The truth about the world's most argued flyover

Reinforced concrete is one of the most essential and most commonly used construction material during the last century, but it didn't always have reinforcement in it, the combination of concrete and reinforcing steel is quite new. Concrete alone existed for a long time, it actually existed a millennium ago, to be specific.

In this series of articles we will get to know more about concrete, how it started?, what are it's components?, how do these components react?, ... etc., so let's begin.

What is concrete?

Concrete is the world's most heavily consumed material; eight billion cubic meters of concrete poured each year that's equivalent to more than one cubic meter per person on the planet every year. Concrete is made from three principal ingredients from cement, aggregate, and water and those materials are mixed together to produce a liquid that can be poured into almost any shape and with time, turns into a rock like material.

The word “concrete” is derived from the Latin "concretus", meaning “to grow together.” Concrete is a composite material composed of coarse granular material (the aggregate or filler) embedded in a hard matrix of material (the cement or binder) that fills the space among the aggregate particles and glues them together.

Alternatively, we can say that concrete is a composite material that consists essentially of a binding medium in which are embedded particles or fragments of aggregates. The simplest definition of concrete can be written as:

concrete = filler + binder  

The Pantheon's dome - Italy | Source

Almost two thousand years after it was built, the Pantheon's dome is still the world's largest unreinforced concrete dome.

Why is concrete so popular?

Hoover Dam - US | Source

Concrete is popular for many reasons:

1. Economical: Concrete is the most inexpensive and the most readily available material in the world. The cost of production of concrete is low compared with other engineered construction materials. The three major components in concrete are water, aggregate, and cement. Compared with steels, plastics, and polymers, these components are the most inexpensive, and are available in every corner of the world. This enables concrete to be produced worldwide at very low cost for local markets, thus avoiding the transport expenses necessary for most other materials.
2. Ambient temperature-hardened material: Because cement is a low-temperature bonded inorganic material and its reaction occurs at room temperature, concrete can gain its strength at ambient temperature. No high temperature is needed.
3. Ability to be cast: Fresh concrete is flowable like a liquid and hence can be poured into various formworks to form different desired shapes and sizes right on a construction site. Hence, concrete can be cast into many different configurations. One good example to show concrete castability is the Baha’I Temple located in Wilmette, Illinois, USA. The very complex configurations of the different shapes of flowers in the wall and roof are all cast by concrete.
4. Energy efficient: Compared with steel, the energy consumption of concrete production is low. The energy required to produce plain concrete is only 450–750 kWh/ton and that of reinforced concrete is 800–3200 kWh/ton, while structural steel requires 8000 kWh/ton or more to make.
5. Low technology production: Concrete is a low technology system; complicated equipment isn't required concrete.
6. Excellent resistance to water: Unlike wood (timber) and steel, concrete can be hardened in water and can withstand the action of water without serious deterioration, which makes concrete an ideal material for building structures to control, store, and transport water, such as pipelines, dams, and submarine structures. A typical example of a pipeline application is the Central Arizona Project, which provides water from the Colorado river to central Arizona. The system contains 1560 pipe sections, each 6.7 m long, 7.5 m outside diameter, and 6.4 m inside diameter. Contrary to popular belief, water is not deleterious to concrete, even to reinforced concrete; it is the chemicals dissolved in water, such as chlorides, sulfates, and carbon dioxide, that cause deterioration of concrete structures. 
7. Ability to work with reinforcing steel: Concrete has a similar value to steel for the coefficient of thermal expansion (steel 1.2 × 10^-5; concrete 1.0–1.5 × 10^-5). Concrete produces a good protection to steel due to existence of CH (calcium hydrate) and other alkalis (in normal conditions). Therefore, while steel bars provide the necessary tensile strength, concrete provides a perfect environment for the steel, acting as a physical barrier to the ingress of aggressive species and giving chemical protection in a highly alkaline environment (pH value is about 13.5), in which black steel is readily passivized. Concrete is usually strong in compression and stiff but it's weak in tension and we can get over that by reinforcing it to produce a material that is both strong in compression and tension.
8. Durability: Concrete is durable in a wide range of environments not just the building's we live in, but in nuclear power stations, in offshore structures, in high-temperature furnaces.
9. High-temperature resistance: Concrete conducts heat slowly and is able to store considerable quantities of heat from the environment. Moreover, the main hydrate that provides binding to aggregates in concrete, calcium silicate hydrate (C–S–H), will not be completely dehydrated until 910oC. Thus, concrete can withstand high temperatures much better than  wood and steel. Even in a fire, a concrete structure can withstand heat for 2–6 hours, leaving sufficient time for people to be rescued. This is why concrete is frequently used to build up protective layers for a steel structure.
10. Less maintenance required: Under normal conditions, concrete structures do not need coating or painting as protection for weathering, while for a steel or wooden structure, it is necessary. Moreover, the coatings and paintings have to be replaced few years. Thus, the maintenance cost for concrete structures is much lower than that for steel or wooden structures.

Baha’i Temple - USA 

Has concrete been around for long time?

As early as around 6500 BC, concretes were used by the Syrians and spread through Egypt, the Middle East, Crete, Cyprus, and ancient Greece. However, it was the Romans who refined the mixture’s use. The cements used at that time were gypsum and lime. The Romans used a primal mix for their concrete. It consisted of small pieces of gravel and coarse sand mixed with hot lime and water, and sometimes even animal blood. The Romans were known to have made wide usage of concrete for building roads. It is interesting to learn that they built some 5300 miles of roads using concrete. Concrete is a very strong building material. Historical evidence also points out that the Romans used pozzalana, animal fat, milk, and blood as admixtures for building concrete. To trim down shrinkage, they were known to have used horsehair. Historical evidence shows that the Assyrians and Babylonians used clay as the bonding material.

Concrete has been around for a long time. You might think of concrete as a modern material but in fact it's been used for thousands of years. Perhaps the most famous example of its use from the ancient world was the domed roof of the Pantheon in Rome; a 43 meter diameter un-reinforced concrete dome that is today in perfect condition and is probably stronger now than it's ever been. The use of concrete really picked up around a hundred years ago when steel reinforcing rods became available and that meant that concrete beams could be produced that would span farther than timber and were also more fire resistant.