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
Causal Analysis:
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.
Control measures:
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
Cause Analysis:
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.
Control Measures:
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
Cause Analysis:
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.
Control Measures:
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
Cause Analysis:
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.
Control measures:
Strengthen the management of the factory, standardize lifting, transfer operation procedures ,workers are strictly required to follow the number and location of lifting points specified in the design drawings , Using a professional hoist to lift slowly up and down to avoid collision with other objects, and to ensure that the hook position of the lifting equipment, lifting gear and the center of gravity of the components in the vertical direction , the Horizontal Angle between the sling and the member should not be less than 45 degrees, not less than 60 degrees ; reduce unnecessary lifting times; ensure that the component reaches 75% of the design strength or the strength required by the design drawing, then lift the component.
7. Stacking and transportation of laminated plates
Cause Analysis:
1. In the actual code storage process, there are often many non-standard ways of stacking, for example : Stacking is too high, and in some factories to save space, stacking can be as high as 8-10 layers ; Stacking Plate Code is not regular, Large Plate Pressure Small Plate; pad wood placed at random, not standard, the upper and lower layer pad wood is not in the same vertical line, and not in accordance with the requirements, the super-long and super-wide stack is still only put four pad wood . These behaviors result in uneven forces acting on the composite slab support, which in turn leads to cracks.
2. The reasons for the cracks in the laminated plates caused by transportation are basically the same as the reasons for the cracks caused by stacking. However, it is inevitable that the road will be uneven and the car will bump during transportation. This will lead to dynamic loads. If the way of fixing the laminated plates is not firm, it is difficult to restrain the laminated plates, and the relative displacement between the laminated plates leads to cracks in the laminated plates.
Control measures:
1. The size and specifications of each stack should be unified as far as possible. It is strictly forbidden to press large plates against small ones. Ensure that each layer fulcrum in the same vertical line, to avoid the fulcrum up and down the shear cracks ; The fulcrum shall be placed on the side of the Truss, at both ends of the plate (up to 200mm) and in the middle of the span with a distance of not more than 1.6 m ; No more than 6 layers should be stacked ; The components shall be transported to the site for installation as soon as possible after the completion of production, and the stacking time shall not exceed 2 months.
2.The fulcrum shall be securely fastened to prevent the member from moving or jumping in transit. At the same time, in the bottom of the edge or in contact with the rope of concrete, the application of liner to protect.
Conclusion: With the continuous development of the prefabricated building in China, the quality of assembled laminated plates has become the focus of attention, and it is believed that only from the various links of the production process of laminated plates, at the same time, strengthen the professional skills training of workers, can effectively prevent the occurrence of crack phenomenon of laminated plate.
Post time: Mar-31-2022