Study On Seismic Performances Of Recycled Aggregate Concrete Structures And Concrete Structure Damage Assessment

With the acceleration of urbanization, many such structures are near the end of their service lives and some are damaged by natural disasters in each year, therefore large amounts of construction waste is produced. Transporting waste concrete to landfills for burying, as a conventional disposal method, results in serious environment pollution and vast resource extravagance. On the other hand, the consumptions of concrete maintain rapid growth due to building industry is one of the pillar industries of economy. A lot of stone and rock resources are needed to produce the coarse aggregates in concrete, which can lead to many environmental issues like soil and water loss, vegetation deterioration and so on. Recycled aggregates (RAs) can be obtained by treating waste concrete with the following processes:recycling, crushing, screening and blending etc. Using RAs as aggregates to produce new concrete, i.e. Recycled Aggregate Concrete (RAC), is corresponding to the strategic thoughts proposed in the national "Twelfth Five-year Plan", like resource conservation, environmental protection and sustainable development, also helps the building of a resource conserving and environment friendly society.So far, RAC is used mainly in subgrade and pavement in highway construction, or some non-bearing structures of building construction. In order to apply RAC to main bearing structures of civil engineering, experiments were carried out for better understanding the mechanical properties of RAC, as well as the seismic performances of RAC beam-column joints and RAC frames. The test results can provide experimental theoretic basis for practical engineering application of RAC structures.The basic properties of RAC such as compressive strength, split strength, flexural strength etc. and the changes of these basic mechanic indices at early age were investigated through experiments, and the relationships between various strength parameters were also presented in this thesis. Besides, uniaxial compressive performances and constitutive relations of RAC and natural aggregate concrete (NAC) were studied by comparison tests, and the expression of RAC’s elastic modulus as well as the fitting formula of RAC’s stress-strain curve were given based on these test results. The experiment results also showed that the main causes of RAC’s inferior strength compared with NAC is the old mortar attached to the surface of RAs. Due to the existence of old mortar, the interfacial transition zone (ITZ) between RAs and new mortar has such features as high porosity, high water absorption and low bond strength, which cause this ITZ to become a weak point in RAC.Different types of beam to column connections include interior connections, exterior connections and knee connections were fabricated and tested under the cyclic loadings. The performance indices of these RAC connections were evaluated, including the stress condition, failure process, hysteretic curves, ductility characteristics, energy dissipation, strength and stiffness degradation etc. The failure process of a RAC joint can be mainly divided into the following four stages:initial cracking stage, thorough cracking stage, ultimate stage and failure stage, which is similar to a NAC joint. In RAC, the ITZ between RAs and new mortar is a weak region because of the existence of old mortar attached to the surface of RAs as well as the defects and cracks inside the RAs. When shear failure occurred at the joint core of a RAC connection, rhomb shaped concrete spalls can be observed in the joint core. Most of the cracking appeared at the ITZ between RAs and new mortar, and showed the obvious characteristic of brittleness. The seismic performances of RAC connections are worse than NAC connections.In this study, a RAC frame was subjected to a series of pseudo-dynamic tests, and then quasi-static tests. In the pseudo-dynamic test stage, the frame was subjected to the selected input motion which is the El-Centro (1940NS) ground acceleration time history, and the seismic performances of this RAC frame such as dynamic behavior, strength characteristic, deformation property were studied. The pseudo-dynamic test results showed that the RAC frame exhibited good seismic performances for resisting small earthquakes and the residual deformations were quite small after the frame was yielded. During the quasi-static test stage, the seismic performances of the RAC frame like the failure process, bearing capacity, hysteretic curves, energy dissipation, ductility, strength and stiffness degradation etc. were studied, and the test results indicated that RAC frames possess good seismic performances. It is possible to design RAC frames and apply them in practical structures in some seismic areas by proper design.Based on these experimental studies, different finite element programs, like ABAQUS and OpenSees were adopted in this research for simulating the quasi-static tests of both RAC joints and the RAC frame. Furthermore, a MATLAB based program was used in this paper for the simulation of RAC frame’s pseudo-dynamic test. The simulation results indicated that the ’SimpleJoint’ based model can simulate the responses of the RAC joints and frame when they’re subjected to low reverse loadings, while the simulation results of the frame’s pseudo-dynamic test showed that Takeda restoring force model is appropriate for RAC structures’nonlinear time history analysis.Health monitoring techniques and damage condition assessment measures are effectively guarantee structures safe and efficient work. Now, it is lack of health monitoring techniques and damage condition assessment measures for concrete structures. Accordingly, two damage indices were developed based on frequency response function and wavelet transform, respectively. Both of the two damage indices could be used for estimating the damage condition of structure as a whole. Meanwhile, these damage indices’ feasibilities for damage assessment were validated by using the former seismic experiment data. Furthermore, this paper also presents an experimental investigation on detecting defects or damages in concrete structures based on piezoelectric materials, and the results indicated that the embedded or adhesive piezoelectric materials in concrete structures can be used as both actuators and receivers or sensors and it is feasible to use them to detect different types of defect or corrosion in some local area of concrete structures.