CONCRETE TECHNOLOGY In building the Bavian canal in 690B.C.the Assyrians used a mixture of one part lime, two parts sand, and four parts limestone aggregate to create a crude concrete. In 1824 Joseph Aspdin took out a patent in England on "Portland" cement. He named it Portland because its color resembled the limestone on the Isle of Portland in the English Channel. That patent makes the beginning of the concrete, as we know it today. This manufactured cement consists of limestone and clay burned at temperatures in excess of 2,700F.Concrete become widely used in Europe during the late 1800s and was brought to the United States late in that century. Its use continued to spread rapidly as knowledge about it and experience with it grew.

Portland cement concrete is one of the most widely used structural materials in the world for both civil works and building projects. its versatility , economy, adaptability ,and especially its low maintenance requirements make it an excellent building material , the term "concrete" is applicable for many products but is generally used with Portland cement concrete. It consists of Portland cement, water and aggregates that have been mixed together, placed, consolidated, and allowed it cure.

Portland cement and water form a paste, which acts as the glue or binder, when fine aggregate is added (aggregate whose size range lies between the no.200 mesh sieve and the no.4 sieve); the resulting mixture is termed mortar. Then when coarse aggregate is included (aggregate sizes larger than the sieve no 4 but less than 3 in). Concrete is produced. Normal concrete consists of about three – fourth aggregate and one fourth paste, by volume, the paste usually consists of water- cement ratios between 0.4 and 0.7 by weight.

Admixtures are sometimes added for specific purposes such as:

To improve workability, to impart color. To retard the initial set of the concrete (e.g., for long hauling

distances). To gain rapid hardening and high initial strength (e.g. for

posttensioned elements). To improve flow ability (e.g., for self-compacting concrete). To waterproof the concrete.

Concrete operation

The operation involved in the production of concrete will vary with the end use of the concrete, but in general, the operations include:

1. Batching the materials2. Mixing3. Transporting4. Placing5. Consolidating6. Finishing7. Curing

PROPORTIONING CONCRETE MIXTURES

For successful concrete performance the mixture must be properly proportioned. The American Concrete Institute (ACI) has a number of excellent recommended practices, including one on proportioning concrete mixtures.

SOME PRACTICAL CONSIDERATIONS IN PROPRTIONING CONCRETE MIXES

1. Although it takes water to initiate the hydraulic reaction, as a general rule, the higher the water-cement ratio, the lower the resulting strength and durability of the concrete.

2. The more water that is used (which is not to be confused with the water-cement ratio) the higher will be the slump.

3. The more aggregate that is used; the lower will be the cost of the concrete.

4. The larger the maximum size of coarse aggregate, the less the amount of cement paste that will be needed to coat all the particles and to provide necessary workability.

5. Adequate consolidation produces stronger and more durable concrete.

6. The use of properly entrained air enhances almost all concrete properties with little or no decrease in strength if the mix proportions are adjusted for the air.

7. The surface abrasion resistance of the concrete is almost entirely a function of the properties of the fine aggregate.

AIR ENTRAINMENT

Is the intentional creation of tiny air bubbles in concrete; the bubbles are introduced into the concrete by the addition to the mix of an air entraining agent. A surfactant (surface-active substance) a (type of chemical that includes detergents).

The air bubbles are created during mixing of the plastic (flow able, not hardened) concrete, air entrainment is to increase the durability of the hardened concrete. Especially in climates subject to freeze- thaw; the secondary purpose is to increase workability of the concrete while in a plastic state.

FRESH CONCRETE

To the designer, fresh concrete is usually of little importance, to the constructor, fresh concrete is all-important; it is the fresh concrete that must be mixed, transported, placed, consolidated, finished and cured. To satisfy both the designer and the constructor, the concrete should:

1. Be easily mixed and transported.2. Have minimal variability throughout, both within a given batch

and between batches.3. Be of proper workability so as to enable proper consolidation,

prevent segregation, completely fill the forms, and provide achievement of a proper finish.

