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NICET CMT Concrete practice questions

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Question 1Concrete Materials, Mixture Proportions, and Specifications

A specification requires high-early-strength concrete so a pavement can be opened to traffic within 24 hours. Which ASTM C150 cement type is designated for this purpose?

  1. ASTM C150 Type III Correct answer.
  2. ASTM C150 Type II
  3. ASTM C150 Type V
  4. ASTM C150 Type I

Explanation

Type III is the high-early-strength portland cement, ground finer so it gains strength faster than the other types. It is used when early opening or rapid form removal governs the schedule. Type I is general purpose, Type II has moderate sulfate resistance, and Type V has high sulfate resistance.

Question 2Concrete Materials, Mixture Proportions, and Specifications

What is the primary function of an air-entraining admixture in concrete that will be exposed to freezing and thawing?

  1. Accelerate initial setting in cold weather
  2. Increase compressive strength by filling capillary pores with air
  3. Create closely spaced voids that relieve pressure as pore water freezes Correct answer.
  4. Reduce the amount of cementitious material needed to reach a given strength

Explanation

Entrained air creates a system of small, closely spaced bubbles that give freezing water somewhere to expand, relieving hydraulic pressure in the paste. This greatly improves freeze-thaw durability. Air entrainment generally lowers compressive strength slightly, so it is not a strength-enhancing measure.

Question 3Concrete Materials, Mixture Proportions, and Specifications

For a fixed set of materials with adequate consolidation, what is the primary effect of lowering the water-cementitious materials ratio from 0.55 to 0.45?

  1. Permeability decreases and compressive strength increases Correct answer.
  2. Air content increases because the paste becomes stiffer
  3. Setting time shortens sharply because the paste is drier
  4. Compressive strength decreases because less water is available for hydration

Explanation

Lowering the water-cementitious materials ratio reduces the capillary porosity of the hardened paste, which lowers permeability and raises compressive strength. The relationship is the basis for proportioning mixtures by w/cm ratio. Workability must be maintained by other means, such as admixtures, when the ratio is reduced.

Question 4Concrete Materials, Mixture Proportions, and Specifications

Which statement best describes the contribution of a Class F fly ash to hardened concrete?

  1. It reacts with calcium hydroxide released by cement hydration to form additional calcium silicate hydrate Correct answer.
  2. It reacts directly with mixing water to form calcium silicate hydrate faster than portland cement does
  3. It acts mainly as an inert filler and does not participate in chemical reactions
  4. It significantly increases the heat of hydration during the first 24 hours

Explanation

Fly ash is a pozzolan: it reacts with calcium hydroxide, a by-product of cement hydration, to form additional calcium silicate hydrate. This pozzolanic reaction proceeds more slowly than cement hydration, so early strength is often lower while later strength and durability improve. Fly ash also generally reduces heat of hydration and improves sulfate resistance.

Question 5Concrete Materials, Mixture Proportions, and Specifications

A concrete member has a narrowest dimension of 10 in. and a clear spacing between reinforcing bars of 3 in. According to ACI 211.1 proportioning guidance, what is the largest nominal maximum aggregate size that satisfies these limits?

  1. 3 in.
  2. 2-1/2 in.
  3. 2 in. Correct answer.
  4. 1-1/2 in.

Explanation

The nominal maximum aggregate size should not exceed one-fifth of the narrowest member dimension (10 in. / 5 = 2 in.) or three-fourths of the clear bar spacing (3 in. x 0.75 = 2.25 in.). The smaller governing value is 2 in. The one-third-of-slab-depth limit also applies to slabs but is not the control here.

Question 6Concrete Materials, Mixture Proportions, and Specifications

A ready-mix producer needs to extend the setting time of concrete placed in hot weather and hauled 90 minutes. Which ASTM C494 chemical admixture type is intended for this purpose?

  1. Type A, water-reducing
  2. Type F, high-range water-reducing
  3. Type C, accelerating
  4. Type B, retarding Correct answer.

Explanation

ASTM C494 Type B is the retarding admixture classification; it slows the initial set of the concrete. Retarders are used in hot weather or long hauls to preserve workability and placeability. Type A and Type F primarily reduce water demand, while Type C accelerates setting.

Question 7Concrete Materials, Mixture Proportions, and Specifications

Under ASTM C94, how much may the batched quantity of cementitious materials vary from the specified mass?

