1 Introduction
At present, China is at the peak of urban construction and infrastructure construction. According to relevant data, the construction waste generated in China is about 2.5 billion tons. Most of these huge construction wastes are simply piled up or buried in the open air, which not only occupies limited land space, but also causes great pollution to the environment [1]. In the recycling of construction waste, although China has made a series of research results, the standard of recycling of construction waste is not mature enough [2].
The research on the reuse of construction waste abroad is relatively early, and the relevant research in the United States shows that the particle size of recycled aggregate of construction waste is the most important factor affecting the performance of construction waste. When there are more recycled aggregate with large particle size, more voids lead to lower strength of concrete made of recycled aggregate [2]; In the utilization of construction waste in Japan, the principle of "whoever produces it is responsible" is adopted, and the utilization rate of construction waste is above 97%. At the same time, Japan has systematically studied the mix proportion, strength, durability and construction technology of concrete made of construction waste [3]; Germany has developed a distillation combustion process for construction waste to separate the effective components and use them separately. The generated gas is used to generate electricity, and the remaining broken construction waste is used to fill road subgrade and artificial landscape [4]. Through a series of experimental studies, the road technical indexes such as compressive strength, splitting strength, compressive resilience modulus, frost resistance and water stability of construction waste are systematically analyzed, which provides technical support for the popularization and application of construction waste on roads.
2 characteristics of raw materials
2. 1 cement
Cement adopts 32.5 # ordinary portland cement.
2.2 Construction waste
The road performance of construction waste is mainly determined by its composition. The construction waste used in this paper mainly comes from the demolition of old buildings. The main components of construction waste are soil, broken bricks, concrete blocks, mortar, wood and steel. In the production site of construction waste recycled aggregate, the general equipment mainly includes: air separation and impurity removal equipment, screening equipment, magnetic separation equipment, impact crusher and other equipment. The concrete blocks and broken bricks in construction waste are crushed and screened, and then mixed according to the gradation required by the specification. The crushing value of construction waste is more than 26%, so it can not be directly used as the base of expressway and first-class highway pavement, but it can be used as the base or subbase of second-class and below highway pavement [5]. Compared with ordinary natural aggregate, the surface of recycled aggregate of construction waste absorbs more cement mortar, and the surface has obvious openings. At the same time, when producing recycled aggregate from construction waste, it has a great impact on aggregate, resulting in a certain number of tiny cracks in recycled aggregate, thus reducing the strength of aggregate. However, the content of micropowder in recycled aggregate of construction waste is higher than that of natural aggregate, and there are unhydrated cement particles and some active substances in micropowder, which can improve the road performance of recycled aggregate to some extent.
3 Road performance of cement stabilized construction waste
3. 1 standard compaction test
When there are few fine particles in the construction waste mixture, the structure formed by the mixture is dense skeleton, and the fine particles are suspended in the gaps of the skeleton. At this time, the dry density of the construction waste mixture is small. When the content of fine particles is large, the mixture is difficult to form a skeleton, and the strength of the mixture is small. The main purpose of standard compaction test is to determine the mixture ratio of recycled aggregate of cement stabilized construction waste, that is, to determine the optimal water content of cement stabilized construction waste under the condition of maximum dry density, and finally to determine its mixture ratio. When the water content is relatively small, the dry density of recycled aggregate will increase with the increase of water content. When the water content increases to a certain extent, the dry density begins to decrease, and the curve of water content-dry density appears inflection point. At this time, the dry density at the inflection point is called the maximum dry density, and the water content at the inflection point is called the optimal water content [6].
According to the method in the test specification [6], firstly, determine the dosage of cement, then take 5-6 recycled aggregate from construction waste, prepare recycled aggregate mixture samples according to different water contents, then compact the mixture samples in a test tube according to the specified compaction work, then weigh the compacted mixture samples, measure their water contents, calculate the dry density, and finally draw all points in the water content-dry density coordinate system in turn and connect them into a smooth curve. Five groups of cement with cement contents of 3%, 4%, 5%, 6% and 7% were selected to determine the optimum water content and maximum dry density of cement stabilized construction waste under different cement contents. The greater the cement content, the greater the optimum water content of cement stabilized construction waste, while the maximum dry density has little change. This is mainly because a certain amount of water needs to be consumed when cement mixed into construction waste reacts with water, and the maximum dry density of construction waste does not change much because of the small content of cement in construction waste.
3.2 unconfined compressive strength test of cement stabilized construction waste
According to the strength requirements of cement stabilized material base or subbase in the specification [7], when cement stabilized material is used as the subbase of light traffic asphalt pavement, its 7-day unconfined compressive strength value should be greater than or equal to1.5 MPa; ; However, when it is used as the base of overweight traffic asphalt pavement, its 7-day unconfined compressive strength should be greater than or equal to 3.5MPa. According to the relevant test regulations [8], the cylindrical specimen is molded by static pressure method, and the specimen is wet cured under standard curing conditions for 6 days, then cured in water for 24 hours, and finally tested. The day before the test, put the specimen in water for 24 hours, then take it out of the water before the test, and dry the surface of the specimen with a wet towel. Finally, the specimen is put on the lifting platform of universal pressure testing machine for unconfined compressive strength test, and the loading speed is controlled at 0.1kn/s ~ 0.2 kn/s. In order to ensure the accuracy of the test results, every
In this paper, five groups of cement with cement content of 3%, 4%, 5%, 6% and 7% were selected, and the unconfined compressive strength of cement-stabilized construction waste was measured at 7 days, 28 days and 90 days respectively. According to the unconfined compressive strength test data, the greater the dosage of cement, the greater the unconfined compressive strength of cement stabilized construction waste. This is because more cement reacts with water to generate more cement hydration products, which gradually form three-dimensional network structure cement paste with construction waste aggregate. These cement pastes will eventually directly affect the strength of cement stabilized construction waste, and at the same time, with the continuous extension of age, the strength of cement stabilized construction waste will continue to increase until it tends to be stable.
