Question 1

What is the difference between Quality Assurance (QA) and Quality Control (QC)?

Accepted Answer

Quality Assurance is the process side — quality plans, method statements, training, document control, internal audits. Quality Control is the verification side — cube tests, slump checks, calibration records, inspection logs. QA defines how QC is done; QC produces the evidence that QA is working. Both are needed on a concrete project and both must be documented.

Question 2

How often should cube tests be conducted on an RMC pour?

Accepted Answer

Sampling frequency depends on the quantity of concrete placed. IS 456:2000 Cl. 15.2 and IS 516 (Part 1/Sec 1):2018 specify that for each grade of concrete, samples shall be taken at the rate of one sample per 50 m³ (or part thereof) of each grade placed, with higher frequency for controlled concrete per Cl 15.2.2. For very large pours, samples are also taken at hopper and at discharge. Each sample yields three 150 mm cubes tested at 28 days for acceptance (7-day cubes are additional and informative). Try the calculator: Sampling Planner .

Question 3

What is the acceptance criteria for 28-day compressive strength as per IS 456?

Accepted Answer

Per IS 456:2000 Table 11 (Cl. 16.1 & 16.3), for specified grade M 15 and above, concrete is deemed to comply with the characteristic strength f ck only when both of the following are met simultaneously: (1) the mean of any group of 4 non-overlapping consecutive test results is ≥ f ck + 0.825 × established standard deviation (rounded to nearest 0.5 N/mm²), or ≥ f ck + 3 N/mm², whichever is greater; and (2) every individual test result is ≥ f ck − 3 N/mm². Note 1: until σ is established from 30 prior samples, use the assumed values from IS 456 Table 8 . Note 2 (small-quantity rule): where concrete placed is up to 30 m³ and the number of samples is less than 4, the mean of all such samples shall be ≥ f ck + 4 N/mm² and individual results ≥ f ck − 2 N/mm²; if only one sample is taken, it shall be ≥ f ck + 4 N/mm². Try the calculator: Cube Test Statistics .

Question 4

How many cubes should be cast per sample as per IS 516?

Accepted Answer

Each sample of concrete is divided into three specimens of 150 mm cubes. The average strength of the three specimens represents the test result, provided no individual cube deviates by more than ±15% of the average. The specimens are typically tested at 7 days (informative) and 28 days (acceptance).

Question 5

What is the role of a Quality Plan in construction projects?

Accepted Answer

A Quality Plan (QAP) is a project-specific document that identifies what will be controlled, the standards it must meet, who is responsible, the procedures to follow, and records to retain. For concrete, the QAP typically covers incoming material tests, mix design approval, batching, placement, sampling frequency, testing, non-conformance handling, and final handover documentation.

Question 6

How is compressive strength tested as per IS 516?

Accepted Answer

IS 516 (Part 1/Sec 1):2018 prescribes: cube size 150 mm (or 100 mm for aggregate ≤20 mm), curing at 27 ± 2 °C in clean water, loading rate of 120 to 240 kg/cm² per minute (≈12 to 24 MPa/min) per IS 516 (Part 1/Sec 1):2018, which specifies 0.3 ± 0.1 N/mm²/s, and capping with self-neat cement paste to ensure parallel loading faces. Test age is normally 7 and 28 days; results are reported to the nearest 0.5 MPa.

Question 7

What are common causes of low cube strength and how are they investigated?

Accepted Answer

Investigate in this order: (1) sampling error — wrong sample location or non-representative mix; (2) specimen defect — poor compaction, damaged moulds, improper curing; (3) mix issue — w/c ratio drift, cement quality, admixture overdose, aggregate moisture; (4) testing error — machine calibration, capping, loading rate. Retest before any structural consequence — and check 7-day results on the same sample for a pattern.

Question 8

How should concrete cubes be cured before testing?

Accepted Answer

After casting, keep cubes in the mould for 24 ± 0.5 h at ≥16 °C and shielded from vibration. Strip the cubes, mark them, and fully immerse in clean, fresh water at 27 ± 2 °C until just before testing. The water must be renewed every 7 days; do not allow cubes to dry. Curing is the single biggest controllable variable in cube results.

Question 9

What is the difference between trial mixes and approved mix designs?

Accepted Answer

A trial mix is the investigation — done in a lab using representative materials to achieve the target strength, workability and durability. An approved mix design is the documented selection of one successful trial mix with the actual material quantities, signed off by the structural designer / owner’s engineer for use in production. Trials without approval have no project status; approvals without trials have no evidence base. Try the calculator: Trial Mix Batch .