  1. Plus or minus 2 percent
  2. Plus or minus 1 percent Correct answer.
  3. Plus or minus 0.5 percent
  4. Plus or minus 3 percent

Explanation

ASTM C94 limits the batching variation for cementitious materials to plus or minus 1 percent of the specified mass. Aggregate is permitted plus or minus 2 percent, water plus or minus 1 percent, and admixtures plus or minus 3 percent. These tolerances protect the specified water-cementitious materials ratio and mixture proportions.

Question 8Concrete Materials, Mixture Proportions, and Specifications

Concrete with a 1 in. nominal maximum aggregate size will be exposed to severe freeze-thaw conditions. What total air content does ACI 318 target for this exposure and aggregate size?

  1. 4-1/2 percent
  2. 6 percent Correct answer.
  3. 5-1/2 percent
  4. 7 percent

Explanation

For severe freeze-thaw exposure with a 1 in. nominal maximum aggregate size, ACI 318 targets a total air content of about 6 percent. The required air content increases as the maximum aggregate size decreases, reaching about 7-1/2 percent for 3/8 in. aggregate. Air content is verified by ASTM C231 pressure testing of fresh concrete.

Question 9Concrete Materials, Mixture Proportions, and Specifications

A mixture contains 520 lb of portland cement, 130 lb of fly ash, and 292.5 lb of water per cubic yard. What is the water-cementitious materials ratio?

  1. 0.50
  2. 0.56
  3. 0.31
  4. 0.45 Correct answer.

Explanation

The water-cementitious materials ratio divides water by all cementitious material, including fly ash: 292.5 / (520 + 130) = 292.5 / 650 = 0.45. Fly ash is counted as a cementitious material in the ratio. Dividing by cement alone would ignore the fly ash contribution.

Question 10Concrete Materials, Mixture Proportions, and Specifications

A structure will be exposed to moist conditions, and the coarse aggregate has been identified as potentially alkali-silica reactive. Which strategy best reduces the risk of deleterious alkali-silica reaction?

  1. Increase the cement content to offset dilution of the reactive aggregate
  2. Use a low-alkali cement together with an effective dosage of Class F fly ash Correct answer.
  3. Increase the water-cementitious materials ratio to improve workability
  4. Use Type III cement to accelerate strength gain and reduce exposure time

Explanation

Alkali-silica reaction requires reactive silica, sufficient alkalis, and moisture. Reducing the alkali contribution with low-alkali cement and diluting the available alkalis with a sufficient dosage of Class F fly ash is a recognized mitigation strategy. A nonreactive aggregate source would be preferable where available. Increasing cement content or w/cm ratio would tend to worsen the risk.

Question 11Concrete Materials, Mixture Proportions, and Specifications

A concrete foundation will be exposed to soil containing high concentrations of soluble sulfates. Which ASTM C150 cement type is specifically manufactured to provide high sulfate resistance?

  1. Type V Correct answer.
  2. Type I
  3. Type II
  4. Type III

Explanation

ASTM C150 Type V cement is the high-sulfate-resistance cement. Its tricalcium aluminate content is limited to a maximum of 5%, which reduces the compounds that react with sulfates. Type II provides only moderate sulfate resistance, while Type I is general purpose and Type III is high-early-strength.

Question 12Concrete Materials, Mixture Proportions, and Specifications

What is the primary function of a high-range water-reducing admixture (superplasticizer) in a concrete mixture?

  1. To accelerate the hydration of cement and shorten setting time.
  2. To increase flowability so that a lower water content can be used at the same slump. Correct answer.
  3. To reduce the alkali content of the pore solution.
  4. To entrain microscopic air bubbles for freeze-thaw resistance.

Explanation

A high-range water reducer disperses cement particles and dramatically increases slump or flow without adding water. This allows the producer to place flowing concrete at a lower water-cementitious material ratio. Air entrainment, acceleration, and alkali reduction are functions of other admixtures.

Question 13Concrete Materials, Mixture Proportions, and Specifications

Which aggregate moisture condition exists when the aggregate has absorbed all the water it can but its surface is dry?

  1. Saturated-surface-dry Correct answer.
  2. Wet
  3. Air-dry
  4. Oven-dry

Explanation

Saturated-surface-dry, or SSD, describes aggregate that is saturated internally with no free moisture on the surface. SSD is the reference condition used in concrete mixture proportioning because it separates absorption from free surface water. Oven-dry and air-dry aggregate contain less moisture, while wet aggregate carries free water.

