Zhou Zhangrong
Guizhou Bridge Construction Group Co., Ltd
Abstract: Taking an engineering project as an example, the standardized construction and production management of the precast yard of small components of the highway project are studied. According to the analysis of site planning, greenhouse construction, internal functional zoning and other aspects in the construction of the small component precast yard of this project, and put forward the relevant measures for the prefabrication production management of the relevant small component precast yard, it is hoped that it can provide reference for similar projects.
Keywords: highway engineering; small components; precast yards;
About author:Zhou Zhangrong (1984—), male, from Sansui, Guizhou, engineer, engaged in highway engineering testing, quality management and other work.
0 Introduction
In recent years, more highway projects have selected concrete precast components to complete the construction of ancillary projects, which not only solves the shortcomings of the original slurry sheet stone and block stone in the masonry treatment, but also ensures the uniformity and reliability of the overall appearance of the highway. However, due to many factors, the processing is scattered and the lack of consistent standards affects the quality of the road. Therefore, it needs to be controlled through standardized construction and management.
1 Introduction to the highway project
1.1 Brief description of the project
This project is a highway project, with a total construction length of 123.5km and a north-south direction. In this contract section, the main line contains nine bridges with a total length of 3,386 m, including one extra-large bridge with a length of 1,117 m. In addition, there are ten tunnels with a total construction length of 9899m. Design an interconnected three-dimensional intersection. The main line of the contract section is set up as a two-way four-lane with a speed limit of 100km/h. There are two forms of subgrade, with a monolithic width of 26m and a separate width of 13m. The connecting line is a secondary highway standard, with a speed of 60 km/h and a roadbed width of 8.5 m. According to the actual construction situation of this contract section, two precast yards with small components will be arranged, and the site scale is 20×30m. The site of small prefabricated components is set up in a closed type, and is divided into areas such as steel bar stacking, production, storage, maintenance, mold drying, finished product stacking and component transfer according to its related functions. In the precast yard, obvious signboards and floor plans, safety signs, etc. are installed.
1.2 Quantities
Combined with the actual situation of the project, the small structure precast site needs to be responsible for the cover plate and other prefabricated parts of the drainage system such as culverts and roadbed side ditches, tunnel drainage ditches, tunnel lateral blind ditches, etc. The total quantity of the precast site includes: 1 111m3 of C25 concrete elements, 32 404 pieces; C30 concrete elements 13 191m3, 143 874 blocks.
2 Standardization construction of small component precast yards
In the construction of precast yards, it is necessary to do a good job in overall management. The width of the road in the precast yard should be controlled at 3.5~5m. In the steel bar making and processing area, in order to facilitate construction, it is necessary to leave a 1m wide sidewalk and make floor paint. In the production area, health area and storage area, C20 concrete needs to be used for hardening, the thickness is controlled at 0.2m, and the hardening thickness of the transportation and construction channel should reach 0.15m. In addition, to check the surface drainage slope, it is necessary to be above 1.5%, and rectangular drainage ditches should be built around the site. Based on the horizontal direction of the site, a health pipeline system is arranged every 2m and built above the maintenance and treatment area. On the outside of the site, a three-stage sedimentation tank must be built to ensure normal drainage needs.
2.1 Master Plan
The houses in the precast yard are built with a lightweight steel structure, and the columns are steel pipes with a specification of 165×4.0, arranged every 6m. The horizontal spacing of the sandalwood is 1m, and the roof slope should not exceed 1.5%. In addition, the roof panel material uses color steel tiles, and the overall roof truss structure is a triangular space truss, and the composition material is hot-dip galvanized round pipe.
2.2 Construction of greenhouses
The entire precast yard adopts a completely closed management method, for which it is necessary to erect greenhouses in designated locations. Choose red color steel tiles on the roof of the factory building, and build a canopy made of steel material, and set up white around it. And set up a steam shed in the curing area, adopting a semi-closed structure.
2.3 Functional partitioning
2.3.1 Rebar processing area
Since the precast yard is close to the tender section steel bar room, there should not only be a processing area inside, but also arrange areas such as semi-finished product storage, steel bar lashing, and tool storage. The steel reinforcement material uses the tire mold base to complete the positioning, arrange the concrete block with the concrete label, control the concrete protective layer, and ensure the construction quality.
