ارزيابي مكانيزم فشار منافذ مويينه و مقاومت كششي مخلوط هاي بتني تازه حاوي انواع افزودني‌هاي معدني تحت جمع شدگي خميري

Evaluating Mechanism of Capillary Pore Pressure and Tensile Strength in Fresh Concrete Including Various Types of Mineral Admixtures under Plastic Shrinkage

ترك­هاي خميري در ساعات اوليه پس از بتن ريزي به علت تبخير آب از سطح بتن ايجاد مي شود. هدف اين تحقيق بررسي ارتباط رشد فشار منافذ مويينه، مقاومت كششي در سنين اوليه و كرنش ناشي از جمع شدگي خميري مي باشد. در اين رابطه دستگاه اندازه گيري فشار منافذ مويينه براي اولين بار در كشور ساخته شد و در مرحله­ي ثبت اختراع مي باشد. هشت نمونه بتن در دو دسته معمولي و خودتراكم شامل نمونه­هاي بدون مواد جايگزين سيمان و نمونه هاي حاوي مواد پوزولاني ساخته شد. نتايج نشان مي دهد كه ارتباط مناسبي ميان فشار منافذ مويينه و كرنش جمع شدگي خميري وجود ندارد و ساير پارامترها مانند رشد مقاومت كششي مي تواند نقش مهمي در حالت خميري بتن ايفا كند. البته ميان زمان شروع جمع شدگي و شروع منفي شدن فشار مويينه ارتباط مناسبي بر اين تمامي نمونه ها برقرار مي باشد. نتايج همچنين بيانگر تاثير مثبت مقاومت كششي سنين اوليه در كنترل ترك هاي خميري مي باشد.

Plastic shrinkage occurs in fresh concrete usually within few hours after mixing the concrete and risk of its cracks endangered concrete structures especially in the elements with high surface to volume ratio such as slabs or highway pavement. An experimental investigation on capillary pore pressure, tensile strength and plastic shrinkage of concrete is presented here. The aim of research is to study the relationship between capillary pore pressure build up in concretes, early age tensile strength and plastic shrinkage strain. Capillary pore pressure apparatus was created for the first time in Iran for this research. Test was done in a climate chamber with the constant evaporation rate of 0.7 kg/m²/h .Eight types of concrete in two categories such as normal concrete and self-consolidating concrete were tested including mixture without any mineral admixtures and containing of pozzolanic materials such as silica fume and metakaolin. The results indicated that there is no strong relationship between capillary pore pressure and plastic shrinkage strain and it can be concluded that other parameters such as tensile strength development can play an important role in the plastic state of concrete. However, it should be mentioned that capillary pressure is the main cause and driving force of concrete shrinkage and the onset of capillary pressure is directly related to the onset of shrinkage. The results also showed that early age tensile strength can be effective factor in controlling cracks of concrete. No cracks appeared in the mixtures containing of silica fume and metakaolin because of tensile strength improvement up to 55 and 32 percent respectively, in comparison with the reference mixture. However capillary pore pressure in these mixtures was higher than pressure in the reference mixture.