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NCCER Concrete Finishing practice questions

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Question 1Concrete Finishing

A finisher is placing a slab in hot, windy weather and notices the surface water sheen disappearing almost immediately after screeding. Which action best protects the surface from plastic shrinkage cracking?

  1. Begin floating while the concrete is still bleeding heavily
  2. Apply an evaporation retardant or fog spray and start finishing promptly Correct answer.
  3. Add water to the surface to restore the sheen
  4. Delay all finishing until the concrete has fully set

Explanation

Rapid surface drying under hot, windy conditions causes plastic shrinkage cracking when moisture evaporates faster than bleed water replaces it. Fog spraying or applying an evaporation retardant keeps the surface from drying until finishing can begin. Finishing must be timed to the concrete's set, not forced early or delayed until fully hardened.

Question 2Concrete Finishing

What is the primary purpose of bull floating a freshly screeded slab?

  1. To create the final non-slip texture required on the surface
  2. To level the concrete and embed aggregate while removing slight humps and valleys Correct answer.
  3. To push coarse aggregate down and bring a rich mortar to the surface
  4. To seal the surface so bleed water cannot escape

Explanation

Bull floating follows screeding and works the concrete to remove ridges, fill low spots, and embed coarse aggregate below the surface. It should be done before bleed water is fully gone but after the concrete can support the float. It is a leveling and consolidating step, not a texturing or sealing operation.

Question 3Concrete Finishing

A finisher is using a power trowel and the machine begins to dig in and leave swirl marks while the concrete is still soft. What is the most likely cause and correction?

  1. The concrete is too hard; add water to the surface
  2. The blades are pitched too flat; increase the pitch
  3. The blades are pitched too steeply or the concrete is too green; reduce pitch and wait for the concrete to stiffen Correct answer.
  4. The trowel is turning too slowly; increase the throttle

Explanation

Digging and deep swirl marks occur when blades are pitched too aggressively or when the concrete has not stiffened enough to support the trowel. Reducing blade pitch and waiting for additional set lets the trowel burnish rather than gouge. Blade pitch is gradually increased as the concrete stiffens through successive troweling passes.

Question 4Concrete Finishing

Which joint is installed specifically to control random cracking by creating a weakened plane in a slab?

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

Explanation

Contraction joints are sawed or formed to create a weakened plane so the concrete cracks in a controlled location as it shrinks. They are typically cut to about one-quarter of the slab thickness. Construction and isolation joints serve different structural and movement purposes.

Question 5Concrete Finishing

For a typical 4-inch-thick slab on grade, contraction joints should be sawed to what approximate depth?

  1. One-half of the slab thickness
  2. The full slab thickness
  3. One-eighth of the slab thickness
  4. One-quarter of the slab thickness Correct answer.

Explanation

Contraction joints are normally cut to about one-quarter of the slab thickness, which for a 4-inch slab is roughly 1 inch. This creates a weakened plane deep enough to direct shrinkage cracking. Cutting too shallow risks random cracking elsewhere; cutting full depth is unnecessary for crack control.

Question 6Concrete Finishing

When should saw cutting of contraction joints generally be performed?

  1. Only after the slab has already developed visible cracks
  2. After the concrete has fully cured for seven days
  3. Immediately after screeding while concrete is still plastic
  4. As soon as the concrete has hardened enough to resist raveling but before it cracks Correct answer.

Explanation

Saw cutting must be timed to occur after the concrete is hard enough that the blade does not ravel the joint edges but before shrinkage stresses cause random cracking. This window is often described in hours and depends on mix, weather, and thickness. Cutting too early ravels edges; cutting too late allows uncontrolled cracks.

Question 7Concrete Finishing

A finisher is broom finishing an exterior walkway. Which technique produces the most uniform, slip-resistant texture?

  1. Push the broom back and forth repeatedly until the surface is rough
  2. Pull a broom with consistent pressure and overlap in one direction after bleed water has disappeared Correct answer.
  3. Drag a stiff broom across the surface while the concrete is still bleeding
  4. Broom the surface after troweling it to a hard, glassy finish

Explanation

A uniform broom texture is achieved by pulling the broom with steady pressure and overlapping passes after bleed water has left the surface. Brooming too early tears the surface, and back-and-forth motion creates an inconsistent texture. Exterior surfaces need texture for slip resistance, so troweling to a glassy finish is inappropriate.

