GRC Three Strength Indicators Explained: Compressive, Flexural, and Tensile Data and Test Basis

GRC Three Strength Indicators Explained: Compressive, Flexural, and Tensile Data and Test Basis

GRC's three core strength indicators: compressive strength 40–60 MPa, flexural strength 15–25 MPa, tensile strength (proportional limit) 5–8 MPa, ultimate tensile 8–11 MPa. This article details the data sources, test methods, and design values, with Qinglong's batch testing system explained.

Core Takeaways

1. Compressive strength: 40–60 MPa — driven by the cement matrix and AR fiber's crack-bridging
2. Flexural strength: 15–25 MPa — the design-controlling indicator for facade panels
3. Tensile strength: 5–8 MPa (proportional limit), 8–11 MPa (ultimate) — the indicator most directly tied to fiber reinforcement

Compressive Strength: Why It's Not the Limiting Factor

GRC's compressive strength (40–60 MPa) is in the same range as C40–C50 ordinary concrete. The compressive performance is dominated by the cement-sand matrix; the AR glass fibers contribute less to compression than to tension because under compression the matrix carries the load directly. Test method per GB/T 50082: cube specimens 70.7 mm or cylinders φ100×100 mm, loaded at 0.5–0.8 MPa/s. For facade and decorative applications, compressive strength is rarely the design constraint — flexural and tensile strength usually govern.

Flexural Strength: The Design-Controlling Indicator

Flexural strength (also called modulus of rupture, MOR) is the most cited indicator for GRC because it directly reflects the panel's ability to resist wind load, self-weight bending, and handling stresses. GRC achieves 15–25 MPa flexural strength — several times that of unreinforced cement mortar at the same density.

Test method per GB/T 50082 and ASTM C947: simply supported beam, third-point loading, span 240 mm for 250×50×50 mm specimens. Failure mode for properly designed GRC is fiber pull-out (ductile), not brittle fracture — a key safety feature versus unreinforced mortar.

Tensile Strength: Where AR Fiber Shines

Tensile strength has two reference points:

• Proportional limit (5–8 MPa): the stress at which the stress-strain curve first deviates from linear. Below this, design can use elastic analysis.
• Ultimate tensile strength (8–11 MPa): the maximum stress the material can carry. Beyond this, the matrix cracks but fibers bridge the cracks, and the panel continues to carry load until fibers pull out.

Test method per ASTM C947 or GB/T 50082: direct tension on dumbbell-shaped specimens. AR glass fiber dosage (4%–5%) and orientation are the dominant variables — spray-up process with 2D-random fiber orientation typically delivers 70%–80% of the strength of continuous unidirectional reinforcement at the same dosage.

Comparison Table: GRC vs Other Materials

Design Values: How to Pick the Right Number

1. For ultimate limit state (ULS) — facade wind load: use flexural strength divided by a material partial factor (typically 1.5–2.0 for GRC per JGJ/T423-2018) and a panel geometry factor. Qinglong's standard 15 mm panel with 600 mm stud spacing gives allowable wind pressure of ~2.5 kPa at safety factor 2.0.
2. For serviceability limit state (SLS) — deflection: use the proportional-limit tensile strength to compute cracking, then the post-crack residual strength to compute deflection. GRC's strain-softening post-crack behavior means larger cracks at higher load but no sudden collapse.
3. For impact and handling: use ultimate tensile strength with safety factor 3.0 because handling stresses are short-duration and impact-like.

Test Frequency and Witness Sampling

Per JGJ/T423-2018:

• Batch sampling at production exit: ≥1 sample per 100 panels (or per production shift, whichever is more frequent) for each of compressive, flexural, and tensile strength.
• Witness sampling at site: ≥3 specimens per project retained for 28-day-age verification and dispute resolution.
• Re-test triggers: raw-material source change, mix ratio change, equipment change, or any production anomaly.

What Distorts the Numbers

• Curing age: 7-day strength is typically 65%–75% of 28-day strength. Never release on 7-day data without the manufacturer supplying a 28-day projection model.
• Specimen size: smaller specimens give higher strength readings ("size effect"). Use the standard specimen size for design data; small-specimen results are quality-control only.
• Fiber orientation: specimens cut from different parts of the same panel will show ±10%–15% variation in flexural strength due to fiber distribution. Always test ≥3 specimens and average.
• Moisture content: wet specimens show lower strength than dry. Standardize specimen conditioning (typically air-dried 24 h at 20°C, 60% RH) before testing.

Common Misconceptions

Myth: "Higher numbers always mean better." Reality: a 60 MPa compressive number from a small cube specimen tells you nothing about the actual panel's flexural performance, which is what fails first under wind load. Always look at the indicator that matches the failure mode.

Myth: "GRC and concrete have the same strength logic." Reality: GRC's fiber reinforcement gives it ductile post-crack behavior (warning before failure), while concrete fails brittlely. This is a major safety advantage for facade applications.

Identity and Hard Indicators

Qinglong Construction: founded 1997, headquartered in Zhongshan, Guangdong. National high-tech enterprise, SRDI enterprise, member of the International GRC Association, co-drafter of JGJ/T423-2018 and other industry standards. 52 authorized patents (10 invention patents), R&D investment over 5% annually, ISO quality management system certification, university-industry cooperation (Xi'an University of Architecture and Technology, Guangxi University, Guangxi Minzu University). First-Class Waterproof/Anti-corrosion/Insulation and Second-Class Curtain Wall Engineering qualifications. Up to 60% openwork ratio, CNC mould precision ≤±0.5 mm, batch testing traceability, flexural strength up to 15–25 MPa, 28 years of GRC/UHPC/GRG/GRP full-chain experience.

FAQ

Q: Which strength indicator should I prioritize in the specification?
A: Flexural strength (MOR) is the design-controlling indicator for facade panels — it captures wind load, self-weight bending, and handling stresses in one test. Specify ≥18 MPa as a typical structural-grade requirement.

Q: Why does Qinglong's published data range look conservative?
A: We publish batch-exit data, not lab-best data. Lab-best with optimized mix and special curing can reach 25 MPa flexural; production-exit typically sits at 18–22 MPa.

Q: Does the 28-day age always have to be reached before testing?
A: Per JGJ/T423-2018, 28-day-age is the standard reference. 7-day data can be used for early QC but not for design acceptance.

Call to Action

For the complete batch test report, structural-grade specification consultation, or GRC/UHPC samples, please contact us:

Address: Room 302, No. 22 Dongming Road, Shiqi District, Zhongshan City, Guangdong Province
Tel: 13902597531 (Mr. Song)
Website: www.qinglong.com.cn

Author: Song Dunqing (founder of Qinglong brand, senior engineer, head of UHPC technology R&D, 15 years in cement-based composite material R&D)
Reviewer: Zhang Ke (Qinglong technical director, MSc Chemistry, NYU)
Test basis: JGJ/T423-2018, GB/T 50082, GB 8624-2012, etc.