Barry Hunt began his review of how sandstone is used in construction last month. This month he concludes the investigation. He begins where he left off last time, explaining why sandstone makes an ideal paving material.
We saw last month how the right sandstone makes the ideal paving material, thanks to its hard-wearing properties and its ability in use to present a continually refreshed, slip-resistant surface.
Millstone Grit is possibly the perfect material for paving because of its angular grains that also have some size variation.
Furthermore, the beds tend to be thicker without obvious or significant lamination, which provides more consistent wearing and reduces the potential for surface delamination.
These properties can work against the stone for internal use, where there is no natural wind and rain removal of abraded materials. Tracking marks reflecting pedestrian movement can occur, created by the ground-in remains of shoe soles.
One of the more unusual uses of sandstone is for roofing tiles.
The concept of using a not especially strong and porous stone for roofing seems wrong – it certainly creates a number of problems to overcome.
The most notable problem is the thickness of stone required to ensure it does not break. If it is thick, it is heavy, which means a substantial and expensive structure is required to support it.
There also has to be good ventilation below the stone to allow it to dry so the supporting structure does not rapidly decay, especially if it is wood.
That sandstone successfully performs as a roofing tile at all is because the exposed surface rapidly becomes saturated by rainwater. Once it is saturated, the surface rejects any further water absorption. Instead, some moisture is transferred by capillary action to the lower areas of the stone but moisture flow is unable to develop.
Most sandstones will achieve compressive strengths well in excess of the 40MPa of standard concrete
Problems with sandstone
Sandstone used as a construction material suffers very few problems early in its life, regardless of how it has been used.
Instead, it suffers more from misunderstandings of how it weathers, which can lead to inappropriate attempts to clean it later in its life.
This is particularly true where sandstone has been used for traditional block masonry and ashlar, which typically suffer from contour scaling or spalling sub-parallel to the outer surface contours. Sandstone appears to be more susceptible to contour scaling than other types of stone.
The reasons for contour scaling are still debated, although it seems that the typically consistent porosity characteristics of most sandstones mean that when they undergo surface wetting, moisture tends to migrate a certain distance into the stone by capillary action. As the stone dries, the moisture migrates back towards the surface, possibly taking with it dissolved silica or other materials. Repeated cycles may deplete the stone and weaken it at the point of average infiltration.
In addition (or alternatively) salts deposited on the stone may be drawn into it. Once inside they saturate and crystallize out to cause damage.
Another factor is heating and cooling cycles that expand the surface more than the interior, so a shearing stress is created. The presence of deposited salts might also affect the thermal gradient, adding to the shearing stresses.
Whatever the actual causes of contour scaling, most sandstone will not readily show the damage unless there are locations where frost action is having a more direct influence to disrupt the surface.
Instead, it is often misguided cleaning that upsets or removes the surface and the delicately balanced protection it affords.
A common and brutal cleaning method is grit or sand blasting and, more recently, jet washing at excessive pressures, which can leave façades uneven, with between 10mm and 50mm of surface removed.
More stone tends to be removed close to joints or on edge details so that joints are widened (see Figure 7 on page 33) and carved details can be completely removed in an instant.
Sandstone also suffers more pronounced surface soiling than most other stones because of its rough surface, which allows airborne particulates to adhere more easily. Furthermore, sandstone can saturate and take a while to dry, increasing the potential for dirt to adhere.
The dirt build-up often penetrates the surface very slightly so that even when surface dirt is cleaned away the stone retains the more ingrained matter. It is often this problem that leads to inappropriate and harsh cleaning in an attempt to remove the more ingrained dirt. Such cleaning can upset scaled surfaces and lead to the dramatic damage mentioned above.
One well recognized cause of sandstone masonry decay is its use in combination with limestone, especially where the limestone is above the sandstone.
The limestone dissolves naturally and the salts formed are transported down the building. If there is sandstone below these salts can enter it and wreak havoc.
A peculiar phenomenon that sometimes manifests itself in sandstone is warping, particularly shortly after it has been laid as relatively thin tiles or pavers.
