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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)
Mix Design
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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
Mix Design
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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
Mix Design
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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.
Mix Design
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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.
Mix Design
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Basic Considerations in Mix Design
■ Basic Considerations in Mix Design are Economy, Workability, and Strength & Durability
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.
Mix Design
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Basic Considerations in Mix Design
■ Basic Considerations in Mix Design are Economy, Workability, and Strength & Durability
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.
Mix Design
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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
Mix Design
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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.
Mix Design
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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.
Mix Design
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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
Mix Design
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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)
Mix Design
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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.
Mix Design
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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)
Mix Design
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(4) Estimation of Mixing Water and Air Content
Based on practical experience
Mix Design
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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
Mix Design
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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 :
Mix Design
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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)
Mix Design
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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
Mix Design
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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.
Mix Design
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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
Mix Design
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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?
Mix Design
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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)
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
Mix Design
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ACI Mix-Design Example from Mindess
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)