16-01-2013, 04:29 PM
High-performance concrete
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INTRODUCTION
High-performance concrete is defined as concrete that meets special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practices. Ever since the term high-performance concrete was introduced into the industry, it had widely used in large scale concrete construction that demands high strength, high flow ability, and high durability. A high-strength concrete is always a high-performance concrete, but a high-performance concrete is not always a high-strength concrete. Durable concrete Specifying a high-strength concrete does not ensure that a durable concrete will be achieved.
HPC is a construction material which is being used in increasing volumes in recent years due to its long term performance & better rheological, mechanical & durability properties than cement concrete. HPC possess invariably high strength, reasonable workability & negligible permeability compared to NCC, preparation of HPC requires lower water binder ratio (w/b) & higher cement content. The HPC permits the use of reduced sizes of structural member, increased building height in congested areas & early removal of formwork.
The use of HPC in prestressed concrete construction makes greater span to depth ratio, early transfer of prestress & application of service loads. Low permeability characteristics of HPC reduce the risk of corrosion of steel & attack of aggressive chemicals. This permits the use of HPC in marine/offshore structures, nuclear power plants, bridges, places of extreme & adverse climatic condition. Eventually HPC reduce maintenance & repair cost.
Fiber Reinforced Concrete:-Fiber Reinforced Concrete can be defined as a composite material consisting of mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed suitable fibers. Continuous meshes, woven fabrics and long wires or rods are not considered to be discrete fibers.
Fiber is a small piece of reinforcing material possessing certain characteristics properties. They can be circular or flat. The fiber is often described by a convenient parameter called “aspect ratio”. The aspect ratio of the fiber is the ratio of its length to its diameter. Typical aspect ratio ranges from 30 to 150.
FRC is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers. Within these different fibers that character of fiber reinforced concrete changes with varying concretes grade, type of fibers, geometries, distribution, orientation, densities & aspect ratio.
Steel Fiber (1): - Steel fibers are available unperforated, corrugated or with wide end for better bending. The fibers can placed single or in the form of mats. The main fields of application of steel fibers are gunned concrete, tunnel constructions & high loaded industrial floors. The addition of steel fiber increases the tensile strength of normal & high strength concrete. It also has positive effects on the tension stiffening behavior, the formation of cracks the tightness & long -term deformations.
It is well known that after failure of concrete, between the cracks which carries tension & hence stiffens the response of a reinforced concrete member subjected to tension. This stiffening effect, after cracking, is referred to as “Tension stiffening”.
Steel fiber reinforcement is widely used as the main and unique reinforcing material for industrial concrete floor slabs, shotcrete and prefabricated concrete products. It is also considered for structural purposes in the reinforcement of slabs on piles, tunnel segments, foundation slabs and shear reinforcement in pre-stressed elements. Ensuring the quality and performance of the steel fibers and ultimately the SFRC is critical and the challenge faced by engineers involved in designing these projects is to unambiguously specify the performance required by the SFRC so as to achieve in the finished structure the performance that was assumed in design.
• Properties of Concrete Improved by Steel Fibers
Below are some properties that the use of steel fibers can significantly improve:
• Flexural Strength: Flexural bending strength can be increased of up to 3 times more compared to conventional concrete.
• Fatigue Resistance: Almost 1.5 times increase in fatigue strength.
• Impact Resistance: Greater resistance to damage in case of a heavy impact.
• Permeability: The material is less porous.
• Abrasion Resistance: More effective composition against abrasion and spalling.
• Shrinkage: Shrinkage cracks can be eliminated.
• Corrosion: Corrosion may affect the material but it will be limited in certain areas.
Why is concrete strong in compression and weak under tension?
Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding metal reinforcing bars, glass fiber, or plastic fiber to carry tensile loads.
SALIENT FEATURES OF HPC (2)
• Compressive strength > 80 Mpa , even up to 800 Mpa
• Water binder ratio=0.25-0.35, therefore very little free water
• Reduced Flocculation of cement grains
• Wide range of grain size
• Densified cement paste
• No bleeding homogeneous mix
• Less capillary porosity
• Discontinuous pores
• Stronger transition zone at the interface between cement paste & aggregate
• Low free lime content
• Endogenous shrinkage
• Powerful confinement of aggregate
• Little micro-cracking until about 65-75% of fck
• Smooth fracture surface
ADVANTAGES OF HPC
The advantages of using high strength high performance concretes often balance the increase in material cost. The following are the major advantages that can be accomplished.
