Affected by natural climatic conditions and local special social environment in Africa, the site construction of two diesel engine foundation projects in HIRGIG Power Plant in Eritrea is very difficult. By planning and selecting reasonable construction technical measures, many influencing factors have been successfully overcome, and the construction of large concrete foundation of two-stroke low-speed diesel generator has been successfully completed, ensuring the use and quality requirements of concrete equipment foundation.
Eritrea has a tropical desert climate. The construction area of this project is about 3m above sea level, and the terrain is relatively flat, belonging to seaside sandy land. Among the two 23MW diesel generators newly built in this project, the foundation construction of large concrete equipment is the key core work of this project. The foundation of large-scale concrete equipment adopts reinforced concrete structure, the bottom plate is a raft foundation with a thickness of 1500mm, and the upper part is a special-shaped reinforced concrete structure. The largest outsourcing size of large concrete equipment foundation is 20691mm×11400 mm× 6725mm. The strength grade of concrete is C35.
2. The characteristics and difficulties of site construction
1) The temperature in massawa is very hot. According to statistics, the local outdoor maximum ambient temperature can reach 55℃. In order to avoid the foundation construction of large-scale concrete equipment in the hottest time, the foundation pouring is carried out in1-March (June-10 is the local high temperature season) when the temperature is low.
2) The local material resources are in short supply, and ordinary materials such as steel, fly ash, concrete admixture, mineral powder, asphalt, building doors and windows need to be purchased from China or a third country. When purchasing materials from China or a third country, it is necessary to consider the procurement time, transportation time and customs clearance time, and at the same time, it must be planned together with the project construction progress plan, which makes the material procurement plan of the project department more accurate. Due to the low output of locally supplied materials and the planned economic system, cement, stone, sand, water and other materials can not be supplied in time, which affects the field use. Because the Eritrean government restricts the import of cement from a third country, only ordinary portland cement produced locally can be used (ordinary portland cement has a high hydration heat, and measures should be taken to reduce it).
3) The local fresh water resources are seriously insufficient, and there is no tap water supply system. All fresh water on the site is transported to the site by large water tankers. In order to save the use of fresh water resources, construction technical measures such as adding water reducer in concrete mixing and adopting water-saving curing film in concrete curing have been adopted. Many unfavorable factors in the above-mentioned specific environment bring difficulties to the foundation construction of large concrete equipment.
3. Main technical measures in the process of planning and implementation
In order to make the site construction go smoothly and ensure the construction quality to meet the requirements of specifications and functions, the following targeted construction technical measures are taken in combination with the actual situation of the site construction, such as characteristics and difficulties, and in view of various unfavorable factors:
3. 1, select functional additive materials.
Affected by the local government's policy of prohibiting the purchase of foreign cement, only ordinary portland cement produced locally can be selected on site. The hydration heat of ordinary portland cement is large, and the technical indexes of the foundation part of large concrete mixing equipment are difficult to meet the requirements of specifications and technical quality. Combined with the performance of ordinary portland cement produced locally, functional additives are selected to reduce the dosage of cement, reduce the hydration heat, improve the working performance of concrete and meet its construction conditions in high temperature environment. The technical scheme of concrete mix proportion construction adopts "double admixture" (admixture, admixture) technology, adding superplasticizer, fly ash and mineral powder during concrete mixing to improve the working performance and crack resistance of concrete. Before the design of concrete mixture ratio, the physical and mechanical properties of foreign local cement and the adaptability of additives were specially sampled and tested, and the test results met the requirements of national specifications. —— JN superplasticizer (retarding type), a concrete admixture, has the following remarkable characteristics: high water reduction rate, remarkable reinforcement effect, improved tensile strength, flexural strength and later strength, and improved bond strength, carbonation resistance and impermeability of steel bars; Good pumpability, water retention, cohesiveness and cement adaptability; Adjusting the dosage and prolonging the initial setting time by 3 ~ 10h can reduce the early hydration heat. Cement can save 15% ~ 20%, and more than 30t cement can be saved per ton of products. It has no adverse effect on concrete shrinkage and no corrosion hazard to steel bars. Fly ash and mineral powder can also reduce cement consumption, reduce hydration heat and improve workability. Mixing fly ash mainly has the following functions: because fly ash contains a large number of silicon-aluminum oxides, of which silica accounts for 40% ~ 60% and alumina accounts for 17% ~ 35%, these silicon-aluminum oxides can react with the hydration products of cement twice, which is the source of its activity and can replace part of cement, thus reducing the dosage of cement and the thermal expansion of concrete; Due to the fine particles of fly ash, the interface that can participate in the secondary reaction increases accordingly, and the dispersion in concrete is more uniform; At the same time, the reaction between fly ash and cement further improves the pore structure in concrete, reduces the total porosity in concrete, further refines the pore structure, makes the distribution more reasonable, makes the hardened concrete more dense and reduces the corresponding shrinkage value. It is worth mentioning that, because the density of fly ash is lower than that of cement, low-density fly ash is easy to float on the concrete surface when concrete is vibrated, which makes the upper concrete have more additives, lower strength and prone to plastic shrinkage cracks on the surface. The dosage of fly ash should not be too much. In the project, the content of fly ash should be determined according to the specific situation, and full-time personnel should be arranged to check after concrete pouring is completed to remove the part with more laitance on the upper surface of concrete. Adding proper amount of mineral powder can improve the fluidity of concrete, reduce the hydration heat of cement, improve the impermeability and later strength of concrete, improve the internal structure of concrete, and improve the impermeability and corrosion resistance. Adding ground mineral powder into concrete can delay the hydration speed of cementitious materials and prolong the setting time of concrete, which is beneficial to the transportation and construction of concrete in high temperature season.
