A 312 s1 cement & concrete

AR 312SA: BUILDING TECHNOLGY 3Handouts

A. CEMENT and CONCRETE WORKS

CEMENT

• Binder• Can bind other materials together• A substance that sets and hardens independently

The word “cement” traces from the Romans who used the term opus caementicium to describe masonry resembling concrete made from crushed rock with burnt lime as a binder. Volcanic ash and pulverized brick were added to form a strong hydraulic binder and later called cement.

Types of Modern Cement

1. Portland Cement2. Portland Cement Blends3. Non-Portland Hydraulic Cement

Portland Cement

Cement is made by heating limestone (calcium carbonate) with small quantities of other materials such as clay. The resulting hard substance, called ‘clinker’, is then ground with a small amount of gypsum into a powder to make ‘Ordinary Portland Cement’, the most commonly used type of cement often referred to as OPC.

Portland cement is a basic ingredient of concrete, mortar and most non-specially grout. The most common use for Portland Cement is in the production of concrete. Concrete is a composite material consisting of aggregate (gravel and sand), cement and water. As a construction material, concrete can be cast in almost any shape desired and once hardened can become a structural (load bearing) element. Portland cement may be gray or white.

Portland Cement Blends

1. Portland blast furnace cement 6. Expansive cements2. Portland fly ash cement 7. White blended cements3. Portland pozzolan cement 8. Colored cements4. Portland silica fume cement 9. Very finely ground cements5. Masonry cements

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Non-Portland Hydraulic Cement

1. Pozzolan lime cements 6. ”Natural” cements2. Slag lime cements 7. Geopolymer cements3. Super sulfated cements4. Calcium aluminate cements5. Calcium sulfo-aluminate cements

Setting of Cement

Cement sets when mixed with water by way of a complex series of hydration chemical reactions still only partly understood. The different constituents slowly hydrate and crystallize while the interlocking of their crystals gives to cement its strength.After the initial setting, immersion in warm water will speed up setting. In portland cement, gypsum is added as a compound preventing cement flash setting. The time it takes for cement to set varies, and can take anywhere from twenty minutes for initial set, to twenty four hours or more, for final set.

CONCRETE

• Is an artificial stone made out from the mixture of cement, sand, gravel and water.• This is known as a solid mass or concrete.• Concrete in which reinforcement is embedded in such a manner that the two materials act together in resisting

forces, is called reinforced concrete.

Concrete MixtureDesigning of concrete mixture is based on the Water-Cement Ratio.• 0.68 pound of water per pound of cement will produce concrete capable of developing 2500 psi in 28 days.• Less water produces stronger concrete.• More water produces lesser strength.

Water-Cement RatioConcrete means it is a mixture of cement, sand, gravel and water.• Class “AA” 1:1½:3 For Concrete under water, retaining walls• Class “A” 1:2:4 For Footings, Columns, Beams and RC Slab• Class “B” 1:2½:5 For Slab on fill and non-bearing walls• Class “C” 1:3:6 For Concrete plant boxes, decorative blocks, others.

Slump Test

When freshly mixed concrete is checked to ensure that the specified slump is being attained consistently. A standard slump cone is 12 inches high (30 cm) and 8 inches (20 cm) in diameter at the bottom and 4 inches (10cm) on top which is open on both ends.

The cone is filled in three equal layers, each being tampered or rodded 25 times with a standard 5/8” bullet nosed rod. When the cone has been filled and levelled off, it is lifted carefully and the amount of slump is measured.


Allowable Slump or Deflection in Concrete• Beams and Columns 7.5 cm or 3 inches• Slabs and Tunnel Inverts 5.0 cm or 2 inches• Tops and Walls, Parapet and Curbs 5.0 cm or 2 inches• Sidewalls, Arch in Tunnel Lining 10.0 cm or 5 inches• Canal Linings 7.5 cm or 3 inches• Heavy Mass Constructions 5.0 cm or 2 inches

Compressive Strength Test

Concrete Cylinder Compression Laboratory Compressive Test

Common quality control of concrete based on 7 and 28 days curing periods. Specimens are usually cylindrical with a length equal to twice the diameter. Standard size is 12 inches high and 6 inches in diameter. Filling is done the same way the slump test but taken out from the mould in 24 hours. It is then sent to a Compression Laboratory, by making the cylinder still wet. Some Compression Stresses are 2,000 psi, 2,500 psi or 3,000 psi.

Concrete CuringFor traditional curing method, common concrete curing is usually done by water spraying or ponding. This is done at least within 7-day period or more, 24 hours after the concrete has been poured.

Blankets Ponding Burlap Chemical

Curing Solution Film Curing Aid/ Hardener Spray

Concrete Additives

Other materials are often added to the mixture or applied to the surface of freshly placed concrete to produce some special results. These materials are known as concrete additives and may be used for one of the following reasons;


• To speed up the initial set of concrete• To retard the initial set• To make the concrete more resistant to deterioration due to repeated freezing and thawing cycle• To prevent bleeding of water to the surface of concrete• To improve the workability of the mix• To improve the hardness and denseness of the concrete surface• To render the concrete more water tight• To improve the bond between two concrete surface• To inhibit the set of cement paste• To produce a colored surface• To produce a non-skid surface• To prevent the evaporation of water from the newly placed concrete• To help develop all the potential strength of the given water-cement paste• To decrease the weight of concrete per cubic foot

TYPES OF CONCRETE ADMIXTURE:

ACCELERATORS: An admixture which is used to speed up the initial set of concrete. Such a material may be added to the mix to increase the rate of early strength development for several reasons. For example, this will allow early removal of forms and in some cases reduce the whole curing period.

