Research On Mechanical Performance Of RPC Ribbed Bottom Panel And Composite Slab

Concrete composite slab with precast opened-rib panel is a kind of semi-precast slab,which used invertible T-shaped panel as formwork.Following the assembly of the panels,transverse reinforcements are arranged in the openings and the composite slab is formed after cast-in-place concrete,which is poured and hardens.This kind of composite slab has advantages such as faster speed in construction compared with cast-in-place slab and better integrity in comparison with precast slab,which has led to their widespread use in engineering construction.Because the flange of panel is thin,the number of prestressing tendons is limited in order to prevent nonlinear creep of concrete.Generally,the applicable span of this kind of composite slab is no more than 6.6m,therefore,it is difficult to satisfy large span composite floor construction in modern buildings.Reactive Powder Concrete?RPC?is a kind of super high strength,excellent durability and volume stability cementitious composite material which is heated curing by mixed cement,silica fume,quartz sand,fibers and water in a certain proportion.If the opened-rib panel is prefabricated by RPC,it can be reinforced with more prestressing tendons in panel flange,and will be beneficial to apply in the large span composite floor of modern buildings.In this paper,the following work from five key aspects work have been completed.?1?Collecting test data of RPC mechanical properties and proposing the method of classifying strength grade of RPC.The mechanical indexes of each strength grade and the material partial factor were obtained through statistical analysis.These results will provide evidence for the design of RPC precast opened-rib panel.?2?Precast opened-rib RPC panel should meet the requirements of crack and deflection control during hoisting and transportation,in addition,it should also meet the requirement of bearing capacity while pouring composite layer concrete.For these reasons,the mechanical properties test of 4 precast opened-rib panels was completed.The flexural crack strain of RPC is much higher than that of ordinary concrete,therefore,the tensile stress in longitudinal tensile bar increased evidently.Based on cracking moment test value and M_cr=(?f_tR+?_pc)W₀,the calculation formula of plastic influence coefficient of section modulus,which consider the influence of longitudinal tensile bar was established.Based on the panels deflection test value and the relationship of bending moment and curvature,the calculation formula of flexural stiffness which consider the influence of elastic modulus reduction coefficient for cracked section of precast opened-rib RPC panel's flange was established.According to panels cross section bearing capacity test value and force equilibrium equation,the value of tensile stress equivalent coefficient for precast opened-rib RPC panel's flange was proposed.In addition,based on the test value of crack width at different load levels(M_cr<M<0.8M_u),this paper established the crack width calculation formula which used nominal tensile stress at the bottom of panel as independent variables.?3?The test of 4 full size concrete composite slabs with precast opened-rib RPC panels was completed.Based on M_cr=(?f_tR +?_pc)W.and cracking moment test value,the calculation formula of plastic influence coefficient of section modulus,which consider the influence of longitudinal tensile bar was established.Based on the test value of strain along depth of slabs and stress analysis of cross section,the value taking method of RPC elastic modulus reduction coefficient,which consider the influence of steel fiber tensile stress in the cracked section of precast opened-rib RPC panel's flange was proposed.According to composite slabs cross section bearing capacity test value and force equilibrium equation,this paper proposed the taking value method of tensile stress equivalent coefficient of RPC panel flange.This method will provide evidence for considering the influence of RPC panel flange steel fiber tensile stress to cross section bearing capacity.Finally,according to crack width test value at different load levels(M_cr<M<0.68M_u),this paper established the calculation formula of crack width which used equivalent tensile stress of prestressing tendons,cover thickness,the area of concrete confined by prestressing tendons and the distance between neutral axis and prestressing tendons as independent variables.?4?The test of 1 simply supported on four sides two-way concrete composite slab?calculation span is 5.8m×5.8m?with precast opened-rib RPC panels was completed.The results showed that,this kind of two-way composite slab had good integrity in two different directions;the ultimate load can be well calculated according to yield line distribution combined with principle of virtual work.Based on the test value of crack width at different load levels(P_cr<P<0.65P_u),the calculation formulation of crack width which consider key parameters was established.The moment of inertia at prestressing tendons direction can be calculated by the method in section 5.6.1.The flexural stiffness at the direction which perpendicular to prestressing tendons equals to that of composite concrete layer at the seam of RPC panels.Then,the deflection of this kind of two-way composite slab can be calculated by two-way slab strip superposition method.?5?In the process of 4 one-way composite slabs,1 simply supported on four sides two-way concrete composite slab,32 Cold-Rolled Ribbed Steel Bar?CRB?concrete slabs?all slabs meet minimum reinforcement ratio requirement?tests,prestressing tendons/CRB in most slabs were tensile failured while the concrete at compression zone was not crushed.The failured tensile strain of steel bars in slabs were lower than that in direct tensile test,their ratio was 0.21?0.45.This phenomenon indicated that tensile-bend abruption strain was less than that in direct tensile test.Test results showed that the tensile-bend abruption strain of prestressing tendons/CRB increased with the increased of reinforcement index.Based on the tensile-bend abruption strain test data of 4 one-way composite slabs and 32 CRB concrete slabs,the equation of tensile-bend abruption strain and reinforcement index was established.This equation provide the method to predict the ultimate deflection of this type of slabs.