Question 14Concrete Materials, Mixture Proportions, and Specifications

Which ASTM specification covers air-entraining admixtures for concrete?

  1. ASTM C150
  2. ASTM C618
  3. ASTM C260 Correct answer.
  4. ASTM C494

Explanation

ASTM C260 is the standard specification for air-entraining admixtures for concrete. ASTM C150 covers portland cement, ASTM C494 covers chemical admixtures, and ASTM C618 covers coal fly ash and raw or calcined natural pozzolans. Matching the admixture type to the correct specification is important when reviewing submittals.

Question 15Concrete Materials, Mixture Proportions, and Specifications

Condensed silica fume is added to a concrete mixture at about 8% by mass of cementitious material. What is a typical effect on the fresh and hardened concrete?

  1. It greatly increases permeability and reduces strength.
  2. It retards setting time and lowers compressive strength at all ages.
  3. It increases bleeding and reduces cohesiveness.
  4. It increases cohesiveness and reduces permeability, often requiring a high-range water reducer. Correct answer.

Explanation

Silica fume consists of very fine particles that increase cohesiveness and fill voids, producing a dense, low-permeability concrete. Because of its high surface area, it usually increases water demand and is commonly used with a high-range water reducer. It does not increase bleeding or lower strength at all ages.

Question 16Concrete Materials, Mixture Proportions, and Specifications

How does replacing a portion of portland cement with ground granulated blast-furnace slag typically affect concrete strength development?

  1. It eliminates the need for curing.
  2. It has no effect on strength but increases air content.
  3. It increases early-age strength but reduces ultimate strength.
  4. It reduces early-age strength but can increase ultimate strength and improve sulfate resistance. Correct answer.

Explanation

Slag cement reacts more slowly than portland cement, so concrete typically gains strength more slowly at early ages. At later ages, slag can increase ultimate strength and improve resistance to sulfate attack. It does not eliminate curing requirements or directly increase air content.

Question 17Concrete Materials, Mixture Proportions, and Specifications

For a fixed cement content and slump, how does increasing the nominal maximum aggregate size from 3/4 in. to 1-1/2 in. typically affect the required mixing water?

  1. It increases the mixing water because larger aggregate has more surface area.
  2. It increases water demand because larger aggregate causes more air entrapment.
  3. It has no effect because water demand depends only on cement content.
  4. It decreases the mixing water because less paste is needed to coat the aggregate. Correct answer.

Explanation

Increasing the nominal maximum aggregate size reduces the total surface area of aggregate that must be coated with paste. For the same slump and cement content, this generally lowers the mixing water requirement. Larger aggregate does not have more surface area per unit volume; it has less.

Question 18Concrete Materials, Mixture Proportions, and Specifications

A trial batch contains 1,200 lb of portland cement, 300 lb of fly ash, and 2,700 lb of aggregate; it yields 1.20 cubic yards of concrete. What is the total cementitious material content per cubic yard?

  1. 1,250 lb/yd³ Correct answer.
  2. 1,500 lb/yd³
  3. 1,200 lb/yd³
  4. 1,000 lb/yd³

Explanation

The total cementitious material is 1,200 lb of cement plus 300 lb of fly ash, or 1,500 lb. Dividing by the 1.20 yd³ yield gives 1,250 lb/yd³. The aggregate mass is not part of the cementitious content.

Question 19Concrete Materials, Mixture Proportions, and Specifications

ACI 318 limits water-soluble chloride ion content in reinforced concrete exposed to chlorides in service. Which maximum limit, expressed as percent by mass of cementitious material, applies to normal reinforced concrete?

  1. 0.06%
  2. 0.15% Correct answer.
  3. 0.30%
  4. 1.00%

Explanation

For reinforced concrete exposed to chlorides in service, ACI 318 sets a maximum water-soluble chloride ion content of 0.15% by mass of cementitious material. The 0.06% limit applies to prestressed concrete, and 0.30% applies to reinforced concrete that will be dry or protected from moisture. These limits help control chloride-induced corrosion of reinforcing steel.

Question 20Concrete Materials, Mixture Proportions, and Specifications

A concrete mixture for a marine structure contains 30% Class C fly ash as a replacement for portland cement. Which concern is most directly associated with the higher calcium oxide content of Class C fly ash compared with Class F fly ash?

