Reinforced Cement Concrete is basically Concrete in which steel bars of desirable magnitude are introduced in the casting stage so that the resulting composite material resists the stresses developed due to the external loads.
In flexural members, steel reinforcement is generally provided near the tension face to resist tensile stresses, as the tensile strength of Concrete is approximately one-tenth of its compressive strength. In the case of compression members, the steel reinforcement is distributed uniformly in the cross-section to resist the compressive stresses developed due to the external loads.
The two materials – Concrete and steel – act together in resisting external forces, such as wind, vibrations, earthquakes, and all other causes of tensile and shear stresses, thus preventing the concrete from cracking, crumbling, or breaking altogether.
The revolutionary engineering concept of reinforcing the weak tensile zone of Concrete with steel reinforcement was developed in the mid-nineteenth century. The early 20th century witnessed significant improvements in the development and use of reinforced Concrete, mainly due to the production of good-quality Concrete with improved strength and improved quality of steel with surface characteristics suitable for developing a good bond between Concrete and steel.
The success of reinforced Concrete as a revolutionary material for use in various types of structures is mainly due to the improved quality of Concrete and steel over the years, and also the improved bond characteristic between the two ingredients.
Historical Development of Reinforced Cement Concrete
The current state of development in the field of reinforced Concrete is mainly due to the continuous research conducted by scientists and engineers in this field over the last 150 years. Isaac Johnson first made the prototype of modern cement in 1845 by burning a mixture of clay and chalk until clinkering, so that reactions necessary for forming potent cementitious compounds are complete.
The early 20th century witnessed the development of mass production of good-quality cement. Currently, Ordinary Portland cement of various strengths, designated as C-33, C-43, and C-53, is available for use in multiple types of structures. Other types of cement with specific properties have contributed to the working loads.
Several Investigators, such as Emperger (1936), Whitney (1937), Jenson (1943), Chambaud (1949), and Hognestad (1951), developed the ultimate load theory based on different types of stress blocks. Reinforced Concrete structures designed solely based on ultimate load theory resulted in slender structural elements, and their serviceability characteristics (deflections and cracks) under working loads were not within codified acceptable limits.
The ultimate load method of design ensures the safety of structures against the collapse limit state only. As such, it does not provide any information about the behaviour of the structure at service loads or the range between service and collapse loads. The inadequate ultimate load method did not ensure the serviceability of the structure, resulting in the development of a limit state design.
The philosophy of limit state design was first incorporated into the Russian code in 1955. Basically, limit state design is a method of designing structures based on a statistical concept of safety and the associated statistical probability of failure. Limit state design is based on the idea of probability and involves the application of statistical methods to account for the variations that occur in practice in the loads acting on the structure and the strength of the materials.
The Limit state design overcomes the inadequacies of the working stress and ultimate load methods, ensures the structure’s safety against excessive deflections and cracking under service loads, and provides for the desired load factor against failure. Hence, the British Code, American Code, Australian Code, German Code, Euro Code, and Indian Code have adopted the limit state design concepts.
Philosophy of Structural Design
The main objective of reinforced Concrete structural design is to comply with the following essential requirements.
- Structures designed should satisfy the criterion of desirable ultimate strength, in flexure, shear, compression, tension, and torsion developed under a given system of loads and their combinations. In addition, the stresses developed in the structure under the given system of loads should be within the safe permissible limits under service loads.
- The structure designed should satisfy the criterion of serviceability, which limits the deflections and cracking to be within acceptable limits. The structure should also have adequate durability and impermeability, as well as resistance to acids, corrosion, and frost.
- The structure should have adequate stability against overturning, sliding, buckling, and vibration under the action of loads.
A satisfactory structural design should ensure that it meets the three basic criteria of strength, serviceability, and stability. In addition, the structural Engineer should also consider aesthetics and economy. The structural engineer and the architect should coordinate so that the designed structure is not only aesthetically superior but also strong enough to safely sustain the intended loads without any distress during its lifetime.
Applications of Reinforced Cement Concrete
Reinforced concrete is well established as an essential construction material often preferred to steel construction, mainly due to its versatility, adaptability, and resistance to fire and corrosion, resulting in negligible maintenance costs. The development of higher-quality cement over the last decade has led to stronger and more durable Concrete for use in various types of structures.
