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Question 1Concrete Materials and Mixture Proportions

Which cement compound is primarily responsible for the early strength gain of portland cement concrete?

  1. Tricalcium silicate (C3S) Correct answer.
  2. Tetracalcium aluminoferrite (C4AF)
  3. Calcium hydroxide (CH)
  4. Dicalcium silicate (C2S)

Explanation

Tricalcium silicate hydrates rapidly and produces most of the strength developed in the first weeks. Dicalcium silicate contributes mainly to later-age strength, while C4AF has little effect on strength development.

Question 2Concrete Materials and Mixture Proportions

According to ACI 211 mixture proportioning, what is the primary purpose of the water-cementitious materials ratio (w/cm)?

  1. It determines the air content of the mixture
  2. It establishes the relationship between strength and durability Correct answer.
  3. It fixes the coarse aggregate volume fraction
  4. It controls workability independent of strength

Explanation

The w/cm ratio is the principal parameter linking compressive strength and durability to the paste quality. Lower w/cm generally increases strength and reduces permeability.

Question 3Concrete Materials and Mixture Proportions

A concrete mixture has a total water content of 300 lb/yd³ and a cementitious materials content of 600 lb/yd³. What is the w/cm ratio?

  1. 0.40
  2. 0.75
  3. 0.60
  4. 0.50 Correct answer.

Explanation

The w/cm ratio is total water divided by total cementitious materials. Dividing 300 lb/yd³ by 600 lb/yd³ gives 0.50.

Question 4Concrete Materials and Mixture Proportions

Which aggregate property has the greatest influence on the water demand of a concrete mixture?

  1. Abrasion resistance
  2. Nominal maximum size Correct answer.
  3. Soundness
  4. Bulk specific gravity

Explanation

Larger nominal maximum aggregate size reduces the paste required to coat particles and lowers water demand for a given workability. Specific gravity, soundness, and abrasion resistance do not govern water demand.

Question 5Concrete Materials and Mixture Proportions

Air entrainment is added to concrete primarily to improve resistance to which mechanism?

  1. Chloride-induced corrosion
  2. Sulfate attack
  3. Freeze-thaw damage Correct answer.
  4. Alkali-silica reaction

Explanation

Entrained air provides closely spaced voids that relieve hydraulic pressure from freezing water in the paste. This protects the paste from freeze-thaw deterioration.

Question 6Concrete Materials and Mixture Proportions

In ACI 211.1, the recommended slump for a concrete mixture is used to establish which quantity?

  1. The required mixing water content Correct answer.
  2. The entrained air percentage
  3. The coarse aggregate bulk volume
  4. The fine aggregate fineness modulus

Explanation

The designer selects a slump for the intended placement, then reads the approximate mixing water requirement from the table for the chosen aggregate size and air content. Slump is not used to set aggregate volumes or air content directly.

Question 7Concrete Materials and Mixture Proportions

Which of the following best describes the role of supplementary cementitious materials such as fly ash in concrete?

  1. They act as inert filler that reduces paste volume
  2. They eliminate the need for water reduction
  3. They accelerate set and increase early strength
  4. They react pozzolanically to form additional calcium silicate hydrate Correct answer.

Explanation

Fly ash reacts with calcium hydroxide from cement hydration to form additional calcium silicate hydrate, improving strength at later ages and reducing permeability. It is not inert and generally slows early strength gain.

Question 8Concrete Materials and Mixture Proportions

What does a fine aggregate fineness modulus of 2.40 indicate compared with one of 3.00?

  1. The 2.40 sand is finer and has more surface area Correct answer.
  2. The 2.40 sand is coarser and needs more water
  3. The 2.40 sand contains more entrained air
  4. The 2.40 sand has a higher specific gravity

Explanation

A lower fineness modulus indicates a finer grading with greater specific surface area, which generally raises water demand. A higher fineness modulus corresponds to a coarser sand.

Question 9Concrete Materials and Mixture Proportions

Which factor most directly determines the required cementitious materials content in an ACI 211 mixture design?

