Research On Impermeability Of Fiber Concrete And Its Application In Tunnel Second Lining

This paper aims to problems of the low strength and poor toughness of the tunnel second lining concrete,especially that the leakage water of tunnel engineering is the most serious disease affecting the safe operation of tunnels.The leakage water increases the humidity in the tunnel,aggravating the corrosion of the tunnel structure and accessory facilities.Particularly in cold areas,it will cause repeated freeze-thaw cycles to damage the tunnel entrance with 300 meters,further reducing the reliability and safety of the tunnel life cycle.Based on the above questions,in this paper,the slag powder,silica fume,polypropylene fiber and steel fiber are combined to mix into the second lining concrete to strengthen the toughening and improve the anti-seepage and frost resistance of second lining concrete.In order to verify its validity and rationality,this paper systematically conducts the research on the influence of the mineral admixtures and hybrid fibers on the performance,basic mechanical properties,impermeability and frost resistance of secondary lining concrete.At the same time,this paper makes an theoretical analysis of the above experimental results at the micro-structure level,and the following main conclusions are drawn:(1)In the orthogonal test of second lining concrete,the content of slag powder is the main factor affecting the compressive strength and electric flux,and the content of steel fiber is the main factor affecting the fluidity,splitting tensile strength and flexural strength.The mechanical properties of second lining fiber reinforced concrete at different ages are improved.The splitting tensile strength of 28 d concrete is63.9%~95.4% higher than that of ordinary concrete,and the tensile and toughening properties are significantly enhanced.In addition,among the three types of the steel fibers,the end hook type steel fibre concrete has the best mechanical properties.Especially in splitting tensile and flexural properties,compared with straight steel fibers,the enhancement effect is more than 10%,while corrugated steel fibers are in the middle.(2)In the water permeability test of second lining concrete,the impermeability of concrete is generally improved by an order of magnitude on the basis of the optimum mix proportion of the electric flux test.At the same time,the impermeability of concrete can be further improved by adding steel fibers.Pre-loading impermeability test againverifies the beneficial effect of steel fibers,especially the second lining concrete with cracks under load,which can effectively restrain the deterioration of impermeability of concrete after loading.But the high content of silica fume increases the brittleness of concrete and has obvious adverse effect on the impermeability of concrete under preloading.(3)Polypropylene fiber can significantly improve the frost resistance of concrete in the frost resistance test of second lining concrete,but accompanied by the increase of the steel fiber content,the effect of improving the frost resistance of concrete is not good.In addition,the composite filling effect and pozzolanic activity of slag powder and silica fume can be effectively exerted,which shows better frost resistance than that of single mixing.(4)The bonding characteristics of fiber interface and the micro-morphology of hydration products under different experimental conditions are observed and analyzed by scanning the electron microscopy(SEM).The results show that under the optimum experimental conditions,the micro-structure of concrete has less porosity and higher compactness,at the same time,the hybrid fibers can be dispersed uniformly and bond closely with the cement base.