Q: Bulk density (unit weight) of aggregates: identify the factor that bulk density does not depend upon during standard measurement procedures.

Introduction / Context: Bulk density (unit weight) of aggregates is the mass of aggregate required to fill a container of a specified volume under standard conditions, including the voids between particles. Understanding factors affecting bulk density is essential for batching concrete by volume and for estimating yield and storage needs. Given Data / Assumptions: Standardized containers and rodding/tamping procedures are used. Aggregate moisture and surface condition are controlled. Focus is on inherent aggregate properties versus measurement artifacts. Concept / Approach: Bulk density depends on particle size and shape (which affect packing), grading (distribution across sizes), and specific gravity (mass per particle volume). However, when standardized containers and procedures are used, the size and shape of the container are not variables; they are fixed by the method and so should not influence the result if used correctly. Step-by-Step Solution: Recognize dependencies: better grading → denser packing → higher bulk density.Angular/flat particles pack less efficiently than rounded particles.Higher specific gravity increases mass per given bulk volume.The container is standardized; therefore bulk density does not depend on container size/shape. Verification / Alternative check: Standards prescribe calibrated measures (e.g., 3 L, 15 L) and rodding counts. When procedures are followed, container geometry does not vary across tests, hence not a dependency. Why Other Options Are Wrong: Size/shape of aggregates: directly influence packing. Specific gravity: higher particle density increases bulk mass. Grading: affects void content and compaction. None of these: incorrect since there is a valid factor that does not affect the result. Common Pitfalls: Using non-standard containers or techniques introduces variability; the test must be standardized to isolate material properties. Final Answer: Size and shape of the container

Q: Volumetric stability of ordinary cement concrete: after casting and subsequent drying, how does ordinary concrete generally behave volumetrically?

Introduction / Context: Dimensional stability of hardened concrete is crucial for crack control and serviceability. Drying shrinkage is a common phenomenon that can lead to cracking if joints, reinforcement, and curing are not properly designed. Given Data / Assumptions: Ordinary Portland cement concrete, normal ambient drying. No expansive additives or shrinkage-compensating cement. Typical water–cement ratios and aggregate types. Concept / Approach: After hydration and initial set, concrete loses moisture to the environment. As capillary water evaporates and internal RH drops, the cement paste experiences shrinkage. Aggregate restrains paste shrinkage to some extent, but net behavior is drying shrinkage (volume reduction). Step-by-Step Solution: Observe common field behavior: slabs-on-ground crack without joints due to shrinkage.Understand mechanism: moisture loss and microstructural changes cause volume reduction.Therefore, ordinary concrete on drying → shrinks. Verification / Alternative check: Standards specify shrinkage tests; admixtures and SCMs can mitigate but not reverse basic trend. Why Other Options Are Wrong: Expands: occurs with expansive cement or alkali–silica reactions, not the general case. Mix: not a valid behavioral description. None of these / Swells then stabilizes: not representative of ordinary OPC concrete drying. Common Pitfalls: Confusing early thermal expansion (heat of hydration) with long-term drying shrinkage; neglecting proper curing and jointing. Final Answer: Shrinks

Q: Quality control in compressive strength testing: the preliminary test set is repeated if the range between the highest and lowest strengths among three specimens exceeds approximately how much?

Introduction / Context: Consistency in compressive strength testing of concrete or cement mortars is vital for trustworthy acceptance decisions. Most standards prescribe limits on the spread (range) within a set of nominally identical specimens; exceeding the limit indicates test or specimen anomalies. Given Data / Assumptions: Three nominally identical compressive specimens are tested. Preliminary testing context for mix qualification. Decision criterion is based on the difference between maximum and minimum strength. Concept / Approach: A practical threshold often used in preliminary checks is a maximum allowable range of about 15 kg/cm² among three results. If the spread exceeds this value, the batch is considered inconsistent and the test is repeated to rule out outliers caused by casting, curing, or testing errors. Step-by-Step Solution: Compute range = max - min strength among the three cubes/prisms.Compare with threshold ≈ 15 kg/cm².If range > 15 kg/cm² → repeat preliminary test set. Verification / Alternative check: Laboratory practice frequently uses similar maxima for small specimen sets to ensure reliability before proceeding to full acceptance lots. Why Other Options Are Wrong: 5, 8, 10, 12 kg/cm²: too stringent for typical field variability; would trigger excessive retesting. Common Pitfalls: Ignoring curing uniformity; mixing end effects; capping/grinding errors; and loading rate deviations, all of which can inflate variability. Final Answer: 15 kg/cm²

Q: Equipment selection — compacting a plain concrete road slab (< 20 cm thick) For compaction of a plain cement concrete pavement or road slab of thickness less than 20 cm, which type of vibrator is most appropriate?

