Requirements for concrete quality

Uniform quality of a concrete is of great importance for modern construction. Procedures for obtaining high quality concrete are given in many publications.

Both normal weight and structural lightweight concrete can be made uniform with proper mix proportioning and production controls. Normally concrete for segmental construction will have a slump of 3 to 5 in., and 28-day strength greater than the strength specified by structural design. It is recommended that statistical methods be used to evaluate concrete mixes

The methods and procedures used to obtain the characteristics of concrete required by the design may differ somewhat depending on whether the segments are cast in the field or in a plant. The results will be influenced by curing temperature and type of curing. Liquid or steam curing or electric heat curing may be used.

A sufficient number of test mixes should be made to assure uniformity of strength and modulus of elasticity. Careful selection of aggregates, cement, admixtures and water will improve strength and modulus of elasticity and will also reduce shrinkage and creep. Soft aggregates and poor sands should be avoided. Creep and shrinkage data for concrete mixes should be available or should be determined by tests.

It is better not to use corrosive admixtures such as calcium chloride or those containing substantial chlorides, nitrates, sulfates and fluorides. Water-reducing admixtures which improve concrete resistance to environmental effects, such as de-icing salts and freeze and thaw actions are highly desirable. However, their use should be rigidly controlled in order not to increase undesirable variations in strength and modulus of elasticity of the concrete.

Mixes should be designed to minimize creep and shrinkage. Reliable data on the potential of the mix in terms of strength gain and creep and shrinkage performance should be used as a basis for improved design parameters.

VOCABULARY NOTES

normal weight concrete обычный бетон

lightweight concrete лёгкий бетон

slump оползание грунта, оползень

curing выдержка

creep ползучесть

 

EXERCISES

I. Give the Russian equivalents for the following terms:

concrete; prestressed concrete; precast concrete; mix proportioning; lightweight concrete; aggregate; cement; admixtures; strength; sand; elasticity.

II. Put up questions to the following sentences:

1. Quality of a concrete is of great importance for modern construction.

2. Different statistical methods must be used to evaluate concrete mixes.

3. The results will be influenced by curing temperature and type of curing.

4. Careful selection of aggregates, cement, admixtures and water will improve strength, modulus of elasticity and will reduce the shrinkage and creep.

5. Water-reducing admixtures are highly desirable.

III. Make up a brief summary of the text.

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:

1) air-entraining agents,

2) 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 de-icing. 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.

Air-entraining admixtures. The materials used as air-entraining agents include vinyl resin and tall oil, the latter a by-product of the paper industry. Both are derived from southern pine trees. Petroleum by-products and other materials are sometimes used. These materials have generally been treated by an alkali to form a salt or soap.

Normally, the amount of air entrained should be about 9 per cent of the mortar portion of the mix. The amount of air, expressed as a percentage of the entire mix, will of course be less than this. In a conventional mix with up to 1,5 in. aggregate, 4 to 5 per cent of air is usually sought. For best durability the air bubbles should be small, ranging from about 0,001 to 0,003 in. in diameter. They should be less than 0,01 in. apart, yet not interconnected.

Air entrainment has improved the resistance of concrete to frost action to an amazing degree in laboratory tests. Air-entraining concrete seems to be performing satisfactorily in the field, although there has been some scaling and freeze-thaw damage in severe exposures. It is an oversimplification to say that if air entrainment is used there will be no scaling or freeze-thaw deterioration. Also necessary are durable aggregates and good proportions – a low water/cement ratio and a cement content that will give adequate strength when freezing occurs.

The principal disadvantage of air entrainment is that it reduces the strength of most mixes and is harmful to other properties, such as abrasion resistance, which depend on strength. For normal mixes, 4 to 5 per cent of entrained air will result in a strength reduction of about 20 per cent. In lean mixes, air entrainment may not reduce the strength; in fact it may increase it.

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 make it possible to meet a strength requirement that could not otherwise be met with the cement and aggregate at hand.

