1.Raw materials

The fineness modulus and silt content of sand must meet the requirements, and the water content should not exceed 6% during use. The sampling of sand must not take the surface layer and the oversize particles of the upper gravel, etc. must meet the requirements, especially the silt content (stone powder content). If the requirements are not met, the particle gradation of the gravel should be basically uniform, the continuous water-reducing agent and air-entraining agent have passed the inspection, and the storage time should not exceed 6 months. the 3-day strength of cement has been tested and found to be in compliance with the requirements of the contract, and the fly ash has been tested and found to be in compliance.

2.Construction mix ratio

The approved mix ratio shall be used as a basis for adjustment according to the fineness modulus and water content of the sand and the water content of the gravel. No water shall be added during construction, but the dosage of the water-reducing agent can be adjusted in a small range according to the workability. When it rains, if the water content of the sand and gravel increases (due to the lack of rain protection for the hopper and silo, or an increase in water content during loading, etc.), the amount of water used in the construction mix ratio shall be reduced compared to before the rain, but no other adjustments shall be made lightly.

3.Slump detection and adjustment

When mixing the first two batches, the slump and workability of each batch of concrete should be checked at the outlet of the machine. Normally, the slump is tested at the pouring site. The amount of water reducer can be appropriately adjusted according to the slump. The slump detection requirements shall be carried out in accordance with the prescribed operating procedures.

4.Construction joint treatment

A mortar with a water-cement ratio of 0.03-0.05 lower than that of the concrete, but not lower than that of the same grade, should be spread over the construction joint with a thickness of 2-3 cm. The spread area should be appropriate for the pouring speed. The construction joint should be in a wet state before the mortar is spread. The chiselling of the construction joints must meet requirements and be cleaned to remove any debris. A layer of cement slurry (water-cement ratio 0.03-0.05 lower than concrete) should be applied to vertical joints, and the area to be applied should be appropriate to the pouring speed.

5.Entering the silo

The height of the concrete into the silo should not be greater than 2 m. If a distributor is used to put the concrete into the silo, the distance between the chute tubes should be 2.5-3.0 m. The concrete should be put into the silo in a sequential manner, with the first layer being placed first and the subsequent layers being placed later. However, all the concrete should be covered and vibrated before it sets. Any substandard concrete that has already been placed in the silo must be removed. The angle of the belt conveyor should generally not exceed 6 degrees. The belt conveyor and chute tubes should be kept damp.

6. Leveling the warehouse

The warehouse should be leveled in time and not allowed to pile up inside. When leveling the warehouse manually, the layer thickness should be 30-50cm (pseudo-laying), and it should not exceed 1.25 times the head of the vibrator. When unloading, the concentrated aggregate should be manually distributed evenly in the places with more mortar, and the bleed water should be removed in time without taking away the cement slurry. If the lower part of the floor slab cannot be manually leveled due to constraints such as steel bars, a special person must direct the distributor to enter the silo, control the concrete entry part and the amount of concrete entering the silo, and ensure that the concrete is not too thick to avoid the phenomenon of aggregate concentration. The surface concrete of the floor slab or the top surface of the top slab must be manually leveled, and the surface of the silo should be roughly level and slightly higher than the designed elevation before the slab vibrator is vibrated.

7. Concrete vibration

should first vibrate the area near the formwork (taking care not to hit the formwork) and insert it about 5 cm into the lower layer of concrete. The vibrating points should be in a plum blossom shape, with a spacing no greater than 1.5 times the effective range of vibration. Insert quickly and pull slowly to prevent sloshing and bubbling. The effective range of vibration is generally determined through testing. Generally, the vertical concrete surface is inserted into the concrete and vibrated in turn. If it is slightly inclined, the tilt direction of each insertion point should be the same to avoid missed vibrations. The usual vibration time is 15-20s. Due to differences in slump and workability, the vibration time varies greatly. The vibration time for each value is based on when the concrete does not sink significantly, no bubbles appear and the concrete starts to slump. The first layer of concrete poured or the joint area of two discharges should be vibrated more intensively. The bottom plate or top plate generally needs a plate vibrator for surface vibration, and the surface should be manually flattened. The vibrating thickness of the slab vibrator is small. If the paving material exceeds 15-20 cm, it is necessary to vibrate with a vibrating rod and then vibrate with a slab vibrator. The requirements for vibrating with a vibrating rod are the same as above. This is to say that the moving speed of the slab vibrator should be appropriately accelerated.

8. Vibrating the concrete near the waterstop

Particular care should be taken when vibrating around the waterstop. The vibrating rod must not touch the waterstop. When placing concrete under the horizontal waterstop, someone should manually flip the waterstop up first, and then place it on the surface of the vibrated concrete after vibration. When placing concrete on top of the waterstop, it should not touch the waterstop. It should be controlled to be at least about 50 mm away from the waterstop, and it should also be ensured that the area around the waterstop is vibrated densely. Where the vibrator cannot vibrate densely, supplementary manual vibration should be used.

9. Inspection of interlayer surfaces and control of concrete pouring time

Layers shall be constructed in the pouring sequence determined in the construction plan, and the layer thickness shall meet the requirements. The length of each layer shall be consistent with the plan. The pouring time of each part of each layer shall be recorded to remind the construction unit to cover the upper layer and progress to the next layer before the concrete in that part sets. If the construction unit is found to be too late to cover the concrete before it sets, the construction unit shall be stopped from continuing to pour concrete. Concrete pouring should be continuous. If the interval time (the initial setting time) is exceeded, the upper layer of concrete should not be covered, and it should be treated as a construction joint. The interval time must not exceed the initial setting time. This time is the time from the discharge of the concrete from the outlet to the completion of covering the upper layer of concrete and vibration. When the concrete can be reshaped, construction can continue. (The vibrator vibrates for 30s, with no holes and with fresh concrete within 10cm of the surface.)

