As an advanced rigid pavement structure, cement concrete pavement has been widely used in the world for more than one hundred years. The cement concrete pavement in China has a history of more than 70 years, which was mainly used in 1980s. Pass? Seven or five? 、? Eight-five? 、? Ninth Five-Year Plan? Three five-year research plans, design theory and construction level have reached a higher level. Since 1990s, China has started to use slipform paver to construct cement concrete pavement, and the quality and progress of the project have been greatly improved.
However, with the rapid development of China's national economy, the total amount of road transportation continues to grow, and the construction of transportation infrastructure continues to break through. The traffic flow, traffic load and tire pressure are increasing, and the road use conditions are getting worse and worse. How to build a smooth, comfortable and safe pavement is a difficult problem for every highway builder and a goal that is constantly pursued.
First, the current situation of pavement
(1) Highway traffic conditions. At present, China's expressways are characterized by a large traffic flow, especially the overloading of trucks and the fast running speed. Therefore, higher requirements are put forward for the construction of expressways in China.
(2) The present situation of expressway pavement. Asphalt concrete pavement and cement concrete pavement are two main forms of expressway pavement in China. Relatively speaking, cement concrete pavement has a poor impression on society, which leads to less and less cement concrete pavement used on high-grade pavement in China in recent years. For this reason, the author has tracked and analyzed the concrete pavements of cement roads in some provinces and cities in China, and found that many pavements do have diseases such as boiling, broken slabs and cracking. Some even started repairing just after opening to traffic. The poor quality is particularly serious, which greatly affects the comfort and safety of driving and has a great impact on society.
Second, the common problems and causes of cement concrete pavement
(a) cement concrete pavement cracking, broken plate, wrong platform. Cracking and broken slab of cement concrete pavement in China will directly affect the performance of pavement. Pavement rainwater seeps into the base, leading to mud pumping and emptying of the base, resulting in dislocation and broken slab. Broken slate appeared on the sidewalk, mainly for the following reasons:
1. Construction interruption board: In the construction of slipform paving pavement, there are generally the following factors:
(1) The daily temperature difference is too large or the evaporation is too large. The daily temperature difference is too large, the joint cutting is not timely, and the temperature stress is higher than the tensile strength of concrete pavement, which leads to broken slab. The wind speed is too large, the evaporation is too large, the maintenance is not timely, the surface dries too fast, and shrinkage cracks appear.
(2) The strength of the base is too high, and the cracks at the base are not treated, which leads to the reflection of cracks at the base.
(3) Too high hydration heat or poor cement stability, or mixed use of two kinds of cement, will also lead to early plate breakage. Enterprises in China are collectivized, and numerous small factories have been merged by major manufacturers and listed after the transformation. The original factory supply is not timely, and the two factories supply cement at the same time, and the two kinds of cement are mixed together, which easily leads to large-scale cracking and broken slabs on the pavement.
(4) Improper operation of slipform paver or inadaptability of concrete workability will cause pavement cracking and broken slab.
2. Staggered platform and broken plate after opening to traffic
(1) The broken slab within two years after opening to traffic, first, the subgrade did not completely settle stably, and the subgrade naturally settled and broke slab. Second, the roughness of the base surface is too large. When the panel shrinks, the local friction between the base and the pavement is too large, which causes the tensile stress of the concrete pavement to be too large and the slab is broken.
(2) The broken slab within two to five years after opening to traffic is mainly caused by poor stability of the base, poor anti-scouring ability, poor lateral drainage of pavement structure, mud pumping and emptying of the base.
(3) The dowel bar at the contraction joint was not inserted, and the heavy vehicle repeatedly rolled and bumped, resulting in slight settlement of the plate end and dislocation between plates.
(4) Insufficient reserve of flexural and tensile strength of pavement, and high-speed growth of heavy vehicles is one of the reasons for pavement slab fracture.
