Placing and Curing of Concrete and Quality Checking

The placing of the concrete mix and its distribution in the form or mould is one of the most labour-consuming operations of concreting.

At present placing and distribution of concrete are mechanized and the operations are carried out with the aid of concrete placers or machines of a simpler construction-concrete distributors. Concrete placers differ from concrete distributors in that they permit the processes of both placing concrete and its distribution to be mechanized in a great measure.

The quality of placing concrete is a very important factor in building durable concrete or reinforced-concrete structures.

The concrete mix must be placed in the form in a manner that no air be entrained in the mass; corners and restrictions in the form must be filled with most care. Placement and leveling of the concrete mix are followed by compacting.

The methods of compacting concrete manually by rodding or with the aid of tampers are almost obsolete now.

Mechanized placing and compacting of the concrete mix by vibrating, vibrostamping, centrifuging, vacuum treatment, rolling and vibrolling are widely practiced.

Vibration consists in uninterrupted positive shaking of the concrete mix by imparting frequent vibratory motion to the entire mass to ensure good compacting.

Vibrostamping. In this method of compacting the treated concrete mass is subjected to the simultaneous action of the oscillatory motion of the vibrator and the load exerted by the stamp, i.e. the method ensures vibration under pressure permitting the outlines of the stamp or dye to be reproduced on the surface of the product being treated.

Вариант 23.

Special Properties of Concrete

Concrete is a porous material. Pores may be formed in concrete due to incomplete evacuation of entrained air in the course of compacting the concrete mix. It is impossible to produce absolutely dense concrete even by practicing dense placement of the concrete mix by vacuum treatment and repeated vibrating. Pores are formed in concrete also as a result of evaporation of water which fails to react with the cement constitution in the course of hardening.

The density of concrete can be increased not only by vacuum treatment, repeated vibrating or by reducing the content of evaporating water which fails to react with cement.

The placement of concrete of a high density can be ensured by the following means:

1. by selecting rationally graded aggregates (with a minimum void age) permitting a reduction in the amount of the introduced cement paste and, hence, a reduction in the water content of the mix;

2. by reducing the mobility of the concrete mix and, hence, by diminishing the cement paste constituent but this requires more intensive compacting;

3. by diminishing the water-cement ratio; this results in a smaller water content, increased density of the cement stone, provided more intensive compacting is practiced;

4. by applying cements binding a great amount of water in the course of hardening such as high-strength Portland cement, alumina cement, expanding cement, etc.; by introducing plasticizers such as soap-naphtha, acidol or alkali-treated wood tar, polymers and materials producing similar effects into the concrete mix.

Вариант 24.

Admixtures for Concrete

Concrete can sometimes be improved by an admixture added to the cement, aggregates and water to modify one or more of the properties of the mix. Admixtures are not magic powders that can be added indiscriminately to poor concrete mixes to make good concrete. Neither can it be assumed that they will necessarily make good concrete better. The right admixture for the job must be used if the admixture is to do more good than harm. When a change is made to improve one property of concrete, some other properties will be affected, frequently adversely.

Principal admixtures are: air-entraining agents and water-reducing admixtures. Perhaps the most widely used admixtures are air-entraining agents. Air-entrainment is used to improve the resistance of concrete to damage from freezing and thawing. It also makes concrete slabs much more resistant to scaling where salts are used for deicing. It makes the mix more workable or at least more cohesive. It permits a substantial reduction in the water requirement and consequently the cement content in mass concrete and has helped with the temperature problem by reducing the amount of heat generated during setting of the cement. Air entrainment is generally considered to be the greatest advance in concrete technology in recent years.

Water-Reducing Admixtures. Use of water-reducing admixtures has expanded rapidly in the past few years. The name comes from the ability of these additives to reduce the mixing water required. Also they generally increase strength and they may take it possible to meet a strength requirement that could not otherwise be met with the cement and aggregate at hand.

 

 

Вариант 25.