  1. The curing method to be used
  2. The selected w/cm ratio and water content Correct answer.
  3. The fineness modulus of the coarse aggregate
  4. The color of the finished concrete

Explanation

Once water content and w/cm are fixed, cementitious content follows by division. Aggregate grading and finishing considerations do not determine the cementitious content directly.

Question 10Concrete Materials and Mixture Proportions

A mixture designed at a w/cm of 0.45 must be adjusted to 0.40 while keeping the water content constant at 280 lb/yd³. What cementitious content is required?

  1. 630 lb/yd³
  2. 700 lb/yd³ Correct answer.
  3. 745 lb/yd³
  4. 560 lb/yd³

Explanation

Cementitious content equals water divided by w/cm. Dividing 280 lb/yd³ by 0.40 gives 700 lb/yd³.

Question 11Concrete Materials and Mixture Proportions

Why does ACI 211 limit the nominal maximum size of coarse aggregate relative to section dimensions and reinforcement spacing?

  1. To guarantee a minimum slump
  2. To reduce the cement fineness requirement
  3. To increase the entrained air content
  4. To prevent segregation and bridging of aggregate between bars Correct answer.

Explanation

Oversized aggregate can bridge between reinforcing bars and cause segregation or honeycombing in congested sections. Limits on nominal maximum size relative to clear spacing and section depth prevent this.

Question 12Concrete Materials and Mixture Proportions

Which condition would most likely require a higher entrained air content in an ACI 318 exposure classification?

  1. Buried foundation concrete with no freezing
  2. Severe freeze-thaw exposure with deicing chemicals Correct answer.
  3. Mild interior exposure with no moisture
  4. Mass concrete with low heat generation

Explanation

Concrete exposed to freezing and thawing in the presence of deicing chemicals requires the highest air contents to resist scaling and internal damage. Milder exposures require less or no air.

Question 13Concrete Materials and Mixture Proportions

What is the effect of increasing the coarse aggregate volume fraction while keeping cement paste volume constant?

  1. Workability decreases because of less mortar
  2. Air content is eliminated
  3. Water demand increases because of more surface area
  4. Paste demand decreases and the mixture becomes more economical Correct answer.

Explanation

Coarse aggregate occupies more volume at lower paste demand than fine aggregate, so increasing its fraction reduces the paste required for a given workability. This lowers cementitious content and cost.

Question 14Concrete Materials and Mixture Proportions

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

  1. It increases permeability of the hardened concrete
  2. It eliminates the need for air entrainment
  3. It requires water reducers because of its high surface area Correct answer.
  4. It is used at high dosages as the primary cementitious material

Explanation

Silica fume particles are extremely fine, greatly increasing water demand, so high-range water reducers are typically used. It is added at low dosages and reduces permeability.

Question 15Concrete Materials and Mixture Proportions

In ACI 211.1, after the water and cementitious contents are fixed, how is the fine aggregate content determined?

  1. By taking 50 percent of the total aggregate mass
  2. By assuming it equals the coarse aggregate mass
  3. By the absolute volume method using the remaining volume Correct answer.
  4. By multiplying the cement content by the fineness modulus

Explanation

The absolute volume method sums the volumes of water, cementitious materials, coarse aggregate, and air, then assigns the remaining volume to fine aggregate. This ensures the batch yields one cubic yard.

Question 16Concrete Materials and Mixture Proportions

What is the main reason for limiting the total alkali content of concrete ingredients when a reactive aggregate is used?

  1. To prevent alkali-silica reaction gel expansion Correct answer.
  2. To improve the entrained air stability
  3. To reduce the water demand
  4. To increase the compressive strength

Explanation

Alkali-silica reaction occurs when alkalis in the pore solution react with reactive silica in the aggregate, forming expansive gel. Limiting alkali content or using supplementary cementitious materials mitigates expansion.

Question 17Concrete Materials and Mixture Proportions

A concrete mixture contains 564 lb/yd³ of cement, 94 lb/yd³ of fly ash, and 300 lb/yd³ of water. What is the w/cm ratio?

  1. 0.53
  2. 0.60
  3. 0.55
  4. 0.46 Correct answer.

