Introduction of concrete mixing plant
Concrete is the construction material produced by concrete mixing plant, which solidifies sand, gravel and other materials, and has the properties of stone after hardening.
The types and properties of concrete required for different construction projects are very different. When the time comes for production, if you have an understanding of the properties and uses of the concrete produced, you can adjust the concrete mixing plant accordingly to improve the quality of concrete mixing, and it is not easy to make mistakes.
One of the main signs of the performance of concrete is its strength, which is determined by a compressive test using a square test block with a side length of 150mm after a standard curing period of 28d. The commonly used strength range is 10-60Mpa, which is generally expressed in strength classes C20-C60.
The workability of fresh concrete, or known as the compatibility, refers to whether the concrete mix is easy to construct and operate, and to obtain uniform quality, molding dense performance, is a collective term for the process of concrete properties. Including fluidity, cohesion and water retention three aspects of the meaning. Mobility refers to the ease of flow, easy to fill the corner of the poured formwork. Cohesion refers to the structural properties of concrete is uniform and stable, not easy to appear delamination and segregation. Water retention means that it is not easy to show water secretion.
Workability of concrete construction is the most influential is the fluidity. The fluidity of plastic concrete is expressed in terms of slump, which generally ranges from 10 to 200 mm. the larger the value, the more fluid it is. Dry concrete is expressed in terms of Vibe consistency, which generally ranges from 5 to 35 s. The larger the value, the higher the dry hardness of the concrete.
The workability of the designed concrete is determined by the project structure and construction methods. Nowadays, the construction sites mostly use liquid or large liquid concrete, and only precast concrete elements use dry hard concrete.
The nature and types of sand and gravel, cement and other raw materials
Sand and gravel is generally known as aggregate (also known as aggregates), which refers to the role it plays in the concrete skeleton. Coarse aggregate refers to the grain size greater than 5mm stone, in the mixing plant, the general requirements of the grain size of pebbles is not more than 80mm, gravel is not more than 60mm. fine aggregate refers to the grain size of not more than 5mm sand and gravel material, mainly refers to sand, usually divided into coarse sand, sand, fine sand and extra fine sand, according to its grain size specifications to fine touch number, from 0.7-3.7. sand and gravel aggregates generally require the grain size of the gradation to Good, that is, the head has a large and small, can fill each other’s gaps. In this way, the concrete formed is dense, good compatibility, and not easy to segregate during transportation.
Cement is a water-hardening cementitious material, which mainly plays a cementing role in the concrete. Commonly used in civil engineering is general-purpose cement. In addition to the selection of cement in the concrete ratio according to the characteristics of various cements, the amount is mainly determined according to the strength index, the strength of cement is reflected in the strength grade of cement (formerly called the standard). The strength of cement in concrete should be slightly higher than or equal to the strength of concrete, when the strength of concrete is very low, can be mixed with admixtures to save cement. The maximum amount of cement in each side of the concrete should not exceed 550kg. Drinking water that meets the national standards can be used for the production of concrete. Other water sources can be used only after passing inspection.
AAdmixtures and admixtures are two admixtures used to improve the production process of concrete mixing plant and the performance of concrete to meet special needs. Admixtures are playing an increasingly important role in the production and application of concrete. The admixtures are usually mixed in amounts greater than 5% of the cement, and their fineness is the same or finer than the cement. The amount of admixture is usually not more than 5% of the cement.
The nature of the material has the following effects on the workability of fresh concrete:
Good gradation of aggregates and its workability;
Coarse sand and fine stone, its workability is not good;
Good workability of pebbles and poor workability of more needle-like stones;
Sand rate is the percentage of sand to the weight of sand and gravel aggregate in concrete. If the sand rate is too small, the concrete will be waterlogged and segregated, and if the sand rate is too large and the amount of cement is small, the workability becomes poor.
The finer the fineness of cement, the smaller the water secretion, good workability; when the amount is too much, its dryness and shrinkage increases.
Water-cement ratio is the weight ratio of water to cement in concrete. Its ratio is good workability, however, too much water will produce segregation, water secretion, strength will be greatly reduced, so the amount of water used in concrete must be strictly controlled. In the case of unchanged water-cement ratio, the workability can be improved by appropriately increasing the amount of cement paste.
Operational factors affecting the quality of concrete
The influence of operation on the quality of concrete is mainly in the aspects of metering and mixing process.
When producing, pay attention to the error of measurement, the adjustment of the amount of overshoot, and the grasp of the residual amount of the feeding process to ensure that the accuracy of measurement is in line with the requirements. The following aspects require special attention:
The slump of concrete has a direct impact on the performance and quality of concrete. If the equipment cannot automatically adjust the moisture content of sand and gravel, it is necessary to constantly observe and grasp the dryness and wetness of sand and gravel and the collapse of concrete at the outlet, and adjust the ratio of sand and water when necessary to ensure that the collapse does not have a large deviation.
