Mix Design

Plain and Reinforced Concrete - I

Producing of Concrete

Mix Design

2010 Fall Semester

Instructor: M. Nasir Amin, PhD

2010, Plain and Reinforced Concrete - I

NICE (SCEE)

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Overview of Course Contents

1. Concrete & Ingredients of Concrete

1.1 Concrete

1.2 Binders (Cements)

1.3 Fillers (Aggregates)

1.4 Admixtures

2. Properties of Fresh Concrete

2.1 Workability

2.2 Producing of Concrete

2.3 Mixing

2.4 Transporting

2.5 Placing

2.6 Vibration (compaction)

2.7 Curing

3. Mix Design (Producing of Concrete)

4. Properties of Hardened Concrete

4.1 Strength (comp, tens, flexure)

5. Special Weather Concreting

6. Types of Concrete

7. Non-Destructive Testing

8. Elasticity, Shrinkage, and Creep

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Mix Proportioning

■ Mindess book

• Sec 10.1: Basic Consideration

• Sec 10.2: Fundamental of Mix Design

• Sec 10.3: ACI Method of Mix Design

Mix Design Example

• Handout can be taken from photocopier.

• Reading assignment: Sec 10.1, 10.2, & 10.3

• Lectures available at http://www.mediafire.com/?63dgsoq3hmetjfw

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Mix Design - Proportioning Concrete Mixes

■ The proportioning of Concrete Mixtures, more commonly referred to as“Mix Design”

- Two interrelated steps:

(1) selection of the suitable ingredients of concrete (binder, filler[CA & FA], water, and admixture)

(2) determining their relative quantities (“proportioning”) by volume or weight

To produce, as economically as possible, concrete of the appropriate workability (fresh state),

strength (hardened stage), and durability (serviceability/service life of facility)

Most design procedures are based primarily on achieving a specified compressive strength at

some given workability and age; it is assumed that if this is done, the other properties (except

perhaps resistance to freezing and thawing or other durability problems, such as resistance to

chemical attack) will also be satisfactory.

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Basic Considerations in Mix Design

■ Basic Considerations in Mix Design are Economy, Workability, and Strength & Durability

Economy : materials, labor, and equipment

The material costs are most important in determining the relative costs of different

mixes.

The labor and equipment costs, except for special concretes, are generally

independent for the mix design.

Since cement is more expensive than aggregate, it is clear that cement content

should be minimized.

This can be accomplished by

(1) using the lowest slump that will permit handling,

(2) using a good ratio of coarse to fine aggregate, and

(3) possible use of admixtures.

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Workability : equipment availability, minimizing bleeding & segregation

The good mix design must be capable of being placed and compacted, with minimal

bleeding and segregation, and be finishable.

Water requirements depend on the aggregate rather than the cement characteristics.

Workability should be improved by redesigning the mortar faction rather than simply

adding more water.

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Strength & Durability :

In general, minimum compressive strength and a range of w/c ratios are specified

for a given concrete mix.

Possible requirements for resistance to freeze-thaw and chemical attack must be

considered (durability).

A balance or compromise must be made between strength and workability.

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Mix Design by ACI Method

The most common method used in North America - established by

ACI Recommended Practice 211.1

Provide a first approximation of the proportions and must be checked

by trial Batches

Local characteristics in materials should be considered

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Mix Design –Points to be Noted

Water/cement ratio (w/c ratio) theory states that for a given combination

of materials and as long as workable consistency is obtained, strength

of concrete at a given age depends on the w/c ratio.

The lower the w/c ratio, the higher the concrete strength.

Whereas strength depends on the w/c ratio, economy depends on the

percentage of aggregate present that would still give a workable mix.

The aim of the designer should always be to get concrete mixtures of

optimum strength at minimum cement content and acceptable

workability.

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Mix Design –Points to be Noted

Once the w/c ratio is established and the workability or consistency

needed for the specific design is chosen, the rest should be simple

manipulation with diagrams and tables based on large numbers of

trial mixes.

Such diagrams and tables allow an estimate of the required mix

proportions for various conditions and permit predetermination

on small unrepresentative batches.

