GUN-POINT LTD Technical Document

Abstract Official Technical Document of Gun-Point LTD

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1. Introduction to Building 1.1 General 1.2 Dampness in Walls 1.3 Exposure 1.4 Drawings

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1. INTRODUCTION TO BUILDING 1.1 General In this chapter we are trying to cover three general aspects. Firstly, mainly in the following drawings, we show the use of the technical terms that you are likely to come across in discussions, in writing quotations for work, and from time to time, in reading reports on particular buildings. It is important to understand the terms used by others – if an architect starts talking about ‘voussoirs’ or ‘rusticated quoins’ or ‘the parapet’, these terms are shorthand descriptions that you should try to become familiar with. Similarly from our point of view when preparing a quotation it will enable us to define precisely what we are offering to for the client. Apart from the technical terms, the drawings also show different ways in which walls are constructed as load-bearing and panel walls, solid or cavity walls. Cavity walls are almost universally used in modern masonry construction since the same degree of resistance to weather and level of heat insulation can be achieved with less material than is required in solid wall construction. 1.2 Dampness in Walls We think that there are likely to be occasions where repointing is being considered for an existing building where the external wall shows signs of rain penetration. There are a number of possible causes for dampness in external walls and it is not always easy to determine which particular cause applies, on occasion a number of factors may be operating at the same time. The indications for rain penetration are dampness (discolouration of plastering and decoration) generalised vertically up a wall. In contrast rising dampness is generally worse in the lower parts of the wall and may not rise higher than perhaps three quarters of the way up the lower storey. Another (perhaps the commonest) likely cause of dampness is condensation, this being at its worst during winter months. In all three cases wall construction is likely to be inadequate by modern code of practice requirements. Much housing until the between wars period was built with one brick thick external walls to the main part of the house, the present code of practice recommends a minimum of one and a half bricks for sheltered sites (for “sheltered”, “moderate” and “severe” exposure refer to next section). Very many of these “inadequate” walls do not show any signs of rain penetration, particularly in sheltered urban situations. However, for more exposed sites rain penetration is likely during the winter months when the wall can become saturated with water for long periods of time. The solid wall obtains its resistance to rain penetration by providing an adequate “reservoir” within its micropore structure to absorb rain falling on the surface. The absorbed water eventually evaporates out when drier weather occurs. A wall of inadequate thickness will have insufficient ‘reservoir’ capacity to cope with rainfall in winter when natural evaporation rates are low. The precise mechanism of rain penetration is complex depending upon the absorbency of the brick and mortar and upon the presence, or not, of hairline cracks between the brick and the mortar. This discussion has been in relation to external walls of one brick thickness: in much Victorian housing of the cheaper type back extensions to the main part of the house were often only half a brick in thickness and even in sheltered sites are nearly always damp from rain penetration.

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These walls of one brick (or less) thickness also have high rates of heat flow through them (poor thermal insulation). Apart from making the cost of providing heating high, the other major effect is that the surface temperature of the wall internally will be very low to the point that condensation on the surface is likely. Mould growth (of yeasts etc) associated with condensation can be very severe in areas where natural ventilation is poor (e.g. Upper corners of rooms between ceiling and walls, built in cupboards (internally), etc). Again this type of construction, particularly in the earlier part of the nineteenth century, often has no damp proof course thus allowing damp to rise from the ground in contact with the foundation. Cavity wall construction overcomes the rain penetration problem by providing two separate ‘leave’ or ‘skins’ to the wall (see drawings). The half brick outer leaf is assumed to be saturated in winter conditions probably to the point where water can run down its inner face. However, the dampness cannot pass across the 50mm (normally) air gap between the two leaves. For reasons of structural stability the two leaves are tied together with metal ties (most commonly galvanized steel) : the tie is designed to prevent water wherever there is a “bridging” of the cavity (commonly at window openings), the separation between the leaves should be maintained by suitable dpc’s or walls). Theoretically, and generally in practice, the cavity wall offers better weather resistance than a solid wall. Most defects that occur are due to inadequate design or construction in maintaining the separation between the two leaves of the wall. Other possible causes of dampness in walls that may need to be investigated are leaking rainwater pipes and gutters, leaking service pipes (water) or soil and waste pipes, displaced flashings (leadwork) around chimneys or at abutments, cracked sills, and copings etc., hygroscopic salts used in brickwork. 1.3 Exposure This is used as a technical term to describe the severity of weather conditions to which a building or part of a building is likely to be exposed. Fairly obviously severe weather conditions are going to demand a higher standard of construction in order to exclude weather from the building and to ensure that materials used are going to last a reasonable length of time. The exposure grading for a particular site is based upon two Met. Office measurements – annual rainfall and average wind speed : these are combined to give a “driven rain index” from which a threefold classification is made – “sheltered”, “moderate” or “severe” exposure. Tall buildings in areas normally classified as sheltered will, in their upper parts at least, be graded as of severe exposure. Similarly in any particular building some parts are subject to more severe conditions of exposure than the building in general. In this latter group are placed parapets, copings, cills, chimneys, retaining walls, free-standing walls (eg boundary walls,) and walls below dpc but above ground level. Careful attention is needed to the above “elements” of a building since they are the parts most likely to be saturated for long periods and to be subject to freezing whilst saturated. In your assessment of potential work particular attention should be paid to these elements since they are normally the areas where pointing may first break down and defects in materials show up. Later in discussing selection of mixes for repointing we shall see that mix proportions should be varied to suit degree of exposure. The above information should not be taken as recommendations for any individual contract/project and are guidelines only. Consult your local licensee for advice on the projects in your area.

