Research On Reactive Powder Reinforced Recycling Technology Of Concrete

Recycling of concrete is one of the hot spot issues for sustainable development of concrete in 21st century. Technology of recycled aggregate concrete (RAC) has attracted widely attention due to its distinguished environmental benefits, economic advantage and social returns. Relatively low strength and high dry shrinkage of RAC, however, often limit the extensive application to structural occasions. It is difficult to produce both cheap and quality-satisfying recycled aggregate (RA) by current crushing technology. As such, utilization of inferior or moderate RA (characterized by high crushing value and high water absorptive behavior) for structural concrete, where relatively high strength and low dry shrinkage are commonly required has become a pressing issue. In addition, character of low strength but good bonding between RA and the cement paste matrix makes it possible to develop functional recycled concrete, e.g., pervious recycled aggregate concrete, has become a new research orientation related to concrete recycling.Based on a deep investigation on basic properties of RA and RAC was completed. Grading method of RA and its recommended applications were then proposed. Gradation of binary, ternary and quaternary system of reactive powder (RP) and ordinary Portland cement (PO) was optimized according to Andersen formula. The proportion of RP was determined and verified by actual packing density calculated by compressible packing model (CPM). By pretreatment of ordinary RA (gradeâ…¡,manufactured from C40 cement concrete),adoption of stone-enveloped with RP , etc., high strength and low dry shrinkage of RP reinforced high strength RAC(RP-HSRAC) was achieved. A novel proportioning design method mainly governed by volumetric parameters for pervious concrete was established. By utilization of silica fume, both high strength and excellent permeability of pervious concrete were achieved.The main research components and conclusions of this dissertation are summarized as follows.1. Database of RAC mainly related to compressive strength with or without active powder was established. Factors influencing compressive strength of RAC, high strength RAC(HSRAC) and reactive powder reinforced HSRAC(RP-HSRAC) were analyzed and summarized. Combining the experimental data and references, grading indexes of RA including apparent density, variation coefficient of apparent density, water absorption ratio, old mortar content (or Los Angeles loss) and crushing value were then put forward. Applicable scope of RA was determined: gradeâ… can be used for RC15~RC60;gradeâ…¡is capable of production of RAC with a moderate strength (lower than 40MPa);gradeâ…¢is limited to lower strength concrete(lower than RC30).2. Principles and requirements for selection of RP and cement were determined. By optimizing gradation of RA and RP and adoption of stone-enveloped with reactive powder (SERP) results in modification of interfacial transition zone (ITZ) that leads to the improvement of the strength. By pretreatment of RA, SERP technology and utilization of high strength cement to strengthen the cement mortar matrix of RAC, high strength of RAC with 28d compressive strength of 71.2MPa, flexural strength of 7.9MPa was successfully manufactured in laboratory. Besides, 29% reduction of the dry shrinkage at 90 days'age was observed. XRD, IR and SEM analysis results reveals that less Ca(OH)2 in hydrates were produced in RP-HSRAC. Uniform and dense structure of both hydrates and ITZ can be observed.3. Enlightened by proportioning mix method for stone matrix asphalt (SMA) and open graded friction course (OGFC), a novel proportioning design method mainly governed by volumetric parameters for both no-fines pervious concrete and no-fines pervious recycled concrete(NPRC) was established. NPRC with 28d compressive strength of 19.03MPa, void volume of 19.29% and 8.3mm/s permeability coefficient was produced by the new method.4. It is pointed out that bonding points between RP and RA vital for the strength NPRC, and hence, silica fume is a type of ideal powder for improving strength when compared with fly ash(FA),blast slag(BS). Compared with polymer, addition of polymer and SF, use of high strength cement, SF proved effective for improvement of strength: recycled pervious concrete with 28d compressive strength of 26.37MPa , flexural strength of 3.37MPa and permeability coefficient of 7.8mm/s was manufactured when an optimum dosage of 4% silica fume was added into the mixture. SEM and IR analysis results indicate that addition of SF benefits to further hydration, which results in much more C-S-H and improves the density. Thus improves the strength of NPRC.5. The interfacial structure of new and old concrete was assumed consisting of permeable layer, reactive layer and transitional layer, which was introduced to investigate the interfacial structure of NPRC. The experimental results verified the existence of reactive layer. New paste exchanges icons through the reactive layer with the attached old mortar. Reactive powder contributes to uniform and small hydrates that help to enhance the permeability of icons. Thus, increases the exchange of icons and results in better bonding in the interfacial zone.6. Finite element model of plane circular particle and polygon particle was established, respectively. Analysis and simulation was conducted on compression and flexion state of pervious concrete. Original failure position was simulated for both compression and flexion test. The result shows that new mortar tends to fail first in compression. However, the new mortar or the old mortar can fail when the specimen is bending. Factors influencing compressive and flexural strength of NPRC were simulated and the theoretical strength of NPRC was calculated. The experimental and calculating results indicate that 90.3% and 94.9% of maximum theoretical compressive strength and flexural strength can be achieved, respectively.