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Cert III Concreting study materials
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Cert III Concreting practice questions

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Question 1Core units of competency (common construction industry skills and concreting specialist field)

What is the minimum compressive strength typically required before formwork supporting vertical faces of walls can be removed, so that the concrete surface is not damaged?

  1. 20.0 MPa
  2. 10.0 MPa
  3. 5.0 MPa
  4. 1.2 MPa Correct answer.

Explanation

Vertical formwork can usually be stripped once the concrete has hardened enough to resist surface damage, commonly taken as about 1.2 MPa compressive strength. Soffit and other load-bearing formwork must stay in place until much higher strengths are reached. This low threshold reflects that side forms carry no structural load from the member itself.

Question 2Core units of competency (common construction industry skills and concreting specialist field)

A concrete mix has a water-to-cement ratio of 0.5 by mass and contains 320 kg of cement per cubic metre. How much water is in the mix per cubic metre, ignoring water in aggregates?

  1. 160 kg Correct answer.
  2. 320 kg
  3. 80 kg
  4. 640 kg

Explanation

Water-to-cement ratio is water mass divided by cement mass, so water equals ratio multiplied by cement content. Here 0.5 multiplied by 320 kg gives 160 kg of water per cubic metre. Checking the arithmetic confirms the relationship is a simple product once the ratio and cement content are known.

Question 3Core units of competency (common construction industry skills and concreting specialist field)

Which type of portland cement is most suitable for general concrete work where no special properties such as early strength or low heat are needed?

  1. Type GP (general purpose) Correct answer.
  2. Type SR (sulfate resisting)
  3. Type LH (low heat)
  4. Type HE (high early strength)

Explanation

General purpose cement is the standard binder for ordinary structural concrete where no special performance is demanded. Special cements are chosen when a specific property such as rapid strength gain, low heat of hydration, or sulfate resistance is required. Using a special cement without need adds cost without benefit.

Question 4Core units of competency (common construction industry skills and concreting specialist field)

During placement, concrete is dropped from a height that causes segregation of coarse aggregate from mortar. Which action best prevents this problem?

  1. Use a tremie or drop pipe to keep free fall short Correct answer.
  2. Increase the maximum aggregate size on site
  3. Add more water to the mix at the point of placement
  4. Vibrate the heap after it has landed from height

Explanation

Segregation occurs when concrete falls freely and heavy aggregate separates from the mortar. Using a tremie, drop pipe, or chute keeps free fall within about one to one and a half metres so the mix lands as a coherent mass. Placement method, not later vibration, controls whether separation happens in the air.

Question 5Core units of competency (common construction industry skills and concreting specialist field)

Why is a poker vibrator inserted vertically and withdrawn slowly during compaction of a concrete pour?

  1. To reduce the amount of cement needed in the mix
  2. To speed up the setting reaction through friction heat
  3. To keep the aggregate evenly distributed by stirring the mix
  4. To allow entrapped air to escape and the concrete to close around the void left by the poker Correct answer.

Explanation

A poker displaces concrete and creates a void that closes as the vibrator is slowly withdrawn, while air pockets rise and escape. Rapid withdrawal leaves a hole that may not seal. Vertical insertion also ensures the vibrating head reaches the full depth of each layer.

Question 6Core units of competency (common construction industry skills and concreting specialist field)

A slab is 6 m long and 4 m wide and is poured at 150 mm thick. What volume of concrete is required, in cubic metres?

  1. 36 m³
  2. 3.6 m³ Correct answer.
  3. 2.4 m³
  4. 0.36 m³

Explanation

Volume equals length multiplied by width multiplied by thickness, with all dimensions in metres. Here 6 times 4 times 0.15 gives 3.6 cubic metres. Converting the 150 mm thickness to 0.15 m before multiplying is the key step.

Question 7Core units of competency (common construction industry skills and concreting specialist field)

Which personal protective equipment is essential when working with wet concrete and cement-based products?

