Technical repointing pages from Gun Point Limited

3. Mortar Mixes and Site Practice

3.1 Mortar Mixes

3.1.1 The Strength of Mortar

3.1.2 How Much Water? – Water-Cement Ratio

3.1.3 Mortar Types and Proportions

3.1.4 Sampling and Testing of Mortar

3.1.4a Taking samples of wet mortar
3.1.4b Laboratory testing
3.1.4c Sampling and testing of hardened mortar

3.2 Site Practice – Storage Batching and Mixing

3.2.1 Storage of Materials

3.2.2 Batching of Materials

3.2.3 Mixing of Materials
Site Practice – Repointing
3.3.1 Recommendations

3.1 Mortar Mixes

In the previous chapter we broke mortars down into their raw materials and looked at the choice that there is and at various problems that can arise through careless selection or use of the raw materials. The wide amount of choice in say binders is probably a bit confusing and maybe the various pitfalls that have been mentioned add to the difficulties at this stage in deciding what is best to do for a particular job that comes up. In this chapter we try to pull together the loose ends and, after a look at things like strength, porosity, shrinkage and other general matters, come to the Code of Practice recommendations for repointing and associated repair work.

3.1.1 The Strength of Mortar

Strangely this is in itself not very important provided a mortar is not too strong. For repointing work the small amount of mortar at the outer edge of the joint is not going to play any structural part; any loads that the wall is carrying will continue to be carried by the original mortar in the main part of the wall.

A strong mortar, even in repointing, by its rigidity can restrain small expansions and contractions, settlements and so on and cause stresses in a wall to build up to such a level that cracking occurs. Weak mortars allow these minor movements in the wall and do not cause the build up of stress that strong, rigid mortars do. Many of the so called settlement problems with modern houses have arisen because the mortar has been too strong and makes a very rigid box structure, the slightest movement in the ground or around an opening and the wall cracks. In old brickwork the bricks just rearrange slightly by hairline cracking in flexible mortar joints.

With soft walling materials, particularly stones, a strong mortar in repointing can in the long term lead to erosion of the stone at the joint and again weak mortars must be used in these situations (see recommendations below).

Present day practice inclines to using mortars that are too strong, wrongly for the reasons outlined above. Of course extreme weakness is going to prove unsatisfactory. This arises from excessive porosity in the mortar which may be due to the use of insufficient binder to fill the voids (or of course an unsuitable sand) or of excessive mixing water, remaining in the mortar and evaporating out later. The porosity will make a very weak mortar; in terms of strength alone this does not matter. What may happen is that the mortar will fairly quickly start to disintegrate when frosts occur.

Although strength in itself is not especially important, it is easy to measure. Later on in this chapter we look at sampling and testing for quality control where the making and crushing of cubes is the system most likely to be used. This is not because of the importance of high strength but because it gives an indication of how the mortar is likely to behave in other respects (particularly frost resistance). When a series of tests are carried out (usually on a large job), the tests are probably most useful in that any large variation between individual results may show up some inconsistency in site work and lack of control of the process.

3.1.2 How Much Water? – Water Cement Ratio

It is probably easiest to start back with the hydration process (cement reacting with water): from the chemistry this requires an amount of water roughly one third of the amount of cement – that is a 50kg bag of cement requires about 15kg of water (about 3 gallons). At this stage the amount (or type) of aggregate does not come into the calculation.

As mentioned previously for practical mixes you will always need more than the minimum of water since that minimum amount will give you a very dry and crumbly mix. The excess water is added to allow the aggregate particles to be coated with a liquid which will allow them to slide over each other and give a coherent mix.

This excess water will eventually evaporate leaving voids in the mix. It is vital therefore that, with the Gun-point process, the mortar is compacted after one initial set by rubbing in with the appropriate tool. Unless this is done properly, these voids will tend to reduce the strength of the mortar although as previously discussed strength is perhaps not the most important feature of a mortar (though it usually will be in concrete work). Frost resistance is likely to be more crucial and an excessively porous mix may fail prematurely by allowing too much water absorption with subsequent freezing of the absorbed water.

