Your Answer : (Not Answered)

Correct Answer : B

Bleeding channels remain continuous when the water-cement ratio is more than 0.7.

As fresh concrete settles, surplus mix water rises to the surface and leaves fine channels behind it. In a normal mix the products of hydration grow into these channels and gradually block them. With a water-cement ratio above about 0.7 there is so much water, and so little cement to hydrate, that the channels can never be closed, so they stay continuous and the hardened concrete remains permeable.

So the effect does depend on the water-cement ratio, which rules out option A, and below 0.7, or at the boundary value, the channels are eventually segmented.

Your Answer : (Not Answered)

Correct Answer : A

The test for low workability concrete is the Vee-Bee test.

The Vee-Bee consistometer vibrates a slumped cone of concrete in a cylindrical pot and records the time, in seconds, for the concrete to remould itself fully. A stiff, dry mix takes longer, so the test gives a useful spread of readings exactly where the slump test fails, for mixes that show little or no slump.

The slump test suits medium to high workability, and a very stiff mix simply reads zero. The compaction factor test is also used for fairly low workability, but it is less sensitive than the Vee-Bee test at the very dry end. The flow table test is meant for highly workable, flowing concrete.

Your Answer : (Not Answered)

Correct Answer : C

The required cement content is 325 kg/m\(^{3}\).

The water-cement ratio is the mass of water divided by the mass of cement, and 195 litres of water weighs 195 kg:

\[ \begin{aligned} \text{cement} &= \frac{\text{water}}{w/c} = \frac{195}{0.6} \\ &= 325\ \text{kg/m}^3 \end{aligned} \]

This is above the IS 456 minimum of 300 kg/m\(^{3}\) for reinforced concrete in mild exposure, so it is adopted. Multiplying instead of dividing gives 117 kg/m\(^{3}\), which is not offered; 472, 383 and 294 kg/m\(^{3}\) correspond to other water-cement ratios.

Your Answer : (Not Answered)

Correct Answer : B

IS 456: 2000 gives \(E_c = 5000\sqrt{f_{ck}}\).

Clause 6.2.3.1 of IS 456: 2000 gives the short-term static modulus of elasticity of concrete as \(E_c = 5000\sqrt{f_{ck}}\), with \(E_c\) and \(f_{ck}\) in N/mm². For M20 concrete this gives about 22,360 N/mm².

The older IS 456: 1978 used \(5700\sqrt{f_{ck}}\), which is why nearby numbers appear among the options, but 500, 700 and 5500 are not the code coefficients.

Your Answer : (Not Answered)

Correct Answer : D

The modulus of elasticity of concrete increases with Increase in age.

The modulus of elasticity rises with the strength of concrete (IS 456 relates it to the square root of the characteristic strength), and strength grows with age as hydration continues.

The other three all lower strength and so lower the modulus: a higher water-cement ratio makes weaker, more porous concrete, a shorter curing period stops hydration early, and less vibration leaves air voids.

Your Answer : (Not Answered)

Correct Answer : C

The key's answer is 5 mm.

In the Le Chatelier soundness test, the gap between the indicator points is read before and after boiling the cement specimen, and the difference is the expansion. The Indian cement standards allow at most 10 mm of expansion, a value not among the options.

5 mm is the stricter limit the same standards apply when a cement fails the first test and is retested after aeration (spread out and exposed to air for seven days). The key takes that limit. 15 mm is above any limit in the standards, and 4 mm and 9.5 mm are not specified values.

Your Answer : (Not Answered)

Correct Answer : C

Initial setting is mainly due to tri-calcium aluminate (C₃A).

C₃A reacts with water almost at once and with great heat, and it is this reaction that stiffens the paste first. Left unchecked it causes flash set, which is why gypsum is ground into cement: it slows the C₃A reaction enough to leave time for placing.

Tri-calcium silicate (C₃S) hydrates next and gives the early strength of the first weeks. Di-calcium silicate (C₂S) hydrates slowly and gives later strength. The tetra-calcium compound (C₄AF, an aluminoferrite) contributes little to setting or strength.

Your Answer : (Not Answered)

Correct Answer : B

Rapid Hardening Portland Cement conforms to IS 8041: 1990.

IS 8041 covers rapid hardening Portland cement, which is ground finer and has more tricalcium silicate than OPC, so it gains early strength quickly, roughly its 7-day OPC strength in 3 days. It is used where formwork must be removed early or for repair work.

IS 455 is Portland slag cement, IS 12600 is low heat Portland cement, and IS 12330 is sulphate resisting Portland cement.

Your Answer : (Not Answered)

Correct Answer : B

Le Chatelier's apparatus finds the Soundness of cement.

Unsound cement expands after setting because of free lime or magnesia. In the test, a split brass mould with indicator arms is filled with cement paste, cured, then boiled; the separation of the indicator points measures the expansion, which IS 269 limits to 10 mm for ordinary Portland cement.

Setting times are found with the Vicat apparatus, workability is a property of concrete (slump test), and compressive strength is found by crushing mortar cubes.

Your Answer : (Not Answered)

Correct Answer : B

The method for rapid gain of strength is High-pressure steam curing.

