Question 1

Slump Test Dimensions – Standard top diameter, bottom diameter, and height In the standard slump (Abrams) cone used for assessing concrete workability, what are the nominal dimensions in the order: top diameter, bottom diameter, height?

Accepted Answer

Introduction / Context: The slump test is the most common field test for fresh concrete. To ensure comparable results, the mould must have the standard geometry. Memorising these dimensions helps inspectors quickly verify equipment compliance. Given Data / Assumptions: Standard slump cone (Abrams cone). We seek top diameter, bottom diameter, and height, in that order. Nominal metric dimensions. Concept / Approach: The slump mould is a frustum of a right circular cone. The top opening is smaller than the base, and the overall height is fixed so that the test measures consistent settlement under self-weight when the mould is lifted. Step-by-Step Solution: Recall standard dimensions: top ≈ 10 cm, bottom ≈ 20 cm, height ≈ 30 cm.Match the sequence requested (top, bottom, height) to the options.Select 10 cm, 20 cm, 30 cm. Verification / Alternative check: Practical cones include handles and foot pieces, but the internal geometry must respect the nominal sizes for test validity and correlation to standard slump classes. Why Other Options Are Wrong: They either invert the diameters or misstate the height, which would alter test behavior and invalidate comparisons. Common Pitfalls: Using dented, out-of-round, or incorrect cones; even small dimensional errors can skew measured slump. Final Answer: 10 cm, 20 cm, 30 cm

Question 2

Concrete Transportation in Tunnels – Preferred method for tunnel linings For concreting operations in tunnel linings, which mode of transporting and placing concrete is most commonly adopted to ensure continuous, controlled delivery through confined…

Accepted Answer

Introduction / Context: Tunnel environments impose access constraints, long haul distances, and overhead or curved forms for linings. Choosing the right transport method is crucial for maintaining concrete quality and productivity while minimising segregation. Given Data / Assumptions: Narrow, elongated work zone typical of tunnels. Need for continuous, steady supply to lining shutters/segments. Modern placement practices with pipelines and suitable slump or pumped mixes. Concept / Approach: Concrete pumping through rigid or flexible pipelines is the prevalent method for tunnel linings. Pumps provide controlled, continuous flow over distance and elevation, reduce manual handling, and allow delivery to overhead forms. Mixes are proportioned for pumpability (adequate fines, cohesive grading) to prevent blockages and segregation. Step-by-Step Solution: Identify constraints: confined access, long reach, overhead placement.Select method providing pressure-driven delivery along the tunnel axis → pumps.Note that belt conveyors or agitator cars may feed pump hoppers, but the final delivery for lining is commonly by pump. Verification / Alternative check: Case histories for NATM and segmental linings routinely specify pumped concrete with steel or rubber hoses to reach shutters, ensuring uniform discharge and minimal rebound. Why Other Options Are Wrong: Pans/wheelbarrows: impractical for long distances and overhead forms; risk of segregation and interruptions. Containers: suited to cranes and open sites; less efficient in tunnels. Belt conveyors: useful for bulk transport but often paired with pumps for final placement; belts alone can be awkward around curves and elevation changes. Common Pitfalls: Ignoring pumpability requirements (fines content, slump retention) leads to blockages; always coordinate mix design with pipeline length and temperature. Final Answer: Pumps

Question 3

Cement Types – Slower early strength but comparable long-term strength to OPC Which cement type generally develops a somewhat lower early-age strength than ordinary Portland cement during the first few months but later attains similar strength level…

