Why is Concrete Weak in Tension? (3 reasons)

This article will answer the question “Why is concrete weak in tension?” and will cover topics such as the roles of concrete components in its overall tensile strength. Standard building techniques to increase concrete’s tensile strength will also be tackled in this blog post.

Why is concrete weak in tension?

Concrete is weak in tension because of the presence of a weak link within the concrete matrix known as the Interfacial Transition Zone of ITZ. Concrete is mainly composed of rock aggregates that are glued together by a cement paste which is a mixture of cement and water. The outer surface of the large solid aggregates or the ITZ is normally filled with calcium hydroxide which is a bi-product of concrete’s hydration reaction. This calcium hydroxide layer is weak and porous, contributing to a weak tensile strength of the overall structure.

What is Tension?

Tension is simply defined as the pulling force exerted perpendicularly and away from the surface. The effect can be effectively visualized from the pulling of a rope on both ends. Upon application of tensile force, rope tends to elongate and extend its length along the axis of application while reducing the cross-sectional area of the material. 

Similar effects may be replicated on other materials, although it is important to note that not all kinds will elongate and simply go back to its initial state once exerted force is eliminated. Such is the case for concrete. This building material could not withstand high amounts of tension, and when applied, could result in irreversible deformation and damage.

In the microscopic level, one can imagine that the heterogenous units of concrete components are initially bound to each other by a paste or gluing material. It is only rational to think that the ultimate bond strength is relative to the strength translated by the glue. In other words, weak links result in weak resistance to external forces such as tension. Concrete happens to have this kind of weak link that renders its limitation against tensile stresses.

Compression vs Tension

While tension refers to the pulling force away from the surface of a structure, compression is defined as the pushing force towards the material. It normally results in an increase in the cross-sectional area of the material while a simultaneous reduction in length happens. One could visualize compression deformation though marshmallows.

 When you squeeze a marshmallow on either or both of its ends, it expands through its cross-sectional area while its height decreases. Like in tension, such is the effect when the applied compressive stress surpasses the compressive strength of the material.

Among other building materials, concrete has an impressive resistance to compression. It’s high compressive strength along with its economics allow it to be considered as a well-sought building material in the industry. Although, it is important to note that although concrete has high compressive strength, it is still weak in terms of tensile resistance. This gap in concrete’s property is normally aided by reinforcements using steel which happens to have remarkable tensile strength.

Factors Affecting Concrete Strength

There are several factors that could affect the overall strength of concrete, but the synergistic effects of its components’ properties greatly affect its values among of which are listed as follow:

  • Water- amount and clarity
  • Cement – amount and quality
  • Rock Aggregates – quality, purity, particle size (grade)
  • Admixtures- presence or absence
  • Handling Techniques -pouring, spraying
  • Curing Conditions – temperature, humidity
  • Foreign Material – presence or absence

Concrete Components and their Functions

Concrete is a heterogenous mixture of three major components namely cement, sand or rock aggregates, and water. The properties of individual components as well as the reactions that happen within the mixture result in the overall mechanical properties of concrete, that’s why it is important to understand the function of each component.

Cement

Cement is a fine mixture of minerals such as limestone which is made through thermal processing in an energy-extensive kiln. The main function of cement in a concrete mixture is being a binder that glues the components within the matrix. The most common cement in the industry is Portland cement which is sought after because of affordability and availability in the market. 

Water

Water is extremely important in concrete mixture because it allows cement to fulfill its role as a binding agent. In other words, cement does not work without water, and they go side by side in strengthening the structure throughout the hardening process. When cement is mixed with water, it undergoes a hydration reaction producing crystals that fills the voids within, resulting in a more durable structure.

Aggregates

Sand or rock aggregates constitute a large fraction of the total volume of concrete structures. Because concrete is mostly composed of this component, the main function of sand aggregates is support on the overall structure while they are held together by cement paste. The quality of sand aggregates is critical because it dictates most of concrete’s properties such as density and durability. When sand’s porosity is high, the final concrete structure is weaker in terms of wear resistance. On the other hand, if its hardness is remarkable, the resulting structure is highly resistant to abrasive attacks.

Interfacial Transition Zone of Concretes

The Interfacial Transition Zone (ITZ) of concrete is a weak region in the concrete matrix resulting in weak tensile resistance. ITZ is found between the outer surface of rock aggregates and cement paste. Normally, the zone is made up of a calcium hydroxide layer which is one of the products of hydration reactions other than calcium silicate crystals. The weak properties of CH layer translate to lesser durability of the overall structure.

One of the main causes of the formation of ITZ is bleeding which is the segregation of water and solid components of concrete due to gravity settling. During bleeding, water goes up while denser components settle. Because the size of rock aggregates is larger compared to other components, some of the floating water is trapped during bleeding, resulting in regions with high water-cement ratio which increases the porosity and reduces the strength of the structure.

How to Increase Tensile Strength

To increase the tensile strength of concrete, it is a standard practice in the construction industry to utilize steel reinforcements. Unlike pure concrete, steel is a durable building material that has remarkable resistance to tensile stress. Steel is ubiquitous and relatively economical, that’s why steel-reinforced concrete structures are common in various structures.

In addition, it is also recommended to use secondary cementitious materials (SCM) such as fly ash in creating concrete mixtures. These SCM could consume the undesirable layer present in ITZ over time, resulting in less porosity and higher durability of the overall structure. Other than fly ash, some of the widely accepted secondary cementitious materials in the industry are slag cement and silica fumes.

Conclusion

This blog post answered the question “Why is concrete weak in tension?” It was clearly explained in this article that concrete has low resistance to tensile stresses because of the presence of a weak link between its components in the matrix known as the Interfacial Transition Zone or ITZ. The components of the concrete were taught to have significant contributions in the mechanical properties of concrete which were highlighted in this blog post. Lastly, steel reinforcements are standardized practice in the construction industry to increase concrete’s resistance to tensile stresses.

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 Weak in Tension?

Is Concrete weak in tension?

Yes, unfortunately concrete is weak in tension. Its tensile strength is comparably weaker compared to other building materials, although its compressive strength is one of the best and highly sought after. To mend this gap, concrete is reinforced with steel which is known for its high tensile resistance.

What makes concrete weak in tension?

 Concrete is weak in tension because of the presence of an internal weak link between concrete components known as the Interfacial Transition Zone or ITZ. Imagine your concrete to be composed of weak paste throughout the matrix, and pulling the structure with too much tensile force gives this paste just the right amount of stress for concrete to disintegrate.

How do you increase tensile strength of concrete?

The standard and most common way to increase the tensile strength of a concrete structure is by steel reinforcement. Steel has impressive mechanical properties like tensile strength. Steel reinforcements could provide a synergistic effect on a concrete structure making it more durable and resistant to external forces.

Can a concrete member take tension?

 Yes, concrete structures can take tensile stresses but only to some extent. Beyond this limit, concrete structures may crack or collapse, resulting in significant structural damages that could compromise the safety of a community. Because of concrete’s limitation in tensile stresses, steel reinforcements are done in concrete structures.

Is concrete stronger than steel?

It depends. Concrete is stronger than steel in terms of compressive strength. However, in terms of tensile resistance, it is much weaker. The strength of a building material depends on several mechanical properties, so it’s only fair to consider them before choosing the right material for your project. Nonetheless, steel-reinforced concrete is stronger and widely accepted in the construction industry.

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