CONCRETE

Concrete Masonry Unit

A concrete masonry unit (CMU) [ US], concrete block, cement block or foundation block [US] is a large rectangular brick used in construction. Concrete blocks are made from cast concrete, i.e. Portland cement and aggregate, usually sand and fine gravel for high-density blocks.

Admixture

A material other than water, aggregates, lime, or cement, used as an ingredient of concrete or mortar, and added to t he batch immediately before or during its mixing; used as a water repellent, as a coloring agent, as a retarder or accelerator (to modify its setting rate), etc.

Tremie

A pipe or tube through which concrete is deposited under water, having at its upper end a hopper for filling and a bail by means of which the assembly can be handled by a derrick.

Screed

A screed is a flat board, or a purpose made aluminium tool, used to smooth concrete after it has been placed on a surface. Also used to assist in leveling the application of plaster.

Compressive strength test of concrete

A strength shall be the average of the strengths of two cy linders made from the same sample of concrete and tested at 28 days or at the test age designated for the determination of fc'·

Burlap

A strong, coarsely woven cloth made of fibers of jute, flax, or hemp used to make bags and generally used for curing of concrete.

Air entrained concrete

Air entrainment will help concrete resist damage from both freeze-thaw and sulfate attack by reducing permeability

Plasticizer

An admixture used with concrete or mortar mix to make it workable with relatively little water.

Reinforced concrete

Concrete in which steel is embedded in such a manner that the two materials act together in resisting forces. The reinforcing steel - rods, bars, or mesh -absorbs the tensile, shear, and sometimes the compressive stresses in a concrete structure. Plain concrete does not easily withstand tensile and shear stresses caused by wind, earthquakes, vibrations, and other forces and is therefore unsuitable in most structural applications. In reinforced concrete, the tensi le strength of steel and the compressive strength of concrete work together to allow the member to sustai n these stresses over considerable spans. The invention of reinforced concrete in the 19th century revolutionized the construction industry, and concrete became one of the world's most common building materials.

Pre-stressed concrete

Concrete reinforced by either pretensioning or posttensioning, al l owing it to carry a greater load or span a greater distance than ordinary reinforced concrete. In pretension ing, lengths of steel wire or cables are laid i n the em pty mold and stretched. The concrete is placed and al lowed to set, and the cables are released, placing the concrete into compression as the steel shrinks back to its original length. In post-tensioning, the steel in the concrete is stretched after the curing process. Prestressing places a concrete member in compression; these compressive stresses counteract the tensile bending stresses of an applied load .

Precast concrete

Concrete that has been cast into a form which is l ater incorporated into a structure. A concrete structure may be constructed by casting the concrete in place on the site, by building it of components cast elsewhere, or by a combination ofthe two. Concrete cast in other than its final position is called precast.

Slump test

Determining the consistency of concrete by filling a conical mold with a sample of concrete, th eninverting it over a flat plate and removing the mold; the amou nt by which the concrete drops below the mold height is measured and this represents the slump. Maximum slump is allowed up to 6". The upper diameter of slump cone is 4 inch, lower diameter is 8 inch, and height is 1 2 inch. Freshly mixed concrete is to be placed in 3 layers into t he slump cone and each of the l ayers shall be temped 25 times by a 5/8 inch diameter rod. A procedure using a slump cone for measuring the slump of concrete.

Class A concrete

I :2:3 (4000 psi in 28 days).

Consolidation of concrete

In some types of construction, the concrete is placed in forms, then consolidated. Consolidation compacts fresh concrete to mold it within the forms and around embedded items and reinforcement and to eliminate stone pockets, honeycomb, and entrapped air. It should not remove significant amounts of intentionally entrained air. Vibration, either internal or external, is the most widely used method for consolidating concrete. When concrete is vibrated, the internal friction between the aggregate particles is temporarily dest royed and the concrete behaves like a liquid; it settles in the forms under the action of gravity and the large entrapped air voids rise more easily to the surface. Internal friction is reestablished as soon as vibration stops.

Water-cement ratio

Is the ratio of weight of water to the weight of cement used in a concrete mix. It has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability, but may make the mix more difficu lt to place. Placement difficulties can be resolved by using plasticizer. The water cement ratio is independent of the total cement content (and the total water content) of a concrete mix. Generally used as 0.4.

