Crushed concrete technology is a new concrete construction technology of dry hard concrete using the construction process of earth and rock dams, compacted by vibratory grinding, which breaks through the traditional method of column casting of concrete dams.

It has a high degree of mechanization, shorten the construction period, simplify the construction process, save investment and other characteristics, become one of the preferred dam type of water conservancy and hydropower engineering dams. Brazil, the United States and other crushed concrete dams are mostly built in a rough and fast mode, with wide technical requirements for dam impermeability, temperature control and crack prevention, and level bonding, and lax quality control requirements for crushed concrete. The resulting concrete cracks and leakage problems occur from time to time and are considered routine.

Japan uses interlayer intermittent mode to pour the rise. Inter-layer bonding, which is not limited by the initial setting time, leads to poor bonding between the layers of crushed concrete, with the resulting impact on the integrity of the dam body, and causes water seepage channels between the layers, affecting the stability and aesthetics of the dam body. The main object of foreign research is the application of crushed concrete construction technology, less research on its quality management.

　　Characteristics of Crushed Concrete Construction Technology

　　As a dry mix with no collapse, the construction method of RCC is close to the filling method of earth and rock dams, using thin layers of material laid through the bin and compacted by vibratory rolling. Compared with conventional concrete, it has obvious superiority in terms of material consumption, construction efficiency, and its own performance.

　　First, the construction speed is fast and the construction period is short. rcc can rise in full section, and its rising speed can reach 15-25m/month. Therefore, the construction period can be shortened by l/3 to l/2 compared with that of normal concrete dams.

　　Second, the dam structure is simple and economical. Since RCC eliminates the longitudinal joints and grouting system, there is no need for Phase I cooling and Phase II cooling, and at the same time, the cross joints can reach 30 to 80m a piece. This transverse joint does not require vertical touching, but only needs to be cut at the site with a seam cutter after each layer of RCC has been rolled. So it can save a large amount of material and labor cost.

　　Thirdly, local aggregates are available and generally do not need special treatment. RCC can use any gravel material or artificial gravel material that can be used for normal concrete. Especially the stone powder of artificial gravel, normal concrete can not be used, while RCC can be used. General sand in up to 8% to 17%. If you use limestone stone powder can also save cement, while improving the compressible properties of RCC, such as Puding RCC, after mixing l7% of limestone powder, the amount of cement used in RCC can be reduced to 50kg / m 3.

If the construction process, the bin surface in case of water secretion, can reduce the amount of fly ash, increase the amount of stone powder to improve the performance of RCC

　　Project Case

　　The dam is a full-section crushed concrete gravity dam with a folding arrangement on the dam axis, a crest elevation of 234.0 m, a crest length of 720.0 m, a crest width of 10.0 m, and a maximum dam height of 130.0 m. The dam is divided into 13 sections.

The dam is divided into 13 sections, including four surface holes, three middle holes and one bottom hole in the overflow dam section, the surface hole is 14 m wide and l8 m high, the middle hole is 4 m wide and 7 m high, and the bottom hole is 2 m in diameter.

Downstream of the main dam, there is a reinforced concrete force dissipation pond with a length of 127 m and a width of 88 m. The riverbed downstream of the force dissipation pond is equipped with a berm, and the side slopes are excavated and supported. The main dam has a total of 21.6 million m3 of crushed concrete, which is one of the largest crushed concrete gravity dams under construction in China.

　　The construction process of hydraulic engineering concrete crushing

　　1、Material selection

　　The quality of the raw materials used in the crushed concrete should meet the requirements of the construction quality standards and technical terms of the contract documents. Crushed concrete, cement fly ash slurry, cement net slurry and cement mortar are used 42.5 medium heat cement; the fly ash used is Class I fly ash or quasi Class I fly ash. Additives are slow-setting high-efficiency water reducing agent and air-entraining agent, and their quality standards meet the requirements of the corresponding specifications.

Slow-setting high-efficiency water reducing agent should be used in accordance with the actual needs of the construction of compacted concrete, according to the two formulations of winter type and summer type respectively. Material suppliers should choose air-entraining agent with the dam concrete raw materials, especially fly ash adaptability, good air-entraining effect, low loss of air content of air-entraining agent, to ensure that the air content of crushed concrete out of the machine mouth in 3% to 4%.

