حالت نهايي و شكل پذيري دال هاي بتن خودمتراكم مجوف پيش تنيده‌ي پيش ساخته

ULTIMATE STATE AND DUCTILITY OF PRECAST PRESTRESSED HOLLOW CORE SLABS CONSISTING OF HIGH STRENGTH SELF COMPACTING CONCRETE

نواقص اجرايي اعضاي پيش ساخته‌ي صنعتي از جمله: پوشش ناكافي بتن روي فولاد هاي پيش تنيده، كمبود و يا ناپيوستگي فولادهاي پيش تنيده با پوشش بتني، ضعف در حفظ نيروهاي جك زدن پيش تنيده در تكيه گاه هاي مورداستفاده در كارگاه هاي پيش‌تنيده‌ي پيش ساخته و افت شديد در نيروي جك‌زدن كابل هاي پيش تنيده قبل از اعمال آن به عضو از جمله مواردي است كه مي‌تواند براي قطعات صنعتي خسارات غيرقابل جبراني به همراه داشته باشد. لذا براي بررسي مناسب تر چنين مواردي سه دال مجوف بتن پيش‌تنيده‌ي ساخته‌شده با بتن خودمتراكم به ابعاد 4 متر طول، 2/1 متر عرض، و 2/0 متر ارتفاع در صنعت ساخته و سپس در آزمايشگاه تا مرحله‌ي تخريب بارگذاري شدند. حالت نهايي و شكل پذيري خيز و انحناي آزمايشگاهي دال ها اندازه گيري و نتايج با مقادير تيوري بررسي و گزارش شده است. نتايج نشان مي دهند كه نواقص اشاره‌شده منجر به كاهش شديد افت نيروهاي پيش تنيده و اقتصادي‌نبودن و ناامني اعضا همراه با كاهش شكل پذيري شده است.

The development of self compacting concrete, SCC, is a relatively new type of concrete made with conventional concrete materials and, in some cases, with a viscosity-modifying admixture (VMA). It fills all sections of formwork, encapsulates the reinforcement without any mechanical compaction (vibration) and has reasonable flow-ability, homogeneity, resistance against segregation and mechanical strength. The key point in achieving the self compacting property is providing high flow-ability and deformability, while maintaining a resistance to segregation and sedimentation. This can be achieved by proper balance between constituent materials. Okamura and Ozawa proposed the limiting of the volume of coarse aggregates, along with the use of high amounts of powder, controlling the water-powder ratio by volume and the use of proper super-plasticizers. The lower time and cost of construction, the lower sound pollution and the high homogeneous concrete are some advantages of SCC. In recent years, there have been a number of significant developments in the SCC precast industry. However, very limited reports are available on prestressed SCC elements and, therefore, more research is urgently needed in order to understand the use of SCC in precast prestressed lightweight slabs (hollow core slabs) to formulate design rules. The deficiencies of industrial precast prestressed constructional members, such as the lack or unbounding of concrete cover on prestressing strands, weakness in holding the prestressed jacking forces in precast prestressed end supports in industry, the loss of high amounts of prestressed jacking forces of strands before transferring the forces into the members and etc, are some cases which can cause irreparable damage. It is, therefore, for better investigation of such cases that three precast prestressed hollow core SCC slabs with dimensions of 2m length, 1.2m width and 0.2m height were industrially cast and then experimentally load tested up to failure. Their ultimate state and experimental deflection and curvature ductility are measured and the results are compared theoretically. The results show that production of such industrial products is unsafe and uneconomic.