In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: nss@qmj.co.uk marked for Barry’s attention

Q: I believe the granite I supply to be frost resistant, and certainly have never had any problems. Do I really need to have it tested?

This question has been posed by many stone merchants in response to requests for proof from potential users and purchasers. The general belief of many is that granite and other igneous rock types are essentially frost resistant because problems of any type are relatively rare with such stones. Allied to this is the perception that igneous rocks are not porous, so water is unable to get into them to freeze and cause a problem. 

Most igneous rocks exhibit typically high strength, which makes them more able to resist damaging forces than weaker stones such limestones and sandstone. Igneous rocks are not impervious to water but they typically absorb negligible amounts of moisture, meaning that only a very small amount of ice can be formed – not enough to generate sufficient force to prize apart crystals or other features. 

But as strength decreases and porosity increases, maybe there is a point at which certain igneous rocks do become susceptible to frost attack. Unfortunately, research does not appear to have been carried out to determine whether this is actually the case and what threshold there may be, if any.

I have to agree with the general view that igneous rocks are frost resistant as I have observed such materials at many locations around the world used in the construction of thousands of buildings and road surfaces and never once has there been evidence of significant frost attack. 

Furthermore, having subjected many different types of igneous rocks to freeze-thaw testing, I have never experienced a failure or even anything that has come close to failure. Also, I have not heard reports from other experts or testing houses that igneous rock types have suffered failures during freeze-thaw testing.

The current standards for pavings, kerbs, setts, tiles and claddings do not insist on freeze-thaw test results being declared, though this fact is not easily apparent in the standards. 

The standards ask that the frost resistance shall be stated with reference to classification tables, but also note that if frost testing has not been carried out this fact should be declared. 

If there are regulatory requirements then results will need to be declared whatever your views are. My personal view is that the great majority of freeze- thaw testing of igneous rocks is unnecessary and I would like to suggest a more pragmatic approach.

Granites and other igneous rock types exhibit high strengths when their absorption rates are around 0.4% or less. As they weather and open up, so porosity increases and strength may rapidly decrease, increasing the potential for possible problems. So why not classify all igneous rock types with absorptions less than or equal to 0.4% as frost resistant and only test those that exhibit higher absorptions, probably matched to a reduced level of strength? 

There are igneous materials that perform well with higher absorption rates, but why take a gamble? Especially as there are relatively few such stone types. 

Many of the performance standards are presently up for revision and it would be a good time to try to change them for the better by removing the need to freeze-thaw test low absorption stones.

The one situation where frost testing should be carried out regardless is with thin claddings to very high buildings. Some skyscrapers experience more than a hundred freeze-thaw cycles a year in high, exposed locations, so greater assurance should be sought. 

However, it is of more use to assess the combined performance of the fixing system with the stone, as this could be considered the potential weakest link.

My parting shot on this matter is that freeze-thaw testing is often long and drawn out and potentially uses up a lot of time in addition to a large amount of energy. Thus it is irresponsible of us to blindly insist on such profligate testing in these energy saving times. Some common sense needs to be applied.