stone aggregate are generally found to be very sticky and difficult to use, and can also shrink excessively as they dry if they are poorly graded. Currently, therefore, if porous particulates are used, they are often added into mixes where sand is the main aggregate. Hair is sometimes added into render mixes in order to give them tensile strength. It is important that the hair should bond and grip well in the mix. Horse hair, which appears in some specifications, is unsuitable because it has very smooth edges, and can often be pulled out of the mix after it has set. Cattle and goat hair tend to have rough and barbed edges and grip the render better. In all cases, hair should be thoroughly blended into the mix so that it is evenly distributed and does not clump together causing weak spots in the render. Good practice (checklist for nonhydraulic limes) 1 Lime render placed into deep recesses will take ages to set and may be subject to shrinkage. Recesses in the substrate should, therefore, be filled either by dubbing out or using galleting (i.e small stones or broken tiles).Joints between stones or bricks do not need to be raked out and may need to be roughly flush pointed if they are deeply recessed, in order to achieve more even suction and cl1ying. The cutting out of joints before rendering, to give a better key, seems to originate from cement technology. 2 The substrate needs to be well wetted before the render is applied. This applies to each of the coats, as well as to the masonry. If a substrate is too d1y the moisture will be sucked out of the render before it has a chance to carbonate, and it will consequently fail. 3 Some current practitioners find that the application of a lime sluny coat before application of the render appears to help even out the suction from a wall and provide a 'tack' coat. There is some evidence that this was clone historically, although there is some debate about how effective it is. It is important that the slurry dries a little before applying the render, because if it is too wet it creates a slip plane. 4 The mix should be kept as stiff/dry as possible, as the more water there is in a mix the more shrinkage there will be. Plasters and renders, obviously, have to be used a lot wetter than pointing/mortar mixes, but the temptation to over wet them should be avoided. 5 Lime renders should be applied in a number of coats, each of which should not exceed 9-10 mm in thickness. With very large aggregate, however, it is Context59 September1998 possible to achieve greater thicknesses. The thicker a coat is, the greater the potential shrinkage. Top coats tend to use smaller aggregates and should, therefore, normally be limited to 2-3 mm thickness. 6 Allow for compressing a render to counteract the shrinkage. With roughcasting, harling and other thrown renders the vigorous application compresses the material. With trowel applied non-hydraulic renders the base coat may be allowed to shrink and crack, but consequent coats should be worked over with floats and/or trowels to compress them. In fine quality work the top coat is applied in two or three very thin layers, following on immediately from each other. This helps to control the suction, aids compression and helps to achieve a consistent surface finish. 7 It is important to achieve a good key between coats. If the coats are thrown on, there is a natural key between them. In trowel applied three coat work the first coat was traditionally keyed using pointed wooden laths, in a diagonal pattern, and the second coat would be keyed using a "devil float" (a wooden float with two or more nails projecting approximately 1 mm from the face) or, before the 19th century, a birch broom. 8 In some areas of the count1y, renders are left to stiffen to the consistency of leather before applying the next coat, whilst in other areas they are left to harden and carbonate to a point where they will resist indentation. With the first method less water is needed to moisten the surface before application of the next coat and may give better bonding between coats. The disadvantage of this method is that it will take longer for the under-coats to fully set once they are covered, and a little more compression may sometimes be needed to counteract shrinkage occurring in these coats. There is a slight risk of damage being caused to lower coats if they haven't adequately carbonated when the scaffold is removed. With the second method, a greater degree of carbonation will have occurred and shrinkage in the lower coats will have already happened. The disadvantages with the second approach are that longer time is required between the application of coats and there needs to be more wetting up of surfaces before applying the next coat. Greater care is needed to ensure there is good keying between coats. 9 The immediate application of a limewash provides a protective layer over the surface of a render (giving long term protection) and also fills small surface cracks. Some people, however, feel that this could reduce the openness of the surface and slow clown the carbonation of the render behind. Re-limewashing eve1y five to ten years should lengthen the life of the render. 10 The top coat of a render needs to carbonate before it can be exposed to adverse weather conditions. It needs to be kept in a moist, but not wet, condition until carbonation has occurred. Until this has happened the lime matrix is very susceptible to damage from rapid drying, excessive wetting or freezing. This was the most important factor in all the failures that were discussed at the technical day. Although there are plenty of lime renders which have not failed, even though scaffolding and protection were removed immediately after the work finished, this is as much clue to good luck as good judgement. The rate of carbonation is dependent on humidity, temperature and wind speed, along with other climatic factors. The rate of set gets slower the deeper in from the outside surface. It is difficult to judge how long protection is required, but in average weather conditions, after 4 weeks, carbonation should have occurred to a depth of at least 5 mm. Specifications and programmes of work must allow for leaving scaffolding and protective sheeting in place until carbonation is well advanced. In very wet weather conditions the speed of carbonation may be slower. If in doubt about whether a set has occurred, it is possible to carry out a carbonation test on a core sample. When lime carbonates it becomes less alkaline. This decrease in alkalinity can be detected by applying phenolphthalein which turns bright pink if the lime has not carbonated, but will remain clear if it has. Hydraulic limes Hydraulic limes are ones that are derived from limestones containing impurities in the form of reactive silicates and aluminates (clay compounds). They are known as hydraulic because the presence of reactive impurities enables them to set underwater, rather than relying on a reaction with carbon dioxide from the air. The more reactive clay compounds there are, the more hydraulic the lime becomes. It must be emphasised, therefore, that we are dealing with a whole spectrum of limes (with different qualities and performance characteristics), not just one material. Traditionally, in this country, the spectrum has been divided into the 17
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