Ultra high strength and high strength concrete

HPC uses water-cement ratios below 0.35, while UHPC drops this to 0.20-0.25. You'll need supplementary cementitious materials (SCMs) like silica fume, fly ash, or metakaolin to fill voids between cement particles. Silica fume is crucial for UHPC—it's 100 times finer than cement and creates an incredibly dense matrix.

For aggregates, HPC allows normal coarse aggregate up to 19mm, but quality matters enormously. The aggregate must be stronger than your target concrete strength, so you'll often use crushed granite or basalt. UHPC eliminates coarse aggregate completely, using only fine sand with particles smaller than 600 micrometers.

Cement: You'll use high-grade Portland cement (Type I or III) as your primary binder. For UHSC, cement content ranges from 400-800 kg/m³ - much higher than normal concrete's 300-400 kg/m³. The cement provides the chemical reaction that creates strength, but alone it's not enough for ultra-high performance.

Silica Fume: This is your secret weapon for UHSC. You'll add 15-25% of cement weight as silica fume (typically 100-200 kg/m³). It's incredibly fine - 100 times finer than cement - and fills microscopic voids while providing additional binding through pozzolanic reactions. Without silica fume, you can't achieve ultra-high strengths.

Workability and Flow
Ultra high strength concrete (UHSC) typically has slumps between 150-250mm, but slump alone doesn't tell the whole story. The high paste content and superplasticizers create a sticky, cohesive mix that flows differently than normal concrete. You'll often see flow table tests (ASTM C1437) used instead, targeting 200-300mm spread.

The key challenge is maintaining workability long enough for placement while preventing segregation. UHSC mixes lose workability faster than normal concrete - you've got maybe 45-90 minutes depending on temperature and admixture dosage.