Again in aggressive environments (industrial effluents etc) a porous mortar is going to absorb effluent more readily and provide a greater volume in which the destructive reaction can take place.

A further possible problem with high water contents is that the mortar will tend to show greater shrinkage as it sets and dries out. This is mainly a problem where large thin areas of mortar are laid (eg. In renderings) and is possibly insignificant in terms of the repointing process where the volumes and surface areas are small.

Having discussed the problems associated with high water contents, we now want to look at conditions under which these might occur.

The most likely cause is where poorly graded aggregates are used, in particular those that contain a high proportion of fine material which thus have a greater surface area to be wetted as compare with a well graded aggregate (see also 2.1.3a and 2.1.3b above on sands). Using such sands one is forced to add more and more water in order to get a mix that is of reasonable consistency.

Another aspect of water content that can be looked at here is that of the “water retentivity” of the mortar (ie the degree to which the mortar holds on to water). This can become an important consideration in hot, dry summer weather. If the water content of the mix falls below the one third water : cement ratio mentioned at the beginning of this section, then there will be insufficient water present to allow the cement to hydrate properly. The water content can be reduced by suction from dry bricks or blocks and most repointing specifications will ask for the damping down of the brickwork before the application of the mortar. Lime containing mortars show greater water retentivity as compared with those using plasticizers and in traditional repointing techniques their use may be preferred under very dry conditions.

3.1.1 Mortar Types & Recommendations for Proportions for Specific Applications

Tables 3.1, 3.2 and 3.3 give the recommendations of BS 8221.

Table 3.1 gives the types of mortar most widely used in the country today. Table 3.2 covers types that are probably most likely to be used in restoration work on older buildings. Table 3.3. gives recommended mortar mixes for different walling materials for external walls subject to varying degrees of exposure. For internal walls and for pavings refer to relevant code of practice.

Mortar designation (I) is only recommended for dense, vitreous brickwork in severely exposed situations.

Figure 3.1

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Table 3.1 Mortar Mixes

DESIGNATION OF MORTAR	TYPE OF MORTAR – Proportions by volume lime putty : dry sand.
DESIGNATION OF MORTAR	Cement : Lime : Sand	Masonry Cement : Sand	Cement : Sand with Plasticizer
(i)	1:0 to ¼ : 3
(ii)	1 : ½ : 4 to 4½	1 : 2½ to 3½	1 : 3 to 4
(iii)	1 : 1 : 5 to 6	1 : 4 to 5	1 : 5 to 6
(iv)	1 : 2 : 8 to 9	1 : 5½ to 6½	1 : 7 to 8
(v)	1 : 3 : 10 to 11	1 : 6½ to 7	1 : 8

…….. continues in table 3.2

Note: ‘lime’ refers to non-hydraulic or semi hydraulic lime.

Table 3.2 Mortar Mixes

DESIGNATION OF MORTAR	TYPE OF MORTAR – Proportions by volume putty : dry sand.
DESIGNATION OF MORTAR	Lime : Sand	Hydraulic Lime : Sand	Lime : pfa : sand	Lime : Brick Dust : Sand
(vi)		2 : 5
(vii)		1 : 3	2 : 1 : 5
(viii)				2 : 2 : 5
(ix)	1 : 1		3 : 1 : 9
(x)				1 : 1 : 3
(xi)	0 : 2 : 5

Note:    (1)        Hydraulic lime is lime which will set under water otherwise ‘lime’ refers to non-hydraulic              semi-hydraulic limes.
            (2)        p.f.a – low sulphate content.

