بررسي تغييرات مقاومت فشاري و تخلخل در هسته‌ي سازه هاي بتني حجيم با مقاومت زياد

A STUDY ON THE VARIATION OF COMPRESSIVE STRENGTH AND PROSITY IN THE CENTER OF MASSIVE HIGH STRENGTH CONCRETE

موضوع تحقيق حاضر، بررسي عمل‌آوري حرارتي (مدلي از عمل‌آوري مركز ستون‌هاي حجيم) و عمل‌آوري استاندارد، و مقايسه‌ي تاثير هريك از آنها بر مقاومت فشاري اوليه و دراز‌مدت، و حجم منافذ داخلي بتن‌هاي مختلف است. بدين‌منظور هشت طرح اختلاط با نسبت آب به سيمان 3/0 ساخته شد. علاوه برطرح شاهد، جايگزيني پوزولان طبيعي به مقدار 15% و 30%، خاكستر بادي به ميزان 15% و 25% و ميكروسيليس به ميزان 5%، 8% و 11% وزني سيمان انجام شد. از هر طرح مخلوط بتن دو سري نمونه تهيه شده است: نمونه‌هاي استاندارد و نمونه‌هاي حرارتي. نمونه‌ها را پس از قالب‌گيري داخل محفظه‌ي پر از آب قرار داده و سپس تاريخچه‌ي دماي هيدراسيون بر آنها اعمال مي‌‌شود. مشاهده ‌شد عمل‌آوري حرارتي تاثير منفي بيشتري بر مقاومت فشاري دراز‌مدت نمونه‌هاي حاوي ميكروسيليس دارد. اما جايگزيني انواع پوزولان‌ها (پوزولان طبيعي، خاكستر بادي و ميكروسيليس) در بتن، باعث كاهش ميزان جذب آب در نمونه‌هاي عمل‌آوري‌شده در حرارت (نسبت به بتن شاهد) مي‌شود.

An investigation was carried out to compare the effects of hydration heat and standard curing on the strength development and absorption water capacity of high strength concrete. In this regard, a temperature matched curing system (TMCS) was designed for simulation of the temperature rising of massive concrete within early stages. In this way, the freshly mixed concrete was cast into a cylindrical (150×300-mm) mold and placed in a semi-adiabatic chamber. A thermocouple was dipped in the sample and the history of the temperature rising of the mixtures, via the designed system, was applied to the concrete samples, which were put inside a box to simulate hydration heat cured samples. Eight mixed proportions, with a water-binder ratio of 0.3, were prepared and the specimens were cured under both standard and heat curing conditions. Natural pozzolan, at 15% and 30%, and low-calcium fly ash, at 15% and 25%, by weight of cement, were used as a replacement for cement in the mixtures. Cement was also replaced by silica fume at three percentages of 5, 8 and 11. The results showed that the peak temperature of both natural pozzolan and fly ash specimens decreased compared to those made without pozzolan, whereas silica fume had no impact on the peak temperature of the mixtures. The results also declared that both early and later stages of compressive strength were seriously affected by the curing condition. Hydration heat curing, due to the rising temperature, had a positive impact on the later stage strengths of natural pozzolan and fly ash specimens, whereas heat curing diminished the later stage strengths of silica fume specimens. The later stage strengths decreased as the percentage of silica fume increased. These findings revealed that care should be taken for utilization of silica fume in massive concrete structures. It is also concluded that a later stage of 91 days was more conservative for the specified strength of mass concrete. The results also showed that utilization of natural pozzolan, fly ash and silica fume diminished the absorption water capacity of the heat cured specimens, compared to those cured under standard conditions.