Keywords: bridge engineering; Concrete construction; Maintenance/curing
With the development of expressway, many new technologies and processes have been applied. In order to effectively implement the spirit of "Quality Management Year", under the requirement of "fine construction" of engineering quality, the construction control and measures taken have been greatly improved. Maintenance of precast beam and slab has always been a weak link in precast beam construction, which provides suitable temperature and humidity conditions for concrete and continuously improves its strength. The concrete construction of bridge engineering is greatly influenced by the construction conditions. Maintenance is an essential process of bridge engineering, and the correct bridge maintenance method is the key to the normal operation of the bridge. In order to keep the bridge in good condition after completion, it needs to be solved through maintenance, so as to achieve good operational purposes.
1, general requirements for concrete construction
1. 1 concrete construction in hot climate
The concrete temperature before pouring should not exceed 32℃. The Contractor shall take the following measures to keep the concrete temperature below 32℃:
1. 1. 1 shading or covering and cooling aggregate and other components. Cool the mixed water by freezing or burying water tank or adding crushed ice to some mixed water, but after mixing, all the ice should be melted.
1. 1.2 The surfaces that contact with concrete, such as formwork, steel bars and steel flanges, shall be cooled to below 32℃ before concrete pouring.
1. 1.3 The concrete pouring temperature of bridge deck and bridge deck pavement shall not exceed 26℃. When the evaporation rate is greater than 0. 5 kg/m3 per hour? 2, should not be on the bridge deck, bridge deck pavement or other exposed concrete slab structure.
1.2 concrete winter construction
1.2. 1 When the outdoor daily average temperature is lower than 5℃ for five consecutive days, in addition to the materials and construction requirements, a winter construction plan shall be submitted, detailing the construction methods and equipment to ensure that the temperature is not lower than 10℃ for the first seven days after pouring.
1.2.2 The contractor shall have a sufficient number of thermometers capable of continuous recording, which shall be recorded approximately every 30 meters for the first 7 days. (2) When pouring concrete, put a thermometer, and observe and record it continuously by special personnel.
1.2.3 When mixing concrete, the temperature of each material should meet the temperature required for mixing, and the materials can be heated separately to meet the mixing temperature.
1.2.4 When chloride is added to the heated mixture, the initial setting of concrete shall not be earlier than the end of concrete pouring, and steam curing shall not be used. When pouring concrete on hardened concrete, the temperature of the joint surface should be at least 5℃, and the temperature should be kept at or above 5℃ when pouring concrete. When mixing concrete, the mixing time should be extended by 50%.
1.2.5 The contractor shall be responsible for protecting concrete in cold weather conditions in winter, and any concrete damaged due to poor protection and freezing must be removed and poured again.
2. General requirements for concrete curing
2. 1 After concrete pouring is completed, the surface should be cured as soon as possible after slurry is collected, and the newly poured concrete surface exposed to the atmosphere should be watered in time or covered with wet sacks and wet cotton felt for curing. If conditions permit, water storage or sprinkler maintenance should be adopted as far as possible.
2.2 Used for concrete with water-binder ratio lower than 0. 45 and high-volume fly ash concrete, when pouring large-area components, the exposed working face should be reduced as far as possible, and the exposed working face should be closely covered with plastic film immediately after pouring, and the film can be uncovered for water conservation after final setting.
2.3 In the process of plastering and screeding after concrete pouring, it is forbidden to sprinkle water on the concrete surface, and excessive operation should be prevented from affecting the quality of surface concrete.
2.4 The temperature of concrete entering the mould should be adjusted according to the air temperature, which should not be higher than the air temperature and not more than 30℃ in hot climate and lower than 12℃ in low temperature. In the process of construction and maintenance, the midpoint temperature of key sections and the surface temperature about 5 cm away from the surface are measured, and strict temperature control is implemented.
2.5 When the ambient temperature is lower than the surface temperature of the concrete being maintained by more than 20℃, the concrete surface must be covered with thermal insulation materials to reduce the cooling speed. Cast-in-place concrete should have enough moisture curing time.
2.6 When the structure comes into contact with flowing surface water or groundwater, waterproof measures should be taken to ensure that the concrete will not be washed away by water within 7 d after pouring.
2.7 In addition to considering the concrete strength during formwork removal, the time for concrete formwork removal should also consider that the concrete temperature during formwork removal should not be too high, so as not to be exposed to air and cooled too fast and crack. At this time, cold water curing is not allowed.
3, concrete construction and maintenance methods
3. 1 Sprinkle water for health preservation
3. 1. 1 Automatic sprinkler system and sprayer should be used for sprinkler maintenance, wet maintenance should not be interrupted, and dry-wet cycle is not allowed. The covering materials provided shall be approved by the supervision engineer in advance.
3. 1.2 Sprinkling water for curing should be based on the temperature situation, and the appropriate time interval should be mastered to keep the surface moist during curing. When the temperature is lower than +5℃, it should be covered with heat preservation, and water should not be sprayed for health preservation.
3.2 Waterproof paper and plastic sheets for health preservation
(1) The waterproof paper should be as wide as possible.
(2) The use requirements of plastic sheets are the same as those of waterproof paper.
3.3 steam curing
3.3. 1 When the contractor adopts steam curing, the concrete members with additives shall be confirmed by tests in advance to have no harmful effects after steam curing, and then steam curing can be carried out.
3.3.2 Steam curing shall be carried out according to section 1 1 of J TJ 04 122000 Technical Specification for Highway Bridge and Culvert Construction. 8 and section 14. 2.
3.3.3 To avoid concrete cracking, formwork removal and steam curing should be selected.
3.3.4 Components after steam curing shall not be watered again.
4, reinforced concrete bridge disease treatment
In bridge engineering, concrete is widely used as the main building material, and it has always been considered as a very durable material. In recent years, it is gradually found that it will lose its original strength and be damaged prematurely, which will affect its normal use, just like natural stone being weathered and deteriorated under certain conditions. The design life of concrete bridges is generally 100 years, but the service life of bridges suffering from diseases will be greatly shortened. After several years of completion, the concrete protective layer will peel off and the steel bars will rust, which requires disease treatment and steel bar maintenance.
4. 1. Causes of diseases
Bridge concrete structure is a composite material which takes the hydration product of cement as cementing material, and combines aggregate or other inert materials with steel bars with a certain gradation. In this composite structure, steel bars provide the tensile strength of the structure, while concrete provides the compressive strength of the structure and the protection of steel bars. Therefore, the diseases of concrete bridges include the deterioration of concrete performance and the corrosion of steel bars in concrete. The deterioration of concrete refers to the deterioration of concrete performance due to the physical, chemical and biological effects of the surrounding environment, such as the reactive denaturation, dissolution and precipitation, crystal expansion and matrix cracking of some components in concrete, which mainly includes corrosive medium corrosion, freeze-thaw damage and concrete cracks. Carbonization (neutralization) and solution corrosion of concrete, etc. The deterioration of concrete not only directly reduces the performance of concrete, but more importantly, it loses the protective effect on the steel bars in concrete, leading to the corrosion and failure of steel bars.