Introduction / Context: Pavement-quality concrete (PQC) and thin slab concretes require rapid and uniform compaction over wide areas. Choosing the right vibration equipment improves finish quality, reduces honeycombing, and ensures adequate density without damaging edges or forms. Given Data / Assumptions: Plain concrete slab thickness < 20 cm. Continuous placement along a road bay with guide forms. Goal: uniform compaction and surface finish across the full width. Concept / Approach: A screed vibrator spans across the forms and provides distributed vibration while also striking off the surface to grade. For thin slabs, this method is superior to internal needle vibrators, which are localized and can leave uneven consolidation lines. External form vibrators mainly stiffen forms and are not ideal for full-width slab compaction alone. Step-by-Step Solution: Identify element: thin, wide slab → needs line contact vibration and strike-off.Select equipment: screed vibrator provides continuous, uniform energy and levelling.Conclude: screed vibrator is most appropriate. Verification / Alternative check: Standard road construction practices employ vibrating screeds or fixed-form pavers for slabs up to typical pavement thicknesses, ensuring both compaction and surface tolerance. Why Other Options Are Wrong: Internal vibrators risk over/under-vibration zones and do not level the surface. Form vibrators are supplementary; they do not compact the slab body effectively. “None” is incorrect because an appropriate tool exists. Common Pitfalls: Dragging the screed too fast, reducing effective consolidation. Failing to maintain consistent head of concrete ahead of the screed. Final Answer: Screed vibrator.

Question 1

Cement choice for road pavements: which common type of cement is generally preferred for cement concrete road construction to balance early strength and durability?

Accepted Answer

Introduction / Context: Pavement concretes require reliable strength development, durability under traffic and environment, and compatibility with large-scale batching and curing conditions. Given Data / Assumptions: Conventional rigid pavement design and curing regimes. No special rapid-opening requirement specified. Concept / Approach: Ordinary Portland cement (OPC) is commonly used for road pavements due to predictable strength gain and durability. Rapid hardening cement may be used for early opening but is not the general default; low heat cement suits mass concreting; slag cement suits durability needs but may slow early strength. Step-by-Step Solution: Identify typical pavement requirements.Match to cement with balanced properties and availability: OPC. Verification / Alternative check: IRC and MoRTH specifications frequently list OPC grades for rigid pavements unless specific early opening criteria demand rapid hardening types. Why Other Options Are Wrong: Rapid hardening: specialized for early opening, higher heat. Low heat: for mass pours, not pavements. Slag cement: excellent durability but slower early strength, not universal default. Common Pitfalls: Choosing specialty cements without considering curing and traffic opening requirements. Final Answer: ordinary Portland cement

Question 2

Cement chemistry proportions: identify the correct order and typical percentage proportions of the principal chemical constituents of Portland cement clinker

Accepted Answer

Introduction / Context: The performance of cement is governed by the proportions of its major oxides: CaO (lime), SiO2 (silica), Al2O3 (alumina), and Fe2O3 (iron oxide). Recognizing their typical proportions is basic to cement technology. Given Data / Assumptions: Representative proportions are approximate and vary with manufacturer. The sequence requested is in decreasing typical percentage. Concept / Approach: In Portland clinker, lime is the dominant constituent (~60–65%), followed by silica (~20–25%), then alumina (~4–8%) and iron oxide (~2–4%). The order Lime > Silica > Alumina > Iron oxide matches the ratio 63 : 22 : 6 : 3. Step-by-Step Solution: List typical ranges for CaO, SiO2, Al2O3, Fe2O3.Compare with options and choose the sequence that matches.Answer: Lime : Silica : Alumina : Iron oxide :: 63 : 22 : 6 : 3. Verification / Alternative check: Textbooks on cement chemistry present similar ranges; the selected ratio is a commonly cited representative set. Why Other Options Are Wrong: Other sequences invert the dominant constituents and do not reflect actual clinker composition. Common Pitfalls: Confusing compound percentages (C3S, C2S, etc.) with oxide percentages (CaO, SiO2, etc.). Final Answer: Lime : Silica : Alumina : Iron oxide :: 63 : 22 : 6 : 3

Question 3

In concrete mix design using fineness modulus (FM), if X, Y, and Z are the fineness moduli of the coarse aggregate, the fine aggregate, and the final combined aggregate respectively, determine the percentage P of fine aggregate in the total combined…