  1. Reduced water demand requiring no water reducer.
  2. Increased susceptibility to sulfate attack in the concrete. Correct answer.
  3. Decreased early-age strength at all replacement levels.
  4. Increased air content without an air-entraining admixture.

Explanation

Class C fly ash generally contains more calcium oxide than Class F fly ash. The higher calcium content can make the concrete less resistant to sulfate attack, so Class C fly ash is used cautiously in sulfate exposures. Class F fly ash, with its lower calcium content, is generally more effective for sulfate resistance.

Question 21Concrete Materials, Mixture Proportions, and Specifications

In a typical conventional concrete mixture, which constituent generally occupies the largest fraction of the total volume?

  1. fine aggregate
  2. portland cement
  3. coarse aggregate Correct answer.
  4. entrained air

Explanation

Coarse aggregate typically makes up roughly 40% to 50% of a conventional concrete mixture by volume, more than any other single ingredient. Cement paste and fine aggregate fill the remaining volume around the coarse particles.

Question 22Concrete Materials, Mixture Proportions, and Specifications

Fine aggregate for concrete is generally defined as material passing which standard sieve?

  1. the 75 µm (No. 200) sieve
  2. the 4.75 mm (No. 4) sieve Correct answer.
  3. the 9.5 mm (3/8 in.) sieve
  4. the 25 mm (1 in.) sieve

Explanation

ASTM C33 defines fine aggregate as particles passing the 4.75 mm (No. 4) sieve and predominantly retained on the 75 µm (No. 200) sieve. Material finer than the No. 200 sieve is generally treated as fines or dust rather than as fine aggregate.

Question 23Concrete Materials, Mixture Proportions, and Specifications

What is the primary purpose of an accelerating admixture in concrete?

  1. To retard setting time during hot-weather placement
  2. To reduce the water demand while maintaining slump
  3. To increase the rate of hydration and early strength gain Correct answer.
  4. To entrain microscopic air voids for freeze-thaw resistance

Explanation

Accelerators increase the rate of cement hydration, which shortens setting time and raises early-age strength. They do not primarily entrain air, reduce water demand, or delay setting.

Question 24Concrete Materials, Mixture Proportions, and Specifications

A batch uses 1,800 lb of coarse aggregate at a total moisture content of 6.0%. The aggregate has an absorption of 1.5%. How much free water does the coarse aggregate contribute to the batch?

  1. 81 lb Correct answer.
  2. 108 lb
  3. 54 lb
  4. 27 lb

Explanation

Free water is the moisture above the saturated-surface-dry condition. Subtract absorption from total moisture: (6.0% − 1.5%) × 1,800 lb = 81 lb of free water.

Question 25Concrete Materials, Mixture Proportions, and Specifications

Under ASTM C94, unless a retarding admixture or other approved provision is used, concrete should be discharged from the mixer within what maximum period after batching?

  1. 90 minutes Correct answer.
  2. 30 minutes
  3. 60 minutes
  4. 120 minutes

Explanation

ASTM C94 limits discharge to before 90 minutes after batching and before the mixer drum has revolved 300 revolutions, unless otherwise approved. The limit may be modified when retarding admixtures or specified conditions are used.

Question 26Concrete Materials, Mixture Proportions, and Specifications

Which ASTM C494 admixture type designates a water-reducing admixture?

  1. Type D
  2. Type B
  3. Type C
  4. Type A Correct answer.

Explanation

ASTM C494 Type A is the classification for a water-reducing admixture. Type B is retarding, Type C is accelerating, and Type D is a water-reducing and retarding admixture.

Question 27Concrete Materials, Mixture Proportions, and Specifications

Why is a well-graded aggregate blend generally preferred in concrete mixture proportioning?

  1. It eliminates the need for a water-reducing admixture.
  2. It increases the required cement content for a given strength.
  3. It reduces the void space to be filled with cement paste. Correct answer.
  4. It guarantees that the concrete will meet its air-content requirement.

Explanation

A well-graded blend contains a range of particle sizes, so smaller particles fill voids between larger ones. This reduces paste demand and improves economy while maintaining workability.

Question 28Concrete Materials, Mixture Proportions, and Specifications

A coarse aggregate sample has an oven-dry mass of 1,940 g and a saturated-surface-dry mass of 1,980 g. What is its absorption?

