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Pebbledashing Trade Application Guide

 

Forward

The purpose of this guide is to give all parties involved in the application of Dry-dashing aggregates an indication as to the correct way to use the products. This guide will only deal with issues relating to the aggregates being used and will not attempt to offer guidance or advice regarding the render that they are being applied on to. The current British Standard Code of practice for External Renderings BS EN13914-1:2005 offers little guidance regarding the specific process used to apply dry-dashing and this document is intended to provide good practice methods that when used on all applications will reduce the likelihood of problems. Derbyshire Aggregates Limited has been processing aggregates for use in the Dry-dashing market for over 20 years. During this time the processes used have been developed to ensure a clean consistent product can be supplied at all times. By working with contractors, system designers and specifier's our range has developed into the most comprehensive set of products available.

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Handling


Sealed poly bag of gravel

All aggregates used for dry-dashing should be supplied in sealed polythene bags that comply with the manual handling regulations. Loose or bulk bagged material should not be used for Dry-dashing. Applicators that use this method do so at their own risk. This is due to the possible contamination of the product prior or during application.

The segregation of fines to the bottom of bulk bags or loose loads will create an inconsistent finish given that the fines cannot be distributed equally and this can lead to an inconsistent and patchy finish when applied. In accordance with BS EN 13914-1:2005 aggregates should be stored separately, according to type, on clean hard dry ground that is well drained and protected from contamination by soil, falling leaves or other harmful materials.

Special non standard aggregates mixes should be obtained in sufficient quantities at one time to enable material of the approved colour to be used for the whole of the work. On larger schemes that involve multiple deliveries natural breaks should be used (i.e. another house, corners or different elevations on the same property).

Should the material used require moving from the pallet it is supplied on then care should be taken to prevent over handling. This will cause higher fines content within the bag caused by the attrition of the aggregates within the bag during the handling process. Special care should be taken when transferring the product from the pallet to the working platform or and site storage facility to ensure that the bags are not walked on or climbed on at any point prior to their application.

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Preparation of Background


Clean surface with non-metallic brush

Backgrounds to be rendered should be examined for contamination, deterioration, surface roughness, suction and strength. Dust and contamination such as residues of concrete release agents, gypsum plaster, paint, other coatings, organic growth, salts and efflorescence should be removed prior to rendering. Salts and efflorescence should be removed by dry brushing (non-metallic bristles). Other special precautions may need to be taken if this removal is not achievable.

The line and flatness of the background should also be assessed to determine if the render can be applied to a uniform thickness or if dubbing out is required. The background should be reasonably dry and free of frost, with a temperature of +5 °C or above at the time of rendering or other recommendations by the manufacturer followed. If the suction is very high, low or uneven the use of a preparatory treatment, metal lathing, or special factory made render should have been be considered as part of the design.

It is important for the wall not to be too wet at the time of rendering. Walls that have recently been exposed to heavy rain should be allowed to dry out sufficiently before rendering is attempted. Do not attempt to apply render if any of the materials to be used are frozen or if there is a risk of frost damage before the render has taken an initial set. Do not apply in full sun as normal curing is impossible and fast drying will occur.

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Application


Applying the render

The aggregates used for Dry-dashing are predominantly naturally occurring minerals that will have colour variations that allows little control over colour consistency. At Derbyshire Aggregates care is taken to reduce any colour inconsistencies although no company can guarantee this. It is important to ensure that aggregates used are from the same manufactured batch to reduce any potential colour matching issues. No aggregate should be applied directly from the bag that it was supplied in. Any fines contained in the bag will usually fall towards the bottom of the bag and this can cause patching when applied where an area with a higher percentage of fines will differ in texture, colour and finish to an area dashed with aggregates containing little or no fines. All dashing aggregates should be decanted from the bags into a clean drainable container (i.e. a plasterer's bath) and blended with 3 to 4 other bags ensuring that the colour and any fines are mixed evenly within the container prior to application.


Applying the gravel dashing

Dashing aggregates supplied by Derbyshire Aggregates Limited do not require any further washing at this stage and care should be taken to ensure that the moisture content within the aggregates will not affect the performance of the render to which it is being applied. This blending process should be repeated throughout the application with more aggregate being added as the material is being applied. With dry-dash finish and certain textured finishes it is essential for the final coat to remain soft for a longer time than normal. This can be assisted through reducing the suction of the undercoat by more thorough wetting down, or including a water-retaining admixture in the final render coat.