Accepted Answer

Introduction / Context: Blended cements modify hydration kinetics and microstructure. While some types boost early strength, others trade early gain for improved durability and comparable or better later-age performance. Recognising these trends is important when scheduling formwork removal or specifying mixes for mass concrete and aggressive environments. Given Data / Assumptions: Comparison is with ordinary Portland cement (OPC). “First few months” indicates beyond 28 days, up to about 90 days. Standard curing conditions are implied. Concept / Approach: Portland blast furnace slag cement (PBFS/PBFSC) contains latent hydraulic slag. Early strength gain (1–28 days) is typically slower than OPC, but slag reacts progressively to form additional hydrates, resulting in comparable or superior strengths at 90 days and beyond. PPC (pozzolana) shows a similar but often less pronounced trend; however, the classic statement associating “first three months lower, afterwards equal” is most consistently attributed to slag cement in many standard texts. Step-by-Step Solution: Identify which cement has slower early kinetics but strong later gain → slag cement.Eliminate rapid-hardening cement: it has faster early strength than OPC.Low-heat cement slows early heat and strength and may remain lower longer; the standard equivalence at later ages is more typical of slag cement. Verification / Alternative check: Performance curves in materials handbooks show PBFSC approaching/equaling OPC by 90 days, with added benefits of reduced permeability and better sulphate resistance. Why Other Options Are Wrong: Rapid-hardening cement: opposite trend (higher early strength). Low-heat cement: designed to limit heat evolution; long-term parity is not the textbook emphasis. PPC: also slower early, but the canonical “after three months equal” phrasing is most closely tied to slag cement in many exam contexts. Common Pitfalls: Assuming all blended cements behave identically; mix design, slag/pozzolana content, and curing significantly influence the curves. Final Answer: Portland blast furnace slag cement

Question 4

Timber/Lightweight Fill – Rendering sawdust chemically inert before use Before using sawdust as a lightweight filler or in non-structural composites, it can be rendered relatively inert by boiling it in water containing which additive?

Accepted Answer

Introduction / Context: Organic fines like sawdust may interfere with cement hydration due to sugars and extractives, or may decay. Pre-treatment helps reduce harmful effects when sawdust is used in lightweight blocks, insulating plasters, or as filler in non-structural elements. Given Data / Assumptions: Objective: reduce reactivity/leaching of organics from sawdust. Application in non-structural, low-strength composites. Simple, field-feasible chemical treatment. Concept / Approach: Boiling sawdust in a solution containing ferrous sulphate helps neutralize soluble tannins and sugars that retard cement hydration. The treatment precipitates some extractives and reduces biological activity, improving compatibility with cementitious matrices. Step-by-Step Solution: Select a salt known to complex with organic extractives → ferrous sulphate is commonly cited.Boil sawdust in the solution to leach and neutralize inhibiting compounds.Dry and use the treated sawdust in the intended non-structural mix. Verification / Alternative check: Materials handbooks list ferrous sulphate and similar salts for treating organic aggregates before mixing with cement to limit set-retardation. Why Other Options Are Wrong: Potassium chloride/ammonia: not standard for this purpose in masonry composites. Nitric/sulphuric acid: strong acids are hazardous and can degrade cellulose, making them unsuitable in typical site practice. Common Pitfalls: Expecting structural performance from sawdust concretes; treatment improves compatibility but does not convert them into load-bearing materials. Final Answer: Ferrous sulphate

Question 5

Concrete Finishing – Operation for removing humps and hollows to achieve level What is the name of the finishing operation in which the freshly placed concrete surface is struck off to remove humps and fill hollows, bringing it to the correct level …

Accepted Answer

Introduction / Context: Concrete finishing comprises several sequential operations. Immediately after placement and compaction, the surface is brought to level by a straightedge. Knowing the correct term for each operation ensures clear instructions and quality control on site. Given Data / Assumptions: Freshly placed concrete in slabs, pavements, or toppings. Use of a straightedge, strike-off board, or vibrating screed. Goal: achieve specified level and profile before subsequent finishing. Concept / Approach: Screeding (or strike-off) removes high spots and fills low areas to the form lines or guide rails, establishing level. Floating follows to embed aggregate and close minor voids; trowelling provides a dense, smooth finish where required. “Finishing” is a broad term encompassing all these steps; the specific name for the removal of humps and hollows to grade is screeding. Step-by-Step Solution: Identify the described action: strike-off to level → screeding.Distinguish from floating (subsequent) and trowelling (final smoothing).Select “Screeding”. Verification / Alternative check: Standard finishing sequences: screed → bull float → rest (bleed water) → steel trowel (as specified) → curing. Why Other Options Are Wrong: Floating: aims at consolidating the paste at the surface after screeding. Trowelling: produces a smooth, dense finish; not primarily to remove humps/hollows. “Finishing” is generic, not the specific operation asked. Common Pitfalls: Overworking the surface while bleed water is present, which can cause dusting and scaling; allow proper timing between steps. Final Answer: Screeding

Question 6

Role of Water in Concrete – Hydration, lubrication, and workability linkage to strength Which of the following statements best describes the roles of water in concrete and its indirect link to strength through workability?