Question 10

How are non-conformities (NCRs) documented and closed?

Accepted Answer

Issue a Non-Conformance Report (NCR) as soon as the deviation is observed — describe what happened, where , when, by whom witnessed, the specification or procedure violated, and the immediate containment . The NCR is then dispositioned (use-as-is, rework, repair, reject) with a clear root-cause action. Closure requires both the corrective action taken and the preventive action for the next pour.

Question 11

What is the difference between nominal mix and design mix?

Accepted Answer

Nominal mix (e.g. 1:2:4 by volume) is a prescriptive recipe using assumed material qualities. Design mix (per IS 10262) is a performance recipe derived for the project’s actual materials, exposure condition and target workability. IS 456:2000 requires design mix for all grades M20 and above and for all durable structures; nominal mix is permitted only for small non-structural volumes. Try the calculator: Nominal Mix .

Question 12

What is Water-Cement Ratio and why does it matter for strength?

Accepted Answer

w/c is the mass ratio of free water (excluding aggregate absorption) to cement. It controls strength, permeability and durability — not the cement content alone. IS 456:2000 Table 5 caps w/c for each exposure condition (0.55 for RCC under mild exposure, down to 0.45 for very severe and 0.40 for extreme exposure). Lower w/c → higher strength and lower permeability, but workability suffers unless compensated by plasticizer. Try the calculator: Water–Cement Ratio .

Question 13

How do admixtures affect concrete performance?

Accepted Answer

Plasticizers (PCE-based) reduce mixing water by 12–25% at a given workability; superplasticizers by 25–40%. Retarders delay setting in hot weather or long hauls. Accelerators speed early strength for repairs and pre-stress. Air-entraining agents protect against freeze-thaw. The dosage is brand-specific and must be checked against IS 9103 limits, and the same brand/source should be used throughout the project to avoid incompatibility shocks.

Question 14

What is the role of supplementary cementitious materials (SCMs)?

Accepted Answer

SCMs like fly ash (PFA), GGBS and silica fume replace part of the cement in the binder. They reduce heat of hydration (useful in mass pours), refine the pore structure (lower permeability) and improve long-term durability. The trade-off is slower early-age strength. IS 3812 (fly ash), IS 12089 (GGBS) and IS 15388 (silica fume) prescribe the use limits.

Question 15

What is the difference between PPC, OPC and PSC cement?

Accepted Answer

OPC (Ordinary Portland Cement, IS 269) is neat clinker + gypsum. PPC (Portland Pozzolana Cement, IS 1489) substitutes 15–35% fly ash and gains strength more slowly but is denser and more durable at later ages. PSC (Portland Slag Cement, IS 455) substitutes GGBS, similar profile. For marine / sulphate exposure, PPC or PSC performs better; for early-strength pre-stress, OPC 53 grade is preferred.

Question 16

How is aggregate grading controlled and why does it matter?

Accepted Answer

Aggregate grading — the distribution of particle sizes — controls water demand, paste content and pumpability. IS 383 specifies grading envelopes for fine aggregate (Zone II is the typical target) and nominal single-sized coarse aggregate (10 mm, 20 mm, 40 mm). Incorrect grading increases water demand by 5–15%, raises shrinkage and reduces pump pressure margin. Sieve testing at least once per lot.

Question 17

What is the significance of the slump test?

Accepted Answer

Slump is a workability indicator , not a strength measure. Target slump depends on the placement method: 25–75 mm for chhabeena/manual compaction in slabs, 75–100 mm for normal vibration, 100–150 mm with pumped concrete, 150–180 mm for congested heavily-reinforced sections. Always confirm slump at point of discharge and also at pour when the transit time is significant.

Question 18

What is bleeding in concrete and how is it controlled?

Accepted Answer

Bleeding is the upward migration of free water to the surface after placement. A little is normal (1–2% of mix water); excessive bleeding brings fines and air to the top, producing a weak laitance layer and — under rebar — voids. Control by reducing water content, increasing fines, using finer cement or SCM, accelerating set slightly, and re-vibrating the top layer before initial set.

Question 19

What is the difference between M20, M25, M30, M35 grades and their typical applications?

