First, the purpose of concrete curing
After the concrete is poured, the reason why it can gradually solidify and harden is mainly because of the result of cement hydration, and hydration requires appropriate temperature and humidity conditions, so in order to ensure that the concrete has suitable hardening conditions and its strength continues to grow, the concrete must be cured. After the concrete is poured, if the climate is hot and the air is dry, if the maintenance is not carried out in time, the moisture in the concrete will evaporate too quickly, forming a dehydration phenomenon, which will make the cement particles that have formed a gel body can not be fully hydrated, can not be converted into stable crystals, lack of sufficient cohesion, and will appear flakes or powders on the surface of the concrete. In addition, when the concrete does not have sufficient strength, the premature evaporation of water will also produce large shrinkage deformation, dry shrinkage cracks, and affect the durability and integrity of concrete. Therefore, the curing of the initial stage after concrete pouring is very important, the concrete should be cured immediately after the final setting, and the dry and hard concrete should be cured immediately after the pouring is completed.
Second, the curing requirements of the concrete surface
(1) Plastic concrete should be sprinkled within 6~18 hours after pouring, and low-plastic concrete should be sprinkled immediately after pouring.
(2) The concrete should be cured continuously, and the surface of the concrete must be in a wet state during the curing period.
(3) The concrete curing time should not be less than 28 days.
3. Mold removal
(1) The principle of mold removal
The following principles should be followed when removing the mold:
1) First support and then dismantle, and then dismantle;
2) first dismantle the plate formwork and both sides of the formwork, then dismantle the secondary beam template, and finally dismantle the main beam template;
3) dismantle the non-load-bearing formwork first, and then dismantle the formwork of the load-bearing part;
4) From top to bottom, remove the lateral support first, and then remove the vertical support.
5) Safety precautions: safety monitoring, safety prevention.
During the demolition, full-time safety personnel should be set up on the site to guide, supervise and supervise the demolition and safety celebrations on the site. The main attention should be paid to: (1) the demolition personnel should take care of each other when the upper and lower floors are crossed. (2) The dismantled material shall not be thrown directly from a high altitude, but shall be transmitted from top to bottom or tied down by a remote rope. (3) A safe area should be removed near the lifting port, and dismantled from here to the outside, so as to ensure the safety of material stacking, transportation and dismantling.
(2) Demolition points
1) Attention should be paid to the protection of the product during the demolition: especially the edges and corners of the beams and columns (yang angles) and the joints of the beams and columns.
2) In the process of formwork dismantling, the formwork removal sequence should be strictly followed, and the formwork should be removed slowly, and it is strictly forbidden to pull hard, so as not to damage the edges and corners and surfaces of the formwork and concrete components. The dismantled formwork and wood should be stacked according to the degree of damage and transported out in batches by the lifting port.
3) Each demolition site should be dismantled only after the strength of the concrete test block under the same conditions on site reaches the full strength of the demoulding specification.
4) It is strictly forbidden to dismantle the mold without permission, otherwise severe penalties will be given.
(3) Requirements for dismantling the mold body
For the components in the concrete structure project, the concrete strength reaches a full amount of the standard value of the compressive strength of the designed concrete cube, and the standard value of the formwork can be dismantled.
1) For the cantilever member in the concrete structure engineering, the bottom template can be removed only when the concrete strength reaches (100%) of the standard value of the compressive strength of the concrete cube designed.
2) When the bottom formwork of beams, arches and shells with a component span greater than 8m is removed, the concrete strength should be greater than or equal to (100%) of the standard value of the designed concrete cube compressive strength.
3) When the bottom formwork of the reinforced concrete slab with a component span of 5m is removed, the concrete strength should be greater than or equal to (75%) of the standard value of the designed concrete cube compressive strength.
4) In the formwork removal work, there are two types of strength requirements.
(4) Load-bearing formwork
The detailed time plan is sufficient in Article 4.3.1 of the Code for Construction Quality Acceptance of Concrete Structure Engineering. The percentages of the standard values of the compressive strength of the design concrete cube that need to be achieved by different components are as follows: slab: ≥50% when the span is ≤2; When the span > 2,≤8, it is ≥75%; When span >8 is ≥100%; Beams, arches, shells: ≥75% when span ≤ 8; ≥100% when span > 8; Cantilever members: 100% must be achieved, regardless of the size of the span.
(5) Non-load-bearing formwork
Comprise the side template of foundation, column, top pressing, in addition to the side template of beam and wall, and when it is dismantled, it can be dismantled as long as the strength can ensure that its surface, edges and corners are not damaged by demoulding. However, for the large wall formwork, the strength is required to reach 1N/m at room temperature before it can be removed.
In addition, there is another strength that is often used in the concrete construction process, that is, the concrete can meet the strength requirements of people: this strength is 1.21N/mm2. When filling in the form, you can fill in the load-bearing template according to the planning requirements, and you can fill in 1N/mm2 for the non-load-bearing side template without any problem.
(6) Mold removal time requirements
The dismantling time of the formwork for concrete walls, beams and columns is sufficient according to the concrete strength
The concrete strength of the bottom mold and the bracket should meet the following requirements: plate: when the span is ≤ 2m, the strength ≥ 50%; When the span > 2,≤8m, the strength ≥ 75%; When the span is >1m, the strength ≥ 100%; Beam: When the span is ≤8m, the strength ≥ 75%; When the span is > 8m, the strength ≥ 100%; Cantilever members: independent of the span, must reach 100% ≥.
The non-load-bearing side formwork, including the side formwork of beams and column walls, can be removed as long as the concrete strength ensures that its surface and edges and corners are not damaged by formwork. Generally, the large formwork can be dismantled at room temperature when the concrete strength reaches IN/mm2. "Code for Acceptance of Construction Quality of Concrete Structure Engineering" (GB50204) 4.3.1 After the concrete structure is poured, it can be dismantled only after reaching a full strength. The date of dismantling of the formwork should be determined according to the structural characteristics and the strength of the concrete. The quality of concrete engineering is the result of the joint efforts of designers, supervisors and construction personnel. Every person responsible for quality must pay attention to the prevention of quality defects or as soon as possible to find the defects that may occur in the construction, in order to take remedial measures at the right time, the maintenance and demoulding process can only be done in accordance with the technological process, in order to ensure the final quality of concrete.