Wednesday, September 17, 2014

Review Notes: Building Materials - Concrete & Concrete Works


Portland Cement – made from materials which must contain the proper proportions of lime, silica, alumina and iron components.

Special Cements:
1. White Portland cement – same materials as normal Portland except in color
2. Masonry Cement – designed to produce better mortar than that made with normal Portland cement or with a lime-cement combination.
3. Air-entraining Portland Cement – small amounts of this is added to the clinker and ground with it to produce air-entraining cements, effective use for resistance to severe frost.
4. Oil Well Cement – special Portland cement used for sealing oil wells.
5. Waterproofed Portland Cement – normally produced by adding a small amount of stearate, usually calcium or aluminum to the cement clinker during the final grinding.

Types of Aggregates Used in Concrete:

Concrete – artificial stone made by binding together particles of some inert material with a paste made of cement and water. These inert materials are the aggregate.

Aggregate – sand, gravel crushed stone, cinder, crushed furnace slag, burned clay, expanded vermaculite, and perlite.
Sand – found in riverbends, free of salt and must be washed.
Fine aggregate – smaller than ¼” diameter stones.
Course aggregate – bigger than ¼” diameter stones.

Concrete Mixes:

Class “AA” - 1:1 ½ :3 - concrete under water, retaining walls
Class A - 1:2:4 - footings, columns, beams, RC slabs
Class B - 1:2 ½:5 - slab on fill, non-bearing walls
Class C - 1:3:6 - concrete plant boxes, etc.

Control of Concrete Mixes:

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 (0.30) and 8 inches (0.20) in diameter at the bottom and 4 inches (0.10) on top which is open on both ends.

Compressive Strength Test – common quality-control test of concrete, based on 7 and 28 day curing periods.

Concrete Additives – materials often added to the concrete or applied to the surface of freshly placed concrete to produce some special result.

Accelerators – an admixture which is used to speed up the initial set of concrete. Such a material maybe added to the mix to increase the rate of early-strength development for several reasons.

Retarders – to delay or extend the setting time of the cement paste in concrete.

Air-entraining agents – air-entrained concrete contains microscopic bubbles of air formed with the aid of a group of chemical 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.

Concrete Hardeners – applied on concrete surface to increase hardiness and toughness.

Two Types of Concrete hardeners:
1. Chemical Hardeners – liquids containing silicofluorides or fluosilicates and a wetting agent which reduces the surface tension of the liquid and allows it to penetrate the pores of the concrete more easily.
2. Fine Metallic Aggregate – are specially processed and graded iron particles which are dry-mixed with Portland cement, spread evenly over the surface of freshly floated concrete, and worked into the surface by floating.

Water Reducing Admixtures – 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.

Damproofers – materials used to reduce or stop the penetration of moisture through the concrete. Reduces permeability.

Bonding Agents:

Paste Slurry – often applied to such an old surface immediately prior to pouring new concrete to increase the amount of paste.

Two Types of Bonding Agents:
1. Metallic Aggregate – iron particle are larger, but with same materials as the permeability reducer. Bonding takes place through the oxidation and subsequent expansion of the iron particles.
2. Synthetic Latex Emulsion – consists of highly polymerized synthetic liquid resin dispersed in water.

Set-Inhibiting Agents – prevent the cement paste from bonding to the surface aggregate but will not interfere with the set throughout the remainder of the pour.

Pozzolanic Admixtures – materials sometimes used in structures where it s desirable to avoid high temperature or in structures exposed to seawater or water containing sulfates. Pozzolans maybe added to concrete mixes-rather than substituting for part of the cement to improve workability, impermeability, and resistance to chemical attack.

Concrete Products – made of lightweight and heavyweight materials for use in exterior and interior load-bearing walls, firewalls, curtain and panel walls, partitions etc.

Concrete Block:
1. Hollow load-bearing concrete block – an 8” x 8” x 16” will approximately weigh 40 to 50 lb. Made with heavyweight aggregate and 25 to 35 lb. when made with lightweight aggregate.
2. Solid load bearing block – defines as one having a core area of not more than 25 percent of the gross cross-sectional area.
3. Hollow; non-load bearing concrete block – one in which the core area exceeds 25 percent of the cross-sectional area.
4. Concrete building tile
5. Concrete brick

Common Sizes:
1. 4” x 8” x 16” – for non-load bearing partitions
2. 6” x 8” x 16” – for load bearing walls

1. Hand made – backyard industry
2. Machine made – commonly sold
3. Steam cured – manufactured by big and nationally known factories for load bearing walls. Usually specified for government and multi-storey buildings.

Cellular Concrete Blocks – lightweight block which is outstanding in thermal and sound insulation qualities. Basic ingredients are cement-made from silica-rich sand and lime-water, and aluminum powder.


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