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.