CE 251 Lecture 6

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1

Dr Rajib ChowdhuryDepartment of Civil Engineering

IIT Roorkee

Module CE-251

Building Materials,Construction and Estimation

Lecture 7: Concrete

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CE-251 - 2Lecture 7

Introduction

!

Many types of concrete

! Portland Cement Concrete (PCC) prevalent

" concrete= PC Concrete

! Engineers are directly responsible for the

"

Design of the mix" Final quality of concrete

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CE-251 - 3Lecture 7

Concrete Ingredients

Aggregates

" Fine

" Coarse

Portland Cement (PC)

Water Admixtures

" Paste = PC + Water

" Mortar = PC + Water + Fine aggregate

" Concrete = PC + Water + Coarse and Fine aggregates

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CE-251 - 4Lecture 7

Classification

!Based on Cementing Material - lime concrete, gypsum concrete and cementconcrete

! Based on Perspective Specifications

! Based on Performance Oriented Specifications

! Based on Grade of Cement Concrete

! Based on Density

! Based on Placing of Casting -

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CE-251 - 5Lecture 7

Quality of Concrete

" chemical composition

" aggregate

"

water"

admixtures

" proportions

" mixing

" transporting

" hydration

"

placing" vibrating

" curing

depends on:

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CE-251 - 6Lecture 7

Order of Operations for Concrete

specific operations must be performed in a certain order!

final quality is influenced by every step

I. mix design (proportioning)

II. trial mixes & testing

III. Batching/measurement of materials

------------------------------------------------------------start the clockIV. mixing

V. transporting

VI. pouring (placing)

VII. vibrating (consolidating)

-------------------------------------------------------------initial set here

VIII. finishing

-------------------------------------------------------------final set here

IX. curing

X. maintenance

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CE-251 - 7Lecture 7

Purpose of mix design

!

To ensure the most optimum proportions of the constituent materials tomeet the requirements of the structure being built

! Mix design should ensure that the concrete:

" complies with the specifications of structural strength laid down,

which is usually stated in terms of the compressive strength of

standard test specimen

" complies with the durability requirements to resist the environment

in which the structure will serve its functional life

" be capable of being mixed, transported, compacted as efficiently as

possible without undue labour

" be economical

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CE-251 - 8Lecture 7

Several Methods

Depends on project size:

! Arbitrary volume method (1:2:3 = PC:sand:coarse agg.)

! Weight method easiest design method

! Absolute volume method most accurate

! Small jobs, non-critical

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CE-251 - 9Lecture 7

Mix Design: Volumetric Method

1. Strength requirements

2. Determine W/C

3. Estimate aggregate mass

4. Workability needs

5.

Estimate water content6. Determine cement content requirements

7. Estimate coarse & fine aggregate mass

8. Evaluate admixture needs

9. Moisture corrections

10.

Trial Mix

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CE-251 - 11Lecture 7

Step 1. Strength Requirements Target Strength

Target Strength =fck+ k

fck= Characteristics strength

k= constant

= Standard deviation

k= constant depending on the probability of certain number of results likely to fallbelowfck.

Characteristic strength is defined as that value below which not more than 5% (1 to20) results are expected to fall, in which case the value of kwill be 1.65.

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Step 1. Strength Requirements Target Strength

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Step 2. Determine Water-Cement Ratio

Representative Plot

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Step 2. Determine Water-Cement Ratio

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Step 4. Workability Requirements for different placing conditions

Degree of Workability Slump (mm)

Extremely Low 0

Very low 0-25

Low 25-50

Medium 50-100High 100-175

Slump is ease of placing, consolidating, and finishing.

! highest slump with no segregationor excessive bleeding

" Coarse aggregate migrates to bottom & water migrates to top

! Increase slump with

" admixtures

" rounded aggregates

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Step 5. Water Content

!

