Achieving Ultra-High Performance Concrete by Using Packing Models in Combination with Nanoadditives

Area of Science:

• Materials Science
• Civil Engineering
• Concrete Technology

Background:

• Ultra-high-performance concrete (UHPC) design relies on packing models to simulate particle interactions and achieve desired compactness.
• Discrete packing models offer superior simulation of particle packing, including interactions via loosening and wall coefficients, crucial for nanometric particle sizes.
• The compaction index relates virtual and real compactness, simulating the energy input during particle placement in molds.

Purpose of the Study:

• To evaluate the effectiveness of discrete packing models in UHPC design, considering particle interactions and compactness.
• To investigate the influence of nanoadditives and cement hydration on UHPC properties beyond maximum packing.
• To establish relationships between mixture composition, compactness, and mechanical properties (compressive strength) at different ages.

Main Methods:

• Characterization of aggregates and additions (dry/wet compactness, granulometry, density, absorption).
• Numerical implementation in a polydisperse packing model to determine mixture compactness.
• Establishment of fixed percentages for nanoadditives in compactness calculations.
• Testing compressive strength of UHPC mixes at 7 and 28 days.

Main Results:

• Discrete models accurately simulate particle packing and compactness, enabling low water-cement (w/c) ratios near 0.18.
• Compressive strength at 7 days is directly proportional to calculated compactness.
• At 28 days, mixes with lower cement content, fewer additions, and lower compactness (φ = 0.775) showed superior compressive strength (124.6 MPa) compared to higher compactness mixes (φ = 0.789, 121.7 MPa).

Conclusions:

• While maximum packing is important, cement hydration and nanoadditive effects are critical for UHPC performance.
• The proportion and type of nanoadditives significantly impact results, with optimal ratios potentially yielding better performance at lower compactness levels.
• The study highlights a trade-off in UHPC mix design, where lower cement and addition content can lead to better long-term strength despite initial lower compactness.