Explanation

Total cementitious material is 564 plus 94, or 658 lb/yd³. Dividing 300 by 658 gives approximately 0.456, which rounds to 0.46.

Question 18Concrete Materials and Mixture Proportions

Which aggregate characteristic is measured by the Los Angeles abrasion test?

  1. Volume stability under wetting and drying
  2. Resistance to degradation by impact and abrasion Correct answer.
  3. Amount of clay coating on particle surfaces
  4. Potential for alkali-silica reaction

Explanation

The Los Angeles abrasion test subjects aggregate to impact and grinding in a rotating drum and reports mass loss. It indicates resistance to wear and degradation during handling and service.

Question 19Concrete Materials and Mixture Proportions

What is the purpose of a water reducer (Type A admixture) in a concrete mixture?

  1. To entrain air for freeze-thaw resistance
  2. To reduce the water content required for a given slump Correct answer.
  3. To retard hydration in hot weather
  4. To accelerate the setting time

Explanation

Water reducers disperse cement particles, allowing the same slump with less water, which lowers w/cm and increases strength. Accelerators, air entrainers, and retarders serve different functions.

Question 20Concrete Materials and Mixture Proportions

Two sands have fineness moduli of 2.30 and 3.10. If all else is equal, which statement is most accurate?

  1. The 3.10 sand will require more water because it is coarser
  2. Both sands will produce identical water demand
  3. The 2.30 sand will likely require more water and may need a higher paste content Correct answer.
  4. The 2.30 sand will reduce the air content requirement

Explanation

Finer sand has more surface area per unit mass, increasing the water needed to coat particles and typically requiring more paste. Coarser sand generally reduces water demand.

Question 21Concrete Materials and Mixture Proportions

A trial batch shows excessive bleeding and segregation. Which adjustment is most appropriate?

  1. Reduce the cementitious content and increase slump
  2. Add more coarse aggregate while reducing fines
  3. Increase the coarse aggregate content and reduce water Correct answer.
  4. Increase the water content to improve cohesion

Explanation

Bleeding and segregation often result from excess water and insufficient fine material or paste cohesion. Reducing water and adjusting aggregate proportions improves stability.

Question 22Concrete Materials and Mixture Proportions

In ACI 211.1, the bulk volume of oven-dry rodded coarse aggregate per unit volume of concrete is selected based on which pair of parameters?

  1. w/cm ratio and cement type
  2. Nominal maximum size and fineness modulus of fine aggregate Correct answer.
  3. Specific gravity and absorption
  4. Slump and air content

Explanation

The coarse aggregate bulk volume table is entered with the nominal maximum size of coarse aggregate and the fineness modulus of the fine aggregate. These two inputs capture grading effects on packing.

Question 23Concrete Materials and Mixture Proportions

A mixture has a specified w/cm of 0.42 and a total cementitious content of 700 lb/yd³. If 25 percent of the cementitious material is replaced by fly ash, what is the maximum allowable water content?

  1. 336 lb/yd³
  2. 294 lb/yd³ Correct answer.
  3. 252 lb/yd³
  4. 280 lb/yd³

Explanation

The w/cm ratio applies to total cementitious material, which remains 700 lb/yd³ after replacement. Multiplying 700 by 0.42 gives 294 lb/yd³ of allowable water.

Question 24Concrete Materials and Mixture Proportions

Which situation best justifies using a higher-range water reducer rather than a conventional water reducer?

  1. A mixture needing accelerated set in cold weather
  2. A mixture needing extended set time for hot weather
  3. A mixture requiring entrained air for freeze-thaw
  4. A low w/cm mixture requiring high slump with minimal added water Correct answer.

Explanation

High-range water reducers provide large water reductions and improved flow, enabling low w/cm concrete to be placed at high slump. Accelerators, air entrainers, and retarders address other needs.

Question 25Concrete Materials and Mixture Proportions

A concrete mixture is proportioned with 650 lb/yd³ of cementitious material, 270 lb/yd³ of water, and 5 percent entrained air. Which change would most likely reduce permeability without changing slump?

