1 ensures the best compactness of subgrade.
The settlement of subgrade will weaken its strength, and it is impossible to prevent the settlement from happening. Even if the compaction degree of subgrade reaches 98%, there will still be 2% void ratio, and there will be settlement if there is void. A small amount of settlement will not damage the subgrade. Therefore, in order to ensure sufficient stability of subgrade, it is necessary to minimize the settlement, especially to avoid the uneven settlement which has a serious impact. In the construction of high-grade highway, it is not appropriate to reduce settlement by natural settlement, especially for projects with short construction period. Even if loading preloading is adopted, it is not economical.
There are two kinds of subgrade settlement: first, the foundation is weak and not properly reinforced, and its bearing capacity is lower than the gravity compression deformation of the fill layer covering it; Second, the filling layer is not well compacted, and its compactness is less than the design requirements, so its strength is inevitably insufficient, and it will deform under the action of self-weight and external force. The smaller the compactness, the greater the deformation. In order to reduce subgrade deformation caused by foundation settlement, before subgrade filling, crops, trees, weeds and humus on the surface of subgrade should be removed first, and then the compaction degree of subgrade should be not less than 93% by heavy roller for many times.
There are 300m sections with poor foundation (too wet foundation) and bearing capacity less than 1.2kg/cm2, so vehicles cannot walk on the foundation. When filling subgrade, the foundation should be reinforced first, and the foundation soil should be ploughed to a depth of 30 cm by taking advantage of the time when the groundwater level drops in winter and spring, dried in the sun, and then compacted with 8% lime. After testing, the compaction degree of foundation reaches more than 90%.
In order to make the subgrade have good compactness, improve its strength, reduce the plastic deformation and permeability coefficient of the subgrade, thus increase the stability and minimize the settlement of the filled soil layer, combined with the existing rolling equipment of the construction unit, the subgrade filling adopts the methods of "layered filling" and "thin layer multi-pressure", and the thickness of each layer is not more than 30cm. A test section with a length of 100 m and a thickness of 50cm is filled at k3+000~k3+ 100, and the soil of the filling layer is cohesive soil. Under the optimum water content, the compaction degree is less than 93% after rolling with 18 ton vibratory roller (which fails to meet the design requirements) for 5 times, and then rolling with 10 times and testing again. The adjacent 30 cm thick fill is 200 meters long. When the same cohesive soil and rolling machine are used for the fourth rolling, the compaction degree has reached 93%~95%. Under the condition of 50-ton vibratory roller, dynamic compaction, dynamic compaction and other large-tonnage compaction machinery, the filling thickness of each layer can be appropriately increased, and the specific layer thickness should be determined by experiments according to different machinery. In order to ensure uniform strength, "layered filling" must be emphasized, because different layers have different compaction requirements.
2. Improve the strength and stability of grass-roots units
Concrete pavement base must have high rigidity, strong integrity and good water stability. Commonly used base structures include lime fly ash stabilized macadam, lime soil, industrial waste residue and other semi-rigid base. Lime soil should be used as subbase, not as the base of cement concrete and other advanced pavements. The initial strength and water stability of calcareous soil are low, and cracks are easy to occur during dry shrinkage and cold shrinkage. The water infiltrated from the surface cracks will hydrate the surface of lime-soil base, reduce its strength, and at the same time, it is easy to make the surface slide. Cement stabilized macadam is used in this road project, which has good water stability and is superior to lime stabilized soil. The strength of the road engineering base requires watering for 7 days, and its saturated unconfined compressive strength is > > 0.8mpa, and it should reach 1.2mpa in 28 days. After testing, the lime-soil subbase trial paved in this contract section shows that the compactness, lime dosage and other indicators meet the requirements. Only when the lime soil is saturated with water and a group of specimens are only soaked in water for 1~4 hours, the specimens are all loose, far from strength. Building a pavement on such a base will eventually cause the base to loosen/slip, resulting in potholes and cracks on the surface.
