Experimental Study On Static And Dynamic Mechanical Properties Of Hybrid PP/POM Steel Fiber UHPC

Concrete has incomparable advantages over other building materials,but it also has obvious defects.Since the 20 th century,with the acceleration of modernization,all kinds of super high-rise buildings and long-span structures have appeared.The performance of ordinary concrete cannot meet the actual needs,which makes the concrete must develop towards higher strength and better working performance.UHPC has higher compressive strength and can meet the actual strength requirements of engineering.However,the high strength of UHPC also brings the risk of brittle failure.Therefore,fiber is added into UHPC to improve the tensile strength and toughness of UHPC by using fiber reinforcement method.In this thesis,the influence of the amount of hp3-fiber on the static mechanical properties and dynamic tensile properties of steel with different volume of hp3-fiber is analyzed.This thesis can draw the following conclusions:（1）Based on 8 groups of 24 cylindrical specimens with ?100mm×200mm,the effects of different fiber types and contents on the static axial compressive mechanical properties of hybrid PP / POM steel fiber UHPC are discussed and analyzed.Among them,in terms of the working performance of each group of specimens,the fluidity of UHPC matrix without fiber is 195 mm,which is significantly reduced whether it is single doped steel fiber or hybrid fiber.Compared with the mixture ratio of single fiber,the mixture ratio reinforced by hybrid fiber has better fluidity.The ratio of hybrid PP / POM steel fiber has an obvious effect on the axial compressive strength of UHPC.The axial compressive strength of 1% POMF-2% SF specimen reaches 154.4MPa,while that of 3% POMF specimen is only 125.3MPa.When the volume content substitution rate of steel fiber increases from 1% to 2%,the axial compressive strength enhances by 9.4% and 23.2% respectively compared with that of 3% POMF specimen.The axial compressive strength of UHPC matrix without fiber is 115.2MPa,while in the fiber with 3% volume content,the axial compressive strength of the specimens from large to small is: 3% SF > 3% POMF > 3% PPF.Taking the elastic modulus of hybrid POM steel fiber specimen as an example,the elastic modulus of 1% POMF-2% SF is 46.27 GPa,while that of3% POMF is 42.72 GPa.When the substitution rate of steel fiber increases from 1% to 2%,the elastic modulus is increased by 1.6% and 8.3% respectively compared with that of 3% POMF.The Poisson’s ratio of matrix is 0.19,while the Poisson’s ratio of hybrid fiber PP / POM steel fiber is basically maintained at the Poisson’s ratio of 0.2.The addition of fiber can improve the Poisson’s ratio of matrix,with little increase and basically the same.（2）Through a total of 24 for 8 groups 330mm×60mm×13mm dumbbell shaped specimen,aiming at hybrid PP / POM steel fiber UHPC,the effects of different fiber types and contents on the static axial tensile properties were discussed and analyzed.The research shows that after most of the specimens reach the cracking stress,due to the good fiber bridging ability and interface bonding resistance of the hybrid fiber,the tensile stress increases continuously,and there is an obvious phenomenon of strain hardening until the peak point,and then enter the softening stage.The tensile strength of PP / POM steel fiber hybrid increases with the enhance of steel fiber substitution ratio,showing a positive correlation trend.In the influence of hybrid fiber PP / POM steel fiber types on the tensile strength of UHPC,the tensile strength of POM steel fiber hybrid is better than that of PP steel fiber hybrid and single blending under two different proportions,even when the volume content of POMF is 3%.POM steel fiber hybrid is better than PP steel fiber hybrid.POM steel fiber hybrid can more effectively enhance the tensile properties and crack resistance of concrete matrix.In addition,the ultimate tensile strength can be improved by adding fiber into UHPC matrix.In terms of toughness,the toughness of 2% PPF-1% SF and 2% POMF-1% SF specimens is the largest,which is 117.7%and 103.8% higher than that of 3% PPF and 3% POMF specimens respectively.The hybrid of POM steel fiber and even POMF with a single volume content of 3% are better than that of PP steel fiber in terms of tensile toughness.Compared with the UHPC matrix without fiber,the strain capacity of UHPC matrix can be improved more effectively.（3）Based on 8 groups of 80 round cake specimens with ?100mm×50mm,the effects of different fiber types and content on dynamic impact mechanical properties of hybrid PP / POM steel fiber UHPC are discussed and analyzed.The stress-strain curves of each group show an obvious linear relationship before reaching the peak stress.The stress increases with the enhance of strain and has an obvious elastic stage.When reaching the peak stress,the curve enters the softening stage.In terms of dynamic axial compressive strength,the dynamic axial compressive strength increases with the enhance of strain rate,and presents a certain linear law,which further shows that the dynamic axial compressive strength increases with the increase of strain rate,and the specimen has an obvious sensitive effect on strain rate.In addition,the dynamic axial compressive strength after adding fiber is significantly higher than that without adding fiber.The impact toughness of UHPC specimens in each group has a similar law with the dynamic axial compressive strength.The dynamic impact toughness increases with the enhance of strain rate,and shows an linear growth regular.The impact toughness of the matrix is obviously enhanced by adding fiber,and the dynamic impact toughness of adding fiber is obviously better than that of not adding fiber.Among the specimens mixed with PP / POM steel fiber,the dynamic growth factor（DIF）of the specimen mixed with 1% SF is the largest under the similar strain rate;Among the specimens mixed with 1% SF or 2% SF,the dynamic growth factor of the specimen mixed with PP fiber is better.However,on the whole,the slope of the fitting curve of dynamic enhancement factors with different mix proportions is basically unchanged,which shows that the strain rate effect of hybrid PP/POM-steel fiber has no obvious inhibitory effect.