  1. 2.06% Correct answer.
  2. 1.02%
  3. 0.98%
  4. 2.00%

Explanation

Absorption is the increase in mass from oven-dry to saturated-surface-dry, expressed as a percentage of the oven-dry mass: [(1,980 − 1,940) ÷ 1,940] × 100 = 2.06%.

Question 29Concrete Materials, Mixture Proportions, and Specifications

A 1 yd³ mixture contains 560 lb of portland cement (specific gravity 3.15), 140 lb of fly ash (specific gravity 2.40), 270 lb of water, and 1,800 lb of coarse aggregate (bulk SSD specific gravity 2.65). Air content is 6.0% by volume. Using absolute volume and a fine aggregate bulk SSD specific gravity of 2.62, what mass of fine aggregate is required?

  1. 1,045 lb Correct answer.
  2. 1,120 lb
  3. 980 lb
  4. 890 lb

Explanation

The cement, fly ash, water, coarse aggregate, and air occupy about 20.62 ft³, leaving about 6.38 ft³ for fine aggregate. At a bulk SSD specific gravity of 2.62, that volume corresponds to 6.38 × 2.62 × 62.4 ≈ 1,044 lb.

Question 30Concrete Materials, Mixture Proportions, and Specifications

A coarse aggregate has a dry-rodded unit weight of 102 lb/ft³ and a bulk specific gravity (oven-dry) of 2.68. What is the approximate void content of the compacted aggregate?

  1. 39% Correct answer.
  2. 61%
  3. 33%
  4. 45%

Explanation

The solid volume fraction is the dry-rodded unit weight divided by the density of the solid particles: 102 ÷ (2.68 × 62.4) ≈ 0.61. The remaining fraction, about 0.39 or 39%, is void space.

Question 31Concrete Materials, Mixture Proportions, and Specifications

Which ASTM C150 cement type is intended for use where high early strength is required?

  1. Type II
  2. Type V
  3. Type III Correct answer.
  4. Type I

Explanation

ASTM C150 Type III is high-early-strength cement, produced with a finer grind that accelerates strength gain. Type I is general purpose, Type II provides moderate sulfate resistance, and Type V provides high sulfate resistance.

Question 32Concrete Materials, Mixture Proportions, and Specifications

Which ASTM specification establishes requirements for fine and coarse aggregates used in concrete?

  1. ASTM C494
  2. ASTM C260
  3. ASTM C33 Correct answer.
  4. ASTM C150

Explanation

ASTM C33 is the standard specification for concrete aggregates and addresses grading and quality requirements. ASTM C150 covers portland cement, C494 covers chemical admixtures, and C260 covers air-entraining admixtures.

Question 33Concrete Materials, Mixture Proportions, and Specifications

What is the primary purpose of an air-entraining admixture in concrete?

  1. Improve freeze-thaw durability Correct answer.
  2. Retard the initial setting time
  3. Reduce the required mixing water
  4. Accelerate early strength gain

Explanation

Air-entraining admixtures generate small, closely spaced bubbles that relieve pressure from freezing water and improve freeze-thaw durability. They are not intended primarily to accelerate strength, reduce water demand, or retard setting.

Question 34Concrete Materials, Mixture Proportions, and Specifications

A concrete mixture contains 620 lb of cementitious material and 310 lb of water per cubic yard. What is its water-to-cementitious materials ratio?

  1. 0.60
  2. 0.45
  3. 0.50 Correct answer.
  4. 0.55

Explanation

The water-to-cementitious materials ratio is the mass of water divided by the mass of cementitious material. Here, 310 lb divided by 620 lb equals 0.50.

Question 35Concrete Materials, Mixture Proportions, and Specifications

Which statement correctly distinguishes ASTM C618 Class F fly ash from Class C fly ash?

  1. Class C ash typically has lower CaO and requires added lime.
  2. Class F ash typically has higher CaO and is strongly self-cementing.
  3. Class C ash typically has higher LOI and is non-pozzolanic.
  4. Class F ash typically has lower CaO and is not self-cementing. Correct answer.

Explanation

Class F fly ash generally has a lower calcium oxide content and is primarily pozzolanic, so it needs calcium hydroxide from cement or lime to react. Class C fly ash typically has higher calcium oxide and may exhibit some self-cementing behavior.

Question 36Concrete Materials, Mixture Proportions, and Specifications

Two coarse aggregates have the same grading and are tested in a saturated-surface-dry condition. Which particle property will most directly increase the water demand of a concrete mixture at a given slump?

