ACI Mix Design Method Quiz

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Questions and Answers

What is the maximum allowable free-water to cement ratio specified for the trial mixes?

0.50 (correct)

0.60

0.45

0.55

Which compressive strength is specified for the first trial mix?

50 MPa

30 MPa

15 MPa (correct)

40 MPa

What is the minimum allowable cement content for the specified trial mix?

350 kg/m3

420 kg/m3

200 kg/m3

290 kg/m3 (correct)

In the ACI Mix Design Method, which is the first step in the procedure?

Choice of slump (D) Signup and view all the answers

What is the defective rate specified for the second trial mix?

1% (B) Signup and view all the answers

What type of coarse aggregate is mentioned for the second trial mix?

10 mm crushed (C) Signup and view all the answers

According to the specifications, what is the Fineness Modulus (FM) of the fine aggregate for the second trial mix?

2.50 (A) Signup and view all the answers

What is the maximum aggregate size specified for the first trial mix?

40 mm (A) Signup and view all the answers

What is the primary purpose of determining the volume of fine aggregate in concrete?

To achieve the desired strength and workability (B) Signup and view all the answers

What specific gravity is given for Type I cement?

3.15 (A) Signup and view all the answers

Which factor must be adjusted for in the preparation of a concrete mixture under the ACI method?

Aggregate moisture content (B) Signup and view all the answers

When comparing material costs, which factor is relevant when changing the maximum aggregate size from 19 mm to 37.5 mm?

Overall volume of concrete mix required (B) Signup and view all the answers

What is the required slump range for the concrete mixture?

75 mm to 100 mm (B) Signup and view all the answers

What contributes to the overall variation in the measured strength of concrete?

Inconsistent testing methods. (A) Signup and view all the answers

How is the relationship between standard deviation and characteristic strength described for strengths above 20 N/mm2?

Standard deviation becomes independent. (B) Signup and view all the answers

Which of the following correctly describes the characteristic strength of concrete?

It indicates the strength below which a specified proportion may fall. (A) Signup and view all the answers

What does a higher coefficient of variation indicate regarding concrete strength results?

More variability relative to the average. (D) Signup and view all the answers

In a normal distribution, what does the area beneath the curve to the left of a specified value represent?

The probability of obtaining a result less than the specified strength. (D) Signup and view all the answers

What is the primary goal of mixture proportioning in concrete mix design?

To select the most economical combination of materials for required performance (C) Signup and view all the answers

What is the purpose of the standard deviation in the context of concrete strength?

To indicate variability in test results. (B) Signup and view all the answers

Which variable is NOT a source of variation in concrete strength?

Economic factors. (C) Signup and view all the answers

Which factor is NOT explicitly mentioned as a consideration in mix proportioning?

Environmental impact (C) Signup and view all the answers

What is the formula used to express the coefficient of variation as a percentage?

s / x̅ * 100 (B) Signup and view all the answers

What is the effect of using a low water/cement ratio in concrete?

It enhances strength and reduces permeability (B) Signup and view all the answers

Which characteristic of concrete does the grade designation primarily provide information about?

Characteristic compressive strength (A) Signup and view all the answers

What is one of the expected qualities of a properly proportioned concrete mix?

Uniform appearance of hardened concrete (D) Signup and view all the answers

Which of the following would NOT be a goal when determining the amount of water in a concrete mix?

Increase the total volume of concrete produced (A) Signup and view all the answers

Why is the type of cement significant in the concrete mix design process?

It affects the rate of compressive strength development (B) Signup and view all the answers

What does the term 'acceptable workability' refer to in concrete mix design?

Ease of mixing and transporting concrete (C) Signup and view all the answers

What is the mixing water quantity (in kg/m3) for a 19 mm nominal maximum aggregate size under a slump of 1 inch for non-air-entrained concrete with a water/cement ratio of 75 - 100?

179 (C) Signup and view all the answers

For which nominal maximum aggregate size does the water mixing quantity decrease most significantly as the slump increases from 0.375 inches to 1.5 inches for non-air-entrained concrete?

37.5 mm (A) Signup and view all the answers

What is the recommended air content for structural concrete exposed to mild conditions?

4.5% (C) Signup and view all the answers

How does the mixing water quantity for air-entrained concrete compare to that for non-air-entrained concrete at a slump of 1 inch with a 25 - 50 water/cement ratio?

Air-entrained concrete requires less mixing water. (D) Signup and view all the answers

What factor is primarily considered for determining the required water/cement ratio?

Strength, durability, and finishability (C) Signup and view all the answers

At a slump of 2 inches for a 100 mm nominal maximum aggregate size, what is the mixing water quantity for non-air-entrained concrete?