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2. Materials 2.1 Mortars

2.1.1 What mortars do in a wall

2.1.2 Different binders 2.1.2a Lime and lime mortars 2.1.2b Cement : lime : sand mortars – gauged mortars 2.1.2c A digression on hydraulic cements 2.1.2d Masonry cement mortars 2.1.2e Ordinary Portland cement mortars with plasticizers 2.1.3 Sands for Mortars 2.1.3a Grading of sand 2.1.3b Particle shape 2.1.3c Other physical properties 2.1.3d Chemical properties 2.1.3e The bulking of sand 2.1.4 Admixtures

2.1.4a Accelerators and anti-freeze 2.1.4b Retarder 2.1.4c Waterproofers 2.1.4d Pigments

2.1.5 Water

2.2 Bricks and Ceramics

2.2.1 Fired clay bricks 2.2.2 Calcium silicate bricks

2.2.3 Terra cotta and faience

2.3 Natural and Artificial Stone

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2. MATERIALS 2.1 Mortars 2.1.1 What Mortars do in a Wall Mortars consist of “aggregates” (sand) which provide the bulk of the volume together with a “binder” which, as the name suggests, sticks the aggregate together. The third ingredient, water, gives the mortar is “fluidity” to enable it to be placed as required and, where Portland cements are used, enables the chemical reaction to take place which results in the cement powder becoming a hard mass. These three are the main constituents of mortar; other materials are sometimes added to modify the behaviour of the mortar – these are discussed later on. In masonry walls, the primary function of your mortar is to provide a continuous bed between bricks. In addition the mortar in an external wall will seal all the joints between the units and prevent penetration by wind and/or rain, snow, etc. Recall from the previous chapter how important weather resistance is in walling and the different ways in which this can be achieved. In the longer term, the mortar joints protect the walling material itself (stone, brick, and so on) from attack by weather. This may be either by frost attack or by the dissolving of the wall material, particularly where industrial pollution of the atmosphere gives rise to mildly acidic rain (mainly with stones). The mortar itself is subject to deterioration from either (or both) of these causes and if the joints are allowed to disintegrate will allow progressively deeper penetration of the wall by the weather and possibly attack on the wall materials. 2.1.2. Different Binders As mentioned above, a mortar consists of binder, aggregate and water. For thousands of years the binder used all over the world was lime. In this country it is only really within this century, perhaps since the 1st World War that lime and sand mortars have been replaced by mortars in which Portland cements are used as binders. 2.1.2a Lime and Lime Mortars Lime is manufactured from either chalk or limestone. The manufacturing process is very simple, consisting of heating the crushed chalk or limestone in a kiln to drive off carbon dioxide. This ‘quick lime’ is very caustic (will burn the skin easily) and must be ‘slaked’ with water to give hydrated lime (or slaked lime) which is the substance used in mortar. Quick lime reacts violently with water expanding and even exploding; this would be too unstable apart from anything else to use as a binder in mortar. Slaking is the controlled addition of quick lime to water so that the usable hydrated lime is obtained. Slaking in this country is now usually carried out at the Lime works and the Lime is supplied as a fine powder in 25kg bags (note Lime has about half the density of cement).

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In the days when quick lime was slaked in shallow pits on site, the lime was run to a putty and left with water standing on top of the putty to mature. This maturing of the lime putty produces a mortar which is more “fatty” (easier to spread) than a mortar made by mixing sand with hydrated lime. A lime mortar gains strength fairly slowly by a process known as ‘carbonation’ (that is reaction with carbon dioxide – from the atmosphere). Thus eventually the lime mortar returns to the chalk or limestone from which completely different from (and much slower than) the ‘hydration’ process (reaction with water which is characteristic of Portland cements and other ‘hydraulic’ cements). The water in a lime mortar is there solely to vie the mortar the fluidity necessary to allow it to be spread, eventually it all leaves the mortar by evaporation. There are three problems with lime mortar. (1) Low strength when fully matured. (2) Water solubility and deterioration in continuously damp conditions. (3) Slow rate of strength gain and vulnerability to frost attack during construction. Before leaving lime it should be mentioned that the solution to some of the problems above has always been available in the form of ‘hydraulic lime’. These are limes made from limestone which contains certain clay impurities which make the resulting lime behave like a portland cement and react with the water to produce a strong binder. These hydraulic limes in fact pointed early inventors in the direction of discovering Portland cement. The distinction is often made between ‘hydraulic limes’ and ‘non-hydraulic limes’. Hydraulic limes harden by hydration and carbonation, non-hydraulic limes harden by carbonation only. High calcium limes are a particular group of the latter which are particularly “white”. Finally, before moving on to Portland cement bound mortars, another intermediate type should be mentioned, the lime – pozzolana mortar. Pozzolanas are natural or man made powders, rich in silica, which, when mixed with normal limes, give them hydraulic properties, that is greater strength, low solubility in water and a hydration reaction. The Romans used pozzolanas widely, the name deriving from Pozzuoli, a town north of Naples, where deposits of volcanic earth were mined (and still are). Pozzolanas are also used with Portland cements as a cement substitute when they are cheaper than Portland cement. Apart from natural pozzolanas there are a number of ‘man-made’ materials which react with lime to produce a hydraulic material. The Romans also used crushed burnt clay brick (or tile) dust for much of their masonry work. In recent years fine ash from coal fired power stations has also been found to have pozzzolanic properties. This is known as ‘fly ash’ or ‘pulverized fuel ash’ (pfa) and is obtainable from the electricity generating board. Not all power stations produce pfa that is suitable for use in a mortar : a lower sulphate content (less than 1% is important and specialist advice should be sought if these materials are to be used. 2.1.2b Cement : Lime : Sand Mortars – Gauged Mortars Once Portland cements become accepted as reliable materials after the turn of the century, builders began to use them to overcome some of the shortcomings of lime : sand mortars. Straight Portland cement : sand mortars were not satisfactory for the bricklayer principally because the mortar lacked cohesion and was very crumbly when spread on the wall. Thus lime mortars were gauged with cements, rather like pozzolanas, to give a higher ultimate strength, a more rapid rate of strength increase (thus reducing risk of frost attack during construction) and resistance to water.