Accepted Answer

The M-prefix is “Mix”; the number is the characteristic 28-day compressive strength in MPa. M20–M25 — residential slabs, lightly-loaded beams, non-structural work. M25–M30 — most RCC residential and commercial frames. M30–M40 — high-rise columns, transfer beams, bridge decks. Mix design — not the M-number alone — controls performance; exposure and durability requirements often govern selection. Try the calculator: Nominal Mix .

Question 20

How does temperature affect fresh and hardened concrete properties?

Accepted Answer

Above 32 °C, fresh concrete loses slump rapidly, the initial set accelerates (danger for long-haul RMC), and water demand rises. Below 5 °C, hydration slows and early strength suffers; below 0 °C, free water freezes and disrupts the paste. Use chilled water / ice in summers and insulated curing / hot-water batching in winter, keeping delivery temperature ideally 18–27 °C. Try the calculator: Concrete Temperature .

Question 21

What is the minimum curing period for OPC and PPC concrete as per IS 456?

Accepted Answer

IS 456:2000 Cl. 13.5 specifies a minimum curing period of 7 days for OPC concrete under ordinary conditions, extended to 10 days for PPC / PSC / slag blends , and further extended to 14 days for concrete with mineral admixtures or under hot / arid exposure. Wet curing by ponding, spray, wet hessian or curing compound is acceptable — but the duration must be cumulative, not nominal. Try the calculator: Curing Water .

Question 22

What is the recommended cover for different structural members?

Accepted Answer

Nominal cover is the distance from the outer rebar surface to the nearest concrete face. IS 456:2000 Cl. 26.4.2 & Table 16 prescribe nominal cover by exposure: 20 mm (Mild), 30 mm (Moderate), 45 mm (Severe) and 50 mm (Very severe / Extreme) for 20 mm nominal aggregate; footings get a minimum 50 mm regardless of exposure (Cl. 26.4.2.2). In marine or sulphate exposure, increase cover by 10–15 mm and control surface crack width to 0.2 mm (Cl. 35.3.2, severe/very severe).

Question 23

How is formwork removal time determined for beams, slabs, and columns?

Accepted Answer

IS 456:2000 Cl. 11.3 sets minimum striking times based on member type and ambient temperature. As a working rule: vertical formwork to columns/walls/beams — 16–24 h per the code (extend toward 48 h in cool weather); slab soffit (props left) — 3 days; beam soffit (props left) — 7 days; props of slabs — 7 days; props of beams — 14 days (or 21 days for spans >6 m). Before striking soffits, confirm early strength on companion cubes — the Cl. 11.2 principle is that concrete must be about twice as strong as the stress the member will carry once props are released. Keep props beneath slabs and beams until their soffits are struck.

Question 24

What is the maximum free-fall height allowed for concrete placement?

Accepted Answer

IS 456:2000 Cl. 13.2 advises a maximum free fall of 1.5 m to avoid segregation. For drops greater than 1.5 m, use a tremie pipe, drop chute or closed delivery hose. Pumped concrete (≤ 60 m vertically per standard pump) typically confines free-fall to the hose outlet; the operator must still avoid dropping the hose end through rebar congestion without a deflector.

Question 25

What are construction joints and how should they be treated?

Accepted Answer

A construction joint is the interface between concrete placed at different times — a deliberate stop / start point, not an unplanned cold joint. Per IS 456:2000 Cl. 13.4 (construction joints), the surface must be cleaned of laitance, roughened to expose aggregate (≈ 5 mm amplitude), kept wet for 6–24 h, and a rich mortar coat or bonding agent applied before the next lift. Shear keys are added in slabs and walls to transfer shear across the joint.

Question 26

How should cold joints be prevented during large pours?

Accepted Answer

Cold joints form when the next layer of concrete is placed after initial set of the previous layer — they show as straight horizontal lines of weakness. Plan around (1) pour size — schedule pours to fit within initial-set window; (2) supply rate — keep a buffer in transit time; (3) retarders in hot weather to extend workability; (4) vibrator practice — penetrate the previous layer by 50–100 mm to re-blend the two lifts before they set.

Question 27

What is the importance of compaction and how is it achieved?

Accepted Answer

Compaction (consolidation) expels entrapped air so that the concrete achieves the designed density, strength and bond to rebar. IS 516 expects <5% air for normal concrete, less for high-strength. Use immersion (needle) vibrators in 25–50 mm diameter; insert vertically at 300–450 mm spacing for 5–15 s, withdrawing slowly. Over-vibration segregates the mix; under-vibration leaves honeycomb.

Question 28

What precautions should be taken for concreting in hot weather?

