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    Crack control coatings: elastomeric paints and precast concrete

    Nathan Johnson

    Fiona Stanley Hospital (above) by HASSELL, Hames Sharley and SILVER THOMAS HANLEY uses Dulux Acratex Elastomeric Paint in Spray On 2mm for the tilt up panels and Roll On 00 for the stairways and soffits.


    Precast concrete offers durable, flexible solutions for floors, walls and roofs in almost every type of domestic construction. However, most structures using precast will still require a coating for protection with either conventional or textured finishes.

    The primary aim of any coating is to enhance and preserve the appearance of the substrate, and in terms of precast concrete this entails protection during the expansion and contraction cracking.

    Cracking, splitting, flaking, efflorescence and other failings of rigid masonry renders and coating systems ushered in the creation of new paint technologies, and in the 1970s, flexible acrylic “membrane” or elastomeric coatings were introduced to Australia. Along with other premium products, such as rigid silicate paints, elastomeric paints have been used to great success on precast concrete surfaces since their introduction, and are widely specified by architects in Australia.

    Elastomeric paints are designed to be applied in very high film builds (about 10 times as thick as regular paints) and therefore have a low spread rate and do require additional skill to apply. Because of this, they are tough and flexible, have excellent Ultraviolet (UV) stability, and stretch as cracks underneath open and close, thus bridging the cracks and maintaining weather resistance.

    Typically there are three types of polymers formulated into coatings labelled as elastomeric coatings including 100 per cent acrylic, styrenated acrylic, and vinyl acetate containing copolymers.

    There are differences between these binders in terms of elasticity, resistance to hydrolysis, and ultraviolet stability, but research shows that 100 per cent acrylic elastomeric emulsion polymers, although generally more expensive, offer the best performance of the three on most of these measures.

    In saying that, the performance of elastomeric paint on precast concrete is really in the formula and some suppliers use different polymers that are formulated specifically for Australian conditions and for a more affordable price. For example, suppliers of 100 per cent acrylic elastomeric paints use internal plasticisers which perform better in a more diverse temperature range however suppliers of styrenated and vinyl acetate using external plasticisers can offer a more diverse range of colours and a product that will perform in Australian conditions that don’t reach freezing.

    The internal plasticisers used in 100 per cent acrylic elastomeric paints to get that ultimate elongation, dirt resistance and ultraviolet stability means they will become “chalky”, particularly in sunny exposures. Therefore higher quality elastomeric paints have traditionally been specified in lighter colours to lessen the visual impact of this discolouration.

    The best coating advice for precast concrete is to use lighter colours because they develop less heat stress through to the substrate and can lessen the expansion and contraction cycle which ultimately causes cracking in the first instance.

    “Preparation, Preparation, Preparation,” Andrew Sullivan, Product Manager of Dulux AcraTex

    Elastomeric paints act as a "weatherproofing barrier" to the ingress of moisture and atmospheric pollutants in order to prevent cracking and other concrete failures. Proper prep is therefore essential before any application.

    Painting precast can be a tricky process and because of their high alkalinity, which is frequently in the region of pH 10 to 12, freshly prepared cementitious surfaces require time to age before they can be painted.  Australian Standards 2311 (AS 2311) gives details on the ageing process and other precautions needed to ensure that the substrate is at an acceptable pH level but most paint suppliers state that the substrate alkalinity should be reduced below pH 9.

    Additionally, the tilt-up and precast construction techniques use bondbreaker materials to prevent the concrete from adhering to the building moulds. These release agents could interfere with adhesion of paint systems and must be removed prior to painting.

    SUPPLIERS

    • Astec Paints, “Elastomeric Protective Wall Coatings”
    • Dulux, “AcraTex”
    • Resene, “Resitex”
    • Quickwall, “Aquaseal”
    • APCO, “Apcolite”
    • Ability Building Colours, “Duro Paint” (not an elastomeric paint but an inorganic mineral coating with a 25 year guarantee)

     

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