Research On Compression Constitutive Relationship And Bond-Slip Properties Of Recycled Concrete

As for recycled concrete is a new type of green building materials, there are still many properties needed to study before it’s applied in structural engineering practice. Therefore, on the basis of experimental and theoretical method, the compression constitutive relationship and bond-slip properties of recycled concrete were studied in this thesis. The main contents are following:Firstly, the magnifying glass and SEM were used for observation of composition and microstructure morphology for recycled aggregate concrete from mesoscopic and microscopic level, and the ABAQUS finite element software was used for stress simulation of recycled aggregate concrete. The recycled concrete’s composition structure, interface and loading mechanism were analysed. Through the observation and analysis found that there were much differences between recycled concrete and ordinary concrete, the ITZ of aggregate-new mortar and new-old mortar existed obvious cracks before its loaded. After loading, the crack occurs first in the old interface, then appears in the new interface, and develops to the old mortar, finally the cracks connected, so it’s necessary to study the properties of recycled concrete.Secondly, the whole stress-strain curve test under uniaxial and conventional triaxial stress were conducted, and found that with the increase of recycled concrete replacement rate, the peak strain increase but peak stress and initial tangent modulus decrease gradually under uniaxial stress. In the conventional triaxial stress state, recycled coarse aggregate and ordinary aggregate damage pattern vary greatly, with the increase of confining pressure, the stress-strain curve descending segment was more smooth,the peak stress and peak strain of recycled concrete increases linearly, and its growth rate higher than that of normal concrete.The compressive constitutive relations of recycled concrete under uniaxial and conventional three axis state were established according to the characteristics of stress-strain curves.The compression strain energy density and brittleness index of two strength of recycled aggregate concrete under different confining pressure were calculated, the results showed that, with the increase of confining pressure, the two kind of strength of recycled aggregate concrete brittleness indexs decreased, when the confining pressure reached at about8MPa, its brittleness index achieve the same and remain unchanged.Thirdly, bond-slip properties of reinforced recycled concrete were studied which contains the corrosion condition on the based of the inner-embedded strain-gauge method and electrochemical principle, and the corresponding bond-slip constitutive relationship was established as τ-ψ(x)g(s).Through the study found that:With the increased of recycled aggregate replacement rate, the peak slip and the peak strength of bond-slip curve decreased. And the bond strength between round steel and recycled concrete was about1/4～1/7of the strength between screw steel and recycled concrete. Compared to ordinary reinforced concrete, the bond stress of reinforced recycled concrete transfered faster,and its distribution was more homogeneous, the growth of bond stress in the free end was greater. We presented three lines of bond position function model combined with the Characteristic of reinforced recycled concrete position curve, and g(s) was expressed by piecewise function.The average bond stiffness between corroded steel bar and recycled concrete degraded significantly, but once the recycled concrete cracking, as the corrosion rate and recycled aggregate replacement increased, the average bond stiffness decline rate was not obvious. The bond position function curv of corroded reinforced recycled concrete was difference to ordinary reinforced recycled concrete, and the bond stiffness of anchorage posterior segment increased gradually and exceed the front stiffness, then reach the peak, but the three line model is still valid.Then,12reinforced recycled concrete beam tests were completed which were used to study the bond-slip performance in the actual bending and shear stress state.The test results showed that,the failure modes and bond stress distribution curve of corroded reinforced recycled concrete beams were similar to reinforced recycled concrete and ordinary reinforced concrete beams, but the bond strength of the beam test were less than the simple drawing test,and stirrup played an important role on the anchorage position function of recycled concrete. The ultimate bond strength of reinforced recycled aggregate concrete beams overall declined with the increased of recycled aggregate replacement and corrosion rate.Finally, according to the anchoring reliability design method and principle of statistics, based on the statistics of related experimental data, this thesis calculated the anchorage length of reinforced recycled concrete, and proposed the corresponding suggestions for anchorage length design.