It is imperative to strengthen the understanding of commercial concrete, master the relevant knowledge of commercial concrete, and learn the production, construction and maintenance standards of commercial concrete. According to the relevant standards and specifications, this paper briefly explains the use and maintenance of commercial concrete, and expounds the common problems and responsibilities that are easy to cause disputes in the contract.
First, the characteristics of commercial concrete
Compared with on-site mixing concrete, commercial concrete has the following main features:
1, slump is large, pumping is generally above 200m, chute is above 180m, and tower crane is above 150m, with good fluidity. Therefore, the lateral pressure of commercial concrete on the formwork is large, and it is easy to expand and explode. Especially when the shear wall is poured, it is easy to deform.
2. The amount of cement is large, most of them are mixed with fly ash, and the sand ratio is about 5% higher than that in the field, so there are many pastes and good workability.
The hydration heat is high, the air temperature is high, the wind speed is high (the floor is high), the relative humidity is low, and the dry shrinkage deformation accounts for a large proportion of on-site mixing, especially for flat or thin plane concrete 1 and soil structures.
2, the construction speed is fast. The degree of mechanization is high, and there are many preparations before construction, including the installation of water, electricity, roads and pumps, and the arrangement of pump pipes.
3. The product has strong quality assurance ability, but the product is in a physical state between liquid and solid when it leaves the factory and is delivered, but the final physical state of the product after it becomes a commodity is solid. Therefore, the quality of general commodities is only the responsibility of the seller, and the quality of concrete, a special commodity, is also the responsibility of the buyer. The remarkable feature of concrete is that it can be produced according to the specifications when leaving the factory, but it may not meet the corresponding strength requirements after solidification. Therefore, after the delivery of the goods and the transfer of ownership, the buyer must carry out the construction according to the specifications, and after pouring, it must be reasonably maintained according to the specifications.
Second, the preparation work before commercial concrete pouring
Before the production and construction of commercial concrete, the following preparations should be made:
1, preparation of construction mix proportion (the concrete seller shall design, inspect, test and investigate commercial concrete according to relevant standards based on the engineering situation, design requirements, construction technology level, climate change, transportation distance, raw material conditions and other factors, and on the basis of existing benchmark concrete mix proportion combined with practical experience, as well as all special requirements).
2. Supply plan and confirmation (mainly confirm pouring time, pouring position, pouring method and strength grade)
3. Arrangement and installation of concrete pump delivery pipe or chute (seller's guidance, buyer's installation)
4, water and electricity template, hole column inspection (focus on the stability of the template palm, template joint tightness, water and electricity and its ability to meet the requirements, maintenance facilities preparation).
5, transportation route survey (the focus is on the bearing capacity of the bridge, air obstacles and the smoothness and firmness of the roads entering and leaving the site, the waiting position and cleaning position of transport vehicles, etc.). ).
6. Construction organization and contact (the key point is to ensure the contact information of mixing building, pouring site, transport vehicles and on-site commanders. )
Third, the pumping of concrete.
1 run the pipe. Before pumping concrete, pump a proper amount of water, and then pump 1: 2 cement mortar to wet the pipeline. The mortar for wetting the pipeline shall be prepared by the user. After being pumped to the pouring site, the pipe-wetting mortar should be dispersed in the operation surface and not allowed to enter the beams and columns.
2 concrete pumping should be continuous. If it is necessary to interrupt, the interruption time shall not exceed the allowable duration from concrete mixing to pouring. The time from concrete mixing to unloading shall be in accordance with GB/T 14902-2003 Ready-mixed Concrete. After the concrete is transported to the site, when the temperature is greater than 30℃, it should be poured within 30 minutes and completed within 90 minutes. When the temperature is 20-30℃, pouring should be started within 60 minutes and completed within 120 minutes. When the temperature is lower than 20℃, pouring should be started within 90 minutes and finished within 150 minutes. If this requirement is not met, the slump loss of concrete is too large, and the physical quality of the structure after fluidization with additives is difficult to guarantee.
3. In case of pipeline blockage during concrete pumping, the site construction personnel of the supplier and the user shall * * * remove the blockage.
Before the end of concrete pumping, the on-site construction personnel of concrete suppliers and users should calculate the amount of concrete still needed to prevent waste, and notify the microcomputer operator of the mixing station in time.
5 After the pumping, the concrete user shall clean up the concrete pump, conveying pipe and its accessories in time, and keep them properly under the guidance of the supplier's on-site construction personnel.
6 Pump or re-pump, eliminate blockage, and clean the concrete pump. The outlet of the concrete delivery pipe should be in a safe direction to prevent blockage or mud flying out at high speed to hurt people.
Fourth, concrete pouring
1 Concrete pouring is completed by the user. Before pouring, users and suppliers divide the concrete pouring area and pouring sequence in advance.
2 The pouring sequence and distribution method of pumped concrete shall be implemented according to the provisions of 6.3.2 and 6.3.3 of Technical Specification for Concrete Pumping (JGJ/T10-95).