Accepted Answer

Introduction / Context: Water in concrete serves dual roles: it hydrates cement and it provides the fluidity required for handling and consolidation. The amount of water relative to cement (w/c ratio) overwhelmingly governs strength and durability, while workability governs whether the designed density and cover are achieved in practice. Given Data / Assumptions: Ordinary Portland cement; normal placement methods. No extraordinary admixture effects considered unless stated. Compaction achievable when workability is adequate for the element and reinforcement. Concept / Approach: Hydration consumes only a fraction of the total water usually added; the rest facilitates mixing, transport, and compaction. However, extra water increases w/c and capillary porosity, reducing strength. Therefore, the practical strength achieved on site is heavily influenced by having just enough workability to compact fully without excess water, often achieved using admixtures, proper grading, and vibration. Step-by-Step Solution: Confirm (a): water is essential for hydration reactions forming binding hydrates.Confirm (b): water acts as a lubricant, allowing aggregates to rearrange under vibration.Confirm (c): chemically bound water demand is small (roughly 0.23 * cement by mass) compared to water added for workability.Confirm (d): insufficient workability leads to honeycombing and low density, reducing actual structural strength even if the mix design strength is high. Verification / Alternative check: Site QA demonstrates that two mixes with identical cement content can yield different strengths due to variations in compaction quality driven by workability differences. Why Other Options Are Wrong: Because each statement is accurate, the combined choice “All of the above” is correct. Common Pitfalls: Chasing slump by adding water instead of using water-reducers or adjusting grading; this decreases long-term performance. Final Answer: All of the above

Question 7

Aggregate Moisture Condition – Name for pores full of water but surface dry What is the correct term for aggregates whose internal pores are filled with water while the particle surfaces are dry to the touch?

Accepted Answer

Introduction / Context: Adjusting mix water for aggregate moisture is essential to maintain the selected water–cement ratio. Knowing the standard moisture conditions of aggregates supports correct water corrections in batching. Given Data / Assumptions: Pore structure of aggregates can hold water internally. Surface condition determines whether the aggregate contributes or absorbs water from the mix. We identify the specific condition where pores are full but surface is dry. Concept / Approach: In the saturated surface dry (SSD) condition, aggregate pores are fully saturated, yet the exterior surfaces are dry. In this state, the aggregate neither absorbs mix water nor contributes free water, making it the reference condition for accurate w/c calculations. Step-by-Step Solution: List the canonical states: oven-dry, air-dry, SSD, and wet (surface moist).Match the description “pores full + surface dry” to SSD.Choose “Saturated surface dry aggregate”. Verification / Alternative check: Material standards define SSD as the condition for reporting specific gravity and absorption, underscoring its importance in mix design. Why Other Options Are Wrong: Moist (wet) aggregates have surface moisture; they add free water to the mix. Very dry/dry imply pores are not saturated and will absorb mix water. “None of these” is incorrect because SSD is the standard term. Common Pitfalls: Ignoring SSD corrections leads to unintended w/c variations and strength scatter; always measure moisture routinely. Final Answer: Saturated surface dry aggregate

Question 8

Cement & Finishes – Commercial name associated with white/coloured cement products in India Which of the following is a well-known commercial name associated with white and coloured cement-based finishing products in India?

Accepted Answer

Introduction / Context: In building practice, certain trade names become synonymous with product categories. For white and coloured cement-based surface finishes and paints in India, one commercial name has been widely recognised and colloquially used for decades. Given Data / Assumptions: The question refers to commercial naming familiar in Indian construction markets. Focus is on recognition rather than exact corporate branding history. White/coloured cement-based finishes are intended. Concept / Approach: “Snowcem” has been a prominent brand in India associated with cement-based, often white or coloured, protective finishes. Though technically a paint/coating based on cement chemistry rather than bagged structural cement, it is the best-known commercial name linked by practitioners to white/coloured cement-based finishes. Step-by-Step Solution: Identify the popularly recognised brand linked to white/coloured cement finishes → Snowcem.Note that other listed names are not widely established or may not be commonly associated with the category.Select “Snowcem”. Verification / Alternative check: Trade literature and market usage for decades have referenced Snowcem products for exterior cement-based coatings, reinforcing brand recognition. Why Other Options Are Wrong: Colocrete/Rainbow cement/Silvicrete: not established or widely recognised brand names for this category in common exam contexts. “All of the above”: incorrect because the recognition is specific rather than generic across these names. Common Pitfalls: Confusing cement-based paints with structural white cement; while related, they serve different purposes in construction. Final Answer: Snowcem