Table   3.3        Recommended mixes for different walling materials according to degree of exposure

TYPE OF MATERIAL	DEGREE OF EXPOSURE
TYPE OF MATERIAL	SHELTERED	MODERATE	SEVERE OR MARINE
A1. Stone :	(v) (vi)	(iv) (vi)	(iii) (v)
A1. Highly durable, eg. Basalt, Granite, millstone, grit.	(v) (vi)	(iv) (vi)	(iii) (v)
Flint	(vi) or (vii)	(vi) or (vii)	(vi) or (vii)
A2. Moderately durable, eg. Many limestones and Sandstones.	(v) (vi) (vii) (ix)	(iv) (v) (vi) (viii)	(iii) (iv) (v)
A3. Poorly durable eg some calcanteous sandstones, some fine pored limestone.	(vii) (ix)	(vii) (vii)	(v) (vi)
B. Claybrick in lime mortar:	(v) (vi)	(iv) (v)	(iii) (iv)
B1. Dense, Strong and vitreous	(v) (vi)	(iv) (v)	(iii) (iv)
B2. Medium and low density	(v) (vi) (vii) (viii)	(v) (vi) (vii)	(iii) (iv) (v)
B3. Low density, weak or friable	(vii) (viii) (ix) (x)	(vi) (vii) (viii)	(vi) (vi) (vii) (viii)
Clay brick in cement or cement : lime mortar.	(iii)	(ii) (iii)	(i) (ii) (iii)
C1. Dense, strong and vitreous	(iii)	(ii) (iii)	(i) (ii) (iii)
C2. Medium and low density	(iii) (iv)	(ii) (iii) (iv)	(ii) (iii)
C3. Low density, soft and friable	(iv) (v)	(iii) (iv)	(iii)
Calcium Silicate brickwork in cement or cement : lime mortar	(iii)	(iii)	(ii) (iii)
D1. Class 4 and stronger	(iii)	(iii)	(ii) (iii)
D2. Class 3	(iii)	(iii)	(ii) (iii)
D3. Class 2 and weaker	(iv) (v)	(iv)	(iii)

3.1.4 Sampling & Testing of Mortar

We hope that in practice it will not be necessary to refer to this section very often. Where the testing is carried out to show compliance with specifications is very important that the whole process of sampling and testing and particularly the site work be done to the letter of the requirements of BS 4551 : 1980.

3.1.4a Taking Samples of Wet Mortar

BS 4551 gives detailed procedures for sampling which are given soon. You may be required to take samples of and test the mortar that is being applied to the wall. This would be no more than part of a quality control process as it were, which should not cause problems unless materials are carelessly stored or batched and inadequately mixed. Poor sampling, or more importantly, poor curing of the samples (discussed in more detail below) can lead to the lab tests giving results that appear to show that the mortar is non-compliant. We return to this point later.

Section 4 of BS 4551 deals with sampling of mortars and reduction on site of samples to the suitable quantity for despatch to the laboratory. The clauses most likely to be relevant to Gunpoint Operations are quoted word for word : the scope of other clauses is indicated. Reference should be made to the BS is they are likely to apply.

"..4.2 Freshly mixed mortar
4.2.1 General Samples shall be obtained by taking uniformly distributed increments (preferably from material in motion, provided this can be carried out in safety) and mixed thoroughly to form a combined bulk sample. The number of increments and size of bulk sample
necessary depends on the quality of the material and its variability and the accuracy required of the test results. The bulk sample shall be reduced in accordance with 4.2.4.

4.4.2 Apparatus According to the method being used, the apparatus required is either a metal receptacle or a scoop of not less than 1 litre capacity and air tight containers which shall be clean and dry at the commencement of the sampling operation.

4.4.3 Taking of samples

4.4.3.1 Batch mixers. The mortar shall be sampled at the discharge point of a batch from the mixer. Not less than twelve increments spaced evenly throughout the batch shall be taken at the discharge point of the mixer. The increments shall be taken by passing the receptacle across the stream of mortar in such a manner as to collect a thoroughly representative sample of mortar.

4.4.3.2 Conveyors, Pumps etc (refer to BSS)

4.2.3.3 Large hoppers, bins or heaps (refer to BSS)

4.2.3.4 Small hoppers, bins or heaps. The material shall be sampled by means of the scoop at regular spacings throughout the mass. Increments shall be taken from the material well below the surface in at least twelve different places in the mass, distributed in a regular manner, so as to ensure, when mixed, a thoroughly representative combined sample.