3. The layered thickness of pumping concrete pouring shall be in accordance with Article 6.3.4 of Technical Specification for Concrete Pumping (JGJ/T 10-95).
4 when vibrating and pumping concrete, the moving distance of vibrating rod should be about 400mm, and the vibrating time should be 15-30 seconds. There should be no under-vibration, leakage vibration or over-vibration, and the second complex vibration should be carried out every 30 minutes.
5, there are overweight reserved holes, embedded parts and steel parts, technical measures should be formulated in advance to ensure smooth cloth and vibrating compaction.
4. After the horizontal structure concrete is formed, the concrete surface should be treated three times before or near the initial setting of concrete.
Pressing: the first time 1 pass shall be leveled with a wood scraper (or aluminum scraper) with a length of about 2m, and the second time shall be rolled with an iron roller to lift the slurry, so as to make the upper and lower layers of concrete uniform. After the third time, after the surface bleeding is basically completed, carefully apply pressure with a wooden trowel to bridge the existing cracks. If shrinkage cracks are found, the concrete surface begins to harden. When it is impossible to wipe by hand, a flat vibrator can be used to vibrate to further compact the concrete. When the temperature is higher than 30℃, or when the wind is strong in spring and winter in Guangyuan area, we should take the time to pour, vibrate and plaster, and compact to prevent surface plastering after initial setting. In the initial setting and final setting stages, vibration of the formed concrete should be avoided to prevent artificial cracks in the concrete.
Verb (abbreviation of verb) concrete maintenance
Concrete users should formulate specific maintenance plans and strictly implement them according to the engineering characteristics, environment, materials used and concrete performance requirements. The early strength curing of commercial concrete must be strengthened, especially in Guangyuan area, where the climate is dry, there are many winds in spring, autumn and winter, the wind is strong, and Xia Dong is dry, with small relative strength and large temperature difference between spring and autumn, so it is more important to strengthen heat preservation.
1 After concrete pouring, it shall be plastered for three times, and the surface of plastered concrete for the third time shall be free from bleeding. It should not be too early or too late, which is not conducive to concrete crack resistance. After plastering for the second time, concrete should be covered immediately and cured with moisture, especially high-strength concrete (C40, C45 and above). The protective layer of high-strength concrete should use plastic film to prevent cracks.
2. The time for curing concrete by water spraying should start when the concrete surface is not sticky (the concrete is lightly pressed without fingerprints) (note that water spraying can only be used for curing, especially when the temperature is higher than 30℃ and lower than 5℃), and the watering times should be able to keep the concrete in a wet state. Water curing time of concrete, commercial concrete should be no less than 14 days. It is strictly forbidden to water and cure the exposed concrete directly after direct sunlight to prevent the board surface from cracking due to the sudden drop of temperature. When the temperature is lower than 2℃ in winter, it is forbidden to spray water for maintenance.
3. The concrete cured with film should keep condensed water in the film.
4. For the maintenance of mass concrete, thermal calculation should be carried out to determine its heat preservation, moisture preservation or cooling measures, so that the temperature difference between the inside and outside of concrete does not exceed 25℃.
5 The curing of pouring concrete in winter should make the concrete reach the critical strength before the curing measures can be lifted. In any case, the strength of concrete before freezing shall not be less than 5MPa.
6 In winter construction, when the concrete temperature drops to 5℃, the formwork and thermal insulation layer can be removed. When the difference between the concrete temperature and the outside temperature is greater than 20℃, the concrete after formwork removal should be temporarily covered to make it cool slowly, otherwise it will easily cause temperature cracks.
7 The removal of concrete side formwork shall be carried out according to the current concrete construction and acceptance specifications.
Six, other matters needing attention
1 After the concrete truck arrives at the construction site, the user should send someone to check the concrete supply list and guide the vehicle to reach the designated position to prevent misuse of the concrete label.
2 The measurement of concrete shall be carried out in accordance with GB/T 14902—2003 Ready-mixed Concrete, and shall be calculated based on the total transport capacity of transport vehicles. If you need to check the actual engineering quantity (excluding the volume of steel bars in concrete structures), the error shall not exceed 2%.
It is forbidden to add raw water to concrete at will, including during transportation, pumping and plastering. If the slump of concrete is too small to be pumped, it should be adjusted by adding pumping agent and a small amount of water (about 10kg per vehicle). The dosage of pumping agent should not be added according to the requirements of relevant personnel of the supplier, but cement mortar with the same water-cement ratio should be added.
In the process of concrete supply, if it is necessary to temporarily stop work or change the contents stipulated in the supply plan for special reasons, the concrete user must inform the supplier 2 hours in advance.
5. In the concrete sales contract, the concrete strength evaluation method should be determined. If there is no agreement, the standard GBJ 107-87 shall be implemented according to Article 6. 1 of Ready-mixed Concrete GB/T 14902-2003. The test block shall be based on the standard maintenance specimen marked by the project responsible party, and the non-standard maintenance specimen shall not be used.
6. Matters not covered in this description must be implemented according to the current national and industrial standards and specifications for design, construction and acceptance.
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