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Engineer Concrete practice questions

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Question 1Concrete Materials and Mix Proportioning

For a given cement and aggregate set, which parameter of the Abrams water-cement ratio law is stated correctly?

  1. At constant workability, increasing the water-cement ratio lowers compressive strength Correct answer.
  2. Compressive strength depends only on the cement content per cubic metre, not on the water content
  3. At constant workability, increasing the water-cement ratio raises compressive strength
  4. Compressive strength is independent of the water-cement ratio once the mix is fully compacted

Explanation

Abrams' law states that, for a given set of materials and adequate compaction, strength is governed inversely by the water-cement ratio. More water per unit cement increases capillary porosity and reduces strength. Cement content alone does not control strength; the ratio of water to cement does.

Question 2Concrete Materials and Mix Proportioning

A mix is specified with a water-cementitious materials ratio of 0.45 and a total cementitious content of 400 kg/m³. What is the approximate water content per cubic metre, ignoring aggregate moisture?

  1. 220 kg/m³
  2. 150 kg/m³
  3. 180 kg/m³ Correct answer.
  4. 200 kg/m³

Explanation

Water content equals the water-cementitious ratio multiplied by the cementitious content, so 0.45 × 400 kg/m³ = 180 kg/m³. The other values correspond to different ratios or cement contents and do not follow from the stated figures.

Question 3Concrete Materials and Mix Proportioning

Which supplementary cementitious material is most commonly associated with early-age strength reduction at high replacement levels and requires a lower water demand?

  1. Silica fume
  2. Limestone filler
  3. Ground granulated blast-furnace slag
  4. Fly ash Correct answer.

Explanation

Fly ash is a pozzolan that reacts slowly, so high replacement levels typically slow early strength gain while often reducing water demand through particle lubrication. Silica fume increases water demand strongly and slag is used at high levels but is not the typical example for reduced water demand with slow early strength.

Question 4Concrete Materials and Mix Proportioning

Which aggregate property has the greatest influence on the water demand of a concrete mix at a fixed slump?

  1. Los Angeles abrasion loss of the coarse aggregate
  2. Total void content and grading of the combined aggregate Correct answer.
  3. Sulfate content of the fine aggregate
  4. Bulk specific gravity of the coarse aggregate

Explanation

Water demand is driven by the total surface area and void system of the combined aggregate, so grading and void content are the dominant factors. Specific gravity affects yield and mass, abrasion loss relates to durability, and sulfate content relates to chemical soundness rather than water demand.

Question 5Concrete Materials and Mix Proportioning

Which statement correctly describes the purpose of the fineness modulus of fine aggregate?

  1. It defines the alkali-silica reactivity of the sand
  2. It measures the total clay content of the sand by weight
  3. It gives an index of the average particle size and uniformity of the sand to guide mix adjustment Correct answer.
  4. It specifies the maximum permissible water absorption of the sand

Explanation

The fineness modulus is a single number derived from cumulative retained percentages on a standard sieve series, giving an index of coarseness that helps adjust mix proportions. It does not measure clay, absorption, or chemical reactivity, which require separate tests.

Question 6Concrete Materials and Mix Proportioning

In a concrete mix design, what does the term 'workability' most precisely refer to?

  1. The resistance of hardened concrete to freeze-thaw cycles
  2. The compressive strength achieved at 28 days
  3. The ease with which fresh concrete can be mixed, placed, compacted, and finished without segregation Correct answer.
  4. The total water content of the mix in kilograms per cubic metre

Explanation

Workability describes the ease of handling fresh concrete through mixing, placing, compaction, and finishing without segregation or bleeding. Water content influences workability but is not the definition, and strength and freeze-thaw resistance are hardened properties.

Question 7Concrete Materials and Mix Proportioning

Which of the following is the standard method for determining the consistency of fresh concrete in the field?

  1. Schmidt rebound test
  2. Splitting tensile test
  3. Rapid chloride permeability test
  4. Slump test Correct answer.

