LESSON 214 min read
Fresh & Hardened Testing
ASTM C143 Slump, C39 Cylinder Compressive Strength, and C109 Mortar Cube testing.
In this lesson: ASTM C143 Slump, C39 Cylinder Compressive Strength, and C109 Mortar Cube testing.
Field & Lab Test Methods
Quality assurance in concrete requires testing both fresh properties (before it sets) and hardened properties (after curing). These tests verify that the concrete meets specification and will perform as designed.
ASTM C143: Slump Test (Fresh Concrete)
The slump cone test measures concrete consistency and workability. A truncated cone (12" tall, 8" bottom, 4" top) is filled with fresh concrete in 3 layers, rodded 25 times per layer, the cone lifted, and the concrete allowed to slump. The slump is the vertical drop from the original cone height.
- True slump (0–4"): Normal, workable concrete. Most structural concrete targets 2–4".
- Shear slump: Indicates a harsh, poorly proportioned mix — retest.
- Collapse slump: Too wet — W/C too high, strength will be low.
- True slump (0–4"): Normal, workable concrete. Most structural concrete targets 2–4".
- Shear slump: Indicates a harsh, poorly proportioned mix — retest.
- Collapse slump: Too wet — W/C too high, strength will be low.
Safety · Slump is NOT Strength
Slump measures workability, not strength. High-slump concrete can be strong (if the workability comes from water reducers, not extra water). Low-slump concrete can be weak (if poorly mixed). Never assume high slump = low strength without knowing WHY the slump is high.
ASTM C39: Cylinder Compressive Strength
The compressive strength test is the primary acceptance criterion for structural concrete. Cylinders (6"×12" or 4"×8") are cast, cured (initially on-site, then in a lab moist room at 73°F), and crushed at specified ages — typically 7 and 28 days.
The cylinder is placed in a hydraulic press and loaded at 35 ± 7 psi/sec until failure. The compressive strength = failure load / cross-sectional area. Reported in psi (or MPa).
Typical specifications: 3,000–5,000 psi for residential, 5,000–8,000 psi for commercial, 10,000+ psi for high-performance.
The cylinder is placed in a hydraulic press and loaded at 35 ± 7 psi/sec until failure. The compressive strength = failure load / cross-sectional area. Reported in psi (or MPa).
Typical specifications: 3,000–5,000 psi for residential, 5,000–8,000 psi for commercial, 10,000+ psi for high-performance.
Key Metric · Key Metric
28-day compressive strength is the standard acceptance criterion. But 7-day breaks (typically 65–70% of 28-day) give early indication of mix performance. If 7-day breaks are low, investigate before 28-day — the problem won't self-correct.
ASTM C109: Mortar Cube Strength
The mortar cube test evaluates cement quality (not concrete). It uses a standard mortar: 1 part cement, 0.485 parts water, 2.75 parts graded standard sand. 2" cubes are cast, cured in a moist room, and broken at 1, 3, 7, and 28 days.
C109 tests the cement itself — eliminating aggregate and admixture variables. It's used for cement certification, comparing cement sources, and QC at the cement plant. Minimum 28-day cube strength for Type I: 2,800 psi.
C109 tests the cement itself — eliminating aggregate and admixture variables. It's used for cement certification, comparing cement sources, and QC at the cement plant. Minimum 28-day cube strength for Type I: 2,800 psi.
ASTM C39 (Concrete Cylinder)
- Tests concrete (full mix)
- 6"×12" or 4"×8" cylinders
- 7 & 28 day breaks
- Acceptance for structural concrete
- Affected by all mix components
ASTM C109 (Mortar Cube)
- Tests cement only
- 2" mortar cubes
- 1, 3, 7 & 28 day breaks
- Cement plant QC & certification
- Eliminates aggregate variables
A shear slump — where one half of the concrete shears off and slides down — indicates the mix is not cohesive. It may be too harsh (insufficient fines), poorly proportioned, or lacking cohesion. The test should be repeated. If shear slump persists, the mix design needs investigation.