Properties of Fresh ConcreteEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsSignificance 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.Early AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsProperties at Early Ages Workability Slump Loss Segregation/Bleeding Plastic Shrinkage Time of Set TemperatureEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsWorkability 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)Early AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsImportance If concrete is very dry, it cannot consolidate well and will end up with lots of voids.w/cfclabfieldLabor costs are high when using dry mixesEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsWorkability consistency (slump) --> how easy to flow and  cohesiveness --> tendency to bleed and segregate.Early AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsConsistency Tests Slump testEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsSlump testEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsConsistency testsVebe Test; compacting apparatusEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsWorkability Test No equipment can measure consistency and cohesiveness at the same timeEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsBleeding 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 statisticallyEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsControl (2) Admixtures Water reducing Air entraining Fly ashes, pozzolans (fine)Early AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsTemperature and Time Control If mix for too long, hydration products start to from --> requires more water --> slump goes down when transport time and temperature increasesEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsSlump Loss Premature slump loss; any abnormal loss of slumpelapsed timeSlump2 hEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsCauses 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.Early AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsExample:  Type II cement (low in C3A ~6%) T= 70 FInitial Slump 30 min 60 min 2h 5 in 4 3/8 3 1/8 1 1/2 Type I cement (10% C3A)Initial Slump 30 min 60 min 2h 5 in 3 1/4 2 1/4 1 Increasing the water content Initial 30 min 60 min 2h Type 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 problemEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsTime of SetStrength (psi)Timebeginning of hardeningbeginning of solidification4000500initialfinalFinal and Initial Set --> arbitrary values of strength. Cannot consolidate, vibrate and finish after starting the initial set.Early AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsSetting and HardeningEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsSet timeEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsContEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsEarly 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.Early AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsSegregation 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.Early AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsCauses of bleeding and segregation: Improper slump Excessive amount of coarse aggregate Lack of fines Inappropriate placing and compactingEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsControl of Bleeding Reduction of water Introduction of fines and air Proper Compaction (too much compaction causes bleeding)Early AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsPlastic 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/hrEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsControl of Plastic Shrinkage Moisten subgrade and forms Erection of wind barriers Cool concrete Spray water Membrane coatingEarly AgesP.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and MaterialsRate of moist