Concrete is a composite material composed of coarse aggregate bonded together with a fluid cement that hardens over time. Asphalt concrete, which is frequently used for road surfaces, is also a type of concrete, where the cement material is bitumen.
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What is Concrete?
The most popular artificial material on Earth isn’t steel, plastic, or aluminum — it’s concrete. Thousands of years ago, we used it to build civilizations, but then our knowledge of how to make it was lost.
It’s the most widely-used material on our planet after water. Ton for ton, humans use more concrete today than steel, wood, plastics, and aluminum combined.
Unlike aluminum, steel or plastic, the word ‘concrete’ doesn’t refer to a single material. It can be any number of substances that combine rocks or gravel with some kind of adhesive material.
Basically, concrete is just a bunch of rubble mixed with water and cement. Together, these ingredients form rocky jello that can be poured into a mold and shaped into whatever you like. Liquid stone, it’s sometimes called.
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How is Concrete Created ?
In its simplest form, concrete is a mixture of paste and aggregates, or rocks. The paste, composed of cement and water, coats the surface of the fine (small) and coarse (larger) aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete.
Within this process lies the key to a remarkable trait of concrete: it’s plastic and malleable when newly mixed, strong and durable when hardened.
The key to achieving a strong, durable concrete rests in the careful proportioning and mixing of the ingredients. A mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete is not cost-effective and can more easily crack.
The quality of the paste determines the character of the concrete. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete, allowing it to be properly placed, consolidated, and cured.
Almost any natural water that is drinkable and has no pronounced taste or odor may be used as mixing water for concrete. Excessive impurities in mixing water not only may affect setting time and concrete strength, but can also cause efflorescence, staining, corrosion of reinforcement, volume instability, and reduced durability.
Types of Concrete?
We have different types of concrete. The following are the most common:
- Regular concrete is the lay term for concrete that is produced by following the mixing instructions that are commonly published on packets of cement, typically using sand or other common material as the aggregate, and often mixed in improvised containers. The ingredients in any particular mix depends on the nature of the application. Regular concrete can typically withstand a pressure from about 10/40 MPa, with lighter duty uses.
Typically, a batch of concrete can be made by using 1 part Portland cement, 2 parts dry sand, 3 parts dry stone, 1/2 part water. The parts are in terms of weight – not volume.
- High-strength concrete has a compressive strength greater than 40 MPa. High-strength concrete is made by lowering the water-cement (W/C) ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond. Low W/C ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used. To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes
- Stamped concrete is an architectural concrete which has a superior surface finish. After a concrete floor has been laid, floor hardeners (can be pigmented) are impregnated on the surface and a mold which may be textured to replicate a stone / brick or even wood is stamped on to give an attractive textured surface finish. After sufficient hardening the surface is cleaned and generally sealed to give a protection.
- High-performance concrete (HPC) is a relatively new term for concrete that conforms to a set of standards above those of the most common applications, but not limited to strength. While all high-strength concrete is also high-performance, not all high-performance concrete is high-strength.
Info source: https://en.wikipedia.org/wiki/Types_of_concrete
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What are the stage of Concrete working?
Mixing of concrete is a very important step for achieving good final properties. Mixing distributes the aggregate evenly throughout the cement paste, ensures that all of the cement has been fully saturated in water, and removes large air voids. The lower the workability, the more mixing energy and mixing time is required.
Once the concrete has been adequately mixed, it must be placed into the formwork that defines its final position and shape. If the concrete is to be reinforced, the rebar must already be in place so the concrete can flow around it. Concrete that is to be pumped has more stringent requirements for workability. If the concrete is too dry, it will not pump well, while if it is too wet it will tend to segregate.
Once the concrete is in place, it should be consolidated to remove large air voids developed during placement and to make sure that the concrete has flowed into all of the corners and nooks of the formwork. This process is also called compacting. The two most common methods of consolidation is are vibration and roller compacting. Vibration is a mechanical process that transfers pulses of shear energy to the concrete. Roller compaction is a simpler and more cost-effective technique that is suitable for roads and very large mass concrete structures such as dams. A specialized vehicle with a heavy roller on the front is driven over the fresh concrete to drive it into place and remove excess air. The fresh concrete used is very stiff so that it can support the weight of the machine as it passes over.
Finishing refers to any final treatment of the concrete surface after it has been consolidated to achieve the desired properties. This can be as simple as pushing a wide blade over the fresh concrete surface to make it flat (screeding). Floating and troweling is a process of compacting and smoothing the surface which is performed as the concrete is starting to harden. This would be standard procedure for driveways and sidewalks.
Once concrete has been placed and consolidated it must be allowed to cure properly to develop good final properties. As the concrete hardens and gains strength it becomes less and less vulnerable, so the critical time period is the first hours and days after it is placed. Proper curing of concrete generally comes down to two factors, keeping it moist and keeping it supported. Hydration of cement, as the word itself implies, involves reaction with water. To cure properly, the cement paste must be fully saturated with water. Not only will this prevent the concrete from gaining its full strength, but it will also generate internal stresses that can cause cracking. To keep fresh and young concrete moist, it can be covered with plastic or damp fabric to prevent evaporation, or sprayed periodically with water.
info and image source: http://iti.northwestern.edu/cement/monograph/Monograph2_3.html
What are Concrete characteristics?
Strength and Durability
Locally Produced and Used
Energy Efficiency in Production
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How was Concrete used throughout history?
Over two thousand years ago, at the height of the Roman empire, the port city of Pozzuoli was a buzzing center of military activity and commerce. Every day, ships left Pozzuoli laden with useful goods, including grains, iron, weapons, and pozzolana, an ashy volcanic sand formed in the nearby supervolcano Campi Flegrei.
Why were the Romans exporting volcanic spew to the far corners of the known world? It so happens that this sand was special. Mix it with water, and it would form a mortar strong enough to bind lumps of rock together into an impenetrable, load-bearing material. As Roman philosopher Seneca noted, the “dust at Puteoli [the city’s Latin name] becomes stone if it touches water.” Nobody knew why.
By sheer luck, the Romans had built a city atop a natural cement factory. Turns out, pozzolana is a mixture of silica oxides and lime, two of the three key ingredients in cement (the third being water). It wasn’t until this year that a Stanford geochemist worked out how this unusual ash forms.
The deep interior of Campi Flegrei’s caldera is padded in limestone, a soft, crumbly rock composed of calcium carbonate (CaCO3). As geothermally-heated water washes over the caldera’s limestone walls, it triggers a decarbonation reaction, releasing CO2 gas and leaving behind calcium hydroxide, otherwise known as hydrated lime.
Circulating geothermal fluids inside Campi Flegrei bring some of this lime closer to the surface, where it combines with silica-rich ash to form an impenetrable, cement-like caprock. But eventually, enough pressure builds up inside the volcano that this caprock bends and breaks. When that happens, the same cement-forming ingredients spew skyward, as pozzolana ash.
Geochemist suspects the ancient Romans first watched pozzolana hardening into cement in the seawater surrounding Campi Flegrei. They co-opted the natural process, mixing in small chunks of pumice — a porous volcanic rock that forms when superheated magma is quickly cooled. And just like that, Roman concrete was born. It became an iconic building material of the ancient world, and it’s the reason many Roman structures, including the Colosseum and the Pantheon, have survived to the present day.
After the fall of the Roman empire, the art of concrete-making was all but forgotten. It gradually returned centuries later, but didn’t become widespread again until 1824, when Joseph Aspdin developed and patented Portland cement.
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