Abstract: Based on construction experience, this paper discusses how to control the quality of dry and hard concrete and hydraulic cement brick making machines in the construction of highway slope protection grids.
Keywords: Dry hard concrete; Hydraulic cement brick making machine; quality control
In recent years, China’s automobile ownership has been continuously increasing, especially after joining the WTO, which will accelerate the growth, followed by a rapid increase in traffic flow. Therefore, it is required that highways not only increase in quantity but also in grade, and higher requirements are also put forward for quality and roadbed slope protection and beautification. Almost all high embankments use concrete mesh slope protection and grass planting to ensure the stability of the roadbed, prevent rainwater erosion, and beautify the environment.
From a construction perspective, the prefabrication quantity of grid slope protection is often large. In recent years, most of them still use manual prefabrication, which requires a lot of manpower and a large number of molds, and the appearance quality of the prefabricated blocks produced is often uneven. So, we used hydraulic cement brick making machines to produce grid prefabricated blocks during construction and gained some construction experience. The construction progress, appearance, and quality are relatively ideal, but there are also some shortcomings that need to be gradually summarized and improved.
1. Construction Plan
1.1 construction machinery
A 150t hydraulic cement brick making machine is used to produce a specialized mold and jack for the mesh. The mixer uses a 250 type small forced mixer, which uses dry and hard concrete to form under pressure in one go. The jack is used to push the formed mesh out of the mold and demould, completing the production of a mesh brick. A brick making machine only requires two sets of molds and is not limited by the site. Each brick making machine can be installed on a 4m2 concrete platform. Each work shift requires 10-12 workers and can produce 100-150 pieces per hour.
1.2 Construction procedures
Concrete mixing → loading and molding → pressure forming → jack demolding → horizontal placement → plastic film covering → watering for curing.
1.3 Construction mix ratio
The concrete is made of dry and hard concrete, with a medium sand to crushed stone ratio of 1:1 and a maximum particle size of less than 1.0cm. To ensure the appearance of the grid surface, a 0.5cm cement fine sand pavement is used, with a cement to fine sand ratio of 1:1. The cement dosage should be greater than that of plastic concrete of the same grade, generally not less than 350kg/m3. The cement dosage needs to be adjusted according to the size of the grid. When the grid size is large, the cement dosage should be increased to ensure that the grid is not loose or deformed after demolding.
1.4 curing
After the grid bricks are formed, they are covered with plastic film. After 24 hours, the film is removed and sprinkled with water for curing. After 3 days, they can be stacked and centralized for curing.
2. Quality Control
Due to the use of dry and hard concrete during construction, it is not possible to produce concrete specimens according to the specifications, so there is no practical and feasible testing method. For this reason, during the construction process, we tested and produced a set of concrete test molds that can withstand the pressure of the press. Due to the constraints of the brick machine space, only 10cm can be used × 10cm × 10cm trial mold, but the effect is not ideal. After the specimen was formed, we observed the internal crushed stones of the specimen. Due to the excessive stress that the specimen was subjected to during compression molding, which exceeded 100MPa, some of the crushed stones inside the specimen were crushed after the specimen was formed, which damaged the overall strength of the specimen. Therefore, the strength of the 28d specimen cannot fully reach the design strength.
Since specimens cannot be used to represent concrete strength, how can construction quality be controlled? Therefore, we have summarized that the quality key points of grid slope protection are strength and aesthetics. Whether the strength can meet the design requirements is not reflected in use, but in transportation and installation. After the grid is installed on the slope protection, there is no problem of bearing driving loads. If the strength is not enough, it will cause a lot of damage during transportation and installation, and also cause waste of labor and materials, It will also directly affect the economic benefits of the construction unit. Therefore, quality control must be carried out during the prefabrication process. We have taken the following measures: firstly, to ensure that the cement consumption does not decrease, calculate the cement consumption based on the production per shift and daily production, and promptly correct any problems found. Secondly, due to the dry and hard nature of concrete, which requires less water, the grid bricks should be covered with plastic film in a timely manner after forming to prevent water loss and ensure sufficient hydration reaction of cement.
From a design perspective, the concrete grade for grid bricks is usually C20, which is determined based on convention and does not require structural calculations. Imagine using ordinary red bricks to pave the slope and planting grass for greening, without the phenomenon of rainwater erosion. The strength of ordinary red bricks is much lower than that of C20 concrete, but the appearance quality cannot be compared to the mechanical slope protection grid. Therefore, in terms of concrete quality control of mechanism grid bricks, the main issue is not whether the concrete strength reaches C20, but rather the appearance and installation paving. We believe that the consistent production of specimens should be changed to determine whether they are qualified after obtaining data, and the quality of grid brick prefabrication process and paving should be used to determine whether the quality is qualified. Specifically standardize the quality control methods during the construction process, such as minimum cement dosage and maintenance methods, to ensure the quality of prefabrication and reduce transportation and installation losses.
3. Advantages and disadvantages of mechanical grid slope protection
Advantages: ① Beautiful appearance, no honeycomb or pitted surface, consistent cross-sectional dimensions, and no mold running phenomenon. ② The construction progress is fast, with 24-hour construction and a daily production capacity of 2000-3000 pieces per unit Not limited by the construction site, there is no need for a large concrete platform, only a flat site is needed. ④ The investment in molds is small, and each brick making machine only requires two sets of molds. For different engineering designs and design changes during the construction process, it is only necessary to replace the mold, avoiding a large number of scrapping of the mold due to different design drawings.
Disadvantages: ① Lack of standardized inspection data for quality inspection. ② The construction mix ratio lacks calculation data and formulas, and now construction can only be guided by experience and on-site tests.
4. Epilogue
Practice has proven that the mechanical slope protection grid is relatively ideal in terms of appearance and quality, and it also saves a lot of templates during construction, accelerating the construction progress. However, further exploration is needed in terms of quality inspection standards, experimental calculations, and construction methods, summarizing experience, and gradually improving them.
