| Different basalt fiber composite materials(e.g.chopped fiber,fiber textile and fiber reinforced polymer(FRP)bar)demonstrate good salty corrosion resistance,and they can avoid the corrosion problem like steel bars and ensure the good corrosion resistance of the reinforcements in the vicinity of the crack,thus improving the durability of concrete structures.However,there are still some shortcomings rergarding the basalt fiber:i)The concrete matrix exhibits a strong alkaline environment(pH>12.5),and the OH-will break the Si-O-Si/Al-OAl linkages of basalt fiber;ii)Basalt fiber is a linear-elastic material,and a brittle failure may occur when the tensile strength is reached;iii)Traditional chopped basalt fiber has the small diameter(<20 μm)and low section stiffness,which has an adverse effect on the workability of fresh concrete;iv)Fiber textile is mainly used as a non-structural material,and the utilization rate of the textile strength is low,thus leading to the serious waste of its excellent mechanical properties.In oreder to extend the application of basalt fiber in concrete structure,a systematic study was carried out on the alkali resistance of basalt fiber and the effect of the fiber composites on the mechanical properties of the concrete.This work is supported by the National Natural Science Foundation of China(No.51578109)and the Science and Technology Innovation Fund of Dalian(No.2020JJ27SN108).The main contents and conclusions of this work are as follows:(1)An alkali corrosion test was carried out to study the alkali resistance of basalt fiber through the five aspects of micro morphology,elemental composition,molecular structure,alkali corrosion mechanism and alkali corrosion model and to analyse the alkali resistance of the fiber textile by means of micro morphology and tensile breaking strength retention ratio.Then the accelerated aging test and flexurl test regarding basalt fiber textile reinforced concrete(BFTRC)slabs were conducted to explore the strengthening and toughening effect of the textile after corrosion in alkaline matrix on concrete.The results show that the alkali resistance of basalt fiber/textile is comparable to that of AR-glass fiber/textile(14.5wt%ZrO2).For the basalt fiber after etching in NaOH solution,the diameter becomes smaller,the typical structure is composed of fiber core,gel layer and deposition layer,and the dissolution kinetics can be expressed by zero-order model and contracting cylinder mode.Aging coefficient(Ca)and enhancement coefficient(Ce)were introduced to evaluate the strengthening and toughening effect of fiber textile after alkali corrosion on concrete.It is found that the Ca and Ce of flexural strength and energy absorption of BFTRC slabs are decreased with aging time,but the Ce is greater than 1.(2)A basalt minibar was invented based on the bond-slip mechanism of the fiber in concrete.The influence of the basalt minibar on the workability,compressive strength and flexural performance of concrete was studied.Traditional basalt fiber and steel fiber were studied as references.The results indicate that the maximum allowable amount of basalt minibar determined by the workability of fresh concrete is increased by 246%compared with that of the traditional basalt fiber,and as a result,the toughness of hardened concrete can be obviously improved.The positive effect of twisted minibar with fiber content of 30 kg/m3 on the toughness of concrete is equivalent to that of end-hooked steel fiber with fiber content of 20 kg/m3.(3)The uniaxial tensile test was carried out to study the influence of textile reinforcement ratio,prestress and chopped fiber on the tensile properties of BFTRC,and the enahancement mechanism was also analyzed.The results show that the prestress applied to the fiber textile aids in improving the cracking load,but it has no obvious effect on the ultimate load.Additionally,the ultimate strain is reduced.The hybrid use of chopped fiber and fiber textile has a positive effect on the cracking load,ultimate load and toughness.It can also be found that the tendency of the effective factor of k’ with textile reinforcement ratio approximately satisfies the power function model,and this model can be used to predict the k’ value of TRC with different textile reinforcement ratios.Taking the effective factor into account,the prediction formula for critical textile reinforcement ratio is proposed,and the obtained results calculated by this formula is in good agreement with the test results.The stiffness reduction coefficient expressed by exponential function was introduced into TRC nonlinear tensile constitutive model.The model can be used to similate the stiffness softening behavior of TRC in the stabilized cracking stage,and the peak stress and the peak strain can be also obtained.In addition,this model is little dependent on experimental data and has a wide applicability for precicting tensile behaviors of TRC.(4)A two-way slab test was conducted to study the influence of maco polypropylene(PP)fiber on load bearing capacity,toughness,failure pattern and internal force and stress redistribution of BFTRC slabs.The synergetic effect of basalt fiber textile and macro PP fiber on load bearing capacity and toughness was explored,and then the ultimate load bearing capacity of the slabs werecalculated by means of yield line theory and strip method.The results demonstrate that the addition of macro PP fiber may reduce the brittleness of BFTRC,improve the internal force and stress redistribution capacity,and enhance the load bearing capacity in the post-peak region.The combined use of fiber textile and macro PP fiber in concrete matrix demonstrates a positive synergetic effect on the load bearing capacity and toughness.The load bearing capacity and energy absorption of BFTRC1PP8 slab are 7%and 4.5%higher than those of RC slab with a steel ratio of 0.31%,respectivily.In addition,the ductility coefficient of BFTRC1PP8 is equivalent to that of RC(i.e.19.2).It means that the conventional steel mesh with a steel ratio of 0.31%can be replaced by the combination of one layer of basalt fiber textile and macro PP fiber with fiber content of no more than 8 kg/m3 in view of improving the biaxial flexural behavior of concrete.The differences between the ultimate load bearing capacity obtained from the yield line theory as well as strip method and the test value are-7.6%and 13%,respectively.It indicates that compared with the yield line theory,the strip method is more reliable in calculating the load bearing capacity of BFTRCPP slabs,and the results are conservative.(5)The self-developed impact test device was used to study the impact resistance of BFRP bar reinforced concrete.The influence of macro PP fiber and basalt minibar on firstcrack blows/failure blows,impact energy and failure modes was explored,the synergetic effect of BFRP bar and macro PP fiber/basalt minibar on impact energy was analyzed,and the impact resistance of concrete was statistically evaluated.The results demonstrate that macro PP fiber and basalt minibar can significantly improve the toughness of concrete.The combined use of BFRP bar and macro PP fiber/basalt minibar demonstrates a positive synergetic effect on impact energy.As a result,crack number,penetration depth and indent diameter of the shrived area are significantly increased,and the specimen exhibits a ductile failure pattern with multiple cracks.The impact energy of B-MPF7 is 7%higher than that of RC,and the crack number of B-MPF7 is increased by 1 compared with that of RC.Meanwhile,no circumferential crack appears for B-MPF7.It indicates thatthe hybrid use of BFRP bars with a reinforcement ratio of 1.9%and macro PP fiber with fiber content of no more than 7 kg/m3 offers an alternative substitute to the conventional steel rebar with the same reinforcement ratio in view of improving the impact resistance of concrete.The two-parameter Weibull distribution can be used to analyze the distribution of first-crack/failure blows of concrete and to estimate the failure blows corresponding to different failure probabilities. |
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