Study On Bond Theory And Test Of New And Old Concrete And Its Application In Bridge Strengthening Engineering

The issue of bond between new and old concrete exists widely in the work of repair and strengthening engineering of new or existing concrete structure. With the application of new material and new technique to the practice of structure repair by enlarging section method, the technique of embedded steel bar in the interface of new and old concrete is used more and more popularly. In particular, in the reconstruction or reinforcement of unsafe old bridge, technique of embedded steel bar is applied widely in order to make new and old concrete whole and bear the load together perfectly. However, research either domestic or overseas about bond property, force transfer mechanism and bond collapse mechanism of the interface with embedded steel bars is reported lightly. When the embedded depth of the bar is small for the limit of the original structure dimension, the study about the computation of the bond shear stress and the shear design value is particularly seldom performed. Considering all of above, combined the subsection of 'the study of the repair and strengthening method for existing bridge' with the sustentation fund of Ministry of Education, the bond characteristic, failure mechanism and shear strength computation of interface between new and old concrete and their applications in bridge strengthening are studied through the model test with embedded bar or not. The research works are mainly concerned with the following aspects:Firstly, considering different influence factors, the full-range shear experiment of 6 groups with 18 Z-shape specimens is executed. The bond shear capacity of interface is studied as well as the degree of influence such as interface roughness, agent, strength of new and old concrete, the depth of embedded bar. Also, the failure mode of the interface is examined. Stress-strain variation of embedded bar is studied and the stress-strain relationship curve is obtained and analyzed consequently.Secondly, Five Zone-Three Layer bond model of new and old concrete is proposed based on the interface shear test, which could apply to analysis of bond mechanism either with embedded bar or without. Except the bond and the failure mechanism, the force transfer mechanism of the interface with embedded bars is analyzed. Furthermore, distribution regularity of the bond shear stress and tension stress is studied between embedded bar and bonding agent and old concrete. It is founded that the shear and the tension stress are not uniform and there is a stress peak value along the depth of embedded bar. In this section, the anchoring and failure mechanism of embedded bar system of existing concrete is defined and mechanism of various factors influencing the bond capacity of new and old concrete is analyzed.Thirdly, the interface ultimate shear capacity of new and old concrete is studied based on the model test results and the interface failure mechanism research result. The practical formula of interface shear strength is proposed with the roughness between 1.8mm and 3.0mm, which is consistent with model test results. By the practical formula proposed, the interface ultimate shear capacity and shear strength could be computed and evaluated.Fourthly, for the strengthening structural member by enlarging section method, the interface shear design value under the external load is studied according to the limit state method. Then, how to determine the shear design values for different structural member and different strengthening location is given out, which could be the reference for design and computation of strengthening structural member by enlarging section method.Fifthly, Full-range shear experiment of two reinforcement concrete arch specimens with strengthening at soffit location by enlarging section method are designed and conducted. The interface bond capability and the failure mode of soffit strengthening are studied, as well as the influence of strengthening concrete technique to the interface bond capability. The cracked load, the failure load, the tension stiffness, the stress of the steel bar and the cracks of the strengthening arch are compared with those of the contrast arch. The bond condition of the new and old concrete in practice bridge strengthening is simulated and analyzed by the experiment in this section, which provide the proper construction technique and method practically for repair and strengthening works of existed bridges.Finally, combining the engineering practice of repair and strengthening works of two bridges, the application of reinforcement new concrete to the bridge repair is studied. By the research result of the test and the theoretic analysis, the construction technique of strengthening is consummated and the interface shear of the new and old concrete of two bridges is computed. Through the load test for the bridges after strengthening, the interface bond capacity under test load is analyzed. It is shown that the research results of the thesis could apply to shear computation and interface arrangement of new and old concrete in bridge strengthening, and consequently, bonding the new and old concrete together perfectly could make the repair and strengthening works of the existed bridge achieve the expectant results.