Concrete is a solid composite material with a certain strength, which is made up of cement, sand, stone and water. The hydration reaction of cement forms gel. Its internal structure is that water reacts with cement to form cement slurry, which is wrapped on the surface of sand and filled in the cracks of sand to become mortar, and the mortar is wrapped on the surface of stone and filled in the cracks of sand. Cement paste firmly bonds sand and stone into a whole, making concrete have the required strength, durability and other properties. Because of the special properties of soil itself, coagulation plays an extremely important role in civil engineering fields such as water conservancy and hydropower engineering and bridge engineering. However, the quality of concrete raw materials, concrete mix proportion, concrete mixing and transportation, concrete pouring, curing and formwork removal have great influence on concrete quality, and strict quality control is needed in the construction process. ?

2. Quality control of raw materials?

Raw materials are the foundation of concrete, and the quality of raw materials directly affects the quality of concrete, so the quality of raw materials should be controlled first. ?

2. 1 The strength and volume stability of cement directly affect the quality of concrete. When the strength of cement fluctuates up and down, the strength of concrete will also change; Poor volume stability of cement will cause expansion cracks of concrete. Therefore, to choose a good variety of cement, according to experience, the quality of cement produced by large cement plants is relatively stable and reliable. ?

2.2 Gravel mainly controls gradation, needle flake content and crushing value. Through investigation, the stone gradation of many concrete manufacturers is not very good at present. Therefore, how to ensure the continuous gradation of stone and its feasibility in production deserve further discussion and study. ?

2.3 fineness modulus and silt content are the key factors of sand. Too fine sand or too much silt will increase the shrinkage crack of concrete. In addition, the high silt content in sand and gravel not only affects the strength of concrete, but also affects the frost resistance, impermeability and durability. Therefore, it is best to use medium-coarse sand for concrete, and the content of mud and organic matter must meet the specification requirements. ?

2.4 concrete mixing water should not contain harmful substances that affect cement hydration and concrete quality. If marsh water and seawater containing organic impurities are mixed into concrete, salt frost will form on the surface of concrete. ?

In the process of concrete production, the quality control of raw materials, in addition to regular testing, also requires quality inspectors to grasp the changing law of its content at any time and formulate corresponding countermeasures, such as: when the silt content of sand and gravel exceeds the standard requirements, timely feedback to the production department, timely screening and other effective measures to ensure the quality of concrete; The moisture content of sand is measured by dry explosion method, and the actual water consumption and aggregate consumption in concrete mix proportion are adjusted in time according to the measured moisture content; For the change of cement activity between the same grades, mortar strength test is used to quickly determine, and the concrete mixture ratio is adjusted according to the cement activity results. The performance indexes of cement and sand and gravel must meet the specification requirements. ?

3. Concrete mix ratio?

Concrete mix ratio refers to the weight ratio of each component material in unit volume of concrete. Water cement ratio, unit water consumption and sand ratio are three basic parameters of concrete mix design, which are closely related to various properties of concrete. The basic principles for determining these three basic parameters are: to determine the water-cement ratio of concrete on the basis of satisfying the strength and durability of concrete, to determine the unit water consumption of concrete on the basis of satisfying the workability of concrete construction, and to determine the amount of sand in aggregate on the principle of slightly remaining after filling stone joints. The concrete construction mixture ratio must be tested and meet the design technical indicators and construction requirements, and can only be used after approval. Concrete construction ingredients must be approved and distributed, and the ingredients must be distributed in strict accordance with the distributed concrete construction ingredient list. It is strictly forbidden to change them without authorization. If the construction ingredients are omitted, insufficient or mismatched, the concrete shall not be put into storage. ?

4. Template engineering quality control?

The construction scheme should be based on the structural system, load size, contract period and formwork turnover of the main project, and comprehensively consider the selected formwork and supporting system. Ensuring the correctness of the shapes, sizes and relative positions of various parts of engineering structures and components is of great significance for the rational design of structural joints and special-shaped parts templates (whether to use special templates or not). The formwork has sufficient bearing capacity, rigidity and stability, and can reliably bear the self-weight and lateral pressure of newly poured concrete, as well as the load generated during construction. The formwork joint treatment scheme should ensure no slurry leakage, and the formwork and supporting system should be simple in structure and easy to disassemble and assemble, so as to facilitate the binding, installation and cleaning of steel bars and the pouring and maintenance of concrete. ?

At present, steel composite formwork, wood formwork, plywood formwork and plastic formwork are commonly used in construction. Should check the template quality (including the template under the condition of repeated use), overall size, flatness, surface cleanliness, corresponding accessories (corner template, connecting accessories) and support system to determine the template specifications. Important parts should be pre-assembled. ?

Choosing good quality and reasonable price isolation agent is an important measure to improve the surface quality of concrete structures and components and reduce the cost of formwork engineering. Therefore, whether the drying time of release agent meets the requirements of construction technology should be considered when selecting. The demoulding effect of demoulding agent is related to demoulding time. When the bonding force between the release agent and the concrete interface is greater than the cohesion of the concrete, the surface concrete will often partially bond and fall off, and the specific demoulding time should be determined by experiments. ?

