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.
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.