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ACI MFTT practice questions

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Question 1Masonry materials, units, mortar, and grout sampling

According to ASTM C90, what is the minimum net-area compressive strength required for loadbearing concrete masonry units?

  1. 1,900 psi (13.1 MPa) Correct answer.
  2. 3,000 psi (20.7 MPa)
  3. 1,500 psi (10.3 MPa)
  4. 2,500 psi (17.2 MPa)

Explanation

ASTM C90 sets the minimum net-area compressive strength for loadbearing concrete masonry units at 1,900 psi (13.1 MPa). The strength is computed using the net cross-sectional area, not the gross area. Units intended for nonloadbearing use are covered by a different standard and are not required to meet this value.

Question 2Masonry materials, units, mortar, and grout sampling

Which mortar type has the highest compressive strength?

  1. Type M Correct answer.
  2. Type N
  3. Type S
  4. Type O

Explanation

Type M mortar has the highest cement content and the highest specified compressive strength of the common mortar types. Type S is high-strength but lower than Type M, while Types N and O provide progressively lower strength. The type designation generally follows the order of decreasing cement and increasing lime content.

Question 3Masonry materials, units, mortar, and grout sampling

ASTM C476 requires grout used in masonry to have a minimum 28-day compressive strength of:

  1. 3,000 psi
  2. 1,500 psi
  3. 2,000 psi Correct answer.
  4. 2,500 psi

Explanation

ASTM C476 establishes a minimum 28-day compressive strength of 2,000 psi for grout used in masonry construction. This minimum applies to the grout mixture proportioned for the project. Higher strengths may be specified for particular structural designs.

Question 4Masonry materials, units, mortar, and grout sampling

The standard thickness for mortar bed joints in typical concrete masonry construction is:

  1. 5/8 in.
  2. 1/2 in.
  3. 3/8 in. Correct answer.
  4. 1/4 in.

Explanation

Concrete masonry walls commonly use a nominal 3/8 in. mortar bed joint. The masonry code permits a range, generally from 1/4 in. to 5/8 in., but 3/8 in. is the standard detailing thickness. Joint thickness affects mortar coverage, bond, and compressive behavior.

Question 5Masonry materials, units, mortar, and grout sampling

A standard concrete masonry unit has nominal face dimensions of:

  1. 8 in. by 8 in. by 16 in. Correct answer.
  2. 6 in. by 8 in. by 16 in.
  3. 4 in. by 8 in. by 12 in.
  4. 8 in. by 12 in. by 16 in.

Explanation

The standard concrete masonry unit has nominal dimensions of 8 in. by 8 in. by 16 in. Its actual dimensions are reduced by the mortar joint thickness, typically about 7-5/8 in. by 7-5/8 in. by 15-5/8 in. The nominal size is used for modular coordination.

Question 6Masonry materials, units, mortar, and grout sampling

According to ASTM C476, grout slump at the time of placement should be:

  1. 8 to 11 in. Correct answer.
  2. 6 to 8 in.
  3. 4 to 6 in.
  4. 11 to 13 in.

Explanation

ASTM C476 requires grout slump at placement to be between 8 in. and 11 in. The high slump helps the grout flow around reinforcement and fill the grout space. Grout that is too stiff may not consolidate properly.

Question 7Masonry materials, units, mortar, and grout sampling

Fine grout is required to pass which sieve size?

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

Explanation

Fine grout is proportioned so that all aggregate passes the 1/2 in. sieve. Coarse grout contains aggregate passing the 1 in. sieve, with a substantial portion retained on the 1/2 in. sieve. The distinction affects flow and placement in narrow spaces.

Question 8Masonry materials, units, mortar, and grout sampling

Which mortar type is generally used for general above-grade exterior walls and has moderate compressive strength?

  1. Type N Correct answer.
  2. Type O
  3. Type K
  4. Type M

Explanation

Type N mortar is a medium-strength mortar widely used for general above-grade exterior and interior walls. It balances workability, bond, and compressive strength. Type M is stronger and more cementitious, while Type O is lower strength.

