Sab 2112 - L7 Concrete
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CONCRETE
Topics
Concrete Water-cement ratio Workability tests Bleeding Segregation
Concrete
Is one of the most common construction material Is used in a wide variety applications ranging from piles, multistory buildings, dams, foundations, pavements, storage tanks, bridges, and many other structures. Is one of the most economical materials of construction, very versatile in nature and application
Concrete Constituents
Cement Fine aggregate Coarse aggregate Water Admixtures – chemical and pozzolanic
Important Properties of Concrete
Compressive strength Durability (weather resistance) Impermeability Resistance to environmental attacks Abrasion resistance
Types of Concrete
1) 2) 3)
Based on aggregate: Lightweight concrete Normal weight concrete Heavyweight concrete
Based on strength
Normal strength concrete ( 0 to 50 MPa) High-strength concrete (51 to 80 MPa) Very High-strength concrete ( greater than 80 MPa)
Based on construction
In-situ concrete Precast concrete Prestressed concrete
WATER-CEMENT RATIO
W/C RATIO The ratio of weight of water to weight of cement used in the mix Weight of water = water added + free water Water-cement ratio will have an effect on strength and durability of concrete
Abram’s Law
Assuming full compaction, and at a given age and normal temperature, strength of concrete can be taken to be inversely proportional to the watercement ratio
W/C vs Compressive strength
W/C Ratio
At a given degree of hydration, the water cement ratio determines the porosity of the cement paste (gel pores, capillary pores and entrapped air) Lower w/c ratio will increase concrete strength due to lower porosity With an increase in age, the degree of hydration generally increases so that strength increases
W/C Ratio
One of the most important factors affecting the strength and durability of concrete W/C ratio for normal strength concrete – 0.45-0.6 W/C ratio for high strength concrete – less than 0.45 W/C ratio will also affect the workability of concrete
PROPERTIES OF FRESH CONCRETE
WORKABILITY
The amount of work required in mixing, placing, and compacting the fresh concrete without segregation
1) 2) 3)
Consistency Mobility Compactibility
Factors Affecting Workability 1) 2) 3) 4) 5) 6) 7)
Water content Cement content and fineness Aggregate type and grading Size of aggregates Aggregate-cement ratio Admixtures Weather/temperature
Workability
Higher water content will increase the inter-particle lubrication. Hence increasing the workability of fresh concrete Fineness of cement is of minor influence on workability Higher cement content increases the workability (W/C) Finer particles require more water to wet their larger specific surface
Cont. Irregular shape and rougher texture of an angular aggregate demand more water than rounded aggregate For a constant w/c ratio, the workability increases as the aggregate-cement ratio is reduced because the amount of water relative to the total surface of solids is increased
Cont. The use of chemical admixture increases the workability of concrete Fly Ash also known to increase the workability due to its spherical shape Higher temperature reduces workability and increases the slump loss
WORKABILITY TESTS Slump
test Compacting Factor test Vebe test
Slump Test
The mould for the slump test is a frustum of a cone, 300 mm high. The base of 200 mm diameter is placed on a smooth surface with the smaller opening of 100 mm diameter at the top. The container is filled with concrete in three layers Each layer is tapped 25 times with a standard 16 mm diameter steel rod, rounded at the end
Cont.
The top surface is leveled using trowel The mould must be firmly held against its base during the entire operation The cone is slowly lifted upward, and the unsupported concrete will slump The decrease in the height of the center of the slump concrete is called SLUMP (measured in mm)
SLUMP TEST
Types of Slump True slump - accept Shear slump – need to redo the test, indication of lack of cohesion of the mix Collapse - reject
Types of Slump
Without superplasticizer With superplasticizer
Example Concrete A – slump 150 mm Concrete B – slump 30 mm
Compacting Factor Test The degree of compaction, called the compacting factor, is measured by the density ratio Density ratio – The ratio of the density actually achieved in the test to the density of the same concrete fully compacted
Compacting Factor Test
Cont.