Accepted Answer

Introduction / Context: Fineness modulus (FM) is a single-number index that captures the overall particle size distribution of aggregates. In practical mix design, the FM of the final combined aggregate is adjusted by blending coarse and fine aggregates in suitable proportions. This question tests the core relationship between the fineness moduli of the coarse (X), fine (Y), and combined aggregate (Z) to compute the percentage of fine aggregate required. Given Data / Assumptions: X: fineness modulus of the coarse aggregate fraction. Y: fineness modulus of the fine aggregate fraction. Z: fineness modulus of the combined aggregate after blending. P: percentage (by mass) of fine aggregate in the total combined aggregate. Concept / Approach: The fineness modulus of a blend is the weighted average of the component FMs, weighted by their proportions. Let p be the fraction of fine aggregate (0 to 1). Then the combined FM satisfies Z = p * Y + (1 - p) * X. This linearity arises because FM is computed from cumulative sieve retention which is itself linear with respect to proportioning of components. Step-by-Step Solution: Start from Z = p * Y + (1 - p) * X.Expand: Z = pY + X - pX = X + p * (Y - X).Solve for p: p = (Z - X) / (Y - X).Convert to percentage: P = 100 * p = 100 * (Z - X) / (Y - X). Verification / Alternative check: If Z = X (combined equals coarse), then P = 0% (no fine), which the formula gives. If Z = Y (combined equals fine), then P = 100%, which the formula also gives. These boundary checks confirm correctness. Why Other Options Are Wrong: P = 100 * (X - Z) / (Y - Z): mixes variables incorrectly; fails boundary checks. P = 100 * (Y - Z) / (Y - X): gives 0% when Z = Y (should be 100%). P = 100 * (Z - Y) / (X - Y): sign and placement errors produce negative or inconsistent values. P = 100 * (X + Y - 2Z) / (Y - X): not derived from linear blending law. Common Pitfalls: Confusing which FM is larger. Typically Y (fine) < X (coarse) is not always true; many conventions list FM of fine sand around 2.4–3.2 and coarse aggregate FM higher. Always verify magnitudes. Also, ensure proportions are by mass when using standard FM blending. Final Answer: P = 100 * (Z - X) / (Y - X)

Question 4

Bulk density (unit weight) of aggregates: identify the factor that bulk density does not depend upon during standard measurement procedures.

Accepted Answer

Introduction / Context: Bulk density (unit weight) of aggregates is the mass of aggregate required to fill a container of a specified volume under standard conditions, including the voids between particles. Understanding factors affecting bulk density is essential for batching concrete by volume and for estimating yield and storage needs. Given Data / Assumptions: Standardized containers and rodding/tamping procedures are used. Aggregate moisture and surface condition are controlled. Focus is on inherent aggregate properties versus measurement artifacts. Concept / Approach: Bulk density depends on particle size and shape (which affect packing), grading (distribution across sizes), and specific gravity (mass per particle volume). However, when standardized containers and procedures are used, the size and shape of the container are not variables; they are fixed by the method and so should not influence the result if used correctly. Step-by-Step Solution: Recognize dependencies: better grading → denser packing → higher bulk density.Angular/flat particles pack less efficiently than rounded particles.Higher specific gravity increases mass per given bulk volume.The container is standardized; therefore bulk density does not depend on container size/shape. Verification / Alternative check: Standards prescribe calibrated measures (e.g., 3 L, 15 L) and rodding counts. When procedures are followed, container geometry does not vary across tests, hence not a dependency. Why Other Options Are Wrong: Size/shape of aggregates: directly influence packing. Specific gravity: higher particle density increases bulk mass. Grading: affects void content and compaction. None of these: incorrect since there is a valid factor that does not affect the result. Common Pitfalls: Using non-standard containers or techniques introduces variability; the test must be standardized to isolate material properties. Final Answer: Size and shape of the container

Question 5

Volumetric stability of ordinary cement concrete: after casting and subsequent drying, how does ordinary concrete generally behave volumetrically?

Accepted Answer

Introduction / Context: Dimensional stability of hardened concrete is crucial for crack control and serviceability. Drying shrinkage is a common phenomenon that can lead to cracking if joints, reinforcement, and curing are not properly designed. Given Data / Assumptions: Ordinary Portland cement concrete, normal ambient drying. No expansive additives or shrinkage-compensating cement. Typical water–cement ratios and aggregate types. Concept / Approach: After hydration and initial set, concrete loses moisture to the environment. As capillary water evaporates and internal RH drops, the cement paste experiences shrinkage. Aggregate restrains paste shrinkage to some extent, but net behavior is drying shrinkage (volume reduction). Step-by-Step Solution: Observe common field behavior: slabs-on-ground crack without joints due to shrinkage.Understand mechanism: moisture loss and microstructural changes cause volume reduction.Therefore, ordinary concrete on drying → shrinks. Verification / Alternative check: Standards specify shrinkage tests; admixtures and SCMs can mitigate but not reverse basic trend. Why Other Options Are Wrong: Expands: occurs with expansive cement or alkali–silica reactions, not the general case. Mix: not a valid behavioral description. None of these / Swells then stabilizes: not representative of ordinary OPC concrete drying. Common Pitfalls: Confusing early thermal expansion (heat of hydration) with long-term drying shrinkage; neglecting proper curing and jointing. Final Answer: Shrinks

Question 6

Quality control in compressive strength testing: the preliminary test set is repeated if the range between the highest and lowest strengths among three specimens exceeds approximately how much?