Slump test

RECOMMENDED SLUMPS FOR VARIOUS TYES OF CONCRETE CONSTRUCTION

Slump (in) Types of construction Maximum MinimumReinforced foundation walls and footing 3 1Plain footing, caissons, and substructure walls 3 1Beams and reinforced walls 4 1Building columns 4 1Pavements and slabs 3 1Mass concrete 2 1----------------------------------------------------------------------------------------- BATCHING CONCRETE MATERIALSMost concrete batches, although designed on the basis of absolute volumes of the ingredients, are ultimately controlled in the batching process on the basis of weigh, therefore, it is necessary to know the wright- volume relationships of all the ingredients, then each ingredient must be accurately weighed if the resulting mixture is to have the desired properties, it is the function of the batching equipment to perform this weighing measurement.

CEMENTFor most large projects, the cement is supplied in bulk quantities from cement transport trucks, each holding 25 tons or more, or from railroad cars. Bulk cement is usually unloaded by air pressure from special truck trailers or rail cars and stored in overhead silos or bins. Cement may be supplied in paper bags; each containing 1 cubic foot (cf) loose measure and weighing 94lb net, bagged cement must be stored in a dry place on pallets and should be left in the original bags until used for concrete.

WATERThe water that is mixed with cement to form a paste and to produce hydration must be free from:

All foreign matters. Organic material and oil may inhibit the bond between hydrated

cement and aggregate. Required water properties are cleanliness and freedom from organic material, alkalis, acids and oils.

In general water that is acceptable for drinking can be used for concrete.AGGREGATESTo produce concrete of high quality

The aggregate should be clean, strong, durable and round or cubical in shape.

The aggregate should be resistant to abrasion from weathering and wear.

Weak, friable, or laminated particles of aggregate, or aggregate that is too absorptive, are likely to cause deterioration of concrete.

MIX PROPORTIONS The mix specifications will define specific requirements for the materials that constitute the desired concrete product. Typical requirements include:

1. Maximum size aggregate (e.g., 1.5in)2. Minimum cement content (sacks per cubic yard or pounds per

cubic yard)3. Maximum water-cement ratio (by weight or in gallons per sack of

cement)

When computing the material quantities for a concrete mix, the following are useful factors and information:

1. The average specific gravity of cement is 3.15.2. The average specific gravity of coarse or fine aggregate is 2.65.3. The unit weight of water is 62,4 lb/cf.4. Once cubic foot of water equals 7.48 U.S. gallons.5. One gallon of water weighs 8.33lb.6. Usually the proportion of fine aggregate varies between 25 and

45% of total aggregate volume.

The absolute volume of any ingredient in cf expressed by

Weight of the ingredient (lb) Volume (cf) =-------------------------------------------------------- Specific gravity of the ingredient X62, 4 lb/cf

Weight of the ingredient (kg) Volume (m3) =-------------------------------------------------------- Specific gravity of the ingredient X1000

MIXING CONCRETE There are two types of concrete-mixing operations in use:

1. Transit mixed2. Central mixed

Today, unless the project is in a remote location or is relatively large, the concrete is batched in a central batch plant and transported to the job site in transit-mix trucks, often referred to as ready-mixed concrete trucks, or truck mixers, this type of concrete is controlled by ASTM specification C94[4], and there is a national organization promoting its use ( national Ready Mixed Concrete Association. In the European construction culture for home building and small projects is often produced on the job site using stand-alone mixing machines that range in capacity from small basic units (5cy/hr) to large advanced assemblies capable of 50cy/hr.

