Properties of Fresh Concrete
Early Ages
Significance
The first 48 hours are very important for the performance of the concrete structure. It controls the long-term behavior, influence fc, Ec, creep, and durability.
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Properties at Early Ages
Workability
Slump Loss
Segregation/Bleeding
Plastic Shrinkage
Time of Set
Temperature
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
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, but good workability when placed in a conveyor belt (mass concrete)
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Importance
If concrete is very dry, it cannot consolidate well and will end up with lots of voids. fc
lab
field
w/c Labor costs are high when using dry mi xes
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Workability
consistency (slump) --> how easy to flow and cohesiveness --> tendency to bleed and segregate.
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Consistency Tests
Slump test
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Slump test
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Consistency tests
Vebe Test;
compacting apparatus
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Workability Test
No equipment can measure consistency and cohesiveness at the same time
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
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
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Control (2)
Admixtures Water reducing Air entraining Fly ashes, pozzolans (fine)
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Temperature and Time Control
If mix for too long, hydration products start to from --> requires more water --> slump goes down when transport time and temperature increases
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Slump Loss
Premature slump loss; any abnormal loss of slump
Slump
2h
elapsed time
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
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.
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Example:
Type II cement (low in C3A ~6%) T= 70 F Initial Slump 5 in
30 min 4 3/8
60 min 3 1/8
2h 1 1/2
60 min 2 1/4
2h 1
Type I cement (10% C3A) Initial Slump 5 in
30 min 3 1/4
Increasing the water content Type II Type I
Initial 7 1/2 7 1/2
30 min 7 4 3/4
60 min 5 1/2 3 1/4
2h 2 1 1/2
so, adding more water does not solve the problem P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Time of Set Strength (psi)
4000
beginning of hardening
500 beginning of solidification Time initial final
Final and Initial Set --> arbitrary values of strength. Cannot consolidate, vibrate and finish after starting the initial set.
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Setting and Hardening
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Set time
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Cont
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Early Stiffening
Solution Retempering: adding more water at the job site (note: total water/cement ratio should be kept constant) Use superplasticizers at the job site.
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Segregation and Bleeding
Segregation: coarse aggregate tends to segregate due to gravity. Typical of dry mixes. Bleeding: water rising to the surface. Typical of wet mixes. when excessive, cement particles and water go to the surface (laitance) --> porosity goes up --> paste can easily be abraded --> dusting.
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Causes of bleeding and segregation:
Improper slump
Excessive amount of coarse aggregate
Lack of fines
Inappropriate placing and compacting
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Control of Bleeding
Reduction of water
Introduction of fines and air
Proper Compaction (too much compaction causes bleeding)
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
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
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Control of Plastic Shrinkage
Moisten subgrade and forms
Erection of wind barriers
Cool concrete
Spray water
Membrane coating
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Early Ages
Rate of moist evaporation
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
