What is concrete?
What is concrete?
Undoubtedly, concrete can be seen everywhere! This manufactured material is widely used in urban construction because it contains properties superior to any other material. What material is concrete and why it is so popular with construction people is discussed in this article.
The Romans built a type of concrete in 7000 BC, which can now be seen in remnants of the era, such as the Pantheon. There was little progress in the use of concrete until the 18th century when engineers began testing new cement compounds.
Modern concrete began with the development of Portland cement in the 1820s. Its development took significant steps by introducing steel as a reinforcing member in the mid-1800s and prestressed concrete in the early 1900s. In the last 75 years, we have witnessed the expansion of a wide range of types of cement, the use of additives to improve the mechanical, physical and chemical properties of concrete based on user demand.
The development and use of reinforced concrete became very popular during the 20th century. With the change of cement compositions, concrete types also expanded from low strength to very high strength, light to heavy and others. Therefore, there is one type of concrete for use in almost all structural applications in the present era.
Concrete is a synthetic and composite material that consists of a combination of three essential components as follows:
• Aggregates (sand)
Aggregates make up 60 to 75% of the mixture, while the rest of the composition consists of cement and water. In the meantime, additives may be used in the mix to change the properties of the concrete. The fine sand particles are placed in the empty spaces between the coarse aggregates, and the combination of cement and water acts as a strong adhesive (cement paste) to hold them.
When water is mixed with cement, it starts the hydration (hardening) process and also binds the aggregate. The ratio of water to cement is the most crucial factor in the quality of concrete because a large amount of water in the mixture reduces the compressive strength, and a lack of water makes the concrete unusable. The water-cement ratio is essential for making complete, solid and workable concrete
Its constituent materials determine the properties of concrete. The water-cement ratio is a determining factor in ordinary concrete with lower water content and, therefore, more robust products. However, water depletion reduces the performance of concrete. Performance can be measured using the slump test.
The size, shape, texture and ratio of aggregate can have a similar effect on the concrete mixing design. If necessary, the number of coarse aggregates in the mixture can be reduced to increase the flow. However, cement is a significant cost factor and increasing it in the mix heightens the cost price.
What is the concrete grade?
Carat means the classification of concrete according to the compressive strength of concrete. Cement, aggregate and water are used to make concrete, mixed in a particular ratio and placed in a mold of 150 mm. The concrete formwork is placed in a water bath for 28 days to reach the desired strength. The 28-day compressive strength under test loading is known as the concrete grade, measured in Newtons per square meter.
According to its compressive strength, the concrete grade is indicated by M10, M20, M30, etc. “M” is derived from the first letter of the word Mixer, and the number after M indicates the compressive strength of concrete after 28 days. M10 concrete means that the concrete has a compressive strength of 10 Newtons per square meter in terms of characteristic strength of 28 days.
In the European standard, the grade is indicated by the letter “C” and in the form of C10, C15, C20, C25, etc. The letter C means a class of strength of concrete, and the number after it indicates the 28-day compressive strength in Newtons per square meter and by compression test on a cylindrical formwork with a height of 30 cm.
Durability of concrete
Corrosion of metals, freezing and thawing cycles, chemical attacks, and alkalis reaction are some of the causes of concrete failure. Usually, the idea that concrete is impermeable is wrong! But in reality, liquids can still pass through it. If the mixing scheme is good, water passage is impossible, but water vapour passage is possible.
The key to preventing concrete from deteriorating is its less permeability. This can be achieved by reducing the water-cement ratio, uniform grading and density of the aggregate, and the use of additives (superplasticizers) and vapour barriers. As permeability decreases and air inside the concrete drops, its durability increases. Also, cracks on the slab occur due to shrinkage.
Advantages of concrete
• Its ingredients are readily available in most parts of the world.
• Unlike natural stones, this synthetic material is free of defects.
• It can be economically converted into a desirable resistance.
• Its durability is very high.
• It can be molded into any shape you want.
• Concreting can be done on site, which saves money.
• Its maintenance cost is almost negligible.
• Its deterioration is not significant with increasing time.
• Badr is fire resistant.
• Can withstand high temperatures.
• It is resistant to wind and water.
• It is a sound insulation material.
Disadvantages of concrete
• Compared to other binders, the tensile strength of concrete is relatively low (this problem is solved with reinforcing steel).
• It has less ductility.
• It weighs more than its strength.
• It may contain soluble salts.
• Constant loads in structures cause creep.
