Precast composite panel is an important part of the prefabricated building, and the problem of cracks in composite panels in the process can not be ignored. Based on the engineering application and the production process of the combined component, the causes of the cracks in the laminated slab are analyzed and the corresponding control measures are put forward.
1 . What’s a laminated plate?
Laminated Slab is a kind of laminated member, which is composed of precast concrete member (or existing concrete structure member) and post-cast concrete, and is formed in two stages.
During the construction, the precast concrete slab is first installed on the site, and is used as a formwork, supplemented by supporting supports, and then the concrete superimposed layer (that is, the upper part of the cast-in-place concrete) is poured, to bear the upper part load . There is a obvious advantages for this structure, combining the advantages of the cast-in-place structure and the precast structure, not only guaranteeing the structural integrity, but also meeting the requirements of the component industrialization progress, and saving a large number of formwork support and dismantling, and reducing the construction cost, is a very potential expansion of the floor form.
2. The process of creating a crack
The technological process of the precast layer of the superposed plate is as follows: Mould platform cleaning → mould assembling → coating retarder and releasing agent → steel bar binding → hydropower pre-embedding → concrete pouring → vibration → pre-curing → stretching → curing → demoulding lifting → transportation to the finished product stacking area (water washing is added according to the design requirements) .
According to experience, the main processes that may produce cracks are vibration, pulling hair, maintenance, demoulding, lifting, stacking and so on.
3.The laminated plate is poured, vibrated and stretched
1. After concreting, at present, the PC automatic assembly line, the prefabricate component mainly uses the shaking table to carry on the vibration. Vibration table vibration, vibration frequency, high efficiency, only 15-30 seconds to complete vibration. Due to the lack of experience of equipment operators, there are often over-vibration, segregation phenomenon, resulting in the production of cracks
2. The precast concrete has smaller slump and higher viscosity. When the fixed mould table is used in the production, the vibrating rod is used to vibrate the truss too much, and the vibrating point is less, it is easy to cause serious bleeding or even local segregation of concrete at the exposed tendons of the Truss, resulting in cracks along the direction of the truss tendons.
Vibration table is used to pound concrete to make clear the operation requirements of equipment operators. When manual vibration is used, the vibrator should be placed horizontally, and at the same time, should be paid attention to the vibrating time to avoid local over-vibration and vibrating truss. In the construction process, trample on truss bars is strictly prohibited until the concrete reach the lifting strength.
4.Maintenance of laminated plates
At present, steam curing is mainly used to maintain the components in the factory. Steam curing is divided into four stages: static stop, temperature rise, constant temperature and temperature drop. Concrete hardening gradually and increasing strength are actually the process of hydration reaction, but the hydration reaction has the higher request to the temperature and the humidity. Therefore, when the temperature and humidity can not meet the requirements, it is easy to cause cracks because of concrete shrinkage.
During the period of pre-curing, the temperature of concrete should be controlled not less than 10 °C . The temperature of concrete can not rise until 4 ~ 6 hours after the completion of pouring ; The heating rate should not be more than 10 °c/h ; The internal temperature of concrete should not exceed 60 °C and the maximum should not exceed 65 °c during constant temperature period , the curing time at constant temperature should be determined by testing according to the requirements of demoulding strength, concrete mix proportion and environmental conditions ; During the cooling period, the cooling rate should not be more than 10 °c/h, and the temperature difference should not be more than 15 °C.
5.Demoulding of laminated plate
After the maintenance of the component, if the component strength does not meet the strength requirement of the demoulding, the forced demoulding may cause cracks on the side of the component due to the strength reason, and the cracks will continue to extend after the later storage and the protection of the finished product is not in place, finally, the cracks form in different directions on the plate surface.
The springback instrument should be used to monitor the strength of the laminates before demoulding. The demoulding can not be done until the laminates have reached 75% of the design strength or the strength required by the design drawing. Mold removal should be in accordance with the requirements of the mold assembly process and mold removal requirements, strictly prohibit violent mold removal.
6.Lifting and transshipment of laminated plates
According to the shape and size of the laminated plate, through the stress analysis, bending moment calculation and reference to national standards, Atlas, the final determination of the location of the lifting point of the laminated plate. Since the laminated plate is flat and only 60mm in thickness, in order to prevent uneven loading during lifting and transferring of the laminated plate, need a special balance frame to assist the lifting.
But in the actual operation process, often appears the component direct hoisting does not use the balance frame; the design request six, eight point hoisting but the production still four point hoisting; not according to the drawing stipulation hoisting point position hoisting and so on. These nonstandard operation will cause the member to have the crack because of the excessive deflection in the hoisting way. The irregular operation will deepen the cracks of the composite slab, and eventually the cracks will extend to the whole slab, and even more serious will form through cracks, resulting in the scrap of the whole slab.