Cement macadam; Harm; Causes of cracks; Construction technology; Preventive and control measures
1 Cracks in Cement Stabilized Macadam Base and Its Harm
Cement stabilized macadam base is a semi-rigid structure. The easy cracking of cement stabilized macadam base is one of the key reasons that affect the early damage of asphalt concrete pavement, and it is also a problem that many researchers have been trying to solve. This kind of crack is very regular, generally every 5 m ~ 10 m across the top surface of the base, with a width of 0.5 cm~4 cm. It appeared earlier within one month after the cement stabilized base was paved, and some began to appear within 1 ~ 2 months after the asphalt concrete pavement was opened to traffic, which was caused by the reflection cracks of the cement stabilized base.
There are two hazards of base cracking: (1) reducing the overall strength of base; (2) After development, reflective cracks will be formed, which will lead to regular transverse cracks and arching of asphalt concrete pavement. In the second case, if not treated in time, rainwater will seep down from the cracks, and the cracks between asphalt concrete and base will be filled with free water. Under the repeated impact of vehicle load, asphalt concrete pavement will be pitted, broken and loose, which will lead to the early damage of asphalt concrete pavement and affect its service life and the quality and comfort of road driving.
2. The main causes of cracks in cement stabilized macadam base
2. Impact of1total
Poor aggregate gradation and high fine powder content are one of the factors that can not be ignored in causing cracks in water-stabilized base. Using poorly graded aggregate, the phenomenon of coarse and fine aggregate concentration is widespread, and its structural strength uniformity is poor, the deflection value variation coefficient is large, and the suspension coefficient of water-stabilized base is large. The concentrated part of coarse aggregate has high strength, while the concentrated part of fine aggregate has low strength and large drying shrinkage coefficient, which causes the water-stabilized base to crack from the lower strength section under the action of internal stress or external load. There are many fine powders with particle size less than 0.075 mm, and the powder itself contains plasticity, and the aggregate itself inevitably contains a small amount of soil, which increases the plasticity index. The fine powder content and plasticity index of mineral aggregate are one of the main factors affecting the dry shrinkage strain of water-stabilized base, and the dry shrinkage coefficient increases with the increase of fine powder content and plasticity index. The cement base with too much fine powder and too high plasticity index is more likely to have cracks.
2.2 the influence of cement varieties and dosage
As the only stabilizer, the quality of cement is very important, and its mineral composition and content are the leading factors for whether shrinkage cracks occur in water-stabilized base. It is generally believed that all kinds of cement can be used to stabilize soil. However, practice has proved that the mineral composition of cement is the main factor to determine the strength of water-stabilized aggregate for the same kind of aggregate. Experiments show that the stabilization effect of silicate cement is better, but that of aluminate cement is worse. In particular, the cement produced by some small factories has high content of dicalcium silicate and tricalcium aluminate, or contains high harmful impurities, which will reduce the stability of the water-stable layer and cause excessive shrinkage, thus leading to shrinkage cracks.
The strength of water-stabilized aggregate increases with the increase of cement dosage, but too much cement dosage can increase the strength, but at the same time it will also produce larger shrinkage and more cracks, which is also unreasonable in economy. If the cement dosage is too large, the shrinkage will increase, and the possibility of shrinkage cracks will also increase. In addition, different batches of cement, especially cement from different manufacturers and brands, are likely to have cracks.
2.3 the influence of construction factors
Mixing of (1) mixture
Cracks may occur if the mixture is mixed unevenly, local coarse and fine aggregates are concentrated, or local cement dosage is too large.
(2) Transportation and paving of mixture
After the mixture is mixed, it is not delivered to the construction site in time; Or because of the long transportation distance, the mixture is not covered during transportation, resulting in water loss of the mixture; And the excessive evaporation of water in the mixture caused by the long paving section makes the surface of water-stabilized base course dehydrated and dry, resulting in dry shrinkage cracks. If serious bumps occur during transportation, the mixture will segregate, or coarse and fine aggregates will concentrate when the paving is uneven, which will lead to cracks.
(3) Poor water content control
Water content is an important factor affecting the early shrinkage cracks of water-stabilized base. If the water content is too high, the phenomenon of "wave" and "elastic softness" will appear, which will affect the possible compactness and strength of the mixture, increase the dry shrinkage of the mixture, and make the structural layer prone to dry shrinkage cracks. High water content is prone to rutting, and the flatness is reduced, which will inevitably lead to cracking. In addition, the greater the water content during rolling, the more serious the shrinkage cracks after completion.
