CE 251 Lecture 6
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Transcript of 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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CE-251 - 12Lecture 7
Step 1. Strength Requirements Target Strength
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CE-251 - 13Lecture 7
Step 2. Determine Water-Cement Ratio
Representative Plot
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CE-251 - 14Lecture 7
Step 2. Determine Water-Cement Ratio
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CE-251 - 16Lecture 7
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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CE-251 - 17Lecture 7
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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CE-251 - 18Lecture 7
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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CE-251 - 19Lecture 7
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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CE-251 - 20Lecture 7
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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CE-251 - 22Lecture 7
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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CE-251 - 23Lecture 7
Acceptable Criteria
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CE-251 - 24Lecture 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
------------------------------------------------------------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 - 25Lecture 7
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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CE-251 - 26Lecture 7
PlacingDirectly into form
Wheel barrow/ buggy Bucket
Pump
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CE-251 - 28Lecture 7
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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CE-251 - 29Lecture 7
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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CE-251 - 30Lecture 7
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