All about Concrete

• Concrete is any product or mass made using a cementing medium, generally the product of reaction between hydraulic cement and water.
• Concrete can contain pozzolan, fly ash, blast-furnace slag, micro-silica, additives, recycled concrete aggregate, admixtures, polymers, and fibers.
• Concrete can be heated, steam-cured, autoclaved, vacuum-treated, hydraulically pressured, shock-vibrated, extruded, and sprayed.

Good Concrete

• Good concrete possesses satisfactory qualities in both its hardened and fresh states.
• In its fresh state, the mix should be compactable by available means and cohesive enough to be transported and placed without segregation.
• Desirable hardened state properties include satisfactory compressive strength, density, impermeability, durability, resistance to abrasion, impact, tensile strength, and resistance to sulfates, but not necessarily creep or shrinkage.

Cement

• Portland cement is made by intimately mixing calcareous and argillaceous materials and burning the mixture.
• Portland cement is made from a combination of calcareous material, such as limestone or chalk, and silica and alumina, found as clay or shale.
• The manufacturing process involves grinding raw materials into a fine powder, mixing them intimately, and burning them in a rotary kiln at about 1400°C (2550°F).
• The material sinters and partially fuses into clinker.

Main Compounds in Portland Cement

• Tricalcium Silicate (3CaO.SiO2) is abbreviated as C3S.
• Dicalcium Silicate (2CaO.SiO2) is abbreviated as C2S.
• Tricalcium Aluminate (3CaO.Al2O3) is abbreviated as C3A.
• Tetracalcium Aluminoferrite (4CaO.Al2O3.Fe2O3) is abbreviated as C4AF.

Oxide and Compound Composition of Portland Cement

• CaO makes up 63% with a calculated compound composition of 10.8%.
• SiO2 makes up 20% with a calculated compound composition of 54.1%.
• Al2O3 makes up 6% with a calculated compound composition of 16.6%.
• Fe2O3 makes up 3% with a calculated compound composition of 9.1%.

Fineness of Cement

• Fineness is a vital property of cement because the rate of hydration relies on the fineness of cement particles
• BS and ASTM require the determination of the specific surface (in m²/kg).
• The specific surface of cement can be determined by the air permeability (Lea and Nurse) method (BS EN 196-6: 1992).

Consistence of Standard Paste

• Standard consistence is achieved with a water content that gives a standard consistence between 26 and 33 (expressed as a percentage by mass of dry cement).

Setting Time

• Setting refers to the change from a fluid to a rigid state.
• For the determination of initial set, a minimum time being measured from adding the mixing water to the cement of 45 min is prescribed by BS EN 197-1.
• A minimum setting time of 60 min is prescribed for Portland cements (ASTM C 150-05).
• A maximum of 10 hours are allowed for the final set for Portland cements as per American Standards

Soundness

• After setting, it is essential that cement paste does not undergo large volume changes.
• Le Chatelier's accelerated test is prescribed by BS EN 196-3: 1995 for detecting unsoundness due to free lime only.

Main Types of Portland Cement

• Ordinary Portland corresponds to to Type I in American standards, and Type (CEM) I in European classification.
• Rapid-Hardening Portland corresponds to Type III in American standards and Type IIA in European classification.
• Type IIB-S is a modified cement, corresponding to Type II in American standards.

Compound Composition of Portland Cement

• Type I cement contains 59% C3S, 15% C2S, 12% C3A, and 8% C4AF.
• Type II cement contains 46% C3S, 29% C2S, 6% C3A, and 12% C4AF.
• Type III cement contains 60% C3S, 12% C2S, 12% C3A, and 8% C4AF.
• Type IV cement contains 30% C3S, 46% C2S, 5% C3A, and 13% C4AF.
• Type V cement contains 43% C3S, 36% C2S, 4% C3A, and 12% C4AF.