  1. Ear muffs and a dust mask only
  2. A high-visibility vest only
  3. Waterproof gloves and boots with eye protection Correct answer.
  4. Cotton gloves only

Explanation

Wet concrete and cement are alkaline and can cause serious skin burns and dermatitis with prolonged contact. Waterproof gloves and boots plus eye protection prevent skin and eye contact. Other items such as high-visibility clothing or hearing protection address different hazards.

Question 8Core units of competency (common construction industry skills and concreting specialist field)

What is the purpose of a slump test on fresh concrete?

  1. To measure the compressive strength of the hardened mix
  2. To assess the workability or consistency of the fresh mix Correct answer.
  3. To check the sulfate resistance of the cement
  4. To determine the air content of the mix

Explanation

The slump test measures how far a cone of fresh concrete settles, giving an indication of consistency and workability. It is a simple site check that the mix is close to the designed water content. It does not measure strength, air content, or durability directly.

Question 9Core units of competency (common construction industry skills and concreting specialist field)

Why are control cylinders or cubes cured under standard conditions before testing?

  1. To ensure strength results are comparable and reflect the mix design Correct answer.
  2. To speed up the hardening so results are available sooner
  3. To remove excess water from the surface pores
  4. To make them easier to handle without breaking

Explanation

Standard curing controls temperature and moisture so that strength results are repeatable and comparable between batches and projects. If specimens dried out or overheated, measured strength would be unreliable. Comparability, not handling convenience, is the reason for controlled curing.

Question 10Core units of competency (common construction industry skills and concreting specialist field)

A concrete element must reach 20 MPa before post-tensioning. Which approach best confirms this on site?

  1. Test field-cured cylinders that have been stored alongside the element Correct answer.
  2. Rely on the number of days since pouring alone
  3. Test standard-cured cylinders stored in a laboratory
  4. Estimate strength from the slump of the delivery

Explanation

Field-cured specimens stored with the element experience the same temperature and moisture, so they better represent in-place strength. Standard-cured cylinders show potential strength under ideal conditions, not what the element has achieved. Time alone and slump are unreliable indicators of actual strength gain.

Question 11Core units of competency (common construction industry skills and concreting specialist field)

Which practice helps prevent plastic shrinkage cracking in a fresh slab on a hot, windy day?

  1. Adding water to the surface to keep it wet during finishing
  2. Delaying initial curing and allowing the surface to dry
  3. Starting curing and providing windbreaks as soon as finishing allows Correct answer.
  4. Increasing the time between placing and finishing

Explanation

Plastic shrinkage cracks form when the surface loses moisture faster than bleed water can replace it, especially in hot, windy conditions. Prompt curing and windbreaks reduce evaporation from the fresh surface. Surface water additions and delays in finishing tend to worsen the problem.

Question 12Core units of competency (common construction industry skills and concreting specialist field)

What is the main function of a damp-proof membrane placed under a concrete slab on ground?

  1. To increase the compressive strength of the slab
  2. To prevent moisture rising from the ground into the slab and building Correct answer.
  3. To provide thermal insulation to the slab
  4. To bond the slab to the subgrade

Explanation

A damp-proof membrane is a continuous moisture barrier that stops ground moisture rising through the slab into the building. It does not add strength or act as a bond layer. Insulation, where required, is a separate material with a different function.

Question 13Core units of competency (common construction industry skills and concreting specialist field)

Which type of joint is installed to control where cracks will occur in a large concrete slab?

  1. Contraction (control) joint Correct answer.
  2. Construction joint
  3. Expansion joint
  4. Isolation joint

Explanation

Contraction or control joints are deliberately weakened planes that force shrinkage cracking to occur at a chosen location. Construction joints mark where pours stop and start, while expansion and isolation joints accommodate movement between elements. Each joint type serves a distinct structural purpose.

Question 14Core units of competency (common construction industry skills and concreting specialist field)

A concrete mix has a target mean strength of 40 MPa and a standard deviation of 5 MPa. Using a margin of 1.65 standard deviations, what is the required mean strength?