124 kg/m3 (C) Signup and view all the answers

Which of the following mixtures would generally have higher mixing water quantities for air-entrained concrete when compared at the same slump and nominal maximum aggregate size?

Non-air-entrained mixes (A) Signup and view all the answers

Which nominal maximum aggregate size contributes to the highest mixing water quantity for a slump of 3 inches in non-air-entrained concrete?

50 mm (B) Signup and view all the answers

What is the water-cement ratio by weight for a compressive strength of 34.5 MPa?

0.48 (C) Signup and view all the answers

Which water-cement ratio would correspond to a compressive strength of 20.7 MPa for air-entrained concrete?

0.74 (B) Signup and view all the answers

Which factor can increase the coarse aggregate volume in concrete estimates?

Increased volume for pavement applications (C) Signup and view all the answers

What is the purpose of estimating coarse aggregate content in concrete?

To minimize cement usage (B) Signup and view all the answers

For an aggregate size of 19 mm, what is the volume of coarse aggregate per unit volume of concrete with a fine aggregate fineness modulus of 2.60?

0.64 (C) Signup and view all the answers

What step follows the calculation of cement content in the concrete mixing process?

Estimation of coarse aggregate content (A) Signup and view all the answers

Which of the following statements is true regarding the water-cement ratios in Table 2?

Lower compressive strengths require higher water-cement ratios. (A) Signup and view all the answers

What is understood by the term 'coarse aggregate' in concrete mixing?

Larger particles that provide strength and stability (D) Signup and view all the answers

Study Notes

Concrete Mix Design Definitions

Mix design determines required concrete mixture characteristics, including fresh concrete properties, hardened concrete strength/durability, and ingredient limits. This leads to a concrete specification.
Mixture proportioning uses local materials to achieve specific concrete characteristics. A properly proportioned mix ensures acceptable fresh mix workability, hardened concrete durability, strength, and uniformity, and is economical.
Mixture proportioning selects suitable ingredients and determines proportions to achieve minimum workability, strength, and durability at the most economical cost.

Factors Considered in Mix Proportioning

Workability (Cohesiveness, Slump): Placement conditions influence required workability.
Strength and Durability: Essential for performance.
Appearance: Visual appeal is important.
Economy: Minimize cement, water-cement ratio, and water content to reduce cement content.

Advantages of Low Water/Cement Ratio

Increased strength
Lower permeability
Increased resistance to weathering
Better bond between concrete and reinforcement
Reduced drying shrinkage and cracking
Less volume change from wetting and drying

Basic Data Required for Mix Proportioning

Grade Designation: Characteristic compressive strength of concrete, considering a suitable margin to the target mean strength for quality control purposes.
Type of Cement: Influences the rate of compressive strength development.
Maximum Nominal Size of Aggregate: Based on the section size and reinforcement spacing.
Maximum Water-Cement Ratio: Reflects strength and durability requirements.
Minimum Cement Content: Determined by environmental exposure conditions.
Workability: Dependent upon shape/size of the section, reinforcement density, and transportation/placing methods.
Exposure Conditions: Durability requirements based on the structure's service life.
Type and Properties of Aggregate: Affects workability and strength; factors like grading, shape, size, and surface texture influence these factors.
Method of Transporting and Placing: Affects workability.
Use of Admixtures: Enhance or modify properties of fresh and hardened concrete.

Compressive Strength Grading and Classes

Concrete is classified according to compressive strength.
Indian Standards (e.g., M10, M15, M20, M25, M30, M35, M40): Denote mix design with material proportions (Cement:Fine Aggregate:Coarse Aggregate).

Characteristic Strength and Target Mean Strength

Concrete strength varies between mixes and even within the same mix during a job.
Variations occur in testing methods, constituent material properties/proportions, production and delivery/handling processes, and climatic conditions.
Concrete strength variation follows a normal distribution.
Standard deviation is a measure of the variability in concrete strength.
Coefficient of variation is a non-dimensional ratio of standard deviation to arithmetic mean.
Target mean strength is a higher strength level than the specified characteristic strength, considering a margin factor to ensure the quality criteria set by the client. It is calculated as characteristic strength + (margin factor)*(standard deviation)

Methods of Concrete Mix Design

Most methods rely on empirical relationships, charts, and graphs from experimental programs.
Methods employ similar principles with minor variations in mix proportioning selection.

British Method of Concrete Mix Design (DOE Method)

Method is used for concrete pavements and various other uses, including roads, and is useful for concrete that contains fly ash and/or GGBFS.
Steps involve finding target mean strength and calculating water/cement ratio.