High strength concrete
Today, concrete is known as the most widely used building material. Therefore, many researches are being done every day in order to best match the properties of concrete with the expected and desired properties all over the world, and the result of these projects and researches are new types of concrete according to the existing needs.
One of these types of concrete is high initial strength, which is being developed due to the increasing need for faster use of buildings and facilities under construction. The term strong concrete may refer to concrete that has more efficiency for the intended application than ordinary concrete.
For example, concrete that provides completely better reliability and durability against different service conditions, or concrete that has extraordinary properties at early ages, or concrete that has extraordinary resistance properties more than ordinary concrete, are examples. It is a powerful concrete.
The properties that strong concrete may have include the following.
High ultimate resistance
High initial resistance
High modulus of elasticity
High pumping ability
High payment capability
High ability to pour in hot and cold weather
Concrete with high initial strength (strong concrete)
Complete control of concrete hydration
Usually, more than one of the above properties is required for a particular application. For example, high ultimate strength, high elastic modulus, high pumpability and the ability to pour concrete without compaction are required in skyscrapers.
These concretes may contain materials such as fly ash, blast furnace overburden, silica fume, fibers, chemical additives, various other materials, alone or in various mixtures with each other.
Today, engineers are making increasing use of strong concrete in various applications, including large-scale road works, including large-scale construction of new roads, repair of existing roads, renovation and implementation of new road coatings, construction of skyscrapers and tall structures, etc.
Higher resistance provides more structural design options. The improved properties of concrete at early ages facilitate construction and renovation works. And it improves the quality and higher durability, increases the service period, which may lead to lower cost of the structure during its useful life.
Concrete with high initial strength and concrete with very fast strength
High initial strength concrete (HES) is also called fast track concrete, which achieves its specified strength in a shorter time than normal concrete.
The time frame in which the determined resistance must be achieved may vary from a few hours or even minutes to several days. If the concrete achieves high initial strength in a short period of time and within a maximum of 20 hours, it is called Very Early Strength Concrete.
High initial strength can be achieved by using traditional concrete components and traditional techniques. Also, in some cases, materials with special techniques are required.
Methods of achieving high initial resistance
High initial resistance can be achieved by using one or a combination of the following, depending on the period over which the specified resistance is to be achieved, as well as the working conditions.
Use of type III cement or HE cement with high initial strength
In general, there are six main types of Portland cement, each of which is used in certain cases.
Type I General applications and conventional concrete construction
Type II Concrete exposed to mild sulfates or when heat of hydration is required.
Type III high concrete strength in a short period of time (high initial strength)
Type IV This type of cement is suitable when mild heat of hydration is necessary.
Type V is used when concrete is exposed to strong sulfates.
White cement Architectural applications, when white or colored concrete or mortar is required.
Using high grade cement (400 to 600 kg/m3)
It is important to pay attention to the fact that in different areas with different stone materials of the same grade of cement, Konaguni resistances are obtained.
Using a low ratio of water to cement materials (mass ratio 0.2 to 0.45)
For concrete with high initial strength, it is necessary that the ratio of water to cement does not exceed 0.42 for type I cement and 0.45 for type III cement.
It is important to note that there is a practical limit to reducing the amount of mixed water, which prevents reducing the water until the desired water-to-cement ratio is reached. This is the practical limitation of the efficiency of fresh concrete.
Depending on the type of aggregate used, the minimum water required for the mixture varies between 150 and 170 liters per cubic meter (equivalent to a 40 mm slump).
To achieve a lower water-to-cement ratio in the concrete mix with minimal water consumption, it is necessary to increase the grade of cement or use super-lubricating additives, or a combination of both methods.
Use of higher temperature for freshly mixed concrete
Temperature is a key determining factor in achieving resistance. Hydration is a chemical reaction and is controlled by temperature.
Using a higher curing temperature for concrete
At temperatures below 12 degrees Celsius, cement reacts very slowly. However, concrete cured at a temperature of about 8 degrees Celsius will have a higher final strength than concrete cured at higher temperatures.
Above 40 degrees Celsius, the strength of concrete is very fast, but its final strength is compromised, cements based on molten iron overburden do not show such a feature.
The use of chemical additives such as quick-drying agents or high-level water reducers
Precipitators reduce the initial setting time of concrete and also provide higher initial strength. Accelerators do not act as anti-freeze, but they accelerate the setting and the speed of obtaining resistance. As a result, they make concrete more resistant to damage caused by freezing in cold weather.
