Traditional render is normally applied by hand, but modern renders can also be applied by machine.

Transcription

1 8 chapter External solid plastering OVERVIEW Render provides a weatherproof surface that prevents the penetration of moisture into the outside of the building, and forms a decorative finish that enhances the building s appearance. Traditional render is normally applied by hand, but modern renders can also be applied by machine. Rendering materials have changed dramatically over the years. In the mid 8th century, rendering was carried out using a mortar of hydraulic lime and sharp sand. This provided a breathable render, but it took a long time to set. It was later replaced by mortars of cement and hydrated lime, which had a quicker setting time and so speeded up production essential in the housebuilding boom after the Second World War. Today, new renders are being developed to replace traditional renders with a better quality product to meet today s needs. In order to speed up productivity these new renders are premixed so that they are uniform in colour and have a consistent strength. Traditional lime renders are still used however, especially for restoration work on chapels, churches and listed buildings. Key issues when choosing the type of render are: planning regulations and conservation requirements; fire and insulation properties to comply with building regulations; the background to which the render is to be applied; climatic exposure; the client s requirements; and, of course, cost. This chapter will cover the following: Materials used for rendering Additives used for rendering Preparing backgrounds to receive rendering Types of bonding adhesive Mixing and applying render Plain face render finish Other types of render finish Modern render Metal trims. 69

2 Materials used for external rendering Method Step Place 25 mm of water into the jar, add teaspoon Some rendering products can be harmful. Always read the manufacturers data sheets, and follow their instructions carefully It is important to choose the correct materials to produce render. This will produce high-quality render mixes that will not develop faults such as cracking and blowing of the surface, which affect the appearance and allow moisture to penetrate. Sand Sand makes up the bulk of the render mix. For traditional external rendering mixes the following types of sand are used: pit sand quarried inland, usually red in colour dredged sand from the river or sea bed, usually a dark yellow colour of salt and gradually add the sand until the level of the top of the sand reaches 50 mm. Step 2 Shake the jar for minute. Step 3 Leave to settle for 3 hours. Measure the height of the aggregate and the thickness of the silt layer. To work out the percentage of silt in the aggregate, calculate the following sum: Thickness of silt Total height of aggregate and silt 00 artificially created sand, available in different colours. Modern render mixes are made with artificially created sand. This is formed by crushing stone Step Ingredients being added to water or gravel to the required grade. It is available in different colours, such as white, red or black. When mixed with cement and lime, the sand will form the bulk of the mix; the sand is known as the aggregate. Protect sand when it is not in use, to help prevent contamination. Use a weighted down tarpaulin or similar Poor quality sand will cause the render to crack and become loose and hollow, or it may eventually crumble. In good quality sand, the particles are irregular in shape (not rounded), and well graded. This means that they range from small to large grains, so that they fill all the voids in the mix. The largest grains should not be more than 5 mm. This is assessed using a sieve test to ensure no grains over 5mm are present in the sand. The sand should have been washed to remove impurities. Clay and silt particles in the mix prevent the cement from bonding to the aggregate, so each load delivered to the site should be checked for cleanliness. The amount of silt must not be more than 0% of the volume of aggregate. The silt test is used to check this. Silt test Jar after shaking Jar after 3 hours Materials and equipment sample of sand water salt glass jar or measuring cylinder tape measure. For example, if we measure the sand to be 45 mm and the silt thickness is 5 mm: = 0 Conclusion this sample would be suitable, but only just. 70 7

3 Storage of sand Sand should be stored in a clean bay, and covered with a tarpaulin to reduce contamination from animals, leaves and other aggregates. Cement Portland cement Portland cement is the basic active ingredient of render mixes. It is a closely controlled chemical combination of calcium, silicon, aluminium and iron, plus small amounts of other ingredients. Gypsum is added in the final grinding process to regulate or retard the setting of the cement. Always wear gloves when handling cement or lime to protect against skin burns The manufacture of cement Each step in the manufacture of Portland cement is checked by frequent chemical and physical tests in plant laboratories. The finished product is also analysed and tested to ensure that it complies with all specifications. Stored cement Cement is used to bind and harden the mix for rendering. Therefore it is important to gauge the mix correctly. Mixes that are weak (not enough cement) will crumble. Mixes that are strong (too much cement) may create stresses that could cause the render to blow from the background. Several different manufacturers make cement, but the product must always be manufactured in accordance with British Standard EN 97- Cement: Composition, specifications and conformity criteria for common cements. Two types of cement are commonly used with external renders. Ordinary Portland cement (OPC) consists essentially of 75% limestone and 25% clay. It is grey in colour. White Portland cement contains 75% limestone and 25% white china clay. It is white in colour, and is used when a whiter, lighter appearance is desirable in the render finish. Setting time for cement The setting of cement is a chemical reaction, and so it is affected by temperature. In warmer climates cement will set more quickly than in colder areas. The initial set of cement is 45 minutes, but the final set is not more than 0 hours. Cement should be stacked off the ground in a container or shed to keep dry. Gauging measuring the correct proportions of materials in a render mix to achieve full strength when set Always use bagged material before its sell-by date. All bagged materials such as sand, cement and lime and premixed modern renders should be rotated this is where older stock is brought to the front so that it used before any newly delivered stock 72 73

