Early Age2

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Properties of Fresh Concrete


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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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Properties at Early Ages

Workability

Slump Loss

Segregation/Bleeding

Plastic Shrinkage

Time of Set

Temperature


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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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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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Workability

consistency (slump) --> how easy to flow and

cohesiveness --> tendency to bleed andsegregate.


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Consistency Tests

Slump test


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Slump test


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Consistency tests

Vebe Test; compacting apparatus


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Workability Test

No equipment can measure consistency and cohesivenessat the same time


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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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Control (2)

Admixtures

Water reducing

Air entraining

Fly ashes, pozzolans (fine)


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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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Slump Loss

Premature slump loss; any abnormal loss of slump

elapsed time

Slump

2 h


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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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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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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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Setting and Hardening


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Set time


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Cont


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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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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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Causes of bleeding and segregation:

Improper slump

Excessive amount of coarse aggregate

Lack of fines

Inappropriate placing and compacting


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Control of Bleeding

Reduction of water

Introduction of fines and air

Proper Compaction (too much compaction causes bleeding)


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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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Control of Plastic Shrinkage

Moisten subgrade and forms

Erection of wind barriers

Cool concrete

Spray water

Membrane coating


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Rate of moist evaporation

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