  1. Lowering w/cm by reducing water and adding cementitious material with a water reducer Correct answer.
  2. Increasing the water content to improve consolidation
  3. Increasing the coarse aggregate nominal maximum size only
  4. Reducing the cementitious content to lower heat

Explanation

Lowering w/cm while maintaining slump reduces capillary porosity and permeability. Water reducers allow this without sacrificing workability.

Question 26Concrete Materials and Mixture Proportions

Which property of a Portland cement is primarily responsible for its classification as Type I, Type II, Type III, Type IV, or Type V?

  1. Its compound composition and resulting heat of hydration and sulfate resistance Correct answer.
  2. Its compressive strength at 28 days when tested in standard mortar cubes
  3. Its fineness as measured by the Blaine air permeability test
  4. Its setting time as determined by the Vicat needle test

Explanation

ASTM C150 classifies Portland cement types based on chemical and physical requirements that reflect the compound composition (C3S, C2S, C3A, C4AF). These compounds control properties such as heat of hydration and sulfate resistance, which are the basis for the type designations. Fineness, setting time, and strength are required properties but do not define the type.

Question 27Concrete Materials and Mixture Proportions

According to ACI 211.1, the bulk volume of coarse aggregate per unit volume of concrete is selected primarily based on which two parameters?

  1. The cementitious materials content and the coarse aggregate dry-rodded unit weight
  2. The nominal maximum size of the coarse aggregate and the fineness modulus of the fine aggregate Correct answer.
  3. The air content and the slump of the concrete mixture
  4. The fineness modulus of the fine aggregate and the water-cementitious materials ratio

Explanation

ACI 211.1 provides a table for the bulk volume of oven-dry rodded coarse aggregate per unit volume of concrete. The table is entered with the nominal maximum size of the coarse aggregate and the fineness modulus of the fine aggregate. These two parameters account for aggregate grading and packing effects on the required coarse aggregate volume.

Question 28Concrete Materials and Mixture Proportions

A concrete mixture has a water content of 300 lb/yd³ and a cementitious materials content of 600 lb/yd³. What is the water-cementitious materials ratio?

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

Explanation

The water-cementitious materials ratio is calculated by dividing the mass of water by the total mass of cementitious materials. Here, 300 lb/yd³ divided by 600 lb/yd³ equals 0.50. This ratio is a key parameter affecting strength and durability.

Question 29Concrete Materials and Mixture Proportions

Which of the following best describes the function of an air-entraining admixture in concrete?

  1. It accelerates the setting time of the cement paste.
  2. It creates a system of small, discrete air bubbles that improves resistance to freezing and thawing. Correct answer.
  3. It increases the compressive strength by reducing the void content.
  4. It reduces the water demand for a given slump.

Explanation

Air-entraining admixtures intentionally introduce microscopic air bubbles into the concrete. These bubbles provide space for water to expand upon freezing, thereby improving freeze-thaw durability. They do not accelerate setting, reduce water demand (they may increase it slightly), or increase strength (they typically reduce strength).

Question 30Concrete Materials and Mixture Proportions

A concrete mixture is proportioned with 650 lb/yd³ of cementitious material, 270 lb/yd³ of water, and 5 percent entrained air. What is the approximate cementitious materials content per cubic yard if the air content is increased to 7 percent while keeping the water content constant and the water-cementitious materials ratio unchanged?

  1. 690 lb/yd³
  2. 650 lb/yd³
  3. 670 lb/yd³
  4. 630 lb/yd³ Correct answer.

Explanation

Increasing air content reduces the volume available for other ingredients, so the cementitious content must decrease to maintain the same water-cementitious materials ratio and water content. A 2 percent increase in air (from 5 to 7 percent) reduces the cementitious content by approximately 2 percent of the total volume, which is about 20 lb/yd³. Thus, 650 lb/yd³ minus about 20 lb/yd³ gives approximately 630 lb/yd³.

Question 31Concrete Materials and Mixture Proportions

Which of the following is a primary reason for using supplementary cementitious materials (SCMs) such as fly ash in concrete?