- Reinforced Concrete is ideally suited for the construction of floor and roof slabs, columns, and beams in residential and commercial structures.
- The current trend is to adopt reinforced Concrete for bridges of small, medium, and long spans, resulting in aesthetically superior and economical structures compared to steel bridges.
- Typical applications of reinforced Concrete in earth-retaining structures include abutments for bridges and retaining walls for earthen embankments.
- Reinforced concrete is ideally suited for water-retaining structures like ground and overhead tanks, and hydraulic structures like gravity and arch dams. The material is widely used for the construction of large domes for water tanks, sports stadiums, and conference halls.
- Reinforced Concrete grid floors, comprising beams and slabs, are widely used for covering large areas, such as conference halls, where column-free space is an essential requirement.
- For aircraft hangars, reinforced Concrete shells comprising thin circular slabs and deep edge beams provide an economical solution.
- Reinforced concrete folded plate construction has been used for industrial structures where an ample column-free space is required under the roof.
- In coastal areas where corrosion is imminent due to the humid environment, reinforced Concrete is ideally suited for constructing marine structures such as wharves, quay walls, watchtowers, and lighthouses. For warehouses in coastal areas, reinforced concrete trusses are preferred over steel trusses.
- Reinforced Concrete poles have almost replaced steel poles for power transmission. Tall towers for TV transmission are invariably constructed using reinforced Concrete.
- Multistorey reinforced Concrete buildings are routinely adopted for both residential and office complexes. For heavy-duty floors in factories, reinforced Concrete is ideally suited due to its resistance to wear and tear, as well as its improved durability.
- In atomic structures, reinforced Concrete is preferred over steel for pressure vessel construction due to the superior radiation absorption characteristics of high-strength, high-density Concrete.
- Reinforced Concrete piles, both precast and cast-in-situ, have been in use for the foundations of structures of different types, like bridges and buildings.
- Another novel application of reinforced Concrete is in the construction of pavements for highways and airport runways.
The Twentieth century has witnessed reinforced Concrete as a revolutionary material suitable for the construction of the most simple to complex structures. With significant improvements in the quality of cement and steel, reinforced concrete is expected to continue finding new applications and widespread use in the 21st century.
Reinforced Concrete Structural System
Any structure can be considered an assemblage of various structural elements that perform a predetermined function of resisting multiple types of forces. Essentially, a structure can be constructed using both structural and non-structural components. The structural elements (beams, slabs, columns, etc.) have the primary function of resisting the external loads. In contrast, the nonstructural elements (partitions, false ceilings, doors, etc.) do not support the external loads.
Basically, structural elements can be classified into one-dimensional elements (such as beams, columns, arches, etc.), two-dimensional elements (such as slabs, plates, shells, etc.), and three-dimensional elements (such as thick pipes, walls of nuclear reactor vessels, domes, etc.).
Circular girders, generally used in water tanks, are subjected to combined flexure, shear, and torsion, while the corner columns in a multistorey framed structure are subjected to biaxial bending.
- One-way Slab Systems
The figure below illustrates the floor system, comprising a one-way slab supported at the edges by walls or beams, which in turn support both dead and live loads. The slabs are subjected primarily to maximum flexure at the centre of the span along the shorter direction and maximum shear at supports under gravity loads.
- Two-Way Slab Floor Systems
The figure below shows a typical two-way slab floor system commonly used in buildings. In this case, the slab is supported at the edges and it is subjected to flexure in two principal directions while resisting gravity loads.
- Beam and Slab floor systems
The figure above shows a typical beam and slab floor system generally used in residential and commercial building structures. In this case, the gravity loads are resisted by the flexure of the slab and beams.
- Flat Slab Floor System
The figure above shows a flat slab floor system in which the slab is supported directly on columns without any intervening beams. This type of floor system is generally preferred for large-span office complexes, commercial buildings, and garages, where headroom is less.
- Grid Floor System
The figure above shows a typical grid floor system comprising beams spaced at short intervals running in perpendicular directions and supporting a thin slab. This type of roof is generally used for large conference halls and commercial buildings that require column-free space. The grid floor is supported on solid walls or columns at regular intervals.