Question 9

Concrete Cube Size vs Aggregate – approval for 100 mm test cubes As per standard practice, 100 mm (10 cm) cubes may be used for work-test compressive strength only if the engineer-in-charge approves and the maximum nominal size of coarse aggregate d…

Accepted Answer

Introduction / Context: Concrete compressive strength is commonly determined using standard cube or cylinder specimens. In Indian practice, 150 mm cubes are standard. However, smaller 100 mm cubes are sometimes permitted for work tests to save materials and curing space, provided certain conditions regarding aggregate size and authority approval are satisfied. This question checks the correct aggregate-size limit that allows the use of 100 mm cubes. Given Data / Assumptions: Work test cubes for compressive strength under normal laboratory procedure. Standard 100 mm and 150 mm cube moulds are available. Engineer-in-charge may approve 100 mm cubes under codal limitations. Concept / Approach: The specimen must be sufficiently large compared with the maximum aggregate size to ensure representative sampling, minimal wall effects, and proper compaction without segregation. Codes and handbooks cap the use of 100 mm cubes to mixes where the maximum nominal size of coarse aggregate is not more than 20 mm. For larger aggregate, a 150 mm cube is required so that the specimen contains an adequate number of particles across the section and provides reliable strength values. Step-by-Step Solution: Identify the permitted aggregate size for 100 mm cubes → maximum nominal size = 20 mm.Confirm that engineer-in-charge approval is also necessary for work tests.Therefore, among the options provided, 20 mm is correct. Verification / Alternative check: Laboratory practice aligns with the general rule of keeping specimen dimension ≥ 4 to 5 times the maximum aggregate size. For a 100 mm cube, 100/20 = 5 satisfies this guidance; larger aggregate fractions would violate it for 100 mm cubes but comply with 150 mm cubes (150/25 = 6). Why Other Options Are Wrong: 10 or 15 mm are overly restrictive and not required by practice; 25 or 30 mm exceed the limit for 100 mm cubes and may yield non-representative results due to boundary effects and poor compaction around large particles. Common Pitfalls: Using 100 mm cubes without checking aggregate size; not adjusting compaction energy; comparing strengths from different cube sizes without correlation. Final Answer: 20 mm

Question 10

Cement Chemistry – effects of C3S and C2S contents on hardening and durability Which statements about the influence of tricalcium silicate (C3S) and dicalcium silicate (C2S) on Portland cement behavior are correct?

Accepted Answer

Introduction / Context: The relative amounts of C3S and C2S in Portland cement clinker strongly influence rate of strength gain and durability in aggressive environments. Recognizing these trends helps in selecting cement types for specific site conditions (rapid construction, massive pours, sulphate exposure, etc.). Given Data / Assumptions: Cements compared are ordinary Portland types differing mainly in C3S/C2S proportions. Other phases (C3A, C4AF) are not the focus of this question. Standard curing and test conditions assumed. Concept / Approach: C3S hydrates relatively fast, producing early-age strength and more calcium hydroxide, while C2S hydrates more slowly, contributing to later strength and producing less calcium hydroxide. A higher C3S content thus accelerates hardening but can reduce chemical resistance because more free Ca(OH)2 is available for deleterious reactions (for example, sulphate attack). Conversely, higher C2S content slows early hardening and commonly improves chemical resistance due to lower Ca(OH)2 release and denser microstructure at later ages. Step-by-Step Solution: Option A: True—C3S drives rapid early strength.Option B: True—more Ca(OH)2 and higher permeability reduce chemical resistance.Option C: True—C2S contributes more slowly to strength.Option D: True—greater C2S typically improves resistance in many aggressive environments. Verification / Alternative check: Standard cement chemistry references show early-age strength correlating with C3S; durability often improves when C3S is moderated and C2S proportion is higher, especially when combined with SCMs. Why Other Options Are Wrong: Since A–D are correct, the best choice is “All of the above”. Common Pitfalls: Over-generalizing without considering C3A and permeability; assuming high early strength guarantees better durability. Final Answer: All of the above

Question 11

Set Regulators and Accelerators – identify the correct roles of gypsum and calcium chloride Choose the correct statement(s) about the action of gypsum (calcium sulphate) and calcium chloride in cement/concrete.