  1. To eliminate the need for chemical admixtures
  2. To reduce the heat of hydration and improve long-term durability Correct answer.
  3. To decrease the setting time of concrete
  4. To increase the early-age strength of concrete

Explanation

SCMs like fly ash react pozzolanically and can reduce the heat of hydration, especially at early ages, and improve long-term durability by refining the pore structure and reducing permeability. They generally do not increase early-age strength (often decrease it) and do not eliminate the need for admixtures. They typically retard setting rather than decrease it.

Question 32Concrete Materials and Mixture Proportions

A fine aggregate has a fineness modulus of 2.40. How does this value compare with a fine aggregate having a fineness modulus of 3.00?

  1. The 2.40 sand is coarser and will require more water for a given slump.
  2. The 2.40 sand has a larger nominal maximum size.
  3. The 2.40 sand is finer and will typically require a higher water demand. Correct answer.
  4. The 2.40 sand will produce a higher compressive strength at the same water-cementitious materials ratio.

Explanation

Fineness modulus is an index of the fineness of aggregate; a lower value indicates a finer grading. Finer sands have more surface area and typically increase the water demand of a concrete mixture. They do not have a nominal maximum size (that is a coarse aggregate property) and do not inherently increase strength.

Question 33Concrete Materials and Mixture Proportions

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

  1. It is used primarily to accelerate setting time.
  2. It is a crystalline material that acts as a filler.
  3. It significantly increases the water demand of the mixture.
  4. It is effective in reducing permeability and improving durability. Correct answer.

Explanation

Silica fume is an amorphous by-product of silicon metal production that is highly pozzolanic. It reacts with calcium hydroxide to form additional calcium silicate hydrate, refining the pore structure and significantly reducing permeability, which improves durability. It typically increases water demand, is not crystalline, and is not used primarily as a set accelerator.

Question 34Concrete Materials and Mixture Proportions

A concrete mixture contains 564 lb/yd³ of cement, 94 lb/yd³ of fly ash, and 300 lb/yd³ of water. What is the water-cementitious materials ratio?

  1. 0.46 Correct answer.
  2. 0.53
  3. 0.48
  4. 0.50

Explanation

The total cementitious materials content is 564 + 94 = 658 lb/yd³. Dividing the water content by this total gives 300 / 658 ≈ 0.456, which rounds to 0.46. The water-cementitious materials ratio includes all cementitious materials, not just Portland cement.

Question 35Concrete Materials and Mixture Proportions

A concrete mixture has a specified w/cm of 0.42 and a total cementitious content of 700 lb/yd³. If 25 percent of the cementitious material is replaced by fly ash, what is the approximate mass of fly ash per cubic yard?

  1. 150 lb/yd³
  2. 200 lb/yd³
  3. 175 lb/yd³ Correct answer.
  4. 125 lb/yd³

Explanation

The fly ash mass is 25 percent of the total cementitious content: 0.25 × 700 = 175 lb/yd³. The water content would be 0.42 × 700 = 294 lb/yd³, but that is not asked. The replacement percentage applies to the total cementitious materials.

Question 36Concrete Materials and Mixture Proportions

Which of the following best defines the term 'fineness modulus' of an aggregate?

  1. The percentage of material passing the No. 200 sieve
  2. The cumulative percentage retained on a standard series of sieves, divided by 100 Correct answer.
  3. The total percentage of material passing each of a standard series of sieves, divided by 100
  4. The mass of aggregate per unit volume, including voids

Explanation

The fineness modulus is calculated by summing the cumulative percentages retained on a specified series of sieves (typically 6 in. down to No. 100) and dividing by 100. It is an index of coarseness, not a measure of gradation, density, or fines content.

Question 37Concrete Materials and Mixture Proportions

In concrete mixture proportioning, what is the primary role of the water-to-cementitious materials (w/cm) ratio?

  1. It controls the compressive strength and durability of hardened concrete. Correct answer.
  2. It sets the total volume of paste in the mixture.
  3. It establishes the air content needed for freeze-thaw resistance.
  4. It determines the workability of fresh concrete.