Explanation

The slump test measures the consistency of fresh concrete by the settlement of a moulded cone under its own weight. The Schmidt rebound and splitting tensile tests are hardened-concrete tests, and the rapid chloride permeability test assesses transport properties.

Question 8Concrete Materials and Mix Proportioning

Which factor most directly governs the required cement content when a minimum strength and a fixed water-cement ratio are specified?

  1. The water content needed for the required workability Correct answer.
  2. The curing temperature in the first 24 hours
  3. The colour of the cement
  4. The maximum aggregate size only

Explanation

Once the water-cement ratio is fixed by the strength requirement, cement content is determined by dividing the required water content by that ratio. The water content needed for workability therefore sets the cement content. Aggregate size influences water demand indirectly but does not by itself set cement content.

Question 9Concrete Materials and Mix Proportioning

A concrete mix shows excessive bleeding and segregation on site. Which adjustment is most appropriate to address this?

  1. Reduce the water content and increase the fines or binder content Correct answer.
  2. Increase the water content to improve cohesion
  3. Replace all coarse aggregate with single-sized coarse aggregate
  4. Increase the maximum aggregate size without other changes

Explanation

Bleeding and segregation indicate insufficient cohesion in the paste, often from excess water or a deficiency of fines. Reducing water and adding fines or binder improves cohesion and stability. Adding water worsens bleeding, and single-sized aggregate reduces grading quality.

Question 10Concrete Materials and Mix Proportioning

Which statement about the use of silica fume in concrete is accurate?

  1. It retards setting and is used mainly to extend workability
  2. It acts only as an inert filler with no pozzolanic reaction
  3. It is typically used at 5 to 10 percent by mass of cementitious material and significantly increases water demand Correct answer.
  4. It is typically used at 30 to 50 percent by mass and reduces water demand

Explanation

Silica fume is a very fine pozzolan used at roughly 5 to 10 percent of the cementitious mass. Its high surface area increases water demand, so superplasticizers are commonly needed. It is highly reactive, not inert, and is not used primarily as a retarder.

Question 11Concrete Materials and Mix Proportioning

Which of the following best explains why air entrainment improves freeze-thaw durability?

  1. Entrained air bubbles provide space for water to expand as it freezes, relieving pressure in the paste Correct answer.
  2. Entrained air chemically binds free water in the pore system
  3. Entrained air increases the compressive strength of the paste
  4. Entrained air reduces the permeability of the aggregate particles

Explanation

Entrained air creates closely spaced voids that accommodate the expansion of freezing water, relieving hydraulic pressure in the paste. Air entrainment slightly reduces strength rather than increasing it, does not chemically bind water, and does not act on aggregate permeability.

Question 12Concrete Materials and Mix Proportioning

A trial mix with a water-cement ratio of 0.50 achieves 35 MPa at 28 days. If all other factors remain constant, which outcome is most consistent with Abrams' law when the ratio is reduced to 0.40?

  1. Strength increases because the capillary porosity of the paste is reduced Correct answer.
  2. Strength remains unchanged because cement content is unchanged
  3. Strength increases only if the cement content is also reduced
  4. Strength decreases because less water is available for hydration

Explanation

Lowering the water-cement ratio reduces the volume of water-filled capillary space, producing a denser paste and higher strength. Water is still sufficient for hydration at 0.40, so strength rises rather than falls, and the change does not require a reduction in cement content.

Question 13Concrete Materials and Mix Proportioning

Which aggregate characteristic is most important to control to avoid excessive drying shrinkage in a concrete pavement?

  1. The sulfate soundness loss only
  2. The bulk density of the coarse aggregate only
  3. The water absorption and elastic modulus of the aggregate Correct answer.
  4. The colour of the aggregate

Explanation

Drying shrinkage of concrete is restrained by stiff, low-absorption aggregate, so absorption and elastic modulus of the aggregate strongly influence shrinkage. Colour has no mechanical effect, and soundness or bulk density alone do not describe shrinkage behaviour.

