I. Introduction
At present, China is building a water-saving society, and agricultural irrigation needs a lot of water. Saving water is of great significance to ensure the sustainable development of economy and society. According to the survey, the water leakage from soil canals accounts for the vast majority of the water loss in the canal system, generally accounting for about 30% of the water diversion from the canal system, and it can be as high as more than 50% in relatively large irrigation areas with poor irrigation equipment and areas with poor seepage control in the canal system, which not only wastes valuable water resources, reduces irrigation efficiency, but also increases farmers' water fee burden. The concrete seepage control channel has good seepage control effect, which can generally reduce the water leakage loss by more than 85%.
Second, the foundation treatment
Before the construction of canal seepage control project, construction lofting should be carried out first. In terms of excavation methods, the tunnel should be excavated in advance to make the foundation dry naturally, so as to enhance the strength of the soil foundation and prevent frost heaving damage in winter. The excavation surface may become loose during the excavation, and it must be cleaned, backfilled, leveled and compacted before the impervious body project is laid. When filling the channel, the silt, sapropelic soil and concealed masonry in the trough bed should be cleaned up, and then layered filling and compaction should be carried out. In order to avoid the damage caused by dry dust on the surface, artificial trampling and rain erosion, the slope cutting of the channel should strictly control the elevation and surface smoothness, and should be carried out 1 day before concrete pouring. If excessive slope cutting can't be backfilled with floating soil, it should be filled with concrete of the same grade as cast-in-place or backfilled and compacted with new soil.
Third, concrete materials.
3. 1 water quality requirements
All drinkable water can be used for mixing and curing concrete. Untreated industrial wastewater, sewage and swamp water cannot be used. If there is extreme water shortage, surface water, groundwater or other types of water can also be used, but it must comply with the provisions of the Code for Construction of Hydraulic Concrete.
3.2 Quality control of cement
There are many kinds of cement, which are divided into general cement, special cement and special cement according to their uses and properties. When using cement, we must distinguish the variety and strength grade of cement, master its performance and usage, and reasonably select and use cement according to the specific conditions of the project. Cement should be waterproof and moisture-proof during transportation and storage, and the cement that has been caked with moisture should be treated and inspected before use. Cement warehouse should have drainage and ventilation measures to keep it dry. When stacking bagged cement, a moistureproof layer should be set up, at least 30cm away from the side wall of the ground, and the stacking height should not exceed 15 bags, and a transportation channel should be reserved.
3.3 Quality control of aggregate
Sandstone and aggregate are the most basic components of concrete. The quality of aggregate directly affects the concrete strength, cement dosage and concrete requirements, thus affecting the quality and cost of hydraulic structures. The quality requirements of aggregate include: strength, frost resistance, chemical composition, particle shape, gradation and impurity content.
(1) Quality requirements of coarse aggregate
The maximum particle size of coarse aggregate shall not exceed 2/3 of the clear distance of steel bars, 1/4 of the minimum side length of member section and 1/2 of the thickness of plain concrete. For concrete structures with little or no reinforcement, larger coarse aggregate size should be selected. During construction, coarse aggregate should be divided into the following particle size combinations according to the particle size: when the maximum particle size is 40mm, it is divided into two levels: D20 and D40; When the maximum particle size is 80mm, it is divided into three grades: D20, D40 and D80. When the maximum particle size is 150( 120)mm, it is divided into four grades: D20, D40, D80, D 150(D 120); Whether to adopt continuous gradation or discontinuous gradation should be determined by experiments. The surface of coarse aggregate should be clean, and powder, mud and pollution should be removed.
(2) Fine aggregate quality requirements
Fine aggregate should be hard, clean and well graded; The fineness modulus of artificial sand should be in the range of 2.4 ~ 2.8, and that of natural sand should be in the range of 2.2 ~ 3.0. The use of sand, coarse sand and ultra-fine sand should be demonstrated by experiments. The moisture content of fine aggregate should be kept stable, and the moisture content of saturated surface of artificial sand should not exceed 6%. If necessary, accelerated dehydration measures should be taken.
Four, concrete mixing and transportation
4. 1 concrete mixing
In small channel construction, the amount of sand and gravel can be converted into volume ingredients, but the error cannot exceed. Concrete mixture should have workability suitable for construction conditions. When the construction operation is convenient and the vibrating compaction is ensured, when the section size of the member is small or the steel bar is dense, and manual vibrating is adopted, the slump can be larger according to the structure and construction method. On the other hand, when the cross-section size of the member is large or the reinforcement is sparse, when mechanical vibration is adopted, a smaller slump can be selected. When temperature control is needed or concrete is poured at high or low temperature, its slump can be appropriately increased or decreased according to the actual situation, which can refer to the Code for Construction of Hydraulic Concrete.
4.2 Transportation of concrete
The transportation of concrete is an intermediate link connecting concrete mixing and pouring, and it must be used with the mixing and transportation. The conveying equipment is tight, stable and does not leak slurry. When unloading, the concrete should be unloaded and the carriage should be cleaned at any time. There should be no initial setting, segregation, slurry leakage, serious bleeding and excessive temperature change during transportation, which can ensure the temperature requirements of concrete storage. The whole process from loading to warehousing should be controlled within 30min. If the transportation time is exceeded for some reason, when the concrete has been initially set or lost its plasticity, it should be treated as waste.
Verb (abbreviation of verb) concrete pouring
Preparation before pouring
The operations before concrete pouring include: foundation surface treatment, construction joint treatment, formwork reinforcement and embedded parts installation. Secondly, it must pass the inspection of the supervision warehouse and obtain the quasi-irrigation license before it can be put into storage.
(1) Basic surface treatment
For the gravel foundation, the sundries shall be cleared and the base surface leveled, and then 10 ~ 20 cm low-grade concrete shall be poured as cushion to prevent slurry leakage; For soil foundation, gravel should be paved first, covered with wet sand, compacted, and then concrete should be poured.
(2) Construction joint treatment
Construction joint refers to the temporary horizontal and vertical joints between pouring blocks, and it is also the joint surface of new and old concrete. Before pouring new concrete, the cement film containing free lime on the surface of old concrete must be removed, so that the stones on the surface are half exposed, forming a pitted surface that is conducive to interlayer bonding. The surface of longitudinal joints need not be chiseled, but should be washed clean to facilitate grouting of construction joints. After galling or rinsing, rinse with pressure water to make its surface free of slag and dust, and before pouring concrete, spread cement slurry or small graded concrete on the joint surface to ensure good combination of construction joints.
(3) shuttering
The installation of formwork must be measured and laid out according to the design drawings. More control points should be set for important structures for inspection and verification, and sufficient fixed facilities should be maintained to prevent the formwork from overturning and sliding. Formwork assembly shall be strict and accurate, with no slurry leakage, smooth surface and no excessive deformation, and the allowable deviation of installation size shall not exceed the specifications for hydraulic concrete construction.
5.2 Storage and Laying
Before pouring concrete, if the canal bed is dry and starts to soil, it should be watered first to avoid tiny cracks on the surface of the poured concrete slab due to excessive water loss. Concrete-lined channels are often poured in blocks according to expansion joints, and the bottom and slope of channels are generally poured by jumping warehouses.
5.3 Closed position and vibration
Concrete shall be sealed and vibrated in time after it is put into storage, and it is forbidden to replace sealing with vibrating. If there is coarse aggregate piled up in the warehouse, it should be evenly distributed to places with more mortar, and it cannot be covered with cement mortar to avoid honeycomb. The vibrating time shall be subject to the fact that the coarse aggregate of concrete no longer sinks obviously and begins to pan-slurry, so as to avoid under-vibration or over-vibration. Generally, it can be vibrated twice, the first time until the surface is flooded, and the second time, it can be faster. Artificial vibration or tamping can be used when necessary.
Six, canal seepage control construction and problems that should be paid attention to.
Seepage prevention channel is the development of conventional channel, which is different from conventional channel. Many factors should be considered in the design of any kind of channel, and the construction should be carried out in strict accordance with the relevant norms and design documents of seepage control channel construction, otherwise the seepage control channel may not play its due role. According to the practical experience of building seepage control channels for many years, the following problems should be paid attention to in the construction of seepage control channels.
(1) The construction of seepage control channels shall conform to the overall planning.
Anti-seepage channels are mostly rebuilt and expanded on the original channels, and the operating conditions of the new anti-seepage channels will be different from those of the original channels. If it is not fully considered, there will be problems such as inconsistent convergence.
(2) The construction of seepage control channel should be combined with field drip irrigation project.
Canal seepage prevention and field drip irrigation are both water-saving irrigation measures. While vigorously promoting drip irrigation projects, higher requirements are put forward for the quality of irrigation water. It is particularly important to build anti-seepage channels to lay a good foundation for field drip irrigation projects.
(3) Deal with the relationship between the old channel and the new anti-seepage channel
The old channel has been in operation for many years, and the dikes and dikes have been very stable. When building on this kind of channel, the original channel foundation will be preserved, and some new channel foundation and dike will be built, which will make the new seepage control channel unstable as a whole. Therefore, the design should be fully considered, the construction quality should be strictly controlled, and the weak links should not be left to cause hidden dangers.
(4) Strict on-site supervision
The construction shall be carried out in strict accordance with the drawings, and the design shall not be modified without authorization, and the construction acceptance shall be carried out in accordance with the relevant national norms, and the construction supervision system for seepage control channels shall be established. In order to ensure the construction quality of seepage control channels, it is very necessary to implement strict on-site supervision.
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