Why is Concrete Brittle?

This article will answer the question “Why is concrete brittle?” and will cover topics such as concrete’s tensile and compressive strength and different types of steel reinforcements that improve concrete’s mechanical properties. The factors that affect the strength of concrete structures will also be tackled in this blog post.

Why is Concrete Brittle?

Plain concrete is brittle because it has low tensile strength. Innately, this mechanical property of concrete (just like any building materials) depends on the synergistic contribution of its components which are water, cement, and aggregate. At the appropriate ratio of water and cement, plain concrete structures have high resistance to compression but have low resistance to tension. 

The vulnerability of concrete to damages due to tensile loading increases when excessive cement is used in the mixture. Hence, it is important to have a standard mixture design as well as necessary reinforcements when dealing with concrete as a building material.

Concrete’s Tensile Strength 

Tension is defined as the ratio of the perpendicular internal force exerted away the material to the area of the cross-section. Macroscopically, as tensile stress pulls each material’s particle away from each other, the material results in a deformation known as elongation which is literally the increase of the material’s length.

While plain concrete is a standard building material in the construction industry, it is important to consider that concrete has is weak in terms of tensile strength. Relative to its resistance to compressive stress, its tensile strength only accounts about a tenth of the compressive strength of plain concrete. The lack of ductility and vulnerability to crack damages are associated with this poor resistance to tension. Hence, reinforcements may be recommended depending on the requirement of the project.

Concrete’s Compressive Strength 

As mentioned, plain concrete has innately remarkable resistance against compressive stresses. This implies sufficient strength when the structure is experiencing high loads of impact and compaction forces. Theoretically, the mechanical properties of concrete are linked to its raw material development which happens during the curing process. Curing is a chemical reaction that happens between cement and water. Water is consumed in the process, forming minerals that logarithmically crystallizes and fills matrix voids. The duration of the process frequently lasts a month, although theoretically the process does not stop but reaches an extremely dormant rate asymptotically.

Steel Reinforcements

Steel is a widely utilized reinforcing material for plain concrete. There are various types and steel forms that are available in the market which can be chosen depending on the requirement of the project. Some of these forms are the following:

Deformed Steel Bar

This steel form is considered to be a staple in any concrete construction projects because it enhances the bond strength between concrete and the reinforcing material. The flexibility of this material is excellent enough to allow reforming opportunities that may be necessary to achieve the requirement of a structure.

Deformed steel bars are manufactured in a high-temperature environment. The process starts by preheating steel billets which are steel with a square-shaped cross-section. Billets are then fed into a set of opposing rollers to form steel bars before adding unique and repeating deformations known as ribs. The two types of ribs are transverse and longitudinal which can be made at various shapes such as crescent, herringbone, and spiral shape. The rib pattern continues throughout its length, and the bar is cut to a specific dimension.

Cold-Worked Steel Bars

Cold-worked steel bars are steel forms that are finished in room temperature which is tremendously cooler than the temperature in the manufacturing of hot-rolled deformed steel bars. The production activities done in the cold environment include drawing, turning, grinding, and polishing which improve the properties of the steel product. For example, drawing in a cold environment avoids the consumption of energy-intensive thermal treatments and relatively increases the tensile resistance of the final product. Grinding provides a more tapered range of size tolerance range while both turning and polishing enhances the surface appearance of the product.

Prestressed Steel Bars

Prestressing is done by deliberately exerting a certain amount of internal force in steel bars in such a way that the total internal stresses experienced by material (which is technically just the sum of the added internal force on top of the stress due to external forces) is still within the safer limits.

The two ways to do prestressing are by pre-tensioning and post-tensioning.  Pre-tensioning is done by applying tension in the tendons before concrete pouring while post-tensioning is pressing the tendons when the concrete is already placed and hardened.

The purpose of prestressing is for the building material to experience acceptable levels of stress and strain and, at the same time, counterbalance critical and unwanted levels of stress and strain. There are several advantages of prestressing some which are easier concrete casting and reduced requirement for web thickness.

Factors Affecting Concrete Strength

There are several factors that could affect the strength of concrete, some of which are the following:

Water/Cement Ratio (w/c)

The ratio of water and cement is considered a reliable indicator of strength and durability. It also translates to the porosity of the hardened mixture which implies the presence of voids within the matrix.

