Question 1

Compaction of concrete – primary objectives on site What is the main objective of compacting freshly placed concrete in structural elements?

Accepted Answer

Introduction / Context: Proper compaction transforms a plastic mix into a dense, durable, and strong structural material. Inadequate vibration leaves honeycombing, weak interfaces, and reduced cover protection for reinforcement—all of which compromise durability and capacity. Given Data / Assumptions: Conventional reinforced concrete with appropriate workability for vibration. Use of immersion vibrators or surface vibrators as suitable. Forms are tight to prevent leakage of mortar. Concept / Approach: Fresh concrete contains entrapped air. Vibration mobilizes particles and allows paste to fill voids, expelling air and improving particle packing. The goals are to achieve maximum practical density, eliminate air pockets especially around reinforcement and inserts, and ensure intimate contact with form surfaces for a good finish and cover. Step-by-Step Solution: Eliminate air: vibration reduces air content drastically (except intentionally entrained air).Increase density: particle rearrangement creates a denser matrix with higher strength and lower permeability.Ensure contact: proper vibration allows concrete to flow around rebars and embedded parts, avoiding voids.Therefore, the comprehensive objective is “All of the above”. Verification / Alternative check: Quality indicators such as surface soundness, absence of bugholes, and rebound hammer uniformity reflect adequate compaction practices. Why Other Options Are Wrong: Each individual goal is valid, but none alone captures the complete purpose; option (d) includes all essential outcomes. Common Pitfalls: Over-vibration causing segregation in very wet mixes, or under-vibration in stiff mixes; always match vibrator type and duration to element geometry and slump. Final Answer: All of the above

Question 2

Water demand vs grading – which grading needs the least water for the same workability? For a given target workability, which type of aggregate grading generally requires the minimum mixing water?

Accepted Answer

Introduction / Context: Water demand in concrete is strongly influenced by the total surface area of solids to be wetted by paste. For the same cement content and target workability, optimizing aggregate grading to reduce surface area helps achieve the slump with less water, thereby lowering the water–cement ratio and improving strength and durability. Given Data / Assumptions: Same cement content and mineralogy. Aggregate shape and texture are comparable; focus is on grading effect. Workability specification (e.g., slump) is held constant for comparison. Concept / Approach: A well-graded aggregate pack reduces voids and minimizes the specific surface area to be coated by paste. Less surface area means less paste and water are required to achieve lubrication and flow. Conversely, very fine or poorly graded aggregates increase surface area and water demand, causing higher w/c ratios if cement is not increased or if admixtures are not used. Step-by-Step Solution: Relate workability demand to surface area to be wetted by paste.Recognize that least surface area → least water for the same flow.Therefore, select the grading with the least surface area. Verification / Alternative check: Empirical mix-design methods (e.g., F.M. adjustment, optimized packing curves) consistently show reduced water demand with well-graded blends of sand and coarse aggregate. Why Other Options Are Wrong: Greater surface area increases water demand; total weight is not the controlling factor when composition is fixed. Common Pitfalls: Assuming that adding fines always improves finish: it can raise paste demand and shrinkage if not balanced by coarser fractions. Final Answer: The grading with the least surface area for the given cement and aggregates

Question 3

Soil/aggregate fines identification – particles of about 0.002 mm size In standard soil and fines classification, particles with a nominal size around 0.002 mm belong to which category?

Accepted Answer

Introduction / Context: In both soil mechanics and aggregate fines control for concrete, distinguishing silt from clay is important because clay exhibits plasticity, high surface area, and strong water sensitivity. The conventional boundary between silt and clay is based on particle size coupled with plasticity characteristics. Given Data / Assumptions: We refer to standard geotechnical size bands. Plasticity is a behavioural attribute but size is the quick index. 0.002 mm (2 microns) is the conventional dividing size. Concept / Approach: Clay is defined as the finest fraction, typically smaller than about 0.002 mm. Due to their plate-like crystal structures and large specific surface area, clay minerals strongly interact with water, causing plasticity, shrink–swell, and high cohesion—properties that markedly influence workability and water demand in concrete fines as well. Step-by-Step Solution: Identify the conventional silt–clay boundary at ~0.002 mm.Classify particles near or below this size as clay.Select “Clay”. Verification / Alternative check: Unified Soil Classification System and related standards use 0.002 mm as a reference divider; Atterberg limits further confirm clay behaviour. Why Other Options Are Wrong: Sand: ranges roughly from 0.075 mm to several millimetres—much coarser. Gravel: coarser still, above the sand range. “None” is incorrect because clay is the recognized category. Common Pitfalls: Assuming all fines behave like clay; silt is finer than sand but lacks clay’s plasticity and surface activity. Final Answer: Clay

Question 4

Standard reference age for full (characteristic) strength of concrete as per IS practice At what standard age is the characteristic compressive strength of concrete specified and accepted for most design and quality control purposes in IS practice?