4.2.3.5 Bulk transport vehicles (refer to BSS)

4.2.4 Reduction of bulk sample. The increments or sub-samples taken in accordance with any of the methods described shall immediately be combined and thoroughly mixed and reduced to a sample of not less than 15kg (if chemical analysis only required 1kg) by taking sufficient Scoopfuls from random positions throughout the mixed material. The reduced sample shall be placed in one or more airtight containers. If consistence or air content tests are to be made on a sample, arrangements should be made to carry out the tests at the point of sampling.

4.2.5 Packaging and certificate of sampling. Each sample to be despatched to a laboratory shall be placed in one or more airtight containers, and suitably labelled so that its origin can be identified at the Laboratory. The sample shall be accompanied by a certificate from the person responsible for taking the sample stating that sampling was carried out in accordance with the requirements of the British Standard.

The certificate shall include as much of the following information as is appropriate:

(a) type of material
(b) the date, time, place and method of sampling
(c) the quantity of the batch and consignment, or the period of production represented by the sample
(d) tests required…..”

Before going on to look at the testing, we would like to bring out a few things from the above.

1. If you have to do tests, do everything properly even if you think it doesn’t matter : it can save hiccoughs later on.

2. Therefore samples are taken at the mixer while it is being tipped, not off the spot board, not out of the Gunpoint pump (unless the architect specifically asks for this), nor scraped out of the wall nor made up from the leftovers after a mornings pointing.

3. The sample you take is much bigger than the one you will send off. It may seem wasteful to struggle to scoop out a pile of mortar and then throw half of it away. It is however necessary to do this to achieve a sample that is statistically correct.

4. Make sure the containers are clean and airtight, decent tins or new polythene sacks securely sealed; don’t use old fertilizer sacks.

3.1.4b Laboratory Testing

The Laboratory chosen for testing should be independent to avoid any possibilities of bias. Some ready mix companies carry out routine testing of the material they supply; this should be taken as no more than an internal (to the supplier) quality control check. Many architects will require independent testing as well.

BS 4551 gives detailed procedures for chemical analysis:

(see BS 4551                free water content
Section two)                  lime content
                                    cement content
                                    sand, silt and clay content
                                    sand grading

Physical testing             dropping ball test for consistence
(see BS 4551                consistence and water retentivity
Section three)                flow

                                    air content
                                    stiffening rate
                                    strength

A straight chemical analysis is more likely to be required on a hardened mortar. This could occur where a dispute has arisen after completion of the work as to the quality of material used (see section 2.1.7d below).

The most likely test is to be that of strength. The test is carried out on cubes of hardened mortar (usually of 100mm size) and will be demonstrated in one of the practical sessions.

The cubes are normally made on site and on this course you will do this yourselves in the Laboratory – a detailed description of the procedure will be given to you. Again the importance of following the procedure given in BS 4551 is emphasized. Purpose made steel moulds must be used to make sure that the cubes are of the correct size and that opposite faces are truly parallel. Home made ply or timber moulds should on no account be used. The cubes will be made from the sample of wet mortar taken as previously described. Unless instructed differently by the architect, six cubes should be made for each test – 3 will be tested at 7 days and 3 at 28 days. The cubes should be made as soon as possible after the taking of the sample and never late than 1 hour after the addition of water to the mix, (except in the case of retarded mixes). Once the cubes are made, place the mould carefully into a clean plastic bag (this is to stop the moisture evaporating), seal it and store at 20 + 2°C. For most of the year this temperature requirement will mean provision of special curing facilities. On no account should the cubes be left in an unheated hut or in the back of a van that is going to be left out in the cold. After one to three days the cubes are carefully taken out of the moulds and transferred to appropriate conditions for further curing until testing. Before describing these conditions some explanations of the importance of these temperatures and humidity requirements may help.