Question 8Concrete Finishing

What does the term "laitance" refer to on a concrete surface?

  1. A weak layer of fines and cement paste brought to the surface by overworking Correct answer.
  2. A type of joint filler used at isolation joints
  3. A curing compound applied after troweling
  4. Coarse aggregate exposed by excessive finishing

Explanation

Laitance is a thin, weak layer of cement paste and fines that rises to the surface when concrete is overworked or finished while bleed water is present. It reduces surface durability and must be avoided by proper finishing timing. It is not aggregate exposure, a curing product, or a joint material.

Question 9Concrete Finishing

A slab shows map cracking (a pattern of fine interconnected cracks) shortly after finishing. Which finishing error most likely caused it?

  1. Curing the slab with wet burlap
  2. Saw cutting joints too deep
  3. Using too large a coarse aggregate in the mix
  4. Finishing the surface while bleed water was still present Correct answer.

Explanation

Map cracking is often caused by finishing while bleed water is still on the surface, which produces a weak, high-shrinkage surface layer. The surface paste shrinks more than the concrete below and cracks in a map pattern. Proper timing of floating and troweling after bleed water disappears helps prevent it.

Question 10Concrete Finishing

Which curing method is generally most effective for retaining moisture in a newly finished concrete slab on grade?

  1. Sprinkling the surface once immediately after troweling
  2. Applying a curing compound or covering with wet burlap and plastic Correct answer.
  3. Covering the slab with dry sand
  4. Leaving the surface exposed to ambient air

Explanation

Effective curing keeps the concrete moist and at a favorable temperature for the required period. Curing compounds form a membrane that limits evaporation, while wet burlap covered with plastic maintains continuous moisture. A single sprinkling or dry cover does not provide sustained moisture.

Question 11Concrete Finishing

A finisher must place concrete against a vertical face and wants to avoid a cold joint. What is the best practice?

  1. Spray water on the hardened face and place new concrete without bonding agent
  2. Allow the first placement to fully harden before adding new concrete
  3. Place the next lift while the previous concrete is still plastic and work it together Correct answer.
  4. Place the new concrete only after applying a curing compound to the old face

Explanation

A cold joint forms when fresh concrete is placed against concrete that has already stiffened and cannot be integrated. Placing the next lift while the previous concrete is still plastic allows the two to be consolidated together. If a cold joint is unavoidable, the hardened surface must be prepared and bonded properly.

Question 12Concrete Finishing

What is the purpose of an isolation joint in a slab on grade?

  1. To separate the slab from fixed elements so they can move independently Correct answer.
  2. To bond two separate concrete placements together
  3. To provide a decorative line in the finished surface
  4. To create a weakened plane for shrinkage cracks

Explanation

Isolation joints separate a slab from columns, walls, or other fixed elements so the slab can move independently and avoid cracking or damaging those elements. They typically extend the full depth of the slab. Contraction joints, by contrast, control shrinkage cracking within the slab.

Question 13Concrete Finishing

A finisher is edging a slab before final troweling. What is the primary reason for edging?

  1. To create the contraction joint pattern
  2. To compact the slab interior
  3. To remove laitance from the surface
  4. To produce a rounded, durable edge that resists chipping Correct answer.

Explanation

Edging with an edger tool compacts and rounds the slab edge, producing a dense edge that resists chipping and spalling. It is done after floating and before troweling the main surface. Edging does not control interior shrinkage or remove laitance.

Question 14Concrete Finishing

Which factor most strongly determines the timing window for initial floating of a concrete slab?

  1. The color of the concrete mix
  2. The number of finishers on the crew
  3. The rate at which bleed water disappears and the concrete supports the float Correct answer.
  4. The type of curing compound to be used

Explanation

Initial floating should begin once bleed water has largely disappeared and the concrete is stiff enough to support the float without leaving deep marks. This timing depends on mix design, temperature, wind, and humidity. Starting too early works water to the surface; starting too late makes the surface difficult to close.

Question 15Concrete Finishing

What is the recommended practice for finishing concrete in cold weather to reduce the risk of surface damage?

