Experimental Study And Nonlinear Analysis Methods Of Bond-retarded Prestressed Systems

Combining the advantages of both the bonded and non-bonded post-tensioned presstressed systems through a special set-retarded material, the bond-retarded prestressed system is really a renovation of the post-tensioned prestressed system. According to the set-retarded materials, the bond-retarded prestressed systems can be classified as two types, i.e., mortar type and coating type. With a systematical introduction of the status quo of the bond-retarded and non-bonded prestressed concrete systems, the author develops a series of studies in four aspects, i.e., the physical and mechanical characteristics of super-duty retarded mortar, the pull out friction, the static characteristics of structural members and the nonlinear analysis methods, for the mortar type bond-retarded prestressed concrete systems.The first aspect deals with the physical and mechanical characteristics of the super-duty retarded mortar, which is classed as type A, B, and C according to its mixing ratio. On the basis of experiments, the variations of the main physical and mechanical characteristics such as flow feature, plasticity index, compressive strength, fracture strength, elastic module with the age are studied respectively in the low temperature zone(5℃～7℃), intermediate temperature zone (16℃～22℃) and high temperature zone (≥35℃). Meanwhile, according to electrochemical analysis and long-period experimental observation, it is concluded that set-retarded mortar is not corrosive to prestressed steel bars embedded in it.The second aspect of the paper describes the study on the pull out friction. The bond-retarded tendons made of plain steel bars and ribbed steel bars, respectively are stretched under different age. For the tendons made of plain steel bars, the pull out friction keeps comparatively steady with the unit static friction being about 3.0～4.5 N/cm~2, and the unit slipping friction being around 1.77～3.5 N/cm~2, when the age doesn't exceed 30d,while the pull out friction increases dramatically, when the age is over 30d and can reach 145N/cm~2,when the age is over 60d. Similarly, for the tendons made of ribbed steel bars, the pull out friction is between 2.47～4.5N/cm~2, when the age is less than 30d, while it would also increase sharply when the age is larger than 30d. Through comparing the stretching frictions of the two types of tendons, it is concluded that the unit pull out friction of plain steel bar is nearly equal to that of ribbed steel bar and the pull out friction is not much correlative with the bars' surface shape of the bars ,which provide basis for the calculation of pull out friction of the new bond-retarded prestressed concrete systems.The third aspect is about the static characteristics of structural members. A group of tests were conducted on 20 flexural members, among which are 16 bond-retarded prestressed ones, 2 bonded prestressed ones, 2 non-bonded prestressed ones and the results were compared to conventional prestressed members with bonded post-tensioning and non-bonded post-tensioning in cracking moment, ultimate moment, cracking forms and load-deflection curves, etc. A conclusion is reached that after a 4-months the drawing, the bond-retarded prestressed members has almost the same working performances as that of the bonded post-tensioned members, which provides an important reference for the design of this bond-retarded prestressed system.The fourth aspect focuses on nonlinear analysis. First of all, the post-tensioning prestressed concrete beam is regarded as a composite beam which consists of prestressed steel bars and reinforced concrete beam, with bond-slip considered. A nonlinear analysis model which can be applied to non-bonded, bond-retarded and bonded post-tensioning prestressed concrete beam is drawn. According to the bond-slip constitutive law for prestressed steel bars and reinforced concrete at the interface, a nonlinear differential equation for the beam, expressed in the form of the tensile force of prestressed steel bars, is derived. Moreover, A nonlinear analytical solution and a section coordination coefficient are obtained under the loading condition of double symmetric concentrative load, uniformly-distributed load and single concentrative load. A practical nonlinear method for the post-tensioning prestressed beam is established and several influence factors which can affect the coordination coefficient of the beam are discussed. Furthermore, with the homogeneous solution to the differential equation expressed in the form of deflection taken as the element shape function of post-tensioning prestressed beam, the element stiffness matrix and equivalent node loads are given which integrate prestressed steel bars, concrete and bond stiffness together. A nonlinear finite element analysis model of post-tensioning prestressed concrete system is established, which provides an effective tool to perform nonlinear analysis of complicated systems which is madeup of post-tensioning prestressed concrete beams(non-bonded, bond-retarded andbonded).