Please use this identifier to cite or link to this item: http://hdl.handle.net/1959.3/222048

Title

Adverse effects in high strength concrete when exposed to fire

Author(s)

Sanjayan, Jay

Abstract

High strength concrete is often chosen when high strength, low permeability and/or high elastic modulus are required. However, it is less known that high strength concrete suffers high risk of spalling in a fire. Spalling of concrete is loss of concrete section due t concrete falling off the fire exposed surface.. Rick can be minimised by optimisation of concrete mixture and testing samples of concrete. Certain aggregates and silica fumes are known to exacerbate the spalling, while polypropylene at certain dosages is effective in reducing spalling. There are three known failure modes by which spalling of concrete can occur in fire: (1) Moisture clog spalling due to the steam pressure build up in the pore system of concrete; (2) Spalling due to restrained thermal dilation; and (3) thermal incompatibilities between cement past and aggregates. Spalling occurs at the early stages of the fire (about 30 minutes into the fire). Therefore, it has been known to hamper rescue efforts. In the first 30 minutes, temperature in hydrocarbon fires rises at much higher rate than the standard fire; therefore, causes higher steam pressures and thermal gradients with increased spalling risk. the Australian Concrete Structures code does not provide any guidance on the sampling of concrete. Hence, Australian engineers normally resort to Eurocode for guidance. However, Eurocode design provisions for spalling are not applicable for hydrocarbon fire exposures or unreinforced concrete. There are simple test methods developed and presented in the literature for spalling. The tests are designed to expose the specimens to rapid temperature rise similar to standard or hydrocarbon fires. Since the risk of spalling is highly influenced by the type of concrete and aggregates used, these tests, if performed as part of the mixture trials, will help identify concretes which are not suitable for this type of construction. Failure of concrete slabs may occur due to thermal warping of the concrete due to large thermal gradients. The failure may occur by two modes: (a) Buckling of the slabs, particularly in longitudinal direction, due to large restraining compressive forces. This occurs when the slab exposed to fire expands while the remaining lengths of the slab in lower temperatures provide restraint. (b) If buckling does not occur, continued exposure to fire can cause large deflections. These large deflections cause the compressive forces to subside, however tensile membrane forces develop. Concrete suffers chemical and physical damage at high temperature, with permanent loss of strength and stiffness. For example, a 60% loss occurs after exposure to 600 degrees C temperature. This permanently reduces the load carrying capacity of the structural members.

Publication type

Journal article

Research centre

Swinburne University of Technology. Faculty of Engineering and Industrial Sciences. Centre for Sustainable Infrastructure

Source

Concrete in Australia, Vol. 37, no. 2 (2011), pp. 23-30

Publication year

2011

FOR Code(s)

0905 Civil Engineering

Keyword(s)

Australia;  Concrete properties;  Fire resistance

Publisher

Concrete Institute of Australia

ISSN

1440-656X

Publisher URL

http://www.concreteinstitute.com.au/Concrete-in-Australia.aspx

Copyright

Copyright © 2011 Concrete Institute of Australia.

Peer reviewed