- Concrete is a composite material composed mainly of cement, sand and water along with other additives such as aggregates and gravel. In modern age, concrete is the most vital and significant building material, especially if steel is enforced into to it to result in a type called reinforced concrete.
- The Romans were the first to use plain concrete in the history of Man, that is, about two thousand years ago. Concrete was used in most of the Romans' buildings because of its malleability and workability and ability to handle it by simple trained workmanship.
- Concrete is the result of mixing raw materials such as sand, gravel and cement and adding water to such mass. When they are mixed properly, the process of their bonding is the setting time.
- Concrete is featured by many properties that make it special and excel other types of mixed masses. By time, concrete becomes solid and hardens. Concrete starts to become harder and solider from the initial setting to the final setting. Whereas concrete is strong in compression, it is weak in tension. Thus, plain concrete ( non-reinforced one) should not be used in structures with tensile stress or loads ( such as beams).
- To overcome this problem, steel bars are placed in the concrete. Steel is an excellent resistant to tension and compression. While long steel bars have high strength in tension, concrete cannot stand compressive actions if its section are slinder; as such, the result is concrete buckling.
- Therefore, we do realize that a fine and proper composite mixture of concrete and steel bars would result in an ideal product that resists all sorts of applied or exercised stresses. This product is known as reinforced concrete.
Most Commonly Significant Concrete Types
In the modern era are various types of concrete as a result of its components and applications; Some include but not limited to the followings:
- Mortar Concrete: It is the result of mixing gravel, sand and cement.
- Concrete: It is the type of concrete composed of a mixture of natural gravel (crushed stone) with sieved, impurity-free and clean sand and very soft powder and cement. The mixture is composed of technically well-known ratios.
- Plain Concrete: It is the type of concrete that might be called self-cleaning concrete and should not be placed in structures with loads. It is usually used to fill in cracks and spaces/voids and to stabilize the soil/ground under or around a reinforced construction or structures.
- Reinforced Concrete: It is the type of concrete reinforced with steel bars. This type of reinforcement is designed by specialized civil engineers. This is in order to produce a structure composed of reinforcing materials usually steel bars that could be stronger and capable of withstanding large weights (such as bridges, roofs, high-rising buildings ...).
- There are also other types of reinforced concrete that have special uses and properties such as the followings:
- Underwater Reinforced Concrete Pour
- Fireproof Reinforced Concrete.
- Reinforced Concrete Against Atomic Radiation.
- Reinforced Concrete for Dams.
- Reinforced Concrete Resistant to Bombs.
- Anti-Seismic Reinforced Concrete
- Colored Reinforced Concrete.
Mixing of Concrete:
- Prior to mixing of concrete ingredients, it is important to ensure that they are clean and free from any impurities. Sand and gravel must be cleaned and freed from any irrelevant materials by shaking them in a sieve or screening them. They must be washed before mixing them. Excessive impurities such as clay, organic matter, sulfates, and phosphates will cause the corrosion of reinforcement and reduce its durability and strength. There are two manners to follow in mixing the ingredients to have a concrete product. These are
- Manual Concrete Mixing Process: After cleaning the sand and gravel, ingredients should then be stored in a safe and protected place to avoid humidity. The concrete is then mixed manually by using a shovel to produce small amounts of concrete..
Mechanical Concrete Mixing Process:
Concrete is mixed in the desired ratios by the use of concrete mixers having certain and special capacities whose size or volume suits the process of pouring and transporting the required quantity.
The number of concrete mixers at work site must meet the type and nature of work and the amount of concrete needed. While some types of concretes are produced mechanically by mixers at site, others are made by Ready Mix plants mixes all ingredients ( except water) and dispenses the concrete into a concrete mixer truck.
Properties of Hardened Concrete:
Having a resistance to compressive stresses/ compressive resistance is one of the most important features of hardened concrete. This property reflects a concrete's high quality and durability. As such, this is the major resistance of typical concretes. Other properties and resistances such as resisting tension, bending and shearing, and enhancing bonding are augmented and improved by embedding reinforcement steel bars and increase by increasing resistance to compression and vice-versa.
Thus, concrete compression testing is done to obtain quality control of concrete production. This sort of testing is used by civil engineers to determine the type of civil design with the ultimate resistance of concrete working stress in the compression. This is taken as a percentage of the ultimate compressive strength. In addition, this testing is used to determine the validity of aggregates and the water used for mixing in order to see the effect of any existing impurities on concrete resistance to compression.
Commonly known now is that compressive resistance of concrete used in conventional structures ranges between 250-350 kg / cm² . But special designed concrete and precast concrete for certain structures have a bigger compressive resistance amounting up to 500 kg / cm² . Yet, prestressed concrete structures must have a compressive resistance exceeding 400 kg / cm² and at times, amounting up to 600 kg / cm².
Tensile strength varies depending on the life cycle of the concrete:
- Hardened plain concrete withstands higher compression. , it is weak in tension. Thus, it is designed to resist compressive stresses. But it is not strong enough to resist tension (whether direct or indirect tension).
Such concretes are regarded as having weak tensile strength if compared to its compressive strength. This is so because concrete is a fragile product. Thus, researchers have carried out many studies to examine and overcome this concrete defect ( tensile strength of concrete). This is because most cracks and fractures happening in concrete are the outcome of its week tensile strength. Concrete tensile strength ranges between 7% to 14% of its overall resistance to any stresses and compression.
This means an average rate of 10% which varies in accordance with a concrete life cycle. It also depends on the type and grade or rank of a concrete.
- It is noted that as the higher the concrete's compressive strength, the lower its relative tensile strength becomes. This continues compressive strength accounts for about about 800 kg / cm² ; only then, tensile strength reaches its maximum value ranging between 60 to 70 kg / cm².
Bending Resistance :
When a concrete beam undergoes some sort of bending force or loads, the bending strengthen could be calculated (which is also considered as gauging for indirect tensile strength). This is also called the flexure measurement of bending. The values of flexure stresses of bending range between 12% - 20% out of total compressive strength.
Therefore, bending strength is higher than tensile strength of the concrete by a percentage ranging between 60 to 100%. In general, the tensile strength of concrete is 60% of the total value of bending resistance.
Thus, it is plain that bending resistance is higher than tensile strength by about 40%. Bending tests are performed to determine the resistance of hardened concrete, study the behavior of concrete beams when exposed to bending loads and the form of flexure resulting from the failure of beams.
Direct Shear Strengths:
These are two types of parallel and equal strengths influencing two surfaces or levels within a small distance between them. These strengths are accompanied by bending moments, that is, tensile stresses and compression. It is therefore rare to perform a test on direct shear resistance of concrete. This is so because in application, a concrete is hardly exposed to pure shear. A concrete, instead, is exposed to shear associated with bending. It has been proven that concrete shear resistance is higher than its tensile strength by a percentage ranging between 10 to 12% of the total compressive resistance.