The hydration process has already been discussed: in this, cement reacts with water to form a hard mass. It takes about 12 months for hydration to be fully completed, though in the first months about 90% of the process is complete. If we measure the strength of a cement mortar at time intervals, we can plot a graph as in fig. 3.1, which can be taken as a way of displaying the hydration process. There are two curves shown here, one shows the way that hydration proceeds under summer conditions, the other giving a picture of the winter situation. Note that in either case the final strength achieved is the same. However, for any particular times, there are significant differences in strength right up past 28 days. They may not look very great on paper but in reality may mean the difference between mortar being accepted and being rejected.

Now the architect asks you to make cubes so that he can compare the strengths that you are giving him with standard values for mixes of various proportions (see table 3.4). These standard 28 day strength values are based on mortars that have been kept at 20°C and 100% humidity.

When a cube test result is below standard, from what has been said above, this may be because the cube has not been stored at the proper temperature. In a properly stored cube, a below standard result can arise because of insufficient binder in the mix (or perhaps because of retardation of the cement by contaminants).

The architect is bound to accept the second cause since in getting you to make cubes to BS 4551 he is saying make and store them in the approved way.

Thus incorrect cube making and in particular curing can lead to the rejection of mortar that is otherwise satisfactory.

BS 4551 gives two modes of curing – hydraulic curing and moist air curing. In the first, the cubes are kept in a tank of lime saturated water at a temperature of 20 + 1°C until 2 hours before testing. With moist
air curing the cubes are kept in an air tight container over water with temperature maintained at 20 + 2°C. Cubes will be immersed in water for 4-6 hours immediately before testing.

For either curing method specialist equipment is needed and it is probably safest to take the cube to the testing laboratory or removing them from the moulds. Remember however that you will be responsible for seeing that the right temperature occurs during the 2 or 3 days until you demould the cubes.

3.1.4c Sampling and Testing of Hardened Mortar

We think that this is most likely to arise in two situations. Firstly, you may be asked to analyse an existing mortar, almost certainly in a historical building, in order to find out what went into it originally. Secondly you may be faced with a situation in which the quality of your own work is being questioned, probably in terms of the likely life (or shortness of it) of material that you have put into a wall. This may arise because a mortar cube has failed and you can perhaps raise doubt as to whether the cube truly represents the mortar that was actually put into the wall. You may, therefore, have suggested further tests on the hardened mortar to demonstrate its suitability.

In the first case you would expect the client to pay for the sampling and testing and you should make it clear that such sampling and testing is an additional charge to normal repointing charges.

In the second case the situation is slightly complicated by the “politics” of a dispute between you and your client. Theoretically if the further tests on the hardened mortar show your material to be acceptable, there is a case to be made out for charging the client for the extra costs of testing. On the other hand if it arises because you made a mess of a cube test then the client could argue that the additional testing is your own fault due to your incompetent cube making etc. The laboratory work in analysing a mortar sample would be about £250.

Unfortunately there are no really satisfactory and sufficiently accepted tests that can be carried out on the mortar in-situ. Various tests using steel probes (needles) fired into the surface have been applied to concrete work but they are only of limited acceptability there. These tests do not appear to have been applied to mortar joints. Thus no direct testing of the mortar can be carried out and we must use a laboratory assessment. This consists in analysing chemically the constituents of samples of hardened mortar taken from the wall.

Section 4.3 of BS 4551 covers sampling procedures and the requirements are summarised here. Firstly you must be clear in your mind as to what you are trying to find out and demonstrate.

(1) There may be a question about variability between different parts of the work. In this case obviously you will keep separate (and well labelled) the samples from the different areas.

(2) There may be a question about the average composition over the whole of a façade (or building). In this case sub-samples should be taken from representative areas (making sure you obtain good coverage); these sub-samples are then merged to form a single composite sample.

(3) There may be a question about pointing in a particular area in which case of course the sample is taken from that area alone.