2.3.2 Cloth area
According to the requirements of the cloth process, this site needs to have a strong bearing capacity, and the ground should be flat to provide basic conditions for the smooth operation of the production unit. The strength of the foundation under the automatic spreader should reach C25 and the thickness should be at least 0.3m. Lay out production lines in the production area, equipped with the required spreaders and shakers to create an integrated production line.
2.3.3 Conservation areas
The maintenance area also needs to have a certain bearing capacity to ensure the smooth storage of tools and components used in maintenance, and there are also high standards for the levelness of the ground. Set up automatic spray system, the curing water is Ca(OH)2 saturated solution, the temperature is maintained at 15~25 °C, and the humidity needs to exceed 90%. When the outdoor temperature is low, a steam shed is set up, and the relative humidity should exceed 95%. Each batch of prefabricated components needs to be stacked separately. In addition, an independent water circulation system is installed in the conservation area to ensure the continuity of the conservation process.
3 Prefabrication production management of small component precast yards
Before the construction of prefabricated components, design parameters and technical disclosure need to be reviewed. In the process of disclosure, it is required to clearly indicate the concrete mix ratio and curing, steel bar laying distance, and the appearance quality of components. Complete the inspection and acceptance of all materials and mechanical equipment. The units in the prefabrication plant of small components of the project should be assembled and accepted according to the regulations, and temporary facilities and personnel arrangements should be in place.
3.1 Mold handling
When prefabricating highway guardrails, the formwork is made of mirrored stainless steel, and the rest of the components are made of plastic formwork. The plastic mold used in the construction should be kept dry at all times, and the surface should not appear yellow, discolored, etc. The surface of the component mold needs to be brushed with a special release agent, the thickness is moderate and uniform, and it can only be applied after it is placed quietly into a film. When assembling the mold, it is necessary to review the geometry and specifications first to avoid using molds of substandard quality. The same release agent should be used for exposed concrete formwork, and it is forbidden to use oil-based release agents such as waste engine oil. In this project, the concrete mix ratio and mold material were combined to finally select the release agent of emulsion and paraffin. The paraffin release agent used in this project contains a variety of raw materials, the specific name and content are: rosin 5kg; Sodium carbonate 1.5kg, soap emulsion 15kg, thickener 0.5kg, silicate 0.75kg, emulsion 2.4kg, inorganic silicone glass nano 4kg, whitening agent 3.64kg, surfactant 5.5kg, pearl bright powder 3.52kg, methylcellulose 1.5kg, water 6.69kg.
3.2 Rebar processing
The roadbed cover steel bar of this project is mainly divided into three sections:
(1) C14 has a single length of 95cm and C8 has a single length of 51cm, five each. The former is set at 12.5cm, the latter is 22.5cm at both ends, and the middle distance is 20cm. According to the latter control requirements, the shaft of the steel bar is 4cm from the concrete surface.
(2) There are five C14 and C8 of the same specification, the former is 15cm apart, the latter is 5cm at both ends, and the middle distance is 6cm. Also in accordance with the latter control requirements, the axis of the rebar on both sides of the roadside trench is 1.5cm from the concrete surface and 2.5cm from the concrete surface at the bottom of the gutter.
(3) Prepare 10 pieces of C12 steel bar with a length of 113.5cm, and use 16 pieces of C8 steel with a length of 46cm. The former spacing is 10cm, the latter has a front spacing of 11cm, and the rear distance is 10cm. According to the former control requirements, the protective layer on both sides is 4cm, and the bottom corresponds to 2cm.