1) Reduction in member size, resulting in increase in plinth area/useable area and direct savings in the concrete volume.
2) Reduction in the self-weight and super-imposed DL with the accompanying saving due to smaller foundations.
3) Reduction in form-work area and cost with the accompanying reduction in shoring and stripping time due to high early-age gain in strength.
4) Construction of High –rise buildings with the accompanying savings in real-estate costs in congested areas.
5) Reduced axial shortening of compression supporting members.
6) Reduction in the number of supports and the supporting foundations due to the increase in spans.
7) Reduction in the thickness of floor slabs and supporting beam sections which are a major component of the weight and cost of the majority of structures.
8) Superior long-term service performance under static, dynamic and fatigue loading.
9) Low creep and shrinkage.
10) Greater stiffness as a result of a higher modulus of elasticity of concrete.
11) Higher resistance to freezing and thawing, chemical attack, and significantly improved long-term durability and crack propagation.
12) Reduced maintenance and repairs.
POLYPROPYLENE FIBER (3)
In the past several years, an increasing number of contractors have placed concrete containing polypropylene fibres. Fibre manufacturers have promoted the material as a practical alternative to the use of welded wire fabric for control of shrinkage and temperature cracking. Addition of fibres in the concrete reduces shrinkage, inhibits shrinkage cracking, reduces permeability and improves impact and abrasion resistance. There is, however, conflicting data concerning the effects of polypropylene fibres’ on the properties of concrete. This article reviews some of the suggested applications for concrete reinforced with fibres and surveys recent studies concerning properties of the fibre- reinforced concrete. We limited our survey to data obtained from tests on concretes containing either 1.5 or 1.6 pounds of collated fibrillated polypropylene fibres per cubic yard of concrete. These are dosage rates recommended by the two major polypropylene fibre manufactures. Results of the testing are fragmentary because there have been a limited number of tests and test conditions investigated. Few of the studies involved field mixing of the concrete containing fibres.
Polypropylene fiber was first used to reinforce concrete in the 1960s. Polypropylene is a synthetic hydrocarbon polymer, the fiber of which is made using extrusion processes by hot-drawing the material through a die. Polypropylene fibers are produced as continuous mono-filaments, with circular cross section that can be chopped to required lengths, or fibrillated films or tapes of rectangular cross section.
Monofilament polypropylene fibers have inherent weak bond with the cement matrix because of their relatively small specific surface area. Fibrillated polypropylene fibers are slit and expanded into an open network thus offering a larger specific surface area with improved bond characteristics.
Reasons for Using Polypropylene Fibres (3)
An unrestrained concrete member will shorten in all directions when it dries or cools. Because most concrete structural members are at least partially restrained, tensile stresses build up when the concrete dries or cools. The stresses are about the same as those that would occur if the concrete had been allowed to contract freely and had then been pulled back to its original length. When these stresses exceed the tensile strength of the concrete, the member cracks. Measures that can be taken to control this cracking include reducing the potential shrinkage of the concrete, providing joints to control crack location and adding non-structural reinforcement. Even if joints are used to control crack location, cracks may still occur between joints. And in structural reinforced concrete, added measures may be needed to control shrinkage and temperature cracking. Goals for the engineer and contractor are to reduce the number of cracks and to keep ones that do form from opening up too wide. Adding polypropylene fibres to the concrete has been suggested as one way of achieving these goals. Other suggested applications for concrete containing polypropylene fibres include structures such as median barriers that are subjected to impact loads, placements where all materials must be non-metallic and areas requiring materials that are resistant to alkalis and other chemicals.
Effects of Fibres on Fresh Concrete (3)
Slump effects
When fibres are added to the concrete slump will decrease. According to one fibre manufacturer’s representative, the reduction in slump depends on the length of fibre used; longer fibres cause a greater slump reduction. Data from recent laboratory and field tests indicate slump losses ranging from 12.5mm to slightly > 75mm, but there is little correlation between slump reduction and fibres length (Table 1). Contractors are cautioned not to add water to restore lost slump. Even at the lower slump, workability of fibre reinforced concrete is said to be adequate for placing, compacting and finishing the concrete.
