Why is Concrete Strong in Compression? (5 reasons)

In this article, we are going to discuss why concrete is strong in compression. We will also identify the concrete components and its role in concrete strength, discuss why concrete is strong in compression but weak in tension, enumerate the factors affecting concrete strength, and discuss examples of concrete with improved strength such as reinforced concrete and ultra-high performance concrete. 

Why is concrete strong in compression?

Concrete is strong in compression because of its composition and structure. Concrete is a mixture of gravel, sand, crushed rocks, and other aggregates which are held together by cement paste (cement and water). This mixture has Interfacial Transition Zone (ITZ) or simply “interface” zone which is the weakest link in the concrete. Under compression, the ITZ tends to transfer the compressive stress from one aggregate to the other, in which the strength requirement is minimal. Thus, concrete can resist compressive forces regardless of the weak ITZ.

What are the components of concrete?

Concrete is a mixture of gravel, sand, crushed rocks, and other aggregates that are held together by a binder which is cement and water. To further understand each component and how it affects the integrity of concrete, the five concrete components are listed and discussed below:

Cement

Cement serves as the hydraulic binder of the other components of the concrete. It hardens and takes form when mixed with water. It fills up all the gaps and spaces to form a solid structure. There are several types of cement available in the market. By European standard, the five main types are the CEM I Portland Cement, CEM II Composite Cements primarily made of Portland Cement, CEM III Blast Furnace Cement, CEM IV, Pozzolan Cement, and CEM V Composite Cement.

The most common and widely used type of cement is the Portland Cement. It is made by heating the base materials (ground limestone, clay, marl) in a kiln to a temperature of about 1,450 degrees Celsius. The resulting product is called a clinker, which is then ground to achieve the fineness of the cement.

Concrete Aggregates

Concrete aggregates, which are sand and gravel, make up 70%  of the concrete volume and 80% of the concrete weight, They serve as the grain skeleton of the concrete mixture, which cavities are to be filled up by cement paste (mixture of cement and water).

The properties of the concrete aggregates should be highly considered so it will not have a negative impact on the integrity of the concrete, as well as on the hardening process of the cement. Likewise, the proportions, size, and quality of each concrete aggregate should be optimized to obtain maximum concrete strength and quality. 

Concrete Admixtures

Concrete admixtures are substances that are added in small quantities to the concrete to modify its properties. They are added together with the cement, cement aggregates, and water usually prior or during mixing.

Concrete admixtures may be used to reduce the building cost, reduce the water requirement, dampen the setting rate of concrete, reduce curing time of concrete, delay the corrosion of reinforcing steel, add color, entrain air in the concrete to resist freeze-thaw degradation, water-proofing, enhance the hardness, etc.

Concrete Additions and Supplementary Cementitious Materials (SCM)

These are materials that are added to the concrete to contribute to the hydraulic and/or pozzolanic activities. SCMs contribute to sustainable construction since using these materials reduce the energy requirement in producing concrete and improve its strength as well.

Water

Water is used in conjunction with cement to form cement paste. This is used as a binder of all the other components of concrete and fills all the gaps and empty spaces of the concrete mixture. The ratio of water and cement highly affects the strength of hardened concrete. Thus, optimizing the water-cement ratio will improve the quality and strength of concrete.

Each component of the concrete contributes to its overall strength, durability, and versatility. The components work together to display its strength especially in compressive forces experienced by the concrete.

Concrete Strength: Strong in Compression but Weak in Tension

Concrete is widely considered as a strong material that most infrastructures are made of it – relying on its strategic composition and structure to withstand compressive forces or the weight placed on it.

There are different types of strength, and the magnitude of each type of strength in concrete differs. This is attributed to the form and structure of concrete, and how the concrete reacts to each type of stress.

Compressive Strength

Compressive strength of concrete is a measure of the ability of the concrete to withstand perpendicular forces towards it (compressive force). Simply put, it is a measure of how well the concrete can withstand weight loads that will cause its shrinkage or volume reduction.

Concrete has high compressive strength. It can resist high weight loads with little to no volume reduction. Compared to concrete tensile strength, the compressive strength of concrete is usually ten times higher. This is attributed to the composition and structure of concrete. As mentioned, concrete is a mixture of sand, gravel, rocks, and other aggregates bound together by hardened cement and water.

