Research On The Effect Of Steel Fibers On The Properties Of Internal Curing (SAP) Concrete

Internal curing concrete is the addition of a water-absorbing and water-retaining material to concrete.When the material is dispersed inside the concrete,internal curing of the concrete occurs,preventing rapid loss of internal humidity and maintaining fully hydrated concrete.Since the material has good water retention,it can slowly release the absorbed water into the concrete,thereby effectively improving the early shrinkage of the concrete.However,the incorporation of internal curing materials also leads to certain defects,such as poor frost resistance,poor toughness,and tiny holes left after the internal curing materials release water,and the northwest region is also an earthquake-prone area.Therefore,it is necessary to improve the defects of internal curing concrete and study its dynamic mechanical properties.Studies have shown that the incorporation of steel fibers into concrete can have a good effect on the improvement of concrete performance.Therefore,in this paper,steel fibers are added to the internal curing concrete at the dosage of 0,0.5%,1.0%,1.5%and 2.0%to study the influence of the steel fiber on the working performance,static and dynamic mechanical properties and frost resistance of the internal curing concrete.Combined with nuclear magnetic resonance test,scanning electron microscope test and thermogravimetric test,the influence mechanism was explored from the microscopic level.The main research results of this paper are as follows:(1)The effect of steel fiber content on the slump of internally cured concrete was analyzed.The slump of ordinary concrete and internal curing concrete with different fiber content were tested respectively.The research shows that the addition of internal curing material(SAP)will reduce the slump of ordinary concrete,and the slump value of internal curing concrete decreases with the increase of steel fiber content.(2)The effects of steel fiber content and curing age on the static mechanical properties of internally cured concrete were studied.Based on four-point flexural test and uniaxial compressive test,the effects of age and fiber content on the compressive strength and flexural toughness of internally cured concrete were analyzed.The research shows that the uniaxial compressive strength and flexural toughness of the internally cured concrete at the same age first increase and then decrease with the increase of the steel fiber content.For internal curing concrete with the same fiber content,the axial compressive strength and flexural toughness increase with age.When the steel fiber content was 1.5%,the compressive strength and flexural toughness of the internally cured concrete specimens improved most obviously.(3)The influence rule of steel fiber content on the dynamic mechanical properties of internal curing concrete is revealed,and a prediction model of the peak compressive strength of steel fiber internal curing concrete is established according to the test results.The effect of steel fiber content on the dynamic compressive strength of internally cured concrete was studied under different strain rates(10-5/s,10-4/s,10-3/s and 10-2/s).The research shows that for the same fiber content,the peak strength and dynamic factor DIF both increase with the increase of strain rate.For the internally cured concrete at the same strain rate,the peak strength and dynamic factor DIF both increase first and then decrease with the increase of fiber content.At lower strain rates,the concrete specimens had sufficient time to deform and most cracks developed along the mortaraggregate bond.When the strain rate is large,the morphology of the failure surface changes,and the concrete does not have time to deform,and the failure surface directly passes through the coarse aggregate.The peak intensity and the dynamic factor DIF were fitted and found to increase linearly with the strain rate.(4)The effect of steel fiber content on the freeze-thaw damage of internal curing concrete was explored,and the freeze-thaw damage model of steel fiber internal curing concrete was proposed.First,the water absorption rate of the steel cured concrete was measured,and then the mass loss,relative dynamic modulus and compressive strength loss of the specimens under different freeze-thaw cycles were measured,and the appearance changes were observed.Finally,the change of relative dynamic modulus was used to fit the freeze-thaw damage degree of internally cured concrete.The research shows that the water absorption rate of internal curing concrete increases rapidly in the early stage and slows down in the later stage,and then gradually tends to be flat.The addition of steel fibers improves the apparent morphology of the internally cured concrete and reduces the mass loss rate,relative dynamic modulus loss and compressive strength loss.When the content of steel fiber is 1.5%,its frost resistance is optimal.Logarithmic function,linear function and quadratic function were used to fit the freeze-thaw damage,and then the fitted coefficient was used to obtain the relationship between it and the steel fiber content by Lagrangian interpolation function.Finally,the relationship between the number of freeze-thaw cycles n and the amount of steel fiber m on the freeze-thaw damage of internal curing concrete is obtained,which provides a certain reference for practical engineering.(5)The improvement mechanism of steel fiber inner curing concrete is revealed from the microscopic level.Scanning electron microscope was used to observe the microscopic morphology,nuclear magnetic resonance analyzer to analyze the internal pore structure of concrete,and thermogravimetric analyzer to study the hydration process.The research shows that more hydration products are attached to the surface of the steel fiber,which makes the cement matrix and the fiber more closely combined,and the hook shape of the fiber also increases the friction between it and the cement matrix,thereby enhancing the toughness of the internal curing concrete.The volume of internal pores increases gradually with the increase of freeze-thaw cycles,and the internal curing concrete increases faster than ordinary concrete.Compared with ordinary concrete cement paste,the internal curing material delays the hydration of cement and alleviates the carbonization of cement paste.