The uniformity and smoothness of base strength have great influence on the quality of concrete slabs. If the mixing is uneven and different soils are mixed, the lime soil will not pass through the sieve or be badly rolled, resulting in more caking, poor flatness and poor treatment of the joint between new and old subgrade. , it will cause uneven strength of the base, poor flatness, and different thickness of the concrete panel, resulting in uneven internal stress of the panel and increased friction at the bottom of the concrete panel, which is easy to cause the panel to break under the action of temperature stress. Therefore, when accepting the delivered base, in addition to the items specified in the Standard for Quality Inspection and Evaluation of Highway Engineering issued by the Ministry of Communications of People's Republic of China (PRC), other related items should be added to check the deflection value, mixing uniformity and water content of the base, so that the base can provide uniform and stable support for the concrete slab and prevent the adverse effects such as frost heaving and frost heaving. Ensure the overall strength and smoothness of the pavement, and achieve the purpose of prolonging the service life of concrete pavement. 3 the influence of construction temperature on concrete slab
3. 1 Causes of temperature cracks
In the process of concrete hardening, cement releases a lot of hydration heat, the internal temperature rises continuously, and the surface produces tensile stress. In the later cooling process, due to the constraint of foundation or old concrete, tensile stress will appear in concrete. The decrease of air temperature will also produce great tensile stress on the concrete surface. When these tensile stresses exceed the crack resistance of concrete, cracks will appear. The internal humidity of many concrete changes little or slowly, but the surface humidity may change greatly or violently, such as poor maintenance, sometimes dry and sometimes wet, and the surface shrinkage deformation is constrained by the internal concrete, which often leads to cracks. Concrete is a brittle material, and its tensile strength is about110 of its compressive strength. The ultimate tensile deformation under short-term load is only (0.6 ~ 1.0) × 104, and under long-term load is only (1.2 ~ 2.0 )×. Due to uneven raw materials, unstable water-cement ratio and segregation during transportation and pouring, the tensile strength of the same piece of concrete is uneven, and there are many weak parts with low tensile capacity, which are prone to cracks. In reinforced concrete, tensile stress is mainly borne by steel bars, while concrete only bears compressive stress. If the tensile stress occurs in plain concrete or at the edge of reinforced concrete structure, it must be borne by the concrete itself. In general design, no tensile stress or only a small tensile stress is needed.
However, when the concrete cools from the highest temperature to a stable temperature during operation, it often produces considerable tensile stress inside the concrete. Sometimes the temperature stress will exceed the stress caused by other external loads, so it is extremely important to master the variation law of temperature stress for reasonable structural design and construction.
3.2 Temperature stress analysis
According to the formation process of temperature stress, it can be divided into the following three stages:
(1) Early stage: Generally, it takes about 30 days from the beginning of concrete pouring to the end of cement exothermic. This stage has two characteristics, one is that cement releases a lot of hydration heat, and the other is that the elastic modulus of concrete changes dramatically. Due to the change of elastic modulus, residual stress is formed in concrete during this period.
(2) Mid-term: the time from the end of the exothermic action of cement to the cooling of concrete to a stable temperature. During this period, the temperature stress is mainly caused by the cooling of concrete and the change of outside air temperature. These stresses are superimposed with the residual stresses formed in the early stage, during which the elastic modulus of concrete changes little.
(3) Late stage: the operation period after the concrete is completely cooled. Temperature stress is mainly caused by the change of outside air temperature, and these stresses are superimposed with the first two residual stresses.
According to the causes of temperature stress, it can be divided into two categories:
(1) autogenous stress: the temperature stress of a structure without any constraint or completely static boundary is generated due to the mutual constraint of the structure itself when the internal temperature is non-linearly distributed. For example, piers have relatively large structural dimensions. When concrete is cooled, the surface temperature is low and the internal temperature is high. Tensile stress appears on the surface and compressive stress appears in the middle.
(2) Constraint stress: the stress caused by the fact that all or part of the boundary of the structure is constrained by the outside world and cannot be deformed freely. Such as box girder roof concrete and guardrail concrete.
These two kinds of temperature stresses often work together with the stress caused by concrete drying shrinkage. It is a complicated work to accurately analyze the distribution and magnitude of temperature stress according to the known temperature. In most cases, it depends on model test or numerical calculation. The creep of concrete greatly reduces the temperature stress. When calculating the temperature stress, the influence of creep must be considered, and the specific calculation is not repeated here.
3.3 Temperature control and measures to prevent cracks
In order to prevent cracks and reduce temperature stress, we can control the temperature and improve the constraint conditions.
(1) Measures to control temperature
1) Measures such as improving aggregate gradation, adding air entraining agent or plasticizer to hard concrete, etc., to reduce the amount of cement in concrete;
2) When mixing concrete, add water or water to cool the macadam to reduce the pouring temperature of concrete;
3) When pouring concrete in hot weather, reduce the pouring thickness and use the pouring elevation to dissipate heat;
4) Water pipes are buried in concrete, and cold water is introduced to cool down;
5) Determine a reasonable time for formwork removal, and conduct surface insulation when the temperature drops suddenly to avoid a sharp temperature gradient on the concrete surface;
6) During construction in cold season, thermal insulation measures should be taken for the surface of concrete pouring blocks or thin-walled structures exposed for a long time;
(2) Measures to improve the constraint conditions
1) Reasonable split and block;
2) Avoid excessive foundation fluctuation;
3) Arrange the construction process reasonably to avoid excessive height difference and long-term side exposure;
(3) Adding additives
In order to ensure the quality of concrete engineering, prevent cracking and improve the durability of concrete, the correct use of additives is also one of the measures to reduce cracking. For example, the use of water reducing agent and anti-cracking agent, the author summarizes its main functions in practice as follows:
1) There are a lot of capillary channels in concrete. After water evaporates, capillary tension is generated in the capillary, which makes the concrete shrink and deform. Increasing the pore size of capillary can reduce the surface tension of capillary, but it will reduce the strength of concrete. This theory of surface tension was recognized internationally as early as 1960s.
2) The water-cement ratio is an important factor affecting the shrinkage of concrete, and the water consumption of concrete can be reduced by 25% by using water reducing and crack preventing agent.
3) Cement dosage is also an important factor affecting the shrinkage of concrete. The concrete added with water reducing agent and anti-cracking agent can keep the strength of concrete, while the cement consumption can be reduced by 65,438 05%, and its volume can be supplemented by increasing the aggregate consumption.
4) Water reducing and anti-cracking agent can improve the consistency of cement slurry, reduce concrete bleeding and reduce settlement and deformation.
5) Improve the cohesive force between cement slurry and aggregate, and improve the crack resistance of concrete.
6) Concrete is restrained to produce tensile stress when it shrinks, and cracks will occur when the tensile stress is greater than the tensile strength of concrete. The water reducing and anti-cracking agent can effectively improve the tensile strength of concrete and greatly improve the anti-cracking performance of concrete.
7) Admixtures can make concrete have good compactness, effectively improve the carbonation resistance of concrete and reduce carbonation shrinkage.
8) Appropriate retarding time of concrete mixed with water reducing and anti-cracking agent can effectively prevent the rapid hydration and heat release of cement and avoid the increase of plastic shrinkage caused by long-term non-coagulation of cement.
9) The concrete mixed with admixture has good workability, and the surface is easy to smooth, forming a micro-membrane, reducing water evaporation and drying shrinkage.
Many additives have the functions of retarding setting, increasing workability and improving plasticity. In engineering practice, we should do more experimental comparison and research in this field, which may be simpler and more economical than simply improving external conditions.
4 Strictly control the mixture ratio and water cement ratio of concrete.
The mix proportion and water-cement ratio of concrete play an important role in strength. Generally, a reasonable mixture ratio can be calculated and selected according to the relationship curve between water cement ratio and strength, but the key is to strictly implement the mixture ratio used in actual construction and resolutely put an end to feeding without weighing.
In road engineering, according to the cement from different manufacturers and the sand and gravel actually collected on site, the mix proportion of each construction unit is inspected by means of on-site sampling and trial matching. Any mixture ratio without the approval of the supervision office shall not be used. It is stipulated that ordinary portland cement produced by Shandong Cement Factory should be used uniformly, and slag cement should not be used to cast panels at will. The cement number is not less than 425, the concrete unit dosage is not more than 320kg/m 3, the water cement ratio is not more than 0.45, the concrete unit weight is not less than 2400kg/m3, and the sand content is controlled between 27-30%. The test strength of concrete is higher than the design strength 10%, that is, the compressive strength is greater than 33mpa and the flexural strength is greater than 5mpa.
It is found that the 28-day flexural strength of several broken slabs is mostly less than the design strength, and the unit mass of some concrete is less than 2400kg/m 3, and some specimens contain yellowish brown spots, indicating that the sand is unclean with soil particles. Sand and gravel piled in the open air will be contaminated with dirt and sundries, and the quality of concrete will be affected if it is not removed. To this end, the supervision room clearly stipulates that sand should be sieved and stones should be blown clean by a blower before entering the mixing equipment. If the impurity content in the soil exceeds 5%, it is not easy to blow clean, so it should be washed with water to make the impurity content less than 1%.
Try to choose the same cement produced by the same manufacturer between the two expansion joints. Especially in expansion joints, it is best to use the same batch of cement to avoid the influence of different cements on the strength uniformity of concrete.
Make at least two groups of specimens on each working face every day to check the mixture ratio and strength. In rainy weather in summer, it is necessary to check the water content of sandstone materials at any time and adjust the water consumption of concrete.
5. The joints of concrete pavement should be carefully made.
Joints have different structures according to their properties. The quality of joints is directly related to the use effect of concrete pavement.
5. 1 expansion joint
When the temperature rises, the concrete expands, which is easy to damage the expansion joints. According to relevant data, when the linear expansion coefficient of concrete is a= 1× 10-5 (℃) and the elastic modulus of concrete is e=3× 105kgf/cm2, when the temperature of concrete slab is higher than that of pouring 10℃, the stress σ at expansion joint is a.
The construction technology of expansion joint is complex and requires high quality. Unqualified expansion joints will aggravate the damage to panels, so care must be taken in the construction of expansion joints. Expansion joints shall be perpendicular to the center line of the pavement. When crossing bridges and culverts obliquely, a gradual section with a length of not less than 3 slabs (about 15m) should be set to gradually adjust the joint direction, and a steel mesh and force-transmitting steel bars should be set in the gradual section. The seam wall must be vertical up and down, and the seam width should be consistent.
The dowel bar of expansion joint should be positioned correctly in both horizontal and vertical directions, and the fixed dowel bar must be parallel to the board surface and the road center line. The fixing method shall be in accordance with the top formwork fixing method and small support method specified in Article 4.6. 1 of the Code for Construction of Cement Concrete Pavement. The movable end of the dowel bar should be free to move, so that the panel will not be damaged or torn when expanding or contracting. When preparing dowel bar, both ends should be sawed off, and should not be cut off, so as to maintain a complete circular section and achieve a smooth and stab-free angle at the end. The movable end shall be made of petroleum asphalt with good stability. After heating to 1 10℃, soak one end of the rod in hot asphalt for more than 2 minutes. The hot asphalt film attached to the rod end should not be too thick, and then roll it with talcum powder to prevent adhesion. During installation, it is required to put the movable end of the dowel bar into the casing, and then wrap it with plastic cloth and wrap it around the dowel bar to prevent concrete mortar from entering the casing.
5.2 longitudinal joints
It is very simple to make horizontal and vertical joints. The longitudinal joints at the two plates are connected by tie rods made of threaded steel bars, and the position of the tie rods is at 1/2 plate thickness. The side of the longitudinal joint should be evenly coated with petroleum asphalt for 2-3 times, but hot asphalt should not be coated on the pull rod of the threaded steel bar.
5.3 Shrinkage joint
In order to resist the cold shrinkage stress of concrete, slit method should be adopted. Sometimes the joint pressing method is also used together, that is, first press a joint every 25 meters, and then cut a joint every 5 meters to prevent the concrete panel from cracking prematurely. For example, once a batch of panels were poured and a piece was broken. Three days later, core sampling showed that the basement was flat and undamaged, and the face plate reached the design thickness and was uniform. After curing for 28 days, the strength of the specimen was tested and calculated to meet the design requirements. According to the field report, the seam cutting was not timely.
If the construction technology of pressure joint is not fine, it will easily lead to poor flatness at the pressure joint and easy damage to the concrete at the seam edge, which is generally not recommended. The Building Code clearly stipulates that the slit method must be adopted in airports and expressways.
All projects in this section adopt the slit method to make contraction joints.
The slotting method mainly uses slotting machine to cut. During operation, the joint width should be consistent. Generally, the seam is 6-8 mm wide. If the seam is too narrow, it is difficult to fill the seam. However, at present, the saw blade with a thickness of 4mm is sold on the market, which makes the seam width tend to narrow. The joint depth shall be not less than 4cm (it is suggested that the depth shall be not less than the thickness of 1/3), and the depth shall be consistent to prevent uneven depth at both ends. The joint cutting time should be timely, the concrete will be damaged too early and the panel will be damaged too late. In general, it is advisable for concrete to reach 25 ~ 30% of the design strength. At present, some units take 250 ~ 300 temperature hours to master the slotting time (the product of the time after pouring and the daily average temperature). The appearance should be beautiful and neat, the adjacent contraction joints should be aligned, and the horizontal curve should be divided into equal parts according to the degree of loneliness.
5.4 Construction joints
Its position should be consistent with the design position of expansion joint and contraction joint, and its setting method and requirements are the same as those of expansion joint. Construction joints should be minimized. In the absence of mechanized construction at present, most of them are manual operations, and the daily progress of each working face is generally 150 ~ 200 meters, so this construction joint has actually become the form of expansion joint.
5.5 joint filling
Joint filling is a detailed process, and its quality directly affects the use of pavement and the effect of driving. In recent years, with the increase of concrete pavement construction, its importance has been gradually recognized by people.
Joint filling shall be carried out immediately after the end of concrete curing period. When caulking, the gap should be blown clean and dry, so as to promote the close combination of the joint material and the joint wall and prevent water seepage.
Commonly used caulking materials are: PVC cement, asphalt rubber, asphalt mixture; In addition, there are polyurethane tar, polyurethane whole skin microcellular foam and hollow rubber caulking tape, which are not widely used because of their high prices. In the reconstruction project of Changchao Road, it is recommended to use PVC cement and asphalt rubber as joint filler. PVC cement has good heat resistance and low temperature plasticity. According to relevant data, there is still a certain elongation at -40℃, and the price is moderate. The finished product is sold in Zhenjiang, Jiangsu Province, which is more convenient to use. Asphalt rubber is easy to use and moderate in price, but its low temperature ductility is less than that of PVC rubber. Asphalt mixture needs to be prepared by itself, which often leads to overflow at high temperature in summer and low filling amount at low temperature because of improper proportion and poor effect.
Generally, the fillet at the lower part of expansion joint adopts cork board and asphalt rubber fillet which have been corroded and do not produce joint splitting deformation. The cork non-extrusion convex seam extension filler developed by Xi Anlin Chemical Plant has a high expansion rate and has been appraised and popularized by relevant departments. The 4cm gap above the expansion joint is filled with PVC cement. We all use joint filler in this section of the project. 6 Pay attention to the early curing practice of concrete. It is proved that the most common cracks in concrete are surface cracks with different depths, and the main reason is temperature gradient. The sudden drop of temperature in cold areas is also easy to form cracks. Therefore, the thermal insulation of concrete is particularly important to prevent early cracks on the surface.
From the point of view of temperature stress, thermal insulation should meet the following requirements: ① to prevent the temperature difference between inside and outside concrete and the gradient of concrete surface, and to prevent surface cracks; (2) In order to prevent concrete from freezing, the lowest temperature during concrete construction should be no lower than the stable temperature during the service life of concrete; (3) Prevent the old concrete from supercooling, so as to reduce the constraint between the new and old concrete.
The main purpose of early curing of concrete is to maintain suitable temperature and humidity conditions, so as to achieve the following two effects. On the one hand, it protects concrete from unfavorable temperature and humidity deformation and prevents harmful cold shrinkage and dry shrinkage. On the one hand, the hydration of cement goes smoothly to achieve the designed strength and crack resistance.
Appropriate temperature and humidity conditions are interrelated. Thermal insulation measures of concrete often have a moisturizing effect.
Theoretically, the water content of fresh concrete can completely meet the requirements of cement hydration. However, due to evaporation and other reasons, it often causes water loss, thus delaying or preventing the hydration of cement, and the surface concrete is most easily and directly affected by this adverse effect. Therefore, the first few days after concrete pouring is the key period of maintenance, and more attention should be paid in construction.