LITS -The peculiar behaviour of heated concrete

Like most materials, when you heat a piece of concrete, it expands.  Unlike most materials, when you heat a piece of concrete with a load on it, it expands less, or not at all, or it may even contract, depending on how big the load is – diagram below.  This unusual behaviour goes by a number of names such transient thermal strain, transient thermal creep, or load-induced thermal strain (LITS).   Here I will use LITS.

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LITS – in simple terms.  Concrete thermal expansion is stress dependent.

My interest in LITS began about eight years ago when then PhD student, Angus Law, started investigating its role in concrete-framed structures under fire.  Angus implemented a LITS model in Abaqus and showed that LITS can affect the behaviour of certain types of structural element substantially.   He also demonstrated the importance of correctly representing the multi-axial behaviour of LITS.

More recently two more PhD students working with me have started looking at LITS from very different angles:

Giacomo Torelli’s project, supported by EDF, is looking at the behaviour of nuclear reactor pressure vessels under fault conditions.  These are very large prestressed concrete structures that may be subject to internal high temperatures for long periods, possibly weeks.   The effect of LITS in these conditions may be considerable.  Existing finite element software doesn’t capture LITS well in triaxial conditions so Giacomo has implemented a new model in the Code_Astor software that for the first time correctly models the effect of hydrostatic confinement on LITS behaviour.  This work will be presented next month at the 2016 SiF conference.  Giacomo has also written a very thorough review of previous work on LITS that will (subject to final review) appear in Engineering Structures soon.  This shows there are still clear gaps in knowledge, for example how LITS in multi-axial conditions develops above 250C, and whether LITS in tension is significant.

Rwayda al Hamd is studying punching shear in fire numerically for her PhD.  She has been interacting with Holly Smith who has recently completed an experimental PhD   at the University of Edinburgh on punching shear in fire that threw up some puzzling results.  Holly tested specimens (conceptually) like those in the diagram below.  When heated these might be expected to deflect away from the heat source as the lower surface expands, but they didn’t – they deflected towards it.

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The odd behaviour in the slabs tested by Holly Smith.

The results were puzzling but similar to results from Liao et al working in Taiwan.  After some thought, Rwayda and I guessed LITS was the source of this odd behaviour.  Unlike the situation with most heated slabs or beams, the heated part of the slab was in compression in Holly’s tests, and would therefore be subject to LITS. The results of two numerical models – one with and one without LITS included– are in  shown in the graphs below..  Quite convincing!    Rwayda’s work will  also be published at the Structures in Fire conference next month, and in due course in a journal.

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Rwayda’s al Hamd comparison of numerical models with and without LITS included against experimental results from Holly Smith.

LITS is clearly a phenomenon that may have big effects on the behaviour and possibly safety of heated structures.  It has been studied since the 1970s, with interest coming and going roughly in line with interest in heated concrete structures, but there are still big gaps in knowledge about both fundamental material behaviour and how LITS affects structural behaviour.  With increasing interest in areas such as extending the life of nuclear power stations and high strength concretes in possibly post-tensioned structures, there are clear avenues for further study.

 

 

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