Accepted Answer

Introduction / Context: Consistency in compressive strength testing of concrete or cement mortars is vital for trustworthy acceptance decisions. Most standards prescribe limits on the spread (range) within a set of nominally identical specimens; exceeding the limit indicates test or specimen anomalies. Given Data / Assumptions: Three nominally identical compressive specimens are tested. Preliminary testing context for mix qualification. Decision criterion is based on the difference between maximum and minimum strength. Concept / Approach: A practical threshold often used in preliminary checks is a maximum allowable range of about 15 kg/cm² among three results. If the spread exceeds this value, the batch is considered inconsistent and the test is repeated to rule out outliers caused by casting, curing, or testing errors. Step-by-Step Solution: Compute range = max - min strength among the three cubes/prisms.Compare with threshold ≈ 15 kg/cm².If range > 15 kg/cm² → repeat preliminary test set. Verification / Alternative check: Laboratory practice frequently uses similar maxima for small specimen sets to ensure reliability before proceeding to full acceptance lots. Why Other Options Are Wrong: 5, 8, 10, 12 kg/cm²: too stringent for typical field variability; would trigger excessive retesting. Common Pitfalls: Ignoring curing uniformity; mixing end effects; capping/grinding errors; and loading rate deviations, all of which can inflate variability. Final Answer: 15 kg/cm²

Question 7

IS 456 concrete grades: approximately how many standard grades of concrete mixes are designated in the code (e.g., M10, M15, M20, …)?

Accepted Answer

Introduction / Context: Indian Standard IS 456 designates concrete grades by characteristic compressive strength (e.g., M10, M15, M20…). Awareness of the range of grades is foundational for specifying concrete in structural and non-structural applications. Given Data / Assumptions: The question references the commonly taught count of standard grades used in general practice. Lower grades (M10–M25) were historically linked with nominal mixes; higher grades require designed mixes. Concept / Approach: Educational summaries often present a set spanning about seven commonly referenced grades for routine building work (e.g., M10, M15, M20, M25, M30, M35, M40). Modern editions list additional higher grades, but the classic count frequently cited in exam contexts is seven. Step-by-Step Solution: Identify the standard lineup used in basic design education.Recognize the exam convention of seven principal grades.Select 7. Verification / Alternative check: While current practice covers many grades beyond seven, the question’s options point to the classical exam reference set. Why Other Options Are Wrong: 3–6 underestimate the widely used range; they do not match the conventional teaching scope. Common Pitfalls: Confusing historical exam conventions with the full modern list of grades; always consult the latest IS 456 for project specifications. Final Answer: 7

Question 8

Measuring workability of fresh concrete on site: which standard test is primarily used to assess workability (consistency) quickly and reliably?

Accepted Answer

Introduction / Context: Workability of fresh concrete governs ease of placement, compaction, and finish. Site personnel require a quick, widely standardized test to confirm consistency against specified limits before pouring. Given Data / Assumptions: Ordinary structural concrete without special rheology (e.g., SCC). On-site assessment prior to placement. Standard equipment available to field teams. Concept / Approach: The slump test uses a standard cone to gauge the vertical subsidence of fresh concrete once the cone is lifted. The measured slump correlates with consistency/workability and is fast, simple, and repeatable, making it the default site test. Step-by-Step Solution: Select a test recognized globally for rapid workability checks.Vicat apparatus measures cement setting, not concrete workability.Minimum void/Talbot–Richart are not standard on-site workability tests.Therefore, the slump test is correct. Verification / Alternative check: Codes and specifications routinely specify slump ranges for different structural elements, confirming its ubiquity. Why Other Options Are Wrong: Vicat apparatus: for cement paste setting times. Minimum void/Talbot–Richart: academic/older methods, not standard site practice. Kelley ball: used in some regions but far less common than slump in exam contexts. Common Pitfalls: Over-reliance on slump alone; true workability also depends on cohesiveness and segregation resistance. Final Answer: Slump test

Question 9

Bulking and moisture in sand: Which statements are correct regarding causes, magnitude, and the saturated condition?

