Study On Flexural And Fire Behaviors Of U-shaped Steel Beams Filled With Demolished Concrete Lumps And Fresh Concrete

It is indicated that the compound concrete made of fresh concrete(FC)and coarsely crushed demolished concrete lumps(DCLs)is an effective approach in recycling waste concrete.Some preliminary studies about the creep behaviors of the compound concrete,mechanical behaviors of the compound concrete at room temperature and after freeze-thaw cycles have been conducted by our research group.The mechanical behaviors(compressive,flexural and shear behaviors),seismic properties and fire performance of the structural members cast with the compound concrete have been also experimentally investigated.To further extend the species of such structural members,a new kind of structural member named U-shaped steel beams filled with DCLs and FC is presented,and studies on the flexural and fire behaviors of such beams has been carried out in this dissertation.The main work and conclusions include as follows:1.Twelve U-shaped steel beams filled with DCLs and FC were tested under sagging bending.The influences of the replacement ratio of DCLs,the thickness of U-shaped steel,and the longitudinal reinforcement ratio on the flexural behaviors of the specimens have been investigated.Simple calculation formula for the flexural capacity of the specimens has been presented,and a comparison between the flexural capacity of the specimens and that of conventional reinforced concrete beams with identical total steel weight is conducted.It can be found that:(1)the influences of the replacement ratio of DCLs within a range of 0~40%on the initial stiffness,ductility,flexural capacity,bond-slip trend,and strain developing of the specimens are very limited;(2)in comparison with the increasing of the thickness of the U-shaped steel,the adding of longitudinal bars at beam bottom is a more effective way to improve the flexural capacity of the U-shaped steel beams filled with DCLs and FC,and the longitudinal bars is helpful to enlarge the pre-yielding stiffness of the specimens to some extent;(3)The relative slip between the U-shaped steel and the in-filled concrete can be reduced significantly by adding longitudinal bars at the bottom of the beams,and in this case the end section of the beam is the cross section with the minimum slip;(4)A simple formula derived from the plastic theory can be used to predict the flexural capacity of such beams;and(5)the flexural capacity of such beams is almost the same as that of the conventional reinforced concrete beams with identical total steel weight.2.Five U-shaped steel beams filled with DCLs and FC were tested in fire.The effects of the replacement ratio of DCLs,the longitudinal reinforcement ratio,the thickness of fire insulation and the load ratio on the fire behaviors of the specimens were investigated.The value and trend of the temperature drop at the interface between the U-shaped steel and the in-filled concrete were revealed.It is found that:(1)using DCLs has a very limited effect on the fire behaviors(including failure process,temperature distribution,mid-span deflection,and fire resistance)of the specimens;(2)embedding longitudinal reinforcements can significantly increase the fire resistance of such beams;(3)the thermal resistance at the interface between the U-shaped steel and the in-filled concrete near the mid-height of the beam is greater than that at the bottom,and the interface thermal resistance can generate a temperature drop of up to 280°C;(4)with the increasing of the heating time,the temperature drop at the steel-concrete interface increases gradually at first,and then decreases slowly.The temperature drop decreases with increasing the thickness of the fire insulation.3.Finite-element models in which the thermal resistance at the interface between the U-shaped steel and the in-filled concrete is considered are developed by using SAFIR to simulate the fire tests in Chapter 3.The influences of the interface thermal resistance,the in-filled concrete and the method for determining the thermal conductivity of the fire insulation on the fire behaviors of the U-shaped steel beams filled with DCLs and FC have been investigated.It can be concluded that:(1)the finite-element models established in this paper are capable of predicting the temperature field,mid-span deflection,and fire resistance of such beams;(2)neglecting the interface thermal resistance between the U-shaped steel and the in-filled concrete will underestimate the temperature of the the U-shaped steel and overestimate the fire resistance of the beams.Therefore,the interface thermal resistance should be considered in the numerical simulation;The thermal resistance at the interface between the web of the U-shaped steel and the in-filled concrete is suggested to be 0.03 m~2K/W,and that at the interface between the bottom flange of the U-shaped steel and the in-filled concrete is recommended to be 0.02 m~2 K/W;and(3)due to the in-filled concrete,the rise in the temperature of the U-shaped steel has been remarkably slowed and the nonuniformity of temperature field increases.The in-filled concrete enlarges the stiffness of the beams at elevated temperatures,making the fire resistance significantly increases.4.A parametric study including 480 cases is carried out to reveal the influences of the thickness of fire insulation,load ratio,sectional dimensions of U-shaped steel,effective width and thickness of concrete slab,bottom reinforcement coefficient,strength grade of concrete,grades of U-shaped steel and longitudinal reinforcement on the fire resistance of the U-shaped steel beams filled with DCLs and FC,and a practical calculation formula is proposed to determine the fire endurance of such beams.It is found that:(1)the influences of the thickness of fire insulation,load ratio and bottom reinforcement coefficient on the beam's fire resistance are remarkable,and the effects of the sectional dimensions of U-shaped steel are moderate;(2)The effects of the width and height of U-shaped steel on the beam's fire resistance can considered simultaneously through one parameter;(3)the beam's fire resistance almost increases linearly with the increasing of the bottom reinforcement coefficient,but the slope of this linear relationship is affected by the thickness of fire insulation and load ratio;and(4)the practical calculation formula for the fire resistance of the U-shaped steel beams filled with DCLs and FC has better prediction accuracy,which can be used for fire design.5.Using the SAFIR software,the fire performances of the restrained U-shaped steel beams filled with DCLs and FC are investigated.The influences of some parameters(i.e.,axial and rotational restraint stiffness ratio,thickness of fire insulation,bending capacity ratio of beam end,bottom reinforcement coefficient,load ratio,sectional dimensions of U-shaped steel,etc.)on the axial deformation,axial load ratio,bending moment at the beam end and mid-span deflection are analyzed.Research results show that:(1)the fire behaviors of such beams can be significantly improved by adding rotational restraints;The beams with rotational restraints can satisfy the requirements of fire resistance class R120,even though the fire insulation is not applied;(2)with the increasing of the thickness of fire insulation,the peak values of the beam's axial deformation and axial load ratio increase,but the peak value of bending moment at the beam end gradually decreases;(3)in the earlier stage of heating,with the increasing of the bending capacity ratio of beam end,the bending moment at the beam end increase,but the axial deformation,axial load ratio and mid-span deflection of the beams decrease;in the later stage of heating,the opposite tendencies are obtained;(4)in the earlier stage of heating the axial deformation,axial load ratio and mid-span deflection of the beams reduce as the bottom reinforcement coefficient increases,but the opposite trends are acquired during the later stage of heating.Increasing the bottom reinforcement coefficient will enlarge the peak value of the bending moment at the beam end;(5)Increasing the width of the U-shaped steel will reduce the beam's axial deformation and axial load ratio evidently,and has little effect on the bending moment ratio at the beam end;as the thickness of the U-shaped steel rises,the axial deformation,axial load ratio and bending moment at the beam end increase clearly,but the bending moment ratio at the beam end changes little.