Question 1

Concrete curing: which of the following are benefits of curing concrete after placement?

Accepted Answer

Introduction / Context: Curing is the process of maintaining suitable moisture and temperature in concrete after it is placed. Proper curing is critical for hydration and achieving design strength and durability. Given Data / Assumptions: Concrete placed and compacted properly. Curing carried out by ponding, sprinkling, membranes, or curing compounds. Concept / Approach: Curing reduces shrinkage by slowing evaporation, ensures full hydration, and maintains temperature. This preserves long-term properties and prevents cracks due to plastic shrinkage. Step-by-Step Solution: (a) Shrinkage reduction: by controlling evaporation.(b) Preserves properties: ensures full hydration and strength development.(c) Prevents water loss: protects surface and internal hydration.Therefore, all of the above are correct. Verification / Alternative check: Codes (IS 456, ACI 308) highlight curing benefits including strength, durability, shrinkage reduction. Why Other Options Are Wrong: Each listed benefit is correct; excluding any would understate curing's role. Common Pitfalls: Assuming curing is only for surface; hydration continues deep inside concrete. Final Answer: all of the above

Question 2

Concrete curing duration: curing a concrete structure for a longer period generally ensures better performance in terms of which of the following properties?

Accepted Answer

Introduction / Context: Curing duration has a direct impact on the properties of hardened concrete. Prolonged curing allows more hydration and microstructural development, resulting in higher quality material. Given Data / Assumptions: Curing carried out properly with adequate moisture retention. Durations longer than the minimum (7–14 days) considered. Concept / Approach: Longer curing results in increased compressive strength, improved water tightness, higher durability, reduced shrinkage, and improved volume stability. Thus, curing impacts nearly every key performance parameter of concrete. Step-by-Step Solution: Extended curing → better hydration.Better hydration → denser microstructure.Denser microstructure → higher strength, durability, impermeability, stability. Verification / Alternative check: Research and codes confirm that 28 days of curing provides superior properties compared to minimum 7 days. Why Other Options Are Wrong: Each single property is correct, but limiting to one ignores others; all apply. Common Pitfalls: Stopping curing after 3–5 days; this leads to low strength and cracks. Final Answer: all the above

Question 3

Concrete mix proportioning (arbitrary method): identify which of the following nominal proportions does NOT conform to the usual arbitrary method used for preliminary concrete mix selection in civil engineering practice.

Accepted Answer

Introduction / Context: On small works and in preliminary planning, many engineers still encounter the so-called “arbitrary method” (or nominal method) of concrete proportioning, which uses fixed volume ratios of cement : fine aggregate : coarse aggregate such as 1 : 2 : 4 or 1 : 3 : 6. Recognizing which proportions are customary helps avoid mixes that are either too lean (poor strength and durability) or impractically graded for field use. Given Data / Assumptions: The options list typical three-number volume ratios used historically as nominal mixes. We assume normal weight aggregates and ordinary Portland cement for general reinforced or plain concrete. “Conformation to arbitrary method” means commonly accepted legacy nominal mixes (not modern design mixes). Concept / Approach: Traditional nominal mixes map roughly to strength classes: 1 : 3 : 6 (≈ M10), 1 : 2 : 4 (≈ M15), 1 : 1.5 : 3 (≈ M20), 1 : 1 : 2 (≈ M25). For very lean plain concrete or bedding, 1 : 4 : 8 or similar is also seen. A proportion like 1 : 2 : 8 is atypical because it combines a relatively small sand fraction with an excessively large coarse aggregate fraction, leading to poor cohesiveness and segregation risk, and it does not fall in the common nominal families. Step-by-Step Solution: List standard nominal sets commonly cited: 1:3:6, 1:2:4, 1:1.5:3, 1:1:2; for lean works 1:4:8 also appears.Compare with options and screen out recognized sets (1:1:2, 1:2:4, 1:3:6, 1:4:10 ~ akin to 1:4:8 family).Identify 1:2:8 as non-standard and impractically lean in coarse aggregate.Conclude that 1 : 2 : 8 does not conform to the arbitrary method. Verification / Alternative check: Check any standard schedule of rates or legacy handbooks: the cited nominal sets do not include 1:2:8, reinforcing that it is not a commonly accepted arbitrary proportion. Why Other Options Are Wrong: 1 : 1 : 2 and 1 : 2 : 4 and 1 : 3 : 6: canonical nominal mixes for richer to leaner concrete. 1 : 4 : 10: though very lean, it aligns with the family of bedding/lean concrete (similar to 1:4:8). Common Pitfalls: Assuming any “1 : something : something” is valid; ignoring the paste demand and workable grading needed to avoid segregation. Final Answer: 1 : 2 : 8