Accepted Answer

Introduction / Context: Set control is essential for practical placing and early strength development. Two commonly referenced additives are gypsum in cement clinker grinding and calcium chloride in concrete admixture practice. Understanding their roles prevents mis-specification and site delays. Given Data / Assumptions: Ordinary Portland cement containing interground gypsum to regulate set. Calcium chloride discussed as a traditional accelerator in non-reinforced or carefully controlled reinforced concrete (recognizing corrosion concerns). Concept / Approach: Gypsum (calcium sulphate) retards the flash set caused primarily by C3A by forming ettringite initially, thus giving workable time. Calcium chloride, by contrast, accelerates early hydration (especially of C3S), increasing early strength and reducing setting time; however, its use with steel reinforcement is restricted because chlorides promote corrosion unless strictly limited by codes. Step-by-Step Solution: Evaluate gypsum: it is a retarder → Option C is correct.Evaluate CaCl2: it is an accelerator → Option D is correct.Therefore, the combined correct choice is Option E (“Both (c) and (d)”). Verification / Alternative check: Specifications for OPC mandate a controlled SO3 content for proper set regulation; admixture datasheets list CaCl2 as an accelerator with limits or prohibition for reinforced concrete unless corrosion risks are addressed. Why Other Options Are Wrong: Options A and B reverse the functions; individually choosing C or D ignores that both statements are simultaneously true. Common Pitfalls: Using chloride accelerators with prestressed or galvanized steel; assuming gypsum increases strength—its primary role is set regulation. Final Answer: Both (c) and (d)

Question 12

Nominal Mixes for RCC – commonly used proportion for slabs, beams, columns and walls (historical practice) For general reinforced concrete work such as slabs, beams, columns, and walls (where nominal mixes were historically specified), which proport…

Accepted Answer

Introduction / Context: Before widespread adoption of design mixes for specific strength grades, many specifications used nominal volumetric mixes. For ordinary RCC elements, a widely cited nominal mix is 1 : 2 : 4 (cement : sand : coarse aggregate), roughly corresponding to lower-middle strength grades with proper water–cement ratio and curing. Given Data / Assumptions: Volumetric batching context with standard aggregates. Workability adjusted by water and, if allowed, admixtures. Modern practice increasingly prefers design mixes by target mean strength; the question asks historical/common nominal proportion. Concept / Approach: Among listed options, 1 : 2 : 4 is the standard nominal mix for RCC, whereas 1 : 3 : 6 and 1 : 4 : 8 are lean mixes used for blinding or mass concrete of lower strength. Proportion 1 : 1 : 2 is a rich mix suited to special cases, not general RCC economy. Step-by-Step Solution: Identify typical RCC nominal mix → 1 : 2 : 4.Cross-check other options as leaner or richer extremes.Conclude Option C is correct. Verification / Alternative check: Traditional schedules and handbooks list 1 : 2 : 4 for RCC where design mixing is not mandated, with cube strength acceptance at 28 days. Why Other Options Are Wrong: 1 : 3 : 6 and 1 : 4 : 8 are insufficient for most RCC structural demands; 1 : 1 : 2 is unnecessarily rich and uneconomical for general use. Common Pitfalls: Using nominal mixes without controlling water–cement ratio; ignoring durability requirements; not switching to design mixes when project demands specific strengths. Final Answer: 1 : 2 : 4

Question 13

Slump Test – what property does it primarily measure? In fresh concrete testing, the slump test is primarily a measure of which characteristic?

