1. Construction preparation
During the concrete mixing process, materials that can improve and adjust the properties of concrete as required are called concrete admixtures. The product quality of admixtures should meet the requirements of current national standards, and their dosage should be determined through testing.
1. Materials
⑴Additional materials are classified according to their use effects as follows:
1) Reducing agent: It has the ability to reduce the workability of concrete without affecting its workability. and additives that enhance the effect.
2) Air-entraining agent: During the concrete mixing process, it can cause a large number of evenly distributed micro-bubbles to reduce the bleeding and segregation of the mixture, improve workability and significantly improve the freeze-thaw resistance of hardened concrete. Durable admixtures.
3) Setting adjusting agent: an admixture that can adjust the setting time and hardening properties of concrete.
⑶The applicable scope of commonly used admixtures is shown in Table 5─30
2. Tools
⑴Ready-mixed concrete mixing station should adopt an automatic metering system for admixtures device.
⑵ When using powder at the construction site, the following tools should be equipped
a. Two large buckets that can hold more than 100kg and have movable iron lids.
b. Two small buckets that can hold 5kg of water.
c. 1 bamboo stick and 1 bamboo hand sweep.
d. One 100kg platform scale and one 10kg steelyard.
⑶ When using solvents at the construction site, the following tools should be equipped
a. Two large buckets that can hold more than 100kg of water and have movable iron lids.
b. 1 small bucket that can hold 5kg of water.
c. One 100kg scale.
3. Operating conditions
⑴Select the type of admixture according to the concrete performance requirements
⑵The amount of admixture must be determined through trial mixing.
⑶ Provide technical operation briefings to workers before starting work.
⑷Designate a special person to supervise the measurement work.
⑸ Prepare enough admixture materials for this operation.
2. Operation process
1. Mixing of admixtures
Admixtures can be divided into solid and liquid. The percentage of concrete admixtures is based on solids.
⑴Aqueous agent, the liquid admixture supplied on the market has a high concentration and is not suitable to be directly incorporated into the concrete mixture. It needs to be diluted into an aqueous solution with a concentration of 20% to 30% first.
1) The water mixing ratio is determined by the difference between the concentration added according to the construction requirements and the original concentration.
2) Weigh out the quantitative admixture.
3) Store the amount of water that should be added in a large bucket.
4) Pour the admixture into a large bucket and stir evenly.
⑵ Powder: First prepare the powder into an aqueous solution with a concentration of 20% to 30% before use.
1) According to the mix ratio, first prepare a small bucket (weight 5kg) of admixture solution according to the amount of admixture powder required for each plate (trough) of concrete.
2) Calculate the amount of admixture powder that should be mixed into a large bucket (weight 100kg) of admixture aqueous solution. (That is, 20 times the volume of the small barrel).
3) Place the admixture on the bamboo and soak it in the water of the large bucket.
4) Brush the bamboo hand back and forth on the bamboo until all the powder is filtered into the large bucket.
5) Before the first bucket is used up, start preparing the second bucket of solution.
2. Operation method
⑴Use a small bucket of admixture aqueous solution for each plate (trough) of concrete.
⑵ The water reducing agent solution should be added to the mixer 30 seconds before unloading, and then discharged after stirring for 30 to 60 seconds. The water consumption of the concrete should be deducted from the water content of the water reducing agent solution.
⑶ The amount of water reducing agent is 0.15% to 0.35% of the cement weight, and the retardant amount is 0.1% to 0.5% of the cement weight.
⑷Requirements for using the solution: The remaining solution used every day can be saved for use the next day, but it must be covered tightly to prevent rain or other liquids from penetrating. Stir evenly with a water rod before use the next day. If the solution cannot be tightly covered, do not use it again.
⑸The curing period of concrete mixed with retarder and expansion agent should not be less than 14 days.
⑹The transportation, pouring and curing of concrete with admixtures are basically the same as those without admixtures.
⑺In steam-cured concrete and prestressed concrete, it is not advisable to mix air-entraining agents and air-entraining water-reducing agents.
⑻Concrete mixed with expansion agent is only allowed to be poured from one direction, and is not allowed to be poured from more than two directions.
3. Examples
Question meaning: It is known that the amount of cement per plate (trough) of concrete is 50kg, and the amount of additive powder is 0.2% of the cement amount. Calculate and prepare a large bucket of water. (100kg) The quantity of admixture powder mixed with the admixture aqueous solution.
1. The amount of admixture powder for each plate (trough) of concrete is 50×0.2%=0.1kg.
2. Make sure to use a small bucket of 5kg admixture aqueous solution and 0.1kg admixture powder for each plate (trough) of concrete.
3. It is estimated that when preparing a large bucket (100kg) of admixture aqueous solution, the amount of admixture powder should be 100÷5×0.1=2kg
Therefore, add 98kg of water to the large bucket. Stir 2kg of powder admixture evenly to prepare a large bucket of admixture aqueous solution.
100÷5×0.1＝2kg
4. Put 98kg of water and 2kg of admixture in a large bucket.
IV. Construction precautions
1. Admixtures must have factory certificates.
2. The dosage must be determined through trial preparation.
3. The proportion of ingredients is accurate, the allowable error is 2%, and no leakage is allowed.
4. When preparing solutions, the minimum dosage is 50kg. It is strictly prohibited to prepare non-integer solutions to prevent mismatching.�
5. Stir the solution in the vat frequently to prevent precipitation and caking.
6. The amount of water used for concrete should be deducted from the amount of solution.
7. The solid additives are pre-weighed according to the required amount of each vat of solution and packaged in plastic bags for later use.
8. The storage of admixtures shall be carried out according to the product instructions. It should be strictly distinguished from other easily confused materials and marked.
9. Every time a solution is prepared, a weighing record must be made for verification.
10. All measuring instruments must pass metrological verification.
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