Question 9Masonry materials, units, mortar, and grout sampling

By volume, ASTM C270 Type S mortar consists of:

  1. 1 part portland cement, 1 part hydrated lime, 6 parts sand
  2. 1 part portland cement, 1/2 part lime, 4-1/2 parts sand Correct answer.
  3. 1 part portland cement, 1/4 part hydrated lime, 3 parts sand
  4. 1 part masonry cement, 3 parts sand

Explanation

ASTM C270 gives the Type S portland cement-lime mortar proportion as 1 part cement, 1/2 part hydrated lime, and 4-1/2 parts sand by volume. This ratio produces a high-strength mortar with good bond. Type S is often specified for structural masonry.

Question 10Masonry materials, units, mortar, and grout sampling

Under ASTM C780, field mortar samples should be taken at least once for every:

  1. 10,000 sq ft of wall
  2. 1,000 sq ft of wall
  3. 2,500 sq ft of wall
  4. 5,000 sq ft of wall Correct answer.

Explanation

ASTM C780 requires field mortar samples at least once for each 5,000 sq ft of wall area. Additional samples are taken when materials or proportions change or when results are questionable. The sample frequency supports verification of mortar quality.

Question 11Masonry materials, units, mortar, and grout sampling

A cone penetrometer is used in field testing of mortar to determine:

  1. compressive strength
  2. air content
  3. consistency Correct answer.
  4. water retention

Explanation

A cone penetrometer measures the depth that a standard cone penetrates a mortar sample, which indicates consistency. It is a field index test, not a direct measure of compressive strength. Penetration results help control water content and workability.

Question 12Masonry materials, units, mortar, and grout sampling

Mortar specimens for compressive strength are typically cast in molds that produce:

  1. 2-in. cubes Correct answer.
  2. 3-in. cubes
  3. 3-in. by 6-in. cylinders
  4. 2-in. by 4-in. cylinders

Explanation

Compressive strength of mortar is commonly determined using 2-in. cube specimens, as described in ASTM C109. Cubes are cast in molds, cured under specified conditions, and tested in compression. The small cube size is standard for mortar.

Question 13Masonry materials, units, mortar, and grout sampling

ASTM C1019 grout samples are required at least once for every:

  1. 10,000 sq ft of wall
  2. 2,500 sq ft of wall
  3. 5,000 sq ft of wall Correct answer.
  4. 20,000 sq ft of wall

Explanation

ASTM C1019 requires grout samples at least once for each 5,000 sq ft of wall area and at least once per project. The samples are used to verify the compressive strength of the grout. More frequent sampling may be required by the project specifications.

Question 14Masonry materials, units, mortar, and grout sampling

When calculating compressive strength of a hollow concrete masonry unit, the load is divided by:

  1. gross area
  2. net area Correct answer.
  3. bed joint area
  4. face area

Explanation

The compressive strength of a hollow concrete masonry unit is calculated by dividing the failure load by the net cross-sectional area. Net area excludes the void areas and reflects the solid material carrying load. Using gross area would understate the unit strength.

Question 15Masonry materials, units, mortar, and grout sampling

ASTM C140 requires a sample for compressive strength testing to consist of at least:

  1. six units
  2. ten units
  3. twenty units
  4. three units Correct answer.

Explanation

ASTM C140 sampling for compressive strength requires a sample of at least three units. The units are selected to represent the lot and are prepared and tested according to the standard. Additional units may be required for other tests.

Question 16Masonry materials, units, mortar, and grout sampling

ASTM C270 requires mortar to have a minimum water retention of:

  1. 60%
  2. 85%
  3. 70%
  4. 75% Correct answer.

Explanation

ASTM C270 requires mortar to retain at least 75% of its mixing water when tested for water retention. Adequate water retention helps the mortar remain workable and develop bond, especially when units are absorptive. Lower retention can lead to premature stiffening.

Question 17Masonry materials, units, mortar, and grout sampling

For portland cement-lime mortar, what is the maximum air content generally permitted by ASTM C270?

  1. 8%
  2. 22%
  3. 18%
  4. 12% Correct answer.

Explanation

ASTM C270 generally limits air content for portland cement-lime mortar to 12% by volume. Excessive air can reduce compressive strength and bond, although some entrained air improves workability. The limit is part of the property requirements for mortar.