Accepted Answer

Introduction / Context: The slump test is one of the simplest and most widely used field tests for fresh concrete. It provides a quick indication of how easily a mix can be placed and compacted without segregation. Distinguishing what slump actually measures prevents misuse of the result for properties it does not represent. Given Data / Assumptions: Abrams cone apparatus with standard dimensions and procedure. Normal-weight, non-flowing concretes without special self-compacting behavior. Test performed on level, non-absorbent surface. Concept / Approach: Slump quantifies consistency/workability by measuring subsidence of the concrete after removing the cone. Higher slump indicates greater ease of placement, all else equal. It does not measure compressive or tensile strength directly, nor is it an impact test; correlations with strength are weak because admixtures can change slump without changing water–cement ratio. Step-by-Step Solution: Recognize test purpose → measure of consistency/workability.Eliminate strength-related options (b, c) and impact value (d).Select Option A. Verification / Alternative check: Standards specify slump ranges for different placement methods (paving, pumped concrete, heavily reinforced sections) precisely as a workability control. Why Other Options Are Wrong: Strength must be checked by cube/cylinder tests; impact value pertains to aggregate tests; setting time is determined by Vicat apparatus, not slump. Common Pitfalls: Using extra water to increase slump and inadvertently lowering strength; not re-mixing the sample before test; performing the test on a porous base. Final Answer: Consistency (workability)

Question 14

Typical Void Ratios (fractional) for aggregates – identify the correct combined statement Choose the correct statement regarding approximate void ratios (e) for common aggregate conditions (values are fractional, not percentages).

Accepted Answer

Introduction / Context: Void ratio (e) represents the volume of voids divided by the volume of solids. In concrete materials, knowing typical void ratios of aggregates helps assess paste demand, workability, and potential bleeding/segregation tendencies. While exact values vary with grading and packing, some approximate figures are widely used for preliminary estimates. Given Data / Assumptions: Aggregates are in loose to moderately compacted condition as used in batching bins. Values are indicative averages; actual site values depend on particle shape and grading. Void ratio is expressed as a fraction (for example, 0.45), not a percent. Concept / Approach: Single-size coarse aggregates leave larger voids because particles of similar size do not fill interstices effectively, giving e around 0.45 (about 45% voids by volume). Well-graded coarse aggregates pack better, reducing void ratio to roughly 0.40. Clean sands often show e in the 0.40–0.55 range depending on grading and compaction, so 0.45 is a representative order-of-magnitude value for many practical sands. Step-by-Step Solution: Assess Option A: consistent with typical single-size packing behavior → true.Assess Option B: gradation reduces e compared to single-size → true.Assess Option C: common values for sands place e near 0.45 under moderate packing → true.Therefore Option D (“All of the above”) is correct. Verification / Alternative check: Mix design handbooks tabulate similar void contents/ratios for preliminary paste demand estimates; laboratory measurements by rodded bulk density support these ranges. Why Other Options Are Wrong: “None” contradicts common data; selecting any single statement omits the fact that all three are acceptable approximations. Common Pitfalls: Confusing “percentage voids” with “void ratio”; ignoring shape effects (angular vs rounded) that can shift these values significantly. Final Answer: All of the above

Question 15

Water–Cement Ratio Law – conditions required for valid comparison Which of the following is NOT an applicable condition when using the water–cement ratio law to compare strengths of different concrete specimens?

Accepted Answer

Introduction / Context: The water–cement ratio law (Abram’s law) states that, for concretes made and cured identically, strength is primarily a function of the water–cement ratio. However, the qualifier “made and cured identically” is crucial. This item asks which stated condition does not comply with that requirement. Given Data / Assumptions: Comparisons are made between mixes intended to follow the w/c law. Specimens are prepared and cured using standardized methods. Strength is measured at a defined age. Concept / Approach: To apply the w/c law properly, variables other than water–cement ratio must be controlled. That includes age at test, curing regime, temperature during curing and testing, specimen geometry, compaction, and materials. Changing test temperature alters measured strength independently of w/c because hydration kinetics and modulus are temperature-sensitive. Therefore, “tested at any temperature” violates the identical-conditions requirement. Step-by-Step Solution: Review Option A: Adequate internal moisture is necessary for hydration → applicable.Review Option B: “Any temperature” breaks standardization → NOT applicable.Review Option C: Same age avoids maturity differences → applicable.Review Option D: Comparable geometry helps ensure similar stress distribution and rate effects → applicable. Verification / Alternative check: Standards require curing at controlled temperature (for example, 27 ± 2 °C) and testing at standardized temperatures. Deviations can significantly shift measured strengths irrespective of w/c. Why Other Options Are Wrong: They are necessary controls for a fair application of the w/c law; removing them introduces confounding factors. Common Pitfalls: Comparing strengths from different labs or curing rooms without temperature records; conflating maturity with age; ignoring size/shape effects on measured strength. Final Answer: Concrete specimens may be tested at any temperature

Question 16

Aggregate Shape – which is unsuitable for high-strength concrete and tensioned pavements? Identify the type of aggregate that is generally not suitable for producing high-strength concrete and for pavements subjected to tensile stresses.

