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5 May 2011

Thursday 5th May 2011; Mortars- Mixes & Applications

Week 4 of 12
Module 3: Material Characteristics and as a Building Material

We had our final day (for now) on mortars with David Odgers, on the topic 'Mortars- Mixes and Applications'. We had theory in the morning, then went to assess our pointing attempts on the roof- carried out by group 1 on Tuesday and group 2 on Wednesday, and this afternoon we had a lab session.

Mortars are designed to fail. Possibly a confusing message, and not one that architects/ homeowners/ contractors want to hear, and not something we want to happen in a short period of time, but essentially mortars are in place to protect the structure/stonework in a building. They do this by being a 'sacrificial' material and ultimately failing. When they fail we are able to replace the mortar (render/ bedding mortar/ plaster/ ...) and continue to protect the structure. The 5 key points David has given us to think about when specifying replacement mortars are performance requirements, permeability, authenticity, appearance and ease of use. It is often the case that we want our modern mortar to have a different function to our historic mortar and therefore in a high number of cases simply trying to replicate the original mortar is inappropriate; e.g. an exposed core wall mortar in a ruin will no longer function in the way it was originally intended.

The pointing mortar test panels we did on Tuesday (group 1- see Day 14 (03.05.2011)) and Wednesday (group 2) being assessed by David Odgers.

The location of the wall on which we carried out test panels of pointing mortars is a good example of an environment that requires a high level of protection to prevent the new mortar from drying out too quickly and therefore preventing carbonation. There is a small roof covering the wall, preventing it from gaining moisture and it is East-facing meaning that it gets the strong morning sun (in fact you can see that even at mid-day the sun was strong and direct on the wall). In a real situation this work should have been protected by being covered by damp hessian and continuously dampened (not soaked) to enable the carbonation process to continue. An additional protection tip is that if plastic sheeting is put over the outside of the hessian, any moisture evaporated from the mortar will at least remain in the close environment, creating a humid condition and allowing the carbonation process to continue. We had been up to the roof a few times to spray the mortar with water since placement, but we could already see the problems forming by the mortar cracking. If a mortar cracks along the interface between the stone and the mortar and then dries out or carbonates, it will never form a bond with the stone and this will always be a location of problems in future.

This afternoon we were each tasked with creating a sample of mortar to match either a piece of mortar or stone of our choice using a combination of any type of lime binder, pozzolanic material and aggregate that was available to us. It was a good opportunity to put more of the theory into practice and be a bit creative.

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