Question 18Masonry materials, units, mortar, and grout sampling

Field-mixed mortar should be machine mixed for a minimum of:

  1. 3 to 5 minutes Correct answer.
  2. 8 to 10 minutes
  3. 1 to 2 minutes
  4. 12 to 15 minutes

Explanation

ASTM C270 requires machine mixing of mortar for 3 to 5 minutes after all ingredients are in the mixer. Proper mixing distributes cement, lime, and sand uniformly and develops workability. Overmixing or undermixing can affect consistency and strength.

Question 19Masonry materials, units, mortar, and grout sampling

Grout should be placed within what maximum time after mixing when conditions are normal?

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

Explanation

Under normal conditions, grout should be placed within 1-1/2 hours after mixing. Delays allow the grout to stiffen and lose flowability, which can cause voids. Retempering grout with water is not permitted after the time limit.

Question 20Masonry materials, units, mortar, and grout sampling

A masonry prism compressive strength is generally lower than the unit compressive strength because:

  1. grout adds no strength
  2. prisms are tested dry
  3. mortar joints are weak planes Correct answer.
  4. units are weaker in the prism

Explanation

A masonry prism is a composite assemblage of units, mortar, and sometimes grout, so its strength is influenced by the mortar joints and bond planes. These joints are generally weaker than the units and govern failure. Therefore prism strength is usually lower than the unit strength.

Question 21Masonry materials, units, mortar, and grout sampling

Type O mortar is best described as:

  1. low-strength mortar for interior walls Correct answer.
  2. moderate-strength mortar for general use
  3. high-strength mortar for severe exposure
  4. grout for reinforced masonry

Explanation

Type O mortar is a low-strength mortar, typically used for interior nonloadbearing walls. It has a high lime content and good workability but limited compressive strength. It should not be used where high strength or severe exposure is required.

Question 22Masonry materials, units, mortar, and grout sampling

Mortar sand should be:

  1. recycled concrete fines from demolition
  2. rounded particles with no angular faces
  3. well graded and free of harmful fines Correct answer.
  4. all particles passing a No. 4 sieve only

Explanation

Mortar sand should be well graded and free of harmful amounts of clay, silt, organic matter, and other deleterious substances. Proper gradation improves workability and reduces the water demand. Single-size sand or contaminated fines can impair mortar performance.

Question 23Masonry materials, units, mortar, and grout sampling

Coarse grout aggregate must pass which maximum size sieve?

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

Explanation

Coarse grout is proportioned with aggregate that passes the 1 in. sieve and has a substantial fraction retained on the 1/2 in. sieve. The 1 in. maximum size is consistent with ASTM C476. Fine grout, by contrast, passes the 1/2 in. sieve entirely.

Question 24Masonry materials, units, mortar, and grout sampling

ASTM C1019 grout compressive-strength specimens are typically:

  1. 3-in. by 6-in. cylinders
  2. 3 by 3 by 6 in. prisms Correct answer.
  3. standard 6-in. by 12-in. concrete cylinders
  4. 2-in. cubes

Explanation

ASTM C1019 uses grout specimens that are 3 in. by 3 in. in cross section and 6 in. long. These prisms are cast in molds and tested in compression to evaluate field grout strength. The specimen geometry is specific to grout testing.

Question 25Masonry materials, units, mortar, and grout sampling

Initial rate of absorption of masonry units is important because high absorption can:

  1. improve retention
  2. increase strength
  3. dewater the mortar Correct answer.
  4. reduce grout slump

Explanation

High initial rate of absorption means a masonry unit draws water rapidly from the fresh mortar. This dewatering stiffens the mortar and can reduce bond strength at the unit-mortar interface. Controlling unit absorption or prewetting can help mitigate the effect.

Question 26Masonry materials, units, mortar, and grout sampling

In typical masonry construction, the nominal thickness of mortar bed and head joints is:

  1. 5/8 in (16 mm)
  2. 1/4 in (6 mm)
  3. 1/2 in (13 mm)
  4. 3/8 in (10 mm) Correct answer.

Explanation

Standard practice calls for mortar joints about 3/8 in (10 mm) thick. Joints much thicker reduce compressive strength, while joints that are too thin make it difficult to achieve full bedding and bond.

