Influence Of Defects In Recycled Aggregate On Mechanical Properties Of Recycled Aggregate Concrete

In this thesis, an experimental investigation was conducted to study the influence of the recycled aggregate defects such as attached mortar and gravel damage on the properties of recycled aggregates, mechanical properties of recycled concrete and recycled concrete’s microscopic structure.Water absorption, apparent density, crush index and soundness were conducted on four kinds of coarse aggregates (natural aggregate NA, recycled aggregate RA obtained by the process of crashing waste concrete, recycled aggregate RA-H and natural aggregate NA-H which both were soaked by3mol/L sulfuric acid solution). Mechanical properties tests (compressive strength, tensile splitting strength, fracture energy and elasticity modulus) were conducted on three kinds of concretes (water-binder ratios were0.26and0.60) made with NA, RA and RA-H respectively. Influence of gravel damage due to crashing processing and attached mortar on the properties of recycled aggregates and mechanical properties of recycled concrete were obtained from analysis of the results of tests above. Meanwhile, recycled coarse aggregate, attached mortar and interfacial transition zone in recycled aggregate and recycled concrete were observed by microscope. RA defect characteristics and its effect on the mechanical properties of RAC were analyzed from the microcosmic angle.The results of the water absorption test, apparent density test, crush index test and soundness test indicate that attached mortar and gravel damage due to crashing process both have a certain negative effect on the properties of recycled aggregate. Attached mortar is the main factor which affects the apparent density, crush index and soundness of the recycled aggregate and the apparent density of recycled aggregate is the most affected. However, the main factor which affects the water absorption of recycled aggregate is the gravel damage due to crashing processing.The results of the mechanical property test of the natural concrete and recycled concrete indicate that attached mortar is the main factor inducing the decrease of compressive strength, tensile splitting strength, fracture energy and elasticity modulus of recycled concrete at a low water/binder ratio.While at a high water/binder ratio, attached mortar is the main factor inducing the decrease of compressive strength of recycled concrete; gravel damage due to crashing process is the main factor inducing the decrease of fracture energy and elasticity modulus of recycled concrete; attached mortar and gravel damage due to crashing process have about the same effects on the tensile splitting strength of recycled concrete. By comparing high and low water/cement ratio concrete, attached mortar has a greater influence on low water/cement ratio recycled concrete, while gravel damage due to crashing process has a greater influence on high water/cement ratio concrete.The results of the microstructure observation of recycled concrete indicate that both new cement mortar and old cement mortar in the recycled concrete fracture surface exist cracks and pores at a0.26water/binder ratio, while the new cement mortar is very dense and old cement mortar exists many cracks and pores at0.60water/binder ratio, which shows the old attached mortar is one of the main factors effecting the mechanical properties of recycled concrete. The cracks occurred in the original coarse aggregate during the crushing and producing process, and they are further intensified in mechanics experiments, which affect the mechanical properties of recycled concrete. The interfacial transition zone between recycled aggregate and new cement mortar contains a lot of defects and it is the weakest interface.