The surface of the concrete structure of a new construction is covered with straw/gunny bags by wetting. This is done to:
- A.prevent fast evaporation, until hydration has proceeded well
- B.protect the concrete structure from contamination by dust particles
- C.prevent development of any fungus on the surface
- D.give smoother and cleaner surface over cement structure
Correct answer
Explanation
The correct answer is A. Wet straw or gunny bags keep the fresh concrete moist, so water does not evaporate before the cement has hydrated properly.
Cement sets and gains strength through hydration, a chemical reaction between its compounds and water that goes on for days and weeks. If the surface dries out early, hydration stops, the concrete stays weak and shrinkage cracks appear. Keeping it damp in this way is called curing, which is why builders spray water and cover new slabs with wet sacking for about a week or more.
B is wrong: the bags are kept wet to hold moisture, not to keep dust away.
C is wrong: damp coverings would, if anything, favour fungus; preventing it is not the aim.
D is wrong: the finish of the surface is done by trowelling before curing begins.
Exam tip: curing of concrete means keeping it moist so that hydration of cement continues and strength develops.
Practice Questions
View allWhich of the following properties is true for a tooth paste?
- A.It is acidic
- B.It is neutral
- C.It is basic
- D.It is made up of Calcium phosphate, the material of tooth enamel
Show answer
Toothpaste is mildly basic so that it can neutralise the acid that causes tooth decay. Bacteria in the mouth break down sugar and food particles left on the teeth and produce acids.
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- A.P' = P
- B.P' = 2P
- C.P' = P/2
- D.P' = P/4
Show answer
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Read the full explanation →In which of the following pairs are the ions isoelectronic?
- A.Mg²⁺, Ar
- B.Na⁺, O²⁻
- C.Al³⁺, Cl⁻
- D.K⁺, Ne
Show answer
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- A.The disc
- B.The sphere
- C.Both have the same moment of inertia
- D.The information provided is not sufficient to answer the question
Show answer
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Read the full explanation →Numerically two thermometers, one in Fahrenheit scale and another in Celsius scale shall read same at
- A.−40°
- B.0°
- C.−273°
- D.100°
Show answer
At this one temperature the Celsius and Fahrenheit scales show the same number. The two scales are linked by F = (9/5)C + 32.
Read the full explanation →