FIGURE 7-8. Truck mixer. (Courtesy of Kenworth Truck Company)

PLACING CONCRETEOnce the concrete arrives at the project site, it must be moved to its final position without segregation and before it has achieved and initial set. This movement can be accomplished in several ways, depending on the horizontal and vertical distance of the movement and other constraints. Methods include:

Buckets buggies and wheelbarrows Chutes drop pipes belt conveyors concrete pumps

BUCKETSNormally, properly designed bottom-dump buckets enable concrete placement at the lowest practical slump. Care should be exercised to prevent the concrete from segregating as a result of

Discharging from too high above the surface. Allowing the fresh concrete to fall past obstructions (such as the

forms themselves in the case of columns). Gates should be designed so that they can be opened and closed at any time during the discharge of the concrete.

MANUAL OR MOTOR-PROPRLLED BUGGIES

Hand buggies and wheelbarrows are usually capable of carrying from4 to 9cf of concrete, and thus are suitable on many projects, either as the sole or main concrete-placing equipment for small works, or as a complementary means on projects utilizing cranes, or pumps as the major concrete- placing equipment.

CHUTES AND DROP PIPESChutes are often used to transfer concrete from a higher elevation to a lower elevation. They should have a round bottom, and the slope should be steep enough for the concrete to flow continuously without segregation. Truck mixers are normally equipped with built-in swing (and often extendable) chutes.

CONCRETE PUMPA concrete pump is a tool used for transferring liquid concrete by pumping. There are two types to concrete pumps.

1. First type of concrete pump is attached to a truck.it is known as a trailer-mounted boom concrete pump because it uses a remote-controlled articulation robotic arm (called a boom) to place concrete with pinpoint accuracy. Boom pumps are used on most of the larger construction projects as they are capable of pumping at very high

volumes and because of the labor saving nature of the placing boom. They are a revolutionary alternative to truck mounted concrete pumps.

Trailer-mounted boom concrete pump.

2. The second main type of concrete pump is either mounted on a truck and known as a truck – mounted concrete pump or placed on a trailer. And it is commonly referred to as a line pump or trailer-mounted concrete pump. This pump requires steel or rubber concrete placing hoses to be manually attached to the outlet of the machine. Those hoses are linked together and lead to wherever the concrete needs to be placed. Line pumps normally pump concrete at lower volumes than boom pumps and are used for smaller volume concrete placing applications such as swimming pools, sidewalks and single family home concrete slabs and nose ground slabs.

Line pump Basic pumping tullesSuccessful pumping of concrete requires good planning and quality concrete. A common fallacy is to assume that any place able concrete can be pumped successfully. The basic principle of pumping is that the concrete moves as a cylinder of concrete. To pump concrete successfully, the following rules should be carefully followed:

1. Use a minimum cement factor of 517lb of cement per cubic yard of concrete (5.5 sacks per cy)

2. Use a combined gradation of coarse and fine aggregate that ensures no gaps in sizes that will allow paste to be squeezed through the coarser particles under the pressures induced in the line. This is the most often overlooked aspect of good pumping. In particular, it is important for the fine aggregate to have at least 5% passing the no.100 sieve and about 3% passing the no 200 sieve. Line pressures of 300 psi are common and they can reach as high as 1000 psi.

3. The use of a minimum 5-in pipe diameter is desirable.4. Always lubricate the line with cement paste or mortar before

beginning the pumping operating. 5. Ensure a steady, uniform supply of concrete, with a slump of

between 2 and5 in. as it enters the pump.

6. Always presoak the aggregate before mixing them in the concrete to prevent their soaking up mix water under the imposed pressure. This is especially important when aggregate are used that have a high absorption capability (such as structural lightweight aggregate).

7. Avoid the use of reducers in the conduit line. One common problem is he use of a 4-to5in.reducer at the discharge end so that workers will have only a 4-in .flexible hose to move around. This creates a constriction and significantly raises the pressure necessary to pump the concrete.

PRECAST CONCRETEReinforced concrete can be classified as pre cast or cast in- situ concrete.Precast concrete is a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and lifted into place.In contrast, standard concrete (cast in-situ concrete) is poured into site-specific forms and cured on site.By producing precast concrete in a controlled environment (typically referred to as a precast plant), the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees.Advantages of precast concrete over cast-in-situ concrete.

