Test III

Select the best possible answer out of the choices from the list to the statements (multiple choice):

1 Portland cement, being a hydraulic type of cement,. … .

a. evaporates in great extent after curing and shrinks.

b. depends on the temperature variations.

c. needs water for developing strength.

d. can result in hundreds of tiny cracks.

2. In case you have selected wrong or weak cement, ... .

a. the resulting increase in tension will be too much for the concrete.

b. concrete may get dramatic and deep cracking.

c. your concrete won’t have enough strength and will develop cracks.

d. it will obviously cause random cracks in concrete.

3. Spalling typically occurs when … .

a. there are extreme temperature variations.

b. too much water is added to the concrete mixture.

c. the strength requirement of the concrete is violated.

d. the concrete surface is restrained by formwork.

4. Concrete, that lacks structural support, … .

a. can suffer catastrophic failure.

b. can cause some portions of the slab to expand.

c. can obtain increase in tension too much for the concrete.

d. can acquire hairline cracks.

5. After the metal has started to rust, … .

a. concrete won’t have enough strength and will develop cracks.

b. it will give room for shrinkage.

c. the resulting cracks will allow more water into the concrete.

d. it will cause the concrete to crack in a straight, even line.

Test IV

Complete the sentences by filling in the correct word that fits a statement (gap fill). Choose the appropriate words from the word bank given below.

A. change B. reinforcement C. hydration D. proper

E. term F. strength G. corrosion H. caused I. sufficient

J. mixture K. flowability L. dries

1. Most of the time cracks could be avoided by using …… concrete mix proportions.

2. After the wet concrete mix is spread, it takes a few weeks to complete the whole …… process.

3. If the concrete does not get …… water for hydration, it tends to develop cracks.

4. The most common cause of cracks in concrete is the shrinkage, that occurs as the material …… .

5. Temperature …… is a major cause of cracks in large concrete applications.

6. Some of the most severe cracks in concrete are …… by inadequate structural support.

7. Many items made from concrete contain metal …… , which helps increase the material's tensile strength.

8. Spalling is the …… given to concrete that has a rough, flaky surface.

9. Spalling typically occurs when too much water is added to the concrete …… .

10. Galvanized metals should be used for the ability to resist …… and rust.

UNIT 13

Working with Versatile Lightweight Concrete

Read, translate the following text and be ready to fulfil the tests:

While still not light and resilient enough for a vehicle tire, properties of lightweight concrete lend creative architectural design possibilities to the realm of structural engineering. The properties of lightweight concrete have some remarkable advantages. While the term “lightweight concrete” sounds strange, the material itself is anything but contradictory. Regular Portland cement concrete production consists of loading aggregates (gravel and sand), cement, and water, then mixing to a thick slurry consistency. The mixing process usually incorporates a small amount of air into the concrete as well. Once the mix has cured it weighs in at around 145 pounds per cubic foot, give or take 5 pounds or so.

Most of this variation is due to the amount of air present and minor differences in the weight of the aggregates. Clearly, to make concrete lighter, more air and/or lighter aggregates need to be used. When more air or gas is incorporated, the result is referred to as lightweight aerated or foamed concrete. When lighter aggregates are loaded, the material is generically referred to as lightweight aggregate concrete. Lightweight aerated concrete typically weighs in between 3 pounds per cubic foot up to around 70 pounds per cubic foot. While this is still not suitable for a vehicle tire, it is tremendously useful for a wide variety of architectural applications. Due to the entrapped gases, it also has good insulating properties.

Roofing, walls, fences, ornaments, vaulted ceilings, sculptures, stairs, doors, even canoes are good examples of the usefulness of this material. It can be cast into blocks or decorative panels to create the impression of stone without the massive weight. And of course it still retains much of the durability of regular weight concrete. The major departure from regular concrete is lower structural and compressive strength.

Lightweight aggregates can consist of naturally occurring gas expanded materials such as vermiculite, pumice, certain volcanic rocks, and some diatomaceous earths. They can also be manufactured by heating some types of clay, shale, slate, and obsidian in a rotary kiln, or by using air quenched cinders and blast-furnace slag. These expanded aggregates incorporate air into a porous structure which retains some degree of structural strength. When used in structural concrete the mix has a density ranging from 90 pounds per cubic foot up to about 115 pounds per cubic foot.

However with a suitable mix design strengths similar to regular weight concrete can be attained. The major advantages of this material over regular concrete are lower densities, better fire resistance and greater insulating properties. Less reinforcing steel is required and a lower dead load of the structure can be achieved. This in turn yields smaller supporting columns, footings, and other engineering elements. It also results in improved seismic characteristics, longer achievable spans, thinner cross sections, and generally smaller sized structural members.

Read and memorize the active vocabulary to the text: