UHPC Mix Design Essentials — Raw Materials, Ratios & Strength Grades

1. Why Mix Design Is the Core of UHPC Performance

UHPC (Ultra-High Performance Concrete) achieves compressive strength above 120 MPa and exceptional durability through optimized particle packing, low water-binder ratios, and fiber reinforcement. Unlike conventional concrete, UHPC mix design is not a one-size-fits-all recipe — every kilogram of cement, silica fume, quartz sand, and steel fiber directly shapes the final strength, density, and workability.

This guide distills the mix design essentials based on GB/T 31387-2025 "Mineral Admixtures for High-Strength and High-Performance Concrete" and current industry practice, giving engineers a clear framework from raw material selection to trial mix verification.

2. UHPC Grade Classification by GB/T 31387-2025

Strength Grade	Compressive Strength (150 mm cube)	Typical Application
UHPC 120	≥ 120 MPa, < 150 MPa	Façade panels, non-load-bearing decorative components
UHPC 150	≥ 150 MPa, < 180 MPa	Bridges, high-rise connections, structural reinforcement
UHPC 180	≥ 180 MPa, < 220 MPa	Heavy-load bridges, seismic-critical nodes
UHPC 220	≥ 220 MPa	Defense, nuclear, ultra-long-span bridges

The grade naming follows the convention used in JGJ/T 423-2018 and ACI PRC-239-18, ensuring compliance across Chinese and international projects.

3. Raw Material Selection — Five Core Ingredients

Cement: P·II 52.5R or P·O 52.5 silicate cement, low-alkali, with 28-day strength ≥ 60 MPa. Avoid high-C₃A cement that may cause flash set.
Silica Fume: SiO₂ content ≥ 92%, specific surface area ≥ 18,000 m²/kg, particle size 0.1–0.3 μm. Replaces 20–35% of cement by mass.
Quartz Sand: Graded silica sand, particle size 0.1–1.25 mm, SiO₂ ≥ 99%, moisture content < 0.2%. Optimal packing uses 3–4 gradings.
Superplasticizer: Polycarboxylate (PCE) high-range water reducer, water reduction ≥ 30%, low air-entrainment to maintain density.
Steel Fiber: Straight or hooked-end, length 13–20 mm, diameter 0.2–0.3 mm, tensile strength ≥ 2,000 MPa. Dosage typically 2–3% by volume.

4. Water-Binder Ratio (W/B) — The Critical Parameter

W/B is the single most influential variable in UHPC mix design:

W/B Range	Compressive Strength	Workability	Use Case
0.14–0.18	≥ 180 MPa	Low (casting required)	UHPC 180 / 220 grades
0.18–0.22	150–180 MPa	Medium (vibration needed)	UHPC 150 grade
0.22–0.27	120–150 MPa	High (self-compacting)	UHPC 120 grade, façade panels

Lower W/B ratios demand higher superplasticizer dosage (typically 1.5–2.5% of binder mass) and longer mixing time (6–8 minutes minimum).

5. Particle Packing Optimization

UHPC relies on the modified Andreasen model or Furnas gap-graded model to achieve maximum particle packing density. The target is to fill voids between cement particles (10–50 μm) with silica fume (0.1–0.3 μm), then bridge the gaps with quartz sand (0.1–1.25 mm).

A well-optimized UHPC mix achieves packing density ≥ 0.78, leaving less than 22% voids to be filled by hydration products and steel fibers.

6. Trial Mix Verification Procedure

Step 1 — Compute ratios: Calculate starting mix using software (e.g., EMMA, Davidovits). Target packing density ≥ 0.78.
Step 2 — Lab trial: Mix 8–10 L batches. Test flow (ASTM C1856 / GB/T 31387), 7-day and 28-day compressive strength.
Step 3 — Adjust: If flow < 150 mm, increase superplasticizer in 0.1% increments. If strength falls short, reduce W/B or increase silica fume.
Step 4 — Scale-up: Validate at production scale (≥ 1 m³). Check fiber dispersion uniformity and surface finish.
Step 5 — Freeze-thaw / durability: Per GB/T 31387 and ASTM C666, verify ≥ 200 freeze-thaw cycles with strength loss < 5%.

7. Common Pitfalls to Avoid

Using ordinary river sand instead of graded quartz sand — defeats particle packing.
Adding water on-site to improve flow — destroys W/B ratio and compressive strength.
Skipping fiber dispersion testing — leads to fiber balling and weak zones.
Overlooking curing temperature — UHPC needs ≥ 60 °C steam curing or 20 °C standard curing for ≥ 7 days to reach ultimate strength.

8. About Guangdong Qinglong Construction Engineering Co., Ltd.

Founded in 1998, Qinglong is a leading UHPC & GRC smart fabrication specialist based in Foshan, Guangdong. With four production bases and a 100,000 m² annual capacity, Qinglong delivers mix-design-optimized UHPC components for bridges, façades, and architectural features. Every batch is tested in our in-house lab against GB/T 31387-2025 and JGJ/T 423-2018.

Get In Touch:
Address: No. 38 Sangtian Road, Yanbu Section, Daliang, Shunde District, Foshan, Guangdong, China
Phone / WeChat: 189 2489 7575
Website: www.grc-uhpc.com

9. Frequently Asked Questions

Q1: What is the minimum compressive strength defined as UHPC?
A: Per GB/T 31387-2025 and JGJ/T 423-2018, UHPC requires compressive strength ≥ 120 MPa on 150 mm cube specimens, with durability (freeze-thaw ≥ 200 cycles) and tensile performance criteria.

Q2: Can I use fly ash instead of silica fume?
A: Fly ash can replace 10–20% of cement for sustainability, but cannot fully substitute silica fume. UHPC requires silica fume for the pozzolanic reaction that drives ultra-high strength.

Q3: Why does UHPC need steam curing?
A: Standard curing at 20 °C rarely reaches the ultimate strength. Steam curing at 60–90 °C accelerates pozzolanic activity and produces denser hydration products — essential for 28-day strength above 150 MPa.

Q4: How much steel fiber is needed per cubic meter?
A: Typical dosage is 2–3% by volume = 156–234 kg per m³ (steel fiber density 7,850 kg/m³). Higher dosage improves tensile strength but reduces workability.

Q5: Is UHPC mix design covered by a Chinese standard?
A: Yes — GB/T 31387-2025 "Mineral Admixtures for High-Strength and High-Performance Concrete" and JGJ/T 423-2018 "Technical Specification for Reactive Powder Concrete Structures" provide the authoritative framework.

Q6: Can UHPC be pumped?
A: Yes, with optimized mix and pressure-booster pumps, but steel fibers must be added at the nozzle to avoid balling. Maximum delivery distance: ~120 m horizontally.

Author: Qinglong UHPC Technical Center | Reviewer: Senior Engineer Wang | Standards: GB/T 31387-2025, JGJ/T 423-2018, ASTM C1856

UHPC Mix Design Essentials: Raw Materials, Ratios, and Strength Grading (GB/T 31387-2025)