Fasteners are also used in rapid construction, which requires rapid removal of formwork, rapid opening to passing traffic, or early application of load to the structure.
There are two types of quick setting additives.
Chloride-based quick-setting additives, such as calcium chloride
Non-chloride quick-setting additives
One of the most effective and economical quick-setting additives is calcium chloride, which exists in the form of liquid or fine scale-like grains and must meet the requirements of ASTM D 98. For unreinforced concrete, calcium chloride up to 2% can be used.
Due to concerns about the corrosion of reinforcing steel due to the presence of calcium chloride, lower limits for chloride are applied to reinforced concrete. Prestressed concrete and concrete that has been used in aluminum or galvanized metals, due to the increasing ability of metal buried in concrete to corrode, should not be used with any materials whose main constituents are chlorides.
In general, the use of calcium chloride is not recommended in order to achieve high initial resistance, but if it is used, it should not be used more than 2%. Occurrence of immediate seizure is very likely if more than 2% of calcium chloride is used, and even in many cases it occurs with any amount less than 2%. In general, the use of calcium chloride greatly reduces the effectiveness of the concrete mixture.
Addition of silica fume
Use of fly ash, silicon soot, or the overburden of the lava iron furnace, which is finely ground. Addition of silica fume is useful in concrete in two ways.
First, fine particles of silica fume physically reduce the void space in the cement matrix.
Silica fume increases the compressive strength and the speed of obtaining it, as well as the durability of concrete. Concrete made with silica fume can achieve a very high initial and ultimate compressive strength if it is mixed with the correct proportions. Precast concrete with a compressive strength of about 135 MPa has been produced in the United States using silica fume.
Using steam treatment or autoclave treatment
The application of this method is in the production of prefabricated concrete parts. In addition to this treatment, it is effective in increasing the final compressive strength, reducing surface erosion, and adding more resistance to wear. Curing allows more water to be available for the hydration reaction of the cement paste, which leads to an increase and development of better strength.
Using insulation and insulation in order to maintain the heat of hydration
Insulating coatings or other measures taken to insulate and isolate the concrete from the outside environment can be used in the first 24 hours to help achieve strength by retaining the heat of hydration.
In order to maintain the heat of hydration and processing, insulating covers made of polystyrene foams are used.
However, when removing these covers, in order to avoid thermal shock, necessary precautions should be taken. Thermal shock may cause premature cracks in concrete.
Use of quick-hardening cements, such as calcium aluminate cements
Calcium aluminate cement is not based on Portland cement. This cement is used in special applications in order to obtain high initial strength, resistance to high temperatures, and resistance to sulfates, weak acids and sea water.
A mixture of Portland cement and calcium aluminate cement has been used to produce quick-setting concretes and mortars.
Typical applications for calcium aluminate cement concrete include chemical-resistant, heat-resistant and corrosion-resistant industrial floors, fireproof coatings, and repair applications.
Concrete applications with high initial strength
Concrete with high initial strength is used in the following cases.
in prestressed concrete in order to create suitable conditions for early application of stress
in precast concrete for the rapid production of concrete members and components
Rapid construction of prefabricated buildings and structures (cast-in-place)
Rapid formwork for re-use of formwork in case of cast-in-situ concrete
Construction and concreting in cold weather
Accelerated laboratory tests
In shotcretes that require high initial resistance.
Rapid repair and restoration of existing concrete pavements for re-use and rapid construction of new concrete pavements or creating reparative concrete coatings on existing damaged asphalt pavements.
Reconstruction and repair of airplane runways at the airport
Minimum requirements for concrete with high initial strength
The minimum conditions that concrete with high initial strength should provide, according to the type of work in which it is made of concrete with high strength
The high initial strength used will be different, and various institutes and directives have determined a series of minimum conditions for concrete with high initial strength, which must be accepted in order to be accepted. fulfill them.
Tips on ready-mixed concrete with high initial strength
Since the setting time of concrete with high initial strength is short and setting occurs in these fast concretes, in order to speed up the construction time, transporting this concrete to the workshop and work site should be done as quickly as possible.
This usually requires proper preparation of the work site to facilitate the access of mobile construction equipment
and concreting and concrete transport trucks to the desired locations for fine concrete. It may also be necessary to use retarding additives to reduce the rate of hydration.
In addition, the transportation time may need to be managed well because it will affect the efficiency and the need to use retarding additives.
Another point that is important is to avoid the accumulation and concentration of trucks containing delivered concrete cargo at the unloading point. Because there, in the second shipment and after that, the slump occurs while waiting for unloading.