Improving Fire Resistance of Reinforced Concrete Columns

Abstract

Fire has become one of the greatest threats to buildings. Concrete is a primary construction material and its properties of concrete to high temperatures have gained a great deal of attention. Concrete structures when subjected to fire presented in general good behavior. The low thermal conductivity of the concrete associated to its great capacity of thermal insulation of the steel bars is the responsible for this good behavior. However, there is a fundamental problem caused by high temperatures that is the separation of concrete masses from the body of the concrete element " spalling phenomenon ". Spalling of concrete leads to a decrease in the cross section area of the concrete column and thereby decrease the resistances to axial loads, as well as the reinforcement steel bars become exposed directly to high temperatures. With the increase of incidents caused by major fires in buildings; research and developmental efforts are being carried out in this area and other related disciplines. This research is to investigate the behavior of the reinforced concrete columns at high temperatures. Several samples of reinforced concrete columns with Polypropylene (PP) fibers were used. Three mixes of concrete are prepared using different contents of Polypropylene ;( 0.0 kg/m³, 0.5 kg/m³ and 0.75 kg/m³). Reinforced concrete columns dimensions are (100 mmx100 mm x300 mm). The samples are heated for 2, 4 and 6 hours at 400 C°, 600 C° and800°Cand tested for compressive strength. Also, the behavior of reinforcement steel bars at high temperatures is investigated. Reinforcement steel bars are embedded into the concrete samples with2 cmand3 cmconcrete covers, after heating at800°Cfor 6 hours. The reinforcement steel bars are then extracted and tested for yield stress and maximum elongation ratio. The analysis of results obtained from the experimental program showed that, the best amount of PP to be used is 0.75 kg/m³, where the residual compressive strength is 20 % higher than of that when no PP fibers are used at400 Cfor 6 hours. Moreover, a3 cmof concrete cover is in useful improving fire resistance for concrete structures and providing a good protection for the reinforcement steel bars, where it is 5 % higher than the column samples with2 cmconcrete cover at 6 hours and 600 C°.