In any case the sample sent to the laboratory should be not less than 100g (about 1/41b). Where sub-samples are to be merged to form a sample representing the ‘average’ mortar (no 2 above) then BS 4551 Recommends the following:

(a) Sub-samples to be of not less than 50g (about 2oz), each to represent not more than 10m of wall.

(b) Main samples made from merged sub-samples to represent not more than 50m of wall.

Ideally samples should be obtained by carefully extracting a brick and cleaning off the ‘mortar’, avoiding contamination with brick material. Where the testing is to be on repointing work then clearly only the repointing material should be cut out and this should not require the removal of a brick or bricks.

As an alternative, BS 4551 allows sampling by drilling with a masonry drill, particular care being needed to make sure that all the fine material is collected.

The sample should be put into one or more containers and labels to show:

(a) Date, time, place and method of sampling.

(b) The location in the building of the area sampled.

(d) Reason for investigation and specified mix, if known.

The analysis of the hardened mortar samples is likely to be expensive and may not produce very conclusive results. It may possibly be justified if you are reasonably certain that you can prove the acceptability of material that would otherwise be rejected and which may have to be cut out and replaced at considerably greater expense. The importance of an independent laboratory is again emphasised.

In the other case (analysis of old mortar for matching purposes), it seems likely that matching can be achieved more economically by trial mixes of different proportions carried out on site.

3.2 Site Practice – Storage, Batching and Mixing.

This section sets out Code of Practice and normal specification requirements which should be followed. A careless approach in this case could create (unnecessarily) criticism of your work.

3.2.1 Store of Materials

Storage areas should be kept tidy at all times. Sand should be kept so that it is not contaminated with soil or with falling leaves, other vegetation or other organic material; as mentioned previously these may affect the hardening of the mortar. In the absence of a suitable (ideally paved) hardstanding, old doors, corrugated sheets etc should be put down for storing sand.

In winter and autumn when frosts are likely, the sand should be covered at night with a tarpaulin to avoid freezing of the sand stockpile. This is really only common sense since you may lose an hour or two’s work in the morning, waiting for the sand to thaw out. It is quite acceptable to heat the sand to thaw it if you do get caught out – spread the sand thinly on a corrugated iron sheet over a wood fire. You must be careful not to mix while there are frozen balls of sand, since generally these do not thaw out in the mixer and only become apparent as pockets of raw sand in the mortar on the spot board (or in the hopper of the pump).

Cement must be kept dry and bags should be used in order. Stack bags clear of the ground with sheet polythene or a tarpaulin over the top. Bagged lime again should be kept dry since it will go lumpy if it becomes damp and lumps of lime do not mix easily with the sand. If cement has gone lumpy, it should not be used since the small lumps of cement will not break up and disperse throughout the mix. There is little point in breaking up the lumps since any powder produced (at great cost in time) will be of partially hydrated cement and is not suitable for making a mortar.

A more workable mix is obtained if the powdered lime is run to putty some time before mixing. The dry hydrated lime should be added to water in a tank (if you add water to the lime the mixture goes lumpy, like making gravy) and mixed until you have a thick cream. This should be left to stand for as long as possible (ideally not less than 16-24 hours) : excess water may rise to the surface which will prevent carbonation of the lime.

It is unlikely that you will want to run lime putty from quick lime, details are given in CP121 : it must be remembered that the slaking of quick lime can be a very dangerous process if the correct procedures are not followed.

Ready mixed coarse stuff should be stored as for sand to avoid contamination. In addition keep the stock pile covered with polythene or a good tarpaulin to prevent the air getting to the lime.

3.2.2 Batching of Materials

It is probably worth making an effort here to show your efficiency and competence by insisting on careful measurement of ingredients using gauge boxes for measuring out sand, if necessary for cement and lime use marked buckets if batch size requires a quantity of less than half a bag. Do not measure by the shovelful, you know the difference between a shovelful of damp sand and a shovelful of dry sand and it is relatively simple to do things more accurately.