Question 14Concrete Materials and Mix Proportioning

A mix design specifies a slump of 100 mm, but the trial mix produces 40 mm at the same water content. Which action is most appropriate before adjusting the cement content?

  1. Increase the water content and keep the water-cement ratio unchanged by adding cement Correct answer.
  2. Increase the maximum aggregate size without changing the water content
  3. Add more coarse aggregate to the mix
  4. Reduce the sand content only

Explanation

When slump is below target, water is increased to reach the required workability, and cement is added in the same proportion to hold the water-cement ratio and hence strength. Adding coarse aggregate or reducing sand alone changes grading and does not reliably correct the slump.

Question 15Concrete Materials and Mix Proportioning

Which statement about the role of the water-reducing admixture in mix proportioning is correct?

  1. It increases the water demand of the mix to improve workability
  2. It replaces the need for cement in the mix
  3. It acts only on hardened concrete and has no effect on fresh concrete
  4. It allows a lower water content at the same slump, which can reduce cement content while holding strength Correct answer.

Explanation

Water reducers disperse cement particles, allowing the same slump with less water. The reduced water content permits lower cement content at a fixed water-cement ratio, preserving strength while reducing binder. They do not replace cement and act on fresh concrete.

Question 16Concrete Materials and Mix Proportioning

Why is the water-cementitious materials ratio used instead of the water-cement ratio when fly ash is included in the mix?

  1. Because fly ash reduces the density of the concrete
  2. Because fly ash is inert and should be excluded from the calculation
  3. Because fly ash increases the water demand in proportion to its mass
  4. Because fly ash contributes to strength development through pozzolanic reaction and must be counted in the binder Correct answer.

Explanation

Fly ash reacts pozzolanically to form additional calcium silicate hydrate, so it is counted as part of the cementitious binder. The ratio of water to total cementitious material therefore better reflects the paste porosity and strength relationship than water to portland cement alone.

Question 17Concrete Materials and Mix Proportioning

A concrete mixture has a paste volume of 0.28 m³ per cubic metre and an aggregate void content of 22 percent. Which statement best describes the role of the paste in this mix?

  1. The paste must exceed the aggregate void volume to provide workability and bond Correct answer.
  2. The paste volume should be less than the aggregate void volume to reduce shrinkage
  3. The paste volume has no relationship to aggregate void content
  4. The paste volume should equal the aggregate void volume exactly

Explanation

Paste must fill the voids between aggregate particles and provide a lubricating excess for workability, so paste volume generally exceeds the aggregate void volume. Too little paste causes harsh, unworkable mixes, while the exact equality is neither required nor sufficient.

Question 18Concrete Materials and Mix Proportioning

Which test is used to detect the potential for deleterious alkali-silica reaction in an aggregate?

  1. Slump test
  2. Accelerated mortar bar expansion test Correct answer.
  3. Schmidt rebound hammer test
  4. Vebe consistometer test

Explanation

The accelerated mortar bar expansion test measures expansion of mortar bars containing the aggregate under alkaline conditions, indicating alkali-silica reactivity. Slump and Vebe tests measure fresh consistency, and the rebound hammer measures surface hardness.

Question 19Concrete Materials and Mix Proportioning

A specification requires a maximum water-cementitious ratio of 0.40 for a bridge deck exposed to chlorides. Which mix adjustment best satisfies this while maintaining workability?

  1. Increase the water content and add more cement
  2. Use single-sized coarse aggregate with no admixture
  3. Reduce the cement content and increase the water content
  4. Use a high-range water reducer and a supplementary cementitious material at a practical dosage Correct answer.

Explanation

A low water-cementitious ratio reduces permeability to chlorides, and a high-range water reducer restores workability at low water content. Supplementary cementitious materials refine the pore structure and improve chloride resistance. Raising water or reducing binder would violate the specified ratio.