Accepted Answer

Introduction / Context: Aggregate shape influences packing, interlock, and the quality of the interfacial transition zone in concrete. For high-strength concrete and slabs/pavements experiencing tensile stresses, selection of aggregate shape is important for both strength and toughness. Given Data / Assumptions: Aggregates are clean and properly graded. High-strength concrete aims for low w/c and dense microstructure. Pavements under tension need reliable crack resistance and fatigue performance. Concept / Approach: Flaky particles have small thickness compared with their other dimensions, leading to poor load transfer, higher surface area (thus higher paste demand), and a tendency to break under compaction or load. They align during placing, creating planes of weakness. Angular aggregates, while increasing paste demand relative to rounded particles, provide better interlock and strength. Rounded aggregates improve workability but may reduce interlock; still, they are preferable to flaky particles for strength and fatigue resistance. Step-by-Step Solution: Define flaky → least dimension is small (thickness < specified fraction of mean size).Relate to performance → poor interlock and breakage → reduced strength.Select the unsuitable type → flaky aggregate. Verification / Alternative check: Specifications limit flakiness index for high-performance concrete; rigid pavement guidelines discourage flaky/elongated particles to avoid cracking and raveling. Why Other Options Are Wrong: Rounded/irregular/angular particles can be used with proper design; none has the inherent structural weakness of flaky particles. Common Pitfalls: Confusing flaky with elongated; both are limited but flaky is especially detrimental for high-strength requirements. Final Answer: Flaky aggregate

Question 17

Bulk Density vs Grading – find the incorrect statement Which of the following statements about bulk density of aggregates and grading is incorrect?

Accepted Answer

Introduction / Context: Bulk density of aggregate depends on how well particles pack together. Grading (distribution of sizes) determines how smaller particles fill voids between larger ones. Understanding this relationship is important for proportioning concrete and asphalt mixtures efficiently. Given Data / Assumptions: Aggregates are clean and dry in a loose to rodded condition typical for bulk density tests. Terms: “single-size” means narrow gradation; “properly graded” means a good spread of sizes. Concept / Approach: Properly graded aggregate packs more efficiently because fines occupy voids between coarse particles, producing higher bulk density and lower void content. Single-size aggregate leaves many unfilled voids, giving lower bulk density. Hence, any statement claiming maximum bulk density for single-size aggregate is incorrect. Step-by-Step Solution: Option A: True—better packing raises bulk density.Option B: True—single-size leaves more voids, reducing density.Option C: False—contradicts packing logic.Therefore, Option C is the incorrect statement. Verification / Alternative check: Mix design guides show lower paste demand with well-graded aggregate (a consequence of higher packing density) compared with single-size aggregate blends. Why Other Options Are Wrong: They correctly express the relation between grading and packing density. Common Pitfalls: Equating particle angularity with grading effects; both influence bulk density but grading typically dominates. Final Answer: In single-size aggregates, bulk density is maximum

Question 18

Making Concrete Test Specimens – essential preparation steps When preparing concrete specimens for strength testing, which of the following steps are necessary before and during casting?

