| With the development of the Chinese economy and society,the pace of urbanization continues to accelerate.Thus,achieving coordinated development on both sides of urban railways has been an irresistible trend.The box culvert jacking construction under the railway tracks group has become an effective method to connect the traffic crossing the railway due to its rapid construction speed,minor environmental influences,and low construction cost.Since concrete cracks would weaken the strength and durability of the concrete terribly,concrete crack has become a severe problem in constructing long frame box culverts under the railway.This paper studies the crack resistance of concrete in long frame box culverts under the railway based on the box culvert project in North Tongling Road,Hefei City.The study adopts several methods:theoretical analysis,laboratory experiments,in-situ tests,and numerical calculation.The main findings of the study are as follows.(1)The crack arresting mechanism of short-cut fibres to inhibit early plastic cracking of concrete is verified.The basic types of cracks and their main characteristics were analysed and the cracking mechanism of concrete in underpassing railway grown-up frame box culverts was discussed in depth.On this basis,the effects of different fibre lengths and admixtures on the early plastic crack resistance of concrete were analysed,and the optimum early crack resistance length and content of fibres were obtained,and the technique of preparing fibre crack resistant concrete for underpassing railway long frame box culverts was proposed.(2)The law of exothermic heat of hydration of concrete for underpassing railway frame box culverts was investigated.The heat of hydration temperature of concrete for underpassing railway frame box culvert shows an overall trend of rapid increase,then decrease,and finally consistent with the outdoor temperature after pouring.The temperature of concrete hydration heat rises to the highest value within 45h～50h after casting,the highest temperature of box culvert top plate is about 52℃,the highest temperature of bottom plate is about 55℃,and the temperature drops in line with outdoor temperature about 10 d time after concrete casting.(3)The distribution law of concrete cracks in the underpass railway frame box culvert was summarized.A total of 23 concrete cracks were measured in 8 sections of the concrete of the underpassing railway frame box culvert,among which 5 cracks with width less than 0.1mm accounted for 22% of the total cracks;3 cracks with width0.1mm～0.2mm accounted for 13% of the total cracks;6 cracks with width0.2mm～0.3mm accounted for 26% of the total cracks;5 cracks with width0.3mm～0.4mm accounted for 22% of the total cracks;3 cracks with width0.4mm～0.5mm accounted for 22% of the total number of cracks;3 cracks with a width of 0.4mm～0.5mm,accounting for 13% of the total number of cracks;only 1 crack with a width of more than 0.5mm,accounting for 4% of the total number of cracks.(4)The mechanism of the cracks caused by the heat of hydration of the concrete in the long frame box culvert under the railway was revealed.Concrete hydration heat causes tensile stress in both sides of the span of the box culvert bottom plate,and with the increase of concrete hydration heat release time decreases,which in turn causes tensile stress in the box culvert concrete;both sides of the front and rear ends of the top plate are the easy cracking parts of the box culvert concrete,and certain measures should be taken to protect them in the box culvert concrete project.The research results of the thesis provide important guidance for the control of concrete cracks in grown-up frame box culverts of the relying project,ensuring the safe and smooth implementation of the relying project,and at the same time provide important reference value and case support for the control of concrete cracks in similar frame box culverts,which in turn can provide theoretical reference and example basis for the revision and preparation of relevant standards,codes and guidelines. |
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