| Currently, the strengthening techniques with carbon fiber sheets have been incorporated into national standard “Design code for strengthening concrete structure”(GB50367-2006) and industry standard “Technical specification for strengthening concrete structures with carbon fiber reinforced polymer laminate”(CECS146：2003). Carbon fiber sheet has good high-temperature resistance at1000℃or less in anaerobic conditions, and its strength does not decrease with increasing temperature. But the glass transition temperature (Tg) of conventional epoxy resin adhesive used in strengthening is only60℃~82℃, and its ignition point is600℃. Because the fire is a high-frequency disaster, and the ambient temperature is up to several hundred degrees to thousands of degrees when the fire broke out, which has seriously hampered the development of the strengthening technique with carbon fiber sheets. View of the high-temperature resistance epoxy resin adhesives with expensive price and the process of pasting carbon fiber sheets required in the high temperature environment, it is difficult to large-scale promote and apply high-temperature resistance epoxy resin adhesives in the strengthening techniques with carbon fiber sheets. Designation and development the high-temperature resistance inorganic cementitious materials become an urgent need for the industry. In addition, the high-temperature resistance inorganic cementitious materials may also be considered to substitute for concrete applied to the construction of high-temperature environments. It is learned from the relevant literatures that the geopolymer has good high temperature performance, and it is qualitative judged that Alkali-Activated Slag Cementitious Material (AASCM) should have similar performance with geopolymer. Therefore, AASCM is chosen as a focal point to carry out research work.（1）Six kinds of mix schemes were attempted, such as ground granulated blast furnace slag and potassium silicate, ground granulated blast furnace slag and sodium hydroxide，ground granulated blast furnace slag and cement and a small amount of sodium carbonate，slag and fly ash and potassium silicate, slag and fly ash and sodium hydroxide, slag and fly ash and cement and a small amount of sodium carbonate. Two better mix proportions of AASCM were gotten, namely, slag is used as raw material, the potassium silicate with the modulus Ms=1.0is used as alkaline activator, the dosage of potassium silicate accounts for12%of the slag mass, the dosages of water account for35%and42%of the slag mass, respectively. When the better mix proportion with the dosage of water accounting for35%of the slag mass is cured for28d, the compressive strength of cement-mortar specimen in size of40mm×40mm×160mm is90.16MPa, and the compressive strength of cubic specimen in size of70.7mm×70.7mm×70.7mm is71.75MPa, and the compressive strength of prism specimen in size of70.7mm×70.7mm×228mm is48.44MPa, and the tensile strength of dumbbell specimen is3.47MPa. When the better mix proportion with the dosage of water accounting for42%of the slag mass is cured for28d, the compressive strength of cement-mortar specimen in size of40mm×40mm×160mm is80.88MPa, and the compressive strength of cubic specimen in size of70.7mm×70.7mm×70.7mm is64.53MPa, and the compressive strength of prism specimen in size of70.7mm×70.7mm×228mm is44.90MPa, and the tensile strength of dumbbell specimen is3.24MPa. AASCM has relatively high strength, The price of AASCM is about330yuan per cubic meter, so its price is relatively low. The better mix proportion of AASCM with the dosage of water accounting for42%of the slag mass has better working performance. Hence, the better mix proportion of AASCM with the dosage of water accounting for42%of the slag mass is chosen for subsequent applied research object.（2）In order to investigate the mechanical properties of AASCM at room temperature, the compression tests about60cement-mortar specimens in size of40mm×40mm×160mm and60cubic specimens in size of70.7mm×70.7mm×70.7mm and60prism specimens in size of70.7mm×70.7mm×228mm were completed, the flexural tests about60cement-mortar specimens in size of40mm×40mm×160mm were completed, the tensile tests about60dumbbell specimens were completed, and the splitting tensile tests about60cubic specimens in size of70.7mm×70.7mm×70.7mm were completed. The highest compressive strength of cement-mortar specimens is121.18MPa, and the highest compressive strength of cubic specimens is96.43MPa, and the highest axial compressive strength of specimens is59.86MPa, and the highest flexural strength of specimens is16.56MPa, and the highest tensile strength of dumbbell specimens is4.45MPa, and the highest splitting tensile strength of cubic specimens is4.15MPa. The uniaxial compression tests about36prism specimens in size of70.7mm×70.7mm×228mm were completed, and the compressive stress-strain curve equation of AASCM is gotten. Comparative analysis shows that, the ascending part of the compressive stress-strain curve equation of AASCM is similar to that of ordinary concrete, they are both quadratic parabola. The descending part of the compressive stress-strain curve equation of AASCM is oblique line form, which has considered strain gradient effects. By using SEM and XRD analysis technique, the amorphous phases calcium silicate hydrate gel, hydrotalcite and tetracalcium aluminate hydrate were determined as the hydration products of AASCM.