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When masonry cements (see later section) became popular in say, the 1950’s, gauged mortars became less and less used because of the inconvenience of mixing lime and cement on site (one bag was preferred to two). More recently they have become popular again where a pre-mixed lime : sand “coarse stuff” is delivered by lorry to the site from a mortar plant, which can operate fairly good quality control procedures. The coarse stuff is gauged in the site mixer with the appropriate quantity of cement. There are particular advantages in using this procedure where coloured mortars are specified since the mortar plant is capable of producing more consistent colour in the mortar as compared with on site mixing using pigments. For repointing work on its own the use of pre-mixed lime : sand coarse stuff may not be economically justified since the quantities of mortar needed are likely to be small and to attract a part load surcharge on the basic price. 2.1.2c A Digression on Hydraulic Cements Portland cements are the most widely used of hydraulic cements (that is those that harden by reacting with water). Without going into details Portland cements are made from a mixture of chalk (or limestone) and clay which is fired in a rotary kiln to produce clinker, this is subsequently crushed to form cement powder. Gypsum (as used for plastering) is added to modify the early reaction of the cement with water. The characteristics of Portland cements can be varied by the cement maker mainly by control of the raw materials or of the grinding process. The normal cement is known as ‘ordinary portland cement’ (o.p.c.) and is used for most jobs. It is the cheapest of the cements. ‘Rapid hardening portland cement (r.h.p.c.), (Blue Circle, ‘Ferrocrete’) is chemically the same as o.p.c but is more finely ground thus allowing the water to react more quickly with the cement powder (the hydration process, also called the ‘hardening’ of the cement). We do not think that there are likely to be many situations in which you would need to use this type of cement. Here we should distinguish between terms used – ‘hardening’ describes the process of strength increase in cement, ‘setting’ is used to describe the change in the physical nature of cement and water when it changes from behaving as a liquid to behaving as a semi-liquid or ‘gel’. The ‘initial set’ of cement is an important stage and once it has occurred then the mortar containing the cement should not be disturbed. After a time, mortars will become stiff and unworkable, in this condition they should not be “knocked back” with further water to make them more workable. Now we must return to the various kinds of Portland cements that are likely to be used in pointing. ‘Sulphate resisting Portland cement’ (s.r.p.c) (“Sulfacrete”) may be specified for pointing in exposed elements of a building (exposed in the ‘weather’ sense of the word described in chapter 1. Mortars containing Portland cements are liable to deteriorate when sulphate salts, dissolved in water, are absorbed by the mortar and react with the cement in the mortar. The product of the reaction occupies a greater volume than the original cement, so that it swells and disrupts the physical structure of the mortar. Two things are necessary, sulphates water.

Sulphates occur naturally in clay soils and are not eliminated in the brick manufacturing process; some makes of brick can contain quite high amounts of sulphates. Present Codes of Practice recommend that these bricks with high sulphate content should not be used in positions of severe exposure to the weather

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(for example, chimney stacks, parapets and boundary walls) : bricks with moderate sulphate content should be bedded in mortars made with s.r.p.c (see also later notes on building defects and remedial treatments). White Portland cement (‘Snowcrete’) is chemically the same as o.p.c in its active constituents; it does, however, contain far less of the impurities which give o.p.c its typical grey colour. White Portland cement is produced from china clay and very pure chalk through a more rigorously controlled manufacturing process. It is thus considerably more expensive than o.p.c. In pointing its use may be required in producing pale or pigmented mortars (in the latter case in conjunction with pigments, it should however be noted that because there is very much more sand in the mortar, and colour tends to be the dominant factor in determining mortar colour. Masonry cements are modified Portland cements and are discussed further below (2.1.2d). Before ending this digression on hydraulic cements, we should mention ‘High Alumina Cement’ (h.a.c), ‘Fondu Cement or ‘Ciment Fondu’. This is not a Portland cement and is manufactured from bauxite and limestone which are fused together at very high temperatives and again crushed. Mortars (and concretes) made with h.a.c differ from previous mortars in three particular characteristics.

(1) Good resistance to sulphates. (2) Very rapid hydration (full strength in 24 hours or so). (3) Resistance to high temperatures (kilns, furnaces etc).

Unfortunately h.a.c concretes were misused in structural applications in the 60’s and early 70’s and after a period of frantic checking, propping and strengthening, virtually all references to them were withdrawn from codes of practice and building regulations. When used with Portland cements a “flash set” can occur depending upon the ratio of the two cements used. This is generally considered undesirable but can be useful in situations (eg plugging leaks) where rapid setting is wanted. 2.2.2d Masonry Cement Mortars You will recall that straight Portland cement and sand mortars are not very satisfactory from the bricklayer’s point of view. One way round this is to gauge a lime mortar with cement to produce the desired combination of properties. Another way is to use a masonry cement, such as ‘Walcrete’, as the binder. Masonry cements are Portland cements, o.p.c, modified by the addition of a filler which helps to retain water in the mix and a “plasticizer” which makes the mix workable for spreading by trowel. Thus with one bag on site instead of two (lime and o.p.c) a suitable mortar can be produced. Plasticizers work by dispersing minute bubbles throughout the mix which allow the sand grains to slide more easily over each other and thus produce a cohesive mix that is easily spread. The introduction of minute bubbles into the mortar will reduce the strength of the mortar (though this is not often in itself very important) but seems to give improved resistance to frost attack. Where masonry cements are used plasticizers should not be added at the mixer.