4 When cement is mixed with water an alkaline solution is produced. Take precautions to avoid skin contact with wet cement, fresh concrete or mortar, and prevent dry cement from entering the eyes, mouth or nose. Wear suitable protective clothing, gloves and eye/face protection Special-purpose cements Sulfate-resisting Portland cement (SRPC) is used in situations where sulfates are present in concentrations that would damage normal Portland cement mortar. This type of damage results in white salt stains appearing on the face of backgrounds that could penetrate through the render. Rapid-hardening Portland cement (RHPC) is finer than OPC, and is used in concrete to ensure a higher rate of early-age strength development. Lime Lime is manufactured by crushing limestone, which is mainly calcium carbonate, and heating it in a kiln to drive out the carbon dioxide content It improves adhesion to the background. It prevents a dark sanding appearance being drawn to the surface when consolidating plain face render. It adds suction in the background to prevent sagging when applying Tyrolean finish to a plain face background. Hydraulic lime Hydraulic lime is produced by slaking lime made from limestone that contains clay and other impurities. Unlike hydrated lime, which has no set, hydraulic lime sets like cement once it is mixed with water, but far more slowly. Hydraulic lime is preferred for use on older buildings that were built using lime mortar. It allows the building to breathe ; cement mixes are too dense and brittle for this type of background. suction this is when a background absorbs moisture from the render mix to produce calcium oxide. This results in the material known as burnt lime, lump lime, quicklime or just lime. FAQ Two types of lime are used in rendering hydrated lime and hydraulic lime. What types of lime would you use when rendering, and when? Hydrated lime Stored lime Hydraulic lime is used mainly for listed building work, whereas hydrated lime is used on modern buildings. slaking the process of mixing calcium oxide (lime) with water Hydrated lime is produced by slaking calcium oxide (lime) with water to produce calcium hydroxide. It is used in sand and cement rendering mixes for application on all Additives Additives are commonly used in cement-based render mixes, whether for the dubbingout coat, scratchcoat or topcoat. Additives are liquid or powder ingredients, and should be carefully measured in accordance with the manufacturer s instructions before adding them to the mix. There are several types of additive that can be added to cement-based render: types of background. Plasticiser improves the workability of the mix and its ease of application The process of slaking lime Hydrated lime offers the following advantages: It prevents shrinkage cracks in the render mix. It improves the workability/spreading of the render mix on the background. Waterproofer improves workability, and forms a waterproof barrier in the render Retarder this can be added to the mix in warm conditions to slow down the setting process Accelerator this can be added in cold conditions to speed up the setting process. Some modern premixed renders have additives included during manufacture. This will eliminate problems caused by adding too little or too much when mixing by hand

5 On the job: Using additives Peter the labourer mixed my render, but used too much plasticiser. This resulted in the mix not achieving its correct strength, and later it crumbled. What should Peter do to prevent this happening in future? Preparation of backgrounds Some backgrounds may still have old render. This will need to be prepared by hacking off Most modern houses and other buildings are built from blocks, which require little preparation. Older buildings will require some type of preparation before rendering. This is essential to ensure a good bond between the render and the background. New blockwork only requires damping down of the surface before rendering, whereas all other backgrounds require preparation. Table 8. identifies the procedure for preparing backgrounds prior to rendering. Table 8. Preparation of backgrounds Type of background Hack off old work Rake out joints Brush and dampen Apply slurry adhesive New brickwork Old brickwork New blockwork Old stonework Concrete surface 76