Plain Concrete I. *Definition 1. Concrete: a hard, strong construction material consisting of a hardened or set mixture with such aggregates as sand, gravel,

Plain ConcreteI. *Definition

1. Concrete: a hard, strong construction material consisting of a hardened or set mixture with such aggregates as sand, gravel, pebbles, broken stone, or slag in a mortar, asphalt or cement matrix

2. Components of concrete

3. Cement concrete v.s. asphalt concrete

4. Plain concrete: a hardened, stone-like material without any reinforcement or without airentrain-ing, of a carefully proportioned mixture of cement, sand, gravel or other aggregate, and water (sometimes with admixture)

II. *Properties of Plain concrete in contrast to other materials

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Plain ConcreteIII.* Factors determining concrete’s properties

1. The proportions of the mix: cement-aggregate ratio; water-cement ratio; and the like (a standard slump test)

2. The thoroughness of the intermixing

3. The conditions of humidity and temperature, i.e., the application of the curing methods

IV. *Placing Concrete

1. On earth: sufficiently moisten the earth to prevent it from robbing the concrete of its water

2. On or adjacent set concrete: thoroughly clean the surface of the existing concrete, with a small quantity of cement grout over the entire area, and place the fresh concrete on or against the mortar.


Plain ConcreteIV. Placing Concrete3. In deep forms/boreholes: a tremie to limit the freefall

not over 3 or 4 ft to prevent segregation.

4. In slab construction: The concrete should first be place around the perimeter at one end with each batch dumped against previously placed concrete.

5. In wall /beam construction: the first batch should be placed at both ends of the section, and the concreting should progress toward the center.

6. Location of mixing plant: as nearly as practicable in its final location to reduce the segregation from movement after it is placed.

7. Thickness of layers placed: permitting uniform compaction


Plain ConcreteIV. Placing Concrete

8. Time lapse between placing of layers: limited to assure perfect bond between the fresh and previously placed concrete

9. Compaction: Immediately performed after placing by hand puddler or a mechanical vibrator to eliminate voids

10. Placing temperature: 40~80ºF (Low temperature reduces the rate of setting, while excessive high temperature reduces the ultimate strength).


Plain ConcreteV. *Advantages of vibrating1. Permitting the use of a drier concrete which has a hig

her strength due to the reduced water content

2. The reduced water content permits a reduction in the cement and fine aggregate because less cement is needed.

3. The lower water content reduces shrinkage and voids.

4. The lower water content increases the strength of the concrete.

5. The drier concrete reduces the cost of finishing the surface.

6. Mechanical vibration can replace three to eight hand puddler

7. The drier mix permits the removal of some forms more quickly, which may reduce the cost of forms.


Plain ConcreteVI. *Curing concrete (control of humidity and tempera

ture):

1. Curing means maintaining a satisfactory moisture content in concrete for a long enough time during its early stages so that its desired properties develop.

2. Curing methods:

1) Curing by ponding/flooding

2) Curing by membrane

3) Steaming curing or curing by fog spraying or sprinkling

4) Curing by wet coverings

5) Air-curing v.s. in-house curing6 Wednesday, April 19, 2023

Plain Concrete Admixtures, added before or during the mixi

ng of concrete, are used to strengthen concrete, to speed up or slow down the set-up time, and to help protect concrete against the effects of temperature changes and exposure to chemicals such as deicers.

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Plain Concrete Slump is used to describe the consistency, stiffness

and workability of fresh concrete. The results of a slump test are stated in inches. It is influenced by the amount of water in fresh concrete. More water means higher slump, but water is not the only influence. Admixtures can be used to increase slump without increasing the water in the concrete. The type of aggregate, the air content, the admixtures, temperature and the proportions of all the ingredients affect slump. The typical slump used for external flatwork is four inches, plus or minus one inch. For a concrete foundation the preferred slump is five inches, plus or minus one inch.

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Plain Concrete

A good indication of the water content of a mix (and thus the workability) can be had from a standard slump test. In this test a metal cone 12 in tall is filled with fresh concrete in a specified manner. When the cone is lifted, the mass of concrete "slumps" downward (Figure 4) and the vertical drop is referred to as the slump. Most concrete mixes have slumps in the 2- to 5-in range.


Plain Concrete

Ponding is accompanied by built earth or sand dikes around the perimeter of the slab to retain a pond of water within the enclosed area. The method requires considerable labor and supervision, and may not be practical except for small jobs.

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Plain Concrete

Liquid curing compounds/liquid membrane-forming compounds (also called seal coats) are used most often because of their versatility, ease of application, convenience, and economy. Most of the compounds have a resin in a solvent. After the application, the solvent evaporates, leaving behind a thin continuous film of resin on the surface of concrete, which seals in most of the moisture. The compounds are generally applied by either brushing, rolling, or spraying.

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Plain Concrete

An excellent method of curing when the temperature is above freezing. However, this method is costly and requires an adequate amount of water. In addition, a process should be in place to prevent the concrete from drying out between applications of water.

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Plain Concrete

Covering the concrete with wet burlap, wet cotton mat, and a 2-in. layer of moist sand, earth, hay, or sawdust (wetted once a day) can be an effective curing technique for pavement and floors. Burlap must be free from any substances that are harmful to concrete or cause discoloration. The moisture-retaining fabric should be kept continuously moist so that a film of water remains on the surface through out the curing period.

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Plain Concrete I. *Definition 1. Concrete: a hard, strong construction material consisting of a hardened or set mixture with such aggregates as sand, gravel,