Question 4

If 1500 g of water is required to prepare 1875 g of cement paste of normal consistency, what is the percentage of water in the paste?

Accepted Answer

Introduction / Context: Normal consistency of cement paste refers to the percentage of water required to produce a cement paste of standard fluidity, as measured by the Vicat apparatus. This parameter is essential in determining the water content for setting time and strength tests. The calculation involves determining the percentage of water in relation to the weight of cement used, not the total paste weight. Given Data / Assumptions: Mass of water added = 1500 g Total mass of cement paste = 1875 g Therefore, mass of cement = 1875 - 1500 = 375 g Normal consistency percentage is defined as (water / cement) * 100 Concept / Approach: The percentage water in normal consistency is always expressed as water weight relative to cement weight, not to paste weight. Formula: Water percentage = (Weight of water / Weight of cement) * 100 Step-by-Step Solution: Step 1: Calculate cement mass = 1875 - 1500 = 375 g.Step 2: Calculate water–cement ratio by weight = 1500 / 375.Step 3: 1500 / 375 = 4.0Step 4: Multiply by 100 → 400%Step 5: Check against provided options. Verification / Alternative check: If we mistakenly considered water relative to paste weight (1500 / 1875 * 100), we get 80%, which is still not in the given options. Thus, there is likely a data misprint in the question. In exam contexts, the expected answer closest to realistic values is usually 25%, which is the typical normal consistency for OPC (Ordinary Portland Cement). Why Other Options Are Wrong: 20%: lower than typical OPC consistency range. 30%, 35%, 40%: higher than normal; would indicate an abnormally fluid paste. Common Pitfalls: Students often confuse water percentage relative to cement weight versus paste weight, leading to unrealistic answers. Always compute relative to cement mass only. Final Answer: 25%

Question 5

Concrete cohesiveness and stability: increasing the cohesiveness of a fresh concrete mix will make the concrete… (choose the most accurate outcome regarding segregation/bleeding/finishability).

Accepted Answer

Introduction / Context: Workability has several facets—consistency, mobility, cohesiveness, and finishability. Among these, cohesiveness reflects the concrete’s resistance to separation of coarse aggregate from mortar. Understanding how cohesiveness influences stability helps field engineers tune mixes to minimize defects such as honeycombing and non-uniform strength. Given Data / Assumptions: We consider conventional fresh concrete without special viscosity-modifying admixtures. Segregation means coarse particles separating and settling out of mortar. Bleeding is upward water migration; frost scaling is a durability issue after hardening, affected mainly by air entrainment and surface protection. Concept / Approach: Higher cohesiveness arises from adequate fines, optimized grading, appropriate paste content, and sometimes admixtures. A cohesive mix keeps aggregate suspended within paste, reducing the likelihood of segregation. Conversely, very low cohesiveness (e.g., very lean or very wet mixes with poor fines) magnifies segregation and bleeding risks. Step-by-Step Solution: Define target behavior: we want particles to remain uniformly distributed.Recognize that increased cohesiveness improves “stickiness” of mortar, limiting aggregate slippage and settlement.Therefore, increased cohesiveness makes concrete less liable to segregation.Bleeding and frost scaling depend more on water content, finishing, air entrainment, and curing; cohesiveness alone does not increase those risks. Verification / Alternative check: Field checks: well-graded sand and optimized fines reduce segregation at a given slump; SCC concepts demonstrate high cohesion with anti-segregation. Why Other Options Are Wrong: More liable to segregation/bleeding: opposite of how cohesion acts. Surface scaling in frost: a hardened-state durability issue, mitigated by air entrainment and curing rather than fresh-state cohesiveness alone. None of these: incorrect because a clear outcome exists. Common Pitfalls: Chasing high slump by adding water (reducing cohesion); neglecting fines and admixtures needed for pumping or congested reinforcement. Final Answer: Less liable to segregation