Compacting factor = Uncompacted concrete Compacted concrete
Value – maximum is one (1) Example: Concrete A – 0.8 Concrete B – 0.4
Vebe test
Slump test is performed inside the container The disc rider is placed on top of the unsupported concrete The vibrating machine is switch on and the concrete will be compacted inside the container Compaction is complete when the transparent rider is totally covered with concrete The time taken for the whole process is known as VEBE TIME
Vebe Test
Example
Concrete A – Vebe time is 10 seconds Concrete B – Vebe time is 5 seconds
Factors Affecting Consistency and Workability of Fresh Concrete
Water content –
higher water content will increase the workability Fineness of cement – workability decreases as the fineness increases Chemical admixture – increase workability Pozzolanic admixture – PFA increase workability Aggregates – depends on the shapes and sizes of aggregates Temperature – higher temperature reduces workability
Test 1
Wednesday 19th Feb 2008 8.30 – 9.30 pm C09-407, 408 Two Questions – topic up to concrete on site
SEGREGATION
SEGREGATION Is defined as the tendency for separation of large and fine particles in a fresh concrete mix Results in a non-homogenous mix that affects the strength and durability of the hardened concrete Segregation one of the causes of pores and honeycombed surface
Segregation occurs due to: Dry
mix Very wet mix Coarser mix
Types of segregation During placing and compaction – concrete mix contains too many coarse aggregate Cement paste segregate from concrete mix – the mix is too wet
Effect of Segregation Lower
compressive strength – strength not uniform
Lack
on durability – higher
porosity
Methods to minimize segregation
Concrete has good workability Concrete not over compacted Correct placing of concrete Nearby construction (piling) should be avoided
BLEEDING
water
concrete
BLEEDING
Is defined as the process of separation of water from the fresh concrete Happens when the concrete mix does not possess the proper consistency that makes it unable to hold the mixing water
Bleeding results in the movement of water and the finer particles to the top of the form and produces a nonhomogenous mix Over vibration, over troweling and lean mixes increases the potential for bleeding
Effect of Bleeding
Weaker concrete Causing fine cracks below large aggregate particles
Minimize Bleeding 1) 2)
3)
4)
Reduce water content Increase finer particles in the mix Use air-entrainment admixture Proper compaction
FACTORS AFFECTING CONCRETE STRENGTH and DURABILITY
Water-cement ratio –
Good mix design –
higher w/c ratio will decrease concrete strength of concrete constituents
Cement content –
proportions
higher cement content increases compressive strength by the production of more C-S-H gel
Cont.
Types of aggregates –
Use of chemical –
Pozzolanic admixture –
size, grading, strength of aggregates will affect concrete strength air-entraining admixture reduces compressive strength gel, make concrete denser
Proper Compaction –
Curing –
increase C-S-H
good compaction reducing the amount of voids improve concrete strength by enhancing the hydration process of cement
Xie Xie DeSiRe!!!!!!!
Good Health…sleep
Topics
Concrete Water-cement ratio Workability tests Bleeding Segregation
Concrete
Is one of the most common construction material Is used in a wide variety applications ranging from piles, multistory buildings, dams, foundations, pavements, storage tanks, bridges, and many other structures. Is one of the most economical materials of construction, very versatile in nature and application
Concrete Constituents
Cement Fine aggregate Coarse aggregate Water Admixtures – chemical and pozzolanic
Important Properties of Concrete
Compressive strength Durability (weather resistance) Impermeability Resistance to environmental attacks Abrasion resistance
Types of Concrete
1) 2) 3)
Based on aggregate: Lightweight concrete Normal weight concrete Heavyweight concrete
Based on strength
Normal strength concrete ( 0 to 50 MPa) High-strength concrete (51 to 80 MPa) Very High-strength concrete ( greater than 80 MPa)
Based on construction
In-situ concrete Precast concrete Prestressed concrete
WATER-CEMENT RATIO
W/C RATIO The ratio of weight of water to weight of cement used in the mix Weight of water = water added + free water Water-cement ratio will have an effect on strength and durability of concrete
Abram’s Law
Assuming full compaction, and at a given age and normal temperature, strength of concrete can be taken to be inversely proportional to the watercement ratio
W/C vs Compressive strength
W/C Ratio
At a given degree of hydration, the water cement ratio determines the porosity of the cement paste (gel pores, capillary pores and entrapped air) Lower w/c ratio will increase concrete strength due to lower porosity With an increase in age, the degree of hydration generally increases so that strength increases
W/C Ratio
One of the most important factors affecting the strength and durability of concrete W/C ratio for normal strength concrete – 0.45-0.6 W/C ratio for high strength concrete – less than 0.45 W/C ratio will also affect the workability of concrete
PROPERTIES OF FRESH CONCRETE
WORKABILITY
The amount of work required in mixing, placing, and compacting the fresh concrete without segregation
1) 2) 3)
Consistency Mobility Compactibility
Factors Affecting Workability 1) 2) 3) 4) 5) 6) 7)
Water content Cement content and fineness Aggregate type and grading Size of aggregates Aggregate-cement ratio Admixtures Weather/temperature
Workability
Higher water content will increase the inter-particle lubrication. Hence increasing the workability of fresh concrete Fineness of cement is of minor influence on workability Higher cement content increases the workability (W/C) Finer particles require more water to wet their larger specific surface
Cont. Irregular shape and rougher texture of an angular aggregate demand more water than rounded aggregate For a constant w/c ratio, the workability increases as the aggregate-cement ratio is reduced because the amount of water relative to the total surface of solids is increased
Cont. The use of chemical admixture increases the workability of concrete Fly Ash also known to increase the workability due to its spherical shape Higher temperature reduces workability and increases the slump loss
WORKABILITY TESTS Slump
test Compacting Factor test Vebe test
Slump Test
The mould for the slump test is a frustum of a cone, 300 mm high. The base of 200 mm diameter is placed on a smooth surface with the smaller opening of 100 mm diameter at the top. The container is filled with concrete in three layers Each layer is tapped 25 times with a standard 16 mm diameter steel rod, rounded at the end
Cont.