The problem is not well researched but is believed to be related to the sometimes considerable volume changes between the wet and dry states of the stone.
Once a slab of sandstone is laid, the top surface can dry out while the underside remains damp for quite some time against the bedding materials.
In drying out, the top surface contracts and the stone warps if it is thin enough not to be able to resist the contraction forces.
A metre long slab 30mm thick requires a differential expansion / contraction between the outer and inner surfaces of just 0.1% to cause the end edges to lift by 5mm.
When you consider that some sandstones exhibit dry to wet expansion of up to 0.7%, it is a wonder more sandstones do not suffer from warping.
Unlike marble when it warps, the problem in sandstone can be remedied by turning the slabs over.
Warping does not occur when slabs have been properly bonded to the bedding materials.
When sandstone paving has a diamond sawn finish it needs to be ‘trafficked’ so the stone does not suffer from the build-up of organic matter that can reduce slip resistance to potentially dangerous levels.
Regular cleaning may not be economically viable if affected areas are large and the problem manifests itself regularly (it can be as often as every six months – once after the autumn leaf fall and again in the spring as sap falls from trees. Nesting birds and summer fruit falls are also potential problems).
Ironically, the problems of organic build-up on sandstone are probably at least partly due to less pollution in the atmosphere. The acid rain that used to be the result of burning fossil fuels acted as a biocide, preventing the growth of organic films feeding off leaf, sap and other organic detritus.
One unusual problem occurred in Stratford-on-Avon, where the sandstone paving was bombarded with discarded chewing gum from the hordes of tourists treading the Shakespeare trail.
The chewing gum was being removed by pressure washing using water heated to around 85° Celsius, which softened the gum and made removal easier. Unfortunately, the cleaning also removed as much as 25-30mm of the stone in places, so the surface suddenly became badly pock-marked.
The pressure washing of the granular structure knocked out a succession of grains from the cementing matrix – this would not have happened with a crystalline stone.
The problem was resolved (through trial and error) by adjusting the pressure to a point where it could not damage the stone.
Finding the best sandstone
Sandstones, probably more than any other stones, exhibit an exceptionally large range of properties, although the differences may be difficult to perceive.
In the simplest terms, as water absorption decreases the strength increases, which might simply be a function of increased siliceous cementation filling up the pores and voids.
This simple view is muddled by the presence of iron oxide in the cementing matrix, which tends not to affect strength as much as it does abrasion resistance – individual grains are easier to remove under abrasive action when there is iron oxide in the matrix.
The presence of mica increases the potential for delamination and most micaceous sandstones do not quite reach the level of strength that purely quartz sandstones exhibit because mica reduces the grain interlock.
Table 2 (left) provides typical dimension stone test values for sandstone and quartzitic sandstone. Most sandstones will demonstrate compressive strengths well in excess of the 40MPa usually associated with standard concrete.
Testing the compressive strength in both wet and dry conditions will provide an immediate idea of the likely soundness of the sandstone. If the values obtained vary by more than 20%, potential unsoundness is indicated and if the stone is still to be considered for use, further testing is advisable.
The absorption test is useful to obtain a quick sense of sandstone quality. If the absorption rate exceeds 10%, proceed with extreme caution.
Strength testing in different directions can also help to determine the presence of any anisotropy (different strengths in different orientations).
However, if sandstone contains potential splitting planes, these would be more easily identified by carrying out flexural strength testing. The limits given by ASTM suggest that the minimum compressive strength should be greater than 13.8MPa in any condition. Additionally, sandstone should not exhibit absorption greater than 7.5%.
As there are so many tests available it is worth undertaking the simplest ones first to determine whether more exhaustive investigation is necessary.
In conclusion, sandstone competes admirably with both limestone for block masonry and granite for paving, and in many instances might be considered a more appropriate choice, even though the results of some tests might be interpreted as indicating an inferior performance.
Sandstone is not an enigma. It is just being a natural material. It is the one stone we can actually see forming around the coastlines that we love to enjoy.
With a little imagination and understanding, sandstone has the opportunity to become the designer’s stone of choice to invigorate buildings and urban landscapes.