The aggregate should be applied in a damp but not soaked state as this helps with the adhesion to the render. Care should be taken to ensure that the moisture content of both the aggregate and the render is consistent as differentials caused by atmospheric conditions can create problems such as dashing the same elevation in changeable weather on the same day. Derbyshire Aggregates Limited does not recommend the re-use of aggregate that has been collected up following the first application. Any aggregate that has been walked on or exposed to any substance that could contaminate it should be disposed off in the responsible way and not re-used as a dashing aggregate

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Production


Washed & screened

To ensure a continuous quality material must be selected from reliable and consistent quarries. All dashing aggregates supplied by Derbyshire Aggregates are washed and screened at least twice prior to being packaged. It is normal for many products to be washed and screened again as part of the blending process used to create the large range of colours and textures demanded by our customers.

It is important that the products produced are carefully handled during the washing, blending and bagging process to minimise the attrition created when aggregates rub together which can cause a higher percentage of fines in the finished product.

The bagging and shrink-wrapping which effectively seals the product until it reaches its site is done using robotic packing lines that prevent manual handling of the product. Colours of aggregates occur naturally and some variation can be present within the quarry. At Derbyshire Aggregates we deal with this by holding large stocks which are constantly rotated and blended to control colour consistency to keep any variation to a minimum. This is an important key factor in our quality control and at any time we stock in the region of 20,000 tonnes of materials to ensure consistency of grade and colour.

The manufacture of standard products is usually carried out in production runs of 100 tonnes or more to ensure a high quality and consistent result with all batches.

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Iron Staining

Rust staining Iron staining is a natural problem that can occur when specific individual stones within the product that have high iron content are used for dry-dashing are exposed to the environment. This problem is often increased if the site is in an exposed or coastal region. It is not unusual for the problem to take a number of years to be exposed and there is no way of identifying if the aggregate is affected prior to application.


It is not possible to screen the iron content out of the product by using magnets as it is part of the chemical makeup of the aggregate. The aggregates affected by this problem are mostly flint or gravel based type products or aggregates that have usually been quarried from glacial or river deposits. Using Calcined flint aggregates will not eliminate this problem if the raw flint used already has high iron content.

Given that iron staining is a natural occurrence no guarantee can be given or liability accepted for areas affected by iron staining. Derbyshire aggregates only select aggregates from quarries that have proven records of low iron content but should it be required that a zero risk of staining is required we can recommend a number of products that have no history of staining over the last twenty years. Products that may contain iron traces are marked on our "Colours Available" page. Advice can be given as to the best course of action required to treat any problematic areas. Whilst iron staining can cosmetically spoil the appearance of the affected areas it has no detrimental effect on the render itself.

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Uneven Appearance and Irregularity of Surface Texture

Uneven towelling and application of chippings can cause patchy appearance on walls. The absence of a suitable waterproofing agent can lead to marked differences in suction resulting in difficulties in embedding the chippings evenly to the mortar. This results in the chippings being more pronounced in some areas that others resulting in irregular overall appearance. Unless both the mortar and the chippings are applied evenly to the wall and with the same intensity, the wall appearance will be irregular. Care should be taken to obtain a wide and even spread in order to distribute the aggregate and give a uniform appearance.

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Temporary Protection of Dashed Elevations

When scaffold is used to apply dashing aggregates in poor or exposed weather conditions temporary protection of the elevation should be used. Driven rainwater can bounce or be deflected onto the dashed area creating scaffold lift lines on the wall that are often not seen until the scaffold is removed. Sheeting off the elevation can prevent this as it is not possible to rectify this problem without replacing the entire affected area. Consideration should be given to scaffold protection prior to any work commencing on site and ideally at the tender stage of any scheme. Bear in mind that other parts of the building may require protection from the render application, particularly if cement polymers are included in the mixes.

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Rendering Defects – What Can Go Wrong?

The main culprits in render defects are;

  • Bad Materials. This could be the wrong grade or contaminated sand used. Substandard or 'dead' cement caused by careless storage or handling. Not up to standard lime used. The use of non suitable or unproven additives. Dirty or contaminated aggregates that could be the wrong size or shape.
  • Bad Mixing. Poor, inadequate or unserviceable plant/mixers. Careless proportions of materials in batching. Over mixing allowing the introduction of too much air into the mortar. Incorrect use of plasticer and waterproofer by simply not following manufactures guidelines.
  • Bad Substrate. Unprepared surface, no mechanical key. To high sorptivity (sunction) Chemical contamination (sometimes activated by long periods of rainfall.
  • Bad Application. Unskilled labour used. Good practice instruction ignored. Skimping on render thickness. Inadequate keying of coats. Curing time to short between coats. Lack of wetting down.
  • Weather. Prevailing weather conditions ignored – too hot or too cold or too wet.
  • Bad Design. Poor detailing generally, no overhangs, no DPC's where required, raw edges exposed. Movement joints that are inadequate or badly positioned too may beads used. Poor specification, lack of thought, bad descriptions, wrong information and technically irrelevant.

