Experimental Research On The Shrinkage Performance About New And Old Concrete Specimens

After maintenance, reinforcement and reconstruction, the existing concrete structure will always become new and old concrete composite structure. The uneven shrinkage resulting from the difference of age between new and old concretes make s the tensile strength of new concrete and the shear strength of the interface between new and old concretes worse. Mastering the development law of restrained shrinkage between new and old concretes is the premise to calculate the uneven shrinkage correctly. However, because of the complicated new and old concrete bondin g mechanism and shrinkage mechanism which is affected by many factors, it seems rare that people study th e influence of the specimen size and the interface treatment on new and old concrete restrained shrinkage at home and abroad.This thesis focuses on the restrained shrinkage of new and old composite concrete specimens with different specimen sizes and inte rface treatments. With the National Natural Science Fund project “Research on the basic theory of the existin g concrete bridge widening design”, the thesis involves mainly the following investigations and corresponding results :(1) With integration of researches at home and abroad, at the aspect of new and old concrete bonding mechanism, introduced types and development history of bonding models, discussed the interlayer force and failure mechanism between new and old concrete in detail, summarized signifi cant factors about bonding performance and proposed related measures to improve bonding performance; at the aspect of concrete shrinkage mechanism, summarized types and principal factors of concrete shrinkage, especially the drying shrinkage mechanism.(2) Through 180 days observation of shrinkage on 6 sets of new and old concrete composite specimens and 3 sets of free shrinkage specimens, the development of shrinkage with different specimen sizes and interface treatments were achieved. The results showed that: the restrained shrinkage strain values close to the free boundary of new concrete was 12% more than the free shrinkage strain values while the ones close to the interface of new concrete was 48% less, which ind icated the free boundary of new concrete were in press and the interface of new concrete were in tension; With the specimen size and the interface constraint increasing, the contrast of strain between different measuring points of composite specimens and free shrinkage specimens became larger; With the theoretical thickness of specimens increasing, both free shrinkage strain and restrained shrinkage strain decreased; With the interface constraint increasing, the strain value of new concrete close to free boundary became larger while the one close to interface became smaller, and the strain value of old concrete became larger.(3) On the basis of equivalent temperature method and age-adjusted effective modulus method, the restrained shrinkage finite model of new and old concrete with connection stif fness were built, the result of which were in good agreement with the test result. The parameter analysis on new and old concrete restrained shrinkage were conducted, and the results showed that: when the creep coefficient of new concrete increased from 0.5 to 3.0, the maximum tension stress of new concrete reduced 48.7%; When stiffness per unit area of the interface was less than 105kN·m-1/m2, the composite specimens were close to free shrinkage; With stiffness per unit area of the interface between 106kN·m-1/m2 and 108kN·m-1/m2, the stress changed substantially; With stiffness per unit area of the interface over 109 kN ·m-1/m2, the composite specimens were close to full bonding; With the strength ratio between new and old concrete increased from 0.6 to 1.2, the maximum tension stress of old concrete reduced 25% and the maximum shear stress of the interface reduced 17%.