Connecting with the engineering practice, this paper analyzes the causes of the damage of the cement concrete pavement slab, and advances some engineering measures for preventing the formation of cracks on the cement concrete pavement.

Key words: cement mixed soil pavement; Destroy; Cause of formation; Preventive and control measures

With the use of cement concrete pavement in the middle and late stage, it is easy to gradually appear local damage, such as cracking, broken slab, subsidence, dislocation and so on. In order to further improve its quality, the following focuses on the diseases caused by cracks and expansion and contraction of cement concrete pavement.

1 Damage causes of cement concrete pavement

The first is the crack in the road surface. The main reasons are the load stress, the stress caused by the shrinkage of concrete itself caused by temperature change, and the strong friction between concrete face slab and base course exceeding the tensile strength of concrete face slab.

Second, early surface cracking and early plate breaking. The reason of early surface cracks is incomplete surface cracks caused by excessive water loss and dry shrinkage of early concrete surface, which mostly occur in the early stage of concrete pavement paving and molding, and the crack law is not obvious. The causes of early slab breaking are complicated, mainly occurring in the early stage of concrete pavement formation, and the fracture law is obvious, mostly transverse cracks, which generally run through the bottom of slab. The causes of cracks and expansion-contraction joints are unreasonable reserved expansion joints, poor performance of joint filling materials, or rain, snow and water pouring into joints after shrinkage cutting, which leads to diseases at the bottom of concrete slabs. In some areas, due to uneven soil quality, high humidity, expansive soil, frost heaving, poor drainage facilities and other reasons, subgrade stability is insufficient, resulting in settlement, and excessive bending and tensile stress at the bottom of the pavement under load, leading to the destruction of concrete pavement.

Thirdly, the stress change of temperature is another reason for the formation of cracks and expansion cracks. At the temperature of 30℃ ~ 40℃ in summer, when the surface temperature of 4m*5m concrete slab reaches about 6 0℃, due to the temperature stress in the slab, the four corners of the slab are tilted, the slab is basin-shaped, and the caulking materials around the slab are disturbed. Such a long period of high temperature weather, the change of temperature difference between day and night, the plate is constantly changing. In the deformation state of the basin, when heavy trucks pass by, seesaw jump will occur and joint filler will be squeezed out. When it enters the rain on rainy days, the bottom of the concrete slab will be washed away and the mud will be washed away.

Fourthly, during the service life of cement concrete pavement, caulking materials are mainly subjected to two functions: stretching and cutting. The stretching of joint filling material refers to the shrinkage of cement concrete pavement slab due to the decrease of temperature, which leads to the opening of joints. Shear force refers to the difference in vertical displacement between the loaded plate and the adjacent unloaded plate when the car passes through the joint, which also causes the joint to open. When pavement and joint filling materials are seriously damaged, under the action of high speed and heavy load, the cushion under the slab is subjected to water freeze-thaw erosion for a long time, forming a kind of running water erosion. Due to the repeated action of erosion, the lime soil under the concrete slab is separated from the slab joints, that is, mortar appears, and the granular materials are ground into mud and gradually pumped out under the repeated action of heavy trucks, and hollowed out under the slab. After hollowing out to a certain area, the board of 4mx5m appears springboard phenomenon or the heavy truck breaks when passing through, which will seriously affect the driving.

2 cement concrete pavement cracks and expansion and contraction prevention measures

In order to avoid cracks, the construction should be carefully organized in strict accordance with the design, construction and acceptance specifications.

2. 1 position joint

2. 1. 1 The construction joints of concrete pavement should be located on the structure as far as possible. When it is inevitable, the concrete of the panels on both sides of the construction joint should be vibrated and compacted, and there should be enough time interval to form strength.

2. 1.2 When connecting with the original concrete pavement, the joints of the original pavement shall be cut neatly by machinery, and the floating dust and loose stones on the surface shall be removed, and the joints shall be cleaned with water.

2. 1.3 cement concrete pavement is rigid, and longitudinal and transverse joints shall be filled with materials in time. The filler should have strong adhesion with the concrete slab to adapt to the shrinkage of the concrete slab. Such as neoprene, asphalt mortar, etc. Strengthen maintenance management, prevent water seepage and prevent sundries from entering the joints.

2.2 Construction joints

2.2. 1 avoid high-temperature operation, the temperature should be controlled below 35 degrees, pay attention to the temperature difference between day and night, add additives such as water reducer, and keep proper moisture.

2.2.2 Avoid construction in windy weather, and prevent the surface water from losing too fast.

2.2.3 After the final set of cement concrete, cover it with straw, etc. And fully spray curing or curing agent on a large area to prevent shrinkage cracks caused by excessive water loss. The curing time depends on the strength growth of cement concrete, generally it should be about half a month to 20 days, not less than 7 days.

2.2.4 Control the mixing quality and speed of cement concrete, do the test section, get the construction reference value, and learn from the construction control of other completed highways.

