This article will answer the question “Why is concrete reinforced by steel” and will ponder on topics such as types of stresses experienced by concrete, the reason why steel is used in concrete reinforcement, and
the limitations of ageing steel-reinforced concrete while discussing current technologies that attempt to solve this.
Why is concrete reinforced by steel?
Concrete needs steel reinforcement to improve its weak tensile strength and brittleness. Yes, concrete is a strong material, but it is not strong enough to withstand all external forces without steel reinforcements. Both materials are ubiquitous, and concrete happens to have weak mechanical properties that steel suffices as shown in the table below.
|Strength in Compression||Good||Good|
|Strength in Tension||Poor||Good|
|Strength in Shear||Fair||Good|
In other words, they are an ideal match in the age of growing infrastructure demand where structural integrity is set to the highest priority.
The idea can be further simplified by relating the human body as a structural entity. Muscles may be sturdy and well-built, but how long can the human body remain still without the bones and just the flesh, fat, and muscles alone? Without the skeletal system, the human body is vulnerable to collapse. Corollary to this, our buildings and homes are prone to structural failure without steel reinforcements.
Concrete vs Cement
Some people happen to use concrete and cement interchangeably, disregarding the significant difference between the two. Concrete is a mixture of cement, sand or rock aggregates, and water. On the other hand, cement is a powdered mixture of minerals manufactured by exposure in a high-temperature kiln. Both concrete and cement are mixtures, although cement is just a component of concrete. In other words, cement is a mixture within a mixture.
Types of Stresses Experienced by Concrete
Like all materials, concrete experiences compressive, tensile, and shear stresses that could result in structural collapse when built without sufficient reinforcements. Hence, it is a prerequisite to understand these types of stresses to know why concrete needs to be steel-reinforced.
- Compressive Stress
Compressive stresses or compression exerts a perpendicular force towards a material that could ultimately result in a shrinking deformation. Concrete happens to have high compressive strength; hence, pushing an appreciable amount of force to a block of concrete will not make it smaller in volume.
- Tensile Stresses
Tensile stresses or tension applies a perpendicular force away from the material which leads to its elongation. A perfect example for this type of stress is the end-to-end pulling of a rubber band. Because of its elasticity, rubber bands return to their original state once tensile forces are relieved. However, pulling much harder will come to a point of breakage. Concretes happen to have a weak tensile strength, so it cannot withstand high tensile loading.
- Shear Stress
Unlike compressive and tensile stress, shear stress applies force parallel to the plane of deformation. It can be visualized by cutting an apple in half using a knife. The sharp kitchenware introduces a parallel shear stress to the apple, resulting in its cutting in half.
Concrete may have an impressive compressive strength, but its low tensile strength could risk the overall integrity of the structure. Concrete is strong but not strong enough, so to increase its resistance to excessive tension, steel reinforcements are necessary.
Steel is a mixture of iron and carbon – or in a more technical term, an alloy of these two elements. It is commonly utilized in the construction of several structures because it is cheaper, easier to produce, and has excellent mechanical properties such as high tensile strength.
Because concrete happens to be lacking in tensile strength, steel transpires to be a good match. Using steel as reinforcement to concrete transforms the brittle material into something ductile. In addition, steel and concrete have similar thermal expansion coefficients implying synchronous reaction to thermal stress. Hence, tolerable temperature change will not result in undesirable thermal cracking.
Limitation of Steel Reinforcement
Pure concrete requires steel reinforcement to make it stronger. However, it is important to remember that this technique does not imply permanent protection from structural failure. Like any other structures, steel-reinforced concrete has limits that entail risks growing over time.
One of the ways to reduce this risk is allowing a proper curing for the concrete. The whole hardening process of concrete does not mean simple drying or water evaporation over a period of time. In fact, the process of adding water to the mixture until it completely solidifies is a chemical reaction that releases heat – an exothermic reaction.
The heat generated from this reaction may cause initial cracks on the structure that’s why it is a common technique to maintain the moisture of the structure during curing. This allows continuous hardening of the concrete making it more resistant to future damages.