Mix proportions for mortars are normally specified as dry volumes : where lime is specified this assumes that lime putty is used. If you are using dry hydrated lime than you should add up to 50% more to achieve the specified mix proportions.

Table 3.5 gives batch proportions to be used where ready mixed coarse stuff is being gauged on site with cement.

3.2.3 Mixing of Materials

Mechanical mixing is preferred since this usually gives a more thoroughly mixed mortar and is also quicker giving you more time for pointing. The main point here is to keep the mixer clean, in warm weather this may mean cleaning the mixer during the day as well as when you pack up in the evening.

Hand mixing is normally acceptable and for traditional pointing techniques is probably preferred since the quantities of mortar required per morning, say, are fairly small. A batch from a mixer is likely to have started to go off before it has all been used. If you hand mix there are two main points:

(1) Use a banker board or hardstanding (not the street) to mix on in order to avoid contamination.

(2) Turn over the dry materials twice before adding water and then, when wet, as much as is necessary to achieve a uniform mix.

3.3 Site Practice – Repointing

This section sets out requirements of Code of Practice (BS 8221) and most specifications where work is carried out by traditional methods. The Gunpoint process differs only in its method of applying the mortar to the joint and therefore in the speed with which the operation is carried out.

3.3.1 Summary of Recommendations in Repointing in BS 8221 – ‘Cleaning & Surface Repair of Buildings’

If cleaning of the façade is to be carried out this should be done before repointing (see also chapter 5). Normally the repointing will be struck to give the same joint profile as the original work (see fig 3.2).

Flush pointing : if the colour and texture of the aggregate are to be shown the joint can be rubbed after the initial stiffening of the mortar with a stick, piece of hose pipe or coarse sacking. Alternatively the face may be stippled with a fibre brush or a water spray.

Pointed finish – this is used on brickwork and is formed by drawing an iron tool (‘jointer’) or the edge of a trowel along the centre of the joint using a straight edge to make the work uniform.

Bucket handle finish – again generally used for brickwork with the recessed curve formed with a piece of bent steel rod of the appropriate diameter.

Struck weathered joint – a very common form of joint for brick and stone (does not look too well on random rubble work) where the slope sheds water.

‘V’ jointed (weather jointed) – generally used for stonework particularly random rubble work where it reduces the apparent size of the generally somewhat thick joints between the stone. The undercutting should not leave too great a ledge for water to lay on.

Recessed jointing – sometimes used externally in brickwork for visual effects or again where arrises in old work have eroded and other types of pointing could make the joints very wide and clumsy.

Tuck Pointing – this type of pointing is generally criticised as being very vulnerable to decay.

Although not covered in BS 8221 ‘strap pointing’ should be mentioned as the equivalent in stonework of tuck pointing in brickwork. It is generally considered undesirable in that a water-trapping ledge is formed.

‘Galletting’ is sometimes used to improve the appearance of wide mortar joints (particularly in rubble stonework) : pieces of stone are pressed into the stiffened joint and left exposed on the surface thus reducing the apparent wide of the joint.

The selection of mortar type and mix proportions has already been discussed (3.1 and 3.2 above).

In general joints should be raked out square to a depth of about 20mm for an effective key : this may be reduced on fine jointed work to give a depth into wall slightly greater than the joint width. Great care is needed to avoid damaging the arrises of the brick or stone work. On hard jointing plugging chisels or other parallel faced cold chisels should be used (the normal wedge bladed cold chisel is more likely to damage the bricks or stones).

After removal of the original pointing, the joints should be cleaned thoroughly of all dust before filling. Joint should be fully filled with the mortar pressed in well so that it bonds to existing mortar at the back of the joint. Where arrises are eroded (as mentioned above) recessing of the pointing is desirable so that feather edges on the mortar are avoided. Repointing should be carried out from the top downwards. The Code of Practice finishes by emphasising the importance of accurate batching of materials in order to achieve consistent quality and in particular colour.

The above information should not be taken as recommendations for any individual contract/project and are guidelines only. Consult your local licencee for advice on the projects in your area.

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