High Performance Concrete
High Performance Concrete
High Performance Concrete is that concrete which meets special performance and uniformity requirements that cannot always be achieved by conventional material, normal mixing, placing and curing pra...
High Performance Concrete is that concrete which meets special performance and uniformity requirements that cannot always be achieved by conventional material, normal mixing, placing and curing practices.
Architects, engineers and constructors all over the world are finding that using HPC allows them to build more durable structures at comparable cost. HPC is being used for building in aggressive environments, marine structures, highway bridges and pavements, nuclear structures, tunnels, precast units.
This reports aims to discuss the application of HPC particularly for bridge structures. The use of HPC was found to have added advantages compared with normal concrete in areas of strengths, service life, construction time, economy, etc.
Concrete is considered as durable and strong material. Reinforced concrete is one of the most popular material used for construction around the world. Reinforced concrete is exposed to deterioration in some regions especially in costal regions. There for researchers around the world are directing their efforts towards developing a new material to over come this problem. Invention of large construction plants and equipments around the world added to the increased use of material. This scenario led to the use of additive materials to improve the quality of concrete. As an out come of the experiments and researches cement based concrete which meets special performance with respect to workability, strength and durability known as” High Performance Concrete” was developed.
HIGH PERFORMANCE CONCRETE
High performance concrete (HPC) is that which is designed to give optimised performance characteristics for the given set of materials, usage and exposure conditions, consistent with requirement of cost, service life and durability.
The American Concrete Institute (ACI) defines HPC ‘‘as concrete which meets special performance and uniformity requirements that cannot always be achieved routinely by using only conventional materials and normal mixing, placing, and curing practices.”
High performance in a broad manner can be related to any property of concrete. It can mean excellent workability in the fresh state like self-levelling concrete or low heat of hydration in case of mass concrete, or very rigid setting and hardening of concrete in case of sprayed concrete or quick repair of roads and airfields, or very low imperviousness of storage vessels, or very low leakage rates of encapsulation containments for contaminating material.
HPC is composed of the same material as normal concrete, but it has been engineered to achieve enhanced durability or strength characteristics, or both, to meet the specified demands of a construction project. The main ingredients of high performance concrete are cement, fine aggregate, coarse aggregate, water, mineral admixtures and chemical admixtures.
If the structure of normal strength concrete (NSC) is compared with high performance concrete (HPC) one notes several differences: The matrix stiffness of HPC is larger than NSC and approaches the stiffness of the aggregate, the bond strength between matrix and aggregate is higher for HPC, matrix tensile strength is higher, Reduced internal cracking in terms of number of cracks and size of intrinsic cracks before loading. These aspects show that HPC is more elastic and more brittle than NSC.
HPC has a greater Young’s modulus than NSC and the post-peak softening branch is steeper. High Performance Concrete (HPC) is more homogeneous than normal strength concrete (NSC).
HPC does not simply mean high strength concrete (HSC), but also includes other enhanced material properties such as early-age strength, increased flow ability, high modulus of elasticity (MOE), low permeability, and resistance to chemical and physical attack (increased durability). HPC is usually high strength concrete (HSC), but HSC may not always be of high performance.
HIGH PERFORMANCE CONCRETE CHARACTERISTICS
High-performance concrete characteristics are developed for particular applications and environments; some of the properties that may be required include:
• High strength
• High early strength
• High modulus of elasticity
• High abrasion resistance
• High durability and long life in severe environments
• Low permeability and diffusion
• Resistance to chemical attack
• High resistance to frost and deicer scaling damage
• Toughness and impact resistance
• Volume stability
• Ease of placement
• Compaction without segregation
• Inhibition of bacterial and mold growth
PREPERATION OF HPC
High-performance concretes are made with carefully selected high-quality ingredients and optimized mixture designs; these are batched, mixed, placed, compacted and cured to the highest industry standards. Typically, such concretes will have a low water-cementing materials ratio of 0.20 to 0.45. Plasticizers are usually used to make these concretes fluid and workable. Table 1 lists materials often used in high-performance concrete and why they are selected.
HIGH PERFORMANCE CONCRETE BRIDGES
High performance concrete bridges include two key elements: total precast bridge systems that can dramatically improve construction speed and high performance concrete that can improve durability and structural efficiency. In HPC bridges, these improvements are achieved at no cost premium and often at a reduced initial cost.
Designing with HPC components can drastically reduce construction time because various precast components can be combined to allow a truck-to-structure systems approach without waiting for site forming and curing. Full depth precast decks are being used on both new and rehabilitated bridges. The cost for this approach can result in overall savings due to more efficient designs that permit longer spans or fewer girders and/or piers. HPC can be used effectively in virtually all bridge components to aid in minimizing construction and future maintenance. HPC components can include piles and pile caps, piers and column bents, abutments, decks, and rails and barriers. HPC uses the same materials as typical concrete but is engineered to provide higher strength and better durability. These attributes can be varied to align with the design’s needs. They will be affected by environmental and geographic conditions and the specific bridge components (that is, substructure, beams or deck).
ADVANTAGES OF HPC BRIDGES
Overall, the advantages accruing from higher durability and/or additional strength include a variety of benefits:
• Longer service life thanks to higher durability and lower chloride penetration. When needed, bridge life can extend to 100 years or even more.
• Lower maintenance and inspection requirements, especially since the bridge requires no painting or rust protection. This savings grows with the bridge’s longer service life.
• Longer spans, which can reduce costs by eliminating piers or allowing the use of concrete beams instead of steel beams.
• Wider beam spacing, reducing the number and cost of beams.
• Shallower beams due to higher concrete strength.
• Improved mechanical properties such as greater tensile strength.
• Rapid construction due to the ability to factory-cast components while site work is underway and the ability to erect pieces upon delivery. These benefits cut the time necessary for disruptions to local traffic.
• Predictable performance and close tolerances for precast members due to the high quality achieved through PCI certification and casting under controlled conditions in the plant.
In general, HPC components can produce lighter, longer precast pieces and smaller-diameter columns that creep less. This means span lengths can be lengthened and under clearances can be maximized.