1. The production process for precast concrete is performed on ground level which helps with safety throughout a project.

2. There is a greater control of the quality of materials and workmanship in a precast plant rather than on construction site.

3. Financially, the forms used in a precast plant may be reused hundreds to thousands of times before they have to be replaced which allow cost of formwork per unit to be lower than for site-cast production.

BREIF HISTORY OF PRECAST CONCRETE

1. Ancient Roman builders made use of concrete and soon poured the material into molds to build their complex network of aqueducts, culverts and tunnels.

2. Modern uses for pre-cast technology include a variety of architectural and structural applications featuring parts of or an entire building system.

3. In the modern world precast paneled buildings were pioneered in Liverpool, England in 1905. A process was invented by city engineer Johan Alexander Brodie. Whose inventive genius also had him inventing the football goal net.

PRECAST CONCRETE PRODUCTS

The following is a sampling of the numerous products that utilize precast/ pre stressed concrete. The majority of precast / pre stressed products can fall under one or more of the flowing categories.

1. Agricultural products

Precast concrete products can withstand the most extreme weather conditions and will hold up for many decades of constant usage.Products include bunker silos; cattle feed bunks, cattle grid, agricultural fencing, H bunks, J- bunks, livestock slats, and livestock watering trough, feed troughs, concrete panels, slurry channels and more.

Cattle feed bunks

Building and site amenitiesPrecast concrete building components and site amenities are used architecturally as fireplace mantels, cladding, trim products, accessories, and curtain walls. Structural applications of precast concrete include foundation, beams, floors, walls and other structural components. It is essential that each structural component be design and tested to withstand both the tensile and compressive loads that the member will be subjected to over its lifespan.

Fireplace mantels

Cladding Retaining wallsPrecast concrete provides the manufacturers with the ability to produce a wide range of engineered earth retaining systems.Products include: commercial retaining wall, residential retaining walls, sea walls, mechanically stabilized earth (MSE) panels, modular block systems, segmental retaining walls, etc.Retaining walls have different types which include: gravity retaining wall, semi gravity retaining wall, cantilever retaining wall, counterfort retaining wall, and buttress retaining wall.

Sanitary and storm waterSanitary and storm water management products are structures designed for underground installation that have been specifically engineered for the treatment and removal of pollutants from sanitary and storm water run-off.These precast concrete products include storm water detention vaults, catch, basins and manholes.

TRANSPORTATION AND TRAFFIC RELATED PRODUCTS

Precast concrete transportation products are used in the construction, safety and site protection of road, airport and railroad transportation systems.

Products include: box culverts, 3 sided culverts, bridge systems, railroad crossings, and railroad ties, TVCBs, central reservation barriers and other transportation products. Used to make underpasses, surface passes and pedestrian subways. So that traffic in cities is disturbed for less amount of time.

Utility structures

For communications, electrical, gas or steam systems, precast concrete utility structures protect the vital connections and controls for utility distribution. Precast concrete is nontoxic and environmentally safe.

Products include: hand holes, hollow core products, light pole bases, meter boxes, panel vaults, pull boxes, telecommunications structures, transformer pads, transformer vaults, trenches, utility buildings. Utility

Manhole

vaults, utility poles, controlled environment vaults (CEVs) and other utility structures.

Water and west water products

Precast water and wastewater products hold or contain water, oil or other liquids for the purpose of further processing into not-contaminating liquids soil products.

Products: aeration system. Distribution boxes, dosing tanks, dry wells, grease interceptors, leaching pits, sand- oil/ oil-water interceptors, septic tanks, water/sewage storage tanks, wet wells, fire cisterns and other water and wastewater products.

In-situ concrete

Concrete which is deposited in the place, where it is required to harden as part of the structure, as opposed to precast concrete.