UHPC stands for ultra-high-performance concrete, a class of concrete with high mechanical, rheological, and durability properties compared to traditional concrete. It can achieve high compressive strength, tensile strength, ductility, toughness, and resistance to abrasion and diffusion. It is usually made of cement, silica fume, fine aggregates, superplasticizers, water, and fibers. The proportions of these ingredients are carefully adjusted to maximize the packing density and reduce the water-cement ratio. It also requires special curing and mixing methods to ensure its performance. This article delves into the mix design of UHPC as per the Indian Standard Code, including the ingredients, recipe foundation, and key suppliers of UHPC and its admixtures.
Fig 1: UHPC
Courtesy: RISE
Ingredients for Ultra-High-Performance Concrete (UHPC)
The ingredients of UHPC, as per Indian standard code, are similar to those of normal concrete, except that UHPC uses finer aggregates (usually quartz sand), higher amounts of silica fume and superplasticizer, lower water-cement ratio and the addition of fibers (usually steel fibers).
The following are the primary components used in the mix design of UHPC:
- Cement: Ordinary Portland Cement (OPC) is commonly used as the binder material in UHPC. However, the Indian Standard Code allows using other cementitious materials like silica fume and fly ash to enhance the performance.
- Aggregates: UHPC typically employs fine silica sand as its primary aggregate. The particles should be well-graded and free from impurities.
- Water: High-quality potable water with low levels of impurities is essential to maintain the desired workability and strength of UHPC.
- Fibers: UHPC incorporates steel fibers to enhance its tensile strength and ductility.
Fig 2: Fibers in UHPC
Courtesy: Unique UHPC Infra LLP
- Superplasticizer: A high-range water-reducing superplasticizer achieves the required workability without compromising strength.
- Mineral Admixtures: Supplementary cementitious materials like silica fume, fly ash, and metakaolin are often added to improve the microstructure and long-term durability of UHPC.
- Chemical Admixtures: Various chemical admixtures, such as retarders and accelerators, may be used to control the setting time of UHPC as per project requirements.
Steps to Achieve Ultra-High-Performance Concrete (UHPC)
To achieve UHPC concrete, the following steps are recommended:
- Select suitable materials that meet the quality and grading requirements for UHPC. For example, use ordinary Portland cement (Type I), ground granulated blast furnace slag (GGBS) cement (slag cement), silica fume, two types of silica sand (fine and coarse), and short steel fibers.
- Use a mixed design method that considers particulate matter’s packing density and size distribution. For example, use the linear density or continuously graded particle packing models. Alternatively, use a machine learning algorithm such as artificial neural network (ANN) or Gaussian process regression (GPR) to develop a high-accuracy model for predicting the properties of UHPC based on the mixture proportions.
- Use a superplasticizer that can provide sufficient workability and flowability for UHPC without compromising its strength and durability. For example, a polycarboxylate-based superplasticizer can reduce the water-cement ratio and increase the slump flow.
- Use a special mixing method to ensure a homogeneous ingredient distribution and avoid segregation or bleeding. For example, use a high-shear or planetary mixer that can provide sufficient mixing energy and time.
- Use a special curing method that can accelerate the hydration and strength development of UHPC. For example, use steam curing or hot water curing at elevated temperatures (60-90°C) for several hours or days.
Mix Design of Ultra-High-Performance Concrete (UHPC)
The mix design of ultra-high-performance concrete (UHPC) is a complex and challenging task requiring careful selection and optimization of the ingredients and proportions. UHPC is a concrete class with exceptional mechanical, rheological, and durability properties compared to conventional concrete. UHPC can achieve very high compressive strength (above 150 MPa), tensile strength (above 10 MPa), ductility, toughness, and resistance to abrasion, corrosion, and penetration. UHPC comprises cement, silica fume, fine aggregates, superplasticizers, water, and fibers. The main principles of the mix design of UHPC are:
- To achieve a high particulate matter packing density and reduce the matrix’s voids and water content.
- To use a low water-cement ratio (below 0.25) and a high amount of superplasticizer (above 2% by mass of cement) to enhance the workability and flowability of the fresh concrete.
- To use a high amount of silica fume (above 20% by mass of cement) to improve the pozzolanic reaction and fill the micro-pores in the matrix.
- To use fine aggregates (usually quartz sand) with a maximum size of 1 mm or less to avoid interlocking and increase the homogeneity of the matrix.
- To use fibers (usually steel fibers) with a volume fraction of 1-3% to improve the post-cracking behavior and toughness of the concrete.
Suppliers of UHPC and Admixtures for UHPC in India
There are several suppliers of UHPC in India, such as:
- HiPer Fiber Solutions provides nonproprietary UHPC mixes with locally available materials and fibers.
- LafargeHolcim offers Ductal®, a proprietary UHPC product with various applications and solutions.
There are also several admixture suppliers of UHPC in India, such as:
- Sika offers a Sika® ViscoCrete® range of superplasticizers for UHPC with high flowability and stability.
Fig 3: Sika® ViscoCrete®
Courtesy: Aplitek SRL
- Fosroc delivers Auramix420SR SP range of superplasticizers for UHPC with high water reduction and slump retention.
Conclusion
Ultra-High Performance Concrete (UHPC) is a revolutionary construction material renowned for its exceptional strength, durability, and versatility. The formulation of UHPC involves a meticulous selection of key ingredients, including top-quality cement, fine silica sand, steel fibers, and various admixtures, aimed at achieving unparalleled properties. The foundation of a successful UHPC mix lies in optimizing particle packing, setting minimum cement content, and maintaining a low water-cement ratio. This approach ensures the densest possible matrix and robust structural performance.