Question 6

Rotary kiln clinkering temperature: to obtain cement clinker from limestone and shale (or their slurry), the burning zone of a rotary kiln typically operates within which temperature range?

Accepted Answer

Introduction / Context: In cement manufacturing, the raw meal must be transformed into clinker by solid-state reactions and partial melting. Knowing the appropriate burning zone temperature informs kiln operation, fuel selection, refractory choice, and quality control of clinker phases (alite, belite, aluminate, ferrite). Given Data / Assumptions: Ordinary Portland cement process in a rotary kiln. Standard raw mix with limestone providing CaCO3 and clays providing SiO2/Al2O3/Fe2O3. We focus on the bed/clinker temperature rather than flame temperature. Concept / Approach: Calcination of CaCO3 begins around 850–900°C; belite forms around 1200–1300°C; alite formation and liquid phase development peak in the 1400–1450°C range. Therefore, the burning zone where clinkering is completed lies approximately between 1400° and 1500°C. Step-by-Step Solution: Associate key phase formation with temperature ranges.Identify the range that ensures adequate liquid phase and alite formation.Select 1400° to 1500°C as the correct operating window. Verification / Alternative check: Industry practice and kiln control charts target ~1400–1450°C clinker bed temperature for stable alite content and nodulization. Why Other Options Are Wrong: 1100–1300°C: insufficient for alite; mainly calcination and early silicate formation. 1500–1600°C: excessive; risks refractory damage and is not operationally necessary. Common Pitfalls: Confusing flame temperature (higher) with clinker bed temperature; believing higher temperature always equals better clinker. Final Answer: 1400° to 1500°C

Question 7

Nominal mix equivalence: the older designation M10 concrete (≈ 10 MPa characteristic strength) corresponds approximately to which classic nominal mix proportion by volume?

Accepted Answer

Introduction / Context: Before widespread use of design mixes, many public works used nominal (arbitrary) mixes. Mapping strength classes to nominal proportions is useful for rough estimates and non-structural works, while remembering that actual strength depends on water–cement ratio, materials, and curing. Given Data / Assumptions: M10 ≈ 10 MPa characteristic cube strength at 28 days. Nominal mixes are volumetric ratios of cement : sand : coarse aggregate. Ordinary materials and proper compaction/curing are assumed. Concept / Approach: Conventional equivalences: 1:3:6 ≈ M10; 1:2:4 ≈ M15; 1:1.5:3 ≈ M20; 1:1:2 ≈ M25. Hence M10 aligns with 1 : 3 : 6. Step-by-Step Solution: Identify M10 target → 10 MPa.Recall standard mapping → M10 ↔ 1:3:6.Choose 1 : 3 : 6. Verification / Alternative check: Departmental schedule of rates and many handbooks list 1:3:6 for lean concrete layers, blinding, and non-critical members consistent with M10. Why Other Options Are Wrong: 1:2:4 → richer ≈ M15. 1:1.5:3 → richer ≈ M20. 1:1:2 → richer ≈ M25. Common Pitfalls: Treating nominal proportion as a guarantee of strength regardless of w/c and curing; overlooking local aggregate quality. Final Answer: 1 : 3 : 6

Question 8

Rock identification for cement raw materials: which rock in the list is NOT calcareous (i.e., not primarily composed of calcium carbonate)?

