Why is Concrete Mix Design Required?

This article will answer the question “Why is concrete mix design required?” and will cover topics such as the definition of concrete mixture design and the types of concrete mixtures. Factors that affect the design of concrete mix such as strength, durability, and the economical aspect will also be discussed in this blog post.

Why is concrete mix design required?

A concrete mix design is required in a construction project in order to attain the desired structural strength and durability without compromising the budget constraints of the project. For instance, one may use raw materials that are of  utmost quality and grade to come up with a concrete strength that may exceed the minimum requirement of the project. That may be acceptable in terms of mechanical properties, but the cost of the project may balloon to a critical level that may consider the choice impractical.

 In other words, concrete mixture design is simply finding the middle ground between attaining the required strength of the structure as well as going within the approved budget of the project.

What is concrete mix design?

Concrete mix design is an intricate decision-making process of selecting raw materials and knowing their appropriate proportions in order to attain the desired strength and durability of a construction project. The raw materials of concrete are water, cement, aggregate, and a couple of admixtures (when necessary) which have individual properties that need to be assessed first to come up with a sound and fitting design.

It is important to understand that mixture design is a laborious activity, and it requires careful consideration of various factors. Furthermore, concrete’s raw materials that are available in the market are made in various grades and variants which make the overall process more strenuous. Location-wise, the properties of these raw material varieties vary, so the test result of one variety may not be applicable to another.

Types of Concrete Mix

There are different types of concrete mix depending on the required strength of the project. Normally, the difference with the amount of cement used in the mixture as well as the presence of admixture defines the distinction between each type.

Normal Concrete

Normal or regular concrete is the most ubiquitous concrete type among the pool of concrete mixes because the design is standardized to be manufactured large-scale. This concrete mixture is made by carefully following the predetermined mixing design instructions indicated by the manufacturer. In terms of mechanical properties, normal concrete has impressive resistance to compressive stress but is totally vulnerable to tensile attack. Its strength ranges up to 40 MPa, but is normally not capable of withstanding critical environmental conditions such as freezing temperatures.

High-Strength Concrete

High-strength concrete is designed for construction projects that require high resistance against stresses such as buildings and sidewalks. Typically, this type of concrete could withstand compressive loads more than 40 MPa which could be attributed to a low water to cement ratio. For high strength concrete, w/c ratio is normally less than 0.35 which is imposing a great disadvantage in terms of attained brittleness. 

Low w/c ratio could drastically increase the strength of the structure while also reducing its ductility. This is because the amount of cement used in the mixture is proportional to the amount of calcium hydroxide that may form and crystallize within the matrix. These crystals increase the porosity of the structure and reduce the bond strength between cement and aggregate.  For this reason, high strength concrete is often added with silica fumes to avoid the formation of calcium hydroxide crystals during curing.

Aside from the impending disadvantage from crystallization of calcium hydroxide, high strength concrete also has reduced workability. Hence, aside from silica fumes, the mixture is added with plasticizers to be able to attain the desired casting capability. Also, the quality of aggregate used in the mixture could affect the overall strength of the structure. As the mixture achieves high strength from the added cement, the mechanical property of aggregate to withstand high loading may be a limiting factor in this case. If the strength of the aggregate could not resist high loads, the strength gained from a low w/c ratio may be put to waste.

Air-Entrained Concrete

Air-entrained concrete is most applicable to environments that have freezing temperatures. Water is a primary component of concrete which expands at very cold temperatures; and the absence of voids within the concrete matrix implies no room for this expansion, creating stress that could lead to cracks. The addition of air-entraining chemicals or admixtures provides the needed space for the expected internal expansion, thus avoiding impending damages. On the downside, air-entrained concrete has low density and strength. Therefore, the use of this type of concrete mix is recommended only to structures that do not experience high loading.

Factors Affecting Concrete Mix Design

There are various factors to consider when designing concrete mixtures, but the most important ones are discussed below.

Concrete Strength

Every structural engineer believes that the strength of a concrete structure should not be compromised when designing a concrete mixture. All of the possible causes of failure are carefully analyzed and studied in parallel with the properties of the raw materials used to avoid unwanted catastrophic events involving structures.

The strength of concrete is known to have an inverse relationship with the ratio of water and cement. Although, as mentioned previously, simply adding too much cement could compromise the ductility and the workability of the structure. Quality tests are required to be done in order to assure that the mixture attains the required strength of the project as well as the other properties that define the project’s purpose.

Durability

Durability is defined as the ability of the material to resist the effects of environmental conditions, chemical attacks, and external forces such as abrasion. The design of a concrete mixture should consider the synergistic effects of such conditions that could deteriorate the structure in the long run. Typically, the addition of chemicals or admixtures are done in order to achieve required durability of the project.

Economy

Every construction project is limited by budget constraints. The economic aspect of selecting raw materials is always considered on top of meeting the required strength and durability of the structure. One may encounter a raw material that shows remarkable strength and durability, but the cost is expensive to not qualify on the selection criteria. Limiting the amount of cement used in the mixture may be a typical response to minimize costs but this may compromise the structural integrity. In other words, designing concrete mix is literally finding the sweet spot between the economy of raw materials and the strength that it can provide.

Conclusion

This blog post answered the question “Why is concrete mix design required?”, and it was clearly explained that mixture design is important in order to allocate appropriate amounts of raw materials to attain the required properties of a concrete project. The selection of material by assessing its grade and quality is included in the scope of mixture design. The types of concrete mixtures were also discussed in this article. Some of the types covered in were normal concrete, high-strength concrete, and air entrained concrete. Finally, the factors that affect the design of concrete mixture such as strength, durability, and the economical aspect were also elaborated in this blog post.

For any questions and suggestions about this article, please feel free to submit your thoughts in the comment section below.

Frequently Asked Questions (FAQs): Why is Concrete Mix Design Required?

What is the purpose of concrete mix design?

The purpose of concrete mixture design is to determine the appropriate amount of concrete raw materials to create a mixture that will attain the desired properties of the structure. Mixture design is finding the middle ground between the economic aspect and the structural requirement of the project.

What is concrete mix design?

Concrete mix design is a decision-making process that involves careful considerations and selection among a pool of concrete raw materials as well as determining the proportion to be used in the mixture in order to attain the strength defined by the project without compromising its budget constraints.

What are the major factors that must be considered while designing concrete mixes?

There are various factors to consider when designing concrete mixes, but the major ones are the following:

1.     Ultimate Strength of the Concrete

2.     Workability

3.     Durability

4.     Cement Quality

5.     Aggregate Quality

6.     Available Quality Control Equipment and Measures

What is a good slump for concrete?

A four-inch slump of a standard concrete weight is considered a good slump for pumping activities. Slump is an indicator of concrete mixture’s consistency and workability to be casted in different forms and shapes. As more water is present in the mixture, the higher the slump is.

What is the strongest concrete mix ratio?

By convention, the amount of aggregate used in a concrete mixture assumes the bulk of its total weight. This means that among the raw materials of concrete, aggregate fraction is highest. The standard ratio of solid raw materials is 1:3:5 (Cement:Sand:Gravel). 

What is concrete workability?

The workability of concrete describes the ease of performing routine tasks on the building material such as transporting, pouring, casting, and compacting. Slump test could also serve as an indicator that signals when a certain concrete batch is mixed incorrectly; hence, the uniformity within the matrix is assured by this test.

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