Accepted Answer

Introduction / Context: Bulking affects batching by volume for mortars and concrete. If not accounted for, water and cement proportions become inaccurate, leading to weak mixes. Understanding how moisture changes apparent sand volume is a core construction management topic. Given Data / Assumptions: Natural sand with varying grading from fine to coarse. Bulking arises at low to moderate moisture contents. At full saturation, water fills voids and surface tension effects vanish. Concept / Approach: (a) True: a surface moisture film causes grains to repel slightly due to menisci, increasing apparent volume. (b) True: fine sands have more surface area and smaller pores, so a given moisture content creates larger relative separation and thus greater bulking. (c) Not generally true: maximum bulking typically occurs around 4–6% moisture for fine sand, with volume increases often in the range of 20–35%, not as high as 50% at 10%. (d) True: at full saturation, meniscus forces vanish and the sand settles; the volume approaches that of dry loose sand. Step-by-Step Solution: Confirm cause → thin moisture film leads to apparent volume increase.Compare grading → finer sands bulk more.Evaluate 10% moisture claim → inconsistent with typical bulking curves; reject.Assess saturated condition → bulking disappears; volume ~ dry loose.Therefore, (a), (b), and (d) are correct together. Verification / Alternative check: Standard bulking charts show peak bulking at a few percent moisture, then decreasing toward zero at saturation. Why Other Options Are Wrong: Option including (c) is incorrect because the stated magnitude and moisture level are unrealistic for most sands. Common Pitfalls: Batching by volume without bulking correction; assuming constant bulking irrespective of grading. Final Answer: Only (a), (b), and (d) are correct.

Question 10

Concrete materials — definitions of free water and total water in mix design In concrete technology, select the correct statement(s) regarding the terms “free water” and “total water” used in batching and water–cement ratio calculations.

Accepted Answer

Introduction / Context: Understanding water accounting in concrete is essential to control the water–cement ratio, which governs strength, durability, shrinkage, and permeability. Terms like free water and total water appear in mix design, site adjustments, and moisture corrections for aggregates. Given Data / Assumptions: Aggregates may be oven-dry, air-dry, saturated surface-dry (SSD), or wet with surface moisture. Water added at the mixer and surface moisture on aggregates both contribute to effective mixing water. Aggregates absorb some water internally up to their absorption capacity. Concept / Approach: Free water is the water that remains available in the paste for workability and hydration after deducting aggregate absorption. Practically, it is the water added at mixing plus any surface moisture contributed by wet aggregates. Total water includes free water and the portion of water that is absorbed into the aggregates during mixing to reach SSD condition. Step-by-Step Solution: Define free water → water added at mixer + surface moisture on aggregates.Define total water → free water + water absorbed by aggregates.Therefore, statements (a) and (b) are both correct. Verification / Alternative check: Site moisture correction sheets compute batch water by starting from target free water, then adjusting for aggregate moisture (deduct if dry, reduce if wet) to keep the intended water–cement ratio constant. Why Other Options Are Wrong: (c) contradicts standard definitions. (e) ignores that total water necessarily includes absorbed water. Common Pitfalls: Confusing SSD condition with free water. SSD has zero surface moisture but contains absorbed water. Not compensating batch water for wet or dry aggregates, causing unintended strength loss or poor workability. Final Answer: Both (a) and (b) are correct.

Question 11

Equipment selection — compacting a plain concrete road slab (< 20 cm thick) For compaction of a plain cement concrete pavement or road slab of thickness less than 20 cm, which type of vibrator is most appropriate?

Accepted Answer

Introduction / Context: Pavement-quality concrete (PQC) and thin slab concretes require rapid and uniform compaction over wide areas. Choosing the right vibration equipment improves finish quality, reduces honeycombing, and ensures adequate density without damaging edges or forms. Given Data / Assumptions: Plain concrete slab thickness < 20 cm. Continuous placement along a road bay with guide forms. Goal: uniform compaction and surface finish across the full width. Concept / Approach: A screed vibrator spans across the forms and provides distributed vibration while also striking off the surface to grade. For thin slabs, this method is superior to internal needle vibrators, which are localized and can leave uneven consolidation lines. External form vibrators mainly stiffen forms and are not ideal for full-width slab compaction alone. Step-by-Step Solution: Identify element: thin, wide slab → needs line contact vibration and strike-off.Select equipment: screed vibrator provides continuous, uniform energy and levelling.Conclude: screed vibrator is most appropriate. Verification / Alternative check: Standard road construction practices employ vibrating screeds or fixed-form pavers for slabs up to typical pavement thicknesses, ensuring both compaction and surface tolerance. Why Other Options Are Wrong: Internal vibrators risk over/under-vibration zones and do not level the surface. Form vibrators are supplementary; they do not compact the slab body effectively. “None” is incorrect because an appropriate tool exists. Common Pitfalls: Dragging the screed too fast, reducing effective consolidation. Failing to maintain consistent head of concrete ahead of the screed. Final Answer: Screed vibrator.