- Multistorey Vertical Framing System
The figure shows a multistory vertical frame comprising columns, beams, and slabs, forming a three-dimensional structure. The gravity loads are transmitted from the slab to the beams, which in turn transfer the loads to the columns and ultimately to the foundations. The rigid column and beam frame can resist lateral loads due to wind.
- Shear Wall System
This system features solid concrete walls that cover the entire height of the building. Generally, the shear wall box is located in the lift/ staircase regions. Sometimes the shear walls are located as Exterior or interior walls placed along the transverse direction of the tall building to resist lateral loads due to wind.
Design Codes
- Objective of Codes
Based on extensive research and practical knowledge, various countries have evolved their national codes, which serve as guidelines for the design of structures. The main objectives of the codes are
- To provide adequate structural safety by ensuring strength, ability, and durability.
- To specify simple design procedures, design tables and formulae for easy computations.
- To provide legal validity and to protect structural engineers from any liability due to failures of structures caused by inadequate design, improper materials, and a lack of proper supervision during construction.
- To provide a uniform set of design guidelines to be followed by various structural designers in a particular country.
National building codes are periodically revised to reflect improvements in the quality of materials and design procedures that have evolved as a result of comprehensive research investigations conducted in institutions both in the country and abroad.
2. Design Code
All reinforced Concrete structural designs in India should conform to the recently revised Indian Standard Code IS:456-2000, Code of Practice for Plain and Reinforced Concrete (Fourth Revision). The corresponding national codes of other countries are often referred to as the American Concrete Institute Code (ACI-318) and the British Code BS 8110 of 1985 and 1995, as well as the Eurocode.
Loading Standards
- Dead loads
These are loads that remain constant over time. The dead loads acting on the structure include the self-weight of the structural elements, partitions, and finishes, which depend upon the type of material used in the structure.
- Live loads
These are loads that change over time. Live or imposed loads include the loads due to people occupying the floor and those due to materials stored or vehicles on garage floors.
- Wind loads
Wind loads must be considered in the design of multi-story buildings, towers, and poles.
- Snow Loads and Local Combinations
Structures subjected to snow loads must be designed suitably, considering the prevailing snow loads in the region and various load combinations.
- Earthquake Loads
Seismic or earthquake forces have to be considered in the design of structures located in seismic zones.
Advantages of Reinforced Cement Concrete
The following are the advantages of Reinforced Concrete
Strength: Reinforced Concrete has outstanding strength in tension as well as compression.
Durability: Reinforced Cement Concrete structures are durable if designed and laid correctly. They can last up to 100 years.
Mouldability: Reinforced Cement Concrete sections can be given any shape easily by properly designing the formwork. Thus, it is more suitable for architectural requirements.
Ductility: The steel reinforcement imparts ductility to the Reinforced Cement Concrete structures.
Economy: Reinforced Concrete is cheaper than steel and prestressed Concrete. There is an overall economy in using R.C.C. because its maintenance cost is low.
Transportation: The raw materials required for Reinforced Cement Concrete, i.e., cement, sand, coarse aggregate, water, and steel, are readily available and can be transported easily. Nowadays, Ready Mix Concrete (RMC) is used for faster and better construction. (RMC is Concrete that is manufactured in the factory and transported to the site in a green or plastic state).
Fire Resistance: Reinforced Cement Concrete structures are more fire-resistant than other commonly used construction materials like steel and wood.
Permeability: Reinforced Cement Concrete is almost impermeable to moisture.
Maintenance: Concrete structures require low maintenance after completion of work as compared to structures made of steel and timber.
Rigidity; As reinforced Concrete is stiff, it provides good rigidity.
Disadvantages of Reinforced Cement Concrete
Reinforced Concrete has the following disadvantages :
- Reinforced Concrete structures are heavier than structures of other materials like steel, wood, glass, etc.
- Reinforced Concrete needs a lot of formwork, centring, and shuttering to be fixed, thus requiring a lot of site space and skilled labour.
- Concrete takes time to attain its full strength. Thus, unlike steel structures, R.C.C. structures can’t be used immediately after construction.
We hope this article helped you learn about Reinforced Cement Concrete. You may also want to know about Home Furniture | Meaning and Type, Best Engineering Books For Engineers, Home Furniture | Meaning and Type, and The Best Garden Hoses of 2022
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