(2) The cement concrete pavement has poor flatness and high noise. Compared with asphalt concrete pavement, the smoothness of cement concrete pavement is lower, but the attenuation is slower. But at present, the smoothness of cement concrete pavement in China is poor, and the author thinks that there are mainly the following factors:
1. The technical starting point of cement concrete pavement in China is low, and the construction level of cement concrete pavement is relatively low due to the low degree and short time of using machinery instead of construction. Design units and construction units pay insufficient attention to the study of pavement, and shorten the construction period at will, which leads to the construction unit rushing to work, which is one of the main reasons for the poor smoothness of pavement.
2. The staff of the construction unit is weak, and the technical personnel do not know much about pavement. They can't adjust each link of paving according to different construction environment, but copy the successful experience in the past. Mechanical operators have low cultural quality and do not know all aspects of pavement construction thoroughly, so the smoothness of pavement construction is not ideal.
3. The design unit did not consider the influencing factors of the smoothness of concrete pavement in the later period, such as insufficient reserve of flexural strength of pavement and the possibility of dislocation when dowel bar is not inserted into contraction joint.
4. The flatness of the base is very poor. The construction unit and the construction unit pay insufficient attention to the smoothness of the grass-roots unit, resulting in uneven pavement thickness and inconsistent shrinkage, resulting in poor pavement smoothness.
5. Insufficient design thickness or construction thickness of pavement affects the service life of pavement. Short-lived Broken boards. Foreign researchers believe that in a certain range, the service life of pavement will be increased by 3 ~ 5 years for every increase in thickness of 1cm.
The noise of cement concrete pavement is higher than that of asphalt pavement, which is mainly caused by poor smoothness, staggered floors and broken slabs. Secondly, the lack of control over the spacing and depth of pavement anti-slide structures in China is also one of the reasons for the noise. Many anti-sliding structures made of soft tension in China are extremely difficult to meet the requirements of the code, and the width or depth of hard notches are also different. In addition, in Germany, France and other countries, slipform paver is used to wet double-layer pavement at one time. The upper layer is high-strength and low-noise fine-grained cement concrete pavement, and its noise level is equivalent to that of asphalt pavement.
(3) The surface of cement concrete pavement is hollow, peeling and exposed. The peeling and void of cement concrete pavement are mainly affected by the following factors:
1. Material problem. Gravel, sand, mud content is too large, or there are mud blocks, potholes appear after opening to traffic, or the surface mortar strength is not enough, peeling and exposure appear.
2. The slump of concrete is not well controlled. Honeycomb pits appear during construction, and artificial repair with concrete or mortar is easy to cause potholes or peeling.
3. When it rains during construction, the rain washes away the road surface that has not been initially set, resulting in peeling and exposure.
Third, further improve the construction quality of cement concrete pavement.
Many expressways and local highways in China often have pavement cracks, broken slabs, frost boiling and a lot of rework. Because of the difficulty in repairing cement concrete pavement, it is often opened to traffic while constructing, which has great security risks, great economic losses and extremely bad social impact. How to further improve the construction quality of cement concrete pavement, reduce or eliminate the occurrence of common faults and improve the driving comfort is a problem that every engineer and technician is very concerned about. After years of study and accumulation, the author talks about how to improve the quality of pavement construction.
(A) to strengthen the choice of raw materials for cement concrete pavement mixture. The basic material condition of constructing cement concrete pavement of expressway is that the raw materials of cement concrete pavement should be excellent, and the quality control of raw materials before entering the site is very important.
Cement mainly controls the flexural strength of cement. There are many domestic cement production enterprises with different production technologies. Therefore, selecting qualified cement is the primary factor to ensure the strength of concrete. In addition, the stability and free calcium chloride content of cement, fineness of cement, drying shrinkage and setting time, alkali content of cement, the dosage of mixed materials in cement, and the construction temperature of cement should also be controlled in summer.
Fly ash should use first-class ash and second-class ash, mainly to control the carbon content and ignition fineness of fly ash.
The choice of additives is the key to the science of pavement concrete materials. Appropriate additives should meet the adaptability requirements, as well as the high flexural strength and low cement content required for concrete pavement construction. The third is to meet the requirements of the mechanical construction methods used for constructability, and the fourth is to meet the requirements of small deformation and durability.
