Concrete mix

The type of concrete mix and the properties of the hardened concrete may be altered. In order to do that the proportions of the ingredients may be varied, the type of cement and aggregates may be altered and admixtures may be used. The aggregates must be strong and durable materials in order to take the loads that are applied and ought not to react chemically with the cement paste. Aggregates help to reduce the volume changes in the concrete due to shrinkage or temperature changes. Aggregates can be selected in order to provide a concrete with properties such as low weight, low shrinkage, high thermal insulation, good fire resistance.

Various kinds of cement are available, for example, those which harden rapidly or those which resist chemical attack, such as sulphates in the soil. Blended cements are also available which give low heat output during the curing period of concrete. The blended cements consist of mixtures of Portland cement and admixtures which are carefully graded. These may replace up to about 70% of the Portland cement which would have been required.

The mix proportions of the concrete, and admixtures, will generally control the strength, permeability, frost resistance and resistance to chemical attack. The strength of the hardened concrete depends greatly on the water to cement ratio. A typical value of this ratio should be about 0.5 by weight. An increase in the value of this ratio would produce a concrete with more voids and result in lower compressive strength. A large number of properties, such as tensile strength, durability, chemical resistance and density are connected with the compressive strength of concrete.

Notes:

attack n. атака, разрушительное воздействие

blended cement многокомпонентный портландцемент

frost resistance морозостойкость

output n. выработка

react v. реагировать

replace v. заменять

sulphate n. сульфат

Ответьте на вопросы к тексту.

1. How can the properties of concrete be altered?

2. What kind of material must aggregates be?

3. How do aggregates influence the volume changes in the concrete?

4. How can aggregates be selected?

5. What kinds of cement are there?

6. What do the blended cements consist of?

7. What is the amount of admixtures in the blended cements?

8. What will the mix proportions of the concrete control?

9. What does the strength of the hardened concrete depend on?

10. What properties are connected with the compressive strength of concrete?