  1. 41.65 MPa
  2. 48.25 MPa Correct answer.
  3. 51.65 MPa
  4. 45.0 MPa

Explanation

The required mean strength is the target plus the margin, where the margin is the standard deviation multiplied by the statistical factor. Here 1.65 times 5 MPa equals 8.25 MPa, and adding this to 40 MPa gives 48.25 MPa. This margin ensures most test results exceed the target.

Question 15Core units of competency (common construction industry skills and concreting specialist field)

Why is a release agent applied to formwork faces before placing concrete?

  1. To seal the formwork against water loss from the concrete
  2. To increase the bond between concrete and formwork
  3. To chemically accelerate the setting of the concrete surface
  4. To allow clean stripping of forms without damaging the concrete surface Correct answer.

Explanation

Release agents create a barrier so the concrete does not adhere to the form face, allowing forms to be stripped cleanly and the surface to remain undamaged. They do not bond, seal, or accelerate setting. Correct application keeps the finished surface sound and the forms reusable.

Question 16Core units of competency (common construction industry skills and concreting specialist field)

What is the primary reason for compacting each layer of concrete with a poker vibrator shortly after placement?

  1. To lower the temperature of the fresh concrete
  2. To remove entrapped air and achieve full density around reinforcement Correct answer.
  3. To increase the water content at the surface
  4. To mix the cement and aggregate more thoroughly

Explanation

Vibration removes entrapped air and helps concrete flow around reinforcement and into corners, producing a dense, void-free mass. It does not mix the constituents or change temperature. Density from proper compaction is essential for strength and durability.

Question 17Core units of competency (common construction industry skills and concreting specialist field)

Which factor most strongly increases the risk of sulfate attack on buried concrete?

  1. Rapid drying of the concrete surface after placing
  2. High ambient humidity during curing
  3. Sulfate-bearing soils or groundwater in contact with the concrete Correct answer.
  4. Use of a high slump at placement

Explanation

Sulfate attack occurs when sulfates from soil or groundwater react with cement hydration products, causing expansion and deterioration. The risk is controlled by using sulfate-resisting cement or a low-permeability mix. Humidity, slump, and drying rate do not drive sulfate attack.

Question 18Core units of competency (common construction industry skills and concreting specialist field)

What does a concrete slump of 20 mm typically indicate about the mix?

  1. A mix that has already begun to set
  2. A mix with excessive air content
  3. A stiff, low-workability mix Correct answer.
  4. A very wet, highly workable mix

Explanation

A small slump means the cone settles only a little, indicating a stiff mix with low workability. Such mixes suit mass concrete or pavement work where a drier consistency is acceptable. High slump, by contrast, indicates a wetter, more workable mix.

Question 19Core units of competency (common construction industry skills and concreting specialist field)

Which action is correct when a concrete delivery shows visible segregation in the truck drum?

  1. Add water to recombine the constituents
  2. Pour it immediately before it separates further
  3. Reject the load or have it remixed before discharge Correct answer.
  4. Add extra cement to thicken the mix

Explanation

Segregated concrete may not meet the specified mix and should be remixed in the drum or rejected. Adding water or cement on site alters the approved mix and is not permitted without authorization. Placing segregated concrete risks weak, non-uniform elements.

Question 20Core units of competency (common construction industry skills and concreting specialist field)

What is the main purpose of a screed rail or edge form when placing a concrete slab?

  1. To insulate the slab edges thermally
  2. To provide a guide for striking off the concrete to the correct level Correct answer.
  3. To reinforce the slab against bending
  4. To act as a permanent expansion joint

Explanation

Screed rails and edge forms give a fixed reference so the concrete can be struck off to the required level and fall. They are temporary aids for accurate placement, not structural reinforcement. Reinforcement, insulation, and joints are provided by separate elements.

Question 21Core units of competency (common construction industry skills and concreting specialist field)

A contractor needs to estimate the number of 20 kg bags of cement required for 1 m³ of concrete with a cement content of 300 kg/m³. How many bags are needed?

  1. 30 bags
  2. 15 bags Correct answer.
  3. 20 bags
  4. 10 bags

Explanation

Dividing the cement content by the bag mass gives the number of bags: 300 kg divided by 20 kg equals 15 bags. This assumes the bags contain only cement and no other binder. Checking by multiplying 15 by 20 confirms the 300 kg total.