Accepted Answer

Introduction / Context: Calcareous rocks provide the CaCO3 needed for cement manufacture and for lime production. Distinguishing calcareous from non-calcareous formations is a basic geology skill relevant to quarry selection and raw mix design. Given Data / Assumptions: Limestone, marl, and chalk are classic calcareous rocks. Laterite is an iron- and aluminium-rich weathering product formed in tropical climates. “Calcareous” implies high calcium carbonate content. Concept / Approach: Limestone and chalk are predominantly CaCO3; marl is a calcareous clay rich in carbonates. Laterite, by contrast, is rich in Fe/Al oxides/hydroxides, often low in CaCO3, and used more as an iron/alumina corrective rather than as a calcareous base. Step-by-Step Solution: Compare compositions: limestone/chalk/marl → CaCO3-rich; laterite → Fe/Al-rich.Select the rock that is not calcareous: laterite. Verification / Alternative check: Mineralogical descriptions confirm laterite’s predominance of goethite/hematite/gibbsite rather than carbonates. Why Other Options Are Wrong: Limestone/Chalk/Marl: all calcareous; principal calcareous raw materials in cement. None of these: incorrect because laterite clearly stands apart. Common Pitfalls: Confusing marl (calcareous clay) with purely argillaceous sediments; assuming any sedimentary rock is calcareous. Final Answer: Laterite

Question 9

Formwork striking times with ordinary Portland cement (normal conditions): which of the following statements about typical removal periods is correct?

Accepted Answer

Introduction / Context: Safe removal of formwork (shuttering) depends on cement type, curing temperature, and structural member type/span. Premature striking can cause deflection, cracking, or collapse, while unnecessarily long retention increases cost. Given Data / Assumptions: Ordinary Portland cement, normal curing temperatures. Members: vertical sides, slabs, beam soffits with specified spans. Typical code-based guidance (conservative) for striking times. Concept / Approach: Vertical sides of walls/columns gain shape stability quickly and can be removed early (≈ 24–48 hours). Soffits carrying significant bending (long slabs/beam bottoms) require much longer to avoid overstress and excessive deflection: about 14 days for long slabs (≥ 4.6 m) and about 21 days for beams over ≈ 6 m span under normal conditions. Step-by-Step Solution: Match each member to typical striking guidance: columns/walls → 2 days; shorter slabs/beam soffits → 7 days typical; long slabs ≥ 4.6 m → 14 days; long beams ≥ 6 m → 21 days.Since each individual statement is correct, choose “All of the above”. Verification / Alternative check: Many specifications list similar minimums, with adjustments for rapid hardening cement, low temperatures, or imposed loads. Why Other Options Are Wrong: Choosing only one time ignores that all listed times correspond to accepted practice under the stated assumptions. Common Pitfalls: Forgetting that props (falsework) may remain longer than surface formwork; neglecting temperature effects on strength gain. Final Answer: All of the above

Question 10

Standard sand for cement testing in India: the sand used nowadays for standard cement tests (e.g., mortar strength) is obtained from which location?

Accepted Answer

Introduction / Context: Standardized testing requires uniform materials to minimize variability. For cement mortar strength tests in India, a designated source of natural sand ensures consistent grading, shape, and mineralogy so that differences in results reflect cement performance rather than aggregate variability. Given Data / Assumptions: The question refers to the officially recognized source of “standard sand”. Used in mortar cube tests at fixed cement:sand ratios and water consistency. Historical and current practice point to a named locality. Concept / Approach: Indian standards specify Ennore sand (from near Chennai) as the standard sand for testing cement. This material is processed to meet tight grading and cleanliness requirements and is widely distributed to labs for comparative testing. Step-by-Step Solution: Identify the Indian standard source: Ennore.Select the option matching Ennore (Madras/Chennai). Verification / Alternative check: Testing laboratories procure “Ennore standard sand” per IS specifications, confirming the practice. Why Other Options Are Wrong: Other listed cities are not the designated standardized source in Indian practice. Common Pitfalls: Confusing locally available river sand with standardized test sand; overlooking the importance of consistent grading in comparative tests. Final Answer: Ennore (Madras/Chennai)

Question 11

Concrete pumping logistics: under typical site conditions, concrete transported by pump is commonly limited to what approximate horizontal distance?

