Experimental Study On Axial Compression Performance Of Hybrid Fiber Reinforced Lightweight Aggregate Concrete Confined By Spiral Stirrups

Lightweight aggregate concrete(LWAC)has the advantages of light weight,high strength,and excellent good heat preservation,while it exhibits higher brittleness compared with the normal weight concrete.The brittleness of LWAC can be effectively improved by fibers and stirrups.At present,one of the most widely used single fibers is steel fiber(SF),and the hybrid fbers are steel-polypropylene fiber(SF-PPF).But SF will increase the weight of LWAC and cannot reflect the lightness of LWAC.Mixing carbon fiber(CF)and PPF into LWAC is a reasonable way to maintain the advantages and improve the performance of LWAC.The stress-strain relationship model of fiber reinforced LWAC confined by stirrups needs to be studied.Hence,the axial compression performance of hybrid fiber reinforced LWAC under unconfined and confined by spiral stirrups was studied in this paper,and the main contents were as follows:1.9 groups of fiber reinforced LWAC with different volumetric contents(0%、0.2% and0.4%)of CF and PPF were tested under axial compression.The influences of fiber types and contents on the characteristic points of stress-strain curves,toughness and hybrid effects were studied.The results showed that the initial elastic modulus of fiber reinforced LWAC were reduced by about 1.3%～18.1%,but the peak strain and ultimate strain of fiber reinforced LWAC were increased by 1.9%～54.8% and 29.2%～122.8%,respectively.The degree of influences of different fibers on the toughness improvement of LWAC was CF-PPF>CF>PPF.When the volume contents of CF and PPF were both of 0.4%,the improvment of fibers to the toughness of LWAC was better.2.18 hybrid fiber reinforced LWAC columns confined by spiral stirrups were tested under axial compression.The failure forms,peak loads,and characteristic points of the stress-strain curves of the columns were studied were studied with different stirrup spacings(30mm and 45mm),fiber types(CF and PPF)and fiber volume contents(0%、0.2% and 0.4%).The relationship between steel strain and axial strain was discussed.According to the test results,the axial compression bearing capacity predicted by various standards were evaluated.The results showed that reducing the spacing of stirrups could increase the bearing capacity of the specimen by about 0.21%～6.1%.Fibers have significant influence on the peak stress,peak strain and post-peak ductility of confined LWAC.Compared with the specimens without fiber,the increases of the ductility index for single fiber specimens and hybrid fiber specimens were ranged from 6% to 67.3% and 52.7% to 488%,respectively.When the contents of CF and PPF were the same,the ductility indexes of columns reached the maximum.When the content of CF increased,the axial strain of the confined concrete corresponding to the yield point of the stirrup was advanced,and it was less affected by the volumetric content of PPF.The bearing capacity predicted by GB 50010-2010 was more accurate than those by JGJ 12-2006 and ACI 318-14.It was suggested that the contributions of spiral stirrups should be reasonably considered in the design of axial compression members.3.Based on the spiral stirrups confined mechanism and the fiber toughening mechanism,the calculation methods of effective lateral confined force of stirrups and fibers were proposed.After selecting reasonable restraint efficiency indexes,according to the database with 105 axial compression performance tests and 18 tests in this paper,the calculation model of for peak points of confined fiber LWAC was established by regression analysis.Based on the fitting results of 4 typical models such as Mander model and Lim model predicting the stress-strain curve of the columns,3 types of ascending section and 4 types of descending section were selected.The key parameters and characteristic points of the curve were revised,and the axial compressive stress-strain relationship model suitable for fiber reinforced LWAC confined by spiral stirrups was established.Through this paper,it is of great significance to understand the axial compressive performance and constitutive relationship model of hybrid fiber reinforced LWAC columns confined by spiral stirrups,and provides theoretical reference for the engineering design and application of fiber reinforced LWAC.