This mixture has Interfacial Transition Zone (ITZ), or simply “interface zone”, which is the weakest link in the concrete structure. Under a compressive force, the ITZ only transfers the compressive force from one aggregate to the other, which only requires minimal strength. Thus, concrete can withstand compressive load regardless of the weak ITZ.

Tensile Strength

Tensile Strength of concrete is a measure of the ability of the concrete to withstand perpendicular forces away from it (tensile force). This measures how much “stretching” force can a concrete withstand to the point of elongation, and eventually, breakage.

When concrete experiences tensile strength, the aggregates of the concrete tend to pull away from each other. The ITZ has to hold the aggregates together. However, since the ITZ is the weakest link in the concrete, it would break “sooner” or at a lower applied stress than in compression.

Flexural Strength

Flexural strength is an indirect measurement of tensile strength. It is also referred to as the Bending Strength since it is a measure of the resistance of a concrete to bending. The flexural strength is a fraction of the compressive strength, the magnitude of which depends on several factors such as concrete mixture.

Other Factors Affecting Concrete Strength

Aside from the individual quality and form of each component, there are other factors affecting the strength of concrete.

 Coarse/fine aggregate ratio and aggregate/cement ratio

The right proportion of the components or aggregates of the mixture will give the desired quality and strength of the concrete. In baking cakes and pastries, for exmaple, the proportions of the components will determine the quality of the baked goods. Thus, optimizing the proportions to achieve best results is desired.

 Water and cement ratio

The ratio of the water to cement will determine both the concrete strength and workability of the concrete. Increasing water may increase workability due to the lower viscosity, but too much water may also lower the final strength of concrete. On the other hand, decreasing the water may increase the concrete strength, but will result in difficulty in handling due to high viscosity. Optimum water/cement ratio is desired to attain balance between concrete strength and workability.

Mixing Time

The strength of concrete increases with increasing mixing time. However, at a certain maximum value or threshold, the strength of the concrete will reduce with further mixing due to evaporation of water in the mixture.

Temperature and Humidity

Both temperature and humidity affects the hydration of the concrete. At higher temperature, the concrete will have higher strength faster than at lower temperature. However, the strength of the concrete at the end of  the curing period at higher temperature will be lower.

High humidity in the curing environment will result in high hydration, while low humidity will result in low hydration.

 Curing Time and Curing Method

Curing of concrete is essential to achieve the best properties of concrete. In curing, the concrete is given proper moisture, time, and temperature. A good combination of these factors will result in higher concrete strength and better concrete quality.

These are just some of the multitude of factors affecting the strength of concrete. Several researches have already been made to further increase the strength of concrete and form it to its desired use. Some have used concrete in conjunction with other materials to aid in its limitations, while other concrete products have altered properties to fit in its desired use.

Reinforced Concrete

Concrete has high compressive strength and low tensile strength. To increase its tensile strength, concrete is reinforced with materials that have high tensile strength such as steel. With this, steel-reinforced concrete can withstand considerably higher compressive and tensile forces acting on its body.

Ultra-High Performance Concrete (UHPC)

Ultra-high performance concrete has almost the same components as the traditional concrete, only that it has added fibers which comprise about 20 to 25% of its composition. Added fibers tend to increase the strength of its end product. These fibers can be basalt, polyester, fiberglass, steel and stainless steel.

In terms of compressive strength, UHPC can have up to ten times higher compared with traditional concrete. With regards to resistance to tensile stress, UHPC can have up to 1,700 psi, while the traditional concrete only has 300 to 700 psi. For flexural stress, UHPC can have up to 2000 psi versus 400 to 700 psi of the traditional concrete.

FAQs: Why is concrete strong in compression

Why concrete is weak in tension and strong in compression?

The main reason why concrete is weak in tension and strong in compression is because of its composition and structure. Under compressive stress, the Interfacial Transition Zone (ITZ), which is the weakest link in concrete, tends to transfer the stress from one aggregate to the other. The strength requirement in this is low.  

However, under tensile stress, the aggregates tend to pull away from each other. With this, the ITZ has to hold the aggregates together. Since it is the weakest link, the stress applied prior to its breakage will be lower.Thus, weak in tension.

Does concrete have high compressive strength?

Yes, concrete has high compressive strength. It does not easily shrink or crack under weight. This is because of its components and structure. Other factors such as cement-water ratio and curing time affects the compressive strength of the concrete.

Why is concrete 10 times stronger under compression than it is under tension?

Concrete is 10 times stronger under compression than it is under tension because of the concrete’s components and structure.