Accepted Answer

Introduction / Context: Concrete continues to gain strength with time, but codes adopt a standard reference age to define characteristic compressive strength for design and acceptance. In Indian Standards and most international practice, the 28-day age is the benchmark. Given Data / Assumptions: Standard curing conditions in water at controlled temperature. Ordinary Portland cement or blended cements typical of structural concrete. Characteristic strength defined statistically (e.g., fck). Concept / Approach: Hydration proceeds over months and years, but early-age strengths are used for construction scheduling while 28-day strength provides a consistent, comparable basis for mix design and structural calculations. Acceptance testing commonly relies on average strengths at 28 days meeting or exceeding specified characteristic values with defined variability margins. Step-by-Step Solution: Identify the standard reference age adopted by IS codes → 28 days.Recognize that earlier ages (7 or 14 days) are sometimes used for preliminary checks but not as the final acceptance strength unless specified otherwise.Select “28 days”. Verification / Alternative check: IS 456 and related testing standards specify 28-day cube tests for acceptance criteria; rapid methods (maturity, accelerated curing) are correlated back to the 28-day benchmark. Why Other Options Are Wrong: 7, 14, 21, or 35 days may be used for progress estimation or special cements, but the universally adopted design strength age is 28 days unless the specification states otherwise. Common Pitfalls: Confusing early stripping strength with design strength; formwork removal may occur earlier depending on cement type, curing, and ambient temperature. Final Answer: 28 days

Question 5

Aggregate Bulk Density – identify the factors that influence it In concrete materials engineering, the bulk density (also called unit weight in a container) of aggregates depends primarily on which of the following influences?

Accepted Answer

Introduction / Context: Bulk density of aggregates is the mass of aggregate that fills a container of known volume, including the solid particles and the voids between them. This apparent density matters because it influences batching by volume, paste demand, and the economy of concrete mixtures. Understanding what controls bulk density helps technicians choose grading and compaction procedures that minimize voids and improve concrete quality. Given Data / Assumptions: Standard container and procedure (loose or rodded/compacted condition) as per lab practice. Normal mineral aggregates used in concrete. Evaluation is comparative, not an absolute number for a specific source. Concept / Approach: Bulk density is governed by how efficiently particles pack. Three major factors dominate: particle shape, grading, and compaction method. Angular, rough particles interlock but create more voids than rounded ones. Well-graded aggregates allow small particles to fill voids between larger ones, reducing overall void content. Finally, the compaction method (shoveling, rodding, vibration) influences how particles rearrange and settle, strongly affecting the measured bulk density in the container. Step-by-Step Solution: Relate shape to packing: rounded/regular shapes pack denser than flaky/angular ones, raising bulk density.Relate grading to packing: a continuous distribution fills interstices better than single-size aggregate, raising bulk density.Relate compaction to packing: rodding or vibration settles particles and increases bulk density relative to a loose fill.Therefore, all three listed factors substantially influence bulk density. Verification / Alternative check: Laboratory comparisons show rodded bulk density exceeding loose bulk density for the same aggregate; similarly, a well-graded blend exhibits higher bulk density than a single-size fraction. Shape corrections are recognized in packing and void content tests. Why Other Options Are Wrong: Choosing only one factor ignores the combined nature of packing; “None of these” contradicts established test results and field experience. Common Pitfalls: Mixing loose and rodded values; assuming bulk density equals specific gravity (it does not); overlooking moisture corrections which change apparent mass in the container. Final Answer: All of the above

Question 6

Concrete Practice – choose the incorrect statement (workability, controlled concrete, and sand bulking) Identify the single incorrect statement among the following site-practice observations.

Accepted Answer

Introduction / Context: Many site decisions hinge on correct understanding of fresh concrete behavior and aggregate characteristics. This item asks you to separate one false statement from a group of otherwise acceptable descriptions regarding workability, controlled concrete, and bulking of sand. Given Data / Assumptions: No special water-reducers unless stated; observations refer to ordinary concretes. “Controlled concrete” implies that trial mixes and preliminary tests govern proportions. Bulking is the apparent volume increase of moist fine aggregate due to a water film. Lean mixes such as 1 : 6 : 12 are associated with mass concrete where high strength is not the primary goal. Concept / Approach: Workability generally increases with additional moisture (to a point), because more paste fluidity allows easier placement and compaction. Thus, claiming that workability decreases with an increase in moisture is incorrect. The other statements reflect accepted practice: controlled concrete is based on trials and tests; bulking depends on fineness with finer sands showing greater bulking; and very lean mixes like 1 : 6 : 12 are used for mass concreting purposes rather than structural RCC elements. Step-by-Step Solution: Assess (a): False—added water typically increases workability though it may harm strength if w/c rises excessively.Assess (b): True—trial mixes determine proportions; hence “controlled concrete”.Assess (c): True—finer sand exhibits higher bulking for the same moisture.Assess (d): True—1 : 6 : 12 is a lean mass-concrete proportion used in non-RCC works like foundations/piers. Verification / Alternative check: Slump typically rises with water content; bulking curves show peak bulking for fine sands around 4–6% moisture; construction handbooks list lean mixes for mass works. Why Other Options Are Wrong: (b), (c), and (d) align with standard guidance; “All of the above” cannot be right because (a) is false. Common Pitfalls: Confusing “workability increases” with “strength increases”—they are not the same; ignoring that excessive water harms durability even though it may improve slump. Final Answer: Workability of the concrete mix decreases when moisture content increases

Question 7

Fine Aggregate Upper Size – IS practice In standard concrete and mortar grading, the maximum size that still classifies as fine aggregate (sand) is taken as not exceeding which sieve size?

