its compressive strength can have a significant effect on the performance of the paver during service.
Based on the requirements of the standard, a 200 mm x 400 mm x 40 mm paver subjected to pedestrian traffic in a public space must have a minimum breaking load of 2 kN (204 kg). This equates to a minimum 3-point bending strength (modulus of rupture) of 3.2 MPa. Based on this strength requirement, the paver could be produced from a broad range of stone types including, slate, bluestone, granite most limestone and some sandstone types. In comparison a paving flag with dimensions of 800 mm x 400 mm x 40 mm that requires a minimum breaking load of 5 kN needs to be made from a stone with a minimum modulus of rupture of 8.8 MPa, limiting the range to quality slate, bluestone, granite, high-density limestone products and quartzitic sandstone.
Because of this increase in flexural load with unit size the standard does not provide any specific design requirements for flags to be used in areas subjected to high traffic load and frequency. The breaking load and thickness requirements for these locations need to be determined on a project specific basis taking into account local conditions.
As the design of most paving installations is based on a desired aesthetic governing the appearance and size of the paving units, the thickness may be the only variable that can be used to produce a serviceable pavement. The effect of thickness on performance of a paving unit should not be underestimated. Any increase or reduction in thickness has an exponential effect on the breaking load of the unit. Doubling the thickness of the paver increases the breaking load fourfold; while the inverse is also true.
Igneous or metamorphic stone types such as granite, bluestone, slate and porphyry are generally strong and dimensionally stable materials that maintain their strength in wet and dry conditions. Sedimentary stone types such as limestone and sandstone are popular for use as paving around the home and have been used successfully as pedestrian paving in public spaces. These stone types usually contain clay and can subsequently lose significant strength when wet. In this case it is important that the breaking load of the paving unit in a wet condition is taken into account when determining fitness for purpose.
External paving is exposed to the elements and undergoes regular heating-cooling and wetting-drying cycles. To be used successfully, the structure and composition of the stone must be able to withstand these regular changes without decay or significant loss of strength. As successful design requires the use of a strong stone type, the issue of durability (or resistance to decay) is not normally an issue as careful selection of these paving materials based on strength properties usually brings with it the benefit of resistance to decay.
Due to the presence of clay, some sedimentary stone types are more sensitive to dimensional instability and decay when exposed to a cyclic wet-dry environment. Durability is usually evaluated in accordance with the Australian standard AS/NZS 4456.10, which evaluates the stone’s resistance to salt attack by measuring the percentage weight loss after 15 cycles of immersion in a salt solution followed by drying in an oven.
While the test was designed to evaluate sedimentary stone types, the method is also useful in providing information on the mode of decay or change in appearance of other stone types and surface finishes. As an example, while a honed granite paver may have a very low weight loss, examination of the honed surface may reveal loss of a particular mineral type resulting in fine pitting. In this case, although the results may indicate the stone is very durable, the development of these fine pits may be considered to detract from the appearance and be unacceptable on aesthetic grounds.
External paving is continually bombarded by a wide range of staining agents ranging from vegetation, general grime, beverage, food and oil spills and the ubiquitous chewing gum. The ability of a paver to resist staining is governed by a complex relationship between its colour, composition, texture and absorption characteristics.
While the porosity of the paver is the key determining factor in a stone’s ability to absorb stains, the colour and surface finish of the paver determine how conspicuous the stain will appear. Oil stains are most conspicuous on materials with a mid-tonality such as greys and mid-browns as the deposition of an oily film changes the optical characteristics of the material producing a darker colour. Coarser finishes such as sandblasted or flamed will assist in making the appearance of the stain more diffuse although these textured finishes also tend to trap grime making cleaning more difficult. Stone types that present a variation in tonality or figuring have a greater ability to hide stains than those with a uniform appearance.
The use of impregnating or topical sealers can improve the stain resistance of external paving although their use presents additional costs related to application and maintenance. The decision to apply a sealer should be made on a project specific basis, taking into account the inherent stain resistance of the stone, an assessment of the risk and consequences of staining and an appraisal of the cost versus benefit of sealer application.
If you decide to seal your paving it is important to select a product that will provide the type of protection required, how and when to apply it and how the sealer will affect the appearance and performance of the paver. Correct sealer selection requires review of the project requirements and environment of the final installation. What type of staining agents are likely to be present? Are they mainly water-based, oil-based or both? Do I want to maintain the natural appearance or a colour enhanced wet-look? Do I want to use a sealer that also helps consolidate the stone?
With risk minimisation being the overarching aim on all projects, the provision of adequate slip resistance has become the ultimate deciding factor in acceptance of a stone type and surface finish.
