Barry Hunt continues the examination of granite that he started in the October issue of NSS. Granite is dense, hard and strong, which makes it an ideal material for everything from plinths protecting buildings where they meet the ground to attractive and durable kitchen worktops.

Often the principal desire of those using granite on the outside of buildings is that the appearance will not change. Granite is also often chosen in such applications because of its typically monotone appearance from afar. Any variation, even subtle shading, is usually unwelcome.

For large projects, the architect might wish to consider whether sufficient stone is available that is within an acceptable range of variation, with the principal issues tending to be the presence of random xenoliths, veining and discolouration.

Often the criteria should be simply to exclude stones displaying such features, no matter how small, as there will usually still be plenty of material available due to the way it was formed by molton rock forcing its way up through the Earth’s crust. It is probably (there is some dispute about it) the slow cooling of the rock that leads to the formation of large crystals typical of granite. It is the interlocking of the crystals that gives granite its great strength.

As granite weathers it does not normally significantly lose that strength. It will lose its surface very gradually over many years (often centuries) and the net result is that the thinner stone will break at a lower stress, but not normally significantly so.

If discolouration occurs this can indicate mineral breakdown – and granite can rapidly reduce in strength (by 50% or more) if the crystal bonds are disrupted (as they have been in the photograph below).

However, the initially high strength of granite usually means that even these reductions in strength still leave the stone capable of continuing to perform its intended use successfully.

The most significant property for the engineer to consider is flexural strength, especially when the granite is to be used for external claddings. As with marble (see the August issue of NSS), four point flexural testing is preferred to ensure any potentially weak features can be identified.

It is believed by some that granite suffers from fatigue, with daily weathering cycles and constantly changing wind loadings rapidly reducing strength. However, the research is limited and most stones show no more than 10-20% loss in strength after weathering cycles, which is most likely caused by the release of internal stresses.

Considering the number of buildings clad in granite and the small number of problems reported, rapid granite fatigue is unlikely, although in the longer term it must not be discounted and it could prove to be an issue in the future.

When used for flooring, slip resistance is probably the only other factor that must be determined when using granite. This depends on the finish – and polished granite, especially if it gets wet, will naturally have a greater potential for slip than when it is textured.

However, polished granite is rarely actually necessary for flooring as typically it is not so very different in general appearance to a textured finish.

Polished granite may be sufficiently slip resistant for internal floors but not for external pavings and road surfacings, including blocks, setts and pavers. What then becomes important is whether the textured finish can be retained.

A flame-finish is often chosen for granite when it is used for paving but a flame-finish may exhibit cracking sub-parallel to the surface and an initially rough surface can rapidly deteriorate to one that is considerably flatter and more liable to be polished in-service by all the shoes or other traffic going over it. With both the macro and micro textures reduced, the potential for slip or skid can dramatically increase.

The strength and hardness of granite makes it ideal for use in lightweight cladding systems. This is where the stone acts as a veneer and is not required to perform any function other than to provide an aesthetically desirable finish that is durable.

In essence, a thin (5-15mm) sheet of stone is glued to a lightweight structural backing such as honeycombed aluminium.

The makers of such panels say they offer excellent flexural strength while being considerably easier to install than much heavier hand-set granite. Being much lighter also means they can be fixed to a lighter framework, saving on material costs.

The final main consideration is stain protection. Even though granite has low absorption, it remains susceptible to potential staining media such as grease and acidic ingredients used in kitchens. Once these have entered the granite they are difficult to remove, which is why kitchen worktops are usually sealed.

Otherwise, granite is relatively easy to clean as it can be subjected to high pressure washing, scrubbing, some chemical cleans and other methodologies without fear of damage to the stone or its finish.

Problems involving granite

There are few problems recorded with the use of granite and those that there are tend to be the result of initial poor selection of material.

Following the bomb blasts in the City of London in 1992 and 1993, new granite claddings were, for the first time, investigated to ascertain how they had been affected by blast impact.

It was found that one type of granite away from the main area of blast damage of the later bomb had a considerably higher concentration of damage than the surrounding granites.

This granite was found to exhibit significantly lower strength than its neighbours – well below the minimum specification requirements – as a result of original geological weathering.

Elsewhere, panels that had been partially flame-textured were found to be more susceptible to breakage. An explanation given has been the differential expansion caused by the flame texturing.

Cracking was also found to have been caused by both over- and under-tightening of the fixings, leading to stressing.

Granite, like all stone, is considerably weaker under tensional stresses. With the right support, granite is able to resist considerable forces and the bomb blast in London’s Docklands in 1996 revealed the benefits of kerf fixing systems. These involve an extruded aluminium frame into which panels are placed using continuous grooves or kerfs in the panel edges.

With buildings devastated up to several hundred metres from the blast, in the immediate vicinity many panels survived intact. In this instance help was provided by the frame having buckled, absorbing some of the blast energy.

An unusual instance of granite cladding failure occurred in London in the early 1990s. The granite had been fixed to a steel frame and during the summer the high temperature caused the frame to expand more than the granite could tolerate. The cladding panels were gradually ripped apart.

