Mechanical Properties Of Compound Concrete Containing Demolished Concrete Lumps At Room Temperature And After Fire Exposure

In the past decade,compound concrete(CC)has been demonstrated as an efficient method for recycling of the waste concrete.To improve the wide application of this concrete,a considerable amount of investigations have been conducted on the material and structural performances of the compound concrete at room temperature.On the bases of these studies,a series of material tests,component experiments,and distinct element analyses were conducted in this thesis,with the aim to further understand the mechanical responses of compound concrete at ambient and high temperatures.The main works and achievements of this study can be summarized as follows:1.A total of 93 cubic,cylindrical and prismatic samples made with CC were cast and tested under axial compression,and the effects of the relative strength of new and old concrete,the replacement ratio of demolished concrete lumps(DCLs),and the specimen shape and size on the mechanical properties of the CC were investigated.Experimental results show that:(1)The ratio of cubic strength to cylindrical strength for the CC is about 8.3%higher than that for traditional concrete;and the ratio of prismatic strength to cylindrical strength is about 1.08 for the CC.Equation is proposed for estimating the compressive strengths of CC cubes,cylinders and prisms with a high predictive accuracy.(2)The elastic modulus of the CC is generally lower than that of traditional concrete when these two concretes possess a same strength.Equations suggested in this study can well predict the elastic modulus of the CC.(3)As the stress-strength ratio is smaller than 0.8,the lump replacement ratio often shows a negligible influence on the CC's Poisson's ratio.However,the growth rate of Poisson's ratio of the CC is obviously higher than that of the traditional concrete when the stress level is higher than 0.8.2.Mechanical properties of compound concrete would be adversely affected if too large-size of coarse aggregates are employed in the new concrete in real constructions.To consider such an effect,as well as the influences of the specimen shape,the specimen size and the lump size,a total of 81 CC cubes,cylinders,and prisms were manufactured and tested.The main results suggest that:(1)As the new concrete's maximum aggregate size increases to35 mm,the measured strengths of the CC samples are lower than the predicted ones on the whole.A modified strength formula is thus proposed to consider this effect.(2)When the lump characteristic ratio is located within the range of 0.22~0.44,the prism's strength is limited influenced;but as this ratio increases to 0.55,an approximately 10% decrease in the compressive strength is observed.(3)Due to the dual effects of the large-size coarse aggregates and the specimen size,the strength of the CC increases at first and then declines gradually as the prism size grows.3.For actual application of the CC,little information is known about the influence of the spatial locations of DCLs and coarse aggregates on the concrete strength.Moreover,the mechanical responses of such a concrete containing various shapes of DCLs is also not well-illustrated.To add knowledge related to these topics,two-dimensional mesoscale simulations of CC containing DCLs under axial compression were performed using the discrete element method.The main variables of interest were the relative strength of the new and old concrete,the distribution of the lumps and other coarse aggregates,and the shape of the lumps.In addition,the differences in compression behavior between CC and recycled aggregate concrete were also predicted.The numerical results indicate that:(1)The influence tendency of the spatial locations of DCLs and coarse aggregate pieces on the compressive stress-strain curves for CC is similar to that of the locations of coarse aggregates for ordinary concrete.The strength variability of CC is generally higher than that of ordinary concrete,regardless of the relative strength of the new and old concrete included,but that variability has no monotonic trend with an increase in the lump replacement ratio.(2)The mechanical properties of CC in compression are little influenced by the geometric shape of DCLs as long as their long axis-to-short axis length ratio is smaller than 2.0.(3)The compressive strength and elastic modulus of CC are always superior to those of recycled aggregate concrete designed with a conventional mixing method.4.A total amount of 21 plain CC samples and 13 reinforced CC beams were tested under flexural loading.Test results indicate that:(1)When the old concrete's strength is smaller than the new concrete's strength,the adverse impact of the weak DCLs on the flexural-tensile strength is observed to be more significant than that on the compressive strength.The proposed equation can well predict the flexural-tensile strength of tested CCsamples.(2)The cracking loads of the reinforced CC beams with a 30% replacement ratio of DCLs only reach 71.5%~78.0% of those of traditional concrete beams,but the yield loads and ultimate loads of the former beams are comparable to those of the latter beams.The lump size almost has a negligible impact on the CC beam's cracking load.(3)The spacing and width of the primary cracks in CC beams are smaller than those in traditional concrete beams.And the influence of the cover thickness is more prominent on the properties of reinforced CC beams as compared to those of traditional concrete.Additionally,the crack spacing and width of the CC beams are both less influenced by the lump size.5.A total number of 159 CC cubes,cylinders,and prisms were fabricated and tested after high temperature exposure.Then the influences of the replacement ratio of DCLs,the specimen shape,and the heated temperature on the residual mechanical properties of the CC were analyzed.Test results suggest that:(1)Cubes generally spall more seriously than cylinders and prisms.Moreover,the fresh concrete samples are more susceptible to the spalling damage than the CC.(2)Overall,the relative compressive strength and elastic modulus of the CC are quantitatively similar to those of conventional concrete.(3)As the heated temperature raises,the cube-to-cylinder strength ratio increases monotonically,while the prism-to-cylinder strength ratio does not have a similar monotonic relationship with the temperature.In addition,both these two ratios increase with an increase in the replacement ratio.(4)For all exposed temperatures,the ratio of the elastic moduli of prismatic and cylindrical CC specimens generally increases with the replacement ratio,while the peak strain ratio shows the opposite tendency.(5)The incorporation of DCLs has an impact on the CC's dilation behavior under axial load.The ratio of the critical load to the peak load exhibits a decreasing trend with the DCL content.But such effect becomes less important after 600 °C exposure.(6)Equations to predict the residual compressive strength,elastic modulus,and peak strain of CC cubes,cylinders and prisms after fire exposure are proposed,and they generally present a good prediction accuracy.