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2.1.2e Ordinary Portland cement Mortars with Plasticizers Masonry cements, because of their lower cement content as compared with o.p.c etc, should not be used for making concrete. In order to reduce variety of cements kept on site many builders prefer to stick with o.p.c and use this for concrete and mortars. In order to obtain the required cohesiveness in the mortar, plasticizers (usually liquid) are added at the mixer. As mentioned in the last section these generate minute bubbles which lubricate the aggregate particles. In masonry cements the quantity of plasticizer (and, therefore, the percentage of air entrained into the mortar) is controlled carefully at the cement works. Site addition of plasticizers has given rise to problems with over-dosed mortars due to careless batching at the mixer or a failure to appreciate the importance of not exceeding the manufacturer’s recommended dosage of plasticizers. Control of dosage on site is probably the major problem with this type of mortar.

2.1.2 Sands for Mortars At first glance all sands may seem to be the same. On further reflection, you might agree that there can be quite a variation in colour or again that there might be differences in the “feel” of a handful of one sand as compared with another. These differences between sands can have quite a marked effect on the mortar produced, especially bearing in mind that perhaps two thirds of the mortar consists of sand. We now look at the properties of sand and see how these can affect the mortar. 2.1.3a Grading of Sand This is perhaps the most important physical property of the sand. The term ‘grading’ is used to describe the way in which differing proportions of small, medium or large individual grains are mixed together to produce a sand. A coarse sand will contain a high proportion of large sand particles and will look and feel gritty. Sands of this type are known as “sharp sands” or “concreting sands”. At the other extreme are “soft sands” (also called “building sands” or “bricklayer’s sands”) which contain a high proportion of fine particles. Sieve analysis of the sand allows the grading to be measured in a more precise way than the descriptions above – “coarse” or “soft”, etc. The practical importance of grading lies in its effect on the “workability” of the mortar. This property is significant mainly in the early ‘wet’ stages of the mortar in its application to the wall. The mortar should be cohesive (stick together), if it is crumbly and friable it will be difficult to apply. Coarse sands will have relatively large voids between the individual sand particles and the normal one-third proportion of binder may not fill the voids adequately and hold together the sand particles. At the other extreme a sand with a high proportion of fine (silt) material, whilst presenting a smaller volume of voids (between the particles) will at the same time present a larger total surface area, more water will be required in the mortar in order to wet the surface of the aggregate. The additional water will tend to weaken the mix (by increasing the water : cement ratio). Alternatively, if further cement is now added to compensate for the increased water content the high cement content will lead to greater shrinkage stressed in the mortar as it sets and hardens and this could give rise to cracking in the mortar or between the mortar and the walling units (bricks, stone etc).

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The sands discussed so far have shown a continuous grading through all particle sizes though in one there was an emphasis on coarse and in the other on fine particles. Sands may also occur in which there is a deficiency to a greater or lesser extent of particles say, over the middle and lower range of sizes – single sized sands. These behave rather like coarse sands only to a greater extent since there is now virtually no fine material at all to occupy the voids between the large particles; extremely unworkable mixes are likely to result in more results. 2.1.3b Particle Shape Grading of sand (as discussed in the last section) is the main factor in determining workability. The shape of the individual sand grains also has some effect on the workability achieved. Natural sand derives from solid rock by a process of weathering, frost attack, transportation in streams and rivers and, over millions of years, grains become fairly well rounded. At the other extreme in certain parts of the country, natural sands do not occur at all and sand has to be manufactured by crushing suitable hard rock. The aggregates produced by crushing tend to be angular in shape since they have not been subjected to millions of years of rolling about in a stream or in the sea. For a given amount of solid material the shape which has the least surface area is the sphere. Thus the more nearly a sand approaches the sphere in its particle shape, the smaller for a given bulk volume is the surface area to be wetted by the binder/water. Also from the geometry of sphere packing (like a greengrocer piling apples on a stall) the voids ratio with spheres is about 30%, which ties up very well with our binder : aggregate ratio of 1:3. In contrast angular aggregates present:-

(i) a greater surface area for a given volume, and (ii) In general a greater voids ratio since the particles do not pack as closely together.

There are other particle shapes intermediate between the two extremes described above but these are more important in considering coarse aggregates for concrete work. 2.1.3c Other Physical Properties The strength of the aggregate itself is not usually significant and its determination is difficult. The specific gravity of aggregates can vary over a large range, say from about 0.6 up to 3.5-4. For mortars again this is not a significant property – natural sands or crushed rock fine aggregate will have a specific gravity of about 2.5.

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2.1.3d Chemical Properties The most important consideration here is the need to ensure that the aggregate used does not contain contaminants that will adversely affect the hydration of the cement. The sand should be supplied washed and should be stored on site preferably on a hard standing or boards to avoid contamination with top soil or other organic matter (leaf and plant debris). Where material is stored on site for a longish period, cat’s urine etc, accumulating in the sand can make it useless.