  1. Angular, rough-textured particles Correct answer.
  2. High dry-rodded unit weight
  3. Low bulk specific gravity
  4. Rounded, smooth-textured particles

Explanation

At saturated-surface-dry condition, absorption is already satisfied, so particle shape and surface texture control how much water is needed for workability. Angular, rough-textured particles increase interparticle friction and generally require more water at a fixed slump than rounded, smooth particles.

Question 37Concrete Materials, Mixture Proportions, and Specifications

A concrete mixture has a specified compressive strength f'c of 4,000 psi, and the producer has no acceptable prior test record. According to ACI 318, what is the required average compressive strength f'cr for proportioning?

  1. 4,000 psi
  2. 4,800 psi
  3. 5,400 psi
  4. 5,200 psi Correct answer.

Explanation

For no acceptable prior test record and a specified strength between 3,000 and 5,000 psi, ACI 318 requires f'cr to exceed f'c by 1,200 psi. Adding 1,200 psi to 4,000 psi gives 5,200 psi.

Question 38Concrete Materials, Mixture Proportions, and Specifications

Polypropylene fibers are added at a low dosage to a concrete slab mixture. Which cracking problem are they primarily intended to reduce?

  1. Plastic shrinkage cracking Correct answer.
  2. Alkali-silica expansion
  3. Sulfate attack
  4. Freeze-thaw scaling

Explanation

Low-dosage synthetic fibers bridge microcracks while concrete is still plastic and are used mainly to reduce plastic shrinkage cracking. They do not provide the primary control for alkali-silica expansion, freeze-thaw scaling, or sulfate attack.

Question 39Concrete Materials, Mixture Proportions, and Specifications

A coarse aggregate source is classified as potentially alkali-silica reactive. Which mixture change is most appropriate to reduce the risk of damaging expansion?

  1. Increase mixing water to dilute the reactive silica.
  2. Increase cement alkali content to stabilize the reaction.
  3. Add calcium chloride to accelerate early strength.
  4. Use low-alkali cement or adequate Class F fly ash. Correct answer.

Explanation

Alkali-silica reaction requires reactive silica, available alkalis, and moisture; reducing alkalis or using sufficient supplementary cementitious material such as Class F fly ash is a standard mitigation strategy. Increasing alkalis or adding calcium chloride can worsen durability, and extra water does not prevent the reaction.

Question 40Concrete Materials, Mixture Proportions, and Specifications

A mixture has 270 lb of water per cubic yard and a water-to-cementitious materials ratio of 0.45. Fly ash replaces 25% of the cementitious material by mass. What are the cement and fly ash contents per cubic yard?

  1. 500 lb cement, 100 lb fly ash
  2. 480 lb cement, 120 lb fly ash
  3. 450 lb cement, 150 lb fly ash Correct answer.
  4. 400 lb cement, 200 lb fly ash

Explanation

Total cementitious material is 270 lb divided by 0.45, or 600 lb per cubic yard. Fly ash at 25% of 600 lb is 150 lb, leaving 450 lb of portland cement.

Question 41Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

When performing the slump test, concrete is placed in the cone in three layers and each layer is rodded 25 times. For the second and third layers, the tamping rod must penetrate into the layer below by approximately what amount?

  1. 3 in. (75 mm)
  2. 1 in. (25 mm) Correct answer.
  3. 2 in. (50 mm)
  4. 1/2 in. (13 mm)

Explanation

ASTM C143 requires three equal layers, with each layer rodded 25 times using the specified tamping rod. For the second and third layers the rod penetrates about 1 in. (25 mm) into the layer below so the layers are integrated. The bottom layer is rodded through its full depth.

Question 42Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

A concrete temperature is measured with a calibrated thermometer in accordance with ASTM C1064. To what precision should the final temperature be recorded?

  1. Nearest 0.1 °F
  2. Nearest 5 °F (2.5 °C)
  3. Nearest 1 °F (0.5 °C) Correct answer.
  4. Nearest 10 °F (5 °C)

Explanation

ASTM C1064 requires the thermometer reading to be recorded to the nearest 1 °F, which corresponds to 0.5 °C. The thermometer must be immersed at least 3 in. (75 mm) and left in the concrete until the reading stabilizes, for at least 2 minutes.

Question 43Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

When a sample of fresh concrete is taken from a transit mixer for acceptance testing, which portion of the discharge should be used?