Explanation

The w/cm ratio is the key parameter affecting strength and durability because it determines the porosity of the cement paste. Lower w/cm ratios generally lead to higher strength and lower permeability, which improves durability.

Question 38Concrete Materials and Mixture Proportions

A concrete mixture has a total cementitious materials content of 600 lb/yd³ and a water content of 270 lb/yd³. What is the water-to-cementitious materials ratio?

  1. 0.45 Correct answer.
  2. 0.50
  3. 0.40
  4. 0.55

Explanation

The w/cm ratio is calculated as the mass of water divided by the total mass of cementitious materials. Here, 270 lb/yd³ ÷ 600 lb/yd³ = 0.45.

Question 39Concrete Materials and Mixture Proportions

Which of the following statements accurately describes the effect of using a supplementary cementitious material (SCM) such as fly ash on the properties of concrete?

  1. Fly ash increases the early-age strength of concrete compared to plain Portland cement concrete.
  2. Fly ash reduces the permeability of concrete and improves its resistance to sulfate attack. Correct answer.
  3. Fly ash increases the heat of hydration and the risk of thermal cracking.
  4. Fly ash eliminates the need for air entrainment in freeze-thaw environments.

Explanation

Fly ash reacts pozzolanically with calcium hydroxide to form additional calcium silicate hydrate, which refines the pore structure and reduces permeability. This improves durability, including resistance to sulfate attack, though early-age strength may be lower.

Question 40Concrete Materials and Mixture Proportions

An aggregate blend is composed of 60% coarse aggregate (bulk specific gravity 2.70) and 40% fine aggregate (bulk specific gravity 2.65). What is the bulk specific gravity of the combined aggregate?

  1. 2.68 Correct answer.
  2. 2.69
  3. 2.66
  4. 2.67

Explanation

The bulk specific gravity of a blend is calculated as the weighted harmonic mean: 1 / (0.60/2.70 + 0.40/2.65) = 1 / (0.2222 + 0.1509) = 1 / 0.3731 = 2.68. Using a simple weighted arithmetic mean would be incorrect because specific gravity is a ratio of density to water density and volumes are additive, not masses.

Question 41Concrete Batching, Mixing, and Delivery

Under ACI 318/301 practice, what is the primary purpose of the specified air content in a batch of concrete that will be exposed to freezing and thawing?

  1. To accelerate the rate of cement hydration during cold weather
  2. To increase the compressive strength by reducing the paste volume
  3. To provide a network of closely spaced voids that relieves hydraulic pressure from freezing water Correct answer.
  4. To reduce the slump loss during long-haul delivery

Explanation

Entrained air creates a system of small, closely spaced bubbles that gives freezing water a place to expand, relieving internal hydraulic pressure and improving freeze-thaw durability. Air entrainment generally reduces strength slightly because it lowers the cementitious paste fraction per unit volume.

Question 42Concrete Batching, Mixing, and Delivery

A ready-mix producer must measure the water added at the jobsite during a slump adjustment. Which record-keeping practice does ACI 301 require for such water additions?

  1. The added water must be recorded only when it exceeds 40 L per truck
  2. The added water may be estimated by the truck driver and need not be recorded
  3. The added water must be measured and recorded, and the total water-cementitious ratio must not exceed the specified limit Correct answer.
  4. The added water is recorded only if the concrete is designated for a structural element

Explanation

ACI 301 limits the total water-cementitious materials ratio including water added at the point of placement, so any water added must be measured and documented. The field addition cannot push the mix above the specified w/cm.

Question 43Concrete Batching, Mixing, and Delivery

A batch is designed with 320 kg of cementitious material and 160 kg of water at the plant. If 15 kg of water is added at the jobsite with no other changes, what is the resulting water-cementitious materials ratio?

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

Explanation

The total water becomes 160 + 15 = 175 kg, and the w/cm is 175/320 = 0.547, approximately 0.55. All water added at any point in the process counts toward the ratio.

Question 44Concrete Batching, Mixing, and Delivery

Which statement best describes the role of the slump test in controlling batching and delivery?

