اثر افزودنيهاي معدني بر ضريب انبساط حرارتي بتن و خمير سيمان

Effect of mineral admixtures on coefficient of thermal expansion of concrete and cement paste

تغيير در دماي بتن سبب ايجاد كرنشهاي انبساطي در بتن شده كه تبديل به تنش كششي ميشود. باكاهش ضريب انبساط حرارتي بتن، خطر تركخوردگي آن تحت تغييرات دما كمتر ميشود. در اين پژوهش به منظور بررسي اثر افزودنيهاي معدني بر ضريب انبساط حرارتي، نمونههاي بتن و خمير سيمان حاوي افزودنيهاي معدني دوده سيليس، متاكائولن و سرباره با نسبت آب به مواد سيماني و مقدار مواد سيماني ثابت ساخته شده و آزمايشهاي مقاومت فشاري، تخلخل كل، توزيع اندازه منافذ و تعيين ضريب انبساط حرارتي روي نمونهها انجام شده است. همچنين ضريب انبساط حرارتي تا سن 120 روز به فاصله هر 7 روز بررسي شده است و اثر روند هيدراسيون و واكنش پوزولاني بر آن بررسي شده است. طبق نتايج به دست آمده، ضريب انبساط حرارتي بتنهاي حاوي دوده سيليس و متاكائولن 14 % كمتر از بتن بدون افزودني معدني بوده است درحاليكه بتن حاوي سرباره، كمي بيشتر از بتن بدون افزودني معدني بوده است. با افزايش سن نمونه تا سن 60 روز، ضريب انبساط حرارتي بتن بدون افزودني معدني 12 % كاهش و پس از آن تا سن 120 روز ثابت شده است. در بتنهاي حاوي دوده سيليس و متاكائولن، اين مقدار كاهش برابر با 8% بوده است و مانند بتن بدون افزودني معدني پس از 60 روز ثابت شده است. در بتن حاوي سرباره، تا سن 60 روز تغيير چشمگيري مشاهده نشده است و از سن 60 تا 120 روز، 10 % كاهش مشاهده شده است. با استفاده از دادههاي آزمايشگاهي، مدلي براي پيشبيني ضريب انبساط حرارتي بتن در سنين مختلف با استفاده از ضريب انبساط حرارتي بتن در سن 7 روز ارائه شده است. در ادامه، ارتباط بين ضريب انبساط حرارتي با مقاومت فشاري، تخلخل كل و قطر ميانه منافذ بررسي شده است كه بيشترين ارتباط بين ضريب انبساط حرارتي با قطر ميانه منافذ بوده است.

Change in the concrete temperature causes the expansion of the concrete to become tensile stress especially in structures such as concrete pavements which are long. Length change in concrete structures is caused by thermal change is related to length, change in temperature, and coefficient of thermal expansion (CTE) of concrete. The risk of concrete cracking decreases with decreasing CTE under temperature changes. In this research, in order to investigate the effect of mineral admixtures on the coefficient of thermal expansion, five concrete and cement paste samples (reference, containing silica fume, containing metakaolin, containing silica fume and metakaolin, and containing slag) with a constant ratio of water to cement materials (0.45) and the amount of cement materials (400Kg/m3) are made, and compressive strength and total porosity on 7, 28, 56, and 120 days, pore size distribution on 28 days age, and coefficient of thermal expansion of the samples from 7 days to 120 days each 7 days are examined. All specimens were kept in water tank for curing until the time for each test. For testing CTE, specimens were returned to water tank after each test. The results indicated that by ageing, in both paste and concrete samples, compressive strength increased and porosity decreased. This rate of change was higher in reference concrete and concrete containing slag. In concretes and pastes containing silica fume, better porosity and compressive strength was observed. It also was seen that although the paste containing metakaolin had more porosity than reference paste, reverse trend was observed in concrete samples. It shows that metakaolin did not have much effect on paste but it was very effective on the interfacial transition zone in concrete. In addition, by looking on CTE of concretes and pastes until 120 days, CTE of reference concrete had a descending trend up to 60 days, and diminished by 12%, after which it remained constant. On the other hand, in the slagcontaining concrete, after 60 days, the descending trend started and declined by 10% up to 120 days. For the concrete containing silica and metakaolin, at the very early ages, it had 14% less CTE as compared to reference concrete, while with the ageing of the sample, its CTE dropped by around 8%. Across all concretes, reduction of CTE was associated with lowered total porosity. Utilize the experimental data, two models in order to different behavior of concrete with and without mineral admixtures for predicting the coefficient of thermal expansion of them at different ages is presented using coefficient of thermal expansion of concrete at 7-days age. For evaluating models, 3 data for each mix design separated and models were examined with them. It was shown that CTE of concrete can be well estimated. In the following, the relationship between CTE and total porosity and the average diameter of pores has been investigated, which was observed that by decreasing the total porosity and the average diameter of pores, CTE was reduced too. The highest correlation was also found between the coefficient of thermal expansion and the diameter of pores.