EXPERIMENTAL STUDY ON FIRE RESISTANCE OF 200-300 MPA HIGH STRENGTH CONCRETE WITH SYNTHETIC FIBER AND STEEL FIBER

Abstract

 High strength concrete columns are largely used for high-rise reinforced concrete buildings. Such concrete with low water to cement ratio has a dense structure and may experience explosive cover spalling when subjected to fire. The decrease of column cross-section could cause the steel reinforcements exposed to a high temperature at an early stage, and consequently, reduce the load-carrying capacity of the column. For 80 – 150MPa high strength concrete, we developed a method of suppressing the spalling behavior by adding a small number of synthetic fibers in 2000. Moreover, in 2009, we applied hybrid fibers of synthetic fibers and steel fibers to 120 – 200MPa high strength concrete. It is now common knowledge that adding both synthetic and steel fibers is indispensable for improving the fire resistance of the concrete with compressive strength more than 150MPa. However, there is a lack of study concerning the effects of fiber content and fiber geometry. This study carried out several heating tests on 200 – 300MPa high strength concrete using the ISO834 standard fire curve, in which various fiber types and fiber contents were considered. The effects of fiber type, fiber content, and geometry of both synthetic fiber and steel fiber on spalling depth were discussed. The following conclusions can be derived from this study.

 (1) Adding 2.0%.vol synthetic fibers and 0.25 – 0.5%.vol steel fibers can greatly suppress the fire-induced cover spalling for 200 – 300MPa high strength concrete.

 (2) A critical amount of synthetic fibers Vof_cr was found for avoiding the appearance of cover spalling. The average spalling depth seems to be inversely proportional to synthetic fiber content (Vof) with a proportional coefficient of α.

 (3) The parameters of Vof_cr and α are influenced by several factors including fiber material, geometry, the water content of concrete, and concrete strength. Within the test range, it was found that the polypropylene fiber was the most effective to suppress the fiber-caused spalling.

 (4) As the diameter of synthetic fiber decreases and fiber length increases, Vof_cr and α become smaller, which is favorable for fire-resistance. These parameters are proportional to the power of fiber diameter and inversely proportional to the fiber length.

 (5) The steel fiber with a smaller diameter is preferable for improving fire resistance. Even the same fiber content can lead to a larger number of fibers per unit volume that can bridge more cracks during the heating process.