Accepted Answer

Introduction / Context: Aggregates are broadly classified into fine and coarse fractions for proportioning and quality control. The upper sieve size defining fine aggregate is a standard reference that determines which grading limits apply and how mix designs are computed. Given Data / Assumptions: Sieve series used in common IS/metric practice. “Fine aggregate” refers to material commonly called sand. Ordinary concrete and mortar specifications. Concept / Approach: Fine aggregate comprises particles that pass the 4.75 mm sieve. Material retained on larger sieves belongs to coarse aggregate fractions for proportioning and grading purposes. This boundary ensures a clear separation of roles: fines influence workability and paste demand, while coarse particles contribute to skeleton and economy. Step-by-Step Solution: Identify the boundary sieve used to classify sand → 4.75 mm.Confirm that material passing 4.75 mm fits fine aggregate grading envelopes.Choose option D accordingly. Verification / Alternative check: Mix design tables and grading specifications consistently use the 4.75 mm sieve as the cutoff between fine and coarse aggregates. Why Other Options Are Wrong: Other sieve sizes listed do not correspond to the standard fine/coarse boundary and would misclassify aggregate fractions for design. Common Pitfalls: Confusing nominal maximum size with the fine/coarse boundary; ignoring that well-graded sands still must comply with limits across multiple sieves below 4.75 mm. Final Answer: 4.75 mm

Question 8

Bulking of Sand – definition in concrete technology What does the term “bulking of sand” specifically refer to in the context of fresh mortar and concrete proportioning?

Accepted Answer

Introduction / Context: Bulking is a practical site phenomenon that directly affects volumetric batching. When moist, sand occupies more volume than in the fully saturated or dry state due to the formation of a water film that pushes particles apart. If not corrected, this leads to under-sanded mixes, poor cohesiveness, and strength loss. Given Data / Assumptions: Natural sand used for mortar or concrete. Moisture contents around 2–8% where bulking is prominent. Volumetric proportioning by gauge boxes or wheelbarrows. Concept / Approach: A thin water film increases interparticle repulsion and arching, creating an apparent volume increase at given mass. Bulking peaks at moderate moisture, then declines toward zero as the voids fill and particles are submerged (saturated). The correct response is to measure bulking and adjust gauged volume of sand accordingly, or to batch by mass instead of volume. Step-by-Step Solution: Define bulking: swelling of moist sand that increases apparent volume at the same mass.Recognize practical effect: volumetric batching underestimates true sand content unless corrected.Therefore, option D is the accurate definition. Verification / Alternative check: Cylinder tests show a typical bulking of 20–35% for fine sands at around 4–6% moisture, returning to the saturated volume when inundated. Why Other Options Are Wrong: Options A, B, C, and E describe different processes (grading, lime addition, maximum water, compaction) and are not the definition of bulking. Common Pitfalls: Ignoring bulking when using gauge boxes; assuming saturated sand has maximum bulking (it does not). Final Answer: Increase in apparent volume (swelling) of sand when it is wetted

Question 9

Cement Composition – major chemical ingredient by percentage Which oxide is present in the highest percentage in ordinary Portland cement (OPC) clinker chemistry?

Accepted Answer

Introduction / Context: Understanding the principal oxides in cement is fundamental for predicting hydration behavior and performance. OPC clinker is chiefly composed of calcium oxide (lime), silica, alumina, and iron oxide, which combine to form clinker minerals such as C3S and C2S that control strength development. Given Data / Assumptions: Ordinary Portland cement without special additions. Typical oxide ranges from standard cement chemistry references. Concept / Approach: Lime (CaO) is the dominant oxide, typically in the range of about 60–65% by mass. Silica (SiO2) commonly contributes around 17–25%, alumina roughly 3–8%, iron oxide about 1–5%, and magnesium oxide is usually limited to a few percent to avoid unsoundness. Since CaO is the highest, option D is correct. Step-by-Step Solution: Identify dominant oxide in clinker: CaO (lime).Compare with other oxides: SiO2, Al2O3, Fe2O3, MgO occur at lower percentages.Select option D accordingly. Verification / Alternative check: Oxide composition tables for OPC list CaO as the largest constituent, consistent with its role in forming C3S and C2S, the primary strength-giving phases. Why Other Options Are Wrong: Silica and alumina, while important, are present in smaller proportions; Fe2O3 and MgO are minor constituents with limits to prevent unsoundness. Common Pitfalls: Confusing oxide percentages with compound percentages (for example, C3S vs CaO); overlooking that excessive free lime (uncombined CaO) can cause expansion. Final Answer: Lime (CaO)

Concrete Technology Questions