STUDY ON COLD ON-SITE RECYCLING CONSTRUCTION PROCESSES AND TECHNIQUES FOR OLD ASPHALT PAVEMENT

Transcription

1 STUDY ON COLD ON-SITE RECYCLING CONSTRUCTION PROCESSES AND TECHNIQUES FOR OLD ASPHALT PAVEMENT Jun SHI (1), Xiaofei LIANG (1) (1) School of Architectural Engineering, Shandong University of Technology, China Abstract This paper takes DongHong road in LinYi ShanDong province as an illustration, to present the main principle and application prospects of cold on-site recycling technology for old asphalt pavement. Laboratory experiment and construction process of this technology are particularly introduced such as the old road milling, mixing the initial shaping, watering nausea materials, paving cement, the second mixing, shaping, rolling, joint treatment, curing, etc. Based on trial research and construction practices, it provides cold-place recycling construction technology for the old asphalt pavement, which is especially suitable for northern part of china. The results will play an active action on drawing up the construction standards and on the extensive application of cold on-site recycling technology for the old asphalt pavement for the future and also become the necessary way to reduce costs, save resources and develop circling economy. Keywords: Cold on-site recycling; Asphalt pavement; Compressive strength 1. INTRODUCTION Chinese current situation with increasing vehicle load and volume of traffic sets up largescale highway maintenance and general upgrading of the road to be a main task for highway construction. Cold on-site recycling technology for old asphalt pavement has been widely used abroad and formed a complete technology standard and norm. China began to attach importance to research asphalt mixture cold-recycling technology in the 1980s and put lots of hot on-site recycling technology, but progress has been slow. This paper presents cold on-site recycling construction processes and techniques for old asphalt pavement by DongHong road maintenance engineering project in LinYi ShanDong province. 122

2 2. PRINCIPLES AND SIGNIFICANCE OF COLD IN-PLACE RECYCLING Cold on-site recycling exploits special recycling machine to complete some procedures at the scene such as milling, crushing, adding new material (cement, emulsified asphalt, recycling agent) to mix, paving etc. within a certain depth of the old asphalt pavement, followed by leveling and rolling to form a special graded aggregate road base (or subbase) eventually. The goal of cold-recycling technology to save resource, protect ecological environment and reduce project cost is achieved by utilizing asphalt mixture(chiefly including sand &stone and asphalt) repeatedly. Firstly, cold-recycling technology for the old asphalt pavement makes the most effective use of the waste and then saves a lot of gravel and asphalt material resources, at the same time, land resources can be chosen without exploitation of sand and gravel materials and taking up by waste materials. Secondly, this technology can prevent asphalt concrete waste from polluting discard sites and their surrounding environment by reusing asphalt material and effectively protect the forest, maintain the natural landscape and ecological environment by reducing the exploitation of stone. Thirdly, comparing with the traditional construction method, cold-recycling technology can save costs because the old road material is fully utilized. The cost has a decreasing roughly 20% -40% along with the regeneration thickness increasing. The deeper thickness reuses, the lower cost reduces. Practice proves that cold-recycling technology is an important component of implementation of sustainable development strategy and also has great practical significance in our country present stage 3. COLD RECYCLING TECHNOLOGY CONSTRUCTION PLAN According to the existing road structure and the new design of the different technologies cold recycling technology can place a variety of construction program. To use cement as additive in cold on-site recycling asphalt concrete pavement is the most common option in terms of the road structure in North China. Generally, the content of cement is appropriate at 3% to 5% among total weight. Practical data needs identified through the test. In order to obtain good compaction results, an appropriate amount of water is as well as added. There are two ways of adding cement. One way is scattering cement beforehand on the road that needs regeneration by artificial or special machinery. Another method is to use cement slurry mixing truck, which mixes cement and water into the cement slurry and then transports them to mixing storage through the control system of regeneration machine. The two mentioned methods have advantages and disadvantages. The first method is simple and easy usage of cement and water control. The disadvantage is that it is prone to dust in the construction process. Although it can avoid the production of dust and wind with cement slurry mixing truck, this method is not easy to control water consumption. Regenerating device segments working trips into several stages in construction to complete a section pavement milling, while the final rolling of regeneration layer can be done after completing all the operations section of the construction process, therefore, the time has great difference 123

