| This project was from of high technology research and development program of China("863" project No. 2005AA332010, named Material & Structure Design of High Impermeability Long Service-life Large Dimension Segment of Shield tunnel and its Engineering Application) and studied special properties of shield tunnel segment concrete at high temperature. Major studied content included four parts: testing method of concrete performances at high temperature, concrete performances at high temperature, applications of ultrasonic wave detecting technology on concrete damage at high temperature, mechanism of concrete performances' deterioration at high temperature. The main works and achievements are listed as follows:1. Testing method of concrete performances at high temperatureAccording to deteriorated characteristics of concrete performances at high temperature, and analyzing designed principles of limit of fire resistance, this paper proposed a new testing method: at 800℃, the time when the temperature at the designed cover of concrete reinforcement arrived at 250℃, had been defined for limit of fire resistance. Afterwards, deteriorated performances of different concretes at limit of fire resistance were detected according to a series of indexes, including variation of temperature inner concrete, physical and mechanical properties after high temperature, ultrasonic wave detecting of concrete specimen, and concrete specimen spalling degree.2. Concrete performances at high temperature(1) At high temperature, overall performances of concrete(J2) prepared by adding 25% (in mass) ground granulated slag were better than pure cement concrete(J0) and adding 20%(in mass) flying ash concrete(J3).J2 and J3's limit of fire resistance were better than J0's, the increases were separately 26.7% and 13.3%. J2's spatling degree was lower than J0's and J3's: loss of quality was 5.87%, number of spalling specimen was one piece, spalling form was surface and part, average maximum spalling area was 7.06cm2, maximum spalling depth was lower than 1mm.(2) Fire resistive coating for tunnel effectively improved concrete performances at high temperature. When plastering fire resistive coating for tunnel, concrete's limit of fire resistance was improved at large, and all concrete specimens were intact. As for as compressive strength and elastic module after high temperature concerned, concrete's performances with fire resistive coating for tunnel were better than concrete's with no fire resistive coating.3. Applications of ultrasonic wave detecting technology on concrete damage at high temperature.(1) Concrete's sonic parameters of ultrasonic wave were deteriorated at large after high temperature, so structure inner concrete was severely damaged.Ultrasonic velocities of J0, J2 and J3 before high temperature were about 5.5 km/s, while ultrasonic velocities of J0, J2 and J3 after high temperature were about 2.5 km/s. Concrete's loss of ultrasonic frequency after cooling 4h and 36h when arriving at limit of fire resistance, showed that damaged degree of J2 and J3 increased with cooling time increasing. After high temperature, ultrasonic initial wave of concrete became abnormal, width of wave were broadened, ultrasonic transit time were increased, ultrasonic wave shape varied from regularity and order to disorder state.(2) after cooling 36h when arriving at limit of fire resistance, depth of damaged concrete layer was 56～100mm.4. Mechanism of concrete performances at high temperature(1) Deteriorated forms of hardened cement paste were hydration products' decomposition, expansion and shrinkage at high temperature.DSC curves of hardened cement paste prepared by different mineral admixtures showed three endothermic peaks: 70～110℃, dehydration of free water, ettringite and gypsum; 430～450℃, dehydration of Ca(OH)2; 650～690℃, decomposition of CaCO3. Enthalpy of decomposition about Ca(OH)2 in hardened cement paste prepared by different mineral admixtures, was lower than pure hardened cement paste. In addition, the more active mineral admixtures, the more lower enthalpy of decomposition about Ca(OH)2.Characteristic peak of Ca(OH)2 in XRD pattern(d=2.6292) gradually grew small with temperature increasing. 25～800℃, Characteristic peak of Ca(OH)2 in XRD pattern widened from single peak to non-crystalline.At high temperature, characteristic of hardened cement paste' variation was expansion before shrinkage. Proper quality of mineral admixtures improved deformation of hardened cement paste at high temperature, and enforced volume stability. Moreover, thermal expansion and shrinkage of pure cement specimen(Sample 0), adding 20%(in mass) ground granulated slag specimen (Sample 2), and adding 10%(in mass) silica fume and 20%(in mass) flying ash specimen(Sample 4) included four phases: 20℃～130℃, micro-expansion; 130℃～420℃, sharp shrinkage, linear expansion ratios of Sample 0 and Sample 4 were higher than Sample 2; 420℃～500℃, continuous shrinkage, linear expansion ratios of Sample 0>Sample 4>Sample 2; 500℃～1000℃, slow shrinkage, linear expansion ratios of Sample 0>Sample 2>Sample 4.(2) Spalling was different for specimens prepared by different mineral admixtures at varied temperatures. Effect of polypropylene fiber aspect ratio, content and hybrid fiber on spalling was eminent.At 100℃and 300℃, specimens prepared by different mineral admixtures weren't spalling; at 600℃, all the specimens were shattered spalling. At 100℃, the specimens' loss of compressive strength were decreased with activity of mineral admixtures increasing. The specimens' compressive strength and flexural strength were higher at 300℃than 100℃.Effect on improving specimens' spalling for high fiber aspect ratio was better than the low one. When fixing fiber aspect ratio, critical content of high fiber aspect ratio for improving spalling was lower than the low one. At 300℃, adding polypropylene fiber specimens' compressive strength and flexural strength decreased in different degrees after high temperature. At 600℃, high content of polypropylene fiber specimens' loss of compressive strength and flexural strength were higher than the low one. In addition, hybrid fiber(different fiber aspect ratio hybrid) effectively improved spalling of specimens.(3) Considering interior factors of concrete, analyzing spalling mechanism from materialogy angle about hardened cement paste, hardened cement paste+fine aggregate, and hardened cement paste+coarse aggregate. Moreover, this paper proposed a spalling model, which was synthesis consideration of the thermal stress, the vapor pressure build-up mechanism and size distortion, and qualitative explain of spalling in experiment.
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