Reinforced concrete design is a process of selecting suitable concrete and steel materials in their right proportions following the engineering specifications to create different structural elements that are safe, serviceable, economical, and functional. Reinforced concrete is a combination of two different but complementary materials known as concrete and steel to form a composite material capable of supporting structural loads.
Concrete is known for its considerable crushing strength, its resistance to fire, and its durability, on the other hand, offers little or no strength in tension but is considered fair in shear resistance. Steel offers good tensile properties and poor resistance to fire due to rapid loss of strength under high temperatures and is very good in both shear and compressive resistance. Therefore the combination of these materials in the right proportions provides a good tensile and compressive strength, durability, and perfect resistance to fire and shear stress. Plain concrete itself, is a composite material that comprises cement, sand, coarse aggregates, and water. Due to its good workability, it can be easily used in many shapes and forms ranging from bulky dam walls to very thin shell roofs.
For example, when a simply supported structural member is loaded, it tends to bend, and the bottom of the remember is subjected to tensile stress and the top to compressive stress, in the case of a cantilever member, the tensile stress occurs at the top and compressive at the bottom. In the case of a simply supported member, since steel is good in tension, the reinforcement is applied at the bottom part of the member to take care of the tensile stress while the compressive stress at the top will be taken by the concrete material, this type of member is simply known as a reinforced concrete member. The process of combining these materials in their right proportions for safety, durability, serviceability, affordability, and functionality is known as reinforced concrete design.
Concrete
Concrete is a composite inert material that comprises of binder course(cement), aggregates or mineral fillers, and water. Aggregates are usually classified into fine sand or crushed stones of say 20mm in diameter depending on the job to be executed. There are two major types of concrete which include:
- Dense concrete: is the most common form of concrete for reinforced concrete work with an average density of 2,400kg /mm3
- Lightweight concrete: concrete weighing less than 1,920kg/mm3 and is made in density as low as 160kg/mm3. The different types of lightweight concrete include:
- Aerated concrete
- Lightweight aggregate concrete
- No-fines concrete
Concrete Quality Control
Just as with all other engineering operations, concrete mix at the site requires special quality control measures which should be carried out during preparation, mixing, batching, and placement of the concrete mix. The quality control of concrete should be aimed at producing a uniform material that provides properties particularly desirable for the work.
The main reason for quality control is to ensure that workmanship does not fall below a certain specified standard, therefore, limiting the overall variations in the quality of the concrete. Variations in concrete quality are due to a large number of factors such as variations in batching, variations in the quality of cement, variations in the degree of compaction, and variations in curing.
After mixing, concrete must be tested for workability either by the Slump Test or by the Compacting Factor Test. The crushing strength of the concrete is obtained by a cube test method. The cube dimensions are usually 15cm by 15cm by 15cm or 10cm by 10cm by 10cm and are adequately compacted and cured under laboratory conditions which are then crushed at the age of 7 days, 14 days, and 28 days; and the various strengths are recorded.
Steel Reinforcement
Steel reinforcement is steel bars used in reinforced concrete design to provide the required strength needed in combination with plain concrete to make it reinforced concrete. Therefore structures that have undergone this process form steel-reinforced cement concrete structures (R.C.C). Steel reinforcement is commonly known as rebars.
Reinforcement should be kept clean by stacking them off the ground, and before usage reinforcements should be free from oil, paint, mud, and loose rust which may weaken the bond with the concrete.
The Objectives of Reinforced Concrete Design
A good reinforced concrete design must satisfy the following functional objectives which include:
- The reinforced concrete structure under the worst system of loads must be safe.
- Under working load, the deformation of the structure must not damage the aesthetics or the appearance, durability, and performance of the reinforced concrete structure.
- The structure must be economically viable, such that the factor of safety is not excessively large to the extent that the cost of the structure is practically unaffordable or not economically viable.
These reinforced concrete design objectives call for a proper load assessment, the right choice of materials, and skillful workmanship. To achieve these, all the various components forming the reinforced concrete and the concrete itself must pass the various tests as specified in the standard code of practice.
The determination of the sizes of the structural members, the sizes, and the quantity of the reinforcement required to enable the structure to withstand the forces and other effects to which it will be subjected, is the main reason for a detailed reinforced concrete design.
Reinforced Concrete Design Methods
The design philosophy of reinforced concrete over the years has undergone three major stages of modifications as follows:
- The modular ratio method: in which the loads are assessed as the working or actual loads but limiting the permissible stress in the concrete and the reinforcement fraction of their actual stresses to provide an adequate factor of safety. This reinforced concrete design method is considered an alternative method and is also known as the elastic theory method. Structures such as water-retaining structures are still designed today using this method.
