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Blocks of any type or grade are not waterproof or weatherproof and must be rendered or cladded if used on an external leaf.
Rendering onto our concrete blocks is a proven, simple and cost-effective way in achieving a waterproof, durable and attractive external finish. The ease and results of the rendering process is significantly affected by the correct selection of render application to the block / background type and the correct preparation of the block surfaces prior to applying the render. In this section we offer a number of simple common sense and proven guidance points in order to achieve repeatable, successful results time after time.
Standard texture Ultralite, Insulite and Dense Concrete blocks provide an excellent key for adhesion. These blocks have moderate suction and no special pre-treatment of the surface is normally required other than ensuring that all dust and debris is removed.
Traditional renders should be applied in 2 coats. The first coat should not exceed 15mm thickness and the second coat should be 5-7mm. The first coat should be slightly stronger than the second coat. Render designation M4/iii should be used.
The bonding and durability of rendered finishes is affected by the weather conditions at the time of application.
In hot weather or where the wind can dry out the render too quickly, the applied render should be kept damp for the first 3 days with the use of protective sheeting. Avoid applying render during strong, direct sunlight and protect the render in such conditions. If necessary, lightly spray the curing render periodically with clean water to prevent rapid drying out.
In cold and bad weather, the same protection is needed from driving rain and freezing conditions. Where sheets are used, ensure that they are hung so as not to contact the applied render and cause chafing and scuffing. The air temperature should be at least 2°C and rising and the background should be unsaturated and free from visual signs of frost.
Where the air temperature is at, below or likely to fall below 5°C, protect the render with sheeting e.g. Hessian.
Airtec, Insulite Premier and Fairtex blocks have a close texture and therefore have less mechanical key for the render to adhere to than a Standard texture block. Although not necessary in all circumstances, the following steps can assure render adhesion even on the smoothest surfaces.
Pre-treatments or raking back mortar joints are advised on paint quality blocks using traditional renders and also may be necessary when using proprietary renders (See render manufacturer's literature).
Aggregate blocks should not be wetted prior to the application of coatings whereas Airtec blocks should be wetted prior to applying renders in dry, warm conditions to overcome their natural suction and prevent the render from drying out too quickly. Proprietary pre-rendering treatments such as Rend-Aid can be applied if desired. Stipple or Spatterdash coatings applied prior to rendering offer the very best solution for rendering onto close-textured blocks.
A stipple coat mix should be prepared using one part of cement with one and a half parts of sharp sand made into a consistency of a slurry with water and a bonding agent such as styrene butadiene rubber (SBR). The mixture should be pushed into the surface with a coarse brush and then dabbed with a refilled brush to give a coarse finish which should be protected from rapid drying out for a day and then left for an addition day or two to harden before applying the first render coat.
A spatterdash coat mix should be prepared using one part of cement to 2 parts of coarse sand with just sufficient water containing a bonding agent such as styrene butadiene rubber (SBR) to form a thick slurry. The spatterdash should be thrown against the background with a small scoop to form a layer of 3-5mm thickness. The mix should be stirred regularly to prevent settlement. The spatterdash layer should be protected from rapid drying out for a day and then left for an additional day or two to harden before applying the first render coat.
Scratch Coat - Serpantine Pattern:
Experience has shown that the use of a wavy 'serpantine' scratch finish on the first render coat, as opposed to the traditional horizontal or vertical scratches, actually lessens the likelihood of hairline cracks appearing, particularly on rendered aerated block walls. This serpantine pattern better dissipates the stresses as the render and the block background dries out and shrink at different rates.
We recommend the use of traditional renders wherever possible due to their proven history and success.
For proprietary two coat renders the manufacturer’s literature should be consulted for base coat and final coat recommended thicknesses, but these are generally similar to those for traditional renders.
Proprietary single coat renders may also be used on aggregate block backgrounds. These have often worked well but on several occasions single coat proprietary renders have failed and caused horizontal and vertical cracking through the blocks.
The cause of the failures is most likely due to the strength of the render being too high for the background and can result in cracking through the block. We therefore urge particular caution if rendering onto 3.6N strength blocks.
It is important that the correct strength render is chosen for a given background and the render supplier would need to verify this prior to commencement of rendering.
We therefore recommend that the render manufacturer is contacted to establish:
a. Whether there is any history of problems with their product on masonry, and
b. Their exact preparation and application guidance recommendations, and
c. That the product strength is no greater than the background blocks which are to be rendered and that they can supply a lower-strength product if necessary.
Thomas Armstrong (Concrete Blocks) Ltd
Part of the Thomas Armstrong Group
T: 01900 68211
F: 01900 602672