• The mixing design requires precision and engineering expertise.
• Prone to cracking.
Concrete is one of the most durable materials in construction that resists rot, fire and rust, providing a stable and secure foundation for high-rise buildings. It lasts twice as long as other building materials, including wood. Because of its durability, it is used to build roads and streets. It helps save asphalt and helps the environment by reducing maintenance costs.
Concrete is a stable building material and requires little maintenance. For this reason, it is used in residential, office, and commercial buildings, and also with its features, it reduces construction and operation costs. For residential buildings, fire resistance means fewer accidents in the event of a natural disaster.
Other uses are as follows:
• Watersheds and sewers
• Concrete stairs
• Concrete dams
• Offshore construction
• Underground tunnels
• Concrete paving
• Use of concrete as embankment
• Road construction and road construction
• Concrete processing
After concreting, concrete is processed. Processing is the care and maintenance of poured concrete while maintaining the level of humidity and temperature. The longer the curing or hydration time, the stronger the concrete.
This depends on the schedule between 3 and 14 days, depending on weather conditions. For example, it is better to reduce the processing time in cold weather due to high humidity. Conversely, in summer, care should be taken with concrete by constantly spraying water and twisting structural members (such as columns) using a sack.
The processing depends on knowing how to set and the process of obtaining concrete strength. These trends include:
• First 48 hours: Do not remove the molds.
• After 48 hours: The concrete is strong enough that it can be walked on without damage.
• After 7 days: The concrete is strong enough to be ready for loading.
• After 28 days: Concrete reaches its maximum compressive strength.
• The minimum processing time is 7 days. During this period, the proper temperature and humidity should be at least 80%.
Types of concrete
Concrete is divided into the following types:
• Ordinary concrete
• Polymer concrete
• Reinforced concrete
• Prestressed concrete
• Precast concrete
• Concrete glass
• Self-compacting concrete
• Fibre concrete
• Smart concrete
• Sponge concrete
The strength of concrete is affected by the quality of cement, the distribution of aggregates used, and the amount of water in the mixture. Processing methods also play an essential role in strength. Portland cement is a mixture of limestone carbonates (limestone, gypsum or marl) and clay, available in several types depending on the type of application, including type one, type two, type three, etc.
Aggregates are sands that are determined by specific grain size. Sand aggregates can be identified by size, sharpness or roundness, roughness, texture, and more. Sharp corner aggregates often have a high impact on compressive strength. A lower water-cement ratio increases strength, but efficiency and fluidity must also be considered.
The performance of concrete is generally determined by the slump test, which uses a 30 cm high conical mold. Slump measures the distance of concrete from its original height. For example, the concrete is filled to the edge of the formwork, and then the concrete is reduced in height as the formwork is removed. The difference between the original height and the amount of height reduction is called a slump.
The higher the slump, the smoother the concrete but, the lower the strength. Slump is determined by the type of application, weather conditions and generally under the graded laws in the national building regulations. However, it is necessary to establish a proper relationship between the desired strength and the flow rate of concrete.
Concrete is poured into metal and wooden molds using a pump or manually on site. After entering the mold, it is vibrated using an electric or gasoline vibrator to remove air bubbles from the mixture.
Vibration should be done very quickly, so that coarse aggregates do not fall to the bottom of the mold. The higher the water to cement ratio, the more likely it is that bubbles and aggregates fall. For this purpose, a mixture with low water to cement ratio is more suitable for vibrating.
Additives are added to the cement mix to improve the special properties of concrete. Most important are air-absorbing agents, which produce small bubbles in the mixture and improve the concrete’s ability to withstand freezing and thawing cycles and wetting agents.
Accelerators such as calcium chloride increase the setting speed, which are suitable for cold weather. Slowers have the opposite effect and are used in hot weather. Other additives include those that reduce shrinkage and shrinkage of concrete. Polymers are sometimes added to Portland cement mixtures to produce concretes with high adhesion and low permeability.
In general, the cost per cubic meter of concrete depends on the following factors:
• Prices of raw materials
• Concrete grade (increasing cement increases the cost)
• Order amount
• Distance from the concrete plant to project site (greater distance means the higher concrete price)
• Add-ons used
• Selected concrete type
Concrete is a building material that is made of cement, water and fine and coarse aggregates. Ingredients of this synthetic material can be found all over the world, and anyone with a bit of technical skill can make it for everyday use! But to build a multi-storey building, you must be an experienced engineer who specializes in concrete.