  1. A single scoop from whichever part is most convenient
  2. A composite from the middle portion of the discharge Correct answer.
  3. The last portion of the discharge after the drum is empty
  4. The first 10% of the discharge

Explanation

ASTM C172 requires that the sample be taken after the first 10% and before the last 10% of the discharge, normally as a composite of portions from the middle of the load. Sampling only the first or last concrete can misrepresent the batch because of segregation or drum variation.

Question 44Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

A pressure air meter gives a gauge reading of 6.0% on a concrete mixture. The aggregate correction factor for this mixture is 0.8%. What air content should be reported?

  1. 6.0%, obtained by ignoring the correction factor
  2. 6.8%, obtained by adding the factor to the gauge
  3. 0.8%, obtained by reporting only the correction factor
  4. 5.2%, after subtracting the aggregate correction Correct answer.

Explanation

The pressure meter gauge responds to air in the concrete plus voids associated with the aggregate, so the aggregate correction factor is subtracted from the gauge reading. With a 6.0% gauge reading and a 0.8% correction, the reported air content is 5.2%.

Question 45Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

From the start of filling the slump cone to the completion of the test, the entire slump test should be completed within what time?

  1. 10 minutes
  2. 5 minutes
  3. 15 minutes
  4. 2 1/2 minutes Correct answer.

Explanation

ASTM C143 requires the slump test to be completed within 2 1/2 minutes from the start of filling the cone. Delays allow the concrete to stiffen or lose moisture, which changes the measured slump and makes the result unrepresentative.

Question 46Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

A unit weight test is performed using a 0.25 ft³ (7 L) measure. How should the concrete be consolidated in the measure?

  1. A single layer rodded 25 times
  2. Three layers, each rodded 25 times
  3. Three layers, each rodded 50 times
  4. Two layers, each rodded 25 times Correct answer.

Explanation

ASTM C138 specifies two layers of 25 strokes each for the 0.25 ft³ (7 L) measure and for the 0.1 ft³ (3 L) measure. The larger 0.5 ft³ (14 L) measure requires three layers of 25 strokes.

Question 47Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

In the volumetric air content test (ASTM C173), isopropyl alcohol is added to the water in the top of the meter. What is the purpose of the alcohol?

  1. To react with cement and release dissolved air
  2. To prevent the concrete from freezing during the test
  3. To accelerate the settling of coarse aggregate
  4. To reduce surface tension so bubbles coalesce Correct answer.

Explanation

In ASTM C173, isopropyl alcohol is added to break the foam and reduce surface tension so entrained air bubbles merge and a stable liquid level forms. The meter is then rolled and the air content is read from the graduated neck.

Question 48Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

A 0.25 ft³ unit weight measure weighs 9.8 lb empty and 46.1 lb when filled with consolidated concrete. What is the unit weight of the concrete?

  1. 139 lb/ft³
  2. 145 lb/ft³ Correct answer.
  3. 36.3 lb/ft³
  4. 126 lb/ft³

Explanation

Subtract the empty measure mass: 46.1 lb minus 9.8 lb equals 36.3 lb of concrete. Divide by the measure volume: 36.3 lb divided by 0.25 ft³ equals 145.2 lb/ft³, reported as about 145 lb/ft³.

Question 49Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

A slump test is performed and, after the cone is lifted, the concrete shears noticeably to one side, leaving a markedly asymmetric mass. What is the appropriate action under ASTM C143?

  1. Discard the result and retest a new portion Correct answer.
  2. Raise the cone higher to complete the test
  3. Average the high and low sides and report that height
  4. Record the height and note a sheared slump

Explanation

ASTM C143 treats a shear or collapse slump as an invalid true-slump result because the mass has not deformed uniformly. The test should be discarded and repeated on a new portion of the sample; if it shears again, the slump cannot be reported by this method.

Question 50Sampling and Fresh Concrete Testing (slump, air content, temperature, unit weight)

A concrete has a measured unit weight of 147.0 lb/ft³. The same mixture has an air-free theoretical unit weight of 152.0 lb/ft³. What is the approximate air content by volume?

  1. 5.0%
  2. 7.5%
  3. 1.7%
  4. 3.3% Correct answer.

Explanation

The air-free theoretical unit weight represents the same concrete with zero air. The unit weight deficit is 152.0 minus 147.0, or 5.0 lb/ft³, and 5.0 divided by 152.0 times 100 gives about 3.3% air by volume. This density-based estimate is approximate and is not a substitute for a direct air content test.