  1. It is an index of consistency used to judge whether the batch is workable and within the approved limits Correct answer.
  2. It determines the air content of the delivered concrete
  3. It measures the compressive strength of the plastic mix directly
  4. It measures the setting time of the cementitious system

Explanation

The slump test is a consistency (workability) index, not a strength or air test. It is used to confirm the batch is within the approved range and to decide whether adjustments are needed.

Question 45Concrete Batching, Mixing, and Delivery

A concrete truck arrives at the site with a batch that has been in the drum for 75 minutes and shows a slump of 60 mm against a specified 100 mm. The approved plan allows one water addition to reach the target slump. What is the most appropriate action?

  1. Discharge the load and note the low slump in the record only
  2. Reject the load because it is more than 60 minutes old
  3. Add cement to stiffen the mix
  4. Add water, remix, and verify slump without exceeding the maximum w/cm Correct answer.

Explanation

When the mix is within its allowable time and the plan permits a single water addition, water may be added to reach the target slump provided the maximum w/cm is not exceeded. The batch must be remixed and the slump re-verified.

Question 46Concrete Batching, Mixing, and Delivery

According to ACI 301, when the ambient temperature is high and the concrete is being placed, what is the typical maximum concrete temperature limit at the point of placement unless otherwise specified?

  1. 24 °C
  2. 40 °C
  3. 32 °C
  4. 35 °C Correct answer.

Explanation

ACI 301 specifies a maximum concrete temperature of about 35 °C at the point of placement for hot-weather concreting unless the specifier sets a different limit. Higher temperatures accelerate slump loss and can reduce strength and durability.

Question 47Concrete Batching, Mixing, and Delivery

Which batching practice is required to ensure that the water content of a batch is accurate?

  1. Water is estimated from the moisture content of the coarse aggregate only
  2. Water is added until the mix reaches the desired slump by visual judgment
  3. Water must be measured by weight or by a calibrated volumetric meter within the specified tolerance Correct answer.
  4. Water may be added by volume without measurement if the operator is experienced

Explanation

Accurate batching requires that water be measured by weight or with a calibrated volumetric device within the tolerances of the batching specification. Visual judgment or unmeasured additions prevent control of the w/cm.

Question 48Concrete Batching, Mixing, and Delivery

A mix design calls for 1100 kg of coarse aggregate in a saturated-surface-dry condition. The aggregate stockpile has a moisture content of 3.0% and an absorption of 1.0%. What is the correct batch weight of the coarse aggregate?

  1. 1133 kg
  2. 1100 kg
  3. 1122 kg Correct answer.
  4. 1078 kg

Explanation

The free moisture above SSD is 3.0% − 1.0% = 2.0%, so the batch weight is 1100 × 1.02 = 1122 kg. The absorbed water is part of the SSD mass and is not added again.

Question 49Concrete Batching, Mixing, and Delivery

Which of the following best describes the purpose of the trial batch in establishing a concrete mixture?

  1. To verify that the proposed proportions produce the required fresh and hardened properties before full production Correct answer.
  2. To establish the formwork removal time
  3. To determine the delivery schedule for the project
  4. To select the color of the finished concrete

Explanation

Trial batches confirm that the proposed proportions yield the specified slump, air content, strength, and other properties under actual materials. Adjustments are then made to the production mix as needed.

Question 50Concrete Batching, Mixing, and Delivery

During batching, the fine aggregate moisture is measured at 5.0% while the absorption is 1.5%. How should the batch water be adjusted relative to the SSD design water?

  1. Increase the batch water by the free moisture content of the fine aggregate
  2. Decrease the batch water by the free moisture content of the fine aggregate Correct answer.
  3. Make no adjustment because moisture does not affect batch water
  4. Decrease the batch water by the total moisture content of the fine aggregate

Explanation

Free moisture (5.0% − 1.5% = 3.5%) is water already present in the aggregate, so the batch water must be reduced by that amount to maintain the design w/cm. Absorbed water is part of the SSD mass and does not require adjustment.