3.3 Splitting strength test of cement stabilized construction waste
According to the test method in relevant test regulations [8], the cement-stabilized construction waste materials with cement contents of 3%, 4%, 5%, 6% and 7% were made into standard specimens, which were cured for 90d under standard curing conditions, and finally the splitting strength of the specimens was measured as shown in Table 6. From the data in the splitting strength test, it can be seen that the splitting strength of lime-fly ash soil (generally 0.25MPa[9]) is less than that of cement-stabilized construction waste, while the splitting strength of cement-stabilized construction waste is less than that of cement-stabilized macadam (generally 0.5MPa), because the construction waste itself has a certain strength. The greater the dosage of cement, the greater the splitting strength of cement-stabilized construction waste, and the cement content increases by 65,438 0%. The splitting strength is increased by about 8%~ 18%, because a large number of cement reactions produce more cement hydration products, which has a great influence on the splitting strength of cement stabilized construction waste.
3.4 Cement stabilized construction waste compressive resilience modulus test
When cement stabilized construction waste is used as the base and subbase material of highway, it bears the load from the pavement. At this time, cement stabilized construction waste should not only have sufficient strength, but also have certain rigidity to resist the deformation of base and subbase. According to the test requirements of relevant codes, the compressive resilience modulus of cement stabilized construction waste with different cement content was measured. The compressive resilience modulus of cement-stabilized construction waste is similar to that of lime-crushed stone soil (generally 700 MPa ~ 1 100MPa[9]), but smaller than that of lime-crushed stone (generally1/00 MPa [9]). The greater the cement content, the greater the compressive resilience modulus of cement stabilized construction waste. This is because more cement hydration produces more hydration products with expansive structure, which are filled in the gaps of construction waste, and finally the recycled aggregate of construction waste has certain deformation resistance [10].
3.5 Frost resistance of cement stabilized construction waste
Pavement subbase and subbase in cold areas not only bear loads, but also bear the influence of environmental temperature changes. Therefore, when cement stabilized construction waste is used in pavement subbase and subbase in cold areas, it needs to have certain frost resistance. According to the test method in the relevant test regulations [8], the frost resistance coefficients of cement stabilized construction waste were determined when the cement content was 3%, 4%, 5%, 6% and 7% respectively. The frost resistance coefficient of cement stabilized construction waste is more than 60%, which has certain frost resistance. Therefore, cement stabilized construction waste can be used as base or subbase material when building roads in moderately cold areas. The greater the cement content, the better the frost resistance of cement-stabilized construction waste. Every time the cement content increases, the frost resistance coefficient of cement-stabilized construction waste increases by 9.6% ~ 65,438+03.0%. This is because more hydration products produced by cement hydration fill the pores in the recycled aggregate of construction waste, making it difficult for water to enter the interior of construction waste, and finally making cement-stabilized construction waste have certain frost resistance.
3.6 Water Stability of Cement Stabilized Construction Waste
Water stability is an important index of highway performance, which is generally expressed by water stability coefficient. According to the test method in relevant test regulations [8], cement stabilized construction waste specimens are generally cured for 28d under standard curing conditions, with the last soaking time of 65438±0d, then naturally dried for 48h for five times, and then soaked for 24h for dry-wet cycle, and finally the ratio of compressive strength of the specimens to compressive strength measured at standard curing age of 28d is measured. According to the test method, when the cement content is 3%, 4%, 5%, 6% and 7% respectively. The greater the water stability coefficient of cement stabilized construction waste is, the water stability coefficient will increase by 3%~6% for every increase of cement content 1%.
4 conclusion
In this paper, through a series of systematic studies on cement stabilized construction waste, a series of road performance test results such as unconfined compressive strength, splitting strength, compressive resilience modulus, frost resistance and water stability of cement stabilized construction waste are obtained. ① Cement-stabilized construction waste has high strength and rigidity, and can be used as subbase material of pavement when building light traffic roads below Grade II. If used as pavement subbase material, the cement content in cement stabilized construction waste mixture should be greater than or equal to 6%. ② The frost resistance and water stability of cement stabilized construction waste are good, and the greater the dosage of cement, the better the frost resistance and water stability. Every time the cement content increases 1%, the frost resistance coefficient of cement stabilized construction waste increases by 9.6% ~ 13.0%, and the water stability coefficient increases by 3%~6%. The frost resistance coefficients of cement stabilized construction waste in the test are all greater than 50%.
List of project bidding owners
List of bidding owners of Shanghai project
List of bidding owners of Jiangsu project
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd