According to the quality requirements and construction requirements of impervious face slab concrete, combined with many years of engineering construction practice, this paper analyzes in detail the causes and treatment processes of concrete defects in impervious face slab of masonry arch dam, so as to realize quality control procedures, hoping to have certain reference value for similar projects.
1 project overview
Kafangzi Water Control Project in Yangxian County, Hanzhong City, Shaanxi Province is one of the key projects in Shaanxi Province, and it is a supporting project of the first phase of irrigation from the Yellow River. Mainly irrigation, combined with power generation, breeding and comprehensive utilization of drinking water for people and livestock; The hub is composed of a masonry gravity arch dam and a power station behind the dam, with a dam height of 78m and a storage capacity of 27.55 million cubic meters. Installed capacity of power station behind dam12000kw; . Seepage-proof panel is located in the water surface of the dam, which is the main barrier to prevent dam leakage. The quality of impervious face slab is the key to the quality of the whole dam, and it is also a sign of the technical level of construction enterprises. When the dam height is ▽820.5m, the thickness of impervious face slab is1.5 m; When the elevation is ▽894m, the thickness is 0.5m and the slope ratio is 1:0.0 136.
2 panel concrete quality and construction requirements
The quality of concrete has a great influence on the safety, durability and economy of reinforced concrete structural members. Therefore, absolute quality assurance is strictly required from every link of construction:
2. 1 material requirements
Materials used in concrete construction, such as cement, water, aggregate, additives, etc. , must be strictly controlled in accordance with the procurement procedures and control procedures in the quality management enterprise standards, and at the same time, combined with relevant industry standards, standardized acceptance; On the basis of written proof and inspection test report of materials, actual on-site production test and irregular sampling must be carried out by the laboratory to ensure the stable and qualified quality of materials.
2.2 site construction requirements
In the process of on-site construction, the mixture ratio of concrete, the measurement and mixing of qualified raw materials, and the crushing and mixing of aggregates; Construction joints on pouring surface, surface treatment, steel bar installation, steel bar insertion, embedded pipe fittings, vibrating during on-site pouring, formwork removal, etc. , must conform to the concrete construction acceptance specification; (Strictly implement the above "three inspection systems", that is, initial inspection, re-inspection and final inspection, and form written construction and quality inspection materials; In the process of concrete production, the notice of mixture ratio issued by the quality inspection station shall be implemented, and the slump of the production and pouring place shall be accurately and strictly controlled (the designed slump of the impervious panel is 5-7 mm), and the mixing time, dosage and water consumption shall be strictly controlled.
2.3 Strength requirements
Inspection of concrete strength: The test block (concrete cube with a side length of 150mm) is conventionally cured in a humid environment or water with a temperature of 20 3℃ and a relative humidity of more than 90% for 28 days, and then the compressive strength is tested on a 200-ton pressure testing machine; (The test blocks on the construction site, from the test mode selection to the sampling, molding, numbering and curing of the test blocks, must be managed by special personnel to ensure the representativeness and authenticity of the test blocks, so as to accurately reflect the strength of the concrete structure. )
3 Impermeable concrete quality defects
3. 1 Concrete quality defects in production practice and their causes
3. 1. 1 pits
Template moisture is not enough, not tight; Slurry leakage or insufficient vibration during vibration, and incomplete air bubble discharge; Factors such as failure to maintain in time after pouring are likely to cause pitting on the concrete surface.
3. 1.2 Honeycomb Dog Cave
The material ratio is not completely accurate; Uneven mixing (insufficient mixing time, etc.) ) and the defects in the pouring method and scheme (with the increase of dam body, the panel becomes thinner, and the influence of terrain, horizontal transportation machinery can not be directly put into the warehouse, etc. ) lead to the separation of mortar and stone; There are also factors such as concrete freezing and mud slag mixing, which cause defects such as honeycomb dog holes on the surface and inside of concrete.
3. 1.3 exposure
When pouring, the cushion or solder joint falls off, and the steel bar clings to the template, resulting in uneven thickness or thinning of the concrete protective layer; The protective layer concrete is not vibrated and compacted; The formwork is not wet enough, and the water absorption is excessive, so that the concrete surface of the panel is exposed.
3. 1.4 cracks
Unbalanced or out-of-limit external load during construction or use; Deformation (humidity and temperature changes in the external environment and uneven settlement, etc.). ); Poor construction operation (making and installing templates; Stacking, transportation and hoisting of concrete; Raw materials, additives, etc. Inaccurate use, etc. ) and other factors lead to defects such as cracks in concrete.
3. The strength of1.4 is insufficient.
There are many reasons for insufficient strength, but we must pay attention to the following links: whether the mixture ratio is reasonable and correct; Whether the numbers are accurate when the mixing and batching system inputs the program; Whether the water content is controlled accurately (in the process of test and production practice, there are often errors in the water content of materials); Whether the mixing process is correct (when cement, fly ash or other additives are added manually, the exact quantity is more noteworthy; Control of mixing time; Whether the stirring is uniform; Whether the performance of raw materials is stable); Whether the transportation process is timely (whether it causes initial setting of concrete, etc.). ); Whether the on-site pouring procedure is compact and reasonable (sometimes it is too different from the theoretical requirements in practice); Whether concrete curing is in place in time, etc.
3.2 defect treatment
For concrete surfaces such as pits, honeycomb dog holes, exposed tendons and exposed stones, 1:2— 1:2.5 cement mortar can be used for plastering and plastering; Before plastering mortar, clean the contact surface with a wire brush, and rinse it with pressure water at the same time; After initial setting, strengthen maintenance. For the honeycomb or exposed reinforcement in the water surface of the dam, unconsolidated concrete and prominent aggregate particles nearby should be removed first, washed with pressure water and fully wetted, then filled with fine stone concrete with high original strength and carefully compacted.
For small cracks, the cracks can be washed clean and repaired by pressure injection of cement slurry; In order to restore the integrity and impermeability of the structure, cement grouting should be adopted according to the width and nature of cracks and construction conditions. For cracks with a width less than 0.5 mm, chemical grouting should be adopted.
For the parts with insufficient strength, it is advisable to use hand pneumatic drill combined with manual prying to remove the relevant parts and then re-pour; It is advisable to cut the layer 15-20cm until the strength meets the requirements.
4 Strength analysis of concrete members with local defects
In the process of construction, when there are quality defects in local impervious panel concrete, it often affects the acceptance of unit projects; At this time, in order to strictly ensure the quality of concrete, especially the strength requirements, the construction party will analyze the quality and strength of the system unit; Through the research and negotiation of the owner, supervisor and construction unit, the depth and degree of defects in the whole concrete unit are confirmed by coring. Details are as follows:
4. 1 Reasonable drilling and coring shall be arranged on the concrete surface of this warehouse number.
4.2 Cut the concrete core sample according to the aspect ratio (aspect ratio) not less than 1, and take three specimens as a group for tensile test and splitting tensile test.
4.3 Before the test, the sample should be put into water to reach saturation; Then, the test is carried out according to the methods and regulations of "concrete cube test" and "concrete splitting tensile strength test" in the electric power industry standard "Hydraulic Concrete Test Specification".
4.4 Combined with the tensile and compressive strength values of the specimen, the strength of the whole unit is comprehensively judged.
5 concluding remarks
Engineering construction, quality first; The construction quality of impervious face slab of masonry arch dam can not be ignored; Construction enterprises must strictly control the quality procedures in engineering construction to ensure the construction quality, so as to build a satisfactory project and win the reputation for enterprises.
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