Accepted Answer

Introduction / Context: Standard preparation of test specimens is critical for obtaining reliable and comparable strength results. Poor mixing sequence, un-oiled moulds, or non-uniform water addition can introduce variability unrelated to the mix itself, undermining quality control decisions. Given Data / Assumptions: Cubes or cylinders are to be cast according to established standards. Materials are proportioned by a qualified mix design and measured accurately. Compaction will be carried out by tamping or vibration as specified. Concept / Approach: The usual sequence is: dry-mix cement and sand for uniform distribution of fines; add coarse aggregate and mix again to achieve a consistent dry blend; then add measured water (and admixtures if used) and mix until uniform. Moulds must be clean and lightly oiled to prevent bonding to the sides and to obtain smooth faces without defects. Proper stepwise preparation minimizes segregation and ensures consistent specimen integrity. Step-by-Step Solution: Option A: Correct—dry blending of paste constituents reduces clumping.Option B: Correct—coarse aggregate must be evenly distributed.Option C: Correct—controlled water addition achieves the target w/c ratio.Option D: Correct—oiling moulds facilitates demoulding and preserves edges.Therefore, Option E “All of the above” is correct. Verification / Alternative check: Laboratory SOPs and codes specify this sequence and require clean, lightly oiled moulds; deviations increase scatter in test results. Why Other Options Are Wrong: Each individual step is necessary; choosing fewer than all omits essential parts of the standard procedure. Common Pitfalls: Adding water first, which causes cement balling; failing to oil moulds; inconsistent mixing times; neglecting aggregate moisture corrections. Final Answer: All of the above

Question 19

Effect of elevated temperature on concrete: Considering hydration kinetics and microstructure, how does sustained high temperature during early age and curing influence the final (28-day and beyond) compressive strength of ordinary Portland cement c…

Accepted Answer

Introduction / Context: Temperature strongly influences cement hydration and the pore structure of concrete. While contractors may observe faster stiffening on hot days, the long-term mechanical performance is governed by the microstructure that develops during curing. This question asks how high temperature exposure (especially at early age) affects the eventual compressive strength of ordinary Portland cement concrete. Given Data / Assumptions: Ordinary Portland cement, normal aggregates, conventional mixes. “High temperature” refers to curing/early-age heat that accelerates hydration (e.g., hot weather concreting or high internal temperatures in mass concrete). Strength evaluation is at 28 days and beyond. Concept / Approach: Elevated temperature speeds early hydration, giving higher early strength but coarser capillary pore structure due to rapid precipitation of hydration products. This coarser, more discontinuous C-S-H network increases permeability and microcracking risk, which reduces long-term strength and durability. The general trend is: higher early strength, lower ultimate strength compared with identical concrete cured at moderate temperature. Step-by-Step Solution: Recognize that heat accelerates reaction rates → early strength rises quickly.Rapid hydration causes non-uniform product deposition → coarser pores and internal microcracks.Coarser microstructure translates into lower 28-day and ultimate strength compared with cool, moist curing.Therefore, the net long-term effect is a decrease in strength. Verification / Alternative check: Hot-weather concreting guides recommend temperature control, slowed setting (admixtures), and extended moist curing to mitigate strength loss and shrinkage cracking. Mass concrete specifications limit temperature rise and gradients for the same reason. Why Other Options Are Wrong: Increases strength/No effect: contradicts observed long-term microstructural outcomes.“Early strength unchanged”: incorrect; early strength often increases, but the question asks about final strength. Common Pitfalls: Confusing faster set with higher long-term strength; neglecting additional curing needs in hot weather. Final Answer: Decreases the strength of concrete.

Question 20

Aggregate shape and packing: Among aggregates of the same nominal size, which particle shape generally yields the least void content when compacted, thus improving workability and reducing paste demand?

Accepted Answer

Introduction / Context: Aggregate shape controls packing density, workability, and required paste content. For a given gradation and nominal size, shapes that pack tightly minimize voids, cutting down on cement paste demand and shrinkage. Given Data / Assumptions: Aggregates have the same nominal size and similar gradation. Compaction is performed to typical site standards. We compare rounded, irregular (angular), flaky, and elongated shapes. Concept / Approach: Rounded particles roll and rearrange under compaction to occupy less void space. Angular irregular particles interlock, increasing voids and requiring more paste for the same workability. Flaky and elongated particles stack poorly, creating high void ratios and poor flow. Therefore, rounded shapes achieve the lowest void content. Step-by-Step Solution: Target: minimize voids under compaction.Rounded particles → best packing → least paste demand.Irregular/Flaky/Elongated → higher voids and harsher mixes.Choose the rounded option. Verification / Alternative check: Workability tests and mix design charts reflect lower sand/ paste needs when rounded sands and gravels are used. Why Other Options Are Wrong: Irregular, flaky, and elongated particles typically increase voids and reduce workability. Common Pitfalls: Ignoring the trade-off: angular particles may improve bond and strength but at the cost of higher paste demand. Final Answer: Rounded (spherical/ellipsoidal) particles.

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