Preparation of raw materials for concrete

1.1 Selection of raw materials for concrete

Before the concrete is poured, the material selection should be done, and the reasonableness and standardization of the material selection has a direct impact on the quality of building construction. The following aspects should be done.
① Strict inspection of the cement used in construction, the inspection includes the outer packaging, type, production date, factory certificate, factory inspection report, etc. of the cement. And in accordance with the national inspection standards for each test;

② in the construction design drawings to indicate the details of the construction raw materials.
③ after the raw material inspection and re-inspection, and then take samples from the site to the laboratory for inspection, the raw material indicators are up to the national standards before the concrete pouring construction.

1.2 Mixing of concrete

Construction personnel should strictly control the amount of cement, concrete aggregates and additives and feeding order; mixing time to control the time, especially note that the mixing time of different varieties of materials is not the same. Strictly according to the proportioning requirements of various materials of concrete, mixing should be uniform.

1.3 Conveying of concrete

The construction site should be set up a mobile pump and a TO pump, the construction personnel should understand the operation and function of the pump thoroughly, and prepare the pump easily damaged parts, so that the pump failure can be timely repair; carefully check whether the pump pipe is firmly connected with the pump machine, to prevent the loss of pressure caused by the pump pipe leakage, before pumping to the pump pipe and pump machine for wetting work.

The site preparation before the concrete pouring

Concealed works acceptance is needed before concrete pouring to ensure the stability of formwork supports and correct elevation control line. And make relevant emergency warning plan. Trash, wood chips, shavings, sawdust, dirt and debris such as oil and scale fallen iron on reinforcement should be removed from the formwork. Mixers, transporters, hoppers, stringers, vibrators and other machinery and equipment are prepared adequately as needed, and repair time is considered in case of failure. Important projects, there should be spare mixers and vibrators. Especially for pumping concrete, there must be a spare pump. All the machines and tools used should be checked and tested before pouring, and be equipped with full-time mechanics to overhaul them at any time. Before pouring, it must be verified that the project materials are finished at one time or before pouring to a certain construction joint, so as to avoid stopping work and waiting for materials.

Strengthen the contact work of weather prediction and forecast. In the concrete construction stage should grasp the change of weather, especially in the thunderstorm typhoon season and the occasion of the sudden attack of cold, more attention should be paid to ensure that the concrete continuous pouring ground smoothly, to ensure that the concrete will not be washed by rain and frost damage after pouring, in order to ensure the quality of concrete pouring.

On-site Concrete Pouring Program Development

Concrete construction is demanding and the integrity of the concrete structure should be taken into account. When layered concrete construction is carried out, it should be ensured that the next layer of concrete can be poured only after the initial setting of the next layer of concrete. The concrete placing process is a step-by-step process and should be coordinated with the mixing, transportation, placing and pounding of concrete to ensure the integrity of the concrete structure and the continuity of the construction as well as the construction quality.

Before the concrete is placed, a concrete placing plan should be made, and the placing method, placing order and layering thickness should be reasonably selected according to the actual situation of the construction site. The so-called comprehensive layered pouring refers to the pouring of the second layer of concrete after the pouring of the first layer, until the pouring is completed. Usually in the process of pouring are poured from the short side, then along the long side direction, but also from the middle to the surrounding or from the surrounding phase in the middle of the way, should be based on the actual situation of construction reasonable choice of pouring method. This pouring method is generally applied in the case where the concrete pouring area is not very large.

If the concrete mixer and transporter at the construction site cannot meet the construction requirements, the method of segmental layering can be used for concrete pouring. The segmental layering starts from the bottom layer of concrete construction, and the second layer of construction is carried out after the pouring is completed, from bottom to top, layer by layer. The segmental layering method is generally applicable to structures with a small thickness and a large area and length.

Concrete pouring should ensure the continuity of pouring, the height of pouring should be determined by the actual structural characteristics of the institution, the strength and density of concrete reinforcement, usually the height of the pouring layer is 1.5 times the radius of action of the vibrator; when using the insertion type vibrator, the vibrator should be fast in and fast out, vibrating can not have missed, point by point and column by column vibrating. The distance that the pounding point moves should be less than 1.5 times of the pounder’s radius of action.

Demoulding of concrete

The time of demoulding directly affects the quality of concrete and the turnover rate of formwork. The demoulding time should be determined according to the design requirements, temperature and concrete strength level. For non-load-bearing members, the formwork can be removed only when the concrete strength reaches above 5mpa and its surface and corners will not be damaged by demoulding. For load-bearing members, the formwork can be removed only when the concrete strength reaches the specified percentage of the concrete design grade. For post-tensioned prestressed concrete structural members, the side mold should be removed before the prestressing tension; the removal of the bottom mold bracket should be carried out according to the construction technology plan, and when there is no specific requirement, it should not be removed before the establishment of prestressing in the structural members.

In the process of concrete casting construction, the process of mold removal is an important construction process. In the process of demoulding is not only the process of demoulding concrete, but also to check the various quality problems in the process of concrete construction and the various defects that are likely to appear in it, and to carry out timely repair measures in the process of finding defects. To avoid the impact and disadvantages of future maintenance.

Concrete maintenance and prevention of cracks

5.1 Concrete maintenance

In the curing process, the concrete should be controlled in the temperature and humidity environment that is conducive to hardening and strength growth, so that the hardened concrete has the necessary strength and durability.

If there are no special requirements, the natural curing is generally the main purpose. When curing concrete naturally, the maximum and minimum temperature of atmospheric temperature and the change of weather should be recorded daily, and the curing method and system should be recorded. For the concrete cured by film or curing agent, the integrity of the film or curing agent and the moisturizing effect of the }kuncrete should be checked frequently.

Maintenance of bulk concrete, should be thermally calculated to determine its insulation, moisturizing or cooling measures, and should set up temperature measurement holes or buried thermocouples, etc. to determine the temperature inside and on the surface of the concrete, so that the temperature difference is controlled within the range of the design requirements, when there is no design requirements, the temperature difference should not exceed 25 ℃.

Concrete poured in winter, should be maintained to have the critical strength of frost resistance before removing the maintenance measures. In any case, the strength of concrete before freezing shall not be less than 5 MPa. during winter construction, formwork and insulation shall be removed only after the concrete has cooled to 5℃. When the difference between the concrete temperature and the outside temperature is greater than 20℃, the concrete should be temporarily covered after demoulding to make it cool slowly.

5.2 Crack prevention

(1) Control temperature cracks

In order to minimize the heat of hydration in the process of concrete solidification, you can try to use dry hard concrete, reduce the proportion of cement in concrete, add the right amount of coal ash to reduce the amount of cement, appropriate addition of other additives can be in the concrete for the concrete has special requirements should strictly comply with the requirements prohibit the addition of other substances; in the process of concrete solidification, water plays a decisive role, in order to reduce the impact of water on the concrete solidification, to strictly control the amount of water. When the construction temperature is relatively high, the pouring thickness should be reduced when pouring concrete, and the next layer should be poured after the poured concrete layer has cooled.

(2) Strengthen construction organization measures

Strictly manage the staff at the construction site to ensure that the construction personnel of each post do their duties and responsibilities, clearly divide the work, and arrange special personnel to be responsible for concrete pouring and later rain and frost prevention in the concrete construction process. Develop a scientific and reasonable assessment, reward and punishment system for construction personnel to reduce the influence of human factors on concrete cracks as much as possible.