The following examples are for guidance only and are based on wide experience of defective render surfaces.

  • Heavy spalling of all render coats, exposing bare substrate, with flush joints with more than 20% already off or coming off. Look for extensive random crazing, moss growth in fissures (small hairline cracks), poor key, bad substraight, insufficient thickness of render coats, bad detailing. Usual causes are frost attack due to poor weather resistance with water getting behind render. Chemical attack affecting substraight.
  • Light spalling is caused by the same issue above but tends to be more localised and affecting less than 20% of the elevation. Look for localised cracking, structural movement or impact damage near to the defect area. Look for signs of previous repair and any bad detailing around the fault. Usual causes are frost attack on an over wet area such as a running overflow. Bad substraight such as different materials in back ground. Chemical attach.
  • Delaminating. One coat from another leaving a lightly keyed or un-keyed surface. Usual causes are poor mechanical key such as first coat not properly scratched, or too rapid dehydration of top coat that can be because the first coat had sorptivity that was too high through lack of waterproofer or wetting down. Top coat was a stronger mix than base coat.
  • Heavy Cracking. Cracks that are over 1mm wide, often around openings, sometimes reflecting brick or block pattern. Look for settlement or structural cracking, shrinkage cracking in brick or block base. (Could be uncured material not fully shrunk or dried.) Different materials used in background such as brick, block, concrete, steel or timber. Heavy cracking is not usually confined to the rendering and normally reflects the substrate.
  • Light Cracking or fissuring. Less than 1mm wide. Look for random pattern of cracks often around door or window apertures. Examine all surfaces carefully and methodically. Look for tell-tail signs such as moss growth in cracks due to water being retained along with dirt in cracks. This problem is usually more obvious after rain when surface is drying out. Usually caused by base coat not fully shrinking prior to top coat application. Wrong materials such as sand with too high a shrinkage co-efficiency. Material used is too fine. Drying out was too rapid due to hot weather or the top coat was not weak enough with too much cement.
  • Balding. This issue applies to dry- dashed buildings. With a loose of surface texture or dashing aggregate leaving bald or pitted areas. Check the shape of the aggregate used. More angular aggregates key into the render better than rounded products. Can be caused by severe exposure to weather, particularly wind and usually affects older render. If the top coat used was too weak this will affect the bond with the aggregate.
  • Friable Surface. Render coat or dashing aggregate easily rubbed away by hand over sizable areas. Usual causes are too weak a top coat, old age of render. Too rapid dehydration (lack of water proofer in the scratch coat) inadequate wetting down. Weather to hot during application or coat exposed to frost before curing. Miss-use of plasticiser or over mixing with too much air entrapment.
  • Wrinkled Surface. Look for 'fluid' effect on rendering, wrinkled or slumped surface. Can be caused by using too soft a mix when applied or dry-dashing too soon after top coat is applied.
  • Lift Marks. Look for marks at scaffold board levels that are often only viable when the scaffolding is removed. Usual causes are a lack of care in forming joint in work being carried out on two levels. On dry-dashing the texture can appear different due to the application being done both upwards and downwards causing a build up of aggregate. Dirt or water mark caused by rain splashing off a dirty scaffold board on to new rendering.
  • Scud or whip marks. Fan marks mainly in wet dashing caused by poor application.
  • Bleaching or Patching. Gradual paling of coloured mortar finishes in isolated spots or all over surface. Usually caused by inferior pigments, lime bloom or rain damage before the render has set. Badly proportioned or badly mixed mortar or dirty water used in mix. Can also be caused by not using render from the same batch or not blending dry-dashing aggregate bags together prior to application such as dashing straight from bag. This can also create problems when aggregates of smaller or larger sizes are concentrated in one area as they have not been blended prior to application or dirty aggregates are used.
  • Rust Staining. Although already covered in detail iron staining can also be caused by using inferior beads that can rust if exposed to elements. Internal plaster beads incorrectly used externally will create this problem.

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Rectifying Problems with Render Applications

The long term effectiveness of any treatment requires the correct identification of the defect and the removal of the source of that defect be it bad design, workmanship or whatever. Removal of the defective material if required and effective preparation of the substraight. There are three courses of action that are appropriate.
Patching

In many cases this would be the most economic solution in the short term. Patching involves cutting out the defective areas to a sound substrate to produce a suitable key and re-rendering in two coats with a matching das or textured finish. Visually this would not be the most appealing finish as it is highly unlikely that it will match the original finish. Dry-dashing patching despite careful matching of aggregates will stand out against the weathered appearance of surrounding render. You may also experience shrinkage of the new area leaving a troublesome perimeter fissure that could be problematic allowing a ready route for water to get behind the render.