The interval time depends on the temperature and cement used when pouring the concrete. Due to the differences between site conditions and laboratory conditions, the initial setting time of concrete measured in the laboratory cannot be used to control the pouring process.

10. Judging initial setting

Judging by appearance: If you tap the concrete with a shovel a few times, there is no sign of bleeding or softening, or a skin forms, or the concrete no longer spatters (within a 10 cm range) when vibrated, there are holes after the vibrator is removed, or there are slight marks when pressed with your finger, etc., then it may have set. Judging by time: If it has been left to set for too long, it may have set. This is related to the weather, etc., and is not very accurate. The laboratory test is more accurate, but it cannot be used to determine the setting time during on-site construction

11. Inspection of the formwork

During the concrete pouring process, the shape and position of the formwork should be checked and adjusted frequently. If the formwork is deformed, the construction unit should be immediately instructed to take effective measures to rectify it, otherwise they should be ordered to stop pouring the concrete. Any leakage should be promptly blocked. To avoid concrete roots, the tightness of the bottom corners is particularly important. (The tightness of the mould when erected should be controlled, the outside of the mould should be blocked with mortar, etc., and the mortar at the construction joint during pouring can be intentionally thickened appropriately at the corners, but it should not be greater than 5 cm. During the inspection of the warehouse, the evenness of the formwork, the tightness and firmness of the joints should be checked. The gap between the reinforcement and the formwork should also be blocked, and the joints between the formwork should be sealed with double-sided tape.

12. Reinforcement reset and binding

Considering the needs of construction, some reinforcement bars may need to be moved during the construction process, but attention should be paid to timely reset and tie.

13. Concrete plastering

If there is a requirement for troweling, the construction unit should be supervised to carry out troweling and smoothing in a timely manner. Troweling generally needs to be carried out in three stages. The first time is carried out after vibration, generally with a wooden trowel, and the large surface should be leveled, with the evenness meeting the requirements. The second time is carried out before initial setting, generally with an iron trowel, and the third time is carried out during initial setting, also with an iron trowel (smoothing). Care should be taken not to disturb the concrete when troweling for the second time.

14. Timing of chiseling

The surface of the construction joint should be roughened using high-pressure water. If the surface of the construction joint has been in contact with concrete for too long, it should be roughened using an air-powered sandblaster instead. If necessary, the surface of the construction joint can be roughened manually to remove all floating concrete, loose material and contaminants. The roughened surface should reveal coarse sand or small stones, but should not damage the internal aggregate. Chiselling of the construction joint should be carried out after the concrete has gained strength, which is generally not less than 5MPa. It should not be done too early, and is generally carried out on the second day after pouring, with an appropriate delay in the low temperature season.

15. Maintenance

After the concrete has been poured, it should be covered and sprinkled with water as soon as possible after the slurry has been collected. This is usually done for 12-18 hours, and the time should be brought forward in the high temperature season. The concrete surface must not be damaged or contaminated during covering. When the concrete surface is covered with a formwork, the formwork should be kept wet frequently during the curing period.

When the temperature is below 5℃, the formwork should be covered with insulation, and no water should be sprinkled on the concrete surface. The requirements for water used for curing concrete are the same as those for mixing water.

The duration of sprinkling for concrete curing is specified in the technical contract terms and may be extended or shortened as appropriate depending on the air humidity, temperature, cement type and admixtures used. During the curing period, the number of sprinklings per day should be sufficient to keep the concrete surface constantly wet.

When the structure comes into contact with flowing surface water or groundwater, waterproofing and drainage measures should be taken to ensure that the concrete is not washed away by water within 7 days of pouring. When the surrounding water has an erosive effect, the concrete should be protected from water immersion until it reaches 70% of the designed strength within 10 days. The concrete should not be subjected to loads from pedestrians, vehicles, formwork, scaffolding or other structures until it reaches a strength of 2.5 MPa. If steam curing is required, it should be carried out in accordance with the relevant requirements of this chapter, and a steam curing inspection record should be completed in a timely manner.

16. Demoulding inspection

After the concrete has been poured, the construction unit shall submit an application for formwork removal, and the formwork shall only be removed collectively if the requirements are met. The construction unit shall submit a strength report for the concrete test blocks that have been cured under the same conditions, and the formwork shall only be removed if the requirements are met. When the formwork is removed, the situation after the formwork is removed shall be inspected on site, including honeycombs, pitted surfaces, incorrect platforms, cracks, etc. The construction unit shall not be allowed to arbitrarily smear or cover up various quality problems or defects in the concrete. Any major quality problems or defects shall be reported in a timely manner. Pay attention to frequently inspecting the concrete for cracks, urge the construction unit to make good observation records, and frequently check the observation records.

17. Quality defect handling procedures

If a quality defect is found during inspection, the construction unit and the supervising unit shall promptly make inspection records and find the cause. The supervising unit shall review and approve the defect handling plan according to its authority, and shall stand by during defect handling to supervise the construction unit’s construction according to the approved plan, promptly filling out supervising records.

For category I defects, records shall be kept before the defects are dealt with, and inspections shall be carried out before the defects are dealt with and acceptance inspections carried out after the defects are dealt with. For category II defects, joint inspections shall be carried out before the defects are dealt with and joint acceptance inspections carried out after the defects are dealt with. If necessary, relevant experts shall be invited to review the defect treatment plan.