In high-pressure steam curing (autoclaving), concrete products are cured in steam at around 175°C and 8 to 10 atmospheres. The high temperature speeds up hydration so much that concrete reaches in about a day the strength normal curing would give in 28 days, and it also improves resistance to sulphates and reduces shrinkage. It is used for precast products.

Sprinkling water and membrane curing only keep the concrete moist at ordinary temperature, so strength develops at the normal rate. Infrared radiation curing is a form of accelerated curing used in cold regions, but it is not the standard method recommended for rapid strength gain.

Your Answer : (Not Answered)

Correct Answer : B

The covers are 50, 40, 25, 20 mm for footing, column, beam and slab.

IS 456 requires a minimum of 50 mm cover in footings, because they are in contact with soil, and 40 mm to the longitudinal bars of columns. For beams and slabs under mild exposure, the usual values in design practice are 25 mm and 20 mm respectively.

The options starting with 75 mm overstate the footing value, 35 mm is not a standard beam cover, and the first option's 20 mm for beams is below the 25 mm conventionally used for main beam bars.

Your Answer : (Not Answered)

Correct Answer : A

Statement 1 is false and statement 2 is true. A higher compaction factor means that the concrete is more workable (it compacts more under its own weight), and a more workable mix has a larger slump, so the slump increases with the compaction factor and does not decrease. Statement 2 is correct: both the slump and the slump flow are lengths, read off in millimetres or centimetres. The other options, which call the first statement true, are rejected.

Your Answer : (Not Answered)

Correct Answer : B

A low water/cement ratio gives a dense concrete of low permeability, so water and aggressive chemicals cannot penetrate easily, which makes the concrete durable. High early strength does not itself make it durable, flaky aggregates give a poor, harder to compact mix and more voids, and high permeability is the opposite of what durability needs, since it lets water and harmful agents in.

Your Answer : (Not Answered)

Correct Answer : C

IS 456:2000 allows a suspended matter limit of 2000 mg/l in water used for mixing concrete (and for curing), as the permissible limit for suspended matter. This is the most generous of the limits in the code. The same table gives organic matter 200 mg/l, inorganic matter 3000 mg/l, sulphates 400 mg/l and chlorides 2000 mg/l for plain concrete (500 mg/l for reinforced concrete). The options 200, 20 and 1000 mg/l do not match the limit set for suspended matter.

Your Answer : (Not Answered)

Correct Answer : B

IS 383 limits the aggregate crushing value of coarse aggregate to 45 percent for concrete other than wearing surfaces, and to 30 percent for aggregate used in concrete for wearing surfaces such as runways, roads and pavements. A wearing surface is exposed to abrasion and traffic, so a stronger aggregate is needed. The values of 20, 40 and 50 percent are not the limits in the code.

Your Answer : (Not Answered)

Correct Answer : A

IS 2386 (Part II) - 1963 covers the estimation of deleterious materials and organic impurities in aggregates for concrete (clay, silt, coal, shale, organic matter and so on). Part I of IS 2386 deals with particle size and shape, and Part IV deals with mechanical properties such as crushing and abrasion values. IS 383 gives the specification for coarse and fine aggregates from natural sources, and sets limits for these materials but is not the test method.

Your Answer : (Not Answered)

Correct Answer : C

Bloated clay (expanded clay) is a manufactured lightweight aggregate. Clay is heated in a rotary kiln until gases are given off and make it swell into light, cellular pellets. Diatomite, tuff and pumice are naturally occurring lightweight materials: diatomite is formed from the fossil shells of diatoms, tuff from volcanic ash and pumice from volcanic froth. They are used as found, and so are not manufactured aggregates.

Your Answer : (Not Answered)

Correct Answer : A

The incorrect statement is the first one, because shotcrete is projected at a high velocity, not a low one. The high speed of the pneumatic jet compacts the material against the backing surface as it hits. The other statements are correct: shotcrete is the more formal name of pneumatically applied mortar or concrete, it is used for thin, lightly reinforced sections such as shell roofs and linings, and it is used to stabilise rock slopes and tunnel faces.

Your Answer : (Not Answered)

Correct Answer : C

The correct matching is 1-C, 2-D, 3-A, 4-B. Portland slag cement has good resistance to chemical attack, so it protects reinforcement from corrosion (1-C). Low heat Portland cement gives off less heat of hydration and is used in massive water-retaining structures such as dams (2-D). Rapid hardening cement gains strength quickly, which makes it suitable for cold weather concreting (3-A). Super sulphated cement resists sulphates and sea water, so it suits offshore structures (4-B). The other codes swap these pairs.

Your Answer : (Not Answered)

Correct Answer : B

Tricalcium silicate (C3S, about 40 to 50 percent) and dicalcium silicate (C2S, about 25 to 30 percent) together make up the greatest part, around 70 to 75 percent, of ordinary Portland cement. They are responsible for its strength. Tricalcium aluminate (C3A) is about 8 to 12 percent and tetracalcium aluminoferrite (C4AF) is about 8 to 12 percent, so any combination that includes one of those two is smaller. Hence the C3S and C2S pair has the largest share.