In coastal areas sea dredged sands may be available. These should be thoroughly washed to remove salts, since although these do not adversely affect the hydration of the cement (they will possibly have a mild accelerating affect), they could cause problems after the completion of the work with efflorescence. We have already talked about sulphate attack on mortar (sulphates being a particular type of salt). In general salts present in either the bricks or the mortar will dissolve in absorbed water when the wall becomes wet, particularly in winter when the wall will be very wet. When the bricks/mortar dry out the water evaporates from the surface leaving behind on the surface the salts that had been dissolved from the body of the wall. This salt deposit has a white fluffy appearance and is known as “efflorescence” (literally ‘a flowering out’): it is most noticeable in the first dry spells of spring and early summer. ‘Efflorescence’ normally does no permanent harm to the brickwork, but clients will often be unhappy about the ‘blotchy’ appearance. If “remedial” action is necessary the salt deposits should be brushed off with a soft brush, not a wire brush. On no account should the wall be washed since this merely transports the salt back into the wall. Acid treatments are of no benefit. The brushing may need repeating at intervals until all soluble salts have come to the surface. 2.1.3e The Bulking of Sand The expression, bulking of sand is used of a sand to refer to the peculiar way in which the volume of the sand varies according to the amount of water contained (mainly contained in the space between the sand particles). In most sands the spaces between particles are small and water absorbed into these spaces can force apart the sand particles, thus “puffing” up the original volume. Beyond a critical moisture content (usually about 7% by weight), the system collapses and the volume occupied by the wet sand is the same as that occupied by dry sand. The critical feature here is that the bulk volume of wet sand can increase by 20-30% over the bulk volume of the same sand in a dry condition. The proportion of mortars (see later) is usually expressed as a ratio based upon volumes of dry material. Account should be taken therefore of ‘bulking’ in the measuring out of the sand for a mortar, both in order to ensure that the specified proportions are obtained and, perhaps more importantly, to ensure that successive batches of mortar mixed, say in drying weather, do not vary excessively in their mix proportions. Ideally measuring should be done by weight since the bulk weight of the sand will only increase by about 5% between dry and saturated states. Realistically, for the small batch quantities likely to be used in repointing, careful volume batching with gauge boxes should satisfy most architects, surveyors and contractors (see section later on mixing).

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2.1.3 Admixtures (Note: This section only applies to mortars containing Portland cements.) ‘Admixture’ is the correct term for substances other than binder/aggregate and water which may be added to a mix on site. The term ‘additive’ is also used loosely in this sense but should really only be used for substances added to a cement during manufacture. We have already come across pozzolanas and plasticizers and seen the way in which they modify the properties of lime or cement. In this section we shall be looking at other chemical admixtures which you may wish to use. Before going into detail I should explain why there is a reluctance to allow admixtures on the part of some architects; at the very least nowadays a specification will include a clause to the effect that – “Admixtures may not be used except with the express consent of the architect in writing”. Hopefully you will remember that two golden rules if you are using plastiscizers on site with o.p.c are:

(i) To measure accurately (not in half milk bottles nor in sprinkles or handfuls), and (ii) On no account to use more than the manufacturer states.

It should also be said that only admixtures from reputable manufacturers should be used: you will find bricklayers using Fairy Liquid (5 squirts or whatever) as a plasticizer. It works of course, but it is hardly worthwhile laying yourself open to all kinds of “incompetence”, “amateurish” accusations is problems arise on a job. Excessive use of plasticizer will entrain large numbers of air bubbles into the mortar : when this occurs in a mortar in which careless batching has meant that the binder content is very low, rapid deterioration of the mortar by frost action and general weathering occurs. This has happened sufficiently often for people to be wary about using plasticizers added on site. Another major problem area with admixtures concerns disintegration of concrete in reinforced concrete structures where accelerators have been used to speed up hydration so that, in winter, early strengths are higher when compared with those that could be obtained with unaccelerated concrete. Unfortunately the accelerator used is calcium chloride, a hygroscopic (ie draws moisture to itself) salt. Again overdosage is often at the bottom of the problem : the chloride salt remains in the concrete, close to the steel reinforcement, and attracts moisture to itself. The combination of salt, moisture and mild steel leads to rusting of the latter. The volume of the rust is greater than the original volume of the steel so that expansion takes place which the surrounding concrete cannot resist and the concrete falls away exposing the steel directly to the atmosphere so that corrosion can then proceed more rapidly. Note the way in which embedded steel disrupts the surrounding material/structure by the expansion that occurs on rusting : we shall return to it again much later when diagnosing defects in walls. We shall also very soon be coming back to accelerators in mortars. Both the problems outlined above are due to human failures rather than to defective technology, particularly in that the man responsible for batching the mix has never been told the effects of incorrect proportioning. On a Murhphy’s law basis architects, surveyors and even main contractors want to have things done in the simplest way and eliminate the certainty that some time, somewhere the wrong thing will be done.

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2.1.4a Accelerators and Anti-Freeze Frost is the major cause for your having repointing work to do. Frost is also one of the major enemies in your being able to work continuously during winter and produce satisfactory, durable work. Frost attack in its first aspect will be looked at later in diagnosis of defects; here we are concerned with frost and its effect on fresh mortar during perhaps the first few weeks of its life. Firstly, as is well known, when water freezes its volume increases. This expansion is the force that leads to disruption of materials and the problems of frost attack. In making a mortar we have to add more water than as needed for hydration to occur in order to make the mix workable. This excess water occupies the spaces between the aggregate particles and eventually evaporates. Until it can evaporate the water exists as free water within the body of the mortar (some water is of course busy combining with the cement). If the temperature of the mortar falls below zero then the free water will freeze and may disrupt the physical structure of the mortar : when the water thaws the disrupted mortar tends to crumble into lumps.

Apart from water freezing the other factor is the strength of the mortar. In a fully matured mortar, the expansion caused by the freezing of water is resisted by the (low) tensile strength of the mortar. Until the mortar has matured there is going to be even less strength available to resist the expansion of the ice. The sooner the mortar reaches an adequate strength the better is it able to resist freezing. This increase in strength is one way of looking at hydration. The rate at which hydration (and strength increase) takes place depends upon temperature : the higher the temperature (up to about 25-30C) the more rapidly hydration progresses. At zero, hydration ceases (and of course, near zero proceeds at snail’s pace). So the problem is now compounded – free water freezes in a mortar whose binder is weak and not increasing in strength. There is a third factor, we tend to measure hydration in terms of strength increase. However, the reaction between cement and water generates heat (chemists call it an ‘exothermic’ reaction, which merely means ‘gives out heat’). For a given quantity of cement there is a fixed amount of heat “locked in” which is released when water reacts with the cement. That fixed amount can be released slowly or rapidly depending upon the rate at which hydration is taking place. When the air temperature drives down the mortar temperature, the rate of hydration drops, and the internal heat generation in the mortar also drops (for practical purposes ceases) and again in a bad situation worsens. Now enter the accelerator : it speeds up hydration : we gain strength more rapidly : we generate internal heat more rapidly and, added bonus, the accelerator is a salt and salt water freezes at a lower temperature than fresh water. However, we need to be careful. The mortar joints in a brick wall may occupy a small proportion of the total volume (maybe about 15%). In long spells of low temperatures, the mass of brickwork is going to be chilled and we will have a “storage heater in reverse” effect. The heat that is being generated in the mortar is being drawn away into the cold bricks probably more rapidly than it can be generated. After a while therefore despite the accelerator the mortar temperature is going to start to drop. The accelerator will probably provide ‘anti-freeze’ properties where there is going to be an overnight frost (not too severe) with daytime temperatures rising to perhaps 5-10C. The lowering of the freezing point of the water to about –1 to –2C is going to help here. In periods of continuous frost (day and night) or when severe frosts occur overnight, extreme caution is needed in relying on an accelerator to protect the mortar against freezing. One final twist to this tale is the need to appreciate that salts are being introduced into the construction. For repointing of external clay brickwork, there is a probability that efflorescence (see notes on sands above) will occur on the joints. Apart from the need to clean it off to keep the client happy, no permanent damage should result. However, certain non-clay materials may deteriorate due to the presence of the

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salts (see below – calcium silicate bricks and natural stone). Where potentially vulnerable materials are present it may be unwise to use a calcium chloride accelerator. In the wake of the calcium chloride/reinforced concrete panic, the Building Research Station at Garston developed accelerators based on formats and these are now available : they appear to avoid most of the difficulties suggested above, although bear in mind that a small amount of mortar (especially pointing on the outer face of the wall) is not going to generate much heat itself even if accelerated. Generally speaking, accelerators are not recommended for repointing works. 2.1.4b Retarder Retarders have an effect opposite to that of accelerators, hydration is slowed down for a period of time dependent upon the amount of retarder used. This slowing down (retardation) is seen in the delay in the initial set of the mortar occurring (recall that “initital set” is the word used for the physical change that occurs when the cement and water cease to be liquid and become a ‘gel’). Again careful dosage is important since hydration can be killed by an excess of retarder. You should also note that there are two ways in which retarders are produced for the building work – ‘an integral retarder’ to be mixed in throughout the mortar (this is the kind that we are dealing with here) and ‘a surface retarder’ which is of paste like consistency and to be applied to the face of moulds where exposed aggregate finishes are wanted on concrete. You are most likely to think of using a retarder in very hot weather when initial set is taking place very quickly. The rapid application by the pointing gun (as compared with repointing by hand) is unlikely to give rise to difficulties. However, it may be that the tooling of the joints is having to be carried out sooner than is desired, a retarder could then be used to keep the mix open for a longer period giving more flexibility in organising the work between gunning in the joint and tooling it up. The other use of these admixtures that you are likely to come across is in retarded ready mixed mortars. These are a relatively new product and often prove more economical and more reliable than site prepared mortars for new brickwork. The mortar is supplied in large plastic containers for handling on site by crane or forklift and generally must be used within a day of delivery (before the retarding effect wears off). It is likely that for most repointing jobs the quantities of mortar used per day would not justify the buying of mortar in this form. 2.1.4c Waterproofers “Waterproofing” admixtures are designed to fill the pores of the mortar with a water repellent substance. To be effective the mortar itself must be proportioned to produce as dense a mix as possible. It is difficult to think of a situation in which their use would be beneficial. For example in situations where one brick thick walls are damp due to rain penetration, one would be cautious in recommending repointing in waterproof mortar as a means of curing the rain penetration. Again in retaining walls, where possibly joints and bricks or stonework are eroded due to water percolating under pressure from the back of the wall, repointing with a dense mortar of low water permeability may only worsen the effects on the bricks themselves (or stone). Water repellents for application to the wall surface are dealt with later in Chapter 5.

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2.1.4d Pigments The ‘normal’ colour of a mortar depends mainly upon the type (colour) of sand used and to a lesser extent upon the type of cement used (in particular o.p.c., white cement and sulphate resisting cement). However, the range of colouring thus produced is limited and on occasion you may be asked to produce a mortar with either a contrasting or complimenting colour to the bricks. You should remember that in most cases walls are seen from a distance such that the jointing is not distinct from the brickwork. The overall colour of the wall as seen is therefore a combination of the brick colour and the mortar colour. The number of combinations is quite large but perhaps the most surprising result is the lightening of the colour of a wall of dark bricks by use of pale coloured mortars. Alternatively the joint between the bricks can be made to “disappear” by use of mortar of the same colour as the bricks. Pigments can be mixed into the mortar to produce these colour effect. Pigments used should comply with BS 1014 to ensure compatibility with cement and quantities added should not exceed 10% of the weight of the cement (in the case of carbon black the amount should not exceed 5% of the weight of cement). In order to produce consistent colouration for the mortar particular attention must be paid to the batching of materials including the effect of bulking of the sand. Alternatives to the use of pigments are:-

(1) The use of Cullamix /Tilcon or other approved (see 2.1.2d above) (though here you are limited to the colours that Blue Circle manufacture).

(2) The use of ready mixed coarse stuff or ready mixed retarded mortars in each case with the pigment gauged in bulk at the mortar plant.

2.1.4 Water The general rule is that if you can drink the water, it is suitable for making mortar. In the majority of cases mains water is likely to be used and no problems should arise. If for some reason mixing water is taken from a rain water butt, say, or an old tank or cistern as long as the water is clear it can be used. If there is scum or algae or living organisms in the water then it may adversely affect the mortar. Sea water or brackish water will accelerate slightly hydration of the mortar (some reduction in setting time is possible), but the major problem is likely to be that of efflorescence (see 2.1.3d above).