To assist in evaluating and managing this risk, Standards Australia has published handbook HB198:2014 “Guide to the specification and testing of slip resistance of pedestrian surfaces”. Table 3B within this document provides minimum recommended slip resistance classifications for a range of locations including external pavements and ramps. According to the table, external colonnade and walkways and ramps with a gradient of less than 1 in 14 should have a minimum wet-pendulum classification of P4 or R11 using the oil-wet ramp test. External ramps with a gradient steeper than 1 in 14 should have a minimum classification of P5 or R12.
When specifying slip resistance it is important to remember that there is poor correlation between the two test methods and oil-wet ramp test can only be carried out in the laboratory. If you do decide to specify an ‘R rating’ the testing must be done prior to installation and it will not be possible to confirm compliance of the installed product.
The versatility of stone comes to the fore once again providing a range of suitable slip resistant finishes. Igneous stone types such as granite can be produced with a wide range of finishes including flamed, water jet, grit-blasted, bush-hammered, chiselled and split. Sandstone and bluestone are both naturally slip resistant and may meet the requirement in a sawn or grit blasted finish. Slate is popular for external paving use in a split and sawn finish.
Whether the product will maintain the required slip resistance throughout the project life may be an issue that needs to be considered. While over-specifying the slip resistance rating may be an answer, this conservative approach may add cost and not meet aesthetic requirements. Performing an accelerated wear test will provide information on the change in slip resistance when subjected to traffic. The test provides useful information on the likely ‘worst-case’ scenario and whether the slip resistance will stabilise in the long term.
Exposure of stone to moisture can affect how the pavers look and behave in service. The major aesthetic issue is related to a change in tonality of the stone as the paver absorbs water. If this occurs in a uniform manner it may not be noticeable, but where it occurs in localised areas such as the tile perimeter, the effect can have a significant impact on the appearance of the paving installation. This peripheral darkening is commonly called ‘picture-framing’ and most commonly occurs on pavers with a light to mid-tonality and relatively low porosity (e.g. a light grey granite).
Although it may sound counterintuitive, the picture-framing phenomenon mainly occurs on stone types with low water absorption capacity as they only require a small amount of water to become saturated, leading to a change in the optical properties of the stone and subsequent darkening. Sensitivity to picture-framing is an inherent feature of the stone although this can be controlled to some degree by selection of a substrate and grout that has minimal water absorption capacity. Six-sided sealing with a water repellent impregnating sealer may also mitigate the risk although it is important to ensure the sealer will not impede the bond between the paver and substrate.
Dimensional stability is also an issue that needs to be assessed particularly when installing large format flags. Pavers with a large length to width ratio that have been produced from materials with a low porosity and very fine pore size may be sensitive to warping particularly during installation. This problem has been found to occur in natural stone, particularly some dense basaltic pavers as well as engineered products. Pavers sensitive to warping should be installed onto a solid sub-base (e.g. concrete slab) using a high strength, fast-setting adhesive. Avoiding installation during temperature extremes and when the freshly-laid pavers are directly exposed to the sun also helps reduce the risk of warping.
You may have diligently carried out a detailed evaluation of the load bearing capacity, durability, moisture sensitivity, stain and slip resistance of the paving stone, but without proper installation practices the pavers are unlikely to provide optimal once in service. Installation is a complex and detailed issue that can’t be covered fully within this article although a few important issues to take into account are discussed briefly below:
Whether the pavers are to be laid on sand or a concrete slab, it is important that the surface is sound and stable and able to receive the paving without significant movement. The substrate must also be free-draining to minimise the risk of waterlogging that can lead to picture-framing and efflorescence.
– Fixing Methodology
The fixing method, whether a wet or dry screed or a specialty mortar, must be able to achieve an adequate bond with the pavers. Sawn faces of some granite products can produce a very smooth surface that can restrict adhesion and can be reduced further by the presence of dust. If a tile adhesive is proposed for use, ensure that an adequate thickness of adhesive is maintained especially when finished levels need to accommodate falls. Avoid laying pavers in extreme weather conditions.
– Joints and Grouting
The joint width between pavers needs to be sufficient to ensure that grout extends to the full depth. Top-filling of joints will result in early failure of the grout and possible disbondment of the paver. Grout strength should be similar to the strength of the paver to present a monolithic unit.
– Control and Expansion Joints
Control joints in slabs must be extended through the paving. Ensure sufficient expansion joints are included to accommodate thermal expansion of the paving installation. These joints must be maintained throughout the life of the installation and should be ensured they are not grouted during any repair or modification works.
– Partial Pavers
Partial paving units are often a weak link in the installation as they are more prone to breakage and disbondment. Give careful thought to where partial units are used avoiding high traffic areas and installation near expansion joints. Consider the use of a header course to support partial units where they must be installed along expansion joints that are exposed to heavy traffic flow.