Thursday, 14 January 2010

Repointing


With exposure to wind, rain, frost etc, the mortar holding bricks (or stone) together on an external wall will often start to crumble away and fall out.
On older properties, the pointing will actually be a harder mortar than the bonding mortar, so once this is dislodged, the softer bonding mortar will become exposed and wear away quite quickly.
This repointing process begins by removing damaged pointing to a depth equal to or slightly more than the with of the joint, or to the point where sound mortar is reached.
On very old buildings with soft materials, such as under-fired brick, removal by hand is often the most effective to avoid damage.
Hard Portland cement mortar is usually removed with a grinder or power circular masonry blade, taking care not to damage the masonry units. Vertical joints in most cases are always done by hand or with small power chisels.
Poor repointing work often raises the level of the mortar joint above the face of the masonry unit, which causes the mortar edge to feather.

Repointing materials
It is essential that the mortar used for repointing have similar performance characteristics to the original mortar used in a building.
Mortar used for re-pointing should normally be based on hydraulic lime and clean sharp sand or other appropriate aggregates. The colour and size of the sand particles determine the appearance of the mortar.
The existing older mortar should be carefully examined so that aggregates used in re-pointing can be selected to match existing mortar for texture and colour.
The strength of the mortar should reflect the strength of the stone or brickwork and the exposure to weathering.
Sandstone requires a weak mix while whinstone will allow a firmer mortar bed but not so hard as to cause cracking and capillary pathways for water. Exposed chimneys will need a more durable mortar than a sheltered area of walling.
Lime is a general term for calcium-containing inorganicmaterials, in which carbonates,oxides and hydrohides predominate.
Materials are still used in large quantities as building and engineering materials (including limestone products, concentrete and mortar) and as chemical feedstocks, among other uses.
The rocks and minerals from which these materials are derived, typically limestone or chalk, are composed primarily of calcium carbonate. They may be cut, crushed or pulverized and chemically altered.

No comments:

Post a comment