Question 20Concrete Materials and Mix Proportioning

Which statement correctly describes the effect of aggregate moisture correction on batching?

  1. Only oven-dry aggregate requires correction
  2. Free water in the aggregate must be subtracted from the added water to maintain the design water-cement ratio Correct answer.
  3. Aggregate moisture has no effect on the batch water
  4. Total moisture in the aggregate is added to the batch water

Explanation

Aggregate as batched contains moisture that contributes to the mixing water, so the free water above the saturated surface-dry condition must be deducted from the added water. Ignoring this changes the effective water-cement ratio and the resulting strength.

Question 21Concrete Materials and Mix Proportioning

Which statement about the effect of cement fineness on concrete properties is accurate?

  1. Finer cement hydrates faster, increasing early strength but also increasing heat release and water demand Correct answer.
  2. Finer cement hydrates more slowly, reducing early strength
  3. Cement fineness has no effect on heat of hydration
  4. Finer cement reduces water demand because of its lower surface area

Explanation

Grinding cement finer increases the surface area available for reaction, accelerating hydration and raising early strength. The trade-off is higher early heat release and greater water demand. Finer cement does not hydrate more slowly or reduce water demand.

Question 22Concrete Materials and Mix Proportioning

A concrete mix has a total water content of 175 kg/m³, a water-cementitious ratio of 0.45, and 25 percent of the binder is fly ash. What is the approximate portland cement content per cubic metre?

  1. 292 kg/m³ Correct answer.
  2. 389 kg/m³
  3. 430 kg/m³
  4. 510 kg/m³

Explanation

Total binder is 175 ÷ 0.45 ≈ 389 kg/m³. With 25 percent fly ash, portland cement is 75 percent of the binder, or about 292 kg/m³. The value 389 kg/m³ is the total binder, not the portland cement fraction.

Question 23Concrete Materials and Mix Proportioning

In a mix with a very low water-cementitious ratio of 0.25, which combination of measures is most necessary to achieve adequate compaction?

  1. Additional water without any admixture
  2. Reduced cement content with the same water content
  3. Larger coarse aggregate only, with no change in admixture or compaction
  4. A high-range water reducer combined with careful vibration and extended mixing Correct answer.

Explanation

At very low water-cementitious ratios the paste is stiff and difficult to compact, so high-range water reducers are used to achieve workability without adding water, together with thorough vibration. Adding water defeats the purpose, and aggregate or cement adjustments alone do not solve the compaction problem.

Question 24Concrete Materials and Mix Proportioning

A concrete producer observes that a mix with 30 percent fly ash replacement shows adequate 28-day strength but fails the specified 7-day strength. Which explanation is most consistent with the chemistry of fly ash?

  1. Fly ash increases the water-cementitious ratio automatically
  2. Fly ash chemically retards the hydration of portland cement permanently
  3. Fly ash reduces the total cementitious content of the mix
  4. The pozzolanic reaction is slower than portland cement hydration, so early strength gain is delayed while later strength catches up Correct answer.

Explanation

Fly ash reacts with calcium hydroxide released by cement hydration, and this pozzolanic reaction proceeds more slowly than cement hydration. Early strength is therefore lower while later strength can match or exceed the reference mix. The effect is a delay rather than a permanent retardation, and fly ash does not by itself change the binder content or water ratio.

Question 25Concrete Materials and Mix Proportioning

Which situation best justifies specifying a gap-graded aggregate blend rather than a continuously graded blend?

  1. When the concrete must have the highest possible workability with no admixture
  2. When a dense, low-void coarse fraction is desired and the fine fraction is controlled separately to reduce paste demand Correct answer.
  3. When the cement has unusually high fineness
  4. When the aggregate source has high sulfate soundness loss

Explanation

Gap grading omits intermediate sizes to create a coarse skeleton with low void content, with fines added separately to fill remaining voids and provide workability. It is a deliberate proportioning choice, not a response to sulfate soundness, cement fineness, or a demand for maximum workability without admixtures.

