ارزيابي زمان گيرش و روند كسب مقاومت فشاري بتن‌هاي داراي مواد پوزولاني

Assessing the Setting Times and Strength Development of Concrete containing Pozzolanic Materials

گيرش بتن به عنوان مرز شناسايي بتن تازه و سخت شده تلقي مي‌شود دانستن زمان‌هاي گيرش بتن در برنامه ريزي عمليات اجرايي آن بسيار مهم است. زمان گيرش اوليه تعيين كننده محدوده زماني جابه‌جا كردن بتن و زمان گيرش نهايي تعيين كننده شروع توسعه مقاومت مكانيكي است. با پيشرفت صنعت فناوري بتن و لزوم به كارگيري انواع پوزولان‌ها در بتن، تشخيص زمان گيرش بتن از اولويت خاصي برخوردار است. زيرا تغييرات زمان گيرش بتن به دليل استفاده از پوزولان‌ها مي‌تواند در ارزيابي اثر پوزولان‌ها نقش مهمي داشته باشد. در اين مقاله با استفاده از روش مقاومت در برابر نفوذ (ASTM C403) براي تعيين زمان‌هاي گيرش بتن، سعي شده است مقايسه‌اي روي زمان‌هاي گيرش و روند كسب مقاومت بتن با سه نسبت آب به سيمان كه حاوي انواع پوزولان ها است صورت پذيرد. نمونه‌هاي بتني با سه نسبت آب به سيمان 35/0 ،45/0 و 55/0 ساخته شدند. ميكروسيليس به مقدار 7%، خاكستر بادي با مقادير 15% و 25% و سرباره با مقادير 25% و 35% به عنوان مواد سيماني جايگزين وزني سيمان شدند. نتايج آزمايش‌ها نشان مي دهد كه پوزولان‌ها باعث تاخير در زمان‌هاي گيرش (اوليه و نهايي) مي‌شود. نتايج اين پژوهش همچنين حاكي از آن است كه ميزان مقاومت فشاري كوتاه مدت متاثر از زمان گيرش نهايي بتن بوده و روابط تجربي به دست آمده در اين پژوهش مي‌تواند در پيش بيني مقاومت فشاري بر حسب زمان گيرش نهايي بتن كاربرد داشته باشد.

Initial and final setting times are considered as identification boundaries of fresh and hardened of concrete, which seriously affect the quality of concrete at early and later stages. Determination of setting times is an important task of engineers. This is due to the fact that transportation, casting, compacting and finishing of concrete must be managed before initial setting; on the other hand it is expected that compressive strength is developed after final setting time. These are the reasons that evaluation of setting times for concrete structures, particularly in concrete made by different pozzolan materials which are recently increased, are unavoidable. In the present work, the effects of silica fume, fly ash and slag, as partial replacement for cement, on the initial and final setting times and strength developments of different concrete mixtures are examined. The setting times were conducted accordance to ASTM C403. In this method the penetration resistance of the sieved concrete mixtures is evaluated. Three water-cement ratios of 0.35, 0.45 and 0.55 were used. In the predetermined specimens silica fume, fly ash and slag were used as partial replacement for cement. Silica fume at 7%, fly ash at 15% and 25%, slag at 25% and 35%, by weight of cement, were used. The results showed that the setting times of the concrete increased as water-cement ratio increased and the lowest setting times corresponded to the specimens with 0.35 water-cement ratio. The setting time of the specimens with water cement ratios of 0.45 and 0.55 increased by 10% and 18%, respectively compared to that for 0.35 water cement ratio specimens. Using silica fume, fly ash and slag, as replacement for cement, retarded the setting times of the mixtures. The more replacement contents led to increase of setting times. The setting time of the fly ash specimens at 15% and 25% increased by 6% and 12%, whereas addition of slag at 25% and 35% retarded the setting time by 11% and 18%, respectively compared to those for reference specimens with 0.45 water-cement ratio. Silica fume caused a slight increase of setting time by 5.5% compared to those without pozzolan materials. These may attribute to the low cement content in the pozzolanic specimens which resulted to the low friction surface between cement particles and on the other hand enhancement of effectiveness of the super-plasticizer in the mixtures which led to the increase of setting times. The results also indicated that silica fume, as a very fine supplementary cementitious material, enhanced both the early and later ages strength. Fly ash and slag, as replacement for cement, increased the later stage strength of the specimens, but had a negative impact on the early- age strength. This may attributed to the low pozzolanic activity of these cementing materials in the early age, however in the later ages which calcium hydroxide content increased in the mixtures the secondary induced gel enhanced the compressive strength. Subsequently, equations were found by fitting analysis, which correlated the early age strength of the concrete at 1, 2, 3 and 7 days with the final setting time.