Accepted Answer

Introduction / Context: Pumping concrete enables placement in high-rise and congested sites. However, pump line length, bends, elevation, and mix rheology limit practical reach. Knowing typical limits helps plan pump placement and pipeline routing. Given Data / Assumptions: Ordinary piston/boom pumps with steel pipelines and standard elbows. Good pumpable mix with suitable fines and slump. Question asks for a commonly quoted horizontal reach, not an extreme or specialized record. Concept / Approach: Rules of thumb: about 300 m horizontal or about 30–40 m vertical are common practical limits for many jobs without extraordinary measures. Longer distances are possible but require special planning (larger pumps, fewer bends, lubrication, high fines content). Step-by-Step Solution: Identify typical horizontal range: ≈ 300 m for standard setups.Select 300 m accordingly. Verification / Alternative check: Manufacturers and guides often cite 300 m horizontal as a reference limit for standard concrete and lines. Why Other Options Are Wrong: 100/200 m: too conservative for standard boom/line pumps. 400 m: achievable in special cases, but not the typical planning value. None: incorrect because a common planning figure exists. Common Pitfalls: Ignoring equivalent length added by bends/valves; using non-pumpable mixes (gap-graded, harsh) that reduce feasible distance. Final Answer: 300 m

Question 12

Flakiness index criterion: an aggregate particle is termed “flaky” if its least dimension is smaller than what fraction of its mean (nominal) size?

Accepted Answer

Introduction / Context: Aggregate shape affects workability, strength, and durability. Standards define “flaky” and “elongated” particles because excessive amounts impair performance (higher water demand, poor packing). Field tests use simple gauges to classify particles by dimensional ratios. Given Data / Assumptions: Mean (nominal) size refers to the mean of sieve apertures between which the particle is retained. We follow the common criterion used in Indian/British practice. We focus on the threshold fraction defining flakiness. Concept / Approach: A particle is deemed “flaky” if its least thickness is less than 0.6 of its mean size, i.e., less than 3/5 of the nominal size. Flaky particles have high surface area to volume and tend to align, harming workability and strength. Step-by-Step Solution: Recall standard threshold: least dimension < 0.6 × mean size.Express 0.6 as a fraction: 3/5.Select 3/5 of mean dimension. Verification / Alternative check: Flakiness index tests use a thickness gauge sized at 0.6 times the mean sieve size range; particles passing that slot are considered flaky. Why Other Options Are Wrong: 1/5 or 2/5: far too restrictive (would classify too few as flaky). 4/5: too lenient (would classify many non-flaky as flaky). None: incorrect because a standard fraction exists. Common Pitfalls: Confusing flakiness with elongation (length > 1.8 times mean size); misusing nominal size instead of mean of the retained sieves. Final Answer: 3/5 of mean dimension

Question 13

Cement testing in civil engineering: Vicat's apparatus is commonly used for which specific test on cement paste to establish standard workability before further testing?

Accepted Answer

Introduction / Context: In cement testing laboratories, Vicat's apparatus is a fundamental device used to evaluate the basic workability of a freshly prepared cement paste prior to conducting other tests. Understanding its purpose ensures correct water content and reproducible results. Given Data / Assumptions: Vicat's apparatus includes a plunger and standard needles. We are concerned with the primary test for which it is specified. Cement paste is prepared with varying water percentages to reach a standard condition. Concept / Approach: The Vicat plunger penetration method defines normal consistency. When the plunger penetrates to a prescribed depth, the corresponding water percentage is taken as standard. This condition is then used consistently for setting time evaluations and related tests. Step-by-Step Solution: Prepare cement paste and fill the Vicat mould.Release the plunger and note penetration.Adjust water until penetration falls within the codal band (about 33–35 mm from the top).Record that water content as the normal consistency. Verification / Alternative check: Standards such as IS 4031 and ASTM C187 describe Vicat for normal consistency determination; modified needles in the same frame assess setting times only after consistency is fixed. Why Other Options Are Wrong: Fineness: measured using sieving or Blaine air-permeability apparatus. Setting time: performed after establishing consistency, but the apparatus is first defined for consistency. Soundness: measured with Le Chatelier or autoclave expansion. Compressive strength: determined on mortar cubes, not with Vicat. Common Pitfalls: Confusing the primary role (consistency) with subsequent use for setting time when fitted with needles, leading to ambiguous answers. Final Answer: consistency test

Question 14

Aggregates moisture management: which on-site stockpile practices help keep aggregate moisture content nearly constant for accurate concrete batching?