Question 22Core units of competency (common construction industry skills and concreting specialist field)

Why should concrete not be placed into standing water?

  1. The water cools the mix too quickly for proper setting
  2. The water causes the reinforcement to bond too strongly
  3. The water dilutes the cement paste and increases the water-to-cement ratio at the base Correct answer.
  4. The water prevents the aggregate from settling

Explanation

Placing concrete into standing water allows water to mix into the paste, raising the local water-to-cement ratio and weakening the lower zone. It can also cause segregation and washout of fines. Water should be removed or a tremie method used under water.

Question 23Core units of competency (common construction industry skills and concreting specialist field)

Which curing method is most appropriate for a large horizontal slab exposed to sun and wind?

  1. Applying a curing compound or covering with wet hessian and plastic Correct answer.
  2. Covering with a single layer of dry sand
  3. Leaving the surface exposed to air after finishing
  4. Spraying the surface with water once only

Explanation

Effective curing keeps the concrete moist and at a suitable temperature for the required period. A curing compound or wet hessian covered with plastic retains moisture and shields the surface from sun and wind. One-off wetting or dry coverings do not maintain moisture.

Question 24Core units of competency (common construction industry skills and concreting specialist field)

A concrete mix design is being reviewed because the specified characteristic strength is not being achieved consistently, although average results meet the target. What is the most likely issue?

  1. The mean strength is too high
  2. The variability of results is too great Correct answer.
  3. The slump is too low
  4. The cement content is too low

Explanation

Characteristic strength depends on both the mean and the spread of results; high variability means some results fall below the target even when the average is acceptable. Reducing variability through better batching and control is the appropriate response. Raising the mean or changing slump alone does not address the spread.

Question 25Core units of competency (common construction industry skills and concreting specialist field)

What is the main reason for keeping the free fall of concrete within a limited height during placement?

  1. To limit the amount of formwork needed
  2. To prevent segregation of the mix Correct answer.
  3. To speed up the finishing operation
  4. To reduce the noise generated on site

Explanation

Free fall over a large height allows heavier aggregate to separate from mortar, causing segregation. Keeping the drop short, or using a tremie or chute, keeps the mix coherent as it lands. Noise, formwork quantity, and finishing speed are unrelated to fall height.

Question 26Core units of competency (common construction industry skills and concreting specialist field)

Which document should be consulted before starting a concrete pour to confirm the specified concrete strength and slump?

  1. The project specification and drawings Correct answer.
  2. The site diary from the previous week
  3. The safety data sheet for cement
  4. The supplier's delivery docket only

Explanation

The project specification and drawings are the authoritative source for the concrete properties required, including strength grade and workability. Delivery dockets confirm what was supplied but do not define the design requirements. Relying on past records or safety data alone would not verify compliance with the design.

Question 27Core units of competency (common construction industry skills and concreting specialist field)

When stacking cement bags on site, which practice best protects the cement from moisture damage?

  1. Keep bags in the back of a steel truck tray
  2. Stack bags against an external wall for ventilation
  3. Store bags directly on a concrete slab in the open
  4. Stack bags on a raised timber pallet under a waterproof cover Correct answer.

Explanation

Cement reacts with moisture, so bags must be kept dry and off damp surfaces. A raised pallet prevents ground moisture from being drawn up, and a waterproof cover protects against rain. Good air circulation also helps, but the priority is excluding water.

Question 28Core units of competency (common construction industry skills and concreting specialist field)

What is the primary purpose of a concrete pump line primer (such as a slurry or sponge) before pumping begins?

  1. To clean the line after the previous pour
  2. To cool the pump line before hot concrete arrives
  3. To lubricate the line and prevent blockages Correct answer.
  4. To increase the concrete's compressive strength

Explanation

Priming the line with a lubricating slurry or sponge reduces friction between the concrete and the pipe wall. This helps the concrete flow and reduces the risk of a blockage at the start of pumping. It does not alter the strength of the concrete or serve as a cleaning step.

