Investigation On The Performance, Cracking Mechanism And Micro-properties Of Concrete Incorporated With Mineral Admixtures

The early age cracking of the concrete occurred before the normal working ofthe members can affect the appearance and the durability of the structure seriously,which had been thought one of the chief causes that lead to the failure ofengineering structures. The optimization of the concrete mix proportions to resistcracking and the research on the early age cracking problem of concrete in theoryand experiment will make good sense in reducing the arise of the early age crack ofconcrete structure, which will also improve the durability of engineering, cut theservice cost down, extend the service life of the structures, guarantee the long-termquality and safety of concrete structures.Previous research paid more attention on the single factor, such as shrinkage,on concrete early age cracking. While most of the work is qualitatively analyzedinstead of quantitatively analyzed which need further investigation. The mainresearch of this paper includes:1. The research started from the analysis of the early age cracking mechanismand the development of basic mechanic properties, static elastic modulus, shrinkage,ultimate tensile strain, thermal field and creep with age were taken intoconsideration. Based on the experiment results, an incremental calculation methodwas established to predict the cracking age of concrete ring. A model in analyzingconcrete early age cracking was established and the feasibility and the reliability ofthe model were tested in an engineering project.2. The durability of concrete was analyzed with carbonization test andchloride penetration test.3. The microstructure and hydration products of concrete at different ageswere studied with scanning electron microscopy test and X-ray diffractionexperiments the concrete double-combined mineral admixtures with goodperformance in resisting cracking was studied in microstructure.From the research, the main conclusion can be drawn as:1. Fiber addition will decrease the compressive strength and elastic modulusagainst no fiber addition, while fiber addition can also reduce the drying shrinkageof concrete obviously. The development of shrinkage with age can be taken as afive-parameter exponential equation.2. The development of ultimate tensile strain in flexure can be divided in tothree parts: falling period, rising period and undisturbed period. The ultimatetensile strain in flexure develops with age in an exponential relationship which leadto the concepts of final ultimate tensile strain in flexure and critical age. Theexperiment results show that the fiber addition can decrease the critical age ofconcrete which means improve the properties in resisting cracking of concrete.3. With the introduction of the cement particle model into hydration kineticstheory, a new cement hydration model was established which took the energy as thecontrol rules of hydration process in kinetic lows. The model was applied innumerical simulation of temperature field of the concrete wall. The numerical results have a good agreement with the experiment results.4. The relationship between stresses of concrete rings with age was analyzed.In considering basic mechanic properties, static elastic modulus, shrinkage,ultimate tensile strain, thermal field and creep, the maximum tension stress theorywas applied to predict the cracking age of concrete rings. The predicted resultsmatch closely with the experiment results. Based on the engineering project, thecracking curves and cracking resistance curves were built upon time history. Thetheoretically calculated results have good agreement with experiment results. At thesame time, a good performance of the double-combined mineral admixturesconcrete in resisting cracking can be proved.5. It can be shown from the investigation that the double-combined mineraladmixtures concrete has a good long-term performance in resisting carbonation andchloride permeability. Double-combined minerals will optimize the microstructuresof concrete and the shape of Ca(OH)₂.This research will afford reliable support to the design, construction andmanagement of infrastructure project such as tunnel and underground engineeringin theory and experiment.