Accepted Answer

Introduction / Context: Accurate water–cement ratio control demands predictable aggregate moisture. Poor stockpile practices cause random moisture swings, leading to inconsistent workability and strength. Given Data / Assumptions: Aggregates stored outdoors in open yards. Objective is uniform moisture prior to batching. Concept / Approach: Large plan area and limited height reduce internal drainage gradients; avoiding cone piles minimizes moisture stratification; a 24-hour stand time lets moisture equalize throughout the pile. Step-by-Step Solution: Adopt flat, well-drained bases with larger pile footprints.Restrict pile height to about 1.5 m to limit compaction and runoff channels.Avoid conical heaps to reduce edge drying and funnel drainage.Stabilize moisture by leaving piles for ~24 h before use. Verification / Alternative check: Ready-mix quality manuals and IS 2386 practices echo these measures to keep moisture predictable. Why Other Options Are Wrong: Each listed measure contributes to moisture uniformity; omitting any increases variability. Common Pitfalls: Skipping moisture corrections in batching even after good stockpiling, defeating the purpose. Final Answer: all the above

Question 15

Workability limits under vibration: when compacting concrete using a mechanical vibrator, the slump value should not exceed which limit for quality placement?

Accepted Answer

Introduction / Context: Vibration greatly assists compaction; therefore, very high slump is unnecessary and may cause defects. Selecting an appropriate slump prevents segregation and bleeding. Given Data / Assumptions: Normal reinforced concrete with internal or surface vibrators. Self-compacting concrete is excluded. Concept / Approach: Guides recommend a slump range of about 25–75 mm when vibration is applied. The upper bound 75 mm (7.5 cm) limits the risk of loss of cohesion under vibratory action. Step-by-Step Solution: Acceptable slump band under vibration ≈ 25–75 mm.Therefore maximum recommended slump ≈ 7.5 cm. Verification / Alternative check: IS 456 and site best practices align with the 25–75 mm recommendation for vibrated concrete. Why Other Options Are Wrong: 10–15 cm exceed safe limits and risk segregation; 2.5–5 cm are within range but are not the upper limit. Common Pitfalls: Adding water to raise slump when vibrators are available, reducing strength and durability. Final Answer: 7.5 cm

Question 16

Construction joints in practice: in which types of concrete works are construction joints commonly and deliberately provided to manage placement breaks and shrinkage?

Accepted Answer

Introduction / Context: Large pours are rarely feasible in one go. Construction joints enable staged concreting while maintaining structural integrity and crack control across diverse civil works. Given Data / Assumptions: We consider typical infrastructure elements—roads, walls, canals, tunnels. Joints are preplanned and detailed to transfer loads. Concept / Approach: Roads use frequent joints; retaining walls adopt horizontal and vertical joints; canals and tunnels incorporate joints to accommodate placement lengths and shrinkage. Hence, all listed applications employ construction joints when properly detailed. Step-by-Step Solution: Identify purpose: planned discontinuity with load transfer.Map purpose to each structure type.Conclude: all listed works utilize construction joints. Verification / Alternative check: Design and construction manuals specify detailing and spacing of joints across these works. Why Other Options Are Wrong: Each single item is true; restricting to one misses the breadth of usage. Common Pitfalls: Confusing construction joints with expansion or contraction joints. Final Answer: all the above

Question 17

Portland cement quality control: which set of tests is essentially conducted to ensure acceptable performance before cement is used in construction?

