- Introduction to Concrete
Concrete is an artificial building material made by solidifying sand and gravel with cement as the binder. After hardening, it has the properties of stone and is referred to as concrete.
The types and properties of concrete required by different construction projects vary greatly. When producing, if you have some understanding of the performance and use of the concrete produced, you can make corresponding adjustments to the equipment to improve the quality of concrete mixing, and it is not easy to make mistakes.
One of the main indicators of concrete performance is its strength, which is measured by a compressive test after a cubic test block with a side length of 150mm has been cured for 28 days. The commonly used strength range is 10-60Mpa, generally expressed by strength grade C20-C60.
The workability of fresh concrete, or workability, refers to whether the concrete mixture is easy to construct and operate, and whether it has uniform quality and compact molding. It is a general term for the process performance of concrete. It includes three aspects: fluidity, cohesion and water retention. Fluidity means that it is easy to flow and fill the corners of the casting formwork. Cohesion refers to the uniform and stable structural properties of concrete, which is not prone to stratification and segregation. Water retention refers to the lack of water seepage.
The greatest influence on concrete construction in workability is fluidity. The fluidity of plastic concrete is expressed by slump, which generally ranges from 10-200mm. The larger the value, the stronger its fluidity. Dry hard concrete is expressed by Vebe consistency, which generally ranges from 5-35s. The larger the value, the higher the dry hardness of the concrete.
The workability of the designed concrete is determined by the engineering structure and construction method. Currently, fluid or highly fluid concrete is mostly used on construction sites, and only precast concrete components use dry hard concrete.
- Properties and types of sand, gravel, cement and other raw materials
Sand and gravel are generally called aggregates (also called aggregates), which means that it plays the role of skeleton in concrete. Coarse aggregate refers to stones with a particle size greater than 5mm. In a mixing plant, it is generally required that the particle size of pebbles is not greater than 80mm and the particle size of crushed stone is not greater than 60mm. Fine aggregate refers to gravel with a particle size of no more than 5mm, mainly sand, which is usually divided into coarse sand, medium sand, fine sand and extra fine sand. The particle size specifications are expressed by the fineness modulus, ranging from 0.7-3.7. Sand and gravel aggregates generally require good gradation of particle size, that is, the particles are large and small, and can fill the gaps with each other. In this way, the formed concrete is dense, workable, and not easy to segregate during transportation.
Cement is a hydraulic cementitious material that mainly plays a cementing role in concrete. General cement is commonly used in civil engineering. In addition to being selected according to the characteristics of various cements, the amount of cement in the concrete mix is mainly determined according to the strength index. The strength of cement is reflected in the strength grade of cement (formerly known as the grade). 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, admixtures can be added to save cement. The maximum amount of cement in each cubic meter of concrete should not exceed 550 kg. Drinking water that meets national standards can be used in concrete production. Other water sources can be used only after passing the inspection.
Admixtures and admixtures are two additional materials used to improve the production process and performance of concrete to meet special needs. Admixtures play an increasingly important role in the production and application of concrete. The amount of admixtures is usually greater than 5% of the cement dosage, and its fineness is the same as or finer than that of cement. The amount of admixtures is usually not more than 5% of the cement dosage.
The properties of materials have the following effects on the workability of fresh concrete:
The aggregate is well graded and its workability is good;
The sand is coarse and the stone is fine, and the workability is poor;
The workability of pebbles is good, and there are more needle-shaped stones, and their workability is poor;
The sand ratio refers to the percentage of the weight of sand and sand and gravel aggregates in concrete. If the sand ratio is too small, the concrete will bleed and segregate. When the sand ratio is too large and the cement dosage is small, the workability becomes worse.
The finer the cement, the less water bleeding and the better the workability; when the dosage is too much, its shrinkage increases.
Water-cement ratio refers to the weight ratio of water to cement in concrete. The larger the ratio, the better the workability. However, excessive water use will cause segregation and bleeding, and the strength will be greatly reduced, so the water consumption of concrete must be strictly controlled. When the water-cement ratio remains unchanged, the workability can be improved by appropriately increasing the amount of cement slurry.