Concrete can withstand compressive forces better because under compressive stress the “interface” zone of the concrete mixture only transfers the stress from one aggregate to the other. However, under tension, the “interface” zone, which is the weakest link in the concrete, has to hold the aggregates which tend to pull away from each other. 

To address this low resistance of concrete in tension, concrete may be reinforced with steel, since steel is strong in tension.

What does strong in compression mean?

Strong in compression means that the material resists loads or stresses that tend to cause shrinkage or volume reduction. Simply put, the material is strong in compression when it does not easily break under weight.

Can a concrete member take tension?

Concrete is very weak in tension. It would easily break under tensile forces. Generally, the tensile strength of concrete is assumed to be zero, and the tension forces are mainly accommodated by steel if the concrete is reinforced by steel.

Conclusion

In this article, we discussed why concrete is strong in compression. We also identified the concrete components and its role in concrete strength, discussed why concrete is strong in compression but weak in tension, enumerated the factors affecting concrete strength, and discussed some examples of concrete with improved strength such as reinforced concrete and ultra-high performance concrete. 

If you have questions or comments on the content, please let me know in the comments section below.

Bibliography 

A C Davis, A Hundred Years of Portland Cement, 1824-1024, Concrete Publications Ltd, London (1924)

ACI Committee 318, Building Code Requirements for Structural Concrete Commentary. Amer. Conc. Inst. 318-8 (2008)

Admixtures in Concrete (2020, October 6). Retrieved from https://www.designingbuildings.co.uk/wiki/Admixtures_in_concrete   

A.M. Yousef, M.A. El-Mandouh, Dynamic Analysis of high-strength concrete frame buildings for progressive collapse. Case Studies in Construction Materials 13 (2020) 

Blezard, R.G. (1998). The History of Calcareous Cements in P C Hewlett. Lee’s Chemistry of Cement and Concrete 4th Ed, Arnold

Concrete Components. (n.d). Retrieved from https://sikaconcrete.co.uk/technical-information/concrete-components/ 

Etienne, Girardet. No Title. 2017. Unsplash, https://unsplash.com/photos/sgYamIzhAhg

Everything You Need to Know About Concrete Strength. (2019, March 31). Retrieved from https://cor-tuf.com/everything-you-need-to-know-about-concretestrength/#:~:text=Concrete%20psi,it%20is%20usually%20more%20expensive.&text=Traditional%20concrete%20walls%20and%20columns,psi%20is%20needed%20for%20pavement.

G. Neville. Concrete Manual. International Code Council (2015)

I. Hussain, B. Ali, T. Akhar, M.S. Jameel, S.S. Raza, Comparison of mechanical properties of concrete and design thickness of pavement with different types of fiber-reinforcements (steel, glass, and polypropylene). Case Studies in Construction Materials 13 e00429 (2020)

K. Jones, Density of Concrete. The Physics FactBook (2016)

M. Qianmin, G. Rongxin, Z. Zhiman, L. Zhiwei, H. Kecheng, Mechanical properties of concrete at high temperature-A review. Constr. Build. Mater. 93, 371-383 (2015)

McLendon, Shane. No Title. 2018. Unsplash, https://unsplash.com/photos/q-V11EA3ZzU

Portland Cement Concrete Pavements Research: Thermal Coefficient of Portland Cement Concrete. (2016,March 8) Retrieved from https://www.fhwa.dot.gov/publications/research/infrastructure/pavements/pccp/thermal.cfm

S. Al-Sabah, S N A Sourav, C. Mcnally, The post installed screw pull-out test: Development of a method for assessing in-situ concrete compressive strength. Journal of Building Engineering 33, 101658 (2021)

Supplementary Cementitious Materials. (n.d). Retrieved from https://www.lafarge.ca/en/supplementary-cementitiousmaterials#:~:text=Supplementary%20Cementitious%20Materials%20(SCMs)%20are,hydraulic%20and%2For%20pozzolanic%20activity.

Understanding asphalt pavement distresses – five distresses explained. (n.d). Retrieved from http://asphaltmagazine.com/understanding-asphalt-pavement-distresses-five-distressesexplained/#:~:text=Transverse%20cracks%20occur%20roughly%20perpendicular,Reflection%20Cracking.&text=The%20cracks%20form%20because%20of%20movement%20of%20the%20old%20pavement.

Y.X. Zhang, X. Lin, in Toughening Mechanisms in Composite Materials (2015)

What was missing from this post which could have made it better?

Leave a Comment