All Spray Process GRC requires an application of a "mist" or "facing" coat to provide an aesthetically pleasing surface. Whilst mist coats use exactly the same matrix as the GRC backing facing coats have different formulations and typically contain decorative aggregates etc.
Importance of Development and Testing
Given the difference in mix design it is absolutely essential that where facing coats are used these have been fully developed and tested to ensure suitability for use. There is no requirement for such testing in either the GRCA Specification or BS EN 1169:1999 – General rules for factory production of GRC. The importance of such testing is however recognised in the USA and covered in depth within the Precast/Prestressed Concrete Institute published document MNL-130-09 Second Edition – Manual for Quality Control for Plants and Production of GFRC
Various issues can be attributed to the use of GRC facing mixes where no verification testing has been carried out. These can include edges which are easily chipped due to insufficient compressive strength and high water absorption causing deterioration of the supplied appearance. The most critical however is compatibility in respect of differential movement with the GRC backing. This can cause surface crazing or in extreme cases structural failure of the GRC element.
Syntec GRC Facing Coats
All Syntec GRC finishes which deviate in any way from the structural GRC Backing have been fully tested by a UKAS accredited laboratory to ensure compatibility with the GRC backing. Prior to release for production all mixes are tested in accordance with PCI MNL 230-09 requirements. Note AST testing standards detailed within the publication are substituted where practical for the equivalent BS/EN standard. Where this is not possible because comparable testing does not exists the ASTM methods are used
As it’s rare for untested facing coast to cause structural failure finished projects are primarily judged by their durability and appearance. For this reason it’s essential architects, specifiers and buyers of GRC products can have complete confidence in not only what isn’t seen but what is. As earlier neither the GRCA Specification nor BS EN standards cover this important aspect. It’s therefore easily possible that standard compliant GRC products may develop unattractive qualities without any contractual recourse to the manufacturer. Our approach to the development of these face coats mitigates that risk on any project we are involved with.