PROPERTIES OF FRESH CONCRETE BASIC AND TUTORIALS

FRESH CONCRETE PROPERTIES BASIC INFORMATION
What Are The Properties Of Fresh Concrete?

Concrete workability is the relative ease with which a fresh mix can be handled, placed, compacted, and finished without segregation or separation of the individual ingredients. Good workability is required to produce concrete that is both economical and high in quality.

Fresh concrete has good workability if it can be formed, compacted, and finished to its final shape and texture with minimal effort and without segregation of the ingredients. Concrete with poor workability does not flow smoothly into forms or properly envelop reinforcing steel and embedded items, and it is difficult to compact and finish.

Depending on the application, though, a mix that has good workability for one type or size of element may be too stiff or harsh for another, so the term is relative. Each mix must be suitable for its intended use, achieving a balance among required fluidity, strength, and economy.

Workability is related to the consistency and cohesiveness of the mix and is affected by cement content, aggregates, water content, and admixtures. Concrete workability is increased by air entrainment.

Entrained air is different from entrapped air. Entrapped air usually accounts for about 1 to 2% of the volume of fresh concrete and its inclusion is not intentional. Small amounts of air are inadvertently entrapped in the concrete mixing process.

Air content can be intentionally increased by a controlled process called air entrainment, which uses either a special cement or a chemical admixture to introduce evenly distributed, microscopic air bubbles. In fresh concrete, the tiny air bubbles act almost like ball bearings or a lubricant in the mix, and in hardened concrete they increase winter durability.

Too much air reduces the strength of concrete, though, so air content is generally recommended to be within the ranges shown in Figure 2-1.

Consistency is the aspect of workability related to the flow characteristics of fresh concrete. It is an indication of the fluidity or wetness of a mix and is measured by the slump test. Fresh concrete is placed in a metal cone.

CIVIL CONSTRUCTION PROJECT COST CONTROL BASIC AND TUTORIALS

CIVIL CONSTRUCTION PROJECT COST CONTROL BASIC INFORMATION
How To Control  Civil Construction Project Cost?


It is during the design stage that measures to keep the cost of a project within a budget figure are most effective. All possible savings in design need to be sought, not only because this is manifestly in the interests of the employer, but because there are sure to be some unforeseen extra costs that need to be offset by any savings that can be made.

Alternative designs of layout or of parts of the works have often to be studied before the most economic solution is found; hence completion of all design before starting construction makes a major contribution to controlling project cost.

The most prolific causes of extra cost are:

• not completing the design of the works in all essentials before the contract for construction is let;
• not allowing adequate site investigations to take place;
• encountering unforeseen conditions;
• making changes to the works during construction.

The first two listed above can be avoided by taking the appropriate measures. The third, however, is not avoidable even if the site investigations have been as reasonably extensive as an experienced engineer would recommend.

The last – changes during construction – can be minimized by ensuring designs are complete before construction commences, and that the employer takes time to assure himself that the works as designed are what he wants. But some changes are unavoidable if, during construction, the employer finds changed economic conditions, new requirements or more up-to-date plant, or new legislation forces him to make a change.

The designer should keep aware of possible changes to the employer’s needs and other technical developments, and not so design the works that possible additions or alterations are precluded or made unacceptably expensive.

If tenders are received which exceed the budget estimate by so large a sum that the employer cannot accept any tender, means of reducing the cost may have to be sought. Generally speaking, down-sizing a part or the whole of the works is usually not as successful in reducing costs as omitting a part of the works.

Reducing the output of some works or the size of a structure by 25 per cent, for instance, seldom results in more than 10 per cent saving in cost, and can make restoration at a later date to the full output or size an expensive and uneconomic proposition.

If the employer can find some part of the works which can be omitted, this is a more secure way of reducing the cost of a project, and it should be possible to negotiate such an omission with the preferred tenderer.