Experimental Study On Fractal Characteristics Of Crack Of Steel Fiber Reinforced Alkali-activated Slag Concrete Beams

Alkali-activated slag concrete is a new kind of building material formed by mixing slag and fly ash with coarse aggregate and fine aggregate excited by a strong alkali solution.Alkali-activated slag concrete not only has excellent characteristics such as early strength,high temperature resistance and low hydration heat,but also effectively utilizes industrial waste and reduces the consumption of cement products.However,due to the large drying shrinkage,Alkali-activated slag concrete is easy to crack during use,which limits its application in practical engineering.Steel fiber has excellent properties such as good ductility,high toughness,and tensile strength.When steel fiber is mixed into concrete,it not only plays the role of "secondary reinforcement",but also plays the role of bridging and cracking resistance.Based on this,steel fiber is mixed into alkali-activated slag concrete in this paper to improve its toughness.The influence law of steel fiber on cracking cracks of alkali-activated slag concrete beams is explored from three angles experiment,theory,and simulation.The main research contents and achievements are as follows:(1)Through the bending test of 8 groups of alkali-activated slag concrete with different steel fiber content,the distribution law of crack morphology,ultimate load,mid-span deflection,and maximum crack width of the test beams during a failure is analyzed.The results show that: With the increase of steel fiber content,the cracks on the surface of the test beam are obviously reduced,and the distribution pattern is more uniform.The height and width of the cracks are obviously inhibited by steel fiber.The addition of steel fiber significantly reduces the cracking load of the test beam,and the ultimate load is increased,but the increased range is small.(2)The short-term stiffness changes of the test beam before and after damage are used to reflect the damage degree of the test beam,the damage variables of the test beam during bending are calculated,the short-term stiffness test values and theoretical values are compared,and the existing standard formula is used to calculate the longitudinal tensile reinforcement strain non-uniformity coefficient between cracks.It is concluded that under the same load,with the increase of steel fiber content,the short-term stiffness of the test beam gradually increases,the damage variable gradually decreases,and the strain non-uniformity coefficient of the tensile steel bar at the crack gradually decreases.The mechanism is that the steel fiber at the crack and the longitudinal tensile steel bar share the tensile force,which increases the stiffness of the test beam and reduces the strain unevenness of the steel bar at the crack section.(3)The fractal theory is introduced to explore the influence of steel fiber content on the cracking of alkali-activated slag concrete beams,and the fractal dimension is used to quantify the crack distribution pattern of the test beams.and the fractal dimension was used to quantify the crack distribution of the beams.The results showed that the fractal dimension of the eight groups of test beams was between 1.024 and 1.291.Under the same load,the larger the steel fiber content was,the smaller the fractal dimension was.The mathematical relationship between fractal dimension and test beam load,mid-span deflection,and maximum crack width is established.It is found that the fractal dimension increases linearly,quadratically,and logarithmically with load,mid-span deflection,and maximum crack width,respectively.(4)Based on the plastic damage constitutive model of concrete,the bending process of steel fiber reinforced alkali-activated slag concrete beams is numerically simulated by Abaqus software.The results show that the error between the simulated values of cracking load and ultimate load of the member and the experimental values is small,and the development of the load-deflection curve is basically consistent with that obtained from the experiment.The development of cracks is similar to that of the experimental cracks by using the tensile damage and stiffness reduction rate.