For a given slump it depends on maximum size and shape of aggregates

! Considers SSD condition

! Never let workers add water in truck or at the jobsite

Oven Dry Air Dry SSD Moist

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Step 6. Cement Content

w/c = 0.45; For maximum size aggregate of 20 mm, the quantity of water will be186 liter (kg), and the sand content 35% of the absolute volume of total aggregates.The cement content will therefore be:

186/0.45 = 413.3 kg

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Step 7. Fine and Coarse Aggregate Content

For fine aggregate:

V = W +C

Sc+

1

p

fa

Sfa

!

"##

$

%&&'

1

100

For coarse aggregate:

V =

W +

C

Sc+

1

1! p( )

Ca

SCa

"

#$$

%

&''(

1

100

V = absolute volume of fresh concrete; i.e., gross volume, m3volume of entrapped air.

W = mass of water per m3of concrete, kg or litter.

C = mass of cement per m3of concrete, kg.

Sc= specific gravity of cement.

p = ratio of fine aggregate to total aggregate by absolute volume.

fa, Ca= total quantity of fine and coarse aggregate per m3of concrete, respectively, kg.

Sfa, Sca= specific gravity of saturated surface dry fine and coarse aggregates.

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Step 8. Admixtures

! follow instructions from manufacturers

! generally small quantities

" volume or mass should be considered in mix proportioning

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Step 9. Moisture Corrections

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Step 10. Trial Mix

! The mix proportions arrived at from the preliminary mixes are then usedto make a trial mix.

! Generally one trial mix is sufficient but to ensure the designed strength ofconcrete at 7 and 28 days more than one trial mix is prepared in such away that the water-cement ratio is increased or decreased from that

obtained from the trial mix by 10%.

! Necessary adjustments have, of course, to be made in the cement contentand quantities of coarse and fine aggregates keeping the total quantity ofwater per m3of concrete constant in all the so obtained three mixes

! Test on Trial Mixes:

#

Slump and / or Vee-Bee test

# determination of unit weight, kg/m3, of fresh concrete, and

# making and curing of test cubes for the compression test

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Acceptable Criteria

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Order of Operations for Concrete

Specific operations must be performed in a certain order final quality is influenced by every step

I. mix design (proportioning)

II. trial mixes & testing

III. batching

------------------------------------------------------------start the clockIV. mixing

V. transporting

VI. pouring (placing)

VII. vibrating (consolidating)

-------------------------------------------------------------initial set here

VIII. finishing

-------------------------------------------------------------final set here

IX. curing

X. maintenance

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Mixing, Placing, & Handling of PC

Batching! Measuring correct proportions of components and placing in the mixer

! By weight is more accurate because air voids don't matter

Mixing! Until uniform appearance

! Usually batch mixers (one at a time), but sometimes continuous (conveyors

automatically feed components into mixer)

! Usually start with 10% of the water in the mixer, then solids with 80% of thewater, and then remaining water

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PlacingDirectly into form

Wheel barrow/ buggy Bucket

Pump

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Vibration of Concrete

!

Consolidation (compaction)complete before initial set

! Manually by

ramming

tamping

!Mechanically using vibrators

Internal poker

" 5 sec to 2 min in one spot

"

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Finishing Concrete

Screeding strike concrete off todesired level

Bullfloating eliminates highand low spots and embedslarge aggregate particlesimmediately after strikeoff.Power float

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Curing of Concrete(The process of keeping concrete damp for this purpose is known as curing)

! M a i n t a i n m o i s t u r e a n d

temperature in the concrete to

promote continued hydration

and strength gain

!

hydration will resume if curingis stopped and resumed

! Curing must be done for at least

three weeks and in no case for

less than ten days.

! Curing affects:

" durability

" strength

" water-tightness

"

abrasion resistance" volumetric stability

" resistance to freezing and

thawing

" r e s i s t a n c e t o d e - i c i n gchemicals

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Curing: Approach 1. Maintaining Presence of Water

Spraying

Wet Covering

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Curing: Approach 3. Heat

!

insulate

! steam

" good for early strength gain and in freezing weather

! heating coils, electrically heated forms or pads

" usually in precast plants only

CE 251 Lecture 6

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