Question 29Core units of competency (common construction industry skills and concreting specialist field)

Which test is commonly used on site to check the uniformity and consistency of a fresh concrete mix?

  1. Compressive strength test on cubes
  2. Schmidt hammer test
  3. Slump test Correct answer.
  4. Cover meter survey

Explanation

The slump test measures the workability and consistency of fresh concrete and can indicate variability between batches. It is quick and performed on site before placement. Strength tests and cover meters are used on hardened concrete.

Question 30Core units of competency (common construction industry skills and concreting specialist field)

When placing concrete in a deep wall form, what is the recommended maximum free-fall height to avoid segregation?

  1. 0.5 m
  2. 1.5 m Correct answer.
  3. 3.0 m
  4. 5.0 m

Explanation

Allowing concrete to fall freely from too great a height causes the heavier aggregate to separate from the mortar. A common limit is about 1.5 m, beyond which a tremie or chute should be used. This keeps the mix cohesive during placement.

Question 31Core units of competency (common construction industry skills and concreting specialist field)

Which action is required before placing concrete against previously hardened concrete to ensure a good bond?

  1. Apply a bonding agent or cement slurry to the prepared surface Correct answer.
  2. Place a layer of gravel between the old and new concrete
  3. Coat the old surface with release oil
  4. Allow the old surface to dry completely in the sun

Explanation

A bonding agent or cement slurry helps new concrete adhere to a clean, prepared substrate. The old surface should be roughened, cleaned, and dampened but not left with standing water. Release oil or a gravel layer would prevent bonding.

Question 32Core units of competency (common construction industry skills and concreting specialist field)

What is the main reason for wearing gloves when handling cement-based products?

  1. To keep the hands warm in cold weather
  2. To protect the concrete from contamination by sweat
  3. To prevent skin contact with alkaline cement and avoid burns Correct answer.
  4. To improve grip on reinforcing steel

Explanation

Wet cement is highly alkaline and can cause serious skin burns with prolonged contact. Gloves provide a barrier between the skin and the cementitious material. They are not primarily for warmth, grip, or protecting the concrete.

Question 33Core units of competency (common construction industry skills and concreting specialist field)

Which practice is correct when finishing a concrete slab to avoid a dusty surface?

  1. Trowel the surface while bleed water is still present
  2. Add extra water to the surface during trowelling
  3. Cover the surface with plastic immediately after screeding
  4. Wait until bleed water has evaporated before trowelling Correct answer.

Explanation

Trowelling while bleed water is present brings excess water and fines to the surface, creating a weak, dusty layer. The correct practice is to wait until the bleed water has evaporated or been removed. Adding water or covering too early can also cause surface defects.

Question 34Core units of competency (common construction industry skills and concreting specialist field)

A concrete mix is specified as 32 MPa. What does this number typically represent?

  1. The slump in millimetres
  2. The characteristic compressive strength at 28 days in megapascals Correct answer.
  3. The maximum aggregate size in millimetres
  4. The cement content in kilograms per cubic metre

Explanation

Concrete strength grades are usually expressed as the characteristic compressive strength at 28 days, in megapascals. This is the strength below which no more than a small percentage of test results should fall. It is not a measure of workability, aggregate size, or cement content.

Question 35Core units of competency (common construction industry skills and concreting specialist field)

During a slump test, the concrete slumps to 30 mm less than the specified target. What is the most appropriate first action?

  1. Check the test method and repeat the test on a fresh sample Correct answer.
  2. Proceed with the pour and note the result
  3. Add water to the truck to increase slump
  4. Reject the load and request a replacement

Explanation

A slump result outside the target may indicate a testing error or a genuine mix issue. Before rejecting the load, the tester should confirm the correct procedure and repeat the test on a fresh sample. Adding water on site is not permitted without approval as it alters the mix design.

Question 36Core units of competency (common construction industry skills and concreting specialist field)

Why is a poker vibrator inserted vertically and withdrawn slowly during compaction?