3 between the first and later milling pavement to the final rolling.with the passage of time the surface water of first milling evaporates more but that of later milling has less evaporation under drier climate environment. If the same amount of water is added in advance, the water content in all parts of road will different in final rolling. Cement and water are separate in pre-spreading cement way so that adjusting the amount of water will not affect the amount of cement. We will adopt pre-spreading cement method in this construction. 4. CONSTRUCTION PRACTICE The original structure of DongHong road is asphalt concrete surface 5 centimeters thick and lime-flyashed 16 centimeters thick. Cold Recycled Pavement is treated as the bottom basic unit of basic level structure. 4.1 Laboratory experiment The screening experiment to the first mixing mixture of different sections shows that there exists quite different phenomenon in macadam content. It is clearly unrealistic to test cement content of every section during construction, therefore, even mixing all mixture took back from the every construction site and screening out the cement of 4.75mm break stone in diameter is up to 47%. The compaction test for different cement content of this mixture can determine the optimum moisture and maximum dry density of cement mixture in the case of different cement content and then unconfined compressive strength test carried out (see Table 1). Table 1: Compaction test for different content of additives Cement content (%) Optimum moisture (%) Maximum dry density (g/cm 3 ) Unconfined compressive strength (MP a ) days unconfined compressive strength of cold recycling treatment layer is required to reach1.5 MPa. According to the analysis of past examples of the use of recycled materials, meanwhile, adverse effects of various factors are taken into consideration as well; the amount of cement was determined 5.0%. In the site construction process, from the beginning of secondary mixing with cement to roller compacted molding needs time. Time delay test aimed at 5.0% cement content has been done in order to control this time. The results of 2 hours, 4 hours, 6 hours compaction test and the unconfined compressive strength test is listed in table

4 Table 2: Time delay test Delay time ( hour ) Cement content (%) Optimum moisture (%) Maximum dry density (g/cm 3 ) Unconfined compressive strength (MP a ) Based on the above data, the time from the beginning of secondary mixing with cement to roller compacted molding is less than two hours within four hours. 4.2 On-site construction technology 1. Old road milling and mixing Using Wirtgen2000DC cold recycling machine, the old pavement is broken, milled and cold mixed to its intended depth in the light of measurement, the design elevation and cold recycled base thickness requirements. During this period, specially-assigned person follows the cold-mixing machine, checks the mixing depth at all times and cooperates operator to adjust the depth to ensure the design thickness of the cold recycled base and preclude the phenomenon that cold mix is not in place. 2. Initial shaping Materials near the site can be directly deployed by loader or grader while loader cooperate with the dump truck to adjust superfluous material above the elevation to lower part longitudinally at a distance from the site. After rough shaping grading with loader or coarse the centre and border stakes are resumed again and measured elevation that is controlled initially by hung measurement. Marked the site elevation between location and loose paving with white ash on spot, the road is shaped with loader at greater elevation site firstly, dig more fill less, and then the second grader leveling. The above-mentioned process is repeated until the surface formation and the surface achieves the required road camber and loose paving elevation basically. In the leveling process, the road width and height should be controlled strictly and local parts of frost boiling are replaced with quality packing. In order to accurately control the height of cold processing layer, based on experience factor, this loose paving coefficient of first level is predetermined Sprinkler boring material The optimum moisture content of cold treatment is 10.5% by testing. The testers measure the on-site sample water content of mixing section ahead of 24-hour, if the water content is too small, the builders need watering boring material so that it is kept within 1~1.5% large than the optimum water content. Watering should be uniform and sprinkler is prohibited in sprinkler paragraph u-turn and detention with the purpose of preventing local excessive water. Specific amount of water spray can be calculated by formula (1): 125

5 L = 100T w h ( ω + 1 ω) 0 ρ d ( m) (1) where L is the length of the spraying water per sprinkler, m; T is carrying the weight of water per sprinkler, kg; w is road width, m; h is the thickness of cold mixing, m; ω is optimum moisture content, %; 0 ω is actual moisture, %; ρ d is maximum dry unit weight, kg/m Cement Paving According to the thickness of cement stabilized soil, preconcerted dry density and the cement dose determined by the test, cement content of each square meter of cement stabilized soil is calculated and thus the spreading area of cement per bag, placed vertical and horizontal spacing is obtained. Then, drawing the border with mortar and unloading cement evenly to the inside border, the builders spread cement evenly with a scraper manually and pay attention to the spreading area of cement per bag in same area After cement paving, the surface should be no blank spaces and no excessive concentration of cement location. Cement delivery trucks should be equipped rainproof equipment. The amount of cement is calculated by formula (2): N = 100G s h ( t + 1) ρ d (2) where N is spreading area per sack of cement, m 2 ; G is weight per sack of cement, kg; s is unit area, 1m 2 ; h is the thickness of cold mixing, m; t is concrete quantity in terms of theory, %; ρ d is maximum dry unit weight, kg/m Second mixing After cement spreading evenly, the workers mixes them with two stabilized soil mixing machine as echelon formation, when mixing mixture, ensure mixing to the side note and mixing depth, strictly prohibit soil sandwich retained in the bottom of mixing layer. After mixing, the mixture is rolled with rubber-tired roller above all and identified potential irregularities, then leveling with graders. To ensure uniform mixing formation, the materials should be the second wet mix. As the cement hydration reaction, the second mixture after 126