The mixing process is mainly based on the characteristics of the concrete produced to adjust the timing of feeding and mixing and release time to ensure that the concrete is mixed and blended evenly to meet the design requirements and performance. When the concrete produced is mixed with admixtures, the mixing time should be extended appropriately. When dry mixing, the mixing time should be extended by 10-30 seconds for fly ash concrete and 30-60 seconds for concrete with other admixtures than normal concrete.
Principle and Composition of Concrete Mixing Equipment
Concrete mixing plant is the equipment that mixes the various raw materials that make up concrete in a certain proportion, then mixes and stirs them according to a certain process, and finally produces concrete with certain properties.
Although there are various concrete mixing plants produced by various manufacturers, their basic forms and composition structures are similar. Nowadays, the popular mixing plants basically use computer as the core of control, which controls the automatic batching, lifting, mixing and discharging of various materials, and some of them have auxiliary functions such as data statistics and report printing.
In terms of function, various concrete mixing plants are generally composed of the following parts:
1. Storage silo: storing materials such as sand, stone, cement and water to feed the batching mechanism;
2. Batching mechanism: measuring sand, stone and other materials according to a certain ratio, mainly composed of various scales, but also the use of volumetric flow measurement equipment, etc;
3. Lifting mechanism: mainly used to lift sand and gravel aggregates;
4. Mixer: mixing various raw materials and finally forming concrete;
5. Control system: control the coordination of various parts to complete a certain production process.
Be familiar with the functions and performance of the various components of the concrete mixing plant
You should be familiar with the performance and operation of the mixing plant to be operated in order to operate and use it correctly, and to find and eliminate problems in time.
Understand the operation principle of each part of the concrete mixing plant, and master the maintenance situation and the degree of maintenance and the state of use of the equipment. For example, the working pressure range of the air compressor, the intact condition of the batching machine, and the runout of the belt. Only by knowing the condition of the equipment can we make the production safe and stable.
Knowing the function and operation of each control component
The concept of the function of each control part of the concrete mixing plant must be clear, and the correct sequence of operation and the provisions of the operating procedures must be mastered. Only in this way can you handle the plant correctly in case of emergencies and avoid operational errors that can cause equipment damage and material loss.
Mixing process and production flow of concrete
The basic mixing process operation points are as follows:
1. The site is kept clean, with smooth drainage and smooth communication of relevant signals;
2. The feeding system should prevent the aggregates from entering the running mechanism; generally, sand and cement should not be fed into the same pipe slot; the materials in the hopper and pipe slot should be unloaded every time and should not be left for the next feeding.
3. Feeding can be used once feeding and multiple feeding, a feeding is the material in accordance with a certain process order into the mixer, but do not allow the first cast cement, so as not to cement sticky shaft; a feeding in order to avoid the formation of cement balls wrapped in water, the cement and gravel should be mixed first, so that the cement is dispersed, and then water; multiple feeding and mixing concrete, also known as shell concrete, its feeding operation It is more complicated and requires special procedures in the concrete mixing plant and mixing building to achieve.
4. During the operation, firstly, we should control the measuring accuracy and mixing time to ensure that the process requirements are met; secondly, we should check for abnormalities and faults, such as water and slurry leaks at the discharge port, and if there are any abnormalities and faults, we should repair them in time.
5. Before shutting down must be out of the net concrete; before shutting down after work is completed, apply a small amount of stone and water to wash the mixer; at the same time, clean up the mixing site. When a sudden power failure or failure to stop, should be careful to operate in time to clean out the concrete in the host.
6. Safety Any personnel must comply with the operating procedures and safety precautions to ensure safe production.
Operators should comply with the following production process:
1. Before production, production task notice must be received and executed strictly according to the content of the task order; when there are multiple task orders at the same time, be clear-headed and the concrete to be mixed must correspond to the delivery order;
2. Check whether the types of various materials correspond to the controlled parts before mixing, and whether the quality of materials can meet the requirements; check whether the state of the equipment is normal and ready.
3. Whether the ratio of sand and gravel needs to be adjusted according to the moisture content; whether the amount of overshoot, feeding timing and mixing time need to be adjusted.
4. The first tank because there is mortar adhering inside the mixer, consider whether to do the compensation of adding sand, cement and reducing stone.
The setting of mixing time needs to be adjusted according to the requirements of the testers, generally speaking from 30 seconds to 3 minutes, when mixed with admixtures or additives, according to the mixing method and process requirements to increase the mixing time appropriately to ensure the quality of concrete to meet the requirements.