Early Age2
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Transcript of Early Age2
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Properties of Fresh Concrete
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Early Ages
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Significance
The first 48 hours are very important for the performance
of the concrete structure. It controls the long-termbehavior, influence fc, Ec, creep, and durability.
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Properties at Early Ages
Workability
Slump Loss
Segregation/Bleeding
Plastic Shrinkage
Time of Set
Temperature
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Workability
Definition: Effort required to manipulate a
concrete mixture with a minimum of segregation.
It is not a fundamental property of concrete
f(conditions, equipment, type of concrete, mix).
e.g., dry mix --> poor workability if pumped, butgood workability when placed in a conveyor belt(mass concrete)
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Importance
If concrete is very dry, it cannot consolidate welland will end up with lots of voids.
w/c
fc lab
field
Labor costs are high when using dry mixes
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Workability
consistency (slump) --> how easy to flow and
cohesiveness --> tendency to bleed andsegregate.
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Consistency Tests
Slump test
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Slump test
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Consistency tests
Vebe Test; compacting apparatus
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Workability Test
No equipment can measure consistency and cohesivenessat the same time
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Bleeding and Segregation
Control:
Water content
Cement content
Friction between aggregates is reduced by increasing
cement paste
Aggregate gradient has a big effect
Fine/Coarse Aggregate Ratio.
Change fineness modulus of the sand --> improve
pumpability and cohesiveness.
measured visually and statistically
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Control (2)
Admixtures
Water reducing
Air entraining
Fly ashes, pozzolans (fine)
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Temperature and Time Control
If mix for too long, hydration products start to from -->
requires more water --> slump goes down whentransport time and temperature increases
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Slump Loss
Premature slump loss; any abnormal loss of slump
elapsed time
Slump
2 h
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Causes of Slump Loss:
1) Use of an abnormal setting cement
2) unusual long time for mixing, transporting or finishing
3) High T due to excessive heat of hydration.
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Example:
Type II cement (low in C3A ~6%)
T= 70 F
Initial Slump 30 min 60 min 2h5 in 4 3/8 3 1/8 1 1/2
Type I cement (10% C3A)
Initial Slump 30 min 60 min 2h5 in 3 1/4 2 1/4 1
Increasing the water content
Initial 30 min 60 min 2hType II 7 1/2 7 5 1/2 2
Type I 7 1/2 4 3/4 3 1/4 1 1/2
so, adding more water does not solve the problem
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Time of Set
Strength (psi)
Time
beginning of hardening
beginning of solidification
4000
500
initial final
Final and Initial Set --> arbitrary values of strength.
Cannot consolidate, vibrate and finish after starting the initial set.
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Setting and Hardening
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Set time
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Cont
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Stiffening
Solution
Retempering: adding more water at the job site(note: total water/cement ratio should be keptconstant)
Use superplasticizers at the job site.
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Segregation and Bleeding
Segregation: coarse aggregate tends to segregate due
to gravity. Typical of dry mixes.
Bleeding: water rising to the surface. Typical of wetmixes.
when excessive, cement particles and water go to thesurface (laitance) --> porosity goes up --> paste can
easily be abraded --> dusting.
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Causes of bleeding and segregation:
Improper slump
Excessive amount of coarse aggregate
Lack of fines
Inappropriate placing and compacting
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Control of Bleeding
Reduction of water
Introduction of fines and air
Proper Compaction (too much compaction causes bleeding)
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Plastic Shrinkage
In slabs, rapid drying of fresh concrete causes plastic
shrinkage.
Rate of water loss > rate of bleed water
It happens when concrete is not a solid yet.
Loss of bond between concrete and rebar
Crazing (Surface cracks develop)
High temperature, high wind velocity, low RH
Rate of evaporation > 0.2 lb/ft2/hr
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Control of Plastic Shrinkage
Moisten subgrade and forms
Erection of wind barriers
Cool concrete
Spray water
Membrane coating
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P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Rate of moist evaporation