Analysis of bonded post-tensioned concrete flat slab in and after fire

Abstract

Bonded post-tensioned concrete slabs have been widely used in buildings with the advantages of economy and large span-to-depth ratio compared to the traditional reinforced concrete slabs. However, when subjected to elevated temperatures, such slabs may suffer degradation of mechanical properties, loss of bond strength between tendons and the surrounding grout, and concrete spalling, leading to potential structural problems or even potential collapse of part or the entire structure. Irrecoverable damage to such slabs can be caused by fire after cooling down to ambient temperature. Therefore, assessments of the structural behaviour of the slabs during fire and after fire are important issues in performance-based design. Experiments were carried out by an accurate testing system for development of empirical formulae to predict the deterioration of prestressing steel at elevated temperatures and after cooling. Specimens tested included Grade 1860 prestressing strands to GB/T 5224 mostly used in Mainland China, and Grade 1860 prestressing strands to BS 5896 used in European and other countries. Consistent models for reduction factors for various mechanical properties were developed. Some prestressing strands reclaimed from prestressed concrete specimens after fire test were also tested for verification. To address the effect of elevated temperature on the bond strength of prestressing strands in grout, the influence of duct diameter, the concrete cover, and the number of prestressing strands in a duct was investigated. Pull-out tests were carried out on specimens at ambient temperature, elevated temperatures and after cooling from elevated temperatures. A typical specimen comprised of 7-wire prestressing strand(s), grout, corrugated steel duct and a concrete cylinder. Reduction factors for bond strength of various scenarios were obtained and empirical equations were put forward. Fire tests of four scale specimens of two-way post-tensioned concrete flat slab were analysed for performance-based design approach. Three of them were provided with bonded tendons while the remaining one was provided with unbonded tendons for comparison. The investigation of the influence of layout of prestressing tendons, prestressing force, bond condition and concrete spalling on the structural behaviour of two-way post-tensioned concrete flat slabs at elevated temperatures is presented. The fabrication of specimens, the phenomena observed during the tests, the temperature distributions, the deflections and occurrence of concrete spalling are examined. Based on the experimental results, design recommendations for two-way post-tensioned concrete flat slabs are provided. A numerical study of structural behaviour of post-tensioned concrete flat slabs with unbonded and bonded tendons at ambient temperatures, elevated temperatures and after cooling from elevated temperatures was carried out. The numerical model was mainly based on the earlier tests carried out by Bailey and Ellobody. An important objective is to understand the structural behaviour of bonded post-tensioned concrete slabs, taking into account the reduction of bond strength between prestressing strand and the surrounding grout at elevated temperature and after cooling from elevated temperature. The results of the model are compared with those obtained by other researchers.

(475 words)