Re-rendering

Providing the defective rendering can be entirely removed without damaging the substraight re-rendering can be an effective long term solution.

Overcoating

This requires the application of a supplementary coat (or coats) of render on top of the existing render finish. This solution is usually used when the existing render requires renewing if it is bald or having lost is texture over the years through weathering. It may be thin through skimping in the original application or through wear and tear over the years. The building may have lost its decorative qualities and require a new look or updating. It is sometimes used if the building has been extensively patched and a unified surface appearance is required.

Overcoating is technically the most problematic remedial process. It is not possible to over coat a building with additional render and assume that it will perform unless careful thought is given to its specification, materials and application. The quality of the surface preparation is very important. The main issues relating to this process is that the new render is likely to overstress the original, causing shear problems particularly when curing and drying. This can result in not only the new render eventually coming away but in the total delaminating of the entire rendering. The rule that the render must be a weaker coat (i.e. less cement content) in each successive coat is more than ever important in Overcoating work.

There are two methods of Overcoating that can overcome these technical issues. One option would be to use a mechanically fixed metal lath. This would require the preparing the existing surface thoroughly by cleaning with a biocide to remove any moss. Hammer testing and repair any defective areas. Surface cleaning and roughening, preferably by grit blasting. An application of a polymer modified stipple coat with a heavy peaked texture. This would only apply if there is inadequate surface roughness to make the lath stand off the surface and give a mechanical fix. The metal lath would require fixing to all areas in accordance with the manufactures instructions.
By using this method the walls are keyed for the new render by the lath which holds the new coat in position. Problems with shear or delaminating are overcome and should not affect the new rendering. The use of lath allows the new render to perform more or less independently.

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Preparing Surfaces for Rendering


Make sure the surface is clean & solid

Appropriate surface preparation is vital for sound trouble free rendering. Examine all surfaces carefully to determine what if any preparation is needed. If practical try a sample area to establish what will work.

If an area if affected by moss or algae ensure it is removed completely and prevent further growth by killing the spores. Always follow the manufacturers instructs when using any chemical based removal products.

A hammer or tap test can be used to identify areas of loose, hollow or spalling material. Tap testing can be done by simple 'ringing' a painter's scraper (lightly held between finger and thumb) over the wall surface. Defective areas sound dull. Hammer testing uses a chipping hammer on the surface and again a defective area will have a dull sound compared to an area that is sound.

The whole area to be rendered will require testing.

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Render Mixes

The most important rule of rendering is that each coat must be weaker (i.e. less cement) and less thick than that to which it is applied. Not following this rule can lead to shear problems.

There are many variations on mix proportions if hydrated lime is to be used in mixes the cement/lime to sand ratio should be kept to 1 to 3. In other words lime is considered to be part of the cement content of the mix and by adjusting the proportion of cement to lime the strength can be modified without impairing surface hardness and strength. A guideline to on-site mixes (as opposed to factory batched mixes) would be as follows;

Rendering onto masonry substraights

Scatch coat (first coat)
1 part Ordinary Portland (OP) Cement : 4 parts sand mixed with water and plasticiser and waterproofer as recommended.
Or
1 part OP Cement : 0.5 part hydrated lime : 4.5 parts sand mixed with water and plasticiser and waterproofer as recommended.

Top Coat (for floating or dashing finish)
1 part OP Cement : 5 or 6 parts sand mixed with water and plasticiser and waterproofer as recommended.
Or
1 part OP Cement : 1 part hydrated lime : 5 or 6 parts sand mixed with water and plasticiser and waterproofer as recommended.

Use white cement to make white mortar for top coat if required and always follow manufacturer's instructions for use of plasticisers and waterproofers.

External render should always be applied to a carefully prepared substrate. Always be in two coats to a minimum thickness of 16mm as the long term durability of a render is a function of its thickness. It should be thoroughly scratched between coats and be allowed to dry and shrink completely between coats. These mixes are for guidance only and should not be assumed to represent the extent of variations available.

Pre Mixed Dash Receiver

Pre-mixed dash receiver products are available that are pre coloured and require mixing with clean water on site with a paddle mix. These products allow uniformed colours and a mix consistency that cannot be guaranteed using the above cement and sand mixes. Base coats are also available to suit. The product often contains silicone and or polymer additives to give a stronger bond that is less liable to cracking than a sand and cement mix. This type of product is often described as a self-coloured cementitious render. This type of render would be manufactured as a type 11 material under BS 5262:1991. Given the consistency of colour that is often required this type of finish is preferable to a sand and cement mix made up on site. The added polymers bond the finish creating a stronger render.

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