- The load factor method: in which the structural members are analyzed at failure, where the actual strength of a member structure is relative to the actual load causing failure which is determined by applying a factor to the design load. In this method, the ultimate strength of the material is used in the calculation, therefore no variations in materials strength are taken into account, and for this reason, it can be applied for the serviceability states to check for deflection and cracking.
- The limit state method: In this method of reinforced concrete design, the working loads are multiplied by partial factors of safety, and the strengths of the ultimate materials are divided by further partial factors of safety. With this method of design, each member must satisfy two separate criteria:
- The ultimate limit state: ensures that the probability of failure is acceptably low.
- The limit state serviceability: which ensures the satisfactory behavior of a member under working loads.
Design Codes and Specifications
A design code is a formal document that outlines the specifications for the design of developmental structures of a city or a given geographical location. It is an important design guideline used in the planning and design process. It is usually accompanied by a design rationale with a detailed explanation of the objectives. The design code provides vital guidelines and design instructions for the accuracy of detailed design work.
Structures must be designed and constructed according to the design code, which is a formal document containing the guidelines related to structural safety, fire safety, plumbing, ventilation, and accessibility to the physically disabled. A design code has the backing of the law administered by a governmental entity. Even though design codes do not give design procedures, they specify the design requirements and constraints that must be satisfied.
Design Loading
The reinforced concrete design begins with the evaluation of the self-weight of the structural members and the loads to be supported. Such loads vary both in magnitude and position. In general, there are three types of loads which include the following:
- Dead loads
- Live loads
- Environmental loads
Dead Loads:
The dead loads are the weight of the structure itself and all other structural elements that are constant in magnitude and fixed in place throughout the lifecycle of the structure. It includes the entire weight of the structure and any permanent material or equipment placed on the structure, such as roofing, cladding, tiles, wall partitions, etc. They are calculated as the product of the specific weight and the volume of the structure with a high degree of accuracy.
Live loads:
Live loads are those loads supported by a structure that may vary in magnitude and may also change in position. Live loads are usually considered mobile loads that consist chiefly of occupancy loads in buildings and traffic loads in bridges. Live loads at any given time are difficult to determine precisely due to their uncertainty, both in magnitude, distribution, and position.
Environmental loads:
These types of load in reinforced concrete design consist mainly of wind loads, snow loads, suction, and inertial forces caused by earthquake motions; likewise, soil pressure on a subsurface portion of structures, loads from possible ponding of rainwater on flat surfaces, and forces caused by temperature differences all constitute environmental loads. Like live loads, environmental loads at any given time are difficult to determine due to the uncertainty of both their magnitude and distribution.
Design Stresses
In reinforced concrete design the characteristic strength of concrete forms the concrete grade, for example, Grade 20 concrete has fcu=20N/mm2 and Grade 40 has fcu=40N/mm2 for a minimum cube strength at 28 days, while the characteristic strengths of reinforcement for mild steel round bar are 250N/mm2 and that of high yield steels are 460N/mm2.
Reinforced Concrete Design Serviceability
Serviceability for reinforced concrete design requires that:
- Deflections are kept to the barest minimum as possible.
- Cracks if any be kept to a tolerable limit.
- Vibrations are minimized to their lowest.
Advantages and Disadvantages of Reinforced Concrete
Reinforced Concrete is a widely used structural material in different types of structures. It is relatively economical when compared to steel structures if properly designed and constructed. The following are some of the pros and cons of reinforced concrete:
Advantages of Reinforced Concrete
- The compressive strength is relatively high compared to other forms of structural materials
- It is highly resistant to fire than steel
- It is durable with a low maintenance cost
- In structures, such as retaining walls, dams, bridge piers, and footings is highly an economical structural material to use.
- It can easily be cast to take any shape required, which makes it widely used in pre-cast structural elements.
Disadvantages of Reinforced Concrete
- The need to mix, cast, and cure reinforced concrete members, can negatively impact the final strength of concrete when not properly handled.
- The use of forms to cast concrete is relatively high
- It has very low compressive strength when compared to that of steel of which the ratio is about 1:10 depending on the material which leads to large sections and weight in columns and beams of multistorey buildings.
- Due to shrinkage and the application of live loads in reinforced concrete tend to develop cracks that affect the strength of the entire structure.