Mixing, transporting, and handling of concrete

Mixing, transporting, and handling of concrete should be carefully coordinated with placing and finishing operations. Concrete should not be deposited more rapidly than it can be spread, struck off, consolidated, and bull floated. Concrete should be deposited continuously as near as possible to its final position. In slab construction, placing should be started along the perimeter at one end of the work with each batch placed against previously dispatched concrete. Concrete should not be dumped in separate piles and then leveled and worked together; nor should the concrete be deposited in large piles and moved horizontally into final position.

Curing

Once it is placed and compacted, the concrete must cured before it is finished to make sure that it doesn't dry too quickly. Concrete's strength is influenced by its moisture level during the hardening process: as the cement sol id ifies, the concrete sh rink s. If site constraints prevent the concrete from contracting, tensile stresses will develop, weakening the concrete. To minimi ze this problem, concrete must be kept damp during the several days it requires to set and harden.

Slip form

Process of pouring concrete into a moving form. The concrete has a low slump so that the concrete does not l ose the shape of the form.

Workability

That property of freshly mixed concrete, plaster, or mortar which determines the ease and homogeneity with which it can be mixed, applied, compacted, spread, or finished; place abili ty.

Laitance

The accumulation of fine particles on the surface of fresh concrete resulting from an upward movement of water in the concrete; occurs when excessive water is used in the mixing of the concrete.

Cement factor

The quantity of cement contained in a unit volume of concrete or mortar, preferably expressed as weight, but frequently given as bags of cement per cubic yard of concrete, e.g. , a 6 1/2 - bag mix

Retarding agent

Used when pouring concrete in hot weather.

Walking on concrete

You can walk on concrete usually after 24 hours of pouring.

Concrete

is a hardened building material created by combining a chemically inert mineral aggregate (usually sand, gravel, or crushed stone), a binder (natural or synthetic cement), chemical additives, and water. Structural concrete normally contains one part cement to two parts fine mineral aggregate to four parts coarse mineral aggregate, though these proportions are often varied to achieve the strength and flexibility required in a particular setting. Weight of one cubic foot concrete is 150 pounds.

Glass Fiber Reinforced Concrete (GFRC)

is a type of fiber reinforced concrete. G lass fiber concretes are mainly used in exterior building fa9ade panels and as architectural precast concrete. This material is very good in making shapes on the front of any building and it is less dense than steel.

Screeding

is the process of cutting off excess wet concrete to bring the top surface of a slab to the proper grade and smoothness.

Additional information: CONCRETE

• Qualities of light weight concrete: Good thermal insulation, excellent fire resistant. • Maximum deflection for concrete beam = L/ 180.• At 50°F the earliest forms can be removed in 24 hrs. • Fresh concrete surface should be screed ing, floating, brooming, and trowelling. • Concrete that has becomes partly set in the mixer should be discarded and not used at all. • In hot weather, newly placed concrete will set better when it is covered with wet burlap. • The one which will lengthen the setting time of concrete is increased in proportion of water used. • Quick drying of concrete will most likely cause cracks. • The best way to prepare an old concrete surface for a new layer of concrete is to clean it and apply a rich cement mortar. • Tool that is used to finish concrete so that a very smooth surface is obtained is a trowel. • A concrete mix can be made more workable without reducing its strength by adding cement and water to the mix.• forms for concrete are usually oiled to make stripping easier. • The head of a bar that was used to break concrete has been redressed and tempered. This is usually bad practice because it should not be tempered. • Concrete driveways shall have a minimum thickness of7 inches.• Air entraining agent: Improve the resistance of the concrete to freezing and thawing conditions. • If a batch of concrete is very stiff, its main characteristics is that it has a low slump. • The main difference between reinforced concrete and plain concrete is that plain concrete uses larger aggregates for reinforcing. • The main advantage of using large coarse aggregates in a concrete mix is that there is saving of cement. • Concrete shall not be placed exceeding 18 inches in thickness. • Place concrete in layers not exceeding 18 inches in thickness. • Concrete placements of 4 feet thickness or greater shall be considered mass concrete.• With regard to the placing of concrete, the contractor is generally permitted to choose his own method of placing of concrete. • The best reason for using vibrators in concrete construction is to consolidate the concrete. • If there is a small amount of water on the surface of a newly laid concrete sidewalk, the recommended procedure before finishing is to allow it to evaporate. • In the concrete for reinforced concrete, coarse aggregate greater than a specified size is not permitted primarily because large size coarse aggregate may not pass between the reinforced bars. • Capacity of a concrete truck is, the largest usual size is 9 cubic yards, the smaller are about 3 cubic yards. • Vermiculite concrete is a light weight concrete.