It is important to note that the relationship of water/cement ratio and concrete strength is inversely proportional to some extent. This means that as the ratio increases, the strength of the structure decreases and vice versa. High compressive strength can be achieved from a w/c ratio of 0.3 at 28-day curing time which decreases logarithmically to as ratio increases to approximately 0.7. Furthermore, to achieve the target  w/c ratio of 0.3, strategic control in the amount of water and cement may be done. If the goal is to decrease the ratio to 0.3, one may either increase the amount of cement (while water remains constant) or decrease the amount of water (while the cement content remains the same).

Cement Quality

The quality and component of cement used in the mixture also affect the strength of the concrete structure. The chemical reaction during curing involves a sequential reaction of tricalcium silicate and dicalcium silicate. The former is responsible for the initial strength of the concrete while the latter is providing the strength of the hardened structure.

Furthermore, the fineness of cement is an essential factor that dictates the rate of hydration. In fact, the two factors are directly proportional to each other wherein as fineness increases, the rate of hydration also increases and vice versa. In relation to strength, lower rate of hydration results in a high ultimate strength.

Aggregate Quality

Because more than half of a concrete mixture mass is made of aggregates, the remarkable compressive strength is attributed to this raw material. In relation to this, the properties of aggregate such as size distribution, texture, and shape affect the strength of the structure. For example, texture affects the tensile resistance of concrete by its contribution in material bond with the cement paste as well as the amount of stress experienced in regions that are prone to crack.

Conclusion

This blog post answered the question “Why is concrete brittle?” It was clearly explained in this article that plain concrete is brittle because it has innately low resistance against tensile forces. The brittleness of concrete structures may aggravate when the concrete mixture is poorly designed, wherein the water cement ratio is terribly low due to the utilization of high amounts of cement paste. Nevertheless, it was mentioned in this blog post that concrete has a remarkable compressive strength and can withstand high impact loading.

Typically, structural projects specifically require high resistance against all types of stresses, so the application of reinforcements is considered a standard construction practice. Some of the different types of steel reinforcements mentioned in this article were deformed, cold-worked, and prestressed steel.

Finally, this blog post elaborated the different factors that generally affect the structural strength of concrete. Some of these were water/cement ratio, curing time, and aggregates.

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 Brittle

Why do we reinforce concrete?

Reinforcements are done in concrete structures to improve its tensile strength. Plain concrete has weak resistance against tensile loading, although it has impressive resistance to compressive stress. This means that concrete can withstand impact forces but is very brittle when tension is applied. This structural vulnerability is addressed by reinforcements.

What is high strength concrete used for?

High-strength concrete is commonly utilized for the construction of large-scale projects like buildings and skyscrapers. It is normally used as a supporting building material in areas where high loads are present such as in building foundations, walls, and bridges.

How do you make concrete brittle?

Adding too much cement can make a concrete extremely brittle. All properties of concrete structures depend on the synergistic effect of concrete components. The ratio of water and cement affect the quality of the final concrete mixture. For instance, if the ratio is too small, the resulting structure is brille. On the other hand, if the ratio is too high, the structure is too porous.

Should I put rebar in my concrete?

It depends on the requirement of your project. It is important to understand that concrete projects have varying strength requirements. If the structure is expected to experience high tensile loading, then it is a need to reinforce the structure. If not, then one may opt to consider building plain concretes. Again, reinforcements are design-specific and should be decided after careful engineering considerations.

What is the difference between cement and concrete?

Cement is a component of concrete. Concrete is a heterogenous mixture of water, cement, and sand aggregates. There are cases when admixtures are added to concrete mixtures in order to attain a certain concrete characteristic, but the three main components remain as they are. On the other hand, cement is a fine solid mixture of different minerals that acts as a paste to the rest of concrete components.

What happens if you don’t put rebar in concrete?

Without reinforcements, plain concrete has no resistance to tensile loading. Hence, if the structure experiences tensile stress internally, the building material cracks and may cause irreparable damage to the overall structure.

BIBLIOGRAPHY

Jenq YS., Shah S.P. (1991) Features of mechanics of quasi-brittle crack propagation in concrete. In: Bažant Z.P. (eds) Current Trends in Concrete Fracture Research. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-3638-9_8

Rajagopalan, K. A new indirect tension test for concrete and other brittle materials. Mat. Constr. 6, 307–308 (1973). https://doi.org/10.1007/BF02473896

Rajagopalan, K. A new indirect tension test for concrete and other brittle materials. Mat. Constr. 6, 307–308 (1973). https://doi.org/10.1007/BF02473896

S.P. Shah and F.L. Slate, inInternational Conference on the Structure of Concrete and Its Behavior under LoadLondon, 1965, Cement and Concrete Association.

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