#7 Ultra High Performance Concrete (UHPC) | Part 1 | Science & Technology of Concrete

Introduction to Ultra High Performance Concrete (UHPC)

Overview of UHPC and Material Design

• The lecture focuses on Ultra High Performance Concrete (UHPC), covering its material design and properties over the first 30 minutes, followed by a discussion on its characteristics for 20-25 minutes.

• Traditional concrete typically has a strength of about 20-25 MPa, while high-performance concrete can reach up to 50 MPa, and UHPC is designed with compressive strengths around 150 MPa or approximately 25000 psi.

• Achieving such high performance involves extremely dense particle packing in the microstructure, ensuring particles are closely arranged to facilitate effective stress transfer.

• The design also incorporates minimal water-binder ratios to achieve high packing density and low porosity, enhancing flexibility through the addition of steel fibers.

Key Properties Required for UHPC

• Beyond just compressive strength, UHPC must exhibit high ductility, corrosion resistance, and durability for specific applications.

• Traditional concrete requires tightly spaced rebar cages for heavy load-bearing members like bridge girders; however, UHPC's enhanced tensile strength reduces the need for extensive reinforcement.

Advantages of Using UHPC

Structural Benefits

• Fiber reinforcement in UHPC helps maintain tensile strength beyond cracking limits, improving overall structural integrity and reducing crack propagation.

• With higher strength levels in concrete components, the length of rebar development can be minimized, leading to cost savings in materials.

Applications of UHPC

• In the United States, selective applications have included bridges like Mars Hill Bridge in Iowa and Cat Point Creek Bridge in Virginia where UHPC was utilized primarily for bridge decks due to their load-bearing requirements.

Connection Techniques in Precast Concrete

Importance of Connections

• When using precast concrete bridges with traditional high-performance connections on-site casting may lead to weak joints if quality control is inadequate.

• Steel connections were previously used but faced corrosion issues; thus, using UHPC has become a preferred option for joining girders effectively.

Designing with Materials

Material Selection Strategies

• The lecture emphasizes that designing ultra-high performance concrete involves careful selection of materials available locally rather than relying solely on proprietary mixtures costing around $2000 per cubic meter compared to traditional concrete at $125 per cubic meter.

Mixing Considerations

• A low water-cement ratio (less than 0.2), along with proper mixing techniques including self-compacting methods are crucial challenges when creating effective mixtures.

Packing Density Challenges

Particle Size Distribution

• Effective packing requires a range of particle sizes from sub-micron silica fume up to larger aggregates ensuring void spaces are filled adequately which enhances flowability.

Aggregate Selection Criteria

• Aggregates play a critical role in dimensional stability; smaller aggregates are necessary as larger ones may not fit within tight reinforcements found in structures like beams or columns.

Binder Material Choices

Selecting Binders Effectively

• Choosing appropriate binder materials is essential; options include Portland cement alongside supplementary materials like silica fume or metakaolin which enhance performance characteristics.

Conclusion: Summary Insights into Designing UHPC

• Overall strategies discussed highlight how understanding material properties and optimizing mix designs can lead to successful implementation of ultra-high performance concrete across various structural applications.

Understanding Compressible Packing Models

Key Concepts in Packing Density

• The need for effective packing methods is emphasized, particularly using compressible packing models to optimize space.

• Two distinct coefficients, Aij and Bij, are introduced to account for the effects of loosening in larger aggregates on packing density calculations.

• The importance of laboratory testing is highlighted to determine the maximum achievable packing density for various particle sizes.

Random Packing and Its Implications

• Random packing typically results in lower densities than theoretically possible due to the nature of how particles settle without specific arrangement.

• The process of compaction is discussed as a method to improve actual packing density by rearranging particles into more efficient configurations.

Practical Applications in Concrete Mixing

Mixture Optimization Techniques

• MATLAB code can be utilized to plot random packing densities against volume fractions, helping identify optimal mixtures for concrete applications.

• Simplified numerical representations (like 26% or 3.5%) are recommended over complex ratios when communicating with ready-mix operators for clarity and ease of understanding.

Communication Strategies with Operators

• Effective communication with ready-mix plant personnel involves providing clear numerical guidelines based on established grading systems rather than overly detailed specifications.

Incorporating Fibers into Mixtures

Effects of Fiber Addition

• Long fibers integrated into aggregate matrices can displace certain aggregates, potentially reducing overall packing efficiency but enhancing other properties like flexibility.

Finalizing Mixture Compositions

Composition Details and Procedures

• Specific mixture compositions are outlined, including proportions involving fly ash and metakaolin alongside superplasticizers for improved workability.

Mixing Process Overview

• A step-by-step mixing procedure is described that includes adding materials sequentially while ensuring proper saturation and dispersion throughout the mixture.