Essay: Concrete

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  • Published on: December 27, 2019
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  • Concrete
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Concrete

It is a composite material composed mainly of water, aggregate, and cement. Often, additives and reinforcements (such as rebar) are included in the mixture to achieve the desired physical properties of the finished material. When these ingredients are mixed together, they form a fluid mass that is easily molded into shape. Over time, the cement forms a hard matrix which binds the rest of the ingredients together into a durable stone-like material with many uses

3.2 Ingredients

The concrete consisting of cement, sand and coarse aggregates mixed in a suitable proportions in addition to water is called cement concrete. In this type of concrete cement is used as a binding material, sand as fine aggregates and gravel, crushed stones as coarse aggregates.
Admixtures are added before or during mixing of concrete in order to achieve certain goals which are explained below.
The two basic types of admixtures available are:
• mineral admixture
• Chemical admixture
Mineral admixture
They are usually added to concrete in larger amounts for the following reasons:
• to enhance the workability of fresh concrete
• to improve resistance of concrete to thermal cracking, alkali- aggregate expansion and sulfate attack
• to enable a reduction in cement content
Examples of mineral admixture are fly ash, silica fume [SF], and ground granulated blast furnace slag.
Chemical Admixtures
Chemical admixtures are added to concrete in very small amounts mainly for:
• entrainment of air
• reduction of water or cement content
• plasticization of fresh concrete mixtures
• control of setting time
Types of chemical admixtures
The different types of chemical admixtures are as follows:
Accelerating admixture: Accelerating admixtures (also known as accelerators) are added to concrete to reduce the setting time of concrete and to accelerate early strength. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.
Examples: calcium nitrate (Ca (NO3) 2) and sodium nitrate (NaNO3).
Retarding admixtures: Retarding admixtures are often used in hot weather conditions to delay the setting time of concrete. Examples: Calcium Sulphate (CaSO4), sugar, starch.
Water reducing admixture: This type of admixture is added to reduce the water content by around 5% to 12% in a mixture while maintaining certain amount of consistency.
Plasticizers: These are organic or a combination of organic and inorganic substances, which allow water reduction for a given workability.
Examples: hydro carbolic acid and lignosulfonate
Super plasticizers: They are high range water reducers and reduce the water amount in cement by around 12% to 30%
Examples: melamine formaldehyde, naphthalene sulphonate formaldehyde.
Air Entraining Admixtures: Air entraining admixtures must be used whenever concrete is exposed to freezing and thawing. Air entraining agents entrain or trap in microscopic air bubbles in the concrete. When the hardened concrete freezes, the frozen water inside the concrete expands into these air bubbles instead of damaging the concrete.
The air entrainment may be done by
 surface active agents, (E.g. Natural wood resins, Animal or vegetable fats, oils such as o1ive oil)
 chemicals (Ex: zinc or aluminium powder)
 cement dispersing agents (Ex: Calcium lignosulphonate)
Bonding admixtures: They are added by 5% to 20% to concrete for increasing the bond strength between old and new concrete.
Ex: rubber, polyvinyl chloride, polyvinyl acetate, acrylic polymers.
(a) Adhesives should have high tensile strength, cohesive strength and fluidity to bond two surfaces strongly. Adhesives used in construction are called structural adhesives. They are classified based on the source they are obtained from. Following are the three types of adhesives in use.
(b) Natural adhesives ( also known as organic adhesives)
Natural adhesives are made from organic sources such as vegetable starch, natural resins or animals (milk protein)
(c) Synthetic adhesives: These are chemically manufactured adhesives. Vinyl, acrylics, rubbers and epoxy resins are some examples of synthetic adhesives.
All structural adhesives fall under the following groups:
• Thermoplastic
• Thermosetting
Thermoplastic adhesives are used in wood, glass, metals and plastics
Examples of thermoplastic structural adhesives are
• PVA – polyvinyl acetate (wood and general)
• Cyano-acrylate (plastics, metals, glass)
• Acrylics (metal, rubber)
Thermosetting adhesives are used in glass fibre, mortars, metal- to-metal and sandwich panel construction. The following are some common examples of thermosetting structural adhesives:
• Epoxy (mainly concrete, metal-to-metal)
• Polyurethane (sandwich panel construction, semi-structural uses with plastic, metal and wood)
• Phenol-formaldehyde (chipboards and plywood)
• Unsaturated polyesters (fibre-glass, resin mortar)

The above structural adhesives can also be classified on the basis of the materials they are used on
Classification of adhesives based on materials
Metal Concrete & mortar Glass & glass fibre Plastic & rubber Timber & general
Epoxy Epoxy Phenol-formaldehyde Unsaturated Polyester Cyano-acrylate
Acrylic Unsaturated Polyester PVA Cyano-acrylate Acrylic
Cyano-acrylate Polyurethane Polyurethane Polyurethane

Adhesives

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