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2.2 Bricks and Ceramics Bricks have been used for walling in this country since the middle Ages. In the days of hand manufacture, brickmakers travelled from site to site, the first job for any building project being to make the bricks on site. Variations in size, colour, strength and durability were very large. Later brick making no longer went from job to job but was carried out in a fixed spot where suitable clay was available and the bricks were transported from the brickfield to the site of the proposed building. Right through to Victorian times transport problems and costs ensured that local bricks, in general, did not travel far. Since then modern transport systems have simplified distribution and reduced it s costs. The result, as we see it today, has been elimination of the local brickworks and the domination of brickmaking by relatively few manufacturers. In dealing with older brick buildings one of the major headaches is in matching old bricks in repairs. In repointing and repairing the elevations of a building, the bricks will probably be of the variety known as “facing bricks”, having been selected for colour and/or shape and surface texture. For clay bricks the colours range from pale yellow through reds, browns and purples to very dark blues, this variation depending upon the type of clay (particularly oxides that may be present) and the method of firing in the kiln. In the south of England, in particular, a number of multi-colour stock bricks have been and are made in which there is a considerable variation in the colouring of bricks of the same type. The surface texture of the brick also plays an important part in determining the appearance of a wall. Hand made bricks are usually sand faced and the face may be irregular. Many types of machine made bricks have sand facing applied to them to imitate hand made bricks. When a brick is fired at a high temperature the clay starts to melt and form a glass. This glassy surface again gives a characteristic texture to the brickwork. In Victorian times in the Oxford area there was a vogue for Flemish bond to be built with dark blue partly vitrified headers and red stretchers. It is difficult to match the headers nowadays though luckily they are less likely to deteriorate because of the reduced absorption through the partial nitrification of the brick. Glazed bricks proper are produced by applying a potter’s glaze to the green brick and firing this in a cikln. Engineering bricks are characterised by very high strength and low water absorption. The latter in particular, makes these bricks particularly durable when used in situations of severe exposure (for example, retaining walls, bridges and, in buildings, cills and copings). Engineering bricks tend to be uniform in colour (blue or red) and of regular shape and precise dimension. This enables them to be laid with a thinner joint (6mm) than that often necessary for one of the more irregular facing bricks (10mm). Engineering bricks are classified ‘A’ or ‘B’ according to their average compressive strength and minimum absorption (class ‘B’ less and more than class ‘A’). Semi-engineering bricks (in the south typically, Warnhams) do not meet the full requirements of the British Standard Specification for engineering bricks but are superior to commons. ‘Commons’ as the name suggests, are bricks which are used for ordinary work, mainly internal, where particular properties of strength, weather resistance or appearance are not acquired. The most widely used common brick now is the ‘Fletton’, a superior version of this is the Fletton facing brick. Most bricks are produced nowadays to a nominal work size of 215 x 102 x 65mm (length x width x height) on which there are manufacturing tolerances. In the past there has been much wider variation on the size of bricks particularly in the height. Prior to metrication (late 60’s) bricks in the South of England were

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made 2 5/8” deep (67mm) and in the north 2 ¾” deep (70mm). In the Midlands a mixture of both sizes is likely. The present 65mm brick matches the 2 5/8” brick reasonably well in repairs but with the 2 ¾” brick, the bed joints will have to be deeper than those in the original brickwork; this should be discussed with the client and/or his architect in order to avoid possible complaints later on (unjustified) about sloppy workmanship. There has also been a demand for bricks that are narrower on the bed than normal – 2” facing bricks have been and still are available. The other variation that occurs in some (modern) brickwork is the use of “modular bricks”. These are bricks whose dimensions are multiples of 100mm typically 200 or 300mm long x 100 wide x 100 high. Demand has always been limited and difficulties again might be experienced in matching these bricks in repairs. Apart from rectangular bricks there are ‘standard special shapes’ made for some types of brick (see fig2.3). Unfortunately the range made now does not match that which has been made in the past and difficulties can occur in trying to find a reasonable match to an existing special. Some brickmakers will manufacture special shapes to order but this of course tends to be very expensive and manufacturing times are likely to be long. With some bricks it is possible to cut a rectangular brick with a carborundum disc to produce the required shape. The bricks suitable for cutting should be frogless and unperforated and of uniform colour through the body of the brick where cutting is going to expose the inside of the brick. Squints are probably the easiest shape to produce by cutting : cants and plinths are possible but their stops will be difficult. Bullnoses and their variants would be impossible. 2.2.2. Calcium Silicate Bricks These are also known as ‘sand lime’ bricks and are made from a mixture of siliceous sand and lime, mixed together, pressed in a mould to form the brick shape and then subjected to the action of steam under pressure in an autoclave (a sort of giant pressure cooker). The steam pressure treatment causes chemical changes in the mixture, producing a synthetic sand stone – grains of sand bound with calcium silicate. Where crushed flint is used, the bricks are known as “flint-lime” bricks. Pigments are often included in the mix to give the required colour. The bricks are supplied in ‘classes’ (class 1 to class 7) based upon average compressive strength. Class 1, the weakest, should only be used internally, external cavity walls, etc should be in class 2 bricks. For the more severely exposed situations (parapets, boundary walls, cills and copings) classes 3 or 4 should be used. In any patching of sand lime brickwork it is important to ensure that the bricks used for any of the locations just mentioned are at least of the quality noted. The colour of sand lime bricks is more uniform than that of clay bricks and apart from white a number of pastel coloured bricks are made. White sand lime bricks have been used in recent years to provide high daylight reflection in light wells and internal courtyards and have provided a more reasonably priced alternative tot he more traditional white glazed bricks. The shape is very regular and tolerance on size much less than that for a clay brick. One important difference between sand-lime bricks and clay bricks is the difference in the “moisture movement” or each type. For any material, this term is used to describe the change in length that occurs when water is absorbed (length increases as compared with the length of the dry brick) when the