  1. Protect the concrete from freezing and avoid finishing while bleed water is frozen Correct answer.
  2. Finish quickly and leave the surface unprotected to allow freezing
  3. Add calcium chloride to the surface to speed setting
  4. Flood the surface with water to keep it warm

Explanation

Fresh concrete must be protected from freezing until it gains sufficient strength, and finishing should not proceed over frozen bleed water or a frozen surface. Frozen surface water disrupts the paste and causes scaling and weak surfaces. Accelerators and heaters are used in the mix or enclosure, not by flooding the surface.

Question 16Concrete Finishing

A finisher notices that a troweled slab has a dusty, soft surface that wears away under foot traffic. Which cause is most consistent with this condition?

  1. Curing the slab for too long
  2. Using a stiff broom for texture
  3. Excess cement in the mix
  4. Finishing while bleed water was present, creating a weak surface layer Correct answer.

Explanation

Dusting results from a weak surface layer, often caused by finishing while bleed water is present or by overworking the surface. The weak laitance-rich layer wears away under traffic. Proper finishing timing and avoiding surface water addition prevent dusting.

Question 17Concrete Finishing

What is the function of a screed in slab construction?

  1. To cut contraction joints after the concrete hardens
  2. To apply a curing membrane to the surface
  3. To strike off and level freshly placed concrete to the required elevation Correct answer.
  4. To compact the subgrade before placing concrete

Explanation

A screed is used to strike off excess concrete and bring the surface to the required grade and elevation after placement. It establishes the plane that floating and troweling refine. Compaction, curing, and joint cutting are separate operations.

Question 18Concrete Finishing

A finisher is applying a dry-shake hardener to an industrial floor. What is the correct application practice?

  1. Apply the hardener only after the concrete has fully cured
  2. Broadcast the hardener onto dry, set concrete and trowel immediately
  3. Broadcast evenly onto fresh concrete after bleed water has disappeared and float it in Correct answer.
  4. Mix the hardener with water and spray it on the surface

Explanation

Dry-shake hardeners are broadcast evenly onto fresh concrete after bleed water has left and are then floated into the surface to bond. Applying to dry concrete prevents proper bonding, and mixing with water changes the product. Timing with the fresh concrete is essential for a monolithic wearing surface.

Question 19Concrete Finishing

What is the typical consequence of over-finishing a concrete surface with prolonged troweling?

  1. Increased surface durability and abrasion resistance
  2. Reduced shrinkage of the slab
  3. Improved bond between concrete and curing compound
  4. A weak, brittle surface prone to crazing and blistering Correct answer.

Explanation

Prolonged troweling draws paste and water to the surface, producing a weak, brittle layer susceptible to crazing and blistering. Over-finishing also traps air and water beneath a densified skin. Finishing should close the surface without overworking it.

Question 20Concrete Finishing

Before placing concrete for a slab on grade, which subgrade preparation step is most critical for uniform slab thickness?

  1. Wetting the subgrade to create mud
  2. Leaving the subgrade uneven to improve drainage
  3. Compacting and grading the subgrade to a uniform, level surface Correct answer.
  4. Covering the subgrade with a thick layer of loose sand

Explanation

A properly compacted and uniformly graded subgrade ensures the slab is placed at consistent thickness and elevation. Uneven or loose subgrade causes variable thickness, settlement, and cracking. Loose sand or mud weakens support and makes thickness control difficult.

Question 21Concrete Finishing

A finisher observes plastic shrinkage cracks forming on a slab surface shortly after placement. Which set of conditions most favors this type of cracking?

  1. Heavy rain immediately after placement
  2. High humidity, cool temperatures, and calm air
  3. Low humidity, high temperature, and strong wind Correct answer.
  4. Concrete placed with a low water-cement ratio in cool weather

Explanation

Plastic shrinkage cracks form when the rate of surface evaporation exceeds the rate at which bleed water reaches the surface. Low humidity, high temperature, and strong wind all accelerate evaporation. Protection such as fogging, windbreaks, or evaporation retardants is needed under these conditions.

Question 22Concrete Finishing

What is the purpose of a float blade compared with a trowel blade on a power finisher?

  1. Float blades polish the surface; trowel blades remove bleed water
  2. Float blades cut joints; trowel blades cure the surface
  3. Float blades level and open the surface; trowel blades densify and burnish it Correct answer.
  4. Float blades apply hardener; trowel blades remove it

Explanation

Float blades are used earlier to level, consolidate, and open the surface so bleed water can escape. Trowel blades are used later to densify and burnish the surface as the concrete stiffens. The sequence of float then trowel matches the concrete's increasing stiffness.

