High Performance Concrete

About High Performance Concrete :- :- 

        High performance concrete (HPC) is that which is designed to give optimized 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 nominal mixing, placing, and curing practices.” The performance may involve enhancements of characteristics such as placement and compaction without segregation, long-term mechanical properties, and early age strength or service life in severe environments.

        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-leveling 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.

        Also, the concrete must have a durability factor greater than 80 after 300 cycles of freezing and thawing to meet their definition.

        "All high-strength concrete is high-performance concrete, but not all high-performance concrete is high-strength concrete," says Henry G. Russell, consulting engineer and former chairman of the American Concrete Institute's high-performance concrete committee. High-performance concrete (HPC) is not one product but includes a range of materials with special properties beyond conventional concrete and routine construction methods.

        Any concrete which satisfies certain criteria proposed to overcome limitations of conventional concretes may be called High Performance Concrete. It may include concrete, which provides either substantially improved resistance to environmental influences or substantially increased structural capacity while maintaining adequate durability. It may also include concrete, which significantly reduces construction time to permit rapid opening or reopening of roads to Traffic, without compromising long-term serviceability. Therefore it is not possible to provide a unique definition of High Performance Concrete without considering the performance requirements of the intended use of the concrete.

General Characteristics Of High Performance Concrete 

1.   High strength    

2.   High early strength

3.   High modulus of elasticity

4.   High abrasion resistance

5.   High durability and long life in severe environments

6.   Low permeability and diffusion

Durability Characteristics 

        The most important property of High Performance Concrete, distinguishing it from conventional cement concrete is it’s far higher superior durability. This is due to the refinement of pore structure of microstructure of the cement concrete to achieve a very compact material with very low permeability to ingress of water, air, oxygen, chlorides, sulphates and other deleterious agents. Thus the steel reinforcement embedded in High Performance Concrete is very effectively protected. 

As far as the Resistance to freezing and thawing is concerned, several aspects of High Performance Concrete should be considered. First, the structure of hydrated cement paste is such that very little freezable water is present. Second, entrained air reduces the strength of high performance concrete because the improvement in workability due to the air bubbles cannot be fully compensated by a reduction in the water content in the presence of a super plasticizer. In addition, air entrainment at very low water/cement ratio is difficult. It is, therefore, desirable to establish the maximum value of the water/cement ratio below which alternating cycles of freezing and thawing do not cause damage to the concrete. 

The abrasion resistance of High Performance Concrete is very good, not only because of high strength of the concrete but also because of the good bond between the coarse aggregate and the matrix which prevents differential wear of the surface. The absence of open pores in the structure zone of High Performance Concrete prevents growth of bacteria. Because of all the above- reasons, High Performance Concrete is said to have better durability characteristics when compared to conventional cement concrete.

Aggregates

Coarse aggregate is one of the most important materials in HPC. The following are some general guidelines to be considered when selecting a coarse aggregate for use in the production of HPC. These include limiting the maximum size of the aggregate to less than 1 inch, which ensures good compatibility. The use of coarse aggregate with lower percent voids results in the production of high compressive strength concrete because the mixing water can be reduced and still maintain good workability.

Smaller maximum size aggregates are typically needed to ensure a high mortar to aggregate bond. Smaller size aggregate also allows for closer spacing between reinforcing steel. It has been found that the use of a coarser gradation of coarse aggregate often results in the achievement of higher compressive strength concrete as a result of being able to use less mixing water while ensuring the same workability. A general guideline developed by ACI Committee 211 suggests that for concrete less than 60 MPa compressive strength, use ¾ to 1 inch maximum size aggregate. For concrete compressive strength greater than 60 MPa, use 3/8 to ½ inch size aggregate.

Curing

Durability of concrete is dependent on many things materials, batching, handling, placing, finishing, and curing. Curing is the protection provided to new concrete to assure the desirable characteristics of the concrete are maximized. Proper curing provides an environment for the concrete; this means keeping the concrete at the proper temperature and moisture conditions to maximize the hydration of the cementitious materials. Thorough hydration provides many enhancements to concrete properties including improved strength gain, reduced permeability, improved freeze-thaw resistance, and reduced plastic shrinkage cracking. Yet, knowing all this, curing is often treated as a secondary operation. As noted in the Introduction to this Section, the success of HPC is dependent on increased attention to detail. Nowhere is this more evident than with curing. As HPC technology and the increased use of supplementary cementitious materials moves forward, proper specifications for curing HPC, followed by improved curing practices during construction, must be implemented.

Conclusion

The design of HPC is met when materials are optimized to produce a strong durable concrete. 

The water, cementitious materials, aggregates and chemical admixtures all need to be proportioned effectively to deliver the mix with the most desirable properties for placement, finishing, curing, and hardened condition.

 The designs are not cook book and in most cases require that the mix be trial batched to compare the fresh and hardened properties. As mentioned earlier in this section, the designer needs to be innovative with his materials and the proportioning of these materials.