Accepted Answer

Hot weather concreting is governed by IS 7861 (Part 1). Cool aggregates by shade / water spray, use chilled mixing water or ice, schedule pours in early morning or evening, keep transit time short, add a retarder, and start curing immediately after finishing. Use windbreaks and sunshades; avoid plastic-shrinkage cracking by fog-spraying the surface between initial set and final finishing. Try the calculator: Evaporation Rate .

Question 29

What precautions should be taken for concreting in cold weather?

Accepted Answer

Cold weather concreting is governed by IS 7861 (Part 2). Keep surface temperature of placed concrete above 5 °C for at least 48 hours — heat mixing water, use hot-water batching, insulate transit, cover the surface with insulated mats or insulated formwork. Never place concrete on frozen subgrade. Avoid early-age de-shuttering and mechanical shock.

Question 30

What is the recommended procedure for de-shuttering vertical and horizontal members?

Accepted Answer

Sequence: (1) remove side shutters of columns and walls after 24–48 h; (2) remove side shutters of beam after 48–72 h; (3) remove slab soffit after 7 days but retain props for 14 days; (4) remove beam soffit after 14 days but retain props at mid-span for 21 days; (5) for spans >6 m, retain full props for 21 days. Strip carefully without spalling edges; cure immediately if the surface was not pre-wetted.

Question 31

What is IS 456:2000 and what does it cover?

Accepted Answer

IS 456:2000 is the Plain and Reinforced Concrete — Code of Practice (fourth revision) . It prescribes the structural design and detailing of RCC structures in India — materials, workmanship, durability, cover, fire resistance, member design (flexure, shear, axial, torsion) and quality control. Mandatory use for all RCC buildings in India via NBC 2016.

Question 32

What is the role of IS 10262 in concrete mix design?

Accepted Answer

IS 10262:2019 is the Indian Standard for concrete mix proportioning guidelines . It walks through selecting target strength, water content, sand-to-total-aggregate ratio, estimating cementitious content, and arriving at mix quantities for 1 m³ of concrete. The mix must also satisfy the durability requirements of IS 456 — typically w/c ratio and minimum cement content for the chosen exposure.

Question 33

What is IS 383 and how does it classify aggregates?

Accepted Answer

IS 383:2016 specifies coarse and fine aggregates from natural sources for concrete. Aggregates are classified by size (fine 0.075–4.75 mm, coarse 4.75–80 mm) and by grading (Zone I–IV for sand, nominal single-sized 10/20/40 mm for coarse). It also limits deleterious material like silt, mica, clay and organic impurities — critical because organic impurities can inhibit hydration by >50%.

Question 34

What is IS 516 and why is it the basis of cube testing in India?

Accepted Answer

IS 516 (Part 1/Sec 1):2018 is the Indian Standard on hardened concrete — testing methods (supersedes the 1959 edition). It defines cube/cylinder preparation, curing conditions, loading rate and how to compute compressive strength — and it is the test method referenced by IS 456 and IS 10262 for trial and acceptance testing. Without IS 516 cube strength, an IS 456 acceptance decision is not possible.

Question 35

What is IS 9103 and how does it relate to admixtures?

Accepted Answer

IS 9103:1999 specifies concrete admixtures — specifications for accelerators, retarders, water-reducers and air-entrainers. It includes performance requirements (water reduction, setting time, compressive strength, air content) and the testing regimen. Admixtures not conforming to IS 9103 — or without current test certificates — should not be used on a project.

Question 36

What is IS 1199 and what does it specify for fresh concrete?

Accepted Answer

IS 1199 (Part 1):2018 et seq. covers sampling and analysis of fresh concrete (supersedes the 1959 edition; the standard is now split into several parts — sampling, consistency, density, air content, etc.). It prescribes how to obtain a representative sample at the batching plant / truck / site, how to determine density, air content, slump and the temperature of fresh concrete. Every site QC should have access to the relevant part.

Question 37

How does ACI 211 differ from IS 10262 in mix design approach?

Accepted Answer

Both follow the same fundamental sequence — target strength, water content, w/c, coarse aggregate factor, fine aggregate correction. The differences: ACI 211.1 uses nominal maximum aggregate size and dry-rodded coarse aggregate bulk density; IS 10262 uses specific gravity and water content by exposure condition and zone of sand. Either approach is acceptable; consistency with the chosen code (IS 456 vs ACI 318) drives the choice.

