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Mortar and Pointing |
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New Build |
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Mortar and Pointing(a) Cement Mortar
Qualities and Proportions
The choice of quality and mix of mortar should take into account those mixes proved by local practice to be the most suitable for use with the stone elected and for the climatic conditions prevailing. In general terms, the mortar used should not normally be harder or denser than the stone employed. A typical range of mixing proportions is given in the table below. As a general principle, Portland cement should be the predominant cementing agent where the chief consideration is strength and frost resistance. The introduction of lime weakens the mortar and increases its flexibility and porosity. But it takes longer to set and workability is reduced.
Proportions (by volume)
Portland Cement Lime Fine Aggregate Uses
(a) Cement Mortar 1 1 - 3 Strong,dense eg. For below DPC
(b) Cement-Lime Mortar 1 1 5 to 6 Severe exposure eg for parapets
1 2 8 to 9 Normal exposure –walls between DPC and eaves
1 3 10 to 12 Sheltered areas but low frost resistance
c)Hydraulic Lime mortar - 1 2 to 3 Normal to sheltered areas
Lime can take one of the following forms:-
(a) dry bagged hydrated (- semi hydraulic)'lime
(b) lime putty formed by mixing hydrated lime and water to a creamy consistency . This is best left over night to mature and needs then to be gauged with cement
(c) ready mixed lime putty/sand mixed with water - best kept overnight before use
d) 'coarse stuff', ie. lime putty/sand mixed with water - best kept overnight before use.
(e) hydraulic lime which will set under water.
Where restoration work is being carried out ,the original aggregates should be copied as closely as possible in type size and distribution.
Mortar for Masonry
(a) Porous limestones and sandstones
For limestones customary and desirable to use the crushed stone as aggregate.For bedding these stones, a 1:3 hydraulic lime crushed stone mix would be suitable .If a more rapid set is required the mortar should be gauged with cement to proportions 1:3:12 cement/lime putty /crushed
(b) Dense sandstones and granite
For these strong mortars may be used. Suitable mixes are:-
sandstones r.r.6 cement/lime putty/fine aggregate
granite 1:2 or 1:3 cement/fine aggregate
New Work
Jointing & Pointing
The finish to the face of the mortar jointrs may be carried out either by jointing ,as the work proceeds,with the same mortat that was used for bedding ,or by raking out the joints to a depth of 25 mm and later filling in with a specially prepared mortar .jointing is to be preferred as it leaves the mortar undisturbed ,but pointing offers facilitied for introducing into the joints colour which will tone with the walling .Mortar used for pointing should have adequate frost resistance and should be of approximately the same strength as the mortar used for bedding .
The method of finishing the pointing will depend upon the nature of the stone and the degree to which it has beendressed .It is recommended that in general the texture of the l\joints should conform to that of the masonry.Jointrs in rubble walling should therefore be struck off flush with a trowel .The joint should preferably ,not be finished ‘smoothed off’ as with new work.
Repointing
If the surface of the mortar joints has weathered to the extent that the face of the stones is vulnerable to damage ,or water can lodge and penetrate ,then a matching mortar must be introduced.
If the stones have retained their sharp arrises, then the joints should be filled flush again ,unless there is specific evidence that the joint face was profiled in some other way. |
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