Concrete Flashcards
What is Bleeding? And what are the advantages and disadvantages of bleeding?
The upward movement of water in the concrete mix and lies on top surface of the mix
Advantages: it decreases w/c ratio thus the compressive strength increases
Reduced plastic shrinkage
Increases impermeability
Disadvantages: water can be trapped inside , thus it increases permeability of hardned concrete
More water lies on the surface of the concrete mix thus it weakens the surface
What is creep? Advantages and disadvantages of creep
The time- dependent increase of strain of material (deformation) under constant applied stress
Advantages: It imparts the ductile forces in the material
It relieve stresses due to different movement in structural memeber
Disadvantages: it increases the deflection in beams and slabs
Loss of prestress force
Cause of creep buckling in long column
Discuss 28-day compressive strength and give us the factors that affects the compressive strength
It is very standard to determine compressive strength at day 28
The compressive strength of concrete mix increases with time after the setting. Hence, it is very significant to measure compressive strength of concrete at day-28.Water/ cement ratio if it is low, the compressive strength will increase. Cement extenders when applied at day 28 , the expected value of compressive strength of concrete will be higher
Other factors: water/ cement ratio,cement extenders, compaction kf concrete, curing and control of bleeding , aggregate shape and size, rate of loading and age of concrete
Which materials are affected by creep? Factors that are creep
Water/cement ratio.. increase in w/c there will be increase in creep
Aggregate properties
Compressive strength of concrete
Time under applied load. Material: concrete and timber
Give us 2 common cement extenders and point out the advantages and disadvantages of cement extenders in concrete .lastly give us where do they come from
GGBS AND FA
Advantages:improves durability
Cost saving
Reduces industrial waste
Reduces water requirement during setting
Disadvantages:increases setting time which may cause bleeding
Delayed setting causes the weakening of concrete strength
Fly ash(FA) - eletricity generation
GGBS- iron ore
What are causes of bleeding ? How can we overcome bleeding?
Causes: high aggregate content relative density
Lack of fine material content
High water content
Delayed setting
Overcoming: add more fine material to the concrete mix
Reduce water content
Which factors do you need to consider when making material selection?
Factors: functionality
Mechanical properties
Durability
Speed of construction
Costs
Appearance- aesthetic
Which factors influence sustainability of construction materials?
Accessibility of construction materials, Availability of construction materials, Abundance of construction materials,Durability,cost of life cycle, re usability, recyclability,energy consumption, methods of disposal that have minimum impact on environment
Give us the fresh concrete properties?
Consistance,workability,settlement and bleeding , plastic shrinkage, loss of slump,water requirement
Which fresh concrete Properties that can be modified by chemical admixtures?
Workability of the mix
- dispersing admixture such plasticisers and superplasticisers will improve the workability of the mix
Setting
Acceleratoring admixture will reduce the setting time , thus which plays an important role in early age development of compressive strength
Density
Air- entrainment admixture will improve the workability of the mixture, reduce density which allow thermal expansion and contraction when exposed to extreme temperatures
Why is it important to measure the workability of the mix?
And why slump test is always used in construction sites?
It provides relative ease of concrete it can be placed, compacted and finished without separating individual materials. If concrete was to be pumped , it needs to be workable but also cohesive for correct concrete strength.Another significant part is that the mix is allowed to have homogenous distribution of aggregate and cement
Slump test is simple method test to follow and apply
Slump test is relative cheaper compared to other tests
Slump test gives us immediate assessment of workability of the mix
List mechanical properties that may need consideration in structural design of elements and structures. For each of the properties give short explanation.
Compressive strength:some structural members are partly/fully in compression. Thus it is of relevance cause you need to determine the limiting height in compression so that you can know the height of the member it can be under compressive stress before it fails
Tensile strength: some structural members are partly/ fully in tension. Thus, it is very important that you determine the limiting height that structural member can have while under tensile stress before it fails
Flexural strength: it is of relevance since it helps you determine the strength that structural member can have to maintain bending forces applied perpendicular to longitudinal axis before it fails
Fatigue strength: it is of relevance since it helps you to determine the number of cyclic stresses that structural memeber can maintain before it breaks
Elasticity and plasticity
Creep and shrinkage
How can we minimise the corrosion of reinforcement of concrete?
Concrete must be impermeable
Concrete must be fully compacted and cured properly
Cover to reinforcement must adequate for the exposure conditions
Spacers used to maintain the required cover must impermeable
What is elastic modulus? Give us 3 methods that can be used to determine elastic modulus
The ratio of applied to resultant elastic strain
3 methods: intial tanget modulus
Tangent modulus
Secant modulus
Factors affecting elastic modulus
Stiffness of hardened concrete paste
Stiff of the aggregate
Relative volumes of cement paste and aggregate
Characteristics of interfaces between aggregate and cement paste
