WATER REDUCING ADMIXTURES FOR CONCRETE BASIC AND TUTORIALS

WATER REDUCING ADMIXTURES FOR CONCRETE BASIC INFORMATION
What Are Water Reducing Concrete Admixtures?


Water-Reducing Admixtures
These decrease water requirements for a concrete mix by chemically reacting with early hydration products to form a monomolecular layer of admixture at the cementwater interface.

This layer isolates individual particles of cement and reduces the energy required to cause the mix to flow. Thus, the mix is ‘‘lubricated’’ and exposes more cement particles for hydration.

The Type A admixture allows the amount of mixing water to be reduced while maintaining the same mix slump. Or at a constant water-cement ratio, this admixture allows the cement content to be decreased without loss of strength.

If the amount of water is not reduced, slump of the mix will be increased and also strength will be increased because more of the cement surface area will be exposed for hydration. Similar effects occur for Type D and E admixtures. Typically, a reduction in mixing water of 5 to 10% can be expected.

Type F and G admixtures are used where there is a need for high-workability concrete. A concrete without an admixture typically has a slump of 2 to 3 in. After the admixture is added, the slump may be in the range of 8 to 10 in without segregation of mix components.

These admixtures are especially useful for mixes with a low water-cement ratio. Their 12 to 30% reduction in water allows a corresponding reduction in cementitious material.

The water-reducing admixtures are commonly manufactured from lignosulfonic acids and their salts, hydroxylated carboxylic acids and their salts, or polymers of derivatives of melamines or naphthalenes or sulfonated hydrocarbons. The combination of admixtures used in a concrete mix should be carefully evaluated and tested to ensure that the desired properties are achieved.

For example, depending on the dosage of admixture and chemistry of the cement, it is possible that a retarding admixture will accelerate the set. Note also that all normal-set admixtures will retard the set if the dosage is excessive.

Furthermore, because of differences in percentage of solids between products from different companies, there is not always a direct correspondence in dosage between admixtures of the same class. Therefore, it is
important to consider the chemical composition carefully when evaluating competing admixtures.

Superplasticizers are high-range water-reducing admixtures that meet the requirements of ASTM C494 Type F or G. They are often used to achieve highstrength concrete by use of a low water-cement ratio with good workability and low segregation.

They also may be used to produce concrete of specified strengths with less cement at constant water cement ratio. And they may be used to produce self-compacting, self-leveling flowing concretes, for such applications as longdistance pumping of concrete from mixer to formwork or placing concrete in forms congested with reinforcing steel.

For these concretes, the cement content or watercement ratio is not reduced, but the slump is increased substantially without causing segregation. For example, an initial slump of 3 to 4 in for an ordinary concrete mix may be increased to 7 to 8 in without addition of water and decrease in strength.

Superplasticizers may be classified as sulfonated melamine-formaldehyde condensates, sulfonated naphthaline-formaldehyde condensates, modified lignosulfonates, or synthetic polymers.

FIRE AND SMOKE BARRIERS DESIGN IN CIVIL BUILDING CONSTRUCTION BASICS AND TUTORIALS

FIRE AND SMOKE BARRIERS DESIGN IN CIVIL BUILDING CONSTRUCTION 
What Are The Fire And Smoke Barriers In Building Construction?


A major consideration in building design is safety of the community. Hence, buildings should be designed to control fires and smoke so that they will not spread from building to building. One way that building codes try to achieve this objective is to establish fire zones or fire limits that restrict types of construction or occupancy that can be used.

Additional zoning regulations establish minimum distances between buildings. Another way to achieve the objective is to specify the types of construction that can be used for enclosing the exterior of buildings.

The distance between adjoining buildings, fire rating, and stability when exposed to fire of exterior walls, windows, and doors, and percent of window area are some of the factors taken into account in building codes for determination of the construction classification of a building.

To prevent spread of fire from roof to roof, building codes also often require that exterior walls extend as a parapet at least 3 ft above the roof level. Parapets also are useful in shielding fire fighters who may be hosing a fire from roofs of buildings adjoining the one on fire. In addition, buildings should be topped with roof coverings that are fire-resistant.

Fire Divisions.
To prevent spread of fire vertically in building interiors, building codes generally require that floor-ceiling and roof-ceiling assemblies be fireresistant. The fire rating of such assemblies is one of the factors considered in determination of the construction classification of a building.

Also, openings in floors and roofs should be fire-protected, although building codes do not usually require this for one-story or two-story dwellings. For the purpose, an opening, such as that for a stairway, may be protected with a fire-resistant enclosure and fire doors.

In particular, stairways and escalator and elevator shafts should be enclosed, not only to prevent spread of fire and smoke but also to provide a protected means of egress from the building for occupants and of approach to the fire source by fire fighters.

To prevent spread of fire and smoke horizontally in building interiors, it is desirable to partition interiors with fire divisions. A fire division is any construction with the fire-resistance rating and structural stability under fire conditions required for the type of occupancy and construction of the building to bar the spread of fire between adjoining buildings or between parts of the same building on opposite sides of the division. A fire division may be an exterior wall, fire window, fire door, fire wall, ceiling, or firestop.

A fire wall should be built of incombustible material, have a fire rating of at least 4 hr, and extend continuously from foundations to roof. Also, the wall should have sufficient structural stability in a fire to allow collapse of construction on either side without the wall collapsing. Building codes restrict the size of openings that may be provided in a fire wall and require the openings to be fire-protected (Art. 11.55).

To prevent spread of fire through hollow spaces, such spaces should be firestopped. A firestop is a solid or compact, tight closure set in a hollow, concealed space in a building to retard spread of flames, smoke, or hot gases.

All partitions and walls should be firestopped at every floor level, at the top-story ceiling level, and at the level of support for roofs. Also, very large unoccupied attics should be subdivided by firestops into areas of 3000 ft2 or less.

Similarly, any large concealed space between a ceiling and floor or roof should be subdivided. For the purpose, firestops extending the full depth of the space should be placed along the line of supports of structural members and elsewhere, if necessary, to enclose areas not exceeding 1000 ft2 when situated between a floor and ceiling or 3000 ft2 when located between a ceiling and roof.

Openings between floors for pipes, ducts, wiring, and other services should be sealed with the equal of positive firestops. Partitions between each floor and a suspended ceiling above are not generally required to be extended to the slab above unless this is necessary for required compartmentation. But smoke stops should be provided at reasonable intervals to prevent passage of smoke to noninvolved areas.