Presentation on theme: "External solid plastering. Task Look at the picture and identify what has gone wrong Poor batch mixing Incorrect gauging Different types of sand Different."— Presentation transcript:
Task Look at the picture and identify what has gone wrong Poor batch mixing Incorrect gauging Different types of sand Different manufacturers of cement
Mixing and applying render Take Note It is essential to gauge all mix materials correctly with a bucket measure. This will ensure that mix will be of correct strength and colour throughout work. Proportioning - 3 buckets of sand to 1 bucket of cement
External render Take NotesTake Notes External render consists of three main componentsExternal render consists of three main components Sand? The aggregateSand? The aggregate Lime? The PlasticiserLime? The Plasticiser Ordinary Portland Cement ? The BinderOrdinary Portland Cement ? The Binder
Materials used for external rendering Sand Sand is used as an aggregate.Sand is used as an aggregate. The types of sand used are:The types of sand used are: – pit sand (quarried inland; red in colour) – dredged sand (from river or sea beds; dark yellow) – artificially created sand (different colours). Sand should be clean and contain not more than 10% of silt.Sand should be clean and contain not more than 10% of silt.
Silt test To conduct a silt test you need: a sand samplea sand sample waterwater saltsalt a glass jar or measuring cylindera glass jar or measuring cylinder a tape measure.a tape measure.
Silt test Materials and equipment sample of sand water salt glass jar or measuring cylinder tape measure.
Method Step 1 Place 25 mm of water into the jar, add 1 teaspoon 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 1 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 Silt divided by aggregate 50, time 100 Less than 10% is acceptable
Sand Take NotesTake Notes Poor quality sand will cause external renders to crack and become loose and hollow.Poor quality sand will cause external renders to crack and become loose and hollow. The external render may even crumble when attacked by weather, especially in winter.The external render may even crumble when attacked by weather, especially in winter.
Task Look at the picture and identify what has gone wrong Oil pollution staining the face of the render
Sand The Aggregate Take Notes Sand should be: stored in a clean bay covered with tarpaulin to reduce contamination from animals, leaves and other aggregates.
Setting time for cement Take Notes 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 10 hours.
Cement The Binder The part of the mix that sets and binds the aggregate togetherThe part of the mix that sets and binds the aggregate together
Cement The Binder Sulphate-resisting Portland cement (SRPC) Is used in situations where sulphates are present in concentrations that would damage normal Portland cement mortar.
Cement The Binder This type of damage results in white salt stains appearing on the face of backgrounds that could penetrate through the render. Take NotesTake Notes two special types of cement.two special types of cement. Sulphate-resisting Portland cement (SRPC),Sulphate-resisting Portland cement (SRPC), Rapid-hardening Portland cement (RHPCRapid-hardening Portland cement (RHPC
external render Take Notes The type of external render that should be applied to insulation boards would be a specialist lightweight render because sand and cement- based materials would be too heavy.
Additives Commonly used in cement- based render mixes.Commonly used in cement- based render mixes. They are liquid or powder ingredients.They are liquid or powder ingredients. They should be carefully measured before adding to the mix.They should be carefully measured before adding to the mix.
Additive for external cement- based render Take Notes Plasticiser – improves workability and ease of application.Plasticiser – improves workability and ease of application. Waterproofer – improves workability, forms waterproof barrier and equalises suction.Waterproofer – improves workability, forms waterproof barrier and equalises suction. Retarder – added in warm conditions to slow down setting.Retarder – added in warm conditions to slow down setting. Accelerator – added in cold conditions to speed up setting.Accelerator – added in cold conditions to speed up setting.
Task draw this chart the preparation of the background 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
Bonding adhesives for sand, cement and lime mixes Take NotesTake Notes Slurry coat or spatterdash coatSlurry coat or spatterdash coat Spatterdash is a coat applied externally?Spatterdash is a coat applied externally? It is used to produce a good quality key for the external render to bond onto.It is used to produce a good quality key for the external render to bond onto. Application of a slurry coat
Bonding adhesives are designed for use on poorly keyed dense surfaces; They improve adhesion between the background and the first render/scratch coat. They come in either powder or liquid form. Some require mixing, while others are pre-mixed. There are many bonding products on the market. One example is SBR.
This is latex-based. It is used in areas subject to humidity, dampness or continuous water contact To improve the water resistance of cement mixtures. Take Notes SBR should be applied externally?
Lime the Plasticiser Take NotesTake Notes Lime is used as a plasticiser in external rendering.Lime is used as a plasticiser in external rendering. Helps to reduce cracking, which also reduces loss of adhesion.Helps to reduce cracking, which also reduces loss of adhesion. Gives a slight suction which aids the application of further coatsGives a slight suction which aids the application of further coats
Ratios of the render mix: why use them? Ratios are designed to help the plasterer know the right quantity of each material when mixing.Ratios are designed to help the plasterer know the right quantity of each material when mixing. Mixing the render too strong may cause subsequent stress resulting in cracking and blowing.Mixing the render too strong may cause subsequent stress resulting in cracking and blowing. Mixing the render too weak may cause no initial set and subsequent crumble.Mixing the render too weak may cause no initial set and subsequent crumble.
Ratios for dubbing-out mixes 3 sand, 1 cement,3 sand, 1 cement, Ratios for scratch coat 4 sand, 1 cement, waterproof additive4 sand, 1 cement, waterproof additive Ratio for topcoats 5 or 6 sand, 1 cement, 1 hydrated lime, plasticiser additive5 or 6 sand, 1 cement, 1 hydrated lime, plasticiser additive Take Notes
Scratch coat Take Notes Used to straighten out background and is backing coat for render finish. Includes a waterproofer.Includes a waterproofer. Thickness is 9–12 mm.Thickness is 9–12 mm. Should be ruled out as flat as possible.Should be ruled out as flat as possible. Keyed with a comb scratcher.Keyed with a comb scratcher.
Two-coat work Two-coat work comprises: the scratch coatthe scratch coat the topcoat.the topcoat. remember never put a strong mix on top of a weak mixremember never put a strong mix on top of a weak mix
Topcoat This is applied to scratch coat to form finished render. But first, the reveals must be completed. 1.Run trowel over scratched surface to remove any hard sand or cement. 2.Fix/hold timber rule on main face of wall, plumbing or lining to window or doorframe to 10–12 mm. 3.Apply topcoat to reveals. 4.When it begins to set, consolidate surface with plastic or wooden float. 5.Sponge or scour over surface to close face, if necessary. Forming an external angle to a render coat
Method Hold a timber rule plumb on one side of the wall to a thickness of 10–12 mm. Apply render up to the timber rule.
Slide the timber rule upwards and away from the angle. Hold the timber rule on previously laid render to a thickness of approximately 10–12 mm. Apply render up to the timber rule. Slide the timber rule upwards and away from the angle.
Dry piecing ? Take notesTake notes Large areas where the work has to be left for more then 24 hours. Another term for this is dog toothingLarge areas where the work has to be left for more then 24 hours. Another term for this is dog toothing
Plain face/scoured finish Take NotesTake Notes Thickness of topcoat should be as tight as possible, usually 10–12 mm.Thickness of topcoat should be as tight as possible, usually 10–12 mm. Rule out without screeds to a flat smooth finish.Rule out without screeds to a flat smooth finish. Consolidate the face of the render coat.Consolidate the face of the render coat. Finish it by the use of a float and also a sponge float.Finish it by the use of a float and also a sponge float. A plain faced finish
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