Question 12

Strength testing — age for specified compressive strength of concrete (cube test) At what standard age is the specified compressive strength of concrete determined from cube tests for acceptance and design purposes?

Accepted Answer

Introduction / Context: The characteristic or specified strength of concrete is tied to a standard age of testing to enable consistent design and quality control. Codes standardize the acceptance age to harmonize mix design, curing, and structural timing decisions. Given Data / Assumptions: Normal Portland cement and ordinary curing conditions. Standard cube specimens prepared and cured per code. Definition of specified/characteristic strength. Concept / Approach: While early-age tests (3, 7 days) are useful indicators of strength development, the specified strength for design is conventionally taken at 28 days. This correlates with maturity models and typical construction timelines, providing a balance between early assessment and near-ultimate strength for many mixes. Step-by-Step Solution: Identify code convention → specified strength f_ck defined at 28 days.Use earlier tests only for trend monitoring, not acceptance to full specified value. Verification / Alternative check: Most national standards define characteristic strength at 28 days, with provisions for other ages only if explicitly specified by the designer. Why Other Options Are Wrong: 3, 7, 14, and 21 days are interim ages and typically show lower strengths. Common Pitfalls: Assuming 7-day strength guarantees 28-day strength without considering curing and cement type. Final Answer: 28 days.

Question 13

Effects of adding pozzolana to cement — select the correct impact When a pozzolanic material is blended with Portland cement (e.g., in Portland-pozzolana cement), which of the following listed effects is correct in general construction practice?

Accepted Answer

Introduction / Context: Pozzolanic materials (fly ash, calcined clay, silica-rich ashes) react with calcium hydroxide to produce additional cementitious hydrates. Their effects on fresh and hardened properties drive selection for durability and temperature control, especially in mass concrete and aggressive environments. Given Data / Assumptions: Typical Portland-pozzolana blends used in structural concrete. Normal curing and placement practices. Comparison with ordinary Portland cement of similar strength class. Concept / Approach: Pozzolana generally reduces heat of hydration (beneficial for thermal cracking control), often improves workability (spherical fly ash particles, improved packing), may reduce early-age strength but can improve later-age strength and durability. It does not reduce the required curing time; good curing remains critical. Therefore, the listed statements (a)–(d) are not correct as general truths, making “None of these” the correct choice. Step-by-Step Solution: Check (a): workability often increases, not decreases → false.Check (b): strength may be lower early and comparable/higher later, so “increases strength” unqualified is false.Check (c): heat of hydration is typically reduced → false.Check (d): curing time is not decreased; adequate curing is essential → false. Verification / Alternative check: Concrete practice texts note reduced temperature rise, improved impermeability, and enhanced sulphate resistance with pozzolanas, balanced against slower early strength gain. Why Other Options Are Wrong: Each statement misstates the typical effect of pozzolanas in general use. Common Pitfalls: Expecting rapid early strength with high fly-ash contents. Reducing curing duration because of improved later-age strength potential. Final Answer: None of these.

Question 14

Cement choice for marine exposure — structures in sea water For the construction of structures exposed to sea water, which cement type is generally preferred among the following options?

Accepted Answer

Introduction / Context: Sea water contains sulphates, chlorides, and magnesium salts that can attack concrete and reinforcement. Selecting an appropriate cement helps limit permeability and chemical reactivity and reduces thermal cracking in mass sections such as sea walls and piles. Given Data / Assumptions: General marine exposure with adequate cover, low w/c ratio, and good curing. Within the listed choices only; sulphate-resisting Portland cement is not included. Durability prioritized over rapid early strength. Concept / Approach: Portland-pozzolana cement (PPC) lowers calcium hydroxide content and permeability via pozzolanic reaction, improving resistance to sulphate/chloride ingress compared with ordinary or rapidly hardening cements. Quick-setting and rapid-hardening cements favor speed, not durability in aggressive environments. Low heat cement reduces temperature rise but does not, by itself, address chemical ingress as effectively as PPC within the given options. Step-by-Step Solution: Identify durability need: sulphate/chloride resistance and low permeability.Select PPC: pozzolanic reaction refines pore structure and reduces CH content.Reject rapid or quick setting options for marine durability. Verification / Alternative check: Marine durability guidance commonly recommends PPC, blended cements, or SRPC; since SRPC is absent here, PPC is the best choice among the options. Why Other Options Are Wrong: Quick-setting/rapid-hardening: higher heat, faster set, not superior durability. Low heat cement addresses thermal issues rather than ion ingress primarily. Common Pitfalls: Neglecting cover, w/c control, and curing; cement type alone cannot ensure durability. Final Answer: Portland-pozzolana cement (PPC).

