Concrete is categorized into normal concrete and high strength concrete based on its compressive strength. Normal concrete typically has a compressive strength ranging between 20 and 40 MPa, while high strength concrete exceeds 40 MPa. In the past, concrete with a compressive strength of 28 MPa was considered high strength, but today concrete can achieve strengths greater than 800 MPa, known as reactive powder concrete.
High strength concrete is used to address various factors such as reducing weight, minimizing creep and permeability issues, enhancing durability, and accommodating architectural considerations that require elements to bear lighter loads. Despite these advancements, normal strength concrete remains the most commonly used type in construction.
Normal and High Strength Concrete Properties
The type of concrete, whether it’s normal or high strength, needs to have a plastic or semi-fluid consistency when mixed. This allows it to be easily molded by hand or with mechanical tools. In a fresh concrete mixture, the particles of sand and coarse aggregates should be well mixed and remain suspended. It’s important to prevent bleeding or segregation of the mixture during handling and transportation. To achieve this, it’s crucial to ensure a uniform distribution of aggregates in the concrete, as it helps control segregation.
1. Bleeding Factors Normal and High Strength Concrete
Bleeding in fresh concrete occurs when solid particles of cement and aggregate settle, leading to the formation of a water layer on the surface. While minor bleeding poses no significant problems, excessive bleeding can have negative effects on the durability and strength of the concrete. In contrast to normal strength concrete, high strength concrete does not exhibit bleeding. This is due to its lower water content and higher proportion of cementitious materials. Similarly, air entrained concrete also has reduced susceptibility to bleeding.
2. Workability Factors of Normal and High Strength Concrete
Workability is an important factor that measures how easily concrete can be placed, compacted, and finished in its fresh state. When it comes to Normal strength concrete, it generally exhibits good workability as long as all the ingredients are properly proportioned. It is crucial for the aggregates to have the appropriate gradation. On the other hand, High strength concrete mix tends to be sticky and challenging to handle and place, even when plasticizers are used. This difficulty primarily arises from the high cement content present in the mix.
3. What if there is Decrease in permeability?
Improving sulfate and chemical attack resistance, as well as corrosion resistance and resistance to chloride penetration, are important considerations in concrete construction. One way to enhance these properties is by reducing the permeability of the concrete. The permeability of concrete is influenced by factors such as water cement ratio and curing period.
Table-1 presents the results of various permeability tests conducted on different concrete mixes. The table distinguishes between normal strength concrete and high strength concrete based on their respective water cement ratios. It is observed that a lower water cement ratio, coupled with an adequate curing period, can contribute to the production of concrete with lower permeability. Typically, normal strength concrete exhibits permeability within the range of 1 x 10-10 cm/sec.
In summary, to improve the resistance of concrete to sulfate and chemical attacks, as well as corrosion and chloride penetration, it is beneficial to minimize the permeability of the concrete. This can be achieved by reducing the water cement ratio and ensuring sufficient curing time.
The table-1 below shows the test results of various permeability tests conducted on different concrete mix. The table differentiates the normal strength concrete and the high strength concrete in terms of water cement ratio. Lower the water cement ratio with adequate curing period helps in having a concrete of lower permeability. For a normal strength concrete the permeability is found to be in the range of 1 x 10-10 cm/sec.
Table.1: Various Permeability Tests on Concrete as per American Concrete Institute, 1988
The addition of supplementary cementitious materials such as silica fume, fly ash, and GGBFS to concrete reduces its permeability. High strength concrete, which is designed with a lower water cement ratio, exhibits lower permeability compared to normal strength concrete.
Silica fume is commonly used in high strength concrete mixes. The permeability coefficient of high strength concrete ranges from 1 x 10-11 to 1 x 10-13 cm/sec. Due to its lower permeability and higher resistance to chloride attack, high strength concrete is suitable for constructing bridges, parking decks, and other structures that are exposed to seawater or de-icers.
4. Permeability of Normal and High Strength Concrete
Durability concerns in concrete, such as corrosion resistance, resistance to chemical attacks, and creep, are closely linked to its permeability. Damage occurs in concrete when foreign substances enter its structure. The permeability of concrete is determined by the permeability properties of both the paste and the aggregates used in its composition.
5. Carbonation of Normal and High Strength Concrete
Carbonation is a process that occurs on the surface of concrete and is influenced by the concrete’s permeability. When carbon dioxide in the air interacts with the compounds found in hardened cement paste, it leads to the formation of calcium carbonates, also known as carbonates.
Interestingly, high strength concrete exhibits less carbonation compared to normal strength concrete. Engineers also consider the mix criteria for both types of concrete and prescribe a sufficient protective concrete cover for the reinforcement steel. This measure is implemented to limit direct contact with the reinforcement and reduce potential damage.
Read Also: Hardened Concrete Properties with Blast Furnace Slag (GGBFS)
Other Difference Between Normal Strength Concrete and High Strength Concrete
Normal strength concrete undergoes the formation of micro cracks when it reaches 40% of its compressive strength. These cracks interconnect and propagate further once the strength reaches 80 to 90%. This leads to the development of a rough fracture surface in normal strength concrete. This rough zone is situated along the transition zone between the paste matrix and the aggregates. On the other hand, high strength concrete exhibits a smooth fracture surface when it fractures.
FAQs about High-Strength Concrete vs Normal Concrete
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What is normal strength concrete definition?
Normal strength concrete refers to the standard concrete used in most construction applications, typically characterized by its compressive strength range. It typically has a compressive strength between 20 and 40 megapascals (MPa) or 2,900 to 5,800 pounds per square inch (psi). Normal strength concrete is commonly used in buildings, bridges, pavements, and other general construction projects where high strength is not a primary requirement. It is composed of cement, aggregates (such as sand and gravel), and water, with or without the addition of admixtures. Normal strength concrete provides adequate structural stability and durability for a wide range of construction applications.
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What are properties of normal strength concrete?
Normal strength concrete has properties such as adequate compressive strength (20-40 MPa or 2,900-5,800 psi), good workability, satisfactory durability against environmental factors, moderate thermal conductivity, and decent fire resistance. It is cost-effective and widely available for various construction applications.
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What is normal concrete definition?
Normal concrete, also known as ordinary concrete or conventional concrete, refers to a mixture of cement, aggregates (such as sand and gravel), and water. It is the most commonly used type of concrete in construction due to its versatility and cost-effectiveness. Normal concrete typically has a compressive strength ranging from 20 to 40 MPa (2,900 to 5,800 psi), depending on the specific application and mix design. It is suitable for a wide range of construction projects, including residential buildings, commercial structures, bridges, and infrastructure. Normal concrete can be customized with additives or admixtures to enhance specific properties such as workability, durability, or setting time.
Read Also: Fly Ash Effects on Durability of Concrete