  1. To allow air bubbles to escape and avoid leaving voids Correct answer.
  2. To prevent the vibrator from overheating
  3. To mix the aggregate more evenly
  4. To speed up the setting time of the concrete

Explanation

Vertical insertion allows the vibrator head to reach the full depth, and slow withdrawal gives entrapped air time to escape. Rapid removal can leave a void or weak zone. Vibration compacts the concrete but does not alter setting time or mix the aggregate.

Question 37Core units of competency (common construction industry skills and concreting specialist field)

What is the main purpose of a construction joint in a concrete slab?

  1. To allow for movement due to temperature changes
  2. To separate the slab from the subgrade
  3. To provide a planned stopping place between concrete pours Correct answer.
  4. To control the location of shrinkage cracks

Explanation

A construction joint is used when concrete placement must stop and later resume, such as at the end of a day's work. It provides a defined, prepared interface between the two pours. Movement and shrinkage control are handled by expansion and contraction joints.

Question 38Core units of competency (common construction industry skills and concreting specialist field)

A concrete slab is to be poured in hot weather. Which measure helps reduce the risk of rapid moisture loss from the fresh surface?

  1. Use a curing compound or cover the surface immediately after finishing Correct answer.
  2. Increase the water content of the mix
  3. Place the slab in the hottest part of the day
  4. Delay finishing until the surface is dry

Explanation

Hot weather accelerates evaporation from the fresh concrete surface, which can cause plastic shrinkage cracking. Applying a curing compound or covering the surface soon after finishing reduces moisture loss. Adding water or delaying finishing can worsen the problem.

Question 39Core units of competency (common construction industry skills and concreting specialist field)

Which factor is most important when determining the required curing duration for a concrete element?

  1. The ambient temperature and the concrete's strength development Correct answer.
  2. The number of workers on site
  3. The type of release agent used
  4. The colour of the formwork

Explanation

Curing duration depends on the concrete's rate of strength gain, which is influenced by temperature and mix properties. Colder conditions generally require longer curing to achieve the same strength. Formwork colour, workforce, and release agents do not determine curing time.

Question 40Core units of competency (common construction industry skills and concreting specialist field)

When a concrete truck arrives on site, what should be checked on the delivery docket before discharge?

  1. The time the truck left the plant only
  2. The colour of the concrete in the drum
  3. The mix design, slump, and quantity ordered Correct answer.
  4. The driver's licence and the truck's registration

Explanation

The delivery docket should be checked against the specification to confirm the correct mix, workability, and volume are supplied. This prevents placing the wrong concrete. Driver details and departure time alone do not confirm compliance.

Question 41Core units of competency (common construction industry skills and concreting specialist field)

Which method is suitable for curing a vertical concrete wall after formwork removal?

  1. Spraying the wall with release oil
  2. Covering the wall with dry sand
  3. Applying a curing compound or attaching damp hessian Correct answer.
  4. Leaving the wall exposed to direct sun

Explanation

Vertical surfaces lose moisture quickly, so a curing compound or damp covering helps retain water for hydration. Direct sun or dry coverings would accelerate drying. Release oil is not a curing method and could interfere with finishes.

Question 42Core units of competency (common construction industry skills and concreting specialist field)

What is the purpose of a kicker (starter bar) at a construction joint in a wall?

  1. To provide a raised edge that helps align and seal the next lift of formwork Correct answer.
  2. To increase the wall's thermal insulation
  3. To act as a permanent expansion joint
  4. To support the reinforcement cage during pouring

Explanation

A kicker is a small upstand of concrete cast at the joint to help locate and seal the formwork for the next lift. It reduces grout loss and helps maintain alignment. It is not an insulation layer, expansion joint, or primary reinforcement support.

Question 43Core units of competency (common construction industry skills and concreting specialist field)

A concrete mix has a water-cement ratio of 0.55. If the cement content is 320 kg/m³, what is the approximate water content?

  1. 160 kg/m³
  2. 200 kg/m³
  3. 240 kg/m³
  4. 176 kg/m³ Correct answer.