3.2, the use of temperature measurement system
The pouring time of large concrete equipment foundation is between 1-3 months, and the local outdoor maximum daily temperature reaches 40℃. In order to monitor the hydration heat temperature of mass concrete and take effective temperature control measures in time, the wireless temperature monitoring system of mass concrete is adopted as the basic temperature measuring system. The system adopts full digital mode to monitor the temperature change of mass concrete in the process of hydration heat, grasp the temperature difference fluctuation of concrete, and guide the thermal insulation measures of foundation slab. When the temperature difference between the inner and outer surfaces of mass concrete exceeds the limit, the system can provide multimedia alarm modes such as graphics and sounds in time to remind on-site technicians to take corresponding insulation measures in time. The layout principles of temperature measuring points are: it can reflect the temperature field and temperature change law of the whole mass concrete; The measuring points are densely arranged in places with large temperature gradient, and sparsely arranged in places with small temperature gradient. In the process of mass concrete construction, cracks should be effectively controlled. Before construction, the necessary theoretical calculation should be made according to the mix design, construction conditions and construction methods of concrete, and the total tensile stress generated in each cooling stage of concrete should be checked. If this value is less than the tensile strength of concrete, it means that cooling shrinkage will not cause through cracks in concrete structure.
3.3, take reasonable on-site construction measures
The ambient temperature is high, and the control measures for the temperature of concrete entering the mold are as follows: reducing the temperature of pouring materials, setting up awnings in storage tanks, mixing stations and raw material yards; Reduce the inlet temperature of raw materials (mainly coarse aggregate) and water them 2 hours in advance before pouring; Reasonable choice of concrete pouring time, should choose to open the pump when the outdoor temperature is low, according to the statistics of on-site temperature measurement, choose to start in the evening when the temperature is low and end in the early morning. Concrete pouring technology management measures are as follows:
1) Concrete pouring shall be carried out reasonably by sections and layers, so that the concrete will rise evenly along the height, and it shall be implemented by the method of "fixed points by sections, layered pouring, step by step, and reaching the top at one time".
2) Large-scale equipment foundation concrete has large slump and slightly larger fluidity. In the process of concrete vibrating, the upwelling exudation and laitance flow along the concrete slope to the bottom of the slope. Therefore, when pouring the cushion, a certain slope should be set aside in the transverse direction in advance, so that most of the secreted water can be discharged from the reserved hole at the bottom of the template along the slope. A small amount of bleeding water that can't be discharged in time is driven to the top of the foundation pit for artificial induction along with the concrete pouring, and the drainage holes left along the concrete pile cap formwork are discharged from the reserved holes on the upper part of the formwork.
3) When pumping concrete, three vibrators shall be arranged before, during and after each post-cast strip. Concrete shall be vibrated by internal vibrator, and it shall be operated by experienced professional technicians.
4) The secondary vibrating process shall be adopted for concrete pouring, and it shall be vibrated again at an appropriate time and position immediately after concrete pouring and before solidification, so as to eliminate the moisture and pores generated by concrete bleeding in the lower part of coarse aggregate and horizontal reinforcement, increase the compactness of concrete, reduce internal micro-cracks, improve concrete strength and crack resistance.
5) In order to prevent dry shrinkage cracks on the concrete surface and control the appearance and development of non-structural cracks on the concrete surface, secondary plastering shall be carried out before the initial setting of concrete or after the pre-settlement of concrete.
3.4, maintenance measures
High temperature and lack of fresh water are the environmental characteristics of Masawa city. After the comparison and selection of maintenance schemes, water-saving and moisture-saving maintenance film is adopted for concrete foundation maintenance.
1) high water saving, the concrete surface only needs to be poured once during curing, which can save more than 95% of curing water.
2) Long-lasting moisturizing, and the surface energy of concrete will always be moist during curing.
3) Effective thermal insulation: Under the action of hydration heat of concrete, the temperature inside the membrane is higher than that outside (4 ~ 10K), and the temperature difference between the concrete core temperature, surface temperature and ambient temperature decreases.
4) Clean construction, the construction site is clean and tidy, which greatly improves the construction image.
5) It has good dual properties of heat preservation and moisture retention, effectively inhibits the generation of micro-cracks and improves the strength of concrete.
6) Improve the carbonation resistance of concrete, which is much higher than other curing methods.
7) Improve the wear resistance, and the wear amount is110 of relevant national standards. By taking the above measures, the construction of mass concrete foundation was successfully completed with good quality and quantity. At present, two diesel generators are running normally, and the concrete performance indexes of large concrete foundation are good, such as compressive strength, crack resistance and harmful cracks on the surface, which can meet the standards of diesel engine operation and acceptance specification.
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