3. Cracking and Shrinkage Flashcards
When does cracking occur? (3)
Movements eg temp change
Expansion of materials embedded in concrete
Externally imposed conditions eg loads
Factors for cracking (2)
Restrain method
Creep (can be beneficial in reducing crack formation)
What is plastic settlement?
Build up of tensile stresses due to natural settlement of fresh concrete
Characteristics of plastic settlement (3)
Cracks over reinforcement
Crack in narrow columns
Cracks at change of depth
Prevention of plastic settlement - prior
Reduce bleeding and settlement through cohesiveness
By introducing fines
Reduce restraint - rebars and cover
Prevention of plastic settlement during construction
Revibration - provided still in plastic state.
What is plastic shrinkage
Net reduction in volume due to rapid drying
When does plastic shrinkage occur? (4)
Concrete bleeding rate > environment evaporation rate
Rate of evaporation > 1kgm2/h
Early framework removal
Autogenous shrinkage - water reacts with cement - reduction in volume
How to reduce plastic shrinkage - mix design (2)
Avoid retarding admixtures
Fines reduce bleeding
How to reduce plastic shrinkage - curing
Moisture reduces evaporation
Curing membranes 2hrs after placing (when tensile capacity is at maximum)
What is early thermal contraction? (2)
Rate of heat development is greater than heat loss - expansion
Rate of heat development is less than heat loss - contraction
When does early thermal contraction occur? (3)
Concrete cast on previously hardened concrete
Large elements - temp gradient
Varying temp conditions
Factors contributing to early thermal contraction? (5)
Initial temp of materials
Amb temp
Formwork shrinking time
Admixtures
Cement type/content
How to reduce early thermal contraction? (5)
Sizer of pour
Distribution of reinforcing steel
Section thickness
Aggregate/cement type
Avoid cold joints
What is drying shrinkage?
Shrinkage due to loss of water due to exposure to unsaturated air