Question 23Concrete Finishing

A finisher is repairing a spalled area on an existing concrete surface. Which preparation step is most important before applying patching material?

  1. Coat the area with a curing compound
  2. Remove loose material and sound the edges to solid concrete Correct answer.
  3. Apply a layer of dry cement powder
  4. Wet the area only lightly and leave standing water

Explanation

Durable patching requires removing all loose, unsound concrete and undercutting or squaring the edges to sound material. The prepared area is then cleaned and bonded according to the patch material requirements. Curing compound or dry cement on the bond surface prevents adhesion.

Question 24Concrete Finishing

Which statement correctly describes the purpose of a vapor retarder beneath a slab on grade?

  1. It prevents groundwater from entering the concrete mix
  2. It acts as a curing membrane on the finished surface
  3. It limits moisture vapor migration from the subgrade into the slab Correct answer.
  4. It replaces the need for a compacted subgrade

Explanation

A vapor retarder is placed beneath a slab to limit moisture vapor from the subgrade migrating into and through the slab. This helps protect floor finishes and reduce moisture-related problems. It does not replace subgrade compaction or serve as a surface curing membrane.

Question 25Concrete Finishing

A finisher is using a magnesium float on air-entrained concrete. What is the main advantage of a magnesium float over a steel trowel at this stage?

  1. It opens the surface and allows bleed water to escape without overworking it Correct answer.
  2. It seals the surface tightly for a glassy finish
  3. It cuts contraction joints as it passes
  4. It applies a curing compound during floating

Explanation

A magnesium float is used to level and open the surface, letting bleed water escape while embedding aggregate without sealing the surface. Sealing too early traps water and creates blisters. A steel trowel is used later to densify and smooth the surface.

Question 26Concrete Finishing

A finisher is selecting a tool to strike off freshly placed concrete to the required elevation. Which tool is designed specifically for this task?

  1. A groover
  2. A steel trowel
  3. A straightedge or screed Correct answer.
  4. A wood hand float

Explanation

Striking off means leveling and shaping the plastic concrete to the planned elevation, and that is the job of a screed or straightedge worked across the forms or guides. Floats, trowels, and groovers are surface tools used later in the sequence, after the concrete has been struck off and consolidated.

Question 27Concrete Finishing

Concrete is being placed on a slope and the finisher wants to prevent the mix from sliding downhill during finishing. What is the standard practice?

  1. Start finishing at the top of the slope and work downward Correct answer.
  2. Start finishing at the bottom of the slope and work upward
  3. Add extra water to make the mix flow evenly
  4. Wait until the concrete has fully cured before shaping it

Explanation

On sloped surfaces, finishing proceeds from the high side downward so that each pass pushes material against the previously finished area and holds it in place. Working uphill lets the plastic concrete creep and tear under the tool, leaving an uneven surface. Adding water weakens the concrete and does not solve slope movement.

Question 28Concrete Finishing

Which surface texture is produced by dragging a stiff broom across concrete that has been floated but not yet troweled?

  1. A uniformly ridged, slip-resistant surface Correct answer.
  2. A glossy, dense surface
  3. A deeply exposed-aggregate surface
  4. A mirror-smooth surface with no visible marks

Explanation

Brooming is done while the surface is still plastic but firm enough to hold the ridges cut by the bristles. The result is a uniform pattern of shallow grooves that improves traction, which is why it is common on walks, driveways, and ramps. Glossy or mirror-smooth surfaces come from steel troweling, and exposed aggregate requires a separate retarder and washing process.

Question 29Concrete Finishing

A finisher is preparing to cure a new slab and needs to know the basic purpose of curing. Which statement best describes it?

  1. Curing hardens the surface by driving moisture out of the concrete
  2. Curing replaces the need for proper mix design
  3. Curing keeps concrete moist and at a favorable temperature so cement continues to hydrate Correct answer.
  4. Curing removes laitance from the surface layer

Explanation

Curing maintains moisture and a suitable temperature so that cement hydration can continue and the concrete can develop its design strength and durability. Drying the concrete prematurely stalls hydration and causes surface defects. Curing does not remove laitance, and it cannot compensate for a poor mix.