Question 26Concrete Materials and Mix Proportioning

Which property of cement is primarily responsible for the early strength gain of concrete within the first 24 hours after casting?

  1. The free lime content, which carbonates on exposure to air
  2. The tricalcium silicate (C3S) content, which hydrates rapidly and contributes most early strength Correct answer.
  3. The tricalcium aluminate (C3A) content, which reacts almost immediately on contact with water
  4. The dicalcium silicate (C2S) content, which hydrates slowly over months

Explanation

Tricalcium silicate (alite) is the most reactive major clinker phase and its hydration products are largely responsible for strength development in the first days. Dicalcium silicate hydrates much more slowly and governs long-term strength gain rather than early strength. C3A reacts first but contributes little to strength and is mainly associated with setting and heat release.

Question 27Concrete Materials and Mix Proportioning

A batch ticket lists total aggregate mass of 1200 kg with a measured total moisture content of 5.0 percent and an absorption of 2.0 percent. What is the mass of free (surface) water contributed by the aggregate to the batch?

  1. 36 kg Correct answer.
  2. 12 kg
  3. 60 kg
  4. 24 kg

Explanation

Only moisture above the absorption value is free water that adds to the effective water content of the mix. The moisture mass is 1200 × 0.05 = 60 kg and the absorbed water is 1200 × 0.02 = 24 kg, so free water is 60 − 24 = 36 kg, which must be deducted from the added mixing water.

Question 28Concrete Materials and Mix Proportioning

A specification limits the chloride ion content of a reinforced concrete mix. Which constituent is normally the dominant uncontrolled source of chloride in a properly proportioned mix?

  1. The coarse and fine aggregates, which can carry chloride from a marine or contaminated source Correct answer.
  2. The cement, whose chloride level is controlled by the cement standard
  3. The chemical admixture, which is dosed at a very low volume
  4. The mixing water, which is usually potable and low in chloride

Explanation

In normal concrete the aggregates form the largest fraction of the mix and can carry substantial chloride if they come from marine dredging or contaminated stockpiles, so they are the main uncontrolled contributor. Cement chloride is limited by product standards, and potable mixing water and low-dose admixtures contribute little. Chloride limits exist to protect embedded steel from depassivation and corrosion.

Question 29Concrete Materials and Mix Proportioning

A mix is specified with a target mean strength based on a standard deviation of 5 MPa and a characteristic strength of 30 MPa. Using the common k factor of 1.65, what target mean strength should be adopted?

  1. 33.3 MPa
  2. 38.3 MPa Correct answer.
  3. 30.0 MPa
  4. 41.5 MPa

Explanation

Target mean strength is the characteristic strength plus k times the standard deviation, giving 30 + 1.65 × 5 = 38.25 MPa, rounded to about 38.3 MPa. This margin accounts for the statistical spread of test results so that the specified proportion of results falls above the characteristic value.

Question 30Concrete Materials and Mix Proportioning

A concrete element is heavily reinforced and the design calls for a high cementitious content with a low water-cementitious ratio. Which combination of aggregate and binder choices best reduces the risk of inadequate consolidation in this situation?

  1. A larger maximum aggregate size combined with a higher paste volume and a superplasticizer Correct answer.
  2. A smaller maximum aggregate size combined with a reduced sand content and no admixture
  3. A gap-graded coarse aggregate combined with a lower paste volume
  4. A rounded gravel combined with a higher water content to increase slump

Explanation

Congested reinforcement limits the size of the coarse aggregate that can pass between bars, so a moderate maximum size with a higher paste volume improves flow and filling ability. A superplasticizer provides the required flow at a low water-cementitious ratio without adding water, which preserves strength and durability. Together these measures improve consolidation while maintaining the specified binder and water levels.

Question 31Fresh Concrete: Workability, Placement, and Consolidation

For a normal structural concrete placed by pump, which slump range is typically specified for a plastic, workable mix?