The top surface is leveled using trowel The mould must be firmly held against its base during the entire operation The cone is slowly lifted upward, and the unsupported concrete will slump The decrease in the height of the center of the slump concrete is called SLUMP (measured in mm)
SLUMP TEST
Types of Slump True slump - accept Shear slump – need to redo the test, indication of lack of cohesion of the mix Collapse - reject
Types of Slump
Without superplasticizer With superplasticizer
Example Concrete A – slump 150 mm Concrete B – slump 30 mm
Compacting Factor Test The degree of compaction, called the compacting factor, is measured by the density ratio Density ratio – The ratio of the density actually achieved in the test to the density of the same concrete fully compacted
Compacting Factor Test
Cont.
Compacting factor = Uncompacted concrete Compacted concrete
Value – maximum is one (1) Example: Concrete A – 0.8 Concrete B – 0.4
Vebe test
Slump test is performed inside the container The disc rider is placed on top of the unsupported concrete The vibrating machine is switch on and the concrete will be compacted inside the container Compaction is complete when the transparent rider is totally covered with concrete The time taken for the whole process is known as VEBE TIME
Vebe Test
Example
Concrete A – Vebe time is 10 seconds Concrete B – Vebe time is 5 seconds
Factors Affecting Consistency and Workability of Fresh Concrete
Water content –
higher water content will increase the workability Fineness of cement – workability decreases as the fineness increases Chemical admixture – increase workability Pozzolanic admixture – PFA increase workability Aggregates – depends on the shapes and sizes of aggregates Temperature – higher temperature reduces workability
Test 1
Wednesday 19th Feb 2008 8.30 – 9.30 pm C09-407, 408 Two Questions – topic up to concrete on site
SEGREGATION
SEGREGATION Is defined as the tendency for separation of large and fine particles in a fresh concrete mix Results in a non-homogenous mix that affects the strength and durability of the hardened concrete Segregation one of the causes of pores and honeycombed surface
Segregation occurs due to: Dry
mix Very wet mix Coarser mix
Types of segregation During placing and compaction – concrete mix contains too many coarse aggregate Cement paste segregate from concrete mix – the mix is too wet
Effect of Segregation Lower
compressive strength – strength not uniform
Lack
on durability – higher
porosity
Methods to minimize segregation
Concrete has good workability Concrete not over compacted Correct placing of concrete Nearby construction (piling) should be avoided
BLEEDING
water
concrete
BLEEDING
Is defined as the process of separation of water from the fresh concrete Happens when the concrete mix does not possess the proper consistency that makes it unable to hold the mixing water
Bleeding results in the movement of water and the finer particles to the top of the form and produces a nonhomogenous mix Over vibration, over troweling and lean mixes increases the potential for bleeding
Effect of Bleeding
Weaker concrete Causing fine cracks below large aggregate particles
Minimize Bleeding 1) 2)
3)
4)
Reduce water content Increase finer particles in the mix Use air-entrainment admixture Proper compaction
FACTORS AFFECTING CONCRETE STRENGTH and DURABILITY
Water-cement ratio –
Good mix design –
higher w/c ratio will decrease concrete strength of concrete constituents
Cement content –
proportions
higher cement content increases compressive strength by the production of more C-S-H gel
Cont.
Types of aggregates –
Use of chemical –
Pozzolanic admixture –
size, grading, strength of aggregates will affect concrete strength air-entraining admixture reduces compressive strength gel, make concrete denser
Proper Compaction –
Curing –
increase C-S-H
good compaction reducing the amount of voids improve concrete strength by enhancing the hydration process of cement
Xie Xie DeSiRe!!!!!!!
Good Health…sleep
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