ارزيابي تاثير فيلرهاي آهكي در كاهش اثرات تخريبي رطوبت و يخبندان با استفاده از آزمايش‌هاي كشش غير مستقيم و مقاومت فشاري

The Role of Limestone and Hydrated Lime Fillers in Reducing Moisture and Freezing Damages in Asphalt Mixes

هدف از اين تحقيق بررسي تاثيرات دو نوع فيلر آهكي (يكي پودر سنگ آهك و ديگري آهك هيدراته) بر مقاومت مخلوط هاي آسفالتي به خصوص در برابر عريان شدگي بوده است. در صورتي كه چسبندگي قير و مصالح سنگي كاهش يافته و يا از بين رود مخلوط آسفالتي دچار عريان شدگي و اضمحلال مي شود. اين پديده در صورت استفاده از سنگ هاي اسيدي و سيليسي كه داراي بارالكتريكي سطحي منفي هستند تشديد مي شود. در اين تحقيق مخلوط هاي آسفالتي با استفاده از مصالح سنگي كارخانه هاي شرق و غرب تهران و با درصدهاي مختلف فيلرآهكي و آهك هيدراته تهيه شدند و طرح اختلاط بهينه آنها به روش مارشال به دست آمد. سپس مقاومت مخلوط هاي آسفالتي در برابر رطوبت و يخبندان با استفاده از آزمايش لاتمن اصلاح شده تحت استاندارد AASHTO T283 و مقاومت فشاري تحت استاندارد ASTM D1075 و با مقايسه مقاومت كششي غير مستقيم و مقاومت فشاري نمونه ها در شرايط اشباع و خشك مورد ارزيابي قرار گرفت. نتايج نشان داد كه با افزودن حدود 4 درصد فيلر آهكي و 1 تا 5/1 درصدآهك هيدراته به تركيب سنگدانه ها نسبت مقاومت كششي نمونه هاي اشباع به نمونه هاي خشك افزايش يافته و ميزان عريان شدگي سنگدانه ها در مخلوط كاهش يافت. با افزايش بيش تر قيلر آهكي و آهك هيدراته، به دليل سخت شدگي مخلوط آسفالتي، مقاومت نمونه ها در برابر رطوبت نيز دچار كاهش گرديدند.

Stripping is a major distress that occurs in asphalt pavements. It happens mainly in presence of water and particularly in freeze and thaw zone conditions. Hence, it is often referred to as moisture damage. Stripping will finally result in early cracking, raveling and potholing in road surfaces. In fact, moisture will first destroy the bonding between aggregate particles and the asphalt binder. This loss of adhesion causes HMA mixes to ravel under traffic loading. Several parameters affect stripping of asphalt mixes (e.g. aggregates source and type, filler and binder properties, etc). Among these, the role of filler is of great importance. This is due to the fact that the filler acts not only as the fine portion of the aggregates in filling the voids; it also reacts with the binder of the mix. The positive action is to make it tick when it covers the aggregate particles. The negative action would be to make the mix stiff and prone to cracking. In this paper, effective parameters that cause stripping were identified and the methods of increasing asphalt mixture durability are discussed. In the experimental work, two different source aggregates from east and west of Tehran were used to prepare a continuously graded asphalt mix. The aggregates were tested both under standard mechanical testing and chemical analysis. This latter testing was performed using XRD and XRF testing. The latter tests indicated that the aggregates were of siliceous sources and exhibit negative charges on their surfaces. Limestone powder and hydrated lime filler, known as potential anti stripping additives, were replaced at different amounts with the original filler of the aggregates. Mixing design was performed on all mix combinations using Marshall Method of mix design and the optimum binder contents were determined. Properties of new mixes were compared with those of the original mix, applying indirect tensile strength and compressive strength tests in accordance with AASHTO T283 and ASTM D1075 standard methods. Samples were kept at dry and saturated conditions and their tensile and compressive strength parameters were determined. Among the various testing parameters, Tensile Strength Ratio (TSR), from ITS testing, and Remaining Strength, from compression testing, were recognized to be the most effective parameters that identify the potential of mixes against stripping. Among these latter parameters, TSR provided more repeatable values upon testing. The results also showed that using 1.5% hydrated lime or 4% limestone filler (by weight of aggregates) significantly increased asphalt mixture resistance against stripping. At lower amounts of these fillers, no significant improvements were achieved and at greater amounts, mixes showed excessive stiffening and low flexibilities. Hence, it was recommended that the mixing design of samples containing anti stripping additives, such as limestone filler and hydrated lime, should be performed based on their tensile strength parameters.