Stone & Brick Preservations and Repairs
Natural and Sustainable Lime Repairs for Stone & Brick
St.One are a specially formulated range of mortars, based on high quality standards from Saint-Astier® for the conservation and repairing of masonry, brick, or stone.
St.One is a neutral white or pre-coloured powder to match the background on properties, granulometry and colours.
Much of the masonry replaced in the last 30 years or so has resulted in partial or complete removal of predominantly structurally sound masonry that has suffered from little more than face weathering, often through neglect and lack of maintenance of basic building details such as gutters and down pipes.
The arbitrary use of replacement stone in these circumstances has left many buildings with thin skins of new stone, seldom matching the true characteristics of the original, and often leading to highly invasive intervention in what was an otherwise stable structure. Many of the thin skins are backed up in concrete, brick or any number of inappropriate mediums, or indeed with significant voids left behind the new veneers.
Characteristics of materials for successful conservation and surface repairs
For conservation and surface repairs to be successful the materials employed need to have certain characteristics.
The materials employed should:
- Have an adequate modulus of elasticity. The repairing material must be able to support :
- Have characteristics similar to the host masonry.
- Be capable of being dressed with similar tools to the original masonry.
- Remain workable long enough to allow details to be fashioned.
- Adsorb water sufficiently in wet and dry periods and match adjacent masonry.
- Have adequate bond strength.
- Be vapour permeable.
- Be durable.
- Match in colour and texture.
The conservation and repair material must always be reversible. St.One ® did proof during many year it’s a 100% reversible material with the Saint Astier quality behind.
The materials employed should never:
- Become significantly stronger than surrounding masonry.
- Have a significantly lower rate of Absorption or Adsorption than surrounding units.
- Create a barrier to Diffusion.
- For materials to meet these criteria, the materials themselves do not require to match every characteristic of the host masonry.
- The main visual characteristic of masonry is colour, matching of colour can be subjective, different people see colour in different ways, many men for instance are red green colour blind to a greater or lesser degree, so one person may see a colour match differently from another, but it is generally controllable. St.One ® can be accurately colour matched with proven light fast pigments.
The finish of a repair material is subject to some degree of licence from the applicator and is not directly attributable to the material, although if the material can be worked correctly with traditional tools after setting it will afford good physical matching. St.One ® is capable of being dressed with standard masons tools or at time of placing by skilled operatives.
Physical properties of the masonry
The physical properties of the masonry can and do vary from one area of the building to the next.
Absorption, is a process by which the materials take in a fluid to fill its voids. It can vary depending on a number of factors, not least the surface finishing. The finished repair mortar must have similar characteristics. St.One ® can be seen by demonstration to Absorb water on to its set and hardened surface.
Adsorption is the process in which molecules adhere to the surface of the material, and should be similar in the repair material. Surface Adsorption is noticeable in ‘St.One ®’ repairs.
Ageing is due to the changing of properties and appearance of materials with time (natural process) and is often a function of the previous two characteristics. St.One ® does weather as a result of both of the above being sufficiently absorbent to take in pollutants over time, although not unduly.
The extent to which a building material is able to allow moisture to move to the surface and evaporate harmlessly should be at least matched or exceeded by the repair material. The greater the degree of breathe-ability of the repair material the faster the release of moisture from the host masonry. St.One ® has a very high degree of Breathe-ability by virtue of its Vapour Permeability.
The ability of a material that is in contact with water, to raise water through capillary canals (capillaries). This is not a desirable quality of any construction or repair material. Capillary pores occur mainly in dense cement materials and hold water, reducing evaporation, many naturally occurring sandstones do have a degree of capillarity, but it is seldom continuous.
Defines the amount of holes (pores) in the unit of the material volume and can have a bearing on the durability and breathe-ability of the masonry and the repair material. The porosity of the masonry and the repair material is a function of the Pore size.
Pore dimensions cover a very wide range. Pores are classified according to three main groups depending on the access size :
- Micropores: less than 2 microns diameter
- Mesopores: between 2 and 50 microns diameter
- Macropores: larger than 50 microns diameter
Matching the porosity is not therefore essential, what is essential is to maintain good Breathe-ability, this is a function of the extent and nature of interlinked pores. There are two main and important typologies of pores: closed and open pores. Closed pores are completely isolated from the external surface, not allowing the access of external fluids in neither liquid nor gaseous phase. Closed pores influence parameters like density, mechanical and thermal properties. Open pores are connected to the external surface and are therefore accessible to fluids, depending on the pore nature/size and the nature of fluid. Open pores can be further divided in dead-end or interconnected pores.
Suitable repair materials should have interconnected pores-Micropores structure with tiny microscopic holes or pores approximately 5 microns in diameter interlinking the general pore structure. Water droplets are typically 500-5000 microns in diameter ‘drizzle’ droplets can be as small as 200 microns in diameter. Attraction between the water molecules forces molecules to group together in a droplet. Water droplets are too large to penetrate the Micropores without substantial pressure. It is difficult for water droplets to pass through, and therefore the material is reasonably waterproof. Water vapour molecules are much smaller (typically less than 0.0003 microns) and pass easily through the holes, carrying moisture away.
St.One ® does have some Porosity, but this is not linked to Capillarity.