Principle and composition of concrete mixing equipment
Concrete mixing equipment is a device 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 the specific forms of concrete mixing stations produced by various manufacturers are varied, their basic forms and composition structures are similar. The popular mixing stations now basically use computers as the core of control to control the automatic batching, automatic lifting, mixing and discharging of various materials. Some also have auxiliary functions such as data statistics and report printing.
From the functional point of view, various concrete mixing stations are generally composed of the following parts:
(1). Storage silo: Store sand, stone, cement, water and other materials to feed the batching mechanism;
(2). Batching mechanism: Measure sand and stone and other materials according to a certain ratio, mainly composed of various scales, and also use flow metering equipment measured by volume;
(3). Lifting mechanism: Mainly used to lift sand and stone aggregates;
(4). Mixer: Mix various raw materials and finally form concrete;
(5). Control system: Control the coordinated work of various parts to complete a certain production process.- Operational factors affecting concrete quality
The impact of operation on concrete quality is mainly in the aspects of metering and mixing technology.
During production, pay attention to the measurement error, the adjustment of overshoot, and the grasp of the residual amount in the feeding process to ensure that the measurement accuracy meets the requirements. The following aspects need special attention:
Additional materials have a great impact on concrete, and special attention should be paid to the accuracy of their measurement; 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 slump of concrete at the discharge port, and adjust the ratio of sand and water when necessary to ensure that the slump does not have a large deviation.
The mixing process mainly adjusts the feeding sequence and the mixing and discharging time according to the characteristics of the produced concrete to ensure that the concrete is mixed and stirred evenly to meet the design requirements and performance. When admixtures are added to the produced concrete, the mixing time should be appropriately extended. When dry mixing, the mixing time of fly ash concrete should be extended by 10-30 seconds compared with ordinary concrete, and the concrete of other admixtures should be extended by 30-60 seconds.
Be familiar with the functions and performance of each component of the mixing station
Only when the working performance and operation of the mixing station to be operated are well understood, can it be operated and used correctly, and when problems arise, they can be discovered and eliminated in time.
Understand the working principle of each component of the equipment, master the maintenance status and maintenance degree of the equipment and the use status of the equipment. For example, the working pressure range of the air compressor, the intact condition of the batching machine, and the deviation of the belt. Only by knowing the condition of the equipment can the production be safe and stable.
Specific knowledge will be described in the operating instructions.
Mixing process and production process of concrete
The basic operation points of mixing process are as follows:
(1). Keep the site clean, drain smoothly, and related signal communication smooth;
(2). The feeding system should prevent aggregates from entering the operating mechanism; generally, sand and cement cannot be fed in the same pipe trough; the materials in the hopper, pipe trough and other parts should be unloaded every time and shall not be left for the next feeding.
(3). The feeding can be done once or multiple times. Once is to put the materials into the mixer according to a certain process sequence, but cement is not allowed to be put in first to prevent cement from sticking to the shaft. In order to prevent cement from being wrapped by water to form cement balls, cement and sand should be mixed first to disperse the cement before watering. Multiple feeding and mixing concrete, also known as shell concrete, is more complicated to operate, and requires special processing in the mixing station and mixing building.
(4). During operation, the metering accuracy and mixing time must be controlled to ensure that the process requirements are met; the second is to check for abnormalities and faults, such as whether there is water leakage or slurry leakage at the discharge port, and if there are abnormalities and faults, they must be repaired in time.
(5). Clean concrete must be discharged before stopping; before stopping after the work is completed, a small amount of stone and clean water should be used to wash the mixer; at the same time, the mixing site should be cleaned. When there is a sudden power outage or a fault, the concrete in the main machine should be cleaned out in time.
(6). Safety Any personnel must abide by the operating procedures and safety precautions to ensure safe production.