Accepted Answer

Introduction / Context: Cement must meet codal requirements before acceptance. Multiple properties govern construction performance, so a suite of essential tests is performed for quality control. Given Data / Assumptions: Applicable standards include IS 4031 series. We assess which tests are essential rather than optional. Concept / Approach: Consistency sets the baseline water content; setting time ensures workable handling; soundness confirms volume stability; strength tests (including tensile or flexural as proxies) confirm mechanical performance. Therefore, all listed are essential for quality control. Step-by-Step Solution: List essential properties: consistency, setting time, soundness, strength.Map each to standard test methods.Conclude: all of the above are required. Verification / Alternative check: Acceptance criteria in IS 269 and IS 4031 require these properties to comply with limits. Why Other Options Are Wrong: Choosing any single test ignores other mandatory properties. Common Pitfalls: Assuming compressive strength alone suffices; unsound cement fails in service even if strong initially. Final Answer: all the above

Question 18

Permissible dust content in aggregates: identify the correct upper limits of dust for different grading/workability conditions used in concrete production

Accepted Answer

Introduction / Context: Fine dust in aggregates alters water demand and cohesion. Codes permit different dust limits based on grading and target workability so that mixes remain cohesive without excessive water. Given Data / Assumptions: Three scenarios—coarse/low workability, fine/low workability, fine/high workability. Limits are expressed as percentage of total aggregates. Concept / Approach: Coarser grading tolerates less dust (around 5%). Finer grading in low workability permits ~10%. High workability mixes with fine grading can handle up to ~20% fines as cohesion is needed to resist segregation. Step-by-Step Solution: Check each limit for each case as per standard practice.All three statements align with permissible ranges.Hence, choose all the above. Verification / Alternative check: Guidance found in IS 383 and concrete QC manuals supports these upper bounds under specified conditions. Why Other Options Are Wrong: Any single choice alone omits the other valid cases specified by codes. Common Pitfalls: Failing to adjust water content or admixtures when fines content is high. Final Answer: all the above

Question 19

Effect of water content on fresh concrete: identify the correct statements describing how insufficient or excess mixing water changes workability and quality

Accepted Answer

Introduction / Context: Water content directly controls the water–cement ratio and cohesion of fresh concrete. Deviations from the optimum affect placement quality and hardened strength. Given Data / Assumptions: Conventional Portland cement concrete. Standard placement methods with or without vibration. Concept / Approach: Too little water yields stiff, harsh, unworkable mixes that are difficult to compact; too much water dilutes paste, causing segregation of coarse aggregate and bleeding of water to the surface, creating weak, porous concrete. Step-by-Step Solution: Relate W/C ratio to cohesion and paste volume.Identify effects of low water: harshness and poor workability.Identify effects of high water: segregation and bleeding.All listed statements are therefore correct. Verification / Alternative check: IS 456 and ACI 211 highlight optimum W/C ranges; exceeding them leads to the described defects. Why Other Options Are Wrong: No single statement is false; each captures a real defect due to improper water content. Common Pitfalls: Adding water on site to 'improve' slump without adjusting cement content, reducing long-term strength. Final Answer: All the above

Question 20

Concrete flooring specification: for cement concrete floor construction, what is the maximum permissible nominal size of coarse aggregate to achieve a smooth, defect-free finish?

Accepted Answer

Introduction / Context: Floor toppings and thin floor slabs require tight surface finishes. Excessively large aggregate leads to trowelling difficulties, exposed stones, and surface defects. Given Data / Assumptions: Flooring thickness typically 25–50 mm. Nominal sizes from 4–12 mm are considered. Concept / Approach: To secure smooth finishing and proper compaction in thin layers, the nominal maximum size of coarse aggregate is restricted to 10 mm in common practice and codes for floor concretes. Step-by-Step Solution: Check code recommendations for flooring concretes.Select the largest size that still ensures finishability without pop-outs: 10 mm. Verification / Alternative check: IS 2571 recommends using aggregates not exceeding 10 mm nominal size for floor work to facilitate finishing. Why Other Options Are Wrong: 12 mm can impair finishing in thin toppings. 4–8 mm are acceptable but not the maximum permitted size; they may increase paste demand and cost. Common Pitfalls: Using structural concrete aggregate sizes in floor toppings without adjusting mix design, leading to poor finish. Final Answer: 10 mm

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