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absorbed water dries out, the brick length reduces again, a process that will continue. In clay bricks moisture movement is not very high (except when the bricks are first drawn from the kiln, when they take up moisture, usually from the air, and expand). Calcium silicate bricks move more and cracking can occur in brickwork built with them, particularly where there is limited freedom to expand. Generally the limitation will arise either because the mortar used is too strong or because the brickwork is tied to, say, a structural concrete frame or to perhaps clay brickwork (eg. One and a half brick wall faced in sand limes and backed with flettons). Current practice is to include movement and separation joints, but many older buildings have been built without a full appreciation of this point. One other peculiarity of sand lime bricks is the gradual breakdown of the calcium silicate when chloride salts are present and the brickwork becomes wet. This is most likely to occur in buildings fairly near the sea where salt spray can be blown onto the building. 2.2.4 Terra Cotta and Faience Both terms come from Italy as no doubt originally did the techniques for which they are used. ‘Terra Cotta’ is merely Italian for ‘cooked earth’ and is used to describe a ceramic facing, often elaborately moulded or shaped, made from a fine clay, fired but not glazed. The panels or tiles were often fixed to facades using wire cramps (as for marbles and polished granite) with joints pointed up. Much terra cotta is a characteristic browny red but buff and yellow examples are also found. Chimney pots and air bricks are the commonest examples of terra cotta... Faience (derived from the town of Faenza in northern Italy) is used to describe what is essentially a glazed terra cotta (ie large glazed tile facing). 2.3 Natural and Artificial Stones

Natural stone is used to describe a stone that has formed naturally over millions of years in the ground and has been quarried to produce blocks of material for building. You will recall from the introduction that for walling purposes we distinguish between ashlar work and rubble stone work, purely depending upon the amount of work done to shape the quarried stone and in no way reflecting on the quality or type of stone. Artificial stone (also reconstructed stone and cast stone) is not really stone at all, but a fine concrete made with aggregates (often of crushed stone) and cement with pigments so as to imitate the colour and appearance of natural stone. Thus artificial stone can be cast in blocks and then built into a wall exactly as though it were a natural ashlar : it can be cast in mould to give the appearance of rubble stone work (eg Bradstone) : it can be cast in slabs which are then hydraulically split to give a less obviously man made texture and appearance. Natural stones are generally classified as sedimentary, metamorphic or igneous for building purposes. The sedimentary group is probably the most important, certainly in the south and covers limestones and sandstones. These stones are relatively soft (as compared with say granite), easily worked and used extensively in ashlar work and in rubble stone work (across the whole of the cotswolds for instance). Stones are named from the general quarrying area and sometimes also the individual quarry or even level from which the stone is dug. For example all Portland stone comes from the Isle of Portland in Dorset but

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may be further described as “whit bed” or “shelly bed” etc. Few stones in this group polish and they are generally left with a slightly grainy texture. With much stonework, the quarry from which stone originally came is often no longer being worked and an acceptable substitute must be found. In the case of minor ‘patching’ repairs to rubble stone work, sound second hand stone (already weathered and proven) is probably as good a replacement as any. Chalk is also a sedimentary rock though generally too soft and likely to soften in the rain for use as external walling. At certain points on the ‘chalk belt’ harder chalks have been worked in the past (‘clunch’ in the Cambridge area). Flints (non crystalline siliceous material) are found in bands in chalk and are widely used in and near the chalk belt for walling. In particular flints can be knapped to expose the glassy surface within the rind and are then roughly squared on the face before being built into a wall. Brick lacing courses and quoins are often used to stabilise the flints. Metamorphic stones will probably have started life as a sedimentary type and then been changed by volcanic or other violent movement which subject the stone to great heat and/or pressure changing the stone physically and chemically. In this group slate and marble are widely used in building work (the former of course not solely restricted to thin sheets in roof covering). Marble because of its cost is generally used as a thin facing veneer, probably ¾” to 1” thick, fixed to the background wall with wire cramps; joints between the pieces of marble are pointed often in a putty or mastic. The igneous rocks are those produced by volcanic action; the best known of these is granite. In granite areas (Cornwall, parts of Scotland, for example), granite will be widely used in every type of building. Elsewhere in the country it is occasionally used for public buildings particularly where resistance to atmospheric pollution was felt to be important.

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

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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.

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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.

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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 the relevant Code of Practice for the cleaning and surface repair of buildings. 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.

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Table 3.1 Mortar Mixes

DESIGNATION OF

MORTAR

TYPE OF MORTAR – Proportions by volume lime putty : dry sand.

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.

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.

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

TYPE OF MATERIAL DEGREE OF EXPOSURE

SHELTERED MODERATE SEVERE OR MARINE

A1. Stone :

(v) (vi)

(iv) (vi)

(iii) (v)

A1. Highly durable, eg. Basalt, Granite, millstone, grit.

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

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)

C. Clay brick in cement or cement : lime mortar.

(iii)

(ii) (iii)

(i) (ii) (iii) C1. Dense, strong and vitreous

C2. Medium and low density (iii) (iv)

(ii) (iii) (iv)

(ii) (iii)

C3. Low density, soft and friable (iv) (v) (iii) (iv) (iii)

D. Calcium Silicate brickwork in cement or cement : lime mortar

(iii)

(iii)

(ii) (iii) D1. Class 4 and stronger

D2. Class 3 (iii) (iii) (ii) (iii)

D3. Class 2 and weaker (iv) (v) (iv) (iii)

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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)

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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.

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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,

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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

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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. (c) The state of the mortar at the time of sampling, eg. Wet or dry : soft or hard. (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.

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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 sa