High performance concrete (HPC) is produced in order to meet several requirements which are crucial for the successful construction of buildings in special circumstances. The performance requirements of HPC may involve improvement of concrete placement and compaction without segregation, long-term mechanical properties, early age strength, toughness, volume stability, and service life.
HPC is an engineered concrete which meets special performance and uniformity requirements that cannot be always achieved routinely by using only conventional materials and normal mixing, placing and curing practices as per ACI code. It addresses most deficiencies of conventional concrete for instance durability in severe environments, time of construction, energy absorption capacity for earthquake-resistant structures, and repair and retrofitting jobs.
So, high performance concrete is far superior to conventional cement concrete as the ingredients of HPC contribute most optimally and efficiently to the various properties. Finally, it should be known that all high performance concrete has high strength capacity but all high strength concrete may not of high performance. It is also worth mentioning that all requirements might not be obtained at the same time.
Requirements of High Performance Concrete
1. High-early Strength
High early strength is an important characteristic which high performance concrete needs to possess in order to be classified as HPC. This property enables HPC to obtain its specified strength at an earlier age compared with normal concrete. The time period in which a specified strength should be achieved may range from a few hours to several days.
High early strength property makes this type of concrete to be used for prestressed concrete to allow for early stressing, precast concrete for rapid production of elements, high-speed cast-in-place construction, rapid form reuse, cold-weather construction, and many other uses. High early strength can be produced using one or combination of the following materials and/or practices:
- High-early-strength cement
- High cement content (400 to 600 kg/m3)
- Low water-cementing materials ratio (0.20 to 0.45 by mass)
- Higher freshly mixed concrete temperature
- Higher curing temperature
- Chemical admixtures
- Supplementary cementing materials
- Steam curing
- Insulation to retain heat of hydration
2. High Strength
High performance concrete can have higher compression strength compared with conventional concrete. if the compressive strength of concrete is greater than 40Mpa then it is considered as high strength in accordance with ACI Code.
High compressive strength concrete is produced under careful control in concrete plants. Production of high-strength concrete may or may not require the purchase of special materials. The producer must know the factors affecting compressive strength and know-how to vary those factors for best results.
3. High Durability
Deficiencies in the durability of conventional concrete lead to many problems in aggressive environments. However, high performance concrete needs to address most of these problems due to abrasions, freezing and thawing, carbonations, sulphate attack, blasts, and alkali-aggregate reactions otherwise it does not deliver the expected performance during its life service.
The resistance to abrasion is associated with concrete strength that is why HPC is ideal answer for abrasive conditions. Such concrete is suitable for the construction of spillways and stilling basins, and concrete pavements or concrete pavement overlays subjected to heavy or abrasive traffic
As far as blast resistance requirement is concerned, the HPC should have compressive strength greater than 120MPa and contain steel fibers. Blast-resistant concretes are often used in bank vaults and military applications.
Both durability and life span of concrete is dependent on the concrete cover permeability. Commonly, high performance concrete should have considerably low permeability to air, water, and chloride ions. Low permeability achieved through good compaction, low water to cement ratio, and sometimes using special types of cement.
High performance concrete possesses great resistance to diffusion on its surface due to harmful ions like chloride because water to cement ratio is low. The lower the water to cementing ratio the lower the diffusion coefficient. Supplementary cementing materials, particularly silica fume, further reduce the diffusion coefficient.
High performance concrete should have good resistance against carbonation. This can be achieved through low permeability. By and large, the resistance of high performance concrete to carbonation is well above the life span of the structure provided that adequate concrete cover is provided.
Control of temperature during production, placement, and curing play crucial role in the quality, strength, and durability of high performance concrete. It is recommended to practice a construction and placing method that yield a low temperature at the time of delivery, the smallest possible maximum temperature after placing, minimum temperature gradients after placing and a gradual reduction to ambient temperature after maximum temperature is reached.
Excessively high temperatures and gradients can cause excessively fast hydration and micro- and macro-cracking of the concrete. So, the use of liquid nitrogen, ice, or chilled water as part of the mix water can control temperature.
Commonly, HPC should be able to resist scaling and break up caused by freezing and thawing. This is because of low water to cement ratio. Freeze-thaw resistance can be improved significantly through the use of air entraining agent.
HPC need to provide great resistance against chemical attacks. This can be achieved primarily by the use of a dense, strong concrete of very low permeability and low water-to-cementing materials ratio.
Reactivity between certain siliceous aggregates and alkali hydroxides can affect the long-term performance of concrete. This is avoided in HPC due to very low water to cement ratios which dry out to a level that does not allow ASR to occur and the use of significant amounts of supplementary cementing materials that may have the ability to control alkali-silica reactivity.
The service life of high performance concrete is usually longer than that of conventional concrete because of superior durability and high strength. That is why the life service of structures constructed with HPC is greater than buildings constructed with conventional concrete.
High performance concrete shall have satisfactory toughness. This can be achieved by using suitable aggregate such as basalt and appropriate fiber like polypropylene fiber in concrete mixture.