  1. 200 to 250 mm
  2. 75 to 125 mm Correct answer.
  3. Over 250 mm
  4. 0 to 25 mm

Explanation

Ordinary pumped structural concrete is normally specified in the 75 to 125 mm slump band, which gives adequate cohesion for transport yet flows readily around reinforcement. Slumps below about 25 mm are stiff and hard to place, while values above roughly 200 mm indicate a highly fluid mix that usually requires a superplasticizer and tighter control.

Question 32Fresh Concrete: Workability, Placement, and Consolidation

A concrete mix shows visible bleeding water rising at the surface shortly after placement. Which adjustment most directly reduces bleeding?

  1. Using a coarser, single-sized coarse aggregate
  2. Increasing the water content of the mix
  3. Increasing the cementitious fines and reducing free water Correct answer.
  4. Reducing the fine aggregate content

Explanation

Bleeding occurs when free water migrates upward faster than the solids settle, which is aggravated by low fines content and high water demand. Raising the proportion of cementitious fines and cutting free water improves water retention and cohesion, so the bleed channels are suppressed.

Question 33Fresh Concrete: Workability, Placement, and Consolidation

Vibration of a concrete layer should stop when which condition is observed?

  1. Water begins to appear on the surface
  2. The vibrator has been held in place for a fixed 60 seconds
  3. The surface turns glossy and air bubbles cease Correct answer.
  4. Coarse aggregate starts to settle at the bottom

Explanation

The accepted criterion for ending vibration is when the concrete has visibly compacted, air bubbles stop rising and a thin glistening film of mortar appears at the surface. Continuing beyond this point causes segregation and over-vibration, while stopping early leaves entrapped voids.

Question 34Fresh Concrete: Workability, Placement, and Consolidation

Which placement practice best prevents segregation when concrete is dropped into a deep wall form?

  1. Using a flexible drop pipe or tremie to keep free fall short Correct answer.
  2. Dropping the concrete from the top of the form
  3. Placing in one deep lift with prolonged vibration
  4. Discharging against the reinforcement cage

Explanation

Limiting free fall with a drop pipe, elephant trunk or tremie keeps the mix coherent and stops coarse aggregate separating from the mortar. Free fall is generally restricted to about 1 to 1.5 m, so a deep wall needs a controlled delivery method.

Question 35Fresh Concrete: Workability, Placement, and Consolidation

A concrete mix is described as cohesive. What does this property primarily indicate?

  1. The mix resists segregation and stays homogeneous during handling Correct answer.
  2. The mix has a high modulus of elasticity
  3. The mix reaches its design strength faster
  4. The mix has a low heat of hydration

Explanation

Cohesiveness is the ability of the fresh mix to remain uniform and resist segregation and bleeding during transport, discharge and compaction. It is a workability property of the fresh state and is independent of hardened strength or thermal characteristics.

Question 36Fresh Concrete: Workability, Placement, and Consolidation

Which test is most appropriate for assessing the workability of very stiff, low-slump concrete such as a pavement mix?

  1. Slump test
  2. Vebe consistometer test Correct answer.
  3. U-tube test
  4. Flow table test

Explanation

The Vebe test measures the time for a slumped cone to be remoulded under vibration, which makes it sensitive in the very dry to stiff range where the slump test loses resolution. Slump and flow tests are suited to more plastic or flowing mixes.

Question 37Fresh Concrete: Workability, Placement, and Consolidation

What is the main purpose of compacting fresh concrete?

  1. To remove entrapped air and bring particles into close contact Correct answer.
  2. To increase the cement content per cubic metre
  3. To accelerate the chemical reaction of cement
  4. To reduce the water demand of the mix

Explanation

Compaction expels entrapped air voids and packs the solid particles tightly together so the paste can bind them, raising density and strength. It does not change the mix proportions or the water demand, and only indirectly influences hydration by improving contact.