（3）In order to contrast pasting effects,90concrete cubic specimens were strengthened with fiber sheets bonded with AASCM on the concrete surface (Two opposite sides of concrete cubic specimens in size of100mm×100mm×100mm were strengthened with70mm width and100mm length fiber strips), and double-shear tests of90specimens were completed by double-shear test methods at room temperature. The failure modes that the concrete adjacent adhesive layer was torn and stripped were obtained, the interfacial shear strength of double-shear specimens is1.09~1.61MPa, and the strengthening effects of AASCM are comparable to that of conventional epoxy resin adhesive.20concrete prism specimens in size of160mm×160mm×1000mm were strengthened on two opposite sides with70mm width,120mm to300mm length carbon fiber strips, and the carbon fiber strips of adjacent specimens had20mm length difference. By the bond anchorage properties tests, the distribution of shear stress between carbon fiber sheets and concrete at all levels of loading and effective bond length were measured, the bond lengths during carbon fiber sheets are pulled off at the same time when the concrete is torn and stripped were obtained (ie anchorage length), the calculated formulas of effective bond length and anchorage length were obtained by fitting. The effects of failure load, the effects of axial stiffness of carbon fiber sheets bfEftf and the effects of loaded end slip were considered in calculated formula of effective bond length. The effects of tensile strength and calculated thickness of carbon fiber sheets, and the effects of bond stress between carbon fiber sheets and concrete were considered in calculated formula of anchorage length.（4）In order to investigate the mechanical properties of AASCM at high temperature and after high temperature, the compression tests about96cement-mortar specimens in size of40mm×40mm×160mm and96cubic specimens in size of70.7mm×70.7mm×70.7mm were completed at100℃~800℃and after100℃~800℃, the flexural tests about96cement-mortar specimens in size of40mm×40mm×160mm were completed at100℃~800℃and after100℃~800℃high temperature, the tensile tests about96dumbbell specimens were completed at100℃~800℃and after100℃~800℃high temperature. Experimental results show that the compressive strengths of cement-mortar specimens at600℃and after600℃are81.5%and103.5%of compressive strength at room temperature, respectively; the compressive strengths of cubic specimens at600℃and after600℃are85.2%and105.5%of compressive strength at room temperature, respectively; the flexural strengths of specimens at600℃and after600℃are44.6%and52.5%of flexural strength at room temperature, respectively; the tensile strengths of dumbbell specimens at600℃and after600℃are40.1%and48.3%of tensile strength at room temperature, respectively; the compressive strengths of cubic specimens at800℃and after800℃are41.3%and66.7%of compressive strength at room temperature, respectively. The AASCM and Ordinary Portland Cement (OPC) were prepared in same size of70.7mm×70.7mm×70.7mm at same curing conditions, the compressive strengths of cubic specimens of OPC at800℃and after800℃are41.3%and66.7%of compressive strength of OPC at room temperature, respectively. It is proven that AASCM has superior high-temperature resistance than OPC. By regression analysis, the calculated formulas about compressive strength, flexural strength and tensile strength and other mechanical indexes variation with temperature were fit. It is seen that the compressive strengths of cement-mortar specimens and cubic specimens decrease with increasing temperature at20℃~200℃, the compressive strengths rebound at200℃~500℃, the compressive strengths decrease with increasing temperature at500℃~800℃again. The compressive strengths of cement-mortar specimens and cubic specimens increase with increasing temperature at20℃~400℃, the compressive strengths decrease with increasing temperature at400℃~800℃. The tensile strengths of dumbbell specimens and the flexural strengths of specimens in size of40mm×40mm×160mm decrease with increasing temperature at100℃~800℃and after100℃~800℃. Comparison analysis shows that the mechanical indexes of AASCM at high temperature are slightly lower than that of AASCM after high temperature. By using SEM and XRD analysis technique, it is revealed that the hydration product calcium silicate hydrate gradual decomposes and akermanite generates between600℃~800℃, and the phase composition translates from amorphous phase to crystalline phase, which is the root causes of mechanical properties of AASCM decline.（5）In order to investigate the bond-anchorage properties between carbon fiber sheets and concrete using AASCM as adhesive and anaerobic sealing layer at high temperature and after high temperature,20concrete specimens in size of160mm×160mm×1500mm were strengthened on two opposite sides with70mm width,225mm to400mm length carbon fiber strips, and the carbon fiber strips of adjacent specimens had25mm length difference. By double-shear tests at100℃~500℃, the bond failure (concrete is torn and stripped, or some concrete is torn and stripped and some adhesive layer slips) is shown but carbon fiber sheets are not pulled off, carbon fiber sheets are pulled off but bond failure is not shown, carbon fiber sheets are pulled off at the same time when the bond failure is shown and so on. The failure loads and anchorage lengths were measured, and the calculated formulas of anchorage length were obtained by fitting at high temperature. It is seen that the anchorage lengths of carbon fiber sheets increase with increasing temperature at20℃~100℃, the anchorage lengths of carbon fiber sheets decrease with increasing temperature at100℃~500℃. In order to investigate bond-anchorage properties after high temperature,20concrete prism specimens in size of160mm×160mm×1500mm were strengthened on two opposite sides with70mm width,200mm to340mm length carbon fiber strips, and the carbon fiber strips of adjacent specimens had20mm length difference; and20concrete prism specimens in size of160mm×160mm×1500mm were strengthened on two opposite sides with70mm width,350mm to500mm length carbon fiber strips, and the carbon fiber strips of adjacent specimens had25mm length difference. By double-shear tests after100℃~500℃, the bond failure is shown but carbon fiber sheets are not pulled off, carbon fiber sheets are pulled off but bond failure is not shown, carbon fiber sheets are pulled off at the same time when the bond failure is shown. The distribution of shear stress between carbon fiber sheets and concrete at all levels of loading and effective bond length were measured from first20specimens. The failure loads of related specimens, effective bond length and anchorage length measured values after high temperature were obtained. The calculated formulas of effective bond length and anchorage length were obtained by fitting after high temperature. |
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