Question 30Concrete Finishing

What is the primary purpose of a bull float on freshly placed concrete?

  1. To push coarse aggregate down and bring paste to the surface
  2. To level and smooth the surface and embed aggregate just after strikeoff Correct answer.
  3. To cut control joints to depth
  4. To apply a dry-shake hardener evenly

Explanation

A bull float is used immediately after strikeoff to smooth the surface, remove slight high and low spots, and embed the coarse aggregate below the paste. It is a leveling and consolidating tool, not a joint cutter or hardener applicator. Working the surface too aggressively with it can bring excess paste and water upward.

Question 31Concrete Finishing

A finisher must place a construction joint in a slab. Which practice best ensures the joint will transfer load and remain tight?

  1. Leave the joint face rough and free of laitance, and use a keyway or dowels Correct answer.
  2. Leave a wide gap and fill it later with sand
  3. Place the adjacent pour before the first pour has begun to set
  4. Coat the joint face with a curing compound before placing adjacent concrete

Explanation

A sound construction joint requires a clean, rough, laitance-free face so the new concrete can bond, plus a keyway or dowels to transfer load across the joint. Curing compound on the joint face would act as a bond breaker. Placing the adjacent pour while the first is still plastic risks disturbing it and does not create a defined joint.

Question 32Concrete Finishing

Which tool is used to round the exposed edges of a slab so they resist chipping?

  1. A float
  2. An edger Correct answer.
  3. A trowel
  4. A jointer

Explanation

An edger has a curved profile that compacts and rounds the edge of a slab, producing a dense, chip-resistant arris. A jointer cuts grooves for control joints, while trowels and floats work the flat surface. Edging is done while the concrete is still plastic enough to hold the rounded shape.

Question 33Concrete Finishing

What does the term "slump" describe in a concrete mix?

  1. The final compressive strength of the hardened concrete
  2. The measure of consistency or workability of fresh concrete Correct answer.
  3. The depth of a control joint
  4. The thickness of the slab

Explanation

Slump is measured with a standard cone and indicates the consistency and workability of fresh concrete. It is not a strength value, though very high slump achieved by adding water can reduce strength. Joint depth and slab thickness are separate construction dimensions.

Question 34Concrete Finishing

A finisher is deciding when to begin floating a slab. Which surface condition indicates the concrete is ready for initial floating?

  1. The concrete has set enough that a footprint leaves only a shallow impression Correct answer.
  2. The concrete has reached full design strength
  3. The surface is dry and light gray with no bleed water
  4. Water is still sheening on the surface and footprints fill with water

Explanation

Initial floating should begin after bleed water has disappeared and the concrete has stiffened enough to support the float without rutting. At that stage a footprint leaves only a shallow impression and the surface no longer works up water. Floating while bleed water is present drives water into the surface and weakens it, and waiting until full strength is far too late.

Question 35Concrete Finishing

A power trowel is being used on an interior slab. Which practice helps prevent the machine from digging in and creating waves?

  1. Keep the blades flat and tilted slightly for the first passes, then increase pitch gradually Correct answer.
  2. Start with the blades at maximum pitch for the deepest burn possible
  3. Add water to the surface to lubricate the blades
  4. Run the machine at full speed immediately after strikeoff

Explanation

Early troweling passes should use nearly flat blades so the machine glides over the surface without digging. Pitch is increased gradually as the concrete stiffens to produce a denser, harder surface. Starting at high pitch or high speed on soft concrete creates ridges and tears, and adding water weakens the surface.

Question 36Concrete Finishing

A finisher needs to estimate when a slab can be saw cut for contraction joints. Which guideline is most reliable?

  1. Cut only after the slab has reached design strength
  2. Cut as soon as the concrete can be walked on without marring the surface Correct answer.
  3. Cut when the surface first turns from dark to light gray
  4. Wait at least seven days after placement

Explanation

Contraction joints should be sawed as soon as the concrete is hard enough to support the saw without raveling the cut edges, often within the first day. Cutting early relieves shrinkage stresses before random cracks form. Waiting a week or until full strength allows shrinkage cracks to develop first.

Question 37Concrete Finishing

A finisher is placing concrete in hot, windy weather and wants to reduce rapid surface drying. Which action is most effective?