Question 15

IS 456 (1978) grade notation — which grade is NOT recommended in that code? Considering the grade notation used in IS:456-1978 (legacy kg/cm²-based naming), identify the grade that was not recommended.

Accepted Answer

Introduction / Context: Older editions of IS:456 (1978) used grade labels such as M100, M150, M200, M250, M300, M350, and M400 (based broadly on kg/cm²). Later revisions adopted MPa-based grades like M20, M25, M30, etc. Recognizing which legacy grades existed avoids confusion when reading older drawings and texts. Given Data / Assumptions: The question references IS:456-1978 specifically. Within that edition, listed grades up to M400 were common. Higher grade M500 was not part of the recommended series then. Concept / Approach: IS:456-1978 commonly cited concrete grades up to M400 for reinforced concrete. M500 belonged to higher-strength concretes beyond coverage of typical prescriptive provisions at that time and was not recommended as a standard grade in that edition. Step-by-Step Solution: Recall legacy series: M100–M400 customary → includes M100, M200, M300, M400.Identify outlier: M500 not recommended in IS:456-1978. Verification / Alternative check: Historic tables and handbooks referencing IS:456-1978 align with this grade band, prior to MPa-based modern designations. Why Other Options Are Wrong: M100, M200, M300, and M400 were part of the conventional range then. Common Pitfalls: Confusing older kg/cm² labels with modern MPa designations (e.g., M20, M25). Final Answer: M 500.

Question 16

Concrete defects — outcomes commonly caused by segregation Segregation of fresh concrete (coarse aggregate separating from mortar) commonly leads to which of the following defects after setting/hardening?

Accepted Answer

Introduction / Context: Segregation occurs when components of fresh concrete separate due to improper proportioning, excessive vibration, high drop heights, or inadequate cohesiveness. The resulting non-uniformity compromises durability, strength, and appearance. Given Data / Assumptions: Conventional vibrated concrete. Poor handling practices or unstable mixes (e.g., very low fines, high slump without VMA). Normal placement conditions with avoidable errors. Concept / Approach: When segregation happens, mortar may run off and coarse aggregate clusters, leaving voids and creating honeycombing; stratified layers form, and surface finish suffers, producing scaling and sand streaks. Preventing segregation maintains designed gradation through the section and ensures proper paste coating and cohesion. Step-by-Step Solution: Recognize mechanism: differential movement of constituents.Map to defects: voids → honeycombing; paste loss → porous layers; non-uniform fines → streaks and poor finishes.Therefore, all listed defects can result from segregation. Verification / Alternative check: Site QA records link high drop placements and over-vibration with pronounced honeycombing and laitance layers, validating the cause–effect relation. Why Other Options Are Wrong: Each individual defect may occur, but segregation can cause all of them simultaneously; hence the comprehensive choice is correct. Common Pitfalls: Confusing bleeding (water rising) with segregation; both can co-occur but are distinct. Ignoring the role of appropriate grading and admixtures in cohesion. Final Answer: All of the above.

Question 17

Concrete floor finishing — name of the final operation In finishing concrete floors, after screeding and floating, what is the name of the final operation that produces a hard, smooth surface?

Accepted Answer

Introduction / Context: Concrete floor finishing follows a sequence to achieve level and durable surfaces. Identifying each step ensures proper timing and avoids defects like dusting, craze cracking, or delamination. Given Data / Assumptions: Standard slab-on-grade or floor topping. Use of straightedges, bull floats, and steel trowels/power trowels. Proper timing based on bleed water and set. Concept / Approach: The usual order is: screeding (strike off to level), floating (embed coarse aggregate and close surface), and trowelling (final densification and smoothing). Trowelling may be done manually or with a power trowel to achieve the desired finish class. Step-by-Step Solution: Perform strike-off → screeding.Do initial consolidation/levelling → floating.Complete the surface → trowelling for smooth, hard finish. Verification / Alternative check: ACI finishing guides and standard practice manuals show this sequence as the baseline method for typical interior floors. Why Other Options Are Wrong: Screeding and floating are earlier steps; “finishing” is a generic term, not a specific operation name. Common Pitfalls: Trowelling over bleed water causes delamination; always wait until bleed water has evaporated. Final Answer: Trowelling.

Question 18

Producing impermeable (low-permeability) concrete — key quality measures Which of the following measures are necessary to produce concrete with low permeability and good watertightness?

