Language of glassfibre-reinforced concrete, The

language of glassfibre-reinforced concrete, The

Ferry, Richard

Every industry inevitably develops its own jargon, speeding communication between those people operating within it. However, such jargon does radically diminish the ability of a non-specialist to fully understand the subject and may dissuade potential clients from finding out more.

The glassfibre reinforced concrete (GRC) industry is no exception. The terminology common within the GRC industry is often in the form of acronyms and these, together with the fact that the behaviour of fibrous reinforcement differs from that of conventional steel reinforcement, may confuse the non-specialist. The purpose of this short article is to try to explain as simply as possible what the language of GRC actually means.

An important difference between traditional concrete and GRC is that the former is assessed by compressive strength and the latter usually by flexural (bending) or tensile tests. GRC is used in thin sections, normally less than 20mm. Concrete, by comparison, is used in thick sections.

The flexural or bending strength is known as the modulus of rupture (MOR). This is measured in MPa or N/mm, as for compressive strength. One of the most important characteristics of the behaviour of GRC is not its MOR but its limit of proportionality (LOP). Confusingly, this is known as proportional elastic limit (PEL) in the USA, the same country also choosing to use the abbreviation GFRC instead of GRC!

The LOP describes the flexural or bending stress at which the stress-strain curve departs from linearity. It is not easy to measure manually and best practice is to use computer-aided testing. It may be thought of as the strength you might have got if there had been no fibres present.

‘Matrix’, a word peculiar to the GRC industry, simply means the cement /sand mix reinforced by the fibres. It is also referred to as ‘slurry’ during manufacture. (I have also heard it called ‘mud’ which, while vividly descriptive of a typical GRC mix, is not very technical!)

It is known that, providing the stresses in the GRC are kept below the LOP, GRC does not suffer from cyclic fatigue. This has relevance to cladding panels or other products that have to resist repeated stresses over their lifetime.

The MOR is easy to measure and is a good indicator of general ‘quality’; in this context, it is similar to compressive strength for conventional concrete. Neither gives the whole picture, but each is a good guide. A huge number of factors determine the actual MOR and the field attracts much research. For our purposes, MOR increases with fibre content, curing and density. The MOR depends on, among others, fibre geometry, fibre length, orientation, alignment, mix design, time and exposure. An appropriate MOR indicates well-made GRC that has been adequately cured.

There is other jargon in the GRC industry, but brevity is a route to clarity and understanding. I think that this is sufficient at present.

Richard Ferry, GRCA International

Copyright The Concrete Society Sep 2002

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