Research On Mechanics Performance Of Hybrid Fibres Concrete After High Temperature

Wuhan Yangtze Rive tunnel is the first tunnel crossing Yangtze Rive, it connects Wuhan citizens of two sides with each other, and it will become important traffic hinge of Wuhan city. Therefore, the security of Wuhan Yangtze Rive tunnel is concerned by millions of people. However, fire is a severe security problem to tunnels, once the fire occures and brings about structure collapse, the loss of lives and properties would be great. In order to improve the fire resistance, concretes are incorporated with hybrid fibres (long and short firbre composite). Using property change mechanism of fibres to improve the properties of concretes is very necessary to the study about propeties of fibre concrete under high temperature.This paper is under the support of Wuhan Yangtze Rive tunnel project. In this investigation, concrete materials of tunnel liners are tested on residual mechanical properties. Concretes are incorporated with polypropylene fibres and hybrid fibres (polypropylene fibres and steel fibres) respectively. Moreover, after exposure to high temperature, the effects of hybrid fibres which incorporated into HPC are investigated on residual mechanical properties including compressive strength and tensile splitting strength. Three hybrid fibre concretes which are mixed into different content of steel fibres are investigated both on mechanical performance and damage degrees with temperature elevated up from normal to 800℃. After analyzing the results, the main conclusions are as follows:1. The mechanics performance of hybrid fibres concrete is systematically studied after high temperature, results show that the performance of concrete which incorporated with hybrid fibres is obviously superiorer than PP fibres concrete and NSC, it is represented that NSC is tend to explosive spalling at 400℃, but after high temperature, the explosive spalling resistance of hybrid fibres concrete is better, it's integrality is kept well, and the cracks are even less than PP fibres concrete; after exposure to 800℃, the residual compressive strength of hybrid fibres concrete is 51%, and residual tensile splitting strength is 32%, which are higher than the values of PP fibres concrete's respectively.2. Three specimens with different steel content are systematically studied and contrasted, the data results show that, when cubage rate is between0.5%～1.2%, with the increase of steel fibres, its compressive strength, fracture energy, residual compressive energy and residual fracture energy are also improved. But, when cubage rate is more than 0.8%, the puddlling ability is far from good, and the distribution of fibres is not equal. Therefore, from this study, the proper proportion is recommended that the cubage rate should be 0.8%.3. Ultrasonic method is used to test and analyse the performance of hybrid fibres concrete before and after high temperature, the intensity curve is worked out, and the formula of compressive strength, fracture energy and velocity of sound is also presented as below: f_cu= 31.725 ln v + 31.885 , f_t = -0.1628v² + 2.29.9v -1.2469. All the results are within the project error areas. It will definitely provide some data supports to the fire security estimate about the Yangtse Rive tunnel which is about to construction completed.The performance of fibre reinforced concrete is excellent than normal concrete in various aspects. Investigating on such subject is really all-around field, moreover, the study on fire resistance is related to many influence factors, so it's difficult to continue in-depth study, even in engineering practices, how to determine the proportions is really one of the problems that concerns all the people. In this investigation, the concrete material of pipes in Yangtze tunnel is tested on mechanical properties and damage degrees. Some results are achieved, and after regressive analyses, the formula about the relation between sound speed and strength was computed. Besides, effective fibre proportions are determined. All the achievements can be offered as references for studies in fibre reinforced concrete both on theories and engineering applications.