Technical Performance Of Foamed Asphalt And Foamed Asphalt Cold Recycled Mix

Recycling of asphalt pavements can effectively enhance the use of resources, protect ecological environment and accords with the national strategy of developing cycling economy and realizing sustainable development. Therefore, research on the technology of asphalt pavement recycling is of great significance.Foamed asphalt is a kind of cold recycling stabilizing agent, which gains more and more acceptance in recent years. Recycled mixes stabilized with foamed asphalt possess advantages in terms of performance, economy and environment protection. However, studies about the behaviors and distress mechanisms of foamed asphalt cold recycled mix have still been limited at present, and mix design also mainly depends on experience, which hold back the popularity of foamed asphalt cold recycling technology.From foamed asphalt and foamed asphalt cold mix composition point of view, this paper, through a lot of laboratory experiments and relative analysis, aims to systematically study the mechanical properties and pavement performances of foamed asphalt cold mix, so as to optimize its design, improve its various performances and promote relative research and practice for the technology.Foamability of asphalt is a key factor influencing properties of foamed asphalt cold recycled mix. The paper selected asphalts with different penetration grades to carry out foaming experiments, so as to determine the evaluation indexes and method for asphalt foamablity, and the relationship between basic characteristics of asphalt and its foamablity. It was confirmed that increase of foam index (FI) within a certain range reflects the improvement of foaming properties. However, it does not necessarily mean that the bigger the value of FI, the better the foam characteristics is. Not be used as a substitute for expansion ratio and half-life, FI should be considered as a complementary index for the design and evaluation of foamed asphalt together with expansion ratio and half-life. Foamant additive can significantly extend half-life of foamed asphalt, but has little bearing on expansion ratio. Dynamic viscosity of asphalt was found to have a comparatively good correlation with its foaming characteristics. Whereas other basic properties, such as PI, penetration, softening point, elongation, specific gravity, characters after aging, penetration grade and etc, show a little correlation with foaming properties.The dispersion of foamed asphalt within cold mix was studied by sieve analysis. The results showed that if some factors, such as asphalt foamability, temperature and gradings of mineral aggregates, mixing moisture content and etc, cannot be reasonably controlled in mix design, asphalt may fail to disperse evenly or conglomerate in mix, leading to an adverse effect on mix properties. Among the mix where asphalt disperses evenly, asphalt mainly coats fine aggregates passing through 1.18mm sieve, especially those passing through 0.3mm sieve and form foamed asphalt motar, which adhere to larger aggregates and bond them together in a manner of "point".Mixing moisture content is one of the pivotal parameters in foamed asphalt mix design. The paper designed different kinds of foamed asphalt mixes with original aggregates and recycled asphalt pavements as well to study the proper moisture contents in mixing of foamed asphalt mixes. The analysis showed that mixing moisture content increases with the increase of filler content within a certain degree. Reasonable ranges of mixing moisture content and optimum moisture contents for foamed asphalt mixes with various kinds and gradings were proposed accordingly.Grading of mineral aggregate is another key factor which affects foamed asphalt cold recycled mix performance. The paper designed foamed asphalt mixes with different fine and coarse fraction gradings and studied their tensile, compression, shear strength separately. It is indicated that optimum foamed asphalt content is closely related to filler content and foamed asphalt mix has a higher filler to asphalt ratio than HMA. Enhancement of fine fraction density can remarkably improve the mechanical properties of foamed asphalt mix. However, change of coarse fraction grading has little bearing on optimum asphalt content and mechanical properties of mix. The grading envelope recommended by Wirtgen was modified accoding to the results concluded in the paper. Besides, the variation sensitivity of key sieves were investigated and the allowable variation ranges in construction were suggested accordingly.Strength and rigidity properties of foamed asphalt recycled mix are influenced by many factors. The paper carried out relative studies from internal and external aspects. It was shown that foaming characteristics, mineral aggregate temperature, amounts of recycled mix, foamed asphalt and cement, curing and compaction method, determing temperature and etc, all have significant influence on strength characters and compression resilient modulus of foamed asphalt recycled mix. In addition, temperature sensitivity of mechanical properties for foamed asphalt recycled mix is less than HMA.Permanent deformation is one of the important performance of foamed asphalt recycled pavements. The paper adopted dynamic compression creep test to analyze the effect of mix composition on its permanent deformation response. It was found that increasing amounts of cement and fine fraction or decreasing foamed asphalt content at a certain degree, can improve the deformation resistance of foamed asphalt recycled mix. By reasonably designing mix grading, materials of every layer in pavement structures can be recycled with foamed asphalt with satisfied deformation resistance. Foamed asphalt recycled mix has superior deformation resistance over HMA. Loading stage of creep testing for foamed asphalt recycled mix was also found to be well simulated by Burgers model, with the visco-elastic parameters for recycled mix can be acquired.Fatigue resistance is a paramount performance needed to be optimized for foamed asphalt recycled mixes, which are used for base or subbase layers in pavements. The paper chose indirect tensile fatigue test to study the influence of different material compositions on fatigue characteristics of recycled mixes. It was indicated that increase of cement at a small amount is beneficial to the improvement of fatigue resistance for recycled mixes. Optimum foamed asphalt content can make the recycled mixes have relatively good fatigue resistance, strength and flexibility. Enhancement of foaming properties can also improve fatigue characteristics of recycled mixes. Gravel treated with lime and fly ash forms recycled mix with inferior fatigue reponse, but mixing with RAP can effectively ameliorate its performance. Fatigue resistance for foamed asphalt recycled mixes fall between semi-rigid material and HMA.At last, the paper studied the construction technics of foamed asphalt recycled pavements. The corresponding inspection methods and standards for construction quality are suggested. An actual project demonstrated that reasonably designed foamed asphalt recycling layers have favorable structural capability and can be used for base or subbase layers of high class pavements.