6 mixing water content will decrease; therefore, sprinkling is needed in time and evenly, not too much. 6. Shaping The grader is used to shape pavement after mixture evenly mixing immediately. Scraping is done from both sides to the center of the road straight section while inside-out in the flat curve. Rubber tire roller presses through pavement initial shaped rapidly to expose potential uneven. For local low recesses, harrowing its over 5cm surface with tooth harrow and leveling with fresh mixture are shaped by graders which will be scrape material above off-road. In the course of shaping, no vehicular can be allowed to pass and no obvious aggregate segregation phenomenon occurs. Dressing to smooth transverse joint is required as far as possible vertical joints. The cross slope, roughness, and elevation are measured for the mixing forming mixture in time and promptly deal with and problem once found. 7. Rolling compaction Rolling compaction is the key to cold recycling, so we must strictly control the mixture of water and rolling methods. When the water content of mixture is 1% ~ 1.5% larger than the optimum water content after rolling, the road should immediately roll. According to the results of test sections rolling method as follow: the CA25 rolling first with 92% degree of compaction, and then double-drum vibration roller shock pressure 2 times with compaction to 98% or more, then CA25 rolling again, the final compaction with smooth rubber tires. As the cement is hydraulicity material, rolling should take full account of the initial set time and final set time, and rolling should be completed within the delay time determined by tests, no obvious wheel track and the required density. In the process rolling, the surface of cement stabilized soil should always be kept moist, for example, the water evaporates too quickly to be completed in time sprinkle a small amount of water. In the rolling process, the sand cone method is adopted to track testing compaction in a timely manner to track testing compaction to ensure compaction to meet the design requirements. 8. Joint treatment The joints of two construction work on the same day should adopt appropriate overlapping method. After the previous section mixing and shaping, 5 ~ 8 meter long zone without rolling will be left to mix again with a reasonable amount of cement and then compacted together with the posterior segment. Mixed and shaped cement stabilized soil should be completed rolling within the time determined by test. Longitudinal joints should be avoided and overlap mixing in the event that the road must be divided into two parts. 9. Curing The main components of influencing strength of cement is tri-calcium silicate (C 3 S) and dicalcium silicate (C 2 S), in particular, tri-calcium silicate has great influence on the early strength of cement, therefore the conservation of cold recycled post-forming will directly affect the strength and apparent quality of structure layer and for which specialist personnel and equipment should be equipped with. 127

7 Each section of the road should be fully covered with straws for curing after rolling and passing through inspection. This project is to be equipped with two sprinklers and more to long curing sections, concrete measures are as follows: First, the road after passing inspection shall be promptly covered with straws, sprinkler maintenance and curing deadline not less than 7 days and must be kept moist structural layer. Second, the road is closed to traffic in real earnest except sprinklers in the curing period (7 day) with dedicated staff to cut off the traffic entrances and exits of all curing section. 5. PROBLEMS IN CONSTRUCTION First, Emphasis would be laid on the control of water content in construction so that construction personnel should add water to the material in the light of site water content timely to ensure the site water content is about 1% higher than optimum water content Second, appropriate amount of water is added according to the measured water content, so as to ensure the amount of water is greater than 10% before paving cement. Finally, during the construction, we found that the 3cm thickness of the early mixing reserved leveling is excessive, therefore it s advisable that in future for actual construction the thickness should be about 2cm. 6. CONCLUSIONS Cold recycling technology of old asphalt pavement shows strong advantages in road rebuilding engineering for resource conservation, environmental protection, low cost, simple construction, etc. A large number of highways in China have been or are about to enter the overhaul period and plenty of low-grade highways also need to be upgraded. Taking old cold recycled asphalt pavement structure as a new layer in the base (sub-base) is proved to be safe and reliable, and it can be widely used in China. This article may contribute to the development of cold recycling technology in the future. REFERENCES [1] JIANG Xingwen1, Application of Emulsified Asphalt Cold Recycling Technology in Major Repair of Expressway, Road Machinery & Construction Mechanization, No.9 pp.48-49, 2009 [2] LING Tianqing, HE Liang, MA Yu, A Cold Recycling Technique for Foamed Bitumen Mix, Journal of Civil,Architectural & Environmental Engineering, Vol 31, No.1 pp , 2009 [3] GENG Jiuguang, CHEN Zhongda, LI Long, DAI Jingliang, Mixing Design of Cement-Emulsified- -Asphalt- Cold-Recycled-Mixture(CEACRM), Journal of Chang an University(Natural Science Edition), Vol. 29, No.1 pp.10-14, [4] SHA Qinglin, Semi Rigid Base of High Class Highway, ISBN: , China Communications Press, Beijing,