This mirrored problems with the granite columns of Westminster Bridge (built between 1854 and 1862). The columns had been slowly ripped apart as a result of movement of the inner metal framework.

Even though the blocks were 450mm thick and more than 2m high, large vertical cracks had been induced over the life of the bridge, which is reported to be London’s most heavily trafficked.

In the UK during the late 1980s and early 1990s there were significant discolouration problems involving grey Sardinian granites.

Under the action of persistent moisture, brown patches developed related to the release of iron from within the biotite crystals in the granite.

Although not certain, it is possible that the problem was most prevalent in granite from higher, more geologically weathered levels of the quarries from which the stone came.

A problem with using geologically weathered granite has been identified in Dublin. Ballyknockan granite from Wicklow has been used extensively in Dublin for many of the Regency and younger buildings.

When viewing the older buildings it is apparent that lower quality, geologically weathered stones have sometimes lost between 2-5cm of surface over the 200 years or so that they have been there.

This is of no real consequence as the blocks are 20-30cm thick. It became relevant on more modern buildings that used 20-30mm granite as cladding. They could lose up to 1mm of surface every four years, which must be considered significant as the loss of just 6mm would reduce the breaking load by as much as 50%.

In modern streetscaping the most common use of granite has been as setts – small, rough hewn, cubes of stone.

Setts began to replace cobbles on London’s streets in the first half of the 19th century when ‘cassies’ of Aberdeen granite, sometimes also referred to as ‘causeway stones’, were used.

Under vehicular braking the setts tends to rock, and it is up to both the bedding and joints to prevent this. However, the bedding and joints are made from considerably weaker materials than the setts and most problems in this kind of road surface are related to their failure rather than the failure of the setts themselves.

The patterns used for laying setts are not just for aesthetic purposes, they also help lateral loads to be absorbed and spread by ensuring that joints are not aligned and have good adhesion to the sett sides.

Trafficking the surface too early also can lead to collapse of the bedding, something that is difficult to avoid given the need to keep many city centre roads open.

There is currently some debate regarding the nature of the edges of granite setts and whether these should be sawn and then textured or simply cropped. The principal problem with cropped setts is that they can be out of square, which can lead to hidden wide joints with an increased potential for debonding as the jointing mortar can have a greater potential for shrinkage.

In Oxford in 2001, the laying of large blocks in Cornmarket Street, one of the main city centre streets, resulted in the newly laid surface apparently humping and then collapsing once trafficked, probably as a result of excessive mortar shrinkage and joints that were too wide.

This particular problem would not have occurred if the bedding and joints had been made flexible using compacted sands, although clients often want to avoid that solution because of the on-going cost of maintenance involved.

Incidentally, CABE, in conjunction with BBC Radio 4, commissioned a survey in 2002 into the best and worst streets in the UK and Cornmarket was voted the second worst.

Granite kitchen worktops, now almost ubiquitous, should be of the highest quality granites as even slight alterations of minerals can lead to pitting appearing in polished surfaces.

Granites must be expected to exhibit some slight degree of feldspar alteration and surfaces must be carefully checked under different lighting conditions to ensure that they are defect free. All too often a complaint occurs after the granite has been installed because the lighting makes visible problems that had not been apparent during manufacture.

Regardless of granite’s strength and durability, these properties should not be relied on solely in construction as there may still need to be restraint and, of course, effective maintenance.

This was amply demonstrated by an old castle in Ireland where a large, solid masonry wall with a substantial granite facing suffered partial collapse. Why? Simply because ivy and other vegetation had been allowed to grow between the facing and backing brick masonry. It could so easily have been avoided.

Good granite or bad?

It is usually simply the presence of trace minerals that colour the constituent crystals and variations in mineral proportions and grain size that create the widely variable aesthetics seen in granites. These changes do not normally alter the properties of the granite. But sometimes they can.

If they do, a tell-tale sign is a rapid increase in water absorption and decrease in compressive strength as grain boundaries are opened up.

Part 12 of BS 7533 for the construction of roads using setts sensibly sets a tight maximum water absorption limit of 0.25% to ensure the most suitable material is used, with 0.35% set for less onerous conditions. This is a good rule of thumb for any use of granite, but do not follow it slavishly or you might exclude granites that are acceptable for a wide variety of uses, especially interiors.

Anisotropy (greater strength in one direction than another) should also be determined as it can suggest alterations that could have changed the expected properties. If anisotopy is discovered yet all other indications remain good, the granite should be used with the direction of greatest strength parallel to the principal stress.

Strength testing in both wet and dry conditions can also rapidly identify potential problems that secondary minerals, particularly clays, might cause. Decreases of 30% suggest a need for caution and of 50% indicate an almost certain problem.

To conclude: granite is the all-rounder of building stones, able to rise to most construction challenges. Problems usually result from it being misunderstood – after all, it is still a stone.

Without granite, much of the early written history of the world would probably have been lost, weathered to dust. We owe much to this hardest of stones and will continue to do so far into the future.