Fire Rating Questioned on Post Tensioned and Prestressed Concrete Slabs

A recent United Kingdom fire test of a post tensioned slab designed for a 2 hour fire rating achieved only a 66 minute fire resistance. Spalling began after 11 minutes and after 20 minutes spalling exposed the tensioning ducts which resulted in a significant loss of strand strength. While care must be taken in extrapolating test results from one form of construction to another without assessing differences in thermal and restraint conditions (Kelly, Purkiss, 2008), it also raises concern over the elevated temperature performance of prestressed concrete floor systems used in New Zealand. Factors which affect spalling at elevated temperatures are concrete moisture content, permeability and the presence of compressive stresses. (Kelly, Purkiss, 2008) The prestressed products in New Zealand have a higher strength of concrete than the concrete strength of the post tensioned slab in the fire test. Concrete permeability deceases with an increase in concrete strength. Therefore high strength concrete is more susceptible to spalling at elevated temperatures. Further investigation is required to allay these concerns. The fire test is presented in an article on ‘Reinforced concrete structures in fire: A review of current rules’ by Fergal Kelly from Peter Brett Associates and John Purkiss formally of Aston University and was published in The Structural Engineer magazine on 7th October 2008. This article describes the fire test carried out on the post-tensioned concrete slab, the conclusions of the test and subsequent UK discussions following the fire test.

FIR1002.pdf

Kevin Cowie