Coarse aggregate mainly controls its maximum particle size, hardness, silt content and particle shape. Igneous rocks and metamorphic rocks are generally selected instead of sedimentary rocks, which can ensure the strength of rocks and thus ensure the flexural and tensile strength of concrete. The processing adopts impact, impact or hammer two-stage crushing equipment, and it is difficult to meet the requirements of needle content only by jaw crushing;
Fine aggregate mainly controls the requirements of silt content and fineness modulus in sand. The fineness modulus of slip-form paving cement concrete pavement sand is between 2.0 and 3.5, and the silt content should be less than 2%.
(2) Optimize the mix design of cement concrete. The design and optimization of cement concrete mixture ratio is the key technology of slipform paving pavement. When designing the mixture ratio, we should not only meet the physical and mechanical indexes of cement concrete pavement, but also consider the workability and economy of concrete. Therefore, we should pay special attention to the following aspects.
1. Selection of water reducing agent. In order to prepare concrete with high flexural and tensile strength and good durability, the water-cement ratio of concrete is required to be small, so superplasticizer must be used to ensure good workability of concrete.
2. Composition principle of mineral aggregate gradation. Because of successful practical experience, the current theoretical and empirical data are relatively mature and easy to select. However, coarse aggregate is generally mixed with secondary aggregate. Because the current situation of coarse aggregate processing is not ideal, it is extremely important to determine a relatively reasonable mixing coefficient. According to the author's experience and consulting relevant data, the composition of coarse aggregate is the optimal curve, which is more conducive to the control of pavement smoothness and the improvement of flexural and tensile strength. In addition, the silt content of coarse aggregate is generally not more than 65438 0%, and the silt content of sand is not more than 2%, otherwise it will seriously affect the flexural strength of pavement and increase shrinkage deformation.
3. Quality requirements of cement and fly ash. Before the mix design, the pavement performance of cement should be tested. If any problem is found, it should be discussed with the manufacturer in time to adjust the cement composition to ensure that the cement quality is completely qualified. Fly ash must reach Grade II or above, and the material of each vehicle must be tested before it can be put into the tank, otherwise fly ash above Grade III will do great harm to the pavement.
4. Comprehensive coordination of influencing factors of mix design
(1) Type and maximum particle size of coarse aggregate. Gravel and gravel can be used to build high-quality cement concrete pavement, but the maximum particle size of gravel concrete is not more than 20mm, and gravel concrete should be not more than 30 mm.
(2) unit cement consumption. The unit cement dosage of pavement concrete should not be too large. If it is too large, the amount of cement slurry will increase, the locking force between aggregates will decrease, and the flexural strength will not increase obviously. Moreover, due to the increase of cement slurry, the shrinkage deformation will increase, which will affect the smoothness of pavement, crack easily and have poor economy. 52.5# cement dosage is generally not less than 300Kg, should not be greater than 360Kg.
(3) Gas content. At present, the specification has required the detection of air content in concrete. The author thinks that the use of air entraining agent is not only beneficial to the frost resistance of pavement, but also greatly improves the flexural strength of pavement, reduces the stiffness of pavement and reduces deformation. Therefore, detecting the air content of pavement is a necessary progress of pavement design and construction theory.
(3) Cement concrete pavement construction
1. Do the test part carefully. The experimental section of paving cement concrete pavement is divided into two stages: trial mixing and trial paving. Through the test of the test section, the following objectives should be achieved.
The experimental section is implemented under the same conditions as the construction conditions. By organizing construction, calculation and summary in designated sections, all necessary working parameters and construction experience are obtained to guide formal production.
Through the implementation of the test section, the following contents shall be determined:
Applicability of raw materials, workability of water-stabilized mixture and its mechanical indexes.
Verify whether the mixture ratio (cement dosage, mineral aggregate gradation) meets the specification requirements.
The number of mixers, transport vehicles and pavers, their working efficiency, the best combination mode and the rationality of personnel arrangement.
Paving technology, loose paving coefficient. Treatment of longitudinal and transverse joints.