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Mix Design by ACI Method – Sequence of Steps

(1) Required information – related to job parameters

(2) Choice of slump (Processibility suitable for mixing, transporting, and placing)

(3) Maximum aggregate size

(4) Estimation of mixing water and air content

(5) Determination of w/c (or w/cm or w/b)

(6) Calculation of cementitious material content (C+FA+SF+Slag+…)

(7) Estimation of coarse aggregate content (G)

(8) Estimation of fine aggregate content (S)

(9) Adjustment for moisture in the aggregates

(10) Trial batch

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Pre-determined Parameters, Step (1) ~ (3)

(1) Required information

- Required structural performance Required material performance

: strength and durability requirements, structural size and dimension, the minimum space between reinforcing bars, and other performance requirements

- Characteristics of raw materials

: aggregates (sieve analysis results (CA and FA), unit weight, bulk specific gravity, and absorption capacity), cementitious material (particle size distribution, specific gravity, and setting property if required…), Chemical admixtures (concentration… )

(2) Choice of slump (or any other measure representing suitability for chosen processing technique)

- Considering job-site condition & process employed for placing concrete

Concrete to be consolidated by vibration (normal process)

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Pre-determined Parameters, Step (1) ~ (3)

(3) Maximum aggregate size

See “Design Code” or “Specifications”.

The largest maximum aggregate size that will conform to the following limitations:

Maximum size should not be larger than 1/5 the minimum dimension of structural members,

1/3 the thickness of a slab, or 3/4 the clearance between reinforcing rods and forms. These

restrictions limit maximum aggregate size to 1.5 inches, except in mass applications.

Current thought suggests that a reduced maximum aggregate size for a given w/c ratio can

achieve higher strengths. Also, in many areas, the largest available sizes are 3/4 in. to 1 in.

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(4) Estimation of Mixing Water and Air Content

Estimation of the amount of water required for air-entrained and

non-air-entrained concretes can be obtained from Table 10.2.

One major disadvantage of concrete is its susceptibility to damage by

single or multiple freeze-thaw cycles.

However, concrete can be made frost-resistant by using air-entraining

admixtures (Discussed in previous lecture of air-entraining admixtures).

Approximate mixing water kg/m3 (lb/yd3) and air content for different

slumps and nominal maximum sizes of aggregates (see Table 10.2 on next

slide)

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(4) Estimation of Mixing Water and Air Content

Based on practical experience

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(5) Determination of w/c

■ Based on Compressive Strength

- This table is available when OPC is used

- Other cases use the data obtained

from experiments

■ Based on Durability

- Table 10.4 ~ Table 10.7

(see handouts of Mindess’ book)

■ Governed by strength and durability

requirements

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(6) Calculation of Cementitious Material Content

■ Cement Content

- C = W / (w/c)

■ Replacement of cement with mineral admixtures (in weight fraction)

1) Total weight of cementitious material :

2) Weight of replaced mineral admixtures :

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(7) Estimation of Coarse Aggregate Content, G

■ Coarse Aggregate Content

- For the same dmax, Vg depends on F.M. of sand in a workable mix

- For the same workability, Vg depends on F.M. of sand and dmax

OD (oven-dry)

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(8) Estimation of fine aggregate content – Weight Method

1st estimate

exact calculation

weight of fresh concrete

Weight of fine aggregate The difference between total weight of fresh concrete and the weight of other ingredients

weight of fresh concrete

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(8) Estimation of fine aggregate content – Volume Method

There are two standard methods to establish the fine aggregate content,

the mass or weight method (as discusses in previous slide) and the volume

method.

"Volume" Method is preferred in some cases, as it is a somewhat more

exact procedure

The volume of fine aggregates is found by subtracting the volume of cement,

water, air, and coarse aggregate from the total concrete volume.

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(9) Adjustment for moisture in the aggregates

Heating at 105℃ to constant weight

No moisture on surface Pores partially saturated

No film of water on surface

All pores fully saturated with film of water on surface

SSD (saturated surface dry) - Close to field condition - Reference state usually adopted

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(10) Trial Batch

Using the proportions developed in the preceding steps, mix a trial

batch of concrete using only as much water as is needed to reach

the desired slump (but not exceeding the permissible w/c ratio).

The fresh concrete should be tested for slump, unit weight, yield,

air content, and its tendencies to segregate, bleed, and finishing

characteristics. Also, hardened samples should be tested for com-

pressive and flexural strength.

Trial batch Does this concrete satisfy performance requirements?

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(10) Trial Batch

1) Slump : an increase of W = 6 kg/m3 an increase by 15mm in slump

2) addition of AE admixture : an increase in the amount of entrained air by 1% decreased water requirement by 3 kg/m3

……………

Practice Problems:

Mix-Design Example given in “Mindess”book (see handouts)

Exercise Problems 10.1 to 10.11 (see handouts)

Home work# 2, Problem 10.7 (due: 26/11/2010)

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ACI Mix-Design Example from Mindess

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Practice Problems:

Mix-Design Example given in “Mindess”book (see handouts or slides)

Exercise Problems 10.1 to 10.11 (see handouts)

Home work# 2, Problem 10.7

(Sec B due: 29/11/2010)

(Sec C due: 01/12/2010)

(Sec A due: 02/12/2010)

Documents

Mix Design