Question 38Fresh Concrete: Workability, Placement, and Consolidation

A slump test on a sample gives 90 mm, but a second sample taken from the same truck 20 minutes later gives 55 mm. Which explanation is most consistent with this result?

  1. The aggregate has absorbed part of the cement paste
  2. The concrete is losing workability through slump loss over time Correct answer.
  3. The slump cone was filled in the wrong number of layers
  4. The cement has fully hydrated and set the concrete

Explanation

Slump loss is the normal reduction in workability as time passes after mixing, driven by early hydration and evaporation. A drop from 90 mm to 55 mm over 20 minutes is typical and does not indicate set concrete or a faulty test.

Question 39Fresh Concrete: Workability, Placement, and Consolidation

In a deep beam, a poker vibrator is inserted and left in one position until the surface glistens. What is the most likely consequence of continuing to vibrate?

  1. An increase in the entrapped air content
  2. Improved bond between concrete and reinforcement
  3. Segregation of coarse aggregate from the mortar Correct answer.
  4. A reduction in the water-cement ratio

Explanation

Over-vibration fluidises the mix so that heavy aggregate settles while paste and water rise, producing segregation and a weak, non-uniform section. Vibration should stop as soon as air bubbles cease and the surface glistens.

Question 40Fresh Concrete: Workability, Placement, and Consolidation

Concrete is being placed in a tall column. Which insertion pattern for the poker vibrator gives the most reliable full-depth compaction?

  1. Inserting horizontally through the form face
  2. Inserting at an angle and dragging the vibrator along the surface
  3. Inserting vertically at close centres, penetrating the layer below Correct answer.
  4. Inserting vertically at wide intervals and vibrating only the top layer

Explanation

Vibrators should be inserted vertically at regular close spacing and allowed to penetrate the upper part of the layer below so the two lifts knit together. Wide spacing leaves unvibrated pockets, and horizontal or surface dragging does not reach the full depth.

Question 41Fresh Concrete: Workability, Placement, and Consolidation

Which factor most strongly increases the risk of plastic shrinkage cracking in a freshly placed slab?

  1. Low ambient temperature with high humidity
  2. Using a low-slump mix with a high fines content
  3. High ambient temperature with wind and low humidity Correct answer.
  4. Placing the slab at night with calm air

Explanation

Plastic shrinkage cracking occurs when the rate of evaporation from the surface exceeds the bleed water reaching it, which is driven by high temperature, wind and low humidity. Curing or evaporation control must be applied promptly in those conditions.

Question 42Fresh Concrete: Workability, Placement, and Consolidation

A slump test result is 40 mm lower than expected on a mix that has been in the truck for 45 minutes. Which action is most appropriate on site?

  1. Reject or retest and seek approval before adding a plasticizer Correct answer.
  2. Vibrate the concrete more heavily to compensate
  3. Add water and cement in equal amounts
  4. Add water directly to the drum to restore slump

Explanation

Adding water on site raises the water-cement ratio and reduces strength and durability, so it must not be done to restore slump without authorisation. The correct response is to retest, report the low result and obtain approval for an approved admixture or rejection of the load.

Question 43Fresh Concrete: Workability, Placement, and Consolidation

Which test measures the filling ability of self-consolidating concrete by timing its flow through a V-shaped funnel?

  1. Slump flow test
  2. L-box test
  3. V-funnel test Correct answer.
  4. Vebe test

Explanation

The V-funnel test times how long self-consolidating concrete takes to flow through a V-shaped funnel, giving an indication of viscosity and filling ability. The slump flow test measures spread, the L-box assesses passing ability, and the Vebe test is for stiff mixes.

Question 44Fresh Concrete: Workability, Placement, and Consolidation

A contractor proposes placing a 2 m deep foundation in a single lift with internal vibration. What is the main technical concern?

  1. The mix will set too quickly at the base
  2. The formwork pressure will be lower than for layered placing
  3. The lower concrete may not be reached by the vibrator, leaving voids Correct answer.
  4. The aggregate will float to the surface

Explanation

In a deep single lift the vibrator cannot reliably reach the bottom, so lower zones may remain uncompacted and contain voids or honeycombing. Layered placing with systematic vibration of each lift ensures the full depth is consolidated.

Question 45Fresh Concrete: Workability, Placement, and Consolidation

Which measure best reduces the risk of honeycombing on a heavily reinforced section?

  1. Increasing the maximum aggregate size to the largest available
  2. Reducing the cement content to lower heat
  3. Placing the concrete in one rapid pour without vibration
  4. Using a well-graded mix with adequate fines and careful vibration Correct answer.

Explanation

Honeycombing arises when mortar cannot fill the spaces between reinforcement because the mix is harsh or poorly compacted. A well-graded, cohesive mix with sufficient fines and systematic vibration lets the mortar flow around the steel and fill the voids.

Question 46Fresh Concrete: Workability, Placement, and Consolidation

A concrete mix is to be pumped through a 100 m horizontal line. Which property is most important for the mix to avoid blockage?

  1. High coarse aggregate content with gap grading
  2. Good cohesion with adequate fines and controlled slump Correct answer.
  3. Maximum aggregate size close to the pipe diameter
  4. Very low slump with a stiff consistency

Explanation

Pumpable concrete needs a cohesive, well-graded mix with enough fines and paste to lubricate the pipeline and keep the aggregate in suspension. Harsh, gap-graded or oversized aggregate mixes tend to form plugs and block the line.

Question 47Fresh Concrete: Workability, Placement, and Consolidation

Why is a maximum free-fall height commonly specified for placing concrete?

  1. To control the rate of hydration
  2. To limit the impact energy that causes segregation Correct answer.
  3. To ensure the formwork is fully loaded
  4. To prevent the concrete from setting in the chute

Explanation

A free-fall limit of roughly 1 to 1.5 m keeps the falling stream coherent so coarse aggregate does not separate from the mortar on impact. Beyond that height, segregation and splashing against formwork and reinforcement become significant.

Question 48Fresh Concrete: Workability, Placement, and Consolidation

A site engineer observes that a poker vibrator is being used to spread concrete horizontally across a slab. Why is this practice unacceptable?

  1. It prevents the formation of a curing membrane
  2. It can cause segregation and does not compact the concrete properly Correct answer.
  3. It increases the concrete temperature excessively
  4. It reduces the entrained air content only

Explanation

A vibrator is a compaction tool, not a spreading tool; dragging it through the concrete fluidises and separates the mix without properly compacting the layer. Concrete should be placed close to its final position and then vibrated vertically at close spacing.

Question 49Fresh Concrete: Workability, Placement, and Consolidation

In a mix design, the water-cement ratio is held constant while the paste volume is increased by adding cement and water together. What is the likely effect on workability and strength?

  1. Workability increases and strength remains broadly unchanged Correct answer.
  2. Workability decreases and strength increases
  3. Workability and strength both decrease
  4. Workability is unchanged and strength increases

Explanation

Adding cement and water in the same ratio keeps the water-cement ratio constant, so strength is broadly maintained, while the larger paste volume lubricates the aggregate and improves workability. This is a common way to increase workability without sacrificing strength.

Question 50Fresh Concrete: Workability, Placement, and Consolidation

A trial mix using crushed angular aggregate requires more water than an otherwise identical mix with rounded gravel. What is the main reason?

  1. Rounded gravel absorbs more mixing water
  2. Angular particles have greater interparticle friction and surface area Correct answer.
  3. Angular aggregate has a higher specific gravity
  4. Angular aggregate reacts chemically with cement

Explanation

Crushed angular particles interlock and have a rougher, larger surface area, so more paste and water are needed to lubricate them and achieve the same workability. Rounded gravel flows more easily because of lower friction between particles.