Explanation

Water content equals the water-cement ratio multiplied by the cement content: 0.55 × 320 = 176 kg/m³. This calculation is used to check that the mix complies with durability requirements. The other values do not match the given ratio.

Question 44Core units of competency (common construction industry skills and concreting specialist field)

What is the main reason for compacting concrete in layers rather than as a single deep pour?

  1. To allow each layer to cure before the next is placed
  2. To make the concrete set faster
  3. To reduce the total volume of concrete required
  4. To ensure entrapped air can escape and full compaction is achieved Correct answer.

Explanation

Deep pours make it difficult for the vibrator to reach the full depth, so air can be trapped. Placing in layers allows each layer to be compacted effectively before the next is added. Layers are placed while the concrete is still fresh, not cured separately.

Question 45Core units of competency (common construction industry skills and concreting specialist field)

Which test is used to determine the hardened concrete's ability to resist penetration by chloride ions?

  1. VeBe test
  2. Slump test
  3. Rapid chloride permeability test Correct answer.
  4. Sieve analysis

Explanation

The rapid chloride permeability test assesses how easily chloride ions can move through hardened concrete, which relates to durability. Slump and VeBe tests measure fresh concrete workability. Sieve analysis characterises aggregate grading.

Question 46Core units of competency (common construction industry skills and concreting specialist field)

A slab on ground is to be poured over a plastic membrane. Why is a blinding layer sometimes placed over the membrane before placing reinforcement?

  1. To reduce the amount of reinforcement required
  2. To increase the slab's thermal insulation
  3. To protect the membrane from damage and provide a working surface Correct answer.
  4. To act as a curing blanket for the slab

Explanation

A blinding layer, such as a thin sand or concrete layer, protects the membrane from puncturing during reinforcement fixing and provides a stable surface. It does not provide insulation, cure the slab, or replace reinforcement. Its main role is protection and workability.

Question 47Core units of competency (common construction industry skills and concreting specialist field)

A concrete element has a specified characteristic strength of 40 MPa with a standard deviation of 4 MPa. Using a margin of 1.65 times the standard deviation, what target mean strength should the mix be designed for?

  1. 48.0 MPa
  2. 46.6 MPa Correct answer.
  3. 40.0 MPa
  4. 44.0 MPa

Explanation

Target mean strength = characteristic strength + (1.65 × standard deviation) = 40 + (1.65 × 4) = 46.6 MPa. This margin reduces the risk that test results fall below the characteristic value. The other values do not apply the given margin correctly.

Question 48Core units of competency (common construction industry skills and concreting specialist field)

A concrete pump is blocked during a pour. What is the safest first step?

  1. Disconnect the pipe immediately at the pump
  2. Hit the pipe with a hammer to dislodge the blockage
  3. Increase the pump pressure to clear the blockage
  4. Stop pumping and relieve pressure before investigating Correct answer.

Explanation

Stored pressure in a blocked line can cause sudden, dangerous movement of concrete or pipe components. The pump should be stopped and pressure relieved before any inspection or work on the line. Increasing pressure or striking the pipe can worsen the blockage or cause injury.

Question 49Core units of competency (common construction industry skills and concreting specialist field)

Why is the slump test not suitable for very dry or very wet mixes?

  1. The test requires hardened concrete
  2. Those mixes are always too strong to test
  3. The cone is too small for those mixes
  4. The test lacks sensitivity outside a moderate workability range Correct answer.

Explanation

The slump test is most reliable for concrete with moderate workability. Very dry mixes may not slump at all, and very wet mixes may collapse, so the result does not discriminate well. Other tests, such as the VeBe test, are better for very dry mixes.

Question 50Core units of competency (common construction industry skills and concreting specialist field)

A contractor notices that a concrete mix is bleeding excessively. Which adjustment to the mix design is most likely to reduce bleeding?

  1. Decrease the sand content
  2. Increase the cement content or use a finer cement Correct answer.
  3. Increase the coarse aggregate content
  4. Increase the water content

Explanation

Bleeding occurs when water rises to the surface because the solids settle. Increasing cement content or using finer cement improves cohesion and water retention. Adding water or reducing fines generally increases bleeding.