Question 27Masonry materials, units, mortar, and grout sampling

Which mortar type is known for the highest compressive strength and is often selected for high-stress or below-grade masonry?

  1. Type N
  2. Type O
  3. Type S
  4. Type M Correct answer.

Explanation

Type M has the highest cementitious content among the common mortar types and develops the highest compressive strength. It is used where high compressive stress or severe exposure demands a strong mortar.

Question 28Masonry materials, units, mortar, and grout sampling

What is the main benefit of hydrated lime in portland cement-lime mortar?

  1. It accelerates the setting time of the mortar.
  2. It raises compressive strength above that of Type M mortar.
  3. It improves water retention and workability. Correct answer.
  4. It acts as a pigment to control mortar color.

Explanation

Hydrated lime plasticizes the mortar, helping it retain water and remain workable so it bonds well to absorptive units. It is not a strength-enhancing replacement for cement.

Question 29Masonry materials, units, mortar, and grout sampling

Mortar specimens for compressive strength are typically tested at an age of:

  1. 28 days Correct answer.
  2. 14 days
  3. 7 days
  4. 3 days

Explanation

Compressive-strength requirements for mortar are conventionally referenced to 28-day curing. Earlier tests may be used for information but do not establish the specified 28-day strength.

Question 30Masonry materials, units, mortar, and grout sampling

Which brick grade is intended for use where severe weathering, such as contact with soil or below-grade exposure, is expected?

  1. Grade MW
  2. Grade FBX
  3. Grade SW Correct answer.
  4. Grade NW

Explanation

ASTM C216 Grade SW denotes severe-weathering resistance. Grade MW is for moderate weathering, and Grade NW is for negligible weathering; FBX is a brick type designation, not a weathering grade.

Question 31Masonry materials, units, mortar, and grout sampling

The density threshold that separates normal-weight from medium-weight concrete masonry units is:

  1. 125 lb/ft³ (2000 kg/m³) Correct answer.
  2. 105 lb/ft³ (1680 kg/m³)
  3. 85 lb/ft³ (1360 kg/m³)
  4. 145 lb/ft³ (2320 kg/m³)

Explanation

ASTM C90 classification treats units with densities of 125 lb/ft³ or more as normal weight. Medium-weight units fall between 105 and less than 125 lb/ft³, and lightweight units are below 105 lb/ft³.

Question 32Masonry materials, units, mortar, and grout sampling

Why should sand for mortar be measured in a damp, loose condition and adjusted for moisture content?

  1. To make the sand particles rounder and easier to compact.
  2. To increase the entrained air content of the mortar.
  3. To reduce the amount of cement needed in the batch.
  4. To compensate for sand bulking in the measured volume. Correct answer.

Explanation

Moist sand bulks, meaning a given mass occupies more loose volume than when dry. Adjusting the batch for moisture keeps the intended cementitious-to-aggregate proportion and avoids a lean, weak mortar.

Question 33Masonry materials, units, mortar, and grout sampling

Why is grout mechanically vibrated or puddled during placement?

  1. To speed up the initial set of the cementitious material.
  2. To increase the grout's slump before it leaves the mixer.
  3. To reduce the amount of water needed in the grout mixture.
  4. To remove entrapped air and fill voids and cores. Correct answer.

Explanation

Consolidation drives out entrapped air and works grout into corners and small core spaces. Proper consolidation is essential for composite action between units and grout.

Question 34Masonry materials, units, mortar, and grout sampling

The flow table test for mortar is used primarily to determine:

  1. the consistency or flow of fresh mortar. Correct answer.
  2. the water retention of fresh mortar.
  3. the compressive strength of hardened mortar.
  4. the air content of fresh mortar.

Explanation

ASTM C1437 uses a jolting flow table to measure how much a mortar pat spreads, which reflects consistency and workability. It is not a strength, air-content, or water-retention test.

Question 35Masonry materials, units, mortar, and grout sampling

Which water is acceptable for mixing masonry mortar and grout?

  1. Any water that is clear, even if it contains dissolved salts.
  2. Clean, potable water free of harmful salts and organic matter. Correct answer.
  3. Recycled wash water without any testing or approval.
  4. Water with visible organic material if it is filtered through sand.