[attachment=47236]
INTRODUCTION
High-performance concrete is defined as concrete that meets special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practices. Ever since the term high-performance concrete was introduced into the industry, it had widely used in large scale concrete construction that demands high strength, high flow ability, and high durability. A high-strength concrete is always a high-performance concrete, but a high-performance concrete is not always a high-strength concrete. Durable concrete Specifying a high-strength concrete does not ensure that a durable concrete will be achieved.
HPC is a construction material which is being used in increasing volumes in recent years due to its long term performance & better rheological, mechanical & durability properties than cement concrete. HPC possess invariably high strength, reasonable workability & negligible permeability compared to NCC, preparation of HPC requires lower water binder ratio (w/b) & higher cement content. The HPC permits the use of reduced sizes of structural member, increased building height in congested areas & early removal of formwork.
The use of HPC in prestressed concrete construction makes greater span to depth ratio, early transfer of prestress & application of service loads. Low permeability characteristics of HPC reduce the risk of corrosion of steel & attack of aggressive chemicals. This permits the use of HPC in marine/offshore structures, nuclear power plants, bridges, places of extreme & adverse climatic condition. Eventually HPC reduce maintenance & repair cost.
Fiber Reinforced Concrete:-Fiber Reinforced Concrete can be defined as a composite material consisting of mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed suitable fibers. Continuous meshes, woven fabrics and long wires or rods are not considered to be discrete fibers.
Fiber is a small piece of reinforcing material possessing certain characteristics properties. They can be circular or flat. The fiber is often described by a convenient parameter called “aspect ratio”. The aspect ratio of the fiber is the ratio of its length to its diameter. Typical aspect ratio ranges from 30 to 150.
FRC is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers. Within these different fibers that character of fiber reinforced concrete changes with varying concretes grade, type of fibers, geometries, distribution, orientation, densities & aspect ratio.
Steel Fiber (1): - Steel fibers are available unperforated, corrugated or with wide end for better bending. The fibers can placed single or in the form of mats. The main fields of application of steel fibers are gunned concrete, tunnel constructions & high loaded industrial floors. The addition of steel fiber increases the tensile strength of normal & high strength concrete. It also has positive effects on the tension stiffening behavior, the formation of cracks the tightness & long -term deformations.
It is well known that after failure of concrete, between the cracks which carries tension & hence stiffens the response of a reinforced concrete member subjected to tension. This stiffening effect, after cracking, is referred to as “Tension stiffening”.
Steel fiber reinforcement is widely used as the main and unique reinforcing material for industrial concrete floor slabs, shotcrete and prefabricated concrete products. It is also considered for structural purposes in the reinforcement of slabs on piles, tunnel segments, foundation slabs and shear reinforcement in pre-stressed elements. Ensuring the quality and performance of the steel fibers and ultimately the SFRC is critical and the challenge faced by engineers involved in designing these projects is to unambiguously specify the performance required by the SFRC so as to achieve in the finished structure the performance that was assumed in design.
• Properties of Concrete Improved by Steel Fibers
Below are some properties that the use of steel fibers can significantly improve:
• Flexural Strength: Flexural bending strength can be increased of up to 3 times more compared to conventional concrete.
• Fatigue Resistance: Almost 1.5 times increase in fatigue strength.
• Impact Resistance: Greater resistance to damage in case of a heavy impact.
• Permeability: The material is less porous.
• Abrasion Resistance: More effective composition against abrasion and spalling.
• Shrinkage: Shrinkage cracks can be eliminated.
• Corrosion: Corrosion may affect the material but it will be limited in certain areas.
Why is concrete strong in compression and weak under tension?
Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding metal reinforcing bars, glass fiber, or plastic fiber to carry tensile loads.
SALIENT FEATURES OF HPC (2)
• Compressive strength > 80 Mpa , even up to 800 Mpa
• Water binder ratio=0.25-0.35, therefore very little free water
• Reduced Flocculation of cement grains
• Wide range of grain size
• Densified cement paste
• No bleeding homogeneous mix
• Less capillary porosity
• Discontinuous pores
• Stronger transition zone at the interface between cement paste & aggregate
• Low free lime content
• Endogenous shrinkage
• Powerful confinement of aggregate
• Little micro-cracking until about 65-75% of fck
• Smooth fracture surface
ADVANTAGES OF HPC
The advantages of using high strength high performance concretes often balance the increase in material cost. The following are the major advantages that can be accomplished.
