what is UHPC ( Ultra-High Performance Concrete ) ?

Ultra-high-performance concrete (UHPC) is defined as ‘concrete that has a minimum specified compressive strength of 150 MPa with specified durability tensile ductility and tough­ness requirements; fibers are generally included to achieve specified requirements”, as per ACI 239R-18.

Commonly, the UHPC consists of cement, silica fume, fine quartz sand, high range water reducing admixtures, steel fibers, and low water to cementitious materials ranging from 0.15 to 0.25. It is possible to employ different mixture with different constituent materials such as coarse aggregate and supplementary materials to improve a specific property of concrete.

The mechanical properties of UHPC include compressive strength higher than 150 MPa, and sustained post cracking tensile strength greater than 5 MPa. Ultra-high-performance concrete durability is considerably superior to that of ordinary concrete since its pores are discontinuous, which declines liquid penetration. That is why this type of concrete is used in different civil engineering structures such as highway infrastructure applications and structural rehabilitation to address some of the main design, service life, and life cycle costing problems related to the use of concrete.

Specifications of UHPC?

You cannot use any random type of cement for applications that call for Ultra High Performance Concrete. There are various specifications set in place that a concrete should pass to be used as UHPC.

Strength Properties

The strength of UHPC should fall between the following limits:

• The compressive strength should be greater than 17,000 pounds per square inch or 120 MPa. French organizations have this minimum limit increased to 150 MPa.
• The flexural strengths of UHPC should be between 2200 to 3600 psi (about 15 to 25 MPa).
• The modulus of elasticity of UHPC should be between 45 to 50 GPa (about 6500 to 7300 ksi)

Durability Properties

Durability is a critical factor that makes UHPC incredibly useful. In order to qualify as UHPC, the concrete should show the following durability characteristics:

• It should retain 100% of its properties after 300 freeze and thaw cycles. This characteristic is tested by immersing a UHPC prism in a water bath. The water is then continuously taken through multiple cycles of freezing and thawing.
• It should have a Relative Volume Loss Index (for abrasion resistance) of 1.7. This factor is measured by calculating the amount of concrete brushed off a surface by a rotary cutter.
• The loss of residue (salt scaling) of the UHPC matrix should be less than 0.013 pounds per cubic foot (about 60 g/m2).
• The Oxygen Permeability of UHPC should be less than 10^-19 square feet (10^-20 square meters).

Strategies for UHPC Production

1. Improve concrete homogeneity through the removal of coarse aggregate in the mixture.
2. Optimize gradation and mixture proportions between principal components of the mixture to reduce void between particles of mix components, and hence the density of the concrete is improved.
3. Increase concrete ductility by the introduction of steel fibers to the concrete mixture. Minimum 2% by volume of steel fibers are added to the mixture to overcome the brittleness of concrete. Factors such as fiber aspect ratio, shape, and UHPC production issues like workability control the maximum fiber content.

Materials and Mixture Proportions

The ultra-high-performance concrete produced from the following materials:

1. Portland cement
2. Silica Fume
3. Limestone and or quartz flour
4. Fine sand
5. High-range water reducers
6. Water
7. Steel fibers

Table 1 presents two possible mix proportions to produce ultra-high-performance concrete: Table 1 Two Ultra-high-performance concrete mixture proportions by mass

UHPC component	Mixture Proportion 1	Mixture Proportion 2
Cement	1	1
Silica fume	0.325	0.389
Sand	1.432	0.967
Quartz powder/silica flour	0.300	0.277
High-range water-reducing admixture	0.027	0.017
Water	0.280	0.208
Steel fibers	0.200	0.310

Placement and Curing of UHPC

The pouring and curing procedure of UHPC is similar to those already established for use with some HPCs. The fluid mix is virtually self-placing and requires no internal vibration. If needed, external form vibration causes the mix to flow smoothly into place. Following an initial set of 24 hours, the curing process requires at least an additional 48 hours, including a vapor bath at a constant 88 °C. Hence, it is available for loading within three days as compared to almost 30 days in the case of conventional concrete.

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Applications of UHPC

1. Highway bridges
2. Field-cast closure pours for prefabricated bridge elements (Joint-Fills)
3. Piles/foundations
4. Security and blast mitigation applications
5. Seismic retrofit
6. Thin-bonded overlays on deteriorated bridge decks
7. Pedestrian bridges
8. Rehabilitation
9. Spent nuclear fuel storage
10. Facades
11. Impact resistance
12. Aggressive environments
13. Canopies/shells

Advantages of UHPC

1. Simplified construction techniques
2. Speed of construction
3. Improved durability
4. Reduced maintenance
5. Reduced out-of-service
6. Minimum interruption
7. Reduced element size and complexity
8. Extended life span
9. Improved resiliency