  1. Increase the water added at the jobsite
  2. Use windbreaks, shade, or evaporation retarder and cure promptly Correct answer.
  3. Delay all finishing until the next day
  4. Remove the vapor retarder beneath the slab

Explanation

Hot, windy conditions accelerate evaporation from the plastic surface, causing plastic shrinkage cracks and crusting. Windbreaks, shade, fogging, evaporation retarders, and prompt curing reduce the evaporation rate. Adding water at the jobsite raises the water-cement ratio and weakens the concrete, and removing the vapor retarder does not address surface drying.

Question 38Concrete Finishing

A finisher is evaluating why a troweled floor has a blistered surface with hollow-sounding bumps. Which cause is most likely?

  1. Cutting control joints too early
  2. Curing the slab with wet burlap
  3. Excess bleed water trapped under a prematurely sealed surface Correct answer.
  4. Using a magnesium float instead of a steel trowel

Explanation

Blistering occurs when air or bleed water is trapped beneath a surface that has been troweled and sealed too early. The trapped moisture or air expands and lifts the densified skin, creating hollow bumps. Proper timing of floating and troweling, and allowing bleed water to dissipate, prevents the problem.

Question 39Concrete Finishing

A finisher is checking formwork before a slab pour. Which condition most directly risks a bulged or out-of-line edge?

  1. Forms that are wet before placement
  2. Forms set slightly above the finished slab elevation
  3. Inadequate staking and bracing of the formwork Correct answer.
  4. Forms that are clean and coated with release agent

Explanation

Formwork must resist the lateral pressure of plastic concrete, so stakes and braces must be adequate to hold line and grade. Weak bracing allows forms to bow outward, producing bulged edges and varying slab width. Clean, coated forms and dampened forms are normal good practice, and forms set to the correct elevation are expected.

Question 40Concrete Finishing

A finisher is applying a curing compound to a fresh slab. Which practice gives the best moisture retention?

  1. Apply uniformly at the specified rate as soon as the surface water disappears Correct answer.
  2. Apply only around the slab edges
  3. Apply after the slab has been saw cut
  4. Apply a thin coat after the surface has fully dried

Explanation

Curing compound works by forming a membrane that limits evaporation, so it must be applied uniformly at the manufacturer's specified coverage rate. Application should occur as soon as the surface sheen disappears, before the concrete dries. Thin coats, partial coverage, or late application allow moisture loss and reduce curing effectiveness.

Question 41Concrete Finishing

A finisher is repairing a honeycombed area on a wall. Which step is essential before placing patching material?

  1. Apply a release agent to the patch area
  2. Coat the area with a curing compound
  3. Seal the area with a vapor retarder
  4. Remove loose material and saturate the prepared surface with clean water Correct answer.

Explanation

A durable patch requires a sound substrate, so loose and honeycombed concrete must be chipped out to solid material. The prepared area should be saturated with clean water so it does not draw water from the patch mix, and a bonding agent may be used. Curing compound or release agent on the substrate would prevent bond.

Question 42Concrete Finishing

A finisher is inspecting a slab for a defect where the top layer separates from the body of the concrete and sounds hollow under tapping. What is this defect called?

  1. Curling
  2. Scaling
  3. Delamination Correct answer.
  4. Segregation

Explanation

Delamination is the separation of a horizontal top layer from the concrete beneath, often caused by finishing too early over bleed water or by overworking the surface. It is detected by hollow sounds under tapping. Scaling is surface flaking, curling is edge warping, and segregation is the separation of aggregate from mortar in the plastic mix.

Question 43Concrete Finishing

A finisher is asked to produce a non-slip finish on an exterior ramp. Which combination of practices best meets that goal?

  1. Float the surface, broom perpendicular to traffic direction, and cure properly Correct answer.
  2. Steel trowel to a hard burn and seal the surface
  3. Finish with a wood float and leave it open and dusty
  4. Apply a curing compound before brooming

Explanation

A slip-resistant ramp finish is achieved by floating to a uniform surface and then brooming while the concrete is still plastic enough to hold the ridges. Brooming across the direction of travel gives the best traction, and proper curing preserves the surface. Hard troweling produces a slick surface, and applying curing compound before brooming would interfere with the texture.

Question 44Concrete Finishing

A finisher is evaluating a slab that has curled upward at the joints. Which condition most contributes to this behavior?