Accepted Answer

Introduction / Context: Permeability governs durability, corrosion risk, and leakage in liquid-retaining structures. Low-permeability concrete results from controlled water–cement ratio and disciplined construction practices throughout mixing, placing, compaction, and curing. Given Data / Assumptions: Normal structural concretes without intentional air entrainment (unless specified). Objective: minimize connected capillary pores and voids. Quality aggregates and proper grading available. Concept / Approach: Permeability decreases when the paste is continuous and dense, voids are minimized, and hydration proceeds sufficiently. Each listed measure directly contributes: thorough mixing for uniformity, proper compaction to remove entrapped air, adequate curing to allow hydration products to refine pores, and suitable aggregates to reduce internal porosity and paste demand. Step-by-Step Solution: Adopt low w/c ratio appropriate for strength/durability.Ensure mixing uniformity → consistent paste and fine distribution.Compact effectively → fewer voids; use vibration correctly.Cure adequately → continued hydration reduces permeability.Select proper aggregates → minimize permeability pathways. Verification / Alternative check: Permeability tests (e.g., water penetration depth) show significant improvement with these practices versus poorly mixed/compacted/uncured concretes. Why Other Options Are Wrong: They are not wrong individually; the comprehensive, correct choice is to implement all. Common Pitfalls: Focusing only on admixtures while neglecting compaction and curing. Final Answer: All of the above.

Question 19

Movement joints per IS:456 (1978) — basic provisions for expansion joints According to the recommendations of IS:456-1978, which of the following statements about expansion joints in reinforced concrete buildings is/are correct?

Accepted Answer

Introduction / Context: Expansion (movement) joints accommodate thermal and shrinkage movements, preventing cracking, distortion, or distress in long or irregular buildings. IS:456-1978 provides guidance on their location and detailing to allow free movement while maintaining structural integrity on either side. Given Data / Assumptions: Reinforced concrete framed or wall–slab buildings. Joints designed to interrupt continuous structural action as needed. Proper water- and fire-stopping treated separately from structural detailing. Concept / Approach: Joints are placed at plan irregularities and at spacing limits dictated by exposure and materials to reduce restraint. Across an expansion joint, each side must be structurally stable by itself and supported on its own columns or frames. Reinforcement is intentionally discontinued across the joint to avoid locking the two segments together; dowels or sliding bearings, if any, are detailed to permit movement, not force continuity. Step-by-Step Solution: Locate joints → at abrupt plan changes and where movement demands are significant.Detail joints → minimize restraint with suitable gap and filler/sealants.Provide separate supports → independent columns/frames on each side.Discontinue reinforcement → no bars carried across expansion joints. Verification / Alternative check: Typical IS-based detailing manuals show free-movement joints with compressible fillers, water bars, and no continuity reinforcement across the joint—confirming all listed statements. Why Other Options Are Wrong: The individual statements (a)–(d) are each correct; hence the inclusive choice (e) is correct. Common Pitfalls: Accidentally lapping mesh or slab bars across the joint, defeating its purpose. Omitting water stops in wet areas, causing leakage through the joint. Final Answer: All of the above.

Question 20

Concrete maturity by Plowman — datum temperature definition In cement-concrete maturity analysis (Plowman/Nurse–Saul approach), the datum temperature is the hypothetical temperature below which cement hydration is assumed to cease. According to Plow…

Accepted Answer

Introduction / Context: Maturity methods relate the combined effect of time and temperature on concrete strength development. The Plowman/Nurse–Saul concept uses an equivalent age or maturity index to estimate in-place strength without destructive testing. A key parameter is the datum temperature, the threshold below which hydration is considered negligible. Given Data / Assumptions: Maturity is calculated as a time–temperature integral. Datum temperature represents the lower cutoff for effective hydration. Standard practice often follows Plowman’s recommended datum value. Concept / Approach: The basic Nurse–Saul maturity function is M = Σ (T_a − T_d) * Δt, where T_a is the average concrete temperature during interval Δt, and T_d is the datum temperature. Only temperatures above T_d contribute positively to maturity. Selecting an appropriate T_d makes the correlation between maturity and compressive strength more accurate over normal curing ranges. Step-by-Step Solution: Define objective: choose Plowman’s datum temperature for practical maturity computations.Identify standard value: many references adopt approximately −11.7°C (about 10°F below 0°C) as T_d.Conclude that −11.7°C is the accepted datum temperature per Plowman. Verification / Alternative check: Field correlations between maturity (with T_d ≈ −11.7°C) and cylinder strengths show good agreement for typical Portland cement concretes, validating the datum selection for most temperate curing conditions. Why Other Options Are Wrong: 23°C: This is a standard curing temperature, not a datum cutoff. 0°: Hydration continues below 0°C in protected mixes; 0° is not the traditional datum in Plowman’s method. −5.6°C: Used in some alternative calibrations, but not the classic Plowman datum. None of these: Incorrect because −11.7°C is widely cited. Common Pitfalls: Using a single datum temperature for all binders; high-SCM or blended cements may require recalibration. Confusing datum temperature with activation energy parameters from Arrhenius-based maturity models. Final Answer: −11.7°C

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