The steel bars of the cover plate of the drainage system of the highway tunnel are also divided into three parts. The first is the cover plate reinforcement of the central drainage, 5 C14 steel bars with a length of 256cm, 6 steel bars with a single length of 26cm in C10, and 20 steel bars with a length of 25cm in C8. The distance between C14 and C10 is 10cm; The C8 pitch is designed to be 22cm. The protective layer is 14 according to the standard and has a thickness of 5 cm. Followed by the steel bars of the tunnel roadside trench cover, 7 C12 steel bars with a length of 59cm, and 11 steel bars with a length of 94cm in C8. The former is 12.5cm apart, the latter is 22.5cm apart at both ends, and the middle is 20cm apart. According to the C8 standard, the thickness of the protective layer is 4cm. Finally, the cover plate of the lateral blind ditch of the tunnel uses 10 steel bars with a single length of 138cm for C14 and 28 bars with a length of 100cm for C8. The spacing between the two is 10cm, the protective layer is subject to C8, and the thickness is 5cm.
3.3 Concrete construction
The concrete construction steps mainly include pouring and vibrating. This project applies an automatic spreader for loading, and retests the concrete slump before discharging to ensure its working performance. Before the concrete enters the warehouse, the tank truck is required to speed up and rotate for 60s to make the material fully mixed evenly. During the discharge period, it is necessary to always pay attention to the changes of concrete, discharge the material in batches, make a good time arrangement, and cannot stop the tank and wait. The concrete should be divided into the mold, and the distribution point should be determined according to the technical parameters of the first article, so as to avoid the problem of excessive single feeding, resulting in different thicknesses of prefabricated small components. The vibrating time of concrete material is at least 30s, and the vibration force in the edge area is low, which can increase the vibration of the vibrator, and its radiation radius is 8~9 times the length of the radius of the vibrating rod, and the single movement distance needs to be within 1.5 times the radiation radius. In the vibrating process, can not be in contact with the mold, steel bar, the vibration point should always be separated from the template 50~100mm distance, if necessary, you can use the mallet to gently tap the outer wall of the mold periphery.
Under high temperature conditions, the maximum discharge temperature of concrete should not exceed 30 °C, and the temperature difference between the inside and outside of the material should be within 20 °C to ensure the subsequent pouring quality. In order to control the temperature difference of the material, the pouring time period should be selected before 6 am and after 5 pm. In a high-temperature construction environment, temperature, plastic shrinkage and dry shrinkage cracks are easy to form. In this regard, we should start with concrete materials to ensure the sand and gravel gradation and the mud content of raw materials, and grasp the water consumption, slump and water-cement ratio to enhance the compactness of concrete. Finally, after the pouring is completed, arrange maintenance in time to avoid exposure to the sun. During the first week of curing, the concrete can be kept moist by sprinkling water and cured continuously.
3.4 Mold removal process
After completing the above construction steps, the components are transported to the curing area as a whole with the help of forklifts. When the actual strength of concrete meets 70% of the design value, or the strength reaches 2.5MPa, the mold can be removed. In addition, it should be noted that the components must not be subjected to any load during mold curing. When removing the mold, apply automatic mold removal equipment, avoid forced disassembly, and control the position of the relatively concentrated stress of the component to avoid problems such as corner drop.
3.5 Mold cleaning
After mold removal, the components need to measure their specifications and appearance shapes, and screen out qualified parts. Then the mold is cleaned, and neutral and weakly acidic solvent cleaning can be applied.
3.6 Finished product health
The prefabricated components of the dismantled mold are transferred to the health area, and the temperature and humidity and the maintenance time are controlled according to the requirements, which can be comprehensively determined according to the size of the components, concrete performance, environment, etc.
3.7 Inspection of storage
After the curing period, quality inspection should be arranged according to the formed batch, the structure and specifications of components, concrete appearance, protective layer and concrete strength should be measured, and for defective products, it is necessary to arrange stacking and packaging treatment. According to the specific type of component, different storage racks are used for storage and moisture-proof measures are implemented.
4 Conclusion
In this project, two precast yards are set up and various forms of quality inspection are adopted to ensure that all processes are completed to standard. The construction method of prefabricated components can make the highway achieve the expected results in terms of performance, reliability and economy.
bibliography
[1] ZHANG Yongxuan. Experimental and applied research on mechanical properties of impermeable concrete of small precast components of expressway[J].Transportation World,2021(1):48-50.)
[2] Sheng Jichong, Fang Jian. Research on construction technology and quality control of small prefabricated components[J].Science and Technology Innovation and Application,2020(13):121-123.)
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