Precast concrete components maximize the compactness of concrete

Most of the domestic building structures today are designed as cast-in-place concrete structures. The more common structural forms include frame structures, shear wall structures, frame-shear structures, core tube structures, tube-in-tube structures, etc. Their common feature is that most of the components are cast-in-place, and only a few secondary components such as door and window lintels are used. In addition to prefabrication, precast concrete component technology has brought a brand new topic. It is proposed that the components in the above structural form, including exterior walls, stairs, kitchens and bathrooms, beams, slabs and even columns, use prefabricated components, which are assembled together during construction and the nodes are processed to form a complete building design and construction technology.

(1) The steel bars must have sufficient protective layer thickness. Under normal circumstances, it takes a certain amount of time for the carbonization of concrete to reach the surface of the steel bars, that is, the depassivation time. Obviously, the greater the thickness of the protective layer, the longer the depassivation time. If the deactivation time exceeds the design service life of the building, the requirements can be met. Therefore, the "Specifications" make the following provisions on the thickness of the protective layer of steel bars: the thickness of the concrete protective layer of longitudinal stressed steel bars and prestressed steel bars, steel wires, and steel strands (the distance from the outer edge of the steel bar to the outer edge of the concrete) should not be less than the nominal diameter of the steel bar or the equivalent diameter of the parallel bars, and should comply with the regulations on the minimum thickness of the concrete protective layer in the appendix.

(2) Reasonably design the mix ratio of concrete. First of all, there must be sufficient cement dosage, generally not less than 225kg/mm2, see the regulations in Table 9.3, to maintain the alkalinity of concrete; at the same time, try to reduce the water-cement ratio to reduce the amount of free water. For this, water-reducing agents can be used to reduce water consumption while meeting construction requirements; use high-quality admixtures and strictly control the salt content of raw materials.

(3) Concrete prefabricated components should try to improve the density of concrete and enhance the impermeability. For general concrete, the density of concrete should be ensured during design and construction, and the vibration should be sufficient and dense. It should be carefully maintained according to the requirements of the regulations. The surface of newly poured concrete should always be kept moist. Curing liquids, covering curing materials and other measures can be used to reduce water evaporation and avoid surface cracks.

(4) Control the amount of concrete admixtures When admixtures are used in concrete, the excess substitution method is used to design the mix ratio if the strength conditions are met.

(5) Use a covering layer. The covering layer can isolate the direct contact between the concrete surface and the atmospheric environment, which is very beneficial to reducing the carbonization of concrete, especially under unfavorable or even harsh environmental conditions. The effect is obvious; at the same time, it prevents the intrusion of water, carbon dioxide, chloride ions and oxygen, and reduces the corrosion of steel bars.

(6) Use steel bar rust inhibitors to prevent chlorine salt corrosion; use anti-corrosion steel bars, the types of which include: epoxy-coated steel bars, galvanized steel bars, stainless steel steel bars, etc.; use cathodic protection methods for steel bars, including sacrificial anode methods and input current methods.