In their basic formulation, these materials usually retard the set of the concrete. This property is useful in warm-weather construction to delay the initial set and help prevent cold joints. Some state highway departments specify retarders in bridge decks so that the concrete will remain plastic until all of it is in place, thus preventing cracks as the forms sag under the gradually increasing weight of the concrete deposited during the pour. The admixture manufacturer may modify the basic formulation with accelerators and other additives to change the setting time and other properties.

VOCABULARY NOTES

admixture примесь; добавка

aggregate заполнитель

bubble пузырёк; раковина (в металле)

degree степень; градус

to assume принимать на себя; допускать

entrainment погружение; проникновение

cohesive способный к сцеплению; связующий

to thaw таять

scaling образование окалины; отслаивание

to occur случаться, происходить

lean скудный

additive присадка; добавка

retarder замедлитель

to prevent предотвращать; предохранять

to result in иметь результатом

to result from происходить в результате, проистекать

EXERCISES

I. Translate into Russian:

1. Water-reducing admixtures can be used for achieving the desired effects.

2. This admixture improves one property of concrete only.

3. It permits an essential reduction in water requirement.

4. These materials have generally been treated by an alkali.

5. Increase the cement content of the mix!

6. It makes the mix more workable.

7. In some mixes air entrainment will reduce the strength.

8. These materials usually retard the setting of the concrete.

II. Answer the questions:

1. Speak about the limitations in using admixtures for concrete.

2. Which are the two basic groups of admixtures?

3. Which are the effects of air entrainment?

4. Name some materials used as air-entraining agents.

5. Describe some disadvantages of air entrainment.

6. What effect do water-reducing admixtures have?

III. Translate the text into Russian in written form and make a back translation orally.

GAS CONCRETE

Lime and silica are ground together to very fine limits. The siliceous material can vary considerably in its composition. Much use is made of such waste materials as fly ash from power stations, shale ash, blast furnace slag, as well as natural pozzolana, pumice, etc.

The degree of foaming in the gas concrete, and thus its specific gravity, is determined by the amount of alumina powder or other agent added. The practical limits of the final density are between 13 and 90 lb. per cu ft. If the gas concrete is allowed to harden on its own, it usually takes about three weeks before the final strength is achieved. It is more customary to accelerate the setting of the gas concrete by steam hardening it in autoclaves with superheated steam at about 140 lb. per sq. in. The steam hardening process takes about 15–20 hr.

Gas concrete can be used as thermally insulating floor screeds or as an additional thermally insulating layer on top of a concrete roof.

The density of the gas concrete mix can be varied according to the degree of thermal insulation required, but at the same time it must not be forgotten that the lower density grades, that have the highest degree of insulation, are also the softest.

Air-cured gas concrete can be used for the manufacture of special components for the refrigeration industry. Such blocks are cast to special dimensions.

Gas concrete can be cast horizontally to form room-sized outer wall units. It is possible to incorporate electric conduit pipes, piping for the cold and hot water systems and also drainage pipes. The units usually include windows and doors, and are reinforced by embedding steel mesh in the mix.

Cast gas concrete is often used as the thermally insulating layer in “sandwich wall” units.

Gas concrete is often used as a thermally insulating layer when casting buildings by a continuous casting technique.

VOCABULARY NOTES

silica кремний

siliceous кремнистый

fly ash летучая зола

blast furnace slag доменный шлак

pumice пемза

foam пена

foaming пенообразование

density плотность

cure вулканизация; выдерживание (бетона)

cast лить, отливать (металл)

to incorporate соединять; помещать, включать

to embed заливать; вставлять; внедрять

mesh отверстие, ячейка

EXERCISES

I. Read the text and answer the following questions:

1. Which materials are used for the production of gas concrete?

2. How can the setting of gas concrete be accelerated?

3. Can you name the main purposes for which air hardening gas concrete is used?

4. Where can gas concrete be successfully used?

5. What can you say about cast gas concrete?

II. Ask questions to which the words printed in italics are the answers:

1. Steam hardening accelerates the setting of gas concrete.

2. Gas concrete can be sawn into all desired shapes.

3. After foaming the material is treated in autoclaves.

4. Experts watched the process.

5. The strength of concrete depends on several factors.

6. The architect’s design necessitated the laying of pipes.

7. These concrete blocks come from a modern factory.