Question 38

What is BS 8500 and how is it used in international projects?

Accepted Answer

BS 8500 is the British Standard for concrete — complementary British Standard to BS EN 206 . It addresses designed, prescribed, standardized prescribed and proprietary concrete, with strength, consistency, durability and embodied-carbon classes. For projects with international consultants (UK/UAE exposure), BS 8500 is the equivalent companion to IS 10262 + IS 456.

Question 39

How are IS specifications updated and how often?

Accepted Answer

BIS revises standards on a 5–8 year cycle. IS 456 itself is on its fourth revision (2000); IS 10262 is now 2019; IS 383 is 2016. Always quote the latest revision on the project. Confirm currency through the BIS website or directly via the standard — older revisions may have been withdrawn and should not be referenced as current.

Question 40

What is the rebound hammer test and when is it used?

Accepted Answer

The Schmidt rebound hammer (IS 516 Part 5/Sec 4) is a surface-impact test that estimates in-situ concrete strength by measuring surface hardness. The hammer strikes the surface at a fixed energy and the rebound distance correlates with compressive strength. It is useful for: (1) uniform-strength surveys across large slabs; (2) comparing suspect zones against sound zones; (3) curing-monitor spot checks. It is not a substitute for cube tests. Try the calculator: Rebound Hammer .

Question 41

What is the Ultrasonic Pulse Velocity (UPV) test?

Accepted Answer

UPV (IS 516 series, UPV section) sends an ultrasonic pulse through the concrete between two transducers; the travel time yields the pulse velocity, which correlates with elastic modulus and indicates internal quality. Velocity > 4.5 km/s generally indicates “excellent” concrete; 3.5–4.5 km/s “good”; 3.0–3.5 km/s “questionable”; UPV Velocity .

Question 42

How are cores extracted and tested for in-situ strength?

Accepted Answer

When cube tests are inconclusive, cores are extracted per IS 516 Part 4 / ASTM C42 — typically 100 mm or 150 mm diameter, perpendicular to the surface, taking care around rebar. Cores are capped (sulphur or aluminium) and tested at the same rate as cubes. The estimated in-situ strength must be corrected for aspect ratio and the direction of load, and is then compared to f ck ± margin per IS 456.

Question 43

What is the half-cell potential test for rebar corrosion?

Accepted Answer

The half-cell potential method (ASTM C876 / IS equivalent) measures the electrical potential of the embedded rebar relative to a reference electrode (Cu/CuSO₄). Readings more negative than −350 mV indicate active corrosion; −200 to −350 mV is borderline. It is a screening test — flag zones, then confirm with chloride profile or petrographic analysis. Useful for parking decks, coastal structures and bridge assessments.

Question 44

How is chloride content in concrete measured?

Accepted Answer

Acid-soluble chloride is the standard wet-chemistry method (IS 14959 (Part 2):2001 / ASTM C1152). Powder samples are drilled at depth intervals (typically 0–5, 5–10, 10–20, 20–30 mm) to build a chloride profile. The threshold for corrosion initiation is ~0.4% chloride by mass of cement — above that, evaluate the cover and choose protective measures (coatings, cathodic protection, repair).

Question 45

How is permeability tested and why does it matter?

Accepted Answer

Permeability is the rate at which water (or chloride ions) can move through the cover concrete. Methods include the IS 3085 water-permeability test, IS 516 (Part 2/Sec 3):2022 rapid chloride permeability (RCPT, equivalent to ASTM C1202) and ISAT. Lower permeability → slower chloride / CO₂ ingress → longer service life. Permeability is more important than cube strength for durability and is the basis for w/c ratio limits in IS 456.

Question 46

What is the carbonation depth test?

Accepted Answer

Carbonation depth is measured by spraying a freshly broken concrete surface with a 1% phenolphthalein indicator. Uncarbonated concrete turns pink; carbonated concrete stays colourless. The depth indicates how much of the cover is no longer protective to the rebar. Typical rates are 1–3 mm/year for ordinary concrete — values above 3 mm/year indicate high permeability and accelerated risk.

Question 47

What is the pull-off test and when is it used?

Accepted Answer

The pull-off (bond) test (ASTM C1583 / EN 1542) measures the tensile strength of the near-surface concrete by pulling a steel disc glued to the surface. It is widely used to evaluate (1) substrate strength before applying repairs/coatings, (2) hardened overlay bond, (3) quality of finished surfaces. Typical acceptable values are > 1.5 MPa for repair substrates; lower values indicate weak substrate needing removal.