5. Quality control of concrete mixing and transportation?

According to the size of the engineering quantity and the equipment conditions of the construction unit, select the corresponding mixing equipment and transportation equipment, predict the possible faults and problems of mixing equipment and transportation equipment in advance, and arrange maintenance personnel to inspect and repair the equipment in time. Concrete pouring cannot be stopped due to equipment failure, so as to ensure that the concrete needed by the project can be provided in time during the construction process, meet the engineering requirements and ensure the construction progress. ?

5. 1 Key points of concrete mixing quality control. ?

(1) Minimum concrete mixing time. According to mixing capacity, maximum aggregate particle size, mixing method, etc. ?

(2) When concrete is mixed, the water content of aggregate shall be tested regularly. ?

(3) Dry mixing method should be adopted for concrete admixture on site, and it must be mixed evenly. ?

(4) In any of the following circumstances, the concrete mixture shall be regarded as unqualified. (1) improper use of mixture ratio; (2) When mixing concrete, the measurement of any material is out of control or missing; (3) uneven mixing or entrainment of raw materials; (4) The slump of export concrete exceeds the maximum allowable range. 5.2 Matters needing attention in concrete transportation. ?

(1) No segregation, slurry leakage, serious bleeding, excessive temperature rise and slump loss during transportation. ?

(2) Concrete transportation time. According to the average temperature during transportation, etc. ?

(3) Low temperature weather should avoid the influence of weather, temperature and other factors, and cover or heat preservation facilities should be adopted. ?

(4) The free falling degree of concrete should not be greater than 1.5m, otherwise, slow falling measures should be set to prevent aggregate separation. ?

(5) If concrete is damaged during transportation, it must be handled in time. Before the initial setting of concrete, try to transport the concrete to the pouring position, otherwise it will be treated as unqualified. ?

6. Concrete pouring?

Before pouring concrete, the formwork and its supports, steel bars and embedded parts must be inspected and recorded. After meeting the design requirements, before pouring concrete, sundries in the formwork and oil stains on steel bars should be removed, and cracks and holes should be blocked. ?

(1) Before concrete pouring, the warehouse surface should be cleaned, and concrete pouring can only be carried out after the warehouse inspection of the pouring surface is qualified. ?

(2) In order to ensure the good combination of new and old concrete construction joint surfaces, before pouring the first layer of concrete, cement mortar with the same label as concrete should be spread for 2 ㎝ ~ 3 ㎝, and the mortar area should be adapted to the concrete pouring strength, and the laying thickness should be uniform to avoid being too thick or too thin. ?

(3) Concrete pouring shall be carried out by tiling method or stepping method, and rolling pouring method is strictly prohibited. Should be carried out in accordance with a certain thickness, sequence, direction, layered, pouring level to level off, the wall rises symmetrically and evenly, and the pouring thickness is generally 30cm~50cm. ?

(4) Before concrete pouring, vibrating should be closed. It is forbidden to use vibrating instead of sealing. The vibrating time shall be subject to the fact that the coarse aggregate of concrete no longer sinks obviously and begins to pan-grout, and bubbles in concrete shall be vibrated to avoid under-vibration, vibration leakage and over-vibration caused by too short or too long vibrating time. After vibrating, it should be pulled out slowly, and it is forbidden to be too fast. The vibrating radius of concrete should not exceed 1.5 times the effective radius of the vibrator, and the vibrator should be inserted into the lower concrete about 5cm, not too deep, so as not to cause excessive vibration of the lower concrete. ?

(5) In the process of concrete pouring, if there is more bleeding on the surface, it should be removed in time and measures should be taken to reduce bleeding. It is forbidden to open holes in the template to receive water, so as not to take away the mortar. ?

(6) In the process of concrete pouring, especially when pouring the roof, observation points of displacement and deformation should be set up, and designated persons should always observe whether the formwork deviates, and designated persons should check and reinforce the formwork. ?

(7) The poured concrete must be covered with heat preservation or rain protection. ?

7. Quality control of concrete curing and form removal?

7. 1 concrete curing.

In order to fully hydrate the cement in concrete, accelerate the hardening of concrete, and prevent abnormal shrinkage and crack damage caused by natural factors such as exposure, wind blowing, drying and cold after concrete self-molding. After concrete pouring, water should be sprayed in time to keep the concrete surface moist. ?

Maintenance requirements of concrete surface:?

(1) Plastic concrete should be watered and cured within 6- 18 hours after pouring, and low plastic concrete should be watered and cured immediately after pouring. ?

(2) The concrete shall be continuously cured, and the concrete surface must be kept moist during curing. ?

(3) The curing time of concrete should not be less than 28 days. ?

7.2 Form removal.

Sooner or later, form removal will directly affect the quality of concrete and the turnover rate of formwork. Form removal time should be determined according to design requirements, air temperature and concrete strength grade. For non-load-bearing formwork, the strength of concrete is above 2.5MPa, and its surface and edges and corners can only be removed without being damaged by formwork removal. The bearing formwork can be removed only after reaching the percentage specified in the concrete design label. ?

8. concluding remarks?

The quality of concrete engineering is the result of the joint efforts of designers, supervisors and constructors. Every quality director must pay attention to prevent the occurrence of quality defects or find possible defects in construction as soon as possible, so as to take remedial measures without losing time. All constructors and supervisors shall monitor the preparation, mixing, pouring and curing of concrete at any time.