Week 4 of 12
Module 3: Material Characteristics and as a Building Material
Gionata Rizzi rejoined us today (he was last with us on Day 5- 19th April) to give us lectures on 'Masonry Systems- Stonework & Mortar' and 'Mechanical Properties of Stone in Masonry'. Below are some of the key points I picked up from the presentations.
We were shown some great photographs by Gionata including one of a gateway to Babylon which was constructed using a dark black mortar that was visible seeping out of some of the joints- bitumen! It's a local material and has many of the desireable properties of a mortar including a high flexibility allowing movement in the building and it acts as a damp proof course to protect agains rising damp. Whether it also causes some of the issues we associate with cement in terms of impermeability I don't know. Some repairs have recently been made to this site which sadly (in my opinion) have been carried out using a brittle cement mortar.
Architects, as I understand, are generally taught that when mortar joints are considered in a building it is necessary to incorporate expansion joints every 10 metres (distance may vary). Noone thought to put expansion joints into Hadrian's Wall though, or the Great Wall of China, and that is because lime mortar was used, which allows movement. Free lime within the mortar is also able to be dissolved and redeposited so cracks which develop are often self-healing.
In Edinburgh it isn't uncommon to see a cracked lintel above a window or door and I have heard divided opinion as to whether these can be left or should be replaced. Gionata and David Odgers (also present) were in agreement that in the majority of cases there would be no need to repair or replace this lintel because it would not be possible for any more movement to occur because there is no space for the physical collapse to occur. Also when the lintel is cracked there will be a redistribution of load away from this point. Gionata recommended a book in reference to this type of issue; Jacques Heyman 'The Stone Skeleton'.
As we all know, a bulge in a wall is a dangerous sign as it indicates the potential for collapse, which could occur very quickly or may never occur. The potential danger is due to the shift in the structural load which is no longer evenly distributed.
In a discussion about arches Gionata showed us the type known to be common in Europe which is a curved arch, based on the load being distributed as a catenary (the inverted shape formed when you suspend a chain by both ends). An arch works under compression and a curved arch has its joints aligned to a central point at the base and centre of the arch. The stresses go at 90 degrees to the joint. In South East Asia, however, historic construction techniques were different and arches there were commonly corbel arches composed of horizontal rows of stones. These are far more prone to failure because compression is still in the same direction as with the arch above but this is no longer at 90 degrees to the joints. Most of these are dry stone structures, but if moisture or biological growth reaches the joints it is possible for them to slip and the arch to collapse.
'Traditional construction relies more on equilibrium than on strength'. This is a very important point for me to take from today's lectures, it is all about how the units are constructed and placed rather than about how strong they are. Lime mortar is a very good example of this statement because we do not need a mortar to be very strong (e.g. cement), we need it to fulfil its purpose of distributing load, allowing movement within a structure and having the correct properties for protecting the stone.
This afternoon half of the group had a practical session up on the roof of the ICCROM building with David Odgers, mixing and using lime mortars to construct the two different types of arches mentioned above, the curved arch and the corbel arch. We also each carried out a small section of pointing on a brick wall. It was a very enjoyable afternoon and a chance to attempt to put some of the theory into practice (we may need a little more practice though).
The group with the two arches we have constructed (curved arch on the left and corbel arch on the right). David says he is prepared to stand on our curved arch to test it out once the mortar has fully carbonated. In the photograph (from left); Simon Warrack (course organiser), Susanne (Denmark), Dodie (Sudan), Anita (Australia), Ileana (Romania), Me (Scotland), Michel (Palestine), Rouba (Syria), Valerie (India) and Stefaan (Belgium).