Question 48

Cube strength is consistently low — what should I check first?

Accepted Answer

Triage in this order: (1) Cubes themselves — wrong mould, poor compaction, damaged before stripping, water temperature, water type (use potable), curing cycles. (2) Mix — actually batched cement was the brand/type declared? Aggregate moisture? w/c by volume in transit mixer? Admixture dosing? (3) Structure — was the same concrete placed at site stronger? If so, it’s a cube-specimen problem, not a structural one. (4) Hold the next pour pending root cause; do not re-test and average past failures. Try the calculator: Cube Test Statistics .

Question 49

Concrete is segregating during placement — what can I do?

Accepted Answer

Segregation shows as coarse aggregate separating from mortar, often in the chute or after extended transit. Immediate actions: (1) reduce free-fall — last 30 cm to the pour face is fine, 2 m is too much; (2) re-mix in the truck at slow mixing speed for 1–2 minutes before discharge; (3) move the pump or chute to drop close to final position; (4) check aggregate grading — gap-graded aggregates segregate easily; (5) confirm slump is at target (segregation risk rises with high-slump, high-pump mixes).

Question 50

Excessive bleeding observed on the surface — what is the cause?

Accepted Answer

Excess water rises to the surface, taking fines with it. Causes: high w/c, over-sanded mix, too-fine cement, slow-setting cement, cold subgrade, or excessive vibration that brings water up. Fix by lowering water content, increasing fines or SCM, switching to a finer cement, accelerating set slightly, and re-vibrating the top layer before initial set. Bleed water must evaporate or be re-absorbed before finishing, or it weakens the top surface.

Question 51

Honeycombing visible after de-shuttering — what went wrong?

Accepted Answer

Honeycomb is localised voids with coarse aggregate exposed. Causes: poor vibration (most common), high free-fall causing segregation, congested reinforcement without needle access, tight formwork with trapped air, high concrete temperature or stiff mix. Repair by removing loose aggregate back to sound concrete, cleaning, applying bond coat, packing with a stiffened mortar / micro-concrete, and curing for at least 7 days. Document the location and root cause as an NCR. Try the calculator: Repair Mortar Quantity .

Question 52

Cracks appearing on hardened concrete — how are they classified?

Accepted Answer

Three useful categories: (1) Plastic shrinkage — shallow, irregular, on large exposed slabs within hours of pour; cure early, use windbreaks and fog-spray. (2) Drying shrinkage — corner-to-corner or random fine cracks appearing weeks later; control joint spacing, proper curing, low-shrinkage mix. (3) Structural — through-depth, parallel to reinforcement, near supports; investigate load, restraint and design. Map cracks by location, length, width, depth and time of first observation. IS 456 Cl. 35.3.2 limits flexural crack width to 0.3 mm for moderate exposure (0.2 mm for severe). Try the calculator: Crack Injection Quantity .

Question 53

Slump loss is too rapid at site — what could be the cause?

Accepted Answer

Slump loss > 60 mm between plant and pour within the same vehicle is the warning sign. Likely causes: high ambient temperature / long haul, cement with high C₃A / alkali / early reactivity, delayed admixture addition (so it kicks in too late), warm trucks, or absorption by dry porous aggregates. Counter: schedule delivery for cooler hours, add retarder at plant, add crushed-ice in mix water, and confirm aggregates are surface-saturated. Re-test slump at point of placement , not just at gate.

Question 54

Setting time is much faster or slower than expected — what to check?

Accepted Answer

Fast set: high temperature, high C₃A cement, overdosed accelerator, mismatched cement-admixture chemistry, contaminated mixer with old concrete residue. Slow set: low temperature, retarder overdose, retarding admixture added before cement, very low-alkali cement, SCM-heavy mix, admixture stored too long. Fix the dosage / sequence before the next pour and capture details (cement batch number, admixture lot, ambient temp) in the batch log.

Question 55

Surface dusting on hardened concrete — what is it and how to fix?

Accepted Answer

A powdery, soft layer on the finished surface — usually the result of bleed water trapped under a tightly-trowelled surface, over-finishing, or excessive water during final trowelling. Test by rubbing with a coin — dusting rubs off; sound concrete doesn’t. Lightly affected areas can be hardened with a silicate sealer and re-used; badly affected surfaces should be ground back to sound concrete and re-finished. Prevention: finish after bleed water has evaporated, and avoid over-trowelling.

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