Other technical indicators. Applicability of various detection methods.
After paving the test section, the construction unit and the supervisor will discuss the test results and submit the test summary report.
2. Cement concrete pavement construction. After the pavement test section is completed, it will be used to guide large-scale construction according to the results obtained from the test section.
(1) High-grade pavement must be constructed by slipform paver. At present, the three mainstream pavers used in the world are Wirtgen in Germany, GOMACO and CMI in the United States. The three pavers have their own advantages and disadvantages, and their working principles are different, but Wirt in Germany? The gen model has wider adaptability. However, with the higher demand for engineering quality and more complex materials, three kinds of pavers have made high-quality and high-smoothness pavements in China. On the road where the author participated in the construction, GOMACOGP2600 paver and WirtgenSP850 paver were used to make the road surface have good flatness.
(2) Before paving the road, make all preparations. The first is to clean up and repair the grassroots. Repair the damaged base with high-grade cement mortar, clean up the cracks at the base, then pour asphalt, and paste a single-layer linoleum with a width not less than 1 m on the surface to prevent reflective cracks on the pavement. Second, prepare the baseline of the paver, ensure that the baseline is smooth and comfortable, check the thickness one by one, and fine-tune the baseline where the thickness is inappropriate; Thirdly, determine the position of expansion joints and steel bars in advance and put them in place in advance, so that the installation of steel bars will not affect the continuous operation of paver.
(3) Before paving that day, the water content of materials, especially the water content of sand and small gravel, should be tested. Pay attention to control the mixing quality of concrete when mixing to ensure the uniformity of concrete slump.
(4) According to the weather conditions such as temperature and wind speed, fine-tune the factory slump of concrete to ensure the uniformity of the mixture in front of the machine, and fine-tune the water reducer and water cement ratio according to the change of transportation distance to ensure the workability of concrete.
(5) Before the paver starts construction, adjust the position of vibration frame and screed, and check the elevation angle and plane position of paver. During construction, control the material level in the vibrating bin and in front of the machine, and control the walking speed, not too fast. If it is too fast, the trimming speed of the workers can't keep up, the insertion position of the pull rod is inaccurate, and the nursing before and after the manipulator is prone to defects. Generally, 1.0 ~ 1.5 m/min is appropriate. In addition, according to the road conditions after construction, adjust the vibration frequency of paver in time.
(6) After pavement construction, spray curing agent in time, and determine the spraying frequency and concentration of curing agent according to weather conditions (temperature, wind speed, humidity, etc.). ). After the pavement is opened, it should be covered with moisturizing curing film or film for curing. After 28 days of health care, it was opened to traffic.
(7) Construction of longitudinal and transverse joints of pavement. In general, the setting of expansion joints should be as few as possible. According to the local temperature difference and construction temperature, set the location of expansion joints. Must be located near the building.
Four. conclusion
The author thinks that cement concrete pavement and asphalt concrete pavement have their own advantages and disadvantages, and adopting different pavement forms according to different regions and geological conditions will be more conducive to the rapid and healthy development of expressways in China. To build high-quality cement concrete pavement, the construction unit, design unit, construction unit, supervision unit and government departments need to correctly understand its characteristics and work together to start from the following aspects:
1. Strengthen the research on cement concrete pavement, analyze all kinds of problems in detail, and solve them one by one in a targeted way, fearing neither illness nor medical treatment.
2. Reasonably determine the construction period, and give the subgrade natural settlement time. If we don't rush the work blindly, the construction will be orderly, and it will be easier to ensure the construction quality.
3. Make a correct estimate of China's traffic development and reserve enough pavement strength reserves.
4. Strengthen the on-site control of construction quality, improve the technical quality of construction technicians and operators, and ensure the quality of pavement construction.
5. Track and introduce the advanced technology of cement concrete pavement design and construction in various countries in time to improve the pavement quality.
Based on the investigation of some domestic projects in recent years and my own practical experience, the author puts forward some viewpoints and methods to strengthen the quality control of cement concrete pavement, aiming at discussing with colleagues and experts to achieve * * *.