II. Substitute the attributive clauses for past participles
1) There are different kinds of concrete which are produced for special purposes.
2) The amount of water which is used to produce concrete must be in definite relation to cement. .
3) The aggregates which are graded properly will increase the density of concrete.
4) Concrete which is prepared right at a building site is known as "in-situ".
5) Many buildings which were constructed by the Romans were built of concrete.
III. Find out sentences with the predicate in Passive
Voice and give possible variations of translation.
IV. Form nouns from the following words.
Add, construct, weak, produce, free, dense, reinforce.
V. Answer the following questions:
1. How is concrete prepared ?
2. Is concrete a modern material ?
3. What influences the strength of concrete ?
4. Why are clean ingredients needed to prepare concrete?
5. How does temperature affect the setting of concrete mix ?
6. What is placed concrete covered for ?
7. Why is the retention of moisture in concrete necessary after placing?
MATERIALS FOR HOME-READING.
Types of concretes.
In normal concretes the aggregates are graded in size from fine to coarse in order to reduce the amount of void space to be filled by cement. Recently another type of concrete has been developed in which all fine aggregates are omitted and only enough cement is used to coat the surface of the coarse aggregates sufficiently to cause it to stick together. The result is a concrete having fairly large voids. This is known as "No-fines" concrete and has certain limited range of usefulness owing to its freedom from the fine capillary path along which moisture could travel. It is not a particularly lightweight concrete. There are also "cellular" concretes made bу using materials which form gas during the mixing of concrete, These give a product of very light weight, because when set it contains a large number of small voids. It may be noted here that this reduction in weight is inevitably accompanied by considerable decrease in strength. Another similar type of lightweight concrete of recent development is made by entraining air bubbles into the mix to which a substance has been added to keep the bubbles stable during setting. Concrete may be made on a building site and poured into position as wet mix, or it may be used as the material for making prefabricated units in a factory, so that another classification is "in-situ" and "precast" concrete. One of the most difficult things with concrete is to produce a pleasing surface appearance. This raises so many problems that it almost justifies a further classification into concrete exposed as facing work, and concrete covered up or used where appearance is of little importance.
The effect of water content оn the strength of concrete.
The important thing here is the water cement ratio. The lowest water content will give a concrete sufficient plasticity to place in position. For normal concrete the water content should be 5.5 gal. or 7.5 gal. per hundredweight of cement. The higher figure is used where placing conditions makе a fairly wet mix necessary. For concrete which is to be compacted by vibration a lower water content is used. It should be noted that there may be a considerable quantity of water present in the sand and that reductions must be made for this. The effect of proper mixing upon the strength of concretes: unless concrete is thoroughly mixed it will not develop its full strength. A fair idea of whether a concrete properly mixed can be got from its appearance, an even colour indicating a good mix. But in no case should the mixing time in a mechanical mixer be less than 2 minutes.