1) Reduction in member size, resulting in increase in plinth area/useable area and direct savings in the concrete volume.
2) Reduction in the self-weight and super-imposed DL with the accompanying saving due to smaller foundations.
3) Reduction in form-work area and cost with the accompanying reduction in shoring and stripping time due to high early-age gain in strength.
4) Construction of High –rise buildings with the accompanying savings in real-estate costs in congested areas.
5) Reduced axial shortening of compression supporting members.
6) Reduction in the number of supports and the supporting foundations due to the increase in spans.
7) Reduction in the thickness of floor slabs and supporting beam sections which are a major component of the weight and cost of the majority of structures.
8) Superior long-term service performance under static, dynamic and fatigue loading.
9) Low creep and shrinkage.
10) Greater stiffness as a result of a higher modulus of elasticity of concrete.
11) Higher resistance to freezing and thawing, chemical attack, and significantly improved long-term durability and crack propagation.
12) Reduced maintenance and repairs.
POLYPROPYLENE FIBER (3)
In the past several years, an increasing number of contractors have placed concrete containing polypropylene fibres. Fibre manufacturers have promoted the material as a practical alternative to the use of welded wire fabric for control of shrinkage and temperature cracking. Addition of fibres in the concrete reduces shrinkage, inhibits shrinkage cracking, reduces permeability and improves impact and abrasion resistance. There is, however, conflicting data concerning the effects of polypropylene fibres’ on the properties of concrete. This article reviews some of the suggested applications for concrete reinforced with fibres and surveys recent studies concerning properties of the fibre- reinforced concrete. We limited our survey to data obtained from tests on concretes containing either 1.5 or 1.6 pounds of collated fibrillated polypropylene fibres per cubic yard of concrete. These are dosage rates recommended by the two major polypropylene fibre manufactures. Results of the testing are fragmentary because there have been a limited number of tests and test conditions investigated. Few of the studies involved field mixing of the concrete containing fibres.
Polypropylene fiber was first used to reinforce concrete in the 1960s. Polypropylene is a synthetic hydrocarbon polymer, the fiber of which is made using extrusion processes by hot-drawing the material through a die. Polypropylene fibers are produced as continuous mono-filaments, with circular cross section that can be chopped to required lengths, or fibrillated films or tapes of rectangular cross section.
Monofilament polypropylene fibers have inherent weak bond with the cement matrix because of their relatively small specific surface area. Fibrillated polypropylene fibers are slit and expanded into an open network thus offering a larger specific surface area with improved bond characteristics.
Reasons for Using Polypropylene Fibres (3)
An unrestrained concrete member will shorten in all directions when it dries or cools. Because most concrete structural members are at least partially restrained, tensile stresses build up when the concrete dries or cools. The stresses are about the same as those that would occur if the concrete had been allowed to contract freely and had then been pulled back to its original length. When these stresses exceed the tensile strength of the concrete, the member cracks. Measures that can be taken to control this cracking include reducing the potential shrinkage of the concrete, providing joints to control crack location and adding non-structural reinforcement. Even if joints are used to control crack location, cracks may still occur between joints. And in structural reinforced concrete, added measures may be needed to control shrinkage and temperature cracking. Goals for the engineer and contractor are to reduce the number of cracks and to keep ones that do form from opening up too wide. Adding polypropylene fibres to the concrete has been suggested as one way of achieving these goals. Other suggested applications for concrete containing polypropylene fibres include structures such as median barriers that are subjected to impact loads, placements where all materials must be non-metallic and areas requiring materials that are resistant to alkalis and other chemicals.
Effects of Fibres on Fresh Concrete (3)
Slump effects
When fibres are added to the concrete slump will decrease. According to one fibre manufacturer’s representative, the reduction in slump depends on the length of fibre used; longer fibres cause a greater slump reduction. Data from recent laboratory and field tests indicate slump losses ranging from 12.5mm to slightly > 75mm, but there is little correlation between slump reduction and fibres length (Table 1). Contractors are cautioned not to add water to restore lost slump. Even at the lower slump, workability of fibre reinforced concrete is said to be adequate for placing, compacting and finishing the concrete.