(4) Improper rolling method
Compaction molding is an important technology in the construction of water-stabilized base, and there are many factors that affect the compaction effect, such as water content, compaction machinery and methods. If the tonnage of the roller is too large and the rolling speed is too fast at the initial pressure, construction cracks will often appear. In addition, excessive vibration rolling is easy to cause large cement slurry on the surface of the water-stable layer, thus increasing the surface shrinkage and causing shrinkage cracks.
2.4 the impact of health care
Water-stabilized base needs moisture preservation for health preservation, so that the cement can be fully hydrated, and the water-stabilized strength is continuously increased to prevent shrinkage cracks caused by excessive water loss. If it is not cured in time or the curing period is insufficient, the mixture loses water too quickly, and the water required for cement hydration reaction is insufficient, resulting in drying shrinkage cracks. The number of shrinkage cracks is directly related to the water loss rate of mixture.
3 cement stabilized macadam base crack prevention measures
3. 1 Precautions during construction
3. 1. 1 Control the dosage and quality of cement
(1) On the premise of ensuring the design strength, the cement dosage shall be controlled within the range of 4% ~ 5.5%.
(2) Try to choose Portland cement, which requires small hydration heat and shrinkage, and both compressive strength and flexural strength can meet the requirements.
(3) Add additives such as retarding water reducer and retarding crack inhibitor to prolong the initial setting time of cement, or reduce the water demand of hydration reaction and improve the performance of cement.
3. 1.2 control water content
(1) The compacted water content should be controlled within 0.5% of the optimal water content, but it should be selected according to the specific construction conditions. If the climate is hot, the water volatilization loss rate is fast, or the transportation distance is long, the water loss is more in the process of transporting materials, and the water content will increase. The principle of selection is to reduce the water content as much as possible while ensuring the compaction quality.
(2) When the surface of the mixture is dry during the rolling process, it is best not to spray water with a sprinkler to prevent excessive water spraying, and a small amount of water can be supplemented with the intermittent water spraying device provided by the roller. In addition, in the preparation stage before paving, the sprinkler should not stop on the subbase to avoid too much water dripping on the parking place.
3. 1.3 Selecting aggregate
Adjust the mineral aggregate gradation of inorganic binder aggregate, increase the amount of coarse aggregate, reduce the content of fine aggregate, improve the internal friction of the mixture, form a reasonable gradation, and make the mixture form an embedded structure as much as possible.
3. 1.4 mixing
The mixing shall be sufficient, uniform and stable in quality.
3. 1.5 scrolling
Timely rolling and reasonable selection of roller can appropriately increase the degree of compaction and enhance the compressive strength of the base, thus increasing its ability to resist internal shrinkage and deformation stress.
3. 1.6 scheme
After the construction of water-stabilized base is completed, water shall be sprayed in time for maintenance, and the maintenance period shall be not less than 7d, and the surface shall be kept moist during the maintenance period. After the curing period expires, the next day, the bottom oil is sprayed for protection. Before paving asphalt concrete pavement, traffic should be closed as much as possible to reduce the influence of vehicle load and pave the pavement as soon as possible. Before laying the surface course, attention should be paid to the cleaning of the water-stabilized base course to remove impurities, otherwise the base course will not be well combined with the surface course, which will easily lead to weak points.
3.2 Preventive measures after the formation of cracks
Check the distribution of cracks: conduct a comprehensive inspection of the base, mark the crack position on the roadside, and count the number and total length of cracks. (1) cleaning the base: clean, blow and clean the 1 m area on both sides of the crack. (2) Joint filling: After cleaning, cut a groove with a width of about 1 cm and a depth of about 2 cm along the crack, blow off the dust in the crack with a forest fire blower, and fill the joint with AH-70 hot asphalt. (3) Spraying emulsified asphalt: spray PC-2 emulsified asphalt in the range of 0.3 ~ 0.5 ㎏/m on both sides of the crack. Laying geotextile or glass fiber grating: laying geotextile or glass fiber grating on both sides of the crack 1 m and fixing with nails; For geotextiles, small rollers should be used for rolling.
4 conclusion
Building roads and bridges for the people, creating first-class and high-quality services. With the rapid development of economic construction, the requirements for durability and comfort of road use are getting higher and higher. Cement stabilized macadam, which is widely used as road base, has many crack factors, but as long as we make more efforts in construction organization, quality control, maintenance management and so on, and do every link well, the cracks at the base will be greatly reduced, thus reducing the occurrence of pavement diseases.