Explanation

Mixing water should be clean and potable or otherwise demonstrated not to contain harmful impurities. Salts and organic matter can interfere with setting, cause efflorescence, or reduce strength and durability.

Question 36Masonry materials, units, mortar, and grout sampling

ASTM C270 requires Type N mortar to reach a minimum 28-day compressive strength of:

  1. 750 psi (5.2 MPa) Correct answer.
  2. 2,500 psi (17.2 MPa)
  3. 1,800 psi (12.4 MPa)
  4. 350 psi (2.4 MPa)

Explanation

Type N mortar has a minimum 28-day compressive strength of 750 psi under ASTM C270. Type O is lower at 350 psi, while Type S and Type M are higher at 1,800 psi and 2,500 psi, respectively.

Question 37Masonry materials, units, mortar, and grout sampling

What property does the water retention test of fresh mortar measure?

  1. Mortar's ability to retain mixing water against suction. Correct answer.
  2. The total evaporable water content after mixing.
  3. The amount of bleed water that rises to the surface.
  4. The rate at which mortar absorbs water from masonry units.

Explanation

Water retention reflects how well mortar holds its mixing water when placed against absorptive masonry. Good retention keeps enough water available for cement hydration and bond development.

Question 38Masonry materials, units, mortar, and grout sampling

Mortar cubes cast for field compressive-strength testing are cured in a dry, unsealed room instead of moist conditions. Compared with properly moist-cured specimens, they will most likely have:

  1. lower strength because hydration is incomplete. Correct answer.
  2. higher strength because drying removes excess water.
  3. higher strength because dry curing accelerates cement reaction.
  4. identical strength because curing does not affect mortar.

Explanation

Cement hydration requires adequate moisture; early drying stops hydration and increases porosity. Dry-cured specimens therefore tend to show lower compressive strength than moist-cured specimens.

Question 39Masonry materials, units, mortar, and grout sampling

If mortar that has already begun to set is retempered with additional water and used, the hardened mortar will most likely:

  1. show no change because mortar is insensitive to retempering.
  2. lose strength and durability from a higher water-cement ratio. Correct answer.
  3. gain strength because the added water improves hydration.
  4. develop better bond because the mortar becomes more fluid.

Explanation

Retempering after setting disrupts the cement hydration that has begun and raises the effective water-cement ratio. The result is lower strength, reduced durability, and unreliable bond, which is why set mortar should be discarded.

Question 40Masonry materials, units, mortar, and grout sampling

A mortar batch contains too much sand relative to its cementitious materials. What is the most likely effect on the hardened mortar?

  1. Reduced shrinkage and reduced permeability.
  2. Increased water retention and improved workability.
  3. Lower strength and poorer mortar-to-unit cohesion. Correct answer.
  4. Higher compressive strength and better durability.

Explanation

Excess sand leaves too little cementitious paste to coat the aggregate and bond to the units. The mix becomes harsh, less workable, and lower in strength and durability.

Question 41Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

A cone penetrometer is used in the field to evaluate the consistency of freshly mixed mortar. What does a larger penetration depth indicate?

  1. A mortar with a higher sand-to-cement ratio
  2. A stiffer mortar with less mix water
  3. A wetter mortar with more mix water Correct answer.
  4. A mortar with a higher entrained air content

Explanation

The penetrometer releases a weighted cone that sinks into a sample of fresh mortar, and the distance it travels is the measure of consistency. A greater depth means less resistance to the cone, which corresponds to a softer, wetter mixture. Sand proportion and air content are separate properties that this test does not quantify.

Question 42Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

A technician performs the flow test on a mortar sample. How is the flow table operated?

  1. 15 drops in 15 s
  2. 10 drops in 10 s
  3. 50 drops in 30 s
  4. 25 drops in 15 s Correct answer.

Explanation

The flow table is raised and dropped a fixed number of times at a fixed rate so that results are comparable between technicians and between batches. ASTM C1437 specifies 25 drops completed within 15 s, with each drop falling 1/2 in. Changing either the count or the rate changes the energy delivered to the mortar pat and therefore the reported flow.