　　2、Pouring construction

　　Each bin of crushed concrete must be constructed strictly in accordance with the pouring key diagram, which describes the detailed construction plan, and each bin block is drawn by engineering technicians, and a detailed on-site technical briefing is given to the commanding officer of the bin surface, inducer, quality inspector and site technician before the opening of the bin. The commanding officer, guide, quality inspector and site technician must be familiar with the pouring essentials and organize the implementation according to the requirements of the pouring essentials chart.

　　3、Method of transporting crushed concrete

　　Adopt the way of direct entry by car, belt machine + negative pressure chute or large drainage chute into the warehouse. Except for the crushed concrete between the middle holes of the dam, all the rest of the concrete is put into the warehouse directly by car. The car shall not walk on the surface of the concrete that has been collected, but must drive on the old concrete surface and back up to the discharge point for unloading by the foot of the slope, and shall choose the middle of the upstream and downstream direction of the bin block (less stress) as the climbing point.

　　4、Unloading

　　In order to reduce the separation of aggregate, the self-unloading car unloads the concrete material on the step of the paving layer paving front, and then the concrete is pushed from the step to the lower step by the warehouse leveling machine for shifting type warehouse leveling. Unload the material as evenly as possible.

　　5、Control of the thickness and slope of the flat bin

　　Before pouring, draw the flat bin line of each layer by measuring on the surrounding formwork and the old concrete surface at the bottom. The thickness of each layer is about 34~36cm, and the part that exceeds the specified value must be re-leveled after inspection. The local uneven parts are paved with manual assistance. The direction of flattening is carried out from the top of the slope to the bottom of the slope, with a slope ratio of 1≥10%, and the sharp corners formed at the foot and top of the slope of the inclined layer are compacted by small vibratory mills. The aggregates concentrated on both sides of the flattening process are evenly dispersed on the strip by hand.

　　6、Compacting

　　Use imported BM202AD double-wheel high-frequency vibrating mill, using the lap method of rolling, the lap width is 20cm, the walking speed of vibrating mill is 1~1.5km/h, the number of rounds of rolling is “2+8+2” according to “no vibration + vibration + no vibration”. The number of passes is “2+8+2”. If the concrete is still not flooded after 6 times of vibration, the water tank of the vibrating mill should be opened for water compensation.

If the VC value is too large, 2mm thick cement fly ash net slurry should be spread before the next layer of concrete cover. When rolling the foot of the slope, the rolling construction procedure is: first pave the 3m wide flat layer at the foot of the rolling construction slope (the width of the bottom foot is adjusted accordingly when the slope changes), and then pave the rolling construction concrete. The foot of the grading layer with a slope greater than 1:10 is paved with a 3m wide mortar (the width of the foot is adjusted accordingly when the slope changes).

In the footing range of the horizontal paving grinding concrete material, paving thickness of about 34cm. footing flat layer grinding, grinding number “2 + 8 + 2” implementation. Paving and rolling construction, paving the foot of the lower slope shall not exceed the outer edge of the flat layer, rolling with the outer edge of the flat layer parts together, but the roller shall not exceed the outer edge of the flat layer line. The next layer of unloading before the foot of the slope to excise (digging out the height of about 1Ocm), such as local cut out of the concrete bottom without mortar, should continue to cut backward until the cut out of the concrete bottom.

　　7、Conditioning

　　Moisturizing is the primary link of crushed concrete maintenance, after the completion of the crushing operation, the moisturizing and maintenance of concrete should be carried out quickly, generally by using plastic film to cover the concrete. To ensure the effect of moisturizing, you can first spray water on the surface of the concrete, so that the concrete is in a wet state, and then covered with moisturizing materials, which is also conducive to the hydration process of cement.

　　The concrete should be guaranteed a certain amount of curing time so that the cement can have sufficient time and moisture for hydration reaction, and the curing operation can be stopped after the concrete’s anti-dissociation strength is greater than 3.5Mpa. When cutting the shrinkage joints, it is necessary to grasp the cutting time, which is usually about 8 hours after the completion of the crushing operation.

It is important to note that the strength of the concrete must meet certain requirements, which is a prerequisite for cutting. The cutting depth should be strictly in accordance with the requirements of the specifications, and attention should be paid to the maintenance of the cutting joints after the end of cutting.

　　Conclusion

　　In summary, as an important part of the construction of water conservancy projects, the quality of crushed concrete construction directly affects the quality of the entire water conservancy project. Based on this, we must improve the technical level of crushed concrete construction of water conservancy projects, grasp every aspect of concrete construction, in order to ensure the quality and safety of the construction of water conservancy projects.