RETARDERS: The function is to delay or to extend the setting time of the cement paste in concrete.

AIR-RETAINING AGENTS: Air entrained concrete contains microscopic bubble or air formed with the aid of a group of chemicals called surface active agents, materials that have the property of reducing the surface tension of water intended for use when better resistance to frost action is concerned.

DISPERSAL AGENTS: When cement and water are mixed, the cement particles tend to gather in clumps or to flocculate. As a result water does not reach some of the particles and some are only partially hydrated. Sometimes only 50% of the cement is hydrated. Cement dispersal agents causes cement particles to separate by imparting like electrostatic charges to them.

BONDING AGENTS: When fresh concrete is poured against another concrete surface already set and at least partially cured, it is often difficult to obtain a bond between the two surfaces unless special precautions are taken. Fresh concrete sinks when setting, and unless there is a very good bond thus shrinkage makes the new pull away from the old surface. A cement mix paste or scratch coat is often applied from old concrete surface. Two types of bonding agents are; Metallic Aggregate and Synthetic latex Emulsion.

CONCRETE COLORING AGENTS: 1. Use concrete paint, applied after the concrete surface has been neutralized, either through exposure or by using a

neutralizing agent such as zinc sulfate.2. Integrating color into the surface concrete while it is still fresh.

SET-INHIBITING AGENTS: Certain inhibiting agents will prevent the cement paste from bonding to the surface aggregates but will not interfere with the set throughout the remainder of the pour. Liquid and powder types are used for this type of agent.

To avoid making concrete surfaces slippery.


NON-SKID SURFACES:

SURFACE CEILING AGENTS: Used for two purposes:1. To form water tight coating that will prevent from evaporating from a concrete surface and allow it to retain

hydration.2. To seal the pores of a concrete surface after it has hardened in order to prevent the passage of water and absorption

of spilled materials such as oil, grease or paint.

GAS FORMING AGENTS: Causes a slight expansion in plastic concrete or mortar and thus reduces and eliminates voids caused by settlements.

POZZOLANIC ADMIXTURES: Pozzolans may be added to concrete mixes rather than substituting for part of the cement to improve workability and resistance to chemical attack.

TYPES OF CONCRETE MIXTURE:

CONCRETE HARDENERS: Plain concrete surfaces which are subjected to rolling live loads, the impact action of foot traffic, and other types of wear begin to dust and crumble at the surface after a period of time. This condition worsens with time, finally resulting into destruction of the surface. To prevent this, Chemical Hardeners and Fine Metallic aggregate are used.

WATER REDUCING ADMIXTURE: A material used to reduce the amount of water necessary to produce a concrete of given consistency or to increase the slump for a given water content.

CONCRETE WATERPROOFERS: Water under pressure can force through channels between inner and outer surfaces. Water can also pass through concrete by the action of capillary forces. If one side is exposed to moisture and the other to air, the water reaching the dry side evaporates, resulting in a flow of moisture through the concrete. Materials used to reduce or to stop this type of flow are more properly called Damproofers.

VARIOUS CONCRETE PRODUCTS


SURFACE CEILING AGENTS: Used for two purposes:1. To form water tight coating that will prevent from evaporating from a concrete surface and allow it to retain

hydration.2. To seal the pores of a concrete surface after it has hardened in order to prevent the passage of water and absorption

of spilled materials such as oil, grease or paint.

GAS FORMING AGENTS: Causes a slight expansion in plastic concrete or mortar and thus reduces and eliminates voids caused by settlements.

POZZOLANIC ADMIXTURES: Pozzolans may be added to concrete mixes rather than substituting for part of the cement to improve workability and resistance to chemical attack.

TYPES OF CONCRETE MIXTURE:

CONCRETE HARDENERS: Plain concrete surfaces which are subjected to rolling live loads, the impact action of foot traffic, and other types of wear begin to dust and crumble at the surface after a period of time. This condition worsens with time, finally resulting into destruction of the surface. To prevent this, Chemical Hardeners and Fine Metallic aggregate are used.

WATER REDUCING ADMIXTURE: A material used to reduce the amount of water necessary to produce a concrete of given consistency or to increase the slump for a given water content.

CONCRETE WATERPROOFERS: Water under pressure can force through channels between inner and outer surfaces. Water can also pass through concrete by the action of capillary forces. If one side is exposed to moisture and the other to air, the water reaching the dry side evaporates, resulting in a flow of moisture through the concrete. Materials used to reduce or to stop this type of flow are more properly called Damproofers.

VARIOUS CONCRETE PRODUCTS

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A 312 s1 cement & concrete

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