The cement stabilized aggregate is the most common semi-rigid base material at home and abroad. However, the cement stabilized macadam base does not eliminate the shortcomings of semi-rigid materials, so how to further reduce the occurrence of reflective cracks is still one of the key factors to give full play to the overall performance of the pavement structure. Considering our country as cement production power, wide raw material sources and low price, the cement material will remain as the main building material in the future for a long period of time, so it is necessary to study the cement stabilized macadam base, in order to provide the experience for the future more extensive application.
1 causes of cracks
There are many kinds of cracks in semi-rigid base asphalt pavement, but the main reasons can be divided into two categories, namely, the load type structural failure and the non load type cracks, including the reflection cracks and the corresponding cracks. Load type structural damage crack is produced by the vehicle dynamic load of vertical or horizontal stress, generated more than allowable ultimate tensile stress of the tensile stress caused by base; non load crack is the result of the environment, mainly the influence of humidity and temperature, caused by dry shrinkage, temperature shrinkage and fatigue, individual cases may also be due to the uneven subsidence caused by. In addition, it is possible to find the cracks caused by frost heaving on the asphalt pavement in frozen area.
Causes of cracks in the asphalt pavement of semi-rigid base appear there are 3 possibilities: one is bad surface performance, two is because the dry shrinkage and temperature shrinkage cracking and reflection to the surface cracks, three is due to surface and base caused by the interaction. It is generally believed that the crack of semi-rigid base asphalt pavement is caused by semi-rigid base, which is mainly caused by dry shrinkage crack of semi-rigid base material. This is a domestic load crack of semi-rigid pavement cracks, a temperature shrinkage crack of asphalt layer and semi-rigid base, temperature shrinkage crack or shrinkage cracks caused by the reflection crack or the corresponding crack of asphalt layer produced.
For semi-rigid base, cracks are often not caused by traffic loads. Because of the evaporation of water and the change of temperature, it is easy to produce cracks in cement stabilized macadam base. Therefore, in fact, it is in the state of a crack under the traffic load, the temperature shrinkage and dry shrinkage is the cause of cracks.
For the formation of reflective cracks, the tensile stress at the bottom of the surface is more than the ultimate strength of asphalt concrete. After cracking at the base level, the stress is transferred to the surface layer at the crack location due to the loss of resistance to tensile stress. If at this time plus the load of the main tensile stress (or shear stress), the stress value may exceed the ultimate strength of the material, the surface layer with cracking. The principal tensile stress (or shear stress) of the partial load is the cause of the formation of the reflective crack. Therefore, the pavement reflective cracking is mainly caused by the horizontal and vertical displacement after the cracking of the base.
Including the main influencing factors of asphalt pavement crack severity: the nature of asphalt and asphalt mixture properties, base material, structure and composition, climate conditions (especially temperature in winter and the change of traffic volume and vehicle type), and construction factors. As for the cement stabilized macadam asphalt pavement, the basic material composition and the construction level are particularly important. Reasonable design and good construction of cement stabilized macadam base is not easy to appear in the early stage of load type cracks.
2 control of shrinkage cracks of cement stabilized macadam base
In recent years, in order to improve the performance of asphalt pavement, asphalt pavement has been studied extensively, and many new technologies have been adopted, such as modified asphalt, SMA pavement and so on. However, the pavement structure as a whole, only to improve the performance of surface layer material is difficult to play the effect, as the main load-bearing layer has very important effect and the service life of the nature of the use of properties of base materials and the overall quality of the asphalt pavement. Therefore, the quality of the grassroots is not only conducive to the control of reflective cracks, but also improve the overall performance, is a reasonable and practical technical approach.
As the base of high quality, the biggest problem is how to reduce the shrinkage crack. Reduce the production of cracks can be from 4 aspects:
(1) reduce the shrinkage coefficient of the base material and improve the tensile strength of the base material;
(2) compensating shrinkage measures, such as adding expansion agent;
(3) the use of restricted shrinkage measures, such as the addition of fiber, geotextile, etc.;
(4) improve the constraint conditions, such as pre cuttingcrack, causing many micro cracks and crack first base. As the expansion of chemical material, because of its actual effect affected by environmental conditions, on the road under complex natural conditions may be difficult to use, while adding the reinforced materials will increase the difficulty in construction and improve construction cost. To reduce the constraints should be the actual effect of construction, such as pre cuttingcrack. The stress concentration due to the pre cut seam gap in basic load and environmental factors of the asphalt pavement, it is necessary to pre process the joint. However, how to deal with the control measures in the construction of the surface layer and the stability under the driving load is difficult to control. Therefore, the author believes that the design of material composition, construction and maintenance measures to start the most practical.
It is concluded that a large number of research results from the following 4 aspects should be considered to reduce cracks:
(1) to control the content of fine material and plastic index in order to reduce the clay content of water stable aggregate;
(2) to reduce the amount of stabilizer as far as possible under the premise of ensuring the strength requirements;
(3) to control the water content in the construction, the effect of water content on the dry shrinkage strain is equivalent to 2-3 times the cement dosage of;
(4) to reduce the exposure time of semi-rigid base, after the end of Health (also in the period of Health), immediately paved the cover. Indeed, cement stabilized macadam base shrinkage is mainly due to the composition of the cement and water, the external environment is the temperature and humidity changes. Therefore, the above points are the most important.
Cement stabilized macadam base shrinkage crack prevention measures, specifically, including the following aspects:
(1) material composition design
The first is the optimization of raw materials. Choose the right cement, in general, C3A content, fineness, gypsum content and S03 content of small cement shrinkage. The aggregate with small amount of clay, compact structure, low water absorption and large elastic modulus is selected. In the range of common aggregate, the shrinkage of sandstone aggregate is the largest, and the shrinkage of limestone and quartz are smaller. With good quality, fine particles of fly ash. Adding new additives, such as admixtures to reduce shrinkage of concrete shrinkage agent. The chemical composition of concrete shrinkage reducing agent is mainly polyether or polyether derivative, it has almost no adaptability to cement, and has good compatibility with other additives.
Followed by a good mix design. Through the reasonable adjustment of gradation, the skeleton dense structure can be used as much as possible to reduce the cement content, reduce the water consumption and increase the relative content of coarse aggregate.
(2) construction control
Practice has proved that the probability and degree of cracking of cement stabilized macadam base is significantly affected by the construction control of the site. Only under the premise of good construction, the effect of reducing measures can be reflected. The main contents of construction control is to strengthen the organization and management of aggregate, aggregate and ensure the lab mix ratio match, cement dosage accurately, maintain uniformity of strength of base material, the rolling layer thickness and moisture, improve the degree of compaction and overall stability, and pay attention to reasonable health.
In fact, many shrinkage cracks are caused by the lack of attention to health. Evaporation of water is too large, or when dry wet, and even long exposure. Cement stabilized crushed stone in the process of strength formation, the need to consume water, especially in the early strength of the formation, maintenance is not good shrinkage, while the tensile strength of the material itself is low, easy to crack. Therefore, in the laying of asphalt concrete layer below, to do the maintenance work at the grassroots level.