2.2.5 Strengthen the construction self-inspection, strictly control the smoothness of the base, and meet the specification requirements.

2.2.6 Remove the accumulated water on the surface of the grass-roots unit, and it is forbidden to sprinkle raw water.

2.2.7 The soft foundation treatment shall be specially designed, and the weak parts can be reinforced with steel mesh to strengthen the rolling quality.

2.2.8 Calculate the cement dosage, control the reasonable range of cement content, refer to other pavements, do the test section, and obtain sufficient scientific data.

2.2.9 In order to meet the requirements of heavy traffic on the strength of concrete pavement, special pavement types such as steel fiber concrete pavement can be adopted according to the specifications to improve the strength of pavement concrete and meet the design requirements.

2.2. 10 Pay attention to the cement quality, realize the design purpose and complete the design intent.

2.2. 1 1 Note that the actual selection value of concrete mixture ratio should be obtained in combination with the construction site.

2.3 material joints

2.3. 1 cement should use the same label from the same manufacturer.

2.3.2 When using steel bars with the same performance, the longitudinal and transverse steel bar diameters should be as consistent as possible.

2.3.3 Strictly control the procurement channels of materials, strictly control the quality through experiments, and eliminate unqualified materials.

2.3.4 Use qualified cement, use inspection-free products and use brand-name cement.

2.3.5 Pay attention to the initial setting time of cement concrete, and the distance between the mixing station and the paving site should not be too far, so as to ensure the standardization during transportation.

2.4 Select qualified raw materials.

Choosing cement with high strength, small shrinkage, strong wear resistance and good safety can reduce the shrinkage deformation at the initial stage of concrete forming and improve the flexural and tensile strength of concrete.

Coarse aggregate shall be continuously graded crushed stone produced by impact crushing, with the maximum particle size not greater than 3 1.5mm (square hole sieve) and the needle-like scale content not greater than 15%. Inactive aggregate with high strength and strong temperature difference resistance should be used for macadam to prevent concrete cracks and strength decline caused by alkaline aggregate.

Choose a reasonable concrete mixture ratio. The mix proportion of concrete is the soul of the mixture, which must meet the requirements of strength, durability and economy. During construction, the mix strength of concrete should be 10%- 15% of the dry design strength, the water cement ratio should be 0.4-0.48, the slump should be 1.8-2.0cm, the sand content should be 30%-35%, the 24-hour flexural strength of concrete should not be less than 3.0MPa, and the water content of aggregate should be. In order to ensure the accurate dosage of cement, aggregate and water, electronic automatic metering should be adopted for concrete batching.

2.5 Control the construction technology.

During the construction of cement concrete pavement, due to the evaporation of water when pouring concrete, the volume shrinks after work, which causes the pavement slab to break. In order to prevent pavement cracks and broken slabs, it is very important to control the joint cutting time. According to our experience, it is reasonable to take 10 day for maintenance when the average temperature is about 7 degrees Celsius. When the average temperature is about 65438 04 degrees Celsius, the appropriate curing time is 7 days. When the average temperature is about 20 degrees Celsius, it is reasonable to cut the seam for 3 days. The split joint of cement concrete pavement should be between 4mx5m as far as possible to reduce the damage of temperature stress to the plate.

Select the appropriate paving time. In order to avoid cracking and slab breaking caused by temperature difference stress, it is necessary to choose a time period with low sunshine temperature, small wind force and small temperature change for paving.

In order to prevent cracks from appearing at the edge of the slab, steel bars should be added at the longitudinal and transverse free edges of the concrete slab, and dowel bars should be installed correctly. Generally speaking, two 10 steel bars. The edge reinforcement is selected as 12- 16, which is arranged at the lower part of the plate. The distance from the bottom of the plate is generally 1/4 of the plate thickness, not less than 5cm, and the spacing is 10cm. Both ends of steel bars should be bent upwards, the protective layer of steel bars should be not less than 5 cm, and dowel bars should be parallel to the road centerline. Dowel should use high plastic casing and asphalt interlayer according to the design requirements.

Strengthen the early curing of concrete slab. It should be covered with sacks or straw curtains, and cured with wet drinks, so as to ensure that the concrete slab is always wet during the curing period, and the curing time is not less than 14 days. Strengthen traffic control during health care, and no vehicles are allowed to pass.

In order to prevent caulking, caulking materials should be filled with materials with strong adhesion to concrete slab wall, good resilience, adaptability to expansion and contraction of concrete slab, insolubility and impermeability in water, no overflow at high temperature, no brittle crack at low temperature and good durability. The joint plate should be made of materials that can adapt to the expansion and contraction of concrete panels, and will not deform during construction and have good durability.

To sum up, there are many reasons for cracks, expansion and contraction of cement concrete pavement, but as long as raw materials, concrete mixture ratio, construction technology level and other aspects are strictly controlled and comprehensive prevention measures are taken, cracks and broken slabs of cement concrete pavement can be effectively controlled and avoided.

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