Different Tests on Concrete
By Gaurav Nepal
Concrete is simple to make and fiendishly complex to get right on site. After twelve years of supervising foundations, slabs, beams and high-rise frames across Nepal and India, I’ve learned that the difference between a durable structure and one that gives trouble early on often comes down to disciplined testing and practical know-how. This article explains the most important concrete tests, why they matter, and how a site engineer or contractor can apply each test practically — no lab mysticism, just what works on real projects.
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Quick snapshot — tests you must know
1. Slump (workability)
2. Compressive strength (load capacity)
3. Tensile / split tensile (crack resistance)
4. Flexural (bending / serviceability)
5. Surface hardness / rebound hammer (non-destructive)
6. Durability / permeability tests (long-term performance)
7. Water absorption (porosity)
8. Air content (freeze-thaw & workability)
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1. Slump Test — on-site sanity check
Why: Ensures fresh concrete can be placed and compacted properly.
How I use it: Do it at every new truck or new mix. If slump is out of spec, don’t force placement — adjust on site (mix or water) and record the change.
Practical tip: Always run slump near the point of discharge; temperature and transport time can reduce workability.
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2. Compressive Strength — the fundamental measurement
Why: Tells you whether concrete meets the specified strength (design requirement).
How it’s usually done: Lab specimens (cubes or cylinders) cured and tested at standard ages (7, 14, 28 days).
Site practice: Ensure proper curing of specimens — many failures come from poor specimen curing rather than bad concrete. Track strength history for each concrete pour and keep test certificates with the pour record.
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3. Tensile (Split Tensile) — cracking behavior
Why: Concrete is brittle in tension; this test predicts how it will crack under tensile loads.
Site practice: Use split tensile results to check reinforcement design and crack control strategy, especially for slabs and pavements. If tensile strength is low relative to compressive strength, check mix proportion and aggregate quality.
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4. Flexural Strength — pavements, slabs and beams
Why: Important for pavements, rigid floors, and beams where bending governs failure.
Practical tip: For on-site concrete used in pavements, pay attention to flexural results — if low, review aggregate gradation and curing.
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5. Surface Hardness (Rebound Hammer) — quick NDT
Why: Quick, non-destructive estimate of surface strength and uniformity.
How I use it: For large structures where coring every location is impractical, use the rebound hammer to spot areas that may need coring or further inspection. Always calibrate against lab strengths (correlate a few cores) before relying fully on rebound values.
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6. Durability & Permeability Tests — design for the environment
Why: Strength is one thing; durability is another. Permeability, chloride penetration and freeze-thaw resistance tell you how concrete will perform decades later.
Site focus: For structures exposed to chloride (coastal, marine) or de-icing salts, insist on permeability / chloride penetration testing and stricter cover and concrete quality requirements.
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7. Absorption / Water Penetration — porosity matters
Why: High absorption = higher permeability = faster deterioration.
Advice: Use absorption tests for quality checks on precast members and waterproofing critical elements (basements, water tanks).
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8. Air Content — the small bubble that saves concrete
Why: Entrained air improves freeze-thaw resistance and workability. Too much air reduces strength; too little reduces durability.
Site tip: In cold climates or aggressive environments, control air content and keep records per mix batch.
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Practical, field-tested checklist for every pour
Verify mix design & batch ticket before discharge.
Perform slump test on arrival; record results.
Cast standard lab specimens (label with pour location & time).
Start curing specimens immediately and maintain records.
Use rebound hammer for quick surface checks where necessary — but confirm with cores.
Keep a log of all test results tied to the pour number and date.
If a test fails, stop work in the affected area and follow the engineer’s remedial steps — don’t patch over the uncertainty.
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Common on-site mistakes I see (and how to avoid them)
Poor curing of test specimens — leads to false low strengths. Fix: cure with moist burlap or curing compound as per standards.
Adding water on site to increase slump — this reduces strength. Fix: adjust mix design or use approved admixtures.
Ignoring air content in cold climates — increased cracking later. Fix: check air entrainment at batching plant and on site.
Trusting NDT alone — always calibrate with destructive tests (cores).
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Final thought
Testing concrete is not a ritual — it’s a feedback loop. Tests tell you how your materials, batching, transport, placement, compaction, and curing interact. A disciplined testing routine and honest record-keeping are the backbone of durable, safe structures.
If you want, I can convert this into a printable field checklist, a short training slide deck for site staff, or a LinkedIn article. Reach out to me for consulting or detailed templates.
📧 damugaurav92@gmail.com
🌐 www.gauravn.com.np
🔗 LinkedIn: https://www.linkedin.com/in/gauravnepal1
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