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 — A, prevent fast evaporation, until hydration has proceeded well. Concrete does not harden by drying out. It hardens because the cement reacts chemically with water, a reaction called hydration, which knits the mix into an interlocked solid over days and weeks. The water that reaction needs is the water already mixed into the concrete, so if the surface dries out in sun or wind the reaction near that surface simply stops, leaving the top layer weak, dusty and prone to cracking. Wetted straw or gunny bags laid over the fresh surface hold moisture in and shade the slab, so the reaction can continue in the whole thickness. Practical references say exactly this: concrete must be kept moist during curing to achieve optimal strength and durability, and abnormally fast drying and shrinkage caused by wind during placement raise tensile stresses before the concrete has gained enough strength, producing shrinkage cracks. Ponding water, spraying, wet burlap or hessian and plastic sheeting are the standard ways of doing it, and the first three days matter most.
- (b)protect the concrete structure from contamination by dust particles — Dust settling on a fresh slab is a cosmetic nuisance, not a structural danger, and a wet sack is a poor dust cover in any case. Nothing about wetting the covering would help against dust — it is the moisture, not the sheet, that is doing the work.
- (c)prevent development of any fungus on the surface — Wet, shaded surfaces encourage growth rather than prevent it. Fresh concrete is also strongly alkaline, so it is hostile to fungus without any help; this option gets the direction of the effect backwards.
- (d)give smoother and cleaner surface over cement structure — Finish is produced by the trowelling and floating done while the concrete is still plastic, before any covering goes on. A gunny bag laid over the surface would, if anything, mark it.
Cement, sand, aggregate and water make concrete. The hardening is a chemical reaction between the cement and the water, in which silicate compounds form and grow into an interlocking network — hydration. Keeping the fresh concrete moist and at a reasonable temperature while that reaction proceeds is called curing, and it is a separate step from setting. Over ninety per cent of a mix's final strength is typically reached within four weeks, and the first three days of hydration and hardening are the critical ones.
The whole item turns on one misconception, and it is a common one — that concrete sets by drying. If that were true, wetting the covering would be perverse. Once a candidate knows that water is a reactant rather than something to be got rid of, only the first option makes sense, and the other three can be read off as plausible-sounding but purposeless. The same principle explains several site practices worth recognising: ponding water on a fresh slab, spraying it at intervals, wrapping columns in wet hessian, and spraying a curing compound that leaves a water-retaining film. In hot, dry or windy weather all of these matter more, because that is when the surface loses water fastest and cracks first.
- Concrete hardens by hydration — a chemical reaction between cement and water — not by drying out.
- Concrete must be kept moist during curing to achieve its intended strength and durability.
- Hydration and hardening during the first three days is the critical period.
- Standard curing methods include ponding, spraying, wet burlap or hessian coverings and water-retaining curing compounds.
- Fast drying from wind or sun raises tensile stress before the concrete has gained strength, and causes shrinkage cracking.
Concrete is cured, not dried — the water is a reactant, not a nuisance.
- Believing concrete sets by drying; it sets by reacting with water, which is the whole point of curing.
- Confusing setting with curing — setting is the initial stiffening, curing is the sustained care that builds strength.
- Assuming more water in the mix always helps; excess mixing water weakens concrete, while external curing water helps.
Usually as an everyday-observation item asking why a site practice is followed; the gypsum-in-cement point and the composition of concrete are the neighbouring questions.
No directly related past PYQ was found.
- practice — not a real PYQ
Gypsum is added to cement clinker chiefly in order to
- (a)increase the strength of the finished concrete
- (b)slow down the rate of setting of the cement
- (c)give the cement its grey colour
- (d)make the cement waterproof
Answer(b) slow down the rate of setting of the cement — without it the cement would stiffen almost at once and could not be placed and worked.
- practice — not a real PYQ
The hardening of concrete after it is placed is chiefly the result of
- (a)evaporation of the mixing water
- (b)a chemical reaction between cement and water
- (c)compaction under its own weight
- (d)absorption of carbon dioxide from the air
Answer(b) a chemical reaction between cement and water — the hydration reaction builds an interlocking solid network, which is why the concrete must be kept moist.