Corrosion and Failure
While it is important to maintain moisture during curing, the final form of concrete may be at a disadvantage due to water penetration over time. Concrete is porous; hence, their exposure to humid environments [and rain] will allow water to come inside the structure. When this happens, moisture and air become accessible to the reinforcement and will then result to an oxidation reaction which is commonly known as rusting. This phenomenon could degrade the integrity of the structure resulting in failure.
Nigeria is one of the countries that has a long history of structural failures. Other than a corroded steel-reinforcement that happens over time, a new building may still collapse due to various reasons such as poor construction practices, lack of supervision from a professional, and failure to adhere to government regulations. The integrity of buildings is important because it interests the collective safety and security of a community.
Apart from stringent regulations in the construction industry, the research and development sector is making a difference to avoid this unprecedented event from happening.
The Future: Self-healing Concretes
As mentioned, the water permeability of concretes has been a long-underlying problem in the world of structural engineers. Auspiciously, a fascinating biotechnological technology has been developed to overcome this gap and is known as the self-healing concretes.
Microorganisms have long been utilized to manufacture essential products, some of which are critical to the survival of human life such as antibiotics, enzymes, and vitamins. Today, these tiny creatures are found to be able to fill the initially forming cracks in concrete, avoiding repair and reducing risks for structural failure.
Technology behind Self-healing Concretes
Some microorganisms are known to produce dormant spores which is their way of survival during unfavourable conditions. Sporulation can be thought of like the hibernation stage of microorganisms. Furthermore, dormant spores can be awakened again once the environmental conditions become more forgiving for survival. In relation to the concrete problem, the activation of dormant spores is triggered by the presence of moisture.
As discussed previously, microorganisms are capable of acting like mini-factories of various materials including calcium carbonate which is one of the components of cement. This innate ability of a group of microbiological creatures allows the progression of a sustainable concrete technology.
Dormant spores of carbonate-producing microorganisms are suspended into the mixture. These spores remain dormant until sufficient moisture and oxygen levels are available in the structure through micro-cracks. When they become activated, these microorganisms continuously produce carbonate crystals until all concrete cracks are closed. Filling all the cracks implies scarcity in available moisture and oxygen, so all active microorganisms sporulate and become dormant again. The cycle continues, paying forward a sustainable self-healing process.
This article answered the question “Why is concrete reinforced by steel?” It was clearly pointed out that reinforced concrete has stronger tensile strength and ductility compared to concrete alone. This blog post clarified the distinction between cement and concrete and discussed the types of stresses experienced by concrete structures. Finally, the limitations of reinforced concrete were raised together with the ongoing innovations to address it..
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 reinforced by Steel?
How does concrete protect steel?
The alkalinity of concrete environment protects steel from corrosion, deviating from the expected rusting over time. Having high pH (12-13), a layer of oxide is formed on steel and creates shield against corrosion attacks.
Why is reinforced concrete better?
In comparison to other construction materials, concrete is considerably higher in terms of compression resistance. With the help of further reinforcement, its tensile strength also increases creating a more durable structure. It is also important to note that while it has fair weather and fire resistance, reinforced concrete is the most economical material among the rest.
Disadvantages of Non-reinforced Concrete
- Concrete is Quasi-brittle Material.
- Weak tensile resistance
- Low toughness
- Low specific strength
- Formwork is required
- Longer curing time
- Prone to cracks
- Stringent quality control
Why is concrete weak in tension?
The composition of concrete such as cement, sand or rock aggregates, and water generally dictates the mechanical properties of concrete and hence its weak resistance to tension. When concrete experiences a pulling force, the binder that glues its component together easily breaks resulting in cracks and damages.
What happens if you put too much cement in concrete?
Rule of thumb says that increasing the amount of cement increases the strength of the structure. However increasing it much further will turn the material very brittle since cement particles cannot transfer normal contact force.
What is the strongest concrete mix ratio?
To achieve the maximum strength of concrete, the ratio of cement, sand, gravel, and water is 1:2:3:0.5. This means that the mixture should be 1 part cement, 2 parts sand, 3 parts gravel, and 0.5 part water.
What is the hardest cement?
The ultra-high-strength mechanical property of PFC allows it to be considered as the hardest cement. The manufacturing process of PFC translates to minimal voids which results in a strength of approximately 400 MPa as compared to a standard concrete with a strength of 20-30 MPa.
B. Pang, Z.H. Zhou, P.K. Hou, P. Du, L.N. Zhang, H.X. Xu, Autogenous and engineered healing mechanisms of carbonated steel slag aggregate in concrete. Constr. Build. Mater. 107, 191–202 (2016)
B.Q. Dong, Y.S. Wang, G.H. Fang, N.X. Han, F. Xing, Y.Y. Lu, Smart releasing behavior of a chemical self-healing microcapsule in the stimulated concrete pore solution. Cement Concr. Compos. 56, 46–50 (2015)
C. Joseph, A.D. Jefferson, B. Isaacs, R. Lark, & D. Gardner, Experimental investigation of adhesive-based self-healing of cementitious materials. Mag. Concr. Res. 62(11), 831–843 (2010)
D. Homma, H. Mihashi, & T. Nishiwaki, Self-healing capability of fibre reinforced cementitious composites. J. Adv. Concr. Technol. 7(2), 217–228 (2009)
D. Snoeck, K. Van Tittelboom, S. Steuperaert, P. Dubruel, N. De Belie, Self-healing cementitious materials by the combination of microfibres and superabsorbent polymers. J. Intell. Mater. Syst. Struct. 25(1), 13–24 (2013)
D.R. Ogunsemi, Cost control and quality standard of building projects. In D.R. Ogunsemi (ed.) Proceedings on Building Collapse: Causes, Prevention and Remedies Ondo State, Nigeria: The Nigerian Institute of Building, pp. 88–94 (2002)
H. Reinhardt, M. Jooss, Permeability and self-healing of cracked concrete as a function of temperature and crack width. Cem. Concr. Res. 33(7), 981–985 (2003)
H.M. Jonkers, A. Thijssen, G. Muyzer, O. Copuroglu, E. Schlangen, Application of bacteria as self-healing agent for the development of sustainable concrete. Ecol. Eng. 36(2), 230–235 (2010)
J.A. Akinpelu, The need for code of conduct, building regulations and by-laws for the building industry in Nigeria. The Professional Builder, Nigeria Institute of Building 2 (1): 11–14 (2002)
K. Van Tittelboom, E. Gruyaert, H. Rahier, N. De Belie, Influence of mix composition on the extent of autogenous crack healing by continued hydration or calcium carbonate formation. Constr. Build. Mater. 37, 349–359 (2012)
K. Van Tittelboom, N. De Belie, F. Lehmann, C.U. Grosse, Acoustic emission analysis for the quantification of autonomous crack healing in concrete. Constr. Build. Mater. 28(1), 333–341 (2012)
L. Lv, Z. Yang, G. Chen, G. Zhu, N. Han, E. Schlangen, F. Xing, Synthesis and characterization of a new polymeric microcapsule and feasibility investigation in self-healing cementitious materials. Constr. Build. Mater. 105, 487–495 (2016)
N. Ter Heide. Crack healing in hydrating concrete. Dissertation for the Master Degree, Delft University of Technology (2005)
S.O. Folagbade, Structural failures in domestic buildings in Nigeria: Causes and remedies. In: S.A. Amole (ed.) Proceedings of a National Symposium on The House in Nigeria. Ile-Ife: University Press, pp. 183–187 (1997)
W.H. Mosley & J.H.Bungey, Properties of Reinforced Concrete. In: Reinforced Concrete Design. Palgrave, London (1987) https://doi.org/10.1007/978-1-349-18825-3_1
W.M.K. Roddis, Structural failures and engineering ethics. American Society of Civil Engineering (Structural Div.) 119 (5): 1539–1555 (1993)