Hydration of Concretes

Concrete is a building material which originates by mixing, laying, compacting and treating the mix containing the cement (binders), aggregates, water, ingredients and admixtures. Under certain conditions this mix obtains in time mechanical properties. The set of all chemical, physical and mechanical reactions which are effective at the same time and whose resultant is permeated, is called hydration. There exist a long term efforts to penetrate deep into the knowledge of hydration processes. There are known many theories, many a partial knowledge. It is not the purpose of this paper to give their detailed summary. However, it is possible to generalize that the prevailing part of explanations supposes that the dominant factor is that of the reaction cementing materials and water. This reaction lies in progressive dissolution of the cementing materials (we do not mean cement only, but possible active aggregates or mixes of various cementing materials, etc.) and through the complicated process of heterogeneously forming gel and metastabile crystalloid structure.

Considerably differences exist in the opinions about the fact, what sort of gel and metastabile crystalloid stages are continuously created, about their kinetics of origin and perishing, about what may be considered as a final product of hydration and about the period of duration of hydration processes. In this respect most easily are accepted the opinions being possible much general enabling wide interpretation.

 

Вариант 26.

HOT-WATER SUPPLY

The term “central heating” applied to the heating of domestic and other buildings indicates that the whole of a building is heated from a central source, usually an independent boiler, fired by solid fuel, gas, electricity or fuel oil. In general, a heating system should be designed so that the water will circulate by gravity. In some installations, circumstances are such that a pump or accelerator must be used to achieve a satisfactory circulation. This should be avoided if possible.

When designing a heating system for a large building, it is usual – in the interests of economy and to ensure efficient heating – to first calculate how much heat will be needed to maintain the building at the desired temperature. Then the size of the boiler and the amount of pipe and radiator heating surface required to give out this heat will be estimated. For small systems, “rules-of-thumb” methods and past experience are generally a sufficient guide.

A steam, or a hot water heating plant consists essentially of the radiators, the boiler and the system of piping connecting the former with the latter. Steam or hot water from the boiler is circulated through the piping and radiators: in these the steam condenses giving up its latent heat and the water given up some of its heat, thus warming the rooms. In the usual hot water installation, the boiler, pipes and radiators are kept full of water at all times, an expansion tank being provided to compensate for the increase in volume of water when heated and to prevent explosions in case of generation of too much steam.

 

Вариант 27.

Boilers. The boiler is usually placed at the lowest available point in the building, having regard at the same time to the convenience of stoking and delivery of fuel.

The boiler may be one of a number of types. It may be solid one-piece casting, rectangular in form; it may be sectional; or it may be conical in shape and wrought or cast iron. For smaller systems, the first and last-named types are both cheap and suitable. The sectional boiler has the advantage of the possibility of added sections should more heat be needed subsequent to initial installation.

System of piping. For steam heating the systems of piping usually employed are the ordinary one-pipe system and two-pipe system. In the former, but one connection is made to each radiator, this connection serving both as inlet for the steam and an outlet for the water of condensation. In the latter, there is a supply pipe and a return pipe for each radiator. The two-pipe system is expensive, and hot used generally in steam heating except for indirect radiators which must always have two connections in order to function properly.

In hot-water heating, although one-pipe systems may be used, it is considered the best practice to have a supply pipe and a return pipe for each radiator. Rules and tables for computing the size of pipe for both system and hot water heating will be found in handbooks.

In selecting a heating plant for residences there must be considered the size, the type of building, the climate and the first cost of operation.

Вариант 28.

SYSTEMS OF HEATING

Heating. In order to maintain standard room temperature, the heating apparatus must supply heat to replace the lost through the walls, floors, and ceilings, and, in addition, the heat necessary to warm the cold fresh air used for ventilation. Heat is lost by conduction through cracks around doors, windows, etc.

Systems of heating. Leaving stoves and fireplaces out of consideration, the systems ordinarily employed for heating may be classified as follows:

a) hot air

b) steam

c) hot water

Hot air systems. In a hot air system, heated air from the furnace is introduced through leaders, stacks, and registers into the room. This air is at a higher temperature than the room, and, in flowing across the ceilings and down by the walls, heat is abstracted until it is eventually cooled to the desired room temperature. Fresh warm air from the furnace then forces the air that has been cooled to room temperature out of the room through cracks, fireplaces, etc. A heat balance may therefore be written as follows: the heat given up by the entering air equals the heat lost by conduction.

The force which causes hot air to flow from furnace to room results from the difference in densities of the cold air outside and the warm air inside the furnace and pipes.

Advantages. A hot air system is cheap to install, has a low cost of maintenance, and is not hard to manage, its operating cost is little, if any, greater that of hot water or steam system of equal capacity.

Вариант 29.