Question 43Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

Two mortar batches are tested on the flow table. Batch A gives 96 % flow and Batch B gives 128 % flow. What is the most reasonable conclusion?

  1. Batch B is stiffer and will develop higher compressive strength
  2. Batch B is wetter and more workable than Batch A Correct answer.
  3. Batch A contains more cementitious material than Batch B
  4. Both batches have the same water content because they were tested the same way

Explanation

Flow is the percent increase in the diameter of the mortar pat after the table is dropped, so a larger value means the mortar spread farther and is softer. Water content is the dominant influence on flow, so the higher-flow batch is the wetter, more workable mixture. Flow does not measure cementitious content, and equal test procedure does not imply equal water content.

Question 44Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

Field specimens for mortar compressive strength are molded in what nominal form?

  1. 4 by 8 in. cylinders
  2. 2 in. cubes Correct answer.
  3. 6 by 12 in. cylinders
  4. 3 by 6 in. prisms

Explanation

Mortar compressive strength is determined on 2-in. (50-mm) cubes molded using the procedures of ASTM C109, which the field evaluation methods adopt for specimens prepared from sampled mortar. Cubes test the mortar alone, without the reinforcing effect of the masonry units.

Question 45Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

Grout compressive strength specimens are cast in molds that have what feature?

  1. Sides lined with absorbent masonry material Correct answer.
  2. Sides lined with sealed steel plates
  3. A perforated base for free drainage
  4. A removable cone-shaped insert

Explanation

In the wall, grout loses water to the adjacent masonry units, so grout specimens are cast against absorbent masonry surfaces to reproduce that water loss. Including the absorption effect makes the measured strength representative of grout in the constructed wall.

Question 46Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

What minimum 28-day compressive strength does the masonry grout specification establish?

  1. 1,000 psi
  2. 3,000 psi
  3. 2,000 psi Correct answer.
  4. 2,500 psi

Explanation

ASTM C476 requires masonry grout to develop a minimum compressive strength of 2,000 psi at 28 days, although a designer may specify a higher value for a particular project. The 2,500 psi figure belongs to the mortar specification for Type M mortar and does not apply to grout.

Question 47Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

Which field measurement is the usual way to judge the consistency of freshly mixed masonry grout?

  1. Flow table spread
  2. Slump Correct answer.
  3. Cone penetration depth
  4. Time of setting by Vicat apparatus

Explanation

Grout is a fluid mixture containing coarse aggregate, so its consistency is judged with the slump test rather than with the mortar flow table or cone penetrometer. Cone penetration and flow table testing are applied to mortar, which is a finer, stiffer mixture.

Question 48Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

How is the water content of a freshly mixed mortar sample determined?

  1. By measuring the depth a weighted cone sinks into the sample
  2. By measuring the spread of the mortar on a flow table
  3. By measuring the pressure needed to force air out of the sample
  4. By drying a weighed sample to constant mass Correct answer.

Explanation

Water content is a gravimetric determination: the sample is weighed, dried to constant mass, and weighed again. The mass lost is the water, which is then expressed relative to the dry mass of the mortar. Consistency tests and air meters measure different properties and do not quantify water.

Question 49Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

A mortar sample spreads to a flow of 115 % on a flow table whose top has a 4.00-in. inside diameter. What spread diameter should have been measured?

  1. 5.15 in.
  2. 4.60 in. Correct answer.
  3. 4.35 in.
  4. 4.15 in.

Explanation

Flow expresses the spread diameter as a percentage of the 4.00-in. base diameter of the flow table top, so a flow of 115 % corresponds to 1.15 times 4.00 in., or 4.60 in. The percentage applies to the whole base diameter, not only to the amount it increases.

Question 50Field testing of masonry mortar and grout (consistency, water content, compressive strength specimens)

How many mortar cube specimens are normally molded for each test age when field mortar is sampled for compressive strength?

  1. Six
  2. Four
  3. Three Correct answer.
  4. Two

Explanation

A set of three 2-in. cubes is molded for each test age so that the reported strength is the average of three specimens. Averaging reduces the influence of a single cube that was molded or tested imperfectly. Testing a single specimen gives no indication of variability.