What is concrete?
Concrete is a general term for engineering composite materials in which aggregates are cemented together by cementitious materials. The concrete we usually talk about refers to cement concrete, which is made by using cement as a binder, sand and stone as aggregate along with water (additives can be added if necessary) and mixed together in a certain proportion, also known as ordinary concrete. It is widely used in civil engineering.
Concrete has the characteristics of abundant raw materials, low price and simple production process. Therefore, in recent years, the amount of concrete in the world has been increasing. At the same time, it also has the advantages of high compressive strength, good wear resistance and a wide range of applications.
Type of concrete
According to the binder, concrete can be divided into inorganic binder and organic binder concrete. What is binder? In building materials, any material that has undergone a series of physical and chemical actions by itself or has undergone a series of physical and chemical actions after mixing with other substances (such as water) can change from suspension to solid, and can transform granular materials (such as sand, stones) or block materials (such as bricks) cemented into a single whole, called binders.
According to apparent density, concrete can be divided into heavy concrete, ordinary concrete and lightweight concrete. The difference between these three types of concrete is the difference in the aggregate.
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Heavy concrete is concrete with an apparent density of over 2500 kg/m3, made from particularly dense and heavy aggregates. Ordinary concrete is the concrete we usually use in construction, with an apparent density of 1950-2500 kg/m3. It is mainly made from sand and stone as the main aggregate. The apparent density of lightweight concrete is less than 1950 kg/m3.
According to the function of use, concrete can be divided into heat-insulating concrete, decorative concrete, waterproof concrete, road concrete, structural concrete, refractory concrete, etc.
Based on the mixture, concrete can be divided into dry concrete, semi-dry concrete, plastic concrete, liquid concrete, etc. Mixtures generally refer to freshly mixed concrete. Fresh concrete refers to concrete that has not yet hardened and hardened by uniformly mixing various materials in a certain proportion, also known as concrete mix.
Raw materials for concrete
Concrete, which we often call cement concrete, is made up of cement, aggregates, water and additives.
Cement
Cement acts as cement in concrete. The choice of its grade and quantity directly affects the strength, workability, durability and cost-effectiveness of concrete. Therefore, when preparing concrete, you should choose the appropriate type of cement and strength grade.
The selection of cement types should be carefully considered according to the project characteristics, environmental conditions, construction conditions and the situation of cement suppliers.
The choice of the cement grade by strength must correspond to the concrete strength grade. The higher the concrete strength grade, the higher the selected cement strength grade. And vice versa, the lower the concrete strength grade, the lower the selected cement strength grade.
Aggregate
The aggregate can be divided into coarse aggregate and fine aggregate. The aggregate with a particle size greater than 4.75 mm is called coarse aggregate, i.e. stones. The aggregate with a particle size less than 4.75 mm is called fine aggregate, i.e. sand. Sand also includes natural sand and artificial sand.
Sand and gravel aggregates make up more than 80% of the total volume of concrete, usually play no chemical role with cement and act as the general skeleton in concrete.
Cement, sand and water form a cement mortar, which is wound on the surface of large aggregates and fills the gaps between the aggregates, and before hardening plays a lubricating role, due to which the concrete mixture has good fluidity and plasticity, which is convenient for construction. After hardening, the cement mortar binds the aggregates together, creating strength and forming a strong whole.
Water
The water used for mixing concrete is usually clean fresh water with a pH value of at least 4, such as tap water, river water, well water, lake water, etc., which can be consumed by humans.
The use of water-based concrete containing harmful substances will affect the workability and setting speed of concrete, hinder the strength gain of concrete, reduce the durability of concrete and accelerate the corrosion of steel rods.
Supplements
Admixtures are substances added when mixing concrete to improve the properties of the concrete.
Commonly used additives are: water reducing agent, air entraining agent, setting accelerator, retarder, accelerator, antifreeze, etc.
When selecting admixtures, it should be determined according to the actual situation of the project. For example, if you want to produce impermeable concrete or frost-resistant concrete, you can choose air-entraining admixture or water-reducing admixture.
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Equipment for concrete production
At present, the main equipment for concrete production is the concrete mixer. The concrete mixer can mix concrete independently or together with the batching machine, cement bin and other auxiliary equipment, forming a concrete plant for mass production.
There are different types of concrete mixers, the most common of which are drum mixers, twin shaft mixers, vertical shaft mixers, etc.
Drum mixers are also known as self-falling mixers. There is no agitator on the inner wall of the mixing drum, mainly due to the rotation of the tank, after the material rises to a certain height, the material can fall freely under the force of gravity to complete the mixing.
Twin shaft mixers and vertical shaft mixers are also known as forced action mixers. The mixer body of this type does not rotate, but there are mixing blades in its mixing cylinder to complete the forced mixing of materials.
According to mobility, concrete batching plant is divided into stationary plant and mobile plant. Its main components are batcher, belt conveyor, mixer, cement bin, control room, etc. The mixer commonly used in concrete batching plants is a twin-shaft mixer.
How is concrete produced?
Let's take a concrete plant as an example to briefly describe the concrete production process.
The process of producing concrete in a concrete plant can be simply described as weighing-transporting-mixing.
The loader feeds various aggregates (sand, stones, etc.) into the storage bin of the corresponding batching machine, and then enters the weighing bin for weighing. The weighed aggregate is lifted to the top of the mixer by a belt conveyor or a lifting bin to wait for loading.
At the same time, the cement in the cement bin is conveyed by the screw conveyor to the cement weighing bin above the measuring mixer. Water and additives are pumped to the water weighing bin and the additive weighting bin respectively through the measuring pipeline.
The measured aggregate, cement, additive and water are discharged into the mixer one by one for mixing. After the mixing is completed, the discharge door of the mixer is opened and the finished concrete is discharged into the concrete mixer truck. Finally, the concrete is delivered to the construction site by the concrete mixer truck.
The whole process is controlled by a fully automatic PLC control system, which is simple and convenient to operate, and ensures high production efficiency and precision.
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