  1. Cutting joints with a saw instead of a groover
  2. Use of a broom finish on the surface
  3. Adding air entrainment to the mix
  4. Moisture and temperature differences between the top and bottom of the slab Correct answer.

Explanation

Curling results from differential shrinkage and temperature between the top and bottom of a slab. When the top dries and shrinks more than the bottom, the edges lift. Broom texture, joint cutting method, and air entrainment are not primary causes of curling.

Question 45Concrete Finishing

A finisher is working with a mix that has a low water-cement ratio and stiff consistency. Which finishing challenge is most likely?

  1. The surface will remain plastic for several days
  2. The surface will bleed excessively and delay finishing
  3. The concrete will stiffen quickly, narrowing the finishing window Correct answer.
  4. The concrete will never gain strength

Explanation

A low water-cement ratio mix is stiffer and sets faster, so the window between initial floating and final troweling is shorter and the finisher must work efficiently. Such mixes generally bleed less, not more, and they gain strength well. They do not remain plastic for days.

Question 46Concrete Finishing

A finisher is placing a slab in freezing weather. Which practice best protects the fresh concrete surface?

  1. Place concrete on frozen subgrade to keep it cool
  2. Finish the surface with water to prevent freezing
  3. Add calcium chloride to the mix without limit
  4. Use heated concrete and maintain protection until it reaches adequate strength Correct answer.

Explanation

Cold-weather concreting requires that the concrete and subgrade be protected from freezing, that the mix be placed at an adequate temperature, and that the concrete be kept warm until it gains sufficient strength to resist frost damage. Placing on frozen subgrade causes uneven set and heave. Unlimited calcium chloride can cause corrosion and set problems, and adding water promotes freezing damage.

Question 47Concrete Finishing

A finisher observes that a slab surface has become dusting and soft under traffic. Which condition is the most likely root cause?

  1. Using a steel trowel for the final pass
  2. A low water-cement ratio at the surface
  3. Finishing while bleed water was present, raising the local water-cement ratio Correct answer.
  4. Curing the slab with water

Explanation

Dusting is caused by a weak surface layer, commonly from finishing while bleed water is present or from troweling water back into the surface. That raises the water-cement ratio at the surface and leaves a soft, powdery layer. A low water-cement ratio, water curing, and steel troweling are not causes of dusting.

Question 48Concrete Finishing

A finisher is comparing a hand float and a power trowel for final surface work. Which statement correctly describes the difference in effect?

  1. The power trowel can densify and burnish the surface more uniformly and with greater pressure Correct answer.
  2. The power trowel is used only before strikeoff
  3. The hand float is used only for exterior concrete
  4. The hand float densifies and burnishes the surface more than the power trowel

Explanation

A power trowel applies greater, more uniform pressure over the surface and can produce a dense, burnished finish when used at the correct time and blade pitch. A hand float opens and textures the surface but does not densify it as much. Tool use is governed by the desired finish, not by whether the slab is interior or exterior.

Question 49Concrete Finishing

A finisher is investigating why a slab has a pattern of shallow, interconnected cracks shortly after finishing. Which set of conditions most likely produced this map cracking?

  1. Using a broom finish on an exterior slab
  2. A low water-cement ratio and long moist curing
  3. Adequate joint spacing and prompt saw cutting
  4. Overworking the surface with a trowel while bleed water was present, plus rapid drying Correct answer.

Explanation

Map cracking, also called crazing, results from a weak, shrinkage-prone surface layer. Overworking the surface and troweling bleed water back in, combined with rapid drying, produces a paste-rich skin that shrinks more than the concrete beneath. Low water-cement ratio, proper jointing, and brooming are not causes.

Question 50Concrete Finishing

A finisher must place a slab against an existing wall and wants to prevent random cracking from restrained movement. Which detail is appropriate?

  1. Tie the new slab rigidly to the wall with rebar
  2. Butt the slab directly against the wall with no joint
  3. Install an isolation joint with a compressible filler between the slab and the wall Correct answer.
  4. Extend the wall footing into the slab area

Explanation

An isolation joint uses a compressible filler to separate the slab